Title: An Investigation of Non-Linear Surge Characteristic in a High-Speed Centrifugal Compressor & Pressure Characteristic Rollover of a Transonic Centrifugal Impeller (Lecture Series)
Time: 9:30 to 12:00, Tuesday to Wednesday, August.1 to 2, 2023
Place: F207, School of Mechanical Engineering
Speaker: Dr. Teng Cao (Imperial College London)
Host: YANG Mingyang, Assoc. Professor (Institute of Advanced Energy and Powertrain Technology)

Biography
Dr. Teng Cao is a lecturer in the Termofluids Division, Department of Mechanical Engineering, Imperial College London. He obtained his Ph.D. in 2015 at Whittle Laboratory, University of Cambridge. He continued as Research Associate at the Whittle Lab. In 2016, he became MHI Senior Research Fellow and College lecturer at Girton College. He was appointed as Lecturer by Imperial College in September 2021. His research has been in the field of turbomachinery which is a crucial component for the current and foreseeable future energy systems in the world. Over the years, his work has been devoted to various research areas: (1) Developing high-performance centrifugal compressors, (2) Low-order modeling of Turbomachinery, (3) Fan-intake interactions, and (4) Pulsating flow effects on turbocharger turbine performance. His research outcomes have been largely recognized by peers and published in the world’s premier journals and peer-reviewed conferences. His recent work on centrifugal compressor tip leakage flow instability has been awarded the ‘Turbomachinery Committee Best Paper’ by the American Society of Mechanical Engineering. He has also been working closely with industries on real-world problems. His research has contributed to developing design tools that have been used by industries for design and research purposes.

Abstract
In centrifugal compressors, it is common to observe a rapid reduction in surge margin toward high rotational speed, which shows a non-linear surge characteristic against the change of rotational speed. This research presents a comprehensive experimental and numerical study to understand the flow mechanisms leading to the non-linear surge behavior in a high-speed centrifugal compressor. It shows that for the studied compressor, there are two critical flow coefficients (ϕ = 0.255 and 0.136) where the stability of the compressor stage is significantly weakened. At a higher speed, the impeller rotating stall happens at the first critical point where the diffuser instability is also enhanced. This is caused by the increase of the flow non-uniformity at the impeller exit as well as increased diffuser inflow angle due to the impeller compressibility effect. Therefore, both the impeller and diffuser’s instability are matched and trigger the surge at a high flow coefficient. In contrast, at a lower speed, the diffuser instability is not enhanced by the impeller rotating stall, this mismatch of the two component’s instability allows the compressor to pass through the first critical point and extend the surge limit to the second critical point where the diffuser reaches the stability limit and causes the rotating stall spontaneously. By these different behaviors at each speed, the non-linearity of the surge characteristic is established.