Convective Heat Transfer

The effect of thermal asymmetry on laminar forced convection heat transfer in a plane porous channel with Darcy dissipation has been investigated numerically. The parallel plates making the channel boundaries were kept at constant, but different temperatures. The thermal asymmetry thus imposed on the system, results in an asymmetric temperature field and different heat fluxes across the channel boundaries. Depending on Darcy, Pecle ´t and Reynolds number, the thermal asymmetry may lead to a reversal of the heat flux at a certain position along the flow at least at one of the channel walls. The corresponding Nusselt numbers become zero and might experience discontinuities thereby jumping from infinite positive to infinite negative, or vice versa. This feature is observed not only in the region of thermal development, but also in the fully developed region. In the fully developed region, an analytical expressions for the Nusselt numbers were obtained. From these expressions, analytical equations were deduced for the calculations of the axial positions along the channel where the Nusselt numbers become zero, or experiences discontinuity.

The effect of thermal asymmetry on laminar forced convection heat transfer in a plane porous channel with Darcy dissipation has been investigated numerically. The parallel plates making the channel boundaries were kept at constant, but different temperatures. The thermal asymmetry thus imposed on the system, results in an asymmetric temperature field and different heat fluxes across the channel boundaries. Depending on Darcy, Pecle ´t and Reynolds number, the thermal asymmetry may lead to a reversal of the heat flux at a certain position along the flow at least at one of the channel walls. The corresponding Nusselt numbers become zero and might experience discontinuities thereby jumping from infinite positive to infinite negative, or vice versa. This feature is observed not only in the region of thermal development, but also in the fully developed region. In the fully developed region, an analytical expressions for the Nusselt numbers were obtained. From these expressions, analytical equations were deduced for the calculations of the axial positions along the channel where the Nusselt numbers become zero, or experiences discontinuity.

The influence of large-scale flow on heat transport in turbulent thermal convection is experimentally investigated. Large-scale flow couples the upper and lower thermal boundary layers. This coupling produces a slow coherent oscillation of the temperature field and strongly influences the spatial distribution of temperature fluctuations. Moreover, when the large-scale flow is either suppressed or strongly modified no significant variation of the heat transport across the cell is observed. ͓S1063-651X͑96͒51012-3͔

An iterative method for measuring two-dimensional refractive index fields induced by convective heat transfer phenomena is presented. Starting from the so-called correction parabola, this reconstruction technique takes into account the local second derivative variation of the refractive index field. The efficiency of the method is analytically defined as a function of non-dimensional parameter and numerically investigated using a ray-tracing code for different classic index profiles and a thermal boundary layer case. Finally, as an implementation test, this algorithm is applied to an impinging jet heat transfer experiment using speckle photography measurement data set. The results show a relative dispersion of 20% compared to the parabola reconstruction when the refractive index gradient becomes more severe. Confinement ratio L Thermo-refractive length (along z-axis) [m] Re Jet Reynolds number T Temperature [K] c Defocusing distance [m] e Slot width [m] n Refractive index s Curvilinear coordinate [m] u Unit vector tangent to the light trajectory x, y, z Spatial coordinates [m] Greek a Deflection angle [rd] b Non-dimensional parameter dth Thermal boundary layer thickness [m] v Non-dimensional parameter D Displacement in the speckle pattern [m] k Wave length [m] n Integration variable [m] Index A Relative to A-method B Relative to B-method i Relative to the ith light ray 0 Relative to ambience w Relative to wall

In recent years highly glazed spaces and atrium buildings are seen as a sign of advanced technology. An atrium is the social center of a building where people gather for social activities and also is a significant element of passive building systems when well designed to provide user requirements. The study aims to accomplish a thermal performance simulation of an atrium building by specifying a prototype model of an office building in Istanbul. With the help of building simulation tools-EnergyPlus (Version 1.2.3), FLUENT (Version 6.2.16)-total energy consumption and air stratification of the atrium building is performed.

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The present study explores the heat transfer enhancement induced by arrays of cubic fins. The fin element is either a cube or a diamond in shape. The array configurations studied include both inline and staggered arrays of seven rows and five columns. Both cubic arrays have the same geometric parameters' , i.e., 11/0=1, S/D=)UD=2.5, which are similar to those of earlier studies on circular pin-fm arrays. The present results indicate that the cube element in either array always yields the highest heat transfer, followed by diamond and circular pin-fm. Arrays with diamondshaped elements generally cause the greatest pressure loss than those with either cubes or pin fins. For a given element shape, a staggered array generally produces higher heat transfer enhancement and pressure loss than the corresponding inline array. Cubic Arrays can be viable alternatives for pedestal cooling near a blade trailing edge.

