Experimental Study of Nucleate Boiling Heat Transfer Using Enhanced Space-Confined Structures

Abstract

For narrow space boiling, it is difficult to release bubbles from the narrow space, especially on a large-area surface. To solve this problem, a new structure is designed in the present paper. An experimental study of pool boiling on the novel copper enhanced structure, with the separate ordinary confined spaces and the open channels between them, was conducted with water and ethanol. High-speed visualizations are performed to elucidate the bubble flow. The results show that the boiling performance of both water and ethanol can be enhanced effectively. The visualizations indicated that most active nucleation sites emerged in the confined channels and rarely appeared at the bare surfaces not covered by enhanced structures even at high superheat. The bubble diameter, the bubble departure frequency, and the numbers of nucleation sites are obtained using statistical methods. The results suggest that the magnitudes of bubble diameter of water are almost the same on the smooth and enhanced surfaces. The amount of nucleation sites on the enhanced surfaces is remarkably increased, indicating its key role in the boiling enhancement of water. The bubble departure frequency is increased on one of the enhanced surfaces while not increased on another, showing that it is also a significant factor for heat transfer enhancement under certain conditions. While for ethanol, all the three parameters are increased on the enhanced surfaces.