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The challenge of cooling the stadia

In the words of the writer, Ahmed Omer, the major challenges in outdoor air-conditioning include minimising the invasion of the outdoor hot airstreams in the region to be conditioned and simultaneously reducing the entrainment of the conditioned air in the space with the outdoor streams.

| | Jul 30, 2011 | 12:08 pm
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In the words of the writer, Ahmed Omer, the major challenges in outdoor air-conditioning include minimising the invasion of the outdoor hot airstreams in the region to be conditioned and simultaneously reducing the entrainment of the conditioned air in the space with the outdoor streams.

Challenges

The Al-Saad Stadium in the State of Qatar is a massive area occupying around 18,000m2 and is a frequent venue for major national and international sports events (predominantly soccer) organised by the country. Due to the torrid climate of the country around the year, there was a naturally strong demand to condition this open air stadium and get rid of the discomfort caused to the spectators and the players due to the extremely hot weather. In the year 2006, players in the HVAC industry worldwide were invited by the government to prove their potential and make attempts to condition the massive stadium for outdoor games.

The major challenging task in air-conditioning the enormous Al-Saad stadium was to conquer its huge open-roof area. The outdoor diurnal airstreams around the stadium location generally accompany a huge momentum, thermal power and unexpected temperature variations. It was another predominant problem to minimise the invasion of such outdoor hot airstreams into the stadium and further to reduce the entrainment of those streams with the conditioned air ejected into the stadium space. Additionally, the conditioned air diffused into the stadium premises was found to easily escape from the stadium, thus producing no cooling effect inside altogether. The task of conditioning the entire stadium was, in fact, a baffling problem compelled with a great unprecedented challenge. However, ‘Advanced Technical Solutions’ (ATS), a German innovative solutions provider in HVAC, was successful in proving its caliber and delivering its outdoor air-conditioning solution on-spot.

Concept

‘Diffusion’, specifically in this context, refers to the distribution of air within a space by an outlet(s) discharging supply (conditioned) air in different directions and planes. In traditional air-conditioning systems, diffusion occurs in the unoccupied zone and the conditioned air is discharged from the air-outlets at a velocity substantially greater than the acceptable velocity in the occupied zone and further at temperatures much below the average temperature in the occupied zone.

Proven in 1940s by McElroy, the implemented concept leverages the fact that a jet stream discharged from a diffuser parallel to a solid surface, attaches itself to the surface for a certain distance. The distance depends upon the momentum of the jet stream and its vertical distance from the solid surface at the exit from the diffuser outlet. This phenomenon is inferred from the well known ‘Coanda Effect’. In non-isothermal jets, the exact streamlines are determined by a balance between the buoyancy forces and the Coanda Effect. The moment the jet stream starts separating itself from the solid surface, a region of negative pressure is created between the stream and the surface which retracts the stream. Due to buoyancy effects, the cold air moves down to the bottom layers of the stream and the hot air moves upwards. This keeps the conditioned air proximal to the field ground.

Implementation

The Diffusion system developed and implemented by ATS at Al-Saad stadium in Qatar was absolutely innovative and the first ever of its kind. Instead of mounting vertical diffusers at the top, horizontal jet streams were used for enabling diffusion in the unoccupied space abutting the playground field level. High Kinetic energy swirl-jet diffusers with a throw of about 50 m. have been mounted on opposite sides of the stadium to continuously supply jet stream from both directions and reach the middle of the field. The positioning of the diffuser jet nozzles is feasibly optimised. Mounted at the edges of the playground, the jet nozzles are disposed at a height of about 30 cm above the ground level, and inclined at an angle of about 80 to the horizontal. This keeps the effluent jet streams attached to the ground and creates a negative pressure zone between the stream and the ground.

The diffuser jets mix the conditioned air with the ambient confined air within the stadium by a process called ‘entrainment’. This process minimises the air velocity and maintains a uniform temperature throughout its spread within the stadium premises. The travelling streams entrain a considerable amount of zone air, thus decreasing the zone temperature. The streams have a low humidity and a temperature of about 9ºC at the diffuser outlet, to approximately achieve the field zone temperature of about 22ºC. Customized in conformity to the field dimensions, the diffusers are designed to maintain the desired temperature and velocity for a large distance. A special GRP-alloy has been used for manufacturing the diffusers to minimise the aerodynamic noise substantially.

