Fluid Thermal Conductivity Research Articles

Thermal conductivity of a diamond magnetite composite fluid under the effect of a uniform magnetic field

We study the thermal diffusion through a fluid made of diamond microparticles immersed in a ferrofluid. A thermal conductivity enhancement is observed when a magnetic field is applied to the fluid; this phenomenon is due to a better heat propagation through a tridimensional chain-like structuring formed by the diamond particles in the direction of the field; additionally the thermal conductivity of the material could be controlled in a switchable way and moreover with the intensity and direction of the field. This thermal improvement shows a strong dependence on the diamond volume fraction, presenting a maximum of 76% when the diamond volume concentration is 15% and as the concentration increases, the relative thermal conductivity enlargement decreases, becoming zero at the maximum diamond volume fraction. This complex behavior could be modeled with the use of the Lewis–Nielsen effective thermal conductivity model, with a form factor that depends on the diamond particle concentration.

Dependence of fluid flows in an evaporating sessile droplet on the characteristics of the substrate

Temperature distributions and the corresponding vortex structures in an evaporating sessile droplet are obtained by performing detailed numerical calculations. A Marangoni convection induced by thermal conduction in the drop and the substrate is demonstrated to be able to result not only in a single vortex, but also in two or three vortices, depending on the ratio of substrate to fluid thermal conductivities, on the substrate thickness and the contact angle. The “phase diagrams” containing information on the number, orientation and spatial location of the vortices for quasistationary fluid flows are presented and analysed. The results obtained demonstrate that the fluid flow structure in evaporating droplets can be influenced in a controlled manner by selecting substrates with appropriate properties.

LOCAL THERMAL NON-EQUILIBRIUM AND HETEROGENEITY EFFECTS ON THE ONSET OF CONVECTION IN A LAYERED POROUS MEDIUM WITH VERTICAL THROUGHFLOW

We investigated the combined effects of throughflow and local thermal non-equilibrium on the onset of convection in a porous medium consisting of two horizontal layers. In our analytical study, which is performed using linear stability theory, we considered variations of permeability, fluid thermal conductivity, solid thermal conductivity, interphase heat transfer coefficient, and porosity. It is found that heterogeneity of permeability and fluid thermal conductivity have a major effect, heterogeneity of the interphase heat transfer coefficient and porosity have a lesser effect, while heterogeneity of solid thermal conductivity is relatively unimportant.

Reference correlations for the viscosity and thermal conductivity of fluids over an extended range of conditions: hexane in the vapor, liquid, and supercritical regions (IUPAC Technical Report)

Reference correlations for the viscosity and thermal conductivity of fluids over an extended range of conditions: hexane in the vapor, liquid, and supercritical regions (IUPAC Technical Report)

Thermal Conductivity of Cu-Zn Hybrid Newtonian Nanofluids: Experimental Data and Modeling using Neural Network

With the emerging trends in nanotechnology and development of biodegradable cutting fluids, the need for conventional mineral based oils have reduced. In this contest the, need for increasing the thermal conductivity of vegetable oils has raised the need to develop Hybrid nanoparticles. In the current work Cu-Zn hybrid nanoparticles with combinations (0:100, 75:25, 50:50, 25:75, and 100:0) were used to prepare nanofluids by dispersing them into vegetable oils. Thermal conductivity of the base fluid and nanofluids with various nanoparticle concentrations at different temperatures were measured experimentally. The results showed increase in thermal conductivity of nanofluids with increase in hybrid nanoparticle loading and with rise in temperature. Neural network models and data fit were proposed to represent the thermal conductivity as a function of the temperature, nanoparticle concentration, diameter of nanoparticle and the thermal conductivity of the nanoparticles and base fluids. The experimental data along with the input were tested for various types of models using existing theoretical models, Data fit software and ANN using Matlab nntool. In ANN regression models were also tested for various performance functions. These models were in good agreement with the experimental data. Regression model with data fit software predicted the outputs with 0.8889% confident levels and with ANN the predicted output has 0.999% confident levels. MSE performance function outperformed all models in training the data and with overall outputs.

