Convection In Vertical Channel Research Articles

Numerical Study of the Heat Transfer Characteristics of Convection in Vertical Channel through FEM Simulation

An attempted is made on the free convection flow of an incompressible viscous fluid in the profile in terms of heat characteristics were analyzed. The flow is assumed to be along the axis of the channel. The uniform temperature is considered in the study. The momentum conservation equations for the fully developed flow are based on the Brinkman model. The viscous and Darcy dissipation are taken into account in the energy equation as the modification of the heat flow. By using the finite element method, the fluid and the porous matrix are in local thermal equilibrium and the flow is unidirectional along the direction of the buoyancy. The non-linear equations governing the flow, heat and mass transfer have been analyzed. Various parameters such as the velocity, temperature, concentration, Nusselt number, and Sherwood number are analyzed and their behavior is interpreted in the paper.

Heat Transfer Correlations for Laminar Free Convection in Vertical Channels with Asymmetrically Heated Isothermal Walls

Laminar free convection in a vertical channel with isothermal walls is a classical heat transfer problem. In general, the channel walls may be at different temperatures, leading to a difference in the rate of heat transfer at the walls. Although numerical and experimental results are often presented in terms of average wall Nusselt numbers, separate correlations for heat transfer at each wall are rarely available. The few existing relations are complex in form due to the extra parameters introduced to facilitate the correlation of results. Moreover, the existing correlations do not reflect the physics of the problem in full. In this work, the three-temperature problem of laminar-free convection of air in a tall vertical channel with isothermal walls is studied in a new theoretical framework, namely the extended Newton formulation. This formulation eliminates the need for an effective temperature difference, hence simplifying the presentation and correlation of the results. The dQdT technique was utilized along with computational fluid dynamics to calculate the average paired Nusselt numbers that characterize the extended Newton formulation of the problem. The dQdT results were then used to derive heat transfer correlations which are both simpler in form and more accurate than the existing correlations. The proposed correlations are applicable to cases of both channel walls hotter or both colder than the ambient air.

ОБОСНОВАНИЕ ГИПОТЕЗЫ ПЕРЕХОДА ЛАМИНАРНОГО ТЕЧЕНИЯ В ТУРБУЛЕНТНОЕ ПРИ СМЕШАННОЙ КОНВЕКЦИИ ЖИДКОСТИ В ВЕРТИКАЛЬНЫХ КАНАЛАХ

ОБОСНОВАНИЕ ГИПОТЕЗЫ ПЕРЕХОДА ЛАМИНАРНОГО ТЕЧЕНИЯ В ТУРБУЛЕНТНОЕ ПРИ СМЕШАННОЙ КОНВЕКЦИИ ЖИДКОСТИ В ВЕРТИКАЛЬНЫХ КАНАЛАХ

Natural convection in a parallel-plate vertical channel with discrete heating by two flush-mounted heaters: effect of the clearance between the heaters

In this study, natural convection in a vertical channel is studied experimentally and numerically. One of the channel walls is heated discretely by two flush-mounted heaters while the other is insulated. The effects of the clearance between the heaters on heat transfer and hot spot temperature while total length of the heaters keeps constant are investigated. Four different settlements of two discrete heaters are comparatively examined. Air is used as the working fluid. The range of the modified Grashof number covers the values between 9.6 × 105 and 1.53 × 10.7 Surface to surface radiation is taken into account. Flow visualizations and temperature measurements are performed in the experimental study. Numerical computations are performed using the commercial CFD code ANSYS FLUENT. The results are represented as the variations of surface temperature, hot spot temperature and Nusselt number with the modified Grashof number and the clearance between the heaters as well as velocity and temperature variations of the fluid.

Stochastic Analysis of Natural Convection in Vertical Channels with Random Wall Temperature

This study attempts to derive the statistics of temperature and velocity fields of laminar natural convection in a heated vertical channel with random wall temperature. The wall temperature is expressed as a random function with respect to time, or a random process. First, analytical solutions of the transient temperature and flow velocity fields for an arbitrary temporal variation in the channel wall temperature are obtained by the integral transform and convolution theorem. Second, the autocorrelations of the temperature and velocity are formed from the solutions, assuming a stationarity in time. The mean square values of temperature and velocity are computed under the condition that the fluctuation in the channel wall temperature can be considered as white noise or a stationary Markov process. Numerical results demonstrate that a decrease in the Prandtl number or an increase in the correlation time of the random process increases the level of mean square velocity but does not change its spatial distribution tendency, which is a bell-shaped profile with a peak at a certain horizontal distance from the channel wall. The peak position is not substantially affected by the Prandtl number or the correlation time.

