Introduction

The problem of underground heat hazards is becoming increasingly serious due to the increasing depth of mine mining. High-temperature environments not only pose serious threats to the health of workers but also significantly impact their work efficiency and the economic profitability of mines1,2. Mechanical refrigeration cooling technology is typically employed to manage high-temperature mine heat hazards3. Consequently, enhancing the cooling effect of the refrigeration system and minimizing the system’s energy consumption have emerged as two pivotal concerns within the realm of mine cooling.

As the final link of underground thermal environment control, air distribution directly impacts the thermal health state of underground workers. Additionally, the utilization efficiency of the system’s cooling capacity is directly affected by the advantages and disadvantages of air distribution. Efficient utilization of cooling capacity can lead to savings in both the design capacity of air conditioning and refrigeration equipment and operational energy consumption. In conventional mine cooling practices, the Full air cooling technology (FACT) is typically adopted4. This method, grounded in the concept of uniform environmental control, fails to differentiate between the characteristics of the controlled environmental area. Consequently, it results in low cooling capacity utilization and an inability to guarantee cooling effectiveness in the workers’ location5. On the other hand, non-uniform environmental control in underground settings is more tailored to meet the cooling requirements of the workers’ location. Compared to FACT, non-uniform environmental control is advantageous in enhancing underground air quality and increasing cooling capacity utilization efficiency6,7. This approach aligns better with the actual underground environment’s characteristics, offering a more effective and sustainable cooling solution for mines.

Head-neck local cooling control is of great significance to the control of mine workers, taking into account the spatial characteristics of the long-narrow underground space and the characteristics of workers. Experimental studies by many scholars have shown the cooling effect of the head-neck is the highest among all parts of the body. At normal body temperature or in a quiet state, the head, being thermogenic, requires constant cooling as indicated by its temperature being slightly higher than the body’s rectal and aortic temperatures. Cooling the head has been found to reduce the sweating rate significantly. Studies by Nunneley et al. and Kissen et al. demonstrated that cooling the head, even when only 8% of the body surface area is involved, is more effective than whole-body cooling in suppressing sweating8,9. Cooling the head also leads to a slower acceleration of heart rate, as shown by Shvartz10, who found that head cooling reduces heart rate variability in high-temperature environments. These results underline the significance of head-neck local cooling in enhancing the thermal comfort and productivity of workers, particularly in high-temperature working environments like mines where non-uniform environmental control can be more beneficial. Moreover, while the head represents only 1/10 of the body’s surface area, its cooling effect on the deep body temperature is substantial due to its ability to take away a significant amount of heat from the body. Therefore, head cooling can contribute to improving thermal comfort and productivity outcomes. Research by Nunneley demonstrated that combining head cooling with body heating during power meter exercise in a 30 °C environment enhances head comfort and work efficiency more effectively than other combinations11. Meanwhile, power meter exercise results showed that head cooling with body heating significantly improved work efficiency, approaching the effect of simultaneous cooling. Additionally, cooling the head can increase the body’s heat tolerance limit by 1 ℃, thereby extending the duration of physical activity. Notably, the neck area offers a high potential for heat dissipation and plays a crucial role in local cooling control of the body. Cooling the neck during moderate-intensity activities in hot environments can dissipate and account for 1/3 of metabolic heat production and an overall body heat load of 1/412. Consequently, in the context of local cooling control for the human body, cooling the head-neck holds substantial engineering applications due to their high cooling efficiency and significant impact on thermal regulation and work performance.

The interaction between the jet ventilation of the head-neck and the crossflow (mainstream ventilation) of the mine roadway generates the phenomenon known as Jet in Crossflow (JICF), which involves injecting a jet flow into a crossflow environment through an exit, such as a nozzle or orifice13. This interaction leads to the emergence of complex and diverse flow phenomena. JICF has diverse applications across various engineering domains, including engine air film cooling14,15, combustion mixing in combustion chambers16,17, dispersion of pollutants in rivers or chimneys18,19, fluid-thermal mixing processes in T-tubes20,21, ventilating control in crossflow environments22,23, and biomedical applications24,25, among others. The flow field of JICF is highly complex and influenced by factors such as the jet ratio (Momentum ratio of jet to crossflow), angle of incidence, Reynolds number, the geometric structure of the jet exit and its arrangement, and its spatial scale. For instance, when considering issues such as confined spatial scales and buoyancy, JICF encompasses free jets26,27, semi-confined jets28,29, confined jets30,31, isothermal jets32,33, and non-isothermal jets34,35. Among these factors, the jet ratio plays a decisive role in determining the structure of the JICF flow field when the spatial geometry and physical parameters of the jet and crossflow are fixed32,36,37,38,39. In a long-narrow confined mine space, the interaction between cold air jets, crossflow ventilation, and space walls results in a complex flow field structure, displaying intricate, non-constant, three-dimensional, and diverse characteristics37. This leads to the formation of various flow phenomena such as attached-wall jets, deflected jets, impinging jets, and accompanying flows40,41,42,43. However, current research on JICF has primarily focused on free-space scenarios, leading to a relative lack of understanding regarding the flow pattern and characteristics of JICF in restricted spaces. This study delves into the flow characteristics of a single jet. In practical mine ventilation, multiple jets are often involved, necessitating further investigation into the interactions and superposition effects of multi-jet flows, leading to complex flow phenomena. However, understanding the fundamental flow structure and behavior patterns of a single jet is crucial for studying the superposition effects of multi-jets under more complex conditions. The basic principles revealed in the study of single jets provide a theoretical basis for investigating multi-jet flows and offer a new perspective for optimizing the design of local ventilation systems.

According to the findings shown above, the traditional full-air cooling mode wastes a significant amount of cooling capacity while providing an insignificant cooling effect. This study proposes a head-neck local cooling mode for mine workers to reduce energy, enhance cooling capacity utilization, and meet workers’ thermal comfort needs. Based on the microclimate control range of mine workers’ head-neck, the concepts of effective and ineffective cooling zones are given as air distribution control boundaries for the head-neck cooling zones. This cooling mode employs the JVIC air distribution44 form to provide local cooling to the head-neck, avoiding the waste of cooling capacity in the ineffective cooling zone. The flow visualization technique is used to get JVIC flow evolution characteristics in a confined space, as well as to clarify the application of the air distribution. The flow evolution characteristics of JVIC are determined under isothermal conditions, without considering additional air movements generated by machinery and operators. Finally, the research suggests an evaluation model for non-uniform environmental control that may be used to systematically examine the real effect of cooling mode as well as the degree of cooling capacity usage.

