In this study, flow in a solid fuel ramjet is numerically simulated to calculate the regression rate of solid fuel. In spite of simple geometry of a solid fuel ramjet combustion chamber, due to presence of phenomena including formation of a recirculation zone, turbulent jet, reattachment zone, turbulence boundary layer, combustion, and evaporation of solid fuel in the boundary, a complex flow is encountered. Investigations reveal that the most important reason for discrepancy between the numerical results and experimental data is the weakness of turbulence models. For the first time, a new low Reynolds number k-ε model is used in solid fuel ramjet which is relevant to the flow behind a step. In present work, a new algebraic model for turbulent heat diffusion coefficient is proposed based on the modification of the constant Prt model. This modification is made using an empirical correlation between maximum Nusselt number and entrance Reynolds number. The model includes both the simplicity of Prt=0.9 assumption and the accuracy of the two-equation heat transfer model. Numerical results show better agreement to the experimental data compared with the previous works.