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In graded structure aerogels, change of pores diameter through the thickness affects the effective thermal conductivity. As the pores diameter is reversely correlated to the density, the effective thermal conductivity of aerogel is often normalized to the density and it is expressed as the B parameter. Lower values of B would be the optimum conditions for the resulting aerogel. The objective of this work is to simulate the heat transfer of the homogenous structures and to compare it with structures that pore diameter vary through the thickness. For this purpose, the structure characteristics and properties of silica aerogel along with the effect of coupling thermal conductivity have to be taken into consideration. Using the COMSOLMultiphysics®software, the heat transfer was modeled for a number of cases, including homogenous structures with minimum density (L), maximum density (H) and for an optimum structure (OPT) having a minimum value of the B parameter. The results were compared to thestructurally graded aerogels in which the density was varied in two fashions, from higher values to lower (HtL) and from lower to higher values (LtH). The change of temperature with time was tracked for all the cases. Results indicated that the minimum value of heat transfer was obtained for the structurally graded aerogel of the type of LtH (a 2-percent increase of efficiency for LtH when compared to the optimum structure (OPT)). Therefore, this structure introduce as the best candidate for producing a thermal insulator.

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One of the miniaturization of heat transfer equipment is enhancing the convective heat transfer coefficient. The main aim of this study is design and producing a kind of nanofluid based on water and ethylene glycol. Graphene was synthesized via electrochemical method and its successful production was confirmed with XRD, FTIR spectrum and, SEM and TEM images. By using different amount of graphene i.e. 0.25, 0.5, 0.75, 1, 1.25, and 1.5%, water/ethylene glycol/graphene nanofluid was produced. Sodium dodecyl sulfate (SDS) was used as surfactant to improve graphene stability in the base fluid. The designed experimental setup was composed of spiral tube with constant wall temperature and equipped with flow meter and pressure and temperature indicators. Nusselt number and pressure drop were measured for pure water and compared with those obtained from theoretical relations and it was found that the setup works properly. Convective heat transfer coefficient, Nusselt number, and heat transfer rate were investigated for water/ethylene glycol (60/40 wt.%) and nanofluid with different amount of graphene using experimental setup. The results showed that by adding 1 wt.% graphene into the based fluid the convective heat transfer coefficient increased about 50% while pressure drop was also increased about 50%. Overall, the findings of this research work support the potential of water/ethylene glycol/graphene nanofluid for using in heating/cooling equipment.
 

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Fruit juices are among heat sensitive foods. Vitamins, proteins and other organic materials present in most fruit juices may be easily decomposed during heat processing. Therefore, evaporators with minimum residence time and maximum efficiency for concentrating fruit juices should be applied. So,tubular type falling and climbing film evaporators are suitable devices. In the present investigation, certain important factors in concentration of in digenous orange juice from north region of Iran in a pilot-plant double effect falling-climbing film evaporator have been studied. Variation of liquid film tickness, heat transfer coefficient, hydrodynamic and thermal properties show that evaporation of the fruit juices take places in turbulent regime and required concentration and ratio of the product concentration to acidity can be achieved by six stages evaporation.

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In the present work, investigation on flow boiling heat transfer of R-134a inside a horizontal tube and also the tubes with coiled wire inserts has been done experimentally. The experimental setup which was used in this investigation was a well instrumented vapor compression refrigeration system. This set-up consists a test evaporator which all the experiments were carried out on it. Refrigerant which flows inside the tube of test evaporator is electrically heated by the coils around it. The evaporator tube is a copper tube with 7.5 mm internal diameter. The range of some operating parameters are: refrigerant mass velocities 54-136 kg/m2s, vapor qualities 0.2-1.0 and heat fluxes 2-6 kW/m2. The empirical data were collected for plain tube and tubes with seven different coiled wire inserts (different coil pitches and different wire diameters). The results show that the insertion of a helically coiled wire inside the evaporator tube increases the heat transfer coefficient by as much as 83% above the plain tube values on a nominal area basis. An empirical correlation was developed to predict the heat transfer coefficient during flow boiling of R-134a inside horizontal coiled wire inserted tubes.

