Hydraulics: turbomachines
BACKGROUND
A turbomachine is a device that establishes energy exchange with the flow of a working fluid via the combined action of a stator (fixed element) and a rotor (mobile elements). The process is dominated by an increase or decrease in pressure, which entails a work exchange, positive or negative, with the exterior.
To design and analyse these Computational Fluid Dynamics (CFD) simulation techniques are an essential tool to tackle the physics of the machine itself. SOLUTE has experience in applying these techniques to the simulation of physical processes of the same or similar nature to other software on the market, better known as OpenFOAM ALTAIR-CFD or ANSYS.
Turbomachinery is present in many engineering sectors, especially in electrical power generation, using gas or wind turbines, propulsion in air vehicles or different industrial processes with turbo-compressors and/or hydraulic pumps. In all cases, a proper design and maintenance are essential to carry out the tasks, but the physics involved in the problem is not easy and experimental tests may be expensive or difficult to carry out.
CFD simulation uses numeric methods to simulate the mathematics of the laws of physics that are involved and thus allow analysing the movement of fluids and the transfer of heat. This know-how allows conducting a study in phases of the design, development and even in service maintenance in order to achieve performance improvement objectives, study the sensitivity and reduce weight and costs, among others.
RESULTS
The results of the calculations include many parameters that allow evaluating the behaviour of the machine and judge if they meet the objectives or if there is margin for improving the optimisation. This analysis can be used at the end of the project as well as during the development phases and for this, calculation reports must be drafted that include the entire process that has been followed, the definition of the developed models and the results with their evaluation.
EXPERIENCE
Since 2020, SOLUTE has been gaining specific experience in CFD simulation in the hydraulic turbo-machinery sector, based on a prior solid experience in the application of CFD analysis using different software in problems, mostly focused on the development of cars, inner and outer aerodynamics and engine cooling.
Navier-Stokes equations are the guiding thread of the simulation, where the approach, more or less, will depend on the level of detail that is required.
METHODOLOGY
First of all, the methodology is determined by knowing the problem to be studied in order to determine the best simulation model, type, size and quality of the mesh, as well as the definition of the boundary conditions.
These types of machines involve external and internal fluids, which phenomena are highly complex: the geometries of compressors and turbines are three-dimensional, the flow is not stationary and turbulent, the viscous effects are noticeable, and the temporal and spatial scales are not easy, among other considerations. Also, compressibility issues may be present as well as interaction with other structures, which provides an additional complexity.
Navier-Stokes equations are the guiding thread of the simulation, where the approach, more or less, will depend on the level of detail that is required. For this, solvers allow selecting key aspects such as turbulence models, boundary layer theories, spatial/temporal discretizations or other simplifications. Currently, OpenFOAM is the most widely used software on the market among researchers as it is free, without setting aside other commercial software such as ALTAIR-CFD or ANSYS FLUENT/CFX, which are highly used because of the guarantees they offer. Likewise, other more specific software is available depending on the specific physics we wish to study, such as UNCLE-M, NUMECA or ADPAC.
In any case, a solid and experience based knowledge of the physics of the problem and a set of good practices provided by engineers with experience takes precedence. This allows creating models that simplify the aforementioned phenomena, choosing those that most affect the case of interest. A type of simulation, turbulent modelling, mesh refinement, boundary conditions and characteristics of the flow will be defined in order to collect relevant information about the machine being studied.
To help engineers efficiently and safely design turbo-machines, different guidelines and standards drafted by different institutions are followed such as ANSI-HI from the Hydraulic Institute (HI). The calculations provide a reliable forecast of the movement of a fluid inside a turbo-machine, which allows establishing different design points based on the application we wish to give or study the operating limits in order to maximise the power or the cost. Clearly, internal phenomena are complex and therefore, their modelling is also complex. This entails providing a very clear description of the calculation models to be used and with this, results that closely mirror reality.
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