| OCR Text |
Show Potential advantages of this way to establish gas combustion in boilers m a y be the following. In the first place, with the use of vortex generators, the combustion process and flame stabilization are not confined by the boundaries of the main tract. In this connection, the decrease of the hydraulic resistance of the tract can be expected as compared to the traditional methods of gas combustion and flame stabilization. In the second place, the vortex generators on the side of the burning gas and the convex surfaces on the side of the coolant essentially enhance the heat transfer on the wall compared to the smooth surface. A s a result, the waterwall surface of the boiler will be more compact. To evaluate the efficiency of the gas combustion method, calculations of the heat transfer on the waterwall surface of the boiler were performed. The following assumptions and values of parameters were adopted in the calculations. Natural gas was used as a fuel, while air was chosen as an oxidizer, and water was as a coolant. The effective temperature of the flame radiation was 1820K, the air flow temperature and velocity in front of the dimple constituted 7, 0=300K and U0=30 m/s, the diameter of the dimple was d=50 m m and its depth was h=25 m m , the water temperature was rw=323K, and the water flow velocity was c7w=l m/s. During the calculations, a radiative component of the heat flux from the side of the air tract together with the convective heat-transfer conditions on convex and concave wall surfaces was taken into consideration. The wall was made of brass, and its thickness comprised 0,5mm. The estimations of the heat transfer through the wall displayed the following. For the waterwall surface of the boiler with the size of lm x lm with the staggered arrangement of the dimples on the surface with the step rx=fz=50 m m , the total heat flux would be Q d =298 kW. For comparison purposes, for the same size smooth waterwall surface, the total heat flux equalled C2sm=156 k W . Thus, the use of the vortex generators on the waterwall surface of the boilers allowed essentially to enhance the heat transfer. For the equal amount of the heat transfer, the waterwall surface with the vortex generators would be more compact than the smooth one. CONCLUDING REMARKS On the basis of the results of our investigations, the following conclusions can be made: 1. The spherical dimples on the surface, i.e. vortex generators, not only effectively enhance the heat transfer from the streamlined surface but also can be used for gas combustion establishing in the flow and to stabilize flame. 2. While establishing gas combustion in dimples, it is very important to choose the appropriate places of gas supply. Steady-state gas combustion and reliable flame stabilization are attained when the gas is fed into the rear with respect to the flow direction, region of the dimple. 3. Combustion process exerts influence on the vortex flow structure in the dimple. For combustion, large scale toroidal vortices appear in die dimple, although no alternating vortex ejections out of the dimple take place, as happens without combustion. 10 |
