TED-AJ03-319 DERIVATIONS OF CORRELATION AND MODEL OF TRANSITION BOILING HEAT TRANSFER

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The transition boiling heat transfer is of importance in the many industiral fields. Because of the complexity of the phenomena and difficulty of steady heat transfer experiments, however, the heat transfer characteristics are still unresolved important issues. The objective of the present study is to examine the heat transfer characteristics of the transition boiling through the steady heat transfer experiments. Correlations of the transition boiling heat transfer were derived from the present experimental data, some models were developed. Experiments were conducted using a copper block and saturated water for a pool condition at atmospheric conditions. The test section was 15 x 15 and 60mm in lengths and thickness, respectively. Six electric cartridge-heaters were embedded on the back side of the block. Temperatures in the block were measured by three thermocouples. The heat flux and the wall temperature of the test section were determined by using the measured temperatures and Fourier's law. The test section was held in a frame of Teflon plate, and insulated thermally except for the top side. The surface of the test section was polished with #2000 or #80 emery paper. Experiments were done in the following manner. First, a vessel was filled with saturated water below the heating surface and the block was heated to 300℃ by a.c. current. Then the water level was increased up to 100mm above the block by pouring water into the vessel. Meanwhile adjusting the electric power, a stable film-boiling state was established. After the state was obtained, the power to the block was decreased stepwise maintaining the other conditions up to the minimum film boiling, MHF. After the MHF, the power was increased and controlled to hold a "time-smoothed" steady state of transition boiling. Again the transition boiling heat fluxes and the temperatures under the respective steady-state conditions were measured by increasing the electric power step by step up to critical heat flux, CHF. The following experiments were also performed. The a.c. power to the heaters was increased stepwise maintaining the nucleate boiling to the CHF. After the CHF, the electric current was decreased to attain the "time-smoothed" steady state of the transition boiling. Some correlations of liquid-solid contact fraction in transition boiling were derived by focusing on the dimensionless wall temperature, θ=(△T_<sat>-△T_<CHF>)/(△T_<MHF>-△T_<CHF>). The following correlations were obtained from the present experimental data : Г=1.000-0.7771 θ-0.2695θ^2,where q_<tb>=q_<CHF>Г+q_<MHF>(1-Г). There was no hysteresis on the transition-boiling heat transfer in the present experiments for water. Furthermore, some models of the transition boiling heat transfer were developed and examined by using the macrolayer model and a liquid-solid contact model, respectively.[figure]
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TED-AJ03-319 DERIVATIONS OF CORRELATION AND MODEL OF TRANSITION BOILING HEAT TRANSFER

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