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A Study of the Dispersed Flow Interfacial Heat Transfer Model of RELAP5/MOD2.5 and RELAP5/MOD3 (NUREG/IA-0163)

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Date Published: April 1999

Prepared by:
M. Andreani*
G. Th. Analytis
S. N. Aksan

Thermal-Hydraulics Laboratory
Paul Scherrer Institute (PSI)
CH-5232 Villigen PSI, Switzerland
*ETHZ, Nuclear Engineering Laboratory
Swiss Federal Institute of Technology (ETH)
8092 Zurich, Switzerland

Prepared as part of:
The Agreement on Research Participation and Technical Exchange
under the International Code Application and Maintenance Program (CAMP)

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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In this work, the model of interfacial heat transfer for the dispersed flow regime used in the RELAP5 computer code is investigated. The validity of the 1-D approach used for calculating the heat exchange between the droplets and the vapour in the dispersed flow region above a quench front was shown to be questionable for conditions of low quality at the quench front and low mass flux. Under such conditions, the interfacial heat transfer calculated assuming a uniform distribution of droplets over the cross-sectional area of the channel is necessarily overpredicted, and the vapour superheat is strongly underpredicted. The purpose of the present paper is to show that the limitations of the 1-D approach, obtained from the steady-stare anlyses of slow reflooding experiments, has some impact on the performance of the 1-D transient computer codes like RELAP5/MOD2.5 and RELAP5/MOD3. As an example, the transient analysis of a low flooding rate experiment in a tube was performed. An early completion of the quench process and a fast desuperheating of the vapour at the tube exit was obtained by both codes. The too high quench front velocity (four times higher than in the experiment) could not, however, be put univocally in relation to the underprediction of the vapour temperature, and the consequent increase of the precursory cooling, as many coupled thermal and hydraulic transient effects prevailed. Quasi steady-state anlyses of two runs, where the boundary conditions for the the post-dryout region could be better controlled for a predetermined position of the quench front, were thus performed. These analyses show that the vapour superheat at the tube exit is strongly underpredicted, confirming the limitations of the 1-D model for interfacial heat transfer in the dispersed flow region.

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