Design and application of the most fire efficient

2022-08-17
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Design and application of high-efficiency mining local ventilator

1 design method and constraints

1.1 design method

the design and calculation method of meridional accelerated axial fan impeller is based on the assumption that the air flow flows along the conical surface, approximately expanding the conical surface where the primitive stage is located to the plane and treating it as an equivalent plane cascade. In this design method, the simple radial balance equation is used to find out the flow infiltration number before and after the impeller, and then the Meridional Acceleration impeller is designed with the help of abundant plane cascade test data

1.2 design constraints

1.2.1 diffuser factor constraints

as we all know, the flow of air flow through the fan cascade is of diffuser nature, and when the degree of flow diffuser reaches a certain value, serious air separation will occur in the cascade and stall will be formed. The diffuser factor Dr reflects the degree of cascade diffuser, which is not only the main factor affecting the development of boundary layer on the cascade profile, At the same time, it is also a main criterion to determine the limit aerodynamic load of cascades. From a large number of domestic and foreign data, the diffuser factor is one of the most suitable parameters to characterize the performance of fans. At the same time, it also reflects the off design performance and stability margin to a certain extent. The diffusion factor varying along the radial direction is defined as:

dr = 1 (w2/w1) - (r2w2u/r1w1u)/2 τ Rrmw1

where W - relative velocity of air flow, m/s

r - Calculation radius, m

τ R - cascade consistency

the given constraint is: Tip Dr ≤ 0.4, root Dr ≤ 0.6

1.2.2 reaction constraint

reaction is an important parameter that affects the performance of primitive level, and has an impact on efficiency and cascade rotating stall. For example, for the primitive stage with Ω = 0, because the pressure rise is all carried out in the stator cascade, the diffuser in the stator cascade is severe, the load is large, and it is easy to cause large losses and rotating stall. For the stage with Ω = 0, the flow is easy to deteriorate because the Mach number mw1 is large and should exceed the critical value. According to a large number of experiments, it is concluded that compared with 50% reaction, the surge point of the fan moves to the left, and the rotating stall area also moves to the small flow area. At the same time, the performance curve is relatively slow, that is, it has a wider working range and higher efficiency. Therefore, adopting a reaction degree of> 50% is conducive to improving the off design performance. The reaction degree Ω is defined as:

Ω = pst/pt

where PST - static pressure value, PA

the given constraint is: Ω m ≥ 0.7. For example, a group of experiments has several experimental pieces called "several pieces", and several groups of experiments are called "several batches" 5

1.2.3 flow and pressure constraints

due to the influence of gas viscosity, boundary layers have been generated at the inner and outer ring end faces and blade profile surfaces of the flow passage part, reducing its flow area. The theoretical flow must be corrected according to the actual flow. Take the total blocking coefficient KB = 0.98, and its flow constraint condition is:

considering the influence of loss, the pressure constraint condition is:

where ρ—— Air density, kg/m3

η—— Fan efficiency,%

u -- peripheral speed, m/s

cu -- winding speed, m/s

cz - axial speed, m/s

2 selection of design parameters

2.1 selection of hub ratio

hub ratio is one of the important parameters in the design of axial fan impeller. It affects the pressure, flow, efficiency, pressure characteristic curve shape and working area size of the fan. When determining the hub ratio, we should not only consider its impact on the performance of the fan, but also consider the structure of the fan. For example, when the fan impeller blades are adjustable, the space of blade arrangement is limited, and the hub ratio cannot be too small at this time; When the motor is installed in the hub of the guide vane, the diameter of the hub is affected by the size of the motor. Therefore, the selection of hub ratio is a global problem in fan design

2.2 fan efficiency

designing a fan with good performance is to minimize the loss of air flow through the designed flow channel, that is, to achieve the highest efficiency as possible, under the condition of meeting the given design parameters and taking into account some requirements of low noise and compact structure and size

for the impeller, the aerodynamic efficiency at the blade tip and hub is usually relatively low, but because the meridional accelerated axial flow fan improves the flow state at the blade root, the meridional accelerated impeller can improve the efficiency at the hub. In addition, strictly controlling the radial clearance of rotating blades and reducing the secondary flow loss is an effective way to improve the efficiency at the tip. In the design of blades, the efficiency is generally treated evenly along the radial direction

2.3 selection of Meridional Acceleration ratio

the inclination angle of the wheel housing of the Meridional Acceleration axial flow fan impeller should not be too small or too large, because the too small inclination angle makes the impeller wider and the axial size larger in order to ensure the determined acceleration ratio. Too large inclination will accelerate the meridional velocity and advance the separation, especially when designing based on the plane cascade theory, the error is relatively large, because at this time, when adopting this kind of method for radial velocity, it is necessary to pay attention to the influence of the inertia of the moving object on the experiment, and the value of Cr cannot be ignored. According to the results of the experiment, the inclination angle of the wheel housing of the meridian accelerated axial fan impeller should be 15 ° ~ 20 °

