Fluid Thermal Conductivity Research Articles

Influence of thermophoresis and chemical reaction on MHD micropolar fluid flow with variable fluid properties

This work investigates the effect of thermophoresis and chemical reaction on heat and mass transfer in hydromagnetic micropolar fluid flow over an inclined permeable plate with constant heat flux and non-uniform heat source/sink in the presence of thermal radiation. It is assumed that the transverse magnetic field is a function of the distance from the origin. The analysis accounts for both temperature dependent fluid viscosity and thermal conductivity. Using the similarity transformation, the governing system of equations are transformed into non-linear ordinary differential equations and are solved numerically using symbolic software MATHEMATICA. Numerical results for the velocity, microrotation, temperature and species concentration as well as for the skin friction, heat and mass transfer are obtained and displayed graphically for pertinent parameters to show interesting aspects of the solution.

Effect of grinding speed changes on dispersibility of the treated multi-walled carbon nanotubes in aqueous solution and its thermal characteristics

For the purpose of increasing the dispersibility and thermal conductivity of fluid containing nanoparticles, raw multi-walled carbon nanotubes (MWCNTs) were treated by strong acids. The purified structures were dispersed into aqueous solution by using wet grinding conditions at various rotation speeds (300–600rpm) merged with ultrasonication. The particle size analyzer reveals that the agglomerated size of ground particles at rotation speed of 300rpm was 39.811μm. Nevertheless, the agglomerated size of particles significantly decreased to 11.482μm after grinding at rotation speed of 600rpm. The maximum absorbance (4.0 abs at wavelength of 300nm) and highest thermal conductivity (0.6074W/mk at 30°C) of the suspension corresponds to the grinding speed of 600rpm assisted by ultrasonication dispersion. From the overall results, grinding method with high rotation speed plays can be significantly increased both the dispersibility and thermal conductivity of MWCNTs in aqueous solution.

Prediction of a temperature‐dependent electroosmotically driven microchannel flow with the Joule heating effect

PurposeA convection‐diffusion‐reaction scheme is proposed in this study to simulate the high gradient electroosmotic flow behavior in microchannels. The equations governing the total electric field include the Laplace equation for the effective electrical potential and the Poisson‐Boltzmann equation for the electrical potential in the electric double layer.Design/methodology/approachMixed electroosmotic/pressure‐driven flow in a straight microchannel is studied with the emphasis on the Joule heat in the equations of motion. The nonlinear behaviors resulting from the hydrodynamic, thermal and electrical three‐field coupling and the temperature‐dependent fluid viscosity, thermal conductivity, electrical permittivity, and conductivity of the investigated buffer solution are analyzed.FindingsThe solutions computed from the employed flux discretization scheme for the hydrodynamic, thermal and electric field equations have been verified to have good agreement with the analytical solution. Parametric studies have been carried out by varying the electrical conductivity at the fixed zeta potential and varying the zeta potential at the fixed electrical conductivity.Originality/valueInvestigation is also addressed on the predicted velocity boundary layer and the electric double layer near the negatively charged channel wall.

On the effectiveness of variable properties and thermophoresis on steady MHD heat and mass transfer over a porous flat surface

PurposeThe purpose of this paper is to examine the effects of thermophoresis and magnetic field on steady two‐dimensional laminar hydrodynamic flow with heat and mass transfer over a semi‐infinite permeable flat surface in the presence of viscous dissipation and thermal radiation effects. The fluid viscosity and thermal conductivity are assumed to vary as a function of temperature.Design/methodology/approachThe boundary layer equations are transformed to non‐linear ordinary differential equations using scaling group of transformations and these equations are solved numerically by using the fourth order Runge‐Kutta method with shooting technique for some values of physical parameters.FindingsSome of the results obtained for a special case of the problem are compared to the results published in previous work and are found to be in excellent agreement. Many results are obtained and a representative set is displayed graphically to illustrate the influence of the physical parameters involved in the problem on the velocity, temperature and concentration profiles, as well as the local skin‐friction coefficient, the wall heat transfer and the particle deposition rate.Research limitations/implicationsOne valuable, important observation is that the effect of the variable viscosity parameter is to increase the effect of all studied parameters in the boundary‐layer's flow field. Also, the skin‐friction coefficient, wall heat transfer and wall deposition flux in a fluid of uniform viscosity are higher than in a fluid of non‐uniform viscosity when the surface is permeable.Originality/valueThe paper presents a numerical solution for two‐dimensional boundary‐layer flow with heat and mass transfer over a semi‐infinite permeable flat surface. Numerical results indicate that the combining effects of magnetic field and radiation strongly controls flow and mass transfer characteristics for the thermophoretic hydrodynamic flow. This problem is interesting from the physical point of view and also for its applications in engineering sciences.

