Critical angle for the inclined pipe to distinguish horizontal and vertical character

Safety analysis codes of nuclear power plant have equipped the flow regime map to provide the proper constitutive relations corresponding to the flow regime. Traditionally, the flow regimes for the vertical flow and horizontal flow have been investigated intensively and we have very reliable map and criteria to be useful for the computer code. However, for the inclined pipe, we have lack of experimental works and transition criteria to be used. Actually, in the pressurized heavy water nuclear reactor like CANDU, certain part of pipe line has inclination angle. At this moment the safety code such as RELAP-CANDU has no flow regime map for the inclined pipe, Engineering judgment intuitively suggested to take the reference angle and horizontal flow regime map for the inclined pipe with the lower angle than the reference and vertical flow regime map for the larger angle than the reference angle. Also, as the reference angle it has been suggested that 45 degree may compromise the difference of two dominant flow regime. However, it has no physical bases. Also, it casted fundamental questions on the drift flux model and the effect of gravity on the instability of two-phase flow. Therefore, the present study is designed to find the reference angle to distinguish the horizontal flow regime map and vertical flow regime map. The study was made based on the experimental way with the special intelligent identifier to remove any chance of human subjective perception