Showing papers in "International Journal of Heat and Mass Transfer in 2010"

Boundary-layer flow of a nanofluid past a stretching sheet

Abstract: The problem of laminar fluid flow which results from the stretching of a flat surface in a nanofluid has been investigated numerically. This is the first paper on stretching sheet in nanofluids. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. A similarity solution is presented which depends on the Prandtl number Pr, Lewis number Le, Brownian motion number Nb and thermophoresis number Nt. The variation of the reduced Nusselt and reduced Sherwood numbers with Nb and Nt for various values of Pr and Le is presented in tabular and graphical forms. It was found 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.

An experimental study on the heat transfer performance and pressure drop of TiO2-water nanofluids flowing under a turbulent flow regime

Abstract: Nanofluid is a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100 nm in the conventional heat transfer fluids. Their use remarkably augments the heat transfer potential of the base liquids. This article presents the heat transfer coefficient and friction factor of the TiO 2 -water nanofluids flowing in a horizontal double tube counter-flow heat exchanger under turbulent flow conditions, experimentally. TiO 2 nanoparticles with diameters of 21 nm dispersed in water with volume concentrations of 0.2–2 vol.% are used as the test fluid. The results show that the heat transfer coefficient of nanofluid is higher than that of the base liquid and increased with increasing the Reynolds number and particle concentrations. The heat transfer coefficient of nanofluids was approximately 26% greater than that of pure vol.%, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 2.0 vol.% was approximately 14% lower than that of base fluids for given conditions. For the pressure drop, the results show that the pressure drop of nanofluids was slightly higher than the base fluid and increases with increasing the volume concentrations. Finally, the new correlations were proposed for predicting the Nusselt number and friction factor of the nanofluids, especially.

Fluid flow and heat transfer in wavy microchannels

Abstract: Laminar liquid–water flow and heat transfer in three-dimensional wavy microchannels with rectangular cross section are studied by numerical simulation. The flow field is investigated and the dynamical system technique (Poincare section) is employed to analyze the fluid mixing. The results show that when liquid coolant flows through the wavy microchannels, secondary flow (Dean vortices) can be generated. It is found that the quantity and the location of the vortices may change along the flow direction, leading to chaotic advection, which can greatly enhance the convective fluid mixing, and thus the heat transfer performance of the present wavy microchannels is much better than that of straight microchannels with the same cross section. At the same time, the pressure drop penalty of the present wavy microchannels can be much smaller than the heat transfer enhancement. Furthermore, the relative wavy amplitude of the microchannels along the flow direction may be varied for various practical purposes, without compromising the compactness and efficiency of the wavy microchannels. The relative waviness can be increased along the flow direction, which results in higher heat transfer performance and renders the temperature of the devices much more uniform. The relative waviness can also be designed to be higher at high heat flux regions for hot spot mitigation purposes.

Development of new correlations for convective heat transfer and friction factor in turbulent regime for nanofluids

Abstract: This paper presents new correlations for the convective heat transfer and the friction factor developed from the experiments of nanoparticles comprised of aluminum oxide, copper oxide and silicon dioxide dispersed in 60% ethylene glycol and 40% water by mass. The experimental measurements were carried out in the fully developed turbulent regime for the aforementioned three different nanofluids at various particle volumetric concentrations. First, the rheological and the thermophysical properties such as viscosity, density, specific heat and thermal conductivity were measured at different temperatures for varying particle volume concentrations. Next, these properties were used to develop the heat transfer coefficient correlation from experiments, as a function of these properties and the particle volumetric concentration. The pressure loss was also measured and a new correlation was developed to represent the friction factor for nanofluids.

Heat transfer of nanofluids in a shell and tube heat exchanger

Abstract: Heat transfer characteristics of γ-Al 2 O 3 /water and TiO 2 /water nanofluids were measured in a shell and tube heat exchanger under turbulent flow condition. The effects of Peclet number, volume concentration of suspended nanoparticles, and particle type on the heat characteristics were investigated. Based on the results, adding of naoparticles to the base fluid causes the significant enhancement of heat transfer characteristics. For both nanofluids, two different optimum nanoparticle concentrations exist. Comparison of the heat transfer behavior of two nanofluids indicates that at a certain Peclet number, heat transfer characteristics of TiO 2 /water nanofluid at its optimum nanoparticle concentration are greater than those of γ-Al 2 O 3 /water nanofluid while γ-Al 2 O 3 /water nanofluid possesses better heat transfer behavior at higher nanoparticle concentrations.