Local ventilation mode principles in long-narrow mine working face

Objectives and strategies for regulating the thermal environment in long-narrow mine workings face

Control objectives and specific characteristics of the underground environment

Objectives of environmental underground control

The main objectives of underground environmental control include ensuring safety and comfort in the workplace. Safety encompasses the extensive ventilation network and equipment within the mine to ensure full ventilation of the underground space. This process aims to increase oxygen concentration, effectively dilute, and remove contaminants, as well as reduce dust concentrations, among other factors, to maintain a hygienic and safe working environment. On the other hand, the need for comfort mainly relates to the high temperature working areas underground. In high-temperature environments, the laborer’s need for cooling becomes a key consideration. When traditional non-mechanical cooling means are ineffective in achieving the desired heat removal and cooling effect, mechanical cooling means are usually used. This approach, as shown in Fig. 1a, provides a feasible cooling solution for laborers based on a comprehensive consideration of cooling effect and economy. The underground cooling system is centralized and operates by transporting low-temperature chilled water produced by the refrigeration unit to the air cooler. The hot air drawn in by the fan exchanges heat with the chilled water, and the cooled air is then channeled through air ducts to the inlet air roadway to cool the working face by lowering the inlet air temperature. However, this traditional method is limited to localized cooling at the working face, and its cooling effect is not ideal for long-distance mining working faces.

Fig. 1
figure 1

Thermal environment characteristics and control diagram of mining working face: (a) mechanical cooling system, (b) wall temperature distribution of roadway and air duct, (c) characteristics of underground working environment, (d) schematic diagram of 411-mining working face.

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In the underground environment, safety is of utmost importance and is a primary objective that must be guaranteed in establishing safety and comfort requirements. The total ventilation network facilities throughout the mining cycle of a mine play a crucial role in ensuring the life safety of mine workers. Consequently, a safety-focused roadway lateral airflow known as mainstream ventilation exists in the mine roadway. Mainstream ventilation, as understood in this paper, entails roadway crossflow that impacts jet deflection. The air velocity associated with crossflow ventilation must adhere to relevant normative requirements, such as those outlined in the China Coal Mine Safety Regulations45, which specify a permissible air velocity range of 0.25–4 m/s for the mining working face. This specified range considers the unique conditions of the mining working face to ensure an adequate air supply for moderate ventilation. Accordingly, the design and management of crossflow ventilation are pivotal in underground environmental control, serving as a critical element in ensuring safety and fostering a safe and efficient underground working environment.

To ensure the thermal comfort of workers in high-temperature thermal environments, cooling measures are essential when environmental parameters reach specific thresholds. As per China’s Coal Mine Safety Regulations45, operations must cease if the air temperature in the mining face exceeds 30 °C or if the temperature in the refuge of electromechanical equipment surpasses 34 °C. In situations where the ambient temperature fails to meet the specified criteria, mechanical cooling methods must be implemented. Typically, in mining settings, mechanical cooling involves blending cold air jets with crossflow ventilation to regulate the underground environment. Despite cold air jets having a lower flow rate than crossflow ventilation, the energy consumption associated with their cooling process is significant. Hence, maximizing the cooling capacity of underground spaces holds considerable importance.

Characterization of the underground controlled environment

Spatial characteristics The structure of the underground space shows the form of an open, long-narrow, pipe-like layout, with the cross dimensions of the pipes being significantly larger than the radial dimensions. Within this spatial configuration, crossflow ventilation primarily moves in the cross-direction of the roadway. However, a notable challenge arises from the low-speed crossflow environment within the ventilation area of a mine roadway, particularly about jet ventilation. Furthermore, the continuous mining activities result in the perpetual evolution of the working face, leading to the thermal and humid environmental conditions showing temporal and spatial variations within both the roadway space and its interior. Consequently, the thermal environmental management within the mine must possess the capability to adapt and adjust flexibly to fluctuations in temperature and humidity across different areas and times of the day. This adaptability is crucial for ensuring that the working environment consistently maintains a safe and comfortable state.

Characterization of the thermal environment The combined effect of several factors is the main cause of mine heat hazard. As mining depth increases, the temperature of the surrounding rock gradually rises. Simultaneously, the airflow is influenced by wall heating and humidification during the flow process. Heat dissipation from equipment, materials, and mined-out areas also contributes to the uneven distribution of temperature and humidity as the airflow laterally progresses. When considering only the heating effect of the surrounding rock on the airflow, the temperature of the cooler airflow will steadily increase exponentially and eventually converge with the temperature of the surrounding rock. This trend can be mathematically expressed by Eq. (1).

$$t_{z} = t_{m} - \left( {t_{m} - t_{0} } \right)\exp \left( { - \frac{4hL}{{1000du_{{\text{m}}} \rho C_{{\text{m}}} }}} \right)$$
(1)

where tm is the temperature of surrounding rock (℃), t0 is the crossflow temperature (℃), h is the convective heat transfer coefficient between the crossflow and the wall of the enclosing rock (W·m−2·℃−1), L is the length of the roadway (m), d is the equivalent diameter of the roadway (m), um is the crossflow velocity (m·s−1), ρ is the crossflow density (kg/m3), Cm is the crossflow specific heat capacity (kJ·kg−1·℃−1).

Figure 1b shows the wall temperature distribution of the roadway and air duct at the position of 10m from the head-on in the excavation face of Banshi 52305 in Hunchun City, Jilin Province. The floor temperature here is the highest, attributed to the exothermic heat release from the comprehensive excavator and belt conveyor. Moving further along the roadway, ranging from 70 to 450m from the head-on, the roof temperature is the highest. In this span, the temperature rise of the left and right walls exceeds that of the floor along the length of the roadway. The temperature of the air duct wall is directly related to the air temperature in the roadway. The trend of temperature rise is similar to the left wall temperature before 200 m and gradually aligns with the right wall temperature after 200 m. Beyond the 650 m mark from the head-on, the temperature variation across the four walls of the roadway tends to stabilize, leading to a convergence in temperature differences among the walls.

Worker characteristics Mine workers engage in strenuous physical labor in confined mine spaces, necessitating the use of safety precautions such as work clothes, rain boots, helmets, and gloves to protect themselves from the external environment. Notably, the exposed skin of workers is mainly concentrated on the head-neck. Given the workers’ high metabolic rates and unique demands for adaptability and thermal comfort in this challenging work environment, specialized safety measures are crucial. The movement of these workers is constrained by the mechanical equipment and long-narrow mine space structure. As shown in Fig. 1c, the workers are located on the sidewalk under the hydraulic support, with their trajectories typically following a reciprocating cross-directional motion near-linearly along the roadway.

