Conjugate Heat Research Articles

Transport Phenomena in a Cross-Flow Hollow Fibre Membrane Bundle Used for Liquid Desiccant Air Dehumidification

The transport phenomena in a hollow fibre membrane bundle for liquid desiccant air dehumidification were investigated. In the bundle, the liquid desiccant solution flows inside the fibres and the process air flows across the fibre bundle; where air is dehumidified by moisture permeation through the membrane. This study investigated the fluid flow and conjugate heat and mass transfer in the cross-flow hollow fibre membrane bundle by considering the interactions between the neighbouring fibres. Two regularly packed arrangements: in-line and staggered, were considered. Due to the periodicity of the fluid flow and heat and mass transfer across the bundle, two representative periodic unit cells, which simultaneously include 2 to 3 neighbouring fibres, were selected as the calculation domains. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams (solution and air) in the membranes, were solved together with the heat and mass diffusion equations. The friction factor and the Nusselt and Sherwood numbers on both the air and the solution sides were then calculated and experimentally validated. The results were compared to those available data calculated from the free surface model.

Turbulent Heat and Mass Transfer Across a Hollow Fiber Membrane Tube Bank in Liquid Desiccant Air Dehumidification

The fluid flow and conjugate heat and mass transfer across a hollow fiber membrane tube bundle used for liquid desiccant air dehumidification are investigated. In this process, humid air flows across the fiber bank and salt solution flows inside the fibers packed in a shell. They exchange heat and moisture through the membranes. To overcome the difficulties in the direct modeling of the whole tube bundle, a representative cell, which comprises of a single fiber, a solution stream inside the fiber, and an air stream flowing across the fiber, is selected as the calculation domain. The liquid flow inside the fibers is assumed to be laminar due to the low Reynolds numbers, while the air flow across the bank is considered to be turbulent as a result from the disturbances from the numerous fibers. The governing equations for fluid flow and heat and mass transfer in the two flows and in the membrane are coupled together and solved numerically with a self-built code. Experimental work on hollow fiber membrane-based liquid desiccant air dehumidification is performed to validate the model. The fundamental data on friction factor, Nusselt and Sherwood numbers on both the shell and the tube sides are then obtained for Re = 300–600.

Conjugate heat and mass transfer in a hollow fiber membrane module for liquid desiccant air dehumidification: A free surface model approach

Conjugate heat and mass transfer in a hollow fiber membrane module used for liquid desiccant air dehumidification is investigated. The module is like a shell-and-tube heat exchanger where the liquid desiccant stream flows in the tube side, while the air stream flows in the shell side in a counter flow arrangement. Due to the numerous fibers in the shell, a direct modeling of the whole module is difficult. This research takes a new approach. A representative cell comprising of a single fiber, the liquid desiccant flowing inside the fiber and the air stream flowing outside the fiber, is considered. The air stream outside the fiber has an outer free surface (Happel’s free surface model). Further, the equations governing the fluid flow and heat and mass transfer in the two streams are combined together with the heat and mass diffusion equations in membranes. The conjugate problem is then solved to obtain the velocity, temperature and concentration distributions in the two fluids and in the membrane. The local and mean Nusselt and Sherwood numbers in the cell are then obtained and experimentally validated.

Surface heat and mass transfer coefficients for multiphase porous media transport models with rapid evaporation

Multiphase transport models of food processes require surface heat and mass transfer coefficients as boundary conditions that are traditionally assumed. A conjugate heat and mass transfer model is solved for simple microwave heating of food as a porous medium (from which vapor blows out) with air flow over it, such that there is no need to provide the boundary conditions at the food–air interface. Surface fluxes due to diffusion and flow (blowing) are computed for both vapor and heat from which transfer coefficients are obtained. As evaporation increases with heating, vapor flux due to pressure-driven flow increases but remains small compared with diffusive flux. Vapor accumulation at the interface increases the diffusive flux, leading to an 8% increase in mass transfer coefficient. Heat transfer at the interface occurs through both conduction and flow, and the heat transfer coefficient shows a slight decrease (10%) as blowing increases with heating. The rate of heating has minimal effect on the transfer coefficients.

