Heat Transfer In Laminar Flow Research Articles

Effects of Nanoparticle Migration on Water/Alumina Nanofluid Flow Inside a Horizontal Annulus with a Moving Core

AbstractThe present study is a theoretical investigation of the laminar flow and convective heat transfer of water/alumina nanofluid inside a horizontal annulus with a streamwise moving inner cylinder. A modified, two-component, four-equation, nonhomogeneous equilibrium model is employed for the alumina/water nanofluid, which fully accounts for the effect of the nanoparticle volume fraction distribution. To determine the effects of thermal boundary conditions on the migration of the nanoparticles, two cases are considered: constant heat flux at the outer wall with an adiabatic inner wall (Case A) and constant heat flux at the inner wall with an adiabatic outer wall (Case B). The numerical results indicate that the thermal boundary conditions at the pipe walls significantly affect the nanoparticle distribution, particularly in cases where the ratio of Brownian motion to thermophoretic diffusivities is small. Moreover, increasing the velocity of the moving inner cylinder reduces the heat transfer rate for Case A. Conversely, in Case B, the movement of the inner cylinder enhances the heat transfer rate, and anomalous heat transfer enhancement occurs when the thermophoretic force is dominant (in larger nanoparticles).

CFD simulation of heat transfer enhancement of Al2O3/water and Al2O3/ethylene glycol nanofluids in a car radiator

The present numerical study simulated turbulent and laminar flow heat transfer in nanofluids (Al2O3 particles in water and ethylene glycol-based fluid) passing through a flat tube in 3D using computational fluid dynamics (CFD) for single and two-phase approaches. The advantages over pure base fluids were evaluated. Empirical correlations were used to calculate nanofluid viscosity and thermal conductivity as a function of the volumetric concentration of the nanoparticles. First, the Nusselt numbers of the pure water and pure ethylene glycol in flat tubes were compared with the experimental data. Next, the Nusselt numbers for both approaches were compared with those for experimental data at the same Reynolds number for different concentrations of nanoparticles. A small difference in the friction factors of the tube was observed between the two approaches and the Nusselt number for the two-phase model was markedly different from that for the single-phase model; however, the volumetric flow for the same heat transfer rate decreased and less pumping power was required for the nanofluids.

Brownian motion and thermophoresis effects on slip flow of alumina/water nanofluid inside a circular microchannel in the presence of a magnetic field

The current study is a theoretical investigation of the laminar flow and convective heat transfer of alumina/water nanofluid inside a circular microchannel in the presence of a uniform magnetic field. A modified two-component four-equation nonhomogeneous equilibrium model was employed for nanofluids, which fully accounted for the effect of the nanoparticle volume fraction distribution. Because of the microscopic roughness in circular microchannels and also the non-adherence of the fluid–solid interface in the presence of nanoparticle migration, known as slip condition, the Navier's slip boundary condition is considered at the walls. The results indicated that nanoparticles migrate from the heated walls (nanoparticles depletion) towards the core region of the microchannel (nanoparticles accumulation) and construct a non-uniform nanoparticles distribution. The ratio of the Brownian to thermophoretic diffusivities (NBT) has relatively significant effects both on the distribution of the nanoparticles and the convective heat transfer coefficient of nanofluids. It was further observed that for smaller nanoparticles, the nanoparticle volume fraction is more uniform and abnormal variations in the heat transfer rate vanish. Moreover, in the presence of the magnetic field, the near wall velocity gradients increase, enhancing the slip velocity and thus the heat transfer rate and pressure drop increase.

Transient conjugated heat transfer in microchannels: Integral transforms with single domain formulation

The transient behaviour of conjugated heat transfer in laminar microchannel flow is investigated, taking into account the axial diffusion effects, which are often of relevance in microchannels, and including pre-heating or pre-cooling of the region upstream of the heat exchange section. The solution methodology is based on the Generalized Integral Transform Technique (GITT), as applied to a single domain formulation proposed for modelling the heat transfer phenomena at both the fluid stream and the channel wall regions. By making use of coefficients represented as space dependent functions with abrupt transitions occurring at the fluid–wall interfaces, the mathematical model carries the information concerning the transition of the two domains, unifying the model into a single domain formulation with variable coefficients. The proposed approach is illustrated for microchannels with polymeric walls of different thicknesses. The accuracy of approximate internal wall temperature estimates deduced from measurements of the external wall temperatures, accounting only for the thermal resistance across the wall thickness, is also analyzed.