The purpose of this study is to evaluate an impact of convective mode of heat transfer on the thermal behaviour of a disc brake system during repetitive braking process with the constant velocity using fully three-dimensional finite element model. The transient thermal analysis to determine the temperature distributions on the contact surface of a disc brake is performed. The issue of non-uniform frictional heating effects of mutual slipping of a disc over fixed pads is tested using FE models with the several possible to occur in automotive application heat transfer coefficients. To have a possibility of comparison of the temperature distributions of a disc during cyclic brake application, the energy transformed during time of every analyzed case of braking process and the subsequent release periods was equal. The time-stepping procedure is employed to develop moving heat source as the boundary heat flux acting interchangeably with the convective cooling terms. The difficulties accounted for the accurate simulation of heating during spin of the rotor is omitted by the use of the code, which enable shaping curves responsible for the thermal flux entering the disc at subsequent moments of time. The resulting evolution of temperature on the friction surface reveals a wide range of variations, distinguishing periods of heating and cooling states. It has been established, that during single braking the convective cooling has insignificant influence on the temperature distributions of a disc brake, consequently is not able to prevent overheat problem. However the brake release period after the braking operation, when the velocity of the vehicle remains on the same level, results in considerable decrease of temperature.

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Understanding mixed convection in engineering applications such as heat exchangers, electronics cooling devices, and solar energy collectors have urged researchers to investigate this phenomenon deeper. This study investigates the fluid flow and heat transfer pattern in a two-dimensional (2D) rectangular cavity with sinusoidal heating on the moving top lid numerically. The bottom wall is kept cool while the vertical walls are insulated. The effect of Hartmann number, 𝐻𝑎 on the thermal characteristics and fluid flow are analyzed for Richardson number, 𝑅𝑖 = 1 which indicate mixed convection dominated regime. The governing equations are solved numerically using a SIMPLE algorithm with the finite volume method. The numerical results are displayed in streamlines and isotherms plots. The value of the Nusselt number indicating the heat transfer rate is also discussed. It is found that 𝐻𝑎 has a significant effect on the heat transfer process and fluid flow. It can be seen clearly when the value of 𝐻𝑎 = 30, the rate of heat transfer dropped significantly on the cold wall. Generally, the heat transfer rate decreases with the increase of 𝐻𝑎.

A mathematical model is developed to investigate seabed heat transfer processes under long-crested ocean waves. The unsteady convection-diffusion equation for water temperature includes terms depending on the velocity field in the laminar boundary layer, analogous to mass transfer near the seabed. Here we consider regular progressive waves and standing waves reflected from a vertical structure, which complicate the convective term in the governing equation. Rectangular and Gaussian distributions of seabed temperature and heat flux are considered. Approximate analytical solutions are derived for uniform and trapezoidal currents, and compared against predictions from a numerical solver of the full equations. The effects of heat source profile, location and strength on heat transfer dynamics in the thermal boundary layer are explained, providing insights into seabed temperature forced convection mechanisms enhanced by free-surface waves.

Je remercie tout d'abord vivement les membres de mon jury, qui ont apporté leur culture scientifique et leur oeil critique à la lecture de mon travail. La direction de ma thèse de doctorat a été assurée par le Professeur Souad Harmand, à qui j'adresse de sincères remerciements. Elle a en effet, par son suivi régulier, ses critiques constructives, ses qualités scientifiques et humaines. toujours su entretenir la mot1vation nécessaire à l'accomplissement de ce travaî1. Sans elle, je n'aurais probablement jamais découvert que ma vocation était de devenir tout d'abord un chercheur, mais également un enseignant. Je remercie égaiement le Professeur Bernard Desmet pour rn• avoir accueilli au Laboratoire de Mécanique et Energétique de Valenciennes durant ces trois années de thèse. Je veux aussi adresser mes remerciements à toutes les personnes qui par un mot ou un geste ont permis que ce travail aboutisse dans de très bonnes conditions. Je pense particulièrement à Anne BaL notre secrétaire, dont le dévouement pour 1es autres n'est plus à prouver, à Jean-Michel Dailliet et André Dubus pour m'avoir apporté une aide considérable lors de la mise en place du banc expérimental, et à Didier Saury qui est devenu, pendant les trois années où nous avons cohabité dans le même bureau dans une ambiance sérieuse mais décontractée, mon ami. Rien de tout cela n'aurait été possible sans le soutien de mes parents, c'est une évidence. Ce n'est par contre pas toujours d'une évidence aussi grande de leur montrer que je suis reconnaissant et de leur dire combien je les aime. Qu'ils le sachent. Enfin. mon dernier mot sera pour Aurélie. >-.los chemins se sont croisés alors que nous étions tous les deux étudiants en thèse ct depuis. nous ne nous quittons plus. Je ]a remercie d'abord d'être ce qu'elle est. puis d'avoir supporté mes (nombreux) accès de nervosité qui ont précédé la soutenance. Je la remercie également pour tout le reste ct bien plus encore. Son amour rn 'est cher. Je lui donne le mien.

