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Noise between 25-500 Hz is a common problem in Heating, Ventilating, and Air Conditioning (HVAC) systems. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook lists values of end reflection loss (ERL), a frequency dependent parameter describing energy reflected back up a duct at a termination impedance, to help engineers design and account for noise. The ASHRAE Handbook does not account for common termination variations and only lists ERL values using octave bands down to 63 Hz. This thesis experimentally determined the ERL of a variety of rectangular duct configurations and termination conditions between 25-500 Hz. This research also compared experimental ERL results with analytic predictions and ASHRAE Handbook values. Seven duct sizes were tested, from 6X6 to 18X54 inches. Duct termination baffle hardness was varied between acoustically hard (plywood) and soft (ceiling tiles) for the 6X6, 6X10, and 6X18 ducts. Five duct termination distances above the termination baffle were tested, between flush and 1D for the 6X10 and 6X18 ducts and between flush and 5D for the 6X6 duct, where D equals the duct s effective diameter. Diffusers and flex duct configurations were installed at the end of the rigid duct to test their effect on ERL on the 6X6, 6X10, and 6X18 ducts. ERL was determined using an adaptation of the ASTM E1050 Standard, an application of the two-microphone impedance tube method. Experimental results closely conformed to analytic predictions and are an improvement over ASHRAE Handbook ERL values. The results indicate that baffle hardness has a negligible impact on ERL, which contradicts the ASHRAE assumption that diffusers that terminate in a suspended lay-in acoustic ceiling can be treated as terminating in free space. Termination distance above the baffle has a negligible impact on ERL at distances less than six inches for the 6X6 duct. Termination distances above the baffle greater than six inches exhibit limited free space ERL behavior for the 6X6 duct. The use of flex duct greatly reduces low frequency ERL and this is not accounted for by the ASHRAE Handbook. The impact from flex duct usage also negates any influence from downstream termination variations.

A wooden, perforated, uniform cross-section duct was examined to determine the optimum levels of aperture ratio and fan speed with respect to uniformity of discharge. The optimum aperture ratio for the 8.54 m long duct was 1.0 with a uniformity coefficient of 90.28%. The fan speed had little effect on the uniformity of discharge. The friction factor was experimentally determined to be 0.048 for a non-perforated duct and this value was assumed to be the same for a perforated duct of similar construction. A kinetic energy correction factor was used to analyze the flow in the duct. Values for this correction factor were determined from experimental data. Values of the coefficient of discharge and the total duct energy were calculated. A mathematical model was proposed based on the conservation of momentum and the Bernoulli's equation. The model responded favourably and predicted the duct velocity nearly perfectly and slightly underestimated the total duct energy.

Air distribution ducts are used in the environmental control of livestock and poultry building as well as the conditioning of most agricultural produce. / In order to simplify the approach to the design of ventilation ducts, a mathematical equation has been derived to describe the average air velocity of a duct. / The primary objective of the research work was to test goodness of fit of an equation describing the average air velocity of perforated ventilation ducts, under balanced as well as unbalanced air distribution: $V = H sb{o}{X over L} + (V sb{L}-H sb{o}) {X sp2 over L sp2}$. / This equation was successfully tested using data measured from 14 ducts of constant cross-sectional area, built of wood or polyethylene with outlets of various shapes and aperture ratios. Results indicated that aperture ratio and distance along the duct are the two most significant factors influencing the average duct air velocity values, but material and outlet shape had little effect.

Includes bibliographical references (leaves 287-305) / 310 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1991

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