Analysis of duct networks at high frequencies using two-ports

Abstract

Duct Networks (e.g. HVAC) should be carefully designed to maintain certain pressure drop, flow rate and acceptable noise levels. To accurately analyze the acoustics in a duct network in the high frequency region, the following mechanisms need to be modeled: The sound power injected into the network by sound sources (e.g. Fans), the flow noise generated in different parts in the network (e.g. junctions), and the noise reduction across different parts of the network. Traditionally only transmission of sound power with no reflection is considered in standards, e.g., ASHRAE or VDI, for analyzing noise in HVAC systems. In this paper, a more general approach is considered based on dividing the duct network into two-port elements where each element can be described by a 2×2 scattering matrix. The state variables are taken as acoustic power flow in the up/downstream directions. Junctions are described by multi-ports depending on the number of elements connected to the junction. A source vector is added to each element and junction to handle sound power injection by fans or other aeroacoustic sources. The advantage of this approach is that the same formalism (based on two-port network theory) can be used to analyze both low frequency range and flow distribution / pressure drop as well as the high frequency range. The two-port power based formulation was validated against a detailed HVAC example in VDI 2081: Part 2.