The American Physical Society PIV measurements of a jet impinging on an opened rotor-stator system at low gap spacing THIEN NGUYEN, JULIEN PELL É, SOUAD HAR-MAND, Univ Lille Nord de France, F-59000 Lille, France -The current work experimentally investigates the flow characteristics of an air jet impinging to an opened rotor-stator configuration at a low nondimensional spacing G = 0.02 and very low aspect ratio e/D = 0.25. The rotational Reynolds numbers varied from 0.33 × 10 5 to 5.32 × 10 5 while the jet Reynolds numbers ranged from 17.2 × 10 3 to 43 × 10 3 . PIV measurements were performed at three axial planes for the entire disk diameter. The obtained PIV results agreed with those obtained by LDA measurements and numerical simulation reported in Poncet et al. 2005 (Physics of Fluids 17, 075110). A recirculation flow region, which centered at the impinging point and possessed high turbulent intensities, was observed. The mean flow and turbulent intensities were evaluated with the local heat transfer coefficients measured by Pellé and Harmand 2009 (Applied Thermal Engineering 29: 1532-1543). It is shown that the local peaks and the gradually rising of the radial heat transfer coefficients N u are due to the secondary peaks and the increases near the outer radius of the turbulent intensity distributions respectively. POD analysis was applied to the cases of the impinging jet with and without rotation. It is shown that the first POD mode captured nearly 60% total kinetic energy and the low-order POD modes revealed a spiral structure in the jet-dominated region.

Fully developed mixed convection of a nanofluid (water/Al 2 O 3 ) has been studied numerically. Two-phase mixture model has been used to investigate the effects of nanoparticles mean diameter on the flow parameters. The calculated results demonstrate that the convection heat transfer coefficient significantly increases with decreasing the nanoparticles means diameter. However it does not significantly change the hydrodynamics parameters. Nanoparticles distribution at the tube cross section shows that the non-uniformity of the particles distribution augments when using larger nanoparticles and/or considering relatively high value of the Grashof numbers.

A design of a low charge refrigerating system based on microchannels technology for the condenser, a compact evaporator and a reduced diameter liquid line is presented. The design of the liquid line of a refrigerating system is specially detailed. Usually designed to avoid pre-expansion before entering in the expansion device, it is shown in this study that a drastic reduction of the pipe diameter is possible by cooling progressively the liquid. An adapted design using this principle, called "integrated liquid line" (ILL) has been developed and its impact on refrigerant charge and energy performance has been evaluated experimentally for a cold room refrigerating unit. A numerical model of the ILL, validated on this experimental device for various running conditions, has been developed and can be used as a design tool for other systems. Experimental measurements were performed and compared to a reference system. Coupled with micro-channel exchangers, a significant reduction of the internal volume and consequently of the refrigerant charge can been reached.

Contents 1. Introduction 251 2. Metal printing methods and post-processing 253 2.1 Printing methods 253 2.2 Powdered metals and properties 255 2.3 Design considerations and process parameters 256 2.4 Part accuracy and surface morphology 260 3. Heat transfer and friction factors for small channels 267 3.1 Build orientation effects 273 3.2 Print process parameters effects 276 3.3 Correlations to predict performance for small additively manufactured channels 279 4. Noncircular channel shapes 285 4.1 Scaling friction factor and heat transfer for noncircular channel shapes 286 5. Other types of microchannels and heat transfer augmentation surfaces 294 5.1 Wavy microchannels 294 5.2 Pin fins, lattices, and triply periodic minimal surfaces 297 5.3 Thin fins 301 6. Heat exchangers 310 7. Conclusions and future needs 315 References 316

High freestream turbulence levels have been shown to greatly augment the heat transfer along a gas turbine airfoil, particularly for the first stage nozzle guide vane. For this study, augmentations in convective heat transfer have been measured for a first stage turbine vane in the stagnation region, along the mid-span, and along the platform resulting from an approach freestream turbulence level of 19.5%. In addition to quantifying surface heat transfer, boundary layer measurements have been made to better understand high freestream turbulence effects. Although there are a number of correlations that have been developed for scaling freestream turbulence augmentations to heat transfer, the results of this study indicate that these correlations are not successful in predicting heat transfer for various regions along a turbine vane.

A theoretical model based on the integral formalism approach for laminar external natural convection in the vicinity of a vertical wall is used to be extended to nanofluids. Two kinds of thermal boundary conditions including uniform wall temperature (UWT) and uniform heat flux (UHF) are used for this modeling. Two different nanofluids are tested, namely, Cu/water and CuO/water nanofluids for which both viscosity and thermal conductivity were determined using Brownian motion-based models. A close attention is focused on the influence due to increasing the volume fraction of nanoparticles on both the heat transfer and dynamic parameters. Results are presented only for particle volume fractions up to 4% to ensure a Newtonian behavior of the mixture. It has been found that natural convection heat transfer increases with the volume fraction for a fixed Grashof number, whatever the nanofluid is. Nevertheless, the enhancement of heat transfer is more pronounced in the case of Cu/water than...

