Numerical and experimental investigation of two phase flow during boiling in a coiled tube
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.
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.
References (21)
- et al.
Report of study group on flow regimes in multifluid flow
Int. J. Multiphase Flow
(2003) - et al.
The constrained interpolation profile (cip) method for multi-phase analysis
J. Comput. Phys.
(2001) Evolution, implement, and application of level set and fast marching methods for advancing fronts
J. Comput. Phys.
(2001)Calculation of two-phase Navier–Stokes flows using phase-field modeling
J. Comput. Phys.
(1999)- et al.
Direct numerical simulations of flows with phase change
Comput. Struct.
(2005) - et al.
A volume of fluid based method for fluid flows with phase change
J. Comput. Phys.
(2000) - et al.
Computations of film boiling. Part I, Numerical method
Int. J. Heat Mass Transfer
(2004) - et al.
Simulation of refrigerant flow boiling in serpentine tubes
Int. J. Heat and Mass Transfer
(2007) - et al.
A model for predicating flow regime transitions in horizontal and near horizontal gas–liquid flow
Amer. Inst. Chem. Eng. (AIChE) J.
(1976) Flow pattern and void fraction of refrigerant two-phase flow in a horizontal pipe
Bull. Jpn. Soc. Mech. Eng.
(1983)
Cited by (355)
Convective heat transfer of falling film around the horizontal half-oval tube with reverse airflow in an evaporative condenser
2024, International Journal of Heat and Mass TransferPhase-change transpiration cooling in heterogeneous composite porous plates: Heat transfer characteristics and their prediction
2024, International Journal of Heat and Mass TransferTransient wall temperature response during liquid nitrogen nucleate pool boiling: CFD analysis and experimental validation
2024, International Journal of Heat and Fluid FlowFlash boiling and pressure recovery phenomenon during venting from liquid ammonia tank ullage
2024, Process Safety and Environmental ProtectionInvestigation of boiling heat transfer process in the barrel of a gravity heat pipe type extruder
2024, International Journal of Heat and Mass Transfer