Showing papers in "International Communications in Heat and Mass Transfer in 2008"

Natural convection heat transfer enhancement in horizontal concentric annuli using nanofluids

Abstract: Heat transfer enhancement in horizontal annuli using nanofluids is investigated. Water-based nanofluid containing various volume fractions of Cu, Ag, Al2O3 and TiO2 nanoparticles is used. The addition of the different types and different volume fractions of nanoparticles were found to have adverse effects on heat transfer characteristics. For high values of Rayleigh number and high L/D ratio, nanoparticles with high thermal conductivity cause significant enhancement of heat transfer characteristics. On the other hand, for intermediate values of Rayleigh number, nanoparticles with low thermal conductivity cause a reduction in heat transfer. For Ra=10 3 and Ra=10 5 the addition of Al2O3 nanoparticles improves heat transfer. However, for Ra=10 4 , the addition of nanoparticles has a very minor effect on heat transfer characteristics.

Application of Optimal Homotopy Asymptotic Method for solving nonlinear equations arising in heat transfer

Abstract: We consider one of the newest analytical methods, the Optimal Homotopy Asymptotic Method (OHAM), to solve nonlinear equations arising in heat transfer. Two specific applications are considered: cooling of a lumped system with variable specific heat and the temperature distribution equation in a thick rectangular fin radiation to free space. Results obtained by OHAM, which does not need small parameters are compared with numerical results and a very good agreement was found. This method provides us with a convenient way to control the convergence of approximation series and adjust convergence regions when necessary. The results reveal that the proposed method is explicit, effective and easy to use.

Influence of thermal radiation on the boundary layer flow due to an exponentially stretching sheet

Abstract: The effect of radiation on the boundary layer flow and heat transfer of a viscous fluid over an exponentially stretching sheet is studied. The homotopy analysis method (HAM) is employed to determine the convergent series expressions of velocity and temperature. The physical interpretation to these expressions is assigned through graphs. It is found that the effects of Prandtl and radiation numbers on the temperature are opposite.

Effect of aspect ratio on entropy generation in a rectangular cavity with differentially heated vertical walls

Abstract: In the present study, entropy generation in rectangular cavities with the same area but different aspect ratios is numerically investigated. The vertical walls of the cavities are at different constant temperatures while the horizontal walls are adiabatic. Heat transfer between vertical walls occurs by laminar natural convection. Based on the obtained dimensionless velocity and temperature values, the distributions of local entropy generation due to heat transfer and fluid friction, the local Bejan number and local entropy generation number are determined and related maps are plotted. The variation of the total entropy generation and average Bejan number for the whole cavity volume at different aspect ratios for different values of the Rayleigh number and irreversibility distribution ratio are also evaluated. It is found that for a cavity with high value of Rayleigh number (i.e., Ra = 10(5)), the total entropy generation due to fluid friction and total entropy generation number increase with increasing aspect ratio, attain a maximum and then decrease. The present results are compared with reported solutions and excellent agreement is observed. The study is performed for 10(2) < Ra < 10(5), 10(-4) < 0 < 10(-2), and Pr = 0.7. (Less)

Peristaltic transport in an asymmetric channel with heat transfer — A note

Abstract: The problem of heat transfer for the motion of a viscous incompressible fluid induced by travelling sinusoidal waves has been analytically investigated for a two-dimensional asymmetrical channel. The channel asymmetry is produced by choosing the peristaltic wave train on the walls to have different amplitudes and phase. The flow is investigated in a wave frame of reference moving with the velocity of the wave. The momentum and energy equations have been linearized under long-wavelength and low-Reynolds number assumptions and closed form expressions for temperature and coefficient of heat transfer have been derived. The effect of Hartmann number, Eckert number, width of the channel and phase angle on temperature and coefficient of heat transfer are discussed numerically and explained graphically.

Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid

Abstract: The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail.

Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency

Abstract: The enhancement heat transfer of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the enhancement of heat pipe thermal efficiency with nanofluids is presented. The heat pipe is fabricated from the straight copper tube with the outer diameter and length of 15, 600 mm, respectively. The heat pipe with the de-ionic water, alcohol, and nanofluids (alcohol and nanoparticles) are tested. The titanium nanoparticles with diameter of 21 nm are used in the present study which the mixtures of alcohol and nanoparticles are prepared using an ultrasonic homogenizer. Effects of %charge amount of working fluid, heat pipe tilt angle and %nanoparticles volume concentrations on the thermal efficiency of heat pipe are considered. The nanoparticles have a significant effect on the enhancement of thermal efficiency of heat pipe. The thermal efficiency of heat pipe with the nanofluids is compared with that the based fluid.

Thermal performance assessment of turbulent channel flows over different shaped ribs

Abstract: Experiments are conducted to assess turbulent forced convection heat transfer and friction loss behaviors for air flow through a constant heat flux channel fitted with different shaped ribs The rib cross-sections used in the present study are triangular (isosceles), wedge (right-triangular) and rectangular shapes Two rib arrangements, namely, in-line and staggered arrays, are introduced Measurements are carried out for a rectangular channel of aspect ratio, AR = 15 and height, H = 20 mm with single rib height, e = 6 mm and rib pitch, P = 40 mm The flow rate is in terms of Reynolds numbers based on the inlet hydraulic diameter of the channel in a range of 4000 to 16,000 The experimental results show a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel The in-line rib arrangement provides higher heat transfer and friction loss than the staggered one for a similar mass flow rate In comparison, the wedge rib pointing downstream yields the highest increase in both the Nusselt number and the friction factor but the triangular rib with staggered array shows better thermal performance over the others

Constrained and unconstrained melting inside a sphere

Abstract: The melting of the phase change material (PCM) inside a sphere using n-Octadecane for both constrained and unconstrained melting is investigated. In constrained melting, the solid PCM is restrained from sinking to the bottom of the sphere. For unconstrained melting, the solid PCM would sink to the bottom of sphere due to gravity. The experiments are carried out at three different wall temperatures of 35 °C, 40 °C and 45 °C with a sub-cooling of 1 °C for the unconstrained melting and three different initial sub-cooling of 1 °C, 10 °C and 20 °C at a constant wall temperature of 40 °C for the constrained melting.

Numerical study on heat transfer of turbulent channel flow over periodic grooves

Abstract: A numerical investigation of turbulent forced convection in a two-dimensional channel with periodic transverse grooves on the lower channel wall is conducted. The lower wall is subjected to a uniform heat flux condition while the upper wall is insulated. To investigate turbulence model effects, computations based on a finite volume method, are carried out by utilizing four turbulence models: the standard k − e, the Renormalized Group (RNG) k − e, the standard k − ω, and the shear stress transport (SST) k − ω turbulence models. Parametric runs are made for Reynolds numbers ranging from 6000 to 18,000 with the groove-width to channel-height ratio (B/H) of 0.5 to 1.75 while the groove pitch ratio of 2 and the depth ratio of 0.5 are fixed throughout. The predicted results from using several turbulence models reveal that the RNG and the k − e turbulence models generally provide better agreement with available measurements than others. Therefore, the k − e model is selected to use in prediction of this complex flow. In addition, the results of the heat transfer coefficient, friction factor, skin friction coefficient and thermal enhancement factor are also examined. It is found that the grooved channel provides a considerable increase in heat transfer at about 158% over the smooth channel and a maximum gain of 1.33 on thermal performance factor is obtained for the case of B/H = 0.75. This indicates that the reverse/re-circulation flow in a channel with transverse grooves can improve the heat transfer rate.

