查看更多>>摘要：Due to high energy storage density and nearly constant temperature, phase change materials (PCMs) have become an attractive option for a variety of thermal equipment. But the usage of PCMs is hindered due to their low thermal conductivity. The addition of nanoparticles (NPs) to PCM is one of the promising solutions. A comprehensive understanding is required before the usage of NPs in PCMs. The present review article focus on synthesis, characterization, effect of NPs on the thermophysical properties, stability, and applications of the nanoparticle enhanced PCM (NEPCM) operating in the range of 30-100 ?. Almost 40 different NPs were used in the literature to improve the thermal conductivity of the PCM. Experimental data of various research articles are analyzed to select the promising NEPCMs based upon the change in thermal conductivity and latent heat. The stability of NPs due to the addition of surfactants and the influence of heating/cooling cycles on the stability of NPs and thermophysical properties are also discussed.
查看更多>>摘要：This paper presents a numerical study of stratified flow condensation heat transfer in tubes with novel cross-section designs. A design framework of the novel cross-section geometry was developed by first studying and analyzing the condensate film distribution in a circular tube. It was found that the circular arc diameter and orientation have effects on the condensate film thickness. Based on this understanding, four different cross-section designs (models N1, N2, N3 and N4) formed by connecting circular arcs of different curvatures at various orientations were proposed. The heat transfer performances of these models were compared to a circular tube of the same perimeter. Our simulations show that all models possess higher average heat transfer coefficients than a circular tube when no accumulated condensate layer is present. Among all the models proposed, model N4 exhibits the largest heat transfer enhancement ratio of 1.42 at epsilon = 0.96. The design of model N4 utilizes small circular arcs which are arranged in an orientation that increases the effect of the gravitational force, providing significant condensate film thickness reduction over a large area. This study not only demonstrates the possibility of enhancing condensation heat transfer by simply varying the tube curvature but also provided a design guideline which can be employed for the development of novel cross-section tubes that can be fabricated by advanced manufacturing techniques such as selective laser melting.
Habeeb, Ahmed S.Karamallah, Abdulhassan A.Aljabair, Sattar
16页
查看更多>>摘要：Recently, Nano-fluids have gained great attention as a result of their significant possibility for heat transfer improvement. The heat transfer of Nano-fluids can be numerically investigated employing the single-phase and two-phase models. Nanofluids are a homogenous mixture of the base fluid and nanoparticles with minor slip in between in the single-phase approach. This model is a widely used approach due to its cost-effectiveness and simplicity. Meanwhile, the two-phase models are divided into the Eulerian-Lagrangian and Eulerian-Eulerian depending on the treatment of nanoparticles and fluid phases. The Eulerian-Eulerian approach consists of three numerical approaches: the mixture, volume of fluid (VOF), and Eulerian. Many articles emphasize twophase approaches because they consider the motion between solid particles and fluid and their precise predictions. However, the accuracy of this approach is extensively based on several conditions, including the flow type and nanoparticle properties. Despite this, the accuracy of various computational approaches to simulate Nano-fluids is still debatable. Therefore, this review paper aims to analyze all the published studies about the homogenous, multi-phase Eulerian-Lagrangian and Eulerian-Eulerian (volume of fluid, mixture, Eulerian) approaches.
查看更多>>摘要：The increasing penetration of renewable energy sources (RESs) and the inherent volatility in demand profiles have added another layer of complexity to the management of energy resources in modern active buildings (ABs). Yet, three challenges have been neglected in previous studies: (1) There is no universal systematic method for identifying an AB's general stochastic model; (2) No research has been conducted on the reliability based design optimization for ABs' energy supply; (3) Uncertain sources are not categorized based on their importance in regard to the optimization problem. This article aims to solve these challenges by proposing a probabilistic-based optimization approach for solving the reliability issue of energy supply in buildings with on-site renewable energy sources (RESs), taking into account the uncertainty associated with photovoltaic (PV) production and demand fluctuations. The suggested framework seeks to reduce the overall costs of the system while ensuring high energy supply reliability. The proposed methodology, when applied to a real-world case study, demonstrates a 60% increase in reliability of energy supply as compared to typical deterministic methodologies.
查看更多>>摘要：In this paper, an analytical model for buildings simulation and performance evaluation has been introduced and compared to existing numerical models already used in simulation tools. The proposed analytical model is based on the quadrupole approach, which is used for solving partial differential linear equations, while the numerical model is mainly provided by EnergyPlus simulation tool. To validate the proposed analytical model, results have been confronted to those of the BESTEST standard case 600 FF (Light weighted free-floating building) and 900 FF (Heavy weighted free-floating building). These latter are based on the ANSI/ASHRAE norm standard 140-2014 "Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs". Simulations have been conducted by solving, in transient conditions, the heat equation and assessing then the temperature evolution inside the building's zone. Results are reported and proved the accuracy of the proposed analytical model in terms of transient regime resolution since it considers all the necessary data and dynamics, to model the building and its internal gains without simplifications or hypothesis. Besides, it requires very low simulation time, which is required for large scale buildings' simulations.
