The lab for aerodynamics and acoustics (AA-Lab) at the department of mechanical engineering is one of the first three labs established at the HKU Zhejiang Institute of Research and Innovation (ZIRI) and is led by Professor Lixi Huang. Recently, a novel design of acoustic silencer (patent pending) has been delivered to its industrial partner, Huadian Heavy Industry Co. Ltd (HHI), for the first large-scale application in Hangzhou where Asian Games 2022 will be held. One of the gas-fired power plant (Fig. 1a) is situated in the urban area and cannot be re-located due to energy efficiency (residual heat utilization) considerations. In its drive to modernize the district, the local government spells out a very high requirement for noise mitigation, backed up by a high budget. The new design for the exhaust (Fig. 1b) will be a fan powered cooling tower with silencers installed at both flow inlet (side walls of the three-storey buildings) and the exit (roof). The novel design by the AA-Lab is a result of close collaboration between ZIRI and HHI, with real-size product tested at ZIRI as well as by a third-party lab, certified under relevant international testing standards.
The scientific question is how to attenuate noise over a very wide frequency band, typically from around 80Hz to 8kHz, a ratio of band limits R=8000/80=100. Academic research typically deals with a ratio of R=2 (one octave band) to R=10 (one decade), and so the real-life application with R=10^2=100 (two decades), represents a huge challenge. The novel design by the AA-Lab involves heavy acoustic virtual mass, coupled to a flow passage that avoids major turbulent flow separations. When the product is used in the current form of specified flow pressure allowance and same manufacturing cost, the acoustic performance is increased by 6.8dB (overall). If the product is used under the specification of equal acoustic performance, the pressure loss (energy cost) can be reduced by about 30%.
Fig. 1a: A power-plant in urban Hangzhou before Fig. 1b: New design of fan-powered cooling
renovation tower with silencer installation highlighted by red arrows.