With the heatsink fins horizontal the efficiency drops.

Convection uses the Bar-Cohen & Rohsenow correlation for an array of parallel fins: it accounts for the gap, the channel length and the temperature rise, so packing fins tighter is no longer free. Fin efficiency follows η = tanh(mL)/(mL) with k(Al) = 200 W/m·K. Radiation is taken from the visible envelope rather than the developed area — facing fins sit at almost the same temperature and exchange with each other without net transfer, so only the radiation leaving through the gaps escapes, and a deep channel radiates almost like a black body. σ = 5.67·10⁻⁸. The fan setting is a fixed h ≈ 25 W/m²·K, roughly 1.5–2 m/s. Horizontal channels are estimated less reliably than vertical ones; fins pointing downwards are worse still and are not covered here.