Thermal Regenerator Research Articles

A POROUS MEDIA APPROACH FOR NUMERICAL OPTIMISATION OF THERMAL WHEEL

The experimental investigations of rotating heat exchangers are usually too costly and provide limited understanding for the phenomena of heat and fluid flow within them; hence, a less expensive and more comprehensive method is required to investigate what can affect their overall performance. In the current study, a porous media concept is presented as an alternative way to numerically analyse the fluid flow and heat transport through a rotary thermal regenerator. An aluminum core formed of multi-packed square passages is simulated as a porous medium of an orthotropic porosity in order to allow the counter-flowing streams to flow in a way similar to that inside the regenerator core. The geometric properties of the core were transformed into the conventional porous media parameters such as the permeability and inertial coefficient based on empirical equations; so, the core has been dealt with as a porous medium of known features. Fluid and solid phases are assumed to be in a local thermal non-equilibrium state with each other. A commercial CFD code "STAR CCM+" was used to solve the current problem numerically, where heat is allowed to be exchanged between the two phases and tracked by creating a heat exchanger interface in the core region. The results are presented by means of overall thermal effectiveness, pressure drop, and coefficient of performance COP. Using porous media approach has been found to be sufficient to simulate the current problem, where the currently computed data were found to deviate by up to 2.7% only from the corresponding analytical and experimental data. The data obtained reveal an obvious impact of the core geometrical parameters on both the heat restored and pressure loss; and hence, the overall efficiency of the regenerator system.

A novel hybrid pyroelectric-Stirling engine power generation system

Temperature fluctuations in the thermal regenerator of Stirling Engine SE occur at relatively high frequencies, making them amenable to direct harvesting using Pyroelectric materials. This paper demonstrates the technical feasibility of a novel hybrid Stirling Engine (SE), coupled with inserted pyroelectric generators in its thermal regenerator, to produce extra power through the pyroelectric effect and increase engine performance. Theoretical models have been constructed to analyze the performance of the pyroelectric-Stirling engine (P-SE) system and determine its advantages over conventional SEs. The transient thermal behavior of the pyroelectric regenerator is modeled by subdividing it into multiple lumped sub-regenerators. Replacing the metal regenerator in a conventional SE at 750 K hot chamber temperature with a pyroelectric regenerator increased power production by 2.9%. Furthermore, at 25 Hz fluctuation, 15 mm regenerator length, and regenerator porosity of 0.6, the system efficiency of the hybrid P-SE system increased by 0.5% and 3.5% compared to systems using steel and aluminum regenerators, respectively. Also, the P-SE system operating at 25 Hz yields the highest combined efficiency of 49.8% at the regenerator porosity of 0.6.

Transient CFD Analysis of Thermal Regenerator

The goal of the work is to analyze the impact of various factors on the fixed bed regenerator’s thermal characteristics while also examining the regenerator’s thermal characteristics. The thermal parameters of the regenerator are affected by a number of factors, including Heat storage capacity, switching time, residence time, bed height/length, regenerator diameter, particle diameter, and gas flow direction. The current study examines each of these variables quantitatively. Understanding the thermal regenerator’s temperature and fluid flow variations CFD analysis is conducted. For the analysis, Ansys Fluent is a for-profit program.

