Ventilating a hub (fluid dynamics advice request)

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Ventilating a hub (fluid dynamics advice request)

Postby John in CR » Sat Aug 29, 2009 3:02 pm

Steveo & DrBass have already demonstrated the much greater heat dissipation of ventilating a hub motor. I'm going to try some hub motors outside of the wheel and want to try to maximize the flow of fresh air over the windings and magnets. I plan to do it on both a direct drive hubbie and a geared hub, so the approaches may be different since the geared hub isn't attached to the motor shell at all (other than via the planetary gears). On the geared hub, that makes the flat part that is normally between the left and right spokes available for ventilation holes or slots too. On the direct drive, only the side covers are available for ventilation.

I can really open them up a lot since the shells no longer support the weight of the bike and rider, and won't see the shock loads from uneven roads. I believe I can stimulate sufficient airflow to increase the motors' power potential at least 3 fold just by cutting the shells, and not going to the trouble of adding material to form something like blades or scoops.

Here's where the fluid dynamics comes into play. What will stimulate the greatest air flow?
On the direct drive should I try to make one side cover the intake and the other exhaust, or try to take air in near the greatest diameter with exhaust nearer the axle?
On the geared hub, should I do the same as the DD or do something more like a centrifugal blower (squirrel cage type fan) and intake (or exhaust by creating reverse curve type blades) at the outer circumference with exhaust out of one or both side covers?

We're talking about all aluminum, so my plan is to just go to town cutting angled slots with an angle grinder instead of just drilling holes like Steveo and DocBass. Also, if I can get away with narrow slots, I may not need a housing to protect from debris getting into the motor, but if big slots are a lot better that's no biggie. If it's thought necessary and worth the effort, I could go to the extent of cutting the covers into 4, 6 or 8 spokes and try to give them a bit of a NACA foil shape, but keep in mind that the rotation will be at pretty low rpms (about wheel speed).

Any advice is welcome.

John
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Re: Ventilating a hub (fluid dynamics advice request)

Postby spinningmagnets » Sat Aug 29, 2009 5:05 pm

Something I haven't seen tried yet is using a "wheel disc" (spoke fairing) to create airflow across the hub body.

http://www.recumbents.com/wisil/wheeldisk/wheeldisk.htm

Image

If you contemplate the common front ventilated disc brake on a car, you will understand what I am imagining.

Image

The air inside the disc volume is flung outwards by the action of the radial vanes and forward rotation of the wheel. As a result, air is then drawn into the center.

If a wheel disc was suspended a small amount away from the sides of the hub, and there was an small opening around the center, and also around the rim, and you also had radial vanes installed...there would be airflow from the center, across the sides of the hub, and flowing out near the rim.

That being said, heat transfer is partially a result of the differential temps involved...A warm hub will have very little heat shedding on a warm day, but a hot hub on a cold day will shed a significant amount of BTU's (on behalf of the entire USA, thank you, Miles) from hub-skin air-flow.

Much hub-heat is built up inside, so internal air-flow is beneficial. I can imagine either an air-scoop or a fan-duct, but either way, I would suggest air entering near the center of a hub, and exhaust flow either from the outside diameter or the opposite side.

Air is quite fluid, so do not insist on having a solid duct connection to the hub side panel. A duct outlet that is merely near the hub inlet holes will create internal cross-flow. Ensure there is slightly more outlet hole area than inlet hole area.
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Re: Ventilating a hub (fluid dynamics advice request)

Postby John in CR » Sat Aug 29, 2009 6:01 pm

Thanks Spinning, but due to their design, hub motors don't dissipate heat very well. Other than through the axle, the stator must conduct heat to the air first, then air to the exterior shell, through it and to the outside world from the shell's surface. The shell's surface already gets plenty of air flow. A geared hub is even worse due to it's smaller exterior surface area and the rotor containing the magnets being inside the housing, not part of the housing.

I want to open it up and force as much fresh air through as possible. The cooler air will conduct heat away better, and the more I can force that flow, the greater will be the heat transfer. Hub motors are big heavy outrunners, whose continuous power is severely limited due primarily to their poor heat dissipation. Look at the power guys like Methods and DrBass are running for short periods. Imagine making these silent and cheap but dependable motors able to do that continuously. That's enough for highway speeds, but more importantly to ebikers, to climb long steep hills.

