In this paper we present the results on experimental and numerical investigation of the local turbulent mixed convection heal transfer in a vertical (φ = 90°) flat channel with opposing flows (heated walls with downword orented forced flow). The experiments were performed in air flow of different pressures, from 0.1 to 1 MPa, in the range of Re from 4·103 to 5.6·104 and Grq up to 1·1011 for symmetrical heating of the walls. Analysis showed that at higher pressures for Re numbers below 8000 heat transfer variation along the channel takes non monotonious character. Numerical two-dimensional simulations were performed for the same channel and for the same conditions as in experiments using FLUENT 6.0 code and low Reynolds k−ω turbulence model. Modelling results show that for regimes with not monotonious variation of heat transfer along the channel, upward oriented revers air flow at the heated wall is existing when in the core the main (forced) flow is downward oriented.