Abstract: In order to carry out direct-connected tests of scramjet with flight Mach number 8 and 9, the shock tunnel named FD-14 of China Aerodynamics Research and Development Center was reconstructed to be a supersonic combustor direct-connected test bed with shock heating. Two nozzles were designed for the direct-connected test bed and their outlet Mach numbers are 3.5 and 4.5, which can simulate the combustor entrance conditions of scramjet with flight Mach number 8 and 9, respectively. Using three different chemical reaction models of pure air developed by Park, Gupta and Dunn/Kang, chemical nonequilibrium flows in Mach number 3.5 and 4.5 nozzles are studied by numerical simulation. Besides, differences between the three chemical reaction models are compared and analyzed. Calculation results indicate that, in contraction section of the two nozzles, dissociations of N₂ and O₂ are obvious, but in expansion section of the two nozzles, recombinations of N and O are more remarkable. On the outlet planes of Mach number 3.5 and 4.5 nozzles, mole fractions of NO are respectively 2.3%~2.57% and 4.8%~6.0%, mole fractions of O are respectively 0.04%~0.11% and 0.75%~1.25%, and mole fractions of N are both nearly zero. In the expansion section of two nozzles, the flows are freezing. Among three chemical reaction models, the dissociation degrees of O₂ and N₂ are maximum by using Gupta chemical reaction model, which leads to the maximum mole fractions of NO and O on nozzle outlet. However, distribution laws of each component are same in nozzles and on outlet planes of nozzles acquired from three chemical reaction models.