CRITICAL HEAT FLUX
Lewis Kiely
College of Management and Technology
Introduction
• What is Critical Heat Flux?
• Why is it important?
• What I am doing
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Heat Flux
• Heat flow between
two mediums
• A metal element and
water as a coolant
• Water removes heat
from surface
• Different boiling
regimes occur
Single Phase
Vapour
Drop Flow
Annular Flow
with
Entrainment
Annular Flow
Slug Flow
Bubbly Flow
Single Phase
Liquid
College of Management and Technology
College of Management and Technology
Critical Heat Flux and Nuclear
Reactors
• Reactors designed to avoid CHF
• To avoid CHF, point of CHF needs to be
predicted
• Predicted using empirical correlations and
look up tables
• Correlations are not very accurate
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Previous Work
• Heated elements in pool
• Elements broke as too
hot
• New design: cut out from
brass block
• Heated surface should
not melt as heat
dissipates into
surrounding plate.
College of Management and Technology
College of Management and Technology
Previous Set Up
• Brass block
surrounded by a
polythene block with
flow channel
• Two cartridge heaters
in brass block
controlled by
temperature
controller.
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Previous Set Up
• Surface temperature,
temperatures of the
water entering and
leaving the channel
are recorded on a
computer using a
data logger
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1000
900
Heat Flux (kW/m 2)
800
700
600
500
400
300
200
100
0
0
20
40
60
80
Surface temp (degC)
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100
120
140
Problems With Original Design
• Polythene block starts to soften and
deform at ~120°C
• Perspex window turns cloudy when in
contact with boiling water for an extended
period of time (~20mins)
• Wide channel flowing into a small pipe
disrupts flow of water
• Channel can only be viewed from front
• It leaks!
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New Design
• Allow side view as well as front view of
channel
• More heat resistant materials used,
allowing higher temperature - achieving
Critical Heat Flux!
• Larger outlet, reducing unintentional flow
restriction
• No more leaks!
College of Management and Technology
College of Management and Technology
New Design-The Problems
• Material chosen to replace polythene
block, Tufnol, is not possible to machine
on site
• Machineable ceramic also investigated,
but very expensive!
• Channel will require more heat resistant
material, such as glass
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Current State
• Plate on brass block has been extended
• End pieces of channel made
• Glass for channel needs to be cut and
channel formed
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Acknowledgements
Dr Paul Chard-Tuckey - Supervisor, group
manager
Simon Jewer - Line manager
Dr Ian Giles - UDS project manager, DE&S, Abbey
Wood in Bristol
Samantha Morris- Placement coordinator
Terry McCarthy - Lab assistant
Dr Steve Andrews - Placement tutor
Jamieson Lock - Co-placement student
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Any
Questions?
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