Cooling technology for long-narrow mining working face

Two main technology paths commonly cool the mining working face: FACT and LSCT. Although both modes offer reliable cooling results, each has its distinct advantages and challenges.

FACT

In the underground mechanical refrigeration thermal environment control, refrigeration equipment is typically installed in the inlet air roadway. A cold air jet is injected radially into the crossflow through an air cooler and then mixed with the crossflow into the working face, as shown in Fig. 2a. This method resembles the mixing ventilation approach used in building air conditioning systems that target dilution. However, this FACT does not take into account the specific location of the workers, resulting in a broad air cooling effect and the need for substantial cooling capacity. Consequently, a significant portion of the cooling capacity remains underutilized. Upon the cold air’s entry into the back-mining working face, it heats exchange with the surrounding rock, materials, and equipment, leading to rapid heating and inadequate cooling efficiency. The insufficient utilization of the cooling capacity of FACT mode results in considerable energy wastage. For instance, this issue is evident in the case of the 411-mining working face in Longjiabao mine, China.

Fig. 2
figure 2

Schematic diagram of cooling mode and environmental parameters of mining working face: (a) FACT, (b) long-distance Segmented cooling technology (LSCT), (c) variation of airflow temperature along the airflow direction, (d) variation of relative humidity and moisture content of airflow along the airflow direction.

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Longjiabao Mining Company, situated in Jiutai City, Jilin Province, has varying ground elevations ranging from the highest point at + 244m to the lowest at + 189.3m. The average ground temperature gradient across the region falls between 1.2 to 3 ℃/100 m, with mining operations reaching depths of 930 to 1215 m. The ground temperature spans from + 11.16 to 36.45 ℃. The 411 mining working face is situated within the – 880 m–− 950 m levels in the mine’s fourth mining sections, focusing on extracting coal from the No.2 coal seam with an average thickness of 10 m. Employing a single-strike long-distance integrated mechanized roof-placing coal mining method, the coal mining operation maintains a mining height of 3.3 m and a coal release height of 6.7 m, with roof-placing steps spaced at 0.8 m intervals. The elevation of the working face ranges from – 950 m to – 740 m, spanning approximately 800 m in roadway length, while the average initial coal body temperature stands at 34 ℃. The specific layout of the 411-mining working face is depicted in Fig. 1d, with measurement points spaced 200m apart in the inlet and outlet air roadways, as well as 30m apart within the working face, totaling 19 strategically placed measurement points.

Figure 2c shows the variation of dry-bulb temperature, wet-bulb temperature, and wet-bulb globe temperature (WBGT) index of the airflow along the airflow direction in the 411-mining working face. Figure 2d shows the variation of airflow relative humidity and moisture content along the direction of airflow. Meteorological parameters exhibit the most drastic changes in the working faces among the inlet air roadway, working faces, and outlet air roadway, followed by the inlet air roadway. In contrast, the meteorological parameters in the outlet air roadway least variable. The dry-bulb temperature and relative humidity of the inlet airflow of the inlet air roadway are lower, and the temperature and moisture potential difference between the surrounding rock wall and the airflow is larger. Compared with the external aperture of the inlet air roadway, the dry-bulb temperature of the airflow at the internal aperture increased by 0.4°C, the moisture content increased by 3.168 g/kg, and the WBGT index increased by 1.43 °C. At this point, the relative humidity of the airflow changed the most, increasing from 77.47% to 89.30% at the outer entrance. In the back-mining working face, electromechanical equipment is concentrated, and the temperature of the surrounding rock wall is high. This causes the dry-bulb temperature of the airflow at the external aperture of the outlet air roadway to rise by 3.60 ℃, the moisture content is elevated by 4.817 g/kg, and the WBGT index is increased by 3.42 ℃ compared to the external aperture of the inlet air roadway. The dry-bulb temperature and relative humidity of the airflow at the internal aperture of the outlet air roadway were 30.6 °C and 90.04%, respectively. The temperature and humidity potential differences between the airflow and the wall of the outlet air roadway were relatively small. Moreover, the meteorological parameters in the outlet air roadway tended to flatten out. The operating environment of the 411-mining working face is classified as level III of high-temperature operation based on the WBGT index measurements of the inlet air roadway, the working face, and the outlet air roadway within 310m from the internal aperture. In this area, the WBGT index exceeds 25 ℃ in the inlet air roadway, working face, and outlet air roadway. The working time in this high-temperature environment is approximately 8 h. The temperature of the inlet airflow at the working face has exceeded the relevant provisions of the Coal Mine Safety Regulation in China45, with a dry-bulb temperature of 26.4°C at the internal aperture of the inlet air roadway. As a result, enhancing the cooling efficiency of the mechanical refrigeration system warrants exploring a more intelligent and energy-saving cooling strategy.

LSCT

For extra-deep, long-distance mining working faces with severe heat hazards, it is necessary to address the challenge of reducing the temperature of the intake airflow to 0–5 °C to ensure that the airflow temperature at the end of the workings remains at 26 °C. However, achieving such a significant temperature reduction poses both technical and hygienic challenges. The substantial temperature difference between the airflow and the surrounding rock near the working face can lead to excessive heat release from the rock, necessitating a considerable cooling capacity. To tackle this issue, an LSCT involving the supply of cold air along the working face can be implemented. Figure 2b illustrates this strategy, in which a series of ventilation holes are opened in the air duct that traverses the working face46. This method optimizes cooling efficiency and helps mitigate the thermal challenges associated with deep, long-distance mining working faces.

LSCT demonstrates superiority in control range over FACT by implementing a segmented cooling supply that delivers cold air in a step-by-step manner, forming a cold air lake that covers an extended distance. This approach effectively supplies cold air to the working face, thereby mitigating temperature differentials and preventing issues arising from excessive temperature variations. The segmented cooling supply allows for precise temperature control on the working face, thus enhancing the overall cooling effectiveness. Consequently, LSCT serves as a viable solution for achieving efficient cooling in the high-temperature conditions prevalent in deep mines. Despite its advantages, the current local cooling technology in mines lacks comprehensive research regarding air distribution at the cold air jet’s endpoint. The existing mode fails to account for the impact of crossflow within the mine roadway on jet ventilation and neglects to fully consider the relationship between workers’ movement patterns and air distribution dynamics. Consequently, enhancements in cooling effectiveness and utilization of cooling capacity in the controlled thermal environment post-cooling remain limited. In contrast, LSCT considers the deficiencies of the current cooling mode and solutions to address these challenges.

