The second law of thermodynamics is applied to evaluate the influence of entropy generation on the performances of a cold heat exchanger of an experimental Stirling refrigeration machine by means of three factors: the entropy generation rate $N_S$ , the irreversibility distribution ratio ϕ and the Bejan number ${Be|}_{N_S}$ based on a dimensionless entropy ratio that we introduced. These factors are investigated as functions of characteristic dimensions of the heat exchanger (hydraulic diameter and length), coolant mass flow and cold gas temperature. We have demonstrated the role of these factors on the thermal and fluid friction irreversibilities. The conclusions are derived from the behavior of the entropy generation factors concerning the heat transfer and fluid friction characteristics of a double-pipe type heat exchanger crossed by a coolant liquid (55/45 by mass ethylene glycol/water mixture) in the temperature range 240 K < T_C < 300 K. The mathematical model of entropy generation includes experimental measurements of pressures, temperatures and coolant mass flow, and the characteristic dimensions of the heat exchanger. A large characteristic length and small hydraulic diameter generate large entropy production, especially at a low mean temperature, because the high value of the coolant liquid viscosity increases the fluid frictions. The model and experiments showed the dominance of heat transfer over viscous friction in the cold heat exchanger and ${Be|}_{N_S}\to 1$ and ϕ → 0 for mass flow rates $\dot{m}\to 0.1$ kg.s⁻¹.

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$R{e}_{\omega }$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$R{e}_{\omega }$on the phase shift is more dominant than that of the hydraulic diameter to length ratio $D_h/L$.

This article deals with an application of the endoreversible thermodynamics theory of heat engine applied to microfluid flow power converters (MFPC). An analogy is demonstrated between thermal and fluid flow efficiencies. Maximum power output and efficiency at maximum power are established for the device based upon the higher and lower pressure bounds. The linear and non linear fluid flows are considered with and without fluid friction losses. This paper provides theoretical limits for designing power flow converter. The best performances are obtained for linear fluid flow without flow losses.