Showing papers in "International Journal of Heat and Mass Transfer in 1998"

A novel concept for convective heat transfer enhancement

Abstract: An analog between convection and conduction with heat sources is made to have a further understanding of the mechanism of convective heat transfer. There are three ways to raise the strength of heat sources/convection terms, and consequently to enhance the heat transfer: (a) increasing Reynolds and/or Prandtl number, (b) increasing the fullness of dimensionless velocity and/or temperature profiles, (c) increasing the included angle between the dimensionless velocity and temperature gradient vectors. Some approaches of heat transfer enhancement are suggested based on such a novel concept of heat transfer enhancement.

Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe

Abstract: An experiment was carried out to investigate the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe having an inside diameter of 2.0 mm. The data are useful in designing more compact and effective evaporators for various refrigeration and air conditioning systems. The effects of the imposed wall heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured evaporation heat transfer and pressure drop were examined in detail. When compared with the data for larger pipes (Di ≥ 8.0 mm) reported in the literature, the evaporation heat transfer coefficient for the small pipe considered here is about 30–80% higher for most situations. Moreover, we noted that in the small pipe the evaporation heat transfer coefficient is higher at a higher imposed wall heat flux except in the high vapor quality region, at a higher saturation temperature, and at a higher mass flux when the imposed heat flux is low. In addition, the measured pressure drop is higher for increases in the mass flux and imposed wall heat flux. Based on the present data, empirical correlations were proposed for the evaporation heat transfer coefficients and friction factors.

An experimental investigation of single-phase forced convection in microchannels

Abstract: Turbulent, single-phase forced convection of water in circular microchannels with diameters of 0.76 and 1.09 mm has been investigated. The data show that the Nusselt numbers for the microchannels are higher than those predicted by traditional large channel correlations. Based on the data obtained in this investigation, along with earlier data for smaller diameter channels, a generalized correlation for the Nusselt number for turbulent, single-phase, forced convection in circular microchannels has been developed. The diameter, Reynolds number, and Prandtl number ranges are 0.102–1.09 mm, 2.6 × 103−2.3 × 104, and 1.53–6.43, respectively. With a confidence level of greater than 95%, differences between experimental and predicted Nusselt number values are less than ± 18.6%.

One-group interfacial area transport in vertical bubbly flow

Abstract: In the two-fluid model, interfacial concentration is one of the important parameters. The objective of this study is to develop an interfacial area equation with the source and sink terms being properly modeled. For bubble coalescence, the random collisions between bubbles due to turbulence, and the wake entrainment process due to the relative motions of the bubbles, were included. For bubble breakup, the impact of turbulent eddies is considered. Compared with measured axial distributions of the interfacial area concentration under various flow conditions, the adjustable parameters in the source/sink terms were obtained for the simplified one-dimensional transport equation.

Modeling forced liquid convection in rectangular microchannels with electrokinetic effects

Abstract: The effects of the electric double layer near the solid–liquid interface and the flow induced electrokinetic field on the pressure-driven flow and heat transfer through a rectangular microchannel are analyzed in this work. The electric double layer field in the cross-section of rectangular microchannels is determined by solving a non-linear, two-dimensional Poisson–Boltzmann equation. A body force caused by the electric double field and the flow-induced electrokinetic field is considered in the equation of motion. For steady-state, fully-developed laminar flows, both the velocity and the temperature fields in a rectangular microchannel are determined for various conditions. The flow and heat transfer characteristics with⧹without consideration of the electrokinetic effects are evaluated. The results clearly show that, for aqueous solutions of low ionic concentrations and a solid surface of high zeta potential, the liquid flow and heat transfer in rectangular microchannels are significantly influenced by the presence of the electric double layer field and the induced electrokinetic flow.

Deposition of tin droplets on a steel plate: simulations and experiments

Abstract: Impact and solidification of tin droplets on a flat stainless steel plate was studied using both experiments and numerical simulation. In the experiments, tin droplets (2.1 mm diameter) were formed and dropped onto a stainless steel surface whose temperature was varied from 25 to 240°C. Impact of droplets was photographed, and evolution of droplet spread diameter and liquid-solid contact angle measured from photographs. Substrate temperature variation under an impinging droplet was measured. A complete numerical solution of the Navier-Stokes and energy equations, based on a modified SOLA-VOF method, was used to model droplet deformation and solidification and heat transfer in the substrate. Measured values of liquid-solid contact angle were used as a boundary condition for the numerical model. The heat transfer coefficient at the droplet-substrate interface was estimated by matching numerical predictions of the variation of substrate temperature with measurements. Comparison of computer generated images of impacting droplets with photographs showed that the numerical model correctly modelled droplet shape during impact as it simultaneously deformed and solidified. A simple analytical model was developed to predict the maximum spread diameter of a droplet freezing during impact.

