CFD ASSISTS DESIGNERS

VENTILATION REQUIREMENTS FOR NEW PHOENIX BALLPARKBy Scott Gamble, Principal

CONSULTING ENGINEERS & SCIENTISTS

Computational Fluid Dynamics (CFD) is a computer modelling technique for simulating fluid flow. CFD can predictair speed and direction, temperature, humidity, turbulence levels, and concentrations of contaminants atthousands of points in the space to help design laboratories, cleanrooms, schools, theatres, hospitals, sportscomplexes, and industrial facilities. Designers can easily interpret the predicted flow patterns shown on colourimages. Air quality and ventilation problems are readily identified and solutions quickly tested. Other uses of CFDinclude improving industrial processes, where transport of particles and chemical reactions can interact with thefluid flow.

In this issue of RWDI’s newsletter, we feature typical examples of the use of CFD in design and problem solving.

Bank One Ballpark, being designed forPhoenix as the home of the ArizonaDiamondbacks, will be a state-of-the-artfacility incorporating a number of recenttechnological developments in professionalsports facilities. Most notable will be the useof a retractable roof system to provide shelterfrom sun and rain during events. The ballparkwill also have a natural grass playing surface.

Major technical hurdles that had to beovercome when designing mechanicalventilation for this facility included providingthe grass with sufficient sunlight for propergrowth and damage repair between ball games, while ensuring that patrons will enjoy comfortableenvironmental conditions during events. Grass growth required that the roof be open during the day in a citywhere summer ambient temperatures often reach 110°F and can exceed 120°F. In contrast, comfort requirementsdictated that the mechanical cooling system have a closed roof for proper climate control. The demandingdaylight requirement of the grass severely limited the time available to cool the stadium with the roof closedprior to a game. Ellerbe Becket Architects retained RWDI to assist in the evaluation of the mechanical systemsunder the direction of the project mechanical engineers, M-E Engineers, Inc. Early in the design process, windtunnel tests and CFD were used to test the effectiveness of augmenting the mechanical systems and naturalventilation through openings under the roof at two ends of the stadium.

(Continued on page 4)

Reputation Resources Results

ISSUE NO. 4

Vertical slice through seating areas andplaying field of stadium showing air flow

direction vectors and temperature gradients(blue is coolest, dark pink is warmest)

At Falconbridge Limited, a mining company, hot gases ladenwith particles are discharged from a smelting furnace into aspray cooling tower before entering a cyclone separator.The existing spray cooling tower had a significant build-upof particles on the north inside wall that the companywanted to eliminate.

Four factors contributed to the problem:

• poor uniformity of the gas flow in the tower, caused bythe geometry of the off-take duct connection to thetower;

• fine-mist spray nozzles possibly wetting the tower walls;

• variability of the off-gas flow rate, temperature, andparticle loading; and

• chemistry of the particles causing them to be sticky undercertain conditions.

RWDI worked with Falconbridge Limited to determinemodifications to improve the gas flow distribution withinthe tower and then developed a CFD model to test theeffectiveness of these ideas.

Figure 1 (a) is the before picture which highlights therelatively high speed flow that collides with the south wall.Note the blue coloured zone in the lower part of the toweron the north side, where the flow recirculates in a vortex-like fashion. These patterns agreed well with measured gas-speed profiles and photographs of the build-up on the wallsof the existing tower.

Figure 1 (b) illustrates a sample of calculated trajectories ofwater droplets discharged from the spray nozzles. Thesespray predictions were useful in assessing the potentialimpact on the walls where wetting was known to causebuild-up and also defined the extent of the cooling zones inthe tower.

Figure 2 is the after picture which illustrates the redesignedgeometry of the spray cooling tower that producedacceptable flow uniformity at an acceptable cost. Thegeometric modifications included angling the off-take ductso that the flow would be more aligned with the tower.One turning vane was installed in the angled elbow of theoff-take duct to spread the flue gas flow more uniformlyacross its outlet. A second turning vane was located in thetower to help reduce or eliminate the flow separationbubble on the north side wall and produce a more uniformflow across the tower.

At the top of the tower, an extension with a new transitionshape was added to provide enough time for the waterspray droplets to completely evaporate within the tower atthe future higher gas flow rates and not cause anotherbuild-up problem.

After modifications to the real tower, gas speedmeasurements again showed satisfactory agreement withthe predicted flow patterns. This success was an importantfirst step in upgrading the performance of the furnace off-gas system.

RE-ENGINEERING OF A FURNACE SPRAY COOLING TOWERBy Ray Sinclair, Principal

FIGURE 1: CFD model predictions for the existing spraycooling tower:a) Gas speed distribution (blue is <3mls; dark pink is >17mls),b) Water spray droplet trajectories.

