No abstract

In this work, we studied the effect of radial surface finning and hydrophobic fluoropolymer coating of finned surfaces on the intensity of heat transfer at boiling. Two samples were used: 16 fins and 32 fins. At first, experiments were carried out without a hydrophobic coating. After that, the samples were covered with a layer of fluoropolymer and a series of experiments were carried out. During these experiments, the state of the fluoropolymer coating was analyzed. Experiments on a sample with 16 fins showed that deposition of fluoropolymer leads to significant intensification of heat transfer. For the sample with 32 fins, the opposite effect was observed: hydrophobization negatively affected the efficiency of heat transfer. Maximal enhancement of heat transfer, in comparison with finely finned and smooth surfaces, was achieved on a surface with large fins coated with fluoropolymer.

In this paper, the investigation of pool boiling heat transfer on biphilic micro/nanostructured surfaces is presented. An array of micrococoons from silicon oxide nanowires was synthesized on the surface of a copper heater using the gas-jet electron beam plasma chemical vapor deposition method. The biphilic properties of the surface were achieved by applying fluoropolymer spots by hot wire chemical vapor deposition. Technology of creating biphilic surfaces was developed and boiling curves were obtained for used samples. The advantages of using a biphilic surface to enhance heat transfer were demonstrated in comparison with a smooth surface and a micro-nanostructured surface without local hydrophobic fluoropolymer regions. This technology can be applied to enhance boiling heat transfer.

Flat surfaces with different patterns of hydrophobic spots were employed for experimental investigation of boiling heat transfer. In one case, hydrophobic spots were created on a smooth copper surface and on a surface coated with arrays of micrococoons from silicon oxide nanowires by vapor deposition of a fluoropolymer. In the second case, a hydrophobic coating was deposited on heater surfaces with cavity microstructures formed by laser ablation and chemisorption of fluorinated methoxysilane. Water under saturation conditions at atmospheric pressure was used as the working liquid. The temperature of the heating surface was varied from 100 to 125 °C, and the maximum value of the heat flux was 160 W/cm2. Boiling heat transfer on the test biphilic surfaces was significantly (up to 600%) higher than on non-biphilic surfaces. Surface texture, the shape of hydrophobic regions, and the method of their creation tested in this study did not show a significant effect on heat transfer. The boiling heat transfer rate was found to depend on the size of hydrophobic spots, the distance between them, and hence the number of spots. The highest heat transfer efficiency was detected for the surface with the largest number of hydrophobic spots. After long-term experiments (up to 3 years), the heat transfer coefficient on the obtained surfaces remained higher than on the smooth copper surface. Biphilic surfaces with arrays of cavities formed by laser ablation turned out to be the most stable during prolonged contact with boiling water.

An experimental study of radiation heat transfer during combustion of a flammable liquid in a narrow rectangular channel is performed. The emission spectrum of combustion products and the absorption spectrum of butanol-1 are obtained. The case when the temperature of the combustible liquid is lower than the flash point is investigated. A comparative analysis of emission and absorption spectra shows that they do not intersect well, and the radiative heating of the liquid during diffusion flame propagation along the surface cannot be too large.