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Abstract In the present work, investigation on flow boiling heat transfer of R-134a inside a horizontal tube and also the tubes with coiled wire inserts has been done experimentally. The experimental setup which was used in this investigation was a well instrumented vapor compression refrigeration system. This set-up consists a test evaporator which all the experiments were carried out on it. Refrigerant which flows inside the tube of test evaporator is electrically heated by the coils around it. The evaporator tube is a copper tube with 7.5 mm internal diameter. The range of some operating parameters are: refrigerant mass velocities 54-136 kg/m2s, vapor qualities 0.2-1.0 and heat fluxes 2-6 kW/m2. The empirical data were collected for plain tube and tubes with seven different coiled wire inserts (different coil pitches and different wire diameters). The results show that the insertion of a helically coiled wire inside the evaporator tube increases the heat transfer coefficient by as much as 83% above the plain tube values on a nominal area basis. An empirical correlation was developed to predict the heat transfer coefficient during flow boiling of R-134a inside horizontal coiled wire inserted tubes.

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This paper presents an analytical solution for steady state conductive heat transfer in a cylindrical composite laminate. The results of this solution can be pretty useful in investigating heat transfer in pipes and reservoirs. In this research, tensor of thermal conductivity coefficients for composite materials is presented and the procedure of determination of the coefficients is described based on the properties of fibers and matrix material. Then, the equation of heat transfer of composite materials has been determined in cylindrical coordinates. The research has been done for conditions that fibers are wound around the cylinder and the heat transfer equation has been solved via separation of variables method.

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- Natural convection heat transfer in a square cavity induced by heated plate is investigated using the lattice Boltzmann method. A suitable forcing term is represented in the Boltzmann equation. With the representation, the Navier-Stokes equation can be derived from the lattice Boltzmann equation through the Chapman-Enskog expansion. Top and bottom of the cavity are adiabatic; the two vertical walls of the cavity have constant temperatures lower than the plate’s temperature. The flow is assumed to be two-dimensional. Air is chosen as a working fluid (Pr=0.71). The study is performed for different values of Grashof number ranging from 103 to 105 for different aspect ratios and position of heated plate. The effect of the position and aspect ratio of heated plate on heat transfer are discussed. With increase of the Grashof number, heat transfer rate is increased in both vertical and horizontal position of the plate. The obtained results of the lattice Boltzmann method are validated with those presented in the literature.

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Abstract- In the present paper, three different samples of alumina used as nanoparticles in the ethylene glycol suspension of alumina with volumetric concentration in the range . These samples have different surface properties, shape and size. The use of Al2O3/EG nanofluids as coolants in a double-tube heat exchanger has been studied under laminar flow conditions. The hot solvent inlet heat exchanger must be cooled down with a specified amount. At first, heat transfer relations between hot solvent and nanofluids as coolants have been investigated theoretically. Subsequently, heat transfer area and flow rate of coolant are optimized by using the nanofluids. In the present paper, heat transfer coefficient, overall heat transfer coefficient, friction factor, pressure drop and pumping power for Al2O3/EG nanofluids calculated.

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There is a full connection between the electrochemical quantities of a fuel cell and the curves of the temperature and primary materials at the catalyst region. These quantities are strongly linked to the mass and heat transfer phenomena in the other regions. In the present paper, the lattice-Bolzmann method, as a microscale model with good computational capabilities in the problems such as the fuel cell, has been utilized to simulate the fluids flow and heat transfer in a two-dimensional cross section of a proton exchange membrane fuel cell including the channel, bipolar plate, gas diffusion layer and catalyst of the cathode and the electrochemical characteristics in the catalyst layer have been analyzed. By representing a method for estimation of the changes in the concentration along the channel, the serpentine arrangement has been modeled. The results reveal the essential role of the bipolar plate on the quantities at the catalyst layer.

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This paper is concerned with the conduction heat transfer between two parallel plates filled with a multi-layer porous medium under a Local Thermal non - equilibrium condition. Analytical solution is obtained for both fluid and solid temperature fields in the porous channel incorporating the effects of thermal conductivity ratio, porosity, and a non-dimensional heat transfer coefficient at the pore level. The effects of the variable porosity on the temperature distribution are completely shown and compared with the constant porosity model. The presented method for analysis of heat transfer in a multi-layer porous medium is a generalized solution which is valid for arbitrary number of internal porous layers. The local temperature difference between fluid and solid phases is also calculated for a wide range of parameters. The results confirmed that the conductivity ratio and the porosity of the internal layers have significant role in the thermal modeling of the porous channel.

