Experimental investigation on heat transfer and fluid friction correlations for circular tubes with coiled-wire inserts

Heat transfer and pressure drop data for air flow and water flow in smooth tubes with coiled-wire inserts were measured. The wire diameter-to-tube inner diameter ratio (e/d) and coil pitch-to-tube inner diameter ratio (p/d) are in the ranges of 0.0725 to 0.134 and 1.304 to 2.319 respectively. It is found that the Nusselt number (Nu) increases with the e/d value, whereas it increases with a decrease of the p/d value. As air is the working fluid, the dependence of the heat transfer enhancement of the wire coil on the Reynolds number (Re) is minor; as water is the working fluid, the heat transfer enhancement considerably decreases with an increase of the Re value. Two heat transfer empirical equations, one for air and the other for water, are proposed. For both air and water, a common fluid friction empirical equation is established. To effectively and efficiently enhance the heat transfer, for air, the proposed e/d and p/d values of the wire coil are 0.101 and 2.319, respectively; for water, the proposed e/d and p/d values are 0.101 and 1.739, respectively.