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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?

RE: Brake Heat Rejection???

I am sure you will get plenty of interest here if you want to discuss your concept. With the exception of trucks, brake fade is rarely a problem in road use. Heat rejection will become even less an issue with the emergence of brake energy recovery.

Engineering is the art of creating things you need, from things you can get.

RE: Brake Heat Rejection???

(OP)
Agreed as to normal street use and energy harvesting. However, there would seem to be an opportunity even on these vehicles to lighten the rotor by decreasing the heat sink load. Class 8 trucks (new distance requirements) and competition vehicles would seem to be the more likely. My take is that the brake suppliers serve various users.

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???

This site considers it spam only when you are massively posting in multiple forums. Staying in this one forum should alleviate any concerns with regard to spam. Since you don't appear to be, per se, selling anything, at least, not right now, you are not infringing on other site rules.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

RE: Brake Heat Rejection???

Brake fade may not be a real life problem in some countries, but when towing 1 tonne trailers behind family cars it is an issue. Every manufacturer I have worked for has a brake fade test, and passing it involves non-trivial compromises with the rest of the pad formulation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Brake Heat Rejection???

I've overheated brakes when going downhill in mountainous areas with a trailer in tow. This was on a car with 4 wheel disks and no trailer brakes. You can smell brakes constantly on many roads like that, so yes, it can be an issue!

RE: Brake Heat Rejection???

I will kick off the discussion with an observation. Much of the mass of a conventional rotor exists to maintain dimensional stability/prevent distortion but also to act as a heat sink to store heat energy produced by short duration, high power braking events. Dissipation by convection and radiation is a much lower power process. Videos I have seen of rotors in action clearly show that the high temp "glow" is primarily a surface effect and dims rapidly as heat transfers to the core metal of the rotor. The "glowing skin" effect must present an opportunity for rapid heat rejection during that event - especially radiation which is proportional to T^4.

Engineering is the art of creating things you need, from things you can get.

RE: Brake Heat Rejection???

So if a person gives out a patentable idea on here can others take the credit? I do have an idea for this brake heat rejection issue.
Here is some ones engineering progress.
https://www.youtube.com/watch?v=c_ItwLcBw5Q

RE: Brake Heat Rejection???

(OP)
Some of this is a bit counterintuitive so keep an open mind.

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???

Isn't this what brake cooling ducts do? By forcing cool air onto the rotor it displaces the hot air. Several guys on the Focus ST forum that autocross have devised wings similar to a Porsche to divert air onto the front rotors. For those unfamiliar with the Focus ST, Ford uses the front brakes (it is FWD) as a pseudo limited slip by braking a spinning wheel, so these cars can really heat the front brakes.

RE: Brake Heat Rejection???

You're saying that at high temperatures, the boundary layer is tenacious and insulates the rotor surface from an ordinary flow of cooling air?
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???

Heated brake pads used to off gas and that would try to push the pads off the rotor; holes and slots were made to address pad gassing. Newer pads don't off-gas so much, so holes and slots are primarily for looks while wearing out the pads faster.

RE: Brake Heat Rejection???

(OP)
I’ve tested with multiple vanes with one per swept area working best, at least at high speeds. Understand that the subject vanes are fixed adjacent the moving rotor. Not much in common with internal rotor vanes.

RE: Brake Heat Rejection???

Only one vane - presumably about 180 degrees from the caliper assembly?
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???

(OP)
At higher speeds one vane per swept area located almost immediately leading the caliper works well. At lower speeds the boundary layer would have more time to heat and it perhaps would be better to have more –I’m thinking of a truck with load descending a long grade here. The work hasn’t been done to optimize the concept for varying conditions.

RE: Brake Heat Rejection???

All sounds feasible except "I suspect that the boundary layer largely flows over the pad/caliper and reattaches to the rotor in the conventional setup". Am I missing something here? Are you suggesting a pad-rotor gap (brakes not applied) for the BL to sneak through or a "homing" BL that will do the seemingly impossible to get back to the rotor?

Engineering is the art of creating things you need, from things you can get.

RE: Brake Heat Rejection???

(OP)
Gruntguru, the operative word is “suspect” in that I’m inferring from faint evidence and need to do more testing to find out one way or the other. However, I have observed that, while the rotor cools nicely with the vane, the pads run a bit hotter. I had expected the cooler rotor to conduct heat energy from the pads. A fuller discussion of this can be found at;

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???

When air cooling a metal brake rotor surface, the heat transfer primarily occurs at the boundary layer, as noted by Overrun. And it is well understood that the heat transfer rate can be increased by tripping up the cooling air boundary layer flow at the rotor surface. But due to factors like temperature limits of metals used for brake rotors, there is still a practical limit to the amount of heat transfer that will occur with this set up. The passing air and rotor surface will always have a certain temperature delta, and there is only a given amount of time available for the heat transfer to take place before the rotor surface passes back under the brake pad.

