Page 222 - Managing Global Warming

182 Managing Global Warming

Fig. 4.47 Heat-transfer
coefficients calculated for a
flow of coolants in
Generation IV, AGR, and
PWR reactors in a bare tube
at nominal operating
pressures and at mass fluxes
close to actual mass fluxes
for the respective reactor [1].

(added for comparison purposes) allow heat-transfer coefficients above 60kW
2
1
(m K) . Calculations also showed that in a PWR, heat-transfer coefficients are about
1
2
45kW (m K) . Lead, as expected, has heat-transfer coefficients around 25kW
2
1
(m K) , which is lower than that of another liquid-metal—sodium. Heat-transfer
2
1
2
1
coefficients of SCW (5–15kW (m K) ) and CO 2 (1.8–2.5kW (m K) ) also lie
within the typical ranges of values.
For calculations of subcritical H 2 O, D 2 O, CO 2 , and He the value of heat flux was
not taken into account, while for SCW, Pb, and Na the value of heat flux was assumed
2
to be 970kWm . A hydraulic-equivalent diameter of 8mm was used in the calcula-
tions for all the coolants.
Fig. 4.48 shows heat-transfer coefficients calculated for all coolants (including
1
2
2
1
FLiNaK) for the generic conditions: G¼1000kg (m s) , q¼970kW (m K) ,
and D hy ¼8mm.
It can be seen that at the chosen generic conditions, a sodium coolant has the
2 1
highest heat-transfer coefficients, ranging from 58 to 96kW (m K) , while CO 2
and FLiNaK have the lowest heat-transfer coefficients, ranging from 1 to 4kW
2 1 2 1
(m K) . The heat-transfer coefficient of SCW starts at 5kW (m K) , then goes
through a peak within the pseudocritical region, where its value increases almost
2 1
twice, and after that drops close to 4kW (m K) at temperatures above 450°C.
Heat-transfer coefficients of the gases, water, heavy water, and lead increase slightly
with temperature. Heat-transfer coefficients of the molten salt increase quite signifi-
cantly with temperature. The heat-transfer coefficient of sodium drops linearly with
temperature increase.