Modelling of the Hm1_1 Benchmark in the Mistra Facility

ABSTRACT

A number of international validation benchmarks were organized in order to assess the capability of CFD tools to predict the containment flows in case of severe accidents. In a recent benchmark, HM1_1, which was performed in the MISTRA facility, a hot air jet was injected horizontally into the enclosed space and impinged onto a cylindrical compartment wall. After the impingement, the deflected air flowed towards a stably stratified helium-air mixture set in the uppermost part of the MISTRA vessel. The present study focuses on modelling the HM1_1. Several turbulence models were employed and compared with the experimental results.

The HM1_1 benchmark was performed in the MISTRA facility, a stainless-steel cylindrical vessel with inner diameter of 4.25 m and 7.38 m in height (internal volume of 97.6 m³). The HM1_1 was designed to address the effect of an obstruction on the mixing of a stably stratified helium-air mixture, by an initially horizontal hot jet. A constant mass flow rate of 25 g/s and temperature of 148.6°C were set for the jet. The temperatures of the top, middle and bottom condensers were controlled along the experiment. Throughout the experiment, measurements of velocity, temperature and helium concentration were taken in several positions, see Alengry et al. (2015) for further experimental details.

2. RESULTS AND DISCUSSION

Computation results using the standard k-e model with C_1e of 1.44 and 1.6, as well as k‑w SST model, are presented (for the justification of the modified C_1e constant see Ishay et al., 2015). In addition, the standard k-e model, with C_1e of 1.44 and the standard wall function (SWF), is also presented to study the influence of wall treatment on mixing rate. Computed helium concentrations are compared with the measured ones at locations identified in benchmark specifications. Transient simulations were run until the vessel atmosphere was predicted to be totally mixed.

The computed results show that the probes positioned at a higher elevation are not affected by the air jet during the first 1000 seconds into the transient. Helium concentration at this height is controlled mostly by molecular diffusion until about 1000 seconds. The temporal evolution of helium concentration at the height of 6.077 m shows a pronounced effect of the jet erosion, as the concentration drops sharply from its initial level of 8.3% during the first 50 to 100 seconds after the beginning of the air injection.

Figure 1. HM1_1 – time-dependent helium concentration at various locations.

3. CONCLUSIONS

The present study deals with modelling the HM1_1 experiment. A standard version of the k-e model was employed, along with additional turbulence models. Whereas the qualitative behavior is captured correctly by the standard k-e model with both values of C_1e employed, the results of k-w SST exhibit much faster mixing rates than the measured ones. On the other hand, the absolute concentration values obtained with the standard k-e model and C_1e=1.44 are marginally over-predicted for the higher elevation, suggesting higher mixing rates than those observed experimentally. The concentration is better predicted when the modification in the C_1e is employed. The velocity decay rate and the spreading rate of the emerged jet are expected to be computed correctly when the modified value of C_1e is used. Hence, it may be concluded that the evolution of wall jets right after the impingement is captured in better agreement with the modified value of C_1e.

REFERENCES

1. Alengry, E. Studer and M. Ishigaki, "HM1-1 Benchmark Specifications," Centre de Saclay, DEN/DANS/DM2S/STMF, Saclay, France, 2015.
2. Ishay, U. Bieder, G. Ziskind and A. Rashkovan, "Turbulent jet erosion of a stably stratified gas layer in a nuclear reactor test containment," Nuclear Egnineering and Design, vol. 292, pp. 133-148, 2015.