Reverse Pass Cooling Systems for Improved Performance

Abstract

Total heat transfer between a hot and a cold stream of gas across a nonporous conductive wall is greatest when the two streams flow in opposite directions. This countercurrent arrangement outperforms the cocurrent arrangement because the mean driving temperature difference is larger. This simple concept, whilst familiar in the heat exchanger community, has received no discussion in papers concerned with cooling of hotsection gas turbine components (e.g., turbine vanes/blades, combustor liners, afterburners). This is evidenced by the fact that there are numerous operational systems which would be significantly improved by the application of “reversepassâ€‌ cooling. That is, internal coolant flowing substantially in the opposite direction to the mainstream flow. A reversepass system differs from a countercurrent system in that the cold fluid is also used for film cooling. Such systems can be realized when normal engine design constraints are taken into account. In this paper, the thermal performance of reversepass arrangements is assessed using bespoke 2D numerical conjugate heat transfer models, and compared to baseline forwardpass and adiabatic arrangements. It is shown that for a modularized reversepass arrangement implemented in a flat plate, significantly less coolant is required to maintain metal temperatures below a specified limit than for the corresponding forwardpass system. The geometry is applicable to combustor liners and afterburners. Characteristically, reversepass systems have the benefit of reducing lateral temperature gradients in the wall. The concept is demonstrated by modeling the pressure and suction surfaces of a typical nozzle guide vane with both internal and film cooling. For the same cooling mass flow rate, the reversepass system is shown to reduce the peak temperature on the suction side (SS) and reduce lateral temperature gradients on both SS and pressure side (PS). The purpose of this paper is to demonstrate that by introducing concepts familiar in the heat exchanger community, engine hotsection cooling efficiency can be improved whilst respecting conventional manufacturing constraints.