Showing papers in "International Journal of Heat and Mass Transfer in 2015"
TL;DR: In this paper, the effects of Brownian motion on the effective viscosity and thermal conductivity of nanofluid were investigated. And the results were presented graphically in terms of streamlines, isotherms and isokinetic energy.
Abstract: In this paper magnetohydrodynamics nanofluid hydrothermal treatment in a cubic cavity heated from below is presented. The mathematical model consists of continuity and the momentum equations, while a new model is proposed to see the effects Brownian motion on the effective viscosity and thermal conductivity of nanofluid. The Lattice Boltzmann method is utilized to simulate three dimensional problems. The Koo–Kleinstreuer–Li correlation is also taken into account. Numerical calculation is made for different values of Hartmann number, nanoparticle volume fraction and Rayleigh number. The results are presented graphically in terms of streamlines, isotherms and isokinetic energy as well as Nusselt number. It is observed that the applying magnetic field results in a force opposite to the flow direction that leads to drag the flow and then reduces the convection currents by reducing the velocities. Also it can be concluded that Nusselt number is an increasing function of Rayleigh number and nanofluid volume fraction while it is a decreasing function of Hartmann number.
TL;DR: In this article, a mathematical model is analyzed in order to study the natural convection boundary layer flow along an inverted cone, where the shape of nanosize particles on entropy generation with based fluid is considered.
Abstract: In this paper, a mathematical model is analyzed in order to study the natural convection boundary layer flow along an inverted cone. The shape of nanosize particles on entropy generation with based fluid is considered. Simultaneous effects of porous medium, magnetohydrodynamics, radiation and power law index effects are also taken into account. Hamilton–Crosser model is used for the effective thermal conductivity. The nonlinear coupled equations under the assumption of Boussinesq approximation are solved analytically. The calculations are performed for different governing parameters such as Prandtl number, Rayleigh number, power law index, porosity parameter, radiation parameter and magnetic parameter. The physical interpretations of obtained results are illustrated by graphs and tables. In addition, correlation of Nusselt number and skin friction corresponding to active parameters are also analyzed in this investigation.
TL;DR: In this paper, an FE model on Selective Laser Melting (SLM) fabrication process is presented, which considers powder-to-solid transition together with an effective method to achieve volume shrinkage and material removal.
Abstract: The Selective Laser Melting (SLM) fabrication process is complex and it is crucial to understand the phenomena that will occur to better control it. In this paper, an FE model on SLM that considers powder-to-solid transition together with an effective method to achieve volume shrinkage and material removal has been created. Experiments were conducted to validate the model. A detailed discussion on the progression of the melt pool and the rate of temperature change has been made. Parametric study with the laser power and scan speed as variables has been conducted to identify their relationships with the melt dimensions, melting and evaporation of powder and rates of temperature change.
TL;DR: Li et al. as mentioned in this paper presented a hybrid thermal lattice Boltzmann (LB) model to simulate thermal multiphase flows with phase change based on an improved pseudopotential LB approach.
Abstract: A hybrid thermal lattice Boltzmann (LB) model is presented to simulate thermal multiphase flows with phase change based on an improved pseudopotential LB approach (Li et al., 2013). The present model does not suffer from the spurious term caused by the forcing-term effect, which was encountered in some previous thermal LB models for liquid–vapor phase change. Using the model, the liquid–vapor boiling process is simulated. The boiling curve together with the three boiling stages (nucleate boiling, transition boiling, and film boiling) is numerically reproduced in the LB community for the first time. The numerical results show that the basic features and the fundamental characteristics of boiling heat transfer are well captured, such as the severe fluctuation of transient heat flux in the transition boiling and the feature that the maximum heat transfer coefficient lies at a lower wall superheat than that of the maximum heat flux. Furthermore, the effects of the heating surface wettability on boiling heat transfer are investigated. It is found that an increase in contact angle promotes the onset of boiling but reduces the critical heat flux, and makes the boiling process enter into the film boiling regime at a lower wall superheat, which is consistent with the findings from experimental studies.
TL;DR: In this article, the authors investigated flow and heat transfer of magnetohydrodynamic (MHD) pseudo-plastic nanofluid in a finite film over unsteady stretching surface with internal heating effects.
