According to Accutech 2000 , 2-phase gas/-liquid flow is common in a number of industrial processes, which include petroleum, chemical, nuclear, refrigeration, space and geothermal industries. Specific flow regimes which can be simulated include steam flow in power generation, refrigerant flow with changing quality, phase change in the flow of a volatile hydrocarbon due to heat transfer, flashing and performance of a knock-out pot with a non-volatile mixture of a gas and liquid.
The calculation of frictional pressure loss for two phase gas-liquid flow is complex. The coexistent flow of two phases complicates the available theoretical and empirical approaches. The software package FluidFlow3 available from Accutech 2000, provides six choices of correlations.
The following friction loss correlations are user-selectable within FluidFlow3:
- Beggs Brill
- Chisholm Baroczy
- Drift Flux
- Lockhart Martinelli
- Muller Steinberg Heck (MSH)
Some correlations have specific areas of applicability. MSH is a good approach for refrigerants and single component fluids, but loses accuracy at high vapour quality and Beggs and Brill may not be suitable for vertical upflow. FluidFlow3 addresses this by providing an option to select the appropriate correlation.
FluidFlow3’s method of solution
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 following steps are made:
- A pipe increment is selected based on a small change in pressure P1 and P2. The length of this increment is not yet known.
- Upstream temperature, pressure, quality and physical properties are determined. (Initially, these are known or defined properties at the boundary of the pipe network). The physical properties for each phase and the mixture physical properties are determined.
The FluidFlow3 fluids database contains the thermophysical properties of more than 900 pure fluids.
- A flash calculation is then performed to determine the downstream quality – the proportion of gas to liquid.
- From the downstream properties, FluidFlow3 determines the flow regime and then determines the incremental length of this segment. Determination of the incremental length depends on the particular friction loss calculation method used.
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, interpolating functions based on results from previous segments are used. This method is computer dependent and FluidFlow3’s advanced solution method provides rapid convergence.
The following references were used in developing FluidFlow3’s 2-phase gas-liquid module.
1. Mechanistic Modeling of Gas-Liquid Two-Phase Flow in Pipes - O. Shoham ISBN 978 1 55563 107 9
2. Fluid Flow Handbook - J. Saleh - ISBN 0 07 136372 6
3. A Basic Approach to Wellbore Two Phase Flow Modelling - AR Hasan, CS Kabir and M Sayarpour - SPE 109868
4. Process Heat Transfer Principles and Applications - R Serth - ISBN 978 0 12 373588 1
5. Stromung und Druckverlust - Walter Wagner - ISBN 3 8023 1879 X
6. Two-Phase Flow in Complex Systems - S Levy ISBN 0 471 32967 3
8. Friedel L. Improved friction pressure drop correlations for horizontal and vertical two phase pipe flow. Ispra European Two Phase Flow Group meet, Paper F2 (1979).
9. A Simple Mechanistic Model for Void Fraction and Pressure Gradient Prediction in Vertical and Inclined Gas/Liquid Flow Khasanov et al SPE