Local ventilation mode based on head-neck cooling

An innovative cooling strategy known as segmental local cooling ventilation has been developed for long-narrow mine working faces to align with workers’ movement trajectories in confined mine spaces. This technology involves using air ducts for long-distance segmental cooling. By deflected and superimposed multi-stage cold air jets, a continuous cooling protection zone is created at head-neck height, ensuring that cooling capacity is effectively utilized.

The necessity of head-neck cooling

The head-neck local cooling mode is a control method of air distribution that is based on the concept of non-uniform environmental control for on-demand cooling. Underground workers, particularly those located in the mining working face, tend to a relatively fixed movement trajectory, moving along a straight line parallel to the mining working face while working under hydraulic support. Therefore, by targeted regulating airflow, the cold air jet can be supplied centrally to the workers’ head-neck to achieve local cooling. This approach allows for precise and effective thermal environment control by adjusting the cooling area following the worker’s trajectory, thereby enhancing overall cooling efficiency.

The specific characteristics of mine workers, such as exposed skin areas like the head-neck, are highlighted in Fig. 3a as effective cooling zones where the surrounding airflow can be Targeted cooling to enhance effectiveness. Consequently, the region surrounding the head-neck along the worker’s movement path is identified as the effective cooling zone, while other regions are categorized as the ineffective cooling zone. Given the spatial constraints posed by the arrangement of underground equipment, the actual cooling requirement for workers is relatively confined. Therefore, by precisely regulating the cooling area covered by the cold air jet, it is possible to achieve localized and efficient utilization of the cooling capacity.

Fig. 3
figure 3

Schematic diagram of long-distance local ventilation mode in mining working face: (a) head-neck local ventilation mode, (b) long-distance segmented ventilation mode, (c) visualization of long-distance segmented jet ventilation flow pattern, (d) temperature field simulation of long-distance segmented jet ventilation airflow.

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Head-neck cooling control method for long-narrow mining working face

The cold air jet in the underground roadway is radially injected into the crossflow from the vent of the air duct, where crossflow ventilation leads to a deflection in the flow trajectory of the jet, as illustrated in Fig. 3b. The jet’s deflection zone is strategically harnessed to the coverage area over the cooling zone around the worker’s head. This is achieved by adjusting the control parameters of the jet, thereby enabling the effective regulation of the head-neck microclimate to achieve the goal of local cooling.

Applicability of long-distance local ventilation mode

Figure 3c and d shows the smoke flow mode and temperature distribution characteristics for the long-distance segmented ventilation mode. In Fig. 3d, the simulation conditions include a model size of 45 m × 2 m × 2 m with 10 ventilation openings spaced 4 m apart. The crossflow velocity is 2 m/s, the jet velocity is 2 m/s, the jet temperature is 273.15 K, and the crossflow temperature is 308.15 K. As a result of the crossflow’s influence, the cold air jets in the long-distance ventilation mode are deflected and then superimposed step by step, creating a controlled microclimate zone underneath the air duct. This suggests the feasibility of segmental cooling through air ducts within a long-narrow mine roadway. The main control objective associated with head-neck cooling is the establishment of a stable microclimate zone, as shown in Fig. 4. To achieve this objective, key control strategies include increasing the degree of jet deflection, extending the jet distance, and reducing the rate of diffusion radius growth. These measures are essential to ensure the effectiveness of the microclimate control for head-neck cooling.

Fig. 4
figure 4

Schematic diagram of flow superposition characteristics of long-distance segmented jet ventilation mode: (a) low-speed jets, (b) medium-speed jets, (c) high-speed jets.

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Long-distance segmented ventilation air distribution superposition characteristics

Flow visualization experiments were conducted under different conditions of R = 1, 2, and 3 to investigate the jet superposition flow characteristics for long-distance air duct multi-stage segmented ventilation. The experiments utilized a 1 m long air duct with 8 circular jet orifices of diameter 0.02 m positioned along the duct, resulting in a spacing of 0.1 m between each orifice. The crossflow space height was 0.2 m. The experiments were conducted under normal temperature and pressure conditions with a fixed crossflow velocity of 2 m/s. Figure 4 shows the flow pattern when multiple jets are continuously superimposed in the segmented ventilation mode of a long-distance air duct. Notably, the jet’s initial flow pattern is determined by the first vent. Subsequent vents contribute to increasing the deflection and diffusion effect on the flow pattern formed by the first vent. For low-speed jet ventilation, Fig. 4a demonstrates that the first vent forms a wall-attached jet. Subsequently, as the jet superimposes with the subsequent vents, a slow diffuse, and stable wall-attached flow zone is formed. With an increase in the initial velocity of the jet, the diffusion of the jet accelerates. Once the velocity reaches a certain threshold, the jet’s inner boundary detachment the upper wall and forms a detachment zone forms in the far-field region.

Flow pattern characteristics of local jet ventilation

Control principle of underground local jet ventilation

To provide a foundational understanding of the flow characteristics of long-distance multistage jets, it is essential to first analyze the flow characteristics of a single jet, which serves as the basic ventilation unit. The flow field characteristics of a long-distance multistage jet’s superposition in a long-narrow mine space involve interactions between crossflow and jet, resulting in the formation of a microclimate-controlled zone that is superimposed cross-direction stage by stage. This foundational analysis, as depicted in Fig. 5a, helps in building theoretical support for understanding and optimizing the design of more complex multistage jet systems.

Fig. 5
figure 5

Schematic diagram of head-neck cooling mode and JVIC flow patterns in confined mine space: (a) head-neck cooling control concept, (b) wall-attached jet, (c) deflected jet, (d) impinging jet.

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In the process of underground mechanical refrigeration, the cold air jet is injected into the working face radially along the roadway through the vent. As shown in Fig. 5a, the cold air jet will be affected by the crossflow, resulting in the jet being deflected in the direction of the crossflow to form a jet air lake. This deflection occurs because, according to the requirements of mine ventilation design, there is always a crossflow of 0.25-4 m/s in the working face. Due to the air distribution of the jet ventilation is confined by the walls of the long-narrow roadway mines. Consequently, the ventilation mode in this confined space is defined as jet ventilation in crossflow (JVIC), which is characterized as both deflective and bounded.

