Research

17 pages, 2922 KiB

Open AccessArticle

Study on Heat Transfer Performance and Parameter Improvement of Gravity-Assisted Heat Pipe Heat Transfer Unit for Waste Heat Recovery from Mine Return Air

by Yu Zhai, Xu Zhao, Guanghui Xue and Zhifeng Dong

Energies 2023, 16(17), 6148; https://doi.org/10.3390/en16176148 - 24 Aug 2023

Cited by 1 | Viewed by 596

Abstract

One of the effective methods for energy conservation and emission reduction in coal mines is to utilize waste heat recovery technology to recover mine return air waste heat. The gravity heat pipe is widely used in mine return air waste heat recovery due [...] Read more.

One of the effective methods for energy conservation and emission reduction in coal mines is to utilize waste heat recovery technology to recover mine return air waste heat. The gravity heat pipe is widely used in mine return air waste heat recovery due to its sustainable and economic advantages, but its heat transfer is a complex process influenced by multiple parameters. A single-tube heat transfer resistance model and a heat transfer calculation model based on enthalpy difference were established for the heat exchange tubes. Four typical application cases of a low flow rate and a low number of tube rows were selected, and their heat transfer characteristics were tested onsite and analyzed. It was found that there were problems such as a low overall heat transfer efficiency, a low fresh air outlet temperature, and a risk of icing in the final tube section. The effects of the gravity heat pipe parameters on the heat transfer performance were studied, such as the tube outer diameter, tube spacing, and the finned tube outer diameter. It was found that the air-resistant force of the heat exchanger increased with the increase of the tube spacing and the finned tube outer diameter, the heat transfer resistance increased with the increase of the tube spacing and the decrease of the finned tube outer diameter, and the heat transfer coefficient first increased and then decreased with the increase of the tube outer diameter. A configuration improvement scheme with a high flow rate and a high number of tube rows is proposed here. Taking Case 2 as an example, the temperature distribution of the heat tube before and after improvement is compared and analyzed. The results show that the heat transfer performance of the heat exchange system significantly improved. Without increasing the air resistance of the heat tube, the temperature of the return air outlet after improvement was reduced to 1.1 °C, 4.1 °C lower than that before improvement, further recovering the waste heat of the mine return air. The temperature of the condensate water film was greater than 0.5 °C, avoiding the icing problem of the condensate tube section, the fresh air outlet temperature reached 5.2 °C, an increase of 7.8 °C compared to that before improvement, and the overall heat transfer efficiency increased from 56.7% to 66%. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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17 pages, 11543 KiB

Open AccessArticle

Numerical Investigation of the Enhanced Stirring Characteristics of a Multi-Lance Top-Blowing Continuous Converting Furnace for Lance Arrangement and Variable-Velocity Blowing

by Wenjie Li, Shibo Wang, Jianxin Xu, Jianhang Hu, Hua Wang, Yuling Zhai, Qingtai Xiao, Ge Deng and Dongbo Li

Energies 2023, 16(5), 2412; https://doi.org/10.3390/en16052412 - 02 Mar 2023

Cited by 1 | Viewed by 1197

Abstract

Oxygen lances are key equipment for copper converters. The effect of the lance arrangement on the mixing of a gas–slag two–phase is discussed using numerical simulation and experimental verification with a water model, and the stirring characteristics enhanced by variable–velocity blowing are explored. [...] Read more.

Oxygen lances are key equipment for copper converters. The effect of the lance arrangement on the mixing of a gas–slag two–phase is discussed using numerical simulation and experimental verification with a water model, and the stirring characteristics enhanced by variable–velocity blowing are explored. The results showed that the single–row lance arrangement (SA) increased the average velocity in the slag phase by 17.93% and reduced the disturbance to the metal phase by 27.78% compared to the double–row lance arrangement (DA). Compared to the constant–velocity blowing system (CSB), the sine–wave blowing system (SWB) and rectangular–wave blowing system (RWB) increased the average velocity in the slag phase by 24.03% and 13.96%, respectively, and reduced the proportion of the low–velocity area by more than 46.2%. The velocity imbalance in the SA local area enhances the mixing of the gas–slag two–phase. The variable–speed blowing improves the mass transfer and mixing effect. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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16 pages, 3825 KiB

Open AccessArticle

Statistical Image Analysis on Liquid-Liquid Mixing Uniformity of Micro-Scale Pipeline with Chaotic Structure

by Haotian Wang, Kai Yang, Hua Wang, Jingyuan Wu and Qingtai Xiao

Energies 2023, 16(4), 2045; https://doi.org/10.3390/en16042045 - 19 Feb 2023

Cited by 3 | Viewed by 1246

Abstract

The aim of this work is to introduce a novel statistical technique for quantifying the concentration field uniformity of the liquid-liquid mixing process within a micro-scale chaotic pipeline. For illustration, the microscale liquid-liquid mixer in which the inlet direction is parallel to the [...] Read more.

