This paper presents a numerical study of stagnant conductivity effect on the fully developed non-Darcian mixed convection in horizontal packed-sphere channels. The effect of stagnant conductivity is characterized by the conductivity ratio of solid sphere to fluid (k(s)/k(f)). The results show that the buoyancy effect affects significantly the secondary flow structure and heat transfer rate when k(s)/k(f) and Peclet number are all low. But under a fixed Rayleigh number, the effect of buoyancy will be suppressed when the value of k(s)/k(f) or Peclet number increases. The critical Rayleigh number for the onset of buoyancy effect is shown to increase more than 1 order of magnitude when k(s)/k(f) increases from 1.3 to 26.8 or 38.8 at low Peclet number.
