Pressure drop single phase flow

Rapid approximate predictions of pressure drop for fully developed, incompressible horizontal gas/fiquid flow may be made using the method of Lockhart and MartineUi (Chem. Eng. Prog., 45, 39 8 [1949]). First, the pressure drops that would be expected for each of the two phases as if flowing alone in single-phase flow are calculated. The LocKhart-Martinelli parameter X is defined in terms of the ratio of these pressure drops ... 

The basis for single-phase and some two-phase friction loss (pressure drop) for fluid flow follows the Darcy and Fanning concepts. The exact transition from laminar or dscous flow to the turbulent condition is variously identified as between a Reynolds number of 2000 and 4000. 

Methods for determining the drop in pressure start with a physical model of the two-phase system, and the analysis is developed as an extension of that used for single-phase flow. In the separated flow model the phases are first considered to flow separately and their combined effect is then examined. 

Pressure Drop and Heat Transfer in a Single-Phase Flow 33... 

Velocity Field and Pressure Drop in Single-Phase Flows... 

A common practice is to calculate the pressure drop using the methods for single-phase flow and apply a factor to allow for the change in vapour velocity. For total condensation, Frank (1978) suggests taking the pressure drop as 40 per cent of the value based on the inlet vapour conditions Kern (1950) suggests a factor of 50 per cent. 

When two-phase flow is compared to the single-phase case for the same flow rate of an individual phase, it is an experimental fact that the frictional pressure drop will always be higher for two-phase flow. This higher pressure drop may be caused by the increased velocity of the phases due to the reduction in cross-sectional area available for flow, and also to interactions occurring at the extended gas-liquid interface which exists in most of the possible flow patterns. It is equally true that the heat flux will always be higher for two-phase flow than for the same situation in single-phase flow with the same liquid flow rate. On the other hand, mass transfer will depend upon both the extent of the gas-liquid interface and the relative velocity between the two flowing phases. 

For single-phase flow the momentum balance can be written to give the static pressure drop as the resultant of acceleration, hydrostatic, and wall-friction pressure-drop terms. 

In two-phase flow, most investigations are carried out in one dimension in the steady state with constant flow rates. The system may or may not be isothermal, and heat and mass may be transferred either from liquid to gas, or vice versa. The assumption is commonly made that the pressure is constant at a given cross section of the pipe. Momentum and energy balances can then be written separately for each phase, and with the constraint that the static pressure drop, dP, is identical for both phases over the same increment of flow length dz, these balances can be added to give over-all expressions. However, it will be seen that the resulting over-all balances do not have the simple relationships to each other that exist for single-phase flow. 

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