International Journal of Thermal Sciences最新文献

Solution heat exchanger plays a vital role to recover the heat and improve the system coefficient of performance (COP) in absorption chiller. This study introduces an innovative microchannel membrane-based heat/mass exchanger (MMHX), aiming at replacing conventional heat exchangers. The MMHX notably increases the solution concentration difference between absorber and desorber, leading to an improved COP. This research comprehensively analyzes the heat and mass transfer performance, along with the solution pressure drop characteristics of the MMHX, in both co-current and counter-current flows, comparing these with a traditional microchannel heat exchanger (MicroHX). Due to the lower thermal conductivity of the porous membrane, the MMHX demonstrates a heat transfer capacity that is 3.5 times and 2.1 times lower than the MicroHX in the respective flow directions. However, the absorption chillers equipped with the MMHX outperform those with MicroHX at solution flow rate above 0.03 kg/s, with average improvement in COP of 15.76 %. While introducing a gap between strong and weak solution channels in the MMHX aids mass transfer, it also reduces heat recovery efficiency, impacting the COP negatively. Consequently, a gap-less MMHX is identified as an optimal solution, enhancing COP and advancing the development of efficient, compact absorption chillers for future space cooling.

The present study explores the flow and heat transfer characteristics of couple stress fluid through a rotating circular microchannel under the influence of electromagnetic fields. This analysis considers the angular and axial flows to be actuated by a pressure gradient, electromagnetic force, and rotation of the circular microtube. Initially, the Debye-Hückel approximation is utilized to get an analytical solution of the Poisson-Boltzmann equation for the electric potential within the electric double layer. Next, field equations for couple stress fluid are introduced with two types of boundary conditions, namely Type A (i.e., vanishing of couple stresses) and Type B (i.e., super adherence condition), which were proposed by V.K. Stokes. Then, the solutions are obtained for each case. Subsequently, these solutions are utilized to solve energy equation by employing the finite difference technique with the aid of the Thomas algorithm. It is perceived that the couple stress parameter enhances the axial velocity but retards the angular velocity, whereas the magnetic field parameter diminishes axial velocity and elevates angular velocity. This study examines the impact of the rotational parameter on angular velocity. Furthermore, the magnitude of temperature rises with both couple stress parameters and the Brinkman number, but it drops with the Hartmann number. In addition, an extensive study on Nusselt number, a significant dimensionless heat transfer parameter, is conducted. Overall, Type A condition has a greater impact on velocity and temperature than Type B condition. The current model may be used as a novel approach to manipulate fluid flow at the microscale for designing microfluidic devices such as microdrillers, micromixers, and microreactors, with potential applications in chemical mixing and processing, blood plasma separation, nanoparticle synthesis, drug delivery and screening systems, and drug mixing.

This study examines and optimizes four design parameters of a bus duct conductor's heat sink: fin pitch, fin height, fin thickness, and the number of fin valleys. Average surface temperature and Nusselt number are chosen as the thermal performance criterion of the heat sink. A Definitive Screening Design is employed as a statistical method to reduce the number of optimization runs required while minimizing the aliasing. The regression analysis, analysis of variance, main effect analysis and optimization are conducted to optimize the heat sink design parameter and its thermal performance. The current results provide an ideal heat sink design for the casing of bus duct conductors. A fin pitch of 4 mm, fin height of 6.5 mm, fin thickness of 1 mm, and six fin valleys are determined to be the most optimal combination of design parameters. The optimized responses' average surface temperature and Nusselt numbers are 72.05 °C and 21.59, respectively, with 2.97 % and 6.25 % deviation from the predicted values of the empirical equation. The experimental results are benchmarked against the IEC 60439-1 and IEC 60439-2 standards. The current analysis is expected to provide more insight into the impact of design factors on the thermal performance of a bus duct conductor.

