![Fan stall, surge and wind grab]()
A stall
Axial fan blades are generally streamlined. When operating under design conditions, the airflow angle of attack (ie, the difference between the relative velocity w of the inlet airflow and the blade installation angle) is approximately zero, airflow resistance is small, and the efficiency of the fan is high. When the fan flow rate decreases, the direction angle of w changes, and the air flow angle increases. When the angle of attack increases to a certain critical value, a turbulent zone is generated at the trailing end of the blade, that is, the so-called deviating condition (stall), the resistance increases sharply, and the lift force (pressure) decreases rapidly; the angle of attack increases again, and takes off. The flow phenomenon is even more severe, and even partial obstruction of the leaves may occur.
Due to the installation error of each fan blade, the installation angle is not exactly the same, and the airflow flow field is not uniform. Therefore, the stall phenomenon does not occur at the same time for all the leaves, but first at one or a few leaves. If there is a divergence in the leaf passage 2, the leaf passage is narrowed due to the displacement of the detachment area and the flow rate is reduced, and the air flow enters the adjacent 1 ,3 leaf passages respectively, so that the airflow direction of the 1 ,3 leaf passages is changed. As a result, the angle of flow of the air flowing into the leaf passage 1 is reduced, and the leaf passage 1 maintains normal flow; the angle of attack of the leaf passage 3 increases, and expulsion and obstruction are aggravated. The obstruction of Ye Dao 3 also affects the air flow of the adjacent leaf passages 2 and 4, so that the leaf passage 2 eliminates the desulfurization, and at the same time, the flow of the leaf passage 4 occurs. That is, the de-fluxing zone is rotating, and its rotation direction is opposite to the rotation direction of the impeller. This phenomenon is called rotating stall.
Unlike surge, the fan can continue to run when it stalls, but it causes a large pulsation of blade vibration and pre-impeller pressure, which is often an important cause of blade fatigue damage. From the characteristic curve of the fan, the rotating stall zone and the surge zone are both located in the low air volume area on the left side of the saddle peak point. In order to avoid the fan from falling into the stall zone, a single blower can be used during the ignition and low load of the boiler to increase the fan flow.
Second, surge
Fan surge refers to the phenomena of large fluctuations in the air volume, pressure, and current caused by the fan operating in unstable conditions, noise increase, and severe vibration of fans and pipes.
When the fan works in the one-way descending part of the curve, its work is stable until the operating point K. However, when the fan load falls below Qk, it enters the unstable zone and works. At this point, as long as there is a slight disturbance to the pipeline pressure is slightly increased, because the fan flow is greater than the pipeline flow (Qk> QG), the working point to the right to move to point A, when the pipeline pressure PA exceeds the fan forward delivery maximum When the pressure Pk, the working point is changed to point B, (A, B point pressure), the fan back against the line pressure and work. At this time, the gas in the pipeline is transported in two directions. On the one hand, the load needs to be supplied, and on the other hand, it is fed to the fan, so the pressure is rapidly reduced. Stopping the flow back to point C, the fan flow increases. However, because the flow rate of the fan is still less than the flow rate of the pipeline, QC<QD, the pipeline pressure will still drop to point E, and the wind-like operating point will instantly jump from point E to point F (E, F, etc.). The fan output flow is QF. Since the QF is greater than the output flow of the pipeline, the wind pressure of the pipeline is increased and the operating point of the fan is moved to point K again. The above process is repeated to form the surge of the wind turbine. During surge, the fan's flow rate varies within the QB-QF range, while the output flow rate of the line only changes between much less QE-QA.
Therefore, fan surge can be avoided as long as the operating point does not enter the above-mentioned unstable zone. Axial fan When the installation angle of the rotor changes, K point changes accordingly. Therefore, the corresponding unstable regions under different blade installation angles are different. Large-scale units are generally designed for the surge alarm device of the wind turbine. The principle is that the peak point of the performance curve corresponding to each angle of the moving blade or the static blade is smoothly connected to form the fan surge boundary line, and then the surge boundary line is moved to the lower right by a certain distance to obtain the surge alarm line. In order to ensure the reliable operation of the fan, its working point must be at the lower right of the surge boundary line. Once the operating point at a certain angle changes due to changes in the pipe resistance characteristics or other reasons, when moving up and down the curve to the surge alarm line, an alarm signal is issued to remind the operator to pay attention and move the operating point back to the stable zone.
The wind pressure of the parallel fans is the same, so the fan blades with small loads have small opening degrees and their performance curve peak point (K point) is lower than that of another fan. The lower the load, the lower the K point. Therefore, a fan with a low load will easily fall within the surge area. Therefore, when adjusting the load of the fan, the load of the two parallel fans should not be excessively biased to prevent the low-load fan from entering the unstable surge zone. During operation, the smoke duct is not smooth or the inlet and outlet dampers of the air volume system are closed or incorrectly closed, and the system resistance increases, which will cause the fan to work in the surge area. The situation of inconsistent leaf opening, or inconsistent indication and local failure, self-control failure, etc., will cause changes in the characteristics of the fan, and will also cause the fan to surge. Avoid long-term operation of the fan under low load.
Third, grab the wind
The so-called grab wind refers to the two wind turbines running in parallel. Suddenly, the current (flow) of one wind turbine rises, and the current (flow rate) of one wind turbine drops. At this time, if the throttle of the small high-flow fan is turned off in an attempt to balance the air volume, another small-flow fan will jump to the maximum flow rate. When adjusting the damper to cast automatically, the fan's rotor blade or static blade frequently opens and closes. When it is serious, it may cause the fan motor to over-current and burn out.
The phenomenon of snatching is due to the existence of large unstable conditions in parallel wind turbines. There is an ∞ font area. If two fans are running in the piping system 1, point P1 is the operating point of the system. Each fan is operating stably at point E1. At this time, wind grab will not occur. If, for some reason, the resistance of the piping system changes to 2 (increases), such as when the auxiliary damper suddenly turns off, the blower enters the ∞-shaped work area. We look at the work of P2. The two fans are located at E2a and E2. Large-flow fans operate in stable areas, and small-flow fans operate in unstable areas. The imbalance of the two fans is easily destroyed. As a result, two wind turbines have caught wind.
In order to eliminate the wind-sweeping phenomenon, a single operation mode can be used for the delivery and induced draft fan when the boiler is ignited or under low-load operation. When a single fan output fails to meet the boiler load requirement, another fan is to be operated in parallel. Once the wind grab occurs, manually adjust the two fans to maintain the correct air flow deviation (in this case, the fan's side-by-side characteristics of the ∞ font area contraction) to avoid the wind grab area.