A new heat transfer correlation for condensation in the presence of air and its implementation into Relap5/Mod3.3
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Date
2009
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Türkiye Atom Enerjisi Kurumu
Abstract
In the framework of safety analysis of Light Water Reactors, film condensation problems may be encountered in several situations. The passive heat removal applications in the current and the advanced water cooled reactors rely on the condensation heat transfer mode. Following the Loss of Coolant Accident (LOCA), the generation of steam can lead to rise in temperature and pressure inside the containment. In order to condense this steam and thus to limit the containment temperature and pressure, containment cooling condensers are provided in some advanced boiling water reactors (ABWR). The emergency condensers, which are located in core flooding pool, are also used in ABWR. The circuit of each condenser contains an anti-circulation loop so that practically no circulation of condensate takes place through the open lines to the reactor during normal operation. Only when there has been a drop in reactor pressure vessel (RPV) does the steam enter the condenser, with the resulting condensate being returned to the RPV [2],
To make a qualified design decision for such passive safety systems utilizing condensation, a fundamental question that arises is the behavior of the steam condensation when the noncondensable (NC) gas is present. It has been well established that the presence of NC gases in the vapor can greatly inhibit the condensation process due to buildup of NC at the condensate-mixture interphase leading to a decrease in vapor partial pressure and in the interphase temperature at which condensation occurs.
The theoretical analysis of the in-tube condensation in the presence of NC gas has been studied by many researchers by using different methods involving either the heat and mass transfer analogy or the boundary layer analysis methods. The former approach is generally based on the two-fluid model in which each phase is separately considered in terms of two sets of conservation equations governing the balance of mass, heat and energy. The interfacial friction factor is estimated by the single phase correlations and two phase empirical or semiempirical correlations. Other possible effects such as entrainment, deposition, suction effect, and interfacial roughness could also be taken into consideration by using suitable relations. Since the transport of mass, heat and momentum in the annular film-wise condensation with NC gas is strongly coupled with each others at the liquid-gas interface, for the systematic understanding of these transport phenomena, the boundary layer analysis, which is solving the governing equations in the gas-mixture and liquid film regions, is more helpful [3], However, it should be noted that the boundary layer solutions are not readily usable form neither for design purposes nor system analysis codes.
In this study, a new correlation for vertical flow is introduced for the condensation in the presence of NC gas problem. The model of correlation is based on the Chen [4] type forced convective flow boiling correlations. Examination of Eq. (2) clearly reveals that while the first term on the right hand side is analogous to the enhancement factor, the second term could be treated as the suppression factor. The data extracted from the Middle East Technical University-Condensation Test Facility (METU-CTF) [5] were engaged to estimate the unknown parameters of Eq. (2) and the details of the data are given in Section (3.6) and Section (4). The implementation of the correlation into the R.ELAP5 code was also in the frame of the present study and this new version ofthe R.ELAP5 code is called as modified throughout the report.
The comparison of wall sub-cooling of the modified R.ELAP5 results with experimental data is performed in Section (5.1). At the mid-elevation ofthe condenser tube, the deviation was found in the range of ±1% and -5% for modified R.ELAP5. However, the maximum deviation of the original R.ELAP5 is -47%. This finding implicitly reveals that the axial variation of air mass fraction at both interface and bulk is well predicted by modified R.ELAP5.
The heatflux predictions and comparisons are reported in Section (5.2). Because of the air accumulation at condensate-mixture interface, the decreasing heat flux variation along the condenser tube was achieved as expected. The original R.ELAP5 code gives higher deviation, which is around 40%, than the modified version in which the deviations are hovered around 10%.
The local heat transfer coefficient (HTC) variations (given in Section 5.3) corresponding to 4 bar system pressure are provided in Fig. 8 and Fig. 9 for both modified and original R.ELAP5 codes, respectively. The achieved propensity is appropriated for the theoretical background and decreasing HTC, which is mainly caused by the accumulation of air at interface, were obtained in axial direction. The maximum mean deviations acquired from the modified R.ELAP5 are much lower than the original code and are 20% and 130%, respectively. The overall comparison given in Fig. 10 also shows that the HTC prediction ofthe modified R.ELAP5 is more accurate than that of original code and most of the data points are predicted within the range ofthe uncertainty band (24%) ofthe experimentally evaluated HTC.
The air mass fraction possesses vital importance for the accurate prediction of local heat flux and hence local HTCs. As discussed in Section (5.4), the deviations for the majority of data are below 5% for modified version. On the other hand, the original R.ELAP5 gives relatively higher deviations (> 25%) especially at the bottom ofthe condenser tube. The general conclusion drawn from this study is that the prediction of the modified R.ELAP5 is much better that that of original R.ELAP5.
Description
TENMAK D.N.. 9363
Keywords
Mass transfer, Kütle transferi, Heat -- Transmission, Isı -- İletim, Condensation, Yoğunlaşma, Noncondensable gas, Yoğuşamayan gaz, Heat transfer correlation, Isı iletim korelasyonu
Citation
Ağlar, F. ve Tanrıkut, A. (2009). A new heat transfer correlation for condensation in the presence of air and its implementation into Relap5/Mod3.3. Ankara : Türkiye Atom Enerjisi Kurumu.