In order to obtain the optimized flow condition inside an optical cavity with direct flow configuration, we conducted numerical simulations using commercial computational fluid dynamics (CFD) code Fluent 15.0 solver. The average residence time of particles, the width of particle beam, and particle loss rate were calculated from the particle trajectories that were obtained using Discrete Phase Model (DPM). Also, the Discrete Random Walk (DRW) was used to consider the effect of turbulence on the particle. It was found that particle loss rate and the residence time of particles can change according to the sheath flow rate. Increasing the sheath flow rate has the effect of reducing the particles loss rate and the size of the recirculating particles. It was found that using sheath flow with a flow rate over 0.7 L/min was an effective way to prevent particles from recirculating, which causes particle loss inside an optical chamber. However, an increase in sheath flow rate increases the pressure drop and reduces the residence time of particles. Furthermore, as the size of particle becomes smaller, the average residence time of particles above 3 mu m decreases. By contrast, due to the turbulent dispersion of particles, the residence time of particles below 3 mu m increases as the size of particle becomes smaller. For 1-10 mu m, the Brownian motion of particles can be neglected compared to the turbulent dispersion of particles. In addition, flow visualization experiments were conducted to validate our CFD simulation results and experimental results were in good agreement with numerical predictions. Overall, in the design of particle detection devices, it is important to consider the influence of the sheath flow on the particle trajectory through numerical simulation.