| Summary: | 碩士 === 國立交通大學 === 機械工程系 === 88 === The saturated and subcooled flow boiling heat transfer of refrigerant R-134a in a plate heat exchanger are measured in this study. Besides, the associated bubble characteristics in the plate heat exchanger are also inspected by visualizing the boiling flow. Two vertical counterflow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Upflow boiling of refrigerant R-134a in one channel receives heat from the downflow of hot water in the other channel. The effects of the imposed heat flux, mass flux, system pressure and subcooling of R-134a on the saturated and subcooled boiling heat transfer are explored in detail.
The experimental data for the saturated flow boiling showed that both the boiling heat transfer coefficient and pressure drop increase almost linearly with the imposed heat flux. At a higher mass flux the pressure drop is substantially higher but the boiling heat transfer coefficient only shows slight improvement. Raising the refrigerant pressure from 0.6 to 0.8 MPa, the frictional pressure drop is significantly lower but the change in the heat transfer coefficient is small. Furthermore, it is noted that at the lowest pressure tested here for P=0.5 MPa the boiling heat transfer coefficient is lowest, but the associated rise in pressure drop is highest. Based on the present data, empirical correlations for the saturated boiling heat transfer coefficient and friction factor are proposed.
Next, the results in the subcooled flow boiling are presented in terms of the boiling curves and heat transfer coefficient. The results for the boiling curves show significant change in the slopes of the boiling curves during the onset of nucleate boiling (ONB) especially at low mass flux and high saturation temperature. Besides, the boiling hysteresis is significant at a low refrigerant mass flux. The subcooled boiling heat transfer coefficient is affected noticeably by the mass flux of the refrigerant. However, the inlet subcooling and saturation temperature have little effect on the boiling heat transfer coefficient.
The photos from the flow visualization for the saturated and subcooled flow boiling revealed that at higher boiling heat flux the plate surface is covered with more bubbles and the bubble generation frequency is higher, and the bubbles tend to coalesce into big bubbles. But these big bubbles are prone to break up into small bubbles as they move over the corrugated plate, producing strong agitating flow motion and hence enhancing the boiling heat transfer. We also noted that the bubbles nucleated from the plate were suppressed to a larger degree for higher inlet subcooling and mass flux. At high heat flux the boiling is so intense that there is a two phase flow pattern transition from a bubbly flow to an annular flow in the channel. Finally, empirical correlations for the heat transfer coefficient and the bubble departure diameter in the subcooled flow boiling are proposed.
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