Melting in a vertical cylindrical tube: Numerical investigation and comparison with experiments

Abstract: The present work numerically investigates melting of a phase-change material (PCM) in a vertical cylindrical tube. The analysis aims at an investigation of local flow and thermal phenomena, by means of a numerical simulation which is compared to the previous experimental results . The numerical analysis is realized using an enthalpy–porosity formulation. The effect of various parameters of the numerical solution on the results is examined: in particular, the term describing the mushy zone in the momentum equation and the influence of the pressure–velocity coupling and pressure discretization schemes. PISO vs. SIMPLE and PRESTO! vs. Body-Force-Weighted schemes are examined. No difference is detected between the first two. However, considerable differences appear with regard to the last two, due to the mushy zone role. Image processing of experimental results from the previous studies is performed, yielding quantitative information about the local melt fractions and heat transfer rates. Based on the good agreement between simulations and experiments, the work compares the heat transfer rates from the experiments with those from the numerical analysis, providing a deeper understanding of the heat transfer mechanisms. The results show quantitatively that at the beginning of the process, the heat transfer is by conduction from the tube wall to the solid phase through a relatively thin liquid layer. As the melting progresses, natural convection in the liquid becomes dominant, changing the solid shape to a conical one, which shrinks in size from the top to the bottom.

Thermal convection with the Cattaneo–Christov model

Abstract: We consider the problem of thermal convection in a horizontal layer of incompressible Newtonian fluid with gravity acting downward The constitutive equation for the heat flux is taken to be one of Cattaneo type Since we are considering a fluid one has to be careful with the choice of objective derivative for the rate of change of the heat flux Here we employ a recent model due to Professor C Christov The thermal relaxation effect is found to be significant if the Cattaneo number is sufficiently large, and the convection mechanism switches from stationary convection to oscillatory convection with narrower cells

The peridynamic formulation for transient heat conduction

Abstract: In bodies where discontinuities, like cracks, emerge and interact, the classical equations for heat and mass transfer are not well suited. We propose a peridynamic model for transient heat (or mass) transfer which is valid when the body undergoes damage or evolving cracks. We use a constructive approach to find the peridynamic formulation for heat transfer and test the numerical convergence to the classical solutions in the limit of the horizon (the nonlocal parameter) going to zero for several one-dimensional problems with different types of boundary conditions. We observe an interesting property of the peridynamic solution: when two m-convergence curves, corresponding to two different horizons, for the solution at a point and an instant intersect, the intersection point is also the exact classical (local) solution. The present formulation can be easily extended to higher dimensions and be coupled with the mechanical peridynamic description for thermomechanical analyses of fracturing bodies, or for heat and mass transfer in bodies with evolving material discontinuities.

Pool boiling characteristics of low concentration nanofluids

Abstract: The pool boiling behavior of low concentration nanofluids (⩽1 g/l) was experimentally studied over a flat heater at 1 atm. Boiling of nanofluids produces a thin nanoparticle film, on the heater surface, which in turn is believed to increase the critical heat flux. The present study also indicates that the nanoparticle deposition results in transient characteristics in the nucleate boiling heat transfer. Finally, this study investigates possible causes responsible for the deposition of nanoparticle on the heater surface. Experimental evidence shows that microlayer evaporation, during nanofluid boiling, is responsible for the nanoparticle coating formed on the heater surfaces.

A critical evaluation of force term in lattice Boltzmann method, natural convection problem

Abstract: Few methods have been introduced and used in simulation fluid flows using lattice Boltzmann method (LBM) with external forces, such as buoyancy, surface tension, magnetic, etc. In some problems, the external force is constant, for instance gravitational force with constant density flows, while for other problems the force may vary spatially and/or temporally with non-zero gradients, such as gravitational force with variable density flows. For problems with the variable force term, adding force term to LBM may not be trivial. The paper evaluates mainly three different schemes of adding force term to LBM with BGK method. In this work, natural convection in a closed and an open ended cavities were used as a test platform. The results for the differentially heated cavity are introduced first. For the open cavity, the vertical left hand wall of the cavity is heated and opposing side is opened to the ambient, with other connecting boundaries are assumed to be adiabatic. Prior to the solution, the boundary conditions at the opening are unknown. The results of predictions using LBM are compared with results predicted by using finite volume method (FVM). The results are presented for Ra = 10 6 and for Pr = 0.71 . It is found that most methods suggested in the literature produces similar results, despite that some authors claim that their scheme is more accurate than the other schemes.

