Brake heat rejection concept

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olefud
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Joined: Sat Mar 12, 2011 11:10 pm
Location: Boulder, Colorado USA

Brake heat rejection concept

Post by olefud » Mon Jan 21, 2013 11:31 pm

This thread describing a novel brake heat rejection concept will hopefully provoke critical comments and input as to the realities of practical –or at least various racing- applications. Since the physical embodiment of the invention is deceptively simple with two nonmoving parts, the theory and multidiscipline basis will be developed with a bit of grinding detail. But hey, this forum has been supportive if demanding of inventive efforts.

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.

Image

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.
Last edited by mx_tifoso on Wed Jan 23, 2013 11:33 pm, edited 1 time in total.
Reason: edited title to reflect topic

Tommy Cookers
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Joined: Fri Feb 17, 2012 3:55 pm

Re: Brakes

Post by Tommy Cookers » Tue Jan 22, 2013 5:55 pm

interesting !
doesn't the configuration of drum brakes (and aircraft brakes ?) mean that their boundary layer is already peeled off, though ?
anyway, thrust reversing means aircraft braking is well managed these days ?

I once decided that the way to go with drums was air pumped through the shoes (except this could upset the check valves ?)
modern (ie non-asbestos) brake friction material is too conductive of heat, but 'they' think we haven't noticed
so plan b was to reduce heat flow from the pads to the cylinder (to avoid managing brake fluid boiling)

brake fade is not all bad, though ?
many front wheel drive cars have small rear drums and big front vented discs/rotors
in continued use there is rear fade IMO
this helps the trail braker to keep his car on the road/track

olefud
80
Joined: Sat Mar 12, 2011 11:10 pm
Location: Boulder, Colorado USA

Re: Brakes

Post by olefud » Tue Jan 22, 2013 7:14 pm

Tommy Cookers wrote:interesting !
doesn't the configuration of drum brakes (and aircraft brakes ?) mean that their boundary layer is already peeled off, though ?
anyway, thrust reversing means aircraft braking is well managed these days ?

I once decided that the way to go with drums was air pumped through the shoes (except this could upset the check valves ?)
modern (ie non-asbestos) brake friction material is too conductive of heat, but 'they' think we haven't noticed
so plan b was to reduce heat flow from the pads to the cylinder (to avoid managing brake fluid boiling)

brake fade is not all bad, though ?
many front wheel drive cars have small rear drums and big front vented discs/rotors
in continued use there is rear fade IMO
this helps the trail braker to keep his car on the road/track
The boundary layer is more “piled up” than peeled away by both pads and shoes on disc and drum brakes, respectively. Vanes detach and conduct the hot boundary layer away from the brake proper. It was surprising though that the vanes caused pads to run a bit hotter.

With a long runway and little traffic, thrust reversers can be the main retarders. But brake shuddered is still pretty common it seems. But this is just an impression. Professional pilot forums have a lot of chatter about overheated brakes in the wheel well.

The front-wheel drive brake bias is, I think, more a function of weight distribution. With the greater weight on the front wheels, the rears are less capable of contributing braking force without locking up. This is actually less stable with the resolved braking force forward of the CG, i.e. unsteady equilibrium.

autogyro
64
Joined: Sun Oct 04, 2009 2:03 pm

Re: Brakes

Post by autogyro » Wed Jan 23, 2013 12:46 am

Very interesting and promising development direction IMO.

I have used very similar 'wiper' strips when modifying off road range rovers for desert use. (Dakar etc) 1970's
Our main job was the transmission systems and diffs.
However we had trouble with brake fade caused by the big dust/sand covers over the discs reducing cooling.
Removing the covers to gain airflow resulted in very high pad wear from the sand getting between the pads and the discs.
Similar curved strips were made up to deflect the sand away fitted just ahead of the callipers.
It also stopped all the problems from damaged and rattling disc covers.