查看更多>>摘要：ABSTR A C T Schools are recognized as high-risk spaces in airborne infectious disease transmission due to occupant density and activity level. Therefore, it is critical to conduct studies and provide proper solutions regarding the spread of airborne transmission diseases to the lowest possible in schools. In this study, the performance of five various ventilation systems regarding control the spread of airborne particles in a classroom were studied by Compu-tational Fluid Dynamics (CFD), and then, the best case was selected with regard to the least probability of infection spread. These cases with various locations of inlet and outlet vents are considered, and the spread of particles is studied. A multi-objective optimization process is then performed to understand the effects of design variables, including air inlet width, air change per hour (ACH), and inlet air temperature on the thermal comfort conditions according to suggested standard values and relations for calculating each of these parameters based on the design variables are proposed. Desirability functions are used to determine the range of variables that result in thermally comfortable conditions and maximum desirability of 0.952 with a width of 0.130 m, tem-perature of 296.60 K, and value of ACH between 9.689 and 9.858 results is obtained.
查看更多>>摘要：A 3D CFD modelling approach of a vertical high temperature furnace in a bright annealing line for stainless steel is presented in this study. The observed line has a maximum production rate of 4.5 tonnes per hour and an overall thermal input of 1.5 MW which is provided by gas burners. The muffle furnace is an indirect heating system equipped with flameless burners, which can operate at nominal power and in off-design. The burners are arranged in four heating zones, which can be controlled separately. Pure hydrogen is used inside the muffle as protection gas around the metal. The metal strip is modelled as a highly viscous fluid with temperature-dependent properties including the specific heat, thermal conductivity and emissivity. Good accordance with measurements from an industrial line was achieved with several values from furnace thermocouples and the transient heating characteristic of a selected strip dimension, thus, demonstrating the applicability of the CFD model for the present investigations. Further simulations were performed with air-staged combustion for an evaluation of combustion and furnace efficiency as well as the thermal load on the furnace's components. It was found that the efficiency decreased by about 0.5% using flameless combustion. The results also showed a decreased peak temperature of about 1155 degrees C and a reduced heat flux of about 23.3 kW/m(2) (-5%) on the muffle when Moderate or Intense Low oxygen Dilution (MILD) combustion is applied. The present methodology presents an efficient approach for investigation and optimization of indirect heated muffle type furnaces.
Moriconi, L.Nascimento, T.de Souza, B. G. B.Loureiro, J. B. R....
10页
查看更多>>摘要：We investigate calcium carbonate scale formation at high Reynolds numbers in a large pipe rig facility. The calcium carbonate solution is produced from the injection, at a T-joint inlet, of pH-stabilized sodium carbonate and calcium chloride aqueous solutions. A scanning electron microscopy analysis of the deposited mass along the pipe indicates that after an initial transient regime of ion-by-ion crystal growth, calcium carbonate scale is dominated by particulate deposition. While limescale formation in regions that are closer to the pipe's entrance can be described as the heterogeneous surface nucleation of calcium and carbonate ions driven by turbulent diffusion, we rely upon turbophoresis phenomenology to devise a peculiarly simple kinetic model of deposition at farther downstream regions. Letting Phi and R be the flow rate and the pipe's radius, respectively, the mass deposition rates per unit time and unit area are predicted to scale as Phi(alpha)/R-beta (for certain modeled values of the alpha and beta parameters) with suggestive support from our experiments.
查看更多>>摘要：Effective diffusion coefficients in monolayer and bilayer growth experiments during gaseous nitriding of pure iron have been previously determined by assuming a diffusion zone of infinite size. In this work, finite size corrections to the effective diffusion coefficients are determined by imposing total mass balance of nitrogen in a sample with a diffusion zone of finite size. The proposed model incorporates an equation of motion for the diffusion zone thickness, which is consistent with total mass balance. The model does not have an exact analytical solution; therefore, semi-analytical methods are used to find approximate solutions to the problem. The effective diffusion coefficients are estimated through a double minimization process. Experimental data from other authors are used to define a least squares error function for the layer thicknesses and another error function for the total mass of nitrogen. The steepest descent method is used to determine a set of diffusion coefficients that minimize the error in the layer thicknesses. The effective diffusion coefficients that best approximate the total mass of nitrogen in the sample are estimated by solving the model for each of the local minima previously determined. The values of the effective diffusion coefficients are validated by comparing the numerical and semi-analytical solutions obtained from the proposed model, with the experimental values for the total nitrogen mass at different nitriding conditions. Finally, non-parabolic growth behavior is captured by the proposed model in experiments where the assumption of a diffusion zone of infinite size or saturated substrate is arguable.
查看更多>>摘要：Heating rate and ambient pressure affect the resin matrix pyrolysis process, the pyrolysis gases flow state and the temperature distribution of polymer matrix composites as heatshield of spacecraft. Therefore, a pyrolysis layer model with heating rate and pressure is presented. Accuracy of prediction of heating rate distribution calculated by the presented model is validated by comparing Arcjet wind tunnel test results of phenolic impregnated carbon ablator (PICA). After that, the thermal behavior of PICA under different heating rates is predicted by this presented model. It is indicated that the mass injection rate of pyrolysis gases increases, the bondline temperature declines and the char layer's density decreases as the heating rate rises. Besides, from material bondline to material surface, the heating rate maximum increases exponentially along material thickness. This study contributes to the optimal design of the thermal protection system of spacecraft.
NSTL
Elsevier