Development of Numerical Experimental Model for Heat Transfer in High Quality Region of Flow Boiling in Tube Evaporator with Twisted Tape-Insert

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This research aimed to develop a numerical experimental mode which can simulates behavior of droplets in the post-dryout region of flow boiling in tube evaporator, in order to find the way to the heat transfer enhancement in this region effectively by twisted tape-insert, which contributes to reducing the tube length required. The research was performed by experimental studies on heat transfer and critical heat flux(CHF)of the flow boiling, and by numerical analysis using computer.In the experiments data for refrigerant HFC134a were obtained for the horizontal flow conditions (mass flux 100-600 kg/mィイD12ィエD1s). The following results were clarified : (1)The heat transfer characteristics and the relation between CHF and the critical quality for a smooth tube(O. D. 7mm, I. D. 6.34mm)showed the similar tendencies to the results which had been obtained previously for the different flow conditions and refrigerants by the investigators. (2)Magnitude of the heat transfer enhancement by the twi … More sted tape inserted into the smooth tube in the post-dryout region agreed qualitatively with the experimental results which had been obtained for refrigerant CHF113 by the head investigator.In the numerical analysis a simulation of behavior of a single droplet in vapor flow which collides with heated wall was carried out by the modified VOF method which is taking account of phase change at the vapor-liquid interface. The results showed that the calculation could simulate behavior of the droplet until it collided with the wall, and the definition of temperature of the cell which included the interface affected the complexity of calculation beyond the investigators' expectation. However the most relevant condition at which the droplet departs form the wall has not been found.In addition, a simple method using Thermo-Paint(temperature sensing paint), which changes its color with temperature, was proposed to measure the surface temperature profile outside the tube, and was confirmed its usefulness by another experiment. Less