查看更多>>摘要：Self-heating can cause the ignition of open-circuit Lithium-ion batteries. Current safety literature focuses on the self-heating chemistry of a single cell, ignoring the effects of heat transfer. However, a large ensemble of batteries has a non-uniform temperature distribution and therefore self-heating ignition is dominated by both heat transfer and chemistry. This type of ignition is of importance when batteries are stored for long periods of time and in large ensembles but has been rarely studied to date. This paper studies the effect of the state of charge (SOC) on the self-heating behavior of LiCoO2 prismatic cells. The SOC of 0% (of interest in the safety of waste facilities), 30% (transport), 50% (storage), 80% (aged battery) and 100% (fully-charged battery), and 1, 2 and 4 cells stacked together were studied using oven experiments. Results show that cells at all SOC can self-ignite. Flames were only observed for SOC larger than 80%. We compare two temperature criteria: the temperature of the middle cell using the critical increase rate of 10 celcius/min defined in standard SAE-J2464, and the ambient temperature around the ensemble when triggering ignition. Both temperature criteria decrease with increasing SOC showing that the hazard grows with energy density. The cell temperature criterion is independent of the number of cells, while the ambient temperature criterion decreases as the number of cells increases, which indicates the increased risk of self-heating ignition when cells are stacked together in ensembles. Thus, the ambient temperature criterion should be used to design safe storage rather than the standard cell temperature increase rate, which does not represent well the criticality of ignition. The effective kinetics and thermal properties at different SOCs are extracted based on the Frank-Kamenetskii theory and are used to upscale laboratory results to storage conditions. The results in this work can improve the safety of the storage and provide scientific insight for safety standards.
查看更多>>摘要：Increasing latent heat thermal energy storage system thermal conductivity is of primary importance to take advantage of their capability of storing large amount of thermal energy. For this task, various solutions have been proposed through the years and a throughout comparison depending on the final application is still lacking. In this paper, the melting process of PCMs embedded in a Triplex-Tube Heat Exchanger (TTHX) is investigated numerically by considering three different methods that include separately or together metal foams, nanoparticles addition and finned surfaces. Organic PCMs with different melting points are used as PCMs in the middle shell of the 3D (TTHX) to maximize latent heat depending on local temperatures. Water across inner and outer tubes is considered too as the heat transfer fluid. Results are presented in terms of liquid fraction, temperature evolution as well as charging the energy storage rate. The results show that a composite of PCM/Metal Foam with porosities that vary from 0.98 to 0.92 engenders shorter melting comparing to pure PCM. By inserting metallic foam with different porosities and nanoparticles with 5% volume fraction in the TTHX (Case A), the melting time decrease can achieve a 69.52% when compared with Pure-PCM. Regarding the melting process in pure Multilayer-PCM (Case B), for all metal foam porosities the foam/nano-PCM device shows a shorter melting time even if nanoparticles have minor impact compared to metal foams, reaching a 83.48% in terms of reduction if nanoparticles and metal foams are employed. Finally, for the Case C, melting times are smaller when comparisons are done with Cases A and B for pure PCM. Furthermore, in the finned surfaces of TTHX (Case C), the inclusion of nanoparticles with foam reduced the melting durations by 53.17% compared to the TTHX (Case A) with pure PCM.
查看更多>>摘要：The convective melting process and thermal performance of phase change materials (PCMs) incorporating porous skeleton in the one-side-heated rectangular cavity are investigated numerically in this work. A pore-scale lattice Boltzmann method (LBM) with implicit scheme is developed to explore the solid-liquid phase change process. The interfacial thermal resistance between the skeleton and PCMs is considered by interfacial conditions treatment. The random porous microstructure is generated by quartet structure generation set (QSGS) which is used to compute effective thermal conductivity in LBM. The validations of the current model are employed by the two-phase series conduction model and previous numerical results. The effects of porous structure, skeleton material, and thermal interfacial conditions on heat transfer characteristics are systematically investigated. The computational results show that the melting rate and effective thermal conductivity increase with the reducing porosity and the high thermal conductivity of the skeleton material accelerate the melting process. In addition, the consideration of interactions in the LBM leads to a significant difference. When the thermal interfacial resistance is set to be 1 x 10(-4) m(2) W/K, the temperature drop is very large in two-phase conduction, the melting rate decreases remarkably in the phase change process. The present simulation is also suitable for optimizing the porous structure to prepare various composite PCM in the thermal energy storage field.