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???

"Breaking up the boundary layer" finally clicked for me; I think another way to look at this is that the vanes introduce turbulence. Since the heat transfer mechanism from the rotor to the air is largely through convection, the turbulence makes the heat transfer more effective.

RE: Brake Heat Rejection???

(OP)
Tbuelna, I’m not sure as to what is meant by “tripping up”. What I’m doing is probably not tripping so much as aerodynamically stripping and conducting the coherent boundary in a number of possible directions. The high Reynolds number of the boundary layer makes a great difference. Most experts I’ve consulted with thought the pad would essentially disrupt the heated boundary layer and thus provide a good bit of the advantage of my rig. Not so. For some reason the pad runs a bit hotter with my gizmo when I would think that detaching the boundary layer would shield the pads from the ring-of-fire blast as well as seeing a cooler –by a good bit- rotor. I agree that the initial pad/rotor friction is almost a singularity with regard to the heat energy generated in essentially two dimensions. T-piles buried in the rotor can be misleading in this regard.

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???

Hopefully aircraft brakes only see one large heat loading event per flight. Very few people I know try to autocross or drive a mountain road with an airplane. And one of my sports car buddies is an airline pilot but he always brings the ST when we go cruising.

RE: Brake Heat Rejection???

(OP)
TheBlacksmith, brakes are not much tested in most instances. Aircraft brakes have a difficult duty cycle. Aborting a takeoff at V1 may well toast them rather well as can just taxiing in that engines have pretty good thrust even then. Retracting hot brakes into a close wheel well is problematic –I recall an exploding tire though it would seem that pressure relief valves would avoid that.

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???

Quote (dicer)

So if a person gives out a patentable idea on here can others take the credit?

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???

(OP)
My patent –though the development has come a ways since the pretesting patent disclosure.

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???

Here is my thinking about your concept. Convective heat transfer is always limited by the boundary layer. The problem is to get cold air to actually contact the hot rotor surface at as high a rate as possible. This cannot happen without the warmed air being removed at an equal rate. Mixing of cold and warm fluid will reduce heat transfer, and so should be minimized.
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???

The heat transfer process between the hot, conductive metal rotor surface and the thin boundary airflow layer, that has a tendency to remain attached to the rotor surface, comes to a halt very quickly once the temperature of the cooling air boundary layer becomes close to that of the rotor surface. Once this happens, the hot air of the attached boundary layer simply acts as an insulator. And there is very limited heat transferred between the boundary layer air and the adjacent core air flow. By tripping up the boundary airflow layer, the cooler core airflow will come into direct contact with the hot rotor surface, increasing the heat transfer rate from the rotor to the airflow.

RE: Brake Heat Rejection???

(OP)
Tbuelna, maybe I can clear up a couple of points. My definition of tripping up the boundary layer is inducing a turbulent boundary layer by such as a vortex generator. This induces turbulence (vortices) between the shearing layers in laminar flow boundary layer. My best results -with admittedly limited testing- were gained by placing the vanes immediately leading the caliper, i.e. after an almost complete rotation after the calipers. There’s no time for the boundary layer to react to tripping at this point.

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???

Hi Overrun. I checked the thread on F1 Tech. Interesting issue with the modification causing cooler rotor and hotter pads. Here's one possibility.

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???

I think that the issue of air viscosity is being misunderstood in this thread. Much of what Chemical Engineers are taught is divided into three categories: mass transfer, heat transfer, and momentum transfer. Most people think of viscosity as a type of friction, but it is mainly momentum transfer between surfaces moving a different rates. In liquids there is a strong attraction between molecules, which lessens with increasing temperature. Thus, viscosity decreases. In gases there is no attraction between molecules. When a gas molecule collides with a moving surface, it will rebound with a small amount of momentum it picked-up from the moving surface. This places a drag on that surface, which is what we call viscosity. At higher temperature, gas molecules have higher velocity, and thus the rate of collisions with the surface increases. This is why gas viscosity increases with temperature. For exactly this same reason, thermal conductivity of gases also increases with temperature.

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???

Viscosity effects reduce convective heat transfer of air at 800*C by up to 15% compared to ambient air.
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???

(OP)
Compositepro, the last step of your argument isn’t clear to me so I can’t refute it. However, what high viscosity does do is promote a laminar flow boundary layer. This limits heat transmittal to simple conduction. What Tbuelna suggests (I think) above is “tripping up” the laminar boundary lay to induce turbulence and enhance the heat transfer in local intra-boundary layer forced convection mode.

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???

Gruntguru, your link does not show show how you reached your conclusion so I cannot comment.

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???

Compositepro.
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???

Did you plug in different thermal conductivities as well?

RE: Brake Heat Rejection???

