# Experimental and CFD Simulation Studies on Bell-Type Air Nozzles of CFB Boilers
## 1 Introduction  
As a mature clean coal power generation technology, the circulating fluidized bed (CFB) boiler has become increasingly applied worldwide [1; 2; 3; 4; 5]. The air nozzle is one of the main components of a CFB boiler [6; 7]. The structure and arrangement of the air nozzle have a significant impact on the flow in the furnace. Irrational design of air nozzles could lead to operating accidents, such as bad fluidization [8; 9; 10] or bed material blocking [11; 12]. Therefore, reasonable air nozzle structure and pressure drop are essential to keep the CFB boiler operating well [13].  
The bell-type air nozzle is a structure adopted in CFB boilers [14; 15; 16]. Compared with pig-tail air nozzles, the bell-type air nozzle has a low pressure drop and low energy consumption. When compared with the arrowhead air nozzle, the bell-type air nozzle is not inclined to cause abrasion and slag leakage [17; 18]. Since the pressure drop of the bell-type air nozzle can be adjusted through the structure of the inner tube and cover, the convenience of the design is increased. In addition, the number of bell-type air nozzles is less than that of arrowhead air nozzles or pig-tail air nozzles for a certain air inlet area of CFB boilers. Consequently, less labor is required for nozzle installation and maintenance [19]. Although the bell-type air nozzle has stable performance and obvious technical advantages compared with pig-tail and arrowhead air nozzles, the following problems still exist in the process of application: outlet abrasion, cover blockage, and inner tube fracture. Therefore, more in-depth research and optimization of the air nozzle is necessary.  
[PERSON] et al. [20] studied unequal airflow inside the plenum of a distributor. computational fluid dynamics (CFD) was used as a design tool to obtain a more homogeneous airflow towards the distributor. [PERSON] et al. [21] performed a numerical simulation on the bubble column, and a chamber configuration to study the inlet nozzle size and its location, opening area, and hole diameter. [PERSON] et al. [22] revealed that uneven airflow in an inlet duct would result in uneven distribution in the windbox and distributor. [PERSON] et al. [23] analyzed the influence of exit ports of a distributor in a manifold system of identical outflows through numerical simulation. The results of [PERSON] et al. [24]further indicated that the recirculation flow in the windbox channel and distributed plate nozzle is the main cause of uneven distribution. [PERSON] et al. [25] found that considerable vertical and horizontal pressure gradients occurred in the primary air grid region, which caused the bed material to flow into the nozzle discharge channels, or even into the windbox. Consequently, [PERSON] et al. [26] designed a new arrowhead air nozzle that solved the bed material backflow problem of 460 MW CFB boilers.  
However, there is very limited research work on bell-type air nozzles in the relevant literature. In this paper, the pressure drop and flow characteristics of bell-type air nozzles are studied by cold-state experiments and numerical simulations. The designed bell-type air nozzle has favorable pressure drop characteristics and strong anti-wear ability. The engineering application results show that the new bell-type air nozzle ensures the safe and stable operation of CFB boilers.