查看更多>>摘要：Low temperature dilemma of lithium ion batteries (LIBs) is the critical restriction for electric vehicles (EVs) and LIB energy storage. As an effective internal heating strategy, the pulsed heating method has well-known advantages in heating rate and durability on cells. However, the detailed temperature distribution of the module under pulsed heating still lacks systematical study. This work carries pulsed experiments and simulations for the LIB module and studies the temperature distribution and uniformity under various pulsed parameters, ambient temperature, heat preservation and inconsistent conditions of the state of charge (SOC) and resistance. The results show that higher pulsed amplitude and lower ambient temperature will increase the heating rate and temperature difference. The resistance inconsistency will dramatically deteriorate temperature difference while the SOC change of certain cell has a negligible impact. Specifically, the temperature difference will increase 6 degrees C if the resistance of the peripheral cell is 2.5 times of others. For practical application in EVs, the maximum temperature difference can be controlled within 8.7 degrees C in a heating process from -20 degrees C to 5 degrees C in 300 s, and the change of the maximum temperature difference is controlled within 2 degrees C when the fluctuation range of certain cell SOC or resistance is 50% around the standard value. These indicate pulsed heating of LIB module or pack with wonderful temperature uniformity and therefore promising for long-time employment.
查看更多>>摘要：This paper documents the experimental characterization of a Li-ion battery cell during charging/discharging cyclic operations. The study of the battery cell is conducted in the absence of cooling aid system, and provides thermal and electrical insights. After describing the experimental set-up, the changes in temperature are pre-sented and highlight the nonuniform distribution of the temperature on the battery cell surface. The findings indicate that the maximum temperature difference on the investigated battery cell surface may reach up to 11 C at 3C and 17 degrees C at 5C, at the end of the discharge in the natural convection case. These changes in space come with temporal variations that are also documented. Voltage curves are provided during charging and discharging operations. The impact of the discharge rate, ambient temperature are then investigated together with the ca-pacity fade after 500 cycles, and results showed that ventilation and low ambient temperatures allow to alleviate the battery capacity fade by 3%.
查看更多>>摘要：Exhaust Gas Recirculation (EGR) cooler is a crucial automotive heat exchanger that decreases the temperature of the recirculated exhaust gas fed to engine cylinders to reduce the NOx emissions. The high temperature gradients in the heat exchanger and its varying hot and cold operation makes it necessary to accurately determine the temperature distribution of the EGR cooler to evaluate vulnerability to various thermal failures. This study demonstrates a thermal assessment of EGR cooler using detailed CFD simulation methods developed and validated with comprehensive experimental design and measurements. The experimental data including detailed boiling detection is collected on a gas burner test bench for eight different working conditions of the EGR cooler with conditioned cooling with an average of 3% repeatability error. The detailed and highly resolved CFD model which includes the heat transfer enhancing features of the heat exchanger is validated at these working conditions and resulted in an average of 1% difference in temperature and maximum 5% difference in pressure drop values. The developed detailed CFD methodology reduces the amount of EGR performance and boiling tests significantly and enables assessment of the EGR cooler at engine assembly conditions which affects the hot gas distribution and resulting temperature gradients. Furthermore, the CFD methodology and its correlation with the experiment proved that a coupled CFD methodology can be used instead of the development tests for analytical sign-off purposes as long as the detailed heat exchanger design is included in the CFD model.
查看更多>>摘要：Fused Deposition Modelling (FDM), is an additive manufacturing technology where polymers are extruded using appropriate processing parameters to achieve suitable bonding while ensuring that overheating does not occur. Among processing parameters, polymer inlet temperature, nozzle size, extrusion speed, and air cooling speed are significantly effect on the extrusion process at the distance between the build plate and the nozzle tip (standoff region). This study aims to evaluate the influences of the processing parameters on the thermal behavior and phase change zone of Polyamide 12 (PA12) and Acrylonitrile Butadiene Styrene (ABS) polymers at standoff region. A nonlinear three-dimensional (3D) finite element (FE) model was developed by implementing an apparent heat capacity model using the Heat Transfer Module in COMSOL (R) Multiphysics software. FE results in the standoff region were validated by experimental tests, concerning various nozzle sizes and extrusion speed. The validated numerical results demonstrated that there is a complex correlation between processing parameters and thermal behaviors such as phase change and temperature distribution in the standoff region. The FE results were then employed in training an artificial neural network (ANN). A well-established compromise between the trained ANN and the FE results demonstrates that the trained ANN can be employed in the prediction of further thermal and glass transition behavior using subsequent processing parameters.
