Convection Channel Flow Research Articles

Effects of Viscous Dissipation and Periodic Heat Flux on MHD Free Convection Channel Flow with Heat Generation

Effects of Viscous Dissipation and Periodic Heat Flux on MHD Free Convection Channel Flow with Heat Generation

Direct numerical simulation of the fully developed turbulent free convection flow in an asymmetrically heated vertical channel

We report direct numerical simulations of the fully developed turbulent natural convection flow of a Boussinesq fluid (Pr = 0.71) in a vertical channel. One wall is heated with a constant heat flux and the other is assumed to be perfectly insulated. Simulations were performed at three different Rayleigh numbers (Ra = 5 × 105, 106 and 5 × 106). Predictions of the time averaged velocities, temperatures and intensities of the fluctuations agree with experiments reported in the literature. Under the fully developed flow conditions considered in this study the shear stress is greater on thermally active wall than that on the adiabatic wall, but the turbulent intensities and the turbulent shear stress are greater near the insulated wall. Conversely, the intensities of the temperature fluctuations and turbulent heat fluxes are higher near the thermally active wall than those near the adiabatic wall. Contrary to the vertical channel with constant but different wall temperatures, where buoyancy and turbulent shear dominate the generation of turbulent kinetic energy, in the asymmetrically heated channel the turbulent shear stress is the principal mechanism of turbulent kinetic energy production as in a forced convection channel flow.

Free convection channel flow of couple stress casson fluid: A fractional model using Fourier’s and Fick’s laws

Fractional calculus is a branch of mathematics that develops from the usual definitions of calculus integral and derivative operators, just as fractional exponents emerge from integer exponents. The fractional derivative has been successfully used to describe various fundamental processes, including coiling polymer, viscoelasticity, traffic construction, diffusive transport, fluid dynamics, electromagnetic theory and electrical networks. However, many researchers do not use fractional derivatives to understand the physical properties of a non-Newtonian fluid that flows over a moving plate. The present paper aims to consider the couple stress Casson fluid between the parallel plates under variable conditions. The flow regime is formulated in terms of partial differential equations. Unlike the published work, this model is fractionalized using Fick’s and Fourier’s Laws. The system of dimensionless fractional PDEs is solved by using the joint applications of Laplace and Fourier transforms. The influence of several physical parameters, such as the Grashof number, Casson parameter, couple stress parameter etc., on velocity, temperature, and concentration profiles are represented graphically and explained physically. Furthermore, skin friction, Sherwood and Nusselt numbers are numerically calculated and presented in tabular form. It is noted that the influence of physical parameters on skin fraction is opposite to the influence on velocity. Also, the Nusselt number decreases with increasing values of Pr and the Sherwood number increases for decreasing values ofSc. The results show that the velocity of the fluid is the decreasing function of the couple stress parameter and Casson parameter while the increasing function of the permeability parameter and Grashof numbers. It is also worth noting that, unlike the classical model, the present study provides various solutions in the range of an in-between (0, 1], shown inFigures 2,7,8) which might be useful for the experimental and numerical solver to compare their results.

DNS Of entropy generation rates for turbulent flows subjected to high temperature gradients

Direct Numerical Simulations are performed to analyze the influence of key boundary conditions and flow characteristics on entropy generation rate statistics in a highly anisothermal forced convective turbulent channel flow of a thermo-dependent fluid submitted to asymmetrical heating. Each parameter is varied separately to quantify its influence compared to a reference simulation, the hot to cold wall temperature ratio from 1.5 to 3, the mean friction Reynolds number from 150 to 210, the mean thermodynamic pressure from 1bar to 3bar. Two thermal boundary condition types are also compared (fixed temperatures vs. fixed heat flux density at the walls). For each parameter that is varied, the evolution of the entropy generation rate mean, root-mean-square and correlations with the temperature and the streamwise or wall-normal velocity components are presented and discussed. All simulations have in common the thermal asymmetry and the highly non-isothermal nature of the flow: beyond the changes induced by the variation of each parameter, the common characteristics of the statistical quantity profiles are also analyzed. This work establishes how the spatial distribution of the local entropy generation rate is modified by the parameters studied, in which direction and with which sensitivity. It demonstrates the significant influence of the thermal boundary condition type on the entropy generation rate fluctuations near the walls.

