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B.T. Swapnalee, P.K. Vijayan, M. Sharma, D.S. Pilkhwal
Nuclear Engineering and Design, Volume 245, April 2012
Abstract
For thermodynamically supercritical loops, explicit correlation for steady state natural circulation flow is not available. While using the subcritical natural circulation flow correlation for supercritical data, it was not able to predict the steady state flow accurately near supercritical pressure condition. A generalized correlation has been proposed to estimate the steady state flow in supercritical natural circulation loops based on a relationship between dimensionless density and dimensionless enthalpy reported in literature. Experiments have been performed with supercritical CO2 and water to validate this generalized correlation. The steady state flow rate data with supercritical CO2 has been found to be in good agreement with the proposed correlation. The correlation has also been validated using limited number of supercritical water data. Subsequently supercritical natural circulation data for different fluids reported in literature has also been compared with the proposed correlation. It is observed that the same generalized correlation is applicable for other fluids also.
Sharp change of fluid properties such as density in the critical region gives rise to instability. The instability could be either density wave type or excursive type (Ledinegg or static instability). Several previous researchers have studied density wave type instability in supercritical natural circulation loops, whereas excursive instability is not studied in detail. In the present paper, an analysis has been carried out to predict the threshold of excursive instability for both supercritical water and supercritical CO2. Static instability was not found for CO2 whereas it was found for supercritical water. The effect of pressure is observed to stabilize the loop.
Steady state flow and static instability of supercritical natural circulation loops
Swapnalee, B.T., P.K. Vijayan, Manish Sharma and D.S. Pilkhwal
Reactor Engineering Division, Bhabha Atomic Research Centre,
Trombay, Mumbai 400085 India
Super critical water has excellent heat transfer characteristics and is a candidate coolant for advanced nuclear reactors. Besides enhancing the thermodynamic efficiency, supercritical fluids do not experience phase change thus eliminating the Critical Heat Flux (CHF) phenomenon which could result in larger power density. Apart from forced circulation, natural circulation could also be a viable option for supercritical water cooled reactors. Due to the large change in fluid properties, the steady state equation for subcritical natural circulation flow is not adequate for supercritical fluids. Traditionally, natural circulation analysis is carried out by one-dimensional approach based on constant fluid properties assuming Boussinesq approximation to be valid. Such an approach, although well suited for subcritical fluids is not applicable for supercritical fluids where the density change is quite sharp especially in the pseudocritical region. As a result, modeling of the density variation is central to modeling of natural circulation loop with supercritical fluids. A generalized correlation has been derived for the steady state flow based on the Ambrosini and Sharabi relationship for the density of supercritical fluids. Simple flow equations for supercritical fluids have been provided after subdividing the near critical region into three regions. Experiments have been performed with supercritical CO2 and water to validate these generalized flow equations. The test data were generated for four different loop configurations with supercritical CO2. The steady state flow rate data with supercritical CO2 and water were found to be in good agreement with the proposed equation. Subsequently steady state literature data for supercritical fluids like Freon-114, Freon-12 and CO2 has also been found to be in good agreement with the proposed correlation.
The density change for supercritical systems is comparable to or even more than that of two-phase systems resulting in instability concerns. The instability could be either density wave type or excursive type (Ledinegg or static instability). Although, there is considerable literature on the density wave instability in supercritical fluids, there are not many studies dealing with the static instability of the excursive type. In the present paper, therefore, an analysis has been carried out to predict the threshold of excursive instability for both supercritical water and CO2. Static instability is predictedfor the present loop geometry with supercritical water and not with supercritical CO2. The effect of pressure is found to reduce the unstable region. At higher pressures, static instability is predicted for very low heater inlet temperature.
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