Prediction of heat transfer in an axisymmetric turbulent jet impinging on a flat plate

Abstract: The problem of cooling of a heated plate by an axisymmetric isothermal fully developed turbulent jet has been studied numerically. Computations were performed with the normal-velocity relaxation turbulence model (V2F model). Local heat transfer coefficient predictions are compared to the available experimental data. For comparison, computations have also been carried out with the widely used k-e model. The V2F heat transfer predictions are in excellent agreement with the experiments, whereas the k-e model does not properly resolve the flow features, greatly over-predicts the rate of heat transfer and yields physically unrealistic behaviors.

An experimental investigation of bubble growth and detachment in vertical upflow and downflow boiling

Abstract: A visual study of vapor bubble growth and departure in vertical upflow and downflow forced convection boiling is presented. A vertical flow boiling facility was constructed with a transparent, electrically-heated test section in which the ebullition process could be observed. High-speed digital images of flow boiling phenomena were obtained, which were used to measure bubble growth, departure diameters, and lift-off diameters. Experiments were conducted for flow of FC-87 over a commercially-finished nichrome heating surface, with mass flux ranging from 190 to 666 kg m−2 s−1 and heat flux ranging from 1.3 to 14.6 kW\m2. The flow was slightly subcooled (ΔTsub = 1.0–5.0°C), and boiling occurred at isolated nucleation sites. A major conclusion of this work is that the observed vapor bubble dynamics between upflow and downflow are significantly different. In the upflow configuration, bubbles departing the nucleation site slide along the heater wall, and typically do not lift off. In the downflow configuration, bubbles either lift off directly from the nucleation site or slide and then lift off, depending on flow and thermal conditions. The process of vapor bubble sliding appears to be responsible for enhanced energy transfer from the heating surface, as evidenced by larger heat transfer coefficients for upflow than for downflow under otherwise identical operating conditions.

Momentum and heat transfer from cylinders in laminar crossflow at 10−4 ⩽ Re ⩽ 200

Abstract: A summary of results of numerical investigations of the two-dimensional flow around a heated circular cylinder located in a laminar crossflow is presented. Numerical investigations were carried out for the Reynolds number range 10 −4 ⩽ Re ⩽ 200 and for temperature loadings of 1.003–1.5. The computations yield information on Nu and C D variation with Reynolds number. The temperature dependence of the fluid properties (air) was taken into account and this resulted in a temperature dependence of the Nu – Re and C D – Re results. Information is also provided on the Strouhal number dependence on the Re number and on the critical Re number where vortex shedding starts.

Transient analysis of incompressible flow through a packed bed

Abstract: The present investigation aims at numerically simulating the temporal energy transport for incompressible flows through a packed bed. The governing equations are formulated according to the volume averaging method. The generalized momentum equation which accounts for the inertial and viscous forces was used to carry out this research, whereas the two-energy equation model was used for simulating the energy transport. The implications of the non-Darcian terms and the thermal dispersion effects on the transient energy transport were explored. Error maps were introduced for a wide range of flow conditions and bed configurations. In addition, the investigation also tackles the applicability of the local thermal equilibrium condition and the one-dimensional approach for studying the thermal response in packed bed.

A network thermodynamic analysis of the heat pipe

Abstract: This work provides a unique view into the physics behind the heat pipe operation which was considered a thermal network of various components. Transient heat pipe behavior was described by first-order, linear, ordinary differential equations. The working fluid undergoes a thermodynamic cycle which was analyzed by T-s diagrams. The heat pipe dimensions must be “thermally compatible” with the heat pipe materials to establish the thermodynamic cycle. This was illustrated by a dimensionless number proposed here for the first time. Validated by comparisons with previous experimental and numerical studies, the present thermodynamic theory may lead to simplified heat pipe design schemes.

