Showing papers in "International Journal of Heat and Mass Transfer in 2014"

A critical review of the pseudopotential multiphase lattice Boltzmann model: Methods and applications

Abstract: This article presents a critical review of the theory and applications of a multiphase model in the community of the lattice Boltzmann method (LBM), the pseudopotential model proposed by Shan and Chen (1993) [4], which has been successfully applied to a wide range of multiphase flow problems during the past two decades. The first part of the review begins with a description of the LBM and the original pseudopotential model. The distinct features and the limitations of the original model are described in detail. Then various enhancements necessary to improve the pseudopotential model in terms of decreasing the spurious currents, obtaining high density/viscosity ratio, reducing thermodynamic inconsistency, unraveling the coupling between surface tension and equations of state (EOS), and unraveling the coupling between viscosity and surface tension, are reviewed. Then the fluid–solid interactions are presented and schemes to obtain different contact angles are discussed. The final section of this part focuses on the multi-component multiphase pseudopotential model. The second part of this review describes fruitful applications of this model to various multiphase flows. Coupling of this model with other models for more complicated multiple physicochemical processes are also introduced in this part.

Two-phase flow instabilities: A review

Abstract: An updated review of two-phase flow instabilities including experimental and analytical results regarding density-wave and pressure-drop oscillations, as well as Ledinegg excursions, is presented. The latest findings about the main mechanisms involved in the occurrence of these phenomena are introduced. This work complements previous reviews, putting all two-phase flow instabilities in the same context and updating the information including coherently the data accumulated in recent years. The review is concluded with a discussion of the current research state and recommendations for future works.

Experimental investigation of phase change material melting in rectangular enclosures with horizontal partial fins

Abstract: This paper presents an experimental investigation of phase change material (PCM) melting in a transparent rectangular enclosure with and without horizontal partial fins. The enclosure was heated isothermally from one side while the other walls were thermally insulated. Experiments were performed with wall temperatures of 55, 60 and 70 °C ( 3.6 × 10 8 ⩽ Ra ⩽ 8.3 × 10 8 ) for finned and unfinned enclosures. Visualization of the melting process and the temperature field were performed directly. Both qualitative and quantitative information about the melting phenomena were obtained using digital photographs of the instantaneous melt front evolutions and temperature recordings at the vertical mid-plane of the enclosure. Temperature histories revealed that the thermally stratified region became smaller as the number of fins increased. Experimental data were used to calculate melt fractions, heat transfer rates and Nusselt numbers during the melting process. Furthermore, two correlation equations were developed using the dimensionless parameters to predict the Nusselt number and melt fraction. Also, in order to evaluate the improved thermal performance of the enclosure in the presence of partial fins, two other parameters were defined, melting enhancement ratio and overall fin effectiveness. Experimental results indicated that increasing the number of fins decreased the melting time and increased the total heat transfer rate while the surface-averaged Nusselt number reduced. Melting enhancement ratio and overall fin effectiveness increased with increasing the number of fins and decreased with raising the wall temperature. Melting enhancement ratios decreased with time after reaching some maximum values indicating that partial fins are more beneficial during the initial time of the melting.

Investigation of entropy generation in MHD and slip flow over a rotating porous disk with variable properties

Abstract: In the present study the effects of magnetic interaction number, slip factor and relative temperature difference on velocity and temperature profiles as well as entropy generation in magnetohydrodynamic (MHD) flow of a fluid with variable properties over a rotating disk are investigated using numerical methods. The nonlinear governing equations of flow and thermal fields are reduced to ordinary differential equations by the Von Karman approach and are then solved numerically under the related boundary conditions. The results are compared with previous studies. The profiles for radial, tangential and axial velocities and temperature profiles, average entropy generation rate and average Bejan number are sketched for different values of magnetic interaction number, slip factor, relative temperature difference, suction/injection parameter and the effects of these parameters are discussed.

