Calibration of combined pressure, temperature probes for application in the Rotating Cascade Tunnel (RGG)

Kurzfassung

Several new combined total pressure, total temperature probes called RRT-probes were designed and tested at DLR Göttingen. The probes are capable of measuring the most important steady 2D flow values including total pressure, total temperature, flow angle and Mach number. The calibration was performed in the Probe calibration Facility (SEG) of the Institute of Propulsion Technology at two Reynolds numbers and in the Mach number range of 0.2 to 1.8. To enable an easy application several probe pressure coefficients were plotted against flow angle and Mach number and approximated by polynomials. It was shown that the pressure coefficient for flow angle determination can be approximated by a straight line in the incidence range <15° and it is not dependent on radial angle. The probes will be used in such a way that first the incidence angle is determined and subsequently the probe turned till zero incidence is achieved. At zero incidence then the Mach number will be determined from the coefficient C_Ma, the total pressure and total temperature from the other coefficients, C_p0, C_T0, whose dependence on incidence accordingly plays no role. The dependence of the coefficients on radial angle is rather weak at zero incidence angle. But at larger radial angles there will be an influence which is not always negligible. A separate evaluation for the subsonic flow regime was performed. Transonic / supersonic velocities are expected only when investigating the turbine flow between stator and rotor and there a total pressure range of 100 kPa will prevail. The gradient of the C_Ma-coefficient is near to zero in the vicinity´of Ma = 1. This effect is caused by a straight shock developing ahead of the probe. If the flow Mach number increases above 1 the shock is moving nearer to the probe, but the flow downstream of the shock which is felt by the probe is remaining subsonic. It is expected that the flow Mach number determination by utilization of this curve leads to excessive errors near Ma = 1. Another Mach number coefficient was defined using the probe pressures at zero incidence and from a further measurement with the probe turned around. The result of the calibration is encouraging as the new Mach number coefficient is much steeper in the transonic and supersonic regime compared to the conventional one leading to less errors in the determination of Mach number using the new coefficient.