As part of a refurbishment of Hackney Central Hall, a large building in London, an analysis of the heating and ventilation system was undertaken. The main aim was to gain understanding of the degree to which the temperature in the main auditorium was controlled by each of the ventilation mechanisms. This information enabled the HVAC energy to be utilised more efficiently while ensuring occupant comfort levels. The work was undertaken by CHAM, in conjunction with the consulting engineers for the work.
In the absence of CAD files for the building, a simple but realistic geometrical representation was built up from architectural drawings, using standard shapes assembled using the PHOENICS VR interface. The hall is 33m ´ 22.8m ´ 12.7m (height) in size, but to reduce computation times, thereby enabling a large number of parameterised simulations to be undertaken, only half of the nearly-symmetric chamber was include in the computations. The main feature of the hall is the balcony at the back and sides, which exerts a strong influence on the air flow. The semi-transparent objects in the figure represent the heat sources associated with people in the body of the hall and lighting below the ceiling.
The hall is used in a number of different ways, and the ventilation needs the flexibility to work well (and efficiently) in all of them. Two main modes were considered: concert (or unamplified) and rock (or amplified). In concert mode the HVAC system provides cooling air at the rear of the hall in two ways; vents below the balcony provide a supply to the lower floor, while the balcony has a low-flow mechanism under the seats. In rock mode more people can be accommodated and additional low level displacement terminals are provided within wall panels below the side balconies. Air is removed by natural convection from a line of vents along the centre of the ceiling.
The figures show visualisations from one simulation. In this case the under-seat air supply is active in the rear balcony, at a temperature of 18ºC; cooler (12ºC) air is provided along the side of the auditorium from under-balcony vents and from a larger wall panel. Heat is produced by the people in the lower auditorium and the balconies, at an assumed rate of 145W/m2; the lights produce heat at a rate of 50W/m2 and 25W/m2, above the stage and the public area respectively.
The results show the consequence of the lack of ventilation below the rear balcony. A significant circulation zone is created, drawing the air from the region in front of the stage which then flows over the rear balcony before reaching the high-level vents. This in turn twists the cool air ‘curtain’ from the side ventilation panels, reinforcing the circulation. Temperatures are generally acceptable in the occupied areas, including the stage which is, if anything, cooled too much.
The PHOENICS simulations enabled a good understanding of the air flow in the hall to be obtained, under a wide range of different occupancy levels. The capacity of the ventilation to deal with these conditions could be investigated, and appropriate settings devised for each.
Hackney Central Hall is a large auditorium, 33m ´ 22.8m ´ 12.7m (height). The main features of the geometry were constructed using standard solid shapes, superimposed on a Cartesian grid. The grid chosen was 60 ´ 20 ´ 78 (modelling half of the hall), providing sufficient accuracy to capture the most important aspects of the air flow and heat transfer. The balcony was constructed from rectangular blocks, but these do not represent precisely the number of different levels actually present.
Small gaps between the blocks in the rear balcony enabled the under-seat air supplies to be easily positioned at the edge of the domain; these vents supply 0.7m3/s of low-speed air at 18ºC. A long wall panel on the side wall, below the balcony, provides about 2.5m3/s at a speed of 3m/s and a temperature of 12ºC. Air leaves the hall via a line of ten vents along the centre of the ceiling, each 0.5m ´ 0.25m; these were treated as fixed pressure
Heat sources were included, representing people and lighting. These sources were applied directly to the air in appropriate locations; it would have been possible to include solid obstructions as well, but this was not considered necessary in this study. Assumed rates of 145W/m2, representing people, and 50W/m2 and 25W/m2, above the stage and the public area respectively, representing the lights.
The solid walls and other structures were treated as non-participating in the simulations. In reality, there will be some heat loss from the air in the auditorium by this route, so the assumption can be regarded as equivalent to considering the worst-case conditions.
The simulation was steady-state and incorporated buoyancy and turbulence. Buoyancy was based on density difference, with the air being treated as an ideal gas. The k-ε turbulence model was used in the simulations; in view of the comparatively low temperatures, no buoyancy adjustment was included in the model.
Click here for animated temperature contours
here for animated streamlines.