Abstract
ABSTRACT This study illustrates that downstream diffusers can significantly aid the performance of an induced draught axial flow fan. Two conical diffusers of length 0.2 and 0.4 times the fan diameter and an annular diffuser with a length equal to the fan diameter are tested. At the design flow rate of the fan, the short conical diffuser increases the available static pressure by 17.6 % and the static efficiency by 8.9 %. The medium-length conical diffuser increases it by 21.9 % and 11.7 %, respectively. The long annular diffuser produces a 28.2 % pressure increase and a 14.2 % efficiency increase. The paper also compares the obtained pressure recovery coefficients of the different discharge diffusers using two-dimensional axisymmetric and three-dimensional computations. It shows that the pressure at the outlet of the fan cannot be assumed to be equal to atmospheric pressure, as is prescribed by the fan testing standards. A new method of measuring pressure recovery from two-dimensional computations is proposed. Additional keywords: Pressure recovery, axial flow fan, diffuser.
Highlights
In the power plant cooling industry, it is accepted that exhaust diffusers can aid the performance of induced draught axial flow fans
Eck [2], on the other hand, argues that it is impossible to provide exact rules for diffuser diffusion angles: half-wall angles in the range of 7° < θ < 9° are often recommend, better diffuser performance might be obtained within the wider range of 5° < θ < 20°, depending on the Reynolds number and turbulence quantities
The fan model was validated against experimental measurements and numerical results obtained from other sources
Summary
In the power plant cooling industry, it is accepted that exhaust diffusers can aid the performance of induced draught axial flow fans. Bekker et al [14] investigated pressure recovery for an induced draught axial flow fan They tested various discharge configurations which included a downstream stator blade row, conical and annular diffusers with and without stators at their inlets, and an annular diffuser with a stator row at its outlet. They found that an annular diffuser of length equal to the fan diameter having equiangular walls directed 22° from the axial direction recovered the most pressure over a range of flow rates for the M-fan of Wilkinson et al [15].
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