Reverse-Pass Cooling Systems for Improved Performance

Total heat transfer between a hot and a cold stream of gas across a non-porous conductive wall is greatest when the two streams flow in opposite directions. This counter-current arrangement outperforms the co-current 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 hot-section 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 ‘reverse-pass’ cooling. That is, internal coolant flowing substantially in the opposite direction to the mainstream flow. A reverse-pass system differs from a counter-current system in that the cold fluid is also used for film cooling. Such systems can be realised when normal engine design constraints are taken into account. In this paper, the thermal performance of reverse-pass arrangements is assessed using bespoke 2D numerical conjugate heat transfer models, and compared to baseline forward-pass and adiabatic arrangements. It is shown that for a modularised reverse-pass arrangement implemented in a flat plate, significantly less coolant is required to maintain metal temperatures below a specified limit than for the corresponding forward-pass system. The geometry is applicable to combustor liners and afterburners. Characteristically, reverse-pass systems have the benefit of reducing lateral temperature gradients in the wall. The concept is demonstrated by modelling 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 reverse-pass system is shown to reduce the peak temperature on the suction side and reduce lateral temperature gradients on both suction and pressure sides. The purpose of this paper is to demonstrate that by introducing concepts familiar in the heat exchanger community, engine hot-section cooling efficiency can be improved whilst respecting conventional manufacturing constraints.