Applicability of flow-rate-independent discharge coefficients in purpose-provided, interior natural-ventilation openings. Issue 1 (2nd January 2018) - Bibby, Chris
ABSTRACT: Natural ventilation involves low pressures, necessitating large interior ventilation openings (ventilators) with low airflow resistance in interior partitions, allowing required ventilation airflow. These ventilators are detrimental to the noise isolation between spaces. Design methods often use flow-rate-independent discharge coefficients, which rely on a high-Reynolds-number (Re) assumption. This paper evaluates this assumption for purpose-provided ventilators, based on theory, field measurement data and prediction. If a high-Re discharge coefficient is used to predict airflow at low Re, the flow rate will be over-predicted and the system under-designed. Reynolds numbers in some existing interior ventilators are low enough that flow rates are inaccurately described by a high-Re discharge coefficient. If the ventilator is highly restrictive to flow or if the hydraulic diameter of the flow path is very small, low-Re behaviour may be critical to system design.