Understanding heatsinks

[vsr123]

I know this topic has come up many times but can someone answer this simple question of mine?

In looking at the graphs on this page -

http://www.thermaflo.com/bin/exsrch.pl?PgNum=1&LengthUnits=in&ExLength=&WidthMin=8&HeightMin=&WidthMax=12&HeightMax=&ThermalResist=&CType=Natural&AirFlow=&AirUnits=LFM&Shape1=ON&Shape2=ON&Shape4=ON&Shape3=ON&SearchButton1=Search+for+Extrusions&SortBy=V4&NumPerPage=v2

if I choose the E1335 model heatsink the rating is shown to be approx .27C/W, however when I look at the convection cooling graphs for this heatsink it shows that there will be an approx 20C rise for 10 W of power dissipated. I understand that heatsinks will be more efficient at higher temperatures but given that we are trying to get heatsinks to keep the rise over ambient temperature at around 20 to 25 C realistically what value of C/W should I be looking at?

thanks
vsr

 

[rtirion]

.27 c/w is the forced convection performance for 3 inch length.
i.e. you will have to use a ventilator producing an air
flow with a veloctiy of 100 m/s.

the 20 degree celsius rise over ambient at 10w is with
natural convection for only 3 inch of sink.

For an Aleph 3 you would want something like .2 or .25 c/w
The ones I am using have the following spec's
11" width, 3,1" height and 10" length. Black anodized.
This gives around 25 degrees celsius over ambient.
Fets are mounted using mica and goop. Natural convection off
course.