Numerical simulation of MHD nanofluid flow and heat transfer considering viscous dissipation

Abstract: In this paper nanofluid flow and heat transfer characteristics between two horizontal parallel plates in a rotating system are investigated. The effective thermal conductivity and viscosity of the nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. In this model the effect of Brownian motion on the effective thermal conductivity is considered. The basic partial differential equations are reduced to ordinary differential equations which are solved numerically using the fourth-order Runge–Kutta method. Comparison between the obtained results and previous works are well in agreement. Results show that the magnitude of the skin friction coefficient is an increasing function of the magnetic parameter, rotation parameter and Reynolds number and it is a decreasing function of the nanoparticle volume fraction. The Nusselt number increases with increase of nanoparticle volume fraction and Reynolds number but it decreases with increase of Eckert number, magnetic and rotation parameters.

A study on peristaltic flow of nanofluids: Application in drug delivery systems

Abstract: This paper studies the peristaltic flow of nanofluids through a two-dimensional channel. The analysis is conducted based on the long wavelength and low Reynolds number approximations. The walls of the channel surface propagate sinusoidally along the channel. The Buongiornio formulation for nanofluids is employed. Approximate analytical solutions for nanoparticle fraction field, temperature field, axial velocity, volume flow rate, pressure gradient and stream function are obtained. The impact of the pertinent physical parameters i.e. thermal Grashof number, basic-density Grashof number, Brownian motion parameter and thermophoresis parameter on nanoparticle fraction profile, temperature profile, velocity profile and trapping phenomenon are computed numerically. The results of this study demonstrate good correlation with the Newtonian results of Shapiro et al. (1969) [4], which is a special case ( Gr T = 0, Gr F = 0) of the generalized model developed in this article. Applications of the study include peristaltic micro-pumps and novel drug delivery systems in pharmacological engineering.

Heat transfer and thermal management with PCMs in a Li-ion battery cell for electric vehicles

Abstract: This paper investigates heat transfer with phase change materials (PCMs) in passive thermal management of electric and hybrid electric vehicles where the PCM is integrated with a Li-ion cell. When higher current is extracted from the Li-ion cells, heat is generated due to the ohmic law. Therefore, it is vital to design a successful thermal management system (TMS) to prevent excessive temperature increase and temperature excursion in the battery pack. During the phase change process, PCMs absorb heat and create a cooling effect. In the discharging (solidification) process, stored heat is released and it creates a heating effect. The case considered in this paper includes the use of PCMs with different thicknesses around the cells. Despite the small peripheral surface of the prismatic cell, the orthotropic property of Li-ion cells improves the planar heat transfer and effectiveness of the PCM around the cell. A numerical study is conducted using a finite volume-based method. The results show that the maximum temperature and temperature excursion in the cell are reduced when PCM is employed. The PCM with 12 mm thickness decreases the temperature by 3.0 K. The corresponding value for thinner layers of 3 mm, 6 mm and 9 mm are then obtained as 2.8 K, 2.9 K and 3.0 K respectively. Furthermore, the effect of the PCM on the cell temperature is more pronounced when the cooling system is under transient conditions. When a 3 mm-thick PCM is employed for the Li-ion cell, the temperature distribution becomes about 10% more uniform which is an important result in thermal management systems in electric vehicles.

Experimental investigation of the effect of inclination angle on convection-driven melting of phase change material in a rectangular enclosure

Abstract: This paper investigates the dynamic thermal behavior of phase change material (PCM) melting in a rectangular enclosure at various inclination angles. Lauric acid as a PCM with high Prandtl number (Pr ≈ 100) is used. The enclosure is heated isothermally from one side while the other walls are thermally insulated. Experiments were performed with hot wall temperatures of 55, 60 and 70 °C ( 3.6 × 10 8 ⩽ Ra ⩽ 8.3 × 10 8 ) for different inclination angles of 0°, 45° and 90°. Image processing of melt photographs along with recorded temperatures were used to calculate the melt fractions, Nusselt numbers and the local interfacial heat transfer rates at the solid–liquid interface. Qualitative time-dependent natural convection flow structures were deduced indirectly from the instantaneous shape of the solid–liquid interface which were confirmed by quantitative data from temperature measurements. The results reveal that the enclosure inclination has a significant effect on the formation of natural convection currents and consequently on the heat transfer rate and melting time of the PCM. As the inclination angle is decreased from 90° to 0°, the convection currents in the enclosure increases and chaotic flow structures appear. When melting commences in the horizontally inclined enclosure, the solid–liquid interface line becomes wavy which implies the formation of Benard convection cells in the liquid PCM. For the same hot wall temperatures, a decrease in inclination angle leads to a considerable enhancement in energy transport from the hot wall of the enclosure to the PCM. It is found that the heat transfer enhancement ratio for the horizontal enclosure is more than two times higher than that of the vertical enclosure.

