Enhancement of Pool Boiling Heat Transfer by Biporous Structure Surface
Date Issued
2008
Date
2008
Author(s)
Wu, Sheng-Lung
Abstract
The purpose of this research is to enhance boiling heat transfer capacity by utilizing two pores distribution structure (biporous structure) on the saturated pool boiling heat transfer of R134a. This surface is fabricated with sintered dendritic copper powders and the pore former, Na2CO3, which forms the larger pores in the matrix. By changing the volumetric ratio of pore former, we are able to alter the porosity and the numbers of larger pores in the porous media. The study was conducted based on a statistical method, with a two-level factorial plan involving three variables (coating thickness/particle size of copper: 6 and 10, coating thickness/particle size of pore former content: 16 and 5, and pore former content: 15% by volume and 25% by volume). Finally, the performance of biporous surface, monoporous surface and commercially enhanced surfaces were compared. he preceding statistical analysis of experiment data show that the boiling performance and characteristics are strongly dependent on the pore former contents (56% of percent contribution) and the better performance tend to have less pore former contents, higher thickness/particle size of copper, and higher thickness/particle size of pore formers. This information provides a direction of the potential improvement. The heat transfer coefficients of biporous and monoporous surface are not much different at low heat flux(less than about 150 kW/m2). The heat transfer enhancement ratios are 6~7 times compared to a smooth surface. For high heat flux removal (higher than 150kW/m2), the heat transfer enhancement ratios of biporous surfaces are 5.1~6.3 and 2.4~4.2 times over a smooth and monoporous surfaces, respectively. The critical heat fluxes for each kind are 671 kW/m2 and 631 kW/m2. At high heat flux, the biporous surface can continuously remove heat at high heat transfer coefficient. The larger pores provide more vapor pathways for bubbles generated inside the structure and reduce the liquid-vapor counterflow resistance adjacent to the surface, while the smaller pores continue to function as liquid supply routes. Therefore, biporous surface is very attractive for high heat flux application.
Subjects
Heat transfer
Pool boiling
Biporous surface
Enhanced surface
Type
thesis
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