Brake Heat Rejection???
Brake Heat Rejection???
(OP)
Is there a need for enhanced brake heat rejection? There seems to be an attitude that heat rejection is fully mature with no room for improvement –not even a brake thread in this forum.
What I have is a cross discipline new means –additive to the existing convection/radiation mechanisms -for heat rejection that avoids the conventional heat sinking for delayed cooling. I’d be happy to go into more detail is there’s interest, but my question is essentially to the need and profile of the brake industry. My background is a bit eclectic but includes suspension enhancements and midlevel race engineering (shoestring but the team did win a TV race). But I have few present contacts.
Any thoughts?
What I have is a cross discipline new means –additive to the existing convection/radiation mechanisms -for heat rejection that avoids the conventional heat sinking for delayed cooling. I’d be happy to go into more detail is there’s interest, but my question is essentially to the need and profile of the brake industry. My background is a bit eclectic but includes suspension enhancements and midlevel race engineering (shoestring but the team did win a TV race). But I have few present contacts.
Any thoughts?





RE: Brake Heat Rejection???
Engineering is the art of creating things you need, from things you can get.
RE: Brake Heat Rejection???
I have no qualms about the tech and theory discussion. But I’m walking a bit of a fine line as a new poster as to spam. I’d rather do so with interest pull rather than push by me.
RE: Brake Heat Rejection???
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RE: Brake Heat Rejection???
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RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
Engineering is the art of creating things you need, from things you can get.
RE: Brake Heat Rejection???
Here is some ones engineering progress.
https://www.youtube.com/watch?v=c_ItwLcBw5Q
RE: Brake Heat Rejection???
I observed that a solid-rotor car I raced did not respond to convection cooling as it should. After a good bit of investigation the problem was determined to be a thick, tenacious boundary layer that insulated the swept faces. Air has the property of becoming more viscous with increasing temperature that resulted in the isolating boundary layer. Some idea can be gained from the telltale glowing bits in the ring of fire evident in a stressed brake. The high temperature swept area was insulated by this aggressive boundary layer.
Once the mechanism was determined it was exploited by placing a simple, fixed aerodynamic vane adjacent to but space outside of runout each swept area. The “tacky” boundary layer was continuously diverted, rejected and replaced with cool air each rotor revolution without adding to the rotor heat sink load. Brake dyno testing has confirmed the effectiveness of the concept. It does little at lower temperatures but becomes progressively more effective with increasing rotor/pad temperature.
I haven’t worked out just how the conventional pad affects the boundary layer but I suspect that the boundary layer largely flows over the pad/caliper and reattaches to the rotor in the conventional setup.
Two series of brake dynamotor test have clearly shown the concept is valid. There are material, mechanism and anomalies to be worked out but it would make sense to do so in the context of a particular application.
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
In that case, several vanes (scrapers) might help by allowing the flow of cooling air to reach the surface of the rotor.
I wonder if that's what the holes and slots in racing rotors actually accomplish?
Jay Maechtlen
http://www.laserpubs.com/techcomm
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
Of course, all this is totally apart from any internal venting, vanes, bumps, etc.
Jay Maechtlen
http://www.laserpubs.com/techcomm
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
Engineering is the art of creating things you need, from things you can get.
RE: Brake Heat Rejection???
http://www.f1technical.net/forum/viewtopic.php?f=6...
And a general discussion and more detailed description at;
http://www.f1technical.net/forum/viewtopic.php?f=6...
RE: Brake Heat Rejection???
The friction contact at the pad/rotor interface produces huge amounts of heat energy, and since the metal rotor has far better thermal conduction properties than the brake pad material, most of the friction heat produced is forced into the metal rotor. To improve the capacity of a rotor brake system you can either increase the thermal mass of the metal brake rotor, increase the surface area of the rotor exposed to cooling airflow, or make the rotor from a material with increased temperature capabilities like ceramic or carbon. Ultimately, with commercial vehicle applications, cost is usually the most important factor. And that's why cars still use cast iron brake rotors.
