•The flow field is found to vary greatly downstream of the perforated plate.•The conjugate heat transfer affects NO formation by relocating the flame.•The preferential diffusion is intensified due to ...the two-dimensionality flow.•The coupling effect of these factors above determines the NO generation rate.
The present work examines the NOx emission characteristics of a premixed micro-combustion system with a perforated plate implemented. For this, a three-dimensional (3D) computational model involving a detailed chemical-kinetic mechanism for ammonia-oxygen combustion in the micro-combustor is developed. The model is first validated with the experimental measurements available in the literature before conducting comprehensive analyses. It is found that implementing a perforated plate in the micro-combustion system creates a flow recirculation zone downstream characterized by a low flame temperature and combustion speed. Meanwhile, the conjugate heat transfer between the combustion products and the inner combustor walls is shown to play a key role in the NO generation by relocating the flame in the axial direction and thus changing the chemical reaction rate. Furthermore, the preferential diffusion caused by the variation in the mass diffusivity of different species and the two-dimensionality flow is identified to vary significantly in comparison with the case in the absence of the perforated plate, especially in the vicinity of the recirculation zone. This diffusion effect results in the considerable drop in the N/O atomic ratio, primarily due to the reduction and increase of O2 and H2O, together with less available N2, and consequently affecting the NO generation rate. This work confirms that the flow field, the conjugate heat transfer as well as the preferential diffusion effect could be regarded as the potential mechanisms leading to the NOx emission variation in the recirculation zones.
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Large Eddy Simulation (LES) with Conjugate Heat Transfer (CHT) is used to analyze the impact of H2-enrichment on the flame structure and combustion dynamics of a lean partially-premixed turbulent ...CH4/Air swirling flame. Experimentally, the combustor is operated at atmospheric pressure with H2 fuel fractions of up to 50%, by volume. LES-CHT results are compared and validated against time-resolved stereo PIV, OH* chemiluminescence, OH-PLIF imaging and acoustic pressure measurements. In terms of dynamics, for the pure CH4 and 20% of H2 enrichment cases, no thermoacoustic oscillation is observed in either the experimental or numerical data. As the fuel fraction of hydrogen is increased, the flame length reduces due to the increase in laminar flame speed and the heat release rate distribution becomes more compact. CHT simulations reveal that H2-enrichment leads to higher temperatures at the centerbody tip. At 50% H2, in agreement with experiments, LES predicts a bi-modal thermoacoustic oscillation, with two main frequencies corresponding to the quarter and chamber modes of the system. Dynamic Mode Decomposition is performed on the measured OH-PLIF images and LES 3D fields to extract each mode contribution to the overall flame dynamics. It is observed that both modes are characterized by local variations of equivalence ratio, while only the higher frequency (chamber) mode is characterized by vortices periodically detaching from the backplane and the centerbody walls causing a strong periodic wrinkling of the flame front during the thermoacoustic oscillation.
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To study the seepage and deformation characteristics of coal at high temperatures, coal samples from six different regions were selected and subjected to computed tomography (CT) scanning studies. In ...conjunction with ANSYS software, 3D reconstruction of CT images was used for the establishment of fluid-solid conjugate heat transfer model and coal thermal deformation model based on the microstructures of coal. In addition, the structure of coal was studied in 2D and 3D perspectives, followed by the analysis of seepage and deformation characteristics of coal at high temperatures. The results of this study indicated that porosity positively correlated with the fractal dimension, and the connectivity and seepage performances were roughly identical from 2D and 3D perspectives. As the porosity increased, the fractal dimension of coal samples became larger and the pore-fracture structures became more complex. As a result, the permeability of coal samples decreased. In the meantime, fluid was fully heated, generating high-temperature water at outlet. However, when the porosity was low, the outlet temperature was very high. The average deformation of coal skeleton with different pore-fracture structures at high temperatures showed a trend of initial increase and subsequent decrease with the increase of porosity and fractal dimension. The maximum deformation of coal skeleton positively correlated with connectivity but negatively correlated with the fractal dimension.
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•Three-dimensional electrochemical thermal model of Li-ion battery pack using computational fluid dynamics (CFD).•Novel pack design for compact liquid cooling based thermal management system.•Simple ...temperature estimation algorithm for the cells in the pack using the results from the model.•Sensitivity of the thermal performance to contact resistance has been investigated.
Thermal management system is of critical importance for a Li-ion battery pack, as high performance and long battery pack life can be simultaneously achieved when operated within a narrow range of temperature around the room temperature. An efficient thermal management system is required to keep the battery temperature in this range, despite widely varying operating conditions. A novel liquid coolant based thermal management system, for 18,650 battery pack has been introduced herein. This system is designed to be compact and economical without compromising safety. A coupled three-dimensional (3D) electrochemical thermal model is constructed for the proposed Li-ion battery pack. The model is used to evaluate the effects of different operating conditions like coolant flow-rate and discharge current on the pack temperature. Contact resistance is found to have the strongest impact on the thermal performance of the pack. From the numerical solution, a simple and novel temperature correlation of predicting the temperatures of all the individual cells given the temperature measurement of one cell is devised and validated with experimental results. Such coefficients have great potential of reducing the sensor requirement and complexity in a large Li-ion battery pack, typical of an electric vehicle.
