In my (limited) experience, a fan’s airflow is rated in CFM (Cubic feet per minute). When I read application notes for certain devices, however, they tend to talk about airflow in terms of LFM (linear feet per minute). LFM is apparently equivalent to CFM divided by the cross-sectional area of interest. The larger the cross-sectional area, the smaller the LFM for a given CFM.
So, why do ribbed surfaces act as heatsinks then? Ribbing a surface effectively increases the cross-sectional area, right? If that’s true, then adding ribs to a metallic surface would increase the area over which the air passes, which, if I’m interpreting the above relation correctly, actually reduces the LFM figure. In the app notes I’ve been reading, lower LFM means less cooling. So, adding a heatsink reduces cooling? That doesn’t make sense. I must be interpreting something incorrectly. Does anyone know which aspect I’m misinterpreting?
That, or I’m forgetting to account for the fact that increasing the surface area will also increases the amount of material the heat can conduct to.
Either way, I’m new to the specifics of cooling, and I’d like to get my facts straight. Any help would be appreciated.