Abstract: This paper investigates flow and heat transfer of magnetohydrodynamic (MHD) pseudo-plastic nanofluid in a finite film over unsteady stretching surface with internal heating effects. Four different types of nanoparticles, Cu, Al2O3, CuO and TiO2 are considered with pseudo-plastic carboxymethyl cellulose (CMC)-water used as base fluids. The effects of power law viscosity on temperature fields are taken into account by assuming temperature field is similar to the velocity ones with modified Fourier’s law of heat conduction for power-law fluids. Governing PDEs are reduced into coupled non-linear ODEs and solved numerically by shooting technique coupled with Runge–Kutta scheme and Newton’s method. The effects of Hartmann number, power law index, unsteadiness parameter, thickness parameter and generalized Prandtl number on the velocity and temperature fields are presented graphically and analyzed in detail.
TL;DR: The thermal network approach is a robust engineering tool that is easy to implement and program, is user friendly, straightforward, computationally efficient, and serves as a baseline methodology to produce results of reasonable accuracy.
Abstract: Heat pipes (HPs) and thermosyphons (TSs) are widely recognized as being excellent passive thermal transport devices that can have effective thermal conductivities orders of magnitude higher than similarly-dimensioned solid materials. The integration of heat pipes into heat exchangers (HXs) and heat sinks (HPHXs and HPHSs, respectively) have been shown to have strong potential for energy savings, especially in response to the significant reduction in the manufacturing costs of heat pipes in recent years. This review documents HPHXs applications, general design procedures, and analysis tools based on the thermal network approach. The thermal network approach is a robust engineering tool that is easy to implement and program, is user friendly, straightforward, computationally efficient, and serves as a baseline methodology to produce results of reasonable accuracy. Three practical case studies are presented in which the predicted results reveal potential advantages of heat pipe heat exchangers. Various HPHX and HPHS systems, including a potential thermal energy storage option using a phase change material, are presented along with the corresponding thermal networks. This review also discusses the opportunities and challenges related to current HPHX and HPHS applications.
TL;DR: In this article, a fractal scaling law for length distribution of fractures and the relationship among the fractal dimension for fracture length distribution, fracture area porosity and the ratio of the maximum length to the minimum length of fractures are proposed.
Abstract: Rocks with shear fractures or faults widely exist in nature such as oil/gas reservoirs, and hot dry rocks, etc. In this work, the fractal scaling law for length distribution of fractures and the relationship among the fractal dimension for fracture length distribution, fracture area porosity and the ratio of the maximum length to the minimum length of fractures are proposed. Then, a fractal model for permeability for fractured rocks is derived based on the fractal geometry theory and the famous cubic law for laminar flow in fractures. It is found that the analytical expression for permeability of fractured rocks is a function of the fractal dimension Df for fracture area, area porosity ϕ, fracture density D, the maximum fracture length lmax, aperture a, the facture azimuth α and facture dip angle θ. Furthermore, a novel analytical expression for the fracture density is also proposed based on the fractal geometry theory for porous media. The validity of the fractal model is verified by comparing the model predictions with the available numerical simulations.
TL;DR: In this article, the thermal conductivity of Al 2 O 3 -EG nanofluids has been examined using a KD2-Pro thermal analyzer and two new correlations with very high accuracy were suggested.
Abstract: To get more experimental and fundamental understanding of the thermal behavior of nanofluids, the thermal conductivity of Al 2 O 3 –EG nanofluids have been examined using a KD2-Pro thermal analyzer. The effects of temperature and concentration on thermal conductivity of nanofluid are investigated. The experiments performed at temperature ranging from 24 °C to 50 °C while volume fractions up to 5%. The experimental results exhibited that the thermal conductivity of nanofluids enhances significantly with increase in concentration and temperature. Also, attempts were made to propose new accurate correlations for estimating thermal conductivity at different temperatures and concentrations. For this purpose, two new correlations with very high accuracy were suggested. To estimate thermal conductivity at different temperatures, focusing more on accuracy and usability, several correlations have been proposed. These correlations have been presented separately at different temperatures which can be more accurate.
TL;DR: In this paper, the effects of shear flow and power law viscosity on the temperature field are taken into account according to a modified Fourier law, and approximate analytical solutions are obtained by the homotopy analysis method (HAM).
