SpinOffs

Turbomachinery performance is almost always analyzed and tested with a fixed inflow condition. In other words, the assumption is that the inflow fluid temperature and pressure is defined and unchanging over the map of machine performance. Since varying conditions often exist in practice, the performance maps are sometimes normalized, as shown in the figure below. The pressure ratio of a compressor is plotted versus a corrected mass flow range and rotational speed. 

In a previous blog, Fluid Phenomena Primer: Energy Versus Temperature, Specific Heat, I explained the behavior for gas phase fluids and how the temperature is affected at high energy levels.  In another blog, When Perfect is Good Enough - Perfect Gas Models, we looked at the simple perfect gas model.  In this blog, we’ll explore the next step up in the hierarchy of gas thermodynamic modeling: semi-perfect gas.

Temperature envelopes in the turbomachinery industry are constantly increasing as the state of the art evolves in pursuit of better performance. This means engineers need to design compressors with higher and higher exit temperatures, and turbines and nozzles with continuously increasing inlet temperatures. This rise in temperature greatly impacts the selection criteria for materials used. 

In my blog Flow Coefficient and Work Coefficient, I outlined the basic concept behind the flow and work coefficient. These nondimensional parameters are widely used to characterize axial and radial turbomachinery. Another widely used parameter for radial design is “specific speed”. For something with such a finite name, specific speed is perhaps the most mysterious and non-intuitive parameter in all of turbomachinery. In this blog, I'll lay the ground work for understanding specific speed.

 When designing compressors, engineers often use ruled-surface blades with the goal of making a shape that’s easily manufactured on a 5-axis machine.  Theses blades can be quickly machined in one pass by aligning the side of a cutting tool to the rulings. This process is often referred to as “flank milling.”  The alternative is to make many passes with the tool tip, a process known as “point milling”. For the right application, flank milling is often favored for shorter cutting times and better surface quality, but there are some caveats.

Gifts for Engineers can usually be segmented into a few categories: Things you have to put together, science fiction, gaming, new technology, and witty phrases printed on stuff. A Google search of the term "Best Gifts for Engineers" will quickly validate this claim.

Extending the operating range of centrifugal compressors has been a highly sought-after goal for several decades. In fact, the potential benefits have motivated researchers to develop and put into practice many pieces of technology, including full span inlet guide vanes (IGV’s), complex multistage systems with interstage bleed, passive casing treatment, and many others.

I think we can all agree that designing turbomachinery is hard. There are just so many moving parts (pun intended) in the design process, and they are all interconnected.  When you change the blade shape, it changes the aerodynamics, and could impact manufacturability. Everything you change has a cascading effect across many different areas, because all of the areas are linked; just like a Rubik's^® cube! Only, in turbomachinery design, you are not always trying to get all of the sides to be one color. Heck, even a 3-year old can do that. 

In the mid 1980’s, while serving in the Canadian Air Force, I had the good fortune, on one of my many adventures, to fly into Sondrestrom Air Base in Greenland. The Base is at the head of a beautiful fjord, so the scenery during the flight to Sondrestrom was magnificent. We arrived in the early summer on a beautiful clear day. I got out of the plane and wandered around the base while the aircraft was being serviced. One feature that caught my eye was all of the bright yellow ropes and stanchions that were strung from building to building. I couldn’t figure out what they were for, so I stopped one of the locals and asked, “Why the Yellow Ropes?”  Now, for those who are not students of the geography of Greenland, Sondrestrom is north of the Arctic Circle, and, apparently, the weather is not always as bright and clear as it was that day! As a matter of fact, one of the meteorological phenomena in the area was virtually instantaneous whiteouts, caused by snowstorms funneling up the fjord. Several people had been caught out between buildings and become disoriented during a blinding storm, a dangerous thing during the long darkness of winter. To eliminate this danger, they had put up the yellow ropes to safely guide people to their destination.

Computational fluid dynamic analysis (CFD) has become a standard part of the turbomachinery design process. Within Concepts NREC’s Agile Engineering Design System, FINE/Turbo, from our Partner NUMECA International, is the tool used to accomplish aerodynamic analysis of designs by applying standard methods of three-dimensional analysis. However, arriving at a converged CFD solution in any CFD program can sometimes be a challenge.