Flow boiling of nanofluids in microchannel heat exchangers: A critical review

Nanofluids and microchannel heat exchangers have been studied extensively due to the advantages that using nanofluids can enhance the heat transfer performance for their larger surface area and the microchannel heat exchanger can decrease the coolant charge for their compact structures. The integration of the nanofluids into microchannel heat exchangers, therefore, has been considered an efficient strategy for cooling applications. However, this technology was still restricted by the small operating range, while the latent heat generated during the phase-change flow can partially improve the efficiency. This article critically reviewed some existing results and controversial issues relating to nanofluids and microchannel heat exchangers. Since the microchannel scale transition criteria have not been standardized yet, different transition criteria were summarised and compared in this article for the first time, as well as the influential factors that were emphasized for further consideration. Another reason for the debate is that flow boiling is highly sensitive, possibly leading to an inaccurate experiment result, thus several practically reasonable testing methods have been proposed to increase the accuracy of the experiment. In addition, the formulas for critical heat flux (CHF) and heat transfer rate empirical formulas were summarized, followed by the proposed future work.