I went through some of this stuff back when I was into overclocking…
When comparing copper and aluminum heatsinks you have to ask whether you are considering heatsinks of equal volume or equal mass. If equal volume then the copper heatsink wins handlily but weighs over 3 times as much. If comparing heatsinks of the same mass the Aluminum heatsink will be over 3 times the size of the copper and can have a much greater surface area. In that situation the copper heatsink will get hotter and initially pull more heat out of the heat source, but once it heats up the aluminum heatsink may be more effective at dumping heat to the environment and allow for a lower temperature of the heat source in equilibrium.
All of this depends on specifics of geometry, thermal contact and radiator design, etc. Ultimately to maximize heat transfer you want the outside of your heatsink to get as hot as possible, have as much surface area as possible and have it be in contact with as cold an environment as possible (generally flowing air or coolant).
For a flashlight, though, we may not care about equilibrium temperature, but just keeping the light head cool for a minute or so. Under these circumstances a heatsink with higher thermal conductivity and more thermal mass (i.e. a big old chunk of copper) probably wins (but may get uncomfortable to hold pretty quickly).
It is important to make sure the heat coming into the heatsink also exits it. This can happen via conduction to your hand, convection and radiation.
Emissivity is the measure of how fast thermal energy radiates from a material. Higher emissivity is better as it means heat can get out of the heatsink to atmosphere faster.
My recollection is dark colored anodized aluminum has much better emissivity than polished copper or bare aluminum and slightly better than tarnished copper.
I wanted to share how heat is transferred from head to body and see the differences. Bike lights are just for compare to unibody design. If I want to simulate outdoor condition of bike light I can cool them with fan. But here I wanted to see how heat is spread over body of light. Also you can tell for lights with cooling when they test in reviews are also unrealistic.
Fireflight, your recollection is about right. I’m curious what the breakdown of radiative vs convective cooling occurs in our typical applications. Anodized aluminum, regardless of color, is pretty darn good when it comes to emissivity.
While everything varies with geometry I saw some old data on heatsinks for electronics showing that in most cases radiation accounted for under 10% of cooling. When forced air was used that went down to ‘well’ under 10%. So I’m not sure how much emissivity really matters here.
I recall doing some quick and rough calculations of that years ago…with conclusion that for small flashlights (and small for me is DQG Tiny 18650 and smaller), hand cooling > wind cooling > radiation. And I say wind cooling rather than convection because even small hand movements generate more wind than convection. So improving surface emissivity might help some but it’s not really a big deal.
Yesterday I tried to put cheap silicone thermal paste on threads and its works. Temperature difference between tube and head is lower maybe 5°C or more. Need to test it additional, also thermal convection seems to be better. About emissivity black anodised aluminium will be best overall. Raw alumium in my case of WT3M is worse compared to black body. Now I have two WT3M in raw but second one is with SST40. So if I compare it with another one which is with XHP50.2 I didn’t know if it will be fare comparsion at all for test of cooling. Also yesterday I repair my 50.2 version because there was leak in one 7135 chips and I removed it. I dont have spare chips. By the way I found it very easy with thermal camera which is defective. But there is also problem with charge circuit and it didn’t work anymore but I don’t need it.
Also I am waiting to receive my 3rd WT3M with XHP50.2 and black anodisation. It will be interesting to compare it to another 50.2 in raw with thermal paste and raw body.
This leads to the question - what is realistically the smallest flashlight where an internal fan would be practical and ergonomic?
(Just dreaming of a 2,000lm sustained 1x21700 flashlight).
I’ve actually been kicking around the idea of 3D printing a body and integrating a fan and using a small cpu sink for the emitter that would ‘float’ inside the host. I was thinking of doing this on a soda can sized light, but can also imagine it as a tube light. Aerodynamics would be compromised, however.
