| Summary: | 碩士 === 國立清華大學 === 工程與系統科學系 === 93 === Abstract
The present study focuses on two-phase flow visualization and boiling heat transfer for deionized water flowing in the converging and diverging microchannel with a mean hydraulic diameter of 70.6μm and 116μm, respectively. The effects of the imposed wall heat flux, volume flow rate and mean hydraulic diameter on the boiling heat transfer coefficient, pressure drop, bubble dynamics and two-phase flow pattern are analyzed in detail.
Experimental results indicate that the bubble growth rate and the bubble departure frequency in the converging microchannels are smaller than that in the diverging microchannels. On the other hand, the bubble departure radius in the converging microchannels is bigger than that in the diverging microchannels. Observation of two-phase flow pattern indicates that the flow reversal in the converging microchannels is more violent than that in the diverging microchannels. The new flow pattern is found, called snake flow in the diverging microchannel with a mean hydraulic diameter of 70.6μm and under certain conditions. For single-phase flow, the heat flux and the heat transfer coefficient increase slowly with the flow rate. After boiling begins, heat flux and the heat transfer coefficient are elevated significantly. The mass flow rate has little effect in boiling heat transfer. For single-phase flow, the pressure drop decreases with increasing the wall temperature (or heat flux) increases with the wall temperature. On the other hand, the two-phase pressure drop increases rapidly with the wall temperature (or heat flux). The effect of flow rates does not affect in two-phase is found to be insignificant. Comparison of the heat fluxes and heat transfer coefficients between converging and diverging microchannels with DH=116μm and 70.6μm, respectively, show that the diverging microchannel presents better performance in boiling heat transfer than that of converging microchannels. Comparison of the pressure drop between the converging and diverging microchannel with DH=116μm and 70.6μm, respectively, indicates that there is little different in single-phase pressure drop. On the other hand, the two-phase pressure drop after boiling begins in the converging microchannel is higher than that in the diverging microchannel. It is demonstrated that boiling heat transfer performance of the diverging microchannel is better than that of the converging microchannel with the same mean hydraulic diameter and under similar operating conditions.
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