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Oxford Thermofluids Institute | Research - Research Groups: Oxford Turbine Research Facility Group

Oxford Turbine Research Facility Group

The ILPF is a short duration wind tunnel, capable of testing an engine size turbine at the correct non-dimensional parameters for fluid mechanics and heat transfer. The facility has been used to test both a single HP turbine stage, and a 1.5 turbine stage turbine (Povey et al., 2003-b). During a run M, Re, Tu and Tg/Tw are all representative of engine conditions.

A novel feature of the facility is the aerodynamic turbo-brake, which is on the same shaft as the turbine and is driven by the turbine exit-flow. At the design speed the turbobrake power is matched with the turbine, and thus constant speed is maintained during a run. Air from a high-pressure reservoir drives a piston down a piston tube, isentropically compressing and heating the working gas (air) inside the tube. When the desired pressure is reached, the compressed air is suddenly discharged, by means of a fast acting valve, into the working section. Typically, steady conditions are achieved for 500 ms.

Measurements of temperature at the exit-plane of combustors show large radial and circumferential variations in temperature. The flow is, in addition, highly unsteady. The time-mean temperature field measured at the exit plane of a combustor typical of a modern military engine is highly non-uniform. Peak temperatures are in excess of 2200 K, whilst at the hub and casing endwalls there are relatively cool regions of flow, with temperatures as low as 1500 K. Circumferential variations (hot-spots) arise because of the discrete nature of fuel and dilution air jets. In addition, combustor lining coolant flow causes a strong radial temperature gradient.

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.