Pipe Flow

The Pipe Flow Module contains built-in physics interfaces that define the conservation of the momentum, energy, and mass of fluid inside a pipe or channel system. The Pipe Flow interface is used for computing the velocity and pressure fields in pipes and channels of different shapes. It approximates the pipe flow profile by 1D assumptions in curve segments or lines. These lines can be drawn in 2D or 3D and represent simplifications of hollow tubes.

For users of the CFD Module and Heat Transfer Module, a Pipe Connection multiphysics coupling is available for cases where piping systems open up to a larger fluid volume. This feature couples a 1D pipe segment (modeled with the Pipe Flow interface) with a 3D single-phase flow body to provide continuity in mass flux and pressure, regardless of direction.

Heat Transfer

The Heat Transfer in Pipes interface is used for modeling heat transfer by conduction and convection in pipes and channels of different shapes, where the fluid velocity and pressure are known a priori. The interface uses 1D energy balance to determine the temperature profiles in curve segments or lines. These lines can be drawn in 2D or 3D and represent simplifications of hollow tubes. Functionality for modeling wall heat transfer, including multilayer walls and cladding, is included as an option. The Nonisothermal Pipe Flow interface extends this physics interface by providing equations to compute the velocity and pressure fields when they are unknown. More extensive descriptions of heat transfer, such as 3D turbulent flow models or problems involving surface-to-surface radiation, can be found in the Heat Transfer Module.

Mechanical Analysis of Pipes

The Pipe Mechanics interface is used for computing stresses and deformations in pipes with loads such as internal pressure, junction forces, and axial drag forces. The Fluid-Pipe Interaction, Fixed Geometry multiphysics coupling can be used to model flow-induced loads in pipes, such as pressure and drag forces, centrifugal forces in curved pipes, and fluid loads at bends and junctions. With the add-on Structural Mechanics Module, a Structure-Pipe Connection multiphysics node is available to couple the Structural Mechanics interfaces with the Pipe Mechanics interface.

Water Hammer Analysis

When a valve is closed rapidly in a pipe network, it gives rise to a hydraulic transient known as a water hammer. The propagation of these hydraulic transients can, in extreme cases, cause overpressures that lead to failures in pipe systems. The Water Hammer interface in the Pipe Flow Module can be used for modeling compressible flow brought about by rapid hydraulic transients by taking the elastic properties of both the fluid and pipe wall into account.

Chemical Species Transport

With its capabilities for modeling the transfer of chemical compounds diluted in fluids flowing through thin pipes, the Pipe Flow Module allows for complex chemical reaction modeling. This can include mass transfer, chemical kinetics, heat transfer, and pressure drop calculations in the same model.

The Transport of Diluted Species in Pipes interface solves a mass balance equation for pipes in order to compute the concentration distribution of a solute in a dilute solution, considering diffusion, dispersion, convection, and chemical reactions.

Friction Models

The flow, pressure, temperature, and concentration fields across the pipe cross sections are modeled as cross-section averaged quantities, which only vary along the length of the pipes and channels. For single-phase flow, the pressure losses along the length of a pipe or in a pipe component are described using friction factor expressions.

The available friction models for Newtonian fluids include Churchill, Wood, Haaland, Von Karman, and Swamee-Jain. When one of these friction models is selected, the surface roughness data can be selected from a predefined list.

For non-Newtonian fluids in tubes of circular cross sections, the Irvine and Stokes friction models are available for Power-law fluids, Darby is available for Bingham fluids, and Swamee-Aggarwal for Herschel-Bulkley fluids. For non-Newtonian fluids in tubes of noncircular cross sections, a value or expression can be entered for the Darcy friction factor.

Junctions, Inlets, Valves, Bends, and Pumps

To account for correlations of sudden pressure change for common elements of pipe networks, the Pipe Flow Module includes features to introduce additional pressure losses due to irreversible turbulent friction at a point associated with bends, valves, pumps, or contractions or expansions in a pipe system. An Inlet feature is available to set the velocity, volumetric flow rate, or mass flow rate inlet conditions that describe the fluid flow.

In addition to the continuous frictional pressure drop along pipe stretches, pressure drops due to momentum changes in components are computed through an extensive library of industry-standard loss coefficients. Friction losses in pipe junctions are characterized by many variables, and the geometries may differ by angles, cross sections, and the number of branches. The Pipe Flow Module offers a variety of junction types that can act as a split or merger, such as T-junction, Y-junction, and N-way junction, to specify additional losses due to irreversible turbulent friction.

Non-Newtonian Fluids and Multiphase Flow

For single-phase flow modeling, the fluid can be characterized according to its response and the actions of shear stresses. Newtonian fluids have a linear relation between shear rate and shear stress. In the case of non-Newtonian fluids, the relationship between shear rate and shear stresses can be nonlinear. The Bingham plastic fluid model is available to describe viscoplastic fluids that have a yield stress. For shear-thinning fluids and shear-thickening fluids, the Power law fluid model is available. The Herschel-Bulkley fluid model is used to describe the rheological behavior of non-Newtonian fluids and to simulate the flow of fluids exhibiting viscoplastic behavior. With the non-Newtonian fluid models, phenomena such as water and mineral suspension can be modeled.

The Newtonian fluid type also has two gas–liquid options: the Gas-Liquid, friction factor multiplier, which modifies the single-phase Newtonian Darcy friction factor; and the Gas-Liquid, effective Reynolds number, which uses an effective, adjusted viscosity to calculate the Reynolds number in the pressure loss calculations. Two-phase gas–liquid flow is a common phenomenon in the nuclear, oil & gas, and refrigeration industries, where gas mixtures are transported in pipe systems.

Acoustic Wave Propagation

The propagation of acoustic waves along flexible pipes is a contributing factor to the design, planning, and building of these networks. The Pipe Acoustics interfaces can be used for 1D modeling of the propagation of sound waves in pipe systems.

With the Acoustics Module add-on, it is possible to perform 3D-to-1D acoustics analyses in both the frequency and time domains. To compute the propagation of acoustic waves in fluids at quiescent background conditions, the Pressure Acoustics, Frequency Domain interface is available for time-harmonic analyses and the Pressure Acoustics, Transient interface is available for transient simulations.

The Acoustics Module also provides an Acoustic-Pipe Acoustic Connection multiphysics coupling to combine the Pressure Acoustics interfaces with the Pipe Acoustics interfaces in both frequency- and time-domain simulations. The coupling is defined between a point in the pipe acoustics interface and a boundary in the pressure acoustics interface.