Numerical Study of Laminar Flow and Convection Heat Transfer of Nanofluids inside Circular Tube

This paper presents a numerical study on laminar flow and convective heat transfer of nanofluids in a circular tube under constant wall heat flux boundary condition. Single phase model is used for simulating the heat transfer and flow behaviors of three different nanofluids. The effects of nanoparticle concentrations, nanoparticle diameter, nanoparticle material and Reynolds number on the Nusselt number and wall shear stress of nanofluids are determined and discussed in details. The comparison of Nusselt number of CuO-EG/water, SiO2-EG/water and Al2O3-EG/water nanofluids are presented. The results show that Nusselt number clearly increases with an increase in the nanoparticle concentration and flow Reynolds number, while the nanoparticle diameter has an opposite effect on the Nusselt number. Compared to SiO2-EG/water and Al2O3-EG/water nanofluids, CuO-EG/water nanofluids give higher Nusselt number with the same nanoparticle concentrations. The results also show that wall shear stress increases with increasing nanoparticle volume concentration.

Field synergy analysis for helical ducts with rectangular cross section

This paper proposes a new approach based on VSP method (Vorticity–Stream function–Poisson equation method), to study numerically fully developed laminar flow characteristics and heat transfer behaviors of helical ducts with rectangular cross section. The aim is to guide heat transfer enhancement for helical rectangular ducts. Based on the numerical results, synergies between velocity field and temperature field have been investigated through field synergy principle. The effects of curvature, torsion, aspect ratio, Reynolds number, and Prandlt number on velocity field, temperature field and field synergy have been examined. The results show that the secondary flow of the central regions is weaker than that in other locations of the cross section. With reference to the entire cross section, the best values of synergy between velocity field and temperature field can be found in the center of the section. Increasing the curvature, Reynolds number and Prandtl number causes the mean synergy angles of the cross section to increase. This result indicates that it is more important to improve the field synergy for high Prandlt number fluid, especially at high Reynolds number. As the torsion increases, the average synergy angles decreases. The mean synergy angles reach the maximum value when the aspect ratio is equal to one; therefore, one can conclude that the field synergy of helical ducts with square cross section is the worst. To enhance heat transfer performance of helical rectangular ducts, for small aspect ratio, the main important aspect is to improve the secondary flow in the center of the cross section. On the contrary, for moderate values of the aspect ratio, the most important aspect is to improve the field synergy of the region near the upper and the lower wall. The correlations of friction factor and Nusselt number have been also developed for helical ducts with rectangular cross section.

Magnetic field effect on nanoparticles migration and heat transfer of water/alumina nanofluid in a channel

The present study is a theoretical investigation of the laminar flow and convective heat transfer of water/alumina nanofluid inside a parallel-plate channel in the presence of a uniform magnetic field. A modified two-component, four-equation, nonhomogeneous equilibrium model was employed for the alumina/water nanofluid, which fully accounted for the effect of the nanoparticle volume fraction distribution. The no-slip condition of the fluid–solid interface is abandoned in favor of a slip condition which appropriately represents the non-equilibrium region near the interface at micro/nano channels. The results obtained indicated that nanoparticles move from the heated walls (nanoparticles depletion) toward the core region of the channel (nanoparticles accumulation) and construct a non-uniform nanoparticles distribution. Moreover, in the presence of the magnetic field, the near wall velocity gradients increase, enhancing the slip velocity and thus the heat transfer rate and pressure drop increase.

Flow and Heat Transfer of Nanoencapsulated Phase Change Material Slurry Past a Unconfined Square Cylinder

Numerical solution is carried out to analyze the effect of nanoencapsulated phase change material (NEPCM) slurry on forced convection heat transfer of steady laminar flow past an isothermal square cylinder. The base fluid is water while the NEPCM particles material is n-octadecane with an average diameter of 100 nm. A parametric study was performed for different volume fraction of nanoparticles ranging from 0% to 30%, two melting temperature ranges, i.e., 10 K and 20 K, and different inlet Reynolds numbers ranging from 15 to 45. The governing equations of flow and energy are solved simultaneously using a finite volume method (FVM) on collocated grid arrangement. It was found that for both NEPCM slurry and pure water, local and average heat transfer coefficients increases with increasing Reynolds number. The results of heat transfer characteristics of slurry flow over the square cylinder showed remarkable enhancement relative to that of the base fluid. The enhancement intensifies for higher particle volume concentrations and higher Reynolds numbers. However, utilizing the slurry can cause higher shear stress on the wall due to higher viscosity of mixture compared to the pure water. The melting temperature range of NEPCM particles has slight effect on heat transfer, although with increasing volume fraction and Reynolds number, lower melting range leads to higher heat transfer coefficient.