A new model and analytical solutions for borehole and pile ground heat exchangers

Abstract: The ground-coupled heat pump (GCHP) systems have been identified as one of the best sustainable energy technologies for space heating and cooling in buildings. While the foundation piles of buildings are used to partly take the place of boreholes in the ground heat exchanger (GHE) in recent years, the classical approaches of the line heat source model and the “hollow” cylindrical heat source model for the borehole GHEs fail for thermal analysis and design of the pile GHEs. Evolved from the classical models, a new “solid” cylindrical source model is presented in this paper to consider both the radial dimension and the heat capacity of the borehole or pile. Expressions of the analytical solution are derived for 1-D and 2-D new models by means of the Green’s function method. Results obtained from the new 1-D model are compared with the classical line source and “hollow” cylindrical source models of the borehole GHE, and also validated by a numerical solution of the same model. While the 1-D and 2-D solid cylindrical source models can provide adequate tools for design and simulation of the pile GHEs, improvement can also be achieved in simulating the temperature response of the borehole GHEs, especially for short time steps.

Study of bubble growth in water pool boiling through synchronized, infrared thermometry and high-speed video

Abstract: High-speed video and infrared thermometry were used to obtain time- and space-resolved information on bubble nucleation and heat transfer in pool boiling of water. The bubble departure diameter and frequency, growth and wait times, and nucleation site density were directly measured for a thin, electrically-heated, indium–tin-oxide surface, laid on a sapphire substrate. These data are very valuable for validation of two-phase flow and heat transfer models, including computational fluid dynamics with interface tracking methods. Here, detailed experimental bubble-growth data from individual nucleation sites were used to evaluate simple, commonly-used, but poorly-validated, bubble-growth and nucleate-boiling heat-transfer models. The agreement between the data and the models was found to be reasonably good. Also, the heat flux partitioning model, to which our data on nucleation site density, bubble departure diameter and frequency were directly fed, suggests that transient conduction following bubble departure is the dominant contribution to nucleate-boiling heat transfer.

Heat transfer in phase change materials for thermal management of electric vehicle battery modules

Abstract: The temperature of battery modules in electric vehicles (EVs) must be controlled adequately to remain within a specified range for optimum performance In this paper, thermal management of battery modules with phase change materials (PCMs) is investigated experimentally An electric heater is used to simulate the heat source of a battery cell Two different PCM designs are investigated: one with a PCM cylinder surrounding the heater, and the other with PCM jackets wrapping the heater Both configurations exhibit good effectiveness in maintaining the heater within a desired temperature range This paper also examines the effectiveness of PCM thermal management under variable heating rates and variable ambient temperatures, as well as the effects of buoyancy during PCM melting

Augmentation of nucleate boiling heat transfer and critical heat flux using nanoparticle thin-film coatings

Abstract: Nanoparticle thin-film coatings applied to boiling surfaces using a layer-by-layer (LbL) assembly method demonstrated significant enhancement in the pool boiling critical heat flux (CHF) and nucleate boiling heat transfer coefficient. Up to 100% enhancement of the critical heat flux and over 100% enhancement of the heat transfer coefficient were observed for pool boiling of nickel wires coated with different thin-films of silica nanoparticles. Surface characterization revealed that the surface wettability changed drastically with the application of these coatings, while causing virtually no change in the surface roughness. It is concluded that the nanoporous structure coupled with the chemical constituency of these coatings leads to the enhanced boiling behavior.

Characterization of evaporation and boiling from sintered powder wicks fed by capillary action

Abstract: The thermal resistance to heat transfer into the evaporator section of heat pipes and vapor chambers plays a dominant role in governing their overall performance. It is therefore critical to quantify this resistance for commonly used sintered copper powder wick surfaces, both under evaporation and boiling conditions. The objective of the current study is to measure the dependence of thermal resistance on the thickness and particle size of such surfaces. A novel test facility is developed which feeds the test fluid, water, to the wick by capillary action. This simulates the feeding mechanism within an actual heat pipe, referred to as wicked evaporation or boiling. Experiments with multiple samples, with thicknesses ranging from 600 to 1200 μm and particle sizes from 45 to 355 μm, demonstrate that for a given wick thickness, an optimum particle size exists which maximizes the boiling heat transfer coefficient. The tests also show that monoporous sintered wicks are able to support local heat fluxes of greater than 500 W cm −2 without the occurrence of dryout. Additionally, in situ visualization of the wick surfaces during evaporation and boiling allows the thermal performance to be correlated with the observed regimes. It is seen that nucleate boiling from the wick substrate leads to substantially increased performance as compared to evaporation from the liquid free surface at the top of the wick layer. The sharp reduction in overall thermal resistance upon transition to a boiling regime is primarily attributable to the conductive resistance through the saturated wick material being bypassed.

