Forced Convection Flow Research Articles

Modifications of the turbulent diffusion layer model for the condensation heat transfer under the presence of noncondensable gases

The condensation heat transfer model of the MARS-KS1.5 code tends to under-predict the condensation in the presence of noncondensable gases. In our previous study, we adopted the diffusion layer model and modified it in order to consider the effect on turbulence on diffusive mass transfer in the diffusion layer. In this work, further modifications were made by introducing the boundary layer theory for natural convection flow.The new condensation model has been validated against 442 experimental data collected from eight different facilities. It is shown that the results of the new model for natural convection flow are more accurate result than those of the previous model. It is confirmed that the new model works well for the condensation in forced convection flow. The relative root mean square error of the new model is reduced to 0.193 from 0.242 of the previous model and 0.401 of the original one.

Numerical investigations of turbulent multiple jet impingement on a heated square block in a confined channel

The analysis of fluid flow and heat transfer characteristics of turbulent single and multiple jets impinging on a square block in a confined channel has been studied numerically. The study of jet impingement on a square block is important as it is directly relevant to the cooling process of food products by jet impingement and present results would be useful in understanding this process. Two-dimensional unsteady state incompressible turbulent forced convection flow is considered for the present analysis. Turbulence is modelled by Reynolds Averaged Navier-Stokes (RANS) equation with a standard k-∊ model and enhanced wall treatment at the wall. The governing equations are solved using a finite volume-based commercial solver. The second-order upwind scheme is used for non-linear terms and the SIMPLE algorithm is used for the pressure-velocity decoupling problem. Effects of a single jet, double jet, 3 jets, and 4 jets, the center distance between jets, block size and height between the block and bottom jets on flow and heat transfer characteristics are reported. The better enhancement of heat transfer is observed from all surfaces of the block with multiple jet impingement compared with a single jet due to multiple recirculation zones than a single jet. The enhancement of heat transfer on the right surface of the block for 4 jets impingement is 113.3% compared to single jet impingement. The overall heat transfer rate is increased by 72.74% with an increase in center distance between jets from 2 to 6. Present results are validated with experimental results available in the literature.

Hydrodynamic forces and heat transfer of nanofluid forced convection flow around a rotating cylinder using finite element method: The impact of nanoparticles

A computational approach is utilized to study the nanofluid forced convection flow around a rotating circular cylinder in a horizontal channel. Different types of nanoparticles in the base fluid are considered and investigated such as Silica (SiO2), Copper (Cu) and Alumina (Al2O3). Average Nusselt number (Nuavg) are compared for various conditions of the cylinder, namely adiabatic and isothermal cylinders respectively for two locations of cylinder. A dimensionless system of partial differential equations over a complex computational domain is discretized by applying the higher order stable finite element method. The discretized nonlinear system of algebraic equations is linearized using the Newton method. A geometric multigrid method is implemented for the computation of linearized subproblems in each nonlinear sweep. Impact of the pertinent parameters is investigated such as the Reynolds number (10 ≤ Re ≤ 200), angular velocity (−75 ≤ Ω ≤ 75) and the nanoparticles volume fraction (0.0 ≤ ϕ ≤ 0.04). Numerical results are demonstrated in the form of isotherms, streamlines and suitable graphs for various quantities of interest. It is inferred that the clockwise rotation of the cylinder makes the fluid move over the cylinder while the fluid moves below the cylinder in the anticlockwise case. Moreover, an increase in the nanoparticles volume fraction enhances the average Nusselt number and decreases both of the lift and drag coefficients.

