Hybrid Ventilation Flows

Abstract

Hybrid ventilation is likely to become an increasingly important element of low-energy building design, as it is often perceived to embody both the energy efficiency of natural ventilation, and the control and reliability of mechanical ventilation. However, at the outset of this work there was little fundamental understanding of the behaviour of hybrid flows and only limited quantitative guidance available for designers. The term hybrid ventilation encompasses a whole spectrum of strategies which incorporate both mechanical elements, i.e. fans, and natural elements, i.e. vents which enable a 'stack-driven' airflow. Herein, the focus is entirely on 'simultaneous hybrid ventilation', in which the mechanical and natural components of the system work concurrently to provide ventilation. Stemming from a desire to provide practitioners with the necessary tools to complete a first-order design, this thesis focuses on mathematical models for hybrid ventilation, as such models offer rapid predictive capabilities and fundamental insights. Of primary interest is quantifying and understanding the behaviour of the, previously unstudied, case of the purging of warm air from a room using a hybrid strategy. A mathematical model is developed for this time-varying ventilation scenario, and good agreement is observed between the predictions from the mathematical model and the results of laboratory experiments. Using the mathematical model, solutions are derived for the time taken to complete a hybrid purge, the variation in the airflow rates during a purge, and the manner in which the natural and mechanical components of a hybrid system combine. Driven by a desire to make these research findings accessible to a wider, less-technical audience, the nuanced manner in which the mechanical and natural components of a hybrid system combine is visualised using 'hybrid ventilation triangles'. Continuing in the theme of providing guidance for practitioners, existing works on steady-state hybrid ventilation flows, i.e. flows for which the rate of internal heat generation equals the rate of ventilative heat loss, are summarised. A unified design framework is then developed and explored. It is anticipated that this framework will allow practitioners to expedite the design process, as well as encouraging them to adopt sound design techniques in place of current practices which, in a contradiction of the underlying physics, involve calculating the hybrid airflow rates by the addition of the natural and mechanical components calculated in isolation. Drawing on work from the entire thesis, qualitative, practitioner-focused design guidance is then developed. This guidance is centred around two key maxims for successful hybrid ventilation design.