Research

17 pages, 3689 KiB

Open AccessArticle

Thermohydrodynamic Lubrication Characteristics of Piston Rings in Diesel Engine Considering Transient Heat Transfer under the Parameterized Surface Texture of Cylinder Liners

by Hongyang Zhang, Xiaori Liu, Junzhen Gong, Shuzhan Bai, Ke Sun and Haoran Jia

Energies 2023, 16(24), 7924; https://doi.org/10.3390/en16247924 - 05 Dec 2023

Cited by 1 | Viewed by 611

Abstract

The cylinder liner and piston ring form the most crucial friction pair in the diesel engine, contributing 35–40% of its overall friction losses. Recent research indicates that transient heat transfer significantly affects piston ring lubrication. However, the impact of such a transfer on [...] Read more.

The cylinder liner and piston ring form the most crucial friction pair in the diesel engine, contributing 35–40% of its overall friction losses. Recent research indicates that transient heat transfer significantly affects piston ring lubrication. However, the impact of such a transfer on varying surface textures and lubrication traits remains unclear. This paper takes the piston ring–cylinder liner of a certain diesel engine as the research object, which is based on a two-dimensional averaged Reynolds function and Greenwood–Tripp micro convex body contact model; establishes a numerical calculation model of the transient heat fluid lubrication characteristics of a vertical piston ring–cylinder liner assembly by combining the oil film thickness equation, energy equation, lubricating oil viscosity–temperature, and viscosity pressure characteristics; avoids large errors associated with assuming different temperature values for lubricants; and also uses the cylinder liner surface texturing technique to examine the effects of surface texturing on lubrication properties in the presence of transient thermal fluids. The findings indicate that employing transient thermal fluid for determining the mean value of the oil film temperature in isothermal lubrication calculations yields comparable values for minimum oil film thickness and frictional power consumption, while the friction power consumption calculated by the transient thermal fluid is slightly lower. The depth of the recesses on the surface of the cylinder liner should be minimized, while the radius of the texture should be maximized, taking into consideration the current circumstances. Compared with a cylindrical texture, a spherical texture achieves lower friction with good lubrication indexes. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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13 pages, 5296 KiB

Open AccessArticle

Numerical Simulation Method for Flash Evaporation with Circulating Water Based on a Modified Lee Model

by Bingrui Li, Xin Wang, Yameng Man, Bingxi Li and Wei Wang

Energies 2023, 16(21), 7453; https://doi.org/10.3390/en16217453 - 05 Nov 2023

Cited by 1 | Viewed by 1080

Abstract

Flash evaporation processes are widely adopted in the desalination, food processing, waste heat recovery and other industries for heat extraction or product separation. In this paper, a pressure-driven phase transition model is developed by improving the Lee model and combined with the VOF [...] Read more.

Flash evaporation processes are widely adopted in the desalination, food processing, waste heat recovery and other industries for heat extraction or product separation. In this paper, a pressure-driven phase transition model is developed by improving the Lee model and combined with the VOF (Volume of Fluid) method to numerically simulate the flash evaporation process. In this modified Lee phase transition model, the driving force for the rates of the local phase transition is calculated using the local temperature and static pressure magnitude. Numerical simulations are carried out in a water-circulating flash chamber and compared with the experimental results to obtain the values of the time relaxation parameters. And the non-equilibrium fraction of the outlet water can be effectively obtained under different conditions of flow rate, inlet temperature and initial liquid level height. The time relaxation factor takes values from 0.195 to 0.43 (P_out,v = 19.9 kPa) and from 0.31 to 0.92 (P_out,v = 31.2 kPa) with increasing superheat. In addition, the model can effectively represent the evolution of the unstable flow flash evaporation from the initial rapid boiling state to dynamic equilibrium. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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14 pages, 4720 KiB

Open AccessFeature PaperArticle

Heat Transfer and Reaction Characteristics of Steam Methane Reforming in a Novel Composite Packed Bed Microreactor for Distributed Hydrogen Production

by Jingyu Wang, Zongxin Liu, Changfa Ji and Lang Liu

Energies 2023, 16(11), 4347; https://doi.org/10.3390/en16114347 - 26 May 2023

Cited by 2 | Viewed by 1301

Abstract

The development of efficient and compact reactors is an urgent need in the field of distributed hydrogen production. Steam reforming of methane is the main method to produce hydrogen. Aiming at the problems of high heat and mass transfer resistance of the existing [...] Read more.