Conjugate heat and mass transfer at gas-phase ignition of a cable line under current overload

The nonlinear nonstationary problem of heat and mass transfer at ignition of an overloaded cable line that penetrates from one room to another through a partition is numerically solved. Dependences of the ignition delay time of a mixture of oxidizer and thermal decomposition components of a cable sheath on the current load and also on the geometrical sizes and the partition material are found.

Modelling and simulation of flow boiling heat transfer

PurposeThe purpose of this paper is to describe the development and application of a numerical model for analysis of flow boiling phenomena and heat transfer.Design/methodology/approachFor flow boiling processes, the fluid and vapour flow regimes in connection with the conjugate heat and mass transfer problem for specimen quenching through the entire boiling curve is modelled. Vaporisation and recondensation, the vapour fraction distribution and vapour movement with respect to the liquid are considered in the calculation of the two‐phase flow and heat transfer process. The derived flow boiling model is based on a mixture model and bubble crowding model approach for two‐phase flow. In addition to the conventional mixture model formulation, here special model implementations have been incorporated that describe: the vapour formation at the superheated solid‐liquid interface, the recondensation process of vapour at the subcooled vapour‐liquid interface, the mass transfer rate in the different boiling phases and the microconvection effect in the nucleate boiling phase resulting from bubble growth and detachment.FindingsThe model prediction results are compared with experimental data for quenching of a circular cylinder, showing good agreement in boiling state and heat transfer coefficient distribution. Simulation and experiments lead to a better understanding of the interaction of incident flow in the boiling state and the resulting heat transfer.Research limitations/implicationsFluid temperatures in the range of 300‐353 K and specimen wall temperatures up to 1,000 K are considered.Practical implicationsFlow boiling is an efficient heat transfer process occurring in several technical applications. Application background of the model development is in quenching of complex metallic specimen geometries in liquids subject to fast changing heat fluxes.Originality/valueA general model for the complex two‐phase boiling heat transfer at high wall temperatures and fast flow conditions that can be used in engineering applications does not yet exist. The results provide detailed information describing the non‐uniform phase change during the complete quenching process from film boiling to pure convection.

Simulation of double-stage double-effect metal hydride heat pump

This paper presents the operational characteristics of a double-stage double-effect metal hydride heat pump (DSDE-MHHP) working with LaNi 4.1Al 0.52Mn 0.38/LmNi 4.91Sn 0.15/Ti 0.99Zr 0.01V 0.43Fe 0.09Cr 0.05Mn 1.5 as high/medium/low temperature alloys. The performances of the DSDE-MHHP are predicted by solving the transient, two-dimensional, conjugate heat and mass transfer characteristics between the paired metal hydride reactors of cylindrical configuration using the finite volume method (FVM). The designed operating temperatures chosen for the present analysis are 568, 361, 296, and 289 K as heat driven ( T D), heat rejection ( T H), heat sink ( T M) and refrigeration ( T C) temperatures, respectively. The variations in hydrogen concentrations, hydride equilibrium pressures, and temperatures within the hydride beds, and the heat exchange between the hydride beds with the heat transfer fluids are presented for a complete cycle. The operating cycle of a DSDE-MHHP is explained on dynamic pressure–concentration–temperature (PCT) plot. The variation of temperatures in the reactors during hydriding and dehydriding processes is compared with experimental data and a good agreement was observed between them. For given operating temperatures of 568/361/296/289 K, the average coefficient of performance (COP) and the specific cooling power (SCP) of the system are found to be 0.471 and 28.4 W/kg of total hydride mass, respectively.