Flow visualization of natural convection in vertical channels with opposing buoyancy forces

Natural convection inside an asymmetrically, isothermally heated vertical channel with opposing buoyancy forces was studied with flow visualization and laser interferometry. Opposing buoyancy forces occur inside the channel when the hot wall is warmer than the ambient temperature and the cold wall is cooler than the ambient. These opposing buoyancy forces can cause flow instability inside the channel. The flow- and temperature-fields are of interest for validation of numerical modelling. The flow-field was obtained with flow visualization using a laser sheet and a Dräger tube, which supplied the aerosol. The temperature-field was obtained using a Mach–Zehnder interferometer. Experiments were carried out over a range of temperature ratios between −0.25 and −0.75 using aspect ratios between 13.2 and 26.4. These conditions provided a modified Rayleigh number range of approximately 5–215. Flow- and temperature-field photographs were taken ranging from steady laminar to unsteady turbulent flow.

Numerical model for thin liquid film with evaporation and condensation on solid surfaces in systems with conjugated heat transfer

Condensation and evaporation processes from wetted surfaces are of utmost importance in many technological or industrial applications. In many devices such as home-appliances and air conditioning systems just to name a few, condensation and evaporation processes greatly impact their performance and energy efficiency; The physics of these processes is quite complex, involving conjugate heat transfer among solid–liquid film-gaseous phase together with the change of phase associated with evaporation and/or condensation. In spite of their own importance in technological applications, most physical aspects of this class of multiphase flows still need to be addressed. So far, many studies have been performed focusing on individual issues of full process, and, to the authors knowledge, their mutual influences was not addressed as yet. The aim of the present study is to supplement this field with a reliable numerical model for evaporation and condensation of thin liquid films in buoyant, conjugated heat transfer systems with vapor transport. The model is presented, validated on a simple case and successively applied to a literature relevant case. Validation is carried out considering a simple, insulated, 2-D geometry consisting of a fluid box connected to a solid. This allows to validate the model in a tightly controlled environment. Successively we reproduce the work of N. Laaroussi, G. Lauriat, G. Desrayaud, Effects of variable density for film evaporation on laminar mixed convection in vertical channel, Int. J. Heat Mass Transfer 52, 2009, 151–164, and we highlight the importance of considering the thermal characteristics of the solid part for the accurate reproduction of the complex physics associated to evaporation/condensation processes.

Buoyancy Effects on Laminar Mixed Convection in Vertical Channel With Dissipation

Deals with the buoyancy effects on laminar mixed convection in vertical channel is considered. Solutions of the governing parabolic equations are obtained by the use of an implicit finite difference technique coupled with a marching procedure. The velocity, the temperature and the pressure profiles are presented graphically for different values of governing parameters like Eckert Number Ek, buoyancy parameter Gr/Re and Prandtl number Pr and their behaviour discussed.

Numerical investigation of passive cooling in open vertical channels

Abstract Numerical simulations have been carried out in order to investigate natural convection flow and heat transfer in vertical channels which are relevant to passive cooling of building-integrated photovoltaics (BIPV) systems. The numerical results have been validated against existing experimental data available in literature. It has been found that narrow vertical channels with different aspect ratios exhibit varied heat transfer behaviours, implying its significance in the design of passive cooling applications. In addition, the different behaviours of heat transfer may be explained by the turbulence quantities obtained through large-eddy simulations (LES). Based on the current numerical results, a correlation for turbulent natural convection in vertical channel has been determined to predict the average Nusselt number in terms of the relevant dimensionless parameters for the geometry considered in this study.

On the modeling of aiding mixed convection in vertical channels

This paper aims at showing that to prescribe a flow rate at the inlet section of a vertical channel with heated walls leads to surprising and counterintuitive physical solutions, especially when the problem is modeled as elliptical. Such an approach can give rise to the onset of recirculation cells in the entry region while the heat transfer is slightly increased under the influence of the buoyancy force. We suggest an alternative model based on more realistic boundary conditions based on a prescribed total pressure at the inlet and a fixed pressure at the outlet sections. In this case, the pressure and buoyancy forces act effectively in the same direction and, the concept of buoyancy aiding convection makes sense. The numerical results emphasize the large differences between solutions based on prescribed inlet velocity and those obtained with the present pressure-based boundary conditions.