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Today’s climate and energy challenges are driving the use of decarbonised and renewable alternative fuels in power generation and transportation. Hydrogen as a fuel is a good candidate to meet these ...requirements, as it offers no carbon emissions and can play the role of an energy carrier to store excess energy produced by renewable energy. Nonetheless, the production of NOx needs to be assessed. For this reason, this study proposes high-fidelity Large Eddy Simulations (LES) with detailed NOx analyzes of a partially premixed lean swirling H2-air flame. The chosen configuration is the technically premix hydrogen injector measured at the Berlin Institute of Technology (TUB) in Germany. A novel kinetic scheme for H2-air comprising 15 species and 47 reactions is developed to take into account all NOx pathways. To accurately solve the combustion process and the NOx production level, static mesh refinement (SMR) and conjugate heat transfer (CHT) are applied to the LES modeling and their impact on the numerical predictions is evaluated. A detailed analysis of the preferential diffusion and formation of NO is presented, demonstrating that the proposed numerical model, combined with the novel chemical kinetic scheme, is able to correctly predict complex transport phenomena observed in lean turbulent hydrogen flames and to predict their NO dynamic formation accounting for both primary and secondary (N2O and NNH) NOx pathways.
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This paper presents a set of high-performing heat sinks that exhibit twice the thermohydraulic performance in terms of conductance compared to conventional rectangular fin heat sinks. The heat sinks ...presented here are designed through three-dimensional topology optimization (TO), manufactured using additive manufacturing (AM), and their performance is validated through experimental testing. The heat sink design is governed by steady-state Navier-Stokes equations and the energy equation. The objective is to minimize the average temperature of the heat source surface with a constant heat flux. Our design process incorporates two constraints: the pressure drop constraint and the project undercut perimeter (PUP) based overhang angle constraint. The incorporation of the overhang angle constraint ensures that the optimized heat sink design is self-supported and amenable to additive manufacturing without the need for additional support structures. Post-optimization CFD investigations revealed that the optimized heat sink offers improved thermal performance, attributed to 2 kinds of three-dimensional convection effects, thermal boundary layer re-initialization, and efficient mixing. The optimized heat sink designs are manufactured using laser-powder bed fusion process, an additive manufacturing technique, and their superior performance relative to a conventional rectangular heat sink is validated through experimental measurements. The experimental tests are in good agreement with CFD simulations, confirming a remarkable 100% increase in conductance for the TO designs compared to a conventional heat sink.
•Geometrically intricate heat sink design: achieved through topology optimization and additive manufacturing.•Improved thermohydraulic performance: twice the heat conductance over rectangular fin design.•Cooling mechanism: optimized designs disrupt boundary layer, enables full-field mixing of cold and hot flow.•Smaller surface area: optimized designs possess smaller surface area than rectangular fins.
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The transformation to a truly sustainable energy system will require taking better advantage of the waste heat. Integrating heat exchangers with the triply periodic minimal surface (TPMS) is a ...promising and efficient way to build waste heat recovery systems that harness heat emissions from the low pitch thermal systems. This is mainly due to the low hydrodynamic resistance and pressure drop in the TPMS while securing good heat transfer at low-temperature gradient. This study establishes a computational design and analysis of heat and mass transfer inside a heat exchanger based on the TPMS structure and determine thermal effectiveness, heat transfer coefficient, and pressure drop inside the channel. The non-linearity dependence of results to several design variables makes obtaining the optimal design configuration solely using conventional CFD or experimental study nearly impossible. Hence, a multi-objective optimization workflow based on a Genetic Algorithm for laminar flow is employed to reveal the underlying relationships between design variables for the optimal configurations. The results illustrate the local sensitivity of important parameters such as the heat transfer coefficient, Nusselt number, and thermal performance of the heat exchanger against various design variables. It is shown that the pressure drop is directly affected by gas inlet velocity, viscosity, and density, from high to low, respectively. The Pareto frontiers for the optimal thermal performance are extracted, and the correlation between design objectives is determined. This methodology provides a promising framework for heat exchangers’ design analysis, including multi-objective goals and design constraints.
•Applicability of TPMS materials in building low-temperature waste heat recovery systems.•Proposing an automated workflow for concept design optimization of TPMS-based heat exchangers using the MOGA algorithm.•Analyzing and simplifying the complex interrelationship between TPMS design parameters.•Demonstrating promising functionality of presented workflow in extracting optimal design configurations for the state-of-the-art low pitch heat exchanger.
