An Experimental Study of Sister Holes Film Cooling With Various Secondary-to-Primary Hole Diameter Ratios

Abstract

For increased specific thrust and efficiency, more effective film cooling schemes are developed with each successive gas turbine design. Adding secondary film cooling holes to each primary film cooling hole represents such improvement without significantly increasing cost. Presented is an experimental investigation on the effects of secondary-to-primary hole diameter ratio on film cooling performance and flow structure in the coolant-to-passage flow merge zone. Film cooling effectiveness values and heat transfer coefficients are measured in the vicinity of the hole by the Thermochromic Liquid Crystal (TLC) technique. Measured in-flow temperature fields in the coolant emerging zone identify flow makeup, whether coolant or passage. Furthermore, complementary flow and thermal fields are numerically documented. The Reynolds number based on mainstream velocity and primary hole diameter is 20,300, a representative value. Performance features are compared at three blowing ratios (0.5, 1.0 and 1.5) and two mass flow ratios (3.43% and 5.15%). Secondary holes improve film cooling effectiveness, especially when blowing rate is high. Secondary holes create an “anti-kidney vortex structure” that weakens the main kidney vortex pair which helps keep coolant attached to the surface, allowing more effective laterally spreading. However, adding secondary holes increases heat transfer coefficients, especially at high blowing rates. The secondary-to-primary hole diameter ratio is an important parameter. Larger secondary holes can counteract the detrimental effects of having higher blowing ratios, but with increased blowing ratios this improvement subsides. An optimum diameter ratio is sought.

Numerical investigation of area ratio on sister hole applications at a constant exit area

“…Recently Zhou et al 31 and Zhu et al 32 studied the effects of diameter ratio between primary hole and secondary hole, numerically and experimentally, respectively. Zhou et al 31 showed that the larger secondary holes provide better cooling effectiveness while the experimental results of Zhu et al 32 consistent with these results. The work of Zhou et al 31 made at constant exit area while Zhu did change the exit areas but keeps the blowing ratio constant with the changing mass flow rate.…”