Heat transfer enhancement of transverse ribs in circular tubes with consideration of entrance effect

The heat transfer enhancement of transverse ribs in circular tubes with a length-to-diameter ratio of 87 was experimentally investigated. The mean heat transfer and friction data were obtained for the air flow started from the entrance. An isothermal surface condition was considered. The rib pitch-to-tube diameter ratio (p/d) was in the range 0.304–5.72; the rib height-to-tube diameter ratio (e/d) was in the range 0.015–0.143; the considered Reynolds number (Re) was in the range 4608–12,936. The mean Nusselt number (Nu) and friction factor (f) were individually correlated as a function of p/d, e/d and Re. A critical e/d, equal to 0.057, was found. For e/d < 0.057, the f is proportional to e/d; for e/d ⩾ 0.057, the f is proportional to (e/d)2.55. A performance map, indicating the corresponding heat transfer enhancement index (r1) and efficiency index (r2) for various p/d and e/d, was constructed. This performance map clearly indicates the ranges of p/d and e/d with a high r2.