Modelling of a steam-water two-phase flow regime map in a horizontal tube

The prediction of pressure drop and heat transfer rate in two-phase flow system is not only dependent on the fluid properties and the equipments used, but it is also dependent on the flow regimes. Most of the flow regime maps published in the literature were constructed based on the experimental data of air-water mixture. These data usually cover over a limited range of flow rates, fluid properties, pipe sizes and angle of inclination. The existing flow regime maps derived from both experimental observation and mechanistic model were developed only for the system under adiabatic condition. They may not suitable to be used in the conditions when the mass transfer between the phases takes place.

In this work, an attempt is made to establish a mechanistic model for two-phase flow regime map under the condition in presence of the mass transfer in hor-izontal tubes. Two fluid model is used in the present analysis. The conservation equations for the mass, momentum and energy are written separately for each phase. The balance momentum of the mixture is then obtained by equating the axial pres-sure drop of the vapour and the liquid phases. The expressions for the interfacial mass transfer are derived and added to the conservation equations. These terms are strongly dependent on the interfacial parameters such as interfacial heat transfer. in-terfacial velocity and interfacial temperature. The other constitutive relations for the equations such as interfacial shear stress and wall shear stress were incorporated via existing empirical correlations. The transition criteria were also developed to account the interfacial momentum transfer effect.

The momentum equation of the mixture was then solved using a sequential algorithm implementing an iterative procedure. The effect of the liquid tempera-ture, system pressure, pipe diameter on the flow regime transitions were presented. Numerical results show that the transition from stratified smooth to wavy flow is sensitive to the effect of the mass transfer and hence wall temperature and system pressures. The results also indicate that the mass transfer also affects to the equilib-rium liquid level. Thus, transition from stratified to intermittent flow which depends on this parameter shifts to higher superficial liquid velocity. Also in this transition, it is observed that the contribution of the force generated by the momentum transfer is small compared to the other forces such as gravity and surface tension forces. Effects of the interfacial friction factors to the flow regime transitions are also investigated in the present studies. Most of the transitions boundaries are sensitive to this parame-ter. The investigation of effect of the fluid properties to the flow regime transitions is conducted by comparing the transition boundaries for various system pressures at saturation condition.

To establish the validity of the model, the numerical results obtained in the present study were compared with the existing two-phase flow experiments reported in the literature. Good agreement is achieved between the present model and the results of existing experiments.