Compressible and multiphase flows
We have seen in Chapter 3 that each one of those tools are able to simulate simple incompressible isothermal flow. If it is also the case with the compressible flow or flow with thermal effects, it could be different with multiphase flows.
To analyze a piping system for hydraulic with thermal effects it is necessary to simulate its behavior on three scale levels: pipe cross section, pipe/branch run, and the whole piping network – and then link and solve these models together.
At the cross-sectional scale it is necessary to define the type of fluid flow (laminar, turbulent, choked, or subsonic, flow pattern for multiphase flow – stratified, annular, intermitted, etc.), and the specifics of heat exchange between fluid and environment through pipe wall and thermal insulation. This knowledge is then used when calculating integral parameters of the flow and heat exchange (pressure gradient, heat losses per length, etc.) and following up how all fluid parameters change along the pipe/branch and all over the network. This would be impossible without access to most reliable thermodynamic data provided by software.
At the very beginning, a multiphase model was included into Flowmaster, not anymore present in 1994. Around 2000 Flowmaster moved to target the automotive market and created a HVAC module.
In 2012, Mentor announced the Flowmaster Power and Energy version for system level thermo-fluid simulation. This new product provided a two-phase solution modeling the phase change from liquid to vapor for steam generation or from vapor to liquid when steam is consumed. This new capability was ideal for the power generation markets, accurately describing industry-specific components that must be developed to meet industry modeling requirements with faster and more accurate analysis.
Built on the Flowmaster Rigorous Energy Balance Solver, the new two-phase capability took advantage of the enhanced fluid properties plus solver framework from the existing generation of the Flowmaster product. By combining these two technologies, the Flowmaster Power and Energy software tool began modeling the phase change that occurs in both steam and liquid processes. The orientation, temperature and pressure flow patterns of steam and liquid are very different, and each flow pattern has an impact on the pressure drop and heat transfer. The new Flowmaster product recognizes and models each flow pattern accurately. While the physics of two-phase flow are complex and can be difficult to predict, Flowmaster team has conducted extensive research and has incorporated the best correlations available for accurate simulation modeling.
Ideally suited for power generation applications, this new Flowmaster product addressed the specific thermo-fluid requirements for power plants (i.e. coal-fired power plants, boiler/steam generators, steam turbines, cooling towers, steam traps), solar energy, and automotive (zero flow heat transfer, advanced heat exchanger) applications.
Actually, the Vapor Cycle add-on enables the modelling of complex two-phase systems, primarily concerning the Rankine cycle and Reverse Rankine cycle plus various other thermodynamic cycles that can be modelled using the enthalpy solver. This is achieved through the Vapor Cycle Steam Systems and Vapor Cycle Systems component catalogues along with a wide range of two phase fluids. These provide the tools needed to investigate a wide range of applications including, HVAC, thermal power generation and waste heat recovery systems.
With FloMaster multiphase is for steam and water only.
Fundamental Approach for Thermal-Fluid Network Simulation
The objective of any thermal-fluid network analysis is to determine flow rates, pressures, temperatures and/or heat transfer rates for components in the network. Every component in a thermal-fluid network has to comply with the system specification and the individual components must function correctly as part of the integrated system. When designing thermal-fluid networks it is essential to accurately predict the flow rates through the components, as well as the temperature distributions and heat transfer rates throughout the network. Flownex can be used to assess the performance and operating conditions of components in complex unstructured thermal-fluid networks.
The analysis of thermal systems is often complicated by various factors, for example, the complex nature of fluid flow and heat transfer processes. Some of the characteristics commonly encountered in thermal systems that complicate their analysis are listed below.
- Time-dependent flow.
- Multidimensional flow.
- Complex geometries.
- Complicated boundary conditions.
- Coupled transport phenomena.
- Turbulent flow.
- Structural- and phase- change.
- Energy losses and irreversibility.
- Variety of energy sources.
By using the fundamental modeling approach adopted by Flownex users can take into account most commonly encountered in thermal systems that complicate their analysis.
Flownex’s two phase capabilities provides the user with the capability to simulate, design and analyse two phase systems which cannot be simulated using simple single phase compressible or noncompressible fluids and methods.
The process industry can make significant savings using Flownex to simulate, predict and optimize outputs in some of the following phenomena accounted in steam simulation:
- Vibration root cause analysis.
- Pipe knocking & water hammer prevention.
- Steam line insulation and heat transfer to the atmosphere.
- Pipeline venting, pressure regulating valves.
- Steam/Water filtering.
- Heat transfer.
- Phase separation.
- Flow regime approximation.
- Critical mass flux (choked flow).
Flownex’s two phase capabilities can be divided into the following broad categories:
- Pure two phase fluids: two phase fluid in its pure form.
- Two phase with a noncondensable gas: low noncondensable gas mass fraction, typically systems with unwanted air ingress.
- Psychrometry: low two phase mass fraction, typically HVAC systems.
As with the Flownex single phase capabilities, the two phase capabilities are based on the fundamentals of mass, momentum and energy balance, enabling the fundamental simulation of two phase thermo-hydraulic systems. Flownex employs a homogeneous mixture model approach in its flow elements (pipes, valves etc) assuming that the liquid and gas phases are evenly distributed over the cross-sectional area of the flow path and that the pressure, temperature and velocity of the phases are the same. Additional to the homogeneous mixture model, Flownex also allows for phase separation and level tracking in containers. This enables the separation of the liquid and gas phases (the gas phase accounts for the two phase vapor and noncondensable gas) within the container while the outflow phase from connections at various container heights are determined.