查看更多>>摘要：A cascade-arch-firing low-NOx and high-burnout configuration (CLHC) was developed as a solution for a 600 MWe W-shaped flame furnace suffering from poor burnout under ultra-low NOx combustion conditions. Under the comprehensive low-NOx combustion conditions regulated by the CLHC, overfire air (OFA) was first positioned at the furnace throat in a uniformly flattening OFA port form along the furnace breadth direction. In order to disclose the OFA angle's effect on the furnace performance and meanwhile establish an appropriate angle for the OFA with the above original designs, the in-furnace flow field, coal combustion, and NOx emissions were evaluated at various OFA angles of theta = 10 degrees, 20 degrees, 25 degrees, 30 degrees, and 35 degrees. Increasing theta only changed a little the symmetry of flow field and combustion morphology, i.e., the symmetry generally deteriorating first and then improving. The OFA penetration in the furnace throat zone initially increased but then worsen as theta increased. In the upper furnace strongly affected by OFA, gas temperatures, high-temperature zone area, and levels of O2 and CO all descended first and then increased with theta, while NO generally undergone an increase-to-decrease trend. Trends of furnace outlet's performance indexes with theta showed that the residual O2 and CO emission generally decreased first and then increased, NOx emissions initially increased but then decreased, while carbon in fly ash undergone an increase-to-decrease trend prior to an increase at theta = 35 degrees. In view of the above findings and the gained optimal low-NOx and high-burnout performance (NOx emissions of 667 mg/m3 at 6% O2 and carbon in fly ash of 5.31%), theta = 30 degrees was taken as an appropriate OFA angle for the CLHC furnace. Compared with a currently advanced low-NOx combustion art, the CLHC reduced further NOx emissions by 26.3% while remained a high burnout achievement.
Bo, YaqingWu, HanHernandez, Juan J.Shi, Zhicheng...
10页
查看更多>>摘要：For diesel engines, the distance between the wall and the spray nozzle is an important feature for the matching of spray and combustion chamber geometry. In this study, the effects of the cold wall with different wall distance on the spray and ignition characteristics at low-temperature conditions are systematically analyzed. CFD simulations are used to reproduce and explain the phenomena in optical experiments, and the chemical kinetics analysis is used to investigate the microscopic influence of the cold wall on the chemical reactions. The results show that the cold wall has little influence on the ignition when the wall distance is larger than a critical distance which is determined by the proportion of vapor phase in the impinged fuel and the chemical reaction degree at the time of impingement. As the wall distance reduces, the fuel evolution process is more inclined to the route of low temperature and low concentration, causing the delay of chemical reactions, and the misfire occurs when the time required for chemical reactions exceeds the time of fuel dissipation. Moreover, varying wall distance changes the timing of the local fuel cooling, which leads to diverse effects. During the transition from low temperature reaction to high-temperature reaction, earlier the fuel cooling leads to a more substantial inhibition effect. The fuel cooling after the start of the high-temperature reactions has little impact on the chemical reaction.
查看更多>>摘要：To improve the controllability of combustion and reduce the emissions of HC and CO of the newly developed combustion modes, such as the HCCI, PCCI and RCCI, the evaporation processes and morphological developments of diesel droplets impacting on the aluminum alloy surfaces with different wettability and temperatures are experimentally investigated. The results show that the oleophilic surface is conducive to evaporation of diesel droplets, while the oleophobic surface promotes the formation of the vapor film between the fuel droplets and the test surface at a high surface temperature and reduces the Leidenfrost temperature of the fuel droplets. Also, stronger oleophobicity of the surface is beneficial to the rebound and secondary breakup of the droplets, thereby promoting the evaporation of the droplets in the gas-phase space of the cylinder and improving the air-fuel mixing. Moreover, the stronger the surface oleophobicity, the smaller the spreading factor and the larger the rebound factor of the droplets. At a higher wall temperature, the ability for enhancing the surface oleophobicity of the convex domes, grooves and protrusions structures on the laser-etched surface is better than that of the boss/pits and needle-like structures on the chemically etched surface. Under the conditions of lower surface temperatures, the evaporation rate of the droplet after hitting the wall is closely related to the spreading area of the droplet. As the wall temperature increases, when the droplet is in transition boiling regime, the large heat transfer rate makes the diffusion width, height and diffusion area of the vapor phase region are obviously large.
