Brake heat rejection concept

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

Re: Brake heat rejection concept

The convective heat transfer from disc/pad surface only occurs within the boundary airflow layer. Also remember that air is an excellent thermal insulator. Then consider the heat transfer situation that exists between a laminar boundary airflow and a hot brake rotor surface. Just how much heat transfer will occur once the brake rotor surface and the laminar boundary airflow both assume the same temperatures? Essentially zero, right? Since the heat transfer rate from the rotor-surface-to-boundary-airflow is more efficient than that existing from boundary-airflow-to-core-airflow, the heat transfer benefits of having a turbulent airflow over the rotor surface becomes obvious.
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autogyro
autogyro
64
Joined: Sun Oct 04, 2009 2:03 pm

Re: Brake heat rejection concept

riff_raff wrote:The convective heat transfer from disc/pad surface only occurs within the boundary airflow layer. Also remember that air is an excellent thermal insulator. Then consider the heat transfer situation that exists between a laminar boundary airflow and a hot brake rotor surface. Just how much heat transfer will occur once the brake rotor surface and the laminar boundary airflow both assume the same temperatures? Essentially zero, right? Since the heat transfer rate from the rotor-surface-to-boundary-airflow is more efficient than that existing from boundary-airflow-to-core-airflow, the heat transfer benefits of having a turbulent airflow over the rotor surface becomes obvious.
Surely this is only correct if the turbulent airflow is proven to dislodge the boundary airflow layer?
A 'wiper' strip will do so if it is close enough to the surface.
It would then be useful to blow turbulent air at the disc surface 'after' the wiper.
If there is a quantifiable reduction of temperature using the wipers, then the next stage should be to find the ideal position to fit them.
It might be found that more than one set of wipers works even better.

olefud
olefud
80
Joined: Sat Mar 12, 2011 11:10 pm

Re: Brake heat rejection concept

riff_raff wrote:The convective heat transfer from disc/pad surface only occurs within the boundary airflow layer. Also remember that air is an excellent thermal insulator. Then consider the heat transfer situation that exists between a laminar boundary airflow and a hot brake rotor surface. Just how much heat transfer will occur once the brake rotor surface and the laminar boundary airflow both assume the same temperatures? Essentially zero, right? Since the heat transfer rate from the rotor-surface-to-boundary-airflow is more efficient than that existing from boundary-airflow-to-core-airflow, the heat transfer benefits of having a turbulent airflow over the rotor surface becomes obvious.
I agree entirely with your basics. However, keep in mind that there is a very large ΔT initially that heats the boundary layer despite low conductivity. As the boundary layer is heated, the air becomes more viscous which both adheres the boundary layer more tenaciously –probably precluding turbulence- and increases its thickness. Thus the boundary layer is thick and hot, but likely has not reached temperature equilibrium in the short period before it reaches the vane. By “peeling away” and guiding away this heated layer each rotor revelation very substantial amounts of heat are pumped from the brake system using the kinetic energy of the boundary layer itself. Thus a new, cool boundary layer is form each rotor revolution free of a very substantial heat loading from the previous revolution.

The above assumes heavy braking with a high rate of heat generation. At lesser rates, the vane heat rejection mechanism scales back as a function of temperature/air viscousity along with the requirement for augmented heat rejection.

olefud
olefud
80
Joined: Sat Mar 12, 2011 11:10 pm

Re: Brake heat rejection concept

autogyro wrote:It might be found that more than one set of wipers works even better.
My initial thought was to use an array of vanes. However, testing suggested that –at least at high speed- it took a bit of time for the heated boundary layer to form. Heavy braking at low speed would likely have a different ideal positioning than such braking at high speed. Trucks and competition vehicles for instance.

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

Re: Brake heat rejection concept

autogyro wrote:.......Surely this is only correct if the turbulent airflow is proven to dislodge the boundary airflow layer?
A 'wiper' strip will do so if it is close enough to the surface.......
The problem with using a "wiper" is the fact that boundary air flows, by definition, are very thin and tend to remain attached to the local surface. To be effective the wiper would need to contact the rotor surface. And then how would you get airflow to fill the trailing space behind the wiper?

The most effective method for "tripping" the boundary layer and causing it to become turbulent is to employ a rough flow surface. But this is not practical with a brake rotor friction surface. The only other approach I can think of is to use a thin jet of high-velocity cooling air directed radially outward across the rotor friction faces.
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autogyro
autogyro
64
Joined: Sun Oct 04, 2009 2:03 pm

Re: Brake heat rejection concept

The problem with using a "wiper" is the fact that boundary air flows, by definition, are very thin and tend to remain attached to the local surface. To be effective the wiper would need to contact the rotor surface. And then how would you get airflow to fill the trailing space behind the wiper?