Experimental study of a pilot unit of a ground regenerator for greenhouses

The object of research: the process of heat exchange in a packing in the form of a dense layer of crushed stone as part of a thermal regenerator designed to utilize the low-grade heat of the air in a greenhouse.
 Investigated problem: increasing the efficiency of air heat accumulation in the greenhouse during the daytime by a regenerative heat exchanger with a granular packing. Conducting experimental tests of regenerator models with a packing in the form of a dense layer of granular material under natural conditions is necessary to improve the design and optimize technological parameters.
 The main scientific results: the analysis of temperature curves of air changes at the inlet and outlet of the regenerator installed in the mock-up greenhouse with a volume of 0.25 m3, and the packing material was carried out. During the heating period of the nozzle, the accumulated heat was Q=8•104 J, which can be used when the temperature in the greenhouse decreases. During the pause, the loss from the heat exchange channel to the environment is 48,000 J. The duration of the period was 358 min, while the average heat flux corresponded to 2.2 W. Heat losses during the pause period are significant due to its duration, however, the average heat transfer coefficient has a low value of k=2.3 W/m2K, so strengthening the insulation of the heat exchange channel is irrational.
 The area of practical use of the results of the study: greenhouses, allowing the possibility of installing additional heat exchange equipment.
 Innovative technological product: the innovativeness of the technical solution is determined by the possibility of using the heat of the air in the greenhouse and the implementation of contact heat exchange between the air flow and the packing particles. The design of the regenerator, the simplicity of its manufacture and operation, as well as the efficiency of air heat accumulation in the greenhouse during the daytime, allow to recommend it for use in greenhouse farms.
 The scope of the innovative technological product: regenerative heat exchangers with a dense layer of granular material for the utilization of low-grade air flows generated in food production enterprises, ventilation systems and greenhouses

Harmonic analysis of temperature profiles of active caloric regenerators

The design of a thermal regenerator is initially carried out by considering the fundamental influencing variables. For novel solid-state cooling systems using active caloric regenerators, the non-linearity of the coupled phenomena of material properties, heat transfer, and hydraulic flow can complicate the interpretation of experimental and simulated results. Based on the boundary conditions of a sinusoidal magnetic field and fluid flow, we elucidate the operation of active regenerators by deriving easy-to-manage analytical expressions for the temperature transients of the caloric materials and heat transfer fluid. An internal temperature measurement system with an estimated uncertainty of ±0.3 K for packed bed regenerators has been developed for validation. The derived expressions have acceptable accuracy relative to the experimental and numerical results for temperature profiles in both magnitude and sensitivity, where average and maximum errors are ∼10% and ∼15%, respectively. Useful figures of merit are post-calculated using the derived temperature profiles. We found that the average temperature profiles are linear for passive regenerators and nonlinear for active regenerators, and their transients are nonlinear functions of the configuration and operating parameters.

Thermal plasma-aided chemical looping carbon dioxide dissociation for fuel production from aluminium particles

In the present article, a new thermal plasma-aided process is proposed and analysed that utilises alumina/aluminium particles to dissociate steam/carbon dioxide blends into high-quality synthetic fuel. The proposed system utilises two reactors namely a synthetic fuel reactor and a thermal plasma particle regenerator following the chemical looping gasification principle. In the former, the gas blend reacts with aluminium particles to produce hydrogen-enriched synthetic fuel and alumina. While in the latter, the alumina is dissociated into oxygen and reduced aluminium. Using thermochemical equilibrium analysis, it was identified that the proposed system can offer a self-sustaining factor of up to 0.18, thermodynamic and exergy efficiency of 0.38 and 0.68, respectively. The system was integrated with photovoltaic energy and a solar share of ≤ 0.5 (with low-capacity battery storage ≤ 4 MWh) and > 0.5 (with high-capacity battery storage < 5 MWh) was obtained at photovoltaic capacity ∼ 3–5 MW. The thermodynamic conditions of 1273 K < T < 1573 under lean oxygen conditions and T > 6373 K were identified for the fuel and plasma reactors, respectively. The calculated syngas quality was > 2 for the selected thermodynamic conditions. The integration of the system with photovoltaic energy showed that the installed capacity of photovoltaic panels and battery storage are intertwined representing a trade-off trend. The optimum storage capacity of 4–6 MWh and photovoltaic installation capacity of 3–5 MWe was calculated for the localised production scale of 775 tonnes/year. The proposed system offers resiliency against the continuous operation in both centralised and decentralised arrangements. Based on this proof of concept, the viability of the thermal plasma-aided process for remote small-scale applications was discussed by benchmarking how it can meet the requirements well known for remote gas-to-liquid compact plants, producing gasoline out of syngas.

