Experimental and theoretical studies of convective heat transfer in a cylindrical porous medium

Convective heat transfer at constant heat flux through unconsolidated porous media has been studied both experimentally and theoretically. Heat transfer measurements have been performed for convective heat transfer over a wide range of operational parameters at constant heat fluxes. In addition to heat transfer coefficients, pressure drop and temperature profiles both in radial and axial direction have been recorded. The equations of motion and energy which account for the non-Darcian effect are used to describe the flow and convective heat transfer through the porous medium. Mathematical models for the prediction of heat transfer coefficients and temperature profiles are presented which predict the experimental data with good accuracy.     

Investigation of flow and heat transfer of nanofluid in microchannel with variable property approach

Laminar flow and heat transfer of water-Al₂O₃ nanofluid under constant heat flux have been investigated numerically. Single-phase with temperature dependant effective properties has been assumed for fluid. Enhancement in heat transfer and increase in friction factor have been obtained by the use of nanofluid. Heat transfer enhancement is more obvious by the use of variable properties. Also, effects of temperature variation on nanofluid heat transfer are greater than the pure water.     

Saturated nucleate boiling to binary and ternary mixtures on horizontal cylinder

In this investigation, a large number of experiments have been performed to determine saturated nucleate pool boiling heat transfer coefficients of MEA/water and DEA/water binary mixtures and that of water/MEA/DEA ternary mixtures. These heat transfer coefficients have been measured at atmospheric pressure and over a wide range of heat fluxes and solution concentrations. The heat flux has been varied in 14 different levels from 7 to about 230 kW/m² and amines concentration has been changed in 10 different levels from zero to 84 wt%. Results show that strong reduction of heat transfer coefficient occurs as a result of mass transfer interference in this phenomenon. Furthermore, in this study, all the correlations proposed during the last years for the prediction of nucleate boiling heat transfer coefficient of mixtures have been categorized in three groups. Some experimental results have been compared with the most accurate representatives of these three groups and the corresponding RMS errors have been calculated. Also, impacts of important existing parameters in these correlations like ideal heat transfer coefficient (h_id.) on the prediction have been discussed.     

Numerical simulation of convective heat transfer to a radial free surface jet impinging on a hot solid

The convective heat transfer between a circular free surface impinging jet and a solid surface has been studied numerically. The thin liquid film formed on the surface has been assumed to be in non-turbulent free surface flow. The effects of surface tension, viscosity, gravity and heat transfer between the film flow and the solid surface have been taken into account. The flow structure on a non-heated surface has been investigated first. Next, the steady-state flow structure in the liquid film as well as the heat transfer has been examined. The predicted results have been compared with experimental data for the purpose of validating the analysis. The hydrodynamics of the liquid film and the heat transfer processes have been investigated numerically to understand the physics of the phenomena. Received on 5 October 1998     

Simultaneous heat and mass transfer under unsteady mixed convection along a vertical slender cylinder embedded in a porous medium

Unsteady laminar mixed convection flow (combined free and forced convection flow) along a vertical slender cylinder embedded in a porous medium under the combined buoyancy effect of thermal and species diffusion has been studied. The effect of the permeability of the medium as well as the magnetic field has been included in the analysis. The partial differential equations with three independent variables governing the flow have been solved numerically using a implicit finite difference scheme in combination with the quasilinearization technique. Computations have been carried out for accelerating, decelerating and oscillatory free stream velocity distributions. The effects of the permeability of the medium, buoyancy forces, transverse curvature and magnetic field on skin friction, heat transfer and mass transfer have been studied. It is found that the effect of free stream velocity distribution is more pronounced on the skin friction than on the heat and mass transfer. The permeability and magnetic parameters increase the skin friction, but reduce the heat and mass transfer. The skin friction, heat transfer and mass transfer are enhanced due to the buoyancy forces and curvature parameter. The heat transfer is strongly dependent on the viscous dissipation parameter and the Prandtl number, and the mass transfer on the Schmidt number.     