The aim of this work is to introduce a novel statistical technique for quantifying the concentration field uniformity of the liquid-liquid mixing process within a micro-scale chaotic pipeline. For illustration, the microscale liquid-liquid mixer in which the inlet direction is parallel to the mixing unit is designed by using the chaotic pipeline with Baker map. Meanwhile, the non-uniformity coefficient method is adopted quantificationally instead of qualitatively estimating the concentration field uniformity of the chaotic micromixer based on uniform design theory and image analysis. Results show that the concentration distribution of the chaotic mixing process of liquid-liquid under various working conditions is obtained by solving the steady-state Navier–Stokes and diffusion convection equations. The average contribution ratio of the three basic mixing units of the chaotic Baker pipeline to the concentration field uniformity is approximately 6:3:1, which is calculated aligned with the fluid flow direction successively. The optimal mixing uniformity can be obtained as the initial velocity is 0.05 m/s and the diffusion coefficient is 5 × 10⁻⁹ m²/s, respectively. The reliability of the new method for estimating the concentration field uniformity parameters is explained from three dimensions. The statistical image analysis technique is illustrated to be reliable and effective in yielding accurate concentration field information of the simulated chaotic mixer. Furthermore, it can be adapted to examine a variety of concentration distribution issues in which concentrations are evaluated under distinct scales. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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19 pages, 5425 KiB

Open AccessArticle

Experimental Measurement and Theoretical Prediction of Bubble Growth and Convection Heat Transfer Coefficient in Direct Contact Heat Transfer

by Jun Yang, Biao Li, Hui Sun, Jianxin Xu and Hua Wang

Energies 2023, 16(3), 1069; https://doi.org/10.3390/en16031069 - 18 Jan 2023

Cited by 1 | Viewed by 1321

Abstract

The measurement of the two-phase contact area is very important to determine the heat transfer coefficient in the process of direct contact heat transfer, but the direct measurement of the two-phase contact area is a difficult problem. The experiments are carried out utilizing [...] Read more.

The measurement of the two-phase contact area is very important to determine the heat transfer coefficient in the process of direct contact heat transfer, but the direct measurement of the two-phase contact area is a difficult problem. The experiments are carried out utilizing a cylindrical Perspex tube of 100 cm in total height and 15 cm inner diameter. The active column height throughout the experiments is taken to be equal to 50 cm. Liquid Therminol^®66 with four different initial temperatures (50 °C, 60 °C, 70 °C and 80 °C) is used as a continuous phase, while liquid R245fa at a constant temperature of 23 °C is used as a dispersed phase. In this paper, the empirical correlations between bubble growth and local convection heat transfer coefficient are obtained through modeling and measurement, and its correctness is verified by experiments. The results show that the bubble diameter is positively correlated with continuous phase temperature, flow rate ratio, and height, but the local convection heat transfer coefficient is negatively correlated with continuous phase temperature, flow rate ratio, and height. At the same time, it is found that the maximum error between the actual bubble diameter and the theoretical bubble diameter is 7%, and the error between the heat flux calculated by the local convection heat transfer coefficient and the actual heat flux is within 10%. This study provides theoretical guidance for an in-depth understanding of the direct contact heat transfer process and the development of high-efficiency waste heat recovery systems. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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14 pages, 5342 KiB

Open AccessArticle

Cracking of Waste Engine Oil in the Presence of Fe₃O₄

by Jialin Gao, Bo Li, Yonggang Wei, Shiwei Zhou and Hua Wang

Energies 2023, 16(2), 655; https://doi.org/10.3390/en16020655 - 05 Jan 2023

Cited by 1 | Viewed by 1753

Abstract

Waste engine oil (WEO), as a waste resource, has not been fully exploited. Using WEO as a reductant for copper slag cleaning is quite meaningful. Fe₃O₄ is an important element in copper slag cleaning. So the laws of thermal cracking [...] Read more.

Waste engine oil (WEO), as a waste resource, has not been fully exploited. Using WEO as a reductant for copper slag cleaning is quite meaningful. Fe₃O₄ is an important element in copper slag cleaning. So the laws of thermal cracking of WEO at different temperatures and the effect on thermal cracking of WEO in the presence of Fe₃O₄ were investigated. The results show that the high-temperature cracking of WEO mainly produces H₂, CO, CH₄, CO_2, and small molecules such as C. Raising the temperature is good for the cracking of WEO. When the temperature rises from 700 °C to 1300 °C, the total amount of gases produced by cracking 700 μL of WEO increases from 177.08 mL to 1010.2 mL. At 1300 °C, the total amount of gases produced by the cracking of WEO in the presence of Fe₃O₄ was 1408.11 mL. The result indicates that Fe₃O₄ can promote the pyrolysis of waste oil. This research provides a novel approach to the clean utilization of WEO. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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20 pages, 3514 KiB

Open AccessArticle

Optimization of Miller Cycle, EGR, and VNT on Performance and NOx Emission of a Diesel Engine for Range Extender at High Altitude

by Weichao Wang, Guiyong Wang, Zhengjiang Wang, Jilin Lei, Junwei Huang, Xuexuan Nie and Lizhong Shen

Energies 2022, 15(23), 8817; https://doi.org/10.3390/en15238817 - 22 Nov 2022

Viewed by 1295

Abstract

Due to the increasing sales of extended-range hybrid vehicles and the increasingly stringent emission regulations for light vehicles in China, the performance and emission of diesel engines for range extenders in the plateau region have attracted increasing attention. In order to obtain the [...] Read more.

