Study On Influence Of Surface Properties On Microscale Condensation

Microscale condensation is the key thermophysics process in the micro heat pipes, lab on a chip, micro fuel cells, etc. Due to the great application prospect and academic value of the research on microscale condensation, it has been an important subject in engineering thermophysics. In particular, the treated condensation surface with hydrophobicity or gradient surface energy, can do help to enhance condensation heat transfer. Furthermore, with respect to that in conventional scale, the impact of surface characteristic on the condensation process in microscale is much more prominent. Therefore, it is necessary to further study the mechanisms of the influence of surface characteristics on microscale condensation.Now, the research on the influence of surface characteristics on microscale condensation is still limited. The inherent law, especially, the role of surface wettability or surface energy gradient on condensation flow pattern, and the motion characteristics of condensation droplet in microchannel, is waiting to be explored. In this context, the experiments are conducted to visually study the flow condensation of steam and ethanol-water mixtures in hydrophobic microchannels. The numerical simulation of the droplet motion and coalescence driven by flow steam in rectangular microchennel is conducted by using VOF method. And the free-energy based lattice Boltzmann model is utilized to simulate the steam condensation and self-driven motion and coalescence of condensation droplets on the gradient surface energy surface. The results and conclusions are summarized as follows:(1) With MEMS processing technology, a hydrophobic microchannel array with the hydraulic diameter of 150 μm, effective length of 56 mm, aspect ratio of 3, and contact angle of 96° is fabricated on the silicon chip. A visual experiment is conducted to observe the flow patterns and measure the flow condensation heat transfer performance. The results indicate that: ① Droplet flow, droplet-annular compound flow, droplet-injection compound flow and droplet-bubble/slug compound flow are observed sequentially in flow direction in the hydrophobic microchannels. In droplet flow and droplet-annular compound flow region, droplet would coalesce and detach from the channel wall under the steam shear flow. ② As the inlet vapor Reynolds number and condensate Weber number increase, the droplet-injection compound flow will be postponed towards the channel outlet with increasing injection frequency. ③ The chip wall temperature decreases mildly along flow direction before the injection flow, after which there is a distinct temperature drop on the chip wall. The average condensation heat transfer coefficient increases with the increase of inlet vapor Reynolds number.(2) By using the ethanol-water mixture, a visual experiment is introduced to study the distributions and characteristics of flow condensation patterns in hydrophobic microchannels under different ethanol concentrations. The results indicate that: ① If water steam is the main component, the droplet condensation occupies almost the whole channel surface in the two phase flow region, and droplet flow, annular-streak compound flow, droplet-annular compound flow, droplet-injection compound flow and droplet-bubble/slug compound flow are observed sequentially in microchannels. ② If the ethanol concentration rises to be equal to steam, the droplet condensation on the channel surface downstream to the droplet-injection compound flow disappears, which is replaced by the bubble/slug flow. ③ If the ethanol steam is the main component, droplet condensation almost disappears, and annular-streak compound flow, annular flow, injection flow and bubble/slug flow are observed sequentially in microchannels. The streak flow disappears when pure ethanol steam condensation in the hydrophobic microchannels. ④ Under the same Rev, the dimensionless position of injection flow, Kp, decreases with the increasing ethanol concentration. The injection frequency increases at first and then decreases with the increase of ethanol concentration. The frequency achieves the maximum value when the ethanol concentration is 10%, and ethanol concentration has little effect on the ejection frequency if the ethanol concentration is larger than 60%. ⑤ Reducing the microchannels wall surface energy is in favour of droplet condensation during the flow condensation of vapor mixture.(3) In order to in-depth study the motion characteristic of condensation droplet in microscale on the hydrophobic surface, a three-dimensional model of droplet motion and coalescence under the steam flow based on volume of fluid (VOF) method is developed and numerically analyzed to study the droplet motion and coalescence process and characteristics of two droplets on the rectangular microchannel wall. The results indicate that: ① The coalescence comes out under the conditions of two droplets being with same size and larger droplet being at the front. ② Due to the contact angle hysteresis, the droplet skews to the flow direction. During the coalescence process, the contact area between droplet and channel wall oscillates and decreases to a stable value. And the contact area has a more larger value at a larger steam velocity. ③ With the increase of steam flow velocity, the coalescence time decreases and the coalescence distance increases at first and then decreases. ④ Under the condition of larger droplet being at the front, the front droplet has a larger sliding speed and the distance between two droplets gets longer and longer.(4) A lattice Boltzmann model of steam condensing on surface with gradient surface energy based on free-energy is developed with the focus on condensate motion and coalescence process. The influence of surface energy gradient and surface tension on the condensation droplet motion, the deformation and motion process of condensation droplet and flow field in droplet as well as the comparison between the condensation droplets coalescence on homogeneous surfaces and gradient surface are all investigated. The results indicate that: ① During condensation, the steam condenses and forms to be liquid film on the surface side with high surface energy and droplets on the surface side with low surface energy. With the accumulation of condensate, the condensation droplets grow up, coalescence and spontaneously move to the high surface energy side under the unbalanced surface tension. ② The condensation droplet contact angle in the high surface energy region is bigger than that in low surface energy region. Therefore, an unbalanced surface tension is on the droplet point to the hydrophilic side. If the surface energy gradient is big enough, the condensation droplet is able to overcome the contact angle hysteresis and move to the high surface energy region. ③ After coalescing, droplets on surface with gradient surface energy move to the hydrophilic side. The moving condensation droplet will coalesce with the other droplets on its way forward and move together.In summary, the current work systematically gains an insight into the impact of surface wettability and the surface gradient surface energy on the microscale condensation. The corresponding research results provide an effective theoretical support for the design and optimization of micro energy and fluid transfer devices. It also supplements and improves the theory of microscale condensation.