Assunta Andreozzi

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

Abstract 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 (105–106), 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 102–105 and the dimensionless auxiliary plate height, h/L, is in the range [0,1].

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 a Horizontal Channel With Adiabatic Moving Lower Plate

In this study a numerical investigation of mixed convection in air in horizontal parallel walled channels with moving lower plate is carried out. The moving lower plate has a constant velocity and it is adiabatic, whereas the upper one is heated at uniform heat flux. The effects of horizontal channel height, heat flux and moving plate velocity are analyzed. Results in terms of temperature and stream function fields are given and the mass flow rate per unit of length and divided by the dynamic viscosity is reported as a function of Reynolds number based on the moving plate velocity. For stationary condition of lower plate, a typical C–loop inside the horizontal channel is detected. Different flow motions are observed in the channel and the two reservoirs, depending on the heat flux values and the distance between the heated upper stationary plate and lower adiabatic moving plate. The dimensionless induced mass flow rate presents different increase between the Reynolds number lower or greater than 1000.Copyright

Numerical Analysis of Opposing Mixed Convection in Air in a Vertical Channel With a Moving Plate

In this study a numerical investigation of mixed convection in air due to the interaction between a buoyancy flow and a moving plate induced flow in a vertical channel is carried out. The moving plate has a constant velocity and moves in the opposite direction with respect to the buoyancy force. The channel principal walls are heated at uniform heat flux. The numerical analysis is obtained by means of the commercial code Fluent. The effects of the channel spacing, heat transfer and moving plate velocity are investigated and results in terms of the channel wall and moving plate temperatures and Nusselt numbers are given. The wall temperature profiles allow to observe different behaviors of the flow motion inside the channel, a buoyancy flow, a forced flow and a transition flow related to the velocity of moving plate. The transition velocity increases as the heat flux and the channel gap increase. Dimensionless heat transfer results, Nu/Re0.68 as a function of Richardson number, Ri, present a good agreement with two correlations obtained for the buoyancy dominant flow, at Ri > 10, and forced dominant flow, at Ri < 10−3 .Copyright

Numerical Investigation on Transient Natural Convection in Vertical Channels With Heated and Cooled Walls

A description of transient natural convection in air in a vertical parallel plates channel, with one plate heated and the other one cooled at uniform heat flux, is numerically accomplished. The transient problem is two-dimensional and laminar with constant thermophysical properties. The numerical solution is carried out employing the commercial CFD code Fluent. The computational domain is made up of the physical configuration and two reservoirs, placed downstream and upstream the channel. Results are obtained for Rayleigh number between 103 and 106 and they are presented in terms of wall temperature profiles as a function of time, velocity and temperature profiles along transversal channel sections. The simulation allows to describe the fluid motion structures inside and outside the channel. A complete skew-symmetric motion is detected. For Ra≥105 temperature profiles as a function of time show periodical oscillations. For Ra≥104 overshoots are observed along the profiles and for corresponding average Nusselt number profiles dips are present.© 2007 ASME

Experimental Investigation on the Effect of Longitudinal Aspect Ratio on Natural Convection in Inclined Channels Heated Below

In this paper an experimental investigation on natural convection in air in inclined channels with rectangular transversal section and lower wall heated at uniform heat flux is carried out. Wall temperature measurements and flow visualization are presented. The results allow investigating on the effect of the distance between the two principal parallel walls and of the inclination angle. The experiments are accomplished for two channel gap values: 20 and 40 mm. The inclination angle is equal to 80° and 88°. The flow development and the shape of flow transitions along the channel are visualized. Flow visualization allows to describe the secondary motion inside an inclined channel. Flow separation region along the lower heated plate begins at lower axial coordinate as the wall heat flux, the inclination angle and the channel gap are greater. The flow separation depends also on transversal coordinate. The detected secondary structures pass from thermals to plumes and vortices. Along the plane parallel to the heated wall, the visualization shows that thermal plumes split in V-shaped structures. For the largest considered channel gap value the instability phenomena in the channel are stronger and chaotic motion in the channel outlet zone is observed. When the channel gap value increases wall temperatures become lower because the higher distance between the walls determines a greater mass flow rate and an increase in the heat transfer.© 2006 ASME

