the liquid film friction on the wall), the second term represents the gravitational pressure change (for the downward condensing flow in vertical tube the inclination angle is [theta] = -[pi]/2), and the third term represents the acceleration pressure change (the pressure change due to the acceleration or deceleration of the flow in the tube).
It can be seen that for lower mass fluxes (lower than 60 kg/[m.sup.2]s) the gravitational pressure change is dominant, and as a result of that the pressure increases from the tube inlet to outlet (the gravitational pressure change term, for a down-ward condensing flow in vertical tube, will be [DELTA][p.sub.g] = (-[rho]g sin [theta])L = [rho]gL in the total pressure change and therefore it results in the increase of total pressure change), which gives the reason for the total pressure change being positive in Fig.
To determine the condensing flow rate [m.sub.cw], the mass transfer coefficient [h.sub.D] and the driving potential are needed.
For the wet surface conditions, the driving potential is positive, and the condensing flow rate will be determined based on the driving potential and hence the latent heat transfer rate.
Finally, the condensing flow rate can be solved by applying the heat and mass transfer analogy.
Topics include: two-phase oscillatory thermal-hydraulic instability, oscillatory flow stability boundary, static instability, thermal-acoustic oscillations in heated channels, instability of condensing flows
, and cases of flow instability in pipelines.
The refrigerant mass flow rate, in general, is continuously changing, causing changes in refrigerant distribution in the system and because of two-phase condensing flows
inside the tube; the local heat transfer coefficient varies in a great range at different locations.