Abstract: We study the mixed convection boundary layer heat transfer of power law fluid over a moving conveyor along an inclined plate. The effects of shear flow and power law viscosity on the temperature field are taken into account according to a modified Fourier law. Approximate analytical solutions are obtained by the homotopy analysis method (HAM). Results indicate that heat transfer is strongly dependent on the values of power law exponent, inclination angle, boundary velocity ratio and Prandtl number. Three distinct characteristics are found for power law exponents 0 n , n = 1 and n > 1, especially the nonlinear behavior due to skin friction and local Nusselt number shown in Figs. 4 and 17, which has never been reported before. The decrease of inclination angle causes the loss of velocity boundary layer but the gain of temperature boundary layer. Heat transfer efficiency is enhanced but skin friction is diminished with the increase in velocity ratio (the ratio of conveyor velocity/mean velocity of flow field). Critical ratio (with skin friction zero) is obtained which strongly depends on the power law exponent. The effects of involved parameters on the velocity and temperature fields are analyzed.
TL;DR: In this article, the effects of wave amplitude, wavelength, volumetric flow rate and volume fraction of different types of nanofluids on thermal resistance, pressure drop, and friction factor of a microchannel heat sink were investigated.
Abstract: To improve the heat transfer performances of microchannel heat sink (MCHS), the advanced channel structures and working fluids can be applied. In this paper, the wavy channel structure and application of nanofluids are investigated. The effects of wavy amplitude, wavelength, volumetric flow rate and volume fraction of different type of nanofluids are presented. Three wave amplitudes of 25 μm, 50 μm and 75 μm with two wavelength of 250 μm and 500 μm at volumetric flow rate ranges from 0.152 L/min to 0.354 L/min are considered. Three different types of nanofluids with volume concentration ranges from 1% to 5% are applied. The effect of wavy MCHS is shown on thermal resistance, pressure drop, friction factor. It is found that in case of the pure water is applied as the coolant the heat transfer performance of the MCHS is significantly improved comparing with the traditional straight channel MCHS, while the replacement of the pure water by nanofluids makes the effect of wavy wall unnoticeable.
TL;DR: In this article, the effect of magnetic field on heat transfer of Al 2 O 3 -water nanofluid in a two-dimensional horizontal annulus was investigated using the lattice Boltzmann method.
Abstract: In this study the lattice Boltzmann method is applied to investigate the effect of magnetic field on natural convection heat transfer of Al 2 O 3 –water nanofluid in a two-dimensional horizontal annulus. In this model, the effect of Brownian motion on the effective thermal conductivity is also considered. The effective thermal conductivity and the effective viscosity of nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. The effect of nanoparticle volume fraction for the enhancement of heat transfer was examined for several sets of values of Rayleigh and Hartmann numbers. Also, a correlation of the Nusselt number with physical parameters is presented. The obtained results indicate that the value of the maximum stream function decreases with increasing Hartmann number. Furthermore, we notice that the Nusselt number has a direct relationship with the Rayleigh number; but quite the opposite is true with the Hartmann number. The obtained results indicate that the Lattice Boltzmann method with double-population is a powerful approach for the simulation of natural convection heat transfer in nanofluids in regions with curved boundaries.
TL;DR: In this paper, the authors studied the fluid flow and heat transfer in micro-channel heat sinks with different inlet/outlet locations (I, C and Z-type), header shapes (triangular, trapezoidal and rectangular) and microchannel cross-section shapes (the conventional rectangular microchannel, the microchannel with offset fan-shaped reentrant cavities and the micro channel with triangular reentrants cavities) were numerically studied with computational domain including the entire microchannel heat sink.
Abstract: In the present study, fluid flow and heat transfer in microchannel heat sinks with different inlet/outlet locations (I, C and Z-type), header shapes (triangular, trapezoidal and rectangular) and microchannel cross-section shapes (the conventional rectangular microchannel, the microchannel with offset fan-shaped reentrant cavities and the microchannel with triangular reentrant cavities) are numerically studied with computational domain including the entire microchannel heat sink. Detailed three-dimensional numerical simulations are useful in identifying the optimal geometric parameters that provide better heat transfer and flow distribution in a microchannel heat sink. Results highlight that flow velocity uniformity is comparatively better for I-type and poor for Z-type. The flow distribution is found to be symmetrical for I-type. It is seen from the header shapes analysis that the rectangular header shapes provides better flow velocity uniformity than the trapezoidal and triangular headers. The fluid flow mechanism can be attributed to the interaction of the branching of fluid and the friction offered by the walls of the header. Effects of microchannel cross-section shapes emphasize that the microchannel with offset fan-shaped reentrant cavities and the microchannel with triangular reentrant cavities of the heat sinks enhance the heat transfer compared to the conventional rectangular microchannel. The heat transfer mechanism can be attributed to the jetting and throttling effect, the additional flow disturbance near the wall of the reentrant cavities and the form drag of the reentrant cavities. The heat sink C has better heat transfer characteristic for q v = 150 ml/min and is able to prolong the life of the microelectronic devices.
