The requirement for an effective cooling of thermally high loaded combustor walls at little coolant consumption leads to effusion cooled combustor liners. Since effusion cooling is always a combination of convective cooling on the back of the wall and within the ejection holes and film cooling on the hot surface, the present study concentrates on the contributions of the various cooling methods to the total cooling effectiveness.
In a comprehensive experimental investigation the thermal behaviour of an effusion cooled test plate was analysed under real engine conditions, i.e. realistic Reynolds numbers, blowing rates, and density ratios. For an indepth understanding of the local heat transfer phenomena the test plate was geometrically scaled up. Two effusion hole patterns were investigated with and without impingement cooling on the cold side of the test plate. Moreover, two materials with different thermal conductivities were chosen for the test plates. Detailed effectiveness distributions were gained from highly resolved surface temperature distributions measured with an infrared camera. The effect of blowing rates on local as well as on laterally averaged effectiveness was addressed.
The experimental results show that the lateral averaged effectiveness is comparatively high with low blowing rates at the beginning of the effusion cooled area, i.e. the area of developing cooling film, due to attached coolant flow. Further downstream high blowing rates perform better. They form a thick and long effective layer of coolant. The relative poor preformance right at the beginning of the effusion cooled area in case of high blowing rates can be improved by applying impingement cooling on the back.
To get an instant information on the contributions of the various cooling methods a one-dimensional calculation procedure was developed. The calculation procedure accounts for different effusion geometries described by hole density, ejection angle, and hole length-to-diameter ratio. Aero-thermal parameters like blowing ratio, density ratio, Reynolds number, and thermal conductivity may be varied in a wide range as well. Comparisons between calculated and measured coolant and wall temperatures showed good agreement.