Heat Transfer Project 3
Usage Guidance
Incubator should be placed in a temperature-controlled room at 20C. Additionally, incubator
should be placed away from all fans to ensure proper natural convection occurs on the external
surfaces. The external foil layer should remain in place and be inspected for damage before use.
Before processing biofilm, incubator should be allowed to warm up to the operating temperature
of 65C. All other usage outlined in previous reports should be followed. Failure to comply with
all guidance will result in material loss and voided legal liability.
Design Assumption:
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Material properties stay constant
Conduction through each wall section is identical
Steady State Conditions prevail
Outside surrounds are large enough to neglect any changes in surrounding temperature.
Added Aluminum foil is thin enough to neglect its effects on conductive heat loss.
Box is airtight during operation
Base of incubator is Adiabatic
Internal baffles induce turbulent conditions and do not affect conductive heat transfer
Forced convection is the dominant fluid movement for the internal incubator air.
No forced air flow on external surface only natural convection.
Internal air temperature is uniform.
Surface temperatures are treated as area-averaged values for each wall section
Helium is treated as an ideal gas, and density is adjusted with pressure while other
thermophysical properties are taken at the selected temperature.
Stated velocity is an equivalent wall velocity
Edge and corner heat transfer effects are accounted for using a two-dimensional heat
transfer model developed in Part 2 of Project 1
Part 1 Conditions to maintain Heat transfer Coefficients
Initial design criteria stipulated a 5 W/m2*K external heat transfer coefficient. Through
natural convection calculations adjusted with outside thickness vertical plate heat convection
coefficient was found to be 3.09 W/m2*K with the top plate having a convective coefficient of
3.36 W/m2*K. Due to the two differing convective coefficients a weighted average was taken
based on each faces respective coefficient and the average coefficient was found to be 3.14
W/m2*K. To maintain the internal heat transfer coefficient of 15 W/m2*K an equivalent plate
airflow velocity of 12.48m/s is required to be achieved by the incubator fan. Additionally, baffles
were added to the inside the box to trip the flow and maintain fully turbulent conditions
throughout the entire incubator. Due to the lower external convective coefficient incubator wall
sizes were decreased to maintain 120-watt heat dissipation. The new wall thickness was found to
be 67.75 mm of cellular glass.
Part 2 Radiation Calculations
Customer wants to know thermal radiation effects on the incubator. Initial design
neglected radiation as a mode of heat loss. Research on cellular glass found a nominal emissivity
of .9 warranting a calculation. Calculation showed that substantial heat loss occurred. To
counteract the heat loss a thin layer of aluminum foil is to be added to the outside of the box
allowing the outside emissivity to be .04 while not interfering with conductive heat loss. Through
the addition of foil the radiative heat loss was found to substantially decrease, and total loss was
found to be 9.12 watts which was incorporated into the part 1 redesign for new wall thickness to
ensure proper temperature control for the incubator.
Part 3
Customer wants to use Helium at 5 atmospheres for the internal fluid. From thermal
property at 350 K and an adjusted density for 5 atmospheres helium was found to have a heat
transfer coefficient of 14.28 W/m2*K. Depending on temperature tolerances of the customer
pressurized helium may be utilized inside the incubator. If exact temperatures are desired an
increase of .27mm is required to maintain exact temperatures. The increased thickness also
accounts for the radiation loss due to the increased external surface area.
Appendix
Figure 1: Full excel Calculation sheet.
Figure 2: Air Calculations
Figure 3:Helium Calcs
References
[1]https://belglas.com/wp-content/uploads/2016/02/ta33-comparison-of-elastomeric-vscellular-glass.pdf
[2] https://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html
[3] Bergman, Theodore L. Introduction to Heat Transfer. 6th ed., John Wiley & Sons, Inc.,
2011.