NATURAL CONVECTIVE CONDENSATION WITH NON-CONDENSABLE GASES IN THE REACTOR CONTAINMENT AND ITS ENHANCEMENT

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1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00044 NATURAL CONVECTIVE CONDENSATION WITH NON-CONDENSABLE GASES IN THE REACTOR CONTAINMENT AND ITS ENHANCEMENT Fu Wen 1, 2, Li Xiaowei 1 *, Wu Xinxin 1 1 Tsinghua University, Haidian District, Beijing , China 2 Luoyang Ship Material Research Institute, Luoyang, Henan , China * Presenting and Corresponding Author: lixiaowei@tsinghua.edu.cn ABSTRACT The heat and mass transfer of natural convective condensation with non-condensable gases is an important phenomenon for th e passive containment cooling system. Numerical simulation of the heat and mass transfer processes of steam condensation with non-condensable gases is realized through adding a condensation model in the Fluent software. The simulation results compared well with the scaled AP600 containment condensation experiment. The natural convective condensation with non-condensable gases in AP1000 containment was simulated. The temperature, species, velocity and heat flux distributions were obtained. For natural convective condensation with non-condensable gases, addition of hydrogen will weaken the heat and mass transfer process significantly. When hydrogen contributes to 50% volume fraction of the non-condensable gases, the heat transfer coefficient will be reduced 45%. The enhancement effect of two dimensional roughness for the natural convective condensation in AP1000 containment was also investigated. The results show that two dimensional roughness can enhance the heat transfer coefficient by 19.5% while increasing the heat transfer by 10.2%. Optimum roughness parameters were also investigated. KEYWORDS: Non-condensable gas, Heat and mass transfer, Reactor containment, Two dimensional roughness, Enhanced heat transfer 1. INTRODUCTION The heat and mass transfer processes of steam condensation with non-condensables are the key phenomena of passive containment cooling system. After severe accidents, large amount of air will be present in the containment. Previous investigations showed that very small amount non-condensable gases will degrade the heat transfer of condensation significantly [1]. If severe accidents with core melt happens, hydrogen will be generated by Zr-water reaction. This will make the natural convection condensation of steam, air and hydrogen mixture more complicated. The accumulation of non-condensable gases at the gas-liquid interface inhibits steam diffusing from the gas mixture to the liquid film. In addition, since the total pressure remains constant, the accumulation of non-condensable gases also decreases the steam partial pressure at the interface, reducing the saturation temperature and the driving force for steam condensation. A number of experiments have been done to investigate the condensation with non-condensable gases. Empirical correlations were obtained based on the experimental data [2-6]. These correlations have been demonstrated to be limited for extrapolation under different thermal-hydraulic conditions and at different geometries. Theoretical research has also been conducted. The heat and mass transfer analogy theory [7-9] was developed, however, the relations are semi-empirical, some coefficients deduced from experiments were used. In recent years, condensation with the presence of non-condensable gases has been analyzed using computational fluid dynamics method [10, 11]. A condensation model is usually not included in commercial CFD software and must be implemented by users. Fu et al. [12] simulated the convective condensation of steam-air mixture and steam-helium mixture in ve rtical tubes, through implemented source terms into the conservation equations, the simulation deviations were within 5%. 1