In the confined space of the mine, various factors such as JVIC deflection degree, flow trajectory, diffusion boundary, coverage, and flow field velocity and temperature distribution vary due to different boundary conditions. Figure 5a shows that the cold air lake resulting from jet deflection should cover the region of the head-neck cooling, tailored to the local cooling requirements of mine workers. Simultaneously, to meet the comfort demand of local cooling, parameters including airflow velocity, temperature, and humidity distribution within the jet field must be reasonably regulated and adjusted.

Characteristics of local ventilation air distribution in confined mine spaces

The deflection and diffusion of JVIC in free space are predominantly influenced by the jet ratio. However, the JVIC in confined mine spaces is constrained by the roadway wall, and this confined effect is defined as the confined scale (C). Alterations in the boundary conditions result in the formation of three typical flow patterns within confined spaces: wall-attached jets, deflected jets, and impinging jets.

Wall-attached jet (JVICw)

When the jet velocity is low and the crossflow velocity is high, the jet deflection angle is large, as shown in Fig. 5b. This phenomenon is attributed to the Coanda effect, which causes the jet to rapidly deflect and attach to the upper wall, forming a wall-attached jet. The wall-attached jet’s outer boundary mixes with the crossflow, while the upper boundary is confined by the wall to create the wall boundary layer. Consequently, the jet squeezes the crossflow in the roadway making it attach to the lower wall to flow. The specific flow field can be divided into the following regions: I—Initial section: the jet velocity is low, and the initial section is relatively short in length. II—Deflection section: the jet presents a large-angle slewing, its inner boundary forms a large vortex slewing area (Coanda effect) with the upper wall, and at the end of the deflection section the jet completely attaches to the upper wall. Simultaneously, the outer boundary of the jet becomes progressively mixed with the crossflow, demonstrating a clear manifestation of wall-attachment under confined conditions. III—Wall-attached section: the inner boundary of the jet is completely attached to the upper wall, and the outer boundary is continuously mixed with the crossflow, showing wall attachment in the confined space.

Deflected jet (JVICD)

With the gradual increase of the jet velocity, the deflection angle of the jet gradually decreases, and its obstruction effect on the crossflow increases, as illustrated in Fig. 5c. The flow pattern evolves from a wall-attached jet to a deflected jet when the inner boundary of the jet detaches from the upper wall. The jet separates the crossflow into a low-velocity region and a high-velocity region. The low-velocity region, primarily the flow-around formed after the obstruction of the crossflow, is located between the inner boundary of the jet and the upper wall. On the other hand, the low-velocity region, mainly the crossflow squeezed by the jet, is located between the outer boundary of the jet and the lower wall. Consequently, the mixing of the jet and crossflow predominantly occurs at the inner and outer boundaries. The specific flow field can be divided into the following regions: I—Initial section: In the near-field of the jet exit, a potential flow core area exists, with the initial section of the jet extending from the jet exit to the end of the potential flow core area. The jet’s deflection angle in this section is relatively small, basic agreement with the direction of the jet exit. II—Deflection section: Transitioning from the end of the potential flow core area to gradually becoming parallel to the crossflow characterizes the deflection section. The deflection of the jet trajectory in this section is influenced by the cross-direction pressure gradient, causing faster decay in flow velocity. The obstruction of the crossflow is more pronounced in this mode than in the wall-attached jet mode, amplifying the flow-around phenomenon of the jet. III—Run-through section: the flow direction aligns essentially with the crossflow direction, and the jet and crossflow within a confined space in an accompanying flow state. Mixing between the two fluids results in the gradual convergence of jet velocity to the crossflow velocity, eventually leading to the fading of the jet’s effect on the crossflow until it vanishes.

Impinging jet (JVICI)

When the jet velocity increases to a certain extent, the lower wall becomes the boundary of jet deflection, leading to the rapid impinging of the jet on the lower wall, forming an impinging jet as shown in Fig. 5d. Unlike the impinging jet, which remains unaffected by the crossflow, the jet experiences deflection within an initial range, resulting in a change in the original flow direction that exhibits deflection characteristics. The specific flow field can be divided into the following regions: I—initial section: the impinging jet has a long initial section where the flow is not directly influenced by the wall and exhibits deflection characteristics. II—impinging section: the jet is completely confined by the lower wall, creating the impinging stagnation point. Due to the high-pressure gradient at this impinging stagnation point, the flow is diverted sideways and attached to the lower wall, with pressure gradually returning to static levels. III—wall-attached section: The wall-attached jet, moving in the opposite direction to the crossflow, encounters obstruction of the crossflow, causing its flow direction to progressively adjust and align with the crossflow direction.

Assessment model for JVIC air distribution efficiency in confined mine spaces

Effectiveness of JVIC

Figure 6 shows the air distribution created by the JVIC within a confined mine space, with a focus on providing cooling for the worker’s head-neck. In the context of JVIC air distribution control, defining key parameters is essential for ensuring effective cooling of workers’ head-necks in confined mine spaces. He represents the effective cooling height responsible for meeting the cooling demand at the worker’s head-neck, with Hu denoting the ineffective cooling height from the upper edge of the effective cooling zone to the upper wall, and Hl as the ineffective cooling height from the lower edge of the effective cooling height to the lower wall. Furthermore, De signifies the dimension of the coverage area of the cold air lake formed by JVIC, while Du and Dl respectively refer to the area from the inner and outer boundaries of the jet to the upper and lower of the wall. A relationship is established by the equation (Du + De + Dl) = (Hu + He + Hl). The primary objective of air distribution is to ensure that De covers He adequately while maintaining Du ≤ Hu, De ≥ He, and Dl ≤ Hl along the trajectory of human body movement. By designing the airflow in this manner, the focus is on providing sufficient cooling to the worker’s head-neck while effectively utilizing the crossflow within the mine roadway.

Fig. 6
figure 6

Schematic diagrams of jet ventilation patterns and local cooling regions in confined mine spaces: (a) attached-wall jet mode, (b) deflected jet mode, (c) impinging jet mode, and (d) long-distance multistage jet ventilation mode.