The development of efficient and compact reactors is an urgent need in the field of distributed hydrogen production. Steam reforming of methane is the main method to produce hydrogen. Aiming at the problems of high heat and mass transfer resistance of the existing fixed bed reactors, and the difficulty of replacing the wall-coated catalyst in the microreactors, a composite packed bed was proposed to meet the demand of small-scale hydrogen production. The structure consists of a multi-channel framework with high thermal conductivity, which is filled with Ni/Al₂O₃ catalyst particles in each channel. A three-dimensional numerical model of the steam methane reforming process in the novel reactor was established using ANSYS FLUENT software. The heat transfer and reaction characteristics in the reactor were studied. Firstly, the advantages of the multi-channel skeleton in enhancing the radial heat transfer performance were verified by comparing it with the traditional randomly packed bed without the channel skeleton. Secondly, the influences of inlet velocity, inlet temperature, and heating wall temperature on the heat transfer and reaction performances in the reactor were studied, and a sensitivity factor was adopted to do the sensitivity analysis. The results show that the methane conversion rate is most sensitive to the wall temperature, while the inlet velocity and inlet temperature have less effect. Finally, the effects of two skeleton materials were studied. The results show that when the wall temperature is higher than 1200 K, there is no significant difference between these two reactors, which indicates that the use of cordierite with a lower price, but also with a lower thermal conductivity can significantly reduce the reactor’s cost. The conclusions can be used as a reference for the design of small-scale hydrogen production reactors. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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18 pages, 4348 KiB

Open AccessArticle

Numerical Analysis of Aerodynamic Thermal Properties of Hypersonic Blunt-Nosed Body with Angles of Fire

by Pengjun Zhang, Wenqiang Gao, Qinglin Niu and Shikui Dong

Energies 2023, 16(4), 1740; https://doi.org/10.3390/en16041740 - 09 Feb 2023

Cited by 1 | Viewed by 1218

Abstract

A hypersonic electromagnetic railgun projectile undergoes severe aero-heating with an increase in altitude. The purpose of this study was to investigate the characteristics of the shock layer flow field as well as the thermal environment of the blunt body wall of a hypersonic [...] Read more.

A hypersonic electromagnetic railgun projectile undergoes severe aero-heating with an increase in altitude. The purpose of this study was to investigate the characteristics of the shock layer flow field as well as the thermal environment of the blunt body wall of a hypersonic electromagnetic railgun projectile at different launching angles. The two-temperature model considers the thermal nonequilibrium effect and is introduced into the Navier–Stokes (N-S) equation, and it is solved using the finite volume method (FVM). The reliability of the calculation model in terms of thermal properties and composition production was verified against a blunted-cone-cylinder–flare (HB-2) test case. The surface temperature of the hypersonic blunt projectile was simulated using a radiation balance wall boundary. The thermal characteristics at the emission angles α = 60° and α = 45° were checked within an altitude range of 0–70 km, including the nonequilibrium effect, reaction heat release, aerodynamic heat flux, and wall temperature. The results show that the translational rotational temperature is higher than the vibrational electronic temperature, and the thermal nonequilibrium effect increases with an increase in altitude. Comparing the two launching angles, the nonequilibrium degree and reaction heat release at α = 60° were higher than those at α = 45°. The rates of exothermic reaction decreased with an increase in altitude. The heat flux along the wall of the generatrix decreased sharply from the stagnation point. With an increase in altitude, the heat flux dropped sharply from 7 MW/m² at H = 0 km to approximately 2 MW/m² at H = 70 km. The wall temperature distribution was similar to the heat flux distribution; however, the surface temperature decreased less rapidly than the heat flux. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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21 pages, 11001 KiB