TL;DR: A topology optimization method for a coupled thermal–fluid problem based on the two- and three-dimensional steady-state Navier–Stokes and energy equations is presented and a Tikhonov-based regularization scheme enables qualitative control of the geometric complexity of the optimal configurations.
Abstract: This paper presents a topology optimization method for a coupled thermal–fluid problem based on the two- and three-dimensional steady-state Navier–Stokes and energy equations. In this research, the optimization problem is formulated as a heat exchange maximization problem to obtain structures that function as high-performance cooling devices. Such devices, for example, liquid-cooled heat sinks, have recently attracted considerable attention as an engineering application for thermal cooling devices. The proposed optimization method employs level set boundary expressions and a Tikhonov-based regularization scheme enables qualitative control of the geometric complexity of the optimal configurations. Using the developed methodology, we provide two- and three-dimensional numerical examples that confirm the applicability, from an engineering standpoint, of the optimization method for the design of cooling devices.
TL;DR: In this paper, a novel material with a thermal conductivity as high as ∼5000 W −m−m−1 K−1 was used as a filler for modified epoxy resins.
Abstract: Thermal interface materials (TIMs) play an important role in the electronic components area due to the continued miniaturization and lightweight. As a novel material with a thermal conductivity as high as ∼5000 W m−1 K−1, graphene is regarded as a promising filler to improve the thermal performance of the TIMs. In this study, graphene prepared by varied approaches are employed as filler to modified epoxy resin. All the resulting TIMs not only show excellent thermal conductivity under room temperature (the maximum value reaches 4.9 W m−1 K−1 with 30 wt% loading, thermal conductivity enhancement factor is up to 1900%), but also demonstrate great stability at high temperature. Experimental and calculated results manifest a strong coupling of phonon modes between graphene and the matrix. The influences from graphene on thermal conductivity of composites are discussed. Larger size graphene sheets and surface functional groups would further reduce the Kapitza thermal resistance between the interfaces of graphene and epoxy resin. Moreover, the tested mechanical properties demonstrate that adding of graphene does not influence the outstanding mechanical performance of the matrix.
TL;DR: An experimental investigation of pool boiling heat transfer on multiscale (micro/nano) functionalized metallic surfaces found that the enhancement of the critical heat flux was directly related to the wetting and wicking ability of the surface which acts to replenish the evaporating liquid and delaycritical heat flux.
Abstract: In this paper, we present an experimental investigation of pool boiling heat transfer on multiscale (micro/nano) functionalized metallic surfaces. Heat transfer enhancement in metallic surfaces is very important for large scale high heat flux applications like in the nuclear power industry. The multiscale structures were fabricated via a femtosecond laser surface process (FLSP) technique, which forms self-organized mound-like microstructures covered by layers of nanoparticles. Using a pool boiling experimental setup with deionized water as the working fluid, both the heat transfer coefficients and critical heat flux were investigated. A polished reference sample was found to have a critical heat flux of 91 W/cm2 at 40 °C of superheat and a maximum heat transfer coefficient of 23,000 W/m2 K. The processed samples were found to have a maximum critical heat flux of 142 W/cm2 at 29 °C and a maximum heat transfer coefficient of 67,400 W/m2 K. It was found that the enhancement of the critical heat flux was directly related to the wetting and wicking ability of the surface which acts to replenish the evaporating liquid and delay critical heat flux. The heat transfer coefficients were also found to increase when the surface area ratio was increased as well as the microstructure peak-to-valley height. Enhanced nucleate boiling is the main heat transfer mechanism, and is attributed to an increase in surface area and nucleation site density.
TL;DR: In this article, an enhanced microchannel heat sink with sectional oblique fin is used to modulate the flow in contrast to continuous straight fin, which resulted in better heat transfer and a comparable pressure drop.
Abstract: Enhanced microchannel heat sink with sectional oblique fin is used to modulate the flow in contrast to continuous straight fin. The re-initialization of thermal boundary layer at the leading edge of each oblique due to breakage of continuous fin into oblique sections and the secondary flow due to these oblique cuts resulted in better heat transfer and a comparable pressure drop. Extensive experimental investigations are carried out with silicon test vehicle with hydraulic diameter of 100 μm and 200 μm and de-ionized water as flowing fluid. A parametric study involving the oblique angle, fin pitch is also carried out. Appreciable heat transfer augmentation is also achieved with maximum heat transfer performance enhancement at 47% when Re = 680. Comparable pressure drop to conventional microchannel is maintained up to Re = 500. Parametric study suggests that smaller oblique angle and smaller fin pitch are beneficial for heat transfer enhancement. The performance of the microchannel with 100 μm channel width and 27° oblique angle is found to be optimum.
