Natural Convection In Vertical Channel Research Articles

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 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.

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

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.

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.

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.

Entropy generation in natural convection in a symmetrically and uniformly heated vertical channel

In this study numerical predictions of local and global entropy generation rates in natural convection in air in a vertical channel symmetrically heated at uniform heat flux are reported. Results of entropy generation analysis are obtained by solving the entropy generation equation based on the velocity and temperature data. The analyzed regime is two-dimensional, laminar and steady state. The numerical procedure expands an existing computer code on natural convection in vertical channels. Results in terms of fields and profiles of local entropy generation, for various Rayleigh number, Ra, and aspect ratio values, L/ b, are given. The distributions of local values show different behaviours for the different Ra values. A correlation between global entropy generation rates, Rayleigh number and aspect ratio is proposed in the ranges 10 3 ⩽ Ra ⩽ 10 6 and 5 ⩽ L/ b ⩽ 20.

Effects of variable properties and non-uniform heating on natural convection flows in vertical channels

The effects of variable properties and non-uniform heating on laminar air flows induced by natural convection in vertical channels are investigated numerically. A full-elliptic model which accounts for variations in viscosity and thermal conductivity with temperature and which determines the density from the state equation has been applied to cases in which variable property effects cannot be neglected (including conditions for which flow reversals may occur). The influence of the Rayleigh number and non-uniformity of the wall heat flux distribution on the critical heat per unit time transferred from the walls in symmetric and asymmetric channels is analyzed for a wide range of Rayleigh numbers. It is shown that the maximum wall temperature can be reduced substantially by selecting an appropriate wall heat flux distribution, although at the cost of slightly more restrictive critical conditions.

Natural convection in vertical channels with an auxiliary plate

Research on natural convection in open channels is very extensive due to its role in many engineering applications such as thermal control of electronic systems. In this paper, a parametric analysis is carried out in order to add knowledge of heat transfer in air natural convection for a symmetrically heated vertical parallel plate channel with a central auxiliary heated or adiabatic plate. The two‐dimensional steady‐state problem is solved by means of the stream function–vorticity approach and the numerical solution is carried out by means of the control volume method. Results are obtained for both a heated and unheated auxiliary plate, for a Rayleigh number in the range 103–106, for a ratio of the auxiliary plate height to the channel plate height equal to 0, 0.5 and 1 and for a ratio of the channel length to the channel gap in the range 5–15. Correlations for maximum wall temperatures and average channel Nusselt numbers are proposed.

Thermal and fluid dynamic behavior of symmetrically heated vertical channels with auxiliary plate

Air natural convection in vertical channel configurations is strongly attractive in thermal design and control of devices. This is mainly due to its simplicity, no maintenance costs and reliability. This paper examines air natural convection in vertical channels with an auxiliary plate along the centerline. The channel is symmetrically heated and the walls are at uniform heat flux, whereas the auxiliary plate is either adiabatic or heated at uniform heat flux. The analysis is obtained for a two-dimensional steady state and laminar regime, and the fully elliptic equations are solved numerically by the control volume method on a finite I-shaped computational domain. Results in terms of stream function and temperature fields, velocity and temperature profiles inside the channel and pressure profiles along the centerline are given either for insulated auxiliary plate or for heated auxiliary plate. The adiabatic auxiliary plate along the centerline of the channel produces a chimney effect reduction of the channel while the heated auxiliary plate, at higher Ra values (10 5–10 6), provides an increase in the mass flow rate in the channel. Finally, two correlations between average channel Nusselt number, channel Rayleigh number, Ra ∗ , and dimensionless auxiliary plate height, h/ L, are proposed. One correlation is for the channel with heated auxiliary plate and another is for the channel with unheated auxiliary plate. The channel Rayleigh number range is 10 2–10 5 and the dimensionless auxiliary plate height, h/ L, is in the range [0,1].

Laminar natural convection in vertical channels with periodic wall protrusions

Natural convection flow and heat transfer of a viscous fluid confined between two vertical parallel plates is analytically investigated taking volumetric heat generation into account. It is assumed that one of the plates is flat and the other has periodic protrusions. The short protrusions are assumed to be periodic in span, facilitating a mathematical representation for the arbitrary shape protrusion by a Fourier series. The governing equations of momentum, and energy are solved by regular perturbation method. The flow, and heat transfer characteristics are determined for different values of Grashof number, wall temperature ratio, heat source parameters, protrusion frequency and spacing.

Laminar natural convection between partially heated vertical parallel plates

A numerical analysis has been performed to examine the characteristics of laminar natural convection in vertical channel with unheated entry and unheated exit. The heated section is subjected to uniform wall temperature (UWT) or uniform heat flux (UHF). Theoretical results for average Nusselt number $$\overline {Nu} $$ and induced volume flow rateQ were derived under fully developed condition. Particular attention is paid to investigating the effects of the partially heated section on the induced volume flow rate and Nusselt number for various conditions. Results show that for UWT the induced volume flow rateQ increases with decreasing unheated entry length or increasing total length of heated section and unheated exit. For a fixed unheated entry length, the channel with a longer heated section length causes a greaterQ. Additionally, for both UWT and UHF, the average Nusselt number $$\overline {Nu} $$ under fully-developed condition increases with increasing value ofE 1/E 2.

Natural Convection in a Vertical Heated Tube Attached to a Thermally Insulated Chimney of a Different Diameter

Natural convection is often a convenient and inexpensive mode of heat transfer. It is commonly employed in the cooling of electronic equipment and many other applications. Since the initial work by Bodoia and Osterle (1962) on finite difference solutions of natural convection between vertical isothermal plates, many other researchers have studied natural convection in vertical channels. Specifically Davis and Perona (1971) studied natural convection in vertical heated tubes. A thermally insulated chimney attached to a vertical heated channel induces an increase in the natural convection in the channel and leads to a higher heat transfer rate. This is the well-known chimney effect discussed in the paper by Haaland and Sparrow (1983). If the chimney diameter is larger than the heated tube diameter, the friction loss in the chimney region decreases with increasing chimney diameter. This induces an increase in the mass flow rate and leads to a higher heat transfer rate than the case for a chimney of the same diameter. However, from a geometric consideration it is evident that the chimney effect diminishes in the limiting case of an extremely large chimney diameter compared with its height. Therefore, there exists an optimum diameter where the heat transfer is maximum.more » To investigate the chimney effect computations are carried out for a Rayleigh number of 12.5, based on the heated tube radius, and for a Prandtl number of 0.7. The numerical results are based on a control volume finite difference method. The average Nusselt number results are compared with the numerical results obtained for a chimney attached to a tube of the same diameter.« less

Thermal Design Considerations in Vertical-Channel Natural Convection

Laminar natural convection in vertical channels has received considerable attention in the literature, principally because of its value as a simplified mathematical model for electronics cooling. Bar-Cohen and Rohsenow (BC and R) applied the correlation technique for Churchill and Usagi nto obtain predictions for heat transfer in the vertical channel configuration. The correlating equations obtained in BC and R gave favorable comparison with previous experimental data and numerical solutions. The symmetric uniform wall heat flux case from is considered in the present note. Maximum channel wall temperature rise, as predicted from the correlating equation, is expressed in terms of dimensional variables. This gives an alternate representation of results for design purposes.