John
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Re: Ventilating a hub (fluid dynamics advice request)

Postby Paul.............D » Sat Aug 29, 2009 6:21 pm

Hi John i cut half circles in the casing as mine was a cheap chinese hubmotor that used to overheat at 60volts after cutting at an angle it run at 72 volts without any oveheating. The guy i sold it to says he has had 96 volts through it with no probs but it is cut to draw air through.

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Re: Ventilating a hub (fluid dynamics advice request)

Postby John in CR » Sat Aug 29, 2009 7:06 pm

Paul.............D wrote:Hi John i cut half circles in the casing as mine was a cheap chinese hubmotor that used to overheat at 60volts after cutting at an angle it run at 72 volts without any oveheating. The guy i sold it to says he has had 96 volts through it with no probs but it is cut to draw air through.

Paul

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Thx Paul, yes SteveO and DrBass also confirmed that the approach works. I'm looking for ideas on how to optimize that approach. Which direction did your half circles face?..I assume the rounded part was the leading edge. Did you file an indention in the leading edge, and angle the cut inward on the "scooping" straight edge? What about flow, where does the air enter and where does it exit?

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Re: Ventilating a hub (fluid dynamics advice request)

Postby Paul.............D » Sat Aug 29, 2009 7:36 pm

Ht john
The air was was forced in the pressure forces the hot air out/cold air goes in heat rises so cold air in hot out , i made mistakes on one trying to get air in and out and it did make a bit of difference then a guy i talked to said cold in under pressure will expel hot air quicker, he had been in uni teaching cooling so i tried it he was 100 percent correct. the rounded part was the leading edg was filed to force air in, i was reluctant to try it at first as the first one i did was not much better but i listened as this guy had been dealing with these things for 30 years hope this helps. Ask luke lfp what his veiws are.


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Re: Ventilating a hub (fluid dynamics advice request)

Postby John in CR » Sat Aug 29, 2009 8:19 pm

It's a complex issue with the shell moving relative to the static interior stator. Plus it's not the same as spinning with the bike stationary, because when the slots (if spinning at wheel speed) are stationary relative to the ground when at the bottom of the circle and fast at the top. It definitely can't be simplified to hot air rising, and the whole purpose of my post is to try to maximize intake with shape and placement, along with other holes with proper placement and shape to create a low pressure areas to "suck" the hot air out. I'm game for anything that has some methodology supporting it. BTW, I like the half circle idea, but I'd think some are needed facing the other direction to be the exit. If that's a good approach, which type should be closer to the axle, and why?

Will the mass of the air cause it to act somewhat like a fluid once inside the spinning structure and centrifugal force push it to the perimeter? If that's the case then I should do like the disk brakes spinningmags posted and try to create low pressure vents as exits near the perimeter. I don't know enough to make an educated guess.

John
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Re: Ventilating a hub (fluid dynamics advice request)

Postby rhitee05 » Sun Aug 30, 2009 10:37 am

The optimum solution would be the one that makes best use of the natural forces at work. Convection won't do you much good because the shell is spinning, but centrifugal force from the spinning and air pressure from bike movement are both very useful.

I agree with spinningmagnets that something like a vented disc brake approach would likely be best. Put some inlet holes near the axle and exit holes much further out. You'd probably have better results if you put some vanes onto the sides of the hub with a disc over them, to make a structure like the inside of a brake disc. That should create some vacuum at the exit holes that would help pull air through the inner parts of the hub. Best results probably if you do this on both sides, and do make the outlet area larger than the inlet.

Alternately, you can make use of the forward motion of the bike (and the spinning of the wheels) to generate ram air pressure. One side of the hub is an inlet, with some small scoops facing into the rotation. The other side has scoops facing opposite the rotation for outlet. That should also generate a pressure differential that will force air to move from one side to the other. You might have even better results if you put the inlets closer to the axle than the outlets, and let centrifugal force help out some too. All you need to do is create a pressure differential and the air will flow. Not quite as certain about this, but my guess is with the shell spinning and the stator fixed, there would probably be enough turbulence in the inside to move air around and cool things fairly evenly.