Open AccessArticle

Thermodynamic Behavior and Energy Transformation Mechanism of the Multi-Period Evolution of Cavitation Bubbles Collapsing near a Rigid Wall: A Numerical Study

by Tianhao Wang and Linya Chen

Energies 2023, 16(3), 1048; https://doi.org/10.3390/en16031048 - 17 Jan 2023

Cited by 2 | Viewed by 1467

Abstract

The dynamic behavior and energy transformation mechanism of the multi-period evolution of bubbles collapsing near a wall have been essential considerations in bubble dynamics research. In this study, a compressible two-phase solver considering thermodynamics and phase transitions is developed on OpenFOAM (version v2112). [...] Read more.

The dynamic behavior and energy transformation mechanism of the multi-period evolution of bubbles collapsing near a wall have been essential considerations in bubble dynamics research. In this study, a compressible two-phase solver considering thermodynamics and phase transitions is developed on OpenFOAM (version v2112). This model is validated via comparison with analytical solutions and experimental results. The dynamics of the multi-period evolution of bubbles collapse process at different dimensionless stand-off distances (γ) were accurately reproduced. The results indicate that the shock wave emitted by the collapse of cavitation bubbles impacts the wall, causing the fluid temperature along the wall to increase. Moreover, the liquid jet has a dual effect on the wall temperature increase, depending on the initial stand-off distance between the bubble and the wall. When γ is small, the jet carries the low-temperature fluid to occupy the high-temperature region, and when γ is large, the jet carries the high-temperature fluid to occupy the low-temperature region. Compared with the mechanisms above of wall temperature increase, the collapse process of cavitation, when directly attached to the wall, increases the fluid temperature along the wall more significantly. Additionally, an energy transformation mechanism is proposed considering the internal bubble energy based on the analysis of the internal bubble energy and acoustic radiation energy with different γ values. Both the internal and acoustic radiation energy initially decreased and subsequently increased with increasing γ values. These findings provide deeper insights into the near-wall collapsing cavitation process mechanism. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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12 pages, 3497 KiB

Open AccessArticle

Simulation Analysis of Emptying the Explosives in Projectiles with Electromagnetic Heating

by Zhiming Qiao, Hongjun Xiang, Genrong Cao, Zhibo Qiao, Qing’ao Lv, Xichao Yuan and Lei Chen

Energies 2023, 16(1), 299; https://doi.org/10.3390/en16010299 - 27 Dec 2022

Viewed by 826

Abstract

This paper concerns a new method for projectile disposal by emptying the explosives in projectiles with electromagnetic heating. It explains the basic principles of the emptying technology via electromagnetic heating. A multiphysical analysis model coupled with an electromagnetic, thermal, fluid and phase transition [...] Read more.

This paper concerns a new method for projectile disposal by emptying the explosives in projectiles with electromagnetic heating. It explains the basic principles of the emptying technology via electromagnetic heating. A multiphysical analysis model coupled with an electromagnetic, thermal, fluid and phase transition model is established, and the explosive melting simulation is conducted based on this model. The dynamic phase transition process of the explosive from solid to liquid is simulated, and the electric field, magnetic field and thermal field distribution characteristics during the process are analyzed. Furthermore, the effect of excitation current characteristics on the phase transition of the explosive is given, which shows that the explosive melting process is controllable by setting the excitation current amplitude or frequency. This paper provides a new method for the disposal of end-of-life projectiles, which is more controllable, safe and environmentally friendly. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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11 pages, 3100 KiB

Open AccessArticle

Piezoelectric Harvesting of Fluid Kinetic Energy Based on Flow-Induced Oscillation

by Ya Xu, Jiangqi Yuan, Daming Sun and Dailiang Xie

Energies 2022, 15(23), 9191; https://doi.org/10.3390/en15239191 - 04 Dec 2022

Viewed by 1036

Abstract

Flow-induced oscillations widely exist in pipelines, fluid machinery, aerospace, and large-span flexible engineering structures. An inherent energy conversion mechanism can be developed for fluid kinetic energy utilization or acoustic energy harvesting. Fluid-resonant acoustic oscillation is featured by stability, easy operation, and a simple [...] Read more.