The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus giving insight into the dynamics of plumes and Taylor-Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat along the cooling plate and back to the heated bottom plate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat flux distribution reveals that Taylor-Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes.

Temperature and humidity measurements are conducted in mixed convective humid-air duct flow with condensation. The latent and total heat transfer during the experiment are determined through the thermal balance for inlet temperatures from 27.5 • C to 35.5 • C, relative humidities from 30% to 55% and at four Reynolds numbers (2000-8000). The experimental results are compared with a heat transfer model from the literature. Adjusted in terms of the geometry and surface properties, the model shows partial agreement for the cases with forced convection but has to be further adjusted regarding the influence of thermal convection.

The effects of skin temperature (T S) on the rate of heat loss by cutaneous evaporation (E S) in Holstein cows chronically exposed to sun, considering hair coat colour were studied. Sixteen purebred cows were measured for E S and T S at 01:00 p.m. after 6 hours of exposure to sun, on three body regions (flank, neck and gluteus) and considering dark and white spots separately. Sweating rate (S) and E S were measured by means of a ventilated capsule. Black skin areas presented mean S (138.9 ± 8.5 gm-2 h-1), E S (93.3 ± 5.7 Wm-2), and T S (33.1 ± 0.2°C) higher than those in the white areas (109.5 ± 9.7 gm-2h-1), 73.6 ± 6.5 Wm-2 and 32.6 ± 0.2°C, respectively). There is an exponential relationship among cutaneous temperature and cutaneous evaporation, which can be represented by the equation: E S = 31.5 + exp{(T S - 27.9)/2.19115}, with coefficient of determination r² = 0.68. Cutaneous evaporative heat loss remains almost constant around 48 Wm-2 until T S reaches nearly 31°C.

Diffusion bonding cycle times can be a large factor in the production cost of metal microchannel devices. The challenge is to significantly minimize bonding cycle times through rapid heating and cooling within the bonding process. A novel method is described which takes advantage of the internal flow passages within microchannel devices for convective heat transfer during the bonding process. The internal convective heating (ICH) technique makes use of heated inert gas to provide the microchannel assembly with rapid and uniform heat input. Results demonstrate that the ICH technique is feasible, capable of producing microchannels with higher dimensional integrity and shorter bonding cycle times than traditional vacuum hot press methods. Results suggest that this may be due to smaller thermal gradients within microchannel devices during the ICH bonding cycle.

We obtain the linear instability and nonlinear stability thresholds for a problem of thermal convection in a rotating bidispersive porous medium with a single temperature. We show that the linear instability threshold is the same as the nonlinear stability one. This means that the linear theory is capturing completely the physics of the onset of thermal convection.

In this study, we deal with the problem of a steady two-dimensional magnetohydrodynamic (MHD) flow of a dusty fluid over a stretching hollow cylinder. Unlike the commonly employed thermal conditions of constant temperature or constant heat flux, the present study uses a convective heating boundary condition. The multi-step differential transform method (multi-step DTM), one of the most effective methods, is employed to find an approximate solution of the system of highly nonlinear differential equations governing the problem. Comparisons are made between the results of the proposed method and the numerical method in solving this problem and excellent agreement has been observed. The influence of important parameters on the flow field and heat transfer characteristics are presented and discussed in detail. The results show that both the thermal boundary layer thickness and the heat transfer rate at the wall increases with increasing Biot number Bi, while it has no effect on the skin friction coefficient.

Impinging synthetic jets have been identified as a promising technique for cooling miniature surfaces like electronic packages. This study investigates the relation between the convective heat transfer characteristics and the impinging synthetic jet flow structure, for a small jet-to-surface spacing H/D = 2, dimensionless stroke length 1 < L 0 /D < 22, and Reynolds number 1000 < Re < 4300. The heat transfer measurements show evidence for a power law relationship between the Reynolds and Nusselt number for a constant stroke length. A critical stroke length L 0 /H = 2.5 has been identified. Using phase-resolved particle image velocimetry, vortex quantification is applied to elucidate the influence of the impinging vortex on the timeaveraged heat transfer distribution.

