Parallel-plate Regenerator Research Articles

Volume compensation of large-deformation 3D-printed soft elastomeric elastocaloric regenerators

Elastomeric elastocaloric regenerators have great potential for use in low-stress elastocaloric cooling devices. However, these regenerators display an asymmetric fluid exchange when operating in an active elastocaloric cooling cycle, due to the large required strains and associated volume change. During strain, the fluid volume increases, which passively forces fluid flow into the regenerator; when the strain is released, the fluid volume decreases, which results in a fluid flow out of the regenerator. During a traditional elastocaloric cooling cycle, there are also active fluid flow periods provided by fluid displacers or pumps. Here, we study the passive fluid flow in high-strain regenerators using a numerical model and experiments in two types of regenerators. Hyperelastic models are used to fit the experimentally measured mechanical behavior of thermoplastic polyurethane elastocaloric elastomers, and the model is subsequently used to conduct finite-element simulations predicting regenerator volume changes for an applied strain of 200%–600%. We validated the results using a specifically designed setup for measuring volume changes using pressure differences on a parallel-plate regenerator. For a strain range of 200%–600%, the predicted volume change ratio is 69.5%, closely matching the experimental value of 66.7%. We observed that the middle region of the regenerator experiences a higher volume change, which can be accurately accounted by the numerical model.

Experimental study on cooling performance of active magnetic regenerators with different structures

Using the distilled water as the heat transfer fluid, this paper built an active magnetic regenerator test bench to investigate the influence of crucial parameters on the cooling performance of active magnetic regenerators with different geometries, including the relative angle between Gd plates and magnetic field, the utilization factor, the temperature at the hot side of the regenerator, and the operating frequency. In addition, the performance enhancement potential of the newly proposed perforated parallel-plate regenerator was experimentally studied. The impacts of the perforation ratio and perforation on different thicknesses of plates were discussed. For the parallel-plate active magnetic regenerator with a thickness of 0.4 mm and a plate spacing of 0.35 mm, the 3.4% perforation ratio increases the temperature span between the hot and cold ends of the regenerator by 14.7%∼24.7% at no load condition from the baseline none-perforated parallel-plate regenerator.

Numerical simulation of a foam regenerator for elastocaloric cooling

The severe consequences of global warming lead to increased demand for environment-friendly cooling technologies. Elastocaloric cooling technology is one of the most promising alternatives to vapor-compression refrigeration technology because of its high energy efficiency and zero global warming potential (GWP). However, the commercialization of this technology is still hindered by its limited specific cooling power (SCP). To overcome this obstacle, this paper proposes a new regenerator design using shape-memory alloy (SMA) foams. A numerical simulation model was used to demonstrate that the SCP of the system with foam regenerators is 3.55 and 0.71 times greater than that of the baseline system with parallel-plate regenerators and tube regenerators. The performance of the system with a pair of foam regenerators is investigated in terms of operating frequency, fluid velocity, and geometric parameters. The theoretical SCP of the elastocaloric cooling system with foam regenerators is up to 80Wg-1, which far exceeds experimental and simulation results found in previous literature. Considering the compromise between cooling performance and power consumption, structural parameters with a porosity of 0.9 and a cell diameter of 2 mm and operating parameters with a fluid velocity of 2.2ms-1 and a frequency of 5 Hz are recommended.

Experimental study on regenerative effectiveness and flow characteristics of parallel-plate regenerator in Stirling engine

