Showing papers in "Heat Transfer Research in 2015"

Thermal Radiation and Heat Source Effects on a MHD Nanofluid Past a Vertical Plate in a Rotating System with Porous Medium

Abstract: This article studies the effect of thermal radiation on a MHD free convection flow of a nanofluid bounded by a semi-infinite vertical plate with a constant heat source in a rotating frame of reference. The plate is assumed to oscillate in time with constant frequency so that the solutions of the boundary layer are the same oscillatory type. The dimensionless governing equations for this investigation are solved analytically using the regular perturbation method. The effect of various important parameters entering into the problem on velocity and temperature fields within the boundary layer are discussed for three different water-based nanofluids such as Cu, Al2O3, and TiO2 with the help of graphs. The predicted results clearly indicate that the presence of nanoparticles in the base fluid enhances the heat transfer process significantly. The present work shows the need for immediate attention in next-generation solar film collectors, heat-exchanger technology, material processing exploiting vertical surface, geothermal energy storage, and all those processes which are greatly exaggerated by heat-enhancement concepts. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21101

Heat Transfer in a Casson Rheological Fluid from a Semi-infinite Vertical Plate with Partial Slip

Abstract: The laminar boundary layer flow and heat transfer of Casson non-Newtonian fluid from a semi-infinite vertical plate in the presence of thermal and hydrodynamic slip conditions is analyzed. The plate surface is maintained at a constant temperature. Increasing velocity slip induces acceleration in the flow near the plate surface and the reverse effect further from the surface. Increasing velocity slip consistently enhances temperatures throughout the boundary layer regime. An increase in thermal slip parameter strongly decelerates the flow and also reduces temperatures in the boundary layer regime. An increase in the Casson rheological parameter acts to elevate considerably the skin friction (non-dimensional wall shear stress) and this effect is pronounced at higher values of tangential coordinate. Temperatures, however, are very slightly decreased with increasing values of Casson rheological parameter. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21115

Optimization of Output Power and Thermal Efficiency of Solar‐Dish Stirling Engine Using Finite Time Thermodynamic Analysis

Abstract: This paper presents an investigation on finite time thermodynamic (FTT) evaluation of a solar-dish Stirling heat engine. FTTs has been applied to determine the output power and the corresponding thermal efficiency, exergetic efficiency, and the rate of entropy generation of a solar Stirling system with a finite rate of heat transfer, regenerative heat loss, conductive thermal bridging loss, and finite regeneration process time. Further imperfect performance of the dish collector and convective/radiative heat transfer mechanisms in the hot end as well as the convective heat transfer in the heat sink of the engine are considered in the developed model. The output power of the engine is maximized while the highest temperature of the engine is considered as a design parameter. In addition, thermal efficiency, exergetic efficiency, and the rate of entropy generation corresponding to the optimum value of the output power is evaluated. Results imply that the optimized absorber temperature is some where between 850 K and 1000 K. Sensitivity of results against variations of the system parameters are studied in detail. The present analysis provides a good theoretical guidance for the designing of dish collectors and operating the Stirling heat engine system.

Analytical Solution and Numerical Simulation for One-Dimensional Steady Nonlinear Heat Conduction in a Longitudinal Radial Fin with Various Profiles

Abstract: In this article we consider a model describing the temperature profile in a longitudinal fin with rectangular, concave, triangular, and convex parabolic profiles. Both thermal conductivity and the heat transfer coefficient are assumed to be temperature-dependent, and given by a linear function and by power laws, respectively. In addition, the effects of the thermal conductivity gradient have been investigated. Optimal homotopy analysis method (OHAM) is employed to analyze the problem. The effects of the physical applicable parameters such as thermo-geometric fin, thermal conductivity, and heat transfer mode are analyzed. The OHAM solutions are obtained and validity of obtained solutions is verified by the Runge–Kutta fourth-order method and numerical simulation. A very good agreement is found between analytical and numerical results. Also for investigation of lateral effects on the accuracy of results, numerical simulation (by Ansis software) is compared with the homotopy analysis method (HAM) and numerical solution (by Runge–Kutta) of the energy balance equation. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21104

Non‐Darcian Unsteady Flow of a Micropolar Fluid over a Porous Stretching Sheet with Thermal Radiation and Chemical Reaction

