Adiabatic Square Research Articles

Influence of Thermal Boundary conditions on Natural Convection Heat Transfer in a Wavy Walled Square Cavity with Adiabatic Block

The aim of the present study is to analyze heat transfer characteristics for natural convection flows in a square enclosure with wavy right wall filled with air(Pr=0.71) having uniform heating on bottom wall and adjacent cold walls with a adiabatic square block inside. This geometry and the following results can be used for analysis in heat management of building architecture, solar collection devices, having walls with surface roughness. The design/methodology/approach of these numerical solutions is the finite element analysis by a commercial software COMSOL Multiphysics version 5.6. The choice of consideration of sinusoidal heating gives a better analysis of heat transfer analysis in this square geometry with wavy wall and with adiabatic block inside has not been investigated by numerical or experimental basis before and therefore, it is this motivation that results in this numerical investigation.

Optimal position of square cylinder vortex generator in channel is not a function of ordinate only – A heat transfer and optimization study

Wall heat transfer enhancement of a channel by introducing an adiabatic square cylinder as a vortex generator is investigated numerically and reported. An open-source computational fluid dynamics toolbox, OpenFOAM is used for the present numerical investigation where a base solver icoFOAM is modified by incorporating the energy equation for analyzing the heat transfer characteristics. The cylinder position; axial (in the direction of flow) and transverse (normal to the flow direction) both the locations are varied to analyze the effect of these parameters on the wall heat transfer performance of the channel. In addition, the effect of change of channel length and height along with Reynolds number on the heat transfer characteristics are investigated. The present result indicates that the position of an adiabatic cylinder for enhancing the wall heat transfer in a channel may not be presumed a priori. It is observed that axial and transverse location of cylinder has significant influence on the enhancement of heat transfer rate. An optimum location of cylinder is observed based on maximum Nusselt number. Further, the Nusselt number is noticeably affected by the channel length and height along with Reynolds number. Thus, the location of a cylindrical vortex generator in a channel needs to be optimized for maximum thermal performance and cannot be ascertained a priori. For this purpose, a correlation of thermal performance with cylinder location, channel dimension, and Reynolds number is presented in the present study, and then the optimized location of the adiabatic cylinder is found for maximum thermal performance by using Particle Swarm Optimization (PSO).

Experiments on flow and heat transfer characteristics of a rectangular channel with a built-in adiabatic square cylinder

Experimental investigation of unsteady flow past a square cylinder confined in a channel and the associated heat transfer phenomena in laminar flow regime is carried out using purely non-intrusive diagnostic technique. A Mach-Zehnder interferometer has been employed to make the path-averaged measurements of the quantities of interest. Five different Reynolds numbers are considered and the heat transfer rates in the channel with and without the presence of square cylinder have been compared in the wake region of the built-in cylinder. Vortex shedding frequencies are determined using schlieren technique. Both the temperature field and the vortex shedding frequencies have been completely determined using non-intrusive techniques. The whole field experimental results depict significant augmentation in the wall heat transfer rates in the presence of a cylinder when compared with its counterpart (channel without cylinder). Flow instabilities induced by the square cylinder alter the thermal boundary layer profile and hence temperature gradients near the channel walls become steeper and consequently the wall heat transfer is significantly increased.

Unsteady MHD Natural Convection Flow of Nanofluid in a Cavity Containing Adiabatic Obstacle with Heat Corners

Unsteady natural convection heat transfer of nanofluid within cavities with local heaters occurs in several engineering applications. Therefore, investigation of nanofluid flow and heat transfer processes in such systems has a considerable value for evolution of industry. In the current investigation, unsteady MHD natural convection flow of Cu-water nanofluid and heat transfer behavior in square cavity containing a centered adiabatic square block. The mathematical formulation part for present problem is presented in succeeding section. It has been found the size of the adiabatic obstacle influences the behavior of the nanofluid and conduction becomes dominant when size aspect ratio increases to 0.5. It is also noticed that angle of the applied magnetic field can maximize(minimum)the rate of heat transfer if applied in the stream-wise (normal) direction. The novelty of the present work is to consider cavity containing a centered adiabatic square block as well as unsteady effects in the natural convection of nanofluids.

