To quantify temperature non-uniformity, Temperature Distortion Factors (TDFs) are used. There are several definitions in current use, many of which are essentially the same. Combustor flow is highly turbulent, and, because hot and cold gas streams are subject to aggressive mixing, typical time-mean combustor exit temperature profiles are generally rather smooth spatially. It may be sufficient, therefore, to describe an ITD with a single numerical value. A distinction is sometimes drawn, however, between the Overall TDF (OTDF), which is a measure of the divergence of the hottest gas streak from the mean temperature, and the Radial TDF (RTDF), which is a measure of the non-uniformity of the circumferentially averaged temperature field.
In a typical civil engine, fuel-injector-to-vane-count ratios near 1:2 are not uncommon. Count ratios are often non-integer, however, and therefore the possible benefits associated with hot streak clocking are difficult to realise in practice. HP vane surface cooling systems must be designed for peak combustor exit temperatures. There has been, therefore, considerable interested in the effects of hot streak clocking (relative circumferential position).
The first ITD generator used in the ILPF is described in detail by Chana et al. (2003). Hot streaks, rotatable with respect to the NGV leading edge, were generated by blowing cool air through struts upstream of the HP NGVs. The number of hot streaks was the same as the HP vane count: 32. This was used to study the impact of clocking the hot-streak with respect to the HP vane. The maximum and minimum measured gas temperatures were approximately 480 K and 412 K. The mean temperature was the same for both uniform and non-uniform inlet temperature: 444 K. Because cool air was injected to create an ITD, a higher main flow temperature was required. This was achieved by increasing the pump-tube compression ratio. The peak-to-mean and minimum-to-mean temperature ratios were approximately 1.08 and 0.93.