The coefficient of friction in the Darcy-Weisbach relation is applicable for a single-phase flow. The Colebrook-White formula is generally used for this type of flow. For a given flow, a given pipe diameter, an absolute roughness, the coefficient of friction is a constant.

The calculation of frictional pressure loss for two-phase gas-liquid flow for instance is a complex process. The coexistent flow of two phases complicates the theoretical and empirical approaches which are available. This means that a complete analytical solution is not possible. After 60 years of extensive research, it is rare to find two correlations with exact predictions. In an effort to overcome these shortcomings the FluidFlow Two-Phase Module provides eight choices of correlation that represent some of the most successful approaches to this complex problem.

There are three main approaches you can take to modeling two-phase gas-liquid flow systems in FluidFlow.

1.Define the temperature and pressure of the fluid at the inlet of the system.

2.Define the vapor quality of the fluid at the system inlet.

3.Define multiple inlet boundary conditions with different fluid types.

When using any of the above three approaches, the software will automatically establish the physical properties of the fluid(s) and automatically track the fluid phase state throughout the system. The appropriate pressure loss correlations are also applied automatically for each element in the system.

Fluid mixtures can also be created either dynamically in the model or using the fluids in the database to create a mixture.

The pressure gradient (ΔP/L) for two-phase flow is not constant but varies along the pipe as a function of temperature and pressure. This means that the pressure drop must be calculated by integrating the pressure gradient along the pipe. The calculation approach used by FluidFlow is similar to that used in compressible flow calculations. The following steps are made:

1. A pipe increment is selected based on a small change in pressure P1 and P2. The length of this increment is not yet known.

2. Upstream temperature, pressure, quality, and physical properties are determined. Physical properties for each phase and the mixture physical properties are needed. Crucial here is FluidFlow’s fluids database containing the thermophysical properties of more than 1200 fluids.

3. A flash calculation is then performed to determine the downstream quality.

4. From the downstream properties, FluidFlow determines the flow regime and then determines the incremental length of this segment. Determination of the incremental length depends on the friction loss calculation method used.

5. Steps 1 to 4 are repeated until the end of the pipe is reached. The incremental length step size, therefore, shortens as the calculation moves down the pipe. For the last segment, which will never be the exact length required, we use interpolating functions based on results from previous segments.

Regards

There is not problems using each one of them, but you need to b clear about which one you are reporting. The origen of that fact is historic. If you know one of them, you calculated the other because fanning factor is 0.25 of Darcy factor. In the heat exchanger area, traditionally the fanning friction factor has been used.

Ruben is right on. There are two streams in the literature. These are roughly civil vs. mechanical engineering. Darcy, Moody, whatever… Here's how you tell the difference. On the left side of the graph there's a downward-sloping diagonal line for the laminar region. It will either have f=16/Re or f=64/Re. That's where Ruben's 0.25 comes from. It doesn't matter which you use as long as you are consistent when applying the calculation. I know of several industrial systems designed incorrectly because they didn't make this distinction. I also know of a CCPP in Saudi that has a boiler feed pump that is 9.8 times as big as it should be because somebody didn't know how to calculate pump head in SI units. The auxiliary load is 44% of the gross. What a colossal screw up!