Considering the effects of both surface micro-scale and gas film thermal viscosity

the expansion average Reynolds equation for compressible fluids was modified based on the theories of gas polytrophic process and gas lubrication.The modified Reynolds equation was solved with the finite element method. The effects of micro-scale and thermal viscosity on a spiral groove dry gas seal(S-DGS)performance were investigated with numerical solution of the modified Reynolds equation.When S-DGS was under normal running conditions

the couple effects of gas slip flow and surface roughness were lessened and even disappeared

and only the influence on leakage was at a fixed value with the pressure increased.The influence of surface roughness on sealing performance should be considered when the non-dimensional film thickness Hs<3.5.If the inverse Knudsen number B≥21 or Knudsen number Kn≤0.0476

influence of gas slip flow could be ignored.Sealing dynamic pressure was increased

while leakage decreased due to the effects of surface roughness and thermal viscosity.On the contrary

the influence of gas slip flow increased leakage and reduced loading capacity of gas film.When the S-DGS was operating at low speed and high pressure

the influence of gas sliding flow on sealing performance was less than the influence of surface roughness.When po≥2.02 MPa

thermal viscosity effect dominated in affecting sealing performance.