Large Eddy Simulations Of Heat Transfer In Impinging Jets

Jet impingement with heat transfer plays an important role in academic research and industrial application. It is a widely studied flow configuration. Applications of impinging jets on surface cooling, heating and drying in the industry continue to increase due to the efficiency in heat and mass transfer. In this thesis, large eddy simulations were performed to study the physics of single impinging unconfined jets with heat transfer using software OpenFOAM. Jet nozzle-surface distance was five nozzle diameters.At first, impinging jets at moderate Reynolds numbers (4400,1.0 x 104 and 2.3 x 104) were simulated. The numerical simulation method was verified by comparing the results of calculation with the data from published papers. Comparison and analysis of transient and time-averaged results of three cases shows that, vortex ring doesn’t appear in any of the three cases. Large scale flow structures remains essentially the same despite the various of Reynolds number of impinging jets. Magnitude of mean velocity normalized by block velocity is independent on Reynolds number, which has some influence on mean turbulent kinetic energy. Normalized mean velocity is not sensitive to Reynolds number at the two radius locations. Reynolds number has influence on the component of normalized mean velocity in the axial direction near the axis of the jet.Then, an impinging jet issuing from an ellipse nozzle was studied numerically to study the effect of nozzle shape on flow and heat transfer. An elliptic nozzle of aspect ratio of 1.5 having the same equivalent diameter with the round jet was simulated at jet Reynolds number of 4400. The transient and time-averaged results were compared with the results of the corresponding round jet. It shows that, there isn’t vortex ring in the free jet region of the ellipse jet either. Large scale structures is similar to those of the round jet. Level of turbulent kinetic energy around the shear layer in the free jet is lower than that in the round impinging jet. There is a second peak of local Nusselt number on the long axis of the ellipse. The ellipse impinging jet has a more uniform heat transfer than the round jet. The heat transfer performance around the stagnation point is worse than that in the round jet. However, if the focused area is large enough, the heat transfer performance is the same in those two jets.At last, numerically studies of a round impinging jet with a inflow condition of velocity field active excitation were performed. In order to learn the potential impact of excitation amplitude on flow and heat transfer, two jets with velocity field excitation of amplitudes of 30% and 70% block velocity were simulated at jet Reynolds number of 10,000. The comparison between the transient and time-averaged results and the results of the corresponding jet without excitation indicate that large flow structures differ essentially with and without excitation. Smooth vortex rings appear in jets with excitation. The periodic velocity variation caused by the velocity field excitation corresponds to the periodic annular vortex structure. When the amplitude of excitation is bigger, large-scale vortex structures are more regular, smooth and more difficult to break downstream and the entrainment is stronger. The level of turbulent kinetic energy is significantly higher in jets with excitation and increased whit the excitation amplitude. The distribution of turbulent kinetic energy is significantly changed when excitation is performed on velocity field due to the change of the large structures in the jet. The influence of the excitation on velocity fluctuation and turbulence stress is more significant than that on the mean velocity. For the area around the stagnation point, the performance of heat transfer is better in jet with excitation of amplitude of 30%Wb, than that of 0, and that of 0 than that of 70% Wb. On larger area, however, the performance of heat transfer is better in jet with excitation of amplitude of 70%Wb than that of 30%Wb, and they are both better than that of 0. The influence of velocity field excitation on heat transfer of impinging jets is insignificant when the focused area is large enough.