Numerical investigation of forced convection heat transfer and flow irreversibility in a novel heatsink with helical microchannels working with biologically synthesized water-silver nano-fluid

This paper aims to evaluate the hydrothermal and irreversibility behaviour of a biological water-Ag nano-fluid in a new heatsink with helical microchannels. Two-phase mixture model is applied to precisely simulate the behavior of nanofluid in the nanoadditive concentration (φ) range of 0–1% and Reynolds number (Re) range of 500–1500. The influences of φ and Re on the convective heat transfer coefficient, CPU surface temperature, pumping power, as well as the irreversibilities due to heat transfer and fluid friction are examined. The findings depict that boosting the Re and φ augments the performance of heatsink by intensifying the convective heat transfer coefficient of the working fluid which favourably declines the CPU surface temperature and the heat transfer irreversibility and importantly results in the temperature uniformity of the CPU surface. However, intensification in Re adversely affects the pumping power, the fluid friction and total irreversibilities in the system. Furthermore, it is revealed that the nano-fluid always has a superior cooling performance as compared with the pure water. Finally, it is found that the best hydrothermal performance of the nano-fluid in the proposed heatsink occurs at Re = 1500 and φ = 1%, while the minimum total irreversibility occurs at Re = 500 and φ = 1%.

Heat transfer rate of power law fluid flow with variable thermal conductivity through a porous annular sector duct

The impact of power law velocity gradients dependent thermal conductivity on forced convective flow of power law fluid through an annular sector duct has been studied in this article. The Darcy Brinkman Forchheimer model is taken into account to describe the flow field. Governing non-linear momentum and energy equations have been transformed into a dimensionless form with the help of dimensionless parameters. By using dimensionless quantities such as friction factor, fRe and average Nusselt number, Nu , characteristics of forced convective flow of power law fluid are analyzed for different values of power law index, n.

Analytical solutions for unsteady forced convection pulsating flow in a microchannel in the presence of EDL effects

A forced convection flow driven in a microchannel by an applied pressure gradient that fluctuates with small amplitude harmonically in time in the presence of electrical double layer effects is investigated. An analytical expression for the electrostatic potential is obtained via Poisson’s equation. Based on this solution, we further obtain analytical solutions for velocity and temperature for both the cases Pr ≠ 1 and Pr = 1. Results show that they match each other as Pr → 1− and Pr → 1+. The explicit expression of the transient Nusselt number is derived. We notice that the Debye–Hückel parameter γ and the angular velocity Ω are key factors for flow behaviours. Our proposed study adds some new insights by including the time-dependent pressure term that is usually overlooked in previous works. It is expected that this work could help to understand the transportal mechanisms of forced convection flow in microfluidic equipment and instruments.

Impact of a helical-twisting device on the thermal–hydraulic performance of a nanofluid flow through a tube

Creating greater thermal efficiency is desirable yet challenging in many industrial applications. Employing nanofluids for increasing the thermal behavior of the working fluid and consequently the overall heat transfer rate is an interesting solution that has widely been studied in the literature. This study, however, proposes the use of a special geometry of disturber (i.e., a helical-twisting shape) for a circular duct with nanofluid (water–CuO) as the heat transfer medium and a forced convective flow for this objective. To assess the effect of this instrument, FVM is employed to simulate the hydrothermal performance of the flow through the duct and the disturber. The main parameters of the study include the effects of the width of the disturber blade and inlet velocity on Darcy factor and the heat transfer coefficient. The homogenous model was used for the properties of nanomaterial. The results of the simulations show that better turbulence, i.e., a greater performance, is observed as the width of the flow disturber blade increases and the Reynolds number picks up.

Reference numerical database for turbulent flow and heat transfer in liquid metals

Turbulent heat transfer is a complex phenomenon that has challenged turbulence modellers over various decades. In this regard, in the recent past, several attempts have been made for the assessment and further development/calibration of the available turbulent heat flux modelling approaches. One of the main hampering factors with respect to the further assessment of these modelling approaches is the lack of reference data. In the framework of the EU SESAME and MYRTE projects, an extensive effort has been put forward to generate a wide range of reference data, both experimental and numerical, to fill this gap. In that context, this article reports the numerical database that has been generated within these projects for various liquid metal flow configurations in different flow regimes. These high fidelity numerical data include seven different flow configurations: a wall-bounded mixed convection flow at low Prandtl number with varying Richardson number (Ri) values; a wall-bounded mixed and forced convection flow in a bare rod bundle configuration; a forced convection confined backward facing step (BFS) with conjugate heat transfer; a forced convection impinging jet for three different Prandtl fluids corresponding to two different Reynolds numbers of the fully developed planar turbulent jets; a mixed-convection cold-hot-cold triple jet configuration corresponding to Ri=0.25; an unconfined free shear layer for three different Prandtl fluids; and a forced convection infinite wire-wrapped fuel assembly. These high-fidelity numerical databases will serve the further development of turbulent heat transfer models by providing unique, new and detailed data for the thermal-hydraulic behaviour of liquid metals in various flow configurations.

