exFan aims to investigate and develop a concept for a novel thrust generating heat dissipation and recuperation system, called heat propulsor. This goal assumes that there is about 50% of waste heat of a fuel cell which needs to be dissipated and can be used to produce additional thrust. The underlying concept of the goal is the so-called Meredith effect, which occurs when air flowing through a duct is heated by a heat-exchanger or radiator in forward flight. The duct is designed in a way that the air flowing into the duct meets drag resistance from the radiator surface and is compressed due to the ram air effect. When the air flows through the radiator it is heated and increases its volume. The hot, pressurized air then exits through the exhaust duct, which is shaped to be convergent, i.e. to narrow towards the rear. This accelerates the air backwards and the reaction of this acceleration against the installation provides a small forward thrust. To design such a device for a fuel cell powered geared electric fan propulsion system of mega-watt class and include it inside the propulsion system is one of key milestones of the exFan project.

The "Meredith effect" explained

The Meredith effect (ME) allows to neutralize/counteract the drag created because of the cooling radiator/heat exchanger, containing a hot working fluid, by generating thrust due to added heat to a flow with elevated pressure level.

Required Conditions:

  • The duct must be moving at a significant speed relative to the air for the ME to take place.

Airflow Dynamics:

  • Air entering the duct faces drag resistance from the radiator surface.
  • Compression occurs due to the ram air effect as the duct travels through the air.

Exhaust Duct Design:

  • Convergent duct accelerates and directs heated, pressurized air backward, creating a small forward thrust.

Cycle Processes:

  • Achieves compression, constant-pressure heat addition, and expansion (open Brayton cycle).

Thrust Generation:

  • Depends on pressure ratio and coolant temperature.
  • The higher boiling point of the working fluid increases specific thrust.

Net Aerodynamic Drag:

  • If the generated thrust is less than the aerodynamic drag of the ducting and radiator, it reduces the net aerodynamic drag of the installation.
  • If the generated thrust exceeds the aerodynamic drag, the entire arrangement contributes a net forward thrust to the vehicle.

And today, it's used as the basis methodology for the exFan project.


Thrust vs. Drag
  • trade-off between thrust and the drag
  • high heat transfer and low pressure losses
  • particle accumulation -increase pressure losses and decrease heat transfer
  • demand on high mass flow
Operation conditions

Optimum different to standard aircraft

  • aircraft layout
  • propulsion system layout
  • flight envelope (speed and altitude)

Heat quality
  • Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, work at low temperature (low heat quality)
  • Heat exchanger (HX) rate depends on temperature difference
  • High heat quality is needed for compact and lightweight design
  • Further development needs justification
  • Validation of technical & environmental parameters
  • Validation at low TRL is required (low TRL simulation)
  • Integration of system in aircraft needs to be considered
  • Clean aviation projects investigate configurations, architectures, etc.
  • Further development needs to use synergies

Work structure