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 "antikidney 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.
Bibliographical noteFunding Information:
• Fundamental Research Funds of Shenzhen City of China (Grant No. JCYJ20170306155153048).
• National Natural Science Foundation of China (Grant No. 51676163; Funder ID: 10.13039/501100001809).
This work was supported by the National Natural Science Foundation of China (51676163), by the National 111 Project under Grant no. B18041, by the Fundamental Research Funds of Shenzhen City of China (JCYJ20170306155153048), and by the Fundamental Research Funds of Shaanxi Province (2015KJXX-12). This work is also supported by China Scholarship Council (CSC). Support for the experimental facilities was from the University of Minnesota Heat Transfer Laboratory. The numerical part of this work was carried out using computing resources at the University of Minnesota Supercomputing Institute sponsored by the State of Minnesota.
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