An empirical study on the pressure drop characteristics of nanofluid flow inside helically coiled tubes

► Pressure drop characteristics of MWCNT/heat transfer oil nanofluid flow inside helically coiled tubes are studied experimentally. ► The experiments are implemented under uniform wall temperature condition in the thermal entrance region of the laminar forced-convective tube flow. ► Helical coiled tubes and nanofluids are used simultaneously as the two heat transfer enhancement techniques. ► Simultaneous utilization of the so-called techniques increases pressure drop significantly. ► A rough empirical correlation is proposed to predict the fluid flow friction factor in terms of the Dean number, fluid properties and geometrical parameters.