2 Condensation with non-condensable gases in containment was simulated using the ANSYS FLUENT software in this work. The velocity distribution in AP1000 containment and the effect of hydrogen on steam mixture condensation were analyzed. The enhancement effect of two dimensional roughness on the natural convective condensation in AP1000 containment was also investigated. 2. CONDENSATION MODEL The condensation process was modeled by introducing appropriate source terms into the mass, momentum, energy and species conservation equations in cells most adjacent to the condensation wall. The source terms were incorporated into the FLUENT code through User Defined Functions (UDFs). The condensation film was not modeled in this paper. According to Fick s first law of diffusion and the solution of Stefan flow, the steam condensation mass flux in the cells most adjacent to the condensation wall can be expresses as [7]: D ln n,c ln vnmvp X X total n,w m cond (1) RT r where r is the t hickness of cell most adjacent to the condensation wall, X n,c is the molar fraction of noncondensable gas in the cell most adjacent to the condensation wall, X n,w is the molar fraction of non-condensable gas at the condensation wall, calculated by X nw, ( 1 Pvw, )/ Ptotal, Pvw, Psat ( Tw) is the saturation pressure based on the wall temperature, D is the laminar mass diffusion coefficient, R is the gas constant. vn Since the interface is impermeable to air, the gas mixture mass source term is equivalent to the steam source term: Sv Smass m condacell, wall / Vcell (2) The momentum source term can be expressed as: Smom umcond Acell, wall / Vcell (3) The energy source term is: Senergy mcond Acell, wallhv / Vcell (4) If the molar fraction of non-condensable gas in the cell is larger than the molar fraction of non-condensable gas at the condensation wall, Xn,c Xn,w, condensation takes place and the source terms are calculated according to Eqs. (2)-(4), otherwise, the source terms are zero. 3. RESULTS AND DISCUSSION 3.1 NUMERICAL METHOD VERIFICATION In order to verify the numerical model, Anderson s AP600 containment experiment was simulated using the developed method. Fig.1 shows the calculated and measured total heat transfer coefficient (HTC). The deviations are within 5%. Fig. 1 Comparison of Fluent calculations results with AP600 containment test data 2

3 3.2 GAS MIXTURE VELOCITY DISTRIBUTION IN AP1000 CONTAINMENT The gas mixture velocity contour and streamline is shown in Fig 2. It can be seen that gas mixture flow downward along the condensaiton wall, until reach the bottom evaporation wall, then flow upward along the symmetry axis. This is because the gas mixture density at the condensation wall is larger than that at the evaporation wall. The density difference is the comprehensive effect of temperature difference and concentration differcence. Fig. 2 Gas mixture velocity contour and streamline in AP1000 containment 3.3 EFFECT OF HYDROGEN IN THE NONCONDENSABLE MIXTURE Hydrogen affects the natural convective condensation processes by increasing the steam diffusion and decreasing the buoyance force. When the non-condensable gas molar fraction is 50%, the heat transfer coefficient with hydrogen molar concentration of the total non-condensable gases ranging from 0 to 50% is shown in Fig 3. It can be seen that the heat transfer coefficient decreases with the increasing of hydrogen molar fraction. When the hydrogen molar conentration of total non-condensable gases reaches 50%, the heat transfer coefficient of steam-hydrogen-air mixture is 55% of the steam-air mixture. Fig. 3 Heat transfer coefficient with different hydrogen contents in air-hydrogen mixtures 3.4 ENHANCED MASS AND HEAT TRANSFER WITH ROUGHNESS WALL The steam-air mixture condensation in AP1000 containment with roughness condensation wall was simulated to investigate the enhancement effect of two dimensional roughness on the natural convective condensation. The gas mixture velocity contour and steamline in AP1000 containment with roughness wall is shown in Fig 4. It can be seen that the steam-air mixture natural convection formed in the AP1000 containment. There is one vortex at th e rib windward and one vortex at the rib leeward. The flow separates at the rib and reattaches the condensation wall about six to eight rib heights distant from the rib leeward. Then enhanced effects by the ribs are shown in Table 1. 3

