Showing papers in "International Journal of Thermal Sciences in 2011"

Boundary layer flow of a nanofluid past a stretching sheet with a convective boundary condition

Abstract: The boundary layer flow induced in a nanofluid due to a linearly stretching sheet is studied numerically. The transport equations include the effects of Brownian motion and thermophoresis. Unlike the commonly employed thermal conditions of constant temperature or constant heat flux, the present study uses a convective heating boundary condition. The solutions for the temperature and nanoparticle concentration distributions depend on five parameters, Prandtl number Pr, Lewis number Le, the Brownian motion parameter Nb, the thermophoresis parameter Nt, and convection Biot number Bi. Numerical results are presented both in tabular and graphical forms illustrating the effects of these parameters on thermal and concentration boundary layers. The thermal boundary layer thickens with a rise in the local temperature as the Brownian motion, thermophoresis, and convective heating each intensify. The effect of Lewis number on the temperature distribution is minimal. With the other parameters fixed, the local concentration of nanoparticles increases as the convection Biot number increases but decreases as the Lewis number increases. For fixed Pr, Le, and Bi, the reduced Nusselt number decreases but the reduced Sherwood number increases as the Brownian motion and thermophoresis effects become stronger.

Magnetic field effect on natural convection in a nanofluid-filled square enclosure

Abstract: This paper examines the natural convection in an enclosure that is filled with a water-Al2O3 nanofluid and is influenced by a magnetic field. The enclosure is bounded by two isothermal vertical walls at temperatures Th and Tc and by two horizontal adiabatic walls. Based upon numerical predictions, the effects of pertinent parameters such as the Rayleigh number (103 ≤ Ra ≤ 107), the solid volume fraction (0 ≤ ϕ ≤ 0.06) and the Hartmann number (0 ≤ Ha ≤ 60) on the flow and temperature fields and the heat transfer performance of the enclosure are examined. Prandtl number is considered to be Pr = 6.2. The results show that the heat transfer rate increases with an increase of the Rayleigh number but it decreases with an increase of the Hartmann number. An increase of the solid volume fraction may result in enhancement or deterioration of the heat transfer performance depending on the value of Hartmann and Rayleigh numbers.

Viscosity and thermal conductivity of nanofluids containing multi-walled carbon nanotubes stabilized by chitosan

Abstract: Thermal conductivity, viscosity, and stability of nanofluids containing multi-walled carbon nanotubes (MWCNTs) stabilized by cationic chitosan were studied. Chitosan with weight fraction of 0.1%, 0.2 wt%, and 0.5 wt% was used to disperse stably MWCNTs in water. The measured thermal conductivity showed an enhancement from 2.3% to 13% for nanofluids that contained from 0.5 wt% to 3 wt% MWCNTs (0.24 to 1.43 vol %). These values are significantly higher than those predicted using the Maxwell's theory. We also observed that the enhancements were independent of the base fluid viscosity. Thus, use of microconvection effect to explain the anomalous thermal conductivity enhancement should be reconsidered. MWCNTs can be used either to enhance or reduce the fluid base viscosity depending on the weight fractions. In the viscosity-reduction case, a reduction up to 20% was measured by this work. In the viscosity-enhancement case, the fluid behaved as a non-Newtonian shear-thinning fluid. By assuming that MWCNT nanofluids behave as a generalized second grade fluid where the viscosity coefficient depends upon the rate of deformation, a theoretical model has been developed. The model was found to describe the fluid behavior very well.

A review of thermal rectification observations and models in solid materials

Abstract: Thermal rectification is a phenomenon in which thermal transport along a specific axis is dependent upon the sign of the temperature gradient or heat current. This phenomenon offers improved thermal management of electronics as size scales continue to decrease and new technologies emerge by having directions of preferred thermal transport. For most applications where thermally rectifying materials could be of use they would need to exhibit one direction with high thermal conductivity to allow for efficient transport of heat from heat generating components to a sink and one direction with low conductivity to insulate the temperature and heat flux sensitive components. In the process of understanding and developing these materials multiple mechanisms have been found which produce thermally rectifying behavior and much work has been and is being done to improve our understanding of the mechanisms and how these mechanisms can be used with our improved ability to fabricate at the nanoscale to produce efficient materials which have high levels of thermal rectification.