Turbulent flow heat transfer and pressure loss in a double pipe heat exchanger with louvered strip inserts

Abstract: In the present work, heat transfer and friction characteristics were experimentally investigated, employing louvered strips inserted in a concentric tube heat exchanger. The louvered strip was inserted into the tube to generate turbulent flow which helped to increase the heat transfer rate of the tube. The flow rate of the tube was in a range of Reynolds number between 6000 and 42,000. The turbulent flow devices were consisted of (1) the louvered strips with forward or backward arrangements, and (2) the louvered strip with various inclined angles (θ = 15°, 25° and 30°), inserted in the inner tube of the heat exchanger. In the experiment, hot water was flowed through the inner tube whereas cold water was flowed in the annulus. The experimental data obtained were compared with those from plain tubes of published data. Experimental results confirmed that the use of louvered strips leads to a higher heat transfer rate over the plain tube. The increases in average Nusselt number and friction loss for the inclined forward louvered strip were 284% and 413% while those for the backward louvered strip were 263% and 233% over the plain tube, respectively. In addition, the use of the louvered strip with backward arrangement leads to better overall enhancement ratio than that with forward arrangement around 9% to 24%.

Entropy generation for natural convection by heated partitions in a cavity

Abstract: Entropy generation for natural convection in a partitioned cavity, with adiabatic horizontal and isothermally cooled vertical walls, is studied numerically by both a FORTRAN code and the commercially available CFD-ACE software. Effects of the Rayleigh number, the position of the heated partition, and the dimensionless temperature difference on the local and average entropy generation rate are investigated. Proper scale analysis of the problem showed that, while fluid friction term has nearly no contribution to entropy production, the heat transfer irreversibility increases monotonically with the Nusselt number and the dimensionless temperature difference.

CFD modeling of hydrodynamic and heat transfer in fluidized bed reactors

Abstract: In this study, a gas–solid fluidized bed reactor has been simulated applying CFD techniques in order to investigate hydrodynamic and heat transfer phenomena. Reactor model predictions were compared with corresponding experimental data reported in the literature to validate the model. The results indicate that considering two solid phases, particles with smaller diameters have lower volume fraction at the bottom of the bed and higher volume fraction at the top of the bed. In addition, it was revealed that bed expansion was larger when a bimodal particle mixture was applied compared with the case of mono-dispersed particles. Gas and solid phase temperature distributions in the reactor were also computed, considering the hydrodynamic of the fluidized bed and the heat generated by the solid particles. The results showed that gas temperature increases as it moves upward in the reactor due to the heat of polymerization reaction leading to the higher temperatures at the top of the bed.

The importance of rib shape effects on the local heat transfer and flow friction characteristics of square ducts with ribbed internal surfaces

Abstract: Accurate prediction of ribbed duct flow and heat transfer is of importance to the gas turbine industry. In the present work, a computer code has been developed to study the turbulent heat transfer and friction in a square duct with various-shaped ribs mounted on one wall. The simulations were performed for four rib shapes, i.e., square, triangular, trapezoidal with decreasing height in the flow direction, and trapezoidal with increasing height in the flow direction. The prepared algorithm and the computer code are applied to demonstrate distribution of the heat transfer coefficient between a pair of ribs. The results show that features of the inter-rib distribution of the heat transfer coefficient are strongly affected by the rib shape and trapezoidal ribs with decreasing height in the flow direction provide higher heat transfer enhancement and pressure drop than other shapes.

Flow and heat transfer over an unsteady stretching surface with non-uniform heat source

Abstract: The non-uniform heat source/sink effect on the flow and heat transfer from an unsteady stretching sheet through a quiescent fluid medium extending to infinity is studied. The boundary layer equations are transformed by using similarity analysis to be a set of ordinary differential equations containing three parameters: unsteadiness parameter (S), space-dependent parameter (A⁎) and temperature-dependent parameter (B⁎) for heat source/sink. The velocity and temperature fields are solved using the Chebyshev finite difference method (ChFD). Results showed that the heat transfer rate, − θ′(0) and the skin friction, − f″(0) increase as the unsteadiness parameter increases whereas decrease as the space-dependent and temperature-dependent parameters for heat source/sink increase.