Buoyancy-induced convection from a pair of heated and cooled horizontal circular cylinders inside an adiabatic tilted cavity filled with alumina/water nanofluids

Purpose This paper aims to investigate numerically buoyancy-induced convection from a pair of differentially heated horizontal circular cylinders set side by side in a nanofluid-filled adiabatic square enclosure, inclined with respect to gravity so that the heated cylinder is located below the cooled one, using a two-phase model based on the double-diffusive approach assuming that the Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. Design/methodology/approach The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles, is solved by a computational code based on the SIMPLE-C algorithm. Numerical simulations are performed for Al2O3 + H2O nanofluids using the average volume fraction of the suspended solid phase, the tilting angle of the enclosure, the nanoparticle size, the average nanofluid temperature and the inter-cylinder spacing, as independent variables. Findings The main results obtained may be summarized as follows: at high temperatures, the nanofluid heat transfer performance relative to that of the pure base liquid increases with increasing the average volume fraction of the suspended solid phase, whereas at low temperatures it has a peak at an optimal particle loading; the relative heat transfer performance of the nanofluid has a peak at an optimal tilting angle of the enclosure; the relative heat transfer performance of the nanofluid increases notably as the average temperature is increased, and just moderately as inter-cylinder spacing is increased and the nanoparticle size is decreased. Originality/value The two-phase computational code used in the present study incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity and the coefficient of thermophoretic diffusion, all based on a high number of literature experimental data.

2D buoyant convective flow in a square box containing an adiabatic solid block: Effect of aspect ratio of solid block and various thermal boundaries

A numerical analysis is performed to study the natural convective flow in a square enclosed space filled with air containing an adiabatic square solid block at the center. The convective flow in a square cavity has examined for three different behaviours of heating. The left wall is heated at 1, y and sin(πy) and the temperature of the right wall is frozen to 0. The adiabatic square block that located in the center of the cavity has various aspect ratios associated to the main cavity. The two-dimensional flow and thermal pattern in the square cavity are investigated for various thermal boundary conditions and aspect ratio of the solid block. The averaged energy transport diminishes on raising the aspect ratio of the solid body for all kind of thermal boundary conditions.

Effect of Brownian motion on flow and heat transfer of nanofluids over a backward-facing step with and without adiabatic square cylinder

A mathematical model to predict large enhancement of thermal conductivity of nanofluids by considering the Brownian motion is proposed. The effect of the Brownian motion on the flow and heat transfer characteristics is examined. The computations were done for various types of nanoparticles such as CuO, Al2O3, and ZnO dispersed in a base fluid (water), volume fraction of nanoparticles ϕ in the range of 1 % to 6 % at a fixed Reynolds number Re = 450 and nanoparticle diameter dnp = 30 nm. Our results demonstrate that Brownian motion could be an important factor that enhances the thermal conductivity of nanofluids. Nanofluid of Al2O3 is observed to have the highest Nusselt number Nu among other nanofluids types, while nanofluid of ZnO nanoparticles has the lowest Nu. Effects of the square cylinder on heat transfer characteristics are significant with considering Brownian motion. Enhancement in the maximum value of Nu of 29 % and 26 % are obtained at the lower and the upper walls of the channel, respectively, by considering the Brownian effects, with square cylinder, compared with that in the case without considering the Brownian motion. On the other hand, results show a marked improvement in heat transfer compared to the base fluid, this improvement is more pronounced on the upper wall for higher ϕ.

