Experimental/Numerical Study of Multiple Rows of Confined Jet Impingement Normal to a Surface at Close Distances

Impingement cooling is used in a variety of applications ranging from industrial bakeries, paper processing, heat exchangers and specially gas turbine engines of all sizes to name a few. Convective impingement cooling has been studied numerous times in a variety of configurations. However little work has been conducted regarding impingement between two surfaces separated by less than one impingement jet hole diameter. This configuration is of special interest for gas turbine cooling applications such as in shrouds, combustor liners and airfoils cooling cavities where small holes are used to cool and purge cavities between two adjacent pieces of hardware. In this study, flow and temperature fields as well as heat transfer coefficients for confined jet impingement are being investigated for multiple rows of round jets impinging normal to a target surface less than one hole diameter from the jet origin. The experiments were conducted for five rows of jets with five jets on each row and steady-state liquid crystal thermography for heat transfer measurements were utilized. Numerical results were obtained from a three-dimensional unstructured computational fluid dynamics model with over 4 million hexahedral elements. For turbulence modeling, the realizable k–ε was employed in combination with enhanced wall treatment approach for the near wall regions. Other available RANS turbulence models such as k–ω, v2f and large eddy simulation were tried for selected geometries and results are compared with those of k–ε model. Nusselt numbers on the target areas and discharge coefficients for flow across the jet holes are reported for jet Reynolds numbers ranging from 10000 to 50000, pitch-to-diameter, P/d, values of 2,3 and 4, each for jet distance-to-diameter Z/d, values of 0.3, 0.4, 0.5, 0.6, 0.8, 1, 2 and 3. Comparisons are made between the test and numerically-obtained results in order to evaluate the employed turbulence models and validate the numerically obtained results. Results showed severe reduction in discharge coefficients as the jet holes were brought closer to each other and closer to the target wall. Heat transfer performance for the hole lateral spacing of P/d = 4 was found to be superior to that for P/d = 2 or P/d = 3.