An investigation of CHF fluid-to-fluid scaling and multi-fluid prediction techniques.

Abstract

More than 800 points of critical heat flux (CHF) data were measured from three test rigs (UO, MR-7A and MR-1A loops). The UO loop is a multi-fluid boiling loop located at the University of Ottawa, in which HFC-134a, HCFC-123, HCFC-22, CFC-12 and CFC-11 have been tested. The CHF data of HFC-134a and HCFC-123 are the first ever published for these fluids. HFC-134a and HFC-123 are non-ozone and low-ozone depletion fluids. The CHF data from the UO loop cover the range of mass flux from 1 to 4 Mg m$\sp{-2}$ s$\sp{-1}$, pressure from 7 to 10 MPa of water-equivalent (water-equivalent is defined as the same dimensionless parameters, e.g. $\rho\sb{\rm f}$/$\rho\sb{\rm g}$ and $\Psi\sb{\rm k}$, of flow condition for non-aqueous fluids as those for water) and heated length from 0.5 to 1 meter. The inner diameter of the test section is 4.2 mm. The MR-7A and MR-1A loops are the refrigerant and boiling water loops, respectively, located at the Chalk River Laboratories of AECL. In the MR-7A loop, CHF data were measured using HCFC-22 as a coolant for the following conditions: mass flux: 1-8 Mg m$\sp{-2}$ s$\sp{-1}$, pressure: 7 and 10 MPa (water-equiv.), heated length: 0.67-1.61 m, and inner diameter: 4.38 and 8 mm. The CHF data for water from the MR-1A loop cover mass flux: 2.5-8 Mg m$\sp{-2}$ s$\sp{-1}$, pressure: 7 and 10 MPa, heated length: 1.75 m, an inner diameter: 8 mm. HFC-134a, HCFC-123 and HCFC-22 are found suitable to replace the CFC's in the CHF experiment for water CHF simulation. The CHF fluid-to-fluid and multi-fluid prediction techniques were examined based on the CHF data of various fluids measured in the present work. Two analytical CHF models were assessed and compared to data from the 5 fluids tested in this investigation. The CHF similarity between water and refrigerants was also examined via the analysis of geometric, thermodynamic and hydrodynamic similarities. A new methodology has been established to investigate the limitations of the CHF fluid-to-fluid scaling technique. The analysis of CHF similarity through the thermodynamic relationship leads to the conclusion that, not only the local thermodynamic quality, but also the upstream history (e.g. heating, flashing, friction, changes of kinetic energy and potential energy, etc.) are important factors in determining the CHF scaling accuracy. Finally, the limitations of the CHF fluid-to-fluid scaling technique were determined in terms of the scaling accuracy.