Heat Transfer Project: The “Cooling Soup” Dilemma Mike Mantyla Steve Harston ME 340-1

The Project Test 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

Procedure Boil 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 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: Freezer Description: Measure time for water to cool from initial temp to final temp in freezer. Average time to cool: 5.6 minutes

Method 3: Air Flow over Surface Description: 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 2

Method 4: Blowing/Mixing Description: Measure time for water to cool from initial temp to final temp while constantly stirring with a constant air flow over the surface. Average time to cool: 4.4 minutes 2

Method 5: Mixing Description: Measure time for water to cool from initial temp to final temp while constantly stirring. Average time to cool: 8.7 minutes

Method 6: Ice Cubes Description: 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

Results and Discussion 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 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. Room Temperature Cooling Method Freezer Cooling Method