Parametric Study on Micro-Roughness Shapes for Enhanced Jet Impingement Heat Transfer

Effective and powerful cooling methods are of increasing importance in response to the constant push by the gas turbine industry for higher overall turbine efficiency by facilitating the increase in the turbine inlet temperatures. The introduction of micro-pin fins at the leading edge of a gas turbine blade ensures efficient and high rates of heat removal at the blade. For a specified pumping power, micro pin-fin geometries are to be optimized to ensure maximum heat dissipation through convection from the heated target area.
In the present study, micro pin-fins of three different shapes were studied at a steady state condition, computationally. The heat dissipation performance was characterized via Nusselt number and fin effectiveness. A fixed array jet configuration 5×5 jets with a jet to jet spacing (X/Djet) of 3 and jet plate to target spacing (Z/Djet) of 1 was maintained for Rejet ranging from 3000 to 12000. Preliminary studies on the effect of pin-fin shape established that the concentric micro pin-fin element had the best fin effectiveness of 2.45 at Rejet = 12000. Further, separate pin-fin optimization studies were performed for the concentric cylinder pin-fin shape – on the effect of pin-fin height, effect of the spacing between the pin-fins and the effect of internal to external diameter ratio. Pumping power characteristics were also studied for the fins, and the configuration with the best thermal-hydraulic performance was suggested for applicative purposes.In this study, two sets of micro pin-fins of three different shapes were manufactured via binder jetting and conventional CNC machining. The micro-pin fin shapes comprised of concentric cubic, double concentric cubic and radial concentric isosceles triangles. The elements were  in height, with a base dimension of 1.8 mm, on a target plate of dimensions 50.8 by 50.8 by 3.175 mm. Using non-destructive methods, roughness characterization and the dimensional difference between intended and manufactured designs were compared for both the manufacturing methods. Steady state heat transfer experiments were conducted to study array jet impingement onto these manufactured parts. 5x5 jets were arranged such that jet-to-jet spacing was x/d = y/d = 2, and jet-to-target spacing was z/d = 1, for Reynolds number varying from 2500 to 12000. The thermal hydraulic performance and pressure drop were primarily compared between the two methods of manufacturing. Computational studies, inclusive of conjugate heat transfer effects, were also performed using ANSYS Fluent. Computational studies were performed to understand the heat transfer characteristics and flow structure and determine the optimal pin-fin height and spacing between the pin fins. The k- turbulence model with enhanced wall treatment functions and second order upwind discretization method was used for the numerical study. The numerical model was validated with experiments (Singh, P., Zhang, M., Ahmed, S., Ramakrishnan, K.R. and Ekkad, S., 2019. Effect of micro-roughness shapes on jet impingement heat transfer and fin-effectiveness. International Journal of Heat and Mass Transfer, 132, pp.80-95) for the baseline smooth configuration, by comparing the area averaged Nusselt number and has an error of approx. 2% over the range of Re. In order to quantify the enhancement obtained using roughness features, fin effectiveness analysis was performed. The concentric square shape resulted in a fin effectiveness of 1.5 at Re = 9000. It was also found that there was no discernable increment in pressure losses in the micro pin-fin case when compared to the baseline configuration.