Effect of aggregation on the viscosity of copper oxide–gear oil nanofluids

Abstract: Results on viscosity of the stable nanofluids, prepared by dispersing 40 nm diameter spherical CuO nanoparticles in gear oil are presented. Viscosity of the studied nanofluids displays strong dependence both on CuO loading in the base fluid, as well as, on temperature between 10 and 80 °C. Presence of aggregated CuO nanoparticles in the fluid, with average cluster size ∼7 times the primary diameter of CuO nanoparticles, have been confirmed by DLS data. Viscosity of the nanofluids is enhanced by ∼3 times of the base fluid with CuO volume fraction of 0.025, while it decreases significantly with the rise of temperature. Newtonian behavior of the gear oil changes to non-Newtonian with increase of CuO loading. Shear thinning is observed for nanofluids containing CuO volume fraction >0.005. CuO volume fraction dependence of the viscosity of CuO–gear oil nanofluids is predicted well using the modified Krieger–Dougherty equation derived taking into account the aggregation mechanism. Temperature variation of the nanofluid viscosity agrees very well with the modified Andrade equation, reported by Chen et al.

A moving finite line source model to simulate borehole heat exchangers with groundwater advection

Abstract: Available analytical models for the thermal analysis of ground source heat pumps (GSHPs) either neglect groundwater flow or axial effects. In the present study a new analytical approach which considers both effects is developed. Comparison with existing analytical solutions based on the finite and infinite line source theory is carried out. This study shows that in general the heat transfer at the borehole heat exchanger (BHE) is affected by groundwater flow and axial effects. The latter is even more important for long simulation times and short borehole lengths. At the borehole wall the influence of the axial effect is restricted to Peclet numbers lower than 10, assuming the BHE length as characteristic length. Moreover, the influence of groundwater flow is negligible for Peclet numbers lower than 1.2. As a result for Peclet numbers between 1.2 and 10 the combined effect of groundwater flow and axial effects has to be accounted for when evaluating the temperature response of a BHE at the borehole wall and thus the use of the moving finite line source model is required.

A numerical study on heat transfer and friction factor characteristics of laminar flow in a circular tube fitted with center-cleared twisted tape

Abstract: Twisted tape is a widely used technique for heat transfer enhancement. In the present paper, we proposed a center-cleared twisted tape aiming at acxhieving good thermohydraulic performance. A comparative study between this type and the short-width twisted tape was performed numerically in laminar tubular flows. The computation results demonstrated that the flow resistance can be reduced by both methods; however, the thermal behaviors are very different from each other. For tubes with short-width twisted tapes, the heat transfer and thermohydraulic performance are weakened by cutting off the tape edge. Contrarily, for tubes with center-cleared twisted tapes, the heat transfer can be even enhanced in the cases with a suitable central clearance ratio. The thermal performance factor of the tube with center-cleared twisted tape can be enhanced by 7–20% as compared with the tube with conventional twisted tape. All these demonstrated that the center-cleared twisted tape is a promising technique for laminar convective heat transfer enhancement.

An experimental study of flow friction and heat transfer in wavy microchannels with rectangular cross section

Abstract: Experimental investigation has been conducted on the flow friction and heat transfer in sinusoidal microchannels with rectangular cross sections. The microchannels considered consist of ten identical wavy units with average width of about 205 μm, depth of 404 μm, wavelength of 2.5 mm and wavy amplitude of 0–259 μm. Each test piece is made of copper and contains 60–62 wavy microchannels in parallel. Deionized water is employed as the working fluid and the Reynolds numbers considered range from about 300 to 800. The experimental results, mainly the overall Nusselt number and friction factor, for wavy microchannels are compared with those of straight baseline channels with the same cross section and footprint length. It is found that the heat transfer performance of the present wavy microchannels is much better than that of straight baseline microchannels; at the same time the pressure drop penalty of the present wavy microchannels can be much smaller than the heat transfer enhancement. Conjugate simulation based on the classical continuum approach is also carried out for similar experimental conditions, the numerical results agree reasonably well with experimental data.