Multi-dimensional modeling of thin liquid films and spray-wall interactions resulting from impinging sprays

Abstract: The focus of this work is to formulate and validate a multi-dimensional, fuel film model to help account for the fuel distribution during combustion in internal combustion engines. Spray-wall interaction and spray-film interaction are also incorporated into the model. The fuel film model simulates thin fuel film flow on solid surfaces of arbitrary configuration. This is achieved by solving the continuity, momentum, and energy equations for the two-dimensional film that flows over a three-dimensional surface. The major physical processes considered in the model include mass and momentum contributions to the film due to spray drop impingement, splashing effects, various shear forces, piston acceleration, dynamic pressure effects, gravity driven flow, conduction, and convective heat and mass transfer. In order to adequately represent the drop interaction process, impingement regimes and post-impingement behavior have been modeled using experimental data and mass, momentum and energy conservation constraints. The regimes modeled for spray-film interaction are stick, rebound, spread, and splash. In addition, modified wall functions for evaporating wavy films are provided and tested. The fuel film model is validated through a series of comparisons to experimental data for secondary droplet velocities, secondary droplet sizes, spray radius, spray height, film thickness, film spreading radius, and percentage of fuel adhered to the surface.

Boiling nucleation during liquid flow in microchannels

Abstract: The boiling of liquids in microchannels/microstructures is currently of great interest due to its very unusual phenomena and its many potential applications in a wide variety of advanced technologies. The thermodynamic aspects of phase transformations of liquids in microchannels was analyzed to further understand the boiling characteristics and to determine the conditions under which a portion of such liquids is likely to undergo phase change. A nondimensional parameter and related criteria, that determine the phase transition in microchannels, were derived theoretically. The size of the microchannels results in dramatically high heat fluxes and superheats for liquid nucleation when the microchannel is sufficiently small. The effect that the liquid thermophysical properties have on the nucleation is also described by the analysis.

The use of the Brinkman number for single phase forced convective heat transfer in microchannels

Abstract: From a survey on studies on convective heat transfer in microchannels, the Brinkman number is proposed as a parameter for correlating the convective heat transfer parameters in microchannels. The proposal emerges from a dimensional analysis of the variables influencing the laminar forced convection in microchannels and it can explain the unusual behaviour of convective heat transfer in the laminar regime in microchannels. Its incorporation is also supported by an energy balance across the microchannel. The physical significance of the Brinkman number as applicable to microchannels and the role played in convective heat transfer in microchannels are elaborated. The limited experimental data reported in the literature for the laminar regime heat transfer seem to correlate with the number. A dimensionless geometric parameter is also proposed.

Heat transfer and flow visualization experiments of swirling, multi-channel, and conventional impinging jets

Abstract: Heat transfer and flow visualization experiments were conducted to investigate and compare the performance of swirling and multi-channel impinging jets with that of a conventional impinging jet (CIJ), having the same diameter at the same conditions. Swirling impinging jets (SIJs) employed a 25.4 mm long solid insert at the exit of housing tube to divert the air flow through four narrow channels along the surface of the insert, with the desired swirl angle (θ of 15, 30 and 45°). The multi-channel impinging jet (MCIJ) had same dimensions as SIJs, except that the narrow channels in the solid insert were vertical (θ = 0°). The local and surface average Nusselt numbers of MCIJ were generally much higher than those of CIJ. SIJs demonstrated large increases in both Nusselt numbers and significant enhancement in radial uniformity of heat transfer compared to MCIJ and CU; best results were for θ = 15° and jet spacing of 50.8 mm. Flow visualization experiments using smoke flow, smoke wires and water jet techniques revealed the mechanisms contributing to the higher and enhanced radial uniformity of heat transfer by SIJs. The smoke flow technique provided images of flow field between jet exit and impinged surface, while smoke wires showed details of flow field at and close to impinged surface. The water jet flow, seeded with tiny air bubbles as tracers, revealed details of flow field and induced mixing on the impinged surface.

Laminar mixed convection with viscous dissipation in a vertical channel

Abstract: Combined free and forced convection flow in a parallel-plate vertical channel is analysed in the fully developed region by taking into account the effect of viscous dissipation. The two boundaries are considered as isothermal and kept either at equal or at different temperatures. The velocity field, the temperature field and the Nusselt numbers are obtained by a perturbation series method which employs a perturbation parameter proportional to the Brinkman number. Dimensionless coefficients which allow the evaluation of the dimensionless mean velocity, of the dimensionless bulk temperature and of the Nusselt numbers are determined.

