Running the Simulation
Preparation of the simulation is now complete, and the simulation can be run.
To run the simulation:
-
Click
(Run) in the Solution toolbar.
The solution progress is displayed in the Output window. The Residuals display is created automatically in the Graphics window and shows the progress of the solvers.
While the simulation is running, you can click the tabs at the top of the Graphics window to view scenes and plots.
During the run, you can stop the process by clicking
(Stop) on the toolbar. If you do stop the simulation, you can click
(Run) to resume it.
If left alone, the simulation continues until 2500 iterations are complete.
As well as observing the progress of residuals during convergence, an interesting feature to follow is the development of the heat flux partitioning.
- Expand the Plots node and then open the Wall SuperHeat and Wall Heat Flux plots.
-
In the
Graphics window, click the tab of one of these plots and drag it to the left or right margin of the window.
The two plots are then displayed side by side, as in the example below.
The wall superheat is initially negative (below boiling point), but rises as the wall power is ramped up, and becomes positive just before full power is achieved at 200 iterations. During this period, the whole of the heat flux is directed to the liquid only. Once boiling point is achieved, an evaporation heat flux takes up a share of the applied wall heat flux. As the calculation continues, the liquid convection in the boiling region is replaced with a quenching heat flux due to the agitation or mixing of the liquid by departing bubbles. Both evaporation (latent heat) and quenching (agitation) are major enhancers of heat transfer during nucleate boiling.
At the inception of boiling, single phase heat transfer is the only contributor to the wall heat flux. As the wall superheat rises, more nucleations sites become active, and both the evaporation and quenching contributions rise accordingly. However, the quenching contribution goes through a maximum, because it is also proportional to liquid subcooling which is falling as the liquid temperature rises.
The overall balance between quenching and evaporation fluxes is a function of operating pressure, since quenching flux varies as departure size squared and evaporation flux varies as departure size cubed. The bubble departure size is smaller at higher pressures.
The wall superheat profile is established early thanks to the sensitivity of nucleation site density to a large power of wall superheat. Net vapor production appears a little later, at about 400 iterations, once the condensation capacity of the subcooled water exceeds the wall evaporation rate. At the same time, the flow begins to accelerate, as can be seen from the maximum velocity monitor plot.
True convergence of mass balance and of residuals can be seen to start after 600 iterations. The maximum/minimum plots show small changes after iteration 2000. The calculation is stopped at 2500 iterations to get a good final mass balance.
- When the simulation has finished running, save it.