Jet engine performance: Difference between revisions

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Revision as of 20:43, 6 June 2024

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=== Increased stage pressure ratio ===

Air compression in a gas turbine is achieved by converting a proportion of the kinetic energy (compressor rotor generated, either by a centrifugal impeller or an axial row) of the air into static pressure one stage at a time. Most early jet engines used a single-stage centrifugal compressor with pressure ratios such as 3.3:1 ([[de Havilland Goblin]]). Higher pressure ratios came with the axial compressor because although stage pressure ratios were very low in comparison (1.17:1 [[BMW 003]])<ref>{{Cite journal |url=https://www.jstor.org/stable/44548294," |pages=503–510 [506]|jstor=44548294 |title=BMW-003 TurboTURBO-JetJET EngineENGINE Compared Withwith Jumo004",the JUMO 004 |last1=Lundquist and|first1=W. G. |last2=Cole, p|first2=R. 506W. |journal=SAE Transactions |date=1946 |volume=54 }}</ref> more stages could be used as required for a higher overall pressure ratio. More advanced centrifugal stages are used in small turbofans as the last high-pressure stage behind axial stages ([[Pratt & Whitney Canada PW300]] and others). The same technology level produces 8:1 when used as the only stage in [[Pratt & Whitney PW200]] helicopter engines.<ref>https://engineering.purdue.edu/~propulsi/propulsion/jets/tprops/pw200.html, PW206 8:1</ref> A centrifugal stage consists of an impeller and diffuser vanes,<ref>"The Jet Engine", Rolls-Royce Limited, Publication Ref. T.S.D. 1302, July 1969, 3rd Edition, Figure 3-6 'Airflow at entry to diffuser'</ref> or alternatively diffuser pipes<ref>https://patents.google.com/patent/US3420435,"Diffuser Construction"</ref> which are considered to give less blockage as the static pressure rises with diffusion.<ref>https://asmedigitalcollection.asme.org/GT/proceedings/GT1972/79818/V001T01A053/231014,"A Comparison Of The Predicted And Measured Performance Of High Pressure Ratio Centrifugal Compressor Diffusers", Kenney, p. 19</ref>

An axial compressor consists of alternating rows of rotating and stationary diffusers,<ref>https://archive.org/details/DTIC_ADA059784/page/n45/mode/2up,"All compression in engines requires a diffusion process", section 1.4.2.3</ref> each pair being a stage. These diffusers are diverging as necessary for subsonic flow.<ref>Supersonic flow is slowed in a converging duct as shown from the inlet lip to the shock trap bleed.[[File:J58 airflow at Mach 3.png|thumb|]]</ref> The channel formed by adjacent blades, amount of diffusion, is adjusted by varying their angle relative to tangential, known as stagger angle.<ref>https://ntrs.nasa.gov/citations/19650013744,"Aerodynamic Design of Axial-Flow Compressors", p. 126</ref> More diffusion gives a higher pressure ratio but flow in compressors is very susceptible to flow separation because it is going against a rising pressure (gas naturally flows from high to low pressure). Stage pressure ratio had increased by 2016 such that 11 stages could achieve 27:1 (GE9X high pressure compressor).<ref name="Dynamic Regulatory System" />