Improved fast-response heat transfer instrumentation for short-duration wind tunnels

Abstract

This paper describes the recent innovations associated with the fabrication and application of fast-response heat flux gauges in short-duration wind-tunnel facilities in Oxford. This work has been driven by the need to measure the heat transfer in an environment with large spatial and temporal variations in surface heat flux, caused by a flow-field that has significant high-frequency periodic fluctuations in both recovery temperature and heat transfer coefficient (namely, an axial flow turbine). A new approach to the fabrication of miniature surface heat flux thin-film gauges is described; this involves the use of a pulse laser to ablate (etch) the gauge and connecting track patterns directly onto electrically insulated and platinized components. This automated technique is shown to produce gauges of high geometrical tolerance at precisely known locations, and to create arrays of gauges on the experimental components rapidly. Typically, the platinum resistance elements are 1 mm by 0.08 mm in plan form and have closely repeatable characteristics (for example, resistance and temperature coefficient). A novel technique for measuring the adiabatic wall temperature and heat transfer coefficient in short-duration facilities is also presented; this includes the capability to measure the deterministic component of these unsteady parameters in periodic (pulsating) flows. The combination of the new high spatial density gauge arrays and the capability to measure unsteady adiabatic wall temperature allows unprecedented insight into the flow physics that influences the unsteady heat flux. In particular, it has been possible to establish whether heat transfer rate fluctuations in a turbine experiment are caused by changes in flow recovery temperature or by changes in heat transfer coefficient. Some example measurements taken in a high-speed turbine test facility are described. Finally, some of the recently manufactured heat transfer gauge systems are presented.