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A study on the half-ducted axial flow fan designed by a diagonal flow fan design method was conducted. The rotor which has NACA65 blades was designed, calculated numerically, manufactured and tested experimentally. As a result of the design and CFD, the meridional streamline and three distributions of the meridional, tangential and radial velocity at inlet and outlet go well as designed values of the half ducted fan. On the other hand, the values of the meridional velocity and the tangential velocity are little smaller than the design values at the hub side of the radial distribution. The improvement of the design is prospected for this point, that is, the approach between the design value and the actual flow is prospected if the tangential velocity is assigned small at hub and is assigned large at the tip so as to accord the actual flow in the vortex design of the rotor blade. Then the designed half-ducted rotor with four NACA65 blades was fabricated by a three-dimensional printer and tested in the wind tunnel in order to validate the half-ducted design method. For the comparison between the design values and the experimental values at the design flow rate coefficient of φ = 0.264, the experimental values of the pressure rise coefficient ψ and the efficiency η are rather small than the design values, while the experimental value of the torque coefficient τ is almost the same as the design value. However, the experimental value of approximately 0.45 of the maximum efficiency is comparably large value considering for the limitation of the situation of half-ducted. For the comparison between the experimental values and the CFD values at φ = 0.264, the CFD values are almost the same values as the experimental values for all the values of ψ, τ and η. In addition, the tendencies of the CFD values when the flow rate coefficient changes are almost similar as the experimental tendencies, though the flow rate coefficient for the CFD values when ψ or η takes the peak value shifts toward larger flow rate. For the case at rotor outlet at φ = 0.264, two values of the meridional velocity and the tangential velocity are larger than the design values at the tip side of the radial distribution.

A lot of axial fans, which are small size and high efficiency, are used in our daily life, such as a power unit cooling fan of personal computer, a room ventilation fan, and a radiator fan in car engine room. In most applications of fans non- ducted types are used for the space limitation or cost saving. In comparison with the conventional full-ducted fan mainly used in industrial applications, they are called as half-ducted type, semi-opened type and opened type, respectively. Among these types, a half-ducted axial flow fan is focused in this paper. The corresponding design methods for half-ducted types of propeller fans are not studied by many researchers. Recently, many of fans are designed by the inverse design method [

The quasi three-dimensional flow theory was applied to investigate the flow of the axial flow fans. The meridional flow and the revolutional flow between blades were calculated by the method of streamline curvature. Based on the theory, the meridional flow was calculated by adopting the radial balance equations [

When the compressibility of the fluid is ignored,

where,^{3}],

and

The solution of Equation (1) is given as

where, C is an integral constant, which satisfies the following equation.

where, G: designed flow rate [kg/s], K_{B}: end-wall blockage coefficient. K_{B} was presumed to be 0.96. The total pressure rise is presumed to be able to calculate by the Euler equation as following.

The controlled vortex design method has been applied for half-ducted and ducted propeller fan design by specifying the constant tangential velocity at both inlet and outlet of the fan rotor. Therefore, the meridional velocity and the tangential velocity can be obtained so that the calculation of meridional flow is finished.

The hub ratio has to be set a little larger to avoid the flow focus on the blade tip. Designed pressure rise and efficiency are set 200 [Pa] and 60[%] which are comparably large value than the conventional one for the superior target. The flow rate and pressure rise are represented with no dimensional form of flow coefficient and pressure rise coefficient which are defined as follows.

Tip diameter [mm] | 200 |
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Hub-tip ratio | 0.6 |

Blade number | 4 |

Flow coefficient | 0.264 |

Pressure-rise coefficient | 0.336 |

Rotation speed [min^{−1}] | 3000 |

Pressure rise [Pa] | 200 |

Efficiency | 0.6 |

Blockage coefficient | 0.96 |

where

The blade profile on the revolutional plane was selected by referring to the diagram of NACA65 carpet. In order to use the cascade data, the through flow in cylindrical or cone surface must be considered on an averaged stream surface. The flow on revolving stream surface is projected to a plane. Thus, the NACA65 blade with quadrilateral blade on the meridional plane was adopted. Three- dimensional effect of inclination and thickness variation of stream surface on flow field is substituted by the distributions of vortex and divergence on the potential theory [

The streamlines on the meridional plane of half-ducted fan were shown in

The analysis of the three-dimensional internal flow fields of half ducted fan are conducted comparing to the numeral computation results in the commercial software (Ansys CFX Release 16.0). Simulation conditions and layouts are shown in

The computation domain was extended 300 mm upstream from the leading edge and 500 mm downstream from the trailing edge, respectively.

