Transient Analysis of the Thermal and Hydraulic Behavior of a Cooling Fluid in a Bayonet Tube

Abstract

Analysis of forced convection in transient fluid flows have recently gathered attention due to the development of automatic control devices for precision control of the fluid flow in heat transfer devices of high efficiency. The precise regulation of the fluid flow is very important when the control of heat exchangers has to be assured, thus requiring a better understanding and a precise evaluation of the transient heat behavior. With few exceptions, the calculus methods for the heat exchangers devices are based on the flow patterns using considerations that could be apart from reality and restricted to stationary state conditions. These methods give little information on the possible variations due to the fluid flow changes and temperature distribution that arise due to thermal stresses. This paper deals with the details of a numerical approach to describe the transient hydrodynamic and thermal behavior in a bayonet tube with a fixed geometry using water as the working fluid. The results are reported as a function of the dimensionless Reynolds, Euler and Nusselt numbers. For each zone of the bayonet tube the time required for the fluid flow to be hydro and thermally developed is determined. The effect of the hydrodynamic flow changes on the heat flow local distribution is discussed. For a given Reynolds number a separation vortex is detected in the returning zone and it changes its position as time elapses. It is found that as time approaches the steady state the heat transfer becomes more stable throughout the bayonet.