Ventilation and thermal performance of an integrated system for use in heritage buildings.

Abstract

Providing modern mechanical heat recovery ventilation for heritage dwellings poses the challenge of invasive duct-work and unwanted modifications to the building fabric. A retrofitted ventilation system will be more desirable if it is inconspicuous and hidden within the building fabric. The author’s original contribution to knowledge encompasses the original proposal and development from first principles of a novel natural ventilation heat recovery system utilizing disused chimneys in heritage dwellings. The system consists of an omnidirectional flue-like windcatcher integrated with a plate heat exchanger. This research provided new information on functional design of geometry, operational characteristics, and feasibility in terms of ventilation and heat recovery performance of counter-current natural air flow in the system. Two commercial CFD programs, namely Autodesk CFD 2013 and Ansys Fluent v14 were employed to perform RANS simulation of 3D models of the system. A comparison of flow results gotten from these packages showed a maximum difference of 10%. This proved the corresponding accuracy of the simpler and more user-friendly Autodesk CFD when compared with the more established Ansys Fluent software package. The air flow and thermal performance of the system’s two main parts were investigated at steady state wind speeds of 0.5m/s, 1.5m/s, 2.5m/s, 3.5m/s and 4.5m/s incident on the windcatcher at 90 degrees. Results were evaluated for a 27m³ design room ventilated at 0.7ach. At all wind speeds, the final windcatcher prototype exhibited the desired flow pattern with no short-circuit and provided more than the minimum required air change rate at all incident wind speeds above 1m⁄s. When the windcatcher is integrated with the heat exchanger, the system’s ventilation performance was 0.3ach at 0.5m/s incident wind speed. This increased to 4.5ach at 4.5m/s incident wind speed. Thermal effectiveness of the system peaked at 46% at 0.5m/s wind speed and decreased to 22% at 4.5m/s wind speed. When compared with the heat exchanger solitary performance, thermal effectiveness was reduced by an average of 27% at all incident wind speeds. This performance reduction is due to the non-uniform pattern of supply air flow through the system. Further work is required to provide experimental data on the performance of the system and investigate the effect of unsteady air flow incident at different angles on the windcatcher. Proposals for these are included. Further work is also required to incorporate features that will minimize fouling in operation and improve heat recovery effectiveness.