Synergies between adsorption and energy conversion technologies

The analogy between heat uptake in a thermal regenerator and selective uptake of gas components in an adsorbent bed has inspired development of alternative architectures for adsorptive gas separation processes. An experimental laboratory sorption enhanced reaction approach for low pressure Haber-Bosch ammonia synthesis successfully used a Stirling engine mechanism to generate coordinated pressure swings and flow reversals of a thermally coupled PSA cycle. Attempts to apply similar mechanisms to air separation and hydrogen purification (rapid cycle PSA applications) motivated the development of very high surface area laminated parallel passage adsorbers for process intensification. Further efforts to develop compact PSA equipment for hydrogen purification and biogas upgrading led to practicable multiport rotary valves, enabling great simplification of PSA systems using multiple adsorbers. Higher degrees of simplicity and compactness for PSA and TSA processes were subsequently achieved using the laminated structured adsorbent in rotary adsorbers, now being used for post-combustion CO2 capture. Prospective applications in the energy conversion field include hydrogen recovery from SOFC anode tail gas, debottlenecking of MCFC carbon capture from flue gas, and recovery of tritium from fusion power plant breeder blanket helium purge gas. A major opportunity is identified for sorption-enhanced ammonia synthesis in the context of green hydrogen technologies.

A novel model and design of a MEMS Stirling cooler for local refrigeration

In this paper, we present a new model design and parametric studies of a miniature Stirling cooler machine for on-site refrigeration. The MEMS (Microelectromechanical systems) technology is investigated to design this machine. The concept could be used to provide cooling at chip scale and mitigate hot spots in electronic devices. Whereas numerous works deal with Stirling engines at a macroscopic scale, only a few works concern miniaturized Stirling engines. Therefore, a model analysis giving insights of the impact of the technological choices and downsizing of the machine is needed. A base design model is presented. The model results lead to a cooling power of 10 mW and a Coefficient Of Performance of 1.45. A parametric study is conducted for operational and design parameters. Compared to macro-scale design, the same trend is observed for the influence of the thermal performance regenerator. Different trends from macroscopic engines were observed for hysteresis losses importance, and the choice of the working gas. The raise in power due to the raise in frequency expected for micro-scale devices is counterbalanced by the degradation of the COP due to the increase in thermofluidic losses. Squeeze film damping and finite speed losses can be neglected at this scale.

Numerical analysis of the flow and heat transfer characteristics in serpentine microchannel with variable bend amplitude

PurposeThis study aims to propose the increase of heat dissipation requirements of modern electronic equipment and the fast development of micro-scale manufacturing technologies. The heat transfer mechanism is studied in-depth, especially for its pattern of secondary flow caused by the repeated inversion of centrifugal force. Effects of η on the frictional pressure drop and average Nusselt number are studied and the performance of such microchannel heat sink with various bend amplitudes is comprehensively evaluated. These results can provide important insight into the optimal design of this novel design configuration for microelectronics cooling.Design/methodology/approachA three-dimensional model based on the finite volume approach and SIMPLEC algorithm is performed to test an innovative serpentine microchannel, which behaves differently from conventional serpentine microchannel due to the significant effect of centrifugal force inversion.FindingsThe effect of centrifugal force significantly influences the flow and thermal fields which are responsible for the enhancement in heat transfer coefficient. The number, size and intensity of vortices increase with increasing Re, and the vortices are reformed at every change of the geometry in a periodic fashion. The serpentine microchannel studies more effectively at larger bend amplitude. Pressure fluctuations and temperature variation are greater with increasing bend amplitude.Practical implicationsSeveral techniques have been developed to augment single-phase convective heat transfer in channels. One technique is to use a serpentine channel that enhances the heat transfer due to flow mixing and periodic interruption of thermal boundary layers. This technique has been applied to micro-heat exchangers, thermal regenerators and mini/microreactors.Social implicationsThe optimal design of this novel design configuration for microelectronics cooling can be attained. It will become an effective cooling technology for solving the increasing of heat dissipation requirements of modern electronic equipment.Originality/valueThe flow and heat transfer characteristics are first presented for the circular serpentine microchannel made up of alternate U-bends without interposed straight segments. The present study first examines the effect of such centrifugal force inversion on velocity contour, pressure distribution and temperature distribution. The patterns of secondary flow along the flow passage caused by the repeated inversion of centrifugal force are further studied in depth. The effect of bend amplitude on the flow and heat transfer is explored and the performance of such microchannel heat sink has been comprehensively evaluated.