An experimental investigation of enhanced pool boiling heat transfer from surfaces with micro/nano-structures

Abstract: Experiments on subcooled and saturated pool boiling of ethanol are conducted on horizontal heated surfaces with micro- and nano-sized structures. The effects of structure size on bubble nucleation and departure characteristics as well as the heat transfer coefficient are discussed. It is found that microstructures can enhance bubble nucleation by significantly increasing the active nucleation site density at low heat fluxes, thus reducing the wall superheat and enhancing heat flux; while nano-structures can accelerate bubble departure by decreasing bubble departure diameter and increasing departure frequency. On the other hand, nano-structures will delay bubble mergence and prevent the vapor film from spreading at high heat fluxes during boiling crisis.

Experimental and numerical study on melting of phase change materials in metal foams at pore scale

Abstract: An experimental study on the melting behavior of phase change material (PCM) in metal foams has been carried out at the pore scale. Paraffin wax was used as phase change material, in which aluminum foams were embedded to enhance the heat transfer. The temperature field and the melting evolution of the PCM at the pore scale were studied using an infrared camera and an optical microscope, respectively. The experimental results indicated that the metal foam is capable of enhancing the solid–liquid phase-change heat transfer, mainly because of the thermal conduction in the metal matrix. It was observed that the effect of the metal structure on the heat transfer is significant during the melting of PCM. A thermal lattice Boltzmann model with doubled populations was implemented to simulate the two-dimensional melting of the phase change material in metal foams. The numerical results agree well with the experimental observations qualitatively.

Effect of magnetic field on laminar convective heat transfer of magnetite nanofluids

Abstract: The effect of an external magnetic field on the convective heat transfer and pressure drop of magnetite nanofluids under laminar flow regime conditions (Re < 830) is investigated. Specifically, the influence of magnetic field strength and uniformity on the convective heat transfer coefficient is examined through experiments and supporting simulations of the magnetic flux density distribution and magnetic force acting on nanoparticles. The data show that large enhancement in the local heat transfer coefficient can be achieved by increasing the magnetic field strength and gradient. The convective heat transfer enhancement becomes more pronounced at higher Reynolds numbers, with a four-fold enhancement (i.e., relative to the case with no magnetic field) obtained at Re = 745 and magnetic field gradient of 32.5 mT/mm. The effect of the magnetic field on the pressure drop is not as significant. The pressure drop increases only by up to 7.5% when magnetic field intensity of 430 mT and gradients between 8.6 and 32.5 mT/mm are applied. Based on the simulation results of magnetic field and magnetic force distribution, the mechanisms for heat transfer enhancement are postulated to be accumulation of particles near the magnets (leading to higher thermal conductivity locally), and formation of aggregates acting enhancing momentum and energy transfer in the flow.

A CFD based thermo-hydraulic performance analysis of an artificially roughened solar air heater having equilateral triangular sectioned rib roughness on the absorber plate

Abstract: In this article, a numerical investigation is conducted to analyze the two-dimensional incompressible Navier–Stokes flows through the artificially roughened solar air heater for relevant Reynolds number ranges from 3800 to 18,000 Twelve different configurations of equilateral triangular sectioned rib ( P/e = 714–3571 and e/d = 0021–0042) have been used as roughness element The governing equations are solved with a finite-volume-based numerical method The commercial finite-volume based CFD code ANSYS FLUENT is used to simulate turbulent airflow through artificially roughened solar air heater The RNG k–e turbulence model is used to solve the transport equations for turbulent flow energy and dissipation rate A total numbers of 432,187 quad grid intervals with a near wall elements spacing of y + ≈ 2 are used Detailed results about average heat transfer and fluid friction in an artificially roughened solar air heater are presented and discussed The effects of grid distributions on the numerical predictions are also discussed It has been observed that for a given constant value of heat flux (1000 W/m 2 ), the performance of the artificially roughened solar air heater is strong function of the Reynolds number, relative roughness pitch and relative roughness height Optimum configuration of the roughness element for artificially roughened solar air heater is evaluated

