The Navier-Stokes slip boundary conditions are considered as conditions following from the mass and momentum balances within a thin, shell-like moving boundary layer. A problem of consistency between different models, that describes the internal and external friction in a viscous fluid, is stated within the framework of a proper form of the layer momentum balance. Appropriate constitutive equations for friction forces are formulated. The common features of the Navier, Stokes, Reynolds, and Maxwell concepts of a boundary slip layer are revalorized and discussed. Different mobility mechanisms connected with the transpiration phenomena, important for flows in micro-and nanochannels, are classified as a part of equations for the external friction.

In this paper a three-dimensional model for determination of a microreactor's length is presented and discussed. The reaction of thermocatalytic decomposition has been implemented on the base of experimental data. Simplified Reynolds-Maxwell formula for the slip velocity boundary condition has been analysed and validated. The influence of the Knudsen diffusion on the microreactor's performance has also been verified. It was revealed that with a given operating conditions and a given geometry of the microreactor, there is no need for application of slip boundary conditions and the Knudsen diffusion in further analysis. It has also been shown that the microreactor's length could be practically estimated using standard models.

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