Upload
zia-walton
View
61
Download
5
Tags:
Embed Size (px)
DESCRIPTION
7. Pot-in-Pot refridgerator. Nat ália Ružičková. Task. The ‘pot-in-pot refrigerator’ is a device that keeps food cool using the principle of evaporative cooling . It consists of a pot placed inside a bigger pot with the space between them filled with a wet porous material, e.g. sand. - PowerPoint PPT Presentation
Citation preview
13
Pot-in-Pot refridgerator
Natália Ružičková
7
13
2
Task
The ‘pot-in-pot refrigerator’ is a device that keeps food cool using the principle of
evaporative cooling.It consists of a pot placed inside a bigger pot with the space between them filled with a wet
porous material, e.g. sand.How might one achieve the best cooling
effect?
→ Low temperature→ Fast cooling
13
3
Mechanism
vapour
transferedQ
latentQ
fridgeT
Wet porous material:Evaporation latent heat
cooling
Heat transfer from the environment
tenvironmenfridge TT Equillibrium:
13
4
Preliminary Experiment
• Porous material: sand
0 20 40 60 80 100 12017
18
19
20
21
22
23
fridgeenviroment
Time [min]
Tem
pera
ture
[°C
]
CT 8,1max
Works, but could be better
13
5
Evaporation requirements
Must be wet
(capillarity)
Exposed to air flow
13
Big granules: Better airflow Water drains down
Small granules: Can soak with water Little space for airflow
Sand: a granular material6
13
7
Other kind of porous material:Cloth
• Gets soaked with water
• Can be exposed to air flow on sufficient area
13
8
Cloth vs. sand: the difference
0 20 40 60 80 100 12015
16
17
18
19
20
21
22
cloth + sandsandenviroment
Time [min]
Tem
pera
ture
[°C
]
We havea working refrigerator
13
9
A working refrigerator
Inner pot
Outer pot
Porous material: cloth
Liquid (water) for evaporation
Content
Airflow
13
10
Task“How might one achieve the best cooling
effect?”
Simple model of the cooling Experimentally verify & optimize
Estimate the best possible cooling
→ Low temperature→ Fast cooling
13
MODEL OF THE COOLING
13
12
Model of the cooling effect• Evaporation rate:
related to volatility of the liquid (in air)
• Heat transferredfrom the surroundings:
related to thermal conductivity, capacity of air
Surface areaAir flow speedTemperature difference
13
13
Model of the coolingCalorimetry:
Final temperature drop:
Speed of cooling:
CdTTdtBSvdtASv
tC
BSv
air eB
ATT 1
B
AT
C
BSv
fridge ofcapacity thermalC
heatlatent λ
13
EXPERIMENTAL VERIFICATION& INVESTIGATION
13
15
1. High vs. low outer pot
• Terminal temperature:depends on materials only Equal
• Cooling rate:more water bigger slower coolingC
BSv
B
AT
13
16
High vs. low pot: final temperature
0 10 20 30 40 50 60 70 8016
17
18
19
20
21
With outside potWithout outside potEnvironment
Time[min]
Tem
pra
ture
[°C
]
Both models:the same terminal temperature
13
17
0 200 400 600 800 1000 120012
14
16
18
20
22
24
26
28
Low PotHigh Pot
time [s]
T [
°C]
High vs. low pot: rate of cooling
Low outer pot:Less water smaller thermal capacity
faster cooling
13
18
2. Size and shape of the fridgeTerminal temperature:
Independent of size/shape
Speed of cooling:
Independent of size/shape
B
AT
C
BSv
Empty refrigerator:
Proportional to surface area
13
19
Effect of different sizes
13
20
Effect of different size
0 20 40 60 80 100 12013
14
15
16
17
18
19
20
21
22
Different sizes
EnvironmentRadius 9,5Radius 7Radius 6Radius 4Radius 3,3
Time[min]
Tem
pra
ture
[C]
13
21
Effect of different shapes
0 50 100 150 200 25013
14
15
16
17
18
19
20
21 EnvironmentRoundElongatedElongated - bigger
Time[min]
Tem
pra
ture
[C]
Size and shape of an empty fridge:No effect on cooling
13
22
3. What’s inside
Terminal temperature:
Independent of content
Speed of cooling:
High capacity slower cooling
B
AT
C
BSv
13
23
Changing heat capacity of the fridge
0 50 100 150 200 250 30013
14
15
16
17
18
19
20
21
22
23
EnvironmentWith airWith waterWith sand
Time[min]
Tem
pra
ture
[C]
Air, quick
er
coolin
g
Sand, slower
coolin
g
Water, slowest
coolin
g
Bigger capacity slower cooling
No effect on final temperature
13
24
4. Different liquid
Terminal temperature:
Grows with volatility & latent heat
Speed of cooling:
Independent of liquid(only a small capacity
difference)
B
AT
C
BSv
13
25
Water VS. Etanol:
0 200 400 600 800 1000 12008
10
12
14
16
18
20
22
24
26
ethanolwater
time [s]
T [
°C]
More volatile liquid lower terminal temperature
13
26
Water vs. ethanol: exponential fit
8.14)012.0exp(*6.10 tT 0.10)014.0exp(*4.19 tT
Water Ethanol
Different liquids similar speed of coolingSlight difference: different thermal capacity
13
WHAT IS THE BEST POSSIBLE EFFECT?