Geometrical Evaluation of a Pair of Elliptical Tubes Subjected to a Flow with Forced Convection Heat Transfer

In the present work it is performed a study on the geometric evaluation of a pair of elliptical tubes subjected to external flow with forced convection by means of numerical approach. The objectives are the maximization of Nusselt number (NuD) and the minimization of drag coefficient (CD). The degrees of freedom for the pair of tubes arrangement are: the ratio between the transverse pitch and characteristic length of tubes (ST/D), where D = (A)1/2, the ratio of the main and secondary axes of the elliptical tube (a/b) and the angle of incidence of the flow on the pair of tubes (α). The simulations were carried out considering two-dimensional forced convective flows, in the laminar regime and incompressible conditions. For all configurations, Reynolds and Prandtl numbers are constant, ReD = 100 and Pr = 0.71. The Finite Volume Method (FVM) is used to solve conservation equations of mass, momentum and energy. The software Gmsh is used for creation of the geometries and generation of the meshes. Results showed that the degrees of freedom affected the fluid dynamic and thermal performance of the forced convective flow. According to the objectives outlined in this study, the best performance for the maximization of heat transfer was obtained when α = 0o, a/b = 1⁄2 and ST/D = 3.5. In the case of the fluid dynamics study, the optimal result for CD minimization occurred when α = 0o, a/b = 2.0 and ST/D = 4.0. Thus, the optimal geometry will depend on the indicator performance where the problem is evaluated.

LBM simulation of forced convective channel flow containing multiple obstacles: Effects of obstacles height and position

AbstractWe examine the effects of the obstacles, height, and position on the forced convective flow in a channel having three obstacles on the lower wall of the channel. All the walls of the channel and obstacles are retained at a constant temperature while the fluid with temperature more than the walls are entered into the channel. The flow governing equations, vorticity equation, and energy equation are solved numerically by using the lattice Boltzmann method (LBM) together with the finite difference successive over relaxation method (SOR). The effects of obstacles height, h, and distance, d, between the obstacles on the streamlines and isotherms are presented. To investigate the heat transfer rate for changing the height and position of the obstacles, local Nusselt number distribution and the mean Nusselt number distribution are also presented. It is observed that vortices, produced backward to each obstacle, increase axially with increasing the height of each obstacle. Also vortices, produced between obstacles, change its shape with decreasing the distance between obstacles. It is asserted that heat transfer rate can be increased by extending only the height of first obstacle.

New Empirical Models for Condensation on Banks of Plain Tubes

New empirical models for evaluating heat-transfer coefficients for condensation on banks of tubes were developed using an experimental database of over 4000 individual data points for 7 different condensing fluids and 13 different tube bank configurations. New empirical models within each of the free and forced convection flow regimes for both of the steam and non-steam fluids separately were obtained, including the relevant dimensionless groups that may affect the heat transfer during the condensation. Two models within the free-convection flow regime for the non-steam were proposed due to large volume and variation of data in this regime. While comparing the results of the new empirical models with the recent database; it was proposed that the overestimation for some of the non-steam data of Shah (1978 and 1981) was due to the presence of the non-condensable gas (air) within the flowing fluid. In terms of predicting the tube bank thermal performance and condensation of flowing fluid, the results of the newly developed models were seen to be the most reliable and accurate when compared with the existing models mentioned in this paper. An overall agreement was seen with the existing experimental database within an average absolute of the errors of about 15%. For steam data, the present models were seen to yield the most accurate results for the inline and staggered tube banks when compare with other empirical and analytical models. However, for non-steam fluids (R-12, R-21 and R-113 only) flowing through staggered tube bank, Honda et al. (1989) model is recommended.

