جستجوی مقالات مرتبط با کلیدواژه « heat transfer » در نشریات گروه « مهندسی شیمی، نفت و پلیمر »

Condensation occurs in most heat transfer processes, from cooling electronic devices to heat removal in power plants. The overall heat transfer coefficient of dropwise condensation (DWC) is several times higher than that of filmwise condensation (FWC). Therefore, obtaining a stable DWC is very important for better performance. DWC stability depends on surface hydrophobicity, surface free energy, and surface tension of the condensed liquid. The properties required for DWC may be achieved by micro-scale surface modification. In this review, micro/nanoscale coatings such as noble metals, ion implantation, rare earth oxides, lubricant-injected surfaces, polymers, nanostructured surfaces, carbon nanotubes, graphene, and porous coatings have been reviewed and discussed. Surface coating methods, applications, and potential have been compared with respect to heat transfer ability, durability, and efficiency. In addition, common limitations and challenges for densification enhancement applications are summarized to provide future research directions.

Thermal comfort is one of the important and effective parameters on the health and efficiency of the human body, and textiles play a significant role in providing this parameter. In this regard, many engineered textiles have been proposed by adjusting different thermal mechanisms. In the present study, the recent developments in the field of active and passive temperature control (heating, cooling and two-mode textiles) have been expressed with emphasis on the effective mechanism of heat transfer in the desired textile. Finally, the used methods are compared and the challenges and opportunities in this field are pointed out.

Condensation may occur during various industrial applications, including water collection, desalination, energy generation, and thermal management. This phenomenon affects heat transfer efficiency during energy generation and heat management processes. Film-wise and dropwise condensation were the two types of condensation based on their formation manner. Condenser tubes are one of the energy conversion systems in which the condensation phenomenon dramatically affects the system's efficiency. The high surface energy of the condenser tube results in water film formation on the tube surface, which hinders heat transfer. Therefore, the heat transfer efficiency of the condenser is reduced due to further energy loss. The film-wise condensation should be altered to dropwise ones by reducing the surface energy of the tube to eliminate this issue. Hence, different coatings are applied to increase the condenser tubes' hydrophobicity and heat transfer capacity. In this study, the importance of wettability and heat transfer mechanisms in condenser tubes was determined primarily. Afterward, different coatings used to create dropwise condensation were introduced. Eventually, the applications and attributes of the mentioned coatings based on their cost, strength, durability, and efficiency were introduced.

Rubber is the main element of tires and the main part of the tire consists of rubber. Tire and track heating is caused by hysteresis effects due to the deformation of the rubber during operation. Temperature in tire components depends on many factors, including tire geometry, inflation pressure, vehicle load and speed, road type and temperature and environmental conditions. In this study, using the theory of heat production in tire and relying on the finite element and calculating the stress and strain field of different parts of the tire, the temperature field of a steady-state rolling truck radial tire is predicted. ABAQUS software is used in this finite element prediction. It is shown that the results predicted using ABAQUS software are in significantly better agreement with the experimental results.

Expanded surfaces are considered as the common way to heat transfer increment in the industry, but due to their operating conditions and lack of footprint, their use in some situations is limited. Because of its ability in thermal efficiency increment using increasing available area and change in velocity gradient, the porous medium has been considered as a novel solution in heat transfer increment. During this investigation the effective parameters on heat transfer (Nusselt number (Nu)) in the presence of porous medium (configuration, thermal conductivity, pore gradient, porosity, permeability and Darcy number (Da), thickness, fluid velocity, and heat source) were studied. The studied configurations can be classified in partial and fully porous categories. Among the investigated papers, the fully filled configuration usually has shown the maximum exchanged energy and pressure drop. Because using a porous medium leads to pressure drop increase, an index was reported to compare between different configuration can be applied. Finally, limitations and challenges in this field were investigated.

The effect of structural and geometrical parameters of textiles on their physical and thermal properties can be understood using simulation and modeling methods. Various studies have been conducted to model the heat transfer in textiles to determine their temperature distributions and thermal conductivities. The geometry and composition of materials should be introduced to these models by a network of nodes related to each other by a series of heat transfer equations.  The heat transfer equations are solved simultaneously by numerical methods by considering the boundary conditions. To study the heat transfer through textiles, one can use the corresponding equations for porous materials. The analytical solution is used in relatively simple/simplified problems with acceptable approximations, and the numerical solution is employed for solving more complex problems with finite difference methods, finite components, and finite volume. This study reviews the research on modeling methods to determine the thermal properties of textiles.

Understanding the thermal properties of textiles, such as thermal comfort, thermal protection, and thermal insulation, is one of the topics of interest in textile industry and clothing. The thermal resistance of textiles is a detrimental parameter in heat transferand it can be obtained by experimental, analytical, or numerical methods. The thermal behavior analysis of textiles that have different geometrical and structural properties is possible by experimental methods. Different parameters affect the thermal properties of fabrics including the type of material, morphological properties, fineness, cross-section, porosity, yarn structure, fabric structural, fabric finishing. The aim of this paper is to review the published experimental results on the thermal behavior of textiles. In this paper (part I), we list the measuring methods of thermal properties and discuss the geometrical and structural parameters affecting the thermal conductivity in textiles.