Simulation of mixed convection heat transfer in a horizontal channel with an open cavity containing a heated hollow cylinder

AbstractThe objective of this paper is to numerically investigate the mixed convective flow and heat transfer controlled by a heated hollow cylinder inside an open cavity attached with a horizontal channel. All the boundaries of the channel and cavity are perfectly insulated while the inner surface of the cylinder is heated uniformly by heat flux q. The equations of conservation of mass, momentum, and energy were solved using adequate boundary conditions by Galarkin's weighted residual finite element technique. The solution has been performed in the computational domain as a whole with proper treatment at the solid/fluid interface. Computations have been conducted for Ra = 103–105, Prandtl number Pr varying from 0.7 to 7 and ratio of solid to fluid thermal conductivities from 0.2 to 50. Results are presented in terms of streamlines, isotherms, heat transfer rate in terms of the average Nusselt number (Nuav), drag force (D), and maximum bulk temperature (θmax). © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21002

Measurement of thermal conductivity of fluids using 3-ω method in a suspended micro wire

A thermally controlled, compact device employing the 3-ω technique, used to measure the thermal conductivity of fluids, is designed, developed, and presented in this paper. The 3-ω method, which analyzes temperature oscillations data in the frequency domain, requires a microscopic sample and extremely low heating power. The functionality is derived from the approximate solutions of temperature oscillations of a line heater based on the infinite line-heater model over an empirically and analytically chosen range of frequencies. The method is devoid of errors related to transient measurements, fluid thermal stratification and mobility errors, which pose difficulties in other methods. A platinum (99.99% pure) wire of 50 µm diameter and a length of 30 mm, suspended in a sample volume of 25 µl of the test fluid, serves simultaneously as the heater and thermometer. Structure-wise, the device is designed to support measurements over a range of temperatures and fluid pressures providing modularity and flexibility to the instrument. The device is successfully employed to measure the thermal conductivity of de-ionized water for temperatures between 15 and 35°C with an accuracy of ±1.2% inmeasurement.

Estimation of heat flux bifurcation at the heated boundary of a porous medium using a pore-scale numerical simulation

Laminar steady forced convection through an array of disconnected conducting cylindrical fins is simulated by implementing a pore-scale numerical simulation. Effects of fin array porosity, fin arrangements, pore-scale Reynolds number, and the solid to fluid thermal conductivity ratio, on the heat flux bifurcation at the uniformly heated wall are investigated. The results show that decreasing the Reynolds number or increasing the solid to fluid thermal conductivity ratio monotonically increases the solid to fluid wall heat flux ratio. It is also observed that solid to fluid heat flux ratio increases with increasing the porosity. The obtained results have been compared to available models in the literature. While in some cases the results are close, errors associated with available models are analyzed and accurate correlations for heat flux bifurcation are proposed.

