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The fluid dynamics of an underfloor distribution system


This thesis discusses the fluid dynamics of an under floor air distribution (UFAD) ventilation system in which, in contrast to conventional air conditioning systems, cool air is delivered from diffusers in the floor rather than from overhead vents. In order to produce more realistic models of UFAD systems, we extend previous work on a simplified system consisting of a single heat source at floor level and a single cooling diffuser developed by Lin & Linden (2005), to the case of multiple cooling diffusers and a single heat source located at different heights above the floor; of two and more sources of equal or unequal buoyancy fluxes and of a line heat source in a UFAD system. This is an attempt to provide more guidelines to complete the complicated models of UFAD in reality. We carry out experiments in which the heat sources are represented as buoyant point plumes, and the cooling diffusers are modelled using negatively buoyant vertical jets. The radiation from the sun into the system is represented by a line plume attached to a wall. The experiments show that the properties of the system are determined by the entrainment into the plumes and the negatively buoyant jets. In the spirit of Morton, Taylor & amp; Turner (1956), we characterize these entrainment processes by entrainment coefficients, and develop a theoretical model based on layered models of ventilation flows introduced by Linden, Lane-Serff & Smeed (1990). The model predictions are compared with the laboratory experiments, and used to determine the dependence of these parameters on the cooling load, the ventilation rate and the properties of the cooling diffusers. The tank experiments have been compared with full-scale UFAD room tests by non-dimensional scaling analysis, based on which explicit equations were concluded to use in EnergyPlus code

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