Regenerator is the critical component of the Stirling engine. Its performance directly affects the efficiency of the whole engine. The heat transfer and flow characteristics of the parallel-plate regenerator (PPR) are experimentally investigated, and the influences of material and geometric parameters on regenerative effectiveness and pressure drop are obtained. The results show that the axial heat conduction will reduce the regenerative effectiveness of the PPR. The higher the thermal conductivity of the plate, the lower the regenerative effectiveness. Larger plate thickness and smaller plate length will also increase the axial heat conduction loss. The PPR shows good flow characteristics. The comparative experiment with the wire mesh regenerator shows that its friction coefficient is much lower than that of the wire mesh regenerator. Through the comprehensive index NPH/NTU defined as the ratio of number of pressure heads (NPH) to number of transfer units (NTU), the overall performances of PPR and wire mesh regenerator are compared. The results show that the PPR has better working performance. To reduce the adverse effect of axial heat conduction on the PPR, the optimization experiment was carried out by segmenting the regenerator into several sections. The results show that the regenerative effectiveness of the segmented PPR is increased by 20%, while the increase of pressure drop is within an acceptable range.

Numerical study on regenerative effectiveness of parallel-plate regenerator for Stirling engine

ABSTRACT Due to the excellent heat transfer performance and low pressure drop, the parallel-plate regenerator (PPR) is very suitable for use in Stirling engines. At present, researches on the application of PPR in Stirling engines are rare. The axial heat conduction loss in PPR is a key factor that affects the heat transfer performance. Therefore, in this paper, the theoretical model of PPR operated under the Stirling engine working conditions is studied numerically, the regenerative effectiveness is taken as the target parameter, and the influence of axial heat conduction and other important parameters are explored. The results show that the axial heat conduction is detrimental to regenerative effectiveness, the regenerative effectiveness does not vary monotonically with the increase in the thermal conductivity, the copper regenerator with higher thermal conductivity has a lower regenerative effectiveness because of its higher axial thermal conductivity. The working frequency and the thermal properties of the working fluids have a comprehensive effect on the regenerative effectiveness, for high thermal diffusivity working fluids, such as helium and hydrogen, the regenerative effectiveness is high, while for low thermal diffusivity working fluid, such as carbon dioxide, the regenerative effectiveness is relatively low, and does not vary monotonously with frequency. The regenerative effectiveness increases with the reduction of the plate spacing, the plate spacing should be determined by considering the regenerative effectiveness and the pressure drop comprehensively. It is recommended that the stainless steel be a suitable material for the plates, and helium be the suitable working fluid.

Numerical investigation of temperature span of magnetic refrigerator using geometric configuration of gadolinium based parallel plate regenerator

In this manuscript, a numerical investigation on the temperature gradient of a magnetic refrigerator using different geometric configurations of a parallel plate regenerator is presented. The parallel plate regenerator is made up of gadolinium (Gd) as a magnetocaloric material with rectangular channels. The parallel plate regenerator is modeled and numerically investigated for 3D conjugated fluid convection and conduction heat transfer using Ansys Fluent. Two piston-cylinder displacers drive water as the working fluid through the regenerator loop. The hot and cold end heat exchangers are treated with the e-NTU method. The effect of changing the parallel plate regenerator’s dimensional parameters on temperature span is examined against the utilization factor of 0.1, keeping the regenerator’s porosity constant. The maximum temperature span is predicted by comparing simulated parallel plate magnetic regenerators for two diverse sets of dimensional parameters and surface areas is 36.5 K for magnetic field intensity of 0.8 T.

Study on vorticity diffusion laws under oscillating flow and its influences on dissipation loss

In this paper, the vorticity-stream function method is proposed to study the incompressible oscillating flow in the channel of Stirling regenerators. The vorticity field and velocity field in the channels of the parallel-plate regenerator are investigated. The plate spacing and oscillating frequency and kinematic viscosity of the working medium are unified as a dimensionless number e. The results showed that when e is ∼7, the “velocity overshoot” phenomenon occurs in the flow field, and this phenomenon becomes more evident with the increase of e and it gradually approaches the wall of the channel. The phase relationship between the pressure drop and the flow rate was obtained; the results showed that the phase of the flow rate lags behind the pressure drop, and the phase difference is influenced by spacing, frequency, and kinematic viscosity, with the maximal phase difference being 90°. The viscous dissipation loss in the channel was obtained based on the characteristic relationship between the cumulative flow and the pressure drop, and the wasted power of helium crossing a single channel was calculated to be less than 50 W when the frequency is no more than 100 Hz and the plate spacing is greater than 0.5 mm (given a swept volume of 500 ml). The results obtained in this paper verified the application of the vorticity-stream function method in oscillating flows, and the advantage of low resistance of helium over nitrogen was theoretically demonstrated in the relatively high frequency range.