Effects of variable fluid properties on natural convection of MHD fluid from a heated vertical wavy surface

In the present paper, the influence of temperature-dependent fluid properties, density, viscosity and thermal conductivity on MHD natural convection flow from a heated vertical wavy surface is studied. It is assumed that, the fluid density and the thermal conductivity vary as exponential and linear functions of temperature, respectively. However, the fluid viscosity is assumed to vary as a reciprocal of a linear function of temperature. The model analysis used here is more relevant to liquid flow. Using the appropriate variables, the wavy surface are transformed into a flat one. The transformed boundary layer equations are solved numerically, using implicit-Chebyshev pseudospectral method, for several sets of values of the physical parameters, namely, the temperature dependent fluid properties parameters, the magnetic parameter, the amplitude-wavelength ratio parameter, and the Prandtl number. The numerical values obtained for the velocity, temperature, shearing stress, and the Nusselt number are presented through graphs and tables for several sets of values of the parameters. The effects of the physical parameters on the flow and heat transfer characteristics are discussed. The results were compared with numerical solutions of previous works. The present results are found to be in good agreement.

Radiation effects on heat and mass transfer in MHD stagnation-point flow over a permeable flat plate with thermal convective surface boundary condition, temperature dependent viscosity and thermal conductivity

Abstract We study a steady laminar 2-D MHD viscous incompressible flow over a permeable flat plate with thermal convective boundary condition and radiation effects. The viscosity and thermal conductivity of fluid are assumed to vary linearly with temperature. Similarity representation of the governing partial differential equations is obtained via group method. Similarity equations are then solved numerically by implicit finite difference technique. Effects of convective heat transfer parameter ( b ), radiation parameter ( R ,) magnetic field parameter ( M ), the thermal conductivity parameter ( S ), suction parameter ( fw ), Prandtl number (Pr) and Schmidt number (Sc) on the dimensionless axial velocity, temperature, concentration, wall temperature, the rate of heat transfer and the rate of mass transfer are investigated. Good agreement is found between the numerical results of the present paper with published result for special case.

새로운 채널 배열을 통한 마이크로채널 열교환기 성능 향상 수치 연구

기존 마이크로채널 열교환기는 한 플레이트에 고온 또는 저온, 한 종류의 유체만이 흐르고 있다. 채널 내부를 흐르는 유체의 수직 방향으로는 다른 종류의 유체가 존재하지만, 수평 방향으로는 같은 종류의 유체가 존재한다. 그로 인해 수평 방향의 열전달률은 수직 방향에 비하여 낮게 나타나게 된다. 열교환기 성능 향상을 위하여 한 플레이트에서 고온, 저온 유동이 번갈아가며 존재하는 새로운 채널 배열을 제안하였다. 새로운 채널 배열을 위해서는 특별한 입구 및 출구 설계가 필요하다. 제안된 채널 배열을 통하여 기존 열교환기보다 높은 열전달률을 얻을 수 있다. Reynolds 수와 Prandtl수가 증가할수록 새로운 채널 배열로 인한 열 성능 향상이 증가하고, 고체와 유체의 열전도계수 비가 증가할수록 열 성능 향상은 감소한다.

Effects of Variable Thermal Conductivity and Chemical Reaction on Steady Mixed Convection Boundary Layer Flow with Heat and Mass Transfer Inside a Cone due to a Point Sink

The steady incompressible mixed convection boundary layer flow with variable fluid properties and mass transfer inside a cone due to a point sink at the vertex of the cone have been investigated. The fluid viscosity and thermal conductivity have been assumed to be temperature dependent. The governing fluid flow equations with boundary conditions have been transformed into set of coupled ordinary differential equations with the help of similarity transformations and solved Runge-Kutta method with shooting technique. The effects of Schmidt number, variable thermal conductivity parameter, mixed convection parameter, buoyancy parameter and chemical reaction parameter on velocity distribution, temperature distribution, concentration distribution, heat transfer rate and coefficient of skinfriction have been investigated. It is observed that concentration decreases with increasing Schmidt number and temperature increases with increasing values of thermal conductivity parameter. Also with increasing values of mixed convection parameter, velocity, temperature and concentration decreases. The present study is relevant in conical nozzle and diffuser flow problems exist in petroleum and chemical industries.

Factors Affecting Thermal and Moisture Comfort of Bamboo Fabric

The fluid permeability and thermal conductivity of bamboo, cotton and ramie fabrics were measured and analyzed using variance analysis and principal component analysis. After the evaluation of the heat and mass transfer properties of these fabrics, it was found that the bamboo fabric owns the best thermal and moisture performance, in terms of wearing comfort.

