To simulate primary atomization, the dense zone of sprays has to be addressed and new atomization models have been developed as the Eulerian Lagrangian spray atomization (ELSA) model [Blaisot, J. B., and Yon, I., (2005) Exp. Fluids, 39(6), pp. 977-994]. A transport equation for the liquid/gas interface density is stated and extends the concept of droplet diameter. Several related source terms require modeling attention. This work describes the contribution of collision and coalescence processes. Several questions arise: Is it possible to represent collision/coalescence from an Eulerian description of the flow? What are the key parameters? What are the particular features of collision in dense spray? To answer these questions, a Lagrangian test case, carefully resolved statistically, is used as a basis to evaluate Eulerian models. It is shown that a significant parameter is the equilibrium Weber number. If it is known, Eulerian models are able to reproduce the main features of Lagrangian simulations. To overcome the Lagrangian collision model simplification that mostly considers collisions between spherical droplets, a new test case was designed to focus on the collision process in dense spray. The numerical code, Archer, which is developed to handle interface behaviors in two-phase flow by way of direct numerical simulation (DNS) [Iyer, V, and Abraham, J., (2005), Atomization Sprays, 15(3), pp. 249-269], was used. Thanks to DNS simulations and experimental observations, the importance of nonspherical collisions is demonstrated. Despite some classical drawbacks of DNS, we observed that an equilibrium Weber number can be determined in the considered test case. This work emphasizes the ability of interface DNS simulations to describe complex turbulent two-phase flows with interfaces and to stand as a complement to new experiments.