Analytical solutions of heat storage and heat transfer performance of parallel‐plate regenerators in Stirling cycle

Parallel-plate regenerators (PPR), with flow resistance lower than traditional wire-mesh regenerators, can improve the thermal efficiency of Stirling engines (SEs). However, as working frequency or plate thickness increase, the heat cannot penetrate into the plate effectively, resulting only the surface part of the plates to have substantial temperature variation, while the internal part fails to store and exchange heat energy. In order to obtain high performance of PPR, the heat storage efficiency and the heat transfer coefficient, as well as their influential factors, are theoretically studied. Three parameters are found to play an important role, which are working frequency, plate thickness, and thermal diffusivity of materials. Their roles can be represented by a dimensionless parameter as a whole, which is the relative thickness, e. By the critical value of relative thickness, ecr = 2.4, two distinct working conditions can be divided, thermally penetrated condition as e < ecr and thermally non-penetrated condition as e > ecr. Under thermally penetrated condition, the heat storage efficiency is high, and the heat transfer coefficient is high enough when e > 1.6, while under thermally non-penetrated condition, the heat storage efficiency is low. In conclusion, by comprehensively considering the heat storage efficiency and heat transfer coefficient, it is recommended that the relative thickness e should be chosen within the range [1.6, 2.4]. And the optimal working frequency, plate thickness, and suitable material can be determined accordingly.

Exergy Analysis of a Parallel-Plate Active Magnetic Regenerator with Nanofluids

This paper analyzes the energetic and exergy performance of an active magnetic regenerative refrigerator using water-based Al2O3 nanofluids as heat transfer fluids. A 1D numerical model has been extensively used to quantify the exergy performance of a system composed of a parallel-plate regenerator, magnetic source, pump, heat exchangers and control valves. Al2O3-water based nanofluids are tested thanks to CoolProp library, accounting for temperature-dependent properties, and appropriate correlations. The results are discussed in terms of the coefficient of performance, the exergy efficiency, and the cooling power as a function of the nanoparticle volume fraction and blowing time for a given geometrical configuration. It is shown that while the heat transfer between the fluid and solid is enhanced, it is accompanied by smaller temperature gradients within the fluid and larger pressure drops when increasing the nanoparticle concentration. It leads in all configurations to lower performance compared to the base case with pure liquid water.

Feasibility Study of a Parallel Plate Desiccant Coated Heat and Mass Regenerator for Dehumidification

Our ever-increasing energy consumption has led to disastrous consequences such as global warming. Air-conditioning takes up a relatively large proportion of the total energy consumed. This makes it imperative to make air-conditioning systems more energy efficient. Desiccant assisted air-conditioning has since long attracted the attention of researchers since low grade waste heat or solar energy can be used. But due to practical limitations such as bulkiness of the system, long pay-back periods, etc., its commercial use has been limited. This paper proposes a desiccant coated parallel plate like configuration of a heat and mass regenerator channel that may be retrofitted into air-ducts of an existing air-conditioning system, instead of using desiccant beds or a desiccant wheel. In this paper, numerical simulations are carried out. Simulation conditions are for warm and humid air conditions. It is concluded that parallel plate regenerator configuration can be a viable part of a desiccant based air-conditioning system.