Experimental investigation of shell and coiled tube heat exchangers using wilson plots

Abstract: In this work, an experimental investigation was performed to study the shell and helically coiled tube heat exchangers. Three heat exchangers with different coil pitches and curvature ratios were tested for both parallel-flow and counter-flow configurations. All the required parameters like inlet and outlet temperatures of tube-side and shell-side fluids, flow rate of fluids, etc. were measured using appropriate instruments. Overall heat transfer coefficients of the heat exchangers were calculated using Wilson plots. Heat transfer coefficients of shell and tube sides were evaluated invoking the calculated overall heat transfer coefficients. The inner Nusselt numbers were compared to the values existed in open literature. Though the boundary conditions were different, a reasonable agreement was observed.

Molecular dynamics modeling of thermal conductivity enhancement in metal nanoparticle suspensions

Abstract: A theoretical approach based on molecular dynamics modeling, for the estimation of the enhancement of the thermal conductivity of liquids by the introduction of suspended metallic nanoparticles is proposed. Algorithms are developed for simulating the nanofluid abiding the procedural steps of the molecular dynamics method. The method is presented as a solution to the generic problem of thermal conductivity enhancement of liquids in the presence of nanoparticles, and illustrated using a specific simulation procedure with properties representing water and platinum nanoparticles. The thermal conductivity enhancement estimated using the simulations are compared with existing experimental results and those predicted by conventional effective medium theories. Parametric studies are conducted to obtain the variation of thermal conductivity enhancement with the temperature and the volume fraction of the nanoparticles in the suspension.

Numerical analysis of natural convection for a porous rectangular enclosure with sinusoidally varying temperature profile on the bottom wall

Abstract: Numerical investigations of steady natural convection flow through a fluid-saturated porous medium in a rectangular enclosure with a sinusoidal varying temperature profile on the bottom wall were conducted. All the walls of the enclosure are insulated except the bottom wall which is partially heated and cooled. The governing equations were written under the assumption of Darcy-law and then solved numerically using finite difference method. The problem is analyzed for different values of the Rayleigh number Ra in the range 10 ≤ Ra ≤ 1000, aspect ratio parameter AR in the range 0.25 ≤ AR ≤1.0 and amplitude λ of the sinusoidal temperature function in the range 0.25 ≤ λ ≤ 1.0. It was found that heat transfer increases with increasing of amplitude λ and decreases with increasing aspect ratio AR. Multiple cells were observed in the cavity for all values of the parameters considered.

An experimental study of flow boiling instability in a single microchannel

Abstract: A simultaneous visualization and measurement study has been carried out to investigate stable and unstable flow boiling phenomena of deionized water in a single microchannel having a hydraulic diameter of 155 µm with a bottom Pyrex glass wall. Fifteen platinum serpentine microheaters, bonded on the Pyrex glass wall, were used to measure local instantaneous wall temperatures. At low mass flux, a syringe pump was used to drive the subcooled water passing through the microchannel. Stable and unstable flow boiling modes in the single microchannel are identified, and flow pattern maps in terms of heat flux and mass flux as well as in term of exit vapor quality are presented respectively. It was found that unstable flow boiling occurred in the single microchannel if the exit vapor quality x e > 0.013.

Thermal enhancement in a round tube with snail entry and coiled-wire inserts

Abstract: Influences of the insertion of coiled wires in conjunction with a snail-type swirl generator mounted at the tube entrance on heat transfer and turbulent flow friction characteristics in a uniform heat-flux, circular tube are experimentally investigated. In the present work, the coiled wire used as a turbulator is placed inside the test tube using air as the test fluid while the snail is employed to create a decaying swirl flow at the tube inlet. The effects of the snail entry and insertion of two different wire-sections of coils: square and circular, with a fixed pitch ratio on heat transfer rate in the tube are examined for the Reynolds number ranging from 5000 to 25,000. The experimental results are compared with those obtained from using coiled wire/snail entry alone, apart from the smooth tube. The results reveal that the presence of coil-wires together with the snail leads to a considerable increase in heat transfer and friction loss over the smooth tube. The snail entry with the coiled square-wire provides higher heat transfer rate than that with the circular one under the same conditions. Also, performance evaluation criteria to assess the real benefits in using both the coil-wire and the snail entry of the enhanced tube are determined.

