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Natural and Forced Convection and Radiation Heat Transfer from a Circular Cylinder

Natural and Forced Convection and Radiation Heat Transfer from a Circular Cylinder. Experiment #9. Kemal Koksal, Mike Kelly, Matt Reginski, and Robert Ellenberg. OBJECTIVES.

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Natural and Forced Convection and Radiation Heat Transfer from a Circular Cylinder

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  1. Natural and Forced Convection and Radiation Heat Transfer from a Circular Cylinder Experiment #9 Kemal Koksal, Mike Kelly, Matt Reginski, and Robert Ellenberg

  2. OBJECTIVES • The purpose of this lab is to understand natural and forced convection on a cylinder by measuring surface and ambient temperatures and relating the data to convection heat transfer equations.

  3. THEORY • Natural Convection: Heat transfer through circulation of fluid due solely to gravity • Forced Convection: Heat transfer through circulation of fluid due to forced fluid movement (fan, pump, etc.) • Radiation: Heat transferred by surface photon emission, typically only significant at T>>Room Temp.

  4. Natural Convection – Newton’s Law of Cooling • qc is the rate of heat transfer by convection • hc is the convective heat transfer coefficient • A is the surface area available • Ts is the average surface temperature • Ta is the ambient temperature

  5. Radiation Heat Transfer Equation • qr is the rate of heat transfer by radiation • σ is the Stefan-Boltzmann constant* • ξ is the emissivity of the surface • Fs-a is the view factor of the surface * 5.67x10-8 W/m2K4

  6. Radiation Equation - Simplifications Can also be written as Where hr is the average radiation heat transfer coefficient.

  7. Radiation Equation - Simplifications Using the previous two equations we can determine h:

  8. Total Heat Transfer The total rate at which heat is lost from the cylinder in this experiment will be: The total heat transfer correlates to the sum of convection and radiation.

  9. Energy Balance In this experiment a cylinder is heated electrically so the amount of energy supplied to the cylinder can be calculated using the equation:

  10. is the average heat transfer Coefficient is the thermal conductivity is the length of the object Nusselt Number Convective heat transfer coefficient ,hc is generally expressed as a non-dimensional parameter called the Nusselt number, Nu, defined as:

  11. Nusselt Number Relations For natural convection, Nu depends on the Rayleigh number, Ra. The Rayleigh number can be written in terms of the Grashof and Prandtl numbers, Gr and Pr. Grashof and Prandtl numbers are given by

  12. Nusselt Number Correlations • Nusselt number, Nu, for natural convection as a function of the Rayleigh number Constants c and n are obtained from the table

  13. Film Temperature The properties k and ν are functions of temperature and should be evaluated at the film temperature, Tfilm for calculating the values, Nu, Gr, and Pr where:

  14. Relevant Fluid Properties • g gravitational acceleration ( 9.81 m/s ) • β volume expansion coefficient, ( 1/ Tfilm )( K-1 ) • v kinematic viscosity ( μ / ρ )( m2 /s ) • cp specific heat ( J/kg ·K ) • ρ density ( kg / m3 )

  15. Equipment • Armfield HT10X Heat transfer Unit • Armfield HT14 Combined Convection and Radiation Accessory. • controls the power input to the cylinder and measurea the temperatures of the upstream air in the tube and the surface of the cylinder.

  16. Set-Up • The fan at the base of the tube provided forced air, which cooled the cylinder at the top of the tube. • Fan is off during natural convection • Data collected: input voltage and current, upstream air temperature, and cylinder surface temperature.

  17. Procedure • Natural Convection • Voltage set to 5 V • Cylinder temperature allowed to reach steady state. • After data was recorded, voltage was quickly increased to next voltage setting, and repeated for 8, 12, 15, and 20 V.

  18. Procedure – Cont. • Forced Convection • Throttle plate to 50% of the total opening, giving air velocity of 7.03 m/s • Input Voltage set to 10 volts • Temperature was allowed to reach steady state, data values recorded. • Process repeated with throttle plate 80% open, tube air velocity of 7.74 m/s

  19. Results

  20. Results - Plots

  21. Results – cont.

  22. Results – cont.

  23. Conclusions • Natural convection is more prevalent at lower temperatures whereas radiation is more prevalent at higher temperatures • Possible Sources of error: • conduction from the heated cylinder to its housing tube • possible changes in ambient temperature • Variations in surface temperature

  24. Appendix – NC Calculations

  25. Appendix – FC Calculations

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