Paper 9: The Measurement of Turbulent Velocity Fluctuations and Turbulent Temperature Fluctuations in the Supercritical Region by a Hot Wire Anemometer and a ‘Cold’ Wire Resistance Thermometer

Abstract

It is necessary to develop experimental methods for measuring the local turbulent velocity and turbulent temperature fluctuations in the supercritical region in order to investigate the unusual heat transfer phenomena that occur in heating turbulent flows of fluids in this region.

The performance of the constant temperature hot wire anemometer has been investigated in isothermal turbulent flow of carbon dioxide in a pipe at supercritical pressures from 1075 to 1200 lb/in² (P_c = 1071 lb/in²) and at temperatures from 24·5 to 40°C (T_c = 31°C), including the critical range of rapid variation in physical properties in which the fluid changes from liquid to gas. The wire temperature was varied and linear mean velocity calibrations were obtained which were found to be markedly sensitive to the pressure, the fluid bulk temperature, and the wire temperature. Measurements of turbulent velocity fluctuations agreed with the expected values for this isothermal case. The heat transfer from the wire showed no unusual characteristics and was correlated for variation in pressure, fluid bulk temperature, and wire temperature by using mean values of the physical properties over the wire to bulk temperature range.

Because of the sensitivity of the wire calibrations to temperature, measurement of the local fluid temperature is required to allow velocities to be measured in the presence of temperature gradients in the supercritical region. A probe has therefore been developed with a thin cold wire resistance thermometer, mounted close to a pair of crossed hot wires and with a sufficiently fast thermal response for the measurement of temperature fluctuations as well as mean temperature. Experiments are described with this probe in the presence of temperature gradients in turbulent flow, but so far only in air. Simultaneous measurements were made of both the radial velocity fluctuations and temperature fluctuations in a volume of air that was small compared with the scale of turbulence. The time-correlation of these two quantities was found to be strong near the wall of the tube and to fall to zero towards the centre. Further, the local radial turbulen theat flux calculated from the mean product of these two quantities agreed quantitatively with the known heat flux for various radial positions in the tube.

The remaining problem is to investigate the effects that temperature fluctuations in the supercritical region have upon the measurement of velocity fluctuations and on the local values of the density and specific heat.