I'm not sure if there's anything you can do with the rim part of the hub for airflow. I'm not real familiar with hub construction, but with magnets all along the inside is there any place to cut holes for venting? That would be a great place for outlet if possible, but that doesn't seem like you could get enough outlet area to work very well. It might be a good place to attach some heatsink extrusions, though. Some very simple linear-fin heatsinks attached around the rim would get nice airflow. They'd have good thermal conductivity to cool the magnets, at the very least, and would probably help the innards at least a little bit.
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Re: Ventilating a hub (fluid dynamics advice request)

Postby gogo » Sun Aug 30, 2009 11:13 am

Racing motorcycles with drum brakes might be a good source of ideas.
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Re: Ventilating a hub (fluid dynamics advice request)

Postby John in CR » Sun Aug 30, 2009 11:56 am

Thanks rhitee,

No way to vent on the magnet rim on the direct drive, but on the geared hub the magnets aren't attached there, so I can do something like reverse curve centrifugal fan blades there. Some ram air at the intake should be easy to, though I like in at the axle out at the perimeter on both sides better than flow one side to the other. I think that will get the most airflow out to the windings. Side to side flow will go primarily through the open spokes in the stator, which I wouldn't want to totally block to force the flow through the air gap b/w the stator and mags. Through the gap would be great, except that it's so narrow that I think it would restrict flow. On motors with the laminations stacked to create an angle, then side to side might be best because the lamination stack would act like vanes to stimulate air flow right where it's needed most, thru the gap.

I've got some space between the cover and the stator, so putting some kind of vane on the inside to help stimulate even more spinning action for the air makes a lot of sense. Since the cover spins relative to the stator, I think even just a handfull of straight blades attached to covers angled in the proper direction relative to spin, should stimulate significant centrifugal force flow with intake near the axle and exit near the perimeter.

Hub motor manufacturers should incorporate this stuff into their designs. Original Etek's and Agni's are somewhat open to the environment, and they have brushes, so there's no reason brushless hubs can't be somewhat open too.

John
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Re: Ventilating a hub (fluid dynamics advice request)

Postby gerhardt » Sun Aug 30, 2009 12:18 pm

Hi John,
You may be able to make a small smoke wand or bubble wand to track the airflow. We used these devices to monitor cooling airflow on designs that were too complex or expensive to model with fluid dynamic models. Let me know if you find a small wireless temperature sensor that can be used inside the motor. I am interested in increasing the performance and durability of the Chrystalyte 400 series motor to 3000 watts. Will high temperatures degrade the magnets in the motor? How high a temperature can the magnets withstand? Are their better magnets hat can be used?
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Re: Ventilating a hub (fluid dynamics advice request)

Postby Miles » Sun Aug 30, 2009 1:44 pm

gerhardt wrote: Will high temperatures degrade the magnets in the motor? How high a temperature can the magnets withstand? Are their better magnets hat can be used?
Don Gerhardt


Yes. The maximum temperature depends on the magnet material and rating. Samarium Cobalt magnets can be used up to as high as 300 degrees Centigrade, even. There's still the limit for the armature winding insulation, and magnet bonding, though....... and running at high temperatures lowers the efficiency a fair bit....

Ref: http://www.engconcepts.net/Magnet_Ratings.htm
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Re: Ventilating a hub (fluid dynamics advice request)

Postby rhitee05 » Sun Aug 30, 2009 7:06 pm

John in CR wrote:I've got some space between the cover and the stator, so putting some kind of vane on the inside to help stimulate even more spinning action for the air makes a lot of sense. Since the cover spins relative to the stator, I think even just a handfull of straight blades attached to covers angled in the proper direction relative to spin, should stimulate significant centrifugal force flow with intake near the axle and exit near the perimeter.


Space permitting, some vanes on the inside would probably be optimal. Stirring action plus centrifugal force should work out pretty nice. Even if it doesn't seem like a lot of air is flowing, you'll probably see a big difference from fully sealed.
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Re: Ventilating a hub (fluid dynamics advice request)

Postby John in CR » Sun Aug 30, 2009 7:12 pm

rhitee05 wrote:
John in CR wrote:I've got some space between the cover and the stator, so putting some kind of vane on the inside to help stimulate even more spinning action for the air makes a lot of sense. Since the cover spins relative to the stator, I think even just a handfull of straight blades attached to covers angled in the proper direction relative to spin, should stimulate significant centrifugal force flow with intake near the axle and exit near the perimeter.


Space permitting, some vanes on the inside would probably be optimal. Stirring action plus centrifugal force should work out pretty nice. Even if it doesn't seem like a lot of air is flowing, you'll probably see a big difference from fully sealed.


Definitely, since guys who drilled almost random round holes reported significant benefit, so an educated strategy should tilt the benefit toward the optimum.

Thanks for your input. It helps me visualize what will happen, so I can better take advantage.

John
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Re: Ventilating a hub (fluid dynamics advice request)

Postby rhitee05 » Sun Aug 30, 2009 8:29 pm

If you're feeling really ambitious - figure out how to route some piping through the axle and watercool it with some copper pipe and PC radiators!