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In the case where the equivalent diameter dm of the roadway is close to the sum of He and Hl, it can be used to employ the attached-wall jet ventilation mode for head-neck local cooling, as shown in Fig. 6a. This scenario occurs when Du = 0, making the main control condition the satisfaction of De ≥ (He + Hu) and Dl ≤ Hl. The attached-wall jet ventilation mode cools the head-neck area locally. When the equivalent diameter of the mine roadway, dm, is greater than the sum of He and Hl, the deflected jet, shown in Fig. 6b where the inner boundary of the jet is detached from the upper wall to form a deflected jet, can fulfill the control requirements of the head-neck cooling zone. At this stage, Hu > Du > 0, making the main control condition the satisfaction of De ≥ He and Dl ≥ Hl. On the other hand, as shown in Fig. 6c, an impinging jet pattern is formed under the high jet ratio condition, resulting in substantial impinging cooling at the impinging stagnation point of the lower wall. However, in the wall-attached section within this ventilation mode, De coincides with Hl, leading to ineffective cooling of the head-neck. Furthermore, the impinging zone of the jet is small, and higher jet velocity may reduce human comfort. To satisfy the head-neck cooling demands, a higher jet exit velocity is necessary, leading to excessive cooling capacity wastage. Therefore, the effectiveness of the impinging jet ventilation mode as an effective air distribution for local cooling in confined mine spaces needs further examination compared to the full air ventilation mode.

Figure 6d shows that as the flow distance increases, the jet diffusion width also expands. This expansion results in a reduction of the jet core velocity and temperature due to the mixing effect of jet and crossflow. In cases where the core air temperature from the cold air lake is insufficient to meet the cooling requirements for the head-neck, the implementation of multi-stage vents becomes necessary. By utilizing multi-stage vents, long-distance segmented cooling can be achieved to adequately address the cooling needs of the head-neck. Specifically, in long-narrow mine passages, the use of multi-stage vents proves to be more effective in adjusting and optimizing airflow to establish a stable local cooling zone for meeting the cooling demands of the head-neck. The optimization of vent spacing plays a crucial role in enhancing cooling efficiency and ensuring that each level of vents provides sufficient core air velocity and temperature for effective head-neck cooling.

Partitioned evaluation model for JVIC cooling effect and cooling capacity utilization efficiency

Figure 7 displays the evaluation boundaries for the cooling effect and cooling capacity utilization efficiency of the JVIC mode in a confined space using the head-neck cooling technique. To establish the distance between the center of the ith jet exit and the (i + 1)th jet exit as li, the length of a temperature control evaluation unit is defined as li. The center point of the ith jet exit is the origin of the coordinates. By defining these parameters, the effective cooling zone is delineated, denoted as Ae, where 0 ≤ x ≤ li, Hl ≤ y ≤ (Hl + He). Conversely, the ineffective cooling zone, denoted as Ai, where 0 ≤ x ≤ li, 0 ≤ y ≤ Hl, and (Hl + He) ≤ y ≤ dm.

Fig. 7
figure 7

Partitioned evaluation model for JVIC cooling effect and cooling capacity utilization efficiency in confined mine spaces.

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The distribution characteristics of air velocity and temperature in the effective cooling zone play a crucial role in evaluating the cooling effectiveness of JVIC. The use of jet deflection characteristics to form an effective air distribution in the head cooling region. Similarly, the air velocity and temperature distribution in the ineffective cooling zone serve as an indicator for assessing the efficiency of cooling capacity. The evaluation of the flow field is conducted by zoning using the air diffusion performance index (ADPI).

  1. (1)

    Indicators for evaluating the effectiveness of the cooling effect:

    $$ADPI_{e} = \frac{{n_{e} }}{{n_{e.t} }} \times 100\%$$
    (2)

    where ne is the number of measurement points in the effective cooling zone that satisfy head cooling, ne.t is the total number of points measured in the effective cooling zone. A higher ADPIe indicates a better cooling effect of the jet on the head. The comfort parameters of the head-neck cooling process can be defined according to the results of the research available so far47,48,49,50,51.

  2. (2)

    Indicators for evaluating the utilization efficiency of the cooling capacity:

    $$ADPI_{e} = \frac{{n_{i} }}{{n_{i.t} }} \times 100\%$$
    (3)

    where ni is the number of measurement points low the upper limit of the head-neck control temperature in the ineffective cooling zone, ni.t is the total number of points measured in the ineffective cooling zone. When the temperature of the ineffective cooling zone is lower than the upper limit of the controlled temperature, it indicates that the cooling capacity is not effectively utilized. Therefore, the higher the ADPIi is, the lower the utilization of the cooling capacity in the jet ventilation process.

In this study, an analysis of the air distribution characteristics of JVIC revealed the coverage features of local microclimate regions, which are the primary factors influencing temperature distribution in controlled areas. Since direct measurements of cooling effects were not conducted in this study, ADPIe and ADPIi were not used to assess the cooling effectiveness and cooling capacity utilization efficiency of the local ventilation modes. ADPI serves as a crucial indicator for measuring air distribution and comfort, playing a significant role in a comprehensive evaluation of ventilation effectiveness. In future research, we will integrate ADPI metrics to quantify and validate the cooling effects and cooling load utilization of local ventilation patterns, and further explore the application of the JVIC mode in mining environments.

Visualization of JVIC flow characteristics in confined spaces

In the superposition process of multistage jet superposition ventilation in a long-distance air duct, the first vent determines the initial flow pattern of the jet, and the subsequent vent will form a superposition and expansion effect on the initial flow pattern. By adjusting the spacing of the vent, it is possible to create a single vent control unit, enabling long-distance multistage local cooling. Therefore, analyzing the flow pattern of single-vent JVIC in confined spaces is crucial as it serves as the foundation for investigating long-distance air duct segmental cooling.

Experimental setup

Apparatus

The wind tunnel utilized for the experiments was an open-loop suction-type wind tunnel, featuring a test section constructed from transparent acrylic panels. This test section had dimensions of 1.5 m (length) × 0.12 m (width) × 0.12 m (height), meeting the requirements for diffusing the deflected jet. The ratio of the test section’s height in the z direction to dm as z/dm = 1. To reduce the influence of the wall boundary layer on the flow, the central point of the jet exit was positioned along the central axis of the test section. The circular jet orifice was situated on the upper wall of the test section, perpendicular to the horizontal crossflow direction, and directed into the crossflow at a 90° angle to the plane of the test section’s lower wall, as illustrated in Fig. 8c. These coordinates were defined with the origin at the projection point of the center of the jet exit on the lower wall in a Cartesian coordinate system, with the transverse direction (crossflow direction), the y-coordinate indicating the lateral direction, and the z-coordinate denoting the axial direction (jet direction). Before the experiments, measures were taken to eliminate the influence of the boundary layer on the flow at the wall of the wind tunnel test section. Furthermore, preliminary experiments were carried out to study the flow pattern of JVIC for velocity ratio (R) and confinement scale (C), with the results indicating no impact from the boundary layer on the wind tunnel wall. The schematic configuration of the experimental setup can be seen in Fig. 8.

