Building a System Model of the RL-10 Rocket Engine in Flownex

When we engineers are building a new system or iterating on an existing design it can be expensive.  Simulating a full system-level model in a 3D CFD program can take days.  Making iterative changes to an existing system can be costly or even impossible. Utilizing a one-dimensional system modeler like Flownex allows us to analyze many different designs very quickly, on the order of seconds or minutes.

Flownex is a thermal-fluid network modeler.  It is a simulation tool that allows for 1D fluid modeling and 2D heat transfer.  It uses a variety of flow components, nodes, and heat transfer elements to model the entire system we are interested in analyzing.  It solves conservation of mass, momentum, and energy to obtain the mass flow, pressure, and temperature of fluids and solids throughout the complete network.  Because of this approach we can analyze large, complex networks very quickly, iterate on designs, and even run short or long transient simulations with ease.

In the example today we are looking at a version of the RL-10 rocket engine, which has been a staple in the delivery of satellites into orbit and an essential part of many spacecraft. The specific iteration of the RL-10 we will be using for building our network model is the RL10A-3-3A. A good place to begin with any system model is a system schematic:

Figure 2: RL10A-3-3A Engine System Schematic – Image from https://ntrs.nasa.gov/citations/19970010379

In Flownex we can assign an image (could be from a P&ID diagram, a CAD cross-section, or even a satellite image!) as the background for our drawing canvas. We simply need to right-click on the drawing canvas and select Edit Page to bring up the drawing canvas properties.

Clicking on the action button under Appearance Style brings up the Styles Editor.  Here we can change the fill style to Image and select the appropriate image for our background.

In the case of the RL-10 we can use the image from figure 2 as our background image.  We may want to consider adjusting the opacity of the image so that it blends into the background a little bit more.

In Flownex building a system model is as simple as drag and drop.  We can build our rocket engine using a variety of flow components from the Flow Solver library. To build the RL-10 system model we will be using the following components:

CEA Adiabatic Flame component to model combustion.

Composite Heat Transfer component to model thermal transport through pipe-walls to ambient and to model the regen.

Boundary Conditions to constrain our system at the inlets and outlet.

Basic Valves to model the different valves in the system,

Flow Resistances to model specified losses where appropriate.

Flow Interfaces to model the fluids entering the combustion chamber (to transfer fluid properties as we switch from two-phase O2 and H2 to gaseous fluids for modeling combustion.

Pipes for modeling various flow-paths.

Restrictors with Discharge Coefficient for our injection ports to the combustion chamber.

Restrictors with Loss Coefficient to model both the Calibrated Orifice and the Venturi contraction/expansion.

Basic Centrifugal Pumps for our Fuel and LOX pumps.

Simple Turbine to model the Fuel Turbine

Shafts to connect our different pumps mechanically.

Gearbox is used to connect the shafts between the LOX pump and the Fuel Pump.

Exit Thrust Nozzle to determine total thrust.

Script is used in assigning O2 properties prior to combustion.

The components may be dragged and dropped from the component library onto the drawing canvas to build our system model. We can also copy and paste components that are already on the canvas into different locations. This can be especially useful when the same inputs for say, a pipe, are used consistently throughout the model. All components have both Inputs and a Results associated with them as seen in the figure below. This is how we will define our flow components.

The completed model of the RL-10 Rocket Engine can be seen below. There are a few simplifications; we are using composite heat transfer components to model free convection to a specified ambient temperature (as though this was a land-based test). Rather than tie the actual temperatures and flow conditions in the nozzle to the regen we are using assumed temperatures and convective heat transfer coefficients. For additional fidelity we could model the heat transfer between these two flow paths with calculated convective heat transfer coefficients and we could model cross-conduction along the pipes which deliver the fuel and oxidizer to the combustion chamber. With additional effort, more complex use cases could also be simulated.

For the sake of demonstration we set up a transient action to slowly vary the oxidizer control valve fraction open; starting at 30% and ending at 100% open and observer the change in thrust at the nozzle as a function of this changing transient action.