At the spectators’ area of the stadium, the mounting methodology for the diffusers is reasonably diacritical. Disposed at the back and underneath each seat, the diffusers supply cold air at a temperature of about 20ºC, thus preventing the condensate formation over the staircases. Ejected with a flow-rate of approximately 35 cubic feet per minute (CFM), the system meshes with the seating area and the streams barely interrupt the viewers walking-by through the passages between the seating rows.

Collectively, the novel ‘diffusion process’ creates a relatively uniform air velocity, temperature, humidity and quality within the occupied zone of the field. This results in a huge comfort being felt by the players and the spectators. A complete and thorough CFD analysis was used by ATS to simulate the system performance and optimise the system parameters.

Double Shade Air-stream deflection system | An Innovation at ATS

In the implementation of outdoor air-conditioning techniques, specifically when the space to be conditioned is huge, preventing/minimising the penetration of the impinging outdoor hot airstreams into the space is a big challenge. A typical example is the case of a playground, for instance, a soccer field or a cricket field that normally occupy thousands of square meters. A major hostile challenge in such cases is the high temperature outdoor wind that would unexpectedly keep on blowing over the field from different directions around the year and to keep the conditioned air effective, the invasion of this hot outdoor air within the field premises is desired to be substantially reduced.

ATS team conducted an exhaustive analysis and took measurements around the soccer fields to understand the wind flow patterns and aerodynamics of stream flow over the fields. The simulation engineers thoroughly observed the wind flow patterns for different designs of shielding systems around the field through Computational Fluid Dynamics (CFD), and carried out numerical simulations at different impinging airstream velocities. Eventually, it was found that achieving a lower wind speed inside the stadium is impossible through the use of a single surface shading system around the field.

Advanced Technical Solutions innovated a unique airstream deflection system for such outdoor fields; a shielding system that leverages the collaborative functional effects of two different surfaces attached to each other, and efficiently deflects the outdoor airstreams over the field. The system partially utilises the ‘Coanda effect’ concept, and the deflected stream profiles have been successfully verified through CFD techniques.

The shielding system for covering the peripheral surface of the field, is composed of individual shielding modules that are mounted in mutual abutment with each other around the field. Each shielding module has two functional surfaces, an upright surface and a horizontal surface, both mounted over a frame. The upright surface, being convex upwards, projects inwardly towards and upwardly from the field. NASA aerofoil 0015 is used for this surface to deflect the impinging streams over the field at high velocities. The horizontal surface has a preferably flat surface and is connected to a lower end of the upright surface. This defines a dead-end wind collector (trap zone) between the upright surface and the horizontal surface that deflects impinging airstreams over the surface. The invading streams, blowing from a specific direction, strike the convex part of the upright surface and are redirected to follow their curvature in conformity to Coanda effect. The redirected streams strike the trap zones (wind collectors) of the shielding modules located at the downstream side, and create a high pressure region within those zones. These high pressure regions act as barriers for the upcoming streams, and hence minimise the impact of wind on the field, allowing the participants and spectators to perform and watch in a congenial environment.

As wind keeps on blowing over the field, the pressure in the wind collectors of the downstream side shielding modules keeps on increasing. After substantially high pressure zones have been created in the wind-collectors disposed at the downstream side, any further airstreams impinging into the field from the upstream side, is first guided by the convex parts of the upstream side shielding modules, strikes the wind-collectors (trap zones) of the downstream shielding modules, and eventually gets reflected and moves away from and over the field. This keeps the field substantially free from the turbulence created by the outdoor wind.

For more information, please contact: Ahmed Omer, Managing Director/CEO at Advanced Technical Solutions (ATS) | E-mail: ahmed.omer@ats-corporate.de | Mobile: +974 5553 2815


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