Heat transfer and pressure distributions of an impinging jet on a flat surface

Experiments were conducted to determine the heat transfer and surface pressure characteristics of a round jet impinging normal on isothermal flat plate. Three nozzles of exit diameters 3, 5 and 7?mm have been used. The local heat transfer rates have been estimated from the outputs of three-wire differential thermocouple heat flux sensors. The results cover a Reynolds number range of 3400 to 41?000 and dimensionless separation distances varies from 6 to 58. The static pressure distributions along the impingement surface are found to be similar and closer to the heat transfer variations at the same configurations. A simple correlation is derived for the average heat transfer coefficients within ±10% deviation from the output data covering the complete range of experimental limits. The predicted values of Nusselt number have also been compared with the results obtained from the literature. The agreement was generally good.     

Semi-empirical modeling of pool boiling heat transfer in binary mixtures

Pool boiling heat transfer has been investigated for various binary mixtures, including acetone/isopropanol, water/acetone, water/methanol, water/ethanol, water/isopropanol, water/monoethanolamine, water/diethanolamine and water/triethyleneglycol as test solutions. Many correlations have been developed to predict the pool boiling heat transfer coefficient in mixtures in the past few decades, however the predicted values are not confirming. In addition, the application of many existing correlations requires some individual adjusting parameters that may be not available for every system. In this investigation, a new set of experimental data are presented. These data have been compared to major existing correlations. It is observed that the pool boiling heat transfer coefficients in mixtures are less than the ideal boiling heat transfer coefficient. A new semi-empirical model has been proposed based on the mass transfer resistance to predict the boiling heat transfer coefficient with satisfactory accuracy. The new model does not include any tuning parameter and is applicable to any given binary system. The performance of the proposed model is superior to most existing correlations.     

The influence of multiple tubes on the tube-to-bed heat transfer in a fluidised bed

There have been few studies modelling both flow and heat transfer in fluidised beds. The kinetic theory of granular flow (KTGF) has been used for flow prediction in the past without heat transfer modelling. In the present study, a two-fluid Eulerian–Eulerian formulation incorporating the KTGF was applied first to a tube-to-bed reactor with one immersed tube and compared with the results in the literature. The bed was then modified to introduce two and three heated tubes. The effects on the flow and temperature distribution, local heat transfer coefficients and averaged heat transfer coefficients over a 3.0 s time period were carried out. Results showed that increasing the number of tubes promotes heat transfer from tubes to the particles and flow. The heat transfer coefficients extracted from the single-tube to three-tube cases were analysed in detail, confirming the importance of linking flow/particle and heat transfer calculations.     

Heat transfer experiments in rotating boundary layer flow

The influence of Coriolis force on heat transfer in a rotating transitional boundary layer has been experimentally investigated. The experiments have been conducted for local Görtler numbers up to 150. Heat transfer measurements have been performed for a flat plate with nearly uniform heat flux applied to the surface, where the temperature was measured by the thermochromic liquid crystal method. The results indicate that heat transfer is enhanced when Coriolis force acts towards the wall, i.e., on the pressure surface. The velocity measurements under equivalent conditions show that Coriolis instability induces counter-rotating longitudinal vortices which augment the lateral transport of the fluid on the pressure surface. On the other hand, the heat transfer on the suction surface remains at the same level as compared to the case without system rotation. As a consequence, the heat transfer coefficient on the pressure surface is 1.8 times higher than that measured on the suction surface when averaged over the measured surface.     

Experiments and homogeneous turbulence model of boiling flowin narrow channels

The boiling heat transfer experiments have been carried out in vertical narrow annular channels with pure water. A two-dimensional homogeneous turbulence model of boiling flow has been developed and solved numerically to yield pressure gradient, and velocity, thermal and turbulence fields, together with local heat transfer coefficient along the length of the tube. Predictions are compared with the data of experiments and agreed well with it. The model results show that the heat transfer coefficient increases as the gap size decreases in annular channels. This model can be used to predict heat transfer of boiling flow in narrow channels.     

