The impact of temperature ratio on the overall cooling performance of conjugate cooled configurations

This thesis report presents the results from low-order model, CFD and experimental analysis to be used for parametric studies of adiabatic film and overall cooling effectiveness for fully cooled systems (internal and film) under wide ranges of mainstream-to-coolant temperature ratio variation. The purpose is to improve understanding of the scaling process from typical rig conditions to engine conditions. The interest is in the variation in overall effectiveness when the controlling non-dimensional groups change in a natural co-dependent way with changes in temperature ratio. This is distinguished from the situation in which individual non-dimensional groups are varied in isolation. The design, development and commissioning data from a new high temperature (600 K) test facility is presented together with detailed uncertainty analysis. This test facility operates at engine-realistic Reynolds number, Mach number and mainstream-to-coolant pressure ration across a mainstream-to-coolant temperature ratio from 0.50 to 2.30. For experiments, a cooled flat-plate configuration was designed incorporating a reverse-pass cooling design and 45° V-shaped broken rib turbulators to enhance the coupling of the internal and external systems. From CFD studies, adiabatic film effectiveness was found to increase with increasing mainstream- to-coolant temperature ratio. The percentage change in adiabatic film effectiveness (from reference case with the temperature ratio of 2.00) were from -21.2 % to -1.38 % for temperature ratio from 1.10 to 1.90. Here for matched Reynolds number, Mach number and mainstream-to-coolant pressure ratio, the increase in adiabatic film effectiveness was found to be due to the increase in both specific heat capacity flux ratio (+70 % of change) and blowing ratio (+40 % of change). The overall cooling effectiveness measurements from experiments conducted in the mainstream-to-coolant temperature ratio range between 1.09 to 1.62 were presented. Overall cooling effectiveness was found to increase with increasing temperature ratio, with predicted percentage changes from the reference condition (temperature ratio of 1.60) in external wall overall cooling effectiveness of -5.92 % to -1.38 % for temperature ratio from 1.10 to 1.50 respectively. On the internal wall the percentage change in overall cooling effectiveness was -3.73 % to -0.78 % for temperature ratio from 1.10 to 1.50 respectively. To explain the trends in overall cooling effectiveness, a bespoke low-order conjugate aerothermal model (calibrated against measurements) was used. From the low-order model, the increase in overall cooling effectiveness was found to be driven largely by the increase in external film effectiveness (+68 % of change) and the ratio of mainstream-to-coolant heat transfer coefficients (+46 % of change). Secondary influence was a Biot number effect (-18 % of change). The influence of internal coolant warming factor coolant factor was found to be very small (+4 % of change).