Review of databases and predictive methods for heat transfer in condensing and boiling mini/micro-channel flows

Abstract: The importance of flow boiling and condensing mini/micro-channel devices to a large number of modern applications has spurred an unusually large number of research efforts that culminated in many types of predictive tools. These efforts have inadvertently contributed enormous confusion when selecting a suitable predictive method. This study reviews methods for predicting heat transfer in condensing and boiling mini/micro-channel flows. Systematic assessment of predictive accuracy of individual methods requires the development of consolidated mini/micro-channel databases for condensation heat transfer, dryout incipience quality, and saturated boiling heat transfer. These databases cover numerous working fluids and broad ranges of geometrical and flow parameters. It is shown that, despite the success of previous predictive methods for specific fluids and narrow databases, these methods are incapable of providing accurate predictions against entire consolidated databases. The consolidated databases are used to develop ‘universal’ correlations with very broad application range. These include two separate correlations for condensation heat transfer, one for annular flow and the other for slug and bubbly flows. Also developed are correlations for dryout incipience quality and saturated boiling heat transfer; the later is shown to accurately tackle both nucleate boiling dominated and convective boiling dominated regimes up to the location of incipient dryout.

Study on theory model of hydro-thermal–mechanical interaction process in saturated freezing silty soil

Abstract: The freezing of frost susceptible soils is a dynamic hydro-thermal–mechanical (THM) interacting process. One-side freezing experiments of saturated soil in an open system with no-pressure water supplement are carried out. In these experiments, we analyzed the influence of temperature gradients, overburden pressures and cooling temperatures on moisture migration and frost heave. Based on these experiments, a mathematical model of frost heave is proposed with the variables of temperature, porosity and displacement, in which Clapeyron equation is employed as the phase equilibrium condition of water and ice in soil. Ice lens is one of the major aspects of the frost heave for frost susceptible soils. According to the mechanical and physical characteristics, a comprehensive criterion for the formation and end of new ice lens is presented. To solve the nonlinear equations, the finite element algorithm is applied to solve the general form of governing equations. Finally, numerical simulations are implemented with the assistance of COMSOL. Validation of the model is illustrated by comparisons between the simulation and experimental results. From this study, it is found that, (1) cooling temperature is the necessary condition for moisture migration and frost heave since pore water phases into ice under cooling temperature. Then negative pore water pressure occurs in soil. Pore water pressure gradient is the direct driving force for water migration in saturated soil. However, temperature gradient and overburden pressure have important influence on the pore water pressure gradient. The response of soil sample to the variation of water content lags behind the response to the change of temperature. (2) Discontinuously distributed ice lenses form near the cold front when the accumulated water phases to ice. Temperature gradient, overburden pressure and cooling temperature are key factors to determine the frost heave and moisture migration. (3) Freezing and migration of unfrozen water cause the change of porosity in soil. Ice lens will block the migration of unfrozen water. The discrete ice lenses result in the oscillation in the distributions of water content. (4) Unsaturated phenomenon occurs in the unfrozen zone under the frozen fringe which might be related to the suction effect of large negative pressure in this zone.

Specific heat of nanofluids synthesized by dispersing alumina nanoparticles in alkali salt eutectic

Abstract: In this study, we report large enhancement in specific heat capacity of a eutectic salt mixture on dispersing alumina nanoparticles at 1% mass concentration Eutectic of lithium carbonate and potassium carbonate (62:38 by molar ratio) was dissolved in distilled water with alumina nanoparticles at 10% mass concentration and with a nominal diameter of ∼10 nm The specific heat capacity measurement was performed using a differential scanning calorimeter (DSC) An alternate model involving in situ phase transformation was proposed which was found to be in good agreement with the variations observed in the experimental data for the different types of nanomaterial samples used in this study These salt nanofluids can lead to the development of efficient thermal energy storage systems which in turn can enable significant reduction in the cost of solar power

Optimal concentration of alumina nanoparticles in molten Hitec salt to maximize its specific heat capacity