The falling film flow and heat transfer characteristics are critical to improve the performance of spray heat exchangers in a sewage source heat pump (SSHP), which is influenced by the operating and structural parameters. Therefore, considering the unique flow patterns of wastewater falling film caused by the oily content, and the influence of heat exchange tubes with surface structures on flow and heat transfer, two types of heat exchanger tubes with axial and circumferential ribs were designed and the falling flow over tubes were investigated through CFD method using VOF model. The falling film flow pattern, liquid film coverage and heat transfer coefficient (HTC) outside the ribbed heat exchange tubes were investigated, and the influence of rib structures and Re were analyzed. It was found that oily wastewater liquid film spreaded differently depending on the rib structures, and circumferential ribs of case 2 improved the flow pattern by increasing film coverage, which obviously affected the tube HTC. At higher Re, the ribbed tubes displayed higher potential in better heat transfer performance. Therein, the circumferential ribbed tubes showed highest average HTC, with up to 31.9 % higher than smooth tubes, which can be further enhanced by increasing Re. This study provides a foundation for enhancing the heat transfer performance of spray heat exchangers in sewage source heat pumps through the design and modification of tube surface structures.

The challenge of preventing and removing ice from exposed regions of aircraft is a significant engineering concern. Understanding the melting process of ice due to aerodynamic heating during flight is essential for developing UAV flight strategies in icy conditions. This paper analyzes the heat transfer mechanisms involved in airflow over ice and introduces a theoretical model to estimate the ice melting rate, using principles of turbulent heat transfer at the stagnation point. The study also discusses the impact of environmental conditions on ice melting rates. Findings indicate that the melting speed of the ice surface exhibits a negative linear correlation with the initial temperature of the ice, whereas it shows a nonlinear correlation with airflow velocity and total temperature of the incoming flow. Lower airflow velocity or total temperature of the incoming flow enhances the sensitivity of ice melting speed to changes. Additionally, lower ice density results in a higher melting speed, showing an exponential relationship with factors like average droplet diameter, airflow velocity, and airfoil leading edge diameter in the cloud field during icing. Experiments conducted using a small jet test bench and an icing wind tunnel confirmed the impact of varying conditions on ice melting rates. The deviation between experimental results and theoretical predictions was under 5 %. These conclusions offer valuable insights for flight safety planning of unprotected iced aircraft in challenging environments.

Achieving improved cooling efficiency and control in electronic components with varying heat outputs can be realized through a thorough analysis of different heat transfer modes, focusing on their contributions and interactions within the system. The analysis is conducted within a cavity containing three circular blocks generating varying amounts of heat. The blocks are affixed to an insulated plate, dividing the cavity into two identical sections with different fluids and different cooling mechanisms. In the open portion of the divided cavity, block cooling is achieved through forced convection using a nanofluid, while the closed section dissipates heat through natural convection and surface radiation. The numerical solution of the governing equations is performed using Galerkin's Finite Element Method, with detailed examination of the cooling process considering various parameters, such as block displacement ($1.5\text{cm}\le y_1\le 3.25\text{cm}$) and dimensions ($0.25\text{cm}\le R\le 1.5\text{cm}$), Reynolds number ($10\le \text{Re}\le 1000$), nanoparticles nature and volumetric fraction(0 %–10 %), emissivity ($0\le \epsilon \le 1$), thermal heat ratio(0.125 to 8), and cavity inclination angle(0°–180°). The results show that the combination of natural convection and surface radiation can be highly effective, rivaling forced convection in cooling the blocks. The study shows that an increase in the Reynolds number results in a temperature reduction of up to 6 °C, while increasing the emissivity leads to a more significant drop of around 10 °C. Additionally, miniaturizing the blocks by reducing their radius by a factor of six causes the maximum temperature to rise by over 20 °C.

