This paper presents a comprehensive numerical study of a low-Reynolds-number hypersonic diffuser designed for a 1.6MWArc-Heated Wind Tunnel facility. The numerical study includes the detailed computational fluid dynamics (CFD) investigation of the high-temperature, viscous, chemically reacting nonequilibrium hypersonic flowfield inside the diffuser with Reynolds-averaged Navier-Stokes simulations at a number of diffuser exit pressure conditions. The CFD analysis is used to evaluate the overall performance of the configuration at selected operating conditions and to provide wall heat flux values for the numerical analysis and design of an active cooling system for the thermal management of the diffuser. The CFD results are obtained with noncatalytic and fully catalytic wall assumptions to bound the estimated heat flux to the diffuser walls. The effects of surface catalyticity and thermochemical nonequilibrium on the flowfield are discussed, with the emphasis on diffuser and active cooling system design. A detailed numerical analysis focusing on active cooling of the diffuser is presented. The results of the CFD and thermal analyses presented in this paper are expected to be a valuable addition to the very limited literature on low-Reynoldsnumber hypersonic diffuser design.