Laminar Convection Heat Transfer Research Articles

Numerical investigation on convective heat transfer over two heated wall-mounted cubes in tandem and staggered arrangement

ABSTRACT In this study, laminar convective heat transfer over two heated wall-mounted cubes is investigated. Two cubes, which are under constant heat flux, are placed in different tandem and staggered arrangements on a base plate. This problem is studied for different streamwise and spanwise distances between two cubes in different Renolds number ( Re ), by using finite-volume method. Effects of these parameters are considered on flow and heat transfer characteristics. The results show that the temperature distribution is strongly dependent on flow structure and varies with any change of flow pattern in different arrangements of cubes. In addition, it is observed that the drag coefficient, which is influenced more by pressure forces, in staggered arrangement, is greater than tandem arrangement. Results show that by increasing the spanwise distance the amount of mean Nusselt number ( Nu ) of Cube 2 becomes the same as Cube 1.

Combined Natural Convection and Thermal Radiation Heat Transfer in a Triangular Enclosure with an Inner Rectangular Body

In this paper, a numerical simulation was conducted to investigate the steady laminar natural convective heat transfer and surface radiation from a heated rectangular body in a triangular enclosure. The coupled equations of Navier-stockes and energy are both solved using the finite volume method. The velocity-pressure coupling is insured by the SIMPLER algorithm. The fluid used in this study is a dry air of Prandtl numberPr= 0.71.In such way that, the radiative exchanges are made only through solid walls and which are assumed to be gray and diffuse. The physical parameters characterizing the problem and influencing heat transfer are Rayleigh numberRa, aspect ratioA, heighthand widthwof the body. The results are presented in terms of isotherms, streamlines and average Nusselt number.

Investigation of nanofluids on heat transfer enhancement in a louvered microchannel with lattice Boltzmann method

Numerical studies of laminar forced convective heat transfer and fluid flow in a 2D louvered microchannel with Al2O3/water nanofluids are performed by the lattice Boltzmann method (LBM). Eight louvers are arranged in tandem within the single-pass microchannel. The Reynolds number based on channel hydraulic diameter and bulk mean velocity ranges from 100 to 400, where the Al2O3 fraction varies from 0 to 4%. A double distribution function approach is adopted for modeling fluid flow and heat transfer. Code validations are performed by comparing the streamwise Nusselt number (Nu) profiles and Fanning friction factors of the present LBM and those of the analytical solutions. Good agreements are obtained. Simulated results show that the louver microstructure can disturb the core flow and guide coolant toward the heated walls, thus enhancing the heat transfer significantly. Furthermore, the addition of nanoparticles in microchannels can also augment the heat transfer, but it creates an unnoticeable pressure loss. With both the louver microstructure and nanofluid, a maximum overall Nu enhancement of 7.06 is found relative to that of the fully developed smooth channel.

Experimental and numerical study of laminar mixed convection from a horizontal isothermal elliptic cylinder

The problem of laminar mixed convective heat transfer from a horizontal isothermal elliptic cylinder has been experimentally and numerically investigated. The test elliptic cylinder has major and minor diameters of 70.3 mm and 34.9 mm respectively with an axis ratio of 0.496. The buoyancy due to the temperature difference between the cylinder surface and incoming air produces a Grashof number of 2.1 × 106. Via the incoming cold air velocity, the generated Reynolds number varies from 145 to 1322.8. Hence, the corresponding Richardson number is in the range of 1.2–100. The combined effect of both upward natural convection and the forced convection has been executed for angle of attack from 0° (assisting flow) to 180° (opposing flow) with an interval of 30°. The numerical solution via the finite volume method (FVM) has been accomplished to validate the obtained experimental results. The numerical results are validated by previous experimental results which show good agreement. Results are presented in the form of average and local Nusselt number around the circumference of the elliptic cylinder. In addition, new empirical correlation is obtained for the average Nusselt number as a function of Richardson number and the attack angle.