Numerical investigation on nanofluids turbulent convection heat transfer inside a circular tube

Abstract: In this paper, turbulent forced convection flow of water–Al2O3 nanofluid in a circular tube, subjected to a constant and uniform heat flux at the wall, is numerically analyzed. Two different approaches are taken into account: single and two-phase models, with particle diameter equal to 38 nm. It is observed that convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement increases with the particle volume concentration and Reynolds number. Comparisons with correlations present in literature are accomplished and a very good agreement is realized.

Convective heat transfer in the flow of viscous Ag–water and Cu–water nanofluids over a stretching surface

Abstract: An analysis is carried out to study the convective heat transfer in a nanofluid flow over a stretching surface. In particular, we focus on Ag–water and Cu–water nanofluids, and investigate the effects of the nanoparticle volume fraction on the flow and heat transfer characteristics under the influence of thermal buoyancy and temperature dependent internal heat generation or absorption. The numerical results indicate that an increase in the nanoparticle volume fraction will decrease the velocity boundary layer thickness while increasing the thermal boundary layer thickness, even in the presence of free convection currents and internal heat generation. Meanwhile, the presence of nanoparticles results in an increase in the magnitude of the skin friction along the surface and a decrease in the magnitude of the local Nusselt number. Such effects are found to be more pronounced in the Ag–water solution than in the Cu–water solution; indeed, the Ag–water solution decreases the boundary layer thickness more than that of the Cu–water solution.

Double-diffusive natural convective boundary-layer flow of a nanofluid past a vertical plate

Abstract: The double-diffusive natural convective boundary-layer flow of a nanofluid past a vertical plate is studied analytically. The model used for the binary nanofluid incorporates the effects of Brownian motion and thermophoresis. In addition the thermal energy equations include regular diffusion and cross-diffusion terms. A similarity solution is presented. Numerical calculations were performed in order to obtain correlation formulas giving the reduced Nusselt number as a function of the various relevant parameters.

Falkner–Skan problem for a static or moving wedge in nanofluids

Abstract: The steady two-dimensional boundary layer flow past a static or a moving wedge immersed in nanofluids is investigated numerically. An implicit finite difference scheme known as the Keller-box method and the NAG routine DO2HAF are used to obtain the numerical solutions. Three different types of nanoparticles, namely copper Cu, alumina Al2O3 and titania TiO2 with water as the base fluid are considered. The effects of the governing parameters on the fluid flow and heat transfer characteristics are analyzed and discussed. It is found that Cu-water has the highest skin friction coefficient and the heat transfer rate at the surface compared with the others. The effect of the solid volume fraction of nanoparticles on the fluid flow and heat transfer characteristics is found to be more pronounced compared to the type of the nanoparticles.

Comparative analysis of single and two-phase models for CFD studies of nanofluid heat transfer

Abstract: CFD predictions of laminar mixed convection of Al2O3–water nanofluids by single-phase and three different two-phase models (volume of fluid, mixture, Eulerian) are compared. The elliptical, coupled, steady-state, three-dimensional governing partial differential equations for laminar mixed convection in a horizontal tube with uniform heat flux are solved numerically using the finite volume approach. It is found that single-phase and two-phase models predict almost identical hydrodynamic fields but very different thermal ones. The predictions by the three two-phase models are essentially the same. For the problem under consideration the two-phase models give closer predictions of the convective heat transfer coefficient to the experimental data than the single-phase model; nevertheless, the two-phase models over-predict the enhancement of the convective heat transfer coefficient resulting from the increase of the alumina volume fraction. The results are calculated for two Reynolds numbers (1050 and 1600) and three nanoparticle volume concentrations (<2%). Although single-phase and two-phase models have been used before to analyze mixed convection of nanofluids, this is the first systematic comparison of their predictions for a laminar mixed convection flow which includes the hydrodynamic characteristics and the effect of temperature dependent properties.

Effects of nanowire height on pool boiling performance of water on silicon chips

Abstract: A new technique is developed to directly grow Cu nanowire (CuNW) on Si substrate with electrochemical deposition to produce height-controlled hydrophilic nanowired surfaces for enhancing pool boiling performance. For broader heat transfer applications, CuNW and Si nanowires (SiNW) with various nanowire heights were fabricated and examined under pool boiling with water. The heat transfer performance of the samples with NW arrays is enhanced with increasing NW heights regardless of the NW materials. The surface with the tallest NW structure (35 mm-tall SiNW) yielded a heat flux of 134 W/cm 2 at 23 K wall superheat, about 300% higher than a plain Si surface at the same wall superheat.