Sensitivity analysis and multiobjective optimization of a parallel-plate active magnetic regenerator using a genetic algorithm

This paper analyzes the energetic and exergetic performance of an active magnetic regenerative refrigerator. A 1D numerical model has been developed to simulate a system composed of a parallel-plate regenerator, magnetic source, pump, heat exchangers and control valves. The effects of several parameters like the mass flowrate and the plate and fluid thicknesses are studied. Various types of refrigerants are tested thanks to the CoolProp library, accounting for temperature-dependent properties. A multiobjective optimization based on a genetic algorithm is used to enhance the system performance regarding the Pareto efficiency. Parameters like mutation and crossover fraction are considered in order to determine the optimal results in a fair computational time. The main goals are to maximize the coefficient of performance, the exergetic efficiency, and the cooling power while respecting precise constraints. The best tuning leads to a solution 9 times faster than mapping all possibilities using a rough grid size.

Performance optimisation of room temperature magnetic refrigerator with layered/multi-material microchannel regenerators

A hybrid numerical model of the magnetic refrigerator with multi-material microchannel regenerator has been developed. The magnetocaloric effect was implemented using instantaneous temperature rise/drop (discrete method). Two pipe-in-pipe heat exchangers at two ends of the regenerator were treated using ε-NTU method. The commercially available compounds of LaFe13-x-yCoxSiy as well as hypothetical compounds of Gadolinium were considered as the magnetocaloric materials (MCMs) with different Curie temperatures. The predicted results of the present work for parallel-plate regenerators employing different compounds of LaFe13-x-yCoxSiy were broadly in good agreement with the available experimental data. The cooling capacity increases as the number of MCMs increase. However, for a given length of regenerator, an optimum number of MCMs was seen yielding the maximum performance of the refrigerator. For a given number of MCMs, a smaller Curie temperature difference ΔTCu between the MCMs was found to give higher performance.

Numerical investigation of room temperature magnetic refrigerator using microchannel regenerators

The paper reports numerical investigation of room temperature, active magnetocaloric regeneration (AMR) refrigerators/heat pumps using microchannel regenerator. The microchannel regenerators are made of a magnetocaloric material (MCM), Gd, with diameter of the circular channels ranging from 0.7 mm to 2.0 mm. Water, the working fluid, oscillates in the regenerator loop driven by two piston-cylinder displacers operating in a range of mass flow rates. Three dimensional conjugated fluid convection and conduction heat transfer in the microchannel regenerator was modeled and numerically simulated using ANSYS Fluent. The magnetocaloric effect (MCE) was incorporated into conservation of energy using a discrete method to simulate the magnetization and demagnetization of the MCM. The hot and cold end heat exchangers were treated with the ε–NTU method. Effects of utilization and porosity of the microchannel regenerator and cycle frequency on the cooling capacity and temperature span were examined. When the utilization, porosity and cycle frequency are 0.2, 0.5 and 5.0 Hz, respectively, the predicted maximum cooling capacity was about 22 W for a 0.8 T variation in intensity of magnetic field. The effects of magnetic field intensity, reservoir temperature span and flow rate profile on the refrigeration performance were also investigated. The performance of the microchannel regenerator is compared with that of the parallel plate regenerator for the no-load temperature span, cooling capacity and pumping power. Under specific geometric and operating conditions, the microchannel regenerator shows better performance than the parallel-plate one.

Improved modelling of a parallel plate active magnetic regenerator

Much of the active magnetic regenerator (AMR) modelling presented in the literature considers only the solid and fluid domains of the regenerator and ignores other physical effects that have been shown to be important, such as demagnetizing fields in the regenerator, parasitic heat losses and fluid flow maldistribution in the regenerator. This paper studies the effects of these loss mechanisms and compares theoretical results with experimental results obtained on an experimental AMR device. Three parallel plate regenerators were tested, each having different demagnetizing field characteristics and fluid flow maldistributions. It was shown that when these loss mechanisms are ignored, the model significantly over predicts experimental results. Including the loss mechanisms can significantly change the model predictions, depending on the operating conditions and construction of the regenerator. The model is compared with experimental results for a range of fluid flow rates and cooling loads.