A study of the flow field of a confined and submerged impinging jet

Abstract: The flow field of an axisymmetric, confined and submerged turbulent jet impinging normally on a flat plate was studied experimentally using laser-Doppler velocimetry. Single jets of a perfluorinated dielectric liquid (FC-77) issuing from square-edged, geometrically similar nozzles were used in the experiments with the radial outflow confined between parallel plates. The nozzle (length to diameter) aspect ratio was unity, giving rise to a still-developing velocity profile at the nozzle exit. Experiments were conducted with nozzle diameters of 3.18 and 6.35 mm, nozzle-to-target plate spacings of up to four jet diameters, and Reynolds numbers in the range of 8500–23,000. The toroidal recirculation pattern in the outflow region characteristic of confined jets is mapped. Velocities and turbulence levels are presented over a fine measurement grid in the pre-impingement and wall-jet regions.

Modeling of heat transfer in dropwise condensation

Abstract: A model using the population balance concept is developed to predict the drop size distribution for small drops that grow by direct condensation. The resistances to heat transfer due to the drop (conduction through the drop, vapor-liquid interfacial resistance, drop curvature) and due to the promoter layer and the sweeping effect of falling drops are incorporated into the model and are also included in calculating the heat transfer rate through a single drop. The total heat flux is calculated from the drop size distributions and the heat transfer rate through a single drop. Drop size distribution for large drops that grow by coalescence is obtained from the works of Rose and Glicksman. The work in this paper reveals that to adequately calculate the heat flux, all the resistances to heat transfer due to the drop and the promoter layer have to be included. Considering heat conduction through the drop as the only resistance to heat transfer overestimates the heat flux. The amount of overestimation increases as the temperature difference increases.

Effect of the leakage on pressure drop and local heat transfer in shell-and-tube heat exchangers for staggered tube arrangement

Abstract: Local heat transfer and pressure drop on the shell side of shell-and-tube heat exchangers with segmental baffles were investigated for different baffle spacings. The distributions of the local heat transfer coefficients on each tube surface within a fully developed baffle compartment were determined and visualized by means of mass transfer measurements. Per-tube, per-row and per-compartment average heat transfer coefficients were drawn from the local values. The local pressure measurements allow the determination of the shell-side flow distributions. For same Reynolds number, the pressure drop and average heat transfer are increased by an increased baffle spacing due to a reduced leakage through the baffle-shell clearance. The experimental results were compared with literature values.

Heat and mass transfer for plate fin-and-tube heat exchangers, with and without hydrophilic coating

Abstract: Heat and mass transfer characteristics of fin-and-tube heat exchangers with and without hydrophilic coating were reported in this study. The test exchangers consisted of three different louvre fin patterns and their corresponding plain fin counterpart. For completely dry test conditions, the enhancement level for the enhanced fin pattern decreases with increase of fin pitch. The Gray and Webb correlation considerably under-predicts the present 7.0 and 9.5 mm tube diameter plain fin samples. For dehumidifying test conditions, the effect of hydrophilic coating on the sensible heat transfer coefficients is negligible, and there are no detectable changes of the sensible heat transfer coefficients with change of inlet relative humidity. However, the pressure drops for the hydrophilic coated surfaces are sensitive to the inlet relative humidity.

Convective cooling of a heated obstacle in a channel

Abstract: A detailed investigation of the forced convective cooling of a heated obstacle mounted upon a channel wall is presented. The Navier-Stokes equations are used to characterize the flow field surrounding the conductive obstacle. Special emphasis is given in the systematic analysis to detail the local Nusselt number distributions and mean Nusselt numbers for the individual exposed obstacle faces. The study employs parametric variations in the obstacle height and width, as well as the thermal conductivity ratio k solid / k fluid , flow rate ( Re D 1 ), and heating method, to detail important fundamental and practical results. Comparison with an analytical solution for thermally developing flow in a channel shows reasonable estimates to Nusselt numbers can be made by choosing an appropriate length scale. It is shown that specific choices in obstacle size, shape and thermal conductivity can produce significant effects on the flow and heat transfer characteristics.

Convective flow and heat transfer in a channel containing multiple heated obstacles

Abstract: The present work. details the numerical simulation of forced convective, incompressible flow in a channel with an array of heated obstacles attached to one wall. Three levels of Nusselt numbers are emphasized in this systematic analysis: local distributions along the obstacle exposed faces, mean values for individual faces, and overall obstacle mean values. This study details the effects of variations in the obstacle height, width, spacing, and number, along with the obstacle thermal conductivity, fluid flow rate, and heating method, to illustrate important fundamental and practical results. The periodicity of the mean Nusselt number is established, relative to the ninth obstacle, at the 5% and 10% difference levels (eighth and seventh obstacles, respectively). The periodic behavior of the velocity components and temperature distributions are also explicitly demonstrated for the array. Extensive presentation and evaluation of the mean Nusselt numbers for all obstacles within the array is fully documented. The results pave the way for different applications involving multiple heated obstacles. 0 1998 Elsevier Science Ltd. All rights reserved.

