The objective was to compare cooling of peaches via free and forced convection. Peaches were submerged in ice baths with one stirred (forced convection) and one stagnant (free convection). Temperature was measured every minute until reaching 7.5°C. Forced convection cooled the peach faster, in 25.14 minutes with a convection coefficient of 3,105 W/(m2K), while free convection took 43.17 minutes with a coefficient of 41.38 W/(m2K). COMSOL models showed faster cooling under forced convection. Differences from theory were likely due to approximations in the experimental setup and peach properties.

1. Conclusion
Discussion
The objective of this project was to compare the calculated convection coefficients (h) of
two water baths cooling a peach via free and forced convection. Cooling one peach in an ice
bath with forced convection and the other peach in an ice bath with free convection, the
temperature change at the center of each peach was measured with time. The data collected
was used to find the convection coefficient of the water for each scenario. The time required
to cool a peach from 20℃ to 7.5℃ via free convection was 43.17 minutes with an h value of
41.38 W/(m2K). The time required to cool a peach from 20℃ to 7.5 ℃ via forced
convection was 25.14 minutes with an h value of 3,105 W/(m2K).
• Hoboware Software is set up on computer 1 using device 1, setting probe 1 to monitor the
peach temperature and probe 4 to monitor the water temperature of for the forced
convection scenario.
• Setup was repeated with computer 2 using device 2 for the peach in the free convection
scenario. A container was filled with ice and water to achieve a temperature of ~6℃. From
the container, two 2000 mL beakers were filled with water to ~1500 mL.
• Leftover ice and water from the large container will be added to the beakers throughout the
experiment to help keep constant water temperature.
• One beaker was placed on a stir plate with a stir bar set to 300 rpm for the forced
convection scenario and labeled beaker A. The other beaker was labeled B.
• A peach was placed into each beaker and was suspended with a makeshift prong
mechanism to ~2 inches from the bottom of the beaker.
• Probe 1 from device 1 was inserted into the peach in beaker A, and Probe 1 from device 2
was inserted into the peach in beaker B.
• Cheesecloth was placed overtop of each beaker and set to lay in the water ~2 inches deep
to separate the any added ice from the peach. This insured that no conduction would occur.
• Probe 4 from device 1 was suspended in the water touching neither the glass of the beaker
nor the peach in beaker A. Probe 4 from device 2 was set up the same way in beaker B.
• The set up was held in place with clothes pins.
• Upon insertion of the peaches into the ice baths, temperature measurements were taken
every minute until the peach reached the same temperature of the water at ~6℃. .
• The water was monitored to stay at ~6℃ throughout the experiment. Additional ice and
water were added to the beaker when the temperature started to rise.
1. Drapcho, C. 2019. Heat and Mass Transfer– Lectures 13 & 14. Unpublished Lecture Notes, BE 4120/6120,
Clemson University.
2. Jafarpur, K., & Yovanovich, M. (1992). Laminar free convective heat transfer from isothermal spheres: A
new analytical method. International Journal of Heat and Mass Transfer, 35(9), 2195-2201.
doi:10.1016/0017-9310(92)90063-x
3. Ware, M. (2017, December 20). Health Benefits of Peaches. Retrieved from
https://www.medicalnewstoday.com/articles/274620.php. Used Peach Image from website for poster
aesthetic.
4. Thermal-Fluids Central. (2010). Natural Convection on Cylinders and Spheres. Retrieved from
https://www.thermalfluidscentral.org/encyclopedia/index.php/Natural_convection_
on_cylinders_and_spheres on April 22, 2019.
5. Drapcho, C. 2019. Heat and Mass Transfer– Homework Set 2. Unpublished Lecture Notes, BE 4120/6120,
Clemson University.
Abstract Results and Modeling
Comparison of Free and Forced Convection
for the Cooling of Peaches
Patrick Cusack, Carrington Moore, Shelby Green, Rachel Cron
BE 4120, Biosystems Engineering, Dr. Caye Drapcho
Clemson University, Clemson, SC, 29631
Materials and Methods
Governing Equations and Calculations
Figure 1. The time it took the peach to cool to the same
temperature as the water bath using forced convection.
