Transient air natural convection in asymmetrically heated vertical channels (original) (raw)
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Numerical Heat Transfer Part A-applications, 2007
In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. The computational domain is made up of the channel-chimney system, and two reservoirs, placed upstream the channel and downstream the chimney. The reservoirs are important because they simulate the thermal and fluid dynamic behaviors far away from the inflow and outflow regions. Results are presented in terms of wall temperature and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles over a period show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but it is the best configuration during the transient heating at the first overshoot. As indicated by the temperature profiles, average Nusselt number profiles over a period of consideration show minimum and maximum values and oscillations before the steady state. Stream function fields allow to observe the development of fluid dynamic structures inside the channel-chimney system, particularly how and when the cold inflow is present and develops.
Numerical Heat Transfer Part A-applications, 2007
In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. The computational domain is made up of the channel-chimney system, and two reservoirs, placed upstream the channel and downstream the chimney. The reservoirs are important because they simulate the thermal and fluid dynamic behaviors far away from the inflow and outflow regions. Results are presented in terms of wall temperature and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles over a period show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but it is the best configuration during the transient heating at the first overshoot. As indicated by the temperature profiles, average Nusselt number profiles over a period of consideration show minimum and maximum values and oscillations before the steady state. Stream function fields allow to observe the development of fluid dynamic structures inside the channel-chimney system, particularly how and when the cold inflow is present and develops.
Impact of external surroundings on natural convection in a vertical channel asymmetrically heated
2016
This paper reports a numerical study of natural convection problem between vertical plates with asymmetric heating. Influence of perturbation conditions outside of the channel on flow structure and on the heat transfer rate are investigated. The effect of temperature consists of considering a gradient of temperature between the bottom and top of the channel in order to obtain a thermal stratification. The effect of surface radiation on the laminar air flow with a thermal stratification is investigated by considering temperature of grey bodies. Results show that these weak perturbations outside the channel are a real influence on flow and that the influence of thermal stratification is more important than surface radiation. Numerical simulations have been carried out at modified Rayleigh number Ra=5.10 (laminar regime) and with Prandtl number Pr=0.71.
The present paper is concerned with the results of the numerical investigation of unpermanent laminar, natural convection in an asymmetrically heated inclined open channel (i 1⁄4 0,45,60 and 75+) with walls at uniform heat flux (qw 1⁄4 10,50,75 and 100 W m"2). Two methodological approaches have been adopted to investigate the air flow in these configurations: 2D and 3D description, and four sets of inlet-outlet velocity-pressure boundary conditions have been considered. Significant differences are observed in the flow dynamics between 2D and 3D results. The numerical results are compared with the experi- mental data and a good agreement is obtained when a local pressure boundary condition is applied at the inlet/outlet sections in the 3D case. A generalized correlation for the average Nusselt number is then obtained from numerical results. This correlation covers a wide range of the modified Rayleigh number and aspect ratio values (Ramcos(i) varying from 1.71 104 to 3.60 106 and 6.5 < H/b < 12.8).
Numerical investigation of natural convection of air in vertical divergent channels
The home of the Transactions of the Wessex Institute collection, providing on-line access to papers presented at the Institute's prestigious international conferences and from its State-of-the-Art in Science & Engineering publications. ... Abstract: A numerical transient analysis of natural convection in air in vertical divergent channels is accomplished. ... The channel walls are heated at uniform heat flux, the problem is two-dimensional and laminar and the full Navier-Stokes and energy equations are employed. ... Results in terms of average wall temperature profile, ...
International Journal of Heat and Mass Transfer, 2012
ABSTRACT In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. The computational domain is made up of the channel-chimney system, and two reservoirs, placed upstream the channel and downstream the chimney. The reservoirs are important because they simulate the thermal and fluid dynamic behaviors far away from the inflow and outflow regions. Results are presented in terms of wall temperature and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles over a period show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but it is the best configuration during the transient heating at the first overshoot. As indicated by the temperature profiles, average Nusselt number profiles over a period of consideration show minimum and maximum values and oscillations before the steady state. Stream function fields allow to observe the development of fluid dynamic structures inside the channel-chimney system, particularly how and when the cold inflow is present and develops.
