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Journal of Enhanced Heat Transfer
IF: 1.406 5-Year IF: 1.075 SJR: 0.287 SNIP: 0.653 CiteScore™: 1.2

ISSN Print: 1065-5131
ISSN Online: 1563-5074

Journal of Enhanced Heat Transfer

DOI: 10.1615/JEnhHeatTransf.v14.i3.60
pages 243-256

Experimental Investigation of Dropwise Condensation Heat Transfer on the Surface with a Surface Energy Gradient

Qiang Liao
Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 400030, China; Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
Y. B. Gu
Institute of Engineering Thermophysics, Chongqing University, Chongqing 400044, P. R. China
Xun Zhu
Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 400030, China; Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
Hong Wang
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing 400030, China; Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
Mingdao Xin
Institute of Engineering Thermophysics, Chongqing University, Chongqing 400044, P. R. China

ABSTRACT

An experimental study of motion of a water droplet and dropwise condensation on the surface with a surface energy gradient are reported in this paper. Two tested surfaces with a surface energy gradient are fabricated on a silicon wafer with dodecyltrichlorosilane and octrytrichlorosilane by the chemical vapor deposition (CVD) technology. The contact angles of a fine water droplet lying on the surface of the treated silicon wafer are measured employing the sessile drop method to characterize the surface wettability profile. Visualization of the motion of a water droplet on a horizontal surface with a surface energy gradient is performed using a high-speed video imaging system under ambient conditions. The experiments show that liquid droplets can move from the hydrophobic area to the hydrophilic area on the treated horizontal surface. The experiments on dropwise condensation of steam are conducted on the surfaces with a surface energy gradient at various inclination angles of 0°, 30°, 60°, and 90°, respectively. The growth, coalescence, motion, and detachment of the condensate droplets are visualized by the high-speed video imaging system. The results show that the condensate droplets larger than about 1 mm in diameter can move at a peak speed of 200 mm/s from the hydrophobic area to the hydrophilic area on the horizontal condensing surface with a surface energy gradient. The velocity of the condensate droplet is much higher than that of a droplet on the surface without a surface energy gradient in ambient conditions. A series of parametric studies, including the effects of the heat transfer temperature difference, of the inclination angle of a condensing surface, and the gradient of surface energy on the condensation heat transfer are performed in virtue of the photographic results. The experimental results show that the condensation heat transfer coefficient increases to reach a maximum value and decreases afterwards with increasing heat transfer temperature difference. A larger inclination angle of the condensing surface induces a higher condensation heat transfer coefficient due to the action of gravity on the departure and motion of the droplet while a larger gradient of surface energy leads to earlier departure and faster motion of the droplet, hence to a higher condensation heat transfer coefficient.


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