NUMERICAL INVESTIGATION OF LAMINAR MIXED CONVECTION IN TROMBE WALL CHANNEL

The two dimensional steady, combined forced and natural convection in vertical channel isinvestigated for laminar regime. To simulate the Trombe wall channel geometry properly, horizontalinlet and exit segments have been added to the vertical channel. The vertical walls of the channel aremaintained at constant but different temperature while horizontal walls are insulated. A finitedifference method using up-wind differencing for the nonlinear convective terms, and centraldifferencing for the second order derivatives, is employed to solve the governing differentialequations for the mass, momentum, and energy balances. The solution is obtained for streamfunction, vorticity and temperature as dependent variables by iterative technique known as successivesubstitution with overrelaxation. The flow and temperature patterns in the channel are obtained forReynolds numbers and Grashof number ranging from 25 to 100 and (100 to 1,000,00,) respectively.A computer program ( Fortran 90 ) is built to calculate the fraction factor and the totalaverage Nusselt number (Nu) also the average heat transfer Q in steady state and for Aspect ratio Ar(10) and Grashof number GR (10 2 − 10 5 ), the fluid Prandtl number is fixed at (Pr=0.733) andReynolds number Re (25-100).The results show reasonable representation to the relation between Nusselt number and frictionfactor with other parameters (Ar, GR and Re). Nu is increased with increasing Re and GR but itdecreases with Ar increase and (Q) is increased with increasing Re ,GR and Ar. At the same time, theproduct friction factor(fRe) increased with (GR) and (Ar)increased and (Re )decrease.Comparison of the result with the previous work shows a good agreement

Heat transfer enhancement and pressure drop reduction due to mixed convection between two vertical parallel plates

PurposeThis paper's aim is to examine flow and heat transfer through vertical channels between parallel plates, which is of prime importance in the design of cooling systems for electronic equipment such as that of finned cold plates in general, plate‐and‐frame heat exchangers, etc.Design/methodology/approachNumerical and analytical solutions are presented to investigate the heat transfer enhancement and the pressure drop reduction due to buoyancy effects (for buoyancy‐aided flow) for the developing laminar mixed convection in vertical channel between parallel plates in the vicinity of the critical values of the buoyancy parameter (Gr/Re)crt that are obtained analytically. The numerical solutions are presented for a wide range of the buoyancy parameters Gr/Re that cover both of buoyancy‐opposed and buoyancy‐aided flow situations under each of the isothermal boundary conditions under investigation.FindingsBuoyancy parameters greater than the critical values result in building‐up the pressure downstream of the entrance such that the vertical channel might act as a thermal diffuser with possible incipient flow reversal. Locations at which the pressure gradient vanishes and the locations at which the pressure‐buildup starts have been numerically obtained and presented for all the investigated cases.Research limitations/implicationsThe study is limited to the laminar flow situation.Practical implicationsThe results clearly show that for buoyancy‐aided flow, the increase of the buoyancy parameter enhances the heat transfer and reduces the pressure drop across the vertical channel. These findings are very useful for cooling channel or chimney designs.Originality/valueThe study is original and presents new findings, since none of the previous studies reported the conditions for which pressure buildup might take place due to mixed convection in vertical channels between parallel plates.

Natural Convection in Vertical Channels with Porous Media and Adiabatic Extensions

A numerical investigation on natural convection in air in a vertical heated channel, partially filled with porous medium, with adiabatic extensions downward and collinear the heated plates is accomplished. The fluid flow is assumed two-dimensional, laminar, steady state and incompressible. The porous material is considered as homogeneous and isotropic and the Brinkman-Forchheimer-extended Darcy model is considered. A finite-extension computational domain is employed to simulate the free-stream condition and allows to account for the diffusive effects and the numerical results are obtained using the finite volume method by FLUENT. Results in terms of wall temperature profiles are presented to evaluate the effects of the main thermal and geometrical parameters. The adiabatic extensions determine a wall temperature decrease and wall temperature decreases increasing Darcy number. In full filled heated channels wall temperature presents a significant increase for Darcy number decrease.

Flow Reversal of Fully-Developed Mixed MHD Convection in Vertical Channels

The present analysis is concerned with flow reversal phenomena of the fully-developed laminar combined free and forced MHD convection in a vertical parallel-plate channel. The effect of viscous dissipation is taken into account. Flow reversal adjacent to the cold (or hot) wall is found to exist within the channel as Gr/Re is above (or below) a threshold value. Parameter zones for the occurrence of reversed flow are presented.