Heat transfer enhancement in Rayleigh-Bénard convection of liquids using suspended adiabatic honeycombs

Laminar natural convection of water inside box-shaped enclosures heated at the bottom and cooled at the top, with perfectly insulated sidewalls, is studied numerically with the aim to investigate the enhanced heat transfer effects deriving from the suspension of an adiabatic square honeycomb in the middle of the enclosure so as to leave an upper and a lower clearance allowing for horizontal flows. A computational code based on the SIMPLE-C algorithm is used to solve the system of the mass, momentum and energy transfer governing equations. Simulations are performed using the Rayleigh number based on the cavity height, the overall number of cells in the honeycomb, and the ratio between the heights of the honeycomb and the cavity, as controlling parameters, whose effects are thoroughly analyzed and discussed. It is found that the suspension of the honeycomb inside the cavity normally leads to a significant heat transfer enhancement, that reaches a peak at an optimal layout of the honeycomb structure.

Natural convection from a pair of differentially-heated horizontal cylinders aligned side by side in a nanofluid-filled square enclosure

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection from a pair of differentially-heated horizontal cylinders set side by side in a nanofluid-filled adiabatic square enclosure. The study is conducted under the assumption that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles, is solved by the way of a computational code which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on a wide number of literature experimental data. The pressure-velocity coupling is handled through the SIMPLE-C algorithm. Simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, as well as the cavity width, the inter-cylinder spacing, the average temperature of the nanofluid, and the temperature difference imposed between the cylinders, as controlling parameters, whose effects are thoroughly analyzed and discussed. It is found that the impact of the nanoparticle dispersion into the base liquid increases remarkably with increasing the average temperature, whereas it increases just moderately as the nanoparticle size decreases, as well as the imposed temperature difference and the cavity width increase. Conversely, the distance between the cylinders seems to have marginal effects. Moreover, an optimal particle loading for maximum heat transfer is detected for most configurations investigated.

Laminar forced convection in power-law fluids from two heated cylinders in a square duct

Forced convection heat transfer has been investigated numerically for a pair of identical circular cylinders in side-by-side and in tandem arrangements confined in an adiabatic square enclosure filled with a power-law fluid with single inlet and outlet ports. The governing equations for the laminar, steady, 2-D and incompressible flow have been solved using a finite-element based method, COMSOL Multiphysics (Version 4.3a) for the following ranges of parameters: 5≤Re≤200; 0.7≤Pr≤100, 0.2≤n≤2 and 0.1≤D/L≤0.3. The numerical results obtained in terms of the velocity and temperature fields have been interpreted using streamlines, isotherm contours, plots of the individual and the total drag coefficients and the local and average Nusselt number. Further insights are developed with the help of velocity vectors and shear-rate contours prevailing in the flow domain, especially adjacent to the two cylinders. Owing to the symmetry, the two cylinders in side-by-side arrangement show identical momentum and heat transfer characteristics. The corresponding values for the upstream and downstream cylinders in the tandem arrangement differ from each other depending upon the values of the Reynolds and Prandtl numbers, power-law index and the diameter of the cylinders. In both cases, an increase in heat transfer is obtained at high values of the Reynolds and Prandtl number in shear-thinning fluids. Moreover, the upstream cylinder exhibits a higher drag and heat transfer as compared to the downstream cylinder. The non-linearity in the definitions of Reynolds and Prandtl numbers due to the occurrence of the power-law index leads to non-monotonic trends in the results. The flow and heat transfer phenomena are strongly influenced by the formation of secondary flows, reverse flow and the varying level of fluid acceleration due to the space constriction in the flow domain. Thus, the overall values of the pressure drop and Nusselt number are determined by a complex interplay between the kinematic conditions, fluid properties and geometric details of the system.

Entropy generation analysis in MHD mixed convection of hybrid nanofluid in an open cavity with a horizontal channel containing an adiabatic obstacle

A computational analysis has been performed in a horizontal channel with an open cavity filled with hybrid nanofluid of Al2O3-Cu-water having an adiabatic square obstacle inside. The bottom wall of the cavity is taken as hot while all the rest walls of the channel and cavity are adiabatic except the left end of the channel which is taken as cold. Three different vertical locations of the obstacle are considered. The governing partial differentials equations are solved via Galerkin finite element method in space and the Crank-Nicolson in time. Newton method is utilized to cope with discretized nonlinear systems of equations and the Gaussian elimination method has been applied to solve the associated linear subproblems in each nonlinear iteration. The emerging parameters are Richardson number (0.01⩽Ri⩽20), nanoparticle volume fraction (0.0⩽ϕ⩽0.04),(1⩽Re⩽200) and Hartmann number (0⩽Ha⩽100). Calculations of the streamlines, isotherms, average Nusselt number and entropy generation will be the main focus of interest in this study.