Combined effects of internal heat generation and higher order chemical reaction on the non‐darcian forced convective flow of a viscous incompressible fluid with variable viscosity and thermal conductivity over a stretching surface embedded in a porous medium

AbstractIn this paper, we study the combined effects of internal heat generation and higher order chemical reaction on a steady two‐dimensional non‐Darcian forced convective flow of a viscous incompressible fluid with variable dynamic viscosity and thermal conductivity in a fluid saturated porous medium passing over a linear stretching sheet. Using similarity transformations, the governing nonlinear‐coupled partial differential equations are made dimensionless and solved numerically for similarity solutions using very robust computer algebra software Maple 8. The non‐dimensional velocity, temperature and concentration distributions are presented graphically for various pertinent parameters such as relative temperature difference parameter, Darcy number, porosity parameter, reaction rate parameter and the order of the chemical reaction. The variations of Prandtl number and Schmidt number within the boundary layer are also displayed graphically when the fluid dynamic viscosity and thermal conductivity are temperature dependent. From the present numerical computations it is found that Prandtl number as well as Schmidt number must be taken as variables within the flow domain when the fluid's dynamic viscosity and thermal conductivity are variable. In the presence of internal heat generation, dynamic viscosity and thermal conductivity of the fluid are found to be higher than when it is absent. Increasing Darcy number reduces dynamic viscosity as well as thermal conductivity whereas increasing pore size reduces the Schmidt number and increases the Prandtl number within the boundary layer. For higher order reaction the rate of increase in mass transfer function is less compared to the rate of increase for the lower order reaction. © 2011 Canadian Society for Chemical Engineering

Theoretical study of the effective thermal conductivity of graphite foam based on a unit cell model

A unit cell model aimed at representing the microstructure of graphite foam is developed in this paper. The geometry of this model can be easily changed to accommodate different pore structures in graphite foams. With this model, the effective thermal conductivity and the interior surface area to volume ratio of graphite foam are obtained. Comparisons of the effective thermal conductivity between the current model and published results show good agreement. The investigation show that the increase of the porosity and the diameter of the openings decrease the effective thermal conductivity while the fluid thermal conductivity has negligible effect on the effective thermal conductivity of graphite foam.

Highly Thermo-conductive Fluid with Boron Nitride Nanofillers

We report for the first time how boron nitride (BN) nanotubes and nanospheres may effectively be used to achieve remarkable thermal conductivity improvement of a fluid. Benefiting from high thermal conductivity and high-aspect-ratio of BN nanotubes, at a fraction of 6 vol %, the thermal conductivity of water was remarkably improved, up to ∼2.6-times. With BN nanospheres as fillers, the viscosity of the fluid can be kept decently low and thermal conductivity can also be effectively improved. A combination of BN nanotubes and nanospheres was found to increase the fluid's thermal conductivity while keeping its viscosity low, thus, such mixtures can be promising fillers for highly thermo-conductive fluids. Finally, calculations based on finite element method were used to investigate the regarded nanofluids. On the basis of the results, thermal conductivity was estimated to be more than, or close to 200 W/mK for BN nanotubes and nanospheres, respectively.

Conjugate Mixed Convection with Surface Radiation from a Vertical Channel with Protruding Heat Sources

In this article, the results of a numerical investigation of conjugate mixed convection from a vertical channel with discrete protruding heat sources mounted on the right side wall of a vertical channel have been presented. The flow is considered to be laminar, two-dimensional, hydrodynamically, and thermally developing. All geometric parameters such as the size of the protruding heat sources and the spacing between the protruding heat sources, the spacing between the channel walls, and the thickness of the channel walls are fixed. The parameters such as Reynolds number (Re S ), modified Grashof number , ratio of thermal conductivity of heat sources and thermal conductivity of fluid (k p /k f ), ratio of thermal conductivity of channel walls and thermal conductivity of fluid (k s /k f ), emissivity of heat sources (ϵ p ), and emissivity of channel walls (ϵ s ) are varied to understand the flow and heat transfer characteristics. A correlation for the nondimensional maximum temperature has also been developed for this configuration using the method of asymptotic expansion method.

