| Ventilation and air conditioning systems play a significant role in indoor temperature and humidity regulation and are essential to modern buildings.The noise of ventilation and air conditioning ducts is an important indoor noise source,and the resistance of air conditioning duct systems is one of the most important factors affecting building energy consumption.Accordingly,this paper utilizes a combination of theoretical analysis,numerical simulation and experimentation to analyze the acoustic or resistance characteristics of elbows,spherical tuyeres,plenum chambers,and tees used in airconditioning duct systems and suggests targeted improvement measures.This paper contains the following main research findings:(1)Due to the high noise and resistance characteristics of the 90° rectangular elbow,studied the influence of guide vanes on noise reduction and resistance reduction of elbows,a quarter-circular guide vane is added to the elbow in order to suppress this noise and resistance.The computational fluid dynamics(CFD)method was used to simulate the local resistance or radiation noise of the elbow under the conditions of different guide vane positions,different flow velocities,and aspect ratios.A rectifying effect caused by the guide vanes in the duct promotes airflow mixing.It reduces turbulent kinetic energy intensity and uniformizes velocity distributions in the downstream portion of the elbow.Furthermore,sound pressure level(SPL)in the 0～1000 Hz frequency range are also reduced.According to scientifically designed guide vanes,when r/D=1.063,under duct flow velocity conditions(4 m/s,8 m/s,12 m/s and 16 m/s),local resistance and radiation noise can be reduced under the condition of W:H=1:1、1:2、1:3、1:4、1:5.The maximum local resistance can be reduced by 17.3%,the maximum radiation noise at 1m and 1.5 m can be reduced by 4.41 dB and 5 dB,respectively.(2)Serrations are added to the leading edge of the guide vane in order to further reduce the local resistance of the 90° rectangular elbow,studied the resistance reduction mechanism of serrated leading edge.The effects of sawtooth width,sawtooth angle,and guide vane radius on the local resistance and entropy production of the elbow have been investigated using a combination of CFD and full-scale experiments.The results showes that the serration was able to reduce the intensity of turbulent dissipation at the leading edge of the guide vane,and to improve the uniformity of velocity distribution in the downstream section of the elbow.The elbow resistance decreases first and then increases with the radius of the guide vane,with the minimum value appearing near radius r/D=1.72 and the serrated guide vane reducing the elbow resistance by up to 9%.When the radius of the guide vane is increased,the entropy production of the elbow decreases initially and then increases with the increasing radius of guide vane.The minimum value appears around r/D=1.56,with a serrated guide vane reducing the entropy production of the elbow by up to 11.3%.(3)A spherical tuyere has high noise characteristics in a large and high space,studied the noise reduction mechanism of sawtooth trailing edge,the sawtooth were added to the trailing edge of the tuyere to reduce the noise.Using the CFD numerical method,the effect of sawtooth height and sawtooth angle on the velocity field downstream of the spherical tuyere jet was simulated,along with the analysis of the power spectrum of the turbulent kinetic energy of the spherical tuyere at the downstream measuring point.As a result,the serrated trailing edge of the spherical tuyere nozzle reduces the mixing effect between the high-speed airflow in the axial direction of the jet and the surrounding air,reduces velocity attenuation in the jet direction,and increases the velocity at which the j et reaches the target area.Spherical tuyere radiation noise decreases with increasing sawtooth height.The maximum noise reduction of tuyere noise approximately 3.8 dB,when θ=0.055,h=0.2,which corresponds to sitting height(1.2 m).(4)A large amount of noise and resistance produced by the plenum chamber within the ventilation and air conditioning duct system,studied the noise reduction and resistance reduction mechanism of orifice plate.A low noise and low resistance plenum chamber with perforated plate was designed based on the design principles of an expansion cavity muffler and an orifice plate flow guide(two straight duct of different sizes are connected to form concentric circular sleeves).An analysis of the influence of the porosity of the orifice plate on the velocity field,local resistance coefficient,radiation noise,velocity uniformity distribution,acoustic transmission loss,and sample entropy of the plenum chamber was performed using numerical simulation.The results indicate that the perforated plate alters the turbulent vortex structure in the flow field and reduces the entropy value of the pressure fluctuation on the inner surface of the plenum chamber.A smaller orifice plate porosity results in a smaller local resistance and larger transmission loss.Radiation noise is reduced as the porosity increases.The peak transmission loss can reach 29 dB when the porosity of the orifice plate is ε1=0.14.At ε1=0.54,the perforated plate can reduce the total SPL of the plenum chamber by 2.5 dB.(5)To improve the acoustic performance of a tee,studied the acoustical characteristics of Helmholtz resonant cavity,a new type Helmholtz resonator tee is proposed.CFD numerical analysis is used to evaluate the impact of the structural parameters of the resonant cavity on the aerodynamic noise and acoustic transmission loss of the tee.To predict the aerodynamic noise of the Helmholtz tee,we used the gray model and the genetic algorithm coupled gray model,taking the A-weighted SPL as the evaluation index and the aerodynamic noise computed by CFD as the sample value.According to the prediction results,the average prediction error for gray model is 10.63%,and the average prediction error for the genetic algorithm coupled gray model is 4.15%.This paper proposes the noise reduction or resistance reduction schemes suitable for elbows,spherical tuyeres,plenum chambers and tees in ventilation and air conditioning ducts,as well as providing a scientific reference for engineering applications of noise and carbon reduction for key local components in ventilation and air conditioning systems. |
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