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

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� 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

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Diagram of Baking Process

The bread process c4hrs end to end

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Energy Use in BakeryRelative carbon emissions

from operation

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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%

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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

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