I'm finning / grooving a head. Suggestions on fin / groove thickness and spacing?

Flashlight Modding and DIY Parts
21

Finned heatsinks can radiate heat just fine. The problem is that heat radiates away from the surfaces in all directions so in between the fins, most of that radiant heat is directed to the fins next to it. Convection currents can flow between the fins to carry the heat away but radiant heat only works well when surfaces face more or less outward from the source. An unfinned flashlight shouldn’t run that much hotter than a finned one if radiant heat did a better job of removing heat.

That said, a finned heatsink relies on certain conditions like having properly designed/positioned fins and plenty of cool air to create currents. In extremely hot conditions or if the air is too thin, radiated heat could easily be greater than convected heat. I think radiant heat also works very consistently in any environment.

22

I did some calculations a couple of months ago, and while the radiated heat from a black body is proportional to the 4th power of the temperature, that temperature is measured in degrees Kelvin. So the 4th power thing is not quite as impressive as it would at first seem.

Kelvin = Celsius + 273. So going from 25C to 75C is not (75/25)to the 4th more effective (3^4)= 81 times, but rather (273+75/273+25) to the 4th which is 1.167^4 which is only 1.86 times as affective.
Anyway, I did the rough calc and determined that a P60 flashlight running at a temp to hot to hold could radiate about 3W of heat. That is not enough to dissipate all the heat generated from a highly driven XM-L.
I did these calculations when I was building the light bar for my zero-turn mower. I added 4 - P60’s to the mix. with only 3W radiated and about 10W to dissipate, I am in trouble if the mower is not constantly moving. (sort of like a stationary air-cooled motorcycle)

One reason why this is not so much an issue with flashlights, is that the cells start to sag as soon as the light is turned on. As the entire body of the flashlight is filling up with heat (energy) , the cells are being depleted of energy. In my app, there is a generator that keeps pumping the energy to the lights.

23

dchomak, your calculation seems reasonable. The radiation portion of the heat transfer from our lights is not trivial, thus cannot be ignored. It is just not the primary mode. In fact, in a situation like yours where you are in need of more cooling than you have, it may be reasonable to dull any shiny surfaces that you have. Emissivity for polished Al is .039-.057. "Roughened" Al is around .2 or higher, depending on how rough you go. I'm thinking a nice bead blast surface for Al would put it in the .2 range as well.

Equation for radiation heat transfer of a gray body. q = E*Boltzman Constant *(Thot^4-Tcold^4) *Area

E is emissivity. Temperatures are in Kelvin. Area is in meters squared. Answer is in Watts.

Even being very conservative and rounding up E for polished Al to .06. Rounding E (way) down for roughened aluminum to 0.10, you can see there is a significant difference. Doing the math for a pretty hot light with those values for E gives a 40% increase for radiation heat transfer just with changing the surface finish. Putting a more realistic value of .2 in for the roughened aluminum instead of .1 shows an increase of 70%. So there is a gain to be had.

I bolded "Area is in meters squared" above because our lights have very small surface areas. Just quickly, I measured a Mag D head and calculate that it's area is in the ballpark of 17 square inches That's only .011 meters squared. And it is why radiation heat transfer is the minor player of the two. Just to give you some numbers, a shiny un-anodized mag head will emit about .14 watts while the factory anodized head will emit a whopping 1.79 watts. A bead blasted head would emit about half a watt. These numbers all assume head temperature is an even 130F, which is pretty darn hot. Cut those numbers in roughly half for head temp of 110F.

Convection is a little more murky. The equation is simple:

q = hA(Th-Tc)

q is heat transfer in watts

h is the heat transfer coefficient (the murky part). It has a wide range. For instance, if you tail stand the light it might be 20. Lying on it's side on a table it might be 11.

A is in meters squared again.

Th and Tc can be either Celsius or Kelvin.

For the same .011 meter square mag light at 130F (I did the conversion to C) we get anywhere from 3.5 to 7 watts of heat transfer in still air depending on what value of h is used.

Neither of these calculations account for any conduction of heat to the body and it's subsequent heat transfer from both mechanisms, which would be considerable. Therefore, total estimated heat transfer would be higher, likely considerably.

While I'm at it, I might as well talk a little about paint. Black epoxy has an emissivity of .89. That's pretty high (Anodized is .77). Putting that into my radiation spreadsheet (yeah, I made a spreadsheet) tells me that I can expect 2.06 watts to be radiated from the same Mag D head at 130F. That's nothing to sneeze at. This begs the question; How much does the insulating property of the paint decrease the convective heat transfer?

***Disclaimer***

My math is rusty and my spreadsheet skills are waning. There are probably a mistake or several. I'm sure you will point them out. :)

24

That’s really not an insignificant amount!

About anodizing, I don’t think it is the same as a layer of paint as it is more a dye:

25

The heat loss that you’re saying is being radiated because convection cannot account for it is indeed radiant heat but not from the flashlight housing. The intense heat directly off the phosphors leaves thru the lens along with all those other useful emissions that we want. Put your hand in front of the lens and you feel can feel it. That’s where it’s shedding nearly all it’s radiant heat. The housing is not emmitting anywhere near as much.

