Abstract: To provide the theoretical basis for practical production and the design of two phase separators, this paper simulated the existing region of the stratified flow under different pipe inclined angles and the inclined angle of maximum region for stratified flow in the pipeline operation conditions was presented. The stable criterion for the stratified flow interface was derived from Kelvin Helmholtz instability theory, and the transition from stratified flow to annual flow presented by Barnea was adopted in the present papers. The results show that: the stratified flow in the upward pipelines almost did not exist; with the angle under the downward pipeline increasing, the existing region for
stratified flows gradually increased, and quickly decreased after exceeding the maximum region, and even disappeared; The optimal separating efficiency can be obtained when the inlet inclined angle of two phase separators is about 30 degree.

Key words: fluid flow, flow pattern, stratified flow, pipe inclined angle