Thermochemistry Overview
Thermochemistry Overview
? For chemical rockets, we have already
examined some aspects of propellant storage and ideal nozzle flow
Comb./Rxn. Chamber
? e.g., non-reacting gas
Te To
? But what can happen to propellant Storage
Nozzle
going through nozzle?
? e.g., NH3, N2, H2 products of hydrazine resistojet
? at To=2500 K, po=50 psi we might expect mostly
N2, H2 and H in the products ? ideal calculation of Te (for pe/po~10-3)
? will composition change at these
Te To 103 1
Te 500 700K
lower temperatures (less H, H2; more NH3)?
? will change MW, cp and Te
Thermochemistry Overview - 1 Copyright ? 2017 by Jerry M. Seitzman. All rights reserved.
AE6450 Rocket Propulsion
Thermochemistry Overview
? For chemical rockets, we have already
examined some aspects of propellant storage and ideal nozzle flow
Comb./Rxn. Chamber
? e.g., non-reacting gas ? Also, how do we determine
Te
To
Storage
Nozzle
conditions exiting combustion/reaction chamber ?
? most accurate calculation requires understanding flow field and finite rate chemistry (reaction rates)
? but is there approach that can provide a good estimate of To and composition (MW, cp, ...) ?
Thermochemistry Overview - 2 Copyright ? 2017 by Jerry M. Seitzman. All rights reserved.
AE6450 Rocket Propulsion
1
Equilibrium Thermochemistry
? Answer is yes ? Chemical Equilbrium ? For most rocket combustors
? the exit To and composition (major products) can be reasonably approximated assuming chemical equilibrium
? For nozzles ? the chemical equilibrium assumption provides an upper bound on the nozzle exit velocity and thus the rocket performance
? frozen flow (no composition change, very slow reactions) provides a lower bound
Thermochemistry Overview - 3 Copyright ? 2017 by Jerry M. Seitzman. All rights reserved.
AE6450 Rocket Propulsion
Our Problem: Combustor Example
? What is general form of chemical equilibrium problem for rocket combustor/reactor? Qloss 0
Reactants
m ,
p,
T,
i
Products
m? ,
? p,
? T,
? i
cp , MW
? steady ? e.g., apply mass conservation (m out m in) ? often near constant pressure internally (pout pin) ? often also assume adiabatic
? So generally we need to do find ? product composition (mole or mass Y fractions) ? (adiabatic) product (flame) temperature (e.g, To )
Thermochemistry Overview - 4 Copyright ? 2017 by Jerry M. Seitzman. All rights reserved.
AE6450 Rocket Propulsion
2
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