26

lightme, that proves out with my HD2010…with a tested throw of over 900M you can hold the light at shoulder level and feel the heat in the beam on your bare foot. Not just warm, but hot…from about 5’ away. :wink:

I do remember reading an article that showed a thermal graph where the heat behind the emitter spreads in an almost identical cone as the light out the front. So while there is an intense amount of heat coming through the lens, it’s just about equaled by the cone of heat put into the host behind that tiny square.

27

If anyone wants to do a more scientific analysis to determine what’s really happening, I’ll listen and learn but thru my own (crude) observations, radiant heat transfer doesn’t contribute much to cooling a well-finned heatsink as long as there is plenty of cool air available. Put a flashlight in a vaccum or a gravityless environment where convection won’t occur and all the heat from the housing will be removed by radiation only but it will get very hot. Under those conditions color would make a noticable difference but in the real world with sufficient finning and air, it won’t.

28

While it is true that the housing is not emitting nearly as much, that is an apples to oranges comparison. The heat in the housing is a byproduct of the <100% efficient conversion of electricity to light in the LED die. Almost all of the radiation produced by the light is emitted out the front and is not the dominant factor heating the housing up. The LED die gets hot because it has resistance and current flow. Light and heat are the bye-products. One we want and one we don't.

On to progress:

Head threaded for pill and top of SL-20X

With Pill screwed in

Curly cues and finned head

Closeup of head

Parted off

I'm pretty pleased so far. I had a moment of anxiety or two while parting all of those grooves, but it worked out well. Most of the top part will be turned down to be threaded for the bezel, which will be made last.

29

A thing of beauty! With that copper cored pill plugged in, wow! Awesome work there Buck, love it!

Mail has run today, no package…will be looking for it tomorrow. :wink:

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lightme said: “Finned heatsinks can radiate heat just fine. The problem is that heat radiates away from the surfaces in all directions so in between the fins, most of that radiant heat is directed to the fins next to it. Convection currents can flow between the fins to carry the heat away but radiant heat only works well when surfaces face more or less outward from the source. An unfinned flashlight shouldn’t run that much hotter than a finned one if radiant heat did a better job of removing heat.”

I wonder if there is a way to design the ‘fins’ so that when heat is being radiated, air currente are formed and utilized to transfer the heat away from the fins/head faster?

31

That is done via the shape/spacing of the fins. Heatsinks for passive cooling have thick, widely spaced, sometime tapered fins. Those for forced air cooling (like CPU coolers) have thin, closely spaced fins.

32

Of course not, radiation can only remove heat from the source. That’s why it accounts for whatever heat loss (missing energy) convection and radiation at the housing doesn’t. Maybe “heat loss” wasn’t a good choice of words.

Yeah, looking good!

How deep is that copper? Does the star overlap the copper and aluminum? It’s difficult to judge the size in the pictures.

I took a closeup of my C12 and noticed an acumulation dust and lint. I have plans to cut fins sometime soon but I think I’ll go with a wider groove than what I really want. You’re too fast.

33

USPS tracking says it was delivered today at 11:17 AM. Maybe double check? It's a small box.

34

The copper runs the length of the pill and is 5/8" diameter, coincidentally almost an exact fit for a 16mm star :) I cut the hole for a sliding fit, then press the crap out of it. It flows into the hole pretty good. I've successfully soldered two stars to similar, if smaller heat sinks prior to this and will be attaching this one the same way. It's surprisingly easy less than impossible. I was going to drive single XM-L2 at 7 amps, but am considering a MT-G2 instead at perhaps 9 amps. I don't know how the switch in this light will do in that regard.

One last pic for tonight. 007 will be happy to see the 4 jaw holding the head. The head has been reversed and faced to length. I bored this end to the diameter needed for the reflector and cut the bezel shoulder which has layout dye on it for threading. I don't thread when I'm tired, so that will be tomorrow.

I guess I should give the scale of this.

Head Diameter = 2.982"

Head Length = 2.347"

Pill Diameter = 2.242"

Pill Length = 1.400"

Pill Weight = 262g (.576 lb)

35

262g for the pill? Do you mean the whole assembly or is there a decimal point missing?

36

Dude, it’s got a 5/8” x 1.4” copper rod in it! That copper makes it heavy! And beautiful! And I bet very efficient! I’m jealous…

37

Tofty is making a 10A+ switch if you’d like to ensure it’s equipped to handle the load. Nice solid switch, reverse clicky, should work very well for this light.

Tofty’s 10A+ Switch

38

262g is just the pill. No decimal point. The head is still in the lathe chuck and I really don't want to have to put it back in and indicate it back to where it is now. I'm pretty slow at centering pieces in the 4 jaw. Anyway, I hesitate to estimate the mass of the head. It's just too hard to guess with all of the fins and two different diameters bored in it.

39

WOW! My whole C8 with a battery weighs a lot less.

40

Here is the head on the light. Machine work to it is done except I haven't decided what to do to break the corners on the fins yet.

On to the bezel!