Starting and duration of the boiling period (Cell 1)
In analyzing the behavior of the F&P cells during the boil-off phase, it is essential to understand when boiling begins and its overall duration. These data are not provided in the F&P paper presented at ICCF3 (1), but can be roughly derived from Figure 6, as shown for Cell 1 in the following jpeg.
The beginning of boiling can be deduced from the shape of Tcell, which is the temperature of the water measured inside the cell. Since the temperatures of water and internals are not perfectly uniform, boiling begins before Tcell reaches the boiling point (101.4 °C for the heavy water at 1 atm). In particular, the warmest part of the cell is the cathode, thanks to the paths of the current lines that concentrate on its surface.
The first and smaller vapor bubbles - which are generated when the water bulk temperature is quite far from the boiling point - condense before leaving the water, releasing their latent heat content to the liquid water.
As the water temperature approaches the boiling point and the power available for water heating increases, the bubbles became larger and reach the water surface, escaping the cell and taking away their latent heat, so that the heating rate of the water temperature starts to decrease. This point is marked with an inflection point along the Tcell curve.
Despite the coarseness of the curves plotted on its expanded portion, the graph of Figure 6A allows to locate this inflection point at about 1343000 s, ie at 13:00 on the 16th day, that is about 9 hours before the dry-out time.
Of course, the amount of heat taken away by the vapor is low at the beginning, because the input power is just over the 11 W calculated in (1) as heat losses due to radiation. As the voltage increases due to the formation of even larger bubbles and the consequent reduction of the electrode surface wetted by the electrolyte, the vaporization rate of water increases and Tcell tends to become horizontal, converging toward the boiling point value. In the meanwhile most of the water content is vaporized, well before the time reported by F&P in their paper (1).