Chemical and petro-chemical engineering, cooling of power electronic devices, air conditioning and refrigeration are among the many phenomena in our daily activities that utilize boiling. In the past fifty years, the boiling phenomenon has been studied far and wide. Most of the studies undertaken have, however, concentrated on parameters such as dimension, configuration, heat flux, type of working fluid and the topography of the surface. More so, majority of the researchers have focused on the use of enhanced surfaces in boiling by preparing mechanically deformed structures, sintered wires porous, materials and complex geometries. The importance of surface condition on nucleate boiling cannot be ignored. Fortunately, recent advances in nanotechnology have led to the conceptualization of efforts aimed at enhancing boiling heat transfer by using nanostructured surfaces such as deposited nanoparticles, nanowires, carbon nanotubes and metal oxide structures. Flow boiling enhancement using structured surfaces in micro-channels is a promising method to achieve high heat removal rates. Regardless, there are few studies focusing on boiling heat transfer in high aspect ratio micro-channels.
Researchers (Abdolali K Sadaghiani, Sorour Semsari Parapari, and Professor Mehmet Keskinoz from Sabanci University, Nawzat S. Saadi and Professor Tansel Karabacak from University of Arkansas at Little Rock) led by professor Ali Kosar from the Center of Excellence for Functional Surfaces and Interfaces for Nano diagnostics (EFSUN) and Sabanci University Nanotechnology and Applications Center (SUNUM) at Sabanci University in Turkey, investigated the effect of surface structure size (size scale: micro and nano) on boiling heat transfer characteristics of samples with different surface morphology. The researchers aimed at conducting flow experiments on samples of different surface morphologies: micro and nano structured, in a high aspect ratio micro channel. A new flow map have developed demonstrating the differences in flow morphologies due to structure size. Their work is now published in the research journal, Applied Thermal Engineering.
The research team undertook the experimental procedure where they investigated heat removal capacity of nano-structured, micro structured and micro-nano structured surfaces. They then employed thermal and high speed camera systems to clarify the differences in heat transfer mechanisms. The team then obtained heat transfer coefficients along with associated boiling images. Eventually, based on the visualization study results, two flow maps were constructed for a rectangular microchannel with micro and nano scale structures on copper surfaces.
From the experiment undertaken, the authors of the paper observed that the surface morphology remarkably changed boiling heat transfer mechanisms. According to the obtained thermal images, bubble departure frequency increased with surface structures, and the surface temperature distribution was more uniform for surfaces with nano scale structures (nano-structured and micro-nano-structured) compared to other surfaces (untreated, micro-structured).
In their study, copper surfaces have been used as the test samples. From the results obtained, it has been found that structured surfaces have a better heat removal capacity in comparison to the bare surface. This is good evidence that micro-structured, nano-structured and micro-nano-structured plates can be implemented in innumerable cooling applications to achieve higher energy efficiency. More so, according to the observations higher heat transfer coefficients are obtained from nano scale structured surfaces in comparison to the micro scale structured sample. Therefore, these promising results obtained in Ali Koşar lab reveal the potential of micro and nano scale structured surfaces application that will help achieve improved energy efficiency for electronics cooling systems.
Abdolali Khalili Sadaghiani, Nawzat S. Saadi, Sorour Semsari Parapari, Tansel Karabacak, Mehmet Keskinoz, Ali Kosar. Boiling heat transfer performance enhancement using micro and nano structured surfaces for high heat flux electronics cooling systems. Applied Thermal Engineering volume 127 (2017) page 484–498.
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