MegaFan Case Study:
AUTHORS: DR JAMES FORREST & PROF IEUAN OWEN
Researchers from the University of Liverpool’s Engineering Department have been contracted by MegaFan Technologies Ltd (MTL) to model the airflow and heat transfer in a heated warehouse building to investigate the levels of de-stratification offered by high-volume, low-speed (HVLS) fans. Computational Fluid Dynamics (CFD) has been used to develop a model of a generic warehouse building that includes a HVLS fan, as well as an air heater. Heater power requirement estimates have been derived based on a target ground level temperature of 16°C; this has been performed for both ‘fan on’ and ‘fan off’ conditions. Several different building heights and outside temperatures have been studied to obtain performance metrics for the HVLS fans in a variety of operating conditions. For the case of an outside temperature of 4°C, the analysis has indicated heating power savings of between 40 and 45% when the HVLS fan is deployed.
The scope of this study was as follows:
– Create a computational model of a generic warehouse building having a footprint of 30x30m; with apex heights of 12.5m, 15.0m, 17.5m and 20.0m.
– Implement fan downwash and heater components into the model to account for the effects of heat, mass and momentum transfer due to the heater and HVLS fan.
– Determine the heater power required to produce a ground temperature of 16°C for all building heights with HVLS fan on and off (for an outside temperature of 4°C).
– Determine the heater power required to produce a ground temperature of 16°C for the 17.5m building for outside temperatures of -4°C and 0°C.
– Derive estimates of energy savings due to the use of the HVLS fan.
The work reported here follows an earlier feasibility study carried out by the current authors (report dated 15th March 2010), which showed that results from the stratification/de-stratification model agreed well with anecdotal evidence from existing HVLS users.
Thermal stratification occurs in heated buildings due to the natural buoyancy of warm air, causing heated air to rise and gather at the ceiling. This leads to thermal inhomogeneity characterised by layering, with temperatures typically increasing by 0.5°C to 1°C per metre (floor to ceiling) in the vertical direction. Thermal stratification is most severe in buildings with high-ceilinged rooms which are heated from above, such as warehouses, where the effect causes energy wastage due to the fact that most of the room (above the height of the thermostat) is heated to a higher temperature than the thermostat set-point.
HVLS fans are typically located close to the roof and provide thermal de-stratification by pushing the warm air down towards the ground. This creates convection currents and also encourages mixing between the warm and cold air, leading to a more uniform temperature distribution and less energy wastage.
Anecdotal evidence from existing HVLS users has indicated that, following the installation of the fans, substantial energy savings have been made through the winter months, leading to a reduction in heating bills. Although the way in which these fans work is fairly well understood, very little in the way of scientific research has been carried out to corroborate these in-situ findings. As a first step towards gaining a better understanding of the physical mechanisms at work, this report provides details of a computational study of heat and fluid flow within a generic warehouse building with a single heater and HVLS fan installed.