This is like saying that because a theatre gradually filled with people over two hours it is implausible to believe the same theatre emptied of people in minutes after a fire alarm. However it is only implausible based on the assumption there is only one entrance/exit or the entrance/exit is small.
Harry On Tue, Nov 22, 2011 at 11:46 AM, Joshua Cude <[email protected]> wrote: > > > 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.