In this study, the heat transfer of a single catalyst particle (cylindrical, sg-cylindrical, and tri-lobe) beside the bed wall was studied experimentally and numerically with the bed to particle diameter ration (N) of 4.5, 5.1, 6.7 respectively. In the numerical section, the Finite Element Method (FEM) was applied to solve the momentum, continuity, and Energy. Experimental data was used to validate the numerical results. The results showed that three are the important area around the catalyst particle: low-velocity fluid around the particle, space between the particle and bed wall, and space behind the particle. The wall effects on the heat transfer were studied by reducing the distance of the particle-bed wall. Results showed that cylindrical and sg-cylindrical particles face hot-spot for yc/Dp≤0.143 and θa=60 and θz=90 degrees are the highest probable degrees for hot-spot creation. Finally, the Taguchi method was used to find the optimized particle location around the bed wall.

Thermal conductivity of a partially saturated rock sample depends on several parameters such as matrix properties, pore structure, pore filling fluids and their saturations. Indeed, complex pore structure of the rock causes many problems in prediction of its thermal conductivity at different saturation conditions. Numerous investigators presented different correlations and mechanistic predictive models to predict thermal conductivity of porous media. But, most of these models have focused on the prediction of thermal conductivity of single phase saturated porous medium. Analogy between electricity transmission and heat transfer through a rock can be considered as a basis to develop a model for prediction of thermal conductivity. In this paper, thermal conductivity of six carbonate plug samples from one of the Iranian reservoirs has been measured at vacuum condition. Also, for four samples of these six plugs, thermal conductivity has been determined at fully water saturated condition and four different water saturations using divided bar steady-state apparatus; in addition, the second phase has been air. This apparatus has been designed and constructed in Petroleum University of Technology (PUT). Finally, according to the results, it is obvious that rock thermal conductivity at vacuum condition decreases with an increase in porosity. Furthermore, thermal conductivity of partially saturated rock increases with an increase in water saturation. Moreover, a new model for predicting rock thermal conductivity of partially saturated rock has been presented based on the analogy between electricity transmission and heat transfer through the rock. Our experimental results confirmed the suitability of the proposed model.

In this work, influence of the twisted tape insertion on heat transfer rate and pressure drop characteristics in a double pipe has been numerically investigated. For this purpose, a 3-D double pipe heat exchanger with several types of twisted tape designed with two twist ratio (3, 5). The results obtained from the heat exchanges with twisted tape insert are compared with those without twisted tape that is plain heat exchanger. The numerical results revealed that the increase in heat transfer rate of twisted tape inserts with twist ratio 3 and 5 at Re=20000 is found to be 28% and 14% more than plain tube, respectively. It was found that with decrease twisted ratio, the amount of heat transfer rate and pressure drop increase rapidly. And by the point of pressure drop, at Re=20000, the amount of pressure drop of twisted tape inserts with twist ratio 3 and 5 are 58%  and 34% higher than plain tube, respectively.

Thermal conductivity is defined as the ability of a material to heat transfer. In other words, thermal conductivity is the natural tendency of material to energy dispersion when temperature equilibrium disturbed by the imposition of a temperature gradient. Therefore, it plays a significant role in the issues of heat transfer. Due to the various applications of nanoscale materials in heat transfer and the importance of determining the thermal conductivity of nanofluids, this study, investigates eleven models for predicting the thermal conductivity of nanofluids (includes water and TiO2 nanoparticles) and comparing the results of calculations with the experimental results in the articles. Based on this study, it was found that the effective thermal conductivity ratio (thermal conductivity of the mixture of basic fluid and distributed nanoparticle) to the basic fluid conductivity (keff/kf) for variable Volumetric percentages ranging from 0.01 to 0.03 varies between 1.01 and 1.1. In the other words, addition of nanoparticles in the range of 1 to 3 Volume percentage is able to promote the keff/kf to 1.1 (10%). Therefore, thermal conductivity of the mixture of basic fluid and distributed nanoparticle increases up to 10% compared with basic fluid. Also, increasing the diameter of the nanoparticles reduces the thermal conductivity improvement.

In this paper, single and two-phase CFD models are introduced for predicting the thermo-hydraulic behavior of nanofluids. In single-phase models, behavior of nanofluid is similar to that of observed in conventional fluids and interactions between particles and base fluid are negligible. In two-phase models, interactions between nanoparticles and base fluid is taken into consideration. Both models showed the identical prediction for the heat transfer behavior, however, the temperature distribution of nanofluid was different, since in two-phase model, the parameters affecting the heat transfer are considered in the model. At low concentrations of nanofluid, it is a logical assumption to ignore the influence of dispersed phase (nanoparticle) on the flow field (continuous phase). Thus, the single phase is suitable for low-concentration nanofluids, while two-phase model is appropriate for nanofluids with high concentrations. Overall, one can use single or two-phase models depending on the type of problem, available computational facilities and accuracy.