Numerical investigation of convective heat transfer in a plane channel filled with metal foam under local thermal non-equilibrium

The present work consists on convective heat transfer modeling in a plate heat exchanger filled with metal foam under local thermal non-equilibrium (LTNE). The metal foam was inserted to fill completely the studied channel, which is crossed by a fluid. The modified Brinkman-Forchheimer extended Darcy model is used in the porous layer, while the macroscopic two-energy equation model is used for the thermal field. The channel walls are maintained at a constant temperature and the velocity at the inlet is supposed uniform. A dimensionless formulation is developed to perform a parametric study in terms of certain dimensionless variables, and solved by the finite volume method (FVM). The results include the effect of the interstitial heat transfer coefficient and the solid to fluid thermal conductivity of different type of metal foams. The results were used to estimate the influence of the convective and conductive contributions using open-celled metal foams with high porosity. It has been shown that such supports can bring a significant enhancement for the heat transfer.

CONJUGATE HEAT TRANSFER WITH VARIABLE FLUID PROPERTIES IN A HEATED HORIZONTAL ANNULUS

In the present work, we numerically study the three-dimensional conjugate heat transfer in an annular space between two horizontal concentric cylinders; the outer cylinder is subjected to an internal energy generated by the Joule effect through its thickness while the inner is adiabatic. The thermal convection in the fluid domain is conjugated to the thermal conduction in the solid. The physical properties of the fluid are thermally dependent. The heat losses from the external outside pipe surface to the surrounding medium are considered. The model equations of continuity, momentum, and energy are solved numerically by a finite volume method with a second-order spatial-temporal discretization. The results obtained show the three-dimensional aspect of the thermal and dynamical fields with considerable variations of the viscosity and moderate variations of the fluid thermal conductivity. As expected, the mixed convection Nusselt number becomes more superior to that of forced convection when the Grashof number is increased. At the solid−fluid interface, the results clearly show the azimuthal and axial variations of the local heat flux and the local Nusselt numbers. Following these results, we have tried to model the average Nusselt number as a function of the Richardson number. With the parameters used, the heat transfer is quantified by the following correlation: NuA= 9.9130 Ri0.0816.

Enhancement of heat transfer coefficient multi-metallic nanofluid with ANFIS modeling for thermophysical properties

Cu and Zn-water nanofluid is a suspension of the Cu and Zn nanoparticles with the size 50 nm in the water base fluid for different volume fractions to enhance its Thermophysical properties. The determination and measuring the enhancement of Thermophysical properties depends on many limitations. Nanoparticles were suspended in a base fluid to prepare a nanofluid. A coated transient hot wire apparatus was calibrated after the building of the all systems. The vibro-viscometer was used to measure the dynamic viscosity. The measured dynamic viscosity and thermal conductivity with all parameters affected on the measurements such as base fluids thermal conductivity, volume factions, and the temperatures of the base fluid were used as input to the Artificial Neural Fuzzy inference system to modeling both dynamic viscosity and thermal conductivity of the nanofluids. Then, the ANFIS modeling equations were used to calculate the enhancement in heat transfer coefficient using CFD software. The heat transfer coefficient was determined for flowing flow in a circular pipe at constant heat flux. It was found that the thermal conductivity of the nanofluid was highly affected by the volume fraction of nanoparticles. A comparison of the thermal conductivity ratio for different volume fractions was undertaken. The heat transfer coefficient of nanofluid was found to be higher than its base fluid. Comparisons of convective heat transfer coefficients for Cu and Zn nanofluids with the other correlation for the nanofluids heat transfer enhancement are presented. Moreover, the flow demonstrates anomalous enhancement in heat transfer nanofluids.

