Effect of Coolant Extraction on Cold Side Heat Transfer Enhancement of an Effusion Cooling Plate

Among the available cooling strategies for combustor liners, one of the most effective is the effusion: a large number of extremely small cylindrical holes with shallow inclination are realized on the whole liner surface and fed by the annulus flow. Despite its cooling capability primary derives from the heat removed by the passage of the cooling air in the perforation and from the film protection, the coolant extraction produces a significant enhancement of convective heat transfer coefficient. Most of the studies available in the technical literature deal with the heat transfer on the hot side of effusion cooling plates, while just few contributions focus on the cold side convective cooling. Nevertheless the effect of coolant extraction plays a significant role in lowering the metal temperature. The main purpose of the present work is the experimental characterization of the heat transfer coefficient distributions on the cold side of an effusion cooling plate for lean burn combustor chamber. The experimental setup consists of a planar rig installed in an open loop wind tunnel reproducing the flow within the combustor annulus. The coolant is extracted by an effusion geometry composed by 23 rows of circular holes arranged in a staggered array, while the annulus channel has a rectangular cross section with constant height. The experimental campaign has been carried out using the TLC (Thermochromic Liquid Crystals) steady state technique with a thin Inconel heating foil. The investigated test matrix consists of several points in terms of pressure drop across the perforation and different level of annulus to effusion mass flow ratio (Bypass Ratio - ByR). Since the main effects of the coolant extraction on the heat transfer distributions are located around the holes, an iterative data reduction procedure based on thermal FEM simulations has been developed to take into account the non-uniform heat generation and the heat loss across the liner plate. Results are mainly presented in terms of bidimensional and averaged distributions of HTC along the liner surface. A data analysis has been carried out to propose a correlation for the prediction of the cold side heat transfer enhancement factor (EF) representing the ratio between local Nusselt and a reference flat plate Nusselt number. The proposed correlation expresses the EF values as a function of the Reynolds number of the annulus channel and the Suction Ratio parameter, i.e. the ratio of effusion hole velocity and annulus velocity.