DATA-DRIVEN SPARSE IDENTIFICATION OF GOVERNING EQUATIONS FOR FLUID DYNAMICS

It is a challenging and important issue to establish a nonlinear dynamic model of system by use of limited data. The data-driven sparse identification method is an effective method developed recently to identify the governing equations of the dynamic system from data developed in recent years. In this paper, governing equations for different flows are identified by data-driven sparse identification methods. Partial differential equation functional identification of nonlinear dynamics (PDE-FIND) scheme and least absolute shrinkage and selection operator (LASSO) scheme are used to identify the governing equations of two-dimensional flow past a circular cylinder, liddriven cavity flow, Rayleigh-Benard convection and three-dimensional turbulent channel flow. An over-complete candidate library is constructed by direct numerical simulation flow field data in the process of identification. Variables in the library are retained up to second order, variable derivatives are retained up to second order, and nonlinear terms are retained up to fourth order. By comparing the results from the two methods, we find both methods show good performance in identifying governing equation with no nonlinear terms, i.e., vorticity transport equation, heat transport equation and continuity equation. PDE-FIND scheme correctly identified the governing equations and Rayleigh number and Reynolds number for the flow field. But LASSO scheme failed to identify the governing equations which contain strong nonlinear terms, i.e., Navier-Stokes equations. This is because grouping effect may occur among the items in the candidate library and only one item in the group is chosen in such case in LASSO scheme. So PDE-FIND scheme is more effective than LASSO scheme in sparse identification of strongly nonlinear partial differential equation. It is also found that selecting data from regions with abundant flow structures can improve the accuracy of data-driven sparse identification results.

Experimental and CFD Analysis of Two-Phase Forced Convection Flow in Channels of Various Rib Shapes

This paper investigates numerically and experimentally heat transfer forced convective two-phase flow (i.e. air and water) over a rectangular ribbed channel with a vertical orientation. Three distinct rib–groove shapes have been examined. Ribs - groove shapes are; Triangle, Trapezoid, and Semi-Trapezoid ribs-groove. The present study has been performed with continuous heat flux through range of water and air superficial inlet velocity values between 0.105 – 0.316m/s, and 0.263 – 1.320 m/s, respectively. Continuity, momentum and energy calculations have been formulated using the Finite Volume Approach (FVM). Results indicate that the triangle rib-groove has the high heat transfer coefficient and lower temperature difference than other cases against a different number of Reynolds. The experimental data has been compared to numerical results for ribs –grooved channel with deviation of about 1.0% - 7.5%. The channel fitted with triangle ribs shows the highest heat transfer, which is about 59% higher than the smooth channel; 56% for trapezoidal rib, and 44% for channel fitted by semi-trapezoidal rib. Finally, the triangle rib-groove gives a better heat transfer improvement value in comparison with trapezoidal and semi trapezoidal rib-groove channel at constant pumping power.

Numerical analysis of channel flow with velocity-dependent suction and nonlinear heat source

This paper studies the theoretical analysis and applications of a convergent numerical algorithm to the problem of heat transfer in a convective channel flow. The model equations are derived under the assumption of exponentially varying solution-dependent suction, and temperature-dependent heat source. We propose a finite difference scheme which is upwind in nature, and is detected by the direction of wave propagation. The method is theoretically investigated, and the convergence results are obtained. A model problem with an exact closed form solution is used to numerically verify the theoretical results. The scheme is then applied to investigate the flow. The physical results show that temperature is enhanced by increasing Brinkman number, while the velocity is enhanced by increasing pressure gradient or viscosity parameter.

Numerical Solution of MHD Incompressible Convection Flow in Channels

The purpose of this paper is to study numerically the influence of the magnetic field, buoyancy force and viscous dissipation on the convective flow and temperature of the fluid in a square cavity, lid-driven cavity, and lid-driven cavity with an obstacle at the center. The continuity, momentum and energy equations are coupled including buoyancy and magnetic forces, and energy equation contains Joule heating and viscous dissipation. The equations are solved in terms of stream function, vorticity and temperature by using polynomial radial basis function (RBF) approximation for the inhomogeneity and particular solution. The numerical solutions are obtained for several values of Grashof number (Gr), Hartmann number (M) for fixed Prandtl number Pr = 0:71 and fixed Reynolds number Re = 100 with or without viscous dissipation. It is observed that in the absence of obstacle, viscous dissipation changes the symmetry of the isotherms, and the dominance of buoyancy force increases with an increase in Gr, whereas decreases when the intensity of magnetic field increases. The obstacle in the lid-driven cavity causes a secondary flow on its left part. The effect of moving lid is weakened on the flow and isotherms especially for large Gr when the cavity contains obstacle.