Abstract: An analysis is presented to investigate the effects of a chemical reaction on an unsteady flow of a micropolar fluid over a stretching sheet embedded in a non-Darcian porous medium. The governing partial differential equations are transformed into a system of ordinary differential equations by using similarity transformation. The resulting nonlinear coupled differential equations are solved numerically by using a fourth-order Runge–Kutta scheme together with shooting method. The influence of pertinent parameters on velocity, angular velocity (microrotation), temperature, concentration, skin friction coefficient, Nusselt number, and Sherwood number has been studied and numerical results are presented graphically and in tabular form. Comparisons with previously published work are performed and the results are found to be in excellent agreement. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21090

Peristaltic Transport of a Herschel–Bulkley Fluid in an Elastic Tube

Abstract: In this paper, we investigate the peristaltic transport of a non-Newtonian viscous fluid in an elastic tube. The governing equations are solved using the assumptions of long wavelength and low Reynolds number approximations. The constitution of blood has a non-Newtonian fluid model and it demands the yield stress fluid model: The blood transport in small blood vessels is done under peristalsis. Among the available yield stress fluid models for blood flow, the non-Newtonian Herschel–Bulkley fluid is preferred (because Bingham, power-law and Newtonian models can be obtained as its special cases). The Herschel–Bulkley model has two parameters namely the yield stress and the power-law index. The expressions for velocity, plug flow velocity, wall shear stress, and the flow rate are derived. The flux is determined as a function of inlet, outlet, external pressures, yield stress, amplitude ratio, and the elastic property of the tube. Further when the power-law index n = 1 and the yield stress and in the absence of peristalsis, our results agree with Rubinow and Keller [J. Theor. Biol. 35, 299 (1972)]. Furthermore, it is observed that, the yield stress, peristaltic wave, and the elastic parameters have strong effects on the flux of the non-Newtonian fluid flow. Effects of various wave forms (namely, sinusoidal, trapezoidal and square) on the flow are discussed. The results obtained for the flow characteristics reveal many interesting behaviors that warrant further study on the non-Newtonian fluid phenomena, especially the shear-thinning phenomena. Shear thinning reduces the wall shear stress.

First and Second Law Analysis for the MHD Flow of Two Immiscible Couple Stress Fluids between Two Parallel Plates

Abstract: This paper aims to analyze the heat transfer by the first and second laws of thermodynamics for the flow of two immiscible couple stress fluids inside a horizontal channel under the action of an imposed transverse magnetic field. The plates of the channel are maintained at constant and different temperatures higher than that of the fluid. The flow region consists of two zones, the flow of the heavier fluid taking place in the lower zone. No slip condition is taken on the plates and continuity of velocity, vorticity, shear stress, couple stress, temperature, and heat flux are imposed at the interface. The velocity and temperature distributions are derived analytically and these are used to compute the dimensionless expressions for the entropy generation number and Bejan number. The results are presented graphically. It is observed that the imposed magnetic field reduces the entropy production rate near the plates.

Slip Flow of Casson Rheological Fluid Under Variable Thermal Conductivity with Radiation Effects

Abstract: This paper investigates the radiation and chemical reaction effects on Casson non-Newtonian fluid towards a porous stretching surface in the presence of thermal and hydrodynamic slip conditions. The governing boundary layer conservation equations are normalized into nonsimilar form using similarity transformations. A numerical approach is applied to the resultant equations. The behavior of the velocity, temperature, concentration, as well as the skin friction coefficient, Nusselt number, and Sherwood number for various governing physical are discussed. Increasing the radiation parameter decreases the temperature. An increase in the rheological parameter (Casson parameter) induces an elevation in the skin friction coefficient, the heat and mass transfer rates. The larger the β values the closer the fluid is in behavior to a Newtonian fluid and further departs from plastic flow. Temperature of the fluid was found to decrease with increasing values of the Casson rheological parameter. The most important non-Newtonian fluid possessing a yield value is the rheological Casson fluid, which finds significant applications in polymer processing industries, biomechanics, and chocolate food processing.

Three‐Dimensional Numerical Investigation of Nanofluids Flow in Microtube with Different Values of Heat Flux

Abstract: Forced convective laminar flow of different types of nanofluids such as Al2O3, CuO, SiO2, and ZnO, with different nanoparticle size 25, 45, 65, and 80 nm, and different volume fractions which ranged from 1% to 4% using ethylene glycol as base fluids were used. A three-dimensional microtube (MT) with 0.05 cm diameter and 10 cm in length with different values of heat fluxes at the wall is numerically investigated. This investigation covers Reynolds number (Re) in the range of 80 to 160. The results have shown that SiO2-EG nanofluid has the highest Nusselt number (Nu), followed by ZnO-EG, CuO-EG, Al2O3-EG, and finally pure EG. The Nu for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nu increases with Re. In addition, no effect of heat flux values on the Nu was found.