The most effective method for "tripping" the boundary layer and causing it to become turbulent is to employ a rough flow surface. But this is not practical with a brake rotor friction surface. The only other approach I can think of is to use a thin jet of high-velocity cooling air directed radially outward across the rotor friction faces.
A combination of the two perhaps.
An 'air knife' built into the wiper?

olefud
olefud
80
Joined: Sat Mar 12, 2011 11:10 pm

Re: Brake heat rejection concept

I’m not getting across the fact that air at 400°-500°C is very different than air at 20°C. This should be somewhat evident viscerally in that the heated boundary layer can be seen to adhere to the rotor despite the rather substantial centrifugal force at say 100 mph. Air density decreases by about 55% when heated from 20°C to 400°C. Thermal conductivity doubles while kinetic viscosity –which drives boundary layer thickness and stability- quadruples.

A simple solid aerodynamic vane is entirely satisfactory to effect a very substantial reduction in the rotor temperature, progressively more so as the heat generated by braking increases. The only nasty so far appears to be a result of the rotor temperature being reduced so much that volatile constituents from the pad condense on the rotor and form an unwanted transfer layer. This can be serious in that pad material on transferred pad material is not a workable friction interface. But this challenge is best solved in the context of a specific application

eyalynf1
eyalynf1
11
Joined: Tue May 24, 2011 12:05 am

Re: Brake heat rejection concept

This is a very intersting discussion.

I would assume that the boundary layer air volume that is displace from the rotor surface is transferring heat to the pad, or being collected around the pad and insulating it from convective heat transfer. If so, then the next step is to prevent that hot boundary layer air volume from collecting around the pad/caliper assembly, and/or removing that air volume from the wheel itself.

This could be accomplished by using vanes which push the boundary layer air toward the center of the wheel assembly and away from the circumferential mounted pads and additional vanes which push the air axially outward from the wheel assembly.

godlameroso
414
Joined: Sat Jan 16, 2010 8:27 pm
Location: Miami FL

Re: Brake heat rejection concept

If it could be possible to place a cylindrical wire mesh on the trailing edge of the caliper it could produce the required turbulence to help cool the pad.
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olefud
olefud
80
Joined: Sat Mar 12, 2011 11:10 pm

Re: Brake heat rejection concept

eyalynf1 wrote:This is a very intersting discussion.

I would assume that the boundary layer air volume that is displace from the rotor surface is transferring heat to the pad, or being collected around the pad and insulating it from convective heat transfer. If so, then the next step is to prevent that hot boundary layer air volume from collecting around the pad/caliper assembly, and/or removing that air volume from the wheel itself.

This could be accomplished by using vanes which push the boundary layer air toward the center of the wheel assembly and away from the circumferential mounted pads and additional vanes which push the air axially outward from the wheel assembly.
The vane illustrated was selected to redirect the boundary layer in a more or less tangential direction, the direction it would want to go if it were detached. However, the concept has also been tested with axial redirection. I believe the hot, detached boundary flow is not the problem in that it was redirected away from the pad/caliper. Rather, I suspect that the vane is also redirecting an attached outer boundary layer of cool air that served to cool the pad/rotor.

This “outer layer” theory appears to be a new concept. But it’s only been discover at best, not tested and certainly not proved.

cwb
cwb
5
Joined: Mon Feb 11, 2013 12:21 pm

Re: Brake heat rejection concept

olefud wrote:I’m not getting across the fact that air at 400°-500°C is very different than air at 20°C. This should be somewhat evident viscerally in that the heated boundary layer can be seen to adhere to the rotor despite the rather substantial centrifugal force at say 100 mph. Air density decreases by about 55% when heated from 20°C to 400°C. Thermal conductivity doubles while kinetic viscosity –which drives boundary layer thickness and stability- quadruples.

A simple solid aerodynamic vane is entirely satisfactory to effect a very substantial reduction in the rotor temperature, progressively more so as the heat generated by braking increases. The only nasty so far appears to be a result of the rotor temperature being reduced so much that volatile constituents from the pad condense on the rotor and form an unwanted transfer layer. This can be serious in that pad material on transferred pad material is not a workable friction interface. But this challenge is best solved in the context of a specific application
Could it be that the volatile consituents condensing on the rotor are also condensing on the pad, changing the heat transfer characteristics?