A multiphase formulation for fire propagation in heterogeneous combustible media

Abstract: A general formulation based on a weighting average procedure is developed for describing the fire-induced behaviour of a multiphase, reactive and radiative medium. The complete set of the resulting equations should be used as the basic one for later studies, especially in the framework of wildland fires. For the moment, in order to demonstrate the capability of the general formulation, a simplified model, named zeroth-order model, in which some physical phenomena (such as char combustion, second-order terms, particle motion) are neglected is presented. In the frame of this simplified model, reverse and forward one-dimensional fire propagations through an heterogeneous medium composed of fixed fuel particles are studied numerically.

Oscillatory double-diffusive convection in a rectangular enclosure with combined horizontal temperature and concentration gradients

Abstract: The effect of buoyancy ratio on the flow structure is investigated numerically for a binary mixture gas in a rectangular enclosure subject to opposing horizontal thermal and compositional buoyancies. The following conditions were considered: RaT = 105, Pr = 1, Le = 2 and N = 0.0–2.0 for A = 2. The numerical solution predicts that oscillatory double-diffusive convection with the secondary cell flow structure occurs for a certain range of buoyancy ratio. The key mechanism for oscillatory flow is that the unstably stratified region of species shifts from the central part of the enclosure to the upper and lower parts, and vice versa in a time-periodic sense, due to the interaction of heat and mass transfer with different diffusivities near the vertical walls. Bifurcation structures of the oscillatory flow in the present system are discussed.

Falling film evaporation of single component liquids

Abstract: Heat transfer coefficients for evaporation of single-component liquids in falling films were experimentally measured over an extended range of parameters. By using two different fluids (propylene glycol and water), over a range of absolute pressures, it was possible to extend the existing data base by an order of magnitude in Prandtl numbers. These new data indicate that existing models and correlations were inadequate for fluids with Prandtl numbers greater than five. New models were developed for both the wavy laminar and the turbulent regimes. An exponential interpolation paradigm between the regimes enabled prediction of the evaporative heat transfer coefficient over the entire range of Reynolds and Prandtl numbers with an average deviation of less than 10% from the experimental data.

Transient response of the human limb to an external stimulust

Abstract: The multidimensional transient temperature profiles of the arterial and venous blood flows and of the tissue within a limb are simulated with the bioheat equations developed by means of the heat transfer principle in porous media. The conjugated differential equations are solved numerically. Three different layer models are introduced to treat discrete distributions of the anatomical and thermal properties. Some examples are computed and discussed.

Isolated fluid oxygen drop behavior in fluid hydrogen at rocket chamber pressures

Abstract: A model has been developed for the behavior of an isolated fluid drop of a single compound immersed into another compound in finite, quiescent surroundings at supercritical conditions. The model is based upon fluctuation theory which accounts for both Soret and Dufour effects in the calculation of the transport matrix relating molar and heat fluxes to the transport properties and the thermodynamic variables. The transport properties have been modeled over a wide range of pressure and temperature variation applicable to LO_x–H_2 conditions in rocket chambers, and the form of the chemical potentials is valid for a general fluid. The equations of state have been calculated using a previously-derived, computationally-efficient and accurate protocol. Results obtained for the LO_x–H_2 system show that the supercritical behavior is essentially one of diffusion. The temperature profile relaxes fastest followed by the density and lastly by the mass fraction profile. An effective Lewis number calculated using theory derived elsewhere shows that it is larger by approximately a factor of 40 than the traditional Lewis number. The parametric variations show that gradients increasingly persist with increasing fluid drop size or pressure, and with decreasing temperature. The implication of these results upon accurate measurements of fluid drop size under supercritical conditions is discussed.

An analytical study of heat and mass transfer through a parallel-plate channel with recycle

Abstract: The effects of recycle at the ends on the heat and mass transfer through a parallel-plate channel with uniform wall temperature are studied by an orthogonal expansion technique. The heat and mass transfer problem is solved for fully developed laminar velocity profiles in a parallel-plate channel with ignoring axial conduction, and with fluid properties which are temperature independent. Analytical results show that recycle can enhance the heat transfer rate for high Graetz numbers compared with that in an open conduit (without an impermeable plate inserted and without recycle). The preheating and residence-time are the two effects which are in domination of the heat transfer behavior.