riff_raff
132
Joined: Fri Dec 24, 2004 9:18 am

Re: Brakes

Post by riff_raff » Wed Jan 23, 2013 6:16 am

I don't understand how a brake system dissipates much heat energy by radiation. As I understand the process, the primary heat transfer mechanisms are conduction through the pad and disc structures, and then convection to the passing airflow boundary layer. Convective heat transfer between the disc surfaces and the passing airflow only occurs within the very thin airflow boundary layer. Because air is a very good insulator, there is very little additional heat transferred from the boundary layer to the core airflow. And since the convective heat transfer rate from the disc surface to the boundary layer airflow is highly dependent upon the relative temperature delta, if the airflow over the disc surface is relatively laminar, then the boundary layer will quickly become hot, and the convective heat transfer process will halt. The most efficient way to achieve maximum convective heat transfer is to make the boundary air flow over the disc surfaces turbulent and high velocity.

The concern with carbon brake pads and discs is the very low thermal conductivity of the material. The low thermal conductivity tends to concentrate the heat energy from braking at the friction interface area. Any friction device, such as a brake or clutch, is ultimately limited by the amount of thermal energy the mass of the device can absorb without exceeding its safe thermal structural limit. While carbon brake discs can safely operate at temperatures of around 1500degF, due to carbon's low thermal conductivity, the local surface temperature between a carbon brake rotor and pad can quickly exceed this limit under braking. And this would result in oxidation failure of the carbon materials.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"

olefud
80
Joined: Sat Mar 12, 2011 11:10 pm
Location: Boulder, Colorado USA

Re: Brakes

Post by olefud » Wed Jan 23, 2013 7:27 pm

autogyro wrote:Removing the covers to gain airflow resulted in very high pad wear from the sand getting between the pads and the discs.
Similar curved strips were made up to deflect the sand away fitted just ahead of the callipers.
It also stopped all the problems from damaged and rattling disc covers.
Small world. We were contacted by a government agency (DARPA?) regarding high helicopter rotor clutch wear in the desert. Since the subject brake concept is heat driven, we referred them to K&M and centrifugal separators. I suspect what your guards turned the air sharply and may have functioned somewhat as the latter.

autogyro
64
Joined: Sun Oct 04, 2009 2:03 pm

Re: Brakes

Post by autogyro » Wed Jan 23, 2013 8:51 pm

Small world. We were contacted by a government agency (DARPA?) regarding high helicopter rotor clutch wear in the desert. Since the subject brake concept is heat driven, we referred them to K&M and centrifugal separators. I suspect what your guards turned the air sharply and may have functioned somewhat as the latter
Well we did not have any excess wear or overheating brakes when the deflectors were fitted.

As a matter of interest, our autogyros dont suffer from sand or dust as they have unpowered rotors.
They have no rotor downwash and are almost silent in operation.
They also make useful autonomous drones with a far higher endurance than current types.
We only need a small budget and we could drasticaly cut operating costs.

langwadt
62
Joined: Sun Mar 25, 2012 1:54 pm

Re: Brakes

Post by langwadt » Wed Jan 23, 2013 10:01 pm

Tommy Cookers wrote:interesting !
doesn't the configuration of drum brakes (and aircraft brakes ?) mean that their boundary layer is already peeled off, though ?
anyway, thrust reversing means aircraft braking is well managed these days ?

I once decided that the way to go with drums was air pumped through the shoes (except this could upset the check valves ?)
modern (ie non-asbestos) brake friction material is too conductive of heat, but 'they' think we haven't noticed
so plan b was to reduce heat flow from the pads to the cylinder (to avoid managing brake fluid boiling)

brake fade is not all bad, though ?
many front wheel drive cars have small rear drums and big front vented discs/rotors
in continued use there is rear fade IMO
this helps the trail braker to keep his car on the road/track
Rear or front wheel drive the rear brakes doesn't have as much work to do, drums might just the be the cheapest
but sufficient choice. When people don't brake in anger now and then the rear brakes will just rust a fail from too
little use, not from wear. I believe some cars try to bias the brakes so that when you are just braking gently they use
the rear more to so they get a bit more use in general driving

