Calculation Analysis And Control For Air Conditioning Ystem Aerodynamic Noise Of A Medium Bus

With the research progress on vehicle power, fuel economy and driving safety, peoplehave higher expectation on riding comfort of automobiles. The main content of the ridingcomfort is to reduce the vibration and noise of vehicles. As the performance of mounts andsuspensions is becoming better and better, noise coming from tires, road, engine has beenreduced greatly. Recent years, consumers from cold northern and humid southern areas ofour country demand that riding comfort should have a new content—thermal comfort. Theyhope they can adjust the interior temperature and humidity in their cars. According tostatistics, almost all passenger cars and more than half of buses are equipped with airconditionings. But using air-conditioning device bring new problems. Due to the restrictionsof the overall layout, sometimes, air conditionings become main noise source.Air-conditioning noise consists of fan noise and duct noise which mainly comes fromaerodynamic noise. With small installation space and demands of sufficient air flow, thenoise of high speed centrifugal fans of air conditionings is loud. Besides, with not propergeometry structure, duct will make loud noise and bad temperature distribution. In the earlyresearch, people mainly test and achieve the performance of air conditionings. There aremany parameters in the design of fans and ducts. Traditional R&D methods last long cycle,waste lots of manpower and material resources. However, as the experiment condition isdifferent from that of the car, the methods have some limitations. The Computational FluidDynamics (CFD) can solve this problem. CFD method is based on the idea of the volumeelement and can easily access the fluid motion and predict the noise generated by the product,while it is not subject to the restrictions of test conditions and test equipment. This paperconcentrates on the noise generated by the air conditioning system of a medium-sized bus.The main content of this paper is follows: This paper firstly introduces the basic principle that fluid movement follow, the basictheory of aerodynamic noise and why fans produce noise. Then air-conditioning noise ismeasured in open outdoor to exam contribution of interior noise. The sound pressurefrequency spectrum of test points is obtained only when the air conditioning is working.Based on it we can find possible causes of the air conditioning system noise. Through actualmeasurements we build geometric model of the cavity between the air-conditioning and theinterior wall and calculate its sound mode. By comparing the acoustic mode frequency withthe peak frequency of measured sound pressure, we find the two results are consistent.Finally we decide to change the cavity structure to avoid the acoustic mode resonance.Then high-frequency noise generated by the fan is studied. Through3D scan imagingtechnology the fan’s geometric model is built with Catia. Then the fan’s volume model basedon the computational domain requests is formed with Hypermesh. At last this mesh data isimported into Fluent. Through simulation, the sound pressure spectrum of measure pointsvalidates the correct of model. Firstly the noise reduction effectiveness of several commonmethods in the same calculation conditions is studied. Then the vortex distribution of thesemodels in two-dimensional conditions is contrasted. Lastly we study the fan’s main influenceparameters on the flow, total pressure, sound power in orthogonal experiments.The volume element model of the duct is built. Then instantaneous information of flowfield calculated in Fluent is imported into Sysnoise to calculate the noise propagationthrough interpolation algorithm. Sysnoise shows the distribution of the sound source and theexternal field sound pressure at a certain frequency. We find a better scheme according to thedistribution of the sound source and prove the improvement effect through simulation.In this paper, the sound pressure frequency spectrum obtained from the test identifies themain cause of noise. Different control methods are used for each peak frequency. This paperput forwards new methods and ideas for the air conditioning system noise reduction.