An advanced computer code CONVERT, developed and validated earlier at the University of Manchester for buoyancy-influenced flow in uniformly heated vertical tubes, was used to perform simulations of the present experiments. This code uses a buoyancy influenced, variable property, developing wall shear flow formulation for turbulent flow and heat transfer in a vertical tube in conjunction with the Launder-Sharma low Reynolds number k-s turbulence model [9]. The conditions covered in the simulations ranged from forced flow with negligible influence of buoyancy to buoyancy-dominated mixed convection. In each case, simulations were made for thermal boundary conditions of both uniform wall temperature and uniform heat flux. These show that the computational formulation used does enable observed heat transfer behaviour in the mixed convection region to be reproduced. Buoyancy-induced impairment of heat transfer is predicted and satisfactory agreement with experiment is found for such conditions. The non-uniformities which develop in the distributions of Nusselt number under such conditions are well reproduced. It is known from earlier work done by the present authors that other turbulence models are generally less successful than the Launder Sharma model in reproducing the influences of buoyancy on turbulence and heat transfer found in mixed convection. Thus, it is important in the computational modelling of containment cooling systems that an appropriate turbulence model is used.

(a) Uniform heat flux case at x/D=72.39

Bo =Gr /(Re

(based on wall heat flux)

(b) Uniform wall temperature case at x/D=72.39

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Bo =Gr/(Re2 625 * Pr° 4)

(based on wall temperature)

FIG. 9. Correlation of heat transfer data for large x/D in terms ofNusselt number ratio and buoyancy parameter for (a) uniform wall heat flux, (b) uni form wall temperature

Even though the flow rates achieved in the naturally-induced cooling experiments with uniform wall temperature were such that the flow would have been turbulent in the absence of buoyancy influences, the effectiveness of heat transfer was seriously impaired in relation to that for turbulent forced convection. It is clear that under the conditions of all the experiments performed in this study turbulence was strongly impaired as a result of the influence of buoyancy. The heat transfer behaviour did not change very much as wall temperature was raised and was very similar in corresponding experiments with uniform wall heat flux. The results obtained highlight the need for care to be taken in the design of systems for cooling a steel containment shell by a naturally-induced flow of air over it so as to ensure that the buoyancy-influenced conditions which prevailed in the present experiments are avoided.

Although the results obtained in the pumped flow experiments confirm that the general pattern of behaviour in buoyancy-aided mixed convection with uniform wall temperature is similar to that with uniform wall heat flux, impairment of heat transfer with onset of buoyancy influences was found to occur more readily with uniform wall temperature.

The computational simulations showed that the formulation used does enable observed heat transfer behaviour to be satisfactorily reproduced throughout the mixed convection region. It is known from earlier work (Reference [10]) that other turbulence models are generally less successful in reproducing the influences of buoyancy on turbulence and heat transfer found in vertical passages. Thus, in the computational modelling of containment cooling systems it is important to use an appropriate turbulence model and to be aware of the limitations of such models.

ACKNOWLEDGEMENT

The experiments reported here have been conducted as part of the NICE Work Package of the European Commission Project CONT-DABASCO (Contract No. F14S-CT96-0042) which was carried out under the 4th Framework Programme on Nuclear Fission Safety. The authors gratefully acknowledge the financial support provided by the European Commission which has enabled this study to be undertaken.