Graduate Thesis Or Dissertation
 

An experimental investigation of radiative and total heat transfer around a horizontal tube immersed in a high temperature gas-solid fluidized bed

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  • This study is primarily concerned with the measurement of the local radiative component of total heat transfer around a horizontal tube immersed in a large particle fluidized bed at elevated temperatures.Total heat transfer was also measured in order to assess the relative radiation contribution.The radiation measurement probe employs a silicon window mounted flush with the tube wall to transmit the radiative heat flux. Athin-film thermopile-type heat flow detector placed behind the window sensed the transmitted radiation. The thermal conductivity of silicon is sufficiently large to prevent the conduction error (less than3%) resulting from the convective component of heat transfer. Silicon also has a wide spectral transmission wave band extending from1.3 μm to 12.0 μm. The total heat transfer probe uses a similar heatflow detector bonded to the tube wall and covered tightly with a stainless steel foil to protect the detector against abrasion.The radiation probe was calibrated using a narrow-angle black body source. The purpose of the calibration was to establish the relation between the heat flux detected by the radiation probe and the incident radiative flux to the tube wall. This relation was found to be linear and insensitive to the tube wall temperature variations encountered in this study.The instrument has been used to measure local radiative and total heat transfer at 0, 45, 90, 135, and 180 degree positions around a horizontal tube immersed in a fluidized bed. Measurements were obtained at bed temperatures of 812 K and 1050 K with 2.14 mm and 3.23 mm mean diameter particles at gas velocities up to two times the minimum fluidization velocity.A sharp increase was noted for spatial average radiative, total and convective heat transfer coefficients when the gas velocity exceeded the minimum fluidizing velocity. Generally, these coefficients showed little variation with further increase in gas flowrate. The radiative heat transfer coefficient was increased with an increase in particle size and bed temperature, the latter having a much more pronounced effect, as expected. The radiation contribution to overall heat transfer was found to increase from 9% to 15% for 3.23 mm particles and from 8% to 13% for 2.14 mm particles when the bed temperature was elevated from 812 K to 1050 K at the optimum gas velocity (maximum total heat transfer). Finally, fast sampling rate (25 samples sec) data were obtained using the radiation probe with a measured response time of about 120 msec. This data was treated as instantaneous and an attempt was made to obtain hydrodynamic parameters such as emulsion residence time and bubble contact fraction. However, this analysis did not yield satisfactory results.
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