Numerical Simulation Of Three Typical Wall Jet

As a common flow phenomenon in nature,wall jet is widely used in engineering applications such as window defogging,electronic component cooling and aircraft wing defrosting.In the past,a lot of experiments and numerical simulations have been carried out on the effects of various thermal boundary conditions on the heat transfer characteristics of wall jet in the fully developed turbulent region.However,these studies mainly focus on the dimensionless flow direction of the jet X/H>30,where the jet has been fully developed.For the incompletely developed jet flow field and heat transfer characteristics,scholars have not carried out in-depth numerical simulation research.In this thesis,the flow field and temperature field of the wall jet with boundary layer transition are studied by using Low-Reynolds-Number k-?models and the results were combined with the experimental data provided by R.S.abdulnour et al.In the intersection region of two fluids with temperature difference exist a buoyancy effect opposed to the background flow direction,at the meanwhile where temperature is highly instantaneous.Therefore,the numerical simulation of wall jet with temperature difference and buoyancy has always been a challenge in the field of Engineering research,and there are few related numerical simulation literatures before.In this thesis,the improved RNG k-?model,Realizable k-?model and Low-Reynolds-Number k-?model are selected to study the flow characteristics of different region of wall jet with temperature difference and buoyancy.The results of the calculation are used to evaluate the turbulence models,so as to provide reference for the related engineering research fields.In order to study the local high heat transfer efficiency of impinging jet,many scholars have studied it by means of numerical simulation.However,in the case of large Reynolds number and small width aspect ratio,the calculated Nusselt number always deviates from the experimental data.Aiming at this problem,in order to get more accurate temperature field,this thesis discusses and improves the traditional method to solve the temperature field,in which the key turbulent thermal diffusion coefficient is solved by using the empirical formula of turbulent Prandtl number and the control equation of t²-?_t,and the results are compared with the traditional turbulent Prandtl constant.On this basis,the effects of constant coefficient of turbulence model,turbulence time scale and turbulence dissipation rate on the results of heat transfer calculation are further explored.According to the calculation results of the above three typical wall jet phenomena,the following conclusions are obtained.In the numerical simulation of wall jet with boundary layer transition,the dimensionless velocity field in the main flow region after entering the self-similar region calculated by the standard k-?model with enhanced wall function and the CHC,AB model have certain accuracy and engineering application value,but they are not suitable for the calculation of the flow and temperature field before the initial boundary layer transition.Since the damping function f?in LB and LS models does not meet the wall-limiting behavior,it will bring adverse effects on the calculation results of near wall flow field and heat transfer.The AKN model which can accurately predict the transition position of boundary layer to turbulent flow was recommended to be used in the numerical calculation of the heat transfer of the wall jet.In the numerical simulation of buoyancy-opposed wall jet,the RANS model of k-?series can give the accurate flow field distribution under isothermal condition.The LS model added an additional source term to the turbulence control equation to modify the turbulent viscosity can give the flow field results closest to the experimental data.The YS model predicted a smaller turbulence dissipation due to the damping function f₁ less than 1 have a little bit difference between experimental data.In the buoyant case,the RANS models of k-?series can not accurately predict the velocity distribution in the mixing region due to its time-averaging method and isotropic dissipation.In the meantime,since the experimental conditions can not be completely adiabatic and the turbulent Prandtl number assumption was used,the calculated temperature field distribution in the wall jet and mixing region also has a large deviation from the experimental dat.In the numerical simulation of impinging jet heat transfer based on low Reynolds number k-?model,for the calculation of turbulent thermal diffusion coefficient,it is suggested to use the turbulent Prandtl number formula proposed by Kay et al.and the t²-?_t equation not affected by the Reynolds approximation assumption still needs to be further improved.In the case of large Reynolds number and small width aspect ratio,AB model suggests that the model constants of the standard k-(?)model should still be used.Under the condition that the modified turbulence dissipation rate(?)?code is correct,in the case of low Reynolds number and large aspect ratio of width,the LS model can give a flow direction Nusselt number curve which is approximately similar to the experimental data trend.