We present experimental data on mixed convection heat transfer in wide ranges of air flow parameters (Re = 4×10³ - 6×10, Gr_q= 3×10⁷- 4×10¹⁰ , pressures p = 0,1; 0,2; 0,4; 0,7; 1,0 MPa) in a flat channel with inclination angles from horizontal (φ=0) through 10, 20, 30, 60 degrees to the vertical position (φ= 90 deg). One-side and two-side heating modes were studied for aiding flows in a vertical channel and only two-side heating mode in an inclined channel. The results reveal typical modes of the effects of buoyancy. The experimental data were compared to the results in vertical tubes of various authors.