查看更多>>摘要：The present study aims to investigate comprehensively the performance of various nanofluids in single U-tube borehole heat exchangers (BHEs). Seven common nanoparticles with the volume fraction ranging from 0.1% to 2.0% are selected to be evaluated as the heat carrier fluid. Firstly, a comparative techno-economic analysis is performed in order to highlight the merits and drawbacks of each nanofluid. Then, a sensitivity analysis is performed to optimise the decrement percentage of BHE thermal resistance. Finally, by means of the linear regression of numerical results obtained for different nanofluids, simple equations are proposed allowing evaluation of the outlet fluid temperature for nanofluids. The obtained results indicate that Ag- and Cu-based nanofluids are characterised by the highest heat transfer enhancement, although this improvement is at penalty of a higher pressure drop and up to 31% higher required pumping power. On the other hand, SiO2- and TiO2water nanofluids are the worst cases in terms of thermal performance, but at the same time, they are characterised by the lowest pressure drop. The optimum decrement percentage of thermal resistance yielded in presence of Cu-water nanofluid is equal to 4.31%. Furtheremore, it is shown that employing nanofluids for the purpose of BHE length reduction is not a promising choice. Economic analysis revealed that the cost of electrical energy for nanofluids due to the higher energy consumption of pump is negligible in comparison with the capital cost of nanoparticles. The SiO2 nanoparticles with a capital cost ranging from 5.8 to 17.5 euro/m is the cheapest nanoparticle to employ, unlike the Ag nanoparticles.
查看更多>>摘要：Pulsating heat pipe (PHP) is an efficient heat transfer technology. The micro encapsulated phase change material (MEPCM) suspension is a novel latent heat fluid with high heat storage density. The research on the PHP with MEPCM suspension is of great significance for expanding the types of working fluids and studying the operating performance. An experimental investigation of starting and running performance was carried out on a closed loop PHP with MEPCM suspension at mass concentration of 0.5%. The results show that the PHP charged with MEPCM suspension starts unstably with irregular oscillating under lower heating power of 30-90 W, while it runs stably after starting with heating power increasing to 120 W. The start-up time of PHP charged with MEPCM suspension first drops rapidly, then flattens from 50 s to 20 s with increasing heating power and it still maintains at 20 s with the increasing of heating power from 150 W during experiments, which indicates the influence on the start-up time from further increasing heating power becomes smaller on the condition that the heating power increases to a certain level. Compared with 70% of filling ratio, conditions with 35% and 50% of that showed better performance. Gravity is very important to overcome the viscous resistance of working fluids. The running performance of PHP was slightly affected by inclination angle which was greater than 60 degrees. When it dropped to 30 degrees, the running deteriorated obviously. However, the PHP with 35% of filling ratio couldn't run normally at a small inclination angle of 30 degrees.
查看更多>>摘要：Based on the results of the experimental studies performed, physical and mathematical models were developed to describe the processes of heat transfer in droplets of potential coal-water fuels (CWF) before their fragmentation caused by intense heating. The model is based on the solution of a system of heat transfer equations taking into account specific features of the CWF droplets fragmentation identified in the course of experiments. CWF droplet fragmentation delay times were recorded in the course of heating using the disintegration criterion identified in the experiments performed. In the case of CWF, complete evaporation of water from fuel composition droplet serves as the criterion. A series of experiments were carried out to test the model and to clarify the mechanism for implementing the processes investigated. The effect of a group of the following factors on the fragmentation delay times was determined: the temperature (773-2273 K), the initial droplet sizes (0.05-1.55 mm) and the component composition of the fuel (solid particles concentration varied between 40 and 60 wt%). The conditions for the intensification of the processes of fragmentation and ignition of potential fuel suspensions were singled out.
查看更多>>摘要：A free-piston engine generator is being developed worldwide as a novel means of electrical power generator for light vehicle application, which can provide a complementary solution towards reducing carbon dioxide emissions of road vehicles in general. However, the absence of a crank-slider mechanism in free-piston engines resulted in poor transient operations. Transient real-time modelling and simulation can provide the interaction between in-cylinder combustion with dynamics of piston motion, which has not been fully explored previously. This paper presents a real-time interaction model between dynamics of the piston motion and thermodynamics of the in-cylinder combustion of a two-stroke spark ignition dual-piston free-piston engine generator for transient operation investigations. The simulation model was developed based on the working prototype and comprised of zero- and one-dimensional sub-models interacted in real-time governed by a single timestep. The study focuses on three critical transient modes: motoring, starting and generating for the transient performance investigation. A series of experimental results during motoring was used for validating the simulation model, which showed good agreement between simulation and experiment results with 2-5% errors. The targeted brake thermal efficiency is around 20-30% at 50-60 Hz engine speed which has been shown achievable. Transient speeds of 10 and 25 Hz produced higher combustion pressure around the top dead centre but suffered pumping loss at the end of the expansion stroke. The lower peak pressures at 50 and 60 Hz have contributed to the lower brake mean effective pressure values. Maximum brake thermal efficiency of 25.8% lies in the mid-range of compression ratios between 10:1 to 15:1 while at the cyclic speeds of 50 Hz to 60 Hz. Maximum brake mean effective pressure contour occurs between 5 and 12.5 compression ratios and corresponding speeds of 10 to 30 Hz. It was observed that the excess energy from combustion results in piston overshoot condition while insufficient energy prevents the piston from achieving the targeted stroke and compression ratio. Knock events were observed for high compression ratio cases regardless of engine speed. Predictive stroke control and knock detection capability are the main contribution of the real-time interaction model presented in this paper for realistic transient operation investigation and performance prediction of the dual-piston free-piston engine generator.
