Small Diameter Film Cooling Holes: Wall Convective Heat Transfer

Abstract

The wall heat transfer resulting from small diameter holes drilled at 90° through gas turbine combustion chamber and turbine blade walls is considered. Available information is briefly reviewed and shown to generally omit the hole approach surface heat transfer and to relate only to the internal hole heat transfer. Experimental techniques are described for the determination of the overall heat transfer in a metal plate with a large number of coolant holes drilled at 90°. The results are compared with conventional short-tube internal heat transfer measurements and shown to involve much higher heat transfer rates and this mainly resulted from the additional hole approach flow heat transfer.

Small Diameter Film Cooling Hole Heat Transfer: The Influence of the Number of Holes

“…Both surface area and internal hole areas will be investigated as a basis for the determination of h. The former essentially assumes that any surface heat transfer can be treated as though it occurred inside the hole, whereas the latter assumes that any hole heat transfer can be treated as part of the surface heat transfer. The dominant heat transfer mode will determine the best corelation and previous work of the authors, for various X/D at constant X and various N at constant X/D, has shown that the approach surface heat transfer was dominant (Andrews et al 1986(Andrews et al , 1987…”

“…It has often been assumed that short hole heat transfer correlations were adequate for this purpose, but these neglected the heat transfer on the hole approach surface which was first investigated and shown to be large by Sparrow and Ortiz (1982). Andrews, et al ( , 1985aAndrews, et al ( , 1986Andrews, et al ( , 1987 have shown that for full coverage discrete hole film cooling of combustion chamber walls, the overall heat transfer was dominated by the hole approach surface area. They provided a correlation for a range of hole geometries that included both the hole approach and internal short hole heat transfer and the present work extends the range of geometries investigated to include the influence of N and X/D at constant D and L/D.…”

“…Andrews et al (1987) showed that conventional short hole heat transfer correlations, such as that of Mills (1962), considerably underestimated the overall wall heat transfer. The present work and that of Andrews et al ( , 1985aAndrews et al ( , 1986Andrews et al ( , 1987 used a plenum chamber feed of the cooling air to the array of film cooling holes. This is a geometry applicable to industrial gas turbine combustors which use a plenum chamber air feed.…”

“…The internal hole heat transfer for short holes has been investigated by many workers and these were reviewed by Andrews et al (1986). All the correlations involve the short hole heat transfer as the fully developed flow heat transfer multiplied by a constant which is a function of the hole L/D.…”

Full Coverage Impingement Heat Transfer: Cooling Effectiveness

“…The objective of this work was to combine impingement with full coverage discrete hole or effusion cooling to give very effective cooling at low air utilisation rates. The approach taken was to separately optimise the impingement and effusion geometries (9)(10)(11)(12)(13)(14)(15). The combined impingement/effusion systems have been shown (4,5,13,16,17) to give a very high cooling effectiveness at low coolant flow rates, well below 50% of current gas turbine combustor film cooling air utilisation rates.…”