A review of heat transfer between concentric rotating cylinders with or without axial flow

Abstract: Heat transfer in flow between concentric rotating cylinders, also known as Taylor–Couette flows, constitutes a long-existing academic and industrial subject (in particular for electric motors cooling). Heat transfer characteristics of those flows are reviewed. Investigations of previous works for different gap thickness, axial and radial ratio, rotational velocity are compared. Configurations with axial flow and/or with slots on the cylinders are also considered. For each case, different correlations are presented. Finally, unresolved issues are mentioned for further research.

Natural convection flow of a nanofluid over a vertical plate with uniform surface heat flux

Abstract: Natural convective flow of a nanofluid over a vertical plate with a constant surface heat flux is investigated numerically, following a similarity analysis of the transport equations. The transport model employed includes the effect of Brownian motion and thermophoresis. The analysis shows that velocity, temperature and concentration profiles in the respective boundary layers depend, besides the Prandtl and Lewis numbers, on three additional dimensionless parameters, namely a Brownian motion parameter Nb, a thermophoresis parameter Nt, a buoyancy ratio parameter Nr. In addition to the study of these parameters on the boundary layer flow characteristics (velocity, temperature, nanoparticle concentration, skin friction, and heat transfer), correlations for the Nusselt and Sherwood numbers have been developed based on a regression analysis of the data. These correlations predict the numerical results with a maximum error of 5.5% for the reduced Nusselt number and 3.2% for the reduced Sherwood number.

An analytical solution for boundary layer flow of a nanofluid past a stretching sheet

Abstract: In this paper, the problem of boundary layer flow of a nanofluid past a stretching sheet has been investigated analytically by using the Homotopy Analysis Method. Both the effects of Brownian motion and thermophoresis are considered simultaneously. An analytical solution is presented which depends on the Prandtl number Pr, Lewis number Le, Brownian motion number Nb and thermophoresis number Nt. The results show that the reduced Nusselt number is a decreasing function of each dimensionless number, while the reduced Sherwood number is an increasing function of higher Pr and a decreasing function of lower Pr number for each Le, Nb and Nt numbers like the results presented by Khan and Pop. Contrary the results presented by Khan and Pop, It is found that the reduced Nusselt number decreases with the increase in Pr for many Nb numbers. However for a special Nb, there are conversely interesting results that are clearly discussed in this paper.

Lattice Boltzmann simulation of natural convection heat transfer of Al2O3/water nanofluids in a square enclosure

Abstract: In the present study, mathematical modeling is performed to simulate natural convection of Al 2 O 3 /water nanofluids in a vertical square enclosure using the lattice Boltzmann method (LBM) Results indicate that the average Nusselt number increases with the increase of Rayleigh number and particle volume concentration The average Nusselt number with the use of nanofluid is higher than the use of water under the same Rayleigh number However, the heat transfer rate of the nanofluid takes on a lower value than water at a fixed temperature difference across the enclosure mainly due to the significant enhancement of dynamic viscosity Furthermore, great deviations of computed Nusselt numbers using different models associated with the physical properties of a nanofluid are revealed The present results are well validated with the works available in the literature and consequently LBM is robust and promising for practical applications

Evaluating the influence of thermal dispersion on temperature plumes from geothermal systems using analytical solutions

Abstract: An analytical study is carried out to examine the effect of thermal dispersion on the simulation of temperature plumes in aquifers that evolve from vertical ground source heat pump (GSHP) systems. Analytical solutions for the simulation of heat transport in aquifers often ignore thermal dispersion. In this study an existing two-dimensional analytical approach for transient conditions is used. Moreover, an equation to calculate the length of the temperature plume for steady state conditions is developed. To study the interplay between thermal dispersion and hydraulic conductivity, Darcy velocities are varied from 10−8 m/s to 10−5 m/s and thermal dispersivities are varied based on two assumptions: 1) thermal dispersion is assumed to be only dependent on the Darcy velocity and 2) thermal dispersion is assumed to be scale-dependent. The results are discussed with respect to their implications for typical legal regulations and operation of such GSHP systems. In general, the effect of thermal dispersion on the temperature plume around the borehole heat exchanger (BHE) is minor when thermal dispersion is assumed to be depending solely on the magnitude of groundwater flow (e.g., in a homogeneous aquifer). On the other hand, based on a field scale of 10 m and assuming thermal dispersion to be scale-dependent, thermal dispersion can be neglected only for conditions typical for fine sands, clays, and silts with q 10−8 m/s) dominate, thermal dispersion has a larger effect on the temperature plume distribution around the borehole heat exchanger.