Abstract: The investigation experimentally studies the optimal concentration of alumina nanoparticles in doped molten Hitec that maximizes its specific heat capacity. A simplified model of the interfacial area is developed to explain the optimal concentration. The specific heat capacities of pure Hitec and nano-Hitec fluid are measured using a differential scanning calorimeter (DSC), and the microstructures following solidification are observed using a scanning electron microscope (SEM). A novel sampling apparatus and process for preparing molten Hitec nanofluids were developed to prevent the precipitation of nanoparticles. An optimal concentration of 0.063 wt.% is identified as yielding the greatest enhancement of specific heat capacity of 19.9%. At a concentration of 2 wt.%, the detrimental effect of the dopant nanoparticles on the specific heat capacity is evident at all temperatures. The negative effect is more significant than that predicted by the thermal equilibrium model. The SEM images following the solidification of samples and the developed model reveal the uniform dispersion of nanoparticles with negligible agglomeration at concentrations of under 0.016 wt.%. The agglomeration becomes significant and the particle clusters seem to be inter-connected at high concentrations. Moreover, the optimal concentration is approximately the concentration at which the contributions of isolated particles and clusters of sizes from 0.2 to 0.6 μm in the interfacial area to the specific heat capacity are equal.

Numerical study of MHD mixed convection in a nanofluid filled lid driven square enclosure with a rotating cylinder

Abstract: In this study, a numerical study of MHD mixed convection nanofluid filled lid driven square enclosure was performed The bottom wall of the cavity is heated and the top wall is kept at constant temperature lower than that of the heater Other walls of the square enclosure and cylinder surface are assumed to be adiabatic The governing equations are solved with finite element method The influence of the Richardson number ( 0001 ⩽ Ri ⩽ 10 ), Hartmann number ( 0 ⩽ Ha ⩽ 50 ), angular rotational speed of the cylinder ( - 10 ⩽ Ω ⩽ 10 ) and solid volume fraction of the nanoparticle ( 0 ⩽ ϕ ⩽ 005 ) on fluid flow and heat transfer are numerically investigated It is observed that 17% of heat transfer enhancement is obtained for Ri = 10 when compared to flow at Ri = 1 Averaged heat heat transfer decreases with increasing Hartmann number and 142% of heat transfer enhancement is obtained for Ω = - 10 compared to motionless cylinder case at Ω = 0 When the solid volume fraction of nanoparticle is increased, heat transfer increases

Entropy generation during Al2O3/water nanofluid flow in a solar collector: Effects of tube roughness, nanoparticle size, and different thermophysical models

Abstract: In this paper, an analytical study is performed on the entropy generation and heat transfer due to nanofluid flow in a flat plate solar collector. The working fluid considered in this work is Al 2 O 3 /water nanofluid with four different particle sizes, including 25, 50, 75, and 100 nm and volume concentrations up to 4%. Effects of tube roughness, nanoparticle size, and different thermophysical models are investigated on the Nusselt number, heat transfer coefficient, outlet temperature of the collector, entropy generation, and Bejan number. In addition, the effects of solar radiation and ambient temperature on entropy generation are examined. The results are presented for constant mass flow rates ranging from 0.1 to 0.8 kg/s. It is found that when the mass flow rate is considered to be constant for all working fluids, the Nusselt number and heat transfer coefficient have different trends. It is observed that uncertainties in thermophysical models and tube roughness have considerable effects on the values of heat transfer coefficient and Nusselt number. The findings show that with an increase in the volume fraction of nanofluid, the outlet temperature increases while with increasing the nanoparticle size a very insignificant decrease is observed in the outlet temperature. It is seen that the trend of changes in the outlet temperature is exactly in opposite to the Nusselt number trend. The analysis of entropy generation concludes that the entropy generation decreases with increasing the nanofluid concentration. It is found that the tube roughness increases the entropy generation and its effect is more visible at high mass flow rates while the effects of uncertainties in thermophysical models on entropy generation are not significant in any mass flow rate and volume fraction. Finally, a critical mass flow rate is determined under two different intensities of solar radiation and ambient temperature so that for the values higher than the critical mass flow rate the effects of roughness on entropy generation become important and should be considered.

