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20 pages, 16373 KiB

Open AccessArticle

Study on Phase-Shift Mechanism and Kriging-Based Global Optimization of the Active Displacer Pulse Tube Refrigerators

by Zongtao Geng, Wei Shao, Zheng Cui and Chen Zheng

Energies 2023, 16(11), 4263; https://doi.org/10.3390/en16114263 - 23 May 2023

Cited by 1 | Viewed by 1233

Abstract

Pulse tube refrigerators are widely used in certain special fields, such as aerospace, due to their unique advantages. Compared to a conventional phase shifter, the active displacer helps to achieve a higher cooling efficiency for pulse tube refrigerators. At present, the displacer is [...] Read more.

Pulse tube refrigerators are widely used in certain special fields, such as aerospace, due to their unique advantages. Compared to a conventional phase shifter, the active displacer helps to achieve a higher cooling efficiency for pulse tube refrigerators. At present, the displacer is mainly studied by one-dimensional simulation, and the optimization method is not perfect due to its poor accuracy, which is not conducive to obtaining a better performance. Based on the current status of displacer research, phase-shift mechanisms of inertance tube pulse tube refrigerators and active displacer pulse tube refrigerators were firstly studied comparatively by multidimensional simulation, and then we determined the crucial effect properties that lead to a better cooling performance for the active displacer pulse tube refrigerator at different cooling temperatures. Finally, an efficient optimization method combining the Kriging model and genetic algorithm is proposed to further improve the cooling performance of the active displacer pulse tube refrigerator. The results show that the active displacer substantially improves the cooling performance compared to the inertance tube mainly by increasing the PV power and enthalpy flow in the pulse tube. The Kriging agent models of active displacer pulse tube refrigerator achieve 98.2%, 98.31%, 97.86%, and 97.32% prediction accuracy for no-load temperature, cooling capacity, coefficient of performance, and total input PV power, respectively. After optimization, the no-load temperature is minimally optimized for a 23.68% reduction compared to the initial one with a relatively high efficiency, and the founded optimization methods can also be weighted for multiple objectives, according to actual needs. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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23 pages, 6091 KiB

Open AccessArticle

Study on Heat Transfer Characteristics and Performance of the Full Premixed Cauldron Stove with Porous Media

by Dingming Zheng, Lei Su, Haoyu Ou and Shijie Ruan

Energies 2022, 15(24), 9523; https://doi.org/10.3390/en15249523 - 15 Dec 2022

Cited by 1 | Viewed by 1700

Abstract

The cauldron stoves used in restaurants and canteens usually adopt the combustion mode of blast diffusion. Low combustion efficiency leads to low thermal efficiency and high CO and NOx emissions. To address these problems, a 52 kW fully premixed stove with porous media [...] Read more.

The cauldron stoves used in restaurants and canteens usually adopt the combustion mode of blast diffusion. Low combustion efficiency leads to low thermal efficiency and high CO and NOx emissions. To address these problems, a 52 kW fully premixed stove with porous media is designed, and the heat transfer characteristics of the stove are analyzed by theoretical analysis and numerical simulation. The results show that under the rated power, the thermal efficiency of the stove reaches 68.55%, which is more than twice the thermal efficiency of the traditional blast diffusion stove. Among them, the radiant heat efficiency of the stove reaches 47.16%; thus, radiation heat transfer has become an important way of heat transfer of the porous media stove. Moreover, increasing the diameter and emissivity of porous media will increase the radiant thermal efficiency of the stove, but it will significantly reduce the flame temperature. In addition, the influence of the diameter is greater than the emissivity. The increase of the thickness of porous media can significantly improve the preheating temperature of the premixed gas, thus improving the ignition performance of the stove. Additionally, the stove has an appropriate thickness (approximately 3 mm), which not only ensures the preheating temperature but also does not easily allow for breakage and damage of porous media. Increasing the pore density or reducing the porosity of porous media can enhance the ignition performance of the stove. Moreover, the results of numerical simulation verify the theoretical results to a certain extent and shows that there is an optimal flue position as well. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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15 pages, 4298 KiB

Open AccessArticle

Prospects for Power Generation of the Doublet Supercritical Geothermal System in Reykjanes Geothermal Field, Iceland

by Yu Wang, Tianfu Xu, Yuxiang Cheng and Guanhong Feng

Energies 2022, 15(22), 8466; https://doi.org/10.3390/en15228466 - 12 Nov 2022

Cited by 1 | Viewed by 2251

Abstract

Supercritical geothermal resources are in the preliminary exploration stage as a new type of clean energy and there are no practical utilization projects. The IDDP-2 well at Reykjanes geothermal field in Iceland encountered supercritical geothermal conditions in 2017, with a maximum temperature of [...] Read more.

