Experimental Study On The Flow Boiling In A Large Aspect Ratio Of Small Channel

In this paper,based on the design of the plate steam generator in the secondary circuit of PWR,the researches on the flow boiling heat transfer characteristics of large aspect ratio rectangular and semicircular mini-channel and visualization of rectangular mini-channel were carried out.In this paper,experimental system for flow boiling and visualization of long mini channel under different pressures,heating methods,mass flux and heat flux was built.The main flow patterns and the heat transfer mechanism were studied respectively.In the flow boiling visualization experiments,seven kinds of flow patterns were observed at atmospheric pressure,namely:bubble flow,slug flow,confined bubble,elongated bubble,mixing-annular flow,annular flow and dry flow;In addition to these flow patterns,a more pronounced throat-annular flow was observed at high pressure.The percentages of flow patterns in long channel under different experimental conditions were analyzed respectively.In the study of the flow boiling heat transfer under uniform heating conditions,heat transfer characteristics of rectangular channels under normal pressure(0.1～0.3 MPa)and high pressure(2.5～2.7 MPa)were studied.The effects of heat flux and mass flux on nucleate boiling and convection boiling mechanism were studied,respectively,and the effects of pressure on the flow boiling heat transfer characteristics of two-phase flow are emphatically analyzed.In the analysis to the channel with semi-circular cross section,the experimental data were compared with the prediction of Fang formula,and the heat transfer correlation was modified by introducing the aspect ratio,and the prediction error was controlled within 21.59%.Through the step-changing heating experiments of the rectangular channel,the heating conditions were simulated in order to close to the counter current high temperature water heat flux as far as possible.The similarities and differences between visualization and heat transfer characteristics under uniform heating conditions were studied.Compared with the previous heat exchange correlation,the heat transfer characteristics of long channels were corrected for engineering application.