| Abstract |
INL LDRD funded research was conducted at MIT to experimentally characterize mixed convection heat transfer in gas-cooled fast reactor (GFR) core channels in collaboration with INL personnel. The GFR for Generation IV has generated considerable interest and is under development in the U.S., France, and Japan. One of the key candidates is a block-core configuration first proposed by MIT, has the potential to operate in Deteriorated Turbulent Heat Transfer (DTHT) regime or in the transition between the DTHT and normal forced or laminar convection regime during post-loss-of-coolant accident (LOCA) conditions. This is contrary to most industrial applications where operation is in a well-defined and well-known turbulent forced convection regime. As a result, important new need emerged to develop heat transfer correlations that make possible rigorous and accurate predictions of Decay Heat Removal (DHR) during post LOCA in these regimes. Extensive literature review on these regimes was performed and a number of the available correlations was collected in: (1) forced laminar, (2) forced turbulent, (3) mixed convection laminar, (4) buoyancy driven DTHT and (5) acceleration driven DTHT regimes. Preliminary analysis on the GFR DHR system was performed and using the literature review results and GFR conditions. It confirmed that the GFR block type core has a potential to operate in the DTHT regime. Further, a newly proposed approach proved that gas, liquid and super critical fluids all behave differently in single channel under DTHT regime conditions, thus making it questionable to extrapolate liquid or supercritical fluid data to gas flow heat transfer. Experimental data were collected with three different gases (nitrogen, helium and carbon dioxide) in various heat transfer regimes. Each gas unveiled different physical phenomena. All data basically covered the forced turbulent heat transfer regime, nitrogen data covered the acceleration driven DTHT and buoyancy driven DTHT, helium data covered the mixed convection laminar, acceleration driven DTHT and the laminar to turbulent transition regimes and carbon dioxide data covered the returbulizing buoyancy driven DTHT and non-returbulizing buoyancy induced DTHT. The validity of the data was established using the heat balance and the uncertainty analysis. Based on experimental data, the traditional threshold for the DTHT regime was updated to account for phenomena observed in the facility and a new heat transfer regime map was proposed. Overall, it can be stated that substantial reduction of heat transfer coefficient was observed in DTHT regime, which will have significant impact on the core and DHR design of passive GFR. The data were compared to the large number of existing correlations. None of the mixed convection laminar correlation agreed with the data. The forced turbulent and the DTHT regime, Celeta et al. correlation showed the best fit with the data. However, due to larger ratio of the MIT facility compared to the Celeta et al. facility and the returbuliziation due to the gas characteristics, the correlation sometimes under-predicts the heat transfer coefficient. Also, since Celeta et al. correlation requires the information of the wall temperature to evaluate the heat transfer coefficient, it is difficult to apply this correlation directly for predicting the wall temperature. Three new sets of correlation that cover all heat transfer regimes were developed. |
