Study On Resistance Characteristics Of Air Caps And Maldistribution Of Distributor In Large Fluidized Bed

As a high efficient and clean coal combustion technology, circulating fluidized bed (CFB) boiler technology has developed rapidly in recent years with towards the direction of larger capacity and higher steam parameters. The area of the air distributor increases with the increase of the boiler capacity. In order to ensure the security and stability of the CFB boiler, the appropriate adjustments of the structure size and arrangement of the caps are made. And the new resistance characteristics and maldistribution of the air distributors should be studied to meet the design requirements of the large CFB boilers.Based on the actual air caps of a supercritical circulation fluidized bed (CFB) boiler, cold-state experiments and CFD simulation of the large-scale air caps with inner tubes and down flow were made to investigate the effect of structure scale and back pressure on the resistance characteristics. The effect of flow rate and cap arrangement on maldistribution of distributor was analyzed based on CFD simulation. The main conclusions are as follows.(1) Experimental study on the resistance characteristics of the caps:among the five local resistances (inner tube, small holes at the head, annular gap, turning position and small holes of the inner tube), the inlet velocity of the small holes of the inner tube has the most influence on the resistance; the resistance decreases with the increase of the back pressure, and the arrangement of air caps does not affect the resistance much; based on the experimental data, and combining with the algorithms of Levenberg-Marquardt(LM) and general global optimization, a calculation model from regression analysis for the resistance of air caps in the actual boilers is obtained.(2) Numerical investigation of the flow characteristics of the caps:the main part of the resistance is from the small holes of the inner tube; the arrangement of holes in the inner tube does not affect the resistance much; the resistance decreases with the increase in the hole rotation angle, and firstly increases and then decreases with the hole downdip angle.(3) Numerical investigation of maldistribution of the air distributor: maldistribution exists in different positions in the air distributor; the velocity profiles tend to become weakened distribution with increasing flow rate; increasing the number of the caps closed to the wall can not only reduce the area of the low velocity zone, but also weaken the velocity maldistribution.