For Cooling check out this online calculator:
http://www.novelconceptsinc.com/calc...resistance.cgi
Cree has a page regarding thermal management:
http://www.cree.com/products/pdf/XLa...Management.pdf
And their page about XP-E's:
http://www.cree.com/products/pdf/XLampXP-E.pdf
Sooooo.... I'm planning on a build with 120 LED's. The Thermal Resistance in the LED's are 9°C/W(XP-E Blue/White) and a good junction to the heatsink is 1°C/W per LED i can calcualte the minimum heatsink resistance needed driving them at 750mA at an ambiant 30°C(my house has AC, should never get that high). Thermal resistance is how much hotter something gets to disapate 1W of heat, so lower is better for a heatsink.
Aluminum is about 237 W/m*K at 26 Celcius.(wikipedia)
Use the formula for Heat from cree:
Tj = Ta + Ptotal (Rth j-sp/ NumLED + Rth sp-h + Rth h-a)
P = 120*.750A*3.25V = 292.5W
Rth h-a = (145°C - 30°C –9°C/W x 292.5W/120 – 1°C/W x 292.5W/120)/292.5W = 0.31°C/WIn
I need a min 0.31°C/W heat sink.
Using the calculator on novel i can find the size i need to meet that. I used the slab calculator and the one linked above and came up that a 10"x42" heatsink with 1" fins and .25cms of air flow, asuming .25cms average flow over the entire thing, 1 computer fan is .25, it would probably take 2 to make this consistant. This would have a C/W of : 0.0653926108°C/W, far more than needed.
Couple other pages for info on Cooling LED's:
http://www.qats.com/cpanel/UploadedP...a_0909_web.pdf
http://sound.westhost.com/heatsinks.htm