Effects of the thick walled pipes with convective boundaries on laminar flow heat transfer

Conjugate heat transfer in laminar tube flow with convective boundary conditions is considered analytically. The steady state problem involving two-dimensional wall and axial fluid conduction is solved using separation of variables for a thick walled cylindrical pipe. The effects of the wall thickness, external Biot number and wall-to-fluid thermal conductivity ratio are investigated on the heat flux, fluid bulk and wall temperatures. Results are presented for the cases when the wall thickness is between 0.1 and 2, Biot number ranging between 0.1 and 10, and the ratio of wall-to-fluid thermal conductivity between 3 and 100. These parameters are found to significantly affect the heat transfer characteristics at the thermal entrance region, for instance, increase in wall thickness results in reduced heat flux while increase in Biot number and the ratio of the wall-to-fluid thermal conductivity result in increased heat flux. Decrease in wall thickness, increase in both Biot number and the ratio of the wall-to-fluid thermal conductivity correspond to decreased fluid bulk and wall temperature profiles.

Optimization of a Hybrid Double-Side Jet Impingement Cooling System for High-Power Light Emitting Diodes

Optimization of a hybrid double-side jet impingement cooling system for high-power light emitting diodes (LEDs) was performed using a hybrid multi-objective evolutionary approach and three-dimensional numerical analysis for steady incompressible laminar flow and conjugate heat transfer using Navier–Stokes equations. For optimization, two design variables, i.e., ratios of the diameter of jet holes and the distance from the exit of upper impinging hole to chips to thickness of substrate were chosen out of the various geometric parameters affecting the performance of the cooling system. To evaluate cooling performance and pressure loss of the system, two objective functions, viz., the ratio of the maximum temperature to average temperature on the chips and pressure coefficient, were selected. Surrogate modeling of the objective functions was performed using response surface approximation. The Pareto-optimal solutions were obtained using a multi-objective evolutionary algorithm, and performances of three representative Pareto-optimal designs were discussed compared to a reference design. In the optimal designs, higher level of uniform cooling was generally achieved with higher pressure coefficient. The Pareto-sensitivity analysis between the objective function and design variable was also performed.

Numerical solutions to heat transfer of nanofluid flow over stretching sheet subjected to variations of nanoparticle volume fraction and wall temperature

The numerical analysis of heat transfer of laminar nanofluid flow over a flat stretching sheet is presented. Two sets of boundary conditions (BCs) are analyzed, i.e., a constant (Case 1) and a linear streamwise variation of nanoparticle volume fraction and wall temperature (Case 2). The governing equations and BCs are reduced to a set of nonlinear ordinary differential equations (ODEs) and the corresponding BCs, respectively. The dependencies of solutions on Prandtl number Pr, Lewis number Le, Brownian motion number Nb, and thermophoresis number Nt are studied in detail. The results show that the reduced Nusselt number and the reduced Sherwood number increase for the BCs of Case 2 compared with Case 1. The increases of Nb, Nt, and Le numbers cause a decrease of the reduced Nusselt number, while the reduced Sherwood number increases with the increase of Nb and Le numbers. For low Prandtl numbers, an increase of Nt number can cause to decrease in the reduced Sherwood number, while it increases for high Prandtl numbers.

Numerical Simulation of a Trapezoidal Cavity Absorber in the Linear Fresnel Reflector Solar Concentrator System

The present article describes the numerical study of heat transfer analysis in a trapezoidal cavity absorber of a linear Fresnel reflector solar concentrator (LFRSC) system. The linear absorber is a part of the linear Fresnel reflector solar concentrator system, in which hot fluid is generated. The absorber is an inverted air cavity with a glass cover enclosing an absorber surface. In the present analysis, a linear multi-tube absorber with and without the absorber plate underneath is considered. The ANSYS workbench package with FLUENT 12.0 version is used to develop the two-dimensional, steady-state, and laminar-flow heat transfer model for a trapezoidal cavity absorber. The models with Boussinesq and non-Boussinesq approximations are used for the numerical analysis and the performance is compared. The heat transfer rate, radiation heat loss, and combined heat loss coefficients are predicted at various absorber surface temperatures and for the absorber surface with ordinary black paint and with selective surface coating. For validation, the present computational model is compared with the analytical values and other reported closed cavity models. A better design of the cavity absorber is found out to maximize the heat transfer rate applied to the absorber tubes for better fluid outlet temperature.