Due to the increasing sales of extended-range hybrid vehicles and the increasingly stringent emission regulations for light vehicles in China, the performance and emission of diesel engines for range extenders in the plateau region have attracted increasing attention. In order to obtain the superior performance of diesel engines for range extenders operating at high altitudes, a multi-objective optimization of the optimal economic operating point of the diesel engine was performed at an altitude of 1960 m. A diesel engine system model with MC-EGR-VNT (MEV) technology was developed using GT-Power based on the data of the engine bench to analyze the effects of the Miller cycle (MC), exhaust gas recirculation (EGR), and variable nozzle turbine (VNT) technologies on the power, economy, and emission performance of high-speed diesel engines. The response surface method (RSM) design was carried out with the Miller cycle rate (MCR), EGR value opening, VNT nozzle opening as variable factors and torque, brake-specific fuel consumption (BFSC), nitrogen oxide (NOx) emission as optimization objectives based on Box Behnken Design (BBD). The optimization results showed that the torque and BFSC remained almost constant, and NOx emission decreased by 59.5% compared with the original machine. The proposed multi-objective optimization method could make the diesel engine with a MEV system achieve a good comprehensive performance. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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16 pages, 26382 KiB

Open AccessArticle

Heat Conduction with Krylov Subspace Method Using FEniCSx

by Varun Kumar, K. Chandan, K. V. Nagaraja and M. V. Reddy

Energies 2022, 15(21), 8077; https://doi.org/10.3390/en15218077 - 31 Oct 2022

Cited by 14 | Viewed by 2430

Abstract

The study of heat transfer deals with the determination of the rate of heat energy transfer from one system to another driven by a temperature gradient. It can be observed in many natural phenomena and is often the fundamental principle behind several engineering [...] Read more.

The study of heat transfer deals with the determination of the rate of heat energy transfer from one system to another driven by a temperature gradient. It can be observed in many natural phenomena and is often the fundamental principle behind several engineering systems. Heat transfer analysis is necessary while designing any product. The most common numerical method used to analyze heat transfer is the finite element method. This paper uses the finite element method to demonstrate steady and transient heat conduction in a three-dimensional bracket. The goal here was to determine the temperature distribution and rate of heat flow in the solid. This is crucial in designing machine elements as they are subjected to various thermal loads during operation and also due to fluctuations in the surrounding environmental conditions. The temperature significantly affects stress, displacements, and volumetric strains. Thus, to analyze thermal stresses induced in a machine element, it is necessary to find the temperature field first. The thermal analysis was performed using the open-source package FEniCSx on Python. The program was run using a preconditioned Krylov subspace method for higher-order function spaces. The Krylov subspace solver drastically reduces computational time. The time taken for the execution of each order was recorded and presented. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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25 pages, 6582 KiB

Open AccessArticle

Analytical Solution of Heat Transfer Performance of Grid Regenerator in Inverse Stirling Cycle

by Yajuan Wang, Jun’an Zhang, Zhiwei Lu, Jiayu Liu, Bo Liu and Hao Dong

Energies 2022, 15(19), 7024; https://doi.org/10.3390/en15197024 - 24 Sep 2022

Cited by 1 | Viewed by 1064

Abstract

The regenerator plays an extremely important role in the Stirling circulation. A grid regenerator can be used for inverse Stirling machines at room temperature due to its low flow resistance. This paper proposes a hexagonal grid regenerator to theoretically explore heat transfer properties [...] Read more.

The regenerator plays an extremely important role in the Stirling circulation. A grid regenerator can be used for inverse Stirling machines at room temperature due to its low flow resistance. This paper proposes a hexagonal grid regenerator to theoretically explore heat transfer properties in the inverse Stirling cycle and establishes an approximate analytical model to analyze the effect mechanism of working frequency, thermal diffusivity and wall thickness on the oscillation flow. The results show that the wall thickness is one of the key factors affecting the equivalent heat transfer coefficient. Specifically, too small or too large wall thickness increases the instability of the heat transfer process. The ultimate wall thickness is determined by the equivalent heat transfer coefficient and thermal penetration depth, whose optimal value ensures not only sufficient heat exchange but also the full utilization of materials. With the increase in frequency, heat exchange performance is improved monotonously. Therefore, high–frequency operation can improve the heat exchange performance of the regenerator. In addition, an optimization criterion for the size of regenerator with the specific capacity of heat transfer as the objective parameter is proposed based on the equivalent heat transfer coefficient. The optimal parameters were obtained when relative thickness was set as 0.8 mm and the equivalent heat transfer coefficient was up to 10⁴–10⁵ W/m²·K indicating that the grid regenerator has broad application prospects in the inverse Stirling cycle. Full article

(This article belongs to the Special Issue Advanced Analysis of Heat Transfer and Energy Conversion)

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