Multi-component droplet heating and evaporation: Numerical simulation versus experimental data

Abstract: The earlier reported simplified model for multi-component droplet heating and evaporation is generalised to take into account the coupling between droplets and the ambient gas. The effects of interaction between droplets are also considered. The size of the gas volume, where the interaction between droplets and gas needs to be taken into account, is estimated based on the characteristic thermal and mass diffusion scales. The model is applied to the analysis of the experimentally observed heating and evaporation of monodispersed n-decane/3-pentanone mixture droplets at atmospheric pressure. It is pointed out that the effect of coupling leads to noticeably better agreement between the predictions of the model and the experimentally observed average droplet temperatures. In most cases, the observed droplet temperatures lie between the average and central temperatures, predicted by the coupled solution. For the cases reported in this study, the observed time evolution of droplet radii cannot be used for the validation of the model. It is pointed out that the number of terms in the series in the expressions for droplet temperature and species mass fraction can be reduced to just three, with possible errors less than about 0.5%. In this case, the model can be recommended for the implementation into computational fluid dynamics (CFD) codes and used for various engineering applications, including those in internal combustion engines.

Free convection in a triangle cavity filled with a porous medium saturated with nanofluids with flush mounted heater on the wall

Abstract: Steady-state free convection heat transfer behavior of nanofluids is investigated numerically inside a right-angle triangular enclosure filled with a porous medium. The flush mounted heater with finite size is placed on the left vertical wall. The temperature of the inclined wall is lower than the heater, and the rest of walls are adiabatic. The governing equations are obtained based on the Darcy’s law and the nanofluid model proposed by Tiwari and Das [1] . The transformed dimensionless governing equations were solved by finite difference method and solution for algebraic equations was obtained through Successive Under Relaxation method. Investigations with three types of nanofluids were made for different values of Rayleigh number Ra of a porous medium with the range of 10 ≤ Ra ≤ 1000, size of heater Ht as 0.1 ≤ Ht ≤ 0.9, position of heater Y p when 0.25 ≤ Y p ≤ 0.75, enclosure aspect ratio AR as 0.5 ≤ AR ≤ 1.5 and solid volume fraction parameter ϕ of nanofluids with the range of 0.0 ≤ ϕ ≤ 0.2. It is found that the maximum value of average Nusselt number is obtained by decreasing the enclosure aspect ratio and lowering the heater position with the highest value of Rayleigh number and the largest size of heater. It is further observed that the heat transfer in the cavity is improved with the increasing of solid volume fraction parameter of nanofluids at low Rayleigh number, but opposite effects appear when the Rayleigh number is high.

Multiple solutions of heat and mass transfer of MHD slip flow for the viscoelastic fluid over a stretching sheet

Abstract: In this paper we investigate the magnetohydrodynamic slip flow of an electrically conducting, viscoelastic fluid past a stretching surface. The main concern is to analytically investigate the structure of the solutions and determine the thresholds beyond which multiple solutions exist or the physical pure exponential type solution ceases to exist. In the case of the presence of multiple solutions, closed-form formulae for the boundary layer equations of the flow are presented for two classes of viscoelastic fluid, namely, the second-grade and Walter’s liquid B fluids. Heat transfer analysis is also carried out for two general types of boundary heating processes, either by a prescribed quadratic power-law surface temperature or by a prescribed quadratic power-law surface heat flux. The flow field is affected by the presence of physical parameters, such as slip, viscoelasticity, magnetic and suction/injection parameters, whereas the temperature field is additionally affected by thermal radiation, heat source/sink, Prandtl and Eckert numbers. The regions of existence or non-existence of unique/multiple solutions sketched by the combination of these parameters are initially worked out by providing critical values and then velocity/temperature profiles and skin friction coefficient/Nusselt number are examined and discussed.

