Experimental Thermofluidic Characterization of Different Metallic Regenerators Crossed by Alternating Air Flow

The regenerator is one of the most important elements of a thermal machine operating in an alternating flow regime. The present study constituted an experiment on the dynamic characterization of different metallic regenerators crossed by alternating air flow. During an experiment, gas temperature, velocity, and pressure were measured at both ends of the regenerator. The acquisition frequency was set at 1 kHz for each temperature, pressure, and velocity signal. This enabled us to fully characterize an oscillating flow cycle. Experiments were performed for different regenerators (structures and porosities), frequencies, temperature gradients, and displaced air volumes. The experimental results showed that the density ratio is significant at high frequencies for all structures. The friction coefficient is determined based on a classical correlation at the time of maximum velocity. The friction factor f seems to decrease with the kinetic Reynolds number Re-omega for a 30% porosity regenerator. For the other tested regenerators (35% and 40% porosity), we observed that it is almost constant with a little dispersion. To minimize the dispersion effects, another definition was proposed to calculate the friction factor at the time of maximum pressure drop. The results showed that for all regenerators a single clear trend is the function of the kinetic Reynolds number. A significant phase shift was observed between the velocity and pressure drop. It increases with the increasing kinetic Reynolds number. It was found that the phase shift only depends on frequency. Finally, a correlation equation was proposed to predict the phase shift of different regenerator structures. It was found that the effect of the kinetic Reynolds number Re, on the phase shift is more dominant than that of the hydraulic diameter to length ratio D-h/ L.