Impact of Temperature Ratio on Overall Cooling Effectiveness: Experimental Validation of a Scaling Theory

Abstract In this paper, we present an experimental study of the impact of mainstream-to-coolant temperature ratio on overall cooling effectiveness (or so-called metal effectiveness, θ). Experimental measurements of overall cooling effectiveness of a fully-cooled (internal and film) flat plate were performed in the mainstream-to-coolant temperature ratio (TR) range 1.07≤TR≤1.62. The high-temperature-capable facility used for the study was purpose built for these tests. The experimental data show that overall cooling effectiveness (θ) decreases with decreasing temperature ratio. This result is in accord with earlier modelling and theoretical work, some of which is rehearsed in this paper. We compare the experimental data of this paper to those studies, and find that all three results are in broad agreement so far as the trend with TR is concerned. The typical correction in overall cooling effectiveness between engine (TR = 2.0) and typical rig (TR = 1.2) conditions is approximately Δθ=−0.017 (for surface-averaged overall cooling effectiveness of θ=0.51). The correction is relatively small. By decomposing the results into the individual changes associated with each local surface boundary conditions, we explain the change in overall cooling effectiveness with temperature ratio in terms of changes in: external heat transfer coefficient (responsible for −582% of the overall effect); internal heat transfer coefficients for surface 2–5 (+396%, +11.6%, +69.3% and +1.24%, respectively); film effectiveness of the mixing layer (+111%); wall thermal conductivity (+49.7%); and internal cooling effectiveness (+43.1%). We see that several significant effects are in competition. The results of this paper are—to the authors' knowledge—the first published data validating the theory of scaling of overall cooling effectiveness between different temperature ratio conditions (e.g., between typical laboratory conditions and engine conditions).