Papers by Keyword: Conduction

Abstract: The precision in temperature estimation plays a pivotal role in the design and operational efficiency of CubeSats. This study leverages the capabilities of COMSOL MULTIPHYSICS to model the thermal behavior of a 1U CubeSat, with a focus on evaluating the impact of orientation and beta angle on heat transfer dynamics and the resultant temperature distribution throughout the satellite. By conducting an extensive range of simulations that explore beta angles from 0° to 90° across four distinct satellite orientations, this research uncovers critical insights into the heat transfer mechanisms within the CubeSat framework. These findings illuminate the substantial influence of orientation and beta angle on the satellite's thermal state, highlighting the necessity of incorporating these factors into any comprehensive thermal analysis of spacecraft. The outcomes of this investigation not only contribute to a deeper understanding of CubeSat thermal management but also underscore the importance of meticulous design and analysis practices to optimize satellite performance in the challenging space environment.

Abstract: This work numerically studies the thermal management of a Li-ion battery pack using Phase Change Materials (PCMs) with two different modelling approaches. Specifically, the results obtained with the Enthalpy-Porosity method, implemented in the tool STAR-CCM+, are compared with those yielded by the Apparent Heat Capacity formulation, employed by COMSOL Multiphysics. Both models are first validated against benchmark cases found in the literature. The study then focuses on the thermal behaviour of a battery pack composed of four 21700 Li-ion battery cells, cooled using the paraffinic PCM RT35. The numerical results show that, while natural convection in the liquid PCM accelerates the melting process, it leads to a non-uniform temperature distribution, particularly disadvantageous for cells located in the upper part of the battery pack. In addition, although both numerical approaches show good agreement between their results, especially in capturing the overall thermal behaviour, some minor differences in the temperature profiles during the PCM phase change still emerge.

Abstract: In this study we have synthesized the iron and bismuth co-substituted BaTiO₃ ceramic, with the general formula: Ba_0.95Bi_0.05Ti_1-xFe_xO₃ for x=0.00 to 1.00, by solid state route. The impedance and electrical properties of these materials were investigated. The dispersion in conductivity in these ceramics can be described by Jonscher's power law and suggests a mechanism of conduction that is related to the Correlated Barrier Jump (CBH) model, according to which charge transport occurs between localized states due to a jump of the potential barriers. The conductivity results confirmed the semi-conductor behavior of these ceramics at high frequency region. The Nyquist plots for the different ceramics confirmed the simple electrical relaxation phenomena with the presence of a Debye-type relaxation phenomenon for x<040 of Fe content. While above this rete, the relaxation behavior is transformed into a Non-Debye phenomenon.

Abstract: Despite many desirable properties, most phase change materials (PCMs) undergo timing issues during the phase change process due to a low thermal conductivity, which limits their application in heat storages. Thus, many techniques have been pointed out to overcome these disadvantages and improve heat transfer, such as coupling PCMs with metal inserts, like high porosity open-cell metal foams. Indeed, the presence of a metal foam increases the effective thermal conductivity of the composite medium and speeds up the charging and discharging phases. In the present paper, a numerical model developed in COMSOL ambient has been calibrated by comparison with experimental results on the melting of pure and metal-foam loaded PCMs, placed in a small case and heated from the top by an electric resistance. The numerical model considers the metal foam as a static solid, filled with a phase changing fluid and employs a literature correlation to evaluate the effective thermal conductivity of the composite medium. The performance of two different paraffinic PCMs (RT35 and RT35HC by Rubitherm GmbH, D), loaded either with a copper foam (20 PPI, 95% porosity, by Porometal, China) or with an aluminum one (10 PPI, 96% porosity, by Recemat, NL) has been investigated in terms of speed rate of the phase change front, time required to complete the melting process, temperature distribution and effect of foam porosity. The obtained results clearly evidence the significant heat transfer improvement yielded by metal foams, whose presence increases the effective thermal conductivity of the composite medium (from 0.2 to 7.03 W/mK for copper foam and to 3.52 W/mK for aluminum one), leading to a significant decrease of the charging time and to a lower temperature gradient within the PCM (from about 16 to 3 K).