查看更多>>摘要：In order to solve the problems of easy leakage and low thermal conductivity of paraffin in molten state. In this study, modified phase-change microcapsules with paraffin as the core material, melamine-formaldehyde resin as the wall material, flake graphite (FG), multilayer graphene (GNP) and expanded graphite (EG) as heat conduction additives were prepared by in-situ polymerization. The results show that the thermal conductivity of the composite phase change material gradually increases with the increase of the mass fraction of carbon additives, and the heat transfer enhancement effect of EG/MPCMs composite phase change material is the most obvious, with a thermal conductivity of 1.9 W/(m.K), which is 9.5 times that of pure paraffin. The addition of FG/MPCMs, GNP/MPCMs and EG/MPCMs carbon-based materials have little effect on the latent heat of phase change microcapsules, and the latent heat retention rates are 83.8%, 78.8% and 86.4% respectively. Microcapsules with phase change have good leakage prevention performance, and there are two obvious temperature buffering stages in the process of melting and condensation, so which have good thermal regulation ability. Compared with the previous studies, the prepared composite phase change microcapsules have high thermal storage capacity while ensuring high thermal conductivity, and have potential applications in industry.
Said, ZafarRahman, ShekSharma, PrabhakarHachicha, Ahmed Amine...
16页
查看更多>>摘要：In the present work, Multi-Wall Carbon Nanotubes (MWCNT)/water nanofluids are used to increase the performance of a shell and tube heat exchanger (STHX) while reducing energy consumption and overall cost. MWCNT/water with 0.3% and 0.05% volume fractions were studied for stability and thermophysical characteristics. At a 0.3% volume fraction, a substantial improvement in the heat transfer coefficient of around 31.08 % was found compared to the base fluid. Experiments were conducted on STHX, and the results show that using nanofluid at a volume fraction of 0.3% improves heat exchanger efficacy by 5.49% compared to the base fluid. Good agreement was obtained between experimental and analytical results. Furthermore, a numerical model was developed using ANSYS commercial software to study the inclusion of semicircular baffles with nanofluid. Results suggest that MWCNT/water nanofluid at 0.3% volume fraction, along with semicircular baffles, enhanced the overall efficacy of the shell and tube heat exchanger by 15.4%, according to numerical data. Furthermore, comparisons between the proposed heat exchanger (STHX) with previous literature was also carried out. Results suggest a notable enhancement of 7% and 12.4% on heat transfer coefficient and overall efficiency was achieved compared to the previous literature. The experimentally acquired temperature variation data was utilized to create an artificial intelligence-based prognostic model. The multilayer perceptron type artificial neural network (MLP-ANN) was employed to map and forecast the thermal performance of MWCNT nanofluids on the tube side and water on the shell side. The tube side model had excellent R and R2 values of 0.998 and 0.996, while the shell side model had R and R2 values of 0.994 and 0.988, indicating a robust predictive model. The Kling-Gupta efficiency of the prediction model as 0.9936 and 0.9865 for tube side and shell side models, respectively, further confirms the MLP-ANN based model as an efficient prognostic model. A life cycle study was additionally performed to assess the framework's total energy usage, carbon footprint emissions, and cost over a 25-year life expectancy. The studies eventually indicated that the solar-assisted STHX is both cost-effective and environmentally beneficial.
Sleiti, Ahmad K.Al-Ammari, Wahib A.Al-Khawaja, MohammedSaker, Ahmad T....
21页
查看更多>>摘要：Operating thermos-mechanical refrigeration (TMR) ejector-based and organic Rankine cycle-based refrigeration systems at ultra-low temperature heat source (60 C to 100 C) is challenging and limited by their low coefficient of performance (COP), instability, and high cost. To overcome these limitations, an innovative TMR system consists of a power loop coupled with a cooling loop through an expander-compressor unit (ECU) was introduced. To ensure the efficient operation, reliability, and flexibility, of the ECU-based TMR system, a thorough experimental investigation is presented in this study. In the present setup, an air compressor is used to provide pressurized air to drive the ECU at a desired pressure of 620 kPa. Using R134a as a refrigerant, the performance of the ECU-based refrigeration system is systematically tested for various operating conditions including refrigerant mass, evaporator pressure, temperature and flow rate of the water used for evaporation and condensation loads. All tests are performed at two operating frequencies of the ECU (0.50 Hz and 0.33 Hz). Over a wide range of testing conditions, the results show that the average COP Hz varies from 1.57 to 2.73 at 0.50 Hz and from 1.56 to 2.39 at 0.33 Hz. Moreover, the evaporator temperature reaches less than-10 C at 0.50 Hz and-9.60 C at 0.33 Hz. These experimental results prove that the COP of the ECU-based refrigeration system is three times higher than the ejector-based systems and 2.70 times higher than the organic Rankine cycle-based systems.
NSTL
Elsevier