3 ms·

Based on a quick search, it looks like air impedance (the mysterious air density x speed of sound calculation) affects turbine stability. So needed for engine c

Based on a quick search, it looks like air impedance (the mysterious air density x speed of sound calculation) affects turbine stability. So needed for engine control?

kens 3y ago
Thanks, that makes sense. Do you have a link that discusses this?
- mgsouth 3y ago
  Wow, what a rabbit hole... I started by searching for "turbine air impedance". Seems to be a common topic, but maybe just for design modeling? And maybe an outdated parameter to use? One of the front-page hits is a recent (2021) paper from Whittle Laboratory [1] "Modeling Turbine Acoustic Impedance" [2]. Abstract says in part "Impedance boundary conditions are an influential yet uncertain parameter in predicting the thermoacoustic stability of gas turbine combustors...A parametric study of turbine stage designs using the analytical model shows acoustic impedance is a weak function of degree of reaction and polytropic efficiency. The design parameter with the strongest influence is flow coefficient, followed by axial velocity ratio and Mach number. We provide the combustion engineer with improved tools to predict impedance boundary conditions, and suggest thermoacoustic stability is most likely to be compromised by change in turbine flow coefficient." Many search iterations later [5], I found a recent paper discussing "fuel flow rate fluctuations which depend upon the air side impedance at the fuel injection location" [3]. Maybe it's for controlling inlet shock waves? However, I couldn't find anything obiously linking "Air inlet control systems" to "air impedance". Here's one paper discussing such a system, with an extremely detailed transfer function [4]. Maybe it's using another term for impedence, or something close? Edit: Considering this was cutting-edge control hardware for military equipment, I guess some lack of public details is to be expected. On the other hand, on Ebay there's a Fuel Control Unit for a J-79 engine as used in the B-58 Hustler [6]. [1] Yes, that Whittle. https://en.wikipedia.org/wiki/Whittle_Laboratory https://en.wikipedia.org/wiki/Whittle_Laboratory [2] https://www.mdpi.com/2504-186X/6/2/18 https://www.mdpi.com/2504-186X/6/2/18 [3] "Analytical modelling of flame transfer functions for technically premixed flames" https://journals.sagepub.com/doi/full/10.1177/17568277221094403 https://journals.sagepub.com/doi/full/10.1177/17568277221094... [4] "Complex Control System for an Aircraft Supersonic Inlet" http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS/SYSTEMS75.pdf http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS... [5] Just so you know, "b-58 engine computer" only brings up automotive stuff. [6] https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DITM%26aid%3D777008%26algo%3DPERSONAL.TOPIC%26ao%3D1%26asc%3D20230823115209%26meid%3D5af4cefff0304c96bd723aeb9765b577%26pid%3D101800%26rk%3D1%26rkt%3D1%26sd%3D325676156863%26itm%3D325676156863%26pmt%3D0%26noa%3D1%26pg%3D4375194%26algv%3DRecentlyViewedItemsV2SignedOut&_trksid=p4375194.c101800.m5481&_trkparms=parentrq%3A0c6fc5d618b0ab9711f1f22affff6958%7Cpageci%3Ae1b69473-6567-11ee-bb79-8eca912930b8%7Ciid%3A1%7Cvlpname%3Avlp_homepage https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DI...
  - kens 3y ago
    Thank you for the detailed response! This is way more than I expected :-)
- akira2501 3y ago
  Seems also to be related to https://en.wikipedia.org/wiki/Rayleigh_flow https://en.wikipedia.org/wiki/Rayleigh_flow

Based on a quick search, it looks like air impedance (the mysterious air density x speed of sound calculation) affects turbine stability. So needed for engine control?

kens 3y ago

Thanks, that makes sense. Do you have a link that discusses this?

mgsouth 3y ago
Wow, what a rabbit hole... I started by searching for "turbine air impedance". Seems to be a common topic, but maybe just for design modeling? And maybe an outdated parameter to use? One of the front-page hits is a recent (2021) paper from Whittle Laboratory [1] "Modeling Turbine Acoustic Impedance" [2]. Abstract says in part "Impedance boundary conditions are an influential yet uncertain parameter in predicting the thermoacoustic stability of gas turbine combustors...A parametric study of turbine stage designs using the analytical model shows acoustic impedance is a weak function of degree of reaction and polytropic efficiency. The design parameter with the strongest influence is flow coefficient, followed by axial velocity ratio and Mach number. We provide the combustion engineer with improved tools to predict impedance boundary conditions, and suggest thermoacoustic stability is most likely to be compromised by change in turbine flow coefficient." Many search iterations later [5], I found a recent paper discussing "fuel flow rate fluctuations which depend upon the air side impedance at the fuel injection location" [3]. Maybe it's for controlling inlet shock waves? However, I couldn't find anything obiously linking "Air inlet control systems" to "air impedance". Here's one paper discussing such a system, with an extremely detailed transfer function [4]. Maybe it's using another term for impedence, or something close? Edit: Considering this was cutting-edge control hardware for military equipment, I guess some lack of public details is to be expected. On the other hand, on Ebay there's a Fuel Control Unit for a J-79 engine as used in the B-58 Hustler [6]. [1] Yes, that Whittle. https://en.wikipedia.org/wiki/Whittle_Laboratory https://en.wikipedia.org/wiki/Whittle_Laboratory [2] https://www.mdpi.com/2504-186X/6/2/18 https://www.mdpi.com/2504-186X/6/2/18 [3] "Analytical modelling of flame transfer functions for technically premixed flames" https://journals.sagepub.com/doi/full/10.1177/17568277221094403 https://journals.sagepub.com/doi/full/10.1177/17568277221094... [4] "Complex Control System for an Aircraft Supersonic Inlet" http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS/SYSTEMS75.pdf http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS... [5] Just so you know, "b-58 engine computer" only brings up automotive stuff. [6] https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DITM%26aid%3D777008%26algo%3DPERSONAL.TOPIC%26ao%3D1%26asc%3D20230823115209%26meid%3D5af4cefff0304c96bd723aeb9765b577%26pid%3D101800%26rk%3D1%26rkt%3D1%26sd%3D325676156863%26itm%3D325676156863%26pmt%3D0%26noa%3D1%26pg%3D4375194%26algv%3DRecentlyViewedItemsV2SignedOut&_trksid=p4375194.c101800.m5481&_trkparms=parentrq%3A0c6fc5d618b0ab9711f1f22affff6958%7Cpageci%3Ae1b69473-6567-11ee-bb79-8eca912930b8%7Ciid%3A1%7Cvlpname%3Avlp_homepage https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DI...
- kens 3y ago
  Thank you for the detailed response! This is way more than I expected :-)
akira2501 3y ago
Seems also to be related to https://en.wikipedia.org/wiki/Rayleigh_flow https://en.wikipedia.org/wiki/Rayleigh_flow