Research Of Enhancing Boiling Heat Transfer On Wettability Surface

With the development of science and technology,heat dissipation has become a difficulyy in the electronic devices and power plants and other industries.Boiling heat transfer,as an efficient phase change heat transfer method,has the advantages of high heat transfer coefficient and small temperature difference which is an important means to solve heat dissipation.Therefore,it is of great significance to study boiling heat transfer.Boiling heat transfer is related to the wettability of heat transfer surface.In order to explore the influence of wettability on boiling heat transfer characteristics,different wettability surfaces were prepared at first,then droplet and bubble collision surface experiments were carried out.Finally,pin-fin surface was modified and pool boiling experiments were carried out.Different wettability surfaces were prepared and then characterized.The contact angle of the hydrophilic surface was measured and the superhydrophilic surface contact angle was only 5.7°The contact angle and rolling angle of the superhydrophobic surface were measured.The rolling angle of the superhydrophobic low adhesive surface was less than 10°,and the rolling angle of the high adhesive surface was 180°.An experimental table for droplet impacting on different wettability surfaces was set up and experiments were carried out.Through droplet collision experiments,it was found that droplets mainly exhibit spreading characteristics on hydrophilic and superhydrophilic surfaces,but the extension is more thorough on the superhydrophilic surface.On low adhesive and high adhesive surface,droplets mainly exhibit rebounce characteristics and droplets can rebound entirely on low adhesive surface.Droplet behaviors could be divided into different regions in the coordinates of We versus Re.The droplet spreading coefficient(?)is affected by wettability and inertia.At low We,wettability dominated droplet spread.At high We,the initial stage of the spreading process was affected by inertia,and the final stage of spreading was affected by wettability.The maximum spreading coefficient(?max)On the superhydrophilic surface is greater than that of other wettability surfaces,and ?max on the superhydrophobic surface is the smallest.An experimental table for bubble impacting on different wettability surfaces was set up and experiments were carried out.Through bubble collision experiments,it was found that when bubble collides on low adhesive superhydrophobic surface,bubble can first vibrate or bounce,then spread and at last form a layer of gas film on the surface.When bubble collides on superhydrophilic and high adhesive superhydrophobic surface,bubble can first vibrate or bounce,and stay on the surfaces with ball-lacking shape in the end.With the increasing L,the initial jump heights(hmax)increase first and then decrease on three types surfaces,and they reach the maximum value at L=13.6 mm,which are 6.05,5.53 and 4.37 mm,respectively.For low adhesive surface,both hmax and the time of bubble spreading decrease with increasing D.The collision behavior of droplets and bubbles was comprehensively analyzed.It was find that droplet on the superhydrophilic surface have static and dynamic similarity with bubble on the low adhesive superhydrophobic surface.Droplets have static and dynamic similarity on the low adhesive superhydrophobic surface with the bubbles on the superhydrophilic surface.Droplets and bubbles are self-similar of static and dynamicon on the high adhesive superhydrophobic surface.Droplets and bubbles have a large similarity in surface morphology.The pool boiling test rig was set up to study the effects of different wettability on boiling heat transfer.Through pool boiling experiments,it was found that compared to the hydrophilic surface(HPI),the superhydrophilic surface(SHI)can improve the heat transfer performance and the heat transfer coefficient of superhydrophobic surface decreased(SHO).At the boiling start point,the superheat of HPI is the smallest,and the superheat of SHI andSHO-SHI combined surface increases slightly.While the superheat of SHO and SHI-SHO combination surface increases by 6.3 K and 6.1 K respectively,due to the increasing of convective heat transfer resistance.The heat transfer charcteristics of SHI and SHO-SHI mixing surfaces are similar,and SHO surface have the similarity heat transfer characteristics with SHI-SHO mixing surfaces.The undercooling increasing greatly reduces densities of nucleation points,leading to decrease in the surface heat transfer coefficient.