This paper discusses experimental results from a two cavity test rig representative of the internal air system of a high pressure compressor. Thermal steady state measurements of the time-averaged, local heat fluxes based on surface temperatures on both sides of the mid disc are presented for the case of axial throughflow of cooling air. Additionally, measurements of the air temperature and the static pressure inside the cavities are given. Tests were carried out for a wide range of rotational Reynolds numbers up to 107 and axial Reynolds numbers up to 2×105 with a uniformly heated shroud. The method of heat flux determination and the approach to calculating the uncertainties are described and discussed. The local heat flux results from different rotational frequencies, and mass flows are compared and interpreted in terms of assumed flow structures. Using the results of air temperature and static pressure measurements, simple theoretical models of the density gradient and the mixing mass flow which radially enters the cavity, deliver deeper insight into the flow structure and its influence on the heat transfer.
The results show that the heat flux increases with increasing mass flow. The influence of rotation on the heat flux is weaker and more complex than the effect of mass flow. The flow can be separated into four parts, the existence and strength of which depend on the test conditions: rotating cavity flow, impinged flow and resultant secondary flow, instabilities due to a negative radial density gradient respective to the buoyancy-induced flow, and instabilities of the incoming jet. For the geometry with a small inlet gap tested here, the flow and heat transfer are dominated by the throughflow or rather by the secondary flow for Rossby numbers Roz > 1.5. The buoyancy-induced flow is negligible. For Rossby numbers Roz < 1.5, the rotation dominates the flow structure. Buoyancy-induced flow can increase radial mixing between the throughflow and the cavity flow as well as the heat transfer to and from the disc. However, at high rotational frequencies, the density gradient becomes positive due to the increase in pressure induced by the centrifugal force which reduces mixing and heat transfer to/from the disc. Therefore, the highest heat transfer at the disc is measured at medium rotational frequencies.