TL;DR: In this article, the effects of wettability on saturated pool boiling heat transfer from a smooth superheated substrate, with a finite thickness at constant wall temperatures at the bottom, are investigated based on a recently developed liquid-vapor phase-change lattice Boltzmann method.
Abstract: Effects of wettability on saturated pool boiling heat transfer from a smooth superheated substrate, with a finite thickness at constant wall temperatures at the bottom, are investigated based on a recently developed liquid–vapor phase-change lattice Boltzmann method For a hydrophilic surface, it is shown that bubble departure diameter and bubble departure frequency increase with the increase of contact angle and superheat, and the whole bubble departs from the surface with a waiting period in the ebullition cycle A microlayer of liquid exists between the bubble and the heated hydrophilic surface and this region exhibits a high local heat flux during bubble growth period On the other hand, a residual bubble will be left on the surface when bubble departs from a hydrophobic surface and hence no waiting period exists No microlayer exists on the heated hydrophobic surface, and the three-phase contact line region has the highest local heat flux and lowest local temperature Time histories of the wall temperature on the top of a hydrophilic substrate and a hydrophobic substrate as well as the associated heat flux are studied in details during the nucleate boiling process Effects of wettability, superheat and heater size on number of nucleation sites in saturated pool boiling are investigated Saturated boiling curves (from onset of nucleate boiling to critical heat flux, to transition boiling to stable film boiling) for hydrophilic and hydrophobic heating surfaces are obtained by numerical simulation for the first time
TL;DR: In this article, three surfaces were fabricated with sintered porous coatings on: (i) entire microchannel surface (sintered-throughout), (ii) only the fin-tops, and (iii) only channel walls, and their performance with degassed water at atmospheric pressure was experimentally obtained.
Abstract: Pool boiling performance can be enhanced significantly by generating separate liquid–vapor pathways through selectively coating different regions of a heat transfer surface. In this paper, heat transfer mechanisms are explored by depositing porous coatings on an open microchannel with channel width 762 μm, channel depth 400 μm and fin width 200 μm. Three surfaces were fabricated with sintered porous coatings on: (i) entire microchannel surface (sintered-throughout), (ii) only the fin tops (sintered-fin-top), and (iii) only the channel walls (sintered-channel). Their pool boiling performance with degassed water at atmospheric pressure was experimentally obtained. A critical heat flux (CHF) of 313 W/cm 2 at a wall superheat of 7.5 °C was obtained for a sintered-throughout surface with a 2.4 fold enhancement in CHF over a plain chip. Highest heat transfer coefficient (HTC) of 565 kW/m 2 °C was obtained for this surface which translated into a 6.5 fold enhancement when compared to a plain surface. Three enhancement mechanisms were identified: (i) Area Augmented Enhanced Nucleation, (ii) Bubble Induced Liquid Jet Enhancement – Type-1 and (iii) Bubble Induced Liquid Jet Enhancement – Type-2. These mechanisms were responsible for the enhancement in HTC and CHF for sintered-throughout, sintered-fin-tops and sintered-channels, respectively. Although the current testing indicated the sintered-throughout surface to provide the highest CHF and HTC enhancement for the selected microchannel dimensions, changing the microchannel dimensions is expected to influence the relative merits of these configurations as the liquid and vapor flow mechanisms are influenced in fundamentally different ways.
TL;DR: In this paper, a transient mathematical model was established to examine the influences of varying porosity on the thermal characteristic, considering natural convection and Brinkman-Forchheimer extension to the Darcy law.
Abstract: Structure of porous metal foam with linearly changed porosity was proposed to enhance phase change during thermal energy storage. Taking the melting process of sodium nitrate inside porous copper foam as example, a transient mathematical model was established to examine the influences of varying porosity on the thermal characteristic. Considering natural convection and Brinkman–Forchheimer extension to the Darcy law, the mathematical models were solved numerically and validated against experimental data from literature. The temperature variations and evolution of solid–liquid interface were explored and recorded. Two parameters, including liquid fraction of phase change material (PCM) and energy storage density, were defined to characterize the system performances. The influences of varying porosity in different average porosity and pore density were analysed. The results showed that porosity linearly increased from bottom to top could improve the heat transfer performance and shorten the completely melted time compared to that for constant porosity by enhancing natural convection. Nearly no negative effect on solidification was found by varying porosity because heat conduction dominated the solidified process.
