computational fluid dynamic (cfd) investigation of air...
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School of somethingFACULTY OF OTHER
School of Mechanical EngineeringFACULTY OF ENGINEERING
Thermal Management in Commercial Bread Baking
Computational Fluid Dynamic (CFD) Investigation of air flow and temperature distribution in a
small scale bread-baking ovensmall scale bread-baking oven
Z. Khatir1, J. Paton1, H. Thompson1, N. Kapur1, V. Toropov1, M. Lawes1 and D. Kirk2
1 University of Leeds, 2 Spooner Industries Ltd
SusTEM Conference 2010, Newcastle, 3rd November 2010
� Context
� Baking Industry
� Challenges
� Small-Scale Bread-Baking Oven
Outline
� Small-Scale Bread-Baking Oven
� Experiments
� CFD Modelling
� Conclusions & Future Work
3rd November 2010 2 of 20
� Total annual carbon (CO2) emissions
>500,000 tonnes
� Main product bread
Industrial Baking Sector - Overview
� Primary sub sectors� Industrial, Supermarkets and Local/ small bakers
�Approximately 100 industrial bakery sites
3rd November 2010 3 of 20
Energy Use in BakeryRelative carbon emissions
from operation
3rd November 2010 5 of 20
Relative carbon emissions
from operation
Carbon Trust, 2010, http://www.carbontrust.co.uk
� The SEC (Primary Energy) for the work undertaken
for the prover, oven, cooler) ranged from: �0.3 kWh/kg - 0.9 kWh/kg
�50% - 60% of the total CO2 emissions from an industrial bakery
Specific Energy Consumption Range
Annual Primary Energy v Production
y = 813.41x + 1E+07
R2 = 0.6868
-
10,000,000
20,000,000
30,000,000
40,000,000
50,000,000
60,000,000
70,000,000
80,000,000
- 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000
Production (tonnes)
Pri
ma
ry E
ne
rgy
(k
Wh
)
� Sector average (for total site) – 1.1 kWh/kg
3rd November 2010 6 of 20
Carbon Trust, 2010, http://www.carbontrust.co.uk
� Experiments at Spooner Industries
� Direct-Fired Oven
� Dimensions: 9 m length, 1 m width and 1.5 m height
� Aims
� Understanding of internal operation of the pilot oven
Pilot Oven Bread-Baking Oven
� Understanding of internal operation of the pilot oven
� Assess potential for detailed parameters study
� Air flow and temperature through the oven
� Measurements
� Air temperature
� Air velocity
� CFD Modelling3rd November 2010 7 of 20
� Direct vs. Indirect
� Direct fired - a hot moving gas transfers heat directly
� Indirect fired - hot gas is used to heat metal elements (i.e. radiation is predominant)
Direct-Fired vs. Indirect-Fired Oven
3rd November 2010 8 of 20
Direct Fired Indirect Fired
- Temperature: Tindirect ≈ 2 Tdirect
- Low air velocity
- Temperature: Tdirect
- High air velocity
� Direct-Fired
� Designed so that forced convection is dominant (i.e. Gr/Re2 << 1)
� High-speed nozzle jets
Oven Under Investigation
3rd November 2010 9 of 20
Three-zone direct fired oven: a) Overview of the oven; b) Simplified schematic showing the mechanism for distributing air through the nozzles for a single zone
Hot air
streams
Nozzles
Burners
a)
Return air Gas burner Air recirculation fans
b)
Hot air from nozzles Direction of product
Experiments - Nozzle Temperature along the Oven
� The temperature of the nozzles through the oven remained constant once the oven reached set-point temperature
3rd November 2010 10 of 22
Graph showing air temperature at nozzle orifice for top and bottom nozzles
� Air temperature correlates strongly with burner set-point
Experiments - Air Temperature along Pilot Oven
� Temperature profiles for
different distance from nozzles
� Burner set points according to production bake profile
� Results suggest uniformity
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� Results suggest uniformity of temperature within each of the three zones
� Agreement of the burner temperature set points and recorded temperature is also good; largest variation taking place in zone 1
Normalised air temperature (T/T1) profile through the oven (x/D) for: H/D = 1.43 (––), 2.86 (-.-) and 4.29 (---)
Experiments - Nozzle Temperature across Oven’s Width
1 2 3 4 5
� IR thermal imaging - useful
technique to determine qualitative temperature distribution within an oven
� Calibration of a thermal image is particularly important for materials with low
3rd November 2010 12 of 22
1 2 3 4 5 for materials with low emissivity – such as many metals
� Possible to extract temperature values from thermal images, however careful consideration of the potential inaccuracies is important
IR thermal image showing nozzle temperature variation within an Oven Section on pilot oven.
