Study on transmission characteristics and factors influencing T-elbow aerodynamic noise in ventilation and air-conditioning pipelines

Abstract

Local ductwork components, such as shunt T-elbows, generate aerodynamic noise in ventilation and air-conditioning systems where turbulent air flow dissipates pulsation. The noise generated by airflow propagates along the ductwork to the end that terminates into a room. The incident acoustic energy may affect the working efficiency and health of the occupants. In this study, computational fluid dynamics and acoustic finite element methods were used to solve the Lighthill acoustic equation and simulate the flow field through T-elbows and the noise field generated by the airflow. The goal is to explore generation and transmission of aerodynamic noise through ductwork in a ventilation and air-conditioning system. The effects of inflow velocity, flow distribution ratio and T-elbow geometry on the noise intensity and acoustic energy propagation were investigated. The results show that low-frequency noises at the branch of the T-elbow are dominant and are caused by local unsteady vortex motion when the air flow diverges rapidly. In addition, the noise intensity and the propagated acoustic energy vary with the inflow velocity, branch flow rate ratio and branch geometry. Finally, the optimal structure for reducing noise in specific conditions can be determined by analysing and comparing various T-elbow geometries.

Improvement of airflow uniformity and noise reduction with optimized V-shape configuration of perforated plate in the air distributor

“…The Ffowcs Williams and Hawkings (FW-H) equation provides the general form of the Lighthill sound analogy, which can be used to predict the noise generated by equivalent sound sources (including monopole source, dipole source and quadrupole source). 29 This equation has been widely used in the prediction of mid-field and far-field noise in pipelines. 30,31 The mathematical expression of the FW-H equation is as given by equation ( 4):…”

“…The Ffowcs Williams and Hawkings (FW–H) equation provides the general form of the Lighthill sound analogy, which can be used to predict the noise generated by equivalent sound sources (including monopole source, dipole source and quadrupole source). 29 This equation has been widely used in the prediction of mid-field and far-field noise in pipelines. 30,31 The mathematical expression of the FW–H equation is as given by equation (4):where c is the sound velocity of far field; p′(xi,t) is the sound pressure of the observation point xi at time t ; ρ0 is the fluid density of far field; ui is the component of the fluid velocity in the direction of xi; un is the fluid velocity component perpendicular to the integral plane; vn is the moving velocity component of the integral plane; δ(f) and H ( f ) are the Dirac delta function and the Heaviside function, respectively; and Pij and Tij are the stress tensor and Lighthill stress tensor, respectively.…”