Study On The Flow Boiling Heat Transfer Characteristics In Microchannels Of Refrigeration System

The microchannel heat exchanger has a broad industrial and market prospects whenapplied on micromodule equipment which is characteristic of high heat flux and finiteoccupancy，because they are equipped with the high specific surface area and effectivetransfer efficiency. Therefore, flow boiling heat transfer characteristic in microchannelsbecome one of popular researching hot points. In the paper, flow boiling heat transfercharacteristics and its influence factors in0.6mm×2mm,1mm×2mm and2mm×2mm channelsare studied, using refrigerant R22as the working fluids.The paper analyse the relationship between boiling heat transfer characteristics anddimensionless parameters, such as Re, Bo, Nconfand Ja number. Experimental results revealthat the nuclear boiling was the main dominant mechanism in microchannels, as heat transfercoefficient increases with increasing Bo number which represent the heat flux when fix themass flow rate, while has no significant change with the increase of Re number. The Janumber and heat transfer coefficient increase with increasing heat flux in1mm×2mmand2mm×2mm channels, while in0.6mm×2mm channels, The Ja number and heat transfercoefficient increase with increasing heat flux, but the Ja number decreased in the threshold ofhigh heat flux, indicating that the bubble behavior had close relationship with heat transfercoefficient. Also, heat transfer coefficient increases with increasing Nconfnumber whichrepresent channel size.Furthermore, empirical formula of two phase heat transfer coefficienthas been proposed by considering comprehensive influence of different dimensionlessparameters on heat transfer coefficient.In the experiment, high speed camera is used to observe the phenomenon of approachingboiling critical condition and occurrence of boiling critical condition. Before the occurrenceof boiling critical condition, The upstream replenished liquid to downstream periodically,causing the thickness of the downstream liquid film change periodically, and resulted inperiodical fluctuations of exit wall temperature and pressure drop. When the boiling criticaloccurred, the liquid film near the outlet of test section was evaporated and vanished, leadingto violent fluctuations of the pressure drop and abrupt rising of exit wall temperature. In the meantime,the dry points which located in sidewalls and bottom wall were observed in2mm×2mm channels, Increasingly, the formation mechanism of the dry points in differentlocation were distinctively. When the heat flux values remained in CHF, the dry point shiftedto upstream gradually, leading to expansion of hot spots, the wall temperature near the outletof test section rising continuously, and the pressure drop still fluctuate violently..Lastly, Experimental investigation were performed on CHF characteristics. Theexperimental results show that CHF increases with the increase of the mass velocity andchannels size. When the saturation pressure increase, CHF increase initially but decreasewhen the saturation pressure reaching a critical value.