The flow-field structure and pressure gain performance of a rotating detonation engine with banded distribution of reactants have been studied using two-dimensional numerical simulations. The reactants are premixed H-2/Air mixture. An unsteady reacting flow solver named rhoHLLCFoam is developed based on the open source software OpenFOAM. Unsteady Reynolds Averaged Navier-Stokes (RANS) equations are solved with second order accuracy in space and time with Harten-Lax-van-Leer-Contact (HLLC) Riemann scheme. The solver resolves the combustion phenomena through finite rate chemistry reaction model with Arrhenius form of reaction rate by using O Conaire scheme. After checking the reliability of the solver, two sets of cases with various inlet-area ratios (psi) and equivalence ratios (phi) are conducted. The result shows that with psi< 1.0, the reactants in front of detonation waves present a discretely banded distribution which causes a series of reverse compression waves in flow-field. This paper estimates the specific impulse and specific thrust of combustion chamber. It's shown that these parameters increase with the promotion of psi. By calculating the area-averaged stagnation pressure along axial direction of combustion chamber, the pressure gain ratio (eta) of the rotating detonation engine is estimated. The result suggests that eta decreases dramatically with the reduction of psi. In order to achieve pressure gain, psi must be greater than 0.60. Moreover, the equivalence ratio should be around unity to obtain higher value of eta. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.