Supercritical geothermal resources are in the preliminary exploration stage as a new type of clean energy and there are no practical utilization projects. The IDDP-2 well at Reykjanes geothermal field in Iceland encountered supercritical geothermal conditions in 2017, with a maximum temperature of 535 °C. The system is still in the field experiment stage and no exploitation work has been carried out. Hence, a hypothetical doublet geothermal system was simulated based on IDDP-2 to study the power generation potential and favorable operating conditions for future development of supercritical geothermal resources. A multiphase flow model is established to predict the fluid and heat flow characteristics. Furthermore, sensitivity and economic analyses were performed to evaluate the expected commercial and environmental benefits of the supercritical geothermal system. The results show that the system’s evolution could be briefly divided into three stages according to the temperature variation. The power generation ranges between 5.4 MW~16.5 MW, and the levelized cost of electricity (LCOE) is 0.02 $/kWh. In addition, the system can reduce CO₂ emissions, which are 1.2~7.75 Mt less than that of fossil fuel plants with the same installed capacity. The results prove the great development potential and commercial competitiveness of the supercritical geothermal system. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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21 pages, 3579 KiB

Open AccessArticle

Effects of Cathode Gas Diffusion Layer Configuration on the Performance of Open Cathode Air-Cooled Polymer Electrolyte Membrane Fuel Cell

by Ming Peng, Enci Dong, Li Chen, Yu Wang and Wen-Quan Tao

Energies 2022, 15(17), 6262; https://doi.org/10.3390/en15176262 - 28 Aug 2022

Cited by 5 | Viewed by 2988

Abstract

The design of a gas diffusion layer (GDL) is an effective way to manage water transport, thus improving the performance of air-cooled fuel cells. In the present study, three group designs of GDL with polytetrafluoroethylene (PTFE)—uniformly doped, in-planed sandwich doped and through-plane gradient [...] Read more.

The design of a gas diffusion layer (GDL) is an effective way to manage water transport, thus improving the performance of air-cooled fuel cells. In the present study, three group designs of GDL with polytetrafluoroethylene (PTFE)—uniformly doped, in-planed sandwich doped and through-plane gradient doped—are proposed, and their effects on the performance of air-cooled fuel cells are explored by numerical simulation. The distribution of key physical quantities in the cathode catalyst layer (CCL), current density and the uniformity of current density distribution in the CCL were analyzed in detail. The results show that properly reducing the amount of PTFE in GDL is beneficial to promoting the water retaining capacity of air-cooled fuel cells, and then improving the performance of fuel cells. The performance of the in-plane sandwich GDL design cannot exceed the design with 10% PTFE uniformly doped, and this design will aggravate the uneven distribution of current density in CCL. Compared with the design of GDL with 40% PTFE uniformly doped, the current density can be improved by 22% when operating at 0.6 V by gradient increasing the PTFE content in GDL from the GDL/MPL interface to the gas channel. Furthermore, this design can maintain as good a current density uniformity as uniformly doping schemes. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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12 pages, 2442 KiB

Open AccessArticle

Experimental Study on Renewable Porous Carbon Dioxide Adsorbent Materials for Space Shuttles

by Chun Zhang, Yu Wang, Tao Liu and Hanbing Ke

Energies 2022, 15(14), 4947; https://doi.org/10.3390/en15144947 - 6 Jul 2022

Viewed by 1359

Abstract

Porous adsorbent material is promising to be used to regeneratively remove CO₂ from space shuttles. In this work, the amount and isosteric heat of CO₂ adsorption in solid amine are experimentally studied at pressures ranging from 0 to 6 bar and [...] Read more.

Porous adsorbent material is promising to be used to regeneratively remove CO₂ from space shuttles. In this work, the amount and isosteric heat of CO₂ adsorption in solid amine are experimentally studied at pressures ranging from 0 to 6 bar and temperatures ranging from 20 °C to 60 °C. The amount and isosteric heat of water adsorption in the solid amine is tested at different humidities (relative humidity 30–80%). The effective thermal conductivity of the solid amine at different atmospheres (air, N₂, CO₂ and water), pressures and temperatures is also investigated. The results show that the best temperature for CO₂ adsorption in the solid amine is 45 °C under dry conditions. The amount of water adsorption increases with enhanced humidity, while the isosteric heat of water adsorption remains a constant value. The effective thermal conductivity of the solid amine increases with an increase in pressure. The adsorbed phase (CO₂ and water) in the solid amine makes a contribution to improving the effective thermal conductivity of solid amine particles. The above findings can help design a better adsorption system in space. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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9 pages, 3278 KiB

Open AccessArticle

The Effects of Pore Geometry on Late Time Solute Transport with the Presence of Recirculation Zone

by Peijie Yang, Guangwei Wu, Sha Gu, Qin Guan and Lichun Wang

Energies 2022, 15(13), 4636; https://doi.org/10.3390/en15134636 - 24 Jun 2022

Viewed by 1444

Abstract

The solute transport process in porous media is central to understanding many geophysical processes and determines the success of engineered applications. However, fundamental understanding of solute transport in heterogeneous porous media remains challenging especially when inertial effects are significant. To address this challenge, [...] Read more.