Flow-induced oscillations widely exist in pipelines, fluid machinery, aerospace, and large-span flexible engineering structures. An inherent energy conversion mechanism can be developed for fluid kinetic energy utilization or acoustic energy harvesting. Fluid-resonant acoustic oscillation is featured by stability, easy operation, and a simple mechanical structure. Acoustic oscillation has high intensity and a mono-frequency, which is beneficial for energy harvesting. A simple cavity with appropriate structural dimensions that can induce fluid-resonant oscillations is set and combined with piezoelectric technology to generate electric power. The energy conversion mechanism is studied numerically and experimentally. The effects of flow velocity on the acoustic frequency, the pressure amplitude, and the output voltage of piezoelectric transducer are analyzed. A stable standing wave acoustic field can be generated in the cavity in a certain range of flow velocity. The results show that the higher intensity acoustic field occurs in the first acoustic mode and the first hydraulic mode and can be obtained in the range of flow velocity 27.1–51.1 m/s when the cavity length is 190 mm. A standing wave acoustic field occurs with a frequency of 490 Hz and a maximum pressure amplitude of 15.34 kPa. The open circuit output voltage can reach 0.286 V using a preliminary transducer. The device designed based on this method has a simple structure and no moving parts. It can harvest the fluid kinetic energy that widely exists in pipelines, engineering facilities, air flow forming around transportation tools, and the natural environment. Its energy output can be provided for the self-powered supply system of low-power sensor nodes in wireless sensor networks. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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22 pages, 7320 KiB

Open AccessArticle

Numerical Study on Particulate Fouling Characteristics of Flue with a Particulate Fouling Model Considering Deposition and Removal Mechanisms

by Peng Liu, Wei Liu, Kexin Gong, Chengjun Han, Hong Zhang, Zhucheng Sui and Renguo Hu

Energies 2022, 15(22), 8708; https://doi.org/10.3390/en15228708 - 19 Nov 2022

Cited by 1 | Viewed by 1350

Abstract

Due to a large amount of particulate matter in industrial flue gas, the formation of particulate deposits on the flue wall will increase the instability of equipment operation, which needs to be solved urgently. In this paper, a numerical investigation on the characteristics [...] Read more.

Due to a large amount of particulate matter in industrial flue gas, the formation of particulate deposits on the flue wall will increase the instability of equipment operation, which needs to be solved urgently. In this paper, a numerical investigation on the characteristics of particulate deposition and removal in the furnace flue was carried out for waste heat and energy recovery. This research adopted a comprehensive fouling model combined with the discrete phase model (DPM) which was performed by the CFD framework and extended by user-defined functions (UDFs). Firstly, the particulate deposition and removal algorithms were proposed to develop the judgment criterion of particle fouling based on the Grant and Tabakoff particle–wall rebound model and the Johnson–Kendall–Roberts (JKR) theory. This model not only considered the particles transport, sticking, rebound, and removal behaviors, but also analyzed the deposition occurring through the multiple impactions of particles with the flue wall. Then, the influence of furnace gas velocity, particle concentration, and inflection angle α of the tee section on the particulate fouling were predicted. The results show that the furnace gas velocity, particle concentration, and flue structure have significant effects on particle fouling and distribution, and the particle fouling mainly occurs in the blind elbow section and the tee sections of the flue. In addition, the fouling mass of particles decreases with an increasing furnace gas velocity and the decrease in particle concentration. Lastly, the fouling mass of particles decreases with the increase in the inflection angle α of the tee section, and the location of particle fouling gradually transfers from the blind elbow section to the tee section. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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23 pages, 12809 KiB

