Experimental Investigations of Flow Boiling Heat Transfer and Flow Instability in a Horizontal Microtube with an Inlet Orifice

Flow boiling heat transfer mechanism has been shown to be an attractive method for satisfying the expected cooling loads of microelectronic devices. One of the main challenges associated with flow boiling is the flow instability created by the two-phase flow process. The design of orifice placed at the entrance of microchannels has been proven to be an effective way of reducing or eliminating these flow instabilities in multichannel systems. The present thesis experimentally investigated the effect of inlet orifice in a single microtube on flow boiling heat transfer and associated flow instabilities using FC-72 as working fluid. The hydraulic diameter and length of microtube were fixed at 889 µm and 150 mm, respectively. The area ratios of inlet orifice to main microtube were selected as 50%, 35%, and 20%. The results showed that the ratio of pressure drop by inlet orifice to the total pressure drop reduced as mass flux and vapor quality increased. Inlet orifice did not have significant effects on the forced convective boiling heat transfer and normal critical heat flux, but increased the nucleate boiling heat transfer and premature critical heat flux. A novel flow stability map was developed and two critical boundaries were identified, which divide the flow into stable and unstable regimes. The microtube with 20% inlet orifice had a best performance on flow stabilization since a large upstream pressure was created. A methodology was also developed in order to predict the onset of flow instability in single microtubes with different sizes of inlet orifices. The predicted heat flux at the onset of flow instability was compared with the experiment and showed a reasonable agreement within ± 30%. Overall, the performance of inlet orifices on flow boiling heat transfer and flow instability has been investigated in a single microtube. The present fundamental work is expected to lead to creation and development of a number of miniaturized devices, which are associated with flow boiling