Numerical Simulation of Transient Natural Convection in a Channel-Chimney System

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. Results are presented in terms of wall temperature, mass flow rate 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 vs time 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 the best configuration during the transient heating at the first overshoot. Velocity profiles in the chimney allow for identification of some different fluid dynamic behaviors such as the vortex in lower corner and the cold inflow in the chimney. According to the temperature profiles, average Nusselt number profiles as a function of time show minimum and maximum values and oscillations before the steady state.Copyright

Transient Analysis of Natural Convection Between Inclined Parallel Plates

In the present investigation a transient numerical analysis for laminar natural convection in air between two inclined parallel plates, with the upper plate heated at uniform heat flux and the lower one unheated, is carried out by means of the finite volume method. The full two-dimensional Navier-Stokes equations together with the continuity and energy equations are solved in an enlarged computational domain. Results are presented in terms of air velocity and temperature profiles, wall temperature profiles and thermal fields for different Rayleigh numbers, Ra, and inclination angles, γ. The steady state conditions are attained faster for γ > 0° at the lower analyzed Rayleigh number values while at the higher Ra values, the time at which steady state conditions are attained increases as the inclination angle increases. At the inlet and midplane section, air temperatures decrease as the inclination angle increases for all Rayleigh number values. For inclination angle greater than 5° overshoot and undershoot in wall temperature profile are observed. At the outlet section, the downflow decreases as the inclination angle increases.© 2003 ASME

Thermal Energy Storages Analysis for High Temperature in Air Solar Systems

In this paper a high temperature thermal storage in a honeycomb solid matrix is numerically investigated and a parametric analysis is accomplished. In the formulation of the model it is assumed that the system geometry is cylindrical, the fluid and the solid thermophysical properties are temperature independent and radiative heat transfer is take into account whereas the effect of gravity are neglected. Air is employed as the working fluid and the solid material is cordierite. The evaluation of the fluid and thermal behaviors are accomplished assuming the honeycomb as a porous medium. The Brinkman-Forchheimer-extended Darcy model is used in the governing equations and the local thermal non equilibrium is assumed. The commercial CFD Fluent code is used to solve the governing equations in transient regime. Numerical simulations are carried out with storage medium at different mass flow rates of the working fluid and different porosity values. Results show the effects of storage medium, different porosity values, porosity effect and mass flow rate on stored thermal energy and storage time. Results in terms of temperature profiles and stored thermal energy as function of time are presented.Copyright

Parametric Analysis for Thermal and Fluid Dynamic Management of Natural Convection in a Channel-Chimney System

In this paper a parametric analysis of natural convection in air in a channel-chimney system symmetrically heated at uniform heat flux, obtained by means of a numerical simulation, is carried out. The analysed regime is two-dimensional, laminar and steady-state. The numerical procedure employs the full Navier-Stokes and energy equations in terms of the stream function-vorticity approach. Results are presented in terms of wall temperature profiles in order to show the more thermally convenient configurations which correspond to the channel-chimney system with the lowest maximum wall temperature. The analysis is obtained for a Rayleigh number in the range between 102 and 105 , for a channel aspect ratio equal to 5, 10 and 20 and the extension and expansion ratios between 1.0 and 4.0. Correlations for dimensionless mass flow rate, maximum wall temperature and average Nusselt number in terms of Rayleigh number, aspect ratio, extension and expansion ratios are presented. Geometric optimal configurations, for assigned Rayleigh number and aspect ratio, are estimated as a function of the extension ratio. For considered Rayleigh number the difference between the highest and the lowest maximum wall temperatures increases increasing the channel aspect ratio. This behaviour is as greater as the extension ratio is. These differences decrease significantly for the highest Rayleigh number value. The optimal expansion ratio values depend strongly on Rayleigh number and extension ratio values and slightly on the aspect ratio.Copyright