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•Simulation of conjugate heat transfer and entropy generation in a cavity is studied.•Nanofluid-saturated porous medium fills the cavity which is heated by solid wall.•The CuO nanoparticles enhance ...the heat transfer and increase the entropy generation.•A new criterion is proposed to predict the best performance of the porous cavity.•Largest thickness and lower wall conductivity give best thermal performance.
Entropy generation due to conjugate natural convection–conduction heat transfer in a square domain is numerically investigated under steady-state condition. The domain composed of porous cavity heated by a triangular solid wall and saturated with a CuO–water nanofluid. Equations governing the heat transfer in the triangular solid together with the heat and nanofluid flow in the nanofluid-saturated porous medium are solved numerically using the over-successive relaxation finite-difference method. A temperature dependent thermal conductivity and modified expression for the thermal expansion of nanofluid are adopted. A new criterion for assessment of the thermal performance is proposed. The investigated parameters are the nanoparticles volume fraction φ (0–0.05), modified Rayleigh number Ra (10–1000), solid wall to base-fluid saturated porous medium thermal conductivity ratio Kro (0.44, 1, 23.8), and the triangular solid thickness D (0.1–1). The results show that both the average Nusselt number and the entropy generation are increasing functions of Kro, while they are maxima at some critical values of D. It is also found that the addition of nanoparticles increases the entropy generation. According to the new proposed criterion, the results show that the largest solid thickness (D = 1.0) and the lower wall thermal conductivity ratio manifest better thermal performance.
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The heat generated during operation of microelectronic devices often adversely affects product performance. A remedy for this problem can be through the incorporation of nanofluidic microchannel heat ...sinks. Advanced working fluids and channel structures can be used to improve the heat transfer performance of the microchannel heat sink. First, thermal resistance is treated as a single objective function, the geometry of the microchannel was optimized using a genetic algorithm. The flow and heat transfer characteristics of the optimized microchannel are analyzed. The effect of different nanofluid volume fractions and geometric parameters on the inlet and outlet pressure drop, flow resistance coefficient, substrate temperature, Nusselt number (Nu), and system thermal resistance in the fractal microchannel are investigated. The thermal resistance of Al2O3 nanofluid with a volume fraction of 5% is 12.5–14.7% lower than that of deionized water, and the microchannel substrate temperature is 6.26 °C lower than that of deionized water.
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•Al2O3-water nanofluid is considered as the working coolant.•The structure of microchannel was optimized by genetic algorithm.•The optimal design is shown to be superior to the existing designs in literature.
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In this study, laminar, steady-state microchannel flow and conjugate heat transfer in a microchannel heat sink (MCHS) with micro pin-fins were investigated numerically. Computational Fluid Dynamics ...(CFD) simulations were performed with ANSYS Fluent for heat and fluid flow in MCHSs with circular-, square- and diamond-shaped micro pin-fins of the same hydraulic diameter. Arrangements of micro pin-fins in the MCHS were optimized for each shape using a multi-objective genetic algorithm (MOGA) method called Non-dominated Sorting Genetic Algorithm (NSGA-II). The fluid entered the microchannel at channel Reynolds numbers ranging from 100 to 350, corresponding to circular- and square-shaped pin-fin Reynolds numbers ranging from 20 to 110 and diamond-shaped pin-fin Reynolds numbers ranging from 20 to 190. In all simulations, uniform heat flux of 60 W/cm2 was applied from the bottom surface of substrate. As the cooling fluid, water with temperature-dependent variable viscosity was assigned in the fluid domain and copper with constant thermophysical properties was considered for the entire solid domain. Design variables were selected in order to represent both the pin-fin arrangement and flow-characteristics: porosity number and pin-fin Reynolds number, respectively. As objective functions, pressure drop ratio was defined as the hydrodynamic performance indicator and the Nusselt number was chosen to represent the thermal performance. To represent the trade-off between two objective functions, optimization was conducted after a systematic parametric study and pareto-fronts were obtained. Optimized solutions of each pin-fin shape showed that square-shaped pin-fin was unfavorable compared with the performance of other shapes of pin-fins. Among all configurations, diamond-shaped pin-fins significantly enhanced heat transfer at an increased pressure drop ratio. Depending on the pin-fin shape, optimal configurations emerged over a wide range of porosity values mostly at the upper limit of the pin-fin Reynolds number, with average Nusselt numbers varying between 3 and 12 and corresponding pressure drop ratios varying between 1 and 12.
•Thermo-hydrodynamic behaviour of pin-finned MCHS was numerically investigated.•Multi-objective optimization (NSGA-II) was utilized to obtain optimized configurations.•Objective functions for optimization were Nusselt number and pressure drop ratio.•Optimal configurations were obtained in a broad span of application.•Optimal configurations were expressed in terms of Reynolds number and porosity.
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