Axial fan design

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A axial fan is a fan type that causes gas to flow through it in the axial direction, parallel to the axis at which the blades rotate. The stream is axial when in and out. The fan is designed to produce pressure difference, and hence force, causing flow through the fan. Factors that determine fan performance include the number and shape of the blade. Fans have many applications including in wind tunnels and cooling towers. The design parameters include power, flow rate, pressure rise and efficiency.

Axial fans generally consist of fewer knives (two to six) than doling fans. The axial fan usually has a larger radius and a lower velocity (?) Than the fan channeling (especially at the same power Stress is proportional to r ^ 2).

Video Axial fan design

Perhitungan parameter

Since calculations can not be performed using inlet and outlet velocity triangles, which do not occur in other turbines, the calculations are performed taking into account the average velocity triangle for flow only through very small blade elements. The blades are divided into many small elements and the various parameters are determined separately for each element. There are two theories that solve the parameters for axial fans:

• Slipstream Theory
• The Elements The Blade
theory

Slipstream Theory

In the figure, the thickness of the vane discs is assumed to be negligible. The boundary between moving fluid and fluid at rest is indicated. Therefore, the flow is assumed to occur in an imaginary convergent channel in which:

• D = Diameter Disk Propeller.
• D _s = Diameter in the Out Door.

Untuk mengurangkan persamaan di atas:

${\ displaystyle P_ {2} -P_ {1} = {\ frac {1} {2}} \ rho (C_ {s} ^ {2} -C_ {u } ^ {2})}  ÂÂ$

• Perbedaan dorong karena perbedaan tekanan adalah area yang diproyeksikan dikalikan dengan perbedaan tekanan. Dorong aksial karena perbedaan tekanan muncul:

${\ displaystyle F_ {x} = A (P_ {2} -P_ {1}) = {\ frac {1} {2}} \ rho A \ left (C_ {s} ^ {2} -C_ {u} ^ {2} \ kanan)}  ÂÂ$

Membandingkan dorongan ini dengan dorong aksial karena perubahan momentum aliran udara, ditemukan bahwa:

${\ displaystyle C = {\ frac {C_ {s} C_ {u}} {2}}}  ÂÂ$

Sebuah parameter 'a' didefinisikan sedemikian rupa sehingga -

${\ displaystyle C = (1 a) C_ {u}}  ÂÂ$ di mana ${\ displaystyle a = {\ frac {C} {C_ {u}}} - 1}  ÂÂ$

Menggunakan persamaan sebelumnya dan "a", sebuah ekspresi untuk C _s keluar menjadi:

${\ displaystyle C_ {s} = (1 2a) C_ {u}}  ÂÂ$

• Menghitung perubahan entalpi stagnasi spesifik di cakram:

${\ displaystyle \ Delta h_ {o} = \ Delta h_ {o} d- \ Delta h_ {o} u = \ kiri (h_ {d} {\ frac { 1} {2}} C_ {s} ^ {2} \ right) - \ left (h_ {u} {\ frac {1} {2}} C_ {u} ^ {2} \ right) = {\ frac {1} {2}} \ kiri (C_ {s} ^ {2} -C_ {u} ^ {2} \ kanan)}  ÂÂ$

Sekarang, Nilai Ideal Daya yang dipasok ke Propeller = Massa laju aliran * Perubahan entalpi Stagnasi;

${\ displaystyle P_ {i} = {\ dot {m}} {\ Delta h_ {o}}}  ÂÂ$ di mana ${\ displaystyle {\ dot {m}} = \ rho AC}  ÂÂ$

Karena itu;

${\ displaystyle D_ {s} ^ {2} = \ left ({\ frac {1 a} {1 2a}} \ right) D ^ {2}}  ÂÂ$

Therefore streams can be modeled in which air flows through an imaginary diverging channel, in which the diameter of the propeller disc and the outlet diameter are related.

The blade element theory

In this theory, a small element ( dr ) is taken at a distance r from the root of the blade and all forces acting on the element are analyzed to obtain a solution.. It is assumed that the flow through each part of a small radial thickness dr is assumed to be independent of the flow through other elements.

Menyelesaikan Pasukan dalam gambar -

$            {\displaystyle         \mathrm{?}        F_{            x          }        =        \mathrm{?}        L        \sin                 (        ?        )        -        \mathrm{?}        D        \cos                 (        ?        )      }        \{\backslash \; displaystyle\; \backslash \; Delta\; F\_\; \{x\}\; =\; \backslash \; Delta\; L\; \backslash \; sin\; (\backslash \; beta)\; -\; \backslash \; Delta\; D\; \backslash \; cos\; (\backslash \; beta)\}\;  ÂÂ$

${\ displaystyle \ Delta F_ {y} = \ Delta L \ cos (\ beta) \ Delta D \ sin (\ beta)}  ÂÂ$

Koefisien Angkat (C _L ) dan Koefisien Seret (C _D ) diberikan sebagai -

${\ displaystyle \ mathrm {Angkat} (\ Delta L) = {\ frac {1} {2}} C_ {L} \ rho w ^ {2} (ldr) }  ÂÂ$

