A validated procedure for determining the buoyancy-induced flow in ducts

A procedure is set out for calculating the buoyant flow induced in a duct with heat input at the wall, as in the cooling ducts used behind photovoltaic arrays. In these, buoyancy is opposed by various pressure losses, due to obstructions at the inlet and outlet, fluid friction at the walls and structural support members passing transversely through the duct. New methods are developed for calculating these losses, and each is validated separately by tests in a purpose-built isothermal rig. Measurements are also reported for some further losses, not yet amenable to calculation, due to nets and hoods at the duct ends, as might be used to exclude rain and wildlife. Finally, the whole procedure is validated by measurement of a duct with one heated wall. Practical application: Verified by measurements at every stage of its development, the method reported gives greater confidence in the routine calculation of the flow induced in ductwork where there is heat gain, as in systems for PV cooling and natural ventilation. The new treatments given for the hydrodynamic losses at basic components apply wherever these are used in the field of HEVAC.

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