Assessment of homotopy analysis method and homotopy perturbation method in non-linear heat transfer equation

Abstract: Two new analytical methods to solve nonlinear heat transfer equations are homotopy perturbation method and homotopy analysis method. Here, homotopy analysis method, which gives us a vast freedom to choose the answer type, is applied to solve nonlinear heat transfer differential equations and analytical results are compared with those of HPM and the numerical results. In this study, the procedure of HAM is applied to two cases in different ways according to the physics of the target problem. Comparing the two methods, our attention is focused on the results accuracy; and applicability of different methods in many cases with different limitation is studied. In the two examples of this paper, the effect of small parameter increaser on the accuracy of the analytical results of two methods also has been studied. The first differential equation is the modeling equation of a cooling lumped system with combined convection and radiation. The second one is the modeling equation of heat transfer with conduction in a slab of thermal dependent conductivity.

Optimum thermal performance of microchannel heat sink by adjusting channel width and height

Abstract: A three-dimensional numerical model of the microchannel heat sink is presented to study the effects of heat transfer characteristics due to various channel heights and widths. Based on the theory of a fully developed flow, the pressure drop in the microchannel is derived under the requirement of the flow power for a single channel. The effects of two design variables representing the channel width and height on the thermal resistance are investigated. In addition, the constraint of the same flow cross section is carried out to find the optimum dimension. Finally, the minimum thermal resistance and optimal channel width with various flow powers and channel heights are obtained by using the simulated annealing method.

Entropy generation due to conjugate natural convection in enclosures bounded by vertical solid walls with different thicknesses

Abstract: Entropy generation due to conjugate natural convection heat transfer and fluid flow has been studied inside an enclosure with bounded by two solid massive walls from vertical sides at different thicknesses. Enclosure is differentially heated from vertical walls and horizontal walls are adiabatic. Governing equations which are written in streamfunction-vorticity form solved by finite difference technique for the governing parameters as Rayleigh number, 103 ≤ Ra ≤ 106, length ratio of solid walls as ɛ1 (for left vertical wall) and ɛ2 (for right vertical wall) and thermal conductivity ratio of solid to fluid (k), 1 ≤ k ≤ 10. Entropy generation contours due to fluid friction and heat transfer irreversibility, isotherms, streamlines, Nusselt numbers and velocity profiles were obtained. It is found that entropy generation increases with increasing of thermal conductivity ratio and thicknesses of the walls. Entropy generation due to heat transfer is more significant than that of fluid flow irreversibility for all values of thickness of the solid vertical walls.

An evaluation on thermal performance of CPC solar collector

Abstract: The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 149% higher than that of the stationary CPC solar collector

Turbulence models application on CFD simulation of hydrodynamics, heat and mass transfer in a structured packing

Abstract: In this study, turbulence model applications on two-phase flow simulation in a structured packing are investigated using CFD application. Dry pressure drop, irrigated pressure drop, mass transfer and heat transfer are studied by k–e, RNG k–e, k–ω and BSL turbulence models. The best results obtained by k–ω and BSL models, but k–ω is recommended because it is more robust than BSL. The mean absolute relative error (MARE) between CFD prediction of k–ω model and experimental data for dry pressure drop, irrigated pressure drop, mass transfer and heat transfer are 16.9%, 10.7%, 8.1%, 0.9%, respectively.

