For convective heat transfer involving multiple sources of different temperatures in the flow field such as in film cooling, the adiabatic-wall temperature, Tad, is used as the reference temperature to define the heat-transfer coefficient (HTC). Studies based on computational Fluid Dynamics (CFD) have always obtained Tad by requiring the cooled or heated surface to be adiabatic. Similarly, most experimental studies that measured Tad have sought to mimic adiabatic wall by using solids with very low thermal conductivity. Other experimental studies have obtained Tad by making two assumptions: (1) Tad at any given point on the cooled or heated surface is independent of the surface temperature, Ts, and the surface heat flux, qs, at that point and (2) the HTC, had, which equals qs/(TsTad), depends only on the ratio of qs to TsTad. With these two assumptions, measuring qs at two different Ts or vice versa at a point yields Tad and had at that point. In this study, CFD simulations, based on steady RANS, were performed to assess the assumptions invoked by CFD and experimental studies that seek to obtain Tad and had. The assessment was made by studying film cooling of a flat plate with an adiabatic wall and with isothermal walls, where the temperature of the isothermal wall, Ts, ranged from the lowest to the highest temperatures in the flow. Results from this study show Tad obtained by enforcing an adiabatic wall does not satisfy the requirement: where and when TsTad = 0, qs = 0. Therefore, had approaches infinity where TsTad is either zero or nearly zero, but qs ≠ 0. Also, obtaining Tad and had by measuring two sets of (Ts, qs) was found to yield non-unique values that depended strongly upon the pair of (Ts, qs) chosen. To overcome the shortcomings of existing methods, a new method was developed in this study to obtain Tad that does satisfy the requirement: TsTad = 0 where and when qs = 0. Also, the method developed yields an had that is continuous across TsTad = 0. By using the new method developed, errors in Tad and had obtained by existing methods were assessed.

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