Thanks for the suggestion. Yes, you are correct, I had only used the formula for long fin. So what I did was, instead of declaring convection for both the surface of the engine block as well as the surface of the fin, I declared them seperately, and added a reaction probe result to get the heat flow from fins alone. Coupled with that I calculated fin tip temperature and heat flow for all 3 types of fins (long fin, short fin with insulated end, and short fin with end not insulated) as provided in the databook, with results as shown below:

Based on above values, I find that "short fin, end not insulated" has values closest to ANSYS results, so I'm going to go forward with that.

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Just one other query: Here's a similar engine block but with triangular fins:

Here, I'm planning to use the same formulas for verification. But I need to know how to find the value of cross-sectional area and perimeter of the cross-section to input in the formula, since unlike rectangular fins, the cross-section changes along the fin here. Appreciate the help!