Turbulent heat transfer and friction factor of Al2O3 Nanofluid in circular tube with twisted tape inserts

Abstract: The thermophysical properties like thermal conductivity and viscosity of Al2O3 nanofluid is determined through experiments at different volume concentrations and temperatures and validated. Convective heat transfer coefficient and friction factor data at various volume concentrations for flow in a plain tube and with twisted tape insert is determined experimentally for Al2O3 nanofluid. Experiments are conducted in the Reynolds number range of 10,000–22,000 with tapes of different twist ratios in the range of 0

A general criterion for evaporative heat transfer in micro/mini-channels

Abstract: Experimental results of the saturated-flow boiling heat transfer in micro/mini-channels for both multi- and single-channel configurations were obtained from the literature. The collected database contains more than 3700 data points, covering a wide range of working fluids, operational conditions, and different micro-channel dimensions. The whole database was analyzed by using various existing correlations to verify their respective accuracies. However, none of the existing correlations could predict the data sets precisely. Using the boiling number, Bond number and Reynolds number, a general correlation for evaporative heat transfer in micro/mini-channels was established. In addition, the Bond number in predicting heat-transfer coefficients can be used as a criterion to classify a flow path as a micro-channel or as a conventional macro-channel.

An investigation of the thermal performance of cylindrical heat pipes using nanofluids

Abstract: In this work, a two-dimensional analysis is used to study the thermal performance of a cylindrical heat pipe utilizing nanofluids. Three of the most common nanoparticles, namely Al2O3, CuO, and TiO2 are considered as the working fluid. A substantial change in the heat pipe thermal resistance, temperature distribution, and maximum capillary heat transfer of the heat pipe is observed when using a nanofluid. The nanoparticles within the liquid enhance the thermal performance of the heat pipe by reducing the thermal resistance while enhancing the maximum heat load it can carry. The existence of an optimum mass concentration for nanoparticles in maximizing the heat transfer limit is established. The effect of particle size on the thermal performance of the heat pipe is also investigated. It is found that smaller particles have a more pronounced effect on the temperature gradient along the heat pipe.

Effectiveness–thermal resistance method for heat exchanger design and analysis

Abstract: The equivalent thermal resistance of a heat exchanger is defined based on the concept of the entransy dissipation rate, which measures the irreversibility of heat transfer for the purpose of object heating or cooling, rather than from the heat to work conversion. The relationships between the heat exchanger effectiveness and the thermal resistance (or conductance) are developed, which do not depend on its flow arrangement, and hence useful for the performance comparison among heat exchangers with different flow arrangements. In addition, such relationships bridge a gap between the heat exchanger irreversibility and its effectiveness. The monotonic decrease of the effectiveness with increasing the thermal resistance shows that the heat exchanger irreversibility can be described by its thermal resistance when evaluated from the transport process viewpoint, while the so-called entropy generation paradox occurs, if the irreversibility is measured by the entropy generation number for a heat exchanger.

A coupled volume-of-fluid and level set (VOSET) method for computing incompressible two-phase flows

Abstract: A coupled volume-of-fluid and level set (VOSET) method, which combines the advantages and overcomes the disadvantages of VOF and LS methods, is presented for computing incompressible two-phase flows. In this method VOF method is used to capture interfaces, which can conserve the mass and overcome the disadvantage of nonconservation of mass in LS method. An iterative geometric operation proposed by author is used to calculate the level set function / near interfaces, which can be applied to compute the accurate curvature j and smooth the discontinuous physical quantities near interfaces. By using the level set function / the disadvantages of VOF method, inaccuracy of curvature and bad smoothness of discontinuous physical quantities near interfaces, can be overcome. Finally the computing results made with VOSET method are compared with those made with VOF and LS methods. 2009 Published by Elsevier Ltd.

