Abstract
A performance assessment of active magnetocaloric regenerators using entropy generation minimization is presented. The model consists of the Brinkman-Forchheimer equation to describe the fluid flow and coupled energy equations for the fluid and solid phases. Entropy generation contributions due to axial heat conduction, fluid friction and interstitial heat transfer are considered. Based on the velocity and temperature profiles, local rates of entropy generation per unit volume were integrated to give the cycle-average entropy generation in the regenerator, which is the objective function of the optimization procedure. The solid matrix is a bed of gadolinium spherical particles and the working fluid is water. Performance evaluation criteria of fixed cross-section (face) area (FA) and variable geometry (VG) are incorporated into the optimization procedure to identify the most appropriate parameters and operating conditions under fixed constraints of specified temperature span and cooling capacity.
Highlights
Magnetic refrigeration systems can be built according to different thermodynamic cycles, such as the Carnot, Ericsson, Stirling and Brayton cycles (Kitanovski and Egolf 2006, Kitanovski et al 2015)
The total cycle-average entropy generation rate, Sg, was used as the objective function in an optimization procedure that evaluates the influence of parameters such as the mass flow rate, operating frequency, regenerator cross sectional area, housing aspect ratio, utilization factor and particle diameter according to the Face Area (FA) and variable geometry (VG) Performance Evaluation Criteria (PEC) of Webb and Kim (2005)
As the numerical convergence of the governing equations is obtained for a specific set of simulated parameters associated with a PEC, the Active Magnetic Regenerator (AMR) configurations that yield the target value of Qc can be identified
Summary
Magnetic refrigeration systems can be built according to different thermodynamic cycles, such as the Carnot, Ericsson, Stirling and Brayton cycles (Kitanovski and Egolf 2006, Kitanovski et al 2015). The total cycle-average entropy generation rate, Sg, was used as the objective function in an optimization procedure that evaluates the influence of parameters such as the mass flow rate, operating frequency, regenerator cross sectional area, housing aspect ratio, utilization factor and particle diameter according to the FA and VG PEC of Webb and Kim (2005). In cases where the magnetocaloric effect is reversible, such as in materials with a continuous magnetic transition (e.g., gadolinium), the local rate of entropy generation due to the MCE is directly included in the interphase heat transfer In this case, the ∆Tad resulting from the external magnetic flux density variation creates a finite temperature difference between the solid and fluid phases. As performed in Trevizoli and Barbosa (2015), the searches for the points of minimum entropy generation rate were refined further using 4th-order (or less) polynomial interpolations (R2 > 0.9999) to guarantee the stability of the numerical solutions and provide a better resolution (finer than 0.5 mm for Dh,Reg, 0.05 for ζ and 0.01 mm for dp) for the minimum Sg value at a reasonable computational cost
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