Heat transfer figures of merit for mapping passive regenerator performance to active regenerator cooling power

This study reports specific heat transfer figures of merit for thermal regenerators and applications using the passive characterization, which can rapidly predict the cooling capacity of active magnetic regenerators (AMRs) in magnetic refrigeration. We expand the expression for the thermal effectiveness in terms of the individual contributions of heat storage and parasitic losses, using one-dimensional energy equations and mathematical estimations. Furthermore, a quantitative expression correlating the heat transfer effectiveness and cooling capacity provides a metric for the characterization of AMRs having new porous geometries. We found that the solid heat storage term of the effectiveness has a clearer impact on both the conjugate heat transfer and cooling capacity of AMRs than the entire effectiveness does.

A multiresolution wavelet optimised finite-difference method for simulation of thermal regenerator

The equations demonstrating the heat transfer phenomenon in a thermal regenerator were solved with the finite-difference method (FDM) and the method has been improvised by using wavelets. In the thermal regenerators, hot flue gas enters an initially cold bed, transfer all of its thermal energy in a portion of the bed and leaves at low temperature creating a heat transfer zone which traverses the bed-length with time while establishing the thermal equilibrium on both sides of the zone. The discretization process in conventional FDM takes uniform dense grids for whole domain, however, in actual physical process, the heat transfer happens only in a portion of the domain at a certain time requiring dense grids. Such problems demand a front-tracking method which can adjust the grid pattern according to the temperature-front location for which wavelets are found to be suitable. Wavelets functions have the property of being translated and dilated according to the requirement. An adaptive grid scheme has been developed using wavelets along with the multiresolution analysis. The grid patterns for different forms of Daubechies wavelets i.e. db1, db2, db3, and db4 have been compared. The method based on wavelets has been found accurate and less time-consuming.

Experimental Investigation of Displacer Seal Geometry Effects in Stirling Cycle Machines

This contribution deals with an experimental investigation of the optimization potential of Stirling engines and similar regenerative machines by an enhanced design of the cylinder liner and the seal. The latter is mounted at the bottom end of the gap surrounding pistons and displacers that separate cylinder volumes at different temperature levels. The thermal loss associated with this gap may amount to more than 10% of the heat input into these machines. Mostly, its design is reduced to an estimation of the optimum width by analytical models, which usually do not account for further relevant optimization parameters, such as a step in the cylinder wall. However, a recently developed, enhanced analytical model predicts that this loss may be significantly reduced by such a step. In this work, this design was realized and investigated experimentally according to this prediction by modification of the cylinder liner and the seal of an extensively tested laboratory-scale machine. The results confirm that such a design actually reduces the thermal loss substantially, presumably by reducing the cyclic mass flows through the open end of the gap. Additionally, it even improves the net power output due to a reduced volumetric displacement by the piston or displacer, resulting in smaller flow losses and thermal regenerator losses, whereas the pressure amplitude remains virtually unchanged, contrary to initial expectations. This has led to the remarkable conclusion that the design of most Stirling engines is possibly suboptimal in this respect and may be improved a posteriori by a minor modification; i.e., a reduction of the effective displacer seal diameter.

