Experimental results are presented for the overall heat transfer coefficient within an impingement/effusion wall, using a transient cooling technique. This was previously used for determining the effusion hole heat transfer alone. Two impingement/effusion geometries were used with an 8 mm gap and the same impingement wall with an X/D of 11. The separate impingement and effusion short hole heat transfer coefficients were also determined. The impingement/effusion overall heat transfer was 45% and 30% higher than the impingement heat transfer alone for the two test geometries. The greater increase was for the higher pressure loss effusion wall. It was shown that the combined heat transfer was predominantly the addition of the impingement and effusion heat transfer coefficients but the interaction effects were significant and resulted in an approximately 15% deterioration in the combined heat transfer coefficient. Overall film cooling effectiveness was obtained that showed a significant improvement with the addition of the impingement cooling, but still had a major effusion film cooling contribution.

Factors influencing the design of full coverage drilled plate wall cooling systems for gas turbine combustors are studied. It is shown that the large number of small diameter holes required result in a low Reynolds number operating regime. The physical features giving rise to the hole pressure loss are examined, and it is shown that under hot conditions heat transfer within the hole can appreciably alter the hole mass flow for a fixed pressure loss. It is shown that this effect may be used to estimate the hole outlet temperature and the results show that the heat transfer within the combustor wall may be very significant. The rise in coolant temperature within the wall appreciably alters the blowing rate and hence influences the hot gas side convective heat transfer to the plate. The influence of an impingement plate on hole discharge coefficients is also investigated and shown to be small.

The influence of hole size and hence blowing rate on full coverage discrete hole wall cooling for gas turbine combustion chamber applications was investigated. Two temperature conditions were used, firstly a 750K gas temperature and 300K coolant, and secondly a realistic combustor primary zone condition of 2100K flame temperature and 700K coolant. It was shown that a large hole size resulted in a significant improvement in the overall cooling effectiveness due to a reduced film heat transfer coefficient. At high temperature the cooling effectiveness was reduced due to radiative heat transfer from the flame gases. At low coolant flow large temperature increases of the coolant occurred within the wall and approached the transpiration situation.

The development of a test facility for investigating full coverage discrete hole wall cooling for gas turbine combustion chamber wall cooling is described. A low temperature test condition of 750K mainstream temperature and 300K coolant temperature was used to investigate the influence of coolant flow rate at a constant cross flow Mach number. Practical combustion conditions of 2100K combustor temperature and 700K coolant temperature are investigated to establish the validity of applying the low temperature results to practical conditions. For both situations a heat balance programme, taking into account the heat transfer within the wall was used to compute the film heat transfer coefficients. The mixing of the coolant air with the mainstream gases was studied through boundary layer temperature and CO2 profiles. It was shown that entrainment of hot flame gases between the injection holes resulted in a very low ‘adiabatic’ film cooling effectiveness.

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