Numerical Analysis Of Dry-out Critical Boiling In Vertical Heating Pipes

The critical heat flux phenomenon on the surface of the reactor fuel assembly is related to the safety and economy of the nuclear reactors during operation.With the rapid development of computer technology and computational fluid dynamics,as well as the demand for refinement analysis of reactors,CFD technology has started its functions of numerical simulation of two-phase flow,on the basis of successfully solving single-phase flow problems with more refined grid size,more robust and efficient numerical methods.At present,commercial two-phase CFD software FLUENT based on the Eulerian-Eulerian two-fluid solver and,together with the wall boiling model in the multiphase flow model,already has certain numerical analysis ability of two-phase flow and can accurately analyze heat transfer and critical heat flux phenomenon on the surface of fuel bundles of PWR or BWR.The two-phase CFD numerical simulation was carried out based on Bennett et al.'s experimental study on the gas-water in a vertical circular tube,taking into account the codes immature and lack of the experimental data of two-phase flow within the reactor fuel assembly.The heat transfer under uniform heating heat flux density,and the correlation analysis of the boiling process was performed.In this paper,the commercial two-phase CFD_FLUENT is used to simulate the two-phase flow in the vertical heating pipe.The velocity inlet,the pressure outlet and the symmetrical boundary conditions are used to perform calculations.By comparing with the data of experiment,the most important factors are determined in the Euler two-fluid model: the grid and the turbulence model.Subsequently,the wall boiling model and the two-phase flow pattern are discussed in detail.By comparing with the experimental temperature,a reasonable wall boiling model is selected to simulate the critical drying phenomenon.The calculated results show that the two-phase k-? mixture turbulence model can obtain satisfactory results.Simultaneously,the calculation results of the CHF wall boiling model is more reasonable,which considers the effect of the two-phase flow pattern on the gas-liquid two-phase flow.Secondly,the momentum transfer between two phases directly affects the accuracy of numerical simulation of two-phase flow.Therefore,after determining the baseline combination of interphase forces,the correlation analysis between the two phase drag and non-drag forces was carried out.By comparing the distribution of void fraction,the drag model has no effect on the distribution of bubbles.The models of lift and wall lubrication force have an effect on the radial distribution of bubbles,while the model of turbulent dispersion force has the greatest influence on the distribution of steam.In some sense,the turbulent dispersion force has a great influence on convergence of numerical calculation,and the force makes the distribution of the steam evenly in the whole flow area,which can effectively reduce the peak of the void fraction.Finally,this paper conducts the simulations of the flow boiling and critical heat flux(hereinafter,referred to as CHF)problems in the tube,focusing on the method of determining CHF during the numerical calculation,and the parameter analysis of the critical phenomena in the tube.The results show that the CHF judgment is reliable by the rise of the maximum wall temperature.With the increase of system pressure,the CHF first increases and then decreases.CHF increases with the increase of inlet mass flow rate.The longer of the pipe,the critical heat flux is smaller.In addition,the critical heat flux decreases with the pipe diameter decreases.