A review of heat transfer and pressure drop characteristics of single and two-phase microchannels

Abstract: An impressive amount of investigations has been devoted to enhancing thermal performance of microchannels. The small size of microchannels and their ability to dissipate heat makes them as one of the best choices for the electronic cooling systems. In this paper, a comprehensive review of available studies regarding single and two-phase microchannels is presented and analyzed. 219 articles are reviewed to identify the heat transfer mechanisms and pressure drops in microchannels. This review looks into the different methodologies and correlations used to predict the heat transfer and pressure drop characteristics of microchannels along the channel geometries and flow regimes. The review shows that earlier studies (from 1982 to 2002) were largely conducted using experimental approaches, and discrepancies between analytical and experimental results were large, while more recent studies (from 2003 to 2013) used numerical simulations, correlations for predicting pressure drop and heat transfer coefficients were considerably more accurate. (C) 2014 Elsevier Ltd. All rights reserved. (Less)

MHD boundary layer flow of a nanofluid containing gyrotactic microorganisms past a vertical plate with Navier slip

Abstract: The combined effects of Navier slip and magnetic field on boundary layer flow with heat and mass transfer of a water-based nanofluid containing gyrotactic microorganisms over a vertical plate are investigated. Using Oberbeck–Boussinesq approximation and similarity transformation, the nonlinear model equations are obtained and tackled numerically to obtain the dimensionless velocity, temperature, nanoparticle concentration and density of motile microorganisms together with the reduced Nusselt, Sherwood and motile microorganism numbers. The present numerical results are compared with available data and are found in an excellent agreement. Pertinent results are presented graphically and discussed quantitatively with respect to variation in the controlling parameters. It is observed that the magnetic field suppresses the dimensionless velocity and increases the dimensionless temperature inside the boundary layers. The bioconvection parameters tend to reduce the concentration of the rescaled density of motile microorganisms. It is also found that the reduced Nusselt, Sherwood and density numbers of microorganisms depend strongly upon the magnetic, buoyancy, nanofluid and bioconvection parameters.

Thermal instability in a porous medium layer saturated by a nanofluid: A revised model

Abstract: We develop an extension of our previous thermal instability analysis of a nanofluid-saturated porous layer. The extension is based on a new boundary condition for the nanoparticle fraction, which is physically more realistic. In the previous model we imposed both temperature and nanoparticle volume fractions at the boundaries of the layer. It is now assumed that the value of the temperature can be imposed on the boundaries, but the nanoparticle fraction adjusts so that the nanoparticle flux is zero on the boundaries. The new boundary condition on the nanoparticle volume fraction is made possible by accounting for the contributions of the effect of thermophoresis to the nanoparticle flux. It is shown that, with the new boundary conditions, oscillatory convection cannot occur. The effect of the nanoparticles on non-oscillatory convection is destabilizing.

Radiation effects on Marangoni convection flow and heat transfer in pseudo-plastic non-Newtonian nanofluids with variable thermal conductivity

Abstract: In this paper, we examine radiation effects on Marangoni convection flow and heat transfer in pseudo-plastic non-Newtonian nanofluids driven by a temperature gradient. The surface tension is assumed to vary linearly with temperature. Four different types of nanoparticles; namely, Cu, Al2O3, CuO and TiO2, are considered with sodium carboxymethyl cellulose (CMC)–water used as a base fluid. The effects of power-law viscosity on temperature field are taken into account by assuming that the temperature field is similar to the velocity field and that the thermal conductivity of the non-Newtonian fluids is power-law-dependent on the velocity gradient. The governing partial differential equations are reduced to a series of ordinary differential equations using similarity transformations, the solutions are obtained numerically by the shooting method. The effects of the solid volume fraction, the Power-law Number, the Marangoni Number and the Radiation Number on the velocity and temperature fields are analyzed and discussed in detail.

