# Experimental and CFD Simulation Studies on Bell-Type Air Nozzles of CFB Boilers
## 4 Results and Discussion  
### Cold-State Experiment  
Pressure drop [32; 33] was the main factor considered when designing and using the air nozzle. The pressure \(\Delta P\) of the air nozzle during the experiments could be measured by inlet pressure \(P_{inlet}\) and the outlet pressure \(P_{outlet}\) of the air nozzle. It is expressed as follows:  
\[\Delta P=P_{inlet}-P_{outlet} \tag{1}\]  
The pressure drop of the air nozzle was affected by the flow and density of air, and different air nozzle structures also change the pressure drop, which can generally be calculated using Equation (2):  
\[\Delta P=\frac{1}{2}\zeta\rho u^{2} \tag{2}\]  
\(\Delta P\), pressure drop of the air nozzle, Pa.  
\(\zeta\), resistance coefficient of the air nozzle.  
\(\rho\), density of air, kg m\({}^{-3}\).  
\(u\), inlet velocity of the air nozzle, m s\({}^{-1}\).  
The resistance coefficient \(\zeta\) can be determined by the given temperature, pressure, and flow rate of air from Equation (3).  
\[\zeta=\frac{2\Delta P}{\rho u^{2}}=\frac{2\Delta P}{\rho_{0}\frac{273}{273+T} \frac{101325}{101325+p}\Big{(}\frac{Q}{A}\Big{)}^{2}} \tag{3}\]  
\(Q\), flow rate of air, m\({}^{3}\) s\({}^{-1}\).  
\(A\), inlet area of the air nozzle, m\({}^{2}\).  
\(\rho_{0}\), density of air in the standard condition, kg m\({}^{-3}\).  
\(T\), air temperature, \({}^{\circ}\)C.  
\(p\), ambient pressure, Pa.  
Theory suggests that the resistance coefficient of the air nozzle is the same in cold-state and hot-state [7; 18]. Therefore, the pressure drop of the air nozzle under hot-state can be calculated from Equation (3).  
The test results of resistance coefficients of five air nozzles are shown in Figure 6. It can be seen that the resistance coefficient was basically linear with the inlet velocity of the air nozzle. The average resistance coefficient of nozzle A was 4.482, and the average resistance coefficient of nozzle B was 3.971.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Model** & **Setting** \\ \hline Viscous & RNG k-\(e\) \\ Inlet of inner tube & Velocity-inlet \\ Outlets of cover & Pressure-outlet \\ Wall function & Standard wall function \\ Momentum & Second order upwind \\ Pressure-velocity coupling & SIMPLE \\ \hline \hline \end{tabular}
\end{table}
Table 2: Boundary conditions and model setting.  
Due to the abrasion of the outer cover outlet in nozzle A1, the outlet diameter of nozzle A generally varied by 22.5 mm. However, outlets on many air nozzles show abrasion after 6 to 10 months, and the diameter of some outlets with abrasion will even reach more than 30-50 mm (as shown in Figure 3a). Due to the increase of the circulation area, the resistance coefficient of nozzle A1 dropped to 3.144. As three outlets in nozzle A2 were blocked (as shown in Figure 3b), the outlet flow area on the outer cover was reduced and the resistance coefficient increased to 5.148. In severe cases, all outlets of the cover were blocked, and the air flows could not pass through. Consequently, the air nozzle lost its function.  
The cover and inner tube of nozzle A were installed with screw thread, and there was screw thread deformation and tilting of the inner tube, as the environment in the furnace area is harsh after long-term application. After the inner tube of nozzle A3 was deformed (as shown in Figure 3c), the average resistance coefficient dropped to 3.374 due to the gap (about 5 mm) between the outer cover and inner tube. This difference meant that the cover and the top of the inner tube could not be intimately contacted, and some of the air flowed out from the gap between the cover and the top of the inner tube, changing the pressure drop of the air nozzle (as shown in Figure 7).