Optimization of the blade profile and cooling structure in a gas turbine stage considering both the aerodynamics and heat transfer

Abstract: The need to design high performance of a cooled gas turbine is considered with emphasis made on coupled aerodynamic and heat transfer optimization of the vane, blade, and single stage cooled gas turbine by using a multiobjective optimization method. The aerodynamic profile is designed to have three sections and the cooling structure to consist of a serpentine passage, with a tail transverse channel and trailing edge slots. The optimization platform is built up in an in-house code using a cooling structure parametric method based on MATLAB, as well as automatic grid generation methods, a blade profile parametric program in FORTRAN, the soft ware ISIGHT and ANSYS-CFX. The optimization platform evaluates the aerodynamic effects through the aerodynamic efficiency and presents the cooling effect by the high-temperature coefficient. The pressure drop is described by a pressure drop function. The multiobjective optimization method is accomplished by optimizing the inlet flow angle, installation angle, and the post-corner angle of the vane and blade profiles, while the position of partition is the optimized variable of the cooling structure. The results show that there exists an optimum case in aerodynamic efficiency, high-temperature coefficient, and pressure drop in a Pareto-optimal front. (Less)

Heat Transfer Control of an Impinging Inclined Slot Jets on a Moving Wall

Abstract: This work is devoted to the numerical study of the interaction of an inclined plane turbulent jet with a moving horizontal isothermal hot wall. The inclination of the jet allows the control of the stagnation point location. The numerical predictions based on statistical modeling are achieved using second order Reynolds stress turbulence model coupled to the enhanced wall treatment. The jet Reynolds number (Re), surface-to-jet velocity ratio (Rsj); and optimal inclination angle of the jet (α) are varied. The calculations are in good agreement with the available data. The numerical results show that the heat transfer is greatly influenced by the jet Re and the velocity of the moving wall. The local Nusselt number (Nu) decreases with increasing Rsj (until Rsj = 1). However, the optimal inclination of the jet enhances heat transfer and modifies significantly the stagnation point location. Average Nu is correlated according with the problem parameters as .

Finite Element Analysis of Radiative Mixed Convection Magneto‐Micropolar Flow in a Darcian Porous Medium with Variable Viscosity and Convective Surface Condition

Abstract: A mathematical study is presented for the collective influence of the buoyancy parameter, convective boundary parameter and temperature dependent viscosity on the steady mixed convective laminar boundary flow of a radiative magneto-micropolar fluid adjacent to a vertical porous stretching sheet embedded in a Darcian porous medium. The fluid viscosity is assumed to vary as an inverse linear function of temperature. Using appropriate transformations, the governing equations of the problem under consideration are transformed into a system of dimensionless nonlinear ordinary differential equations, which are then solved with the well-tested, efficient finite element method. The results obtained are depicted graphically to illustrate the effect of the various important controlling parameters on velocity, microrotation, and temperature functions. The skin friction coefficient, wall couple stress, and the rate of heat transfer have also been computed and presented in tabular form. Comparison of the present numerical results with earlier published data has been performed and the results are found to be in good agreement, thus validating the accuracy of the present numerical code. The study finds applications in conducting polymer flows in filtration systems, trickle bed magnetohydrodynamics in chemical engineering, electro-conductive materials processing, and so on.

Effect of Newtonian Heating and Thermal Radiation on Heat and Mass Transfer of Nanofluids over a Stretching Sheet in Porous Media

Abstract: Laminar boundary layer slip flow from a stretching surface in a nanofluid-saturated homogenous, isotropic porous medium is studied numerically. A Newtonian heating boundary condition in the presence of thermal radiation is incorporated and a Darcy model utilized for the porous medium. The model used for the nanofluids include the effects of Brownian motion and thermophoresis. A group theoretical analysis is conducted to generate similarity transformations. The governing transport equations are nondimensionalized and rendered into a set of coupled similarity ordinary differential equations using similarity transformations. The transformed equations are then solved using the Runge–Kutta–Fehlberg fourth-fifth order numerical method with shooting technique. It is shown that the physical quantities of interest depend on a number of parameters. The results are presented in tabular and graphical forms. Comparison of the present numerical solutions with published work shows very good agreement. The study finds applications in high-temperature nanotechnological materials processing.