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This paper investigates suitable approximation for Calculating the thermal radiation flux divergence and effect of errors on performance evaluation of porous radiant burners (PRB).Thus, a single layer and a double layer of buried flame type of the porous radiant burners have been selected and numerically simulated. Due to the significant difference in the temperature of the solid matrix and the fluid passing the burner, the energy equations was considered as a non-local thermal equilibrium. Complete kinetics of methane air was used for combustion modeling. Since the effect of lateral walls should be neglected the problem was solved in 1D to present exact solution of RTE and compares the other approximations. Results show that discrete ordinate as well finite volume approximation of RTE show that eight directional spherical split is the best selection. Lower ordinates have substantial deviation and increasing the number of division enlarges computation cost without any considerable improvement on errors reductions. Furthermore, two flux method and Rosseland approximation are not valid for this kind of modeling.

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This paper discusses about the effects of square wave pulsation on the turbulent flow and heat transfer from slot jet impinging to a concave surface. The RNG k-ε turbulence model is applied for modeling the turbulent flow and heat transfer filed in the present 2-D slot jet flow. The effects of jet Reynolds number, nozzle to surface distance and pulsation frequency on time-averaged Nusselt number distribution are studied carefully. Results show that applying the pulsating jet in the range of 10 Hz to 50 Hz can increase heat transfer from the concave surface in comparison with the steady jet. Increasing jet Reynolds number ranged from 4740 to 9590 significantly increases the time-averaged local Nusselt number. Also, in steady jet, decreasing the nozzle to surface distance, consequences increasing the Nusselt number near the impingement zone. While in pulsating jet, it causes both increasing/ decreasing the Nusselt number all over the concave surface.

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In this study, 2D Asymmetric heat transfer in multi-layer composite cylinder is investigated, analytically. The boundary conditions are the most linear general boundary conditions which can cover all the heat transfer mechanisms consists of convection, conduction and radiation. The fibers are wounded around the cylinder. The angle of fibers and composite materials can be changed layer by layer. The governing equation of orthotropic conduction has been extracted and temperature distribution series have been obtained using the separation of variables method. In order to find the unknown series’ coefficients, three independent set of equations have been constructed by applying the boundary conditions inside/outside of cylinder and temperature/heat flux continuity between the layers. These set of equations have been solved using the orthogonal function relations and Thomas algorithm to find the recursive relation for unknown coefficients. The effect of design parameter, fiber angle and layers’ materials, have been investigated via two functional examples.

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In this research, validity of temperature-independent thermophysical properties assumption of water-Al203 nanofluid in natural convection problems within the enclosures is investigated. The numerical results are obtained utilizing an in-house finite volume code based on the SIMPLE algorithm. In order to do the validation the numerical results and those of existing correlations are compared. In order to evaluate the thermal performance of the enclosure, the average Nusselt number on the hot side wall in both temperature-independent and dependent cases is compared Results show that, in the all considered solid volume fractions, the difference in the Nusselt number in the case of temperature-independent properties is less than 10 percent in comparison with the case in which the properties are temperature-dependent when temperature difference is less than 5 ○C. As the temperature increases, the difference between Nusselt number in both cases increases and the effect of increase in solid volume fraction is to increase this difference. Results also show that the difference between these two cases is dependent solely on temperature differences between the hot and cold walls regardless of the temperature they have.

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In this paper, natural convection heat transfer inside an enclosure which is partially filled with porous layer is reported using lattice Boltzmann method. Generalized equations in modeling flow in porous media have been employed which are coupled with the lattice Boltzmann formulation of the momentum and energy equations. The present study investigates the effect of position of porous layer on heat transfer rate for different dimensionless parameters, such as Rayleigh number, Darcy number and porosity of the porous layer. In addition, a modified Rayleigh number is presented as an effective parameter which affects the degree of penetration of the fluid into the porous layers. The obtained results showed that the heat transfer rate in the case of vertical layer is more than that of horizontal porous layers.

