SpinOffs

Valentine’s Day is February 14, and while some cynics refer to it as a “Hallmark holiday”, most people commemorate the day in some way. One of the biggest challenges is finding a card that perfectly captures the way you feel about someone, while also reflecting who you are.  Well, Concepts NREC has created some turbomachinery-themed Valentine’s Day cards for engineers. These fall into the Art end of our Art-to-Part Solution.

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.

As an engineer in the rotating machinery world, it is my job to design things that work for a very long time. To help ensure this, we have evolved the best analytical tools to calculate the stresses and deflection of the parts we have so carefully designed. But sometimes, we lose track of what matters. We know that material strength, weight, stiffness, toughness, thermal conductivity and thermal growth all matter. They are in a material database, so they must.

Ideally, the exit flow angle for an impeller should be the same as the exit blade metal angle. However, the exit flow angle deviates from the blade guidance at the impeller exit due to the finite number of blades. Correctly predicting flow deviation is a critical task in meanline and through-flow modeling because the exit flow angle is directly related to the work input and the pressure rise across the impeller.

Many gas turbines with radial compressors utilize a radial-to-axial inlet duct upstream of the first compressor stage. Aside from the fact that flow in the duct generates aerodynamic losses, the flow profiles at the duct exit, delivered to the inlet of the first impeller, also affects the performance of the compressor. 

Two often used quantities to characterize turbomachinery are flow coefficient and work coefficient.  The two are generally represented as Φ for flow coefficient and φ for work coefficient.  The mathematical definition for the two quantities are as follows:

The vast majority of turbomachinery textbooks emphasize analysis when the heart of engineering is based in design. Concepts NREC is uniquely qualified to share the nature and process of engineering design. When Concepts NREC decided to write and publish a range of turbomachinery textbooks, the objective was to emphasize engineering design rather than analysis.  

The turbine of an automotive turbocharger is subject to a wide and demanding range of requirements and constraints. High efficiency is required to reduce the pumping work of the engine that can make a small but desirable improvement in fuel economy. The automotive application also requires a good transient response, and a low rotating inertia of the turbine is necessary for this. Durability and the ability to operate for a guaranteed lifetime are required – but such performance is not easy to achieve, given the hot gas environment and the way the turbine is subject to frequent changes in condition. The market is very competitive, and so manufacturing cost is very important. These various factors are in competition with each other, and a successful turbine is one that achieves the right balance between them, rather than one that excels in one particular area.

There are two dominant manufacturing methods used to produce turbomachinery parts – Casting and 5-axis machining. This blog will explore some of the applications and restrictions of these two methods.