In the beginning, a problem was noted with an under-braked race sedan sporting solid front rotors/rear drums that showed no discernable rotor improvement when outfitted with slipstream cooling air in accord with Carroll Smith’s instructions. This was in marked contrast to results with similar cooling flows to the interior of vented rotors. In practice, rotors function as heat sinks storing heat energy during the relatively low duty cycle brake event and rejecting it longer term. But why is the swept area of rotors inefficient as cooling areas?
Cutting to the chase, after a good bit of study and observation, two factors were noted. First, certain idiosyncrasies of the rotor ring of fire were observed. While the red-hot rotor swept area surface was prominent, close inspection also revealed a rather thick layer proud of the surface as marked by telltale minute incandescent particles. It also was clear that this layer was tenaciously adhered to the rotor surface in that the rotor was spinning at high speed with no indication of centrifugal force detaching it. Clearly attempts to do so by blowing air at it were much too feeble. But why?
After poking around in some faintly remembered formula for boundary layer characteristics, and recalling an odd (to me) property of air, I cobbled together a theory.
From calculating the appropriate exhaust header length for a Wankel -which runs a fair amount hotter than a normal engine- I had found that air becomes more viscous with temperature. Put simply, the heated, viscous air in the ring of fire was in a shear relationship with the cool, less viscous ambient air. The hot viscosity alone would lead to a thickened insulating boundary layer. The adjacent less viscous air amplified the boundary layer thickness. Problem identified. What to do?
Learning from a good number of blind alleys, I decided to go with the boundary layer rather than fight it. By positioning a fixed aero vane closely adjacent the rotor, it intercepted, detached at least a potion of the boundary layer and redirected it. Since this was all theory, there was confidence that the positive pressure side of the vane would be effective. However, the convex side of my preferred vane facing the rotor seemed a bit iffy. Given that this surface was acting on a boundary firmly attached to the rotor rather than a free air stream, the effectiveness of the Candela effect was a question.
Initial testing was done on a rig comprising a Ford Interceptor secured to an alignment rack with the rear wheels removed and vanes on one rotor. The wheel speed was set with cruse control and the brakes were applied manually. This was a bit scarier at speed than anticipated, but observation of the vane showed the telltale incandescent particles in a plume detached by the vane. Though a qualitative validation, it was enough to move on from our ad hoc brake dyno.
Next we bit the bullet and signed up for two days of dyno time in Detroit. Many were the trials of this effort. Ambient cooling air varied 30 degrees F between the control and test runs. The rather wimpy standard test routine didn’t challenge our performance rotor/pad setup in that the boundary layer thickness is a function of temperature. The data hinted at a cooler rotor with the vane and –perhaps an anomaly- somewhat higher pad temperatures. Fortunately, after the standard routine, the dyno consultant suggested a “burn down” run by clamping down the pads at high speed. This was supposed to be measured in seconds until failure. However, after several minutes he called the test off since it was stressing the dyno.
http://youtu.be/kHrP7gxBJ_Y
A conversation with the research director of a leading brake company during a SEMA show brought the opinion that, since radiant cooling is the dominant heat rejection mechanism for heat-stressed brakes and has been for over a century, it was improbable that convective cooling could be effective. However, my explanation of the theory gained an invitation to test at their facility –under a strict confidential agreement since race team stuff was there that we shouldn’t see. As per usual, there was a computer glitch while I was there and the testing was done later. Can’t comment on the “confidential” details, though I’m most grateful for the dyno time and will religiously respect the nondisclosure agreement.
The present status of the project is that the concept has been successfully demonstrated. Rotor temperatures are significantly decreased by the vane relative to controls. Heat rejection by the vane becomes progressively more effective with higher temperatures as the rotor boundary layer becomes more viscous. Vane spacing from the rotor can be substantial, i.e. beyond runout, as a result of the Coanda effect. The rotor can be kept at relatively cool temperatures under extreme conditions.
However, the testing developed one negative. The pads run hotter with the vane than in the vaneless control. I had expected the cooler rotor to sink heat from the pads by conduction. However, heat from friction appears to flow to the pad essentially independent of the rotor temperature. Further, it appears to me that the hot boundary layer also carries a significant volume of adjacent cooler air that may normally cool the pads, but which is diverted with the hot air by the vanes. There may not even be a pad problem with less than 100% “burndown” braking duty cycles tested, but this hasn’t been established yet.
A vane can be positioned adjacent a drum swept surface as well as a rotor.
Possible Applications and concerns;
F-1 is a tough application in that the carbon/carbon brakes apparently have exaggerated wear at lower temperatures. The advantage and problem of these brakes is that great quantities of heat are rejected as radiation at the temperatures of 1600°-1800° F. The radiant heat is absorbed by tires, wheels etc. It’s interesting that during a discussion of possible aero use of hot brake air plumes, the knowledgeable Aussiegman expressed concern about ducting materials withstanding such hot air. The same quantities of energy are turned loose within the wheel now. So, big obstacles, possible big, significant advantages.
NASCAR seems to be a natural for vane use with heavy cars, cast iron disks and copious forced convection air available for the pads. While rotor heat checking seems to be better controlled recently, it is unclear if the brakes still have to be rested at say Texas Speedway or on the short tracks.
Aircraft brakes are a bit of puzzlement as to heat. Given the velocities and weight, alternating carbon rotors and stators are apparently used to increase contact area. But this minimizes both cooling air and radiating surfaces that see the outside world. Thus much of the heat remains in the brake which can be troublesome if going around –though if the brakes are fully heated there may not be airspeed for a go around. Still a dragging brake on takeoff could put a lot of energy in a wheel well (as could a soft tire). A Mexicana 737 had a tire explode in its wheel well from brake heat, and there have been a number of other wheel well fires. Rejecting the heat upon generation could well be advantageous, though it would require a blank sheet of paper approach.
Trucks with less- massive drum brakes -relative to rotor brakes- often overheat on long grades are another candidate for vanes that would be of increasing effectiveness as temperatures rise. In the US, trucks are dealing with requirements for shorter stopping distances. Since heat rejection would appear to be the limiting factor in attaining the new standards, this is a new area of interest.
Obviously, my in-depth knowledge of the various brake applications, idiosyncrasies and requirements is limited. Critical comment is invited.