Experiments - Air Velocity along the Oven
� Velocity values computed from
measured pressure with a
manometer in a cold oven
� The arrows show the internal
air distribution pattern: the
recirculation fan is shown by the
3rd November 2010 13 of 22
recirculation fan is shown by the
up-arrow, the return air is shown
by a down arrow
� The peak at the middle of each
section is due to the internal air
flow within the ducts and the
balance of pressure drop down
the plenum that feeds the nozzlesNormalised air velocity distribution through oven: ( ) top nozzles, ( ) bottom nozzles.
Experiments - 3D Plot of Air Velocity at Nozzles
� Air velocity across the width of the oven varies between:
� 93-105% for the top nozzles
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nozzles
� 96-103% for the bottom nozzles
� Compared with the average nozzle velocity
CFD Modelling – ANSYS Gambit/Fluent
� 2D Modelling – Equivalent flow rate
3rd November 2010 15 of 22
Geometry of an oven zone baking chamber, showing a typical CFD mesh And indicating the boundary conditions.
CFD Modelling – Temperature & Velocity Results
Contour plot of normalised temperature (T/T1) throughout Zone 1
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Contour plot of normalised temperature (T/T1) throughout Zone 1
Normalised air flow patterns (v/vref) throughout Zone 1
CFD Modelling – Impact of Jet Velocity on Flow Field
3rd November 2010 17 of 20
Normalised streamlines flow (kg/s) throughout (Left) and velocity at centreline of baking chamber (Right) at various nozzle jet velocities: a) 0.5vref m/s(...) , b) vref m/s (–.–) and c) 3vref m/s(––)
CFD Modelling – Experimental & CFD Results
3rd November 2010 18 of 20
Comparison between CFD predictions (◊◊◊) and experimental measurements (xxx) of the normalised air
temperature profile at various points below the top nozzles within zone 1 of the baking oven located at H/D = : a) 1.43; b) 2.86 and c) 4.29
CFD Modelling - Error Analysis
Distance below the top nozzles
(H/D) 1.43 2.86 4.29
Relative Error 2.54 4.16 3.8
Coefficient of
correlation 0.92 0.92 0.90
� Comparing the modelled temperature profile to the measured temperature to different H/D leads to an average relative error of 3.5%
3rd November 2010 19 of 20
RMS Error 2.8 4.2 3.9
Relative and root mean square (RMS) error compared to the measured temperature [%] together with coefficient of correlation.
� The average coefficient of correlation and RMS error between the modeled values and the measured values are 0.91 and 3.6% respectively
� Complexity of thermal air flows in baking ovens
� CFD models require careful validation (Practical oven design)
� CFD models are able to provide valuable insight into key baking issues
� Temperature, Air flow fields
Conclusions
� Temperature, Air flow fields
� Parameter Study (i.e. Pitch, nozzle diameter, jet velocity, etc)
� CFD embedded into bread baking Design Optimization Tool for practical baking applications
� Economic Model/Cost ���� Energy Efficiency
� Thermodynamic System Level Analysis ���� Product Quality
3rd November 2010 20 of 20
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