制冷剂CO_2两相流动及沸腾传热特点分析

ANALYSIS OF TWO-PHASE FLOW AND BOILING HEAT TRANSFER PERFORMANCE OF REFRIGERANT CARBON DIOXIDE

为了能够设计出高效的CO_2蒸发器,有必要对CO_2两相流动和沸腾传热特点进行研究。通过理论模拟计算发现,CO_2出现液滴夹带时的临界气相速度远小于传统制冷工质,CO_2的液体夹带率比传统制冷剂高,这说明了环状流是CO_2两相流动的主要型式。CO_2的核态沸腾抑制干度值比较大,大约为传统制冷剂的1.5倍左右,表明CO_2的流动沸腾传热机理主要是核态沸腾占主导地位。CO_2出现干涸时的干度比较低,说明发生干涸现象比传统制冷剂较早。

In order to design a high-efficiency CO2 evaporator, it is indispensable to investigate two-phase flow and boiling heat transfer characteristics in CO2. By theoretical simulation, it is found that the critical gas-phase velocity for CO2 liquid droplet entrainment is far smaller than that of the traditional refrigerants, and the CO2 entrainment rate is higher than that of the traditional refrigerants. This shows that annular flow is the predominant flow pattern for CO2 two-phase flow. The depression mass quality value for CO2 nucleate boiling is relatively large and about 1.5 times of that of the traditional refrigerants. It indicates that the flow boiling heat transfer mechanism for CO2is nucleate boiling. In addition, the inass quality for onset of dry-out for CO2 is much lower, and shows that the phenomenon of dry-out for CO2 occurs earlier than that of the traditional refrigerants.