The Effect of Temperature Field on Low Amplitude Oscillatory Flow within a Parallel-Plate Heat Exchanger in a Standing Wave Thermoacoustic System

Abstract

Thermoacoustic technologies rely on a direct power conversion between acoustic and thermal energies using well known thermoacoustic effects. The presence of the acoustic field leads to oscillatory heat transfer and fluid flow processes within the components of thermoacoustic devices, notably heat exchangers. This paper outlines a two-dimensional ANSYS FLUENT CFD (computational fluid dynamics) model of flow across a pair of hot and cold heat exchangers that aims to explain the physics of phenomena observed in earlier experimental work. Firstly, the governing equations, boundary conditions and preliminary model validation are explained in detail. The numerical results show that the velocity profiles within heat exchanger plates become distorted in the presence of temperature gradients, which indicates interesting changes in the flow structure. The fluid temperature profiles from the computational model have a similar trend with the experimental results, but with differences in magnitude particularly noticeable in the hot region. Possible reasons for the differences are discussed. Accordingly, the space averaged wall heat flux is discussed for different phases and locations across both the cold and hot heat exchangers. In addition, the effects of gravity and device orientation on the flow and heat transfer are also presented. Viscous dissipation was found to be the highest when the device was set at a horizontal position; its magnitude increases with the increase of temperature differentials. These indicate that possible losses of energy may depend on the device orientation and applied temperature field.

Numerical Predictions of Early Stage Turbulence in Oscillatory Flow across Parallel-Plate Heat Exchangers of a Thermoacoustic System

“…Hence, the transition model seems to be a better model for predicting the flow at this drive ratio. The difference in amplitude of velocity between experimental and numerical results at ϕ5 and ϕ15 may be due to the moderation constant [21,23,26,27] not being varied in this study.…”

“…It is at 0.17 from the pressure antinode. The electrical heating elements in HHX and cooling water flow in CHX aim to provide flat temperature profiles over the fins: 200 °C and 30 °C, respectively; but in practice, the heat leaks produce a slightly distorted temperature distribution as reported in [17] and [21]. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) techniques provide the oscillatory velocity and temperature field information around the heat exchangers.…”

“…It was found that all models needed to be calculated for at least 70 cycles to meet this condition (approximately 35 h of computational time). The flow solutions presented in this paper rely on the RANS equations used for turbulence modelling, while the laminar model (see e.g., [21]) is solved for comparisons. The four RANS models tested included: k-ε, k-ω and shear-stress-transport (SST)-k-ω and transition-SST provided by ANSYS FLUENT 13.0 [23].…”

Comparative Performance of Thermoacoustic Heat Exchangers with Different Pore Geometries in Oscillatory Flow. Implementation of Experimental Techniques

“…This is due to the fact that the flow is oscillatory and the existing knowledge for steady flow arrangements is of little practical value [11][12][13]. As a consequence, standard heat transfer correlations for steady flows cannot be directly applied to thermoacoustic HXs, for which reliable and unambiguous design criteria are still lacking.…”

Modeling of Heat Transfer and Oscillating Flow in the Regenerator of a Pulse Tube Cryocooler Operating at 50 Hz

“…Saat and Jaworski [12] proposed a friction factor correlation applicable to a larger mesh range for a regenerator in a travelling-wave thermoacoustic system, based on experimental and CFD studies. They also studied the effects of temperature field, gravity and device orientation on the oscillatory flow [13]. Hsu and Biwa [14] carried out an experimental study on the heat flow in different mesh screen regenerators and compared the experimental result with four empirical equations, and concluded that all four empirical equations reproduce the measured heat flow rates to a similar degree.…”