Mixed convection heat transfer of nanofluids in a trapezoidal cavity having an adiabatic square body at its center

Mixed convection heat transfer of nanofluids in a trapezoidal cavity having an adiabatic square body at its center

Natural convection on an open square cavity containing diagonally placed heaters and adiabatic square block and filled with hybrid nanofluid of nanodiamond - cobalt oxide/water

A finite difference based two dimensional simulations on laminar natural convection inside the open square cavity containing diagonal heaters and a central adiabatic square block is presented by vorticity – stream function approach. The enclosure is filled with hybrid nanofluid of Nanodiamond - Cobalt Oxide/Water. The top and bottom walls are considered as adiabatic and the vertical walls have diagonal heaters. The inlet port is placed on the left end of the top wall and the outlet is placed at the bottom of the right wall. The variables considered are Rayleigh number (104 to 106) and volumetric fraction of Nanodiamond - Cobalt Oxide (0 to 6%) particles. The results of fluid flow with single phase model are elucidated with streamlines, Isotherms and Average Nusselt number. The strength of the primary vortex depreciated with the increasing percentage of nano composites for all the Rayleigh numbers. Intensity of heat transfer is high in the right wall than the left wall.

Effect of adiabatic square ribs on natural convection in an asymmetrically heated channel

A 2-D numerical simulation is carried out to investigate the effect of two adiabatic square ribs on laminar flow and heat transfer in an asymmetrically heated channel. The two ribs are symmetrically located on each wall, exactly above the heating zone. The computational procedure is made by solving the unsteady bi-dimensional continuity, momentum and energy equations with the finite volume method. The investigations focused more specifically on the influence of ribs sizes on the flow structure and heat transfer enhancement. The results showed that the variation of ribs sizes significantly alters the heat transfer and fluid flow distribution along the channel, especially in the vicinity of protrusions. Also, the results show that streamlines, isotherms, and the number, sizes and formation of vortex structures inside the channel strongly depend on the size of protrusions. The changes in heat transfer parameters have also been presented.

Modeling of MHD natural convection in a square enclosure having an adiabatic square shaped body using Lattice Boltzmann Method

A steady laminar two-dimensional magneto-hydrodynamics (MHD) natural convection flow in a square enclosure filled with an electrically conducting fluid is numerically investigated using Lattice Boltzmann Method (LBM). The left and right vertical sidewalls of the square enclosure are maintained at hot and cold temperatures respectively. The horizontal top and bottom walls are considered thermally insulated. An adiabatic square shaped body is located in the center of a square enclosure and an external magnetic field is applied parallel to the horizontal x-axis. In the present work, the following parametric ranges of the non-dimensional groups are utilized: Hartmann number is varied as 0⩽Ha⩽50, Rayleigh number is varied as 103⩽Ra⩽105, Prandtl number is varied 0.05⩽Pr⩽5. It is found that the Hartmann number, Rayleigh number, and Prandtl number have an important role on the flow and thermal characteristics. It is found that when the Hartmann number increases the average Nusselt number decreases. The results also explain that the effect of magnetic field on flow field increases by increasing Prandtl number. However, the Prandtl number effect on the average Nusselt number with a magnetic field is less than the case without a magnetic field. Comparisons with previously published numerical works are performed and good agreements between the results are observed.

