【摘要】 针对常规闭式并联微通道内流动沸腾换热存在气泡生长受限产生的堵塞效应以及不同通道内气泡核化生长不同步导致的并联通道传热不稳定性等问题,设计了一种顶部联通型开式并联微通道蒸发器。采用无水乙醇为工质,在入口过冷度为15℃、质量流速为175 kg·m^-2·s^-1及热流密度270～761 kW·m^-2条件下,开展了该新型微通道冷却器中流动沸腾换热的实验研究,发现了传热系数随干度的增加呈现三类典型趋势,即传热系数单调上升、传热系数先上升后下降再上升、传热系数先上升再保持基本不变;结合高速可视化流型研究,发现了与流型密切关联的三类传热机理,即:1)以气泡核化为主的核态沸腾换热;2)上游核态沸腾为主,下游两相强制对流换热主导;3)偏离核态沸腾后的膜态沸腾换热。分析表明,沸腾数Bo是主导三类传热模式的主要无量纲数。更多还原

【Abstract】 In conventional microchannels, bubble growth is confined by the channel walls and bubble is easy to block most of the channel cross section except for the corners. Meanwhile, bubble nucleation and growth in different channels are usually asynchronous, which may canse flow and heat transfer instability due to the pressure difference among parallel channels. In the present study, a new type of microchannel evaporator was designed, which is characterized by the parallel, microchannels inside the evaporator are top-connected. With ethanol as the working fluid, an experimental investigation on flow boiling heat transfer inside the evaporator was performed under the condition of inlet subcooling15℃, mass flux 175 kg·m^-2·s^-1 and heat flux from 270 to 761 kW·m^-2. It was found that the heat transfer coefficients displayed three types of trends with increasing vapor quality i.e. kept rising;increased firstly, followed by a slight declination and then increase again; increased slightly near the inlet and then almost maintained at a constant value. Based on high-speed visualization of flow pattern, three kinds of heat transfer mechanism were identified: 1) the bubble nucleation and growth dominated nucleate boiling; 2) the nucleate boiling for the upstream and two-phase forced convection for the downstream; 3) film boiling after the departure from nucleate boiling. Theoretical analysis shows that the boiling number Bo is the main dimensionless number dominating the three types of heat transfer modes.更多还原