Abstract
A series of numerical studies on the fluid flow and heat transfer characteristics for transient natural convection within vertical finite-length heated channels have been performed. The geometric configurations explored in the present study including a single channel and an array ofchannels. The parameters studied are Prandtl number (Pr) of 0.7, 7.0 and 25.0, channel aspect ratio (A) various from 1 to 8, and steady-state Grashofnumbers (Grs) ranged from 10 to 10 ^8. From the studies of vertical finite-length channels, it is found that theeffective time ton is strongly dependent on the following parameters: steady-state Grashof number, local position on the channel wall, chanel aspect ratioand Prandtl number. At a specific location on the channel wall, the effectivetime ton decreases with increasing steady-state Grashof number, and increaseswith increasing channel aspect ratio or Prandtl number. At a specific case,theeffective time ton increases along the location on the channel wall (x/L). Foran array of channels,the effective time ton on the side channel planes is lessthan that on the center channel planes. Base on the present numerical results, new correlations of the effective time ton for transient natural convection in a single heated finite-length channel and in an array of vertical heated finite-length channels have been proposed, respectively. In addition, transient thermal and flow fields,including isotherms,pressurecontour, streamlines and velocity profiles have been presented. The effects ofrelevant parameters on transient thermal and fluid flow characteristics investigated in the present study include the distribution of local walltemperature, maximum temperature on channels walls, induced Reynolds number,and distribution of local Nusselt numbers. According to the numerical results, new composite correlations of transient induced reynolds number for verticalheated finite-length channels are proposed. The correlation is valid at thetransient time t>ton. As for local heat transfercharacteristics along the channel walls, the distributions of transient local Nusselt number (Nux) are presented. At a specific location of channel wall, the Nux decreases with time in the beginning period of 0<t<ton; while it gradually increases with time in the transient period of t>ton. A rapid switching of heat transfer mode on the Nux distributions can be found for the cases with large steady-state Grashofnumbers, say Grs>10^5. Also the switching time decreases with decreasing channel aspect ratio or Prandtl number. For an array of channels, at a specific location of channel wall,the Nux distributions on the heated planes of the side channel are lower than those of the center channel walls, especially on Plane 3 of the side channel. Finally, new empirical correlation of local steady-state Nusselt number for a single heated finite-length channel and for an array of vertical heated finite-length channels are presented,respectively.