Ultrasonic measurements of unsteady heat transfer in a reciprocating gas spring

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

In this paper, an experiment is reported that was devised to measure the thermodynamic loss associated with the unsteady heat transfer that takes place in the compression space of a gas spring and to differentiate the loss from any mass leakage or viscous or dissipation frictional effects. As the total mass in the system is unknown, due to leakage between the cylinder and the piston, it is necessary to measure three thermo-dynamic bulk parameters in order to completely determine the system state. Pressure, temperature and volume are convenient choices. However there is a challenge in measuring the gas temperature in the compression space using traditional physically invasive methods, such as thermocouples. These methods generally have a number of undesirable side effects; for example their poor response times due to the added thermal mass, their sensitivity to the varying heat transfer in the cylinder, the disruption of the gas flow and gas temperature, and the fact that they only provide a point measurement within the flow. Instead, a novel technique is presented for the estimation of the bulk temperature of the gas by measuring the time of flight of an ultrasonic pulse across the compression space. The technique is based on the principle that the speed of sound in an ideal gas is dependent on the square root of the absolute temperature, based on which the temperature of the gas along the ultrasonic path can be found. The ultrasonic pulse is transmitted and received at 400 kHz using piezo ceramic transducers with a repetition rate of 1 kHz. This instantaneously resolved temperature information, along with the pressure and volume of the compression space, was used to calculate the work and heat flows into the compression space. It was therefore possible to measure the instantaneous rate of heat transfer throughout the compression expansion cycle of the gas spring. The heat transfer rate was seen to be linear for most of the cycle, with the heat flux proportional to the gas–wall temperature difference. There was however a deviation from this during expansion. The thermodynamic loss around the cycle of the gas spring was also measured and reported at different rotational speed. The loss was observed to increase with increasing P´eclet number which was in agreement with the theoretical models of a gas spring