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I have a high power LED (100W) that needs attaching to a heatsink (and presumably fan). Could I just immerse it in an insulating fluid and use natural convection/conduction to cool it?

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It depends on the thermal resistance between the LED and the fluid. A heatsink not only dissipates the heat, it also conducts the heat away from the device itself. A heatsink will probably have a better thermal contact to the LED than a liquid. – FakeMoustache Mar 2 at 15:12
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@FakeMoustache - using Liquid removes (mostly) the case to heatsink thermal junction. Getting almost 100% contact with the case. – Marla Mar 2 at 15:16
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You need to consider the generally high viscosity of natural and synthetic oils which impedes effective thermal convection. With no forced flow, I would still use a heatsink as FakeMoustache said. A better (and more expensive) choice might be two-phase cooling, where a volatile chemical boils at the heat source at low temperature carrying away the heat. The vapour subsequently condenses at the cooler surfaces of the (hermetically sealed) enclosure. – jms Mar 2 at 15:16
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@BrianDrummond Kerosene is more transparent, has similar thermal conductivity, lower viscosity and higher heat capacity, but there might be 'other disadvantages'. – Spehro Pefhany Mar 2 at 15:22
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@SpehroPefhany - True, but it also can serve as a source of backup illumination. – WhatRoughBeast Mar 2 at 15:51
up vote 11 down vote accepted

Liquid cooling can be, and is used in some electronics equipment.

You still end up with the requirement to remove the heat (and you did mention using convection and conduction). So you end up having a heat sink (or other heat removal device) somewhere remotely located from the heat source.

The benefit of liquid cooling really comes into play when the liquid is circulated (moved), moving the hot liquid to the remote heat removal device. Without movement of the liquid, you end up with just a heat reservoir.

Another consideration to take into account is the purity of the liquid. If contaminants get into the liquid, negative results could occur.

Also, liquids have different specific heat and thermal conductivity, which would need to be considered.

Resistivity of the cooling medium could be of concern if your electrical contacts are also submerged.

EDIT 1 : Caution : I am not sure that it is relevant here, but flammability of oil should be considered as potential hazard.

EDIT 2 : Reference Reading Material on liquid cooling

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+1 Consider the liquid CPU coolers that move the heat to a radiator where a fan or fans can exchange the heat to the air. If you just want to illuminate the LED briefly a hefty chunk of aluminum will probably work better and has fewer of the disadvantages that Marla points out. – Spehro Pefhany Mar 2 at 15:12
    
+1. I'll also add that in applications when duty cycle is small, a heat reservoir can be a viable solution. – Nick Alexeev Mar 2 at 16:44

A little aluminum can go a long way, even aluminum with no fins as a heat sink. Anything to increase the surface area. I would recommend trying a piece and see how it goes, even a 1/8" or 1/16" thick piece that is a few square cm's. If you can put fins on it your in even better shape. Digikey also has small heat sinks in stock that would work well, thermal grease also helps.

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The question is about liquid cooling, not heatsink cooling. – Mark Mar 2 at 22:37
    
Yep, and I'm suggesting that liquid cooling is silly – laptop2d Mar 2 at 22:58

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