Heat Transfer Project: Heat Transfer Project: The “Cooling Soup” DilemmaThe “Cooling Soup” Dilemma

Mike MantylaMike MantylaSteve HarstonSteve Harston

ME 340-1ME 340-1

The ProjectTest the effectiveness of cooling a hot liquid using

different cooling methods.

Cooling Methods tested:– Ambient air (small temperature difference)– Freezer (large temperature difference)– Mixing the fluid while cooling– Constant air flow over liquid– Constant air flow with stirring– Adding ice-cubes

Liquids tested: Water

ProcedureBoil water, cool to determined temperature,

measure time.

Assumptions:– Heat transfer through bowl neglected– Change in mass due to evaporation neglected– Bowl can be evaluated as a cylinder– Heat transfer to stirring spoon neglected

See Appendix for Calculations

Method 1: Ambient Air

CTi 95CT f 65CT 28

Description: Measure time for water to cool from initial temp to final temp in ambient air.

Average time to cool: 13.1 minutes

Method 2: FreezerDescription: Measure time for water to cool from initial temp to final temp in freezer.

Average time to cool: 5.6 minutes

CT 16

CTi 95CT f 65

Method 3: Air Flow over SurfaceDescription: Measure time for water to cool from initial temp to final temp with a constant air flow over liquid surface.

Average time to cool: 5.1 minutes

CTi 95CT f 65CT 28

smU 7 2

Method 4: Blowing/MixingDescription: Measure time for water to cool from initial temp to final temp while constantly stirring with a constant air flowover the surface.

Average time to cool: 4.4 minutes

CTi 95CT f 65CT 28

smU 7 2

Method 5: MixingDescription: Measure time for water to cool from initial temp to final temp while constantly stirring.

Average time to cool: 8.7 minutes

CTi 95CT f 65CT 28

Method 6: Ice CubesDescription: Measure time for water to cool from initial temp to final temp while placing ice cubes in the water one at a time.

Average time to cool: 3 minutes

CTi 95CT f 65CT 28

Results and DiscussionTime for Boiling Water to Cool with different Cooling

Methods

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5 6

Trial Number

Tim

e (m

inut

es) Ambient Air

Freezer

Blowing

Blowing/Mixing

Mixing

Ice Cubes

Average Times for Water to Cool with different Cooling Methods

0

2

4

6

8

10

12

14

1 2 3 4 5 6

Aver

age

Tim

e (m

inut

es)

Ambient Freezer Blowing Blow/Mix Mix Ice Cubes

• The fastest way to cool the soup is not to increase the temperature difference as intuition might suggest.

• Combining cooling methods will cool the soup faster.

• Contrary to intuition, just blowing on the soup will cool it faster then putting it in the freezer while blowing and stirring will cool it faster still.

Conclusion

• Without calculations or experiments it is difficult to tell if the convection coefficient (h) is greater for a large temperature difference or greater for mixing and blowing on the liquid.

• Through the experiments it has been determined which method is most effective at cooling a hot liquid.

• If only one method is to be used, the ice cube method is the fastest.

• If ice is not available, blowing while mixing will have the best results.

Appendix: Calculations

121

20 1ln

CTTTTrt i

min2.161143.11

301328301368ln

9408.613.10136.0

27

2

et

Since the Biot > 0.1 an approximate analytical solution was used.

The Fourier number found after the calculations was approximately 0.851, which is greater than 0.2 signifying that these equations are valid to use for this situation.

For simplicity, only two calculations were shown. See authors for other calculations if desired.

min89.61143.11

267328267368ln

9408.613.10136.0

27

2

et

Room Temperature Cooling Method

Freezer Cooling Method