Model tests on the control behaviour of a test air supply system in open or closed-loop operation

Abstract

The Leibniz Universität Hannover is currently establishing a new mechanical engineering campus which includes a new research building "Dynamics of Energy Conversion" (DEW). This building provides a large compressor station for either steady or dynamic (transient) operation of turbomachinery and power plant test rigs (e.g. air turbine, axial compressor, combustion chamber, planar cascade, acoustic wind tunnel). The test air supply system is designed to enable investigations under high load gradients over wide operating ranges with Reynolds and Mach numbers controlled independently in order to fulfil aerodynamic similarity conditions between reality and model experiments. This is achieved by closed loop operation of the test air supply system which allows independent adjustment of pressure, temperature and volume flow rate as well as independence from environmental influences such as temperature or humidity. The compressor station utilizes as first stage two parallel Roots-type PD compressors and as second stage two parallel screw compressors. The test rigs operate at expansion ratios between 1 and 6. Test rig inlet pressures range from 1 bar(abs) to 8 bar(abs) with a maximum mass flow rate of 25 kg/s. At all conditions temperatures can be regulated between 60°C and 200°C. The test air supply system has a maximum electric power input of approximately 6 MW. As stringent demands on stability and reproducibility have to be met and automatic operation was requested, a scaled and simplified but fully functional model of the test air supply system was built, mainly to enable testing of control methods and devices prior to their final implementation on site. The functional model uses DN150 piping and consists of one Roots-type PD compressor as first stage and one screw compressors as second stage. Both compressors are driven by electric motors regulated by frequency converters. A turbine test rig is represented in the model by an adjustable throttle valve. Precise control of the mass flow rate is provided by a cascaded adjustable bypass around the test rig. The paper describes the test air supply system and the scaled model and presents experimental results on the achievable stability of pressure, temperature and mass flow rate at the test rig inlet in steady operation at several operating conditions of the model.