Laminar combined magnetoconvection in a wavy enclosure with the effect of heat conducting cylinder

In the current work, a numerical study of the flow characteristics on combined magnetoconvection in a lid-driven square enclosure, differentially heated, is carried out. This problem is solved by using finite element method of the partial differential equations, which are the heat transfer and stream function in Cartesian coordinates. The tests are performed for different solid–fluid thermal conductivity ratio, cylinder location and Richardson number while the Prandtl number, Reynolds number, magnetic and Joule heating parameters are kept constant. One geometrical configuration is used namely two undulations. The outcome obtained shows that the heat conducting inner square cylinder affects the flow and the heat transfer rate in the enclosure. The trend of the local heat transfer is found to follow a wavy pattern. Results are presented in terms of streamlines, isotherms, average Nusselt number at the heated wavy wall, average temperature of the fluid in the enclosure and dimensionless temperature at the cylinder center for different combinations of the governing parameters.

The effect of Fe3O4 nanoparticles on the thermal conductivities of various base fluids

Conventional heat transfer fluids have intrinsically poor heat transfer properties comparedto solids. Enhancing the efficiency of heat transfer is of great interest for various industrialapplications. Suspending solid particles in a fluid increases the thermal conductivity of theresulting suspension and enhances the heat transfer properties. In this work, changes in thermalconductivities of fluids upon the addition of magnetic nanoparticles have been investigated.Fe3O4 nanoparticles are synthesized using different synthesis methods and are suspended invarious oils. The effect of the base fluid and the type of magnetic particle on the thermalconductivity is investigated in detail. Up to 28% increase in the thermal conductivity isobtained with 2.5 wt% magnetic particles in hexane. The thermal conductivity enhancementis found to depend on the particle concentration, method of preparation and base fluid.The enhancements obtained are higher than those estimated using any theoretical modelpresent in the literature.

Numerical analysis of fluid flow due to mixed convection in a lid-driven cavity having a heated circular hollow cylinder

Mixed convection heat transfer in a lid-driven cavity along with a heated circular hollow cylinder positioned at the center of the cavity has been analyzed numerically. The present study simulates a realistic system such as air-cooled electronic equipment with a heat component or an oven with heater. A Galerkin weighted residual finite element method with a Newton–Raphson iterative algorithm is adopted to solve the governing equations. The computation is carried out for wide ranges of the Richardson numbers, cylinder diameter and solid fluid thermal conductivity ratio. Results are presented in the form of streamlines, isothermal lines, average Nusselt number at the heated surface and fluid temperature in the cavity for the mentioned parameters. It is found that the flow field and temperature distribution strongly depend on the cylinder diameter and also the solid–fluid thermal conductivity ratio at the three convective regimes.

Multifunctional Nanofluids: Synthesis, Aggregation and Thermal Conductivity

We synthesize multifunctional nanofluids by incorporating fluorescent dyes into magnetite nanoparticles. The synthesis is carried out in two different routes, in which nanoparticles with two sizes are coated by two different kinds of stabilizers. The structures of coating layers of the as-synthesized nanoparticles are characterized by the analysis of Fourier transform infrared spectra. Magneticinteraction- induced aggregation still exits and is traced by the DLS. Thermal conductivities of the synthesized nanofluids are measured under day-light, in the dark and under the UV irradiation. The fluid thermal conductivity varies in a wave-like shape as a function of particle concentration due to the existence of aggregation. Furthermore, the measured thermal conductivity under UV light is higher than those with the other conditions. The collapse of aggregates under UV irradiation due to the unidirectional non-radiative energy transfer increases the number of single magnetite particles and is thus assumed to be responsible for the increase of fluid thermal conductivity. Keywords: Multifunctional nanofluids, fluorescence, aggregation, thermal conductivity, non-raidiative energy transfer, magnetite nanofluids, surface properties, stabilizers, coating layers, particle concentration, optical imaging probes, conductivity enhancement