A theoretical and experimental study of oscillating flow and heat transfer in porous media

Abstract

Theoretical and experimental investigations have been performed to study flow and heat transfer in porous media subjected to steady and oscillating flows. The macroscopic unsteady momentum equation in porous media was derived and closure relation constants were determined. The mechanism of longitudinal heat transfer in porous media subjected to oscillating flow was studied.

A low (<4.0 Hz) and a high (up to 30.0 Hz) frequency oscillating flow facilities were constructed. With the low frequency facility, steady flows through packed columns can be also generated. Velocity, pressure drop through packed columns, temperatures in packed columns, and piston displacement were measured in the experiments. Data reduction schemes for oscillating flow and heat transfer were developed.

The results of experiments for steady and oscillating flows show that the boundary layer effect should be accounted in the correlation between pressure-drop and velocity in the range of intermediate Reynolds numbers. For the same packed column, the correlation between pressure-drop and velocity for oscillating flows when r_h/A <0.026 is the same as that of steady flow correlation. Hence, the oscillating flow in the packed column can be regarded as quasi-steady when r_h/A <0.026. For high frequency oscillating flows through porous media, the pressure drop depends not only on Reynolds number but also on the nondimensional fluid displacement. Large deviation of pressure drop from that of steady flow can be found when the oscillating frequency is high.

The theoretical analysis of heat transfer of a packed column subjected to oscillating flows shows that the longitudinal heat transfer consists of three parts: stagnant conduction, thermal dispersion and local mixing. Thermal dispersion due to oscillating flow dominates the longitudinal heat transfer. Experimental results show that thermal dispersion coefficient is linear proportional to Peclet number when Pe_h>>10 and the frequency is less than 4.0 Hz. Significant longitudinal heat transfer enhancement due to oscillating flow was found. The enhancement factor is linear proportional to Peclet number when Pe_h>>10 and the frequency is less than 4.0 Hz.

Interfacial heat transfer coefficients between the solid and fluid phases in a woven screen packed column were determined for both steady and oscillating flows. Experimental results show that the heat transfer coefficients for steady and oscillating flows are the same when the oscillating frequency is less than 4.0 Hz.

Key words:

porous media, heat transfer, oscillating flow, pressure drop, thermal dispersion, heat transfer coefficient, enhancement

Date of Award	1998
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology