CFD-based Throughflow Analysis: Bridging the Gap Between 1D and 3D Analyses

Many of us have come to appreciate the expedience that 1D (meanline) analysis provides when scoping out a new design or revisiting a legacy design for use in a new application. However, we also know that this approach to design is heavily reliant on empirical models. Inherent in this design approach is, or should be, the acceptance that these empirical models are highly useful but obviously limiting and maybe not necessarily universally applicable. We are accepting the tradeoff of fidelity for time.

On the other end of the spectrum, we have 3D CFD. Here, we attempt to remove many of the assumptions that are covered by the empirical models and allow for highly intensive numerical calculations to “run wild”—simulating all the wonderful and complex physical behavior that is present in our turbomachinery designs. Obviously, this comes at a cost. TIME! Especially in the world of axial compressor and turbine design where we commonly have 4, 5, 7, 11, 17, or 20-something stages. Models can grow to tens of millions of cells very quickly and take hours or even days to run a single operating point even with powerful parallel computing. Suddenly, 1D analysis doesn’t look so bad after all?

So, what if we could find a happy middle ground between the 1D and 3D approaches? Enter the wonderful world of 2D throughflow analysis! This is not a new concept, but it does have quite a bit of nuance, which makes it difficult to approach (and get “right”). There are generally two approaches to throughflow analysis: streamline curvature or CFD.

In general, CFD-based throughflow provides advantages when dealing with choked or transonic flows. It also manages injections and extractions naturally whereas streamline curvature would require specific models. Even though CFD-based throughflow is loss and deviation based, it manages spanwise effects naturally rather than needing additional models to make approximations. And maybe the most obvious, since it is CFD-based, direct comparisons to 3D CFD results can be done in the same post-processor and those comparisons can be used to calibrate the 2D analysis, or even use the 2D throughflow analysis to initialize 3D CFD downstream in the design process to improve and speed up convergence of these larger and more complex simulations!

CFD-based throughflow adds additional value in other ways such as quickly running several map points to correlate a 1D-generated map, generating an entire operating map, using throughflow results as part of a 3D geometry optimization, or even using a “design mode” to “reverse engineer” blade angles from desired flow and pressure conditions.

The obvious drawback of this methodology when compared to streamline curvature is solving time. Despite being longer, we still are looking at minutes or hours—nothing dramatically greater than that of a streamline curvature methodology to realize all the benefits! It also makes for a more challenging implementation of a design mode due to the likelihood of “misalignment” between design sections and grid locations, but this is not beyond the realm of possibility.

Here at Concepts NREC we are continuing to fine-tune our CFD-based throughflow solution. This includes development of analysis and design modes of the solver. Ensuring that the gap between 1D and 3D analysis is filled with a dependable, efficient 2D solver will remain a high priority in the years to come. When working in combination with our 3D design tool, AxCent^®, this solution brings a tremendous amount of value to the axial machine design process by striking the perfect balance of analytical fidelity and solve time to allow for more informed design decisions when working with 3D geometry.

We welcome your questions and comments here or contact us at info@conceptsnrec.com.