aka The King, the Mice and the Cheese

Investigation of near field effects of generator plumes using CFD

DMUG, April 2016

Bethan Tuckett-Jones

URBAN DEVELOPMENT2

High rise development in urban environment

Partially open roof with ‘fins’ containing energy plant, heat rejection units, air handling units including intakes, plus communications etc etc 3 x 2.5MW Gas/Oil Boilers 2 x 1.5MW Gas CHP 4 x Life Safety Generators

(<1MW) 5 x 2MW Diesel Generators

Architects/Planners: Flues should not be visible above the fins

PLANT EMISSIONS3

CHP 250mg/Nm3 NOX = 0.29g/s per unit

Boilers ~70mg/Nm3 NOX = 0.06g/s per unit

Generators Initially 6000mg/Nm3 NOX = 7.3g/s per unitAmended on advice to: Emissions Optimised 1600mg/Nm3 NOX = 2.7g/s per

unit

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Initial Commission – Planning Application in April 2012 Modelling undertaken using ADMS Stacks and Buildings at same height

(77m and 57.5m) Impacts from CHP/Boilers small (2µg/m3

for annual mean; 5µg/m3 for hourly mean) at ground level and at height on adjacent buildings

Impacts from generators considered statistically for 100hours operation per year- Max Impact: 150µg/m3 hourly mean

(99th percentile), ~90m above ground

PROJECT HISTORY (1) - PLANNING

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Query on Impact on Air Handling Units – Sept 2012 Modelling undertaken using ANSYS CFD Neutral stability atmosphere only Logarithmic upstream wind profile with

U10 from 3m/s to 10m/s 16 Wind directions Flues modelled with ‘top hat’ profile

- Large Re

Interested in dispersion over 5 – 50m

PROJECT HISTORY (2) – AHU IMPACT

~200µg/m3 hourly mean surface; 35% NOX as NO2

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Conclusions Diesel generator plume likely to be drawn

into roof space of adjacent building Benefits from locating CHP/Boiler flues

discharging above heat rejection units Reduced ingress with increased exit

velocity- Ruled out due to noise impacts

Recommendation that AHUs located at lowest level in roof space and as far as possible from generators

PROJECT HISTORY (2) – AHU IMPACT

~200µg/m3 hourly mean surface; 35% NOX as NO2

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Query on Impact of Thermal Plumes on Building Fabric – May - Dec 2015 Scale of interest reduced to near field (0m

– 10m) Temperature Constraints

- Anodising on the fin profile: 100ºC - 150ºC

- Silicone on the fin profiles 150ºC- Paint on brackets: 80ºC - 100ºC

PROJECT HISTORY (3) – IMPACT ON FINS

Typical operation of CHP/Boilers; contours at 10ºC

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Conclusions CHPs/Boilers not a major problem under

normal or abnormal operations Diesel generators exceed criteria under both

normal and abnormal operations Worst case under moderate to strong winds Conclusions:

- Heat protection required on the fins to a distance of up to 6m from flues

- Raising the flues to top of fins better but does not remove problem

PROJECT HISTORY (3) – IMPACT ON FINS

Worst case operation of generators; contours at 50ºC

PLANT EMISSIONS9

CHP Temperature 98ºC [375ºC] Diameter 0.4m Velocity 20m/s [35m/s]

Boilers Temperature 92ºC [125ºC] Diameter 0.5m Velocity 6m/s [7m/s]

Generators Temperature 460ºC [545ºC] Diameter 0.6m Velocity 27m/s [31m/s]

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Query on Impact of Thermal Plumes on Building Maintenance Unit – Dec 2015 - Jan 2016 Scale of interest reduced to near field (0m

– 5m) Temperature Constraints

- Operation of BMU compromised: 50ºC

- Later increased to 90ºC

PROJECT HISTORY (4) – IMPACT ON BMU

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Conclusions Maximum temperature with operation of

generators significantly above 50ºC at the level of the BMU (2m above fins)

Exclusion zone up to 11m at 50ºC; Reduced to 4.5m at 100ºC

Maximum impacts occur under light/moderate winds with slight bending of plume

PROJECT HISTORY (4) – IMPACT ON BMU

Blue = >50ºC; Green = >100ºC; Yellow = >150ºC; Red = >250ºC

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In particular, is the model underestimating the temperature of the plume

Theory e.g. Morton, Taylor and Turner 1955 Density deficiency (proportional to

temperature) - x is the height above source

Predicts rapid decrease in temperature near the source

But – implicit assumption that entrainment is proportional to velocity

IS THE MODEL REPRESENTING BEHAVIOUR OF REAL PLUMES IN THE NEAR FIELD?

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Prengle, Mahagaokar and Tse, 1985; Jellison and Miller, 2004

Plume temperature monitored using Infrared technology

Decrease in temperature in near field shows a concave down rather than concave up shape

Temperature in plume is constant for ~1 – 5 diameters downwind, depending on exhaust characteritics

REMOTE SENSING

Stack parameters: Temp 507K, Velocity 11.37m/s, Diameter 1.0m

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Conclusions Plume as three zones: Core; Transition

and Fully Developed Core similar to turbulent pipe flow; Fully

developed region has Gaussian distribution

Axial decay of temperature/velocity almost negligible in 1 – 2 diameters

Followed by rapid decay in temperature due to entrainment and mixing between plume and ambient air

The greater the initial plume temperature, the faster the decay of temperature

REMOTE SENSING

From Prengle et al, 1985

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Temperature on streamlines from generators At first viewing appears to follow theory,

with initial rapid decrease in temperature, followed by slower decay

But this hides significant variation Difference identified between streamlines

at centre of plume and those towards edge of plume

MODELLED PLUME

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Model reproduces observations (with ~core region) for maximum plume temperature reasonably well

Temperature at the edge of plume decreases from exit of stack

Caveats Observations under calm winds Length of core region decreases as

temperature difference increases Model formulation not wholly

representative of reality- Plume radial structure?

COMPARISON

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Improve heat resiliance of BMU Prevent use of generators when BMU is

operating Not possible for life safety generators Commercial issues

Retractable stacks Space constrained

Secondary flues Space constrained

Spread out flues Reduced plume rise benefits

Positioning of flues to minimise inaccessible area of façade…But

SOLUTIONS

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Optimum location for flues for BMU operation reintroduced problems of entrainment into adjacent roof space.

Generic lesson? – where there is pressure to reduce the stack height, flues close to building edges might be more influenced by eddies than if placed centrally on roof.

Final solution Compromise on location of flues: Dispersion v BMU Operation

THE KING, THE MICE AND THE CHEESE

aka The King, the Mice and the Cheese

Investigation of near field effects of generator plumes using CFD

DMUG, April 2016

Bethan Tuckett-Jones