FIGURE 2: Predicted gas speed distribution in the modified spraycooling tower (blue is <3m/s; dark pink is >27m/s

20,000 Nm3/hr gas flow rate from furnace

North wall

Water SprayNozzle locations

Extension in towerheight

20’

North Wall

Water SprayNozzlelocations

Angled Off-takewith twoturning vanes

Separatedflow zonewheresignificant build-up hasbeen observed

60,000 Nm3/hr gas flowrate from furnace

FIRE CODES ARE NOT ENOUGHBy Frank Kriksic, Principal

The design for smoke management systems in large indoorspaces such as atria, convention centres, airport terminals,sports complexes and industrial facilities has been governedprimarily by prescriptive fire codes (e.g., a required numberof air changes per hour). However, with an increasingemphasis on performance-based codes and the potential forlitigation, there is a greater need for engineering analysesto demonstrate adequate performance (e.g., maintaining asmoke layer above a certain height). These analyses havetraditionally involved calculation methods based onempirical equations to predict the migration of smokeduring a fire.

Two types of computer models have evolved to assistdesigners with this process: these are zone models and CFDmodels. Zone models, such as Hazard I, divide the indoorspace into two distinct zones with uniform characteristics topredict the smoke layer thickness and average temperatureversus time. CFD models divide the space into a 3-dimensional grid with thousands of discrete volumes. WithCFD, the predicted distributions of air speed and direction,temperature, pressure and smoke concentrations areavailable for the entire space.

The adjacent figure compares the predicted growth of thesmoke layer in a hypothetical atrium with no ventilationand a steady-state fire using the empirical calculationmethods prescribed in NFPA 92B, the Hazard I zone modeland a CFD model. As shown, there is good agreementamong all three methods for this “idealized” case. However,in a more realistic scenario, complex building geometriesand the effects of normal ventilation systems and smokecontrol systems can cause significant variation in the smokelayer depth. CFD modelling methods are well suited topredict smoke migration patterns in these more complexsituations.

Shown below are two illustrations of an office buildingatrium that was modelled using CFD to simulate internal airflow and smoke migration patterns during a fire. Theprimary concern of the design team was smoke obscurationof emergency exit routes. The large size and complexgeometry of the atrium warranted additional analysisbeyond the simpler smoke modelling methods.

Results of the CFD simulation graphically illustrate thatwithin 240 seconds of fire ignition, the upper portion of theatrium was engulfed in smoke obscuring many of the higherlevel perimeter walkways connected to the atrium. Thisindicated that the proposed smoke control system could notmaintain safe egress passages for occupants, even though thetested design exceeded the required number of air changesprescribed by the fire code. A similar conclusion could notreadily be reached with the use of traditional calculationmethods described above. It was further demonstrated inthis project that the distribution and location of the supplyair proved to be critical in the performance of the system.

a) Smoke plume 80 seconds after ignition of fire

PERSPECTIVE VIEWS OF ATRIUM:

b) Smoke plume advance after 240 seconds

Comparison of CFD and traditional calculation methods

Results showed that wind-driven air currents in the stadiumcould adversely affect the distribution of chilled air.Therefore, to improve cooling performance and to reducethe cost of the air cooling system, the under-roof openingswere eliminated.

A second factor in the design required evaluation of thesolar energy stored by the concrete during the open-roofperiods, since this would be a primary factor in cooling thefacility prior to an event. To assess this effect, a numericalmodel was used to predict variations in concrete and airtemperatures at several representative locations in thestadium through a typical midsummer day. Results of thisanalysis showed that time lags in the 4” to 5” thickconcrete were such that only the thermal history over theprevious six hours was significant. Additionally, hollow,lightweight seats provided significant shading for theconcrete, which reduced the temperatures and improvedcomfort.

With the predicted heat load distribution of the concrete,as well as the estimated heat load of spectators andlighting, a CFD model was developed to predict airflowpatterns and temperature distributions in the ballpark.This information was used to assess the effectiveness of thedesign of the conditioned air delivery system, in terms ofproviding uniform, comfortable conditions at game time.Results showed that negative buoyancy of the chilled airrequired re-aiming the supply air jets upward from theiroriginal direction. To provide the best balance betweendelivery of cool air out to the extremes of the seatingdecks and to avoid cold drafts near the jets, they wereaimed horizontally instead of parallel to the seating deck.

The final configuration demonstrated the feasibility of theproposed operation. It also identified certain areas nearopenings to concourses where ventilation may be reducedat times in order to maintain comfort and reduceoperating costs.

Rowan Williams Davies & Irwin Inc. (519) 823-1311 www.rwdi.com

RWDI Anemos Ltd.01582 470250 www.rwdi-anemos.com

Wind and Microclimate Services:• Acoustics, Noise & Vibration • Microclimate• Environmental Engineering • Regulatory Permitting• Hazard & Risk • Industrial Processes• Wind Engineering

Rowan Williams Davies & Irwin Inc. (RWDI) is a leading wind engineering and microclimateconsulting firm - the result of more than 30 years of growth and development. From offices inCanada, the United States and the United Kingdom, our consultants meet the world’s most complexstructural and architectural challenges with experience, knowledge and superior service. In the earlyplanning stages, careful attention to the effects of wind, snow, ventilation, vibration and relatedmicroclimate environmental issues on buildings and structures will save time, save money and reducerisk. Our capable and qualified staff uses advanced engineering tools and carefully defined consultingprocesses to deliver understandable and useful results.