Ensuring well integrity for the life of a well is a crucial challenge faced in the industry. Heat transfer between formation and fluids in the tubing or annulus fluids can result in undesired annular pressure buildup (APB), leading to integrity problems such as collapsed tubing and casing burst. Heat transfer across the wellbore also poses environmental concerns like thawing of surrounding permafrost in arctic environments. Remediation involves costly and unsustainable intervention methods such as killing the well or performing complicated workover operations. Using vacuum-insulated tubing is a conventional practice in the industry to mitigate APB by restricting heat transfer from production flow to casing and surrounding formation, but this mechanical solution used alone has several limitations. Hence, a chemical solution should be included to increase the effectiveness of operations. A chemical solution is placing an insulating packer fluid (IPF) into the wellbore. However, conventional IPF with crosslinking agents does not always provide results that satisfy flow assurance requirements. This leads to the need for developing a more sustainable and robust IPF solution with long-term stability at wide range of temperatures. A novel nonaqueous insulating packer fluid (NAIPF), containing base oil with a newly designed synthetic polymer forms a unique micelle structure chemistry under static conditions. The chemistry provides sufficient viscoelastic properties and yield stress without need of crosslinking agents to minimize convective heat loss in conjunction with low thermal conductivity. The NAIPF exhibits stable rheological properties at high temperatures and provides conductivity superior to water-based IPF systems. With no requirement of any additional solids, it can be recovered back on surface when needed and reused, providing additional sustainability benefit. In this paper, we characterize the micelle structure based NAIPF with different base oils. Validation of chemistry required extensive laboratory study for a period of 2 years prior to being ready for field trials. NAIPF were formulated with different base oils, tested for key properties including thermal conductivity, yield stress, and thermal stability. Extended aging was conducted at temperatures varying from ambient up to 350°F for prolonged periods of time to test fluid stability. Field application required detailed discussion about performance expectations to identify the challenge the fluid intended to address. The NAIPF has been successfully deployed on multiple projects globally including Arctic, South America and West Africa. Performance was further validated during a well reentry to monitor pressures confirming the fluid maintained its properties over extended period. This paper presents the design and implementation of the NAIPF, including methods developed to optimize fluid mixing, transport, and wellbore displacement. The system's ability to limit conductive and convective heat transfer while maintaining stability makes it an economical and sustainable chemical solution to offer maximum protection against well integrity and production complications.

The present paper focuses on the heat transfer of an equilateral triangular duct by employing the CuO/water nanofluid in a laminar flow and under constant heat flux condition. The triangular ducts were used due to their ease of creation and high compaction. They have less pressure drop when compared to the circular and non-circular ducts and their other attributes are very useful in industrial applications. These reasons cause their heat transfer characteristics to be very important. In this paper, to improve the heat transfer of an equilateral triangular duct, a CuO/water nanofluid was employed. The nanofluid was conducted through an equilateral triangular duct with a constant wall heat flux. Results show that the experimental heat transfer coefficient of the CuO/water nanofluid is more than that of distilled water. Also, the experimental heat transfer coefficient of a CuO/water nanofluid is greater than the theoretical one. The heat transfer enhancement of the equilateral triangular duct increases with the nanofluid volume concentration as well as the Peclet number. So a 41% enhancement in the convective heat transfer coefficient for a 0.8% CuO/water nanofluid can be seen when compared to pure water.

Nanofluids refer to a new kind of heat transfer fluids with suspended nanoparticles and can be employed to increase heat transfer rate in various applications. In this investigation laminar convective heat transfer of three series of metallic and non-metallic nanofluids through circular tube under constant wall temperature boundary condition investigated experimentally. The heat transfer coefficients for different concentration of , 3 2 O Al CuO and Cu nanoparticles at various Peclet numbers were determined. The experimental results emphasize on the enhancement of heat transfer by increasing the concentration of nanoparticles as well as Peclet number for each nanofluid but measured results are greater than the values predicted by theoretical correlations.

In this paper the convective heat transfer of Al 2 O 3 /water, CuO/water and Cu/water nanofluids through a square cross-section duct in laminar flow with constant temperature boundary conditions is numerically investigated. Because of the size and volume constraints, it may be required to use square flow-passage geometries. A square cross section duct has the advantage of lower pressure drop, but it has a lower heat exchange rate than that of a circular duct. However, this study shows that this could be compensated by using nanofluids in these systems. A dispersion model was used to include the presence of nanoparticles. Results show that the Nusselt number was enhanced by increasing the concentration and decreasing the size of nanoparticles. Up to 77%, 68% and 59% enhancement in Nusselt number for Cu/water, CuO/water and Al 2 O 3 /water nanofluids at 4.0% vol of 10nm nanoparticles was achieved respectively.

