Thermal Optimisation of Longitudinal Ribs in Variable Geometry Ducts

Abstract

Augmenting the heat transfer rates in the internal flow passages of several components of a gas turbine, such as the turbine blades, vanes and combustor walls is an important pre-requisite for maintaining their structural integrity. This is particularly paramount when higher turbine inlet temperatures and pressure ratios are utilised for enhancing the thermal efficiencies of the gas turbine plant. In this study, the heat transfer enhancement, which can be achieved by longitudinal ribs in a variable geometry duct, has been examined. With the base of the ribs maintained at a constant temperature, it was observed that the optimal rib spacing, which corresponded to the maximum heat transfer from the ribs, was a strong function of the rib height to length ratio and the Reynolds number but relatively insensitive to the amount of clearance above the ribs. A design correlation is proposed which shows the distribution of this optimal rib spacing for a wide range of rib geometrical and operational conditions. Comparisons of the longitudinal ribs with pin fin arrays indicated that at rib height to length ratios of ≥ 0.24, higher heat transfers can be achieved with the longitudinal ribs. The frictional characteristics of the longitudinal ribs is comparable to those of circular pin fins. Measurements of the local heat transfer coefficient for the rib surfaces indicate that it is highly non-uniform along the rib height and length and also significantly influenced by the amount of clearance above the ribs. For all the cases examined, it was observed that developing flow conditions (thermally and hydrodynamically) were prevalent within the longitudinal rib channels.