In this paper an empty cylinder which is as a simulator for a ramjet engine has been studied by a cold flow. The air flow comes into the cylinder from one side and injection operation will be done by four nozzles which are located in the entrance of cylinder. By changing length to diameter ratio and injection angle, we have studied droplet sizing and efficient mass fraction of fuel vapor near and at the exit area. We named the mass of fuel vapor inside the flammability limit as the efficient mass fraction. To fulfill the calculation we used a modified version of KIVA-3V.