An Investigation Into Stratified Co- and Countercurrent Two-Phase Flows

Abstract

Johnston, A.J.; James Cook U. of North Queensland Summary This paper investigates theoretically and experimentally certain stratified co- and countercurrent two-phase flows in circular pipes that have not been studied adequately. The theoretical analysis produced semiempirical relationships based on the momentum-balance concept. These relationships were then experimentally evaluated with two fluid systems of significantly different density ratios. It is suggested that the equations that correlated these results can be accepted for general use. An example is presented for a certain natural-gas/condensate application. Introduction Most of the material published on stratified two-phase flow has concentrated on situations where the gas and liquid phases are in the same direction (i.e., cocurrent) where the gas velocity is considerably greater than the liquid velocity. Such situations are produced in many applications; however, studies related to natural gas pipelines have provided a reasonable understanding of the phenomenon. In contrast, there provided a reasonable understanding of the phenomenon. In contrast, there is a relatively poor understanding of cocurrent flows where the velocities of the gas and liquid phases are similar in magnitude and of countercurrent flows where the liquid and gas phases travel in different directions. One practical situation where such conditions are possible is in the main bottles (or pipes) of a parallel-bottle slug catcher, which is often installed at the terminal of a two-phase natural gas pipeline. Fig. 1 illustrates schematically one such installation. Its principal function is to separate the liquid and gas phases, thereby facilitating both types of co- and countercurrent flow (as described above) in the main bottles when a slug of condensate enters the slug catcher. The flow characteristics in these inlet bottles where countercurrent flow occurs are closely related to the capacity and controlled operation of the slug catcher. To improve the general understanding of these stratified two-phase flows, a simultaneous theoretical and experimental investigation was conducted. Theoretical Analysis Considering the smooth stratified cocurrent flow condition illustrated in Fig. 2A, we can write the following momentum balance equations for each phase. phase. For the liquid phase, ............................(1) For the gas phase, ............................(2) Eliminating, we obtain ............................(3) If the smooth stratified countercurrent flow shown in Fig. 2B is considered, momentum equations for each phase can again be written. For the liquid phase, ............................(4) For the gas phase, ............................(5) Again, eliminating, we obtain ............................(6) In both Eqs. 3 and 6, the wall shear stresses can be represented in the conventional manner; i.e., ............................(7) and ............................(8) where the gas and liquid friction factors can be obtained from ............................(9) and ...........................(10) In these relationships, the hydraulic diameter parameters, and, can be evaluated with the procedure suggested by Argawal et al. ..........................(11a) SPEPE p. 212