DIPLOMA THESIS Experimental Investigation of the Systematic Errors of Pneumatic Pressure Probes Induced by Velocity Gradients

Pneumatic pressure probes are a reliable and robust tool for flow measurement in components of thermal turbomachines. Three-hole probes are used for two dimensional flow fields and five-hole probes for three-dimensional flow fields, respectively. The calibration of the probes is usually performed in a free-jet wind tunnel with constant velocity. However, the flow field downstream of turbomachinery blade rows shows considerable gradients of pressure and velocity. Therefore, systematic errors in the measured quantities -especially in the flow angleoccur. The present diploma thesis is a investigation on the influence of velocity gradients on the flow angle error of pneumatic three-hole probes. The work starts with a literature survey on this topic. The next step is the calibration of the three-hole probe in a flow field of constant velocity magnitude. The hole and calibration coefficients are compared with results obtained from the simple streamline projection method. This method is also used for an estimation of the principle effect of velocity gradients. The strength of the gradient can be described by a nondimensional velocity gradient K and the flow angle error ∆ε is proportional to K. The appropriate constant is determined experimentally. A natural turbulent boundary layer is used for this investigation. The velocity distribution is measured with a single hot-wire probe and the three-hole probe is traversed in the boundary layer Finally, the correction method is applied to the measured flow field downstream of a linear turbine cascade. An appreciate improvement of the measured flow angle distribution is achieved by the application of the correction method.