《td_lec10_2016》.pdf

Turbomachinery Design – Lecture 10 Multi-Stage Compressor Characteristics Compressor System Instabilities Prof. Reza S. Abhari GE90 HP Compressor Surface Pressures • Full Engine Calculation done by NASA: Nacelle inlet – 49 blades rows – exhaust nozzle • 3D, but average passage flow code Thursday, November 22, 2012 Laboratory for Energy Conversion 2 Limits on Diffusion • Boundary layers will only undergo a limited amount of diffusion before the boundary layer separates • Diffusion factors of 0.6 or more imply blade stall (design choice around 0.45) • At high levels of diffusion, separation grows, and no longer reattaches (aerofoil stall) • When one or more blade rows stall: the efficiency of the machine degrades significantly. More importantly the stability of the machine may be compromised. • Axial compressors are less stable than centrifugal compressors Thursday, November 22, 2012 Laboratory for Energy Conversion 3 Compressor Instabilities Compressor stall whilst in reverse thrust Thursday, November 22, 2012 Laboratory for Energy Conversion 4 Compressor Map o Increasing the back pressure P by i 2 t a throttling means R e