Fig. 8
figure 8

Schematic of the experimental setup: (a) test section; (b) selective planes for flow visualizations and green laser; (c) dimensionless for jet exit diameters. (d) boundary conditions for jet ventilation in confined mine spaces.

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The jet velocity (uj) was determined using a flow generated by a variable frequency centrifugal fan, measured through an ASAIR thermal gas flowmeter (AFM07) with an accuracy of ± 3% F.S. The measured flow rate from the thermal gas flowmeter was subsequently used to calculate the uj by dividing the flow rate by the jet’s exit cross-sectional area. Measurement holes for crossflow velocity were positioned on the lower wall at x/dm = − 1. The crossflow velocity was measured using a Pitot tube and pressure outputs were connected to a high-precision electronic pressure transducer. After each velocity measurement, the Pitot tube was retracted to the test-section floor to prevent any disturbance of the flow field. The non-uniformity of the time-averaged axial velocity was monitored using a one-component hot-wire anemometer (TSI-9535), boasting a reading accuracy of ± 3% (± 0.015 m/s) and a resolution of 0.01 m/s. To ensure the precision of the test results, the sampling frequency at each measurement point was set to 1 Hz with a sampling period of 120 s, minimizing the influence of turbulent fluctuations. Using anemometers, the time-averaged flow velocity was recorded at 14 measurement points along the axial direction, spaced at 1/15 dm intervals. The velocity distributions in the test section showed a non-uniformity of around 0.8%, with turbulence intensity of the flow less than 0.25%.

Flow visualization

Figure 8a shows a smoke mixing device constructed in-house was installed in the ductwork connecting the flowmeter to the fan to visualize the flow. By blending smoke with air, this device effectively improved the visualization of the flow pattern. The smoke mixing device, connected to a smoke generator (1500 FOGGER), produced homogeneous smoke with excellent followability and resolution, enhancing visibility under the green laser. The smoke from the smoke generator was uniformly mixed with the airflow from a variable frequency centrifugal fan before being released from the jet exit.

Figure 8b shows a stationary laser positioned below the test section emitting a continuous laser sheet intersecting the central plane of the jet. This laser scattered the smoke flow, enabling the visualization of the airflow pattern resulting from the crossflow and the jet interaction. To facilitate flow visualization, a 532 nm wavelength green laser (13GCN3) was chosen as the light source. This laser emitted a horizontal green light measuring 13 mm × 62 mm with a maximum output power of 400 mW. The laser sheet width was set at 1 mm and expanded into a planar laser sheet with a thickness of about 0.5 mm in the x–z plane, aligned vertically with the centerline of the x–y plane of the upper wall of the test section for side observation of the smoke flow pattern. To ensure clear visualization of the smoke flow pattern, this study employed a high-speed imaging technique to capture the smoke flow pattern rapidly and repeatedly.

A Canon EOS 800D digital camera was used to capture time-averaged flow images by extending the exposure time. Equipped with a built-in CMOS sensor featuring a maximum resolution of 6000 × 4000 pixels, the camera provided a spatial resolution of 0.085 mm/pixel. A long exposure time of 6 s was selected to take the smoke flow pattern. Repetition of smoke visualization experiments and capture of time-averaged flow images were conducted to verify the consistency of the collected flow patterns and ensure the reliability and accuracy of the experimental results.

To determine the inner and outer boundaries of the jet, the binary boundary detection technique was employed 52. Firstly, the grayscale processing and histogram equalization were applied to the time-averaged flow images to enhance clarity and contrast. Shapiro and Stockman (2001) proposed calculating gray-level gradients around visual jet boundaries and using peak local gray-level gradient values to define the boundaries. Preliminary tests indicate that averaging the jet boundary data obtained from fifteen 6 s exposure images converges to a stable value, mitigating the impact of noise signals on the grayscale gradient of the jet boundary shear layer. Therefore, the jet boundaries presented in this study are averaged from twenty long-exposure images. Using the grayscale distribution method, a grayscale of 60 was defined as the threshold, and parts with grayscale below 60 were eliminated. Finally, the air distribution coverage area in the averaged images was enhanced with high brightness.

Test conditions

This study aims to investigate the flow characteristics of JVIC in confined mine spaces through flow visualization experiments. The Reynolds number (Re) of the jet (Rej = ujdj/vj) and the crossflow (Rem = umdm/vm) were defined. The degree of bending of JVIC was found to be primarily associated with the initial momentum ratio. In the case of a pure jet where the kinematic viscosity vj = vm, it was influenced by the velocity ratio R = uj/um, where uj is the jet exit average flow velocity (m/s) and um is the crossflow average flow velocity for roadway entrance (m/s). The flow pattern of the JVIC in free space is predominantly determined by R, while in confined mine spaces, the wall confinement effect also influences the flow pattern. To quantitatively characterize the impact of wall confinement on the JVIC flow pattern, a confinement scale C is introduced, where C = dj/dm, with dj being the equivalent diameter of the jet exit section (m) and dm being the equivalent diameter of the roadway section (m). An increase in R indicates a weaker influence of the roadway crossflow on jet ventilation performance, whereas an increase in C suggests a stronger confinement effect of the roadway on the jet. Buoyancy effects were disregarded during the experiment, as the crossflow velocity exceeded 0.1 m/s53. The experimental conditions included a fixed crossflow velocity of um = 2 m/s and a Reynolds number of Rem = 15,834. The range of jet velocities is from 0.55 to 11.29 m/s and the Rej ranges from 363 to 12,826. This study used the Reynolds number ratio (RNR = Rej/ Rem) as a similarity condition54 and the RNR involved in the experiment ranged from 0.023–0.81. Three different jet exit diameters (dj = 0.01 m, 0.02 m, and 0.03 m) were utilized, resulting in different confinement scales of C = 1/12, 1/6, and 1/4, respectively.

