Periodic impulse generated by detonation waves is a key for a pulse detonation engine. To simulate inhomogeneous two-phase detonation, a new two-dimensional Eulerian-Lagrangian model that includes two-way coupling between gas and droplet phases is presented. Detonation of n-heptane in air/oxygen is simulated using the high resolution monotonic upwind scheme for conservation laws (MUSCLs) scheme to study thermodynamic and dynamic properties of two phase detonation. The computed detonation wave pressure is in agreement with experimental results. Based on the fuel droplets with a smaller particle size (25 $\mu $m), the computed detonation parameters corresponding to the Chapman-Jouguet (C-J) state are close to the theoretical values. It verifies the model and the research method established in this paper. Moreover, effects of droplet sizes on the detonation process are discussed. The results show that droplet break and the evaporation domain is related to the droplet size for uniform premixing. As the initial particle size of the fuel increases, the liquid droplet breaks and the evaporation domain width increases, with linear relationship between them. When the initial particle size of the fuel is increased from 25 $\mu $m to 100 $\mu $m, the width of the reaction zone is increased by 240 %, and the peak pressure of detonation is decreased from 7.08 MPa to 5.14 MPa.