olefud
80
Joined: Sat Mar 12, 2011 11:10 pm
Location: Boulder, Colorado USA

Re: Brakes

Post by olefud » Wed Jan 23, 2013 11:43 pm

riff_raff wrote:I don't understand how a brake system dissipates much heat energy by radiation. As I understand the process, the primary heat transfer mechanisms are conduction through the pad and disc structures, and then convection to the passing airflow boundary layer. Convective heat transfer between the disc surfaces and the passing airflow only occurs within the very thin airflow boundary layer. Because air is a very good insulator, there is very little additional heat transferred from the boundary layer to the core airflow. And since the convective heat transfer rate from the disc surface to the boundary layer airflow is highly dependent upon the relative temperature delta, if the airflow over the disc surface is relatively laminar, then the boundary layer will quickly become hot, and the convective heat transfer process will halt. The most efficient way to achieve maximum convective heat transfer is to make the boundary air flow over the disc surfaces turbulent and high velocity.

The concern with carbon brake pads and discs is the very low thermal conductivity of the material. The low thermal conductivity tends to concentrate the heat energy from braking at the friction interface area. Any friction device, such as a brake or clutch, is ultimately limited by the amount of thermal energy the mass of the device can absorb without exceeding its safe thermal structural limit. While carbon brake discs can safely operate at temperatures of around 1500degF, due to carbon's low thermal conductivity, the local surface temperature between a carbon brake rotor and pad can quickly exceed this limit under braking. And this would result in oxidation failure of the carbon materials.
In that I’ll bediscussing principles in rather basic terms for the casual reader, let me first say that I’ve reviewed a number of riff-raff’s patents and have the highest regard –and a good bit of envy- for his abilities. In other discussions with both with both academic and motorsport PhDs, there has been some inertia resistance to something new in a discipline that has been rather settled for 100 years or so. An open mind and rethinking established knowledge may be useful.

That said, radiation heat transfer is perhaps the most powerful of the three modes, though not much appreciated in motorsport circles. While both conductive and radiation heat transfer are solely ΔT driven, conductive is first order ΔT while radiation is fourth order ΔT of the absolute T. This is not instinctively apparent in that at the usual dynamic equilibrium the considerable energy radiated is balanced by an equal amount absorbed, since emissivity and absorption coefficients being essentially equal. But as a surface heats above its surroundings, the fourth order ΔT term gets to be a very significant driving force. Even though I’m pushing convective heat rejection, radiation has been the important heat rejection mechanism when brakes are under duress –particularly with convective heat rejection going south at higher temps.

Riff-raff’s discussion of convective heat transfer is a good description of the state of the art and germane –particularly the bit about heat transfer ceasing at the disc surface. The hotter the surface the poorer the heat transfer. The disc is generally thought of as a heat sink, storing the heat energy and rejecting it over time with the brakes having a rather low heat generation duty cycle. I do differ with the thought of “...boundary air flow over disc surfaces…” My observations suggest that the boundary layer is in fact firmly adhered to the disc even when subjected to fairly brisk cooling-air forced convection. It is this well attached, viscose hot boundary layer that the vanes detach in a form of forced convection using the boundary layer kinetic energy.

Finally, the information that carbon brakes tend to oxidize above 1500° F is encouraging – I thought the limit was more like 1800° F. Maybe carbon brakes could benefit from convective cooling simultaneous with the heat generation.

thisisatest
18
Joined: Sat Oct 16, 2010 11:59 pm

Re: Brake heat rejection concept

Post by thisisatest » Fri Jan 25, 2013 2:36 am

couldnt these vanes be integrated with the brake pad or caliper?
it seems that with this info, racing teams should be aiming their brake ducts to try to disrupt the boundary layer instead of just blowing on the brake.

olefud
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Joined: Sat Mar 12, 2011 11:10 pm
Location: Boulder, Colorado USA

Re: Brake heat rejection concept

Post by olefud » Fri Jan 25, 2013 4:52 pm

thisisatest wrote:couldnt these vanes be integrated with the brake pad or caliper?
it seems that with this info, racing teams should be aiming their brake ducts to try to disrupt the boundary layer instead of just blowing on the brake.
Good observation! Before the strong Coanda effect was confirmed, I was thinking of just that; moving the vane with the pad for a tight clearance with more clearance when the pad is knocked back. One drawback is pad wear that would alter the vane clearance. But a vane located well outside of fairly large runout cools the rotor most effectively.
Probably the best location for cooling air with a conventional brake is the inlet to the cooler internal fins of vented rotors. Short of an air knife –a form of vane- external blown air mostly cools the pads. The vanes more than adequately cool the rotor but could use some blown air for the same purpose.

