Numerical Simulation Study On Heat Transfer Enhancement Of Unsteady Impinging Jet

Jet impingement technology is widely used in various industries.because of its high heat transfer coefficient.As unsteady jet impingement has better heat transfer potential,the purpose of this thesis is to reveal the influencing factors of heat transfer in unsteady jets and to analyze the mechanism of heat transfer enhancement by which some contradictory phenomena in the literature could be explained.And the corresponding design suggestions are put forward for the application of unsteady jet heat transfer.Compared to the experimental study,the numerical simulation has benefits of low cost,short cycle,and more importantly,it can capture the details of the flow field.In this paper,the numerical simulation method was used to study the impingement heat transfer of unsteady jet.The applicability of the turbulence models was evaluated,and a series of factors such as the average velocity,velocity amplitude,frequency and oscillation waveform were studied.The specific research contents are as follows:Numerical simulations of steady jet were carried out by using a variety of turbulence models and compared with the experimental results in the literature.The influence of velocity parameters on heat transfer was analyzed.The results showed that,compared with Transition SST turbulence model,results of RNG k-? model showed bigger turbulent viscosity and caused the velocity in the jet central line decreased more quickly,which may cause the underestimation of heat transfer on the target wall;and in the impingement region,RNG k-? model over-estimated the turbulent kinetic energy,which may cause the overestimation of heat transfer on the target wall.Thus,the magnitude of the jet Reynolds number directly affects the target surface heat transfer that is dominated by fluid inertial forces or turbulent viscosity.Transition SST model is superior to RNG k-? model in modelling jet exit velocity,velocity and turbulence distribution on centerline,stagnation zone heat transfer.The Unsteady Reynolds Averaged Navier Stokes(URANS)is used to simulate the unsteady periodic flow of periodic sine wave jet.The results showed that,the Transition SST model can well simulate the amplitude amplification effect of Reynolds average velocity during unsteady jet development(The time step and the method of temporal discretization have little effect on the calculation of the time-averaged amount and have a significant effect on the calculation of the jet velocity amplitude).For the heat transfer of the periodic impinging jet,the Transition SST model performs well in the stagnation region,while the RNG k-? model was in good agreement with the experimental results in the downstream of the stagnation region.The effect of amplitude amplification does not result in the enhancement of the heat transfer of the impinging target,but only makes the heat flux of the target surface more violent.The numerical results also show that the increase of the jet velocity amplitude may lead to the weakening of the stagnation point heat transfer and the enhancement of the heat transfer in the downstream region of the stagnation point.The intermittent pulsed(Square wave)jet was simulated mainly by the Transition SST model.Compared to the steady jet,it was found that,in the case of Square wave free jet,the attenuation of the Reynolds-average velocity on the centerline is slower,which also coincides with the performance that the steady-state jet turbulence first began to grow significantly.For the case of Re2800 h/d=6,the steady heat transfer of stagnation zone is stronger than 40Hz,which is stronger than that of 5Hz,consistent with the experiment.From the analysis of the flow field,the reason was that the mean velocity difference among the three is very small,while the turbulence is steady state>40Hz>5Hz.For the case of h/d=10,the mean velocity of impingement to the target is 40Hz>5Hz>steady state.Therefore,the unsteady jet impingement heat transfer is enhanced.Through the analysis of free jet flow field,it can effectively guide the setting of target position in engineering practice to realize the enhancement or weakening of heat transfer.