For a spherical object that satisfies the conditions for the Heisler approach, the time
required to cool the object to a specified temperature through convective heat transfer
is dependent on the initial temperature, radius, thermal conductivity, and thermal
diffusivity of the object, as well as the temperature and convection coefficient of the
bulk fluid. As shown in Figure 1, it took 25.14 minutes for the peach to cool to 7.5℃
under forced convection. As shown in Figure 2, it took 43.17 minutes for the peach to
cool to 7.5℃ under free convection. Theoretically, these times should have been 27.52
minutes and 89.98 minutes, respectively. These differences were most likely attributed
to the inaccurately calculated convection coefficients of the water baths. Fluctuations
in the bulk fluid temperatures, slight differences in the sizes of the two peaches, the
approximation of the peaches to be perfectly spherical, and the assumption that the
temperature gradients within the peaches were relatively negligible led to convection
coefficients that did not perfectly represent the experimental designs.
The convection coefficient is a function of the flow of the bulk fluid, the thermal
properties of the bulk flow and the geometry of the object. The convection coefficient
of the moving water bath was calculated to be 3105 W/m2K, while the convection
coefficient of the stagnant water bath was calculated to be 41.38 W/m2K. Using these
values, among others, the two peaches were modeled using COMSOL. Figure 3
shows the temperature profile of the peach under forced convection and Figure 4
shows the temperature profile of the peach under free convection, both after 2 minutes
of being placed in the water bath. The peach in Figure 4 is cooled less compared to
Figure 3, both after 2 minutes because heat transfer occurs more slowly with free
convection compared to forced. Figure 5 and Figure 6 show the temperature profiles
of both peaches at 7.5℃. Although the two models look almost identical, the time it
took Figure 6 (free convection) to reach this temperature was almost 3X longer than it
took Figure 5 (forced convection).
• Hoboware Device (2) and Software (2)
• Stir plate (1) and stir bar (1)
• Ice water
• Peaches (2)
• Cheese cloths (2)
• 2000 mL beaker (2)
• Clothes pin (~6)
• Pronged suspension device (2)
• Container large enough to hold ~3L of water
• Cooler to hold ice
Figure 7. The experimental set up of peach cooling
with free convection occurring.
In order to preserve peaches after picking, cooling treatments are often employed. These
cooling treatments are meant to inhibit the growth of harmful microorganisms, reduce
respiratory activity, and control moisture loss. After peaches are picked, they still continue
their metabolic activity through respiration. Pre-cooling reduces respiration greatly and
therefore extends the storage life of a peach [Aswaney, 2007]. An ice bath can be used to
pre-cool peaches. By submerging peaches in an ice bath, the heat transfer by convection
causes heat loss from peach, cooling the center. The rate at which convection affects the
heat loss of the peach is represented by the convection coefficient, h. The h value can be
calculated using a series of dimensionless numbers that have defined relationships by
empirical data. The series of calculations used to calculate the h value of the water bath
depends on the type of convection is occuring: free or forced convection [Drapcho, 2019].
The purpose of this project was to compare the time required to cool the center of a peach
from 20℃ to 7.5℃ under free and forced convection. Additionally, the h values of the ice
water under free and forced convection were calculated and compared. Theoretically, it
should take longer to cool the peach by free convection than by forced convection.
References
Introduction
0
5
10
15
20
25
0 10 20 30 40 50 60
Temperature(T)[°C]
Time (t) [min]
Peach
Water
0
5
10
15
20
25
0 10 20 30 40 50 60 70
Temperature(T)[⁰C]
Time (t) [min]
Peach
Water
Figure 3. The COMSOL temperature profile for forced
convection on a peach at t = 120 seconds
Figure 5. The COMSOL temperature profile for forced
convection on a peach at t = 1800 seconds
.
Figure 4. The COMSOL temperature profile for free
convection on a peach at t = 120 seconds
Figure 6. The COMSOL temperature profile for free
convection on a peach at t = 4979 seconds
Figure 2. The time it took the peach to cool to the same
temperature as the water bath using free convection.
As expected, the peach placed in the moving water bath (under forced convection) cooled
faster than the peach placed in the stagnant water bath (under free convection). From the
experimental data, the average convection coefficient of the moving water was calculated to
be 3105 W/m2K; the average convection coefficient of the stagnant water was calculated to
be 41.38 W/m2K. The theoretical time required to cool the center of the peach from 20 to
7.5℃ under forced convection was 27.52 minutes. In this experiment, the center of the peach
reached 7.5℃ at 25.14 minutes. The theoretical time required to cool the center of the peach
from 20 to 7.5℃ under free convection was 89.98 minutes. In this experiment, the center of
the peach reached 7.5℃ at 43.17 minutes.
We would like to thank Dr. Caye Drapcho for her assistance and knowledge of heat transfer.