Aerodynamic And Aeroacoustic Design Of A Centrifugal Fan For Medical Applications

Respiratory and breathing disorders such as obstructive sleep apnea(OSA),upper airways resistance syndrome(UARS)or chronic obstructive pulmonary disease(COPD)can be partially or fully treated at home by the use of continuous positive airway pressure(CPAP).This is done by connecting the outlet of a fan with the nose or mouth of a patient,hence aiding with breathing.However,the size and level of noise of this machines occasionally result in patients’ refusal to use,thus inhibiting them from recovery.Based on a literature review of the pressure requirements for each disorder,a small and quiet fan is designed in this thesis.This two requirements pose challenges to the design,smaller devices are more affected from surface roughness than larger ones,and hence the value of roughness should be limited so it doesn’t affect the fan performance.Aeroacoustic noise is the main source of noise for small fans,keeping it below annoyance levels will ensure that patients are not discourage to use it,especially at night.With the pressure and flow requirements established,a fan was selected based on similarity laws,and a 3D CAD model was generated.The performance of the fan was validated in single passage and full assembly simulations via CFD.With the geometry defined,the influence of the surface roughness on performance was evaluated.Results showed that surface roughness above 3.5m reduce the global isentropic efficiency of the fan,with the effect being more severe at high flow rates.Plastic injection molding was chosen as the manufacturing process and it should guarantee that the fan’s final surface roughness stays within the mentioned value.Aeroacoustic noise was calculated with the FW-H equation,by using the results of a transient flow simulation to find the acoustic pressure;results were then converted to frequency domain.It was found that the blade passing frequency is the responsible for the loudest(58 d B(A))sound,corresponding to the tonal noise of the fan,results also showed that the direction aligned with the outlet of the fan have the largest noise.The last stage of design was the reduction of the aeroacoustic noise.The lambda-2 criterion was used to identify main vortex structures from the flow field,these were in turn related to a particular geometry of the fan.By modifying the trailing edge of the blades,from a sharp edge to a rounded one,the broadband noise of the fan was reduced by 5-10 d B(A)in the midrange bands,while tonal noise was reduced by 3 d B(A).Increasing the radius of the volute by 10% reduced the tonal noise of the fan by 10 d B(A)and yielded noise reductions over the whole frequency spectrum,except for the bands associated with the rotor frequency and its second harmonic,nonetheless the sound level of the latter are still within limits.The increase of the radius of the volute came with a reduction of 7% in aerodynamic efficiency,however given the exceptionally low-noise level,a compromise was made.The final design of the fan then has overall dimensions of less than 8 cm and sound level below 40 d B(A)for all frequency bands.