Numerical and experimental investigation of two phase flow during boiling in a coiled tube

https://doi.org/10.1016/j.ijheatmasstransfer.2007.05.025Get rights and content

Abstract

A numerical simulation, using the VOF multiphase flow model, and the corresponding experiments were conducted to investigate the boiling flow of R141B in a horizontal coiled tube. The numerical predictions of phase evolution were in a good agreement with the experimental observations, and the two phase flow in the tube bends was much more complicated due to the influence of liquid–vapor interaction with the interface evolution. The associated heat transfer was also considered. It was found that the temperature profile in the two phase flow was significantly affected by the phase distribution and higher temperature always appears in the vapor region.

Introduction

Tube/channel evaporators are widely employed in various energy systems, like HVACR and petrochemical industries. As well-known, both associated flow boiling and two phase flow play very important roles in evaporators and are still far from being fully understood. As a result, present evaporators are normally designed with a profuse safety factor, resulting in an unnecessary tube length and high superheat of vapor at the exit. Further understanding of flow boiling could potentially decrease the manufacture cost and energy consumption, and furthermore obtain better performance.

Flow boiling and associated two phase flow in tubes/channels was comprehensively investigated as a classical topic of boiling heat transfer in open literature. Particularly, numerous investigations were conducted for horizontal, adiabatic gas–liquid two-phase flow in tubes [1]. Two-phase flow with phase change is very complicated. In addition to inertia, viscous and pressure forces present in single-phase flow, two-phase flows are greatly affected by interfacial tension forces, the wetting characteristics of the liquid on the tube wall and the exchange of momentum between the liquid and vapor phases in the flow, also by phase distribution, mass exchange among two phases, flow pattern evolution and phase redistribution if with phase change. Usually, traditional liquid–gas flow modes in a horizontal tube includes bubbly flow, plug flow, slug flow, wavy flow and annular flow, and also stratified flow, stratified/wavy flow, intermittent flow, mist flow and so on. Theofanous and Hanratty [2] presented an overview of flow modes of steady, fully developed multifluid flows in tubes/channels. Depending on operating conditions and fluid properties, most of these flow modes are also expected to occur in the two-phase flow systems with phase change of evaporation.

Normally flow pattern maps obtained from experiments are employed to describe two phase flow, like vapor–liquid flow configurations, and boiling heat transfer in tubes/channels. The flow pattern map proposed by Taitel and Dukler [1] is widely applied for adiabatic two-phase flow, which identified the flow mode as a function of Martinelli parameter Xtt. Later, Hashizume [3] found that the boundaries or transitions of flow pattern in the flow pattern map for boiling two-phase flows of a refrigerant differs significantly from those presented in adiabatic two-phase flow. Actually, for flow boiling, mass transfer plays an important role in the dynamic phase distribution and hence flow mode. The adiabatic flow pattern maps, such as Baker map or Taitel and Dukler map, showed very poor accuracy of prediction of flow mode in various cases, even for diabatic two-phase flows. Based on Taitel and Dukler map, Steiner [4] developed a diabatic flow pattern map based on R-12 and R-22 data. Recently, Kattan et al. [5], [6], [7] modified the axes of the map to improve Steiner map for two phase flows with evaporation, and accounted for the influences of heat flux and dryout on the flow mode transition. This work was based on R-134a, R-123, R-402A and R-404A data under evaporating conditions and could be applied to both diabatic and adiabatic two-phase flows. They also proposed a new flow boiling model based on local flow patterns. Thome and Hajal [8] presented a simpler method to obtain the equivalent results of Kattan map. The newest version of Kattan map was proposed by Wojtan [9], which considered dynamic void fraction and cross-sectional locus of the liquid–vapor interface, and the effects of heat flux on the transition to mist flow.

In last decade numerical simulations are increasingly and widely employed to investigate two phase flow during flow boiling in tubes/channels. Recently, a series of so-called “one-fluid” methods was proposed to correctly advent the phase boundary in a number of adiabatic problems, like volume-of-fluid (VOF) [10], CIP [11], the level-set [12], [13], and phase field [14] methods. For problems involving phase-change, the energy equation should be considered to account for the heat absorption/release during evaporation/condensation and the volume change as well. Proper source terms should be added in the governing equations to model the phase-change phenomenon [15], [16], [17].

Coiled tubes is a kind of very important tube type for industrial applications. However, except for the case of single phase flow, very limited experimental investigations and theoretical/numerical analyses have been found in the open literature to understand the two phase and heat transfer during boiling in coiled tubes. Wu et al. [18] applied the Eulerian multiphase flow model to investigate the boiling process in a coiled tube and paid special interests in flow mode. They found that the phase distributions showed a continuous stratification in the horizontal tubes and were influenced by both buoyancy force and centrifugal force in the tube bends. Although a gradually increase of vapor volume along the flow was shown in their work, the most interesting flow details, such as flow mode and bubble motion, were not presented.

In the present paper, an attempt was made to combine the experimental observation with numerical simulation to explore and understand fundamentals of two-phase flow behavior during flow boiling in coiled tubes. Numerical simulations were conducted for flow boiling in a horizontal coiled tube using refrigerant R-141B as the work fluid. Special interests were addressed on the flow mode evolution and exploring the details of local flow and transport phenomena. Meanwhile, an experiment was conducted to validate the simulations.

Section snippets

Basic considerations

The modeling of boiling flow was accomplished by using volume of fluid model (VOF) in CFD software Fluent 6.0 (USA), with a user defined function (UDF) as an evaporation model. In VOF model, volume fractions of each phase in a computational cell are recorded, and volume fractions of all phases sum to unity.αl+αv=1

Information on phase distribution can be directly extracted from the volume fractions. For instance, the computational grid is empty or full of the liquid when αl reaches the minimum

Experiment

To validate the numerical simulations, an experimental investigation was also conducted to visually observe the fundamental phenomena of the flow boiling and associated two-phase flow in the coiled tube. The experimental facility employed is shown in Fig. 2 and the test section was installed horizontally. The working fluid, R141B, was circulated in a closed loop, consisting of a liquid tank, pump, flow meter, pre-heater, pre-mixing chamber, test section, post-mixing chamber, filter and

Flow patterns

The complete development of flow mode in a straight heating tube mainly consists of six different regimes, namely bubbly, churn, slug, stratified, annular and mist flow. Under proper working conditions, e.g. flowrate, heat flux, inlet liquid temperature and pressure, these flow modes can be observed in both simulations and experiments conducted for a coiled tube. In this investigation, six working conditions were investigated both numerically and experimentally, see the details in Table 3. Two

Pressure drop and vapor volume fraction at the outlet

During boiling flow, generation and coalesce of bubbles, and waving of liquid–vapor interface as well, result in great variation in the pressure drop ΔP between the inlet and outlet and the area-averaged volume fraction αv,out at the outlet. The variations of ΔP and αv,out closely depend on the flow mode at the outlet region (the last straight tube). There are three types of flow modes in the outlet region observed in the simulation: continuous waving, bubbly flow and vapor flow with separated

Conclusions

Numerical simulations and experiments were conducted to investigate the flow boiling of refrigerant R141B in a horizontal coiled tube. The comparison indicated that the phase distributions in various cases predicted by the simulations were well consistent with those observed in the experiments. Particularly, the simulations comprehensively exhibited the evolution process of flow mode depending on hydrodynamical and thermal flow conditions.

Flow velocity predicted in the simulations showed a

Acknowledgements

This work is currently supported by Specialized Research Fund for the Doctoral Program of High Education (Contract No. 20040003076) and Advanced Heat Transfer LLC, USA.

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