Convective heat transfer and pressure loss in rectangular ducts with drop-shaped pin fins

It has been experimentally researched that convective heat transfer and pressure loss characteristics in rectangular channels with staggered arrays of drop-shaped pin fins in crossflow of air. The effects of arrangements of pin fins on heat transfer and resistance are discussed and the row-by-row variations of the mean Nusselt numbers are presented. By means of the heat/mass transfer analogy and the naphthalene sublimation technique, the heat transfer coefficients on pin fins and on endwall (base plate) of the channel have been achieved respectively. The total mean heat transfer coefficients of pin fin channels are calculated and the resistance coefficients are also investigated. The experimental results show that heat transfer of a channel with drop-shaped pin fins is higher than that with circular pin fins while the resistance of the former is much lower than that of the latter in the Reynolds number range from 900 to 9000. Received on 20 January 1997     

Metal Foam Heat Exchangers for Heat Transfer Augmentation from a Cylinder in Cross-Flow

A numerical study has been conducted to examine the heat transfer from a metal foam-wrapped solid cylinder in cross-flow. Effects of the key parameters including the free stream velocity and characteristics of metal foam such as porosity, permeability, and form drag coefficient on heat and fluid flow are examined. Being a determining factor in pressure drop and heat transfer increment, the porous layer thickness is changed systematically to observe that there is an optimum layer thickness beyond which the heat transfer does not improve while the pressure drop continues to increase. This has been verified by the application of Bejan’s Intersection of Asymptotes method. Results have been compared to those of a finned-tube heat exchanger to observe much higher heat transfer rate with reasonable excess pressure drop leading to a higher area goodness factor for metal foam-wrapped cylinder.     

Conception for enhanced mass transport in binary nanofluids

Besides their application in enhancing heat transfer, suspended nanoparticles have been found to improve mass transfer process inside binary nanofluids. The concepts of enhanced mass transfer in binary nanofluids are involved. By means of the heat and mass transfer analogy, the approaches for determining the mass diffusivity and mass transfer coefficient are proposed and discussed.     

Flow and heat transfer analysis in porous wick of CPL evaporator based on field synergy principle

In order to optimize the structure of a CPL evaporator and enhance heat transfer, a mathematical and physical model is developed to analyze the flow and heat transfer in the porous wick of the evaporator, whose calculation domain is divided into two parts: vapor-saturated region and liquid-saturated region. The characteristics of flow and heat transfer in the porous wick of a CPL evaporator have been numerically studied according to the field synergy principle. The influences of geometrical structures and heat flux on heat transfer enhancement are analyzed and illustrated by the figures in the present paper.     

A separated flow model for predicting two-phase pressure drop and evaporative heat transfer for vertical annular flow

A separated flow model has been developed that is applicable to vertical annular two-phase flow in the purely convective heat transfer regime. Conservation of mass, momentum, and energy are used to solve for the liquid film thickness, pressure drop, and heat transfer coefficient. Closure relationships are specified for the interfacial friction factor, liquid film eddy-viscosity, turbulent Prandtl number, and entrainment rate. Although separated flow models have been reported previously, their use has been limited, because they were tested over a limited range of flow and thermal conditions. The unique feature of this model is that it has been tested and calibrated against a vast array of two-phase pressure drop and heat transfer data, which include upflow, downflow, and microgravity flow conditions. The agreements between the measured and predicted pressure drops and heat transfer coefficients are, on average, better or comparable to the most reliable empirical correlations. This separated flow model is demonstrated to be a reliable and practical predictive tool for computing two-phase pressure drop and heat transfer rates. All of the datasets have been obtained from the open literature.     