A Numerical Study of the Thermal Performance of Microchannel Heat Sinks with Multiple Length Bifurcation in Laminar Liquid Flow

A rectangular, straight microchannel heat sink with multiple length bifurcation has been designed, and the corresponding laminar flow and thermal performance have been investigated, numerically. Five different cases with multiple length bifurcation and various arrangements are investigated, as well as one case without bifurcations. All laminar fluid flow and surface heat transfer results are obtained using computation fluid dynamics, and a constant heat flux thermal boundary condition is applied to the heated surfaces. The inlet velocity ranges from 0.6 m/s to 1.4 m/s, with the corresponding Reynolds number from 220 to 520. The local pressure, velocity, temperature distributions, and averaged heat transfer coefficient are presented. The overall thermal resistances subjected to inlet Reynolds number are evaluated and compared for six different microchannel heat sinks. Numerical results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink, especially the heat sink with longer length bifurcation. The usage of multiple length bifurcated plates in microchannel heat sink can reduce the overall thermal resistance by a factor of up to 2, and make the temperature of a heated surface more uniform, especially for the heat sink having a short bifurcation in the center and two bifurcations near the sides (case 4). It is suggested that proper design of the multiple length bifurcation could be employed to improve the overall thermal performance of microchannel heat sinks.

3D NUMERICAL STUDY ON LAMINAR FORCED CONVECTION IN V-BAFFLED SQUARE CHANNEL

The article presents a mathematical study of fully developed periodic laminar flow visualization and heat transfer characteristics in an isothermal wall square-channel fitted with V-shaped baffles on one wall. The computations based on the finite volume method together with the SIMPLE algorithm have been performed. The investigation covers a range of Re based on the hydraulic diameter of the channel, Re = 100-1200. To create a pair of main streamwise vortex flows through the tested section, the V-baffles with the attack angle of 30° with the main flow direction are mounted in tandem and pointing downstream on the lower channel wall only. Effects of different baffle heights and pitches on heat transfer and pressure drop in the channel are examined and the results obtained are compared with smooth channel with no baffle. The numerical result shows that the presence of the V-baffle yields a significant heat transfer enhancement compared with the smooth channel. It is visible that the main vortex flows, a pair of streamwise twisted vortex (P-vortex) can induce impingement flows on the walls leading to a drastic increase in heat transfer rate over the channel. In addition, the increase in the baffle height leads to the rise in the heat transfer and pressure loss while that in the baffle pitch provides the opposite trend. The predicted results expose that the maximum thermal enhancement factors for the V-baffles with BR = 0.3, 0.3 and 0.4; and PR = 1, 1.5 and 2 are, respectively, about 2.44, 2.29 and 2.37 at higher Re.

Experimental Study on Heat Transfer of Heated Falling Film Under Gas–Liquid Cross-Flow Condition

With the influence of the different gas Reynolds numbers and liquid Reynolds numbers on heated falling film heat transfer, an experiment was performed by noncontact thermal infrared imaging technology under the gas–liquid cross-flow condition. The results indicated that during the increase of liquid Reynolds number the thickness and thermal resistance of liquid film increased in the determined temperature of the heating water, which weakened the heat transfer of the liquid film. However, the increase of liquid Reynolds number strengthened liquid film turbulence and therefore enhanced heat transfer. Under the synergistic effect of these two factors, there should be an optimal liquid Reynolds number that minimizes thermal resistance and maximizes the heat transfer coefficient of the liquid film. Temperature plays an important role in heat transfer of laminar liquid film flow. However, the heat transfer of turbulent liquid film flow is not sensitive to liquid film inlet temperature.

