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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