Conjugate Heat and Mass Transfer from a Drying Rectangular Cylinder in Confined Air Flow

Heat and mass transfer from a porous body subject to convective drying is investigated numerically based on Luikov's equations. The air flow is assumed incompressible, two-dimensional, laminar, confined in a channel, and parallel to the rectangular-shaped solid. The finite-volume method is used and the computed temporal and spatial variations of moisture content, temperature, concentration, and flow parameters for two different flow rates are analyzed. Two flow configurations are studied: with and without a flow divider upstream of the cylinder in an attempt to eliminate the presence of separation zones and study their effect on drying. It was found that such effects may greatly affect the drying process, along with frontal area stagnation and the thickness of the body.

Modeling of conjugate conduction and heat and mass convection in tube‐fin exchangers

PurposeThe purpose of this paper is to examine the modeling of simultaneous heat and mass transfer under dehumidifying conditions. Moist air cooling in tube‐fin exchangers is investigated using a finite element technique.Design/methodology/approachThe model requires the solution of a conjugate problem, since interface temperatures must be calculated at the same time as temperature distributions in adjacent fluid and solid regions. The energy equation is solved in the whole domain, including the solid region, and the latent heat flux on the surfaces where condensation takes place is taken into account by means of an additional internal boundary condition.FindingsThermal performances for different Reynolds numbers of a typical two‐row tube‐fin exchanger are numerically analysed, for both in‐line and staggered arrangements of tubes. The results justify the great importance that the ratio between latent and overall rates of heat transfer has in the design of compact heat exchangers.Practical implicationsIn this work, the capabilities of the proposed methodology to deal with industrial applications in the field of compact exchangers are outlined.Originality/valueThe paper presents an effective approach to the solution of conjugate conduction and convection problems with simultaneous heat and mass transfer. The formulation is completely general, even if the finite element method is used in the calculations.

A generalized conjugate model for forced convection drying based on an evaporative kinetics

A model describing the heat and mass transfer involved in food drying is presented. The aim is to determine the effect of air temperature on the performance of the drying process applied to fresh-cut vegetable slices, but other effects can be easily incorporated in the model. The model allows to disregard one of the most limiting parameters in such modeling, i.e. the average heat and mass transfer coefficients at the food/drying substrate interface, which are generally taken from the literature. Such assumptions are limiting in the sense that they are referred to average transfer conditions and general geometries. The presented model relies upon a finite-element solution of time-dependent differential equations for simultaneous and conjugate heat and moisture transfer in a two-dimensional domain, without any inference in such empiricism. A special formulation for drying kinetic of the substrate is also exploited, and a treatment of the dependence of the properties upon the residual moisture content is included. After proper validation with the available experimental measurements, the numerical solution is discussed by presenting each involved field variables, emphasizing on the conjugate nature of the drying process. Due to its flexibility and generality, the model can be used in common industrial driers’ optimization, even in the assumption of a laminar flow field.

Conjugate Heat and Moisture Transfer During a Dynamic Thermal Treatment of Food

This work concerns the thermal decontamination of the surface of a food by hot air with the objective of inactivating the bacteria located at the surface. This heat treatment is considered to be intense and short, compared to more conventional drying processes. In order to predict the surface temperature, a coupled heat and mass transfer model was developed taking into account convective and diffusive mechanisms, evaporation, and shrinkage. A pragmatic alternative between Eulerian and Lagrangian formalisms was used in order to perform an efficient model. It was shown that this thermal decontamination process was governed by convection in a first stage, whereas mass diffusion was the limiting phenomenon in the second period. An inverse method was carried out in order to optimize the parameters of the mass diffusivity law.