A NUMERICAL STUDY OF FORCED CONVECTIVE FLOW IN MICROCHANNELS HEAT SINKS WITH PERIODIC EXPANSION-CONSTRICTION CROSS SECTION

This paper aims to study numerically the laminar convective heat transfer of ionized water flow inside rectangular heat sinks with periodic expansion-constriction cross-section; each heat sink consists of parallel microchannels system with 4 mm wide and 0.1 mm deep in constant cross-section segment.  Two-dimensional laminar numerical simulations, based on Navier-Stoks equations and energy equation, are obtained under the same boundary conditions for different microchannels. In this study, the heat transfer and pressure drop inside microchannels with cross-section (cylindrical grooves and triangular cavities) are compared with that of simple smooth microchannel at Reynolds number ranging from 150-1500; an increase in pressure drop of 44% for all microchannels  is observed with Reynolds number increasing.  The obtained results indicate an enhancement in Nusselt number for all microchannels at all Reynolds number values with a maximum enhancement of 36%, these ameliorated thermal parameters attribute to enhance the heat transfer efficiency of proposed microchannels. Which improve the effect of periodic expansion-constriction cross-section on the heat transfer performance for microelectromechanical systems (MEMS) cooling phenomena.  

Laminar convection heat transfer and flow performance of Al2O3–water nanofluids in a multichannel-flat aluminum tube

The heat transfer experiments of Al2O3–water nanofluids were carried out at fixed Reynold number and velocity in a multichannel-flat aluminum tube. The studies we have performed revealed that the convection heat transfer coefficient increased about 5.9% at 0.5vol.% Al2O3 concentration at Re=1732. While at a constant velocity, higher addition of nanoparticles had no evident influence on the heat transfer performance. Compared with theoretical correlations, the data obtained were accorded with Shah equation (deviation <10%) except for those in the entrance. Then a comparative study of Al2O3–water nanofluids and the base fluid was made on the local heat transfer coefficient. The results indicated a higher heat transfer augmentation (11.1%) in the entrance region for 0.5vol.% Al2O3-DW nanofluid, which seems to have a bearing on the thermal conductivity enhancement and the thermal boundary layer thickness reduction. The friction factor and the pressure drop were also studied and the results revealed that the pressure drop slightly increased with the volume concentration of nanoparticles. For 0.5vol.% Al2O3-DW, the average pressure drop increased about 4.4%.

Laminar mixed convective heat transfer enhancement by using Ag-TiO2-water hybrid Nanofluid in a heated horizontal annulus

In the present paper, laminar mixed convection in horizontal annulus filled with a TiO2/water nanofluid and Ag-TiO2/water hybrid nanofluid has been numerically studied. The outer cylinder is uniformly heated while the inner cylinder is adiabatic. The governing equations with the appropriate boundary conditions are discretized by the finite volume method with second order precision, and solved by using the SIMPLER and Thomas algorithms. The numerical simulations are performed for various nanoparticles volume fractions, between 0 and 8% and Grashof numbers between 105 and 106. The results shows that for all studied Grashof numbers, the local and average Nusselt numbers, and the bulk temperature increase with the increasing of the volume fraction and the Grashof number. The heat transfer is very enhancement when using a Ag-TiO2/water hybrid nanofluid compared to the similar TiO2/water nanofluid. Moreover, the exploitation of the numerical results that we obtained enabled us to develop two new correlations, which allow the estimation of the average Nusselt number. The results reveal that the numerical data are in a good agreement with the correlation data. The maximum error for nanofluid and hybrid nanofluid was around 2.5% and 4.7% respectively. Hence, among the multitude of the obtained results in this work, it remains that the new correlations developed, especially for the hybrid nanofluid Ag-TiO2 / water, constitute for their originality, the most significant result of this research.

