Search

Mixing of surface microstructure and wettability is one of the future development trends. Their combination always affects the bubble motion and heat transfer coefficient under pool boiling/flow boiling, but it is difficult to distinguish their respective contributions. This work studies the combination of new microstructure and mixed wettability to improve the flow boiling heat transfer in the channel. The nucleation, growth, coalescence and departure behaviors of R134a boiling bubbles were numerically analyzed. Effects of different heights, gaps and mixed wettability of microstructures and heat fluxes on bubble emission frequency and boiling heat transfer coefficient were discussed. Results showed that this new microstructure provides the bubble with the momentum of lateral motion, which helps the bubble to merge and leave faster. The surface of the hydrophobic groove coupled with the hydrophilicity as the substrate can effectively increase the bubble emission frequency, up to 134.37%. At the same time, the heat transfer coefficient decreases with the increase of the height of the microstructure, and it also increases with the increase of the microstructure gap. Finally, the contribution of each variable was determined by analysis of variance. The research results in this paper provide a reference for heat pipe and vapor chamber. • A creative microchannels with mixed wettability was proposed and optimized. • Synergistic effect of groove microstructure and mixed wettability was studied. • Surface mixed wettability has more influence on boiling heat transfer. • Bubble emission frequency of surface-A is 134.37% higher than surface-B. [ABSTRACT FROM AUTHOR]

To study energy recovery from low temperature wastewater during industrial production, a water-boiling heat transfer experimental device was built. A casing evaporator with a length of 1450 mm, an inner tube diameter of 30 mm, and an annular space gap of 14.2 mm is taken as the main research object. The boiling heat transfer characteristics of water in the annular tube area of the casing evaporator were experimentally studied by adjusting the inlet temperature (60-85 °C) of hot water and the pressure in the annular tube (12.1-57.6 kPa). The results show that, as the pressure of the system decreases, the boiling phenomenon in the annular tube becomes more intense and the convective heat transfer coefficient increases. The boiling flow and average surface convective heat transfer enhancement rates, β, were 2.2 and 1.5 when an initial pressure of 1 kPa was used. When the flow rate of the working medium in the annular tube was 1.69 kg·m²/s, the convective heat transfer coefficient gradually increased with the temperature and then stabilized when the inlet temperature reached between 80 °C and 85 °C. These results reveal the characteristics of boiling-water heat transfer in an annular tube and aid in the design of heat pipe systems for waste-heat recovery. [ABSTRACT FROM AUTHOR]

• Nanomaterial deposition on heating surface are used to attain reduction in wall superheat, enhancing nucleate boiling HTC and CHF. • Different working fluid is used such as de-ionized water, fluorocarbon fluid, hydofluoroethers and refrigeration-based fluid for enhancing CHF. • The surface modification enhanced the CHF due to the reduction in wall superheat temperature, increased nucleation site, porosity rise, enhanced surface wettability, small nano-pores/cavities generation, rise in bubble release frequency. • Nanoparticle size, nanofluid preparation techniques and concentration of nanofluid mainly influenced the CHF. Boiling is the most significant phase change phenomena for various applications such as cooling of electronic devices, heat pipes, cooling in nuclear reactor, different industrial application and plenty more. Due to its wide scope of application, any enhancement in this domain leads to remarkable economic and energy efficient consequences. Augmentations in boiling heat transfer performance are very crucial which establishes the various industrial applications further energy efficient. To reduce the energy dissipation in heat transfer equipment, boiling heat transfer augmentation is one of the significant techniques used by researchers. This paper demonstrates a thorough review and investigation of published articles describing the various surface modification method (nanomaterial coating, nano/micro porous coating, nano/micro structured surface) for pool boiling and flow boiling heat transfer enhancement as well as the effect of surface modification on wettability, porosity, surface roughness, nucleation sites are studied. Various augmentation methods are utilized to attain three apparent heat transfer desire: reducing wall superheat temperature, enhancing pool boiling heat transfer coefficient (HTC) and improving critical heat flux (CHF). [ABSTRACT FROM AUTHOR]

In the present experimental study, nucleate pool boiling heat transfer measurements of two high-flux tubes (sample A and sample B) were conducted at atmospheric pressure with HFE-7200 as the working fluid. Both high-flux tubes were made from a sintered Cu-Ni (high-flux) alloy powder. The porous high-flux surface was coated inside the test tube and it is tested within the heat flux ranging from 2.6 to 86 kW/m2. The major difference between sample A and sample B was the coating thickness, where sample B (0.6 mm) was much larger than that of sample A (0.07 mm). Both tubes showed about three times enhancement in heat transfer coefficient (HTC) when compared to plain tube. Even though sample B contained a higher HTC than sample A, it also revealed a faster level-off phenomenon regarding the HTC vs. wall superheat. The major parameter which characterizes the boiling performance of high-flux tube was the ratio of coating thickness to pore diameter which also yielded different trends upon HTC vs. wall superheat amid sample A and B. It was found that the porous based Nishikawa correlation can well predict the performance of sample A but not sample B. This is because the ratio of coating thickness to pore diameter is far outside the applicable range of the Nishikawa correlation. Hence, a modified Nishikawa correlation is proposed. The predicted capability of the proposed modified Nishikawa correlation against sample A and sample for HTC was within ±28% deviation. The standard mean deviation of the Nishikawa correlation with experimental data for sample A and sample B was 0.302 (12.48%) and 5.64 (73%), respectively. [ABSTRACT FROM AUTHOR]