High temperature latent heat thermal energy storage using heat pipes

Abstract: A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.

Entropy generation due to flow and heat transfer in nanofluids

Abstract: Present study provides a theoretical investigation of the entropy generation analysis due to flow and heat transfer in nanofluids. For this purpose, the most common alumina–water nanofluids are considered as the model fluid. Since entropy is sensitive to diameter, three different diameters of tube in their different regimes have been taken. Those are microchannel (0.1 mm), minichannel (1 mm) and conventional channel (10 mm). To consider the effect of conductivity and viscosity, two different models have been used to represent theoretical and experimental values. It has been found that the reduced equation with the help of order of magnitude analysis predicts microchannel and conventional channel entropy generation behaviour of nanofluids very well. The alumina–water with high viscosity nanofluids are better coolant for use in minichannels and conventional channels with laminar flow and microchannels and minichannel with turbulent flow. It is not advisable to use alumina–water nanofluids with high viscosity in microchannels with laminar flow or minichannels and conventional channels with turbulent flow. Also there is need to develop low viscosity alumina–water nanofluids for use in microchannel with laminar flow. It is observed that at lower tube diameter, flow friction irreversibility is more significant and at higher tube diameter thermal irreversibility is more. Finally, for both laminar and turbulent flow, there is an optimum diameter at which the entropy generation rate is the minimum for a given nanofluid.

Heat transfer characteristics of synthetic jet impingement cooling

Abstract: Synthetic jet is a novel flow technique which synthesizes stagnant air to form a jet, and is potentially useful for cooling applications. The impingement heat transfer characteristics of a synthetic jet are studied in this work. Toward that end, the behavior of the average heat transfer coefficient of the impinged heated surface with variation in the axial distance between the jet and the heated surface is measured. In addition, radial distribution of mean and rms velocity and static pressure are also measured. The experiments are conducted for a wide range of input parameters: the Reynolds number ( Re ) is in the range of 1500–4200, the ratio of the axial distance between the heated surface and the jet to the jet orifice diameter is in the range of 0–25, and the length of the orifice plate to the orifice diameter varies between 8 and 22 in this study. The maximum heat transfer coefficient with the synthetic jet is found to be upto 11 times more than the heat transfer coefficient for natural convection. The behavior of average Nusselt number is found to be similar to that obtained for a continuous jet. The exponent of maximum Nusselt number with Re varies between 0.6 and 1.4 in the present experiments, depending on the size of the enclosure. A direct comparison with a continuous jet is also made and their performances are found to be comparable under similar set of conditions. Such detailed heat transfer results with a synthetic jet have not been reported earlier and are expected to be useful for cooling of electronics and other devices.

Correlations of two-phase frictional pressure drop and void fraction in mini-channel

Abstract: Alternative correlations of two-phase friction pressure drop and void fraction are explored for mini-channels based on the separated flow model and drift-flux model. By applying the artificial neural network, dominant parameters to correlate the two-phase friction multiplier and void fraction are picked out. It is found that in mini-channels the non-dimensional Laplace constant is a main parameter to correlate the Chisholm parameter as well as the distribution parameter. Both previous correlations and the newly developed correlations are extensively evaluated with a variety of data sets collected from the literature.

3D numerical simulation of flow and conjugate heat transfer through a pore scale model of high porosity open cell metal foam

Abstract: A 3D numerical simulation methodology for the flow and heat transfer at the pore scale level of high porosity open cell metal foam is presented. The pore scale topology is directly represented with a 3D numerical model of the geometry, which is discretised using a tetrahedral volume mesh for both its void and solid phases. The conjugate flow and temperature fields are obtained by solution of the Navier–Stokes and energy equations for two different foam pore densities under various flow and temperature conditions. Model validation is performed against macroscopic parameters such as pressure drop and heat transfer coefficient; the results are found in reasonable agreement with the experimental measurements.

Flow-boiling heat transfer of R-134a-based nanofluids in a horizontal tube

Abstract: The influence of nanoparticles on the flow-boiling of R-134a and R-134a/polyolester mixtures is quantified for flows of low vapor quality (x

Enhancement of pool-boiling heat transfer using nanostructured surfaces on aluminum and copper

Abstract: Enhanced pool-boiling critical heat fluxes (CHF) at reduced wall superheat on nanostructured substrates are reported. Nanostructured surfaces were realized using a low temperature process, microreactor-assisted-nanomaterial-deposition. Using this technique we deposited ZnO nanostructures on Al and Cu substrates. We observed pool-boiling CHF of 82.5 W/cm 2 with water as fluid for ZnO on Al versus a CHF of 23.2 W/cm 2 on bare Al surface with a wall superheat reduction of 25–38 °C. These CHF values on ZnO surfaces correspond to a heat transfer coefficient of ∼23,000 W/m 2 K. We discuss our data and compare the behavior with conventional boiling theory.

