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Tham khảo tài liệu 'mass transfer in multiphase systems and its applications part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Laminar Mixed Convection Heat and Mass Transfer with Phase Change and Flow Reversal in Channels 189 boundary layer. As the air moves downstream these forces become weak and the effect of buoyancy forces becomes clear. As both thermal and solutal Grashof numbers are negative buoyancy forces act in the opposite direction of the upward flow and decelerate it near the walls. This deceleration produces a flow reversal close to the channel walls at X 2.31. Buoyancy forces introduce a net distortion of the axial velocity profile compared to the case of forced convection. The flow reversal is clear in Figure 4 which show the evolution of the axial velocity near the plates. Three different temperatures at the channel inlet are represented in this figure To 30 C GrT -0.88.105 and GrM 1.07.104 41 C GrT -1.71.105 and GrM 0 and 50 C GrT -2.29.105 and GrM -1.29.104 . We notice that the axial velocity takes negative values for the last two cases over large parts of the channel length. Along these intervals air is flowing in the opposite direction of the entering flow. That change in the flow direction gives rise to a recirculation cell and to the flow reversal phenomenon. Figure 5 shows the streamlines for the vertical symmetric channel. Two recirculation cells are present close to the channel entrance. Careful inspection of Fig. 5 show that the streamlines contours in the recirculation cells are open near the plates. Indeed these streamlines are normal to the channel walls. Local velocity is then directed to these walls as condensation occurs here Oualid et al. 2010b . a Fig. 3. Axial velocity profiles in the vertical symmetric channel for T0 50 C and Ộ0 30 Oulaid et al. 2010b Fig. 4. Evolution of the axial velocity near the plates of the vertical symmetric channel for Ộ0 30 at Y 1.33 10-4 Oulaid et al. 2010b 190 Mass Transfer in Multiphase Systems and its Applications Case 4 Fig. 5. Streamlines in the vertical symmetric channel for To 41 C and ộữ 43.25 GrT -1.71.105 and GrM .