Open AccessArticle

Enhancement of Heat Transfer in a Microchannel via Passive and Active Control of a Jet Issued from the Circular Cylinder

by Zhiqiang Xin, Xiangyu Kong and Jing Chen

Energies 2022, 15(21), 8287; https://doi.org/10.3390/en15218287 - 06 Nov 2022

Cited by 1 | Viewed by 1131

Abstract

The heat transfer enhancement of a jet issued from the circular cylinder placed in a three-dimensional microchannel at low Reynolds numbers were studied systematically by using the numerical simulation. The effects of the jet on thermal efficiency were evaluated by varying injection ratios [...] Read more.

The heat transfer enhancement of a jet issued from the circular cylinder placed in a three-dimensional microchannel at low Reynolds numbers were studied systematically by using the numerical simulation. The effects of the jet on thermal efficiency were evaluated by varying injection ratios (I) and jet angles (θ). The physical mechanism of heat transfer was revealed through the analyses of vorticity dynamic and temperature field. The results showed that the thermal efficiency was proportional to the injection ratio at Re = 100 and 200. However, at Re = 300, the thermal efficiency did not increase monotonically with the injection ratio, and the local maximum value of heat transfer efficiency, slightly less than the highest thermal efficiency, appeared at I = 1.5. This was a result of the jet inducing the vortex generated on the cylinder to become unstable. Furthermore, the change of jet angle had a better effect on heat transfer performance compared to the increase in the injection ratio. The separation point of the flow over cylinder and the vortices in the near field were adjusted by the change in jet angle. At the appropriate range of the jet angle, the wake vortices in the near field transitioned from quasi-steady to unsteady in the far field. The instability of wake vortices can disturb the thermal boundary layer near the wall so as to improve the heat transfer performance. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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15 pages, 7941 KiB

Open AccessArticle

Investigation on Flow Maldistribution and Thermo-Hydraulic Performance of PCHEs with Spoiler Perforated Boards

by Wei Wang, Mengke Niu, Yufei Tan, Bingxi Li and Yong Shuai

Energies 2022, 15(18), 6518; https://doi.org/10.3390/en15186518 - 06 Sep 2022

Cited by 1 | Viewed by 1131

Abstract

In this study, the effects of the maldistribution coefficient on the thermo-hydraulic performance of discontinuous fin printed circuit heat exchanger (DF-PCHE) entrance head and channels are numerically investigated. To improve the flow uniformity at the entrance head, the flow in the exchanger with [...] Read more.

In this study, the effects of the maldistribution coefficient on the thermo-hydraulic performance of discontinuous fin printed circuit heat exchanger (DF-PCHE) entrance head and channels are numerically investigated. To improve the flow uniformity at the entrance head, the flow in the exchanger with three types of spoiler perforated boards (SPBs) having 3 × 3, 4 × 4, and 5 × 5 holes and three kinds of hole diameters (Φ_d = 30, 25, and 20 mm), respectively, are compared to the flow in an exchanger with no SPB. The results show that a small maldistribution coefficient for the inlet velocity field is beneficial for the thermo-hydraulic performance of the DF-PCHE channels, and a maldistribution coefficient of 0.7 is an acceptable velocity distribution for the PCHE channel inlet. Using the 3 × 3 SPB with Φ_d = 30 mm, the maldistribution coefficient becomes 0.7, the fastest among all the SPB application cases at ΔL = 150 mm. Moreover, its heat transfer coefficient and pressure drop increase by 22.46% and decreases by 47.2% compared to those of the exchanger without SPB, respectively. Full article

(This article belongs to the Special Issue Numerical Simulation on Heat Transfer Technique)

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