Assessment of demagnetization phenomena in the performance of an active magnetic regenerator

The primary objective of this paper is to quantify the impact of the internal magnetic field and reversibility of the magnetocaloric effect on the numerical evaluation of the thermal performance of a parallel-plate active magnetic regenerator. To this end, direct measurements of the adiabatic temperature change of commercial-grade gadolinium samples were carried out for an applied field change of 1.65 ± 0.03 T (for both magnetization and demagnetization) and temperatures between 283 and 303 K. The tests were carried out in a specially constructed apparatus and the results were in good agreement with the literature. The data were incorporated into a mathematical model for solving the fluid flow and conjugated heat transfer in the parallel-plate regenerator. It was found that if demagnetization phenomena (i.e., the demagnetization factor and the reversibility of the magnetocaloric effect) are not take properly into account, the thermal performance of the regenerator can be severely over-predicted.

Numerical analysis of an active magnetic regenerator

A one-dimensional transient model of an active magnetic regenerator has been developed. The magnetocaloric properties are calculated using the Mean field theory and the model is used to study the transient and steady state response of a parallel plate regenerator. A parametric study is performed and highlights the influence of the Number of Transfer Units, the Utilization, and the solid axial conductivity. Finally the magnetic field inhomogeneity and the fluid axial conductivity are found to have no significant influence on the regenerator performance.

Analysis of single blow effectiveness in non-uniform parallel plate regenerators

Non-uniform distributions of plate spacings in parallel plate regenerators have been found to induce loss of performance. In this paper, it has been investigated how variations of three geometric parameters (the aspect ratio, the porosity, and the standard deviation of the plate spacing) affects this loss in a single blow model of a parallel-plate regenerator. Simple analytical functions for the magnitude and the time scale of the reduction of performance are presented and compared to numerical results.

A Monolithic Perovskite Structure for Use as a Magnetic Regenerator

A La0.67Ca0.26Sr0.07Mn1.05O3 (LCSM) perovskite was prepared for the first time as a ceramic monolithic regenerator used in a regenerative magnetic refrigeration device. The parameters influencing the extrusion process and the performance of the regenerator, such as the nature of the monolith paste and the influence of the sintering on the adiabatic temperature change, were investigated. Comparisons between the extruded monolithic structure before and after the sintering showed that an increase of the adiabatic temperature change was seen after the sintering. Furthermore, calculations show that the performance of the monolithic structure is potentially superior to a parallel plate regenerator, indicating the potential cost and structural benefit of using such structure, i.e. a mechanically stable ceramic thin wall structure, which can be produced in one processing step.

Modeling of parallel-plate regenerators with non-uniform plate distributions

A two-dimensional finite element model describing the performance of parallel-plate regenerators with arbitrary channel width distributions has been developed in order to investigate the effect of non-uniform plate spacing on the performance of regenerators. Results for a series of hypothetical plate spacing distributions are presented in order to understand the impact of spacing non-uniformity. Simulations of more realistic distributions where the plate spacings follow normal distributions are then discussed in order to describe the deviation of the performance of a regenerator relative to one with uniform spacing as a function of the standard deviation of the plate distribution. It has been shown that the most significant reduction in performance occurs when a volume of fluid between 100% and 200% of the regenerator void volume is displaced in a single blow.

A comprehensive parameter study of an active magnetic regenerator using a 2D numerical model

A two-dimensional numerical heat transfer model is used to investigate an active magnetic regenerator (AMR) based on parallel plates of magnetocaloric material. A large range of parameter variations are performed to study the optimal AMR. The parameters varied are the plate and channel thicknesses, cycle frequency and fluid movement. These are cast into the non-dimensional units utilization, porosity and number of transfer units (NTU). The cooling capacity vs. temperature span is mapped as a function of these parameters and each configuration is evaluated through the maximum temperature span and exergy. The results show that the optimal AMR should have a utilization in the range 0.2–1 and an NTU higher than 10 and not necessarily more than 30. It is concluded that parallel plate-based regenerators face significant challenges in terms of manufacturability. However, the benefit of parallel plate regenerators is a very low pressure drop, which is needed for high performance.