Flow instabilities in parallel-channel flow systems of gas-liquid two-phase mixtures

Abstract

Parallel-channel two-phase flow systems with compressible capacities and the negative-slope characteristics of the pressure drop vs flow rate curve suffer from a non-uniform flow distribution and/or pressure drop oscillation, depending on the pressure drop characteristics of each channel, under certain operating conditions. The modes of flow distribution are quite similar to those observed in boiling channels. Pressure drop oscillations are subdivided into two types: relaxation oscillation and quasi-static oscillation. The modes of oscillation observed in the parallel-channel system are a single-channel mode, a U-tube mode and a multi-channel mode. These modes of oscillation are closely related to the type of oscillation. Upstream compressibility in the gas feed pipe has a destabilizing effect on the oscillation. On the other hand, upstream compressibility in the liquid feed pipe has a stabilizing effect on the oscillations of the gas flow rate and the pressure drop between the headers of parallel channels, but induces a small oscillation of the liquid flow rate. These non-uniform flow distribution and oscillation patterns are analyzed; the results of the analysis are found to be in good agreement with the experimental results. Thus, the characteristics of the non-uniform flow distribution and the oscillation can be estimated by the method of analysis presented in this paper.