The Porosity Effect of Stainless Steel Balls on Forced Convection Pipe Flow

An experimental study has been carried out to investigate the effects of stainless-steel balls on forced convection flow in pipe under uniform heat flux. Water is used as the working fluid and stainless-steel balls as a porous media. The Reynolds number range from (5000 to 9000) based on the diameter of the pipe. The experiments were conducted on three various numbers of stainless-steel balls (N) with various diameters (dp), which give various porosity (0.33, 0.38 and 0.41). These are (N= 2400, dp=1mm), (N=1600, dp=3mm) and (N= 750, dp=5mm). Results show that, heat transfer coefficient increases with the decrease in the porosity due to the reduction in the space between balls. This led to an increase in turbulence and produced eddies. Furthermore, enhancement in heat transfer coefficient reached its maximum value of (45%) for ball diameter with (dp=1mm) and water flow rate (9 L/min). New Correlation equations for the average heat transfer coefficient were obtained for three different diameters of balls (1, 3 and 5 mm).

Numerical simulation of the magnetic field and Joule heating effects on force convection flow through parallel-plate microchannel in the presence of viscous dissipation effect

The effects of Joule heating, Hartman, Brinkman, and Reynolds numbers on the flow pattern and thermal characteristics of force convection flow through a parallel-plate microchannel are investigated in various nanoparticles volume fraction. Water–Al2O3 is considered as the working nanofluid while taking viscous dissipation effect (VDE) into account. The mid-section of the microchannel is heated with a constant uniform heat flux and influenced by a magnetic field with a uniform strength. The effective thermal conductivity and viscosity of nanofluid are calculated through a new correlation in which the influence of Brownian motion is considered. A control volume finite different scheme, along with the SIMPLE algorithm, is adopted to conduct the numerical analyses and solve the discrete equations. Contour plots of streamlines and isotherms are presented to graphically display the impact of the investigated variables. Furthermore, the values of the Nusselt number for the minimum temperature and maximum velocity are calculated and presented through figures. The results show that all of the Brinkman, Joule, nanofluid concentration, and Hartmann numbers have decreasing effect on the heat transfer. The conclusion is supported by the fact that all the aforementioned factors increase the temperature throughout the flow field. The higher the flow field temperature, the lower the heat transfer from the wall. Higher Brinkman number leads to the friction intensification between flow layers due to considering VDE. It can be said about the Joule heating that, since this term has an inverse relation with the squared velocity, increase in Joule number is followed by a reduction of heat transfer from the walls. Also, an increase in the nanofluid concentration increases the temperature throughout the microchannel leading to heat transfer deterioration.

Experimental characterization of airflow within a pallet of product generating heat: Application for cheese product

This work aimed to investigate the airflow and temperature levels within a pallet of heat-generating product under steady-state conditions. A cheese product was simulated. An experimental device was used in which cheese was replaced by plaster camembert cheeses; each simulated camembert was equipped with a heating resistance to simulate heat generation by the product. The pallet was placed in an experimental room with controlled upwind temperature and air velocity. The air velocity within the pallet was measured by LDV (Laser Doppler Velocimetry). Two pallet orientations, two upwind velocities of 0.31 m s−1 and 0.73 m s−1 and two heat fluxes of 0.05 W and 0.30 W per product item with a mass of 0.25 kg were investigated and compared. This study provides a better understanding of the complexity of the airflow pattern related to natural and forced convection flows as well as the dynamic interactions under mixed convection. It can be applied to other living products such as fruit or vegetables that generate heat via respiration.

GEOMETRIC EVALUATION OF FOUR STAGGERED CYLINDERS ARRAY SUBJECTED TO FORCED CONVECTIVE FLOWS BY MEANS OF CONSTRUCTAL DESIGN