Performance improvement of a nanofluid solar collector based on direct absorption collection (DAC) concepts

Abstract: Nanofluids are engineered colloidal suspensions of nanoparticles in base fluids, which have good properties of radiation absorption and heat transfer and are a kind of potential working fluids for solar collector based on direct absorption collection (DAC) concepts. A simulation model of nanofluid solar collector was built based on DAC concepts by solving the radiative transfer equations of particulate media and combining conduction and convection heat transfer equations. The system efficiency and temperature distributions are analyzed by considering the absorption and scattering of nanoparticles and the absorption of the matrix. The simulation results were in accordance with the experiments’. The nanofluids improved the outlet temperature and the efficiency by 30–100 K and by 2–25% than the base fluid. The photothermal efficiency of a 0.01% graphite nanofluid is 122.7% of that of a coating absorbing collector. The study indicated that nanofluids, even of low-content, have good absorption of solar radiation, and can improve the outlet temperatures and system efficiencies.

An experimental investigation of heat transfer enhancement of a minichannel heat sink using Al2O3–H2O nanofluid

Abstract: The thermal performances of a minichannel heat sink are experimentally investigated for cooling of electronics using nanofluid coolant instead of pure water. The Al2O3–H2O nanofluid including the volume fraction ranging from 0.10 to 0.25 vol.% was used as a coolant. The effects of different flow rates of the coolant on the overall thermal performances are also investigated. The flow rate was ranged from 0.50 to 1.25 L/min as well as the Reynolds number from 395 to 989. The coolant was passed through a custom made copper minichannel heat sink consisting of the channel height of 0.8 mm and the channel width of 0.5 mm. The experimental results showed the higher improvement of the thermal performances using nanofluid instead of pure distilled water. The heat transfer coefficient was found to be enhanced up to 18% successfully. The nanofluid significantly lowered the heat sink base temperature (about 2.7 °C) while it also showed 15.72% less thermal resistance at 0.25 vol.% and higher Reynolds number compared to the distilled water.

Heat transfer characteristics and pressure drop of COOH-functionalized DWCNTs/water nanofluid in turbulent flow at low concentrations

Abstract: In this paper, an experimental study is performed to assess the heat transfer characteristics and pressure drop of low concentrations of a new class of nanotubes, i.e. COOH-functionalized double-walled carbon nanotubes (DWCNTs) suspended in water under turbulent flow in a double tube heat exchanger. First, the thermal conductivity and viscosity of nanofluids at volume fractions of 0.01%, 0.02%, 0.05%, 0.1%, 0.2%, and 0.4% are measured and corresponding correlations are presented. Next, the heat transfer and pressure drop of the nanofluids through the double tube heat exchanger are evaluated. The results indicate that even the use of low concentration of the nanofluid, i.e. 0.4%, leads to a remarkable increase in heat transfer coefficient (by 32%) in comparison with the distilled water. On the other hand, the pressure drop due to using the volume fraction of 0.4% raised by 20%. Finally, analysis of the heat transfer and pressure drop data via thermal performance factor reveals that in spite of the pressure drop penalty, the COOH-functionalized DWCNTs/water nanofluid with volume fraction of 0.4% is a good option to use in the double tube heat exchanger.

Conjugate natural convection in a square porous cavity filled by a nanofluid using Buongiorno’s mathematical model

Abstract: Steady-state natural convection heat transfer in a square porous enclosure having solid walls of finite thickness and conductivity filled by a nanofluid using the mathematical nanofluid model proposed by Buongiorno is presented. The nanofluid model takes into account the Brownian diffusion and thermophoresis effects. The study is formulated in terms of the vorticity-stream function procedure. The governing equations were solved by finite difference method and solution of algebraic equations was made on the basis of successive under relaxation method. Effort has been focused on the effects of seven types of influential factors such as the Rayleigh and Lewis numbers, the buoyancy-ratio parameter, the Brownian motion parameter, the thermophoresis parameter, the thermal conductivity ratio, and solid walls thickness on the fluid flow and heat transfer. Streamlines, isotherms, isoconcentrations, local Nusselt and Sherwood numbers are presented. It has been found that the local Nusselt number at the solid-porous interface ( x = D ) is an increasing function of Ra , Nr and a decreasing function of Nt , Le and D . An effect of K r on Nu and Sh is non-monotonic. Ranges of key parameters for which a non-homogeneous model is more appropriate for the description of the system have been determined.