Simulation study of the adsorption dynamics of cylindrical silica gel particles

Abstract: The paper presents a simulation study of loose cylindrically shaped particles packed within a copper plate and aluminum fins. The model presented solves coupled heat and mass transfer equations using the finite volume method based on ANSY S FLUENT medium. Three different arrangements of cylindrical particles are considered. The model is validated with experimental data. It is found that the arrangements which represented monolayer configurations are only marginally better in heat transfer and uptake efficiency than the tri-layer configuration in the presence of fins. However, there is an appreciable difference in the uptake curve between monoand tri-layer configurations in the absence of fins. Finally, it is found that the fin pitch also plays an important role in determining the time constant for the adsorber design.

Second Law Efficiency Analysis of Heat Exchangers

Abstract: Analytical analysis of unbalanced heat exchangers is carried out to study the second law thermodynamic performance parameter through second law efficiency by varying length-to-diameter ratio for counter flow and parallel flow configurations. In a single closed form expression, three important irreversibilities occurring in the heat exchangers—namely, due to heat transfer, pressure drop, and imbalance between the mass flow streams—are considered, which is not possible in first law thermodynamic analysis. The study is carried out by giving special influence to geometric characteristics like tube length-to-diameter dimensions; working conditions like changing heat capacity ratio, changing the value of maximum heat capacity rate on the hot stream and cold stream separately and fluid flow type, i.e., laminar and turbulent flows for a fully developed condition. Further, second law efficiency analysis is carried out for condenser and evaporator heat exchangers by varying the effectiveness and number of heat transfer units for different values of inlet temperature to reference the temperature ratio by considering heat transfer irreversibility. Optimum heat exchanger geometrical dimensions, namely length-to-diameter ratio can be obtained from the second law analysis corresponding to lower total entropy generation and higher second law efficiency. Second law analysis incorporates all the heat exchanger irreversibilities. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21109

Numerical Investigation of Double Diffusive Mixed Convection Laminar Flow in Two Sided Lid Driven Porous Cavity

Abstract: This paper presents the numerical study of mixed convection in a two-sided lid driven porous cavity due to temperature and concentration gradients. The top and bottom walls are stationary and insulated. The left and right walls are moving at an equal velocity (Vo) in the same direction. The temperature and concentration are kept high at the right wall and low at the left wall. The governing equations are discretized using finite volume method. The pressure–velocity coupling is performed by the SIMPLE algorithm. A third order differed QUICK scheme is applied at the inner nodes and a second order central difference scheme is used at the boundary nodes. The flow behavior and heat transfer are analyzed for different nondimensional numbers, such as, 1 × 10−4 ≤ Ri ≤ 10, 1 × 10−4 ≤ Da ≤ 0.1 and 0.7 < Pr < 10. The present numerical results are compared with the literature and are in good agreement. For the above selected nondimensional numbers, the heat and fluid flow behavior is investigated using local and average Nusselt (Nu) and Sherwood (Sh) numbers. Results show that the convection flow is significant up to Da = 0.1, beyond that the effect of porosity is negligible. The effect of Prandtl number (Pr) on average Nu is found to increase significantly.

Approximate Analytical Solution of Stagnation Point Flow and Heat Transfer over an Exponential Stretching Sheet with Convective Boundary Condition

Abstract: This study is concerned with the stagnation point flow and heat transfer over an exponential stretching sheet via an approximate analytical method known as optimal homotopy asymptotic method (OHAM). The governing partial differential equations are converted into ordinary nonlinear differential equations using similarity transformations available in the literature. The heat transfer problem is modeled using two-point convective boundary condition. These equations are then solved using the OHAM approach. The effects of controlling parameters on the dimensionless velocity, temperature, friction factor, and heat transfer rate are analyzed and discussed through graphs and tables. It is found that the OHAM results match well with numerical results obtained by Runge–Kutta Fehlberg fourth-fifth order method for different assigned values of parameters. The rate of heat transfer increases with the stretching parameter. It is also found that the stretching parameter reduces the hydrodynamic boundary layer thickness whereas the Prandtl number reduces the thermal boundary layer thickness.

Entropy Generation of Natural Convection Heat Transfer in a Square Cavity Using Al2O3–Water Nanofluid

Abstract: Entropy generation of an Al2O3–water nanofluid due to heat transfer and fluid friction irreversibility has been investigated in a square cavity subject to different side-wall temperatures using a nanofluid for natural convection flow. This study has been carried out for the pertinent parameters in the following ranges: Rayleigh number between 104 and 107 and volume fraction between 0 and 0.05. Based on the obtained dimensionless velocity and temperature values, the distributions of local entropy generation, average entropy generation, and average Bejan number are determined. The results are compared for a pure fluid and a nanofluid. It is totally found that the heat transfer, and entropy generation of the nanofluid is more than the pure fluid and minimum entropy generation and Nusselt number occur in the pure fluid at any Rayleigh number. Results depict that the addition of nanoparticles to the pure fluid has more effect on the entropy generation as the Rayleigh number goes up.