4 Fig. 4 Gas mixture velocity contour and streamline in AP1000 containment with roughness wall Table 1 Enhanced heat and mass transfer with roughness wall 10 ribs 20 ribs 40 ribs 80 ribs ( A A )/ A 0.17% 0.35% 0.68% 1.34% ribs total total L / L 1.75% 3.51% 7.03% 14.1% ribs vertical ( Q Q )/ Q 2.05% 3.6% 5.3% 10.2% gribs, g,0 g,0 ( h h )/ h 6.11% 9.11% 13.0% 19.5% g, ribs g,0 g,0 ( h h )/ h 6.28% 9.24% 13.1% 19.3% cond, ribs cond,0 cond,0 ( h h )/ h 5.03% 8.30% 12.7% 21.1% conv, ribs conv,0 conv,0 3.5 EFFECTS OF ROUGHNESS HEIGHTS AND ROUGHNESS PITCH The condensation mass fluxes with various roughenss heights are shown in Fig 5. It can be seen that the condensation mass flux increase with the increasing of the roughness height, until reaches the maximum value when the roughness height is 5mm, then decrease gradually when the roughness height is larger than 5mm. The condensation mass fluxes with various roughenss pitches are shown in Fig 6. It can be seen that when the roughness pitch is about 13 times of the roughness height, the enhancement performance is the best. This is because when the roughness pitch is smaller, there are more roughness elements along the tube, leading to la rger flow resistance and larger heat transfer rate. When the roughness pitch is too s mall, the flow can not reattach the wall between two ribs, the heat transfer will be reduced. Fig. 5 Condensation mass flux and heat flux with various roughness heights 4

5 Fig. 6 Condensation mass flux and mass flux with various roughness pitches 4. CONCLUSIONS Simulation of natural convective steam condensation with non-condensable gases in reactor containment was realized by adding source terms in commercial software. The method was verified using AP600 experiments. The velocity and species contours for the natural convective condensation processes in AP1000 containment were simulated. Hydrogen will decrease natural convective condensation heat transfer coefficient by 45% when it contribute to 50% of the molar fraction of the non-condensable gases. Effects of ribs were investigated for enhancing the natural convective condensation. Ribs can enhance the heat transfer coefficient by 19.5%. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China ( ). REFERENCE 1. Othmer, D.F., The condensation of steam. Industrial & Engineering Chemistry, (6): p Uchida, H., A. Oyama, and Y. Togo, Evaluation of post-incident cooling systems of light water power reactors. 1964, Tokyo Univ. Tokyo, Japan, Report. 3. Dehbi, A.A., The effects of noncondensable gases on steam condensation under turbulent natural convection conditions. 1991, Massachusetts Institute of Technology, PhD thesis. 4. Kuhn, S.Z., Investigation of heat transfer from condensing steam-gas mixtures and turbulent films flowing downward inside a vertical tube. 1995, University of California, Berkeley, USA, PhD thesis. 5. Anderson, M.H., Steam condensation on cold walls of advanced PWR containments. 1998, University of Wisconsin--Madison, USA, PhD thesis. 6. Malet, J., E. Porcheron, and J. Vendel, OECD international standard problem ISP-47 on containment thermal-hydraulics conclusions of the TOSQAN part. Nuclear Engineering and Design, (10): p Kreith, F., R. Manglik, and M. Bohn, Principles of heat transfer, ed. t. ed. 2010, Harper & Row, New York, NY, USA. 8. Corradini, M.L., Turbulent condensation on a cold wall in the presence of a noncondensible gas. Nuclear Technology, : p Peterson, P.F., V.E. Schrock, and T. Kageyama, Diffusion layer theory for turbulent vapor condensation with noncondensable gases. Journal of Heat Transfer, (4): p Li, X.W., Wu X. X., and He S.Y., Numerical simulation of the heat and mass transfer process of convective condensation with noncondensable gas. Journal of Engineering Thermophysics, : p. 027 (in Chinese). 11. Dehbi, A., F. Jan asz, and B. Bell, Prediction of steam condensation in the presence of noncondensable gases using a CFD-based approach. Nuclear Engineering and Design, (0): p Fu, W. Li X, W., Wu X. X., Numerical investigation of convective condensation with the presence of non-condensable gases in a vertical tube. Nuclear Engineering and Design, : p