Flow between two stretchable disks—An exact solution of the Navier–Stokes equations ☆

Abstract: In this work, an exact solution for the steady state Navier–Stokes equations in cylindrical coordinates is presented by similarity transformation technique. The solution involves the flow between two stretchable infinite disks with accelerated stretching velocity. The similarity equation was solved numerically and the effects of disk stretching parameter and stretching Reynolds number were studied. With the increase of the stretching Reynolds numbers, the fluid begins with a creeping type flow at R = 0 to a typical boundary layer type flow for large Reynolds numbers. The pressure parameter β changes from a positive number to a negative value with the increase of non-zero stretching parameter. The upper wall stretching parameter also greatly affects the velocity distribution between the two disks with a downward net flow for γ ≠ 1. The results are also useful as a benchmark problem for the validation of three-dimensional numerical computation code.

Thermal-diffusion and diffusion-thermo effects on combined heat and mass transfer of a steady MHD convective and slip flow due to a rotating disk with viscous dissipation and Ohmic heating☆

Abstract: Thermo-diffusion (Soret effect) and diffusion-thermo (Dufour effect) effects on combined heat and mass transfer of a steady hydromagnetic convective and slip flow due to a rotating disk in the presence of viscous dissipation and Ohmic heating is investigated. The partial differential equations governing the problem under consideration have been transformed by a similarity transformation into a system of ordinary differential equations which are solved numerically by applying the shooting method. For fluids of medium molecular weight (H2, air), profiles of the dimensionless velocity, temperature and concentration distributions are shown graphically for various values of slip parameter γ, magnetic field parameter M, Eckert Ec, Schmidt Sc, Dufour Du and Soret Sr numbers. Finally, numerical values of physical quantities, such as the local skin friction coefficient, the local Nusselt number and the local Sherwood number are presented in tabular form.

Numerical and experimental analysis of moisture transfer for convective drying of some products

Abstract: Numerical and experimental results of moisture transfer in drying process for apple and potato slices are compared in this study. Experimental results are obtained using a cyclone type dryer. Two-dimensional analysis of heat and moisture transfer during drying of objects is carried out solving heat and mass equations using finite-volume approach. Thus, moisture distributions inside the moist objects are obtained at different time steps. Comparison of results showed that there is a considerably high agreement between experimentally measured data and predicted values. Moist distribution also presented inside the products at different time periods.

Steady mixed convection across a confined square cylinder

Abstract: The effects of cross-buoyancy and of Prandtl number on the flow and heat transfer characteristics of an isothermal square cylinder confined in a channel has been investigated here. The numerical results have been presented for the range of conditions as: 1 ≤ Re ≤ 30, 0.7 ≤ Pr ≤ 100 (the maximum value of Peclet number being 3000) and 0 ≤ Ri ≤ 1for a fixed blockage ratio of 0.125. The overall drag and lift coefficients, local and average Nusselt numbers and the representative streamline and isotherm plots are presented to elucidate the role of Reynolds number, Prandtl number and Richardson number. The drag coefficient is found to be less sensitive to the Richardson number than the lift coefficient.

Design and fabrication of coaxial surface junction thermocouples for transient heat transfer measurements

Abstract: Low cost coaxial surface junction thermocouples (CSJTs') have been fabricated in-house and calibrated to measure the transient surface temperature rise within a UNITEN's shock tube wall facility, consisting of K-type coaxial thermocouple elements. The aim of this paper is to explain the design technique of the CSJTs' and the difficulties that have occurred during the fabrication process. The microstructural analysis and the chemical characterization for these types of thermocouples have also been carried out to verify the surface morphology and to qualitatively evaluate the CSJT materials composition. The preliminary testing was performed to demonstrate the performance of these thermocouples to be used for measuring the surface temperatures and heat transfer rates under transient conditions. The preliminary results from shock tube tests have shown that these thermocouples have a time response on the order of microseconds and were suitable for making heat transfer measurements in highly transient conditions. It was concluded that the current construction technique produced gauges that were reliable, reproducible, rugged and inexpensive.