TED-AJ03-264 NON-CONDENSIBLE GAS EFFECT ON CRITICAL HEAT FLUX OF SUBCOOLED POOL BOILING OF WATER
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概要
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Bubble behavior was observed and the critical heat flux was measured on subcooled boiling in water dissolving various concentration of non-condensible gas. The experiment was carried out at atmospheric pressure and mainly at 30K subcooling. The situation is essentially pool boiling-subcooled water is supplied to the boiling surface at very low velocity of about 5mm/s. Two kinds of gases-air and carbon dioxide-were used as a solute. Concentration is controlled from their saturate one to 0.1(dm)^3/m^3 The flow loop consists of a boiler tank to evacuate dissolved gas, a pump and a flow meter to control flow rate, a cooler and a preheater to control subcooling, dissolved gas collector, the test vessel and a condenser. The concentration of solute gas is controlled with the temperature of the boiler tank and of the water near the free surface in the test vessel. It is measured with a dissolved gas collector by heating to the vapor saturation temperature to evacuate the gas after trapping supplying water in the collector. The temperature of evacuated gas is measured when it is released after the measurement of the volume and it is used to compensate density change and the partial pressure of vapor. The test surface is made of 0.5mm diameter platinum wire heated with direct current. Current supplied to the boiling surface was increased stepwise by 2.5% or less and the current, tap voltage and temperatures at various locations in the test loop were measured and recorded. The heat flux just before the sudden increase of wall temperature was regarded as the critical heat flux. The official concentration of the solute gas was measured just after each series of measurement. The measured critical heat flux decreases so much with increasing the concentration of solute gas. The critical heat flux for water dissolving air is 1MW/M^2 lower than that for water dissolving carbon dioxide compared at the same concentration. The critical heat flux seems to be predominant with the relative concentration of solute gas referenced to that of gas-saturation. The concentration of solute gas at which the gas dissolves again freely to the surrounding liquid at the condensing side of bubbles is evaluated under the assumption that (1) heat added to the boiling surface is solely absorbed by evaporation, (2) all of the evacuated gas due to the evaporation of liquid diffuses into liquid free of gas at the condensing side of the bubble, and (3) the partial pressure of evacuated gas in the bubble is due to the surface tension. The relative concentration is derived as a function of Lewis number, the ratio of sensitive heat to latent heat, and the ratio of pressure difference due to the surface tension to the system pressure. This may explain that the bubble behavior then the critical heat flux are predominant with the relative concentration referenced to the saturation[figure]