Mass Transfer Effects on Viscous Flow in an Expanding or Contracting Porous Pipe with Chemical Reaction

Abstract: In this paper, an analysis is performed to study the effects of mass transfer and chemical reaction on laminar flow in a porous pipe with an expanding or contracting wall. The pipe wall expands or contracts uniformly at a time dependent rate. The governing equations are reduced to ordinary differential equations by using a similarity transformation. An analytical approach, namely, the homotopy analysis method is applied in order to obtain the solutions of the ordinary differential equations. The convergence of the obtained series solutions is analyzed. The effects of various parameters on flow variables have been discussed. It is noticed that the wall expansion ratio significantly increases the axial velocity and the concentration for the case of wall expansion and it decreases the axial velocity for the case of wall contraction irrespective of injection or suction. Further, it is observed that the concentration (ϕ) decreases for a destructive chemical reaction () and increases for a generative chemical reaction (). The concentration reduces as Schmidt number () increases. The corresponding problem related to the porous pipe flow with a stationary wall can be recovered from the present analysis in the limiting case where the wall expansion ratio approaches to zero (i.e., ).

An Analytical Solution for Fully Developed Forced Convection in Triangular Ducts

Abstract: In this paper, an exact analytical solution for fully developed convective heat transfer in equilateral triangular ducts under constant heat flux at the walls is presented The previous studies have been performed using numerical methods and to the knowledge of authors, this study is the first EXACT analytical solution about the heat convection in triangular ducts Here, the finite series expansion method is used to derive the closed form of dimensionless temperature distribution The Nusselt number and dimensionless temperature at the center of the cross section were calculated equal to 28/9 and 5/9, respectively The present analytical solution could be useful in analysis of the heat convection in microfluidics and designing compact heat exchangers

Comparative Performance of Different Sector Arrangement in a Desiccant Wheel Using a Mathematical Model

Abstract: A one-dimensional mathematical model has been used for a three-sector desiccant wheel (two sectors with purge) with different flow arrangements. The model considers both gas and solid side resistance and shows a good agreement with experimental results. This model has been used to conduct a comparative performance analysis in both the effective adsorption and effective regeneration sector of a desiccant wheel. It was found that an effective regeneration sector gives better results for the performance parameters (rotation of wheel, regeneration temperature, velocity, and ambient moisture) as compared to an effective adsorption sector. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21103

Modeling and Optimization of Nanofluid Flow in Flat Tubes Using a Combination of CFD and Response Surface Methodology

Abstract: In this paper, modeling and optimization of Al2O3–water nanofluid flow in horizontal flat tubes is performed using a combination of computational fluid dynamics (CFD) and response surface methodology (RSM). At first, nanofluid flow is solved numerically in various flat tubes using CFD techniques and the heat transfer coefficient () and pressure drop () in tubes are calculated. The numerical simulations are performed using two phase mixture model by FORTRAN programming language. The flow regime and the wall boundary conditions are assumed to be laminar and constant heat flux respectively. In the second step, numerical data of the previous step will be used for a parametric study, modeling and optimization of nanofluid flow in flat tubes using the RSM technique.It is shown that the results include important design information on nanofluid parameters in flat tubes. The important design information about the relationship between design variables and responses will not be achieved without the simultaneous use of CFD and optimization approaches.

Numerical Study of Natural Convection in an Enclosure with an Internal Heat Source at Higher Rayleigh Numbers

Abstract: Natural convection is extensively used in cooling of large scale electrical and electronic equipments. This work involves study of flow and heat transfer characteristics in enclosures with partial openings having an internal heat source at higher Rayleigh number (Rah > 106). It involves the numerical simulation of 2D steady state natural convection in enclosures of different aspect ratios (H/W = 2 and 3) for five Rayleigh numbers (Rah = 107, 108, 109, 1010, and 1011). Two different configurations have been considered based on the number and position of vents—diagonal side (DS) and two inlets one outlet (2I1O). The time dependent nature of the flow is characterized by performing a Fast Fourier Transform (FFT) analysis of temperature and velocity at a characteristic location in the enclosure. The global parameters considered are the mass flow rate driven through the cavity by the heater and the average Nu defined over the heater surface. It is seen that with increase in Rah, flow becomes more fluctuating and moves towards chaotic regime and this transition is quicker at lower H/W. For the given configuration both the global parameters increases with increase in Rah and decrease in H/W.