Date of Award

12-1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Abstract

To improve the cooling technology for the future generation of Advanced Turbine Systems (ATS), studies on mist/steam heat transfer of confined slot jet impingement have been performed with a focus to investigate the heat transfer enhancement by injecting water mists into steam jet impingement flow. Efforts have also been made to reveal the enhancement mechanism.

The mist/steam flow was obtained by blending saturated steam at 1.5 bars with fine water droplets generated by a water atomizer. Special attention was paid to the arrangement of the atomizing nozzles in the mixer. The slot jet had a width of 7.5 mm and a nozzle-to-target spacing of 22.5 mm. Five segmented heater elements were heated directly by a DC power supply.

Experiments have been conducted with various Reynolds numbers and heat fluxes. Results indicated that the wall temperature decreased significantly near the stagnation point due to mist injection while the decrease in the wall temperature in other regions was negligible. The heat transfer enhancement decreased with the wall temperature and increased with the mist concentration. To further the understanding of droplet dynamics, a phase Doppler particle analyzer (PDPA) system was employed to measure the droplet size and velocity. The averaged diameter (d10) was about 3∼6 μm in most cases. Observations through the optical windows found that a dry heated surface was maintained even under the lowest heat flux in this study.

To overcome the complicated experimental process and extend the study to a wider range of parameters, numerical studies have been conducted by using a dispersed-phase model. The standard K-ϵ turbulence model was used to simulate the steam flow. A reasonable result was obtained by combining two kinds of boundary conditions for droplets. The effects of the droplet size and liquid concentration on mist/steam impinging jet heat transfer have also been examined.

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