Experimental investigation of chimney-enhanced natural convection in hexagonal honeycombs

The natural convective heat transfer performance of an aluminum hexagonal honeycomb acting as a novel heat sink for LED cooling is experimentally investigated. The concept of adding an adiabatic square chimney extension for heat transfer enhancement is proposed, and the effects of chimney shape, height, and diameter are quantified. The average N uav of a heated honeycomb with straight chimney is significantly higher than that without chimney, and the enhancement increases with increasing chimney height. At a given chimney height, honeycombs with divergent chimneys perform better than those with convergent ones. For a fixed divergent angle, the N uav number increases monotonically with increasing chimney height. In contrast, with the convergent angle fixed, there exists an optimal chimney height to achieve maximum heat transfer.

An experimental study on thermal mixing in a square body inserted inclined narrow channels

An experimental study has been performed on thermal mixing phenomena in a narrow channel by twin-jets at different temperatures. Water was used as working fluid and it is supplied by hot and cold taps. The channel has a circular exit hole to supply continuity of mass. An adiabatic square shaped object, which in the thickness of the channel, is inserted into the channel to control thermal mixing as a passive technique. Other parameters in experiments are ratio of flow rate of inlet fluid, inclination angle of the channel, jet diameter and jet velocities. Finally, a thermal mixing index was calculated from measured values of temperatures for different parameters. Temperature distribution is obtained for whole channel and isotherms are plotted. The obtained results indicated that higher thermal mixing efficiency is observed for ϕ=60o and inserted body can be a control parameter for thermal mixing for the same geometrical parameters.

Natural convection in a square cavity containing a nanofluid and an adiabatic square block at the center

The problem of free convection fluid flow and heat transfer of Cu–water nanofluid inside a square cavity having adiabatic square bodies at its center has been investigated numerically. The governing equations have been discretized using the finite volume method. The SIMPLER algorithm was employed to couple velocity and pressure fields. Using the developed code, a parametric study was conducted and the effects of pertinent parameters such as Rayleigh number, size of the adiabatic square body, and volume fraction of the Cu nanoparticles on the fluid flow and thermal fields and heat transfer inside the cavity were investigated. The obtained results show that for all Rayleigh numbers with the exception of Ra=104 the average Nusselt number increases with increase in the volume fraction of the nanoparticles. At Ra=104 the average Nusselt number is a decreasing function of the nanoparticles volume fraction. Moreover at low Rayleigh numbers (103 and 104) the rate of heat transfer decreases when the size of the adiabatic square body increases while at high Rayleigh numbers (105 and 106) it increases.

Experimental Analysis of Microchannel Entrance Length Characteristics Using Microparticle Image Velocimetry

The study of the entrance region of microchannels and microdevices is limited, yet important, since the effect on the flow field and heat transfer mechanisms is significant. An experimental study has been carried out to explore the laminar hydrodynamic development length in the entrance region of adiabatic square microchannels. Flow field measurements are acquired through the use of microparticle image velocimetry (micro-PIV), a nonintrusive particle tracking and flow observation technique. With the application of micro-PIV, entrance length flow field data are obtained for three different microchannel hydraulic diameters of 500 μm, 200 μm, and 100 μm, all of which have cross-sectional aspect ratios of 1. The working fluid is distilled water, and velocity profile data are acquired over a laminar Reynolds number range from 0.5 to 200. The test-sections were designed as to provide a sharp-edged microchannel inlet from a very large reservoir at least 100 times wider and higher than the microchannel hydraulic diameter. Also, all microchannels have a length-to-diameter ratio of at least 100 to assure fully developed flow at the channel exit. The micro-PIV procedure is validated in the fully developed region with comparison to Navier–Stokes momentum equations. Good agreement was found with comparison to conventional entrance length correlations for ducts or parallel plates, depending on the Reynolds range, and minimal influence of dimensional scaling between the investigated microchannels was observed. New entrance length correlations are proposed, which account for both creeping and high laminar Reynolds number flows. These correlations are unique in predicting the entrance length in microchannels and will aid in the design of future microfluidic devices.

Numerical study of transient natural convection in a cavity with multiple adiabatic blocks

AbstractThis study deals with the solution of a problem of unsteady natural convection in a thermally driven square enclosure with four adiabatic square blocks placed inside. The governing equations are solved numerically using higher order compact schemes. Results are in good agreement with the established results for some special cases. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)