查看更多>>摘要：The experimental outcomes of single bubbles nucleated and growth from a heated surface immersed in an electric field in high-quality microgravity level are presented. Data were obtained between September 2019 and January 2021 from the European experiment known as Reference mUltiscale Boiling Investigation (also multiscale boiling project), in which single bubbles of FC-72 were nucleated on a heated surface, on-board the International Space Station. In the experiments reported here, which are only a part of the complete dataset, an electrostatic field is imposed in the boiling region by means of a washer-shaped electrode, centred above the nucleation site. The experimental apparatus is designed to initiate a single bubble from an artificial nucleation site machined in the surface. Such bubbles are heavily distorted by the electric stresses; in particular, contact angles and contact line length increase with electric field intensity. While in the absence of electric field no detachment occurs, when the electric field is applied the bubbles are continuously and regularly sucked towards the electrode as they are attracted to regions of weaker electric field; a detailed analysis of the electric forces is provided as well. The significant contribution of electro-convection is highlighted by the bubbles growth rate. These experimental observations contribute to the insight of the basics of boiling and the effects of an electric field on the ebullition cycle and show promising opportunities for practical application of electric fields in space.
Nowak, Andrzej I.Ochman, AgnieszkaPietrowicz, SlawomirCzajkowski, Cezary...
15页
查看更多>>摘要：The performance of a very effective and innovative thermal device based on Pulsating Heat Pipes (PHP) in a rotating system is studied and analysed in the paper. The appliance called a Flower Shaped Oscillating Heat Pipe (FSOHP) is developed for use in industrial sectors and is arranged in the specific shape of a flower so as to obtain homogeneous heat reception from technological processes. The capillaries, with an inner diameter of 2.7 mm, were filled and tested, with HFE-7000 as a working fluid. This fluid, according to a confinement criterion based on the Bond or Garimella numbers, places the device in the range of thermosyphon operation, the description and experimental verification of which is addressed in this paper. The influence of rotational speed (0-300 rpm), heat input (300-450 W), and filling ratio (40-90%) on the thermal performance has been studied as an experimental research. Current research shows that a low boiling point liquid such as HFE-7000 can be used as a working fluid in a rotating system, and the thermal resistance of the process is characterised by a value of almost 100% higher compared to water. The maximum value of the heat flux value achieved was equal to 22.1 kW/m(2) for HFE-7000, while for water it was more than four times higher and equal to 98.4 kW/m(2). It was also observed that by increasing the Filling Ratio (FR) by up to 80%, the thermal resistance decreases to the lowest value of 0.05 K/W in speed conditions such as above 200 rpm. Results have shown that with respect to dimensionless numbers, despite going beyond the scope of the confinement criterion, FSOHP achieves a low value of thermal resistance and temperature stabilisation with characteristics very similar to those that describe standard pulsating heat pipe solutions in a stationary system. Furthermore, in a rotating system, an increase in heating power improves the thermal efficiency of the device only for FR > 70%, while in a stationary system, an increase in the power supply range studied is always associated with a decrease in thermal resistance, regardless of the FR.
Caputo, Antonio C.Federici, AlessandroPelagagge, Pacifico M.Salini, Paolo...
18页
查看更多>>摘要：This paper addresses the problem of designing shell and tube heat exchangers operating under uncertain conditions. Traditional design methods assume constant process conditions and known value of thermal design factors. Thus may fail to meet specifications when operational conditions change. Use of safety factors or worst case design often leads to conservative design and equipment oversizing. No guideline is available in the literature for choosing a design approach. In order to provide designers better awareness on this issue, we investigate when uncertainty should be taken into account, what are the advantages and drawbacks of available approaches for design under uncertainty, and which is the preferred method to be applied in specific problem instances. To answer the above research questions, comparative sizing of equipment is carried out under consistent assumptions adopting five alternative design methods; namely design for nominal reference condition with verification of off-design performances, worst case design, use of safety margins, design to optimize a prescribed objective function, and robust design. Either random uncertainty in external process conditions, and epistemic uncertainty in heat transfer coefficients and fouling resistances are considered. Different types of specifications (more/less is better or nominal is better) are accounted for. Probability of meeting specifications and resulting geometrical characteristics are compared. It is the first time that a similar comparison is carried out, and results discussion allows to identify merits and limitations of the considered approaches. It is found that no single approach is superior, as any method can be suited to a design situation but perform poorly in other cases. Justification for explaining the observed behaviour is used in order to provide guidance to designers in choosing the preferable design method for a specific instance resulting in more cost-effective equipment configuration with higher probability of meeting specifications.
NSTL
Elsevier