13
28
Minimal temperature estimationAnalogical phenomenon:
Assman psychrometer
• Wet and dry thermometer
• Wet is cooler due to evaporation
• Equilibrium state:
• Solution: Sufficient airflow
latenttransfered dQdQ
)(])([ fridgeairpairfridgelatm
air TTceTEp
M
13
29
0 2 4 6 8 10 12 14 16 18 2014
16
18
20
22
24
26
28
30
Fridge tempera-ture
wet bulb tempera-ture
time [min]
Te
mp
era
ture
[°C
]
Wet bulb vs. refrigerator
We can get closeto the theoretical
minimum
13
30
Minimal achievable temperature
0 0.090.180.270.360.450.540.630.720.81 0.9 0.99101520253035404550
10 1112 1314 1516 1718 1920 2122 2324 2526 2728 2930 3132 3334 3536 3738 39
CTair
humidityRelative
CT fridge :Colour
13
31
Best cooling effect
tC
BSv
air eB
ATT 1
Sufficient air flow
volatile liquid with low
concentration in air (eg. Ethanol)Minimise thermal
capacity of the fridge (eg. Low outer pot)
Maximize the area
of evaporati
on
speed
Minimal temperatur
e
13
32
0 0.090.180.270.360.450.540.630.720.81 0.9 0.99101520253035404550
10 1112 1314 1516 1718 1920 2122 2324 2526 2728 2930 3132 3334 3536 3738 39
Thank you for your attention
CTair
humidityRelative
CT fridge :Colour
13
APPENDICES
13
34
Assman psychrometer theory
Assumptions:• Air cools to Tfridge; vapor pressure saturates
(thanks to fast airflow)• Equilibrium of heat flow:
• Numerically solve for(complicated function )
latenttransfered dQdQ
fridgeT)(TE
Wet bulb
)(])([ fridgeairpairfridgelatm
air TTceTEp
M
13
EXTRA MEASUREMENTS
13
36
Airflow importance
0 20 40 60 80 100 120 14013
14
15
16
17
18
19
20
21
enviromentno fanfan
Time [min]
Tem
pera
ture
[°C
]
13
37
Water vs. ethanol:Rate of evaporation
0 200 400 600 800 1000 120027293133353739414345
ethanolewater
time [s]
mass [
g]
Ethanol: more volatile
13
38
Do we need cloth or not?
0 20 40 60 80 100 12015.5
16.5
17.5
18.5
19.5
20.5
21.5
Cloth
Cloth, Sand, Wa-ter
Sand and a lot of water
Sand and a little of water
Environment
Only water
Time [min]
Tem
pra
ture
[°C
]
13
39
What is the best material for cloth?
0 20 40 60 80 100 120 14013
14
15
16
17
18
19
20
21
Fluffy clothNormal clothenviroment
time [min]
Te
mp
era
ture
[°C
]
13
40
Why the type of cloth does NOT matter?
1) Water fills gaps
• Water fills gaps.
• The surface of water = the area of evaporation
The same as normal cloth
Area of evaporation
=
area through which heat gets in
• Equilibrium temperature doesn’t change
2) Water soaks
13
41
Effect of different sizes
13
42
Effect of different shapes
13
43
Our refrigerator
tC
BSv
air eB
ATT 1
13
44
Covered VS uncovered areas
Height of
cloth
• Uncovered areas heat gets in
• Covered areas Evaporation + heat gets in
13
45
Does area of cloth matter?
0 1 2 3 4 5 6 7 8 9 1013
14
15
16
17
18
19
20
The height of cloth
Height of cloth[cm]
TER
MIN
AL T
em
pra
ture
[°C
]
Whole fridge must be covered
13
46
Why istn’s the second pot needed?
Area of evaporation
= area through which heat expands
We do not need the second pot
only requirement for fridge’s surface
…ISOLATED AREAS MUST BE UNCOVERED WITH
CLOTH
So that area of evaporation
= area trough heat gets in
13
47
Speed of cooling
tcm
Svk
vair
efftotalek
LkTT
2
12
1
Svdtkdmwater 1
CdTdtTTSvkdtSvLk fridgeairV )(21
efftotalpotpotww cmcmcmC
k1= ability of liquid to evaporate [kg/m3]k2= part of air which cools down [J/Km3]
Temperature difference
1/tauProperties of liquid
13
48
Assman psychrometer: theory
Equillibriumof heat flow:
Air cools to Tfridge; vapor pressure saturates:
fridgeairairairtransfered TTcdmdQ
vwaterlatent LdmdQ
latenttransfered dQdQ
eTEp
nMdQ fridgelatent
fridgeairptransfered TTncdQ
)(])([ fridgeairpairfridgelatm
air TTceTEp
M
13
49
Assman psychrometer: theory
Surrounding air:– Transfers heat
to the cloth:
– Absorbs water vapor(heat used for evaporation)
Equilibrium (terminal temperature):
fridgeairairairtransfered TTcdmdQ
vwaterlatent LdmdQ
latenttransfered dQdQ
13
50
Minimal T: theoretical estimation
Terminal temperature:
Numerically solved for (complicated function )
)(])([ fridgeairpairfridgelatm
air TTceTEp
M
)( fridgeairairairT
transferedlatent
TTcdmdmL
dQdQ
fridgeT
)(TE
13
51
Conclusion – What should the most effective refrigerator look like?
• Minimal temperature:– sufficient air flow, maximize evaporation area– volatile liquid with low concentration in air (eg.
Ethanol)
• Fastest cooling:– Fast air flow– Minimise thermal capacity of the fridge (eg. Low
outer pot)
• Prediction of minimal temperature
tC
BSv
air eB
ATT 1