Abstract: This paper presents the results of mathematical modelling of three–dimensional heat transfer in a closed two-phase thermosyphon taking into account phase transitions. Three-dimensional conduction equation was solved by means of the finite difference method (FDM). Locally one-dimensional scheme of Samarskiy was used to approximate the differential equations. The effect of the thermosyphon height and temperature of its bottom lid on the temperature difference in the vapor section was shown.

Abstract: 1700 V/20 mΩ SiC Junction Transistors (SJTs) were recently released by GeneSiC with specific on-resistance as low as 2.3 mΩ-cm², and current gain > 100. This paper benchmarks the electrical characteristics of the 1700 V SJTs against two best-in-class Si IGBTs. The SJT features 47% and 49% lower on-state voltage drops than the two Si IGBTs, respectively, with the SJT operating at 175°C, and the IGBTs at 150°C. The conduction power loss of the best Si IGBT is 2.2 times larger than the SJT at 25°C, and 1.6 times larger at 150°C. The leakage currents measured on the best IGBT at 1700 V and 150°C is 0.93 mA, as compared to 200 nA for the SJT at 175°C. As compared to the SJT, 3.6x and 3.3x higher (hard) switching energy losses are measured on the best 1700 V Si IGBT, at 25°C, and 150°C, respectively, when switching at a DC link voltage of 1200 V.

Abstract: Energy conservation is a major topic of concern since our energy sources are exhausting exponentially. This paper focuses on waste heat recovery using which scrap preheating is done in metal castings using sand molds. During solidification of molten metal, most of the heat is lost to the sand. The proposal is to prepare the sand mould with aluminium shots surrounding the mold cavity. These shots absorb some of the heat from the solidifying metal in the mold cavity. The heated shots are separated from the mold and they are allowed to transfer their heat energy to the metal scrap by conduction. The experiments indicate that at least 6.4% of heat recovery is achievable. This will be instrumental in reducing the enormous amount of energy spent to melt the metal considering the fact that casting is the most widely used manufacturing process globally.

Abstract: To ensure fresh water cooling system in a certain self-elevating drilling unit to work securely, the pipes of fresh water cooling system, which are immerged in ballast water tanks, are analyzed and designed by means of convective heat exchange and heat conduction. The calculated results show that it’s unnecessary to adopt insulating measures for the pipes of the fresh water cooling system exposed in sea water, and it saves cost for shipyard and ship owner. The references are afforded to the design of fresh water cooling system in the process of self-elevating drilling unit shipbuilding in the future.

Abstract: The present paper reports results of simulation studies on combined conduction – convection – radiation from two dimensional electronic board equipped with three discrete non identical heat sources. The three non identical heat sources are located across the board. The heat generated in the three heat sources is conducted across the board subsequently getting dissipated by convection and radiation. Air, a radiatively transparent medium, is considered to be the cooling agent. The governing partial differential equations for temperature distribution in the entire computational domain are obtained by appropriate energy balance between the heat generated, convected and radiated. The non linear partial differential equations deduced as above are discretized using finite difference method. The resulting algebraic equations are solved using Gauss - Seidel iterative method. A computer code in C⁺⁺ is written to solve the problem. A thorough energy balance test and grid study has been performed to freeze on appropriate grid size. The effect of thermal conductivity, surface emissivity and convection heat transfer coefficient on local temperature distribution and maximum temperature distribution of the electronic board are demonstrated exhaustively. Keywords: Surface Radiation, Conduction, Convection, Electronic Board, identical Heat sources.

Abstract: ZnO film and Cu2S/ZnO bilays on the glass substrate were fabricated by RF magnetron sputtering. We carried out the experiments by adjusting the thickness of Cu2S on ZnO layer. The performance of Cu2S/ZnO on the transparency, conduction and photocatalysis were investigated. The photocatalytic experiments showed a good photocatalytic activity for photodegradation of methyl orange.