Investigation of the Effect of Perforated Sheath on Thermal-Flow Characteristics Over a Gas Turbine Reverse-Flow Combustor—Part 1: Experiment

Abstract Reverse-flow combustors have been used in heavy, land-based gas turbines for many decades. A sheath is typically installed over the external walls of the combustor and transition piece to provide enhanced cooling through hundreds of small impinging cooling jets, followed by a strong forced convection channel flow. However, this cooling is at the expense of a large pressure loss. With the modern advancements in metallurgy and thermal-barrier coating technologies, it may become possible to remove this sheath to recover the pressure loss without causing thermal damage to the combustor chamber and the transition piece walls. However, without the sheath, the flow inside the dump diffuser may exert nonuniformly reduced cooling on the combustion chamber and transition piece walls. The objective of this paper is to investigate the difference in flow pattern, pressure drop, and heat transfer distribution in the dump diffuser and over the outer surface of the combustor with and without a sheath. Both experimental and computational studies are performed and presented in Part 1 and Part 2, respectively. The experiments are conducted under low pressure and temperature laboratory conditions to provide a database to validate the computational model, which is then used to simulate the thermal-flow field surrounding the combustor and transition piece under elevated gas turbine operating conditions. The experimental results show that the pressure loss between the dump diffuser inlet and exit is 1.15% of the total inlet pressure for the non-sheathed case and 1.9% for the sheathed case. This gives a 0.75 percentage point (or 40%) reduction in pressure losses. When the sheath is removed in the laboratory, the maximum increase of surface temperature is about 35%, with an average increase of 13–22% based on the temperature scale of 23 K, which is the difference between the bulk inlet and the outlet temperatures.

Effect of mixed hydrophilic and hydrophobic surface coatings on droplets freezing and subsequent frost growth during air forced convection channel flows

The formation of frost and ice decreases the energy efficiency of refrigeration systems and of air-source heat pump systems. This paper presents new experimental data of frost nucleation and frost growth on cold flat plates operating in frosting conditions with air forced convective flow. The plates surfaces had different wettability: from an uncoated aluminum surface with fine finish roughness and contact angle of 75°, to hydrophobic (θ ≈ 110–116°) and hydrophilic (θ ≈ 19–29°) coatings. Three biphilic coatings with regions of hydrophilic coatings adjacent to areas with hydrophobic coatings were investigated. These coatings manage water droplets movements, reduce freezing temperatures, and delay icing and onset of frost nucleation. A new experimental technique was developed in order to mimic actual field type operating conditions of convective channel flow experienced by fin structures of heat exchangers of typical air-source heat pump systems. Hydrophobic surface and one biphilic surface had early phase changeovers and high thresholds of the frost thickness before switching to the frost growth phase due to the presence of large droplets on the surfaces before they froze into ice beads. Fine-finished aluminum surface and hydrophilic coating had delayed phase changeovers and low frost thicknesses with respect to the other surfaces.

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.

Status and perspectives of turbulent heat transfer modelling in low-Prandtl number fluids

Thermal-hydraulics is recognized as a key safety challenge in the development of liquid metal cooled reactors. At nominal operating conditions, the Prandtl number of liquid metals which are used as primary coolants, such as lead and sodium, is very low: typically of the order of 0.025–0.001. Obtaining an accurate prediction of the turbulent heat transfer at such a low Prandtl number is not an easy task for the standard turbulence models and has challenged the modellers over several decades. In the framework of the EU SESAME project, an effort has been put forward to assess and/or further develop/calibrate different turbulent heat flux closures. In this regard, the present article reports an assessment of four different turbulent heat flux closures for applications involving low-Prandtl fluids. These closures include: (i) the Reynolds analogy based on a constant turbulent Prandtl number (ii) a four-equation explicit algebraic heat flux model (AHFM) (ii) a three-equation implicit AHFM called AHFM-NRG and (iv) a non-linear second-order heat flux model called Turbulence Model for Buoyant Flows (TMBF). The performance of these turbulence models has been assessed in three different test cases against high-fidelity numerical reference data been generated within the SESAME project. The three test cases are: a natural Rayleigh-Bénard convection flow, a mixed convection planar channel flow and a forced convection impinging jet flow. The shortcomings of the classical Reynolds analogy approach for low-Prandtl fluids in all flow regimes are highlighted; hence, more advanced and well-calibrated closures are recommended.

