Droplet vaporization  The vaporizing droplet (droplet vaporization) problem is a challenging issue in fluid dynamics. It is part of many engineering situations involving the transport and computation of sprays: fuel injection, spray painting, aerosol spray, flashing releases… In most of these engineering situations there is a relative motion between the droplet and the surrounding gas. The gas flow over the droplet has many features of the gas flow over a rigid sphere: pressure gradient, viscous boundary layer, wake. In addition to these common flow features one can also mention the internal liquid circulation phenomenon driven by surface-shear forces and the boundary layer blowing effect. One of the key parameter which characterizes the gas flow over the droplet is the droplet Reynolds number based on the relative velocity, droplet diameter and gas phase properties. The features of the gas flow have a critical impact on the exchanges of mass, momentum and energy between the gas and the liquid phases and thus, they have to be properly accounted for in any vaporizing droplet model. As a first step it is worth investigating the simple case where there is no relative motion between the droplet and the surrounding gas. It will provide some useful insights on the physics involved in the vaporizing droplet problem. In a second step models used in engineering situations where a relative motion between the droplet and the surrounding exists are presented. In this section we assume that there is no relative motion between the droplet and the gas, formula_1, and that the temperature inside the droplet is uniform (models that account for the non-uniformity of the droplet temperature are presented in the next section). The time evolution of the droplet radius, formula_2, and droplet temperature, formula_3, can be computed by solving the following set of ordinary differential equations.::  where:  The heat flux entering the droplet can be expressed as:  where:  Analytical expressions for the droplet vaporization rate, formula_7, and for the heat flux formula_11 are now derived. A single, pure, component droplet is considered and the gas phase is assumed to behave as an ideal gas. A spherically symmetric field exists for the gas field surrounding the droplet. Analytical expressions for formula_7 and formula_11 are found by considering heat and mass transfer processes in the gas film surrounding the droplet. The droplet vaporizes and creates a radial flow field in the gas film. The vapor from the droplet convects and diffuses away from the droplet surface.