Influence of transient heat flux on boiling flow pattern in a straight microchannel applied in concentrator photovoltaic systems

• A MCHS to cool CPV under transient heat flux was investigated numerically. • Flow boiling instability affected by the transient heat flux were investigated. • Two-phase flow pattern under different sudden heat flux increase were studied. • With heat flux increase from 97.96 to 580 kW/m2, duration of rapid development period was increased from 0.03 s to 0.066 s. • Fluctuation intensity of pressure drop in the quasi-stable period was increased from 0.025 to 0.193. Concentrated photovoltaic (CPV) overheating due to the high radiation intensity became the critical challenge to preventing its benefits in advancing the photoelectric efficiency. To promote the application of CPV, efficient heat dissipation becomes the key technology. Meanwhile, microchannel flow boiling, as one of the most promising solution to advance cooling of high-flux equipment, has attracted increasing attentions in recent years. However, the fluctuating and non-uniform heat fluxes of CPVs may lead to challenges to microchannel boiling for cooling CPV. Therefore, the transient flow and phase change phenomenon in a microchannel with deionized (DI) water was studied through a numerical modeling on flow boiling. The effects of different sudden heat flux increases on flow boiling two-phase flow pattern were analyzed. It was found that different heat flux increase resulted to different flow pattern evolutions, a much higher sudden increase in heat flux leaded to serious variation of flow pattern, especially the fluctuation of elongated restricted bubbles, which may evolve to annular flow or even dry-out. The vapor phase volume fraction showed a much higher growth rate. When the heat flux increase was raised from 97.96 kW/m 2 to 580 kW/m 2 , the maximum vapor phase volume fraction was increased from 0.104 to 0.8064, and the bubble length increased from smaller than 0.3 mm to 1.5 mm in the dry zone, indicating rapid phase change. Based on analysis on pressure drop and vapor volume fraction variation, the period after receiving sudden heat flux was divided into three stages: initial period, rapid development period, and quasi-stable period. With the rising of heat flux increase from 97.96 to 580 kW/m 2 , the duration of rapid development period was increased from 0.03 s to 0.066 s, the pressure drop increase was obviously enlarged, from 2.43 kPa to 14.51 kPa, and the vapor phase volume fraction increase in the microchannel was raised by 3.3 times to 25.6 times, indicating more intensive boiling, and deeper effect of higher heat flux increase on the phase change. In addition, the fluctuation intensity of pressure drop in the quasi-stable period was obviously increased from 0.025 to 0.193, demonstrating the advancement in flow boiling instability under higher heat flux increase, which should be controlled.