Experimental Study Of Void Fraction And Pressure Drop Characteristics Of Air-water Two-phase In Rod Bundles With Spacer Grids

The void fraction and pressure drop are the basic parameters of the two-phase flow system,and the void fraction is related to the calculation of the two-phase pressure drop.The accuracy of their prediction is related to the accuracy of the reactor thermal analysis program and the safety and economy of the reactor operation.Based on the experiment of air-water two-phase flow in 5×5 rod bundle with spacer grids at atmospheric pressure,the characteristics of the void fraction and pressure drop in rod bundle channel are measured and studied.Based on the experimental data of low pressure and low flow,the relationships between the distribution parameters and the drift velocity,void fraction,mixture superficial velocity are analyzed.The distribution parameter is mainly affected by recirculating flow patterns,and the trend of the distribution parameter and mixture superficial velocity is most regular.The change trend is divided into three parts: the linear increasing part,the critical point and the exponential descending part.On the basis of Clark model,new fitting curve of the exponential descending part are obtained by using present data,and considering the different effects of the bundle scale and the liquid shear,a new correlation of critical point is proposed.The improved Clark model gives relatively better prediction for present data.For bare rod channel,the spacer grid with mixing vane is less significant for flow field in single-phase laminar and turbulent flow,but it has great effect in transition region.Therefore for this situation,it is suggested that the coefficient ? of Cheng model should be 2/3.Through summing up the existing rod bundle two-phase frictional pressure drop experimental data,it is found that the two-phase friction multiplier and the Martinelli parameter can be expressed by a general expression of Chisholm C model.Moreover,the fitting parameters C and n have a certain function relationship with P/D,so as to establish a two phase friction pressure drop model with good applicability and accuracy.The single-phase spacer grid pressure loss coefficient is related to drag coefficient of strip,nugget,spring and mixing vane,generally,it is a function of the Reynolds number.Since only considered the drag coefficient of high Reynolds number in Chun model,it is essential to modify Chun model for present spacer grid.For the two-phase flow,due to the two-phase disturbance,the pressure loss coefficient increases with the increase of gas flux,as well as rapidly increases with the increase of the void fraction.Through Utilizing present two-phase spacer pressure loss data to evaluate the existing separated model,slip model and homogeneous model,it is found that the separated model and the slip model basically satisfy the engineering calculation,while the homogeneous model predicts poorly,and its error will increase with the increase of the void fraction.