Q&A with Professor Bijay K. Sultanian, author of Gas Turbines
Professor Bijay K. Sultanian participates in a Q&A with us about his book Gas Turbines: Internal Flow Systems Modeling – part of the Cambridge Aerospace Series.
Question: What inspired this book?
Answer: In 1988, when I joined the heat transfer and secondary flow group of GE Aircraft Engines (GEAE), now called GE Aviation, in Cincinnati, I came across a new operational term – windage. Windage temperature rise of secondary air flows significantly affected thermal boundary conditions. Although we had design tools to compute windage, its precise definition and clear understanding were missing. It was often confused with either viscous dissipation or thought to be resulting from the friction on both rotor and stator surfaces.
The related concept of a generalized vortex in rotor cavities causing changes in air pressure and temperature was another source of confusion. Rotor disk pumping present in rotor-rotor and rotor-stator cavity was seldom understood, especially the role it plays in hot gas ingestion under the circumferential pressure asymmetry present behind the rows of turbine vanes and blades.
I was truly fascinated by the interplay of these elusive concepts featuring rotation, so much so that I decided to dig deeper into them from first principles (conservation laws of mass, momentum, energy, and entropy). In 1989, I developed at GEAE my most successful design tool (BJCAVT) to quickly and accurately calculate windage and swirl distributions in a complex rotor cavity formed by surfaces with arbitrary rotation, counter-rotation, and no rotation (stator).
My passion for the beautiful world of gas turbine internal (secondary) air systems continued unabated with continuous learning and deeper understanding of related concepts, physics of design, and reduced-order mathematical modeling. To my great surprise, I found that no US universities offered any related graduate-level courses. In 2007, I gave a twenty-hour lecture series within Siemens Energy, Orlando, titled “Physics-Based Secondary Air Systems Modeling.” The response to this series was overwhelming, as more than 60 engineers globally joined these online lectures.
Encouraged by the response at Siemens Energy, I instructed a two-day preconference workshop on “Physics-Based Internal Air Systems Modeling” in conjunction with ASME Turbo Expo 2009 in Orlando. I later taught this workshop in an eight-hour format at ASME Turbo Expo 2016 in Seoul, South Korea. Teaching these workshops and seeing the persistent need to bridge the gap between academic preparation and typical gas turbine design engineering practices in this area inspired me to finally write this book. This unique book will help interested practicing engineers and graduate students to bring about further advances in gas turbine internal air systems. Hopefully, some of them will someday replace this book with a better one.
Q: What do you hope will be the lasting impact of this book?
A: At the current stage of our civilization, gas turbines are ubiquitously needed for aircraft propulsion, land- and marine-based power generation, and various mechanical drives. This book will therefore last for a long time, until of course a new disruptive technology renders the gas turbines obsolete! The book will remain seminal to the development of graduate-level courses in leading US universities. Each gas turbine engineer engaged in the secondary air system design and whole engine modeling will benefit from this book. In addition, the book is ideally suited to support commercialized flow network codes, namely, Rolls-Royce’s SPAN and GE’s Flow Simulator in their user training courses.
For someone to write a competing book will require at least a decade of gas turbine secondary flow and heat transfer design experience at GE, Pratt & Whitney, or Rolls-Royce, and a decade of graduate-level teaching experience at a leading university. Nonetheless, I can’t wait to see another book in this area during my lifetime!
Q: Can you share any interesting anecdotes from the book?
A: Once, using my design code BJCAVT, an engineer was computing windage and swirl in a complex gas turbine cavity. He was not happy with the computed result for windage temperature rise and started suspecting the code. He finally discussed his problem with me and wanted to know the windage temperature rise I should expect in the cavity he was analyzing under the specified boundary conditions. I said, “I expect no windage temperature rise in the cavity.” Surprised by my answer, he immediately retorted, “That’s what your program is telling me, but we should expect significant windage temperature rise in this cavity due to friction on cavity surfaces.” I explained to him, “Because the cavity has no rotor surface, we should expect zero rotational work transfer to the flow, hence no windage temperature rise.”
Once, experienced gas turbine engineers in a design review were discussing how the compressor air for turbine cooling will flow radially inward from point A to point B in the rotor drum cavity shown in the figure below. The vortex structure of this flow significantly influences the drop in static pressure between these points. All expected the coolant flow to behave as a free vortex, passing between two counter-rotating donuts, driven by radially outward pumping along the disks.
A CFD-based visualization of this cavity flow defied everyone’s intuitive understanding and revealed that the coolant flowed along two paths hugging the rotor walls, one along ACB and another along ADB. The total incoming mass flow rate was split at point A into unequal mass flow rates along these paths, joining together at point B. The CFD study also showed that there was little flow activity in the core of the rotor drum cavity, as if the air was in solid-body rotation with the rotating cavity.
Once I conducted a simple experiment at home to demonstrate the magic of radially-inward flow in an Ekman boundary layer. I took a glass cup with a flat bottom, filled it 3/4th with water, and dropped in it a few mustard seeds. With a straw, I vigorously stirred the water, bringing it to a state of solid-body rotation. I pulled the straw out and let the spinning water come to rest. During spinning, all the mustard seeds were near the vertical wall. Everyone watching this experiment predicted that the mustard seeds would land vertically and settled at the bottom periphery when the water stopped spinning. They were surprised to find the seeds collected at the bottom of the cup near its center, not the periphery. No one realized that the mustard seeds were carried there by the radially-inward flow on the bottom surface.
Q: Why is your book important?
A: We currently have a number of titles on gas turbines. These titles primarily deal with the aerodynamic design of compressor and turbine airfoils that directly partake in energy conversion. Only a few titles in the market dwell on the internal flow systems of these complex machines, and they fall short of being directly helpful to practicing gas turbine design engineers. Just as the external power of strong athletes depends on the health of their internal flows of blood, water, and air, so is true for a gas turbine whose operational life depends heavily on the robust design of its internal cooling and sealing flows! Like me, for more than a quarter century, most gas turbine engineers have been waiting for such a book to enhance their physics-based understanding and modeling of complex swirling compressible flows around stator and rotor surfaces of gas turbine internal (secondary) air systems.
Find out more about Gas Turbines: Internal Flow Systems Modeling here.