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Journal of Enhanced Heat Transfer

Theory and Application in High Performance Heat and Mass Transfer

ISSN for PRINT: 1065-5131

Institutional price:	$577.00
Issues per year:	4

Best Paper Award Selection - Editorial Board Site

2007, Volume14

83 pages

DOI: 10.1615/JEnhHeatTransf.v14.i3

Issue price - $160.00

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

Qiang Liao
Institute of Engineering Thermophysics, Chongqing University, Chongqing 400044, P. R. China

Y. B. Gu
Institute of Engineering Thermophysics, Chongqing University, Chongqing 400044, P. R. China

X. Zhu
Institute of Engineering Thermophysics, Chongqing University, Chongqing 400044, P. R. China

H. Wang
Institute of Engineering Thermophysics, Chongqing University, Chongqing 400044, P. R. China

M. D. 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.