The hydrodynamic entrance length in helical coils for single and two-phase bubbly flow was studied experimentally and numerically. Development region length and the detailed characteristics of fluid flow have been investigated by varying helical coil parameters such as tube diameter, coil diameter and void fraction. For the CFD simulation of the two-phase fluid flow, the Eulerian-Eulerian model was employed. To calculate the turbulent fluctuations, the SSTk-omega turbulence model has been used. The experimental and numerical simulation of the local parameters demonstrates that the hydrodynamic developing length (LID) increases when Reynolds number increases in the single and two-phase flows. The obtained results show that the development entrance length increases with the increase of pipe diameter and decreases with the increase of coil diameter. Also, entrance length decreases while curvature ratio of helical coil and void fraction increase. A correlational equation has been suggested to predict the hydrodynamic entrance length as functions of various parameters of the helical coil. (C) 2014 Elsevier B.V. All rights reserved.