Software: Ansys CFX Release 16.0 |
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RANS (Reynolds Averaged Navier-Stokes Simulation) |

Air Temperature: 25˚C (Density: 1.185 kg/m^{3}) |

Turbulence model: SST k-ω model |

Relative Coordinate System (Shaft Speed: 3000 min^{−1}) |

Mass Flow Inlet (0.1971 kg/s, Uniform) |

Pressure Static Outlet (1 atm, Uniform) |

Wall Boundary: Dirichlet boundary, No Slip (Counter Rotating Wall in Shroud) |

the perspective view of the calculated region for the full-pitch calculation. The unstructured, tetrahedral grids were used. The total number of the grids is approximately 5,550,000. The tip clearance of 2 mm is set for the numerical calculation.

The turbulence model was SST. The use of a k-ω formulation in the inner parts of the boundary layer makes the model directly usable all the way down to the wall through the viscous sub-layer. The SST formulation also switches to a k-ε behaviour in the free-stream and thereby avoids the common k-ω problem that the model is too sensitive to the inlet free-stream turbulence properties. Especially, the influence of stall by separated flow at low flow rate can be better simulated in SST turbulence model. Therefore SST turbulence model was adopted.

tangential velocities are normalized by the rotational velocity at the blade tip

The designed and calculated half-ducted rotor with four NACA65 blades was fabricated by a three-dimensional printer shown in ^{−1} driven by the direct-current electric motor. In order to get smaller static pressure rise, the centrifugal fans were set in the outlet of the system. The flow rate of the system can be well controlled by altering the

rotating speed of the booster fan. Axial torque coefficient and efficiency are defined as:

The static pressure of the propeller fan is obtained by averaging the data from 16 static pressure taps setting at downstream of rotor as show in ^{−1} in this paper. Calculated and measured pressure rise, torque and efficiency is shown in

flow rate coefficient of f = 0.264, the CFD values are almost the same values as the experimental values for all the values of ψ, τ and η. In addition, the tendencies of the CFD values when the flow rate coefficient changes are almost similar as the experimental tendencies, though the flow rate coefficient for the CFD values when ψ or η takes the peak value shifts toward larger flow rate.

The half-ducted axial flow fan was designed by a diagonal flow fan design method.

Half-ducted fans have been designed, numerically analyzed, manufactured and tested to confirm the effect of this design method for improving design method on the overall performance and the three-dimensional flow field in design. The conclusions are summarized as follows.

1) For the comparison between the design values and the experimental values at the design flow rate coefficient of f = 0.264, the experimental values of the pressure rise coefficient ψ and the efficiency η are rather small than the design values, while the experimental value of the torque coefficient τ is almost the same as the design value. However, the experimental value of approximately 0.45 of the maximum efficiency is comparably large value considering for the limitation of the situation of half-ducted.

2) For the case at rotor outlet, at the flow rate coefficient of f = 0.264, two values of the meridional velocity and the tangential velocity are larger than the design values at the tip side of the radial distribution. On the other hand, the values of

3) For the comparison between the experimental values and the CFD values at the design flow rate coefficient of f = 0.264, the CFD values are almost the same values as the experimental values for all the values of ψ, τ and η. In addition, the tendencies of the CFD values when the flow rate coefficient changes are almost similar as the experimental tendencies, though the flow rate coefficient for the CFD values when ψ or η takes the peak value shifts toward larger flow rate.

The authors would like to acknowledge the financial support of Saga University for this research project.

Kaji, A., Kinoue, Y., Shiomi, N. and Setoguchi, T. (2017) A Study on the Half-Ducted Axial Flow Fan Designed by a Diagonal Flow Fan Design Method. Open Journal of Fluid Dynamics, 7, 15-25. http://dx.doi.org/10.4236/ojfd.2017.71002