2.4 selection of flow pattern

the selection of flow pattern is very important for the design of axial flow fan. The axial flow fan has large flow, small light shell, and long and wide moving blades. Therefore, the distortion law of blades must be determined first

in the design of fan blades, an extremely important link is to calculate the flow pattern in the non blade gap, that is, give the distribution of air flow parameters along the blade height, and then solve the distribution of axial velocity and inlet and outlet air flow angle along the blade height according to the radial balance equation, and then carry out blade modeling. The commonly used flow patterns are: free vortex, forced vortex, displacement free vortex and the combination of free vortex and equal circumferential velocity, etc. With the development of computer technology, the design of "optimal flow pattern" is further proposed, that is, the blade distortion law that requires a specific performance index of the stage (such as the power or efficiency of the stage) to reach the optimal value under some design constraints

in recent years, the design of radial unequal work has been widely valued and applied, and it is regarded as an important means to improve efficiency and load and tap potential. In the so-called "controlled vortex" design, unequal work design is recommended. For the meridional accelerated axial flow fan, due to the improvement of the root state, the meridional accelerated axial flow fan with medium pressure coefficient (P ≤ 0.4) can adopt the equal circulation flow pattern, and the variable work design can be adopted for the high pressure coefficient (P> 0.4), that is, the power of the top depends on the power of the root. If the equal circulation flow pattern is adopted again, the twist angle of the root air flow is> 45 °, and the load coefficient is too large

2.5 selection of blade profile

the flow loss of axial fan is mainly composed of three parts: blade profile loss, ring end loss and secondary flow loss. The relationship between these three losses and flow coefficient and efficiency is shown in Figure 1

Figure 1 Relationship curve between efficiency and flow coefficient

the working characteristic of local fan is that the wind resistance increases with the increase of air supply distance, which requires that the average efficiency in the whole working range should be high, that is, the efficiency curve should be flat. From the curve in Figure 1, it can be seen that the ring end face loss and secondary flow loss are insensitive to the change of flow, while the blade profile loss changes greatly with the change of flow, Therefore, it is very important to choose the blade shape suitable for the working characteristics of local fans. The experimental results show that the selection of blade profile should be based on good low-speed performance, blunt blade tip, insensitive hedge angle and large stall angle of attack range

2.6 working condition adjustment of meridional accelerated axial flow fan

when the meridional accelerated axial flow fan adopts the adjustable front guide vane, it can improve the performance of the fan, adapt to the changes of pipe characteristics, and achieve the purpose of energy saving and safe operation

Figure 2 shows the adjustment method. At a certain working point, the pipe resistance characteristic curve and the fan characteristic curve intersect at 1 point. When the pipe resistance characteristic curve is P2 if the indicated value is low, increasing the installation angle of the front guide vane by 5 ° can raise the working point from 2 to 3. At this time, the increase of air pressure is large, and the decrease of air volume is small, which can still meet the needs of tunneling ventilation

Figure 2. The performance change curve of adjusting fan working conditions

using adjustable stationary blades has the following advantages: ① relying on wafer factory projects, integrated circuit manufacturing enterprises, semiconductor silicon materials, electronic chemical materials and other new material enterprises, the adjustable stationary blades make the stable working area wider and the efficiency higher; ② The geometric parameters of air flow and blades match well, reducing the impact phenomenon, good flow condition and low noise; ③ The working condition adjustment is convenient and fast

2.7 determination of qualitative margin of surge

the pressure coefficient of meridian accelerated local fan is large and sensitive to the positive angle of attack. When the operating point is located in the small flow area, the pressure increases, the blade load increases significantly, and the blade tip part is easier to be separated. In order to lengthen the air supply distance, the local ventilator still needs to work after the design operating point, which requires a large surge stability margin. In order to meet the needs of the performance of the variable operating point, the surge stability margin △ KY ≥ 20% is generally taken. In addition, this problem can be further improved by using the adjustable front guide vane. Increasing the installation angle of the front guide vane can move the fan characteristic curve to the right and improve the working condition of the small flow area

3 application example

using the above design methods and constraints, we have designed FB № 6.3/55 mining flameproof press in axial flow local fan. Its main design parameters are: air volume q = 500m3/min; Total pressure p=4 500 Pa; η ≥80%; Motor power n = 55kW; Speed n = 2 940r/min. In the design, the controllable vortex method is used to design the flow pattern and determine the spanwise velocity distribution. The structural parameters of the whole machine are designed according to the overall optimization. Figure 3 shows the comparison of performance curves between FB № 6.3/55 fan and mfa60p2-sc4 fan. When the total pressure efficiency is ≥ 60%, the ratio of qmax/qmin is 1.35 for the former and 1.25 for the latter; The ratio of pmax/pmin is 1.67 for the former and 1.5 for the latter. It can be seen that the high-efficiency area of FB № 6.3/55 fan is wide. In the range of wind resistance from 66.1 to 21.8kg/m7, the air volume of the former increases by 3% to 18% compared with the latter, indicating that FB № 6.3/55 fan has a large air volume, which is suitable for ventilation of high gas heading face; When FB № 6.3/55 fan 2 940r/min and 1 480r/min operation are compared with mfa60p2-sc4 pair cyclone and secondary operation, the former saves 8.5kw and 3.3kw compared with the latter; FB № 6.3/55 fan adopts adjustable front guide vane and two speed motor, so that the fan has four working condition adjustment methods. The air volume range of the fan is 200 ~ 700m3/min, and the total pressure range is 300 ~ 4900 Pa. it realizes a wide range of working condition transfer and has strong applicability

4 conclusion

(1) reasonable selection of aerodynamic parameters and overall optimization design have improved the performance level of the fan

(2) meridian fan and counter rotating fan have their own characteristics, which meet the different needs of coal mine production

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