Energy, economic, and environmental analysis of a flat-plate solar collector operated with SiO2 nanofluid

To overcome the environmental impact and declining source of fossil fuels, renewable energy sources need to meet the increasing demand of energy. Solar thermal energy is clean and infinite, suitable to be a good replacement for fossil fuel. However, the current solar technology is still expensive and low in efficiency. One of the effective ways of increasing the efficiency of solar collector is to utilize high thermal conductivity fluid known as nanofluid. This research analyzes the impact on the performance, fluid flow, heat transfer, economic, and environment of a flat-plate solar thermal collector by using silicon dioxide nanofluid as absorbing medium. The analysis is based on different volume flow rates and varying nanoparticles volume fractions. The study has indicated that nanofluids containing small amount of nanoparticles have higher heat transfer coefficient and also higher energy and exergy efficiency than base fluids. The measured viscosity of nanofluids is higher than water but it gives negligible effect on pressure drop and pumping power. Using SiO2 nanofluid in solar collector could also save 280 MJ more embodied energy, offsetting 170 kg less CO2 emissions and having a faster payback period of 0.12 years compared to conventional water-based solar collectors.

Analysis of collimated irradiation under local thermal non-equilibrium condition in a packed bed

Abstract Forced convective heat transfer in a packed bed in the presence of collimated irradiation and under local thermal non-equilibrium is analyzed in this work. Both the collimated and diffusive radiative transfer processes are accounted for using the modified P-1 approximation. Two boundary condition models considering different limiting conditions at the wall which couple radiation and convection under LTNE were constructed. The effect of pertinent parameters such as the porosity φ, pore diameter dp, ratio of the solid to fluid thermal conductivities ζ; radiative properties including optical thickness τ, scattering albedo ω, and the wall emittance ɛw were analyzed. Also, their effects on the temperature and heat flux distributions in the incident direction were analyzed systematically and the limiting interactions between thermal radiation and conduction were revealed. The differences between the two boundary models with the effects of the cited parameters were analyzed. Our analysis demonstrated that an increase in either φ or dp enhances the transfer of radiative energy into the channel.

Investigation of Mixed Convection in a Ventilated Cavity in the Presence of a Heat Conducting Circular Cylinder

The study is aimed to investigate the mixed convective transport within a ventilated square cavity in presence of a heat conducting circular cylinder. The fluid flow is imposed through an opening at the bottom of the left cavity wall and is taken away by a similar opening at the top of the right cavity wall. The cylinder is placed at the center of the cavity. Two cases are considered depending on the thermal conditions of the cavity walls. In the first case, the left and right vertical walls are kept isothermal with different temperatures and the top and bottom horizontal walls are considered as thermally insulated. For the second case, the top and bottom walls are maintained at different constant temperatures and the left and right walls are considered adiabatic. Heat transfer due to forced flow, thermal buoyancy, and conduction within the cylinder are taken into account. Effect of the cylinder size (0.1 ≤ D ≤ 0.5) and the solid–fluid thermal conductivity ratio (0.1 ≤ K ≤ 10) are explored for various values of Richardson number (0 ≤ Ri ≤ 5) at fixed Reynolds (Re = 100) and Prandtl (Pr = 0.71) numbers. The fluid dynamic and thermal transport phenomena are depicted through streamline and isotherm plots. Additionally, the global thermal parameters such as the average Nusselt number and average fluid temperature of the cavity are presented. It is found that the aforementioned parameters have significant influences on the fluid flow and heat transfer characteristics in the cavity.