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
Just in case it was missed, the perhaps most significant advantage of what I’m doing is the heat rejection essentially real time and not adding to the rotor heat sink burden. This gets rid of much of the heat without affecting –other than heat load- the classical sinking/convection/radiation means by which brakes deal with heat.
Cost and cast iron are indeed driving factors. Iron is a bit iffy at the temperatures needed to drive radiant cooling since this is a fourth order Δ absolute T mechanism it can be quite effective at high rotor temps. Pony cars are practically begging for better brakes, though they’re much improved from the one stop wonders they were.
Aircraft brakes are a bit of puzzlement to me. As I understand the design carbon rotors and stators are stacked to gain swept area. But his would defeat much of the radiant cooling potential since the components would only “see” other similarly heated components. At such temps I can see giving up some volume to my vanes to reject the heat energy real time with essentially forced convection of the boundary layers. The high C-C temps would really make my vanes super effective.
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
Basically, heat is wear even for C/C brakes (low temperature can be bad too). If lighter brakes that ran less hot were feasible, it seemingly would be worthwhile.
RE: Brake Heat Rejection???
Or the other way around?
http://www.google.com/patents/US20060011425
Anyway, strip away the boundary layer of hot air, presumably replaced with cooler air. Effective for obvious reasons, but unremarkable.
RE: Brake Heat Rejection???
Under the new law the first to file –as opposed to the first to invent with actual or constructive reduction to practice-gets the patent. But theoretically the filer needs to be an actual inventor.
RE: Brake Heat Rejection???
High velocity air generated by blowers is the usual approach to getting high convective heat transfer. Your vane (scraper?) and rotor appear to be a unique blower design. I question how much mass flow of air you can achieve with a single vane and a rotor turning at wheel rpm. After all, a gram of air can only carry so much heat. However, the air in the boundary layer is actually higher than rotor temperature (since the heat is generated between rotor and pad), so that helps. It seems that multiple vanes would be an improvement and placing one right after the pad would be the most effective. It also seems that the same effect could also be achieved with an appropriate groove geometry in the face of the brake pad.
Whether your invention is a commercial success will be determined by whether the improvements in performance you offer are worth the costs and complexity. This is where many inventions fail. A patent is worse than worthless if you cannot make money with it. Most patents are merely a very expensive plaque on the wall.
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
The vanes are almost certainly aero devices that strip the superheated boundary layers and divert them as coherent flows. The cool air is essentially filling the resulting vacuum.
The effectiveness of the vanes is, from testing, clearly a function of the boundary layer temperature. I don’t know the actual temperature –need more testing- but based upon the glowing particles in the ring of fire it’s well north of what the T-couples in the pad or rotor report. At such temperatures the viscosity is many multiples of that of air at room temperatures. This enables the vanes to extremely efficient aero devices and allows the highly heated air to maintain its flow a distinct flow.
Most simplified calculations for aero air flow deal with inviscid flow. The highly viscous boundary layer yields result that is counterintuitive as indicated by my initial “best embodiment” having multiple vanes. Though perhaps not optimized as to position, at higher speeds one per side seems best.
RE: Brake Heat Rejection???
Brakes released, hot, standard config. Air is dragged through the thin rotor-pad gap. Viscosity roughly constant (high) so velocity profile linear from rotor speed at one side to zero at the pad side. Good heat transfer from pad.
Brakes released, hot, vane fitted. Air is dragged through the thin rotor-pad gap. The rotor and its freshly replaced BL are much cooler therefore low viscosity. Viscosity much higher on pad side so a very thick, slow, BL exists on the pad side. Velocity profile much steeper on rotor side. Poor heat transfer from pad.
Not sure if a similar process could occur with the brakes applied. The only area possible would be the microscopic regions adjacent to the contacting asperities.
You say the modification works "best" with the vane immediately before the calliper. Does this mean best rotor temperature or best combination of rotor and pad temperature? If the mechanism is as I suggested above, moving the vane would improve pad temperature. Immediately after the calliper may be too soon as the BL may not have built-up yet.
Engineering is the art of creating things you need, from things you can get.
RE: Brake Heat Rejection???
