Heat transfer in horizontaly oriented air enclosure

Dereje Yihun Amare

Master's thesis

Heat transfer in horizontally oriented enclosure

Abstract:

The presented diploma thesis deals with the heat transfer in rectangular, horizontally oriented air enclosures. The aim of this work was to determine the "effective emissivity" of horizontal surfaces delimiting air enclosures with vertical walls characterized as re-emitting. Enclosures with different heights of the air layer were created in the measuring space of the measuring device HFM/436/3/1E by means of square polystyrene boards with a nominal thickness of 0.01 m, in which a coaxial square hole with a side of 0.102 m was formed. The size of the hole coincided with the size of the aerial heat flow transducers of the measuring device. The horizontal surfaces delimiting the air enclosures were formed by three materials significantly differing in their electrical resistance, on which the angular distribution of photons emitted from the surface of these materials depends. The thermal conductivity of the enclosure determined by the measurement was compared with the thermal conductivity of the enclosure calculated for the gradually increasing emissivity of the horizontal surface of the enclosure in the range of emissivity forming the surroundings of the tabulated hemispherical emissivity. When equality between measured and calculated thermal conductivities was reached, the corresponding emissivity was designated as the effective emissivity of the horizontal surfaces for a given height of the air enclosure. This solution introduces an error into the value of effective emissivity caused by the fact that the angular dependence of emissivity of real horizontal surfaces was not taken into account.

Abstract:

The presented diploma thesis deals with the heat transfer in rectangular, horizontally oriented air enclosures. The aim of this work was to determine the "effective emissivity" of horizontal surfaces delimiting air enclosures with vertical walls characterized as re-emitting. Enclosures with different heights of the air layer were created in the measuring space of the measuring device HFM/436/3/1E by means of square polystyrene boards with a nominal thickness of 0.01 m, in which a coaxial square hole with a side of 0.102 m was formed. The size of the hole coincided with the size of the aerial heat flow transducers of the measuring device. The horizontal surfaces delimiting the air enclosures were formed by three materials significantly differing in their electrical resistance, on which the angular distribution of photons emitted from the surface of these materials depends. The thermal conductivity of the enclosure determined by the measurement was compared with the thermal conductivity of the enclosure calculated for the gradually increasing emissivity of the horizontal surface of the enclosure in the range of emissivity forming the surroundings of the tabulated hemispherical emissivity. When equality between measured and calculated thermal conductivities was reached, the corresponding emissivity was designated as the effective emissivity of the horizontal surfaces for a given height of the air enclosure. This solution introduces an error into the value of effective emissivity caused by the fact that the angular dependence of emissivity of real horizontal surfaces was not taken into account.

Keywords

Heat Transfer, Conduction, Convection, Radiation, Air enclosure, Thermal conductivity, HFM/436/3/1E Lambda, Blackbody, Emissivity, View factor, Space resistance, Reradiating surface, Wavelength.

Language used: English

Date on which the thesis was submitted / produced: 7. 6. 2021

Thesis defence

• Supervisor: doc. Ing. Jaroslav Šulc, CSc.

Technical University of Liberec

Faculty of Mechanical Engineering

Master programme / field:
Mechanical Engineering / Machines and Equipment Design