Impingement Heat Transfer: CHT CFD Predictions of the Influence of Reduced Crossflow using Large Gaps.

Experimental and Numerical investigations were carried out on impingement jet cooling, for variable gap to diameter ratio Z/D ranging from 0.76 - 6.42 with varied Z, constant D and constant mass flux G of 1.93 kg/sm2bar, which is typical of G for regenerative backside cooling of gas turbine combustors. This is the cooling geometry relevant to reverse flow cylindrical combustors with low NOx burner where air used for film cooling increases the NOx. The geometries investigated were for 10 × 10 square array of impingement jet cooling holes at constant diameter D and pitch X, hence constant X/D ratio. The experimental results used the lumped capacity method to determine the locally surface average heat transfer with thermocouples spaced at 25.4 mm intervals in the direction of the single exit flow 152.4 mm long impingement gap. The target walls were 6.35 mm thick Nimonic-75 alloy materials that were electrically heated to about 353 K with a coolant air temperature of 288 K. Conjugate heat transfer (CHT) computational fluid dynamics (CFD) were applied to the same geometries. The predicted CFD results agreed with the measured pressure loss, which indicates that the predicted aerodynamics were good. Also, the locally X2 and overall surface average heat transfer coefficients (HTC) h were well predicted, apart from at the lowest Z/D. The pressure loss increased significantly for Z/D <3 and h also increased but this was not a practical design due to the excessive pressure loss.