A comparative study of 2-D and 3-D conjugate natural convection from a vertical rectangular fin array with multilayered base subjected to distributed high heat flux

This paper presents a comparative numerical study of 2-D and 3-D steady, laminar conjugate natural convection heat transfer from a vertical rectangular aluminium fin array subjected to a distributed high heat flux related to the cooling of a 105 W LED street lighting luminaire. The effect of radiation is also considered in the aforementioned models. The 2-D model uses the Stream function-Vorticity-Temperature approach. The finite-difference methodology is used to discretize the governing equations. The discretization method in the 3-D model is finite-volume. The solution algorithm is SIMPLE (Semi-Implicit Method for Pressure-linked Equations). A detailed parametric study based on a realistic lamp data is conducted to observe the effect of heat flux, fin spacing, fin height, fin width, fin thickness and fin material (aluminium, brass, copper, mild steel) on the fluid (air) velocity and temperature distributions, heat transfer and LED junction temperature. A novel finding of this work is the revelation of asymmetric flow and thermal fields in the successive channels of the fin array, as predicted by the 3-D model. In contrast with the existing studies in which fin base is treated as isothermal, an optimal fin spacing is found to exist corresponding to the maximum heat transfer coefficient in the present study. The difference between 2-D and 3-D model-predicted average heat transfer coefficients increases with increase in fin height to fin spacing ratio. The contribution of radiation heat transfer is about 8–14% of the total heat transfer in the range of dimensions of the fin array considered in this work. A penalty function based optimization study is carried out to maximize the average heat transfer coefficient while maintaining the LEDs below the safe limit of 353 K. Finally, correlations for predicting the average heat transfer coefficient in the vertical rectangular fin array heat sink, and the maximum LED temperature as a function of luminaire wattage and geometric parameters of the fin array have been presented.

Numerical analysis on enhanced performance of new coaxial cross twisted tapes for laminar convective heat transfer

Owing to the high viscosity of lubricating oil, oil coolers meet the disadvantages of low heat transfer coefficients and thus large volumes. In order to enhance the heat transfer performance, a new type of twisted tape, called coaxial cross twisted tape, is proposed based on the field synergy theory and traditional twisted tapes. The numerical analysis of the heat transfer performance is presented in the article using CFD software STAR-CCM+. The local flow characteristics, and local and average convective heat transfer coefficients are analyzed, when the Reynolds number ranges from 40 to 1050. The effects of the four different twist ratios of 2.0, 3.0, 4.0 and ∞ on the heat transfer performance are also investigated. To evaluate the effect of heat transfer enhancement under given pumping power, the performance evaluation criterion (PEC) is discussed. The results indicate that the local convective heat transfer coefficient increases markedly with the insertion of the new twisted tapes because of the increase of the velocity near wall and thickness reduction of thermal boundary layer. The more flow zones are separated by the coaxial cross twisted tapes, the larger is the temperature gradient near wall and the higher is convective heat transfer coefficient. The Nusselt number and PEC of the heat transfer tube fitted with the new coaxial cross twisted tapes have increased by 151–195% and 90–123% of the traditional twisted tape, respectively.

Study of nanofluid forced convection heat transfer in a bent channel by means of lattice Boltzmann method

In this paper, the laminar forced convection heat transfer of nanofluid through a bent channel was numerically investigated. The lattice Boltzmann method was used for solving the governing equations in the domain. The effect of different parameters such as Reynolds number (50 ≤ Re ≤ 150), vertical passage ratio (2.0 ≤ M ≤ 4.0), and nanoparticle solid volume fractions (Φ = 0, 0.01, 0.03, 0.05) are analyzed in terms of streamlines, isotherms, and local Nusselt numbers. It was concluded from this study that the local and average Nusselt number increased with increasing nanoparticle volume fraction regardless of Re and M. Moreover, the effect of the nanofluid concentration on the increment of heat transfer was more remarkable at higher values of the Reynolds number. Simulations show that by increasing the Reynolds number or decreasing the vertical passage ratio, the local and average Nusselt number increases.