Hydromagnetic Mixed Convective Transport in a Nonisothermally Heated Lid-Driven Square Enclosure Including a Heat-Conducting Circular Cylinder

A two-dimensional numerical study is performed in an effort to understand the fundamental characteristics of hydromagnetic mixed convective transport in a nonisothermally heated vertical lid-driven square enclosure filled with an electrically conducting fluid in the presence of a heat-conducting solid circular cylinder. Additionally, entropy generation due to heat transfer, fluid friction, and magnetic effect is also determined. Simulations are performed for various controlling parameters such as the Richardson number (1 ≤ Ri ≤ 10), Hartmann number (0 ≤ Ha ≤ 50), Prandtl number (0.02 ≤ Pr ≤ 7), Reynolds number based on the lid velocity (Re = 100, 150, and 200), and amplitude of the sinusoidal function (A = 0.25, 0.5, and 1), keeping the solid–fluid thermal conductivity ratio fixed as K = 5. The flow and thermal fields are analyzed through streamline and isotherm plots for various Ha, Ri, Re, and Pr. Furthermore, the drag coefficient on the moving lid and Nusselt numbers on heated surfaces are also computed to understand the effects of Ha, Ri, Re, Pr, and n on them. It is observed that the drag on the moving lid decreases with Re and increases with Ri and Ha, however, remains insensitive with Pr. The heat-transfer rate from the hot right wall increases as usual with Re, Pr, and Ri but decreases with Ha. The sinusoidally heated bottom wall shows a decrease in the heat-transfer rate with increasing Pr, and at higher Pr, it also decreases with Re. Furthermore, increasing magnetic field strength causes an increase in the heat-transfer rate from the bottom wall. It also decreases with decreasing value of the amplitude of the sinusoidal function.

Investigation of Alumina Nano Fluid Thermal Conductivity

The paper studies experimentally the enhancement in thermal conductivity of Nano fluid 3 2O Al   / distilled water (10 nm particle size) at four different volume fractions (0.13, 0.24, 1 and 1.7%). The Alumina powder posses the following specifications: spherical, 99% purity, white appearance, 3700 bulk density, 160m/g specific surface area, 880J/kg.K specific heat, 46W/m.K thermal conductivity. The tests performed at average temperatures (37.9 C  and 44.23 C  ). The effect of both temperature and Nano fluid volume fraction on its thermal conductivity is investigated. The fluid is allowed to flow in annulus cooling channel subjected to constant heat fluxes (20693W/m and 27752W/m) which corresponds to Nano fluid temperature, Tnf (37.9 C and 44.23C). Thermal conductivity of the fluid is measured using Decagon KD2-senser. The experimental results showed an enhancement in the ratio of Nano fluid thermal conductivity to distilled water thermal conductivity, bf nf k k of about (0.34.5%). The experimental results are then compared with those related to other results obtained using correlations elaborated from previous works.

Constructal design of thermoelectric power packages

In this paper we consider the complete thermofluid design and performance of a thermoelectric module. We increase the temperature difference that must be maintained across the module, and at the same time we reduce the pumping power required by the streams that bathe the hot and cold plates of the module. We find that for greater power output the two streams must be configured in parallel, not in counterflow, and not between two well mixed plenums. We also find that the loss of thermoelectric power due to the temperature nonuniformity of the two plates competes with the power lost during the pumping of the two streams, and that from this competition results the optimal mass flow rates of the two streams. At the optimum, the maximized power output of the module is proportional to a group of geometric parameters (Eq. (39)), which can be maximized further by designing vascular flow architectures for the two plates. The vascular designs reveal an optimal ratio of channel diameters, optimal plate aspect ratio, and a channel flow volume fraction that is of the same order as the ratio of the fluid thermal conductivity divided by the solid thermal conductivity. The flow architectures are further illustrated with numerical examples of vascular and serpentine configuration and performance.