Thermal performance comparison of oscillating heat pipes with SiO2/water and Al2O3/water nanofluids

Abstract: Thermal performances of two same oscillating heat pipes (OHPs) charged with SiO 2 /water and Al 2 O 3 /water nanofluids, respectively, were investigated experimentally. Both the average evaporator wall temperature and the overall thermal resistance of the OHPs at different nanoparticle mass concentrations (0–0.6 wt% for silica nanofluids and 0–1.2 wt% for alumina nanofluids) and at the volume filling ratio of 50% were tested and compared. Experimental results showed that different nanofluids caused different thermal performances of OHPs. Within the experimental range, using the alumina nanofluid instead of pure water enhanced the heat transfer of the OHP (reductions in the evaporator wall temperature and thermal resistance of the OHP of about 5.6 °C (or 8.7%) and 0.057 °C/W (or 25.7%), respectively, were obtained), while using the silica nanofluid instead of pure water deteriorated the thermal performance of the OHPs (with the evaporator wall temperature and the thermal resistance of the OHP being increased by 3.5 °C (or 5.5%) and 0.075 °C/W (or 23.7%), respectively). A preliminary analysis was conducted for the different effects induced by the addition of different nanoparticles to pure water, and it was found that the change of surface condition at the evaporator and condenser due to different nanoparticle deposition behaviors was the main reason for the thermal performance improvement or deterioration of the OHPs charged with different nanofluids.

On exergy calculations of seawater with applications in desalination systems

Abstract: Exergy analysis is a powerful diagnostic tool in thermal systems performance evaluation. The use of such an analysis in seawater desalination processes is of growing importance to determine the sites of the highest irreversible losses. In the literature, exergy analyses of seawater desalination systems have sometimes modeled seawater as sodium chloride solutions of equivalent salt content or salinity; however, such matching does not bring all important properties of the two solutions into agreement. Furthermore, a common model that represents seawater as an ideal mixture of liquid water and solid sodium chloride may have serious shortcomings. Therefore, in this paper, the most up-to-date thermodynamic properties of seawater, as needed to conduct an exergy analysis, are given as correlations and tabulated data. The effect of the system properties as well as the environment dead state on the exergy and flow exergy variation is investigated. In addition, an exergy analysis for a large MSF distillation plant is performed using plant operating data and results previously published using the above-mentioned ideal mixture model. It is demonstrated that this ideal mixture model gives flow exergy values that are far from the correct ones. Moreover, the second law efficiency differs by about 80% for some cases.

Natural convection in wavy enclosures with volumetric heat sources

Abstract: In this paper, the effects of volumetric heat sources on natural convection heat transfer and flow structures in a wavy-walled enclosure are studied numerically. The governing differential equations are solved by an accurate finite-volume method. The vertical walls of enclosure are assumed to be heated differentially whereas the two wavy walls (top and bottom) are kept adiabatic. The effective governing parameters for this problem are the internal and external Rayleigh numbers and the amplitude of wavy walls. It is found that both the function of wavy wall and the ratio of internal Rayleigh number (RaI) to external Rayleigh number (RaE) affect the heat transfer and fluid flow significantly. The heat transfer is predicted to be a decreasing function of waviness of the top and bottom walls in case of ( Ra I / Ra E ) > 1 and ( Ra I / Ra E ) 1 .

Analytical and numerical prediction of heat transfer and pressure drop in open-cell metal foams

Abstract: Enhanced cooling methods are needed for advanced power systems. A promising method is using an open-cell metal foam to improve the heat transfer rates. However, the pressure drop induced by the metal foams is relatively higher and thus becomes a critical issue in engineering applications. The focus of this research is the modeling and simulation of heat transfer enhancement and corresponding pressure drop. A simplified analytical model based on diamond-shaped unit cells has been developed to predict the heat transfer capability of a foamed channel. The heat transfer rates predicted by the analytical model have been compared with available experimental data from other researchers and favorable agreements have been obtained. To evaluate the pressure drop in metal foams, a unit-cell CFD model was built using software package Fluent. The model is based on a structure of sphere-centered open-cell tetrakaidecahedron, which is very similar to the actual microstructure of an aluminum metal foam. Flow patterns and grid independence are investigated and simulation results are shown to agree well with experimental data.