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In the present paper,by performing a two-dimensional simulation,the heat transfer from a hot cylinder to a cold square enclosure has been studied parametrically and the consequent effect of changing in cylinder diameter has been investigated. The 2-D governing equations have been solved using the finite volume method and TDMA in an ADI procedure for different diameters of cylinder inside a square enclosure with a constant characteristic length for two different Rayleigh numbers of 104 and 105.Results showed that the patterns of streamlines, isotherms and the Nusselt number values depend strongly on the Rayleigh number and also ratio of cylinder diameter to characteristic length of enclosure (2R/H). In this case, the centers of vortices created around the cylinder appear in bottom half of enclosure in 2R/H=0.4 for Ra=104 and in 2R/H= 0.5 for Ra=105. Moreover, it is observed that increasing the Rayleigh number and 2R/H ratio, the heat transfer rate from the enclosure is also increased.For example,in 2R/H=0.5, by increasing the Rayleigh number from 104 to 105, the average Nusselt enhances about 30 percent of its initial value and in Ra=105, by changing the 2R/H ratio from 0.2 to 0.5, the average Nusselt climbs almost 35 percent of its initial value.

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In this paper, heat transfer in a sinusoidal channel filled with nanofluid under magnetic field effect is investigated numerically. The magnetic field transversely applied to the channel. Water as a base fluid and copper as nano particles were considered .The Maxwell-Garnetts model and Brinkman model for heat conduction coefficient and dynamic viscosity were used respectively. The effects of changing some parameters such as shape ,volume fraction , Hartmann number and Reynods number were considered. The results show that increasing in all mentioned parameters lead to increasing in Nusselt number. Volume fraction is mainly affect on maximum local Nusselt number in each channel’s wave while Hartmann number is affected minimum and maximum Nusselt number.

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Electrohydrodynamic (EHD) is one of the techniques for heat transfer enhancement. In current study, the enhancement of natural convection heat transfer inside a vertical tube is experimentally investigated under applying a strong electrical field (EHD). For this purpose, a wire electrode with positive polarity is used along the pipe axis while the inner surface of tube is connected to the ground. EHD disturbs the thermal boundary layer by generating ionic wind which flows from wire electrode to inner side of tube and causes the heat transfer enhancement. In this study, the effects of wire electrode diameter and also electrical field on heat transfer enhancement are investigated. Obtained data are reported as local Nusselt number along the pipe axis and mean Nusselt number. The results show that decreasing the wire electrode diameter increases the heat transfer of tube. In addition, increasing of electrical current due to strong electrical field, increases the Nusselt number. At the lowest wire electrode diameter, the highest Nusselt number was observed which was 2.03 times more than the case that no electrical field was applied.

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Marine transportation is the most conventional method for transportation of natural gas, mostly liquid form; namely, Liquefied Natural Gas (LNG) to international far market. Hereon provide safe transportation of natural gas is very important. In the event of exterior material contact to LNG, swift boiling and exploding anticipated. The paper, investigates thermo physical water contact (0oC as a fluid with higher temperature) with liquid methane (cause the similarity of thermo physical properties to LNG) at low temperature (-162oC). The intensity of heat transfer between water particle and liquefied methane resulted to swift pressure increase in vapor film. It causes the generation and swift growth of methane vapor film which has been resulted from abrupt evaporation and results to liquid methane explosion. In this situation, the intense vapor explosion phenomena, endangers the safety of system. Mathematical model of these phenomena has been developed by assuming saturation condition on interface phase. Then, the effects of different thermo physical parameter changes on vapor film growth have been investigated. Based on the results, in some cases, the vapor pressure pulse created in the film has been more than 3 times the initial pressure, which can endanger the safety of system.

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In rocket systems, the re-entry speed to atmosphere is very high which leads to compression of air molecules and appearance of strong bow shock waves in the leading edge; consequently, this yields aerodynamic heating. Using ablating-dispensing materials on the leading edge surfaces, it is important to accurately determine heat flux on these moving boundaries. Measuring heat flux directly is very difficult or impossible in some situations. In the present study, the online Kalman filtering is used to determine heat flux accurately. Since the heat flux is estimated in online (non-iterative) fashion, the optimum location of temperature sensors can be effectively determined. In addition, the results of this study can be used to design heat flux sensors. In this paper, the optimum locations of three temperature sensors are calculated on the basis that the disturbances occur due to burning of sensors are reduced. More robust solutions are obtained for heat flux on the ablating surfaces.