J. Paniagua et al., Modeling of thermal hydraulic instabilities in single heated channel loop during startup transients, NUCL ENG DE, 193(1-2), 1999, pp. 207-226
A thermal hydraulics computer code was developed to simulate the geysering
instability in a natural circulation system starting from subcooled conditi
ons and to assess the impact of the system pressure and channel inlet subco
oling on the inception of instability. The formulation of thermal hydraulic
s is inherently general and accounts for both single-phase liquid flow and
nonhomogeneous, nonequilibrium two-phase flow. The computer code is based o
n momentum integral method where the current practice of basing fluid prope
rties on the system averaged pressure has been relaxed and the local proper
ties are based on local pressures estimated using the shape of steady-state
pressure distribution, thereby, improving the predictions while preserving
the computation speed, one of the important strength of the integral metho
ds. This is an important modeling feature since the local vapor generation
rate depends on local saturation temperature The methodology has been valid
ated with the experiments conducted to investigate the instabilities in a l
ow pressure natural circulation loop at low powers and high inlet subcoolin
gs. The numerical simulations predicted periodic channel flow reversal, whi
ch is one of the feature of condensation-induced geysering. Basing local pr
operties on local pressures instead of system. average pressure led to decr
ease in the discrepancy in the prediction of the positive side amplitude fr
om 40% to 6% and in the frequency from - 15% to 5%. In addition, it was obs
erved that the start-up instability can be avoided by increasing system pre
ssure or by decreasing channel inlet subcooling. This study showed that the
integral method coupled with local pressure variation for the vapor genera
tion model is suitable to predict startup or geysering transients. (C) 1999
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