High Gradient Temperature Thermo-Buoyant Flow in a Square Cavity with Magnetoconvection Using a Novel Non-Boussinesq Algorithm

We study numerically the heat transfer of steady laminar flow in a square cavity filled with electrically conducting fluids, in the presence of an external uniform magnetic field. Imposing a large temperature gradient between two opposite vertical walls, there are substantial temperature and density variations in the domain. The fluid is treated as an ideal gas. Indeed, high temperature gradient thermo-buoyant cavity flows result in natural convection flow domains with high Rayleigh number. To implement the temperature variation effect, the fluid properties, including the conductivity and viscosity coefficients, are considered to vary with temperature in accordance to the Sutherland's law. We use a novel non-Boussinesq algorithm to include the density variation in the domain. Of importance, the current results are different from those obtained considering the classical Boussinesq assumption, in which the density variation is replaced with the temperature variation. We use the finite volume method and SIMPLE algorithm to treat the continuity, momentum, and energy equations. The results show that the magnetic field plays a damping role in the flow pattern formations. We show that the increase of Hartmann number would decrease the maximum fluid velocity and Nusselt number magnitudes. Generally, the current study indicates that the computational heat transfer applicants, who need to take into account the effect of MHD in high thermobuoyant flow field treatments, should consider a variable density instead of using the classical Boussinesq approximation if they wish to achieve accurate solutions; specifically in higher Rayleigh number regime applications.

Momentum and Heat Transfer in Laminar Slip Flow over a Cylinder

The vorticity transport and energy equations are solved numerically in a laminar gas flow past a circular cylinder. The slip boundary condition and temperature jump are applied at the cylinder wall. The changes in heat transfer and slip velocity at the cylinder wall due to Knudsen and Reynolds number variations are calculated. The velocity at the wall increases as the Knudsen number increases due to the slip condition. The separation point moves downstream as slip increases. As the Knudsen number increases, the slip increases at the wall and the heat flux between the cylinder wall and the flow decreases. These results show that the heat transfer coefficient and the Nusselt number decrease as the slip increases. When hot wires are used for temperature measurements, failure to include a slip boundary condition will also lead to an error in the temperature measurement.

Enhancement of heat transfer of laminar flow through a circular tube having integral helical rib roughness and fitted with wavy strip inserts

The experimental friction factor and Nusselt number data for laminar flow through a circular duct having integral helical rib roughness and fitted with way strip inserts have been presented. The heat transfer test section was electrically heated and uniform wall heat flux boundary condition was imposed. Friction factor and Nusselt number correlations have been developed by log-linear regression analysis. The performance of the present fin geometry has been evaluated. The major findings of this experimental investigation are that the wavy strip inserts in combination with integral helical rib roughness perform better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain range of parameters.

Effects of an inserted square cylinder on wall heat transfer in laminar channel flows

Effects of unsteady flows past a confined square cylinder on wall heat transfer are investigated numerically. Two Reynolds numbers and several the gap distances of the square cylinder are considered to analyze the geometric effects on the wall heat transfer. The results show the detailed description of flow structure and attendant heat transfer caused by the variation of the gap distance. As the gap distance increases, the flow changes from steady asymmetric flow to symmetric vortex shedding having the inversion of the Karman vortex street. Also, the square cylinder induces intensive flow instabilities and significant alteration of the thermal boundary layer. As a result, the temperature distributions on the channel walls become steeper and wall heat transfer is increased.

Cooling performance of nanofluids in a small diameter tube

This article reports convective single-phase heat transfer performance in laminar flow for some selected nanofluids (NFs) in an open small diameter test section. A 0.50mm inner diameter, 30cm long stainless steel test section was used for screening single phase laminar convective heat transfer with water and five different water based NFs. Tested NFs were; Al2O3 (two types), TiO2 (two types) and CeO2 (one type), all 9wt.% particle concentration. The effective thermal conductivity of the NFs were measured with Transient Plane Source (TPS) method and viscosity were measured with a rotating coaxial cylindrical viscometer. The obtained experimental results for thermal conductivity were in good agreement with the predicted values from Maxwell equation. The local Shah correlation, which is conventionally used for predicting convective heat transfer in laminar flow in Newtonian fluids with constant heat flux boundary condition, was shown to be valid for NFs. Moreover, the Darcy correlation was used to predict the friction factor for the NFs as well as for water. Enhancement in heat transfer for NFs was observed, when compared at equal Reynolds number, as a result of higher velocity or mass flow rate of the NFs at any given Reynolds number due to higher viscosity for NFs. However, when compared at equal pumping power no or only minor enhancement was observed.