A New and Simple Concept to Replace “Film Effectiveness”

Abstract

Film effectiveness (η) is used to describe the adiabatic wall temperature (TAW) which results when a colder or hotter fluid is injected into the boundary layer. Eq (1) is the defining equation for η:(1)(TMS−TAW)=η(TMS−TF)where TF and TMS refer to the temperatures of the film and mainstream fluids. Literature data indicate that η is a constant coefficient in Equation 1 only in applications where TF/TMS is restricted to a narrow band. If TF/TMS varies over a wide band, such as in gas turbines and rockets, then η is a variable in Eq 1. This article demonstrates that, in applications where TF/TMS varies over a wide band, the film effectiveness concept is neither simple nor direct, and it complicates and confuses the solution of practical problems. This article also presents a new and simple concept to replace film effectiveness. The new concept retains the TF/TMS parameter used in film effectiveness methodology, but it replaces η with TAW/TMS. The particular advantage of the new concept is that it describes TAW in a simple and direct way even if TF/TMS varies over a wide band. The end result is that, in applications such as gas turbines and rockets, the new concept is much simpler and therefore much better than the film effectiveness concept it replaces.

The Highly Nonlinear Thermal Behavior of Film Cooling

“…(4) would be written Taw = Tmsn[T fc ,Tms J(Tms -Tfc) 5Thus the current view represents a definite conclusion about film cooling behavior, just as the view that r, is independent of Tf c and T ms represents a definite conclusion. (Adiutori (1991) notes that, when rî s dependent on Tfc/Tms (as in the current view), f[Tfc/Tms] is mathematically unacceptable, and should be abandoned in favor of the parameter (Taw/Tms)[Tfc/Tms].…”