Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 160

Figure 5.48 Flow regimes and heat transfer coefficients in a bubble pump
For the generators investigated only the slug flow regime is relevant. At low heat
flux densities, slug flow is formed by more and more germ cells. The bubbles form
plugs which engage the whole circular tube section. This flow regime must be reached
in order for the operation of the DACM to be able to guarantee the solution lifting and
with that the solution circulation.
For the characterization of the gas bubble pump, five different generators were
constructed and compared. The measured total heat transfer capacities were between
0.29 and 0.12 kWK − 1 , which is too low to obtain small temperature differences across
the generator. The main limitations on increasing the heat transfer power are the costs
of too many tubes, the high pressure drops on the external side and especially leakage
flows around the baffles, which are fixed to the tubes and cannot be closely sealed to
the external wall. In the third prototype the number of tubes was therefore increased
by 30%. The values of the investigated range of generator heating temperatures and
mass flow rates are presented in Table 5.5.
The measured solution concentrations of the first pilot plant were 31% for X Sw
and 44% for X Sr with a degassing width of 13%. At the same pressure, but a lower
degassing width, the specific solution circulation index in prototype 2 is about 1.5 to
2 times higher than that of the generator of prototype 1.
During the measurements of prototype 1, the volume flow for the generator was
determined for different heating inlet temperatures as well as transferred heat ca-
pacities. The measuring method for the acquisition of the volume flow of the lifted