Experimental measurement of venturi discharge coefficient including sensitivity to geometry and flow quality variations

The Arnold Engineering Development �enter (AEDC) offers the aerospace community a number of test facilities for evaluating turbine engine opera}>ility, performance, and durability at simulated altitude conditions. The accomplishment of these tasks generally requires the measurement of the airflow rate used by the engine. Applying the direct-connect method, the AEDC turbine engine test facilities use measurements of the airflow through the air supply duct, typically obtained from venturi installations, to determine the engine airflow rate. The AEDC initiated an airflow measurement technique investigation to address turbine engine test requirements with respect to both airflow measurement accuracy and costs. The accuracy improvement component focused on the venturi discharge coefficient, motivated by observed differences in discharge coefficient between various calibrations as well as a dearth in the understanding of the discharge coefficient sensitivity to parameters known to vary from facility to facility. These include not only the geometry of the particular installation, but the flow quality delivered to the venturi by the facility. The understanding of the bridge between the calibration laboratory and the test facility application is needed to better quantify the accuracy achieved in turbine engine tests. This thesis focuses on three specific objectives that contribute to the accuracy improvement initiative: (1) verify the currently used AEDC venturi discharge coefficients, (2) determine the applicability of the laboratory calibrations of the discharge coefficient to actual turbine test facility installations, and (3) identify any parameters influencing discharge coefficient that should be addressed in the accuracy improvement. The experimental approach consi_sted of two parts. The first centered on repeating experiments and verifying the data set that formed the basis for the currently used AEDC discharge coefficients. The second focused on directly measuring the influence of the flow quality, venturi geometry, and venturi installation parameters on discharge coefficient. Execution of this approach demanded the development of a unique test facility and flow-field probing systems that permit the measurement of the detailed flow field in the venturi throat. The detailed flow-field measurements provided the mass flux distributions which, when integrated, provided the venturi mass flow and a calibration of the discharge coefficient. The calibrations were compared to the historical data that define the currently used AEDC discharge coefficients. Subsequently, two venturis calibrated in that fashion served as reference venturis for the influence coefficient determinations. The influences were investigated by subjecting a test venturi to systematic variations in key flow quality, geometry, and installation parameters and measuring the response relative to the reference venturi. This thesis provides the results of both sets of experiments. These include the comparisons of the detailed flow-field characteristics in the venturi throat as well as the discharge coefficients between the historical data and the present data. The results substantiated the historical discharge coefficients. Next, the thesis provides results of the parametric investigation showing the sensitivity of the discharge coefficient to parameters that vary from facility to facility. These include the flow quality parameters of total pressure, swirl, and turbulence as well as key installation parameters such as the proximities between the venturi inlet and the plenum wall and the plenum bulkhead. The thesis also provides results pertaining to installations characterized by a number of venturis mounted adjacent to each other on a common bulkhead. Finally, the thesis provides the measurements of the effects of venturi surface degradation. The sensitivity measurements showed that the discharge coefficient of a choked venturi is relatively insensitive to many of the parameters that differ between the laboratory environment and the turbine engine test cell substantiating the practice of directly applying the laboratory discharge coefficients. However, the results also revealed parameters that should be considered in improving the accuracy delivered in test installations and the need to preserve venturi surface smoothness.