Hydrosystem provides a calculation engine for all necessary fluid properties and phase equilibrium on the base of fluid composition, for a wide spectrum of fluids and applications.
- The proprietary Properties library provides quick property calculation of ~150 most often used organic and non-organic components and their mixtures.
- The proprietary STARS library is focused on applications in the refining and petrochemical industry and allows for calculation of properties for individual fluid components, oil fractions and their mixtures in liquid and gas phases. This database contains more than 1600 components. STARS can also automatically compute gas-liquid phase equilibrium and check the validity of fluid phase entered.
- The GERG-2008 library allows calculation of natural gas thermodynamic properties and gas-liquid phase equilibrium based on the popular GERG-2008 equation of state.
- The WaterSteamPro module provides accurate calculations of water and steam fluid properties on the basis of IAPWS-IF97 equations.
For more advanced fluid properties and phase equilibrium calculations the program can use the Simulis Thermodynamics library by ProSim. This library covers Oil and Gas, Chemical, Biochemical, and other industries and provides a wide range of modern methods of physical properties, gas-liquid, and liquid-liquid phase equilibrium calculation for mixtures of more than 3000 compounds and oil fractions.
For hydrate formation prediction the program can use the Hydrate Open Structure module of the PVTSim library by Calsep, which can be purchased as a separate product.
Hydraulic analysis of two-phase gas-liquid flow
Two-phase gas-liquid flow can be analyzed ignoring mass transfer between phases (so-called “frozen” flow) or taking into account vaporization and condensation. Frozen flow analysis can be performed both for isothermal flow and as a heat and hydraulic calculation. To analyze flow with mass transfer heat and hydraulic calculation should be made. This analysis requires phase equilibrium calculation, so only STARS, WaterSteamPro or Simulis Thermodynamics libraries may be used.
Gas-liquid flow is considered to be steady, and phases are considered to be in the state of thermodynamic equilibrium and having the same temperature and pressure. “Slippage” of liquid and gas phases can be considered; i.e., their movement at different flow velocity.
During hydraulic analysis of a two-phase gas-liquid flow, the pattern of two-phase flow at different points along the pipeline is found, and for all components the following values are calculated:
- two-phase flow pattern;
- void fraction and true phase velocities, as well as hydrostatic losses in components due to elevation change;
- pressure loss due to friction;
- pressure loss due to local resistance;
- pressure loss due to acceleration of flow;
- Mach number.
Various methodologies can be used for determining flow pattern and performing the analysis. Since analysis methods for two-phase flows are currently developing and at this point no universally accepted approach exists, Hydrosystem utilizes a number of methodologies for different calculations, from which the user can select the most appropriate methods for any given task. There is also a high degree of flexibility in which methodologies should be used for various types of components in different situations, which can be set using special rules of method selection for two-phase analysis. User defined rules may be used as well as one of the predefined rules included in the software.
PSRE is member of TUFFP project, and most modern TUFFP Unified model for multiphase flow is embedded and can be used in Hydrosystem. It is also possible to simulate three-phase (liquid-liquid-gas – oil-water-gas) flow analysis.
Two-phase gas-liquid flow analysis considering mass transfer between phases
Analysis of two-phase flow taking into account mass transfer (boiling/condensation) may be performed for pipelines of various structures without recycles. Individual branches are analyzed using direct calculation method, long pipes are subdivided into parts with slight change of calculated parameters, if necessary.
Change of flow temperature and phase composition for this type of flow is calculated using full energy balance equation. First change of fluid enthalpy is determined, then phase equilibrium on the basis of enthalpy and pressure is calculated and finally fluid temperature and phase composition are obtained.
If positive flow quality is entered in the beginning of pipeline, then the input temperature is ignored. In this case at calculating phase equilibrium fluid temperature in the start node is determined using flow quality and pressure values.
To analyze flow with mass transfer in unbranched pipeline one may enter pressure in the end node of a branch. In this case the fluid temperature is also interpreted as being entered in the end node and the “upstream” calculation is performed that obtains initial flow parameters (quality, temperature, pressure).
The analysis determines where in the pipeline the fluid state changes to one phase (liquid or gas). In these points the automatic switch to one-phase methods is performed. Similarly the reverse transition is recognized (the beginning of boiling up or condensation) with switching to two-phase methods.
For two-phase flow in unbranched pipelines Hydrosystem determines where the choked flow occurs in the pipeline (at the end of straight pipes, at the exit, in reducers and abrupt expansions). These points are characterized by Mach number equal to 1, flow discontinuity and shock waves. At “upstream” analysis the program uses iterations based on energy and material balance equations to determine fluid properties before shock wave. A specific algorithm is used to analyze straight pipes with Mach number close to 1 and pressure, temperature and other parameters changing very fast along the pipe. Such flow is simulated using a system of ordinary differential equations which is solved by Runge-Kutta method and quadrature formulas.
Hydrosystem can do severe slugging prediction and can calculate settling slury flow (i.e. liquid + solid particles) on the base of modern Delft Head Loss & Limit Deposit Velocity Framework (DHLLDV Framework), including flow pattern prediction.
Each situation is different
The needs of your company are not necessarily the same as another society, they may also have changed with years.
Do not hesitate to contact Fluids & Co to have a personalized study of your project.