Abstract
We present an experimental study of the impact of mainstream-to-coolant temperature ratio on overall cooling effectiveness. 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 decreases with decreasing temperature ratio. This result is in accord with the modelling work of Naidu and Povey [1], and the theoretical work of Cartlidge and Povey [2]. 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 0.017; (for overall cooling effectiveness of 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; internal heat transfer coefficients; film effectiveness of the mixing layer; wall thermal conductivity; and internal cooling effectiveness. To the authors' knowledge, this is the first published data validating the theory of scaling of overall cooling effectiveness between different temperature ratio conditions.