Numerical simulation of time‐dependent heat and fluid flows inside and around single rising bubbles using a moving axisymmetric boundary‐fitted mesh system

PurposeThis paper aims to develop a numerical method for analysing the time‐dependent conjugate heat and fluid flows inside and around single bubbles rising in a hot liquid.Design/methodology/approachThe procedure combines the moving mesh method for flows in time‐dependent geometries and the zoned calculation algorithm for conjugate viscous flows. A moving axisymmetric boundary‐fitted mesh is used to track the deformable gas‐liquid interface, while conjugate flows in both gas and liquid sides are calculated by a two‐block zoned method. The interfacial stresses are employed to calculate the velocity value and to decide the time‐dependent bubble shape simultaneously. Governing equations for the rising velocity and acceleration of the bubble are derived according to the forces acting on the bubble.FindingsA calculating procedure for time‐dependent conjugate heat and fluid flows inside and around a rising single bubble has been developed. The algorithm has been verified, and can be employed for further analysing heat, mass and momentum transfer phenomena and their relevant mechanisms.Originality/valueThe paper developed a method to obtain high fidelity results for the heat and fluid flow details in the vicinity of a time‐dependent moderately deformable rising bubble; the physically zero‐thickness of a gas‐liquid interface is guaranteed. The governing equations for the time‐dependent rising velocity and acceleration are derived.

Three-Dimensional Numerical Analysis of Curved Transpiration Cooled Plates and Homogenization of Their Aerothermal Properties

Three different designs of a transpiration cooled multilayer plate—plane, convex, and concave—are analyzed numerically by application of a 3D conjugate fluid flow and heat transfer solver. The geometrical setup and the fluid flow conditions are derived from modern gas turbine components. The conjugate analysis of these designs focus on the influence of the surface curvature, the cooling film development on the plate surface, the fluid structure in the cooling channels, and on the cooling efficiency of the plate. Moreover, to predict the effective thermal properties and the permeability of these multilayer plates, a multiscale approach based on the homogenization technique is employed. This method allows the calculation of effective equivalent properties either for each layer or for the multilayer of superalloy, bondcoat, and thermal barrier coating (TBC). Permeabilities of the different designs are presented in detail for the TBC layer. The influence of the plate curvature and the blowing ratio on the effective orthotropic thermal conductivities is finally outlined.

05/02694 Humidified gas turbines — a review of proposed and implemented cycles

Industrial exhaust flue gas contains massive heat and water. Effective recovery will make positive sense for energy saving and conservation. Traditional condensing heat exchanger is restricted by coolant water temperature and corrosion problem. This paper proposes a new open absorption heat pump (OAHP). The system works on the basis of directly contacting spray heat and mass transfer occurring in three main parts, including generator, condenser and absorber. Driven by high-temperature flue gas directly from a burner, the system generates strongly concentrated LiBr solution prepared for the absorption of moisture and heat from the objective flue gas. A detailed theoretical model is established in Eulerian-Lagrangian formulation to simulate the particle dynamics and the heat and mass transfer characteristics. Extraordinary temperature profiles different from those in common heat transfer are found in the generator and are explained by the conjugate heat and mass transfer. The effects of the driving flue gas temperature and the return water temperature on system performances are studied. The OAHP system achieves excellent heat recovery capacity even at high return water temperature, indicating superiority to the condensing heat exchanger.

05/00603 Stranded gas — Australia's offshore retention leases

This paper presents the operational characteristics of a double-stage double-effect metal hydride heat pump (DSDE-MHHP) working with LaNi4.1Al0.52Mn0.38/LmNi4.91Sn0.15/Ti0.99Zr0.01V0.43Fe0.09Cr0.05Mn1.5 as high/medium/low temperature alloys. The performances of the DSDE-MHHP are predicted by solving the transient, two-dimensional, conjugate heat and mass transfer characteristics between the paired metal hydride reactors of cylindrical configuration using the finite volume method (FVM). The designed operating temperatures chosen for the present analysis are 568, 361, 296, and 289 K as heat driven (TD), heat rejection (TH), heat sink (TM) and refrigeration (TC) temperatures, respectively. The variations in hydrogen concentrations, hydride equilibrium pressures, and temperatures within the hydride beds, and the heat exchange between the hydride beds with the heat transfer fluids are presented for a complete cycle. The operating cycle of a DSDE-MHHP is explained on dynamic pressure–concentration–temperature (PCT) plot. The variation of temperatures in the reactors during hydriding and dehydriding processes is compared with experimental data and a good agreement was observed between them. For given operating temperatures of 568/361/296/289 K, the average coefficient of performance (COP) and the specific cooling power (SCP) of the system are found to be 0.471 and 28.4 W/kg of total hydride mass, respectively.

