Need some justification....
 

Iam simulating a natural convection flow around vertical and inclined tube bundles. The tube bundles consists of 4 pipes of diameter 20mm and length 200mm. The spacing between pipe centres is 40mm. The pipes are at constant temperature of 50 degree. The heat transfer coefficient for individual pipes in inclined arrangement is higher than vertical arrangement. Can anyone explain the reason behind this increment....

Also I gave roughness to the outer surface of pipes. The percentage increase in heat transfer coefficient in vertical pipes is higher than inclined pipes. Pls someone explain me the reason behind this.

Think about how the fluid flows around the pipe when the pipe is inclined vs how the fluid can only flow along the pipe when it is vertical. Also, think about the implications for boundary layer growth when the flow behaves this way. To ask a different way, what is the difference between a vertical pipe vs a horizontal pipe?

The wall roughness (in Fluent) is not very magical but requires some reading from you to understand exactly what the wall roughness option does. Btw, you should read this carefully to make sure you don't misunderstand the wall roughness option and apply the wrong wall roughness. Basically, if I add 1 to 10 versus 1 to 11, then the percentage change from 10 to 11 is bigger than 11 to 12.