Plots may be easily added by dragging a Line Graph from the Visualization > Graphs section of the component library onto our canvas. To choose the characteristics we would like plotted against time we simply need to drag and drop the desired inputs or results onto our newly placed line graph.

RL-10 Transient Thrust Plot

We can plot both the oxidizer control valve fraction open and the thrust versus time to observe the thrust reaction to the opening of the valve. The thrust plot has some jumps that are likely due to numerical singularities – with additional work this could be improved.

As can be seen, setting up complex system models in Flownex is relatively simple with most operations being drag and drop. For ease of sharing models with colleagues or customers adding a background image makes it very easy to see how the flow components in the model correspond with a system schematic. Setting up and plotting the effects of operational transients is a breeze!

For more information on Flownex please reach out to Dan Christensen at dan.christensen@padtinc.com.

New Enhancements to Flownex 2015: Even Better Fluid-Thermal Simulation

987786-flownex_simulation_environment-11_12_13The developers of Flownex have been hard at work again and have put out a fantastic update to Flownex 2015.  These additions go far beyond what most simulation programs include in an update, so we thought it was worth a bit of a blog article to share it with everyone.  You can also download the full release notes here: FlownexSE 2015 Update 1 – Enhancements and Fixes

What is Flownex?

Some of you may not be familiar with Flownex. It is a simulation tool that models Fluid-Thermal networks.  It is a 1-D tool that is very easy to use, powerful, and comprehensive. The technology advancements delivered by Flownex offer a fast, reliable and accurate total system and subsystem approach to simulation that complements component level simulation in tools like ANSYS Fluent, ANSYS CFX, and ANSYS Mechanical.  We use it to model everything from turbine engine combustors to water treatment plants. Learn more here

Major Enhancements

A lot went in to this update, much hidden behind the scenes in the forms of code improvements and fixes.  There are also a slew of major new or enhanced features worth mentioning.

Shared Company Database

One of the great things about Flownex is that you can create modeling objects that you drag and drop into your system model. Now you can share those components, fluids, charts, compounds, and default settings across your company, department, or group.    There is no limit on the number of databases that are shared and access can be controlled. This will allow users to reuse information across your company.

Shared Database
Shared Database

Static Pressure Boundary Conditions

In the past Flownex always used a total pressure boundary condition. Based on user requests, this update includes a new boundary condition object that allows the user to specify the static pressure as a boundary condition. This is useful because many tests of real hardware only provide static pressure. It is also a common boundary condition in typical rotational flow fields in turbo machinery secondary flow.

Subdivided Cavities

Another turbo machinery request was the ability to break cavities up into several radial zones, giving a more accurate pressure distribution in secondary flow applications for Rotor-Rotor and Rotor-Stator cavities.  These subdivisions can be automatically created in the radial direction by Flownex.

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Subdivided Cavity Input Dialog

Excel Input Sheets and Parameter Tables

The connection between Microsoft Excel and Flownex has always been strong and useful, and it just get even better.  So many people were connecting cells to their Flownex model parameters that the developers decided to directly connect the two programs so the user no longer has to establish data connection links.  Now an properties in Flownex can be hooked to a cell in Excel.

The next thing users wanted was the ability to work with tables of parameters, so that was added as well.  The user can hook a table of values in Excel to Flownex parameters and then have Flownex solve for the whole table, even returning resulting parameters.  This makes parametric studies driven from Excel simple and powerful.

flownex_Parametric-Tables
Excel Parameter Tables

Component Enhancements

Users can now create component defaults and save them in a library. This saves time because in the past the user had to specify the parameters for a given component. Now thy just drag and job the existing defaults into their model.

Compound components have also been enhanced by the development team so you no loner have to restart Flownex when you move, export, or import a compound component.

Find Based on Property Values

Users can now search through properties on all the objects in their model based on the value assigned to those properties.  As an example, you can type > 27.35 to get a list of all properties with an assigned value that is larger than 27.35.  This saves time because the user no longer has to look through properties or remember what properties were assigned.

Network Creation through Programming

Users can now write programs through the API or scripting tool to build their network models. This will allow companies to create vertical applications or automate the creation of complex networks based on user input. Of all the enhancements in this update, this improvement has the potential to deliver the greatest productivity improvements.

Automatic Elevations Importing in GIS

Users who are specifying flow networks over real terrain can now pull elevation data from the internet, rather than requiring that the data be defined when the network is specified. This enhancement will greatly speed up the modeling of large fluid-thermal systems, especially when part of the simulation process is moving components of the system over terrain.

Multiple Fluid Interface Component

A very common requirement in fluid-thermal systems is the ability to model different fluids or fluid types and how they interact. With this update users can now model two separate fluid networks and define a coupling between the two. The mass balance and resulting pressure at the interface is maintained.

Static Condition Calculation Improvements

Many simulation require an accurate calculation of static pressures. To do this, the upstream and downstream areas and equivalent pipe diameters are needed to obtain the proper values.  Many components now allow upstream and downstream areas to be defined, including restrictors and nozzles.

flownex_upstream-downstream-area
Dialog for upstream and downstream area specification

Scaled Drawing

The ability to create a scale 2-Dimensional drawing was added to Flownex. The user can easily add components onto an existing scaled drawing that is used as a background image in Flownex. These components will automatically detect and input lengths based on the drawing scale and distance between nodes. This results in much less time and effort spent setting up larger models where actual geometric sizes are important.

Scaled Drawing Tools
Scaled Drawing Tools

How do I Try this Out?

As you can see by the breadth and depth of enhancements, Flownex is a very capable tool that delivers on user needs.  Written and maintained by a consulting company that uses the tool every day, it has that rare mix of detailed theory and practical application that most simulation engineers crave.  If you model fluid-thermal systems, or feel you should be simulating your systems, contact Brian Duncan at 480.813.4884 or brain.duncan@padtinc.com. We can do a quick demo over the internet and learn more about what your simulation needs are.  Even if you are using a different tool, you should look at Flownex, it is an great tool.

Integrating ANSYS Fluent and Mechanical with Flownex

Component boundaries generated in Flownex are useful in CFD simulation (inlet velocities, pressures, temperatures, mass flow). Generation of fluid and surface temperature distribution results from Flownex can also be useful in many FEA simulations. For this reason the latest release of Flownex SE was enhance to include several levels of integration with ANSYS.  

ANF Import

By simply clicking on an Import ANF icon on the Flownex Ribbon bar users can select the file that they want to import. The user will be requested to select whether the file must be imported as 3D Geometry which conserves the coordinates system or as an isometric drawing.

The user can also select the type of component which should be imported in the Flownex library. Since the import only supports lines and line related items this will typically be a pipe component.

Following a similar procedure, a DXF importer allows users to import files from AutoCAD.

This rapid model construction gives Flownex users the ability to create and simulate networks quicker. With faster model construction, users can easily get to results and spend less time constructing models.

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ANSYS Flow Solver Coupling and Generic Interface

The Flownex library was extended to include components for co-simulation with ANSYS Fluent and ANSYS Mechanical.
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These include a flow solver coupling checks, combined convergence and exchanges data on each iteration, and a generic coupling that can be used for cases when convergence between the two software programs is not necessary.

The general procedure for both the Fluent and Mechanical co-simulation is the same:

  1. By identifying specified named selections, Flownex will replace values in a Fluent journal file or ds.dat file in the case of Mechanical.
  2. From Flownex, Fluent/Mechanical will then be run in batch mode
  3. The ANSYS results are then written into text files that are used inputs into Flownex.
  4. When applicable, specified convergence criteria will be checked and the procedure repeated if necessary.

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Learn More

To learn more about Flownex or how Flownex and ANSYS Mechanical contact PADT at 480.813.4884 or roy.haynie@padtinc.com.  You can also learn more about Flownex at www.flownex.com.