The present study deals with natural convection flow in a vertical open-ended channel with wall constant heat flux. The experimental and numerical investigations are both conducted using water as the working fluid. The numerical code is developed using finite differences scheme to solve the Navier-Stokes equations under the Boussinesq assumption. Concerning the experimental apparatus, it consists of two heated walls immersed in water. Temperature and velocity measurements are provided for different modified Rayleigh numbers based on the walls spacing b 7 7
Transient natural convection flow dynamics in a asymmetrically heated vertical channel
2014
An experimental study in an open-ended vertical channel is carried out in order to describe the fluid dynamics during the early stage regime of free convection inside a vertical channel asymmetrically heated at uniform heat flux. The flow dynamics were characterized both by means of flow visualization techniques based on laser tomography and by velocity field measurements in the plane of symmetry of the channel using 2D-PIV (Particle Image Velocimetry) technique. The analysis allowed to detect a complex topological behaviour in the internal flow leading to numerous instabilities whose topological features are described. Vortex formation and shedding, vortex splitting, separation and beating of the boundary layer from one wall to another are identified, leading to an imbalance in the outlet pressure field which is certainly at the origin of a large-scale reversal flow.
Numerical Study of Natural Convection in Vertical Channels with Adiabatic Extensions Downstream
Numerical Heat Transfer Part A-applications, 2005
In this paper natural convection flows in a vertical annulus filled with a fluid-saturated porous medium has been investigated when the inner wall is subject to discrete heating. The outer wall is maintained isothermally at a lower temperature, while the top and bottom walls, and the unheated portions of the inner wall are kept adiabatic. Through the Brinkman-extended Darcy equation, the relative importance of discrete heating on natural convection in the porous annulus is examined. An implicit finite difference method has been used to solve the governing equations of the flow system. The analysis is carried out for a wide range of modified Rayleigh and Darcy numbers for different heat source lengths and locations. It is observed that placing of the heater in lower half of the inner wall rather than placing the heater near the top and bottom portions of the inner wall produces maximum heat transfer. The numerical results reveal that an increase in the radius ratio, modified Rayleigh number and Darcy number increases the heat transfer, while the heat transfer decreases with an increase in the length of the heater. The maximum temperature at the heater surface increases with an increase in the heater length, while it decreases when the modified Rayleigh number and Darcy number increases. Further, we find that the size and location of the heater effects the flow intensity and heat transfer rate in the annular cavity.
International Journal of Heat and Mass Transfer, 2007
An experimental investigation on natural convection of air in horizontal channels with well-insulated lower wall and a heated upper wall is carried out.Flow visualization and air temperature measurements are employed to obtain a phenomenological description of air natural convection inside a horizontal, open-ended cavity with a heated upper plate and an unheated lower one.A laminar flow, with a C-loop shape, is observed inside the open-ended cavity. The penetration length is dependent upon the Grashof number. The penetration length increases particularly by increasing either the distance between the walls or the heat flux. The temperature measurements confirm the flow visualization observations. The air temperature profiles inside the open ended cavity indicate that the temperature gradients along the gap cavity are weak for low heat fluxes values. Scale analysis is carried out and shows that penetration length depends on Ra1/2, in accordance with other authors. Furthermore, the estimation of the penetration length depends on the values of the thickness of the boundary layer, evaluated in terms of distance between the walls. Monomial correlations for average Nusselt numbers and dimensionless wall temperatures are proposed in a range of Rayleigh number from 2.78 × 103 to 3.90 × 105 and for an aspect ratio between 2L/b = 10 and 2L/b = 20. The equations agree highly with the experimental results.