No. Done that now (also changed density & Cp) coefficient increases from 12.7 to 13.5 (400*C).

You were right - apologies.

Engineering is the art of creating things you need, from things you can get.

RE: Brake Heat Rejection???

(OP)
Your point as to conductivity per se is well taken;
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???

They keep putting bigger and bigger brakes on cars (& motorcycles) which drives up cost, rotating mass and unsprung weight (unless inboard) and yet I have read several tests of recent vintage cars that have cooked their brakes on road courses. I would think something simple, light & effective that improves heat rejection from the rotor should have a good market. I think your biggest challenge is going to be finding a way into the brake & car manufactures inner circle and then overcoming the inherent organizational FUD & NIH tenancies.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Brake Heat Rejection???

Overrun-
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???

(OP)
tbuelna, I’m familiar with the use of internal cooling air in the Jumo 004 turbine (Me 262) vanes because of unavailability of high temperature alloying metals. But using the boundary layer as insulation is new to me. It makes sense. A laminar flow boundary layer would transfer heat energy by simple conduction. This is somewhat analogous to internal combustion engine flame travel. Very slow by conduction but increased perhaps 15X with squish turbulence.

RE: Brake Heat Rejection???

The air film cooling approach used with turbine engine nozzle guide vanes is not just a cooling process, it is also a process for using a thin boundary layer of airflow attached to the nozzle vane surface to greatly reduce the heat transferred from the very high-temp exhaust gas flow to the metal nozzle guide vane surface. By using an array of holes on the vane surface to feed air to the boundary layer, and preventing it from separating, the boundary layer airflow functions as a very effective insulator. Basically the opposite process of what you want to achieve with your brake air cooling approach.

RE: Brake Heat Rejection???

I am amazed that no one has mentioned regenerative power as a braking system on gasoline and diesel powered engines. What a waste of energy when using conventional braking systems.

RE: Brake Heat Rejection???

(OP)
Chicopee, regenerative braking and the subject vanes are actually complementary. The former is useful for low speed braking but becomes less attractive for the rare high speed braking event because of the cost and weight of batteries, cables etc. as well as the need to reserve battery capacity the store the regenerated energy. Since the energy load increases with the square of speed, these requirements quickly escalate. High speed events are conventionally dealt with –or not- by heavy rotors that sink and store the heat energy over time.
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???

"High speed events are conventionally dealt with –or not- by heavy rotors that sink and store the heat energy over time."

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???

(OP)
Tmoose, thanks -and thanks again. A bit saddening in that the Riverside tests mentioned I expect are those that killed Ken Miles.

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???

Have you measured any caliper temperatures - or perhaps more importantly, brake fluid temperatures?

Norm

RE: Brake Heat Rejection???

(OP)
Norm, we only measured midpoint rotor and pad temperatures during two brake dyno tests. The first was a “standard” test that didn’t show much at the low temperatures. However, at the end the operator offered a “burn down” that involved clamping down and running at high speed. It was supposed to last maybe 10 to 15 seconds. He stopped the test after a minute or two since it was stressing the dyno.

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???

Going back to the discussion of C-C brakes used on large commercial aircraft, these brake systems use stacks of rotor and stator plates that are clamped by caliper surrounding the entire circumference of the rotor.



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???

Do you suppose that in addition to the vanes 'tripping' the boundary layer off the rotor they are also rendering the leading surfaces of the caliper less effective at shedding their heat gain?

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???

(OP)
Tbuelna, the C-C brakes are highly developed and do the job, particularly with thrust reversers. They require high temperatures to avoid high wear rates. Still, carbon doesn’t have a high specific heat and rejecting heat is better than storing it as in taxiing or a missed approach with braking. Weight is always a concern and rejected heat doesn’t have to have a heat sink. Just theory on my part. Getting something new tested and certified can’t be easy.

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???

Overrun, I think what Norm is saying is the reason the pads run hotter. That is, whatever airflow is cooling the pads and calipers is partially blocked by the vanes.
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???

With auto disc brakes that use calipers and pads, the pad friction surface normally runs much hotter than the rotor surface. The pad material typically has poor conductive properties thru its thickness, so more of the friction heat generated at the contact interface between the pad and rotor is forced into the cooler rotor surface. It is not too difficult to thermally isolate the caliper pistons and body from heat conducted thru the pad body. So the main concern with cooling the caliper is keeping the brake fluid temperature within acceptable limits.

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???

But if the pads are in fact running hotter with the vane(s) in place, so are the calipers and ultimately the fluid.

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???

(OP)
Tbuelna, I agree with your characterization of C-C brakes –though couldn’t open your link. C-C brakes are a bit unique and live well with high temperatures –see below. But if substantial heat could be rejected real time the brakes could be even lighter and avoid retracting a hot brake after taxiing or a go around after an aborted landing attempt. But, as you observe, there’s no crying need.

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.

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