A good analogy is to think of two trains passing each other in opposite directions. If someone throws a brick to someone on the other train, the train receiving the brick will slow down slightly. If the brick is thrown back to someone else, the first train will slow slightly. Keep repeating the process fast enough and both trains will come to a stop. Momentum has been transfered between trains, but total momentum has not changed. The same analogy works for heat transfer if you substitute hot and cold buckets of water passing between tanks.
I'm saying that viscosity is not relevant to this discussion.
RE: Brake Heat Rejection???
Ambient viscosity = 0.018 Cp
800*C viscosity = 0.045 Cp
Link
Engineering is the art of creating things you need, from things you can get.
RE: Brake Heat Rejection???
Actually, I want the boundary layer to be laminar so that it can contain rather large amounts of heat energy thereby becoming more viscous so that he vane can effectively separate and direct it. These air thermoclines tend not to mix even as weather fronts so I expect that an air stream over 1000° F hotter than ambient would maintain its integrity. By rejecting the heat in an aerodynamically separated and directed stream of highly heated air, very effective heat transfer is possible. Each rotation would generate and reject a modest amount of heat energy –but there are a good number of rotations.
RE: Brake Heat Rejection???
Helium and hydrogen have much higher viscosity than air and also much higher thermal conductivity, for the reasons I explained. In closed systems, like sealed, high power electric motors, air is sometimes replaced with helium for better heat transfer. This would indicate that the viscosity of the gas is not by itself of overriding importance in gaseous heat transfer.
To "trip-up" is not a technical term so it isn't terribly useful in a scientific discussion. Perhaps a better way to say it is "to induce turbulence" into the boundary layer.
RE: Brake Heat Rejection???
I supplied two viscosity values for air. Plugging those into the calculator shows a difference in heat transfer coefficient. (12.7 vs 10.9 W/m^2.K)
Engineering is the art of creating things you need, from things you can get.
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
You were right - apologies.
Engineering is the art of creating things you need, from things you can get.
RE: Brake Heat Rejection???
http://www.engineeringtoolbox.com/dry-air-properti...
And there’s a good deal that I don’t understand about the subject heat-rejection mechanism. However, I have empirically observed that the boundary layer clings tenaciously to the rotor at high RPM and resists aggressive blasts of cooling air. Also, testing has shown very significant rotor temperature reduction relative to the same control conditions sans vanes, significantly more so with increasing temperature and viscosity. Most of the remainder of what I’m presenting is an attempt to explain the empirical data. But more testing is needed to better define the mechanism. But, if it continues to prove out, it is a significant new mechanism for brake heat rejection.
My original question as to whether technology has become so siloed and MBA-driven that the door is closed to innovation still stands. This should be important if not interesting to engineers.
RE: Brake Heat Rejection???
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RE: Brake Heat Rejection???
Have you looked at the air film cooling approach used with gas turbine engine nozzles? These nozzles use an array of tiny holes on the surface fed by compressed air to create a boundary layer of airflow that insulates the nozzle surface from the passing high-temp exhaust gas flow. This is a very effective use of a boundary airflow to minimize heat transfer to the metal surface.
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
RE: Brake Heat Rejection???
The vanes deal with the higher heat by rejecting rather than sinking the heat with increasing effectiveness as the rotor temperature increases thereby reducing the parasitic energy loss resulting from accelerating and decelerating the heavier rotors.
RE: Brake Heat Rejection???
Figure 9 on Page 97 here shows rotor temperature rise hitting the brakes in 10 second bursts, followed by 35 second cooling cycles. at a mere 65 mph
http://www.gt40s.com/images/howto/LynnMiner/Refere...
I did not see any mention of relative rotor masses.
The difference between the inboard and outboard surfaces after the first few stops is kind of interesting.
RE: Brake Heat Rejection???
While the results are qualitatively what I would expect, it was heartening to see that the solid rotor showed the same internal and external temperatures. This would seem to support my theory of the insulating high temp boundary layer. The experts have had a reluctance to buy in on this.
The inboard/outboard difference is likely due to testing with the wheel on, though I haven’t yet read enough to confirm this.
It’s interesting that NASCAR was having the same heat checking problems just a few years ago. But they seem to have solved it or stopped talking about it –probably the former since there doesn’t seem to be as many rotors rolling down the track lately.
