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Exercise 600.2 — Turbofan performance

Hydrogen vs kerosene in a conventional turbofan cycle

🧪 Script
Turbofan_ker_vs_h2.py

Reference

This exercise follows the discussion in Section 6.2 — Effect of hydrogen combustion on cycle performance of the lecture notes.

Students are expected to be familiar with:

  • TSFC vs TSEC,
  • thermal vs propulsive efficiency,
  • the Brayton-cycle interpretation of turbofan engines.

How to run

From the script folder (chapters/600_hydrogen_combustion/scripts):

python Turbofan_ker_vs_h2.py

Learning objectives

By completing this exercise, you will learn to:

  • Compare turbofan performance across fuels with different energy densities
  • Understand why TSFC is misleading when comparing hydrogen and kerosene
  • Use TSEC as a physically meaningful comparison metric

  • Interpret how fuel properties affect:

    • exhaust velocities,
    • efficiencies,
    • turbine inlet temperature requirements
    • Distinguish mass-flow effects from thermodynamic effects

What the script does

The script compares a conventional turbofan cycle fueled with:

  • kerosene (reference),
  • hydrogen,

under three different comparison logics.

All cases assume:

  • identical engine architecture,
  • identical bypass ratio,
  • identical component efficiencies,
  • no unconventional cycle features (no intercooling, no recuperation).

Only fuel properties and resulting cycle states are changed.

Comparison modes implemented in the script

1) Same design parameters (same TIT, OPR, BPR)

In this mode:

  • turbine inlet temperature \(T_4\) is fixed,
  • overall pressure ratio and bypass ratio are identical,
  • the hydrogen engine is not retuned.

This isolates the pure effect of fuel substitution.

Observed trends (see the script output):

  • TSFC decreases dramatically (≈ −64%) due to hydrogen’s high LHV
  • TSEC increases slightly (+1.5%)
  • thermal efficiency increases
  • propulsive efficiency decreases
  • specific thrust increases

This confirms a key message from the notes:

Lower TSFC does not imply lower energy consumption.

2) Same TSEC comparison (energy-fair comparison)

In this mode:

  • hydrogen turbine inlet temperature \(T_4\) is reduced
  • TSEC is forced to be equal to the kerosene case

This answers the question:

What hydrogen cycle delivers the same energy efficiency as kerosene?

Key observations:

  • required hydrogen \(T_4\) is lower than kerosene
  • TSFC is still much lower (mass-flow effect)
  • overall efficiency becomes nearly identical
  • thermal efficiency remains slightly higher for hydrogen
  • propulsive efficiency is slightly lower

This result is central to the lecture notes:

For comparable Brayton cycles, TSEC is broadly similar across fuels.

3) Same specific thrust comparison (operational equivalence)

In this mode:

  • hydrogen \(T_4\) is adjusted so that specific thrust is identical
  • the engine delivers the same thrust per unit airflow

This corresponds to a practical engine matching condition.

Observed trends:

  • TSEC becomes slightly lower for hydrogen
  • TSFC remains much lower
  • thermal efficiency increases
  • propulsive efficiency decreases marginally

This illustrates that hydrogen can: - maintain thrust, - reduce turbine temperatures, - without improving energy efficiency dramatically.

Guided questions

1) TSFC vs TSEC

  • Why does TSFC drop by more than 60% in all hydrogen cases?
  • Why does TSEC change only marginally?
  • Which metric is appropriate for cycle efficiency comparisons, and why?

2) Exhaust velocity and propulsive efficiency

  • Why does the core exhaust velocity increase with hydrogen?
  • How does this affect propulsive efficiency?
  • Why does the bypass stream remain unchanged?

Relate your answer to the discussion of exhaust composition and \(c_p\) in the notes.

3) Thermal efficiency gains

  • Why does hydrogen show a systematic increase in thermal efficiency?
  • How is this related to:
  • gas composition,
  • turbine expansion,
  • heat capacity effects?

Explain why this does not automatically translate into lower TSEC.

4) Turbine inlet temperature as a design lever

  • Why can hydrogen achieve the same TSEC or thrust with a lower \(T_4\)?
  • What are the implications for:
  • NOx formation,
  • turbine blade life,
  • material limits?

Student tasks

Task 1 — Comparative table interpretation (core)

Using the script output, summarize in a short table:

  • TSFC
  • TSEC
  • thermal efficiency
  • propulsive efficiency
  • specific thrust

for:

  • same-design,
  • same-TSEC,
  • same-specific-thrust cases.

For each case, write 2–3 lines explaining the dominant physical mechanism.

Task 2 — Efficiency decomposition

For one comparison mode of your choice:

  • explain qualitatively how:

  • mass flow rate,

  • combustion chamber composition,
  • turbine expansion contribute to the observed efficiency changes.

You may refer to the Brayton-cycle interpretation from the notes.

Task 3 — Engineering interpretation

In 10–12 lines, answer:

  • Why does hydrogen not dramatically reduce TSEC in a conventional turbofan?
  • Why is hydrogen nevertheless attractive from a cycle-design perspective?
  • Which limitations of this exercise could be overcome only with unconventional cycles?

Limitations (important)

This exercise assumes:

  • fixed engine architecture,
  • no heat exchangers,
  • no inlet cooling,
  • no recovery of cryogenic fuel exergy,
  • identical component efficiencies.

Therefore:

  • results apply to conventional turbofans only,
  • they do not represent the full potential of hydrogen-enabled cycles.

Key takeaway

Hydrogen fundamentally changes mass flow rates and hot fluid properties,
but energy efficiency (TSEC) remains largely dictated by the Brayton cycle itself.

Meaningful gains require architectural innovation, not fuel substitution alone.