The substrate temperature gradient of electronic devices not only affects their performance, but also reduces their reliability and service life. In order to get a more uniform temperature distribution on the bottom of a microchannel, a wavy microchannel heat sink with staggered inlets and outlets was proposed. With water as the coolant, numerical simulation was adopted to explore the reinforcing impact of the wavy sidewall structure on the temperature uniformity and the heat transfer of the staggered inlets and outlets microchannel unit when the Reynolds number varies at 102–615. The findings reveal that with the increase of the wave amplitude and Reynolds number, as well as the decrease of the wavelength, the intensity and number of Dean vortices increase, and heat transfer of the wavy microchannel with the staggered inlets and outlets is improved. In comparison with the traditional straight microchannel unit with the co-current mode, the temperature difference on the bottom of the wavy microchannel unit with the staggered inlets and outlets is reduced by (90.1–94.5) %.

This study presents a thermal analysis using a self-developed capacitor discharge welding equipment. The addressed process involves a rapid welding of the hot junction of a K-type thermocouple wire. Precise energy input must be provided to achieve an effective junction. The procedure involves solving a three-dimensional nonlinear transient heat transfer equation including phase change, facilitated by COMSOL Multiphysics® software. The Iterative Function Specification Method is employed to estimate the heat rate, solving this inverse heat conduction problem. Experiments were conducted to gather temperature data at 90 ms intervals from accessible regions within the domain. Furthermore, the elemental identification of the k-type thermocouple was accomplished using Scanning Electron Microscopy. The utilization of two thermocouples is instrumental in improving data quality and mitigating measurement uncertainties due to the problem complexity. The efficiency of the welding process is evaluated by determining the energy stored within the capacitor bank, resulting in 30%. The low efficiency is partly attributed to energy losses through light and noise. Results show close alignment between experimental data and numerical temperature. This study not only provides insights into rapid welding processes but also holds potential for various approaches within this field.

Experiments are carried out to understand the flow and thermal behavior of a pulsating jet. The pulsating jet is generated using acoustic excitation. The study considered variations in Reynolds number (Re = 2800, 4900, and 6800), Strouhal number (St = 0–0.84), pulsation amplitude (A = 0–60 %), and nozzle-to-surface distance (z/d = 1–8). Findings revealed that the potential core length of the pulsating jet is shorter compared to the steady jet. The potential core length initially decreases with an increase in St up to 0.42, then begins to increase. Pulsating jets improve thermal performance in the wall jet region due to greater entrainment and mixing from the surrounding fluid. Results demonstrated that pulsating jets could increase average heat transfer rate by up to 58 % at Re = 2800 compared to the steady jet. Although heat transfer rates are higher in pulsating jets, changes in pulsation frequency or amplitude had a minimal effect. The enhancement in average heat transfer rate diminishes as the Reynolds number increases for the same Strouhal number. Each tested Reynolds number showed at least a 10 % improvement in heat transfer in pulsating jet over steady jet. The improved thermal performance of the acoustically pulsating chamber offers the potential for enhanced thermal management in various applications.

At extremely high Mach number (Ma ≥8), kerosene is faced with issues of cracking with a limited heat sink for regenerative cooling. Supercritical CO₂ can be used as additional cooling method for regenerative cooling because of its excellent heat and mass transfer capability and it can easily convert heat into electricity for the engine electric system. In this study, pin-fins are applied to a regenerative cooling channel using sCO₂ to further enhance heat transfer at extremely high heat flux. Heat transfer and fluid flow are analyzed by the k-ω SST model considering effects of pitch ratio, solid materials and accelerations. From this study, compared with a smooth cooling channel, the pin-fin channel (Case 3) obtains a heat transfer enhancement of 3.08, a friction factor of 4.66, thermal performance enhancement of 1.84, and the maximum temperature of the heated surface is decreased by 36 % at Re = 45,000. The maximum velocity is found at the near-wall regions determined by the combined effects of temperature difference and accelerations. When the channel material is Cu with the high thermal conductivity, the maximum temperature is decreased by 37 % compared with a steel channel and the temperature distribution also becomes more uniform.