Suppression of Bubbling in Boiling Refrigerants

Abstract

DURING recent studies of the properties of radicals trapped at low temperatures it has been found that the presence of bubbles in the body of a boiling refrigerant can be eliminated by introducing a stream of a gas which is comparatively insoluble in the refrigerant. In a typical experiment a small, unsilvered Dewar flask was half-filled with liquid nitrogen at atmospheric pressure, vigorous bubbling of the liquid being observed. When a jet of helium, hydrogen, or neon gas was blown over the liquid nitrogen at a rate sufficient to break up the surface, the bubbles at first became smaller and then were no longer visible. The liquid was now completely clear and still, and its temperature had fallen by several degrees. If the gas flow was interrupted, the bubbling eventually resumed its original intensity. When the gas was introduced 10 cm. below the surface of the liquid nitrogen instead of being blown across the surface it was found that the rate of gas flow needed to suppress bubbling was reduced considerably. In the case of helium gas, the flow could be reduced to a value of 1/6 to 1/8 of that required when blown across the surface. The introduction below the surface of nitrogen, air or argon gases, instead of helium, had little or no effect on the bubbling, so that the suppression was not caused merely by a local increase in total pressure above the surface of the liquid nitrogen.