In present study, the effect of magnetic field on heat transfer enhancement and friction factor of Fe 3 O 4 / water nanofluid is experimentally investigated. For this purpose, well-dispersed nanofluids with concentrations of 0.1 and 0.2 % (by mass) in presence of stabilizer are synthesized. Then the experiments are conducted at different Reynolds numbers and magnetic field strengths in an apparatus containing a horizontal circular tube with a length of 1000 mm and diameter of 7 mm. According to the obtained results, the Nusselt number improves with an increase in Reynolds number and concentration of nanopowders. The same result is observed with increasing the magnetic field strength. Moreover, the friction factor of nanofluids is more than that of pure water due to the presence of solid nanoparticles. According to the obtained results, the magnetic field does not impose a significant increase in the amount of friction factor. Considering heat transfer enhancement and friction factor together with a parameter named performance index, it is revealed that privilege of using ferrofluids (improving heat transfer) far outweighs their demerit (increasing friction factor) and the ferrofluids investigated in this paper are capable to be utilized in practical applications.

In present study, the effect of magnetic field on heat transfer enhancement and friction factor of Fe 3 O 4 / water nanofluid is experimentally investigated. For this purpose, well-dispersed nanofluids with concentrations of 0.1 and 0.2 % (by mass) in presence of stabilizer are synthesized. Then the experiments are conducted at different Reynolds numbers and magnetic field strengths in an apparatus containing a horizontal circular tube with a length of 1000 mm and diameter of 7 mm. According to the obtained results, the Nusselt number improves with an increase in Reynolds number and concentration of nanopowders. The same result is observed with increasing the magnetic field strength. Moreover, the friction factor of nanofluids is more than that of pure water due to the presence of solid nanoparticles. According to the obtained results, the magnetic field does not impose a significant increase in the amount of friction factor. Considering heat transfer enhancement and friction factor together with a parameter named performance index, it is revealed that privilege of using ferrofluids (improving heat transfer) far outweighs their demerit (increasing friction factor) and the ferrofluids investigated in this paper are capable to be utilized in practical applications.

According to the response surface methodology (RSM), a statistical analysis of laminar forced convective heat transfer behavior of the multiwalled carbon nanotube (MWCNT)-deionized water nanofluid was carried out using a horizontal circular tube under constant wall heat flux. Sodium dodecyl sulfate, cetyltrimethylammonium bromide, and gum Arabic (GA) were used as surfactants to disperse MWCNT in the deionized water in order to synthesize MWCNT-deionized water nanofluids. MWCNT-deionized water nanofluids in the presence of different surfactants and MWCNT were prepared at various weight ratios (0.5:1, 1:1, and 2:1). The properties of stability, dispersion, and wettability were then studied. According to the results, MWCNT-deionized water nanofluid in the presence of GA at a ratio of 1:1 was selected as the best sample. Nanofluids at concentrations of 0.05 and 0.1% wt% were then prepared. According to the statistical model (RSM), a total of 15 experiments were tested under the laminar flow regime and a flow rate of 171.4-545.4 ml/min, concentration of 0-0.1 wt%, and a magnetic field 0-400 G. The maximum heat transfer coefficient (1,600.1 W/m 2 K) was obtained at a concentration of 0.09wt%wt, a flow rate of 301.3 ml/min, and a magnetic field of 400 G. The experimental results are in agreement with the results obtained by RSM method, confirming the validity and responsiveness of the method.

Nanofluids refer to a new kind of heat transfer fluids with suspended nanoparticles and can be employed to increase heat transfer rate in various applications. In this investigation laminar convective heat transfer of three series of metallic and non-metallic nanofluids through circular tube under constant wall temperature boundary condition investigated experimentally. The heat transfer coefficients for different concentration of , 3 2 O Al CuO and Cu nanoparticles at various Peclet numbers were determined. The experimental results emphasize on the enhancement of heat transfer by increasing the concentration of nanoparticles as well as Peclet number for each nanofluid but measured results are greater than the values predicted by theoretical correlations.

In this paper, the finite volume method is used to investigate the laminar forced convection of water-copper nanofluid between two porous horizontal concentric cylinders. The effects of Reynolds number, the volume fraction of nanoparticles, the geometry, and the porous medium porosity on heat transfer have been studied. The problem is investigated in two different geometries and Re = 10, 25, 50, 75,100, and volume fraction of nanoparticles 0, 0.2, 0.5, 2, and 5% that were related to Copper nanoparticles and the porous medium porosity of 0.5, 0.9. The results indicated that in each geometry, the corresponding Nusselt number increase in the porosity of 0.9 is greater than that of the case with the porosity of 0.5. The results show that the increase in the heat transfer coefficient in the second geometry is greater than the first geometry and in porosity 0.9 is greater than porosity 0.5. These increase values are 6 and 3%, respectively. The increase in the average temperature of the inner cylinder surface in the five mentioned Reynolds values and both geometries is investigated. This increase in temperature in Re = 10 is greater than other Reynolds numbers. The corresponding temperature increases of Re = 10, for the first and second geometries, are 1.8 and 2.2%, respectively. Investigation of the effects of volume fraction of nanoparticles on Nusselt number and heat transfer coefficient shows that both parameters increase by increasing in the volume fraction of nanoparticles. The results show that the increase in the volume fraction of nanoparticles causes the increase in average temperature of the surface. The results show that these increases of temperature that take place in the volume fractions of 0.5, 2, and 5% of nanoparticles are equal to 0.6, 1.14, and 2.3% and relative to the water, respectively. Thermal conductivity coefficient (Wm -1 K -1 ) Nu Nusselt number Re Reynolds number T Temperature (K) u, v, w Velocity components in x, y, z directions (ms -1 ) * Masoud Afrand

