FLOW CONTROL AND HEAT TRANSFER ENHANCEMENT OVER SURFACE MOUNTED OBSTACLE USING SYNTHETIC JET

Abstract

Ribbed surfaces have been used for various applications such as cooling gas turbine blade, heat exchanger, electronic chip cooling, etc. The flow field behind the rib is very complex due to its unsteady nature. Control of flow over ribbed surface is quite challenging due to its unsteady and possibly turbulent nature. Flow over a ribbed surface is separated and forms a shear layer downstream of the rib. A wake zone is created behind the rib where the flow is recirculated and the flow velocity reduced near the wall. The reduced flow velocity near the wall suppresses the heat transfer from the surface. Reduction of the wake zone (recirculation zone) or modification in the flow field behind the rib is the prime motive of the present work. Abstract Present study uses both the active and passive method of flow control to alter the flow field behind rib. Five different geometrical shapes of rib have been used to achieve the passive flow control and observe the modification in the flow field. However a detailed investigation has been done for three geometrical shapes namely square, trapezoidal with chamfer angle 21° and triangle rib. The present study proposes an novel active method of flow control using synthetic jet. A synthetic jet has been applied from the bottom of the surface to achieve the flow control. Experiments have been conducted in an open circuit horizontal low speed wind tunnel. Particle Image Velocimetry, Hot Wire Anemometer and a flow visualization technique have been used to measure the experimental data. A speaker has been used to generate the synthetic jet through a circular orifice created on the bottom surface of the test section. The synthetic jet has been located in different upstream and downstream positions to examine its effect in the flow field. The effect of different operating parameters of synthetic jet for optimum locations has also been investigated for all three geometries. Experiments are performed for three different Reynolds number, however a detailed investigation has been done for a Re=32000 calculated on the basis of hydraulic diameter of tunnel (Re=1400 on the basis of rib height). The present investigation shows that active flow control achieves the maximum reduction of reattachment length and has a maximum positive effect on recirculation zone over passive flow control. The locational study of synthetic jet for a square rib identifies two optimum locations for which maximum reduction in reattachment length is observed. The upstream location of synthetic jet (at x/D=-1) shows complete removal of recirculation bubble and hence a drastic reduction in reattachment length, however x/D=1.5 is found to be the optimum location in the downstream position of rib. Results show that complete removal of recirculation bubble is found at operating parameters of 80Hz frequency and 4Vpp amplitude settings of the synthetic iv jet for the square rib. The high strength vortical structure which develop inside the shear layer diffuse early with the effect of synthetic jet for the same parameters and make the flow comes inside the recirculation zone. Near wall study shows that turbulence increases by introducing the synthetic jet in upstream location, however maximum intensity of turbulence inside the shear layer decreases. The trapezoidal rib with chamfer angle 21° has a maximum reduction in reattachment length for the 50Hz operating frequency of synthetic jet (fixed actuating amplitude 4Vpp) in both the cases either in upstream location of synthetic jet (at x/D=-1) or in downstream location of synthetic jet (at x/D=1.5). However, variation in actuating amplitudes of synthetic jet for fixed operating frequency shows a maximum reduction in reattachment length at 6Vpp amplitude for the downstream location of synthetic jet and at 8Vpp amplitude for upstream location of synthetic jet. The triangle rib shows a maximum reduction in reattachment length for unexcited flow. However, the effect of synthetic jet is less attractive for a triangle rib compared to other ribs. The locational effect of synthetic jet (for fixed actuating frequency and amplitude) does not show any reduction in reattachment length. However, the effect of actuating frequency of the synthetic jet located in the downstream position indicates a remarkable reduction in reattachment length for a 50Hz actuating frequency, while reduction is not seen for the upstream location of synthetic jet. Different actuating amplitudes of synthetic jet (for fixed frequency) do not show any significant reduction in reattachment length for the downstream location of synthetic jet. Reduction in reattachment length for the upstream position of synthetic jet is achieved at a 6Vpp actuating amplitude. Heat transfer is found to be enhanced by using the synthetic jet located in the upstream position for different Reynolds number. The position of maximum heat transfer is slightly moved towards the rib direction. The effect of a synthetic jet on heat transfer enhancement is found minimum for the triangle rib. The heat transfer enhancement achieved by using the synthetic jet for the square rib compared to an unexcited flow in lieu of power consumption of synthetic jet is fairly good.