Such a form of the fluid flow was first observed by Benard in 1901. In a horizontal fluid layer between cold upper (T c1) and hot lower (T h) walls, the fluid flow has a cellular form with hexagonal cells ( Benard cells) in the center of which fluid ascends from the hot surface to the cold one whereas in the periphery, it descends ( Figures 2c, d). A flow in narrow slots between parallel vertical plates is formed in the form of periodic circulations ( Figure 2b). On heating one vertical wall (temperature T h) and cooling the other (temperature T c1) the modes with a common fluid flow are possible through the entire cavity that involve local secondary flows near vertical walls as is exemplified in Figure 2a by FC in a square cavity. In narrow and closed cavities a FC flow is much more complex ( Figure 2), due to the interaction between near-wall fluid flows formed on the heat exchanging surfaces. From hot bodies of small diameters a laminar thermal plume ascends ( Figure 1b) which at some distance from a body becomes turbulent. On bodies of large diameters ( Figure 1c) a turbulent boundary layer develops thus forming an ascending turbulent thermal plume in a trailing edge. The pictures of FC development in the flow past a hot sphere or horizontal cylinder are qualitatively similar ( Figures 1b, c). To this character of flow structure variation there corresponds the change in the coefficient of heat transfer α x which in the case of the developed turbulent FC remains constant along the plate length where the characteristics of thermal turbulence become statistically equal. The layer thickness grows along the flow direction and at a certain distance, corresponding to x c1, the fluid flow becomes unstable changing within the range from x c1 to x c2 from laminar to turbulent. At the beginning of heating of a vertical surface (x = 0) ( Figure 1a) a laminar boundary layer is formed.
#UPPER PLATE MOVING WITH A CONSTANT SPEED VOLUMETRIX FREE#
Figures 1 and 2 schematically illustrate characteristic examples of free convection. A flow past a solid surface, the temperature of which is higher (lower) than that of the surrounding flowing medium, is the most widespread type of free convection. The density variability may also result from nonuniform distribution of concentration of any component in a mixture or from chemical reactions, difference in phase densities or from surface tension forces at the phase interface (in this case concentration diffusion or convection is implied).įree-convective flows may be laminar and turbulent. If density variation Δρ is caused by spatial nonuniformity of a temperature field, then a flow arising in the Earth gravitational field is called thermal gravitation convection.
0 kommentar(er)