Thermal-hydraulic evaluation of 3D printed microstructures

High performance thermal regenerators require matrices with low pressure drop and high thermal effectiveness. To address this topic a new regenerator geometry is designed, manufactured and experimentally tested in a passive regenerator apparatus. Two microstructure configurations based on crossed fibers are examined, one using aligned fibers and another with staggered fibers. A total of four regenerators are tested where the fiber diameter and spacing are varied. All the matrices are made of stainless steel and distilled water is used as heat transfer fluid. The experiments are performed using operating frequencies and utilization factors typical of those in active magnetic regenerators. The regenerators are characterized and compared based on the thermal effectiveness and pressure drop. Effectiveness imbalance and temperature profiles along the regenerator bed are also presented and discussed. The results reveal that 3D printed matrices are promising geometries for active magnetic regenerators.

Influence of the heat capacity of the storage material on the efficiency of thermal regenerators in liquid air energy storage systems

Abstract Liquid air energy storage is an innovative technology for electricity storage, using liquefied air as storage medium. Due to the high energy density of liquid air, the storage volume is smaller than that of similar storage technologies like compressed air or pumped hydro energy storage systems. Air is liquefied by means of a modified Claude-cycle. In the discharging process, liquid air is energy-efficiently compressed (compression of a liquid), vaporized, superheated, and finally expanded from high to ambient pressure using an air expander for power generation. In between, a thermal energy storage device at cryogenic temperature level is used to improve the round-trip efficiency of the system. One possible design is a packed bed thermal energy storage device, consisting of a cylinder and a packed bed of storage material. Investigations on thermodynamic properties show that especially the temperature-dependence of the heat capacity has a major influence on the performance of the thermal energy storage system. In this paper, nine real and further hypothetical storage materials are investigated. The influence of the heat capacity at cryogenic temperature is systematically analyzed and a general formulation in terms of summarizing key figures is developed. It turns out that especially the temperature-dependence is a significant parameter in this context, which is not considered within the majority of investigations in this field.

Wavelet based simulation of thermal regenerator

Thermal regenerators are the heat exchanging devices used for transferring heat from hot fluid to cold fluid via porous matrix of solid material. The transfer of heat energy in the thermal regenerator can be expressed by coupled partial differential equations (solid phase and fluid phase). In the present work, the governing equations for thermal regenerators are solved and compared by the finite difference method and wavelet optimized finite difference method. Validation of previous experimental results has also been done by using the present simulation. A methodology based on Sparse Point Representation (SPR) is used to adapt the effective grid pattern using wavelet transformation. Temperature profiles along the bed length are shown for the heating process by both the methods. The modified grid patterns based upon SPR, at different time levels are represented with the help of appropriate figures, and it is observed that the use of wavelet transformation in numerical solution increases the efficiency of the method by reducing CPU time.

Performance analysis of solar vacuum membrane distillation regeneration

This paper proposes a novel solar vacuum membrane distillation (VMD) regeneration for liquid desiccant air conditioning (LDAC) system, and establishes a mathematical model of the VMD regeneration process and traditional thermal (TH) regeneration process based on the heat and mass balance. The corresponding simulation model was established by using Matlab software, and the model was verified by previous experiments. The results show that: the model results are consistent with the previous experimental results, and the maximum error is less than 6%. Model simulations are carried out to study the regeneration performance under various operating parameters and climatic conditions, meanwhile the comparison the solar power consumption for regenerating one kilogram desiccant between the VMD regeneration and TH regeneration. Results demonstrate that when the air temperature is less than 32 °C, the humidity ratio is higher than 19 g/kg, and the air flow rate is less than 0.13 kg/s, the VMD regenerator solution regeneration effect is better than the conventional thermal regenerator. VMD regeneration is more suitable for solution regeneration in wet areas than TH regeneration. Furthermore, when the solution temperature is lower than 60 °C, VMD regeneration outlet concentration is higher than TH regeneration. VMD regeneration is more suitable for low-grade energy. Analyze the solar power consumption for regenerating one kilogram desiccant found that the VMD regeneration saves 10–37% energy than TH regeneration. Through economic analysis, it is found that the VMD regenerator can save 582.4 kWh of electricity per year compared to the TH regenerator. According to the cost recovery calculation formula, the cost recovery period can be calculated as 13.8 years.