USING OF FLOW ROUTING PLATE FOR COOLING OF PRINTED CIRCUIT BOARDS

Effective cooling of electronic components plays an important role in system design and efficiency. In this study, the effects of using the flow routing plate in cooling printed circuit boards have been investigated. For this purpose, effects of the flow routing plate on the laminar mixed convection heat transfer from protruded heat sources at the side walls of the horizontal channel, were investigated numerically. The air was used as cooling fluid, and protruded heat sources were equipped as rows into the rectangular channel with insulated walls. Numerical investigations were carried out for different plate inclination angles at different Reynolds and modified Grashof numbers. It is observed that the using of flow routing plate increases the heat transfer at different ratios by comparison to the case without plate and enhances the cooling conditions for all values of parameters in the analyses. The highest heat transfer enhancement occurred at values where Reynolds number (Re) was Re = 1000 and plate inclination angle (α) was α = 60 °.The results obtained during the numerical analyses are presented in detail in the form of graphics for the row averaged Nusselt number, heater temperatures, velocity vectors, and temperature contours.

Numerical Study of Heat Transfer Enhancement for Low-Pressure Flows in a Square Cavity with Two Fins Attached to the Hot Wall Using Al2O3-Air Nanofluid

The gaseous low-pressure nanofluid flow of a steady-state two-dimensional laminar natural convection heat transfer in a square cavity of length L with two attached solid fins to the hot wall is numerically investigated. Such flows are found in many engineering applications, such as nuclear reactors and electronic cooling equipment. Physical parameter ranges in this study are as follows: 0 ≤ Kn ≤ 0.1, 103 ≤ Ra ≤106, 0 ≤ ϕ ≤ 0.2, LF/L takes the value of 0.5, HF takes the values of 0.25 to 0.75. Simulation results show that Nusselt number depends directly on the Rayleigh number and inversely on the Knudsen number. In addition, it is found that heat transfer will be enhanced by dispersing the nanoparticles of Al2O3 in the base low-pressure gaseous flow. Moreover, it is found that the Nusselt number of such flows increases as the nano-particle volume fraction increases for the investigated range of volume fractions considered in this study. In addition, a correlation of the Nusselt number among all the investigated parameters in this study is proposed as Nu = 0.2196 Ra0.0829 Kn–0.511 ϕ0.104.

Two-dimensional analysis of low-pressure flows in an inclined square cavity with two fins attached to the hot wall

The gaseous low-pressure flow of a steady-state two-dimensional laminar natural convection heat transfer in a cavity with an attached two solid fins to the hot wall is numerically investigated. Such flows can be found in many engineering applications such as the “evacuated” solar collectors and in the receivers of the solar Parabolic Trough Collectors (PTCs), nuclear reactor cooling and electronic equipment cooling. Buoyancy effect is modeled using Boussinesq approximation. Effects of Knudsen and Rayleigh numbers, location and length of the fins, conductivity ratio and fins porosity, and the tilt angle on the flow and heat transfer characteristics are investigated. Physical parameters ranges in this study are as follows; 0≤Kn≤0.1, 103≤Ra≤106, LF takes the values of 0.25, 0.5 and 0.75 m, HF takes the values of 0.25_0.5, 0.25_0.75 and 0.5_0.75, 1≤Kr≤8000, 0≤ε≤1 and 0∘≤θ≤90∘. It is shown that Nusselt number depends directly on Rayleigh number and inversely on Knudsen number. In addition, it is found that attaching two solid fins to the hot wall will enhance the heat transfer for such flows. Moreover, it is found that the porous fins are superior to the solid fins as far as the heat transfer is concerned. In addition, it is shown that for the geometry where the lower fin height to cavity length ratio is 0.25 and the upper fin height to cavity length ratio is 0.75, the best heat transfer is achieved. In addition, for the fin lengths considered in the study, it is concluded that by increasing the length of the fin, a better heat transfer is achieved. Also, it is found that by increasing the tilt angle of the cavity, a better heat transfer is achieved up to a certain value which was found to be (30°) where beyond this value, Nusselt number will decrease. In addition, it is found that by increasing the conductivity ratio up to a certain value (103), better heat transfer is gained. Finally, a correlation among Nusselt number and the parameters investigated in this research proposed as Nu=0.02197ε0.00932sin(θ)0.00373Kr0.0273(h2/LF)2.814Ra0.13((h2−h1)/LF)1.373(h1/LF)1.615Kn0.37.