TL;DR: In this article, a model of wall-induced fluid flow within an infinite tapered channel has been developed to simulate the transport phenomena due to asymmetric wall displacements, and the analytical solution has been obtained for the temperature and concentration of the nanofluid.
Abstract: This paper deals with a theoretical investigation of the peristaltic transport of a Williamson nanofluid in a tapered asymmetric channel under the action of a thermal radiation parameter. In general, the nanofluids are electrically conducting nature. A model of wall-induced fluid flow within an infinite tapered channel has been developed to simulate the transport phenomena due to asymmetric wall displacements. This problem has plentiful applications. Moreover, it may serve as a model for the intrauterine fluid motion in a sagittal cross-section of the uterus under cancer therapy and drug analysis. The analytical solution has been obtained for the temperature and concentration of the nanofluid. The expressions for the axial velocity, stream function and pressure gradient were also obtained by a regular perturbation technique. Numerical computations have been performed for the pressure rise and the effect of various emerging parameters on the flow characteristics are shown and discussed with the help of graphs. The numerical results shown that the trapped bolus was increased in size and more trapped bolus were also occurred near the right wall with increasing Weissenberg number and thermophoresis parameter but that got decreased for large values of local temperature Grashof number.
TL;DR: In this paper, a stable-state natural convection heat transfer in a three-dimensional porous enclosure filled with a nanofluid using the mathematical nano-fluid model proposed by Buongiorno is presented.
Abstract: Steady-state natural convection heat transfer in a three-dimensional porous enclosure filled with a nanofluid using the mathematical nanofluid model proposed by Buongiorno is presented. The nanofluid model takes into account two important slip mechanisms in nanofluids like Brownian diffusion and thermophoresis. The study is formulated in terms of the dimensionless vector potential functions, temperature and concentration of nanoparticles. The governing equations were solved by finite difference method on non-uniform mesh and solution of algebraic equations was made on the basis of successive under relaxation method. Effort has been focused on the effects of six types of influential factors such as the Rayleigh and Lewis numbers, the buoyancy-ratio parameter, the Brownian motion parameter, the thermophoresis parameter and the aspect ratio on the fluid flow, heat and mass transfer. Three-dimensional velocity, temperature and nanoparticle volume fraction fields, average Nusselt numbers are presented. It is found that low Rayleigh and Lewis numbers and high thermophoresis parameter reflect essential non-homogeneous distribution of nanoparticles inside the cavity, hence a non-homogeneous model is more appropriate for the description of the system.
TL;DR: In this article, the authors studied the smoke behaviors induced by fires in inclined tunnels with different slopes and the upstream maximum temperatures along the tunnel centerline were specifically focused, showing that the longitudinal centerline peak temperature occurs at the downstream region of fire source rather than right above the fire source.
Abstract: Numerical simulations were carried out to study the smoke behaviors induced by fires in inclined tunnels with different slopes and the upstream maximum temperatures along the tunnel centerline were specifically focused. The simulation results show that the longitudinal centerline peak temperature occurs at the downstream region of fire source rather than right above the fire source. Two typical behaviors were found during the quasi-steady state: the upstream smoke layer interface is almost parallel to horizontal level while the downstream smoke layer interface is parallel to the inclined tunnel ceiling. The upstream maximum temperature under the ceiling remain approximately constant near the fire sources and then drop progressively with increasing distance to fire source due to the existence of vortexes, which is fairly different from the downstream maximum temperature distribution. Hence, an empirical correlation is developed by taking into account the tunnel slope, heat release rate and the upstream maximum temperature and using dimensional analysis. The correlation indicates that the dimensionless upstream maximum temperature decreases as the distance from fire source increases and it is proportional to 0.56 power of the dimensionless heat release rate and its relationship with tunnel slope is nonlinear and non-monotonous.
TL;DR: In this article, the performance of microstructured surfaces in enhancing boiling heat transfer (BHT) and critical heat flux (CHF) was investigated with a set of experiments with thirteen prepared samples: twelve with a micro-structured surface, and one with a bare surface.
