A Study On The Mechanism Of Tonal-noise Reduction Within Uneven Cooling Fans

The scheme of applying uneven spacing to the fan-blades within low speed fan has been proved to be effective in reducing the tonal noises that radiates from the fans.Since this practice on the other hand brings little damage to a fan's pumping ability,it is nowadays broadly used in aeroplane situations,helicopters,as well as automobile engines.However,there is a gap between conventional uneven schemes,including MBS,and the increasing demand for quieter cooling fans.In order to seek a novel design with a quieter uneven spacing for fan-blades,an investigation into both the flow features and the acoustic features within uneven fans is taken in the paper,based on which the uneven spacing is optimized to reach a quieter scheme for cooling fans.Transient CFD analysis was conducted to a type of cooling fans,of which the rotors are designed with three different uneven spacings.The features of the flow field and the blade forces were checked based on the CFD results.The tonal noise features of the uneven fans were predicted with Lowson model and the results were then compared to that of an even fan.Based on the results,a series of prediction model for the tonal noises emitting from uneven fans was developed and validated with acoustic experiments.In the end an optimal design was reached.The main points covered in the paper are as follows.1.Structured grids were developed for a type of cooling fans of which the rotors are designed with three different uneven spacings.Transient CFD analysis was done and validated with experiments.Simulation results show that the relative pressure comes to a peak at the mid-span positions on the fan-blades while high values of TKE and intensive axial acceleration of the main flow aggregate at high-span locations downstream the Trailing Edge of the blades.The simulations results were analyzed with DMD.Results show that the frequency features of the interfaces upstream and downstream of the fan blades are alike but not no exactly the same.2.Blade force characteristics of uneven fans were investigated based on the CFD results.Results show that the time-averaged blade loadings are different from blades to blades as the uneven spacing schemes vary.The axial component T was analyzed so as to develop a model between the time-averaged blade forces and the passage angles.As to the fluctuation items of the blade forces,the oscillations of the blade force components for both the even fans and uneven fans are in good consilience.FFT analysis shows that applying uneven spacing to fan-blades does not bring distinct changes to the frequency features of the blade forces while changing the vane's frequency maps by reducing the strength of concentration around the multiples of BPF.3.The acoustic features of uneven fans were studied based on CFD results with the help of Lowson model.Fan blades are proved to be the main source of the tonal noises and the fan configurations appear similar to a forward-facing dipole when the directional features of the noises were checked.The mechanism of the uneven fans' reducing tonal noise was proposed in the paper to be in three aspects,the phase difference caused by uneven spacings,the difference between the force's steady loadings,and circumferential shifting of the equivalent noise sources on blades.4.A semi-mixed method was proposed to provide predictions for tonal noises within uneven fans and an evaluation model for uneven fans' potential in reducing the tonal noises was then developed for uneven fans with arbitrary uneven spacings.The methods save the design process from cost of CFD analysis to formula calculations,which brought an optimal design into reality and helped to get an optimal uneven spacing.The optimized prototype was manufactured and field test shows that it reduce the tonal noise at BPF by about 5 dBs when compared to baseline uneven designs.All in all,the paper conducted a detailed flow analysis as well as noise predictions based on uneven fans by means of numerical simulations,results of which are of instructive meanings to later designs of the low-speed fans alike.