Three-dimensional investigation of the effects of external magnetic field inclination on laminar natural convection heat transfer in CNT–water nanofluid filled cavity

A finite volume-based three-dimensional numerical analysis is conducted to investigate the effects of the angle of inclination of an external magnetic force on the natural convection inside a cubical cavity filled with a carbon nano-tube (CNT)-water nanofluid. The governing equations are solved using vorticity-vector potential formalism. The vertical walls are differentially heated and the horizontal walls are assumed to be adiabatic. The effects of the Rayleigh number (103≤Ra≤105), volumetric fraction (0≤φ≤0.05) of the CNT particles, angle of inclination of the external magnetic force (0°≤α≤90°), and the Hartmann number (0≤H a≤100) are investigated. The results of a fluid flow using a single-phase model are illustrated based on the particle trajectories, velocity vectors, isosurfaces of temperature, and the Nusselt number. The heat transfer is shown to intensify with the increasing percentage of CNT particles and a larger Rayleigh number. The repelling effect of the magnetic force inhibits the heat transfer by 50% when Ha is increased from 50 to 100.

Natural convection analysis through a radiatively participating media within a rectangular enclosure

A computational approach for the modelling of combined gas radiation and laminar natural convection heat transfer within a rectangular enclosure is presented. One wall among the others is maintained at a constant higher temperature (Th) while the others are of a constant lower temperature (Tc). The discrete-ordinate method (DOM) through S12 directions is applied to resolve the radiative transport equation (RTE) while the 'statistical narrow band correlated-k' (SNBCK) model is adopted to provide gas radiative properties. The effect of radiative contribution, the enclosure tilt angle, the boundary and geometry conditions are presented. Special attention is given to the effect scales of these parameters on the average Nusselt numbers. The results show that radiative effect remarkably contributes to the acceleration of the vortexes, improving heat exchanges at walls.

Analysis of Natural Convection Flow in a Trapezoidal Cavity Containing a Rectangular Heated Body in Presence of External Oriented Magnetic Field

The laminar natural convection flow and heat transfer inside a trapezoidal cavity filled with air and containing a rectangular block is investigated numerically. The left and right walls of the cavity are cold, the top and bottom walls are adiabatic and the rectangular body is heated uniformly. Finite Element Method of Galerkin’s weighted residual scheme is used to solve the transport equations with appropriate boundary conditions. The main objective of this study is to explore the influence of pertinent parameters such as Rayleigh number, Hartmann number and orientation of the magnetic field on the flow and heat transfer performance of the fluid while the Prandtl number is considered fixed. Results indicate that the heat transfer rate is significantly affected by increasing the mentioned parameters.

Heat transfer enhancement using U-shaped flow routing plates in cooling printed circuit boards

This study presents a numerical investigation on the effect of the U-shaped flow routing plate to laminar mixed convection heat transfer from protruded heat sources at the side walls of a horizontal channel. The air was used as a cooling fluid and protruded heat sources were equipped as 4 × 8 rows into the rectangular channel with insulated walls. Numerical investigations are carried out for the plate width/channel width ratio (LP/W) of 3/20, 1/10, 1/20 and plate angles (α) of 0°, 30°, 60° at different Reynolds (Re), modified Grashof (Gr*) and Richardson (Ri) numbers. In the study, periodic plate placement was analyzed numerically and the effects of a plate placement array on heat transfer enhancement were investigated. The highest heat transfer enhancement (180%) was observed for the values of α = 30° LP/W = 3/20 at all the Re, Gr* and Ri number values. The predominance of natural convection increases the use of the flow routing plate effectiveness but causes a decrease in the heat transfer enhancement after a certain number of Ri. Therefore, the increase in natural convection needs to be controlled. Theoretical fan power (Nfan) requirements due to pressure loss are also investigated. Depending on the parameters used in the numerical study, as a result of the plate usage, the pressure losses increased according to the case without a plate. It is observed that the most important factor affecting the pressure loss was the Re number and this case indicated that the plate placement was more appropriate for low values of Re. The findings obtained during the numerical studies were presented in detail as graphics of the row averaged Nusselt number (Nurow ave.) and the heater temperatures, surface stream lines, and temperature contours.

