ABSTRACT

The paper presents results of an experimental investigation carried out to study the effect of relative roughness pitch and perforation of the spring roughness (by changing coil pitch of the helical spring) on heat transfer and friction factor for turbulent flow in an asymmetrically heated annular duct (radius ratio = 0.39) with a heated tube having a spirally wound helical spring. The spring diameter to hydraulic diameter ratio is 0.11. The relative roughness pitch ranges from 4.09 to 8.18, while the relative coil pitch is 2.66−4.5. The maximum enhancement in the Nusselt number over the smooth annulus ranges from 95 to 172% for Spring 2 (with a relative coil pitch, p_c/d, of 2.66 arranged at a relative roughness pitch, p/e, of 4) while the lowest enhancement of 54−81% is seen for Spring 12 (p_c/d = 4.5; p/e = 8) in the flow Reynolds number range of about 4000−14,000; the Nusselt number ratio, Nu/Nus, has been found first to increase with an increase in the Reynolds number up to about 10,000 and then decrease. The corresponding enhancement in the friction factor values is 193−307% and 116−137%, respectively. Thermal performance comparison at equal pumping power for the roughened and smooth annuli shows performance advantage of 32−83% for Spring 2. Nusselt number and friction factor correlations have been developed for the most preferred type of roughness.