The present paper is an experimental study of Al 2 O 3 /water nanofluid convective heat transfer through square cross-sectional duct under constant heat flux in laminar flow. The increase of heat transfer coefficient is one of the most important technical aims for industry and researches. Also, the decrease in the pressure drop for systems that generate high fluid pressure drop is very noticeable. Convective heat transfer can be enhanced passively by changing flow geometry and boundary conditions or by improving the thermal conductivity of the working fluid. A square cross section duct has the advantage of lower pressure drop, but it has a lower heat exchange rate than that of a circular duct and it is expected that using of nanofluid as a new heat transfer media may improved the heat transfer performance of this kind of duct. In this study, convective heat transfer coefficients and Nusselt numbers of nanofluid were obtained for different Al 2 O 3 nanoparticles concentrations as well as Peclet numbers. Experiments show that considerable enhancement of heat transfer coefficient is achieved and this enhancement is up to 27.6% at 2.5% volume fraction of nanoparticles comparing to the base fluid (water), also it has been noticed that convective heat transfer coefficient increases with the increment of nanoparticles' concentration in nanofluid especially at high flow rates. The decrement of wall temperature observed using nanofluid.

The present paper focuses on the heat transfer of an equilateral triangular duct by employing the CuO/water nanofluid in a laminar flow and under constant heat flux condition. The triangular ducts were used due to their ease of creation and high compaction. They have less pressure drop when compared to the circular and non-circular ducts and their other attributes are very useful in industrial applications. These reasons cause their heat transfer characteristics to be very important. In this paper, to improve the heat transfer of an equilateral triangular duct, a CuO/water nanofluid was employed. The nanofluid was conducted through an equilateral triangular duct with a constant wall heat flux. Results show that the experimental heat transfer coefficient of the CuO/water nanofluid is more than that of distilled water. Also, the experimental heat transfer coefficient of a CuO/water nanofluid is greater than the theoretical one. The heat transfer enhancement of the equilateral triangular duct increases with the nanofluid volume concentration as well as the Peclet number. So a 41% enhancement in the convective heat transfer coefficient for a 0.8% CuO/water nanofluid can be seen when compared to pure water.

In this study, experiments were performed by six different volume fractions of Al 2 O 3 nanoparticles in distilled water. Then, actual nanofluid Nusslet number compared by Adaptive neuro fuzzy inference system (ANFIS) predicted number in square cross-section duct in laminar flow under uniform heat flux condition. Statistical values, which quantify the degree of agreement between experimental observations and numerically calculated values, were found greater than 0.99 for all cases.

In this article, laminar flow-forced convective heat transfer of Al 2 O 3 /water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometersized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.

Sometimes the need for non-circular ducts arises in many heat transfer applications because of lower pressure drop of non-circular cross section such as square duct compared to circular tube, particularly in compact heat. But square cross section has poor heat transfer performance and it is expected that using a nanofluid as a new heat transfer media may improve the heat transfer performance of this kind of duct. In this work, a nanofluid of CuO nanoparticles and distilled water has been prepared and its heat transfer characteristics have been studied through square cupric duct in laminar flow under uniform heat flux. Experiments revealed that a remarkable enhancement in heat transfer coefficient is achieved compared to the base fluid. Moreover, it has been reported that heat transfer coefficient enhances with increasing nanofluid flow rate as well as concentration of nanoparticles in the nanofluid especially at high flow rates. So, ultimate enhancement of 20.7% in Nu achieved at 1.5% volume concentration of CuO/water nanofluid. The basic reason for lower heat transfer rate of square ducts is existence of a static section for some part of fluid near corners of square duct and the results indicated that the presence of nanoparticles decrease this unmoved static section which consequently increase the heat transfer from the duct wall to the nanofluid.

Nanofluids include suspension of metallic or nonmetallic nanopowders in base liquid and can be employed to increase heat transfer rate in various applications. In the present paper laminar flow forced convective heat transfer of CuO/water nanofluid in a triangular duct under ...

