ARTICLE:

• EXPERIMENTAL INVESTIGATION OF DROPWISE CONDENSATION HEAT TRANSFER ON THE SURFACE WITH GRADIENT SURFACE ENERGY  download article

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

By using the chemical vapor deposition (CVD) technology, two tested surface with gradient surface energy was fabricated on a base of silicon wafer with dodecyltrichlorosilane and octrytrichlorosilane. In virtue of the sessile drop method, the surface wettability profile was characterized by measuring the contact angle of a fine water drop which lied on the surface of treated silicon wafer. Using a high-speed video imaging system, the movement of water drop on horizontal surface with gradient surface energy was visualized and measured under ambient condition. The experiments showed that the liquid drops could be driven to move from hydrophobic side to hydrophilic side on the horizontal treated surface. The heat transfer experiments were conducted on dropwise condensation of steam on the surfaces with gradient surface energy. During experiments, the condensing surface was placed at various inclination angles of 0°, 30°, 60°, and 90°, respectively. The growth, coalescence, motion, and abscission of condensing droplets were visualized by the high-speed video imaging system. The observed results showed that the condensing droplets larger than about 1 mm in diameter can move at a peak speed of 110 mm/s from hydrophobic side to hydrophilic side on horizontal condensing surface with gradient surface energy. The velocity of condensing droplet was far larger than that of droplet on the surface with gradient surface energy in ambient atmosphere. The effects of heat transfer temperature difference, the inclination angle of condensing surface, and the gradient of surface energy on the condensation heat transfer were respectively discussed in virtue of the photographic results. The experimental results showed that with an increase of the heat transfer temperature difference, the heat transfer coefficient increases to a maximum value and then decreases afterwards. As the inclination angle of condensing surface increasing, the condensation heat transfer coefficient increased due to the action of gravity on the departure and the motion of droplet. A larger gradient of surface energy led to a smaller departure diameter and a faster motion of droplet, which resulted in a larger condensation heat transfer coefficient.

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