Again, thanks, good info even from almost fifty years ago is very helpful.
RE: Brake Heat Rejection???
Norm
RE: Brake Heat Rejection???
The second test I can’t discuss because of a confidentiality agreement that was supposed to relate to anything I might see concerning race team testing. After the test the company turned hostile and refused any release of the results. The agreement probably doesn’t actually cover the data but the test was gratis and I really don’t much want to fight about it. But I know a good bit more than can be discussed.
Only the pad temps were measured and they ran a bit hotter with the vanes than without in the first test. Thus I think the brake fluid would also be a bit hotter. But there’s still a lot to learn about vane placement and pad material when the rotor runs substantially cooler than the pad. You want a transfer layer of pad material on the rotor to promote micro welding and shearing of the micro welds. But too much pad transfer could get out of hand if not managed.
While the pad temperature was a bit hotter the amount of energy involved is relatively small compared to a similar temperature rise in the rotor.
RE: Brake Heat Rejection???
These brake systems are designed for a single brake application that requires absorbing huge amounts of energy. The brake system uses multiple rotor/stator plates to provide a large thermal mass capable of absorbing the thermal energy created by braking the aircraft at landing. These C-C brakes don't require any real amount of air cooling.
RE: Brake Heat Rejection???
I can see why you'd place the vanes immediately leading the caliper or close to such a location - it takes a finite amount of time for the heat to flow from rotor to the air, and at 750 or so revs/mile and much road speed at all there isn't going to be a lot of time before the 'new' boundary layer air gets scraped off.
I understand the confidentiality aspects, though the refusal to release data in spite of such an agreement seems a little odd.
Norm
RE: Brake Heat Rejection???
NormPeterson, truthfully, I don’t know from the limited testing. My theory is the close-coupled vane is diverting the hot boundary layer and cooler air being carried with it. My testing was more concerned with extreme failure-certain conditions as a proof of concept. Refinement is still to be done for differing needs such as truck brakes that, with big wheels, low RPM would probably do well with multiple vanes per rotor side.
As to the agreement, I suspect this came from corporate when they got wind that the engineers were doing the testing.
RE: Brake Heat Rejection???
The vanes are an extremely effective way to disturb the boundary layer. Holes and slots are not as effective, but do blow or suck the boundary layer and provide some additional surface area (as well as sites for cracks to form, etc).
Your comment about the pads not benefiting from a cooler rotor is interesting. I think the pads should benefit from a cooler rotor. Is it the caliper-pad assembly or just the pads? You could use air ducted behind the vanes. This should make things better for the caliper and pad.
RE: Brake Heat Rejection???
Going back to the picture of the C-C aircraft brake I posted, you'll note there are no brake "pads" used. The caliper pistons simply bear against two outer stator discs that don't rotate. No provisions for cooling airflow anywhere on the rotor or stator discs. The amount of energy absorbed by these brakes is huge, and they provide extremely precise anti-skid performance.
RE: Brake Heat Rejection???
FWIW and I won't attempt to guess why, episodes of brake fade - either pads fading or fluid boiling - seem to be more frequent occurrences among track day enthusiasts than rotor failures. Rotor cracks that have been "encouraged" to start by drilled thermal discontinuities perhaps excepted.
Norm
RE: Brake Heat Rejection???
The hotter pads were measured by thermocouples and showed the difference with identical runs other than the vanes. For competition this could be addressed with ducted cooling air. For trucks or street vehicles it probably could be addressed with appropriate pad compounds.
Brakes used hard rely on a transfer layer of pad material on the rotor to develop optimum friction. The transfer layer develops a like-material frictional interface that favors the micro fusion of asperities between the rotor and pad that make and shear. This is workable with the normally modestly hotter pad material. However, when the pad gets overly hot, too much material transfers and presents the risk of welding larger portions of the pad to the rotor. It’s sort of the same but opposite of C-C brakes in which the like C pad and rotor materials bond more aggressively at lower temperatures to accelerate low temperature wear.
The anomalies I’m reporting should –till you’ve done so you haven’t- be readily solved with further development. However, the solutions will probably differ for competition and truck brakes, for instance.