${\ displaystyle \ mathrm {Drag} (\ Delta D) = {\ frac {1} {2}} C_ {D} \ rho w ^ {2} (ldr) }  ÂÂ$

Juga dari sosok -

${\ displaystyle \ tan (\ phi) = {\ frac {\ Delta D} {\ Delta L}} = {\ frac {C_ {D}} {C_ {L }}}}  ÂÂ$

Sekarang,

${\ displaystyle \ Delta F_ {x} = \ Delta L (\ cos \ phi - {\ frac {\ Delta D} {\ Delta L}} \ sin \ phi) = \ Delta L (\ cos \ phi - \ tan \ phi \ sin \ phi) = {\ frac {1} {2}} C_ {L} \ rho w ^ {2} ldr {\ frac {\ sin (\ beta - \ phi}} {\ cos \ phi}}}  ÂÂ$

Jumlah Blades (z) dan Spacing (s) terkait sebagai, ${\ displaystyle s = {\ frac {2 \ pi r} {z}}}  ÂÂ$ dan total dorong untuk bagian elemen baling-baling adalah z? F _x .

Karena itu,

${\ displaystyle \ Delta p (2 \ pi rdr) = z \ Delta F_ {x}}  ÂÂ$

${\ displaystyle \ Rightarrow \ Delta p = {\ frac {1} {2}} C_ {L} \ rho w ^ {2} ({\ frac {l} { s}}) {\ frac {\ sin (\ beta - \ phi)} {\ cos \ phi}} = {\ frac {1} {2}} C_ {D} \ rho w ^ {2} ({\ frac {l} {s}}) {\ frac {\ sin (\ beta - \ phi}} {\ sin \ phi}}}  ÂÂ$

Demikian pula, pemecahan untuk? F _y ,? F _y ditemukan menjadi -

${\ displaystyle \ Delta F_ {y} = {\ frac {1} {2}} C_ {L} \ rho w ^ {2} ldr {\ frac {\ cos (\ beta - \ phi}} {\ cos \ phi}}}  ÂÂ$

dan ${\ displaystyle (\ mathrm {Torque}) \ Delta Q = r \ Delta F_ {y}}  ÂÂ$

Finally, thrust and torque can be found for elemental parts because they are proportional to F _x and F _y respectively.

Maps Axial fan design

Performance characteristics

The relationship between pressure variation and volume flow rate is an important characteristic of the fans. The typical characteristics of axial fans can be learned from the performance curve. The performance curve for the axial fan is shown in the figure. (The vertical line joins the maximum drawn efficiency point that meets the Pressure curve at the point "S") The following can be inferred from the curve -

1. Since the flow rate increases from zero, the efficiency increases to a certain point reaches the maximum value and then decreases.
2. The fan power output increases with a nearly constant positive slope.
3. The pressure fluctuations observed at low discharge and at the flow rate (as indicated by the point "S") decrease the pressure.
4. Pressure variations to the left of the "S" point cause an unstable flow caused by two Stalling and wavy effects.

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Unstable flow causes

Redirecting and jumping affects fan performance, blades, and output and are therefore undesirable. They occur due to improper design, the physical properties of the fan and generally accompanied by noise generators.

Stuck effect/Stall

The cause is the separation of the flow from the surface of the blade. This effect can be explained by the flow above the air foil. As the incidence angle increases (during low velocity flows) at the entrance of the air foil, changes in flow pattern and separation occur. This is the first stage of stalling and through this separation point a separate stream leading to the formation of vortices, backflow in a separate region. For more details on kiosks and kiosks to spin, see the compressor surge. The enclosure zone for a single axial fan and axial fan operated in parallel is shown in the figure.

The following can be inferred from the graph:

• For Fans operated in parallel, performance is less when compared to individual fans.
• Fans should operate in a secure zone of operation to avoid stall effects.

VFD is not practical for some Axial fans

Many Axial fan failures have occurred after the blade controlled blade fan is locked in a fixed position and Variable Frequency Drives (VFDs) are installed. VFD is not practical for some Axial fans. Axial fans with severe instability areas should not operate at the angle of the blade, rotation speed, mass flow rate, and pressure that expose the fan to stop the condition.

Wavy/Surge effect

Running should not be confused with stalling. Stalling occurs only when insufficient air enters the fan blades causing the flow separation on the blade surface. Running or unstable flow causes complete damage in fans mainly contributed by three factors

• System spike
• Fan scrolling
• Alignment

System spike

This situation occurs when the system resistance curve and the static pressure curve of the interceptor fan have the same or parallel slant to each other. Instead of intersecting at a definite point, the indentations cut out some spikes in a particular region's reporting system. This characteristic is not observed in axial fans.

Spike fan

This unstable operation results from the development of pressure gradients in the opposite direction of the flow. Maximum pressure is observed in the impeller impeller discharge and minimum pressure on the opposite side of the discharge side. When the impeller blades do not rotate, this negative pressure gradient pumps the flow in the opposite direction of the fan. The result is a fan blow oscillation that creates vibration and hence noise.

Alignment

This effect is s

Source of the article : Wikipedia