Electro-Thermal Measurement of Heat Transfer Coefficients

Abstract

Heat transfer coefficients are very important for the design of the various flow paths found in turbomachinery. Therefore, the measurement of heat transfer coefficients plays an important role in the field of turbomachinery research. An accurate measurement of heat transfer is not a simple task considering gaseous flow in combination with good thermal conductivity of the boundaries along the flow path. The majority of the measurement methods applied has at least one of the following problems. The measurement setup as for instance a heat flux sensor is a thermal barrier in the object of interest or the sensor introduces for measurement reasons a lot of heat into the object of interest. In both cases the main error results from the modification of the system, which is critical for the investigation of any kind of flow influenced by buoyancy. Furthermore, insufficient fluid reference temperature and/or heat flux with changing sign corrupts any attempt to calculate reasonably heat transfer coefficients. The measurement of heat transfer coefficients becomes even more complicated if the flow path of interest rotates at some thousand rpm as for instance in gas turbines or any other fast rotating machine with fluid flow. This contribution presents an experimental investigation of a setup for the direct measurement of heat transfer coefficients in gaseous flow with metallic boundaries. The calibration of a test sample probe is presented for the standard case forced convection on a flat plate. The sensor setup provides low influence of the measurement on the object investigated. It overcomes the problem of a reference temperature and delivers always positive heat transfer coefficients. Furthermore, the measurement setup fulfils the requirements of telemetric application in the rotating system of a machine. The test of the sensor for its strength against centrifugal acceleration is part of the continuation of the work.