| In all heat exchange methods,the efficiency of boiling heat exchange has an order of magnitude change from traditional air cooling and water cooling.At the same time,lowering the boiling start temperature and increasing the heat flow density can strengthen the heat dissipation,and can also reduce the heat dissipation area to a certain extent,so as to achieve the purpose of saving energy.Therefore,the research on heat transfer performance of different surfaces has important research significance.Inspired by the T-shaped heat pipe,one of the four internationally popular heat pipes,and combined with the bionics idea.T-groove microstructured surfaces with different scales were prepared on the copper surface.The effects of different processing techniques and microstructure sizes on heat transfer performance were studied,and possible heat transfer mechanisms were further explored.In this paper,T-shaped microstructure grooves with different structural parameters were prepared on the copper surface with WEDM,which is called single-scale T-shaped surface.In addition,combined with laser processing,doul-scale T-shaped heat transfer surfaces are further prepared.In addition,smooth and laser-machined surfaces were also prepared.Furthermore,the properties of the prepared surface were further characterized by means of scanning electron microscope,laser confocal,and contact angle measuring equipment.Based on the testing of the surface properties,it was found that the prepared surface properties include changes in morphology,wettability,adhesion,chemical composition,etc.compared to smoother surfaces.Based on the existing theoretical basis,the reasons for the surface wettability change are analyzed in detail,and the wetting state is proved to be a typical Cassie model.At the same time,on the premise that the surface has high adhesion,increase the surface droplet volume and quantify the surface viscosity by the maximum droplet volume that the surface can suspend.Then,set up a heating test rig to measure temperature,visualize bubbles,and stabilize voltage and current.And based on this,experimental observations of boiling heat transfer of deionized water on different heat transfer surfaces are achieved.The test bench includes a temperature control system,a visualization system,and a data acquisition system.Among them,the temperature control system consists of a copper heating rod,a regulated DC power supply,a copper-constantan electric heating rod,and a sealant,rock wool,polytetrafluoroethylene plate,thermally conductive silicone grease,and aluminum plate.The visual image acquisition system is mainly composed of high-speed camera,CCD industrial camera,anti-fog glass,image processing software,auxiliary power supply,boiling pool device,etc.The data acquisition system consists of a temperature acquisition instrument,a K-shaped thermocouple,and a computer.The boiling curve of a single-scale T-shaped surface was drawn from the aspects of superheat degree,heat flux density and heat transfer coefficient.The size is 0.9mm> 1.2mm> 0.6mm> 0.3mm in order.The analysis reason is that too small groove spacing will affect the continuous replenishment of the bottom of the groove by the liquid.When the groove depth is large,the groove spacing will be affected by the bubble merger zone.The subsequent impact caused the liquid to not immerse into the nucleation point in time,increasing the frequency of bubble detachment.That is,too large or too small pitch will increase the frequency of bubble separation,thereby reducing heat transfer efficiency.When 0.3mm <slot width <0.9mm,under the same slot width,the heat transfer effect of the surface with a larger slot depth is relatively good,because when the slot depth is larger,the temperature required to generate the gasification core is easier to reach However,once the bubbles are generated and smoothly detached from the surface,it indicates that the nuclear boiling has started smoothly.Therefore,a surface with a large groove depth can smoothly start nuclear boiling at a low superheat degree for efficient heat exchange.When the groove width is 1.2mm,the heat transfer effect on a surface with a groove depth of 1.5mm is not as good as a surface with a groove depth of 0.6mm.The reason for the analysis is that too large a distance will alleviate the impact caused by the coalescence of bubbles in the groove.Complementing the bottom of the trench occupies a dominant position,while the increase in heat transfer area occupies a secondary position.Obviously,the surface with a groove depth of 0.6mm is the easiest to get wet,which reduces the waiting time for liquid inflow and bubble generation,and improves heat exchange efficiency.In addition,the two parameters of the gasification core area enhancement ratio ?1 and the heat transfer area improvement ratio ?2 are introduced.It is found that as ?1 increases,the heat transfer effect increases first and then decreases,that is,the larger the area where bubbles cannot be generated,The better the corresponding heat exchange effect.When the gasification core area improvement ratio is 20%-30%,the heat exchange effect is the best.The relationship between the heat transfer coefficient and ?2 is not large,but when the groove width is constant,the heat transfer coefficient is generally proportional to ?2,because the limitation of the groove width,the bubbles cannot be completely related to all of them during the escape process.The solid-liquid contact surfaces are in contact,and the calculated heat transfer area at this time is not the entire effective area.At the same time,the heat transfer curves of the single(multi-scale)heat transfer surface,smooth surface and laser scanning surface show that the heat transfer effect of multi-scale T-shaped surface is better than single scale,and its average heat transfer coefficient is single scale surface.1.31 times,2.38 times the smooth surface.The heat transfer effect of the laser scanning surface is the worst,which is about 16% lower than that of the smooth surface.The analysis of bubble detachment frequency on multi-scale and single-scale surfaces shows that the former has a higher detachment frequency than the latter.This is because there are more sub-micron or nano-scale pits on the multi-scale surface after two processing.These structures can provide more favorable locations for bubble nucleation,which can further increase the bubble nucleation points,and the laser-processed pit structure can induce liquid to flow into the nucleation points,thereby shortening the cycle from the formation to the surface of the bubbles.In addition,the state of the bubbles on the surface was also observed.It was found that on reaching a saturated boiling point,bubbles would form independently and continuously escape on a smooth surface.On the laser scanning surface,adjacent bubbles will gradually combine to form a gas film and cover the surface.The above-mentioned air affinity phenomenon is also the reason that the heat transfer coefficient is significantly lower than that of a smooth surface.Finally,the existing theoretical formulas are used to analyze the different requirements of microstructure size for the escape of air bubbles and the inflow of liquids.Therefore,based on increasing the core area of gasification,taking into account the effect of capillary force on liquid inhalation,it is the key to improving heat transfer efficiency.Finally,wetting theory explains that the hydrophobicity existing on the laser-scanned surface is the main reason to promote bubble merger. |
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