Brake Heat Rejection???

[Overrun]

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?

 

[TheBlacksmith]

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.

 

[Overrun]

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.

 

[MintJulep]

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.

 

[Overrun]

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.

 

[Compositepro]

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.

 

[tbuelna]

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.

 

[Overrun]

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.

 

[gruntguru]

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.

 

[Compositepro]

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.

 

[gruntguru]

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.

 

[Overrun]

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.

 

[Compositepro]

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.

 

[gruntguru]

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.

 

[Compositepro]

Did you plug in different thermal conductivities as well?

 

[gruntguru]

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.

 

[Overrun]

Your point as to conductivity per se is well taken;

http://www.engineeringtoolbox.com/dry-air-properties-d_973.html

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.

 

[dgallup]

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.

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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.

 

[tbuelna]

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.

 

[Overrun]

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.

 

[tbuelna]

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.