## 4 Discussion  
In the present paper, an experimental and theoretical study of the initial stage of annular gas-liquid flow was conducted as a continuation of work [29]. No systematic experimental studies of the initial waves in annular flow with entrainment were carried out previously. In [27], the closest position of measurements was 15 cm below the inlet. In [31], the measurements were limited by the range of low liquid and gas flow rates. Theoretical stability analysis studies [32; 33; 34] were aimed at predicting the large-scale waves directly, ignoring the wave formation and development stage. Only in [22] a numerical model was taking the flow development into account. The present investigation was focused on the effect of flow orientation and the viscosity of a working liquid on the stabilization of the liquid film after it leaves the distributor slot, and on the characteristics of high-frequency fast growing waves, which appear prior to the stabilization of the liquid film. Thus, this paper was aimed at the expansion of the range of applicability of the earlier [29] results. Upward flow orientation leads to thicker initial film, but also a shorter stabilization length in comparison to downward flow. Both the frequency and increment of the growth of initial waves are lower in the upward flow; the waves' velocity remains unaffected by flow orientation. In most cases, change in liquid viscosity has a minor effect on film and wave properties, provided that the gas and liquid flow rates are the same. For this reason, the dimensional liquid flow rate (or wetting density if different hydraulic diameters are compared) is a better parameter for the generalization of wave properties than the liquid Reynolds number. The parameter most sensitive to viscosity is the wave increment. The weak effect of viscosity is quite unexpected. One could assume that the actual viscosity of the working liquid decreases at large liquid flow rates or shear rates, but this is not true for the employed liquids and range of shear rates [35]. A possible explanation is that the flow of the liquid film just after leaving the slot is essentially different from the stabilized flow of a gas-sheared liquid film. The momentum of liquid when it is just leaving the slot is induced by the hydrostatic pressure of the liquid in the pressure tank, and not yet affected by the action of gravity or gas shear. Thus, the bulk inertia of the liquid at this stage cannot be immediately compensated by the viscosity, and this is the supposed reason of a weaker viscosity effect than it could be expected for stabilized films. It is possible that for even more viscous liquids, the effect of viscosity would be more prominent and thus detectable. The modeled characteristics of waves were compared to the measured ones for downward flow with different viscosities. The model reproduces the main qualitative trends in the behavior of wave characteristics, except for the effect of viscosity on wave frequency. On the quantitative level, there is no systematic quantitative agreement between the model and experiment at present. Figure 11: Comparison of the measured against the calculated increment of amplitude growth: (a) water; and (b) WGS2. The reason for this discrepancy is most likely the same as for the weak viscosity effect as described above. The transient character of the flow is also manifested, e.g., in the independence of film thickness in the area of interest on liquid and even gas (for downward case) flow rate or the independence of wave velocity on gas velocity and flow orientation. It could be expected that the velocity profile inside the liquid film changes essentially between the slot outlet and the point of stabilization of film thickness and that these changes affect the wave generation. Further development of the model should be aimed at taking this aspect of the downstream evolution of the flow into account.