Numerical analysis of lithium-ion battery thermal management system using phase change material assisted by liquid cooling method
Keywords:Lithium-ion battery;Thermal management;Hybrid cooling;Phase change material;Liquid cooling;Cold plate
Abstracts:In this paper, a novel design for hybrid battery thermal management systems (BTMS) is proposed and evaluated from the economic and engineering perspectives. Numerical models are compared with phase change materials (PCM) BTMS. Further, the suggested hybrid cooling system’s thermal performance at the pack level is investigated considering cell-to-cell variation. A three-dimensional thermal model is used for the numerical simulation of the battery cooling system. The probability distributions is utilised for the cell-to-cell variations of a 168-cell battery pack. Results shows that for a 53 Ah lithium-ion battery (LIB) under a 5C discharge rate, a hybrid cooling system with two-sided cold plates can reduce the maximum temperature from ∼64 ∘C to 46.3 ∘C with acceptable system weight and power consumption, which is used for further pack level simulation. It is concluded that the two-sided cold plate hybrid design system can manage the maximum average temperature as well as temperature difference of cells in the desirable range at extreme cases.
A review of the thermal performance of vapor chambers and heat sinks: Critical heat flux, thermal resistances, and surface temperatures
Keywords:Vapor chamber;Phase-change;Wick;Critical heat flux;Thermal resistance;Surface temperature
Abstracts:Critical heat flux (CHF), heat transfer surface temperature, and thermal resistance are arguably the three most important design and performance parameters for any two-phase cooling system. The present study focuses on a review of published articles on the thermal performance enhancements of a vapor chamber, which is one of the most-studied two-phase liquid-vapor-based cooling system for high heat-flux-dissipating systems in the last few decades. Included are both experimental and numerical findings of different geometrical and thermophysical parameters that govern and control different heat transfer phenomena in vapor chambers. Like other two-phase cooling systems, a typical vapor chamber consists of an evaporator, a condenser, a wicking structure, and an adiabatic casing. However, the overall thermal performance of a vapor chamber is mainly dependent on the heat transfer and/or capillary characteristics of the wicking structure, i.e., the evaporation and liquid supply wicks, thus, vapor chambers in this study have been classified based on the type of wick. They include sintered metal powder, axial grooves or channels, and screen/wire mesh-type evaporation wicks. Other less popular evaporation wick designs are also discussed. Moreover, some researchers have carried out comparative studies between vapor chambers of different evaporation wick, and this study has highlighted their respective pros and cons. It is important to mention that this review paper is not designed as an exhaustive study of vapor chambers but to serve as an update on the existing progress on the heat transfer performance enhancement of the technology. Finally, recommendations are provided for future research direction in terms of manufacturing and characterizing vapor chambers for concentrated heat flux dissipation applications.
Optimal hybrid parameter selection for stable sequential solution of inverse heat conduction problem
Keywords:Inverse heat conduction problem;Tikhonov regularization;Hybrid parameter selection;Ridge estimator;Finite element method;Euler time integrator;Morozov discrepancy principle
Abstracts:To deal with the ill-posed nature of the inverse heat conduction problem (IHCP), the regularization parameter α can be incorporated into a minimization problem, which is known as Tikhonov regularization method, popular technique to obtain stable sequential solutions. Because α is a penalty term, its excessive use may cause large bias errors. A ridge regression was developed as an estimator of the optimal α to minimize the magnitude of a gain coefficient matrix appropriately. However, the sensitivity coefficient matrix included in the gain coefficient matrix depends on the time integrator; thus, certain parameters of the time integrators should be carefully considered with α to handle instability. Based on this motivation, we propose an effective iterative hybrid parameter selection algorithm to obtain stable inverse solutions. We considered the Euler time integrator to solve IHCP using the finite element method. We then considered β, a parameter to define Forward to Backward Euler time integrators, as a hybrid parameter with α. The error amplified by the inverse algorithm can be controlled by α first by assuming β=1. The total error is then classified into bias and variance errors. The bias error can be computed using the maximum heat flux change, and the variance error can be calculated using the measurement noise error generated by prior information. Therefore, α can initially be efficiently defined by the summation of the bias and variance errors computed in a time-independent manner. Reducing the total error for better stability of the inverse solutions is also available by adjusting β, which is defined to minimize the magnitude of gain coefficient matrix when spectral radius of the amplification matrix is less than one. Consequently, α could be updated with new β in the iteration process. The proposed efficient ridge estimator is essential to implement the iterative hybrid parameter selection algorithm in engineering practice. The possibility and performance of the hybrid parameter selection algorithm were evaluated by well-constructed 1D and 2D numerical examples.