Performance assessment in a heat exchanger tube with alternate clockwise and counter-clockwise twisted-tape inserts

Abstract: The article presents an experimental study of turbulent heat transfer and flow friction characteristics in a circular tube equipped with two types of twisted tapes: (1) typical twisted tapes and (2) alternate clockwise and counterclockwise twisted tapes (C–CC twisted tapes). Nine different C–CC twisted tapes are tested in the current work; they included the tapes with three twist ratios, y/w = 3.0, 4.0 and 5.0, each with three twist angles, θ = 30o, 60o and 90o. The experiments have been performed over a Reynolds number range of 3000–27,000 under uniform heat flux conditions, using water as working fluid. The obtained results reveal that the C–CC twisted-tapes provide higher heat transfer rate, friction factor and heat transfer enhancement index than the typical twisted-tapes at similar operating conditions. The results also show that the heat transfer rate of the C–CC tapes increases with the decrease of twist ratio and the increase of twist angle values. Depending on Reynolds number, twist ratio and twist angle values, the mean Nusselt numbers in the tube fitted with the C–CC twisted tapes are higher than those with the typical ones and the plain tube around 12.8–41.9% and 27.3–90.5%, respectively. The maximum heat transfer enhancement indexes of the C–CC twisted tapes with θ = 90o for y/w = 3.0, 4.0 and 5.0, are 1.4, 1.34 and 1.3, respectively. In addition, correlations of the Nusselt number and the friction factor for using the C–CC twisted tapes are also determined. Both predicted Nusselt number and friction factor are within ±15% and ±15% deviation compared to the experimental data.

CFD analysis of convective heat transfer at the surfaces of a cube immersed in a turbulent boundary layer

Abstract: Steady Reynolds-Averaged Navier–Stokes (RANS) CFD is used to evaluate the forced convective heat transfer at the surfaces of a cube immersed in a turbulent boundary layer, for applications in atmospheric boundary layer (ABL) wind flow around surface-mounted obstacles such as buildings. Two specific configurations are analysed. First, a cube placed in turbulent channel flow at a Reynolds number of 4.6 × 103 is considered to validate the numerical predictions by comparison with wind-tunnel measurements. The results obtained with low-Reynolds number modelling (LRNM) show a satisfactory agreement with the experimental data for the windward surface. Secondly, a cube exposed to high-Reynolds number ABL flow is considered. The heat transfer in the boundary layer is analysed in detail. The dimensionless parameter y∗, which takes into account turbulence, is found to be more appropriate for evaluating heat transfer than the commonly used y+ value. Standard wall functions, which are frequently used for high-Reynolds number flows, overestimate the convective heat transfer coefficient (CHTC) significantly (±50%) compared to LRNM. The distribution of the CHTC–U10 correlation over the windward surface is reported for Reynolds numbers of 3.5 × 104 to 3.5 × 106 based on the cube height and U10, where U10 is the wind speed in the undisturbed flow at a height of 10 m.

A comprehensive flow regime map for microchannel flow boiling with quantitative transition criteria

Abstract: Due to the critical role of vapor confinement in establishing distinct flow and heat transfer characteristics in microchannels (as distinct from those in larger channels), the conditions under which such confinement occurs in microchannels are of great interest. It is shown in the present work that channel dimensions and flow properties alone, as proposed in past studies, are insufficient for determining confinement effects in microchannel boiling. Hence, a new criterion for physical confinement in microchannel flow boiling, termed the convective confinement number, that incorporates the effects of mass flux, as well as channel cross-sectional area and fluid properties, is proposed. This criterion helps determine the conditions under which a channel qualifies as a microchannel for two-phase flow, needing special treatment, and when a macroscale treatment is adequate. In addition, based on previous work by the authors, a new comprehensive flow regime map is developed for a wide range of experimental parameters and channel dimensions, along with quantitative transition criteria based on nondimensional boiling parameters.