[nived]

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.