Investigation On Heat Transfer And Mixing Characteristics Of Jet Excited By Tabs

The tabs are a kind of vortex generators with simple configurations and wide applications in control or improvement jet flow structures. Although a series of investigations have been made on jet impingement heat transfer and mixing in coaxial jets using tabbed vortex generator, there are some problems being of challenge and innovation, such as the physical mechanism of tabbed jet impinging heat transfer, the influence rules on tabbed jet impingement heat transfer and mixing enhancement in coaxial jets by different tabs configuration, and the interactional characteristics of flow fields between tabbed jet flow and cross flows. The present paper aims on two aspects:(1) For the jet impingement cooling, the numerical calculation and experiment were conducted to investigate the performance of flow and heat transfer of tabbed jet impingement heat transfer for three jet impinging modes, such as single row jet impingement, double rows jet impingement and single row jet impingement with initial cross flow. The influences of tab configurations on heat transfer enhancement were analyzed and the physical mechanism of flow field control and heat transfer enhancement by tabbed jet was made known.(2) For the mixing in coaxial jets, the producing and developing rules of stream wise vortices and the flowing structure changing by the jet flows with tabs were made known, and the effects of tab configuration with the different structure and installation manner on mixing process were researched. At the same time, the optimum tab configuration parameters were put forward.The investigation on tabbed jet impinging heat transfer enhancement mechanism indicates as follows. In jet impingement cooling, because of the tabs inserting the jet at the orifice edge, two effects could be obtained. On the one hand, a pair of counter-rotating stream wise vortices appears on the both sides of the tab and developes along jet flow direction. It exchanges quantity of heat and momentum with the surrounding fluid. The stream wise vortices induced by tabs intensify turbulence motion in the boundary layer depending under their rolling and mixing process. The augment of turbulent kinetic engegy is benefit to the enhanced convective heat transfer. On the other hand, the jet flow passage takes on convergence, making for the increase of jet core velocity. It is propitious to improve the convective heat transfer near the stagnant region.The numerical calculation and experiment investigation on the impingement cooling in the jet excited by the tabs to improve heat transfer indicates as follows. The influence of the tabs number on the impingement heat transfer is related to the diameter of the impingement orifice. The tab declining angle and the tab length inserting the jet also have influences on the impingement heat transfer. For the impingement cooling with double-rows tabbed jet flows, the change of the stream wise space between the orifices has an important influence on the target temperature distribution. Under the presence of the initial cross flow, the inlet mode of initial cross flow (inline or staggered related to jet) has little influence on the jet flow and heat transfer when the initial cross flow is weak, but behaves great difference when the initial cross flow becomes dominant. In a word, the tabbed jet impingement heat transfer is more sufficient than the common jet.During the mixing of the coaxial jets, the surrounding fluid flows toward the mainstream along the radial after the mainstream passes through the tabs. Then the mainstream is disparted by two counter-rotating fluids so that a pair of counter-rotating stream wise vortices is shaped on the both sides of the tab. The stream wise vortices are weakening uninterruptedly and enlarging and mixing with the surrounding fluid and then disappearing finally along with the flow of the mainstream and the mixing between the mainstream and the secondary flow. The presence of the stream wise vortices enhances and reinforces the mixing between the mainstream and the secondary flow, shortening the mainstream core length. The turbulence kinetic energy in the surrounding fluid is increased after the mixing. The shape of the temperature field takes on obvious change at the exit section of the tabbed jet flow. The low temperature area expands and the high temperature area shrinks at the inserting position of the tabs. The temperature field is even much more equably.The numerical calculation and experiment investigation on the strengthening mixing tab in coaxial jets indicates as follows. The appropriate number of the tabs and the appropriate jam proportion are the key factors for make the optimal mixing effect. The angle and length of the tabs also have important influence on the mixing effect. In the conditions given in the present paper, the optimal tab parameters are obtained. The tab number is 8, the tab angle related to the jet axes is 30°, the tab tip angle is 90°and the ratio of tab length to nozzle diameter is 0.1.