PlatinumZealot
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Re: Brake heat rejection concept

Post by PlatinumZealot » Sat Jan 26, 2013 1:14 am

Olefud... so basically you are saying that those vanes peel away the old strongly attached boundary layer to allow a fresh cooler boundary layer to form on the disc?

Also on the radiation heat transfer... When I was doing my gas turbine project back in school, radiation was the highest mode of heat transfer, then there was convection.. but this was for a extremely hot swirling body of combusting gas, with some luminescent particles... Are you suggesting that the brake disk gets so hot that radiation takes over as the primary mode of heat transfer? So in layman terms If I stoop next to a race car wheel right after it has done a few hot laps, I will feel the heat from radiation even in still air conditions?
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tok-tokkie
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Location: Cape Town

Re: Brake heat rejection concept

Post by tok-tokkie » Sat Jan 26, 2013 7:52 am

Why does the brake caliper & pad not strip the boundary layer away in the same way as your vane?
Is the boundary layer normally passing the heat back to the pad & caliper whereas your vane is stripping it away before it does that?
Would it not be better to place the vane close to the trailing edge of the caliper to strip the boundary layer away soon after it is heated?

olefud
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Location: Boulder, Colorado USA

Re: Brake heat rejection concept

Post by olefud » Sat Jan 26, 2013 10:23 pm

n smikle wrote:Olefud... so basically you are saying that those vanes peel away the old strongly attached boundary layer to allow a fresh cooler boundary layer to form on the disc?

Also on the radiation heat transfer... When I was doing my gas turbine project back in school, radiation was the highest mode of heat transfer, then there was convection.. but this was for a extremely hot swirling body of combusting gas, with some luminescent particles... Are you suggesting that the brake disk gets so hot that radiation takes over as the primary mode of heat transfer? So in layman terms If I stoop next to a race car wheel right after it has done a few hot laps, I will feel the heat from radiation even in still air conditions?
That’s basically it, peeling the boundary layer. But, if the vane is closely leading the calipers, just how much of a new boundary layer forms is questionable. So, depending upon conditions i.e. rotor RPM, braking intensity etc., the greater part of heat rejection appears to be with the peeled” initial boundary layer. The somewhat limited testing to date suggests that a single vane per rotor face rather closely leading the pad does the job. Just how close is again still in question regarding pad temps. This has just not been tested yet. What I do see is a very significant reduction in the rotor temperature.

Other than cooling since use you should definitely feel radiated IR. Keep in mind that car parts that “see” the rotor will also be heated by absorbing rotor radiation.

olefud
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Location: Boulder, Colorado USA

Re: Brake heat rejection concept

Post by olefud » Sat Jan 26, 2013 10:47 pm

tok-tokkie wrote:Why does the brake caliper & pad not strip the boundary layer away in the same way as your vane?
Is the boundary layer normally passing the heat back to the pad & caliper whereas your vane is stripping it away before it does that?
Would it not be better to place the vane close to the trailing edge of the caliper to strip the boundary layer away soon after it is heated?
It appears to be a matter of removing the Peeled” boundary layer from the rotor/caliper local. The rotor/pads would seemingly block the boundary layer but “pile it up” such that the heat is not rejected from the brake system as you suggest.

We have tried trailing vanes but only under simulated extreme braking conditions. The primary concern appears to be allowing time for the heat to form a fat boundary layer. Keep in mind that the initial heat formation is sort of a singularity, i.e. essentially two dimensional with no volume. At high rotor RPMs it takes a while for the boundary layer to be heated. Only closely leading and closely trailing iterations have been tested. It would be interesting to try intermediate positions to see if the boundary layer would reform a bit and perhaps carry along some cooler air to also provide a bit of pad cooling. The rotor cooling is so significant that sacrificing a bit to balance the pad/rotor temperatures might be worthwhile.