Critical Heat Flux on Subcooled Boiling (Effect of Concentration of Non-Condensible Gas)

点击翻译按钮获取中文摘要

The objective is to investigate experimentally the effect of non-condensible gas on critical heat flux on subcooled boiling. The concentration of non-condensible gas was measured with a specially designed gas collector in which the test liquid was heated up to vapor saturation temperature to evacuate dissolved gas. The research in 2000 aimed at the establishment of the measurement technique of the concentration of non-condensible gas in water, which revealed the following :(1) Heat flux of heating liquid to evacuate dissolved gas must be high enough to generate vapor on the heater, and the swirl of liquid is effective to minimize the period of collecting - 5 minutes minimum.(2) Since the collected gas contains vapor, the subtraction of the partial pressure of vapor is necessary using the temperature of the gas, measured when the gas is released.Using air or carbon dioxide as dissolved gas, purified water as test fluid, the effects of the concentration of dissolved gas on heat transfer, … More critical heat flux and bubble behavior were studied in 2001. The boiling surface is 0.5 mm in diameter platinum wire horizontally mounted, and heated with direct current. Test liquid, the subcooling and the concentration of non-condensible gas of which were controlled, was supplied at about 5 mm/s. The subcooling of liquid is fixed at 30 K, and the concentration of gas was from 0.2 dm^3/m^3 to that of saturation. The results are summarized as followed.(1) When the concentration of gas is large, heat transfer is enhanced at low heat flux but reduced at high heat flux. Critical heat flux is attained nearly the same superheating independent of the concentration.(2) Critical heat flux decreases very much as the concentration of gas becomes large.(3) Critical heat flux in water dissolving carbon dioxide is about 1 MW/m^2 higher that that dissolving air comparing at the same concentration.(4) The reason why the critical heat flux in water dissolving carbon dioxide is higher is that the evacuated gas is easier to dissolve to the water at the condensation side of the bubbles. It reduces the dumper effect on shrinkage of bubbles on condensation. Less