Mixed convection flow over a vertical wedge embedded in a highly porous medium

 The steady mixed convection flow over a vertical wedge with a magnetic field embedded in a porous medium has been investigated. The effects of the permeability of the medium, surface mass transfer and viscous dissipation on the flow and temperature fields have been included in the analysis. The coupled nonlinear partial differential equations governing the flow field have been solved numerically using the Keller box method. The skin friction and heat transfer are found to increase with the parameters characterizing the permeability of the medium, buoyancy force, magnetic field and pressure gradient. However the effect of the permeability and magnetic field on the heat transfer is very small. The heat transfer increases with the Prandtl number, but the skin friction decreases. The buoyancy force which assists the forced convection flow causes an overshoot in the velocity profiles. Both the skin friction and heat transfer increase with suction and the effect of injection is just the reverse. Received on 21 May 1999     

Thermo-fluid-dynamic analysis of the flow in a rotating channel with a sharp “U” turn

Infrared thermography has been employed to carry out a detailed convective heat transfer measurements at Re?=?20,000 in a two-pass square channel both for the static case (absence of channel rotation) and for the rotating case (Ro?=?0.3). At the same time, the main and secondary flow fields have been measured by means of particle image velocimetry with the aim to investigate how the flow behavior affects the local distributions of the convective heat transfer coefficient for the two cases. The normal-to-wall velocity component (w) and the turbulent kinetic energy, both measured close to the heat exchanging wall, have been used to formulate an empirical heat transfer correlation within an attempt to identify the role performed by these two quantities on the convective heat transfer coefficient distributions. The latter ones have been reported in terms of normalized Nusselt number (Nu/Nu*) maps, where Nu* is the Nusselt number evaluated with the classical Dittus-B?lter correlation.     

A visual flow study of mixed convection in the entry region of a vertical internally heated annulus

The velocity distribution in laminar upward flow of water (Pr 7.25) in the entry of a vertical internally heated annulus (radius ratio 4:1) has been determined by visual observation. Photographic measurements have been made of the motion of hydrogen bubble clusters, which were generated by a carefully controlled process of electrolysis, to assess the effects of free convection effects on the forced flow.For heat fluxes up to 2500 W/m² and at a Reynolds number of approximately 450, local heat transfer coefficients have been obtained in a length of about 23 equivalent diameters. Heat transfer rate in the immediate entry was found to be insensitive to change in heat flux over the range of variables considered. As the distance downstream increased, the heat transfer rate was found to be dependent on the heat flux.     

Natural convection of an alumina-water nanofluid inside an inclined wavy-walled cavity with a non-uniform heating using Tiwari and Das’ nanofluid model

The present study is devoted to numerical analysis of natural convective heat transfer and fluid flow of alumina-water nanofluid in an inclined wavy-walled cavity under the effect of non-uniform heating. A single-phase nanofluid model with experimental correlations for the nanofluid viscosity and thermal conductivity has been included in the mathematical model. The considered governing equations formulated in dimensionless stream function, vorticity, and temperature have been solved by the finite difference method. The cavity inclination angle and irregular walls (wavy and undulation numbers) are very good control parameters for the heat transfer and fluid flow. Nowadays, optimal parameters are necessary for the heat transfer enhancement in different practical applications. The effects of the involved parameters on the streamlines and isotherms as well as on the average Nusselt number and nanofluid flow rate have been analyzed. It has been found that the heat transfer rate and fluid flow rate are non-monotonic functions of the cavity inclination angle and undulation number.     

Numerical simulation of nucleate pool boiling on the horizontal surface for ferrofluid under the effect of non-uniform magnetic field

Nucleate pool boiling heat transfer of ferrofluid on a horizontal plate; has been explored numerically. Extra necessary equations have been used in this model to simulate mass transfer and effect of magnetic field that had not considered in previous researches using mixture model. Also effect of negative and positive gradient of magnetic field on the heat transfer rate and bubble shape has been investigated. Results are in good agreement with experimental data.