Abstract: We study the effectiveness of microstructured surfaces in enhancing the boiling heat transfer (BHT) and critical heat flux (CHF). A set of experiments is designed with thirteen prepared samples: twelve with a microstructured surface, and one with a bare surface. The samples are fabricated using microelectromechanical systems (MEMS) techniques. The samples are tested using pool boiling experiments in saturated and atmospheric pressure conditions. The experimental results show that BHT increases with the surface roughness, defined as the ratio of the rough surface area to the projected area, but this enhancement gradually slows. The heat transfer coefficient of the structured surface is more than 300% that of the bare surface. The increase in the heating surface area due to the roughness ratio improves nucleate BHT due to the enhancement of convective heat transfer. The structured surface shows a 350% improvement in CHF over the bare surface. However, through analysis of the capillary flow rate on the structured surface, a critical gap size that limits the CHF is found. The critical gap size is discussed analytically and compared with experimental data. Designs for optimal boiling performance are proposed by studying the role of microstructured surfaces in both BHT and CHF.
TL;DR: In this paper, the maximum droplet radius and droplet size distribution adjustment with hydrophobic-hydrophilic hybrid surface, and the resultant heat transfer performance are investigated experimentally.
Abstract: The maximum droplet radius and droplet size distribution are significant for dropwise condensation heat transfer. Adjusting the maximum droplet radius and droplet size distribution with vertically patterned hydrophobic–hydrophilic hybrid surface is an effective method to enhance and optimize condensation heat transfer performance. The maximum droplet radius and droplet size distribution adjustment with hydrophobic–hydrophilic hybrid surface, and the resultant heat transfer performance are investigated experimentally in this paper. The results indicate that with the increase of hydrophobic region width, the maximum droplet radius on hydrophobic region increases while the droplet population density decreases. An optimum hydrophobic region width exists and the steam condensation heat transfer performance decreases with the increase of hydrophilic region width. And the performance can be larger than that of complete dropwise condensation for appropriate hybrid surfaces. The steam condensation heat transfer performance on the optimum hybrid surface is about 23% higher than that of complete dropwise condensation at surface subcooling of 2.0 K. Steam condensation heat transfer enhancement factor increases with the increase of hydrophobic region width first and then decreases with its further increase. The optimum hydrophobic region width is about 0.55 mm and the corresponding optimum maximum droplet radius is about 0.25 mm. Heat transfer enhancement factor decreases with the increase of surface subcooling and the optimum heat transfer enhancement factor is also significantly dependent on the surface subcooling. When the surface subcoolings are 2.0 K, 4.0 K and 6.0 K, the optimum heat transfer enhancement factors are about 1.23, 1.11 and 1.07, respectively. Steam condensation heat transfer can be enhanced with hydrophobic–hydrophilic hybrid surface more effectively at low surface subcooling. The experimental results and theoretical analysis agrees well to each other.
TL;DR: In this paper, the melting process of phase change material (PCM) infiltrated in a finned metal foam was numerically investigated using two approaches: (a) pore-scale and (b) volume-averaged numerical simulations.
Abstract: The melting process of phase change material (PCM) infiltrated in a finned metal foam was numerically investigated using two approaches: (a) pore-scale and (b) volume-averaged numerical simulations. The pore-scale simulation modeled the intricate geometry of the open-cell metal foam using sphere-centered tetrakaidecahedron and coupled the heat transfer in foam/fin solids with that in the PCM. The volume-averaged simulation used the Darcy–Brinkman–Forchheimer model to account for the motion of melt PCM as well as the one-temperature model based on local thermal equilibrium assumption. The volume-averaged simulation results were compared with the pore-scale simulation results which were used as the benchmark. Reasonable agreement between prediction results of the two approaches was observed. When using the volume-averaged method, the one-temperature model may be applicable without needing the more complicated two-temperature model. The thermal performance of the finned metal foam was compared with conventional plate-fin and metal foam structures, demonstrating its superiority as thermal conductivity enhancer of PCM.
TL;DR: The impact morphology of millimetric water drops on a polished aluminium surface has been studied experimentally by high-speed imaging, for surface temperatures between 50 and 400°C, and Weber numbers up to 160 as discussed by the authors.
Abstract: The impact morphology of millimetric water drops on a polished aluminium surface has been studied experimentally by high-speed imaging, for surface temperatures between 50 and 400 °C, and Weber numbers up to 160. Five impact regimes are defined based on the final outcome of the impact: three independent regimes (secondary atomisation, rebound, and splashing), and two mixed regimes (rebound with secondary atomisation and splashing with secondary atomisation). Impact regimes are displayed on a quantitative two-dimensional map, having the surface temperature and the impact Weber number at ambient conditions as coordinates. Some characteristics of the transition boundaries between impact regimes are discussed.