Lignin as dispersant for water-based carbon nanotubes nanofluids: Impact on viscosity and thermal conductivity

The viscosity and thermal conductivity of water-based nanofluids containing carbon nanotubes, stabilized by lignin as surfactant, are measured. These experiments were performed at 20°C and we study the effect of nanotube volume fraction which ranges from 0.0055% to 0.55%. In comparison with SDBS, we show that lignin reduces viscosity and shear-thinning behavior of nanofluids at high volume fraction, without penalizing thermal conductivity enhancement. Viscosity enhancement at high shear rate with respect to nanoparticle content is well modeled by the Maron–Pierce equation, suggesting that lignin is better than SDBS at dispersing carbon nanotubes within water at high volume fraction. The impact of surfactant on base fluid thermal conductivity is also reported and thermal conductivity enhancement of nanofluids with nanoparticle volume fraction is finally compared with previous published models.

Flow of variable thermal conductivity fluid due to inclined stretching cylinder with viscous dissipation and thermal radiation

The aim of the present study is to investigate the flow of the Casson fluid by an inclined stretching cylinder. A heat transfer analysis is carried out in the presence of thermal radiation and viscous dissipation effects. The temperature dependent thermal conductivity of the Casson fluid is considered. The relevant equations are first simplified under usual boundary layer assumptions, and then transformed into ordinary differential equations by suitable transformations. The transformed ordinary differential equations are computed for the series solutions of velocity and temperature. A convergence analysis is shown explicitly. Velocity and temperature fields are discussed for different physical parameters by graphs and numerical values. It is found that the velocity decreases with the increase in the angle of inclination while increases with the increase in the mixed convection parameter. The enhancement in the thermal conductivity and radiation effects corresponds to a higher fluid temperature. It is also found that heat transfer is more pronounced in a cylinder when it is compared with a flat plate. The thermal boundary layer thickness increases with the increase in the Eckert number. The radiation and variable thermal conductivity decreases the heat transfer rate at the surface.

Effect of Magnetic Field and Wall Conductivity on Conjugate Natural Convection in a Square Enclosure

Steady, laminar, conjugate natural convection flow in the presence of a magnetic field in a square enclosure is considered. The left vertical wall of the enclosure is thick with a finite thermal conductivity, while the other three walls are taken to be of zero thickness. The enclosure is filed with liquid gallium and subjected to horizontal temperature gradient: the vertical boundaries are isothermal at different temperatures, whereas the remaining walls are adiabatic. Finite volume method is used to solve the dimensionless governing equations. The physical problem depends on seven parameters: Rayleigh number (500 ≤ Ra ≤ 106), the Prandtl number (Pr = 0.020), the wall to fluid thermal conductivity ratio (0.1 ≤ Kr ≤ 10), wall thickness (D = 0.2), the Hartmann number (50 ≤ Ha ≤ 200), the inclination angle of the magnetic field (ϕ = 0°, 45° or 90°) and the wall to fluid thermal diffusivity ratio (α* = 1). The main focus of the study is on examining the effect of both inclination angle and Hartmann numbers on fluid flow and heat transfer. The effect of Rayleigh number and conduction in the left wall is also considered. The obtained results, in the absence of a magnetic field, show that natural convection can be strengthened by the increase of both Rayleigh number and conductivity ratio, because of the increase of the effective temperature difference driving the flow. For poor conducting wall (Kr = 0.1), where the solid part is an insulated material and the thermal resistance is more important, the average Nusselt number is approximately constant and having low values comparing with equal (Kr = 1) and high (Kr = 10) conducting wall, indicating that most of heat transfer is by heat conduction. The interface temperature is found to be quite non-uniform, especially for high Ra and Kr. This non-uniformity tends to make the flow pattern in the enclosure asymmetric. In the presence of a magnetic field, the results show that for a given Ra, as the value of Hartmann number increases, convection is suppressed progressively and the rate of heat transfer is reduced in the enclosure. Convection mechanism is also affected by the direction of the magnetic field. It is found that the rate of convection heat transfer is more reduced with the x direction of the magnetic field (inclination angle ϕ = 0°). Also the results show that for poor conducting wall (Kr = 0.1), where the convection is dominated by heat conduction, the presence of a magnetic field is not important and its effect in this case can be neglected.