Results and discussions

Time-averaged flow characteristics

Figure 9 shows the long-exposure time-average smoke flow evolution images at different R and C conditions. Notably, the smoke vortices (Conda effect)44 formed by the jet deflection with the upper wall all disappeared because they overlapped during a 6 s long exposure time. The jet attaches to the upper wall flow forming a wall-attached jet under the critical R, as shown in Fig. 9 (a, f, and k). The maximum diffusion widths of the jet increase gradually with the increase of C as W/dm = 0.59, 0.77, and 1.11, respectively. This is attributed to the increased cross-sectional area of the jet exit, which allows the jet to carry more smoke mass. Figure 9(b, g, and l) shows the detachment of the inner boundary from the upper wall to form a deflected jet. At x/dm = 2.21, the outer boundary intersects with the lower wall to form a semi-confined point (SP) as shown in Fig. 9h. In the far-field region of the SP after, the deflection of the jet is confined. When SP appears, a reaches the longest in the deflection section. The flow pattern gradually transitions from a deflected jet to a semi-confined deflected jet. The R of jet emergence SP decreases gradually, with R of 2.910, 2.250, and 1.555, respectively. Consequently, the wall confinement effect for deflection and diffusion will manifest earlier. Figure 9 (d, i, and n) shows that the positions of SP appear at x/dm = 0.54, 0.81, and 1.24, respectively. The constant movement of SP to the -x direction leads to a gradual shortening of the deflection section.

Fig. 9
figure 9

Long-exposure time-averaged smoke flow evolution images. Exposure time = 6 s.

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Notably, the ability of the inner boundary to diffuse in the z direction is improved by the lower wall’s confinement effect30. At x/dm = 1.85, the inner boundary intersects with the upper wall to form a confined point (CP) as shown in Fig. 9i. In the far field after the CP, its diffusion ability in the axial direction is completely lost. The jet is fully confined, and the diffusion width reaches a maximum value equal to the height of the roadway (i.e., z/d1 = 12). Figure 9(e, j, and o) shows the impinging jet flow pattern with a long initial section. The jet rapidly impinges on the lower wall forming an impinging stagnation point31, which is overlaid in the long-exposure image. The long-exposure image shows the formation of a significant counterclockwise recirculation flow structure near x/dm = 0. This structure forms from the interaction of the wall-attached jet along the -x direction, the crossflow, and the lower wall. Redirected by the crossflow, this recirculating flow structure enables the wall-attached jets in the -x direction to flow back to the jet imping region. Furthermore, the recirculation flow structure superimposed with the jet-impinging region results in a more pronounced alteration in the flow-field structure. Impinging jets influenced by crossflow create recirculation flow structures that develop more quickly and prominently compared to those not influenced by crossflow43. The formation location of recirculation flow structures shifts from the far-field to the near-field impingement zone16.

Evolution characteristics of flow modes

Figure 10 shows the flow evolution pattern of the JVIC within a confined space. During the evolution of flow patterns under three different C conditions, the R corresponding to the critical flow pattern is the average value recorded throughout the continuous experiment. The bands composed of gray lines in Fig. 10 represent the borders between neighboring characteristic regimes55. The width of the boundary zones in the figure represents the range of uncertainty between adjacent flow patterns. Based on the varying degrees of confinement, the flow patterns of JVIC in confined spaces are classified into five types: wall-attached jet (I), deflected jet (II), semi-confined deflected jet (III), confined deflected jet (IV), and impinging jet (V). This aligns with the three typical flow patterns presented in the study by Kamide et al.56. Notably, the upper limit of R for the wall-attached jet (I) flow pattern increases gradually as C increases, and R is 0.275, 0.425, and 0.595, respectively. However, for the other four flow patterns (II-V), the upper limit of R for the flow pattern decreases gradually as C increases. Furthermore, as the C increases, the required R for the transition of the flow pattern in the confined space decreases. The R required for the transition from flow mode to the impinging jet is 5.645, 4.605, and 3.240, respectively. This is because the diffusion capability of the jet is enhanced as C increases under the same R. The JVIC exhibits various flow modes depending on the level of confinement. The JVIC in free (unconfined) space predominantly produces the deflected jet, without two flow modes: the wall-attached wall jet and the impinging jet26. For JVIC in semi-confined space, the jet mainly forms two flow modes: wall-attached jet and deflected jet or deflected jet and impinging jet. However, the JVIC can form five flow modes in a confined space28. As shown in Fig. 9, the R-range decreases steadily as C increases, highlighting the escalating impact of wall confinement with higher C values. Therefore, it can be inferred that the flow pattern of the JVIC within a confined space is jointly determined by the parameters R and C. In summary, the wall-attached jet and deflected jet flow patterns formed by JVIC are highly suitable for local ventilation control in the head-neck region. However, the impact jet mode requires further experiments and simulations to verify and evaluate its actual cooling effectiveness.

Fig. 10
figure 10

Regimes for characteristic flow modes.

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Conclusion

  1. (1)

    This paper examines the spatial distribution characteristics of long-narrow underground spaces and the thermal environmental control requirements associated with such spaces. A local ventilation mode applicable to the head-neck area of mine workers is proposed as a solution to address cooling needs. Based on this local ventilation mode, an air distribution form applicable to local cooling in mine roadways is proposed to meet the control needs of head-neck cooling. This air distribution form is the JVIC formed by crossflow and jet ventilation in a confined mine roadway.

  2. (2)

    Effective cooling height and ineffective cooling height are proposed as the boundaries of JVIC air distribution control in confined space, based on the concept of head-neck cooling control. Meanwhile, this clarifies the applicability of various flow patterns. Additionally, a partitioned evaluation model is proposed based on the ADPI evaluation concept. This model uses the effective cooling zone to evaluate the local cooling effect and the ineffective cooling zone to assess the utilization efficiency of cooling capacity.

  3. (3)

    Investigating the impact of C and R on flow evolution characteristics and axial diffusion capacity, the five flow patterns revealed in terms of SP and CP boundaries for JVIC in confined spaces: wall-attached jet (I), deflected jet (II), semi-confined deflected jet (III), confined deflected jet (IV), and impinging jet (V). Among them, the attached-wall jet and deflected jet modes have high application prospects. The influence of R and C on the pattern evolution of JVIC is analyzed and a parametric description is given.

This study offers valuable insights into the flow characteristics and effectiveness of the proposed head-neck local ventilation mode under isothermal conditions. However, it does not consider the effects of non-isothermal flows or air movement caused by machinery and miners. Furthermore, this study did not directly assess the cooling effect. Future research will combine the ADPI evaluation index to further investigate the application of multi-jet ventilation systems in actual mine environments, and validate their cooling effectiveness through industrial experiments.