Condensing flows in horizontal pipes

Introduction

Condensation inside horizontal tubes plays an important role in many engineering fields such as refrigeration, petroleum, chemistry, power generation and safety systems of the new generation of power plants. Essentially, condensation can be defined as a process of converting of the saturated vapor into the liquid, which means an energy conversion to latent heat. The enthalpy decreases from the level of steam to the much lower level of the saturated liquid. Condensation takes place when the vapor (in saturation temperature or higher) comes in touch with a fluid interface or a solid surface which is in lower temperature than saturation temperature. In this case, the phase change process occurs. This phase change is an important phenomenon for the heat removal.  The amount of removed heat is crucial for the design and the optimization of heat exchangers. Moreover, condensation is occurring in the safety systems of nuclear power plants. In Fig.1 the design of KERENA reactor is shown. According to the figure the passive heat removal systems include four emergency condensers and these four condensers are responsible for the cooling of the system in the case of emergency break or leakage. The Passive safety systems do not need any external power supplies and they mostly depend on physical phenomena such as natural circulation and gravity driven flows.

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In order to optimize the performance of passive safety systems and their efficiency mostly one-dimensional codes are using such as ATHLET, RELAP and TRACE. These codes are able to calculate most of the phenomena in power plants; however, they cannot reflect the 3D phenomena. Therefore, Computational Fluid Dynamics (CFD) methods should be used to simulate and predict the complex multiphase flow structure. Despite the previous research being done on the two-phase flow behavior, this phenomenon needs much more investigations.

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Fig.2 shows the schematic representation of the flow condensation inside horizontal tubes. Pure vapor exist at the inlet and due to the phase change and heat transfer through the wall the vapor is condensing and a liquid film generates near the wall. By going further along the pipe the vapor is condensing more and different flow morphologies such as stratified flow, slug flow and plug flow occur inside the pipe. Many attempts have been done for the modeling of condensation inside the pipes. The goal of the current work is modeling of the transition between different morphologies which are occurring during the condensation inside the pipe. In order to do that, several CFD models such as IMUSIG, AIAD and GENTOP which have been developed in HZDR in cooperation with ANSYS are available. The Inhomogeneous MUSIG model considers the bubble size distribution and is used for modeling the small-scaled dispersed gas phase [1]. The AIAD (Algebraic Interfacial Area Density Model) is developed for detection of the local morphology and corresponding switch between them [2]. The recently developed GENTOP-model combines both concepts. GENTOP (Generalized Two-Phase Flow) approach is able to simulate co-existing large-scaled (continuous) and small-scaled (polydispersed) structures [3]. All these models are validated for adiabatic cases without any phase change. Therefore, the start point of the current work project is using the available models and integrating phase transition and condensation models into them. As initial stages the AIAD model has been used since in this model 2 continues phases should be considered and it is less complicated compare to GENTOP model which also considers a poly-dispersed phase. In the further investigation the GENTOP model will be used in order to include more comprehensive phenomenon inside the pipe.

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Acknowledgments

This project is an ongoing PhD project in Helmholtz Zentrum Dresden Rossendorf (HZDR), which is funded by Bundesministerium für Bildung und Forschung (BMBF) in Germany.

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References

1. Krepper, E.; Frank, Th.; Lucas, D.; Prasser, H.-M.; Zwart, P.J. “The Inhomogeneous MUSIG model for the simulation of poly-dispersed flow”. Nuclear Engineering Design 238: 1690-1702 (2008).
2. Höhne, T.; Deendarlianto; Lucas, D.  “Numerical simulations of counter-current two-phase flow experiments in a PWR hot leg model using an area density model”. International  Journal of  Heat and Fluid Flow 31 (5): 1047-1056 (2011).
3. Hänsch, S.; Lucas, D.; Krepper, E.; Höhne, T. “A multi-field two-fluid concept for transitions between different scales of interfacial structures”. International Journal of Multiphase Flow 47:171-182(2012).