FEA Based Frequency Optimization Of Duct With Variable Cross Section Through Structural Modifications For Avoiding Resonance

Ducts are used to carry fluid in various applications in HVAC systems, thermal power plants, cement and other process industries. Such ducts are generally subjected to failure due to cyclic stresses either caused by high temperature fluctuations, mechanical vibrations or due acoustic vibrations caused due to the fluid flowing through the ducts. In this paper we study the failure of one such duct in which acoustic vibrations cause resonance leading to weld failure and cracks. A lot of work has been earlier done to improve the frequency of ducts using CFD analysis for finding the nature of the flow, effect of the varying C/S of the duct on the flow and duct vibration. Through the earlier works, the methods like active noise control, and development of mathematical model for finding analytical solutions to the duct acoustic failure have been demonstrated. In this paper different methods to improve natural frequency of variable cross sectioned ducts(was duct) are discussed briefly while the main onus of the paper is on use of structural modifications for frequency improvement. Frequency of fluid flowing in duct under study was(earlier it was ‘is’) measured onsite and was found to be 14Hz.So all modes of vibration needed to be greater than 14 Hz for avoiding resonance and reducing vibrations by great extent. The paper focuses on the structural modification approach in order to improve the frequency and thus presents the various structural modifications and the optimized solution along with the impact of various modifications on the frequency. The duct is modelled in pro-e while the modal analysis is performed in ansys.