The dendritic form is one of the most common forms of crystals growing from supercooled melts and supersaturated solutions. In recent decades, an analytical theory has been developed that describes a stable dendrite growth mode under the conditions of a conductive heat and mass transfer process. However, in experiments, the growth of dendritic crystals is often observed under the conditions of convective fluid flow. In the present work, the theory of the growth of dendritic crystals is developed taking into account the convective mechanism of heat and mass transfer at the crystal-melt interface. A stable mode of dendritic growth in the case of intense convective flows near the steady-state growing dendritic tip is analyzed. The selection theory determining a stable growth mode in the vicinity of parabolic solutions as well as the undercooling balance condition are used to find the dendrite tip velocity and its tip diameter as functions of the melt undercooling. It is shown that the theoretical predictions in the case of convective boundary conditions are in agreement with experimental data for small undercoolings. In addition, the convective and conductive heat and mass transfer mechanisms near the growing dendritic surfaces are compared. Our calculations show that the convective boundary conditions essentially influence the stable mode of dendritic growth.

In this paper, we develop a theory of stable dendritic growth in undercooled melts in the presence of conductive and convective heat and mass transfer boundary conditions at the solid/liquid interface of a dendrite. To simplify the matter and construct the analytical theory, conductive and convective mechanisms are considered separately. Namely, the laws for total undercooling and selection criterion defining the stable growth mode (dendrite tip velocity and diameter) are derived for conductive and convective boundary conditions. To describe the case of simultaneous occurrence of these heat and mass transfer mechanisms, we sew together conductive and convective laws using power stitching functions. The generalised selection theory is compared with experimental data for Al 24 Ge 76 and Ti 45 Al 55 undercooled melts.

This review examines selected mechanistic and empirical models reported in the literature to predict convective heat and mass transfer coefficients in gas-fluidized beds. The role of hydrodynamics in heat and mass transfer is briefly outlined before embarking on the modeling approaches. Both bed to wall and interphase heat transfer, are considered. In bed to wall heat transfer, the main focus of the review is the modeling of particle convective components, based on surface renewal. The concepts of transient and local heat transfer models are also discussed briefly. In the case of mass transfer, only interphase transfer is considered. Emphasis is placed on models based on combustion where mass transfer is seen to occur from a few active particles contained in a fluidized bed of inert particles.

This review examines selected mechanistic and empirical models reported in the literature to predict convective heat and mass transfer coefficients in gas-fluidized beds. The role of hydrodynamics in heat and mass transfer is briefly outlined before embarking on the modeling approaches. Both bed to wall and interphase heat transfer, are considered. In bed to wall heat transfer, the main focus of the review is the modeling of particle convective components, based on surface renewal. The concepts of transient and local heat transfer models are also discussed briefly. In the case of mass transfer, only interphase transfer is considered. Emphasis is placed on models based on combustion where mass transfer is seen to occur from a few active particles contained in a fluidized bed of inert particles.

Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions. One method involves the use of vortex generators to produce streamwise and transverse vortices superimposed to the main flow. This study presents numerical computation results of laminar convection heat transfer in a rectangular channel whose bottom wall is equipped with one row of rectangular wing vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible flow. Three-dimensional numerical simulations are performed to study the effect of the angle of attack α of the wing on heat transfer and pressure drop. Different values are taken into consideration within the range 0°< α < 30°. For all of these geometrical configurations the Reynolds number is maintained to Re = 456. To assess the effect of the angle of attack on the heat transfer enhancement, Nusselt number and the friction factor are studied on both local and global perspectives. Also, the location of the generated vortices within the channel is studied, as well as their effect on the heat transfer enhancement throughout the channel for all α values. Based on both local and global analysis, our results show that the angle of attack α has a direct impact on the heat transfer enhancement. By increasing its value, it leads to better enhancement until an optimal value is reached, beyond which the thermal performances decrease.

An experimental study is performed using an infrared thermography system. The experimental method uses the temperature transient variation of a thin plate (tested fin) in order to obtain detailed quantitative heat transfer coefficients. The method developed is similar to the lumped capacitance method usually used to measure heat transfer coefficient on heat exchanger fin models. But the method presented here exploits the capabilities of infrared thermography to measure surface temperatures in a transient technique in order to take into account errors effects due to tangential conduction and radiation of the tested fin. The method is validated using a two-dimensional channel experiment and its advantages are highlighted using a plate fin and two-tube rows assembly experiment. Moreover, convection coefficient variations with fin pitch and frontal air velocity of an automotive plate fin and two-tube rows assembly are also examined.

In the current context more and more efficient heat transfer system are required. The solution based on turbulent flow through tube with twisted tape inserts systems has proven successful in terms of high heat transfer rate. All past studies on this topic was experimental based. The present approach is based on CFD (Computational Fluid Dynamics) technique. The purpose of this paper is to provide an overview of research works carried out to improve the thermal performance of a heat exchanger by using different parameters like type of fins, orientation, shapes and locations. This review is to help understand how every mentioned parameters influences on improvement of thermal performance.