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AbstractThe effects of pressure on flow pattern transition boundaries for crude oil-natural gas two-phase flow were investigated.Experimental observations of flow patterns were carried out for different pressures (200 and 450 psia) and inclination angles (horizontal, +1 degree and vertical). Special attention was given to the stratified-intermittent transition and the intermittent-annular transition for horizontal and inclined flow, and the transition to annular flow for vertical flow, since these boundaries were known to be sensitive to pressure.The experimental results show that pressure affects the stratified-intermittent transition for horizontal flow and slightly affects the intermittent-annular transition for inclined flow and for vertical flow. The predictions of transition boundaries by existing mechanistic flow pattern prediction models show good agreement for all transition boundaries described above.