9th International PHOENICS User Conference Moscow, September 2002

1. 9th International PHOENICS User ConferenceMoscow, September 2002 A presentation by Dr. Paddy Phelps on behalf of Flowsolve and IAC Ltd September 2002

2. Predicting air flow and heat transfer in an anechoic test chamber for industrial chillers

3. Outline of Presentation • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Simulations performed to date • Presentation of Results • Conclusions

4. Chiller Test Chamber Ventilation study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Simulations performed to date • Presentation of Results • Conclusions

5. Industrial Context • IAC Ltd design and construct a range of bespoke anechoic test chambers. • Their client in this instance was York Ltd, manufacturers of air chiller units for building HVAC systems. • York wish to improve the design of their products by testing them at the limits of their performance envelope

6. Industrial Context • Test chamber design brief calls for air supply temperatures at chiller intakes to be uniform to within 10C . • Chiller unit intakes are located along upper body sides and ends. • Up to 12 ducted fans on top of unit emit highly swirling air extract flow, several degrees different from ambient

7. Test Chamber Geometry - 1

8. IAC Ltd Chiller Test Chamber ventilation study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Simulations performed to date • Presentation of Results • Conclusions

9. Objectives of Study • Use simulation tools to predict mixing of hot swirling extract flow with ambient airflow inside test facility • Provide input to design of chamber air supply / extract arrangements, by predicting likely effect on airflow patterns • Confirm client criteria for uniformity of temperature at chiller intakes can be met

10. Test Chamber Geometry - 2

11. Test Chamber Geometry - 3

12. Simulation Tool Options • Direct Experiment • not applicable - building not yet constructed • use to confirm other predictive tools • Wind-Tunnel Modelling • scale-up and thermal representation difficult • problem with interpretation of results • Numerical Simulation • passive (Gaussian) dispersion models • Computational Fluid Dynamics

13. IAC Ltd Chiller Test Chamber ventilation study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Simulations performed to date • Presentation of Results • Conclusions

14. Benefits of CFD Approach • No scale-up problem • Three-dimensional, steady or transient • Interrogatable predictions • Handles effect of • blockages in domain • recirculating flow • multiple inlets and outlets • multiple interacting heat sources

15. IAC Ltd Chiller Test Chamber ventilation study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Simulations performed to date • Presentation of Results • Conclusions

16. Solution Domain(s) CHAMBER MODEL • Solution domain encompasses the test chamber up to, but not including, the outlet plenum • Domain 15.22m by 18.88m by 8m high PLENUM MODEL • Solution domain encompasses the outlet plenum only • Domain 12.2m by 17.08m by 1.3m high

17. CFD Model Description - 1 • Representation of the effects of • blockage due to the presence of an internal obstacle (chiller unit) • multiple inlets and outlets for chamber air • resistance and mixing in extract silencers • distributed intakes on chiller sides & ends • discrete, swirling outlets on chiller top [ Flow inside chiller not solved for ]

18. CFD Model Description - 2 • Dependent variables solved for : • pressure (total mass conservation) • axial, lateral and vertical velocity components • air/chiller effluent mixture temperature • air residence time in chamber • turbulence kinetic energy • turbulence energy dissipation rate • Independent Variables: • 3 spatial co-ordinates (x,y,z) and time

19. CFD Model Description - 3 • Iterative “guess and correct” solution procedure to convergence of scheme • Typical domain size - 15x8x19 m. • Around 1500 “sweeps” of domain required for convergence • Typical nodalisation level - 207,000 • Convergence involves solution of around 2,500 million simultaneous linked differential equations

20. CFD Model Description - 4 • The set of partial differential equations is solved within the defined solution domain and on a prescribed numerical grid • The equations represent conservation of mass, energy and momentum • The momentum equations are the familiar Navier-Stokes Equations which govern fluid flow

21. CFD Model Description - 5 • The equations may each be written in the form D(rj) /Dt + div (r Uj - Gjgradj) = S{j} • Terms cover transience, convection, diffusion and sources respectively • Equation is cast into finite volume form by integrating it over the volume of each cell

22. IAC Ltd Chiller Test Chamber Ventilation Study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Simulations performed • Results Obtained • Conclusions

23. Supply / Extract Arrangements Studied • Chamber air supply arrangement • Straight supply ducts • Angling of supply end regions • Blocking middle region • Chamber air extract arrangement • Long side outlet ducts • Small additional centre outlet • Large centre outlet • Small vestigial side outlets

24. Chamber Geometry Arrangements Studied • Effect on chiller intake temperatures of: • Friction on walls & ceiling • Silencer pressure losses at inlet & outlet • Mid-height wall “lip” • End “hood” on chiller • Baffles along chiller sides • “Lip” around centre ceiling extract • “Swirl breaker” above chiller

25. Chiller Operating Conditions Studied • Chamber dimensions • Dimensions 2.2 x 8..7 x 2.44 m. high • 12 outlet fans, swirl angle = 30 degrees • Chiller “Hot” Operating Condition • Inlet temperature ~ 35 deg.C • Heat input = 951kW or 1019 kW • Air flowrate = 75 or 67 m3/s • Chiller “Cold” Operating Condition • Inlet temperature ~ 7 deg.C • Heat input = -363kW

26. Chamber Operating Conditions Studied • Chamber Air supply Rate • Initially 110% of chiller throughput ( i.e. 1.1*75 = 82.5 m3/s] • Subsequently increased to 90 m3/s • Hot Operating Condition • Inlet supply temperature = 35 deg.C • Cold Operating Condition • Inlet supply temperature = 7 deg.C