This work presents a numerical study on the geometric evaluation of forced convective flows over four staggered arrangement of four cylinders. The forced convective flow is considered incompressible, two-dimensional, laminar and unsteady. Geometry varies according to Constructal Design method. The objectives are the maximization of Nusselt number (NuD) and minimization of drag coefficient (CD) between the cylinders and the surrounding flow. Simulations were performed considering Reynolds numbers of ReD = 10, 40 and 150 and air as working fluid, i.e., Prandtl number is assumed Pr = 0.71. The problem presents three degrees of freedom: ST/D (ratio between transversal pitch of the intermediate cylinders and the cylinders diameter), SL1/D (ratio between the frontal and intermediate cylinders longitudinal pitch and the cylinders diameter) and SL2/D (ratio between the intermediate and posterior cylinders longitudinal pitch and the cylinders diameter). However, SL1/D and SL2/D measures were kept fixed at 1.5 and ST/D varies in the range 1.5 ≤ ST/D ≤ 5.0. The conservation equations of mass, momentum and energy conservation are solved with the Finite Volume Method (FVM). Optimal results for fluid-dynamic study in all ReD cases occurred for the lowest values of ST/D, i.e., (ST/D)o,f = 1.5. For thermal analysis, NuD behavior was assessed, where optimal results for ReD = 10 and 40 occurred for the highest values of ST/D, whilst, for ReD = 150, the optimal value was achieved for the intermediate ratio of ST/D = 4.0.

Empirical correlation of laminar forced convective flow in trapezoidal microchannel based on experimental and 3D numerical study

In this paper, the fluid flow and heat transfer characterizations in trapezoid shaped microchannel with H1 (axially constant heat flux and circumferentially constant temperature) and H2 (constant axial and circumferential wall heat flux) boundary conditions are studied. A three-dimensional numerical model is developed for trapezoidal geometries with side-angles of 30° and 60° with different aspect ratios ranging from 0.1 to 10 to evaluate the Nusselt number and Poiseuille number for fully-developed and developing flow conditions. Experiments are performed on a trapezoidal microchannel and experimental results are compared with the three-dimensional conjugate heat transfer numerical model. It is found that the experimental results are close to the H1 boundary condition. The Nusselt number and Poiseuille number are found to be increasing with the side-angle and aspect ratio of trapezoidal channel. Generalized correlations for Poiseuille number, Nusselt number and entrance length are developed for trapezoidal channel with side angles in the range of 30°–60° and aspect ratios of 0.1–10. These correlations can be used for designing of trapezoidal microchannel in future heat transfer applications.

Mixed convective flow in a bottom heated lid‐driven cubical cavity: Energy streamlines and field synergy

AbstractAnalysis of three dimensional natural convective lid‐driven cavity flow is carried out numerically. The top wall is assumed to slide in its own plane at a constant speed. Isothermal temperature is maintained at horizontal walls in which the bottom wall is assumed to be at a higher temperature than the top wall. Governing equations of this problem, expressed in dimensionless form are solved by using the finite volume method. Numerical results are computed for the control parameters arising in the system, namely, the Reynolds number (Re) and Richardson number (Ri) in the range of 100 ≤ Re ≤ 1000 and 0.001 ≤ Ri ≤ 10. The contours of isotherms, streamlines, Vortex corelines, energy pathlines, and field synergy are used to visualize the flow and thermal characteristics. The simulated results are corroborated with those available in the literature. When Re = 100 and 400 with growth of Ri there are "free" energy streamlines and they exhibited symmetric nature near the boundaries. The participation of convective thermal energy and kinetic energy is insignificant compared to conductive thermal energy, where the velocity components are modest. When Re = 1000 with increase of Ri, "trapped" energy streamlines are detected. Energy streamlines occupy substantial part. This is due to the result of high Re, with increasing Ri, kinetic energy and convective thermal energy get dominated and hence "trapped" streamlines formed. As Re increases, synergy angle increases for distinct Ri values. So the synergy between temperature and velocity gets worse. The synergy angle of buoyant‐aiding flow is high while the buoyant‐opposing flow is significantly less than that of forced convection flow when Ri = 1. This gives the relation between temperature field and velocity at buoyant‐aiding flow, which is at the worst situation leading to increasing average Nusselt number.

