Experimental Investigation of Free Convection Heat Transfer With Entrance Restriction Placed at Top of a Vertical Circular Tube

A free convection from the inside surface of vertical circular tube with a uniformly heated surface (constant wall heat flux) was investigated experimentally for laminar air flow in the ranges of Ra_L from 6.9 × 10⁸ to 5 × 10⁹. The effect of restriction lengths placed at top position of the heated tube on the surface temperature distribution and the local and average heat transfer coefficient were studied. The experimental apparatus consists of aluminum tube with 900mm length and 30mm inside diameter. The entry restrictions were included a circular tube of same diameter as the heated tube but with different lengths of 60cm, 90cm, 120cm, 150cm, and 180cm. It was found that the surface temperature along the tube surface higher values for restriction with length of 180cm and smaller values for the restriction with length of 120cm. The results showed that the local Nu_x and average Nusselt number Nu were higher values for the restriction with length of 120cm and smaller values for the restriction with length of 180cm. The results obtained are correlated by dimensionless groups as Log Nu_L against Log Ra_L for each case and proposed a general correlation for all cases.