Conjugate Heat Transfer CFD Predictions of Impingement Heat Transfer: Influence of the Number of Holes for a Constant Pitch-to-Diameter Ratio X/D

Abstract

Conjugate heat transfer CFD studies were undertaken on the influence of the number of impingement holes/unit surface area or hole density n (m−2) for n from 1076 to 26910m−2 at a constant X/D of 4.7, with n varied by varying the hole diameter D from 1.31 to 6.54mm and pitch X varied from 6.1mm to 30.5mm. Square array impingement cooling geometries for the jet holes were used with a 152.4 × 152.4mm experimental wall area. The impingement gap had a single sided exit which generated a cross-flow in the gap. The number of impingement holes N in the cross-flow direction was 5, 10, 15 and 25. A coolant mass flux G of 1.93kg/sm2bar was investigated at a constant impingement gap Z of 10mm (Z/D 1.53–7.65 as n was varied). This high coolant mass flow simulated the coolant flow for regeneratively cooled combustors using all the combustor air flow to cool the combustor wall prior to entering the low NOx flame stabiliser. The predictions were compared with experimental results for the heat transfer coefficient h, that used the lumped capacitance method. The predictions of the surface averaged h and pressure loss ΔP/P were in good agreement with the measured results. The predictions showed that increasing the number of impingement jet holes resulted in lower h, due to the impact of cross-flow for large numbers of holes. At the other extreme, a very small number of holes were predicted to have high thermal gradients. The maximum heat transfer was found experimentally and computationally to be 4306 holes per m2 for an X/D of 4.7, with acceptable thermal gradients.