Phonon transport across GaN/AlN interface: Interfacial phonon modes and phonon local non-equilibrium analysis
Keywords:GaN/AlN heterostructure;Interfacial phonon modes;Modal temperature;Wave-resolved spectral heat flux
Abstracts:Interfacial thermal conductance (ITC) is of critical importance for GaN-based device performance. GaN/AlN is a key component in GaN-based devices. Therefore, systematically investigating the interfacial thermal transport between GaN and AlN is of great significance for the thermal management of GaN-based devices. The temperature dependence of ITC across GaN/AlN interfaces are investigated using nonequilibrium molecular dynamics (NEMD) and Monte Carlo simulations based on first-principles calculations. Interestingly, the calculated ITC is much larger than that of GaN/metal, GaN/Si and GaN/SiC. Structural similarity and interfacial phonon modes are used to reveal the underlying mechanism. Moreover, the modal temperature and wave-resolved spectral heat flux are calculated to understand the interfacial thermal nonequilibrium and the dominant thermal transport channel, respectively. This study explores the modal-level and frequency-level mechanisms for interfacial thermal transport between GaN and AlN, which provides effective understanding in thermal management of GaN-based devices.
Mass diffusion characteristics on performance of polymer electrolyte membrane fuel cells with serpentine channels of different width
Abstracts:Variation in performance of polymer electrolyte membrane fuel cells (PEMFCs) is analyzed, focusing on operating pressure and change in the width of the flow channels. At 0 bar, the maximum power densities are 431, 569, and 799 mW/cm2 with the width of 1.0, 0.5, and 0.3 mm, and at 3.0 bar, the maximum power densities are enhanced to 650, 829, and 914 mW/cm2, respectively. A pair of bipolar plates having the largest width exhibit the highest effect of the pressurized operation, and thus the most considerable change rate in maximum power density (50.8%). To investigate the diffusion phenomenon in the flow channels, a correlation is derived by introducing non-dimensional parameters. The dimensionless number associated with vertical diffusion, Sherwood number, decreases with lessening the width of flow channels and increasing the operating pressure, and diffusion characteristics of PEMFCs are generalized through Sherwood number correlation consisting of Reynolds and Schmidt number. Polymer electrolyte membrane fuel cell; Electrochemical impedance spectroscopy; Flow channel; Diffusion; Sherwood number
Design of a compact mesh-based high-effectiveness counter-flow heat exchanger and its integration in remote cooling systems
Keywords:Cryogenics;Heat exchangers;Remote cooling;Counter-flow;Woven mesh;System design
Abstracts:Compact high-effectiveness Counter Flow Heat EXchangers (CFHEX) are crucial components of recuperative coolers, such as Joule-Thomson and Turbo-Brayton coolers and of remote cooling systems realised by a convective loop. This paper presents a design and analysis of a cryocooler-based remote cooling system that extends the cooling capabilities of a two-stage cryocooler. Increased heat exchange between high- and low-pressure channels is established by adding copper mesh material. A compact effective mesh-based CFHEX design covering the 4.5-290 K temperature and 1–10 bar pressure operation ranges is presented. The discretised numerical model of the CFHEX is also presented and covers a wide field of parameters, including axial conduction, variable material and fluid properties based on experimental and theoretical data and wall-mesh thermal contact conductance. In our design the latter has shown to have a significant influence on the effectiveness of the CFHEX based on the analysis of a range of inner tube materials. The sizing of a high-performance CFHEX with a predicted effectiveness of 96.5 % (number of transfer units ( NTU)=27.6) and an accumulated pressure drop of 15 mbar using the model is demonstrated. The outlook for future work and experimental measurements of the parameters to complete the numerical model is presented.
A Fast Computational Method for the Optimal Thermal Design of Anisotropic Multilayer Structures with Discrete Heat Sources for Electrified Propulsion Systems
Keywords:Electric motors;Electronics cooling;Anisotropic multilayer structures;Spreading resistance;Optimization
Abstracts:This work investigates the effect of the anisotropy on the heat transfer properties of layered composite materials with discrete heat sources, that are used in the power electronics of hybrid-electric propulsion systems. An analytical method, based on closed-form expressions structured on Fourier expansion series, is proposed for the solution of the Laplace’s steady-state anisotropic heat equation. The physical presence of electrical components is replaced by externally applied power sources. The method provides an accurate prediction of the temperature distribution and of the heat transfer across perfect layer-to-layer adhesion or finite conductance interfaces; its use is therefore encouraged for the optimization of composite substrates. Code verification has been employed on test cases for which the analytical solution was available.
