For vertical air-water flow, five flow regimes were identified by a co
mbination of visual/ video techniques and pressure loss fluctuation ch
aracteristics in an 0.026 m i.d. tube. Checks against existing how reg
ime maps indicated that it was unlikely that a universal map will be d
erived that will correctly predict phase regimes in vertical flow. Exi
sting theoretical transition equations were shown to have difficulty i
n handling the effect of diameter on the slug to churn transition. The
churn to annular transition proved hard to predict at higher liquid r
ates. Two new relations are presented that predicted the transitions f
or the slug to churn and churn to annular regimes.