Natural convection channel flow of CMC-based CNTs nanofluid

Fractional order derivatives are more efficient compared to classical derivatives to formulate a physical phenomenon having diverse and widespread applications in science and technology. Their results are comparatively accurate and can interpret memory effect. On the other hand, nanofluids are considered as the next generation of fluids having superior thermo-physical properties to enhance heat transfer rate. Therefore, it is important to study nanofluids and fractional derivatives together. This article deals with the natural convection flow of Carboxy-Methyl-Cellulose (CMC) based carbon nanotubes (CNTs) nanofluid in a two parallel plates channel. CNTs are taken in single and multiple wall shapes (SWCNTs and MWCNTs). The non-Newtonian CMC base fluid is presented by the governing equation of the Jeffery fluid and energy equation with effective thermophysical properties of nanofluids. The problem is subjected to static velocity and constant temperature boundary conditions on the plates together with initial conditions. The governing equations are generalized using the Caputo-Fabrizio fractional derivative (CFFD) approach without a singular kernel. Exact solutions are obtained via the Laplace transform method. In the limiting sense, the results of fractional second grade and Newtonian viscous nanofluids and fluid are recovered. Moreover, for $ \alpha\rightarrow 1$ , the solutions for classical Jeffery, classical second grade and classical Newtonian viscous nanofluids regular fluid are obtained. To examine the influences of various pertinent parameters, the parametric study is carried out through graphs. An opposite trend of velocity and temperature is noticed for increasing $ \alpha$ and $ \phi$ for both SWCNTs and MWCNTs whereas the Nusselt number increases with an increase in $ \alpha$ and $ \phi$ .

Mechanism study of departure of nucleate boiling on forced convective channel flow boiling

We carried out a visualization study of Departure from Nucleate Boiling (DNB) of a vertical upward flow boiling condition. In order to evaluate the effects of the convective flow condition on the DNB mechanism, we synchronized three high-speed cameras (bottom view, side view and total internal reflection view) and captured the detailed dry patch dynamics and relevant bubble behavior. A convective flow of 250 kg/m2 s and subcooled 5 °C (water) was controlled on a heating surface (10 mm × 120 mm), which is much larger than the Rayleigh-Taylor instability wavelength (approximately 25 mm). As a brief result, a high heat flux near DNB (1000 kW/m2) produced periodic massive elongated bubbles through numerous bubble coalescence, and a thin liquid film was developed on the heater surface. Local evaporation of the thin liquid film generated a dry patch, which was observed using a total internal reflection technique. Accounting for the dry patch size and lifetime, the local overheated condition of the dry patch was discussed as an important physics of the irreversible dry patch (DNB). This study may provide a deep insight for understanding the DNB mechanism on the relatively low mass flow boiling condition.

Mixed convection flow in a channel with slip in a porous medium saturated with a nanofluid containing both nanoparticles and microorganisms

The fully developed mixed convection channel flow in a porous medium saturated with a nanofluid in suspension of nanoparticles and microorganisms driven by its upper moving stripy wall is investigated. By a set of similarity variables, we reduce the governing equations of the conservation of total mass, momentum, thermal energy, nanoparticle volume fraction and microorganisms to four coupled nonlinear ordinary differential equations. The accurate solutions are obtained analytically. Besides, effects of physical parameters on important physical quantities are examined and discussed in detail. The transport mechanism of heat and mass due to the permeability, the slip parameter, the thermophoresis and Brownian diffusions, as well as the bioconvection diffusion is presented. As far as we know, the channel flow of a nanofluid with accelerating wall(s) is not yet considered before. Hence this investigation is original and new. It is expected that the current analysis can be applied in the petroleum engineering where nanoparticles and microorganisms play a role on exploitation efficiency enhancement and energy conservation.