Enhancement of convection heat transfer using EHD conduction method

Laminar forced convection heat transfer in an annulus is enhanced using electrohydrodynamics (EHD) technique. A novel EHD-enhanced double pipe heat exchanger is designed and incorporated with six pairs of electrodes. An experimental setup is constructed to investigate heat transfer enhancement, pressure drop and power consumption in the double pipe. The finite volume method is employed to solve the governing equations of EHD conduction pumping, Navier-Stokes and energy. The numerical results are in agreement with the experimental results and demonstrated that in the considered range of the parameters, heat transfer enhances up to 50% with low electric power consumption and the method is more effective in lower Reynolds numbers. An effectiveness parameter is introduced which considers both heat transfer enhancement and pumping power consumption. Increasing applied voltage will improve performance until an optimum voltage which generates a cross-stream flow comparable to main-stream inlet flow. The flow patterns of the induced cross-stream flow are investigated using numerical simulation.

Numerical Investigation of Single Phase Gas Flow Through Straight, Wavy and Branched Wavy Minichannels

In this paper, a parametric numerical study was undertaken to understand and analyze the refinement in the heat transfer performances of straight minichannels due to employing wavy design. Three dimensional conjugated heat transfer simulation was analyzed using finite-volume based CFD approach in ANSYS 16.0 to evaluate laminar convective heat transfer characteristics on a minichannel heat sink purely on the basis of geometrical modifications. The results showed that there was a significant improvement in the heat dissipation capacity of heat sinks in wavy minichannels than observed in straight minichannels. Another novel concept of secondary branches is introduced to intensify macroscopic mixing by disrupting and reinitializing boundary layer development periodically across the newly formed branches. This further enhanced the thermal performance of heat sinks.

Numerical investigation of laminar convective heat transfer and pressure drop of water-based Al2O3 nanofluids in a microchannel

The heat transfer and flow of nanofluids in microchannel heat sink is usually numerically investigated by the single-phase model. The thermal conductivity and viscosity of nanofluids are calculated in the whole computational region because the single-phase model, assumes the nanofluids is static and nanoparticles concentration is uniform. However for the actual forced flow process, the concentration distribution of the nanofluids is not uniform. So the thermal properties of the nanofluids vary within different computational regions. With the increase of the velocity of the nanofluids, the deviation between the prediction of single-phase model and experimental results increases. In this paper, a new simulation method is proposed to consider the inhomogeneous and dynamic thermophysical properties of the nanofluids by obtaining the nonuniformly distributed nanoparticle concentration in each numerical cell. At the same time, it takes into account the following factors: the interactions between the nanoparticles, the solid wall and the base fluid, the axial conduction, the slip boundary conditions and fluid-solid coupling. The results show that the proposed numerical simulation method further improve the simulation accuracy for the forced flow and heat transfer process of nanofluids in the microchannel in a wider range. The effects of the nanoparticles concentration, Reynolds number and axial heat transfer on the thermophysical properties of the nanofluilds, the flow and heat transfer characteristics of a rectangular microchannels with hydraulic dimensions of 341μm were analyzed using the proposed simulation model. The main results are as follows: the nanoparticles are enriched at the near wall for the fully developed region; the nonuniformity of the nanofluilds thermal conductivity coefficient distribution at the channel cross section increases with the increase of the mass flow rate and the inlet centration; under the same Reynolds number, the higher the nanofluids concentration the better the heat transfer characteristics, but there is no linear relationship between the enhancement amplitude and concentration; the heat transfer coefficient of nanofluid with 1% and 2% alumina nanoparticles concentration is about 5.86% and 8.49% higher than that of deionized water, respectively; the axial conduction effect cannot be ignored for the microchannel, which significantly affects the inlet and outlet region of the microchannel especially when the Reynolds number is low. The nanofluilds temperature in the microchannel increases with the curve and does not match the conventional linear growth hypothesis.