Oct 7, 2004 Institute of Nuclear Safety System, Inc. Institute of Nuclear Technology

1. Reflux Condensation Heat Transfer of Steam-Air Mixture under Gas-Liquid Countercurrent Flow in a Vertical Tube Oct 7, 2004 Institute of Nuclear Safety System, Inc. Institute of Nuclear Technology Technical Support Project Takashi Nagae

2. Core decay heat is cooled by RHR Loss of RHR function Boiling away of the core at an early stage Purpose • Shutdown PSA evaluation in Japan found that the mid-loop operation showed a high core damage probability. Mid-loop Operation・・・ The RCS inventory is so low that it may decrease to the center line of the reactor coolant piping. Pressurizer Reactor Vessel RHR Pump One of possible alternative cooling method is Reflux condensation by SG Fig 1 Mid-loop operation

3. To estimate realistic availability of reflux condensation • heat transfer we must consider following realistic • conditions • Existence of noncondensable gases • ← degrade heat transfer • Gas-liquid countercurrent flow • ← flow regime effect to heat transfer Reflux condensation ・・・ • Core heat is removed by boiling • Steam flows to the SG and condenses inside tubes • Condensate on the up-flow side flows back to the core Fig 2　Reflux cooing • Default model in RELAP5 • →is not confirmed whether they are applicable for condition (1)(2). （It is reported that the model underestimates heat transfer in NUREG report.） • Suggested models in other researchers • →are applicable only for narrow condition To investigate the reflux condensation heat transfer, we had experiments of reflux condensation and developed the new heat transfer models Fig 3　Condensation with noncondensable gases in U-tube

4. Local heat flux q’’　and interface condensation heat transfer coefficient hi are calculated and evaluated Nusselt numbersNui. New heat transfer models Table 2 Test conditions Experiment ※Condition during reflux condensation • Measured temperatures are • Mixture of steam and air : Tg • Condenser tube outer wall : Tw,o • Coolant water: Tc • at 9 distances from the condenser tube by • thermocouples Fig 4 Test section (Double-pipe, concentric-tube heat exchanger)

5. － Calculation of local heat flux q’’ － Inlet side of test section (2) Outlet side of test section Inner tube Outer tube 外管 内管 Calculations Liquid film Coolant Steam & air －Calculation of interface condensation heat transfer coefficient － 1/K (z) = rw,iln(rw,o/rw,i )/λw(z) + 1/hf (z) + 1/hi (z) Overall heat resistancetube wall liquid film interface 　 q’’　(z) = K(z) (Tg(z) - Tw,o(z)) Nui (z) = hi(z)dw,i /λs(z) K : overall heat transfer coefficient rw,i: tube inner radius rw,o : tube outer radius λw : thermal conductivity of tube hf : heat transfer coefficient for liquid film Nu : Nusselt number dw,i : tube inner diameter λs : thermal conductivity of film Nusselt number for the condensate film is obtained by applying the modified McAdams correlation to the Nusselt analysis for falling laminar film on a cold plate

6. Test condition Experimental result Pressure = 0.1MPa Inlet steam flow rate = 1.23g/s, Inlet air flow rate = 0.06g/s Tg ： Mixture of Steam and air temperature Tw,o ： Condenser tube outer wall temperature Tc ： Coolant temperature • RELAP5 default heat transfer model underestimate the heat transfer coefficients • Moon’s empirical correlation F = htot/ hf = 2.58x10-4Reg0.200Ref0.502Ja-0.642Wair-0.244 　（6119< Reg <66586, 0.140< Wair<0.972, 0.03<Ja<0.125） • No measurement in low temperature region • Exploration of the correlation overestimate the heat transfer coefficient Ｆ　：非凝縮性ガスによる熱伝達の劣化係数 hfpt：非凝縮性ガスを含む場合の凝縮熱伝達率 hf　：純粋蒸気のNusseltによる凝縮熱伝達率の理論値 Reg ：蒸気・空気の混合ガスレイノルズ数、Reg ：液膜のレイノルズ数 Ja : ヤコブ数、Wrir ：局所の空気質量流量比 Fig 5 Temperature profile (at steady state)

