Chap 3 : Fundamentals & Thermodynamique Cycle Flashcards
Eq of state for an ideal/real gas and R value ?
(Given in the formulary)
Ideal : PV=RT=P/rho
Real : PV=ZRT (Z=compressibility factor)
R air = 8.314/molar mass = 287 J/(kg.K)
Name existing process/transformation ?
Polytropic : PV^n=cst
Isochoric : V=cst & n->infinit
Isobaric : P=cst & n=0
Isothermal : T=cst & n=1
Isentropic : S=cst & n=k => isentropic relation
Caloric equation of state for an ideal gas ?
(Given in the formulary)
dh=Cp(T)×dT
du=Cv(T)×dT
1st & 2nd low of thermodynamics ?
(Given in the formulary)
specific heat (q) + specific work (w) = delta specific total enthalpy between in and out
Delta entropy = entropy exchanged + entropy created >= 0
What’s the specificity of a polytropic change of state over a isentropic change of state on a T-S graph ?
For compression & expansion
For compression :
Enthalpy of an isentropic transformation is straight upward
Enthalpy of a polytropic transformation is right upward
For expansion :
Enthalpy of an isentropic transformation is straight downward
Enthalpy of a polytropic transformation is right downward
!!! Always start and finish at the same pressure !!!
What are the 4 change of state in an ideal Joule-Brayton cycle in a h-s diagram
1) Isentropic compression (specific work from compression)
2) Isobaric heat addition
3) Isentropic expansion (specific work from compression + net specific work = specific work from turbine)
4) Isobaric heat removal
What’s the stagnation pressure and how is it measured ?
For incompressible flows :
Stagnation pressure = total pressure for most of gases = static pressure + dynamic pressure
It’s measured with a Pitot flow probe
Differences between real & ideal cycle ?
1) not an isentropic process (slight specific entropy value)
2) not an isobaric process (specific pressure before CC slightly > to specific pressure after CC)
3) same than 1) & the net specific work for a real cycle is < to an ideal cycle
