The overall surface averaged heat transfer was determined for air passing through arras of small diameter holes drilled at 90 through thin metal walls. The influence of the number of holes and hence of the pitch to diameter ratio, X/D, was investigated over the range 4.7 to 21 for a fixed hole size of 1.4 mm and hole L/D of 4.5. A transient cooling technique was used to determine the overall heat transfer coefficient for the cooling due to the air passage through the wall. It was shown that the dominant heat transfer was that on the hole approach surface area due to flow acceleration into the hole. The hole approach surface area was used in the heat transfer correlation. The results of the authors were combined with previous results for the variation of X/D at constant X to give a heat transfer correlation, independent of L/D.

Effusion cooling with 15 film cooling rows of holes was investigated and a comparison of normal and inclined holes was undertaken for laser drilled holes. Overall and adiabatic cooling effectiveness data are presented for a density ratio of 2.5 for N=9688 holes per square metre and an X/D of 11. The main variable studied was the angle of inclination of the 30° film holes to the crossflow, 30°, 90° and 150°. Comparison was made with 90° holes of similar X/D and a smaller X/D of 4.5, which was shown to be as effective as inclining the holes for improving the cooling effectiveness. The opposed film cooling flow of the 150° inclined hole was very effective in providing a good film protection at the leading rows of holes. A cooling effectiveness of 0.7 was demonstrated at a mass flow per unit surface area G of 0.35–0.4 kg/sm2 bar with both 30° and 150° holes. This was equivalent to 15–20% of the combustor air flow for a typical gas turbine combustor. Comparison with previous work on effusion cooling showed reasonable agreement for similar geometries, although very few previous workers have reported overall cooling effectiveness results. Also the present work covered a much wider range of film cooling mass flow, a larger number of rows of holes and a more practical density ratio than in previous work.

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