TL;DR: In this article, an ensemble of 4000 spherical nanoparticles with the material properties of CNT, aluminum, aluminum oxide, copper and gold was simulated in four base liquids and the ensemble-averaged results generated the thermophoretic velocity of these particles in the base liquids.
Abstract: Thermophoresis is the realization of the averaged Brownian motion of particles in a fluid, which is subject to a steady temperature gradient. At sufficiently long times, the stronger molecular impulses in the hotter fluid region drive particles towards the colder region, where the molecular impulses are weaker. The effect of the molecular impulses on the particles is described by a stochastic Brownian force. When this force is applied to an ensemble of particles the thermophoretic velocity is the average velocity of the ensemble. In this study the motion of an ensemble of 4000 spherical nanoparticles with the material properties of CNT, aluminum, aluminum oxide, copper and gold was simulated in four base liquids–water, ethyl glycol, engine oil and R134a. The ensemble-averaged results generate the thermophoretic velocity of these particles in the base liquids. It was observed that the computational results agree very well with the few experimental data available for liquids. The computational method is general and may be applied to all heterogeneous systems of nanoparticles in liquids. The numerical results yield very useful information on the process of thermophoresis in liquids as well as values of the thermophoretic coefficients in nanofluids.
TL;DR: In this article, a set of similarity transformations are introduced to convert the boundary layer equations into self-similar forms, and the solutions have been obtained numerically through shooting method with fourth-fifth-order Runge-Kutta integration technique.
Abstract: This work deals with the three-dimensional flow of nanofluid over an elastic sheet stretched non-linearly in two lateral directions. Suitable boundary conditions showing the power-law variation in the velocities are imposed. Further the recently suggested model for nanofluid is considered that requires nanoparticle volume fraction at the wall to be passively rather than actively controlled. A set of similarity transformations are introduced to convert the boundary layer equations into self-similar forms. The solutions have been obtained numerically through shooting method with fourth-fifth-order Runge–Kutta integration technique. The results reveal that penetration depths of temperature and nanoparticle volume fraction are decreasing functions of the power-law index. We notice that impact of Brownian motion in the temperature and heat transfer rate from the sheet is insignificant.
TL;DR: In this paper, a review of the literature concerning two-phase flow and heat transfer in reduced gravity is presented, where different methods and platforms dedicated to exploring the influence of reduced gravity, including ground flow boiling experiments performed at different orientations relative to Earth gravity.
Abstract: Space agencies worldwide are actively exploring the implementation of two-phase thermal management systems to support astronaut life onboard future space vehicles and planetary bases. Key motivations for these efforts are to increase the efficiency of power utilization and reduce overall weight and volume. These advantages are realized by orders of magnitude enhancement in heat transfer coefficient achieved with flow boiling and condensation compared to single-phase systems. This study will review published literature concerning two-phase flow and heat transfer in reduced gravity. Discussed are the different methods and platforms dedicated to exploring the influence of reduced gravity, including ground flow boiling experiments performed at different orientations relative to Earth gravity, as well as reduced gravity adiabatic two-phase flow, pool boiling, flow boiling and CHF experiments. Despite the extensive data and flow visualization results available in the literature, it is shown that there is a severe shortage of useful correlations, mechanistic models and computational models, which compromises readiness to adopt flow boiling in future space systems. Key recommendations are provided concerning platform, heater design, and operating conditions for future studies to expedite the deployment of two-phase thermal management in future space missions.
TL;DR: In this paper, a highly instrumented condensation module is used to map detailed axial variations of both wall heat flux and wall temperature, which are used to determine axial variation of the condensation heat transfer coefficient.
Abstract: This explores downflow condensation in a circular tube both experimentally and computationally using FC-72 as a working fluid. A highly instrumented condensation module is used to map detailed axial variations of both wall heat flux and wall temperature, which are used to determine axial variations of the condensation heat transfer coefficient. The experimental results are compared to predictions of a two-dimensional axisymmetric computational model using FLUENT. The study provides detailed construction of the model, including choice of interfacial phase change sub-model, numerical methods, and convergence criteria. The model is shown to yield good prediction of the heat transfer coefficient. The computed temperature profiles exhibit unusual shape, with steep gradient near the annular liquid film interface as well as near the wall, and a mild gradient in between. This shape is shown to be closely related to the shape of the eddy diffusivity profile. These findings point to the need for future, more sophisticated measurements of liquid film thickness, and both velocity and temperature profiles, to both validate and refine two-phase computational models.