Experimental and numerical investigation of noise generation from the expansion of high velocity HVAC flows on board ocean going fast ferries

摘要

This thesis details a study of strategies used to limit the flow generated noiseencountered in the outlet diffusers of high velocity heating, ventilation and airconditioning (HVAC) duct systems. The underlying noise rating criterion is drawn fromthe specifications covering ocean going aluminium fast ferries. Although directedprimarily towards the fast ferry industry the results presented herein are applicable toother niche high velocity HVAC applications.Experimental tests have been conducted to prove the viability of a high velocityHVAC duct system in meeting airflow requirements whilst maintaining acceptablepassenger cabin noise levels. A 50 mm diameter circular jet of air was expanded using aprimary conical diffuser with a variety of secondary outlet configurations. Noisemeasurements were taken across a velocity range of 15 to 60 m/s. An optimum outletdesign has been experimentally identified by varying the diffuser angle, outlet ductlength and the termination grill. A 4 to 5 fold reduction in required duct area wasachieved with the use of a distribution velocity of 20 to 30 ms-1, without exceeding theprescribed passenger cabin noise criteria. The geometric configuration of the diffuseroutlet assembly was found to have a pronounced effect on the noise spectrum radiatingfrom the duct outlet.The development of a numerical model capable of predicting the flow inducednoise generated by airflow exiting a ventilation duct is also documented. The modelemploys a Large Eddy Simulation (LES) CFD model to calculate the turbulent flowfield through the duct diffuser section and outlet. The flow-generated noise is thencalculated using a far field acoustic postprocessor based on the Ffowcs-Williams andHawkings integral based formulation of Lighthill’s acoustic analogy. Time varying flowfield variables are used to calculate the fluctuating noise sources located at the ductoutlet and the resulting far field sound pressure levels. This result is then used tocalculate the corresponding far field sound intensity and sound power levels. Thenumerical acoustic model has been verified and validated against the measuredexperimental results for multiple outlet diffuser configurations.