Convective Heat Transfer between Liquid Argon Flows and Heated Carbon Nanotube Arrays using Molecular Dynamics

This paper presents the molecular dynamics simulations of unconfined forced convective flow through the nanostructures at steady state condition. A better understanding of forced convective flow through the nanostructures is important because of its wide range of applications in nano-scale devices. Present work focuses on the heat transfer process of argon flow over a carbon nanotube and carbon nanotube arrays with constant surface temperature using molecular dynamics simulations. We consider two elementary configurations for the case of carbon nanotube arrays based on the unit cell structure. The simulation domain consists of fixed carbon nanotubes surrounded with the flowing argon atoms. An extensive study of momentum and thermal transport between carbon nanotube and surrounded argon atoms are analyzed from its microscopic state. The heat transfer coefficient is found in the order of 108 W/m2K. The method proposed in this paper can be an elementary step for the geometry calculation of nano-structured heat sink in the high heat flux electronic chips.

Concurrent Flame Spread Over Two-Sided Thick PMMA Slabs in Microgravity

Spacecraft fire safety is an important consideration when designing future exploration missions. Concurrent flow flame spread experiments were carried out aboard the Cygnus spacecraft while in orbit to address current knowledge gaps related to solid fuel combustion in microgravity. The experiments used PMMA (polymethylmethacrylate) samples that were 50 mm wide and 290 mm long with two variations—one sample was a 10 mm thick flat slab, while the other was a 10 mm thick flat sample that had a 4 mm thick grooved center section. The thickness variation had a major impact on the flame shape, and the grooved sample developed a deep inverted-V shaped notch as the thin center section burned through. This notch enhanced heat transfer from the flame to the solid through an effectively wider flame base, which is the part of the flame with the highest temperature. Unlike in normal gravity (and also likely in partial gravity), where buoyant flow promotes acceleratory upward flame growth, the microgravity flames reached a steady size (limiting length) for a fixed forced convective flow in agreement with theory (i.e. there is a zero net heat flux at the flame tip). The limiting length implies that the spread rate of the flame will be controlled by the regression rate (burnout rate) of the material because the flames remains anchored to the upstream end of the fuel samples. This is a significant finding for spacecraft fire safety, and makes the probability of flashover in a spacecraft unlikely as the flame size will be small for low convective ventilation flow environments typical in spacecraft. On the other hand, long-burning flames in small vehicles will generate significant quantities of fuel vapor that may reach the lean flammability limit and cause a backdraft. The rapid extinction of the flame when the flow was turned off also supports the existing fire mitigation strategy on the ISS to deactivate the ventilation system in the event of fire alarm. These findings can be applied to improve the safety of future space exploration missions.

The Onset of Instabilities in Mixed Convection Boundary Layer Flow Over a Heated Horizontal Circular Cylinder

AbstractThe purpose of this study is to examine the instabilities of a two-dimensional mixed convection boundary layer flow induced by an impinging ascending flow on a heated horizontal cylinder. A significant amount of works is done in recent years on this problem because of its wide range of applications. However, they did not check the stability of the flow in the face of small disturbances that occur in reality. For this, we adopt the linear stability theory by first solving the steady basic flow and then solving the linear perturbed problem. Thus, the governing equations of the basic flow are reduced to two coupled partial differential equations and solved numerically with the Keller-Box method. The corresponding steady solution is obtained, by varying the position along the cylinder’s surface, for different values of Richardson number (λ) and Prandtl number (Pr), up to, respectively, 3000 and 20. To examine the onset of thermal instabilities, the linear stability analysis is done using the normal mode decomposition with small harmonic disturbances. The Richardson number λ is chosen as the control parameter of these instabilities. The resulting eigenvalue problem is solved numerically by the use of the pseudospectral method based on the Laguerre polynomials. The computed results for neutral and temporal growth curves are depicted and discussed in detail through graphs for various parametric conditions. The critical conditions are illustrated graphically to show from which thermodynamic state, the flow begins to become unstable. As a main result, from ξ = 0 to ξ ≈ π/3, we found that forced and mixed convection flow cases are linearly stable in this region. However, in free convection case (λ > 100), it appears that the stagnation zone is the most unstable one and then the instability decreases along the cylinder’s surface up to the limit of its first third, thus giving the most stable zone of the cylinder. Beyond ξ ≈ 1.2, strong instabilities are noted also for low values of Richardson number, and the flow tends to an unstable state even in the absence of thermal effect, i.e., hydrodynamically unstable Ri = 0, probably due to the occurring of the boundary layer separation.