Optimal and Experimental Analysis of Electrohydrodynamic Enhancement of Water Evaporation

The paper investigates the EHD effect on the evaporating rate of the water surfaces by numerical and experimental methods. Two-dimensional turbulent forced convection channel flow is theoretically analyzed. Parametric evaluation including the applied voltage, electrode location is executed in details. The related optimization of the electrode pitch and height for specified input voltage is investigated numerically along with a simplified conjugated-gradient method (SCGM). The results show the EHD effect on the evaporating rate is increased with increase of voltage and electrode pitch and decrease of electrode height. In addition, based on the per unit power consumption, the best mass transfer performance is obtained at the optimal electrode location for each input voltage case. At last, the numerical results get a good consistency with the experiment within a discrepancy of 27%.

On the Validity of Boussinesq Approximation in Variable Property Turbulent Mixed Convection Channel Flows

ABSTRACTThe present study investigates the effects of fluid thermo-physical property variations on fully developed turbulent mixed convection flow in vertical and horizontal parallel plate channels where walls are kept at different temperatures. The main attention is paid to the validity of the Boussinesq approximation when the temperature differences are enhanced. Reynolds number based on the channel half-width and friction velocity is assumed to be 150. The governing equations are solved utilizing the large eddy simulation and the low Mach number approach on a finite-volume basis. Dealing with the sub-grid scale stress tensor and the heat flux vector, the dynamic Smagorinsky model is used. All the numerical simulations are carried out by developing a solver in OpenFOAM. The present results display that an increase in the walls' temperature difference, applying the Boussinesq approximation and neglecting the dependency of thermal conductivity and dynamic viscosity on temperature, can result in relatively large errors. Deviations of up to 33% and 45% in the prediction of friction coefficient and Nusselt number, respectively, are shown. This is true for both vertical and horizontal configurations.

Natural convection flow of heat generating hydromagnetic couple stress fluid with time periodic boundary conditions

In this paper, analysis has been conducted to investigate the effects of couple stresses and internal heat generation on the magnetohydrodynamic (MHD) natural convection channel flow with steady-periodic heat input. By applying the steady-periodic heating assumptions, the flow-governing equations driving the fluid system are reduced to boundary-value problems and solved by a convergent successive approximation. The result of the computation shows that the skin friction at the lower wall decreases with increasing values of the couple stress parameter while it enhances the heat transfer rate.

Shape effects of MoS2 nanoparticles on MHD slip flow of molybdenum disulphide nanofluid in a porous medium

Magnetohydrodynamic (MHD) mixed convection channel flow of Molybdenum disulphide (MoS2) nanofluid inside vertical porous medium is investigated. Molybdenum disulphide nanoparticles are used in water as based fluid. Fluid slips at the lower wall of the channel. Molybdenum disulphide as solid nanoparticles is used in different shapes such as platelet, blade, cylinder, brick and spherical. The governing equations are modeled and then solved analytically using perturbation technique. Approximate solutions are obtained for velocity and temperature profiles. Graphical results are obtained through a computational software Mathcad and discussed for various embedded parameters. A comparative study for different shapes of molybdenum disulphide nanoparticles is provided graphically. It is found that MoS2 nanoparticles with blade and platelet shapes have shown the highest heat transfer rate compared to cylinder and brick shapes. Porosity and magnetic parameters have shown opposite effects on velocity, whereas velocity has shown increase with increasing slip.

Performance optimization of a channel flow problem using shape functions

This study explores the use of shape functions on the boundary condition optimization of a forced convection channel flow subjected to an axial heat flux distribution. The goal is to determine the heat flux distribution that minimizes the coolant overheating and simultaneously reduces the computational optimization efforts. The calculations are implemented in a 2-D homemade code, which solved the conservation equations of mass, momentum and energy, while coupling the optimization variables, here represented by the multiple coefficients of the shape functions, with a genetic algorithm. Generally speaking, two types of shape functions were used: unbiased and biased. In the former, the optimization procedure is responsible for obtaining the optimal coefficients for Legendre shape functions, while in the latter, scaling-based power laws are used to construct shape functions. The results computed for both biased and unbiased shape functions show that not only higher performance levels (i.e., less overheating) can be obtained with the present formulation when compared with the techniques employed so far in the available literature (e.g., constant discrete heat flux heaters along the channel), but it also significantly reduced computation efforts. More specifically, the biased shape functions outperform the unbiased formulation by lowering the maximum plate temperature as much as 20% with respect to the standard constant heat flux case, while simultaneously reducing the computational time by a factor of over 4.