Numerical Simulation For The Shape Of Drop On The Gradient Surface

Nowadays the whole society has emphasized to advocate to save the energy, the boiling and condensation heat transfer, as one of the most efficient heat exchange method, have been paid more attention. To developing and inventing the new style of the surface material for condensation has been a hotspot problem for the research in the world. The characteristics of the droplet self-propel on the gradient surface energy supply a new method of eliminating the condensing water in expect for the gravity. Therefore, the gradient surface energy could be capable of eliminating the condensation in time and improve dramatically the coefficient of condensation heat transfer as for horizontal surface or other micro-gravity state in effect.In our experiments, we observed the small volume of water drop can keep stable figure on the horizontal gradient surface, and there are different appearances of drops on the different positions along the direction of the gradient surface. In engineering fields, the appearances of drops are very important parameter to reflect the performance of the condensation; therefore, there are some scholars have been carried out the experiments and theoretical work on this problem, but these relevant works only studied the homogenous surface in detail. Base on this point, we built a new numerical model to describe the drop on the gradient surface, including the whole isolated system of forming by the three phase of gas, liquid and solid, then the total potential energy of the system is derived. According to the second law of thermodynamics, when the drop is in balance, the mechanical energy will be minimized. Basing on the above theory, the finite element method is established to solve the total potential energy equation. We calculate the shapes of drops on the horizontal homogeneous surface, horizontal gradient surface, inclined homogeneous surface and inclined gradient surface, and analyze the main factors determining the shape of drop and their effects. Therefore, we provide a basic theory for the research of drop's motion on the gradient surface and its application in the condensation heat transfer. The main conclusions would be described as follows:①The shape of drop on a horizontal homogeneous surface is a rolling body, and the contact line is a circle. The contact angle is the main factor determining the shape of drop. A larger value of contact angle will give a higher drop, making the solid-liquid interface area less. ②Drops on the different positions of a horizontal homogeneous surface have different shapes. In the two end points of the profile, the values of contact angle are maximum and minimum respectively. From the hydrophobic side to the hydrophilic side, the profile of drop will be extended continuously, and its height will be decreased. The contact line extends towards the hydrophilic side, while drop's span enlarging.③On the same position of a horizontal gradient surface, bigger the gradient of surface energy is, making drop's span larger. The effect of the gradient of surface energy affecting drop's shape will be indistinctive when drop's center contact angle is less than 35o.④On the horizontal gradient surface, bigger the gravitation is, making the deformation of drop shrewd.⑤Drop's shape on an inclined homogeneous surface is asymmetry, and extends toward the bottom of the surface. There is some difference between the advancing contact angle and the receding contact angle. The deformation of the contact line mainly occurs around the advancing side of the drop. While the gravitation increasing, drop's deformation becomes shrewd.⑥Drop's deformation on an inclined gradient surface will be shrewd while the gravitation increasing. The deformation of drop's profile and contact line is in a similar way. The difference between advancing contact angle and receding angle becomes more bigger while the drop extending toward the bottom of surface.⑦Drop's deformation on an inclined gradient surface becomes shrewd when the gradient of surface energy is agreement with the component force of gravitation. On the contrary, drop's deformation is reduced.