A DOS-Enhanced Numerical Simulation of Heat Transfer and Fluid Flow Through an Array of Offset Fins With Conjugate Heating in the Bounding Solid

The method of Design of Simulation (DOS) was used to guide and enhance a numerical simulation of fluid flow and heat transfer through offset-fin arrays which form the interior geometry of a cold plate. The basic problem involved 11 independent parameters. This prohibitive parametric burden was lessened by the creative use of nondimensionalization that was brought to fruition by a special transformation of the boundary conditions. Subsequent to the reduction of the number of parameters, the DOS method was employed to limit the number of simulation runs while maintaining an accurate representation of the parameter space. The DOS method also provided excellent correlations of both the dimensionless heat transfer and pressure drop results. The results were evaluated with respect to the Colburn Analogy for heat and momentum transfer. It was found that the offseting of the fins created a larger increase in the friction factor than that which was realized for the dimensionless heat transfer coefficient.

CONJUGATE COMPUTATION OF TRANSIENT FLOW AND HEAT AND MASS TRANSFER BETWEEN HUMID AIR AND DESICCANT PLATES AND CHANNELS

This is a numerical study of dehumidification of humid air in laminar and turbulent flows (333 ≤ Re ≤ 6,000) over desiccant (silica gel)-lined finite flat plates and in channels. The problem is treated as conjugate flow, heat, and mass transfer, and solved by using a finite control-volume method. The effects of the plate thickness (3 ≤ b ≤ 7 mm), the Reynolds number (333 ≤ Re ≤ 3,333), and the turbulence intensity (1 ≤ TI ≤ 10%) on the dehumidification process are investigated. The results show that increasing the desiccant plate thickness decreases the heat and mass transfer coefficients by 25% and 22% at t = 10 s and x = 0.11 m, respectively, in comparison to a thin plate. Mass transport rates increase with Re, e.g., at t = 20 s, Wave increases by 50% as Re is increased 10-fold from 333 to 3,333. Turbulent flow in channel desiccants increases the rate of dehumidification, e.g., an increase in Re from 600 (laminar) to 6,000 (turbulent) results in an increase in Wave by 22% at t = 20 s. Also, increasing the turbulence intensity from 1% to 10% increase Wave by 7%.

Conjugate heat and mass transfer process within porous media with dielectric cores in microwave freeze drying

A conjugate heat and mass transfer model was developed for microwave freeze drying within porous media with dielectric cores. The set of transient governing equations developed was solved numerically using finite volume method with variable time-steps. Analysis of numerical results may lead to following main conclusions: (1) by carefully choosing the loss factor and size of the dielectric cores could dramatically reduce the drying time, (2) the size and the loss factor ε″ of the dielectric core are the two important parameters influencing the drying process, (3) there may be two sublimation fronts in existence within the porous media.

Conjugate Heat and Mass Transfer under Conditions of Motion of a Viscous Incompressible Liquid in an Open Rectangular Cavity and Wall Melting

Numerical simulation is performed of heat transfer and hydrodynamics under conditions of motion of a viscous incompressible liquid in an open rectangular cavity with allowance made for the process of melting of its walls. Temperature distributions for the solid and liquid phases, as well as the velocity fields and the contours of craters being formed, are obtained at different moments of time. The effect of the model parameters and of the liquid flow mode on the pattern of motion and heat transfer of liquid in the cavity is studied.

Conjugate Heat and Mass Transfer in Crystallization from Food Solutions

Conjugate Heat and Mass Transfer in Crystallization from Food Solutions