The solute transport process in porous media is central to understanding many geophysical processes and determines the success of engineered applications. However, fundamental understanding of solute transport in heterogeneous porous media remains challenging especially when inertial effects are significant. To address this challenge, we employed direct numerical simulations in a variety of intrapore geometries at a high Reynolds number (Re = 10) flow regime, where recirculation zones (RZs) are present with significant inertial effects. We find that the volume of RZs depends on pore geometries. Moreover, RZs serve as an immobile domain that can trap and release solutes that lead to non-Fickian transport, characterized by the early arrival and heavy tailing of breakthrough curves and bimodal residence time distributions (RTDs). Lastly, the late time portion of RTDs is fitted to the power law function with determined exponent n, where n depends on the pore geometries and consequently the volume of RZs. Our study sheds light on the mechanisms of an immobile zone on the solute transport, especially improving our understanding of late time transport tailing in pressurized heterogeneous porous media. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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17 pages, 6483 KiB

Open AccessArticle

Numerical Simulation on the Structural Design of a Multi-Pore Water Diffuser during the External Ice Melting Process of an Ice Storage System

by Lei Li, Yude Wu, Yi Lu, Xiao Yang, Qiyang Wang, Xiaoai Wang and Yulin Wang

Energies 2022, 15(6), 2181; https://doi.org/10.3390/en15062181 - 16 Mar 2022

Viewed by 2136

Abstract

A water diffuser is a critical auxiliary equipment for an ice storage system during the external ice melting process. This paper proposes a linear multi-pore water diffuser for an ice storage system with 500 t of refrigeration capacity to enhance the performance of [...] Read more.

A water diffuser is a critical auxiliary equipment for an ice storage system during the external ice melting process. This paper proposes a linear multi-pore water diffuser for an ice storage system with 500 t of refrigeration capacity to enhance the performance of external ice melting. By establishing a three-dimensional two-phase volume of fluid (VOF) model, different structural designs of water diffusers for the ice storage device are numerically examined regarding the degree of turbulence, flow velocity, and pressure drop. The results show that the optimal water diffuser with five rows of trunk pipe and six perforated pores arranged in per row of branch pipe with a 4 mm diameter of perforated pores exhibiting a relatively lower degree of turbulence with a lower pressure drop compared with the other designs in this study. Meanwhile, the influence of the flow velocity on the ice melting process is also investigated by a numerical model of ice melting. It is found that the fed flow velocity from the main pipe inlet exhibits a great impact on the external ice melting process. Compared with the external ice melting process without the water diffuser, the external ice melting process with optimal water diffuser design under flow velocity of 1.0 m s⁻¹ could shorten the overall ice-melting time by 16 h. Additionally, through adjusting the water flow velocity, different output cooling can be realized to provide a fast response speed to the cooling variations in demand of the terminal users with a reduced cost. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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Review

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23 pages, 1250 KiB

Open AccessReview

On the Existence and Applicability of Extremal Principles in the Theory of Irreversible Processes: A Critical Review

by Igor Donskoy

Energies 2022, 15(19), 7152; https://doi.org/10.3390/en15197152 - 28 Sep 2022

Cited by 2 | Viewed by 1636

Abstract

A brief review of the development of ideas on extremal principles in the theory of heat and mass transfer processes (including those in reacting media) is given. The extremal principles of non-equilibrium thermodynamics are critically examined. Examples are shown in which the mechanical [...] Read more.

A brief review of the development of ideas on extremal principles in the theory of heat and mass transfer processes (including those in reacting media) is given. The extremal principles of non-equilibrium thermodynamics are critically examined. Examples are shown in which the mechanical use of entropy production-based principles turns out to be inefficient and even contradictory. The main problem of extremal principles in the theory of irreversible processes is the impossibility of their generalization, often even within the framework of a class of problems. Alternative extremal formulations are considered: variational principles for heat and mass transfer equations and other dissipative systems. Several extremal principles are singled out, which make it possible to simplify the numerical solution of the initial equations. Criteria are proposed that allow one to classify extremal principles according to their areas of applicability. Possible directions for further research in the search for extremal principles in the theory of irreversible processes are given. Full article

(This article belongs to the Special Issue Advances in Heat and Mass Transfer and Reaction in Porous Media)

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