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

Abstract

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 investigation of transient natural convection in a vertical channel-chimney system symmetrically heated at uniform heat flux

“…For Ra = 10 4 the maximum wall temperature for simple channel is equal to 0.493 and the percentage decrease changes between 4.5% for B/b = 4.0 and 21% for B/ b = 2.0 The comparison between the results for the two Rayleigh number values shows that the higher the Ra value, the lower the time spent to attain the steady state condition. For all configurations the time for Ra = 10 4 is significantly lower than the one for Ra = 10 2 , as indicated in [15]. By means of the comparison between maximum wall temperatures, it is interesting to observe that the first overshoot indicates that the simple channel presents the lowest value, although at the steady state regime, its maximum wall temperature is the highest.…”

“…The overheating is a typical effect in natural convection with assigned wall heat flux and it is well documented in [1]. It was detected in vertical channel with parallel plates [15,28] and in convergent and divergent vertical channels [13,19].…”

“…Thus, the time step in the transient calculations was Ds = 10 À3 . More details on this aspect are found in [15].…”

“…It was observed that the transient Nusselt number decreases up to the time when convective effects become relevant, then it gradually increases. A numerical investigation on transient numerical analysis, for laminar natural convection in air, between two vertical parallel plates, heated at uniform heat flux by means of the finite volume method, was carried out in [15]. A composite I-shaped computational domain was employed and the simulation allowed to detect the complex structures of the flow inside and outside the channel.…”

“…The need to improve heat transfer in natural convection explains the reason behind the discovery of find new configurations or to analyze standard configurations to carry out optimal geometrical parameters for a higher heat transfer rate and transient behaviors for a suitable thermal design [2,[6][7][8][9][10][11][12][13][14][15][16][17][18][19]. A very simple method, which allows for improvement of the chimney effect and consequently heat transfer rate in vertical channels and other configurations, is that of placing parallel adiabatic extensions downstream heated configurations [20].…”