Heat Transfer and Fluid Flow Characteristics in Helically Coiled Tube Heat Exchanger (HCTHE) Using Nanofluids: A Review

One of the most common methods used to transfer heat is by using various types of heat exchangers. The efficiency of heat exchangers can be improved through active or passive techniques. Active technique involves the use of external forces such as vibration, rotation, electric field, etc. Where else the passive techniques involve fluid additives or modified surface geometries. Helical coiled tubes have been known to exhibit excellent heat transfer characteristics compared to regular straight tubes and for this reason it is widely used in industrial applications. Another known passive technique to enhance heat transfer is by altering the fluid thermal conductivity. Recent studies have shown that nanofluids (nano sized particles suspended in conventional heat transfer fluids) posses superior heat transfer capabilities due to its high thermal conductivity. The amount of research done in this area is fairly new and limited. Most studies done on helical coiled tubes and nanofluids show enhanced heat transfer capabilities and results that challenge conventional theories and limitations on heat transfer devices and fluids. Several important aspects of HCTHE that affect the performance such as geometry, fluid inlet and outlet arrangement, and types of HCTHE are discussed based on the reported findings from experimental and numerical studies. This review also focuses on the vital aspects of nanofluids such as types, properties and heat transfer characteristics and limitations towards the application of nanofluids.

Combined effects of uniform heat flux boundary conditions and hydrodynamic slip on entropy generation in a microchannel

The effects of wall heat flux boundary conditions, wall to fluid thermal conductivity ratio and slip flow on heat transfer and entropy generation by considering the conjugate heat transfer problem in microchannels are studied, analytically. The heat transfer equations in the fluid and the finite thickness walls of the microchannel are solved analytically using uniform heat flux boundary conditions at the outer surfaces of the walls with appropriate continuity of temperature and heat flux at the fluid-wall interfaces. Exact analytic solutions for the velocity and temperature fields in the fluid and walls of microchannel are utilized to compute the entropy generation rate. The latter is integrated in the whole region of analysis so that the finite dimensions of the device are considered to get the global entropy generation rate. Finally, this quantity is discussed in detail and investigated considering combined effects of wall and hydrodynamic slip. Findings reveal that it is possible to find optimum values of heat flux across the walls of microchannel where the global entropy generation reaches a minimum. Special attention has been given to the effect of the wall heat flux on optimal values of other parameters. The optimum values of both the slip length and wall to fluid thermal conductivity ratio, where the entropy generation is minimum, decrease with the wall heat flux. Also, optimum values of Peclet number with minimal entropy are found for certain suitable combination of geometrical and physical parameters of the system.

Fluid Flow and Heat Transfer over a Permeable Stretching Cylinder

In this paper, we analyze the effects of thermo-physical properties on the axisymmetric flow of a viscous fluid induced by a stretching cylinder in the presence of internal heat generation/absorption. It is assumed that the cylinder is stretched in the axial direction with a linear velocity and the surface temperature of the cylinder is subjected to vary linearly. Here, the temperature dependent thermo-physical properties namely, the fluid viscosity and the fluid thermal conductivity are respectively assumed to vary as an inverse function of the temperature and a linear function of the temperature. The governing system of partial differential equations is converted into a system of coupled non-linear ordinary differential equations with variable coefficients. The resulting system is solved numerically using a second order finite difference scheme known as the Keller-box method. The governing equations of the problem show that the flow and heat transfer characteristics depend on six parameters, namely the curvature parameter, fluid viscosity parameter, injection/suction parameter, variable thermal conductivity parameter, heat source/sink parameter and the Prandtl number. The numerical values obtained for the velocity, temperature, skin friction, and the Nusselt number are presented through graphs and tables for several sets of values of the pertinent parameters. The results obtained for the flow and heat transfer characteristics reveal many interesting behaviors that warrant further study on the axisymmetric flow phenomena. Comparisons with the available results in the literature are presented as special cases.