# Experimental and CFD Simulation Studies on Bell-Type Air Nozzles of CFB Boilers
## 4 Results and Discussion
### Pressure Drop Characteristics  
The pressure drop of the air nozzle must be compatible with the operating conditions. Excessive pressure drop of the air nozzle under hot-state increases unnecessary energy consumption, while low pressure drop of the air nozzle is likely to cause uneven fluidization, high pressure fluctuations in air plenum, and even slag leakage. The pressure drop change in nozzle A and nozzle B under hot-state was calculated by the CFD method. It can be seen from Figure 9 that the pressure drop in nozzle B was smaller, since the flow area of the outer cover was larger than nozzle A, and the changes in the inner tube showed the opposite trend.  
Figure 10 also compares the resistance characteristics of nozzle A and nozzle A3. It can be seen that the airflow from small holes in the inner tube was significantly reduced when there was a gap between the outer cover and inner tube, so that the pressure drop in the inner tube rapidly decreased. The resistance was only 943 Pa at 30% load. In addition, according to the design bed material pressure drop of 5 kPa (30% load), the pressure drop of the air nozzle was only 18.7% of the bed material,  
Figure 8: Computational fluid dynamics (CFD) and cold-state experimental results of nozzle A and nozzle B.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Description** & **Units** & \begin{tabular}{c} **Nozzle A** \\ **(100\% Load)** \\ \end{tabular} & \begin{tabular}{c} **Nozzle A** \\ **(30\% Load)** \\ \end{tabular} & \
\begin{tabular}{c} **Nozzle B** \\ **(100\% Load)** \\ \end{tabular} &
\begin{tabular}{c} **Nozzle B** \\ **(30\% Load)** \\ \end{tabular} \\ \hline Pressure drop & Pa & 5265 & 1253 & 4691 & 1110 \\ Inlet velocity & m s\({}^{-1}\) & 56.5 & 27.1 & 56.5 & 27.1 \\ Outlet velocity & m s\({}^{-1}\) & 54.1 & 25.8 & 40.7 & 19.3 \\ Air temperature & \({}^{\circ}\)C & 200 & 180 & 200 & 180 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of operating results.  
Figure 9: Pressure drop characteristics of nozzle A and nozzle B.  
which is lower than the recommended value of 20% [7]. The overall resistance data of the air nozzle calculated by numerical simulation show that the pressure drop of the air nozzle decreased when the gap increased, which is consistent with the cold-state experimental results. The above results also explain the reason why some CFB boilers are prone to display slag leakage after replacing the air nozzle. Obviously, the flow area of the inner tube will increase if there is a large gap between the newly replaced air nozzle and original inner tube, and the orifices in the inner tube will not generate sufficient pressure drop. The pressure drop characteristics of a bell-type air nozzle will be destroyed, thus causing a series of problems such as slag leakage. Since there is no gap between the cover and the top of the inner tube in nozzle B, the consistency of pressure drop characteristics of the air nozzle can be guaranteed.  
Since the outer cover blockage of air nozzles in CFB boilers is common in the site, the pressure drop and velocity changes after the small holes are blocked in nozzle A and nozzle B under 100% load condition were simulated. It can be seen from Figure 11 that the pressure drop and outlet velocity in nozzle A and nozzle B displayed an increasing trend, but the changes in nozzle B were less than nozzle A. In other words, the operating performance of nozzle B was better than nozzle A as the holes were blocked.