ME 440 Aerospace Engineering Fundamentals

Transcription

1 Fall 006 ME 440 Aerosace Engneerng Fundamentals roulson hrust Jet Engne F m( & Rocket Engne F m & F ρ A - n ) ρ A he basc rncle nsde the engne s to convert the ressure and thermal energy of the workng flud nto knetc energy. here are a varety of dfferent roulson systems. We begn by subdvdng them nto those that are ar breathng and those that are not called rocket engnes. In ar breathng engnes the oxygen for the combuston rocess s rovded by the ambent envronment, whle for rocket engnes, the oxdant s stored wthn the craft as s the fuel. Often rocket engnes roel crafts that fly n ambent condtons that have nsuffcent oxygen n ther atmoshere. Basc hermodynamcs For an deal gas wth a constant secfc heat, we can wrte h - h c ( - ) - s c ln( / ) - Rln( / ) s Work Devces urbnes, Fans, and Comressors Ideal assumtons: no heat transfer (adabatc) no entroy change n the flud (sentroc) urbnes extract energy from the flud by work Fans and comressors add energy to the flud by work. Inlet temerature and ressure are assumed known and the ext ressure s secfed.

2 ME 440 Aerosace Engneerng Fundamentals Fall 006 Isentroc assumton - s c ln( / ) - Rln( s / ) ( γ ) / γ c γ cv Ideal secfc work w h h c ( ) w deal c Ideal ower s then W & m& deal w deal ( γ ) / γ Effcency, η W & act ηw& deal for a turbne W& deal W& act for a comressor or fan η Actual ext temerature W& act,act for a turbne cm& W& act,act + for a comressor or turbne c m& Heat Devces Burner, Combuston Chamber, and Afterburner Ideal heat devce s a constant ressure Inlet temerature and ressure are known Ether the heat transfer or the let temerature are secfed Secfc heat transfer q h h c ( ) Actual heat devce ressure dro s ether calculated or secfed, or a ressure rato s secfed. 0

3 ME 440 Aerosace Engneerng Fundamentals Fall 006 Flow Devces Jet es, Byass Legs, and Mxng anks Ideally adabatc and no drag losses Contnuty equaton ρ A ρa Bernoul s equaton ρ ρ + + Mxng tank Conservaton of mass m& + m& m& byass Conservaton of energy m& c + m& c m& byass byass Conservaton of momentum A + A byass byass (recall that we assume the nternal veloctes to be zero) Actual devces can have heat transfer and drag losses Use heat transfer models resented from ME 40 Calculate ressure loss from a frcton factor. Flow Devces Nozzles and Dffusers Knetc energy converters Dffuser converts flud knetc energy nto ressure/thermal energy Nozzle converts flud thermal/ressure energy nto knetc energy Ideally adabatc and sentroc Conservaton of energy s wrtten: h + h + c + c + Inlet temerature, ressure, and velocty are known Insde the engne we assume that knetc energy s neglgble ake the velocty at the let of a dffuser to be zero Nozzle assumes comlete exanson, reaches ambent ressure A c 3

4 ME 440 Aerosace Engneerng Fundamentals Fall Isentroc condton γ γ ) / ( Ideal dffuser ) ( c c + ) /( γ γ Ideal nozzle ) ( c γ γ ) / ( / ) / ( c γ γ Actual nozzles and dffusers Nozzle Effcency: h h h h a a N η Dffuser ressure Coeffcent: K a Actual nozzle ) ( c a

5 ME 440 Aerosace Engneerng Fundamentals Fall 006 rocedure of Cycle Analyss We must now consder a methodology that wll allow us to aly our thermodynamc devce models to analyze ths comlcated system.. he lant lay s sketched as a block dagram. he devces or rocesses are laced and connected accordng to the lant descrton.. he nodes between the devces or rocesses are numbered. hese nodes reresent the varous states of the workng flud as t asses through the cycle. 3. A table s constructed wth the followng headngs: Assumng an deal gas as the workng flud n a jet lant usng the constant secfc heat equatons Node &m 4. Wth the gven oeratng condtons and lant descrton, all known thermodynamc nformaton s entered on the table. 5. he system s traversed, devce by devce or rocess by rocess, analyzng the flud as t asses through each devce or rocess. hs analyss rovdes addtonal flud roertes, whch when used n conjuncton wth ste #5 systematcally comletes the table. 6. If there s nsuffcent nformaton to fx the state at the node, contnue on to the next node. After the cycle has been traversed, reeat the traversng untl the state at each node s fxed. 7. Wth the comleted table, lant nformaton (such as thermal effcency and work roduced) s calculated. 5

6 ME 440 Aerosace Engneerng Fundamentals Fall 006 erformance arameters hrust: Ft m( & ext nlet ) Secfc hrust: ft (ext nlet ) roulsve ower: W& m( & ext nlet ) roulsve Effcency: W& η Q& n Secfc Fuel Consumton: m& fuel (FA)m& ar SFC F F Ar Breathng Engne yes Ar Breathng Engnes t t arcraft (FA) f t Internal Combuston Engnes Gas urbne Cycle Engnes Constant olume Engnes ston Cylnder Engnes Rotary Engnes urbojet urbofan ulse Jet Engnes (-) Ram Jet urboro 6

7 ME 440 Aerosace Engneerng Fundamentals Fall 006 urbojet Engne After- Dffuser Comressor Burner urbne burner Nozzle he afterburner s otonal. urbofan Engne Inlet Byass Afterburner Dffuser Fan Comressor Combuston Chamber urbne Afterburner Nozzle Exhaust Ether afterburner s otonal Fan may oerate only on byass ar he byass rato s defned as m& byass β m& Recombnng of the two ar streams Searate exhaust streams (no mxng, each wth ts own nozzle) F t F t,byass + F t, F m& ( - ) + m& ( - ) t byass,byass n, Combned exhaust streams (n a mxng tank, tycally a jet e) m& + m& m& byass m& c + m& c m& A byass byass byass byass + A A After mxng use standard nozzle analyss c n 7

8 ME 440 Aerosace Engneerng Fundamentals Fall 006 Ramjet Engne ressure ncrease derved through the ntake of ar caused by the forward movement of the arcraft, the ram affect. Large arcraft velocty requred (above 400 mh or a Mach number of 0.5) No longer ncomressble flow and must account for comressblty effects. Use of Stagnaton of otal roertes Industry uses stagnaton roertes t, + c ρ t, + Ratos of the statc values t, + c t, ( γ t, + -)M ( γ -)M + γ/( γ-) Ratos of the total roertes across a devce τ for temerature π for ressure 8

9 ME 440 Aerosace Engneerng Fundamentals Fall 006 Ratos for deal devcese Ideal Devce τ π Dffuser Comressor ( γ) / γ πc π c Fan ( γ) / γ πf π f Burner τ b urbne τ t γ /( γ) τ t Jet e Afterburner τ ab Nozzle Rato of the total quantty to the statc value of the roerty t θ amb t δ amb Rocket Engnes Ambent ar s not used as art of the roulson system hrust s created by the exulson of mass hree grous chemcal electrc nuclear Chemcal Rockets wo grous Lqud roellant Oxdant Suly ank Oxdant um Combuston Chamber Nozzle Fuel Suly ank Fuel um 9

10 ME 440 Aerosace Engneerng Fundamentals Fall 006 Sold roellant Sold roellant Combuston Chamber Nozzle Chemcal rockets requre a combuston analyss 0