Contrails ! Design of a radiator duct

 

La Pierre Skunk Works

During the design of the cooling radiator system on the MCR 4S, we had not much to start from. Once the radiator core was defined, we still had to design a duct or tunnel, as efficient as possible, while keeping cooling drag to a minimum.

Our most valuable sources of information : the NACA reports in the references, Küchemann & Weber's Aerodynamics of propulsion and Michel Colomban's advice.
Add to that S. Hoerner's book on aerodynamic drag.

Very few recent papers apart from Miley's articles. Many contemporary authors are just repeating what they have read, or inventing things.

With no means to simulate or compute, I had to take measurements and visualize. The experiments took place from 2003 autumn through 2004 spring, in a secret facility in the French Alps.

•   La Pierre Skunk Works ?
•   Cooler block characterization
•   Wind tunnel experiments
•   Excerpts of test report #14

La Pierre Skunk Works?

A secret lab

 

The code-name hides... my father-in-law's barn, where were secretly conducted the radiator duct experiment and the building of Lorem Ipsum.

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Cooler block characterization

 

The first step consists of measuring the resistance coefficient of the cooler block. To this end, air is flown through an element of the unit, and the pressure drop (Δp) between entry and exit is measured, as well as the entry dynamic pressure (q= ρV²/2).

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The setup resorts to PVC pipes, pressure pickups are made of brass tubing.
The air stream is provided by a woodworking dust collector.

Those measurements allow us to compute our cooler block resistance coefficient C = Δp/q. Experimenting with the actual core eliminates the need for scale effect corrections.
Within the velocity range of our setup, the resistance coefficient does not vary significantly.

With this parameter, we can readily size the diffuser opening.

The core permeability is difined by λ = 1/√(1+C).
If the air intake area is greater than or equal to 1,5×λ - 0,5×λ², one finds that the mass flow in the tunnel and radiator block practically depends only on the exit area.
One can thus vary the mass flow across the radiator, through the use of an exit cowl flap.

The outer part of the radiator fairing can be designed by eyeball, to insure a "nice curve" (Michel Colomban), with a sufficient inlet lip radius to avoid external flow separation.

It is now possible to design a standard diffuser according to the articles in the reference section.
Litterature offers some predefined mathematical curves for the diffuser walls, intended to eliminate flow separation.

The result will be acceptable, without ensuring optimum efficiency.
This can suit a homebuilder with no further in-depth experiment capacities.

•   Reference : Bréguet and Küchemann
•   Reference : NACA reports

Wind-tunnel experiments

 

Following Michel Colomban's advice, I constructed a wind-tunnel to design an air intake closer to the optimum, but with less calculations.

It has been established that by designing the diffuser walls according to the radiator-block streamline shape, no flow separation will occur.

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First test of the radiator.
One can see that most of the airstream flows around and past the radiator core.
The picture gives only an indication, as the string tension distorts the streamline visualization. A correct jet exploration requires a shorter and thinner yarn.

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The radiator block is installed between walls in the wind-tunnel for two dimensional tests.

Hot-wired inlet lips smooth the airstream, at the cost of a slight loss of mass-flow. Measurements show that streamline shapes are independant of the flow velocity.

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Installation of a small fence below the core, in order to stabilize the flow at this location.

The test section is equipped with static pressure pickups at different places, and one pitot head at the face of the radiator core.

Stick-and-yarn plotting of the streamline corresponding to the lower part of the block. The plotted curve will be used to shape the diffuser wall.

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The results are transferred to the engine setup plans. This wall shall be the diffuser "ceiling" to clear the exhaust silencer. Small local adjustments are necessary to eliminate sharp turns at the radiator face.

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Next steps

A ceiling corresponding to the plotted curve will be installed in the test section, and the procedure will be repeated for the new lower streamline, as the presence of the curved ceiling affects the airflow.

Tests with different ceiling/floor and lip combinations, with some local modifications to improve airflow near the radiator face. Cross tests with some "area rule" variants.

 

Excerpts of the test report N°14

 

 

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