The effect of short pin fin aspect ratio on thermal characteristics of intermittent impinging jet; An experimental and numerical study

Abstract

BackgroundPin fins are secondary surfaces extending from an object to enhance the heat transfer rate. Pin fin is one of the most effective practical techniques used in many industrial applications and thermal treatments especially for cooling electronic devices. Also, Pulsating flow is extremely used in the machinery industry and it can improve the heat transfer. Therefore, pulsating flow is combined with an extended surface to enhance the heat transfer efficiency. MethodsThe current paper investigates numerically and experimentally the intermittent impingement flow and consequence heat transfer characteristics in the presence of a pin-fin array over a flat plate. The RNG k − ɛ model is employed to simulate unsteady three-dimensional turbulent flow using CFD software. The influences of jet Reynolds number, pulsation frequency, jet-to-surface distance, and aspect ratio on the distribution of the area-time averaged Nusselt number are studied. 36 elliptical pin-fins considered at three rows on the surface, placed at r/ D = 3, 5, and 7 (36, 60, and 84 mm) from the central point of the jet. Considering the influence of the aspect ratio on the streamline and temperature patterns of the flow, numerical simulations were conducted in various pin-fin diameters of 4–12 mm. Significant findingsFrom the experimental data, the results indicated that there is a specific frequency that the heat transfer increases for Reynolds numbers in the range of 10,000–20,000. The results illustrated that the rate of heat transfer is reduced with rising the diameter of the pin fin for AR>1, whereas it is enhanced for AR<1. The area-time averaged Nusselt number is enhanced with rising aspect ratio at various pin fin heights. For Re=10,000, the heat transfer rate decreases with increasing the jet-to-surface distance in steady state while it increases for pulsating flow. Also, the maximum value of turbulent kinetic energy appears nearby the pin-fin owing to the formation of a horseshoe vortex. The vortex structures are strengthened and grow downstream, as the frequency increases.