Experimental investigations of flow boiling heat transfer and pressure drop in straight and expanding microchannels – A comparative study

Abstract: Flow boiling experiments were conducted in straight and expanding microchannels with similar dimensions and operating conditions. Deionized water was used as the coolant. The test vehicles were made from copper with a footprint area of 25 mm × 25 mm. Microchannels having nominal width of 300 μm and a nominal aspect ratio of 4 were formed by wire cut Electro Discharge Machining process. The measured surface roughness (Ra) was about 2.0 μm. To facilitate easier comparison with the straight microchannels and also to simplify the method of fabrication, the expanding channels were formed with the removal of fins at selected location from the straight microchannel design, instead of using a diverging channel. Tests were performed on both the microchannels over a range of mass fluxes, heat fluxes and an inlet temperature of 90 °C. It was observed that the two-phase pressure drop across the expanding microchannel heat sink was significantly lower as compared to its straight counterpart. The pressure drop and wall temperature fluctuations were seen reduced in the expanding microchannel heat sink. It was also noted that the expanding microchannel heat sink had a better heat transfer performance than the straight microchannel heat sink, under similar operating conditions. This phenomenon in expanding microchannel heat sink, which was observed in spite of it having a lower convective heat transfer area, is explained based on its improved flow boiling stability that reduces the pressure drop oscillations, temperature oscillations and hence partial dry out.

Lattice Boltzmann simulation of natural convection dominated melting in a rectangular cavity filled with porous media

Abstract: A thermal lattice Boltzmann model is developed for the melting with natural convection in porous media at the representative elementary volume scale An evolution equation of the temperature distribution function is constructed through selecting the equilibrium distribution function and non-linear source term properly Simulations of melting with natural convection in a cavity with and without a porous matrix are performed using the present model Numerical results show good agreement with previous analytical, experimental and numerical solutions In addition, the analysis of the melting process over a wider range of dimensionless parameters indicates that for conditions of high Darcy number and high porosity the effect of natural convection on the melting becomes stronger, and the Rayleigh number based on porous media (Ram) proposed in the previous studies may not be appropriate to correlate the average hot wall Nusselt number independently The present model is also suitable for simulating freezing and solidification in porous media without modification

Natural convection of nanofluids in a shallow cavity heated from below

Abstract: This paper reports an analytical and numerical study of natural convection in a shallow rectangular cavity filled with nanofluids. Neumann boundary conditions for temperature are applied to the horizontal walls of the enclosure, while the two vertical ones are assumed insulated. The governing parameters for the problem are the thermal Rayleigh number, Ra, the Prandtl number Pr, the aspect ratio of the cavity, A and the solid volume fraction of nanoparticles, F. For convection in an infinite layer ðA[1Þ, analytical solutions for the stream function and temperature are obtained using a parallel flow approximation in the core region of the cavity and an integral form of the energy equation. The critical Rayleigh number for the onset of supercritical convection of nanofluids is predicted explicitly by the present model. Furthermore, a linear stability analysis of the parallel flow solution is studied and the threshold for Hopf bifurcation is determined. Also, results are obtained from the analytical model for finite amplitude convection for which the flow and heat transfer is presented in terms of the governing parameters of the problem. Numerical solutions of the full governing equations are obtained for a wide range of the governing parameters. A good agreement is observed between the analytical model and the numerical simulations.

Convective heat transfer of non-Newtonian nanofluids through a uniformly heated circular tube

Abstract: Three kinds of nanofluids were prepared by dispersing γ-Al2O3, CuO, and TiO2 nanoparticles in an aqueous solution of carboxymethyl cellulose (CMC). The forced convective heat transfer of these nanofluids through a uniformly heated circular tube under turbulent flow conditions was investigated experimentally. The base fluid and all nanofluids show pseudoplastic (shear-thinning) rheological behavior. Results reveal that the local and average heat transfer coefficients of nanofluids are larger than that of the base fluid. Heat transfer enhancement of nanofluids increases with an increase in nanoparticle concentration. Similar trend are demonstrated for Nusselt number of nanofluids. For a given nanoparticle concentration and Peclet number, the local heat transfer coefficient of the base fluid and that of the nanofluids decreases with the axial distance from the tube inlet. A new correlation is proposed to predict successfully the Nusselt number of non-Newtonian nanofluids as a function of the Reynolds and the Prandtl numbers.