ME315: HEAT AND MASS TRANSFER
Meeting 25: A Watched Pot Does Sometimes Boil
Today’s
Objectives
1. Heat changes when bubbles change
2. Matching correlations with bubbles
Thomas Beechem, Ph.D.
25
45
43
1
2
Last Time: Free Convection
velocity & temperature profiles when buoyancy drives flow
Bottom Line Different
Geometry and gravity drive flow….and Correlation
3
Bird’s Eye View of Convection
What’s the flow?
What are the properties?
What’s the correlation?
4
Today….Boiling, Bubbles and Cooling
can I use phase change to get rid of
Driving Question How
as much energy as possible?
Image: Wikipedia
5
Today….Boiling, Bubbles and Cooling
can I use phase change to get rid of
Driving Question How
as much energy as possible?
Image: Wikipedia
Image: Flikr, Pattana Pradistapongs
Today….Boiling, Bubbles and Cooling
Image: Cameco via Nuclear-power.com
Image: Flikr, Pattana Pradistapongs
6
You make the call….Which one moves the most heat?
All Videos Courtesy of
Prof. Issam Mudawar
7
To Maximize Heat Transfer I need…
8
9
Measuring Pool Boiling by Prof. Mudawar
Transparent
Test Cell
Fluorinert FC-72
liquid (Tsat = 56C)
Thermocouple
(TL)
Support post
Bus bar
Alternating
electric current
Spring-loaded
bus bar
(maintains wire
in tension)
Resistive
Alumel wire
Courtesy of Prof. Issam
Mudawar
10
Free Convection Bubbles Up…Onset of Nucleate Boiling (ONB)
Figure: Adapted from R. Hugo
All Videos Courtesy of
Prof. Issam Mudawar
11
Bubble Mixers…Nucleate Boiling
Figure: Adapted from R. Hugo
All Videos Courtesy of
Prof. Issam Mudawar
12
Nucleate Boiling…Bubble Teamwork
Figure: Adapted from R. Hugo
All Videos Courtesy of
Prof. Issam Mudawar
13
Maximizing….Critical Heat Flux (CHF)
Figure: Adapted from R. Hugo
All Videos Courtesy of
Prof. Issam Mudawar
14
Thin-Film Boiling…Boiling Crisis
Figure: Adapted from R. Hugo
All Videos Courtesy of
Prof. Issam Mudawar
15
Thin-Film Boiling…Cooling Down
Figure: Adapted from R. Hugo
All Videos Courtesy of
Prof. Issam Mudawar
16
Minimum Heat Transfer…Leidenfrost Point
Figure: Adapted from R. Hugo
All Videos Courtesy of
Prof. Issam Mudawar
17
Things Change if I Control Surface Temperature
Figure: Adapted from R. Hugo