Also, have you calculated a Reynolds number for the situation to see whereabouts you are on the laminar/turbulent spectrum?

olefud
olefud
80
Joined: Sat Mar 12, 2011 11:10 pm

Re: Brake heat rejection concept

cwb wrote:
olefud wrote:I’m not getting across the fact that air at 400°-500°C is very different than air at 20°C. This should be somewhat evident viscerally in that the heated boundary layer can be seen to adhere to the rotor despite the rather substantial centrifugal force at say 100 mph. Air density decreases by about 55% when heated from 20°C to 400°C. Thermal conductivity doubles while kinetic viscosity –which drives boundary layer thickness and stability- quadruples.

A simple solid aerodynamic vane is entirely satisfactory to effect a very substantial reduction in the rotor temperature, progressively more so as the heat generated by braking increases. The only nasty so far appears to be a result of the rotor temperature being reduced so much that volatile constituents from the pad condense on the rotor and form an unwanted transfer layer. This can be serious in that pad material on transferred pad material is not a workable friction interface. But this challenge is best solved in the context of a specific application
Could it be that the volatile consituents condensing on the rotor are also condensing on the pad, changing the heat transfer characteristics?

Also, have you calculated a Reynolds number for the situation to see whereabouts you are on the laminar/turbulent spectrum?
There may be some cooler volatile material condensing on the pad; but much more is driven off and much more is condensing on the cooler rotor. That’s pretty much the nature of vapor deposition.

Calculating a Re is somewhat arbitrary given the large variation in the velocity and temperature of the rotor, Re being directly proportional to the speed and inversely so to the air viscosity, which increases with temperature. Air temperature immediately adjacent the rotor is not measured in conventional dyno testing.

Just from observation, the boundary layer appears to be laminar away from the pad when the rotor is glowing dark red based on the incandescing particles. On the other hand, some of these heavier ?such particles escape immediately following the pad. My working assumption is that a heat stressed rotor has a laminar boundary layer though it may take a while to form after the pad disruption.

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

Re: Brake heat rejection concept

olefud wrote:I’m not getting across the fact that air at 400°-500°C is very different than air at 20°C. This should be somewhat evident viscerally in that the heated boundary layer can be seen to adhere to the rotor despite the rather substantial centrifugal force at say 100 mph. Air density decreases by about 55% when heated from 20°C to 400°C. Thermal conductivity doubles while kinetic viscosity –which drives boundary layer thickness and stability- quadruples.
I would agree that all other things being equal, air at 400-500degC has different physical properties than air at 20degC. The most obvious differences would be lower density. Of course, a laminar flow boundary air layer is also an effective form of thermal insulation. This is why the air-film cooling of turbine engine exhaust nozzles is so effective.

The most effective type of brake system will always be the one that has the best efficiency at transferring the thermal energy from braking friction to the passing airflow.
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cwb
cwb
5
Joined: Mon Feb 11, 2013 12:21 pm

Re: Brake heat rejection concept

olefud wrote:Calculating a Re is somewhat arbitrary given the large variation in the velocity and temperature of the rotor, Re being directly proportional to the speed and inversely so to the air viscosity, which increases with temperature. Air temperature immediately adjacent the rotor is not measured in conventional dyno testing.

Just from observation, the boundary layer appears to be laminar away from the pad when the rotor is glowing dark red based on the incandescing particles. On the other hand, some of these heavier ?such particles escape immediately following the pad. My working assumption is that a heat stressed rotor has a laminar boundary layer though it may take a while to form after the pad disruption.
Re is arbitrary by definition but it is an important indicator, especially when trying to figure out whether a particular flow structure is laminar or turbulent.

My back of the envelope calcs suggest a Re of the order of 250,000 - 400,000 at ambient air temperatures and 50,000 - 80,000 at temperatures of 400 degC, halving again to 30,000 - 40,000 around 800 deg C depending on what assumptions you make about rotor/air speed and dimensional scales. These numbers do show the dramatic effect of the change in density and dynamic viscosity over temperature that you have often mentioned. However they are all well north of the (yes arbitrary) threshhold between laminar and turbulent flow around Re = 5000, to get to this number vehicle speed would have to be reduced by a factor of 6.

I guess I would like to challenge the assumption that the flow patterns you are observing are laminar. I suggest that what you are observing are sections of relatively stable flow structure, due to favourable pressure gradients and relatively small length scales due to the flow being adjacent to the rotor, but it is still very turbulent.

I realise this doesnt actually shed much light on your situation but to me its an important distinction.

cwb
cwb
5
Joined: Mon Feb 11, 2013 12:21 pm

Re: Brake heat rejection concept

riff_raff wrote:Of course, a laminar flow boundary air layer is also an effective form of thermal insulation.
I disagree, air is only an effective form of insulation when it isnt moving. Yes convective heat transfer coefficients are lower for laminar flow, but they are not zero.
riff_raff wrote:This is why the air-film cooling of turbine engine exhaust nozzles is so effective.