Heat transfer enhancement of a loop thermosyphon with a hydrophobic spot-coated surface

Abstract

Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling is widely used in application of thermal engineering and considerably more efficient than single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circular spots (0.5 -2 mm in diameter and 1.5 -3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and low incipience temperature overshoot with water as working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests under heat loads 30 W to 260 W reveal the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation-site density. The surface with hydrophobic spots with diameter 1 mm and pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. A comparison of three evaporator surfaces with identical wettability patterns but with different surface topographies and coating thicknesses is carried out experimentally. The results show superior heat transfer rates and wear resistance on the surface coated with HNTs spots thanks to the large contact angle, great thickness, and durability of the coating layer. He, Shen, Chen, Hidaka, Takahashi, Kohno and Takata, Journal of Thermal Science and Technology, Vol.13, No.1 (2018) wettability with the contact angle close to 0 o after being exposed to UV light for several hours. Sarwar et al. (2007) performed a sub-cooled flow boiling experiment with microporous TiO2 and Al2O3 surfaces to enhance CHF by 20% to 25% compared to a smooth one. They found that microporous Al2O3 surface had high wettability, which led to water moving in pores easily. Chang et al. (1997) also demonstrated CHF enhancement on DOA (Diamond particles, Omegabomd 101 epoxy, and alcohol)-coated surfaces with different particle sizes. Significant CHF increase was achieved, which delayed the onset of film boiling effectively. Furthermore, nanoparticle deposition was found to be an advanced method to modify the surface wettability significantly and in turn to improve CHF. Forrest et al. (2010) developed a PAH/SiO2 (poly, allylamine, and hydrochloride) nanoparticle coating, whose application on a nickel wire in a boiling experiment brought about surface wettability change that led to 100% enhancement of CHF. Nanoparticle coatings of Al2O3-water/ethanol was studied in pool boiling experiments by Kwark et al. (2010). They found a strong relationship between quasi-static contact angle and CHF. That is, with increasing wettability, CHF increased gradually.Prior studies have demonstrated how to enhance the performance of HTC. The influence of surface wettability on HTC was made clear by . From a comparison between weakly wetted surface and strongly wetted surface, they found that excellent HTC can be achieved both at very low contact angle close to 0 o and at a contact an...

ANALISA TRANSFER PANAS PADA KONDENSASI LUAR DENGAN MENGGUNAKAN REFRIGERANT R-134a

“…Selain itu peningkatan transfer panas dapat dilakukan dengan modifikasi permukaan seperti He et al [6] yang menggunakan lapisan Ni-P-PTFE dengan Teknik photolithography untuk menghasilkan permukaan hydrophobic dan Qi et al [7] yang menganalisa kebasahan permukaan dengan teknik modifikasi permukaan micro-grooved. Salah satu metode yang akhir-akhir ini sering diteliti adalah dengan mengurangi kebasahan permukaan atau surface wettability sehingga kondensat yang biasanya berupa film atau lapisan tipis akan berubah menjadi dropwise yakni kondensat berupa tetesan kecil karena adanya peningkatan sudut kontak antara kondensat dengan permukaan.…”