Actually, I'd be kind of curious what would happen if you could seal the hub sufficiently and then fill it with some sort of thermally conductive and non-conductive fluid. I think mineral oil is non-conductive, but messy. Motor oil would have a nice lubrication side benefit, but not sure about its electrical properties. That would probably give you a nice thermal path to the outside wall, then just add some fins to cool it maybe? An experiment for a brave hub motor owner. :twisted:
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Re: Ventilating a hub (fluid dynamics advice request)

Postby amberwolf » Sun Aug 30, 2009 10:01 pm

I have to agree with those proposing centrifugal air forcing methods, especially if you intend to run the motor faster than it would usually run on the wheels themselves.

The most efficient fan I was able to put on my treadmill motor to cool it (without having to use a separate motor to run it inside a duct or tube) was a vacuum-cleaner fan that internally looks very much like the vented disc brake. It's larger hole on one side of the center faced the motor's own ventilation holes, so it sucked air from inside the motor (which also had vents on it's opposite end), and threw it out the edges, causing a vacuum that continued to suck more air. Even at relatively low speeds it worked very well, and was reasonably quiet.

Unfortunately it was made of very very thin aluminum, and *anything* that touched it while it was spinning would deform it. The one time that it deformed enough to touch the face of the motor was the last time I could use it, because it essentially folded up into a little circle. I'm surprised it didn't just disintegrate and throw bits all over.

I then went back to the original fan I'd tried on it, from an alternator in a car, which works the same way, but is heavier (and stronger), though a lot noisier.
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Re: Ventilating a hub (fluid dynamics advice request)

Postby jsplifer » Sat Sep 12, 2009 1:25 pm

Hey,

Like rhitee05 said, "If your feeling real ambitious." You may want to try this AWESOME idea that I just cooked up.

http://home.ptd.net/~jaspermine/stuff/A ... 203542.jpg

Basically you modify the suspension to pump a fluid through one way valves in order for the fluid to make a loop through the motor, and through a heat sink/radiator mounted somewhere.

Cheers.
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Re: Ventilating a hub (fluid dynamics advice request)

Postby Lock » Sat Sep 12, 2009 1:46 pm

I'd add scoops at the rims and connect the scoops by hose to holes in the hub between the spokes. Drill holes in the sides of the hub too, as intakes. The scoops would face "aft" and create suction as the wheel spins fwd. to create an exhaust flow. Putting the scoops at the rim rather than on the hub would maximize their speed through the air/draw... I don't know what I'm tapping about so get to think outside the box(hub)... :mrgreen:
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Re: Ventilating a hub (fluid dynamics advice request)

Postby amberwolf » Sat Sep 12, 2009 5:35 pm

jsplifer wrote:Basically you modify the suspension to pump a fluid through one way valves in order for the fluid to make a loop through the motor, and through a heat sink/radiator mounted somewhere.

That actually IS a really interesting idea, though not quite as you have it envisioned.

Rather than modifying the shock directly, use a lever and/or gear system to take the very small linear movement of the shock, and convert it into a long linear throw for pumping much more volume, using an external little pump. That way you could just bolt it on any bike, rather than modifying a specific shock fork (the modifications of which would be different for each one, too).

All it would need is to connect between the crown and the U below that where the screw mount is often unused (when they come equipped with linear pull or other types of brakes using the studs).

The compression between those points could act across a lever, or could have teeth on a straight strip that engage a rotating gear, which (if necessary) in turn engages a different sized gear to give the ratio needed for high speed pump movement.

EDIT: Also, one-way valves would be unnecessary, just use a ratcheting gear (freewheel) so it only goes one way, and connect that gear to a regular little motorized fluid pump, just not using any power on the motor--driving the shaft from outside, instead. This way if you really needed to, because of being on a really smooth road, you could still power the pump to cool things.

It's more complicated, but would be even more efficient, as much more coolant could be moved thru the system in much less time.

The same system could be used on the rear shock for a rear-mounted hub motor (which could probably not be done, at least not easily, with the original method).

It is even possible to tie both ends together to make a larger circulation system (but it would get heavier quick due to more fluid in the system).

This system could be used to cool controllers and/or battery packs as well as motors.


Now you have me thinking, and I have some stuff laying around somewhere that could be adapted to test this, to try cooling my non-hub-mounted motor without using an electric pump, using some parts off a PC liquid cooling system.
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