I need a heat sink with a thermal resistance of 0.7 °C/W/3" or less. C/W/3" represents how many degrees Celsius per watt a 3" long section of heat sink can dissipate without exceeding a max temperature, usually specified as 75°C.
I did find thermal resistance calculators online but only for simple rectangular heat sinks.
I'm interested in the large, differently configured heat sinks on eBay but none list the thermal resistance. How can the TR of odd shaped heat sinks be determined?
The photo is typical of the complex shapes I'm interested in.