Numerical Simulation And Experimental Study On Transient Heat Transfer Characteristics By Jet Impingement At Stagnation Point

The local heat transfer efficiency of jet impingement cooling is very high,especially at the stagnation point.But there is not a reasonable theory to explain the mechanism of the high heat transfer efficiency at the stagnation point.So,it is important to explore the transient heat transfer characteristics at the stagnation point to improve the jet impingement cooling theory.In this paper,the liquid unsubmerged jet was the research object.The Fluent,a fluid simulation software,was the research tool,and the original Newton cooling curve distribution characteristics were used for reference.With numerical simulation and experimental research methods,the transient heat transfer characteristics at stagnation point were studied base on thermal convection.In addition,the steady heat transfer processes of single and combined jet impinging vertically and obliquely on the wall were numerically simulated,and the influence of jet flow field on the heat transfer characteristics of the wall was quantitatively analyzed.First of all,the heat transfer mechanism of impingement jet at stagnation point was simulated by numerical simulation.This paper expounds the original description of Newton’s cooling law,interprets Newton’s cooling curve and determines two main distribution characteristics.The stagnation point was presented by a little steel cube.The transient heat transfer processes of vertical and horizontal jets and forced convection cooling of the steel cube were numerically simulated,respectively.The results showed that the distribution characteristics of the transient temperature response curve at the center of the six faces of the steel cube were identical with the original Newton cooling curve,which proved that the heat transfer mechanism at the stagnation point of the jet was convection rather than heat conduction.At the same time,it was found that the transient temperature response curve of the jet impingement cooling cube steel cube was very similar to that of the forced convection,which indicated that the heat transfer mechanism at the stagnation point of the jet was essentially similar to that of the forced convection.In addition,a theoretical explanation was given on how the transient temperature response curve guides the design of the array jet impingement cooling scheme and its physical meaning.Secondly,an experimental device to find the heat transfer mechanism of impingement jet at the stagnation point was designed and built.A 304 stainless steel cylindrical specimen was used to simulate the transient heat transfer process at the stagnation point,and the error of the test results was analyzed.The results showed that the transient temperature response curve at the stagnation point presented an exponential distribution.When the temperature of the specimen was high and the nozzle outlet flow was large,the relative change rate of the temperature difference between the thermal specimen and the cooling water approached a fixed value.Influenced by the accuracy of the temperature sensor and the strong jet turbulence,the lower the temperature of the specimen,the greater the fluctuation of the relative rate of change.Then the experimental results were compared with the simulation results.The overall experimental results were smaller than the results from simulation.On the one hand,the actual temperature measuring point of the temperature sensor deviated from the stagnation point position.On the other hand,due to the size of the temperature sensor,the size of the specimen was too large,so the influence of heat conduction inside the specimen cannot be ignored.Finally,the impact cooling process of the single jet with different angles was simulated numerically,and the influence of the jet flow field on the heat transfer characteristics of the wall was discussed quantitatively.The result of combined jet impingement cooling model indicated that the heat transfer in stagnation zone and wall jet zone was closely related to the velocity component of jet.Meanwhile,the effects of inclined nozzle angle and spacing on the wall heat transfer characteristics in the combined jet impingement cooling model were gotten.When the inclined nozzle was close to the straight one,the overall heat transfer characteristics of the combined jet were similar to that of the single inclined jet,and the cooling effect of the upstream and downstream of the jet was closely related to the inclination angle of the inclined nozzle.The average Nusselt number first grew and then declined with the increase of inclined nozzle angle.It meant that the inclined nozzle angle had an optimal value.When the inclined nozzle was away from the straight one along the horizontal direction,the average Nusselt number rose,the two high temperature zones in the upstream away from the straight nozzle moved to the downstream and the cooling efficiency was increased.When the inclined nozzle was away from the straight one along the longitudinal direction,the average Nusselt number went up,and the two high temperature zones in the upstream away from the straight nozzle decreased until they disappeared gradually.At the same time,the uniformity of the wall temperature in the downstream of the jet became worse gradually.