Influence of inlet flow maldistribution and carryover losses on the performance of thermal regenerators

Void (dead) volumes represent a penalty to the thermal performance of Active Magnetic Regenerators (AMR). While much effort has been put in designing AMRs with small void volumes, in most designs, penalties associated with inlet flow maldistribution and pressure loss may increase with the reduction of the void volumes. In this work, an experimental evaluation of the combined effect of void volume and inlet flow maldistribution is carried out. The analysis is performed based on the regenerator thermal effectiveness determined experimentally in a passive regenerator test apparatus. The results show that the negative impact of inlet flow maldistribution on the regenerator effectiveness is more significant than that of the void volume.

Thermal calculation of heat exchangers with simplified consideration of axial wall heat conduction

The usual thermal design and rating methods [1, 2] for heat exchangers neglect axial wall heat conduction in the separating walls and external shells of recuperators and in the solid matrix of regenerators. This may lead to undesirable undersizing. In this paper a simplified model is developed for the fast estimation of axial wall conduction effects in counterflow, parallel flow and mixed-mixed cross-flow recuperators. The dispersion model [3] is used to describe the performance deterioration of the exchanger with an effective fluid dispersion Peclet number for the correction of the heat transfer coefficients or mean temperature difference. The method is tested against analytical and numerical calculations for counterflow and parallel flow with good results. It is also shown how the method can be adapted to thermal regenerators and the related thermal calculation methods [1, 2]. An alternative approach is suggested for the consideration of lateral heat conduction resistance in the solid matrix.

Thermal-hydraulic behavior and influence of carryover losses in oscillating-flow regenerators

Thermal regenerators that use a liquid as the heat transfer fluid are encountered in active-caloric (e.g., magnetocaloric, electrocaloric, elastocaloric) coolers/heat pumps and other applications. In this study, we present a mathematical model, its numerical implementation and comparisons with an extensive experimental database on the thermal performance of packed bed regenerators composed of stainless steel spheres [1]. The regenerator model consists of the one-dimensional Brinkman-Forchheimer equation to describe the fluid flow in the porous matrix and coupled energy equations to determine the temperatures in the fluid and solid phases. A mathematical model for the so-called carryover (leakage) losses provoked by the dead (void) volumes on each side of the regenerator was implemented to improve the thermal effectiveness prediction. A good agreement between the mathematical model and the experimental data was observed, enabling an accurate quantification of thermal-hydraulic losses. The mathematical model and its results can be extended to and incorporated into performance analyses of active magnetic regenerators.

Thermal–hydraulic evaluation of oscillating-flow regenerators using water: Experimental analysis of packed beds of spheres

Thermal regenerators that use liquid heat transfer fluids are being researched for their application in active caloric cooling/heat pumping systems. The performance of active caloric devices is significantly influenced by the regenerator thermal effectiveness and by viscous losses. The present paper is the first part of a study on the thermal–hydraulic evaluation of packed bed regenerators that use water as a thermal fluid. Here, a detailed experimental analysis of the thermal–hydraulic performance of regenerative matrices composed of packed beds of stainless steel spheres is carried out. A laboratory apparatus was developed to quantify the viscous losses and thermal effectiveness in oscillating-flow regenerators that use water as the heat transfer fluid. Operating parameters such as frequency and mass flow rate were varied to cover broad ranges of utilization and number of transfer units (NTU). The diameter of the spherical particles and the dimensions of the regenerator housing were also changed in order to evaluate their influence on the behavior of the time-dependent and time-average performance parameters.