27. Overview of Workscope 34 simulations performed in 7 “stages” • Stage 1 - Original design concept; effect of swirl; add small central outlet; remove lateral offset; longer central outlet; add wall friction; hot & cold runs • Stage 2 - chamber outflow partitioning sensitivity; effect of inclining and part-blocking some of supply inlets • Stage 3 - revised chiller inflow partitioning; Central outlet lip and vestigial side outlets

28. Overview of Workscope 34 simulations performed in 7 “stages” • Stage 4 - chiller swirl level; outlet silencer resistance; central outlet lip. • Stage 5 - Increase chamber air rate; chiller end and side baffles; increase chiller heat rate and reduce throughput for “worst case”. • Stage 6 - Worst case run with “swirl breaker” • Stage 7 - Air loading run with chiller off.

29. IAC Ltd Chiller Test Chamber Ventilation Study • Industrial Context • Objectives of Study • Benefits of using CFD • Description of CFD Model • Simulations performed • Results Obtained • Conclusions

30. Original Design Concept • Configuration • Two long low-resistance side outlets • No central outlet • Supply • Chamber supply rate = 82.5 m3/s • Chamber supply temp = 35 deg. C • “Hot” Chiller Operating Condition • Chiller throughtput = 75 m3/s • Temperature rise through chiller = 11.17 deg C

31. Original Design Concept:Predictions - 1 • Max temperature difference across chiller intake ports = 7.99 oC • Min intake temperature = 35.1 oC • Max intake temperature = 43.1 oC • Mean intake temperature = 36.65 oC • Mean intake residence time = 7.46 sec • Max chamber residence time = 72.1 sec

32. Original Design Concept: Predictions - 2

33. Original Design Concept: Predictions - 3

34. Original Design Concept: Predictions - 4

35. Original Design Concept: Initial Findings • Hot, highly swirling flow from chiller outlet creates non-symmetric flow patterns in chamber, despite symmetry of inlet, outlet and chiller locations • Hot recirculating flow re-entrained into chiller end intakes, creating a “hot end” and a “cold” end • Intake temperature differences are eight times desired criterion …...

36. Stage 1 Simulations • Effect of chiller outlet swirl level • reducing swirl improves matters … (but this is not an option) • add small central outlet • DT reduced to 4.35 oC • lengthen central outlet • DT increases slightly to 4.85 oC

37. Stage 1 Simulations • Chamber wall friction • DT reduced slightly to 4.52 oC • Hot and Cold Operation • DT for cold operation about half that when hot Hot operating condition will thus be the worst case for achieving the chiller intake temperature uniformity criterion

38. Stage 2 SimulationsSupply/Extract geometry sensitivity • Chamber outflow partitioning sensitivity • Tinkering with outlet resistance does not improve matters. DT in range 4.5 to 6oC • Inclining the outer supply inlets • Directing outer inlet jets towards chiller ends, to sweep away descending hot fluid from intakes, does hot have desired effect. • DT in range 4.5 to 5oC

39. Stage 2 SimulationsSupply/Extract geometry sensitivity • Blocking the lower centre supply inlets • Blocking the central lower inlet increases the incoming momentum of supply jets towards chiller sides . Does hot have very dramatic effect, reducing DT by about 0.1oC

40. Stage 3 SimulationsSensitivity to Chiller Inflow specification Chiller inflow partitioning (ends, sides, base) derived from : • Manufacturers Estimates • For hot operation, DT is about 5.3oC • IAC Experimental Measurements • For comparable run, DT is about 3.1oC • these more reliable data used for subsequent simulations

41. Stage 3 SimulationsExtract geometry sensitivity • Central outlet lip • Adding a deep lip around periphery of central roof outlet should allow capture of more of swirling flow from chiller top. • Unfortunately, it also provides a shortcut for hot air to the ends, leading to a dramatic increase in DT ! Moral: Not all intuitive aids work as one might expect . . .

42. Unexpected Outcomes . . .

43. Unexpected Outcomes . . .

44. Stage 3 SimulationsExtract geometry sensitivity • Further enlarged central outlet with vestigial side extract ducts • Long extract ducts on each side replaced by four smaller apertures at intervals; central outlet further enlarged, but no lip. DT falls to about 2.7oC

45. Stage 4 SimulationsSensitivityto chiller outlet swirl level For a reference geometry & “hot” operation • effect is dramatic . . . . . • for 0% swirl, DT is about 0.6oC • for 30% swirl, DT is about 1.3oC • for 100% swirl, DT is about 6.0oC

46. Stage 4 SimulationsFurther i/o geometry sensitivity • Outlet silencer resistance • Specification of high and low resistance zones in inner and outer regions of central outlet has small (~10% reduction) effect on DT • Increase airflow from 82.5 m3 to 90 m3 • Increasing ventilation rate has a greater effect, reducing DT by about 25%

47. Stage 5 SimulationsSensitivity to internal “baffles” Side and end baffles added, to • channel supply air to intakes at chiller ends; • prevent descending hot air plume being re-entrained into end inlets’ Baffles and shrouds do not perform quite as envisaged . . . • Dead zones form in end shrouds, negating some of supply-air channelling benefit . • However, DT reduced by about two thirds

48. Baffled and Shrouded- - Tried and rejected - -

49. Baffled and Shrouded- - Tried and rejected - -

50. Baffled and Shrouded- - Tried and rejected - -