Solitary Roll Vortices in the Mixed Convection of a Vertical Fluid Layer

AbstractSome earlier experimental and numerical findings from convection in vertical channels suggest that localized convection rolls could play a role in the transition process of free or mixed convection. In the present work solitary convection roll vortices for a vertical fluid layer with stable stratification, differential shear and differentially heated side‐walls have been obtained numerically for a fluid of unit Prandtl number. The solutions appear through saddle‐node bifurcations and in certain parameter ranges they do also exist for linearly stable basic flow. (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mixed Convection in a Vertical Heated Channel: Influence of the Aspect Ratio

In mechanical and environmental engineering, mixed convection is a frequently encountered thermal fluid phenomenon which exists in atmospheric environment, urban canopy flows, ocean currents, gas turbines, heat exchangers, and computer chip cooling systems etc... . This paper deals with a numerical investigation of mixed convection in a vertical heated channel. This flow results from the mixing of the up-going fluid along walls of the channel with the one issued from a flat nozzle located in its entry section. The fluid- dynamic and heat-transfer characteristics of vented vertical channels are investigated for constant heat-flux boundary conditions, a Rayleigh number equal to 2.57 10 10 , for two jet Reynolds number Re=3 10 3 and 210 4 and the aspect ratio in the 8-20 range. The system of governing equations is solved with a finite volumes method and an implicit scheme. The obtained results show that the turbulence and the jet-wall interaction activate the heat transfer, as does the drive of ambient air by the jet. For low Reynolds number Re=3 10 3 , the increase of the aspect Ratio enhances the heat transfer of about 3%, however; for Re=2 10 4 , the heat transfer enhancement is of about 12%. The numerical velocity, pressure and temperature fields are post-processed to compute the quantities of engineering interest such as the induced mass flow rate, and average Nusselt number, in terms of Rayleigh, Reynolds numbers and dimensionless geometric parameters are presented. and natural convection was initiated in the 1960's and mainly based on experimental approaches. Natural convection between heated vertical parallel plates is the most frequently used configuration in convection air cooling of electronic equipment. The passive character of cooling by natural convection makes it very attractive for applications in electronic devices. A review of the literature shows that laminar natural convection in vertical channel has been studied by several authors: Elenbaas (1) led the first experimental study which was used in the follow as reference solution. Bodoi and Osterle (2) have obtained the first numerical solution of the natural convection in vertical channel. Their results were in good agreement with those Elenbaas. In order to increase the cooling requirements, researches for methods to improve the heat transfer parameters or to analyze standard configurations to carry out optimal geometrical parameters for a better heat transfer rate

Numerical simulation for natural convection in vertical channels

The investigation of laminar natural convection in vertical obstructed channels is conducted using an h-adaptive finite element algorithm. The adaptive model uses an L 2 norm based a-posteriori error estimator with a semi-implicit, time-stepping projection technique. The advection terms are treated using an explicit Adams Bashforth method while the diffusion terms are advanced by an implicit Euler scheme. By using the adaptive algorithm, mesh independent studies can be avoided. Results are obtained for thermal and flow patterns including average Nusselt numbers for different parameters (Rayleigh number, aspect ratio and locations of obstructions) in both smooth and obstructed channels.

Transient Natural Convection in Vertical Channels Symmetrically Heated at Uniform Heat Flux

A numerical transient analysis of natural convection in air between two vertical parallel plates, heated at uniform heat flux, is carried out. The problem is two-dimensional and laminar and the full Navier-Stokes and energy equations are employed. The control volume method is used to discretize the equations on a uniform grid. Results are given at different aspect ratio values and Rayleigh number values. The simulation allows detection of complex structures of the flow inside and outside the channel. Temperature profiles as a function of time show an overshoot and undershoot increase at the lowest aspect ratio and highest Rayleigh number. Inside the channel conductive and convective regimes as well as an inverse fluid motion are observed. Transient average Nusselt number presents oscillations before the steady-state.

Entropy generation due to mixed convection between vertical parallel plates under isothermal boundary conditions

This paper presents the results of an investigation of the buoyancy effects on the entropy generation due to laminar mixed convection in vertical channels between parallel plates. The buoyancy effects, in terms of the buoyancy parameter (Gr/Re) as well as the effects of other operating parameters on the entropy generation are obtained and analysed. The analysis revealed the existence of optimum values of the buoyancy parameter Gr/Re at which the total entropy generation assumes its minimum values. These values are obtained analytically for isothermal boundary conditions of first kind at different values of the two plate's temperature ratio, θT.

Numerical modeling of the electrohydrodynamic effect to natural convection in vertical channels

The electrohydrodynamic effect to natural convection inside the vertical channels is numerically investigated by computational fluid dynamics technique. The range of parameters considered are 10 4 = Ra = 10 7, 7.5 = V 0 = 17.5 kV, and 2 = aspect ratio = 10. Flow and temperature distributions are affected with supplied voltage at the wire electrodes, and the heat transfer enhancement is significantly influenced at low Rayleigh number. The augmented volume flow rate of fluid is indicated in relation with the number of electrodes. Moreover, heat transfer enhancement also depended on the electrode arrangement while the number of electrodes is initially fixed. The relation between channel aspect ratio and number of electrodes that performs the maximum heat transfer is expressed incorporating with the optimum concerning parameters.