On Tue, Nov 22, 2011 at 7:33 AM, Berke Durak <[email protected]> wrote:
> > > The behaviour of the fluid during boiling is highly dependent upon > > > the excess temperature, delta T = T_s - T_sat, measured from the > > boiling point of the fluid. Figure 9-1 indicates six different > > regimes for typical pool boiling; the heat flux curve is commonly > > called the boiling curve. > > It seems that a couple of degrees of increase for T_s translates to > a couple of orders of magnitude increase in power transfer. > This is true, but the surface temperature depends on the rate that heat is removed by the vaporization, and the rate that it can be restored from the hotter thermal mass behind it. That's why I mentioned an effective heat differential. When water changes phase, it absorbs a lot of heat, and that heat comes from the surface. The temperature of the surface would then decrease if heat didn't flow from the core heater to replace it. The rate of that heat flow is proportional to the temperature gradient in the ecat. At the onset of boiling, the heat is moving into the water at the total rate of 70 kW, and that's how fast the heat at the surface needs to be replenished from the core. If the rate of vaporization is 675 kg/h (the input flow rate), then the heat is moving into the water at a rate 7 times higher (470 kW), and it has to be replenished from the core at a rate 7 times higher. Heat flow depends on temperature differentials, so the gradient in temperature between the surface and the core would have to be 7 times steeper. To produce that change requires a lot of energy and time for the energy to flow into the thermal mass. Rossi claims the transition from 70 kW (boiling onset) to 470 kW (full vaporization) occurs over the period of a few minutes (or instantaneously), but that is not plausible, given that the transition from 0 kW to 70 kW took 2 hours. The fact that the temperature is constant throughout the second transition is deceiving. Rossi makes use of the latent heat of deception to claim much higher output than the data supports. If he monitored some variable that actually depended on the power transfer, like the output volume flow rate (or steam velocity), or the enthalpy (in a heat exchanger), we would have some idea of the power out as a function of time. But he doesn't, and that allows him to claim that the power out changes discontinuously by a factor of 7, right when boiling begins. Note, that if you look at the heat exchanger data from the Oct 6 demo, there is no discontinuous change in the power output that occurs at the onset of boiling. Those temperatures are not reliable for determining absolute power, but they should give some indication of the time dependence of the output power; certainly a 7-fold change in power out in 3 minutes would give an obvious step in the power output. It's not clear where the onset of boiling occurs in that test, but the apparent power out increases gradually over a period of 3 hours. > That, plus the fact that power transfer is proportional to the > area of contact. If you pump in water, you may cover more of the > heating element if it has vertical surfaces, and thus arbitrarily > increase the power transfer. > You would need to cover 7 times the area in a matter of minutes, also not plausible, and it would still require 7 times the heat transport rate from the core, which doesn't depend as simply on the area of contact.
