ANALYSIS OF A TRANSIENT ASYMMETRICALLY HEATED COOLED OPEN THERMOSIPHON

We present a numerical study of transient natural convection in a rect angular open thermosyphon having asymmetric thermal boundary condition s. One vertical wall of the thermosyphon is either heated by constant heat flux ( ''warmup '') or cooled by convection to the surroundings ( ''cooldown ''). The top of the thermosyphon is open to a large reserv oir of fluid at constant temperature. The vorticity, energy, and strea m-function equations are solved by finite differences on graded mesh. The ADI method and iteration with overrelaxation are used. We find tha t the thermosyphon performs quite differently during cooldown compared with warmup. In cooldown, flows are mainly confined to the thermosyph on with little momentum and heat exchange with the reservoir. For warm up, the circulation resembles that for a symmetrically heated thermosy phon where there is a large exchange with the reservoir. The differenc e is explained by the temperature distributions. For cooldown, the flu id becomes stratified and the resulting stability reduces motion. In c ontrast, the transient temperature for warmup does not become stratifi ed but generally exhibits the behavior of a uniformly heated vertical plate. For cooldown and Ra > 10(4), time-dependent heat transfer is pr edicted by a closed-form expression for one-dimensional conduction, wh ich shows that Nu --> Bi1/2/A in the steady-state limit. For warmup, t ransient heat transfer behaves as one-dimensional conduction for early times and at steady state and for Ra greater-than-or-equal-to 10(5), can be approximated as that for a uniformly heated vertical plate.