A molecular dynamics study on thermal conductivity enhancement mechanism of nanofluids – Effect of nanoparticle aggregation
Keywords:Nanofluid;Thermal conductivity;Non-equilibrium molecular dynamics simulation;Aggregation;Morphology
Abstracts:Aggregation of nanoparticles is crucial in enhancing the thermal conductivity of nanofluids, but the underlying mechanism remains unclear so far. This paper used the non-equilibrium molecular dynamics (NEMD) simulation method to compare the thermal energy transfer characteristics of Ar-Cu nanofluids containing non-aggregated and aggregated nanoparticles. Simulations of thermal conductivity and its component calculations were performed to elucidate how the energy transport terms of molecular motions and intermolecular interactions are related to the nanoparticle aggregation state. The simulation results illustrated that the interaction between Ar atoms dominates the thermal conductivity, and its contribution increases with the volume fraction of nanoparticles, which is mainly caused by the solid-liquid interaction at the interface. The thermal conductivity of nanofluids is closely related to the aggregation morphology. Radial distribution function (RDF) analysis shows that when the nanoparticles change from a fully dispersed state to a compact aggregation state, the density of the nanolayer near the nanoparticles decreases, thus reducing the contribution of Ar-Ar interaction to heat transfer. Nanoparticle aggregates with chain-like structures maximize the thermal conductivity of the nanofluids due to the increased interaction between the copper atoms providing a more efficient heat transfer. Our results will provide essential insights into understanding the effects of nanoparticle aggregation on the microscopic heat transfer of nanofluids.
Experimental and numerical analysis of a three-fluid membrane-based ionic liquid desiccant absorber
Keywords:Membrane-based absorber;Heat and mass exchanger;Ionic liquid desiccant;Dehumidification;Separate sensible and latent cooling
Abstracts:Membrane-based liquid desiccant system is a promising technology for efficient humidity control. Also, in comparison to systems using conventional desiccants, ionic liquid (IL) desiccants enable increased system operational envelope and efficiency. In this study, a finite difference numerical model is developed for an IL-based counter and cross flow internally cooled polymer heat and mass exchanger (i.e. absorber). A super-hydrophobic membrane separates the IL desiccant and air flows while allowing moisture transfer from air to IL. The numerical model determines the outlet conditions of all three absorber fluids (water, desiccant, and air), establishing the absorber heat and mass transfer performance. The model was compared with the experimental data obtained from an IL desiccant absorber under a wide variety of water, desiccant, and air inlet conditions. The maximum discrepancy between the model predictions and experimental data for the air exit temperature, air exit relative humidity, cooling water exit temperature, and solution exit temperature are 4%, 9%, 5%, and 2%, respectively. A comprehensive parametric study is then conducted to evaluate the sensitivity of the absorber performance to different input conditions. This highly accurate model and parametric study of a membrane-based absorber can be utilized in design and performance analysis of emerging liquid desiccant dehumidification and separate sensible and latent cooling (SSLC) systems.
A dynamic method to optimize cascaded latent heat storage systems with a genetic algorithm: A case study of cylindrical concentric heat exchanger
Keywords:Latent heat storage;PCM, Genetic algorithm, Optimization, Thermal analysis
Abstracts:The flow of heat to and from Phase Change Materials (PCMs) is governed by the temperature differences. One of the most viable passive strategy to enhance temperature differences is by using cascaded PCMs. Based on a dynamic heat transfer model coupled with a genetic algorithm, a method for optimizing the performance of cascaded latent heat storage systems is proposed. In this method, the optimization variables and thermal performance based on different objective functions and boundary conditions are investigated. The results show that the mass of the PCM and the number of transfer units (NTU) in each cascaded stage should not always be the same under different objective functions and boundary conditions, unlike in the literature. Additionally, the objective function based on charged exergy is better than that based on charged energy or entransy. Increasing the charging time would increase the charged energy, exergy and entransy, but it will result in a decrease in the efficiencies. As the heat transfer fluid (HTF) has a flow rate greater than 0.2 kg/s, the energy, exergy and entransy efficiencies drop sharply, but have no significant influence on the charged energy, exergy and entransy. For a steady state HTF, an increase of inlet temperature, causes the charged energy, exergy and entransy to increase linearly. However, in this case the rate of temperature increase of the PCMs increase as expected, but the efficiencies decrease slightly. For an unsteady HTF, as the fluctuation in the temperature increases, the charged energy, exergy, entransy along with the efficiencies decrease linearly. In addition, the latent heat capacity of PCMs in different stages will have a significant influence on the optimization variables and thermal performance. In the model in this study, the recommended charging time and HTF flow rate are 3000 s and 0.2 kg/s, respectively.