7. Development of Heat transfer models Turbulent flow • Correlation for the local heat transfer coefficients Eq （１） • Correlation for local Nusselt number is obtained as a function of the steam-to-air partial pressure ratio and plotted in Fig 6. Nui = 120 (Ps/Pa)0.75，(Nu <500)(1) • Eq (1) is not valid for turbulent flow region and we can’t neglect the influence of gas flow Laminar flow Fig 6 Nusselt numbers • To develop the correlation in turbulent flow region, steam Reynolds number is adopted to Eq (1). Nui = 120 (Ps/Pa)0.75max(1.0，aRe,sb) 　　 (Re,s≦5000，a=0.0012，b=1.0) 　　　　　(2) • Comparing between the measurement and calculation from Eq (2) shows good agreement not only in laminar flow region but also in turbulent flow region. Turbulent flow Laminar flow Fig 7 Comparison between measurements and calculation (Nusselt numbers)

8. In low heat transfer area, Eq(2) underestimate the Nusselt numbers • Estimation only by the steam Reynolds number is not enough when air mass flow rate increases • In low heat transfer area, Eq(2) over estimate the Nusselt numbers • Effect of Re,s ｂ (b=1) is too big Improvement of Heat transfer models Additional experiment (increasing air mass flow to 0.2-1.0g/s) to improve the correlation in turbulent flow region □0.2MPa △0.4MPa +50% -50% Fig 8 Comparison between measurements and Eq(2) (air mass flow: 0.2-1.0g/s）

9. Improvement of Heat transfer models The steam Reynolds number Re,s in Eq(2) was changed to steam-air mixture Reynolds number and Eq(3) was derived (a = 0.0035，b = 0.8) (3) Comparing between the measurement and calculation from Eq (3) shows good agreement not only in laminar flow region but also in turbulent flow region including the air mass flow increasing condition ◇0.1MPa □0.2MPa △0.4MPa +50% -50% Fig 9 Comparison between measurements and Eq(3) (air mass flow: 0.03～1.0g/s）

10. Evaluation of Heat transfer models • Temperature measurements by thermocouples may contain errors, so calculated local heat transfer coefficients may have errors • To evaluate the accuracy of calculation, we calculated the mixture of steam and air temperature profile and compared with measurements • It was verified that Eq (3) effectively simulate the temperature profile. • We confirmed the validity of Eq (3) as heat transfer model Eq (3) Eq (3) Eq (3) Eq (3) Eq (3) Eq (3) Eq (3) Fig 10 Comparison between measurements and calculation (temperature profile)

11. We confirmed the validity of Eq (3) as heat transfer model in with Moon’s experiment Evaluation of Heat transfer models Comparing local heat transfer coefficient between measurements and Eq (3) Moon’s experiment (test condition) • Comparing local heat transfer coefficient between measurements and Eq (3) （1/hc = 1/hf + 1/hi） ◇ 0.1MPa □ 0.15MPa △ 0.25MPa +50% -50% Measurement limitation Fig 11 Comparison between measurements and calculation (hc)

12. ７．Summary • To estimate realistic availability of reflux condensation heat transfer we must consider following realistic conditions • Existence of noncondensable gases • Gas-liquid countercurrent flow • An experimental facility was constructed to study reflux condensation heat transfer in the riser section of PWR U-tubes • New heat transfer models were developed 　　① Correlation for local Nusselt number was obtained as a function of the steam-to-air partial pressure in laminar flow region ②In turbulent flow region steam-air mixture Reynolds number was adopted • It was verified that New heat transfer models effectively simulate the temperature profile

13. ８．Future plan • Incorporation of new models into RELAP5 • Validation of new models in RELAP5

14. Reference

15. 乱流域 乱流域 層流域 層流域 Comparison with RELAP5 heat transfer models Turbulent flow hi(W/m2K) Table 3　Calculation with RELAP5 heat transfer models • Comparing local heat transfer coefficient with measurements, Eq (2) and RELAP5 models Laminar flow Eq (2) measurement Psteam/Pair Turbulent flow RELAP5 condensation heat transfer models tend to underestimate the heat transfer coefficient in all region. Laminar flow hi(W/m2K) We will incorporate the new models into RELAP5 as a option and we will be able to calculate SG reflux condensation more accurate than default models. Eq (2) measurement ※Now we are under additional experiment because data in turbulent flow region is not sufficient. Psteam/Pair Fig 9　Interface condensate heat transfer coefficient （Comparison with RELAP5)