Abstract
A regenerator of a Stirling machine alternately absorbs and releases heat from and to the working fluid which allows to recycle rejected heat during theoretical isochoric processes. This work focuses on a milli-regenerator fabricated with a multiple jet molding process. The regenerator is a porous medium filled with a dense pillar matrix. The pillars have a geometrical lens shape. Two metallic layers (chromium and copper) are deposited on the polymer pillars to increase heat transfer inside the regenerator. We performed experiments on different milli-regenerators corresponding to three porosities (ε = 0.80, 0.85 and 0.90) under nitrogen steady and oscillating compressible flows (oscillating Reynolds number in the range 0 < Reω < 60 and Reynolds number based on the hydraulic diameter ReDh,max<6000) for different temperature gradients (ΔT < 100°C). Temperature, velocity and pressure experimental measurements are performed with microthermocouples (type K with 7,6 µm diameter), hotwires and miniature pressure sensors, respectively. We identified a threeterm composite correlation equation for the friction factor based on a Darcy-Forchheimer flow model that best-fit the experimental data. In steady and oscillating flows permeabilities and inertial coefficients are of the same magnitude order. Inertial coefficients decrease when the porosities increase.
Highlights
The regenerator plays a key role on the performances of Stirling machines
The regenerator is a porous medium with a total length L = 60 mm filled with a dense pillar matrix
Two metallic layers with a total thickness of 800 nm are deposited on the polymer pillars to increase heat transfer inside the regenerator
Summary
The regenerator plays a key role on the performances of Stirling machines (engines, coolers, heat pumps). 100 % and the thermal efficiency of the machine corresponds to the Carnot efficiency In this case, all thermodynamic processes are thermodynamically reversible, there is no pressure drop in the regenerator, not heat conduction loss from the warm end to the cold end, there is an infinite rate of heat transfer between the working gas and solid matrix and the solid matrix presents an infinite heat capacity. The regenerator efficiency is less than 100 % and the fluid flow across the regenerator creates a pressure drop and the heat transfer between the gas and the solid matrix are not reversible and infinite. The regenerator design must be optimized for each gas (Nitrogen, Helium, Hydrogen) considering different geometrical parameters (length, porosity, hydraulic diameter) and thermal parameters (temperature gradient between the two ends, thermophysical properties of the solid matrix)
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