Void fraction distributions in helical coils of various diameters are computationally estimated with inlet mass flux, degree of subcooling and wall superheat as inputs. The heat flux wall partitioning method in wall boiling model is used for numerical simulations of subcooled forced convection boiling. The effect of input parameters on axial variation of void fraction is studied. The void fraction in axial direction starts increasing under the presence of subcooled liquid for a tube of large coil diameter. In comparison, the increase in void fraction in axial direction for a small coil diameter tube is less but the rise in liquid temperature is larger. The reasons of the deviations are ascertained by examining the phase distribution contours. The phase distribution is found to be different at constant vapor fraction values for varying mass flux and coil diameters. Empirical relations are given for estimating void fraction based on drift flux model. From the computational results, a correlation for estimating vapor quality for helical coil under varying conditions is proposed. A correction factor that accounts for the curvature effect is incorporated for evaluating distribution parameter and drift velocity. © 2021 Elsevier Ltd