Thermodiffusion or Soret effect: Historical review

Abstract: Ever since the discovery of the Soret effect, numerous experimental and theoretical works have been accomplished to understand the phenomena in terms of scientific interpretation. There is published review articles existed focusing in different aspects of Soret effect such as, experimental techniques, theoretical approaches, and combination of both theoretical and experimental approaches. However, most of the articles focused on recent or short era only. It is essential for the novice researchers to have historical or chronological database of the works already done by others. In the contemporary study, a historical database of experimental and theoretical works has been offered. Major challenges, limitations, and technical hitches of performing experiments on ground (earth) condition has been selected, and cautiously deliberated. Contemporary experimental works performed in microgravity environment have been incorporated. All of the experimental methodologies and theoretical models have been addressed briefly. However, because of its popularity remarkable attention has been focused on the experiments with optical techniques, and theoretical models based on non-equilibrium thermodynamics.

An experimental method to simultaneously measure the dynamics and heat transfer associated with a single bubble during nucleate boiling on a horizontal surface

Abstract: The heat transfer mechanisms of nucleate boiling are associated with how the liquid–vapor phase and the surface temperature are distributed and interact beneath a single bubble on a heated surface. A comparative analysis of the hydrodynamic and thermal behavior of a single bubble may contribute greatly to the understanding of nucleate boiling heat transfer. In this paper, a technique to simultaneously measure the liquid–vapor phase boundary, temperature distribution, and heat transfer distribution at a boiling surface is described. The technique is fully synchronized in time and spatially resolved, and is applied to explore single-bubble nucleate boiling phenomena in a pool of water subcooled by 3 °C under atmospheric pressure. The temperature and heat flux distributions at the boiling surface are quantitatively interpreted in relation to the distribution and dynamics of the dry and wet areas, the triple contact line, and the microlayer underneath the single bubble. The results show that intensive wall heat transfer during single-bubble nucleate boiling exactly corresponds to the extended microlayer region. However, the overall contribution of the microlayer evaporation to the growth of a bubble is relatively small, and amounts to less than 17% of the total heat transport.

Analysis of heat transfer effects on gas production from methane hydrate by depressurization

Abstract: The dissociation of natural gas hydrates is an endothermic process. This dissociation process requires the continuous absorption of heat energy from the sediment and pore fluid. This heat transfer governs the dissociation rate and affects gas production. In this study, a two-dimensional axisymmetric simulator is developed to model the effects of heat transfer on the process of hydrate dissociation in porous media by depressurization. A series of simulations are performed to study sensible heat effects on the sediment, heat flow transfer in the cap- and base-sediment, and the effects of conductive and convective heat transfer on gas production from methane hydrate depressurization. The results show that the porous media material and the water content are two significant factors that affect the sensible heat in gas hydrate dissociation: the porous media material can increase methane hydrate dissociation, but water inhibits the dissociation process by affecting the pressure on the inner sediment. A high thermal conductivity of the sediment can initially positively affect hydrate dissociation but may later partially inhibit the process. Convective heat transfer in the gas flow increases hydrate dissociation markedly compared to that in water flow.

Pool boiling enhancement through microporous coatings selectively electrodeposited on fin tops of open microchannels

Abstract: Open microchannels and microporous coatings have been individually employed by previous investigators for enhancing pool boiling heat transfer. In this paper, their combined effect is investigated by electrodepositing microporous coatings on the fin tops of microchannels. The microporous coatings were applied using the optimal electrodeposition parameters developed in an earlier study. The effect of microchannel geometry on heat transfer performance for water boiling at atmospheric pressure on 10 mm × 10 mm copper chips is reported here. A maximum critical heat flux (CHF) of 3250 kW/m 2 was obtained for Chip 9 with fin width = 200 μm, channel width = 500 μm and channel depth = 400 μm at a wall superheat of 7.3 °C. A maximum value of heat transfer coefficient (HTC) of 995 kW/m 2 °C was achieved for Chip 12 with a different channel width of 762 μm for a heat flux of 2480 kW/m 2 at a wall superheat of 2.5 °C. Bubble growth and heat transfer processes are altered when nucleation takes place preferentially on the fin tops. Visual studies indicate a microconvective mechanism in which bubbles leaving from the fin tops induce strong localized liquid circulation currents in the microchannels. A liquid microcirculation based theoretical model is developed to predict heat transfer under this mechanistic description. The preliminary results are in good agreement with the experimental data.