This is the first stage of stalling and through this separation point the flow separates leading to the formation of vortices, back flow in the separated region. When the angle of incidence increases (during the low velocity flow) at the entrance of the air foil, flow pattern changes and separation occurs. This effect can be explained by the flow over an air foil. The cause for this is the separation of the flow from the blade surfaces. They occur because of the improper design, fan physical properties and are generally accompanied by noise generation. Stalling and surging affects the fan performance, blades, as well as output and are thus undesirable. The pressure variations to the left of the point "S" causes for unsteady flow which are due to the two effects of Stalling and surging.The pressure fluctuations are observed at low discharges and at flow rates(as indicated by the point "S" ) the pressure deceases.The power output of the fans increases with almost constant positive slope.As the flow rate increases from zero the efficiency increases to a particular point reaches maximum value and then decreases.
The following can be inferred from the curve.
(The vertical line joining the maximum efficiency point is drawn which meets the Pressure curve at point "S") The performance curve for the axial fan is shown in the figure. The typical characteristics of axial fans can be studied from the performance curves. The relationship between the pressure variation and the volume flow rate are important characteristics of fans. This Figure shows the Performance Curve for Axial Flow Fan. C 1 = C 2 = C įinally, thrust and torque can be found out for an elemental section as they are proportional to F x and F y respectively. In the figure, across the propeller disc, velocities (C 1 and C 2) cannot change abruptly across the propeller disc as that will create a shockwave but the fan creates the pressure difference across the propeller disc. The Enthalpy will be same at −∞ and +∞ as it depends upon the atmospheric conditions that will be the same Stagnation enthalpy will be different at −∞ and +∞ Velocity will change due to flow across an assumed converging duct Pressure will be atmospheric at both −∞ and +∞
Parameters at −∞ and +∞ and their relationship Therefore, the flow is assumed to be taking place in an imaginary converging duct where: The boundary between the fluid in motion and fluid at rest is shown. In the figure, the thickness of the propeller disc is assumed to be negligible. Variation of Pressure and Velocity of Flow through a Propeller disc.
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