Measurements of the contact angle between R134a and both aluminum and copper surfaces

Measurements of quasi-static advancing contact angles of refrigerant R134a on copper and aluminum surfaces are reported over a temperature range from 0 °C to 80 °C. The metal surfaces tested were aluminum (alloy 3003) and copper (alloy 101) plates. Measurements were done using a direct optical observation technique where the liquid meniscus at the surface of a vertical plate was captured using a high magnification camera system. The contact angle of solid–liquid interface was deduced by enhancing and manipulating the digital image using solid modeling software by drawing a tangent line to the meniscus at the intersection location of the solid, liquid and vapor. Values of the contact angle were found to vary between 8.3° and 5.6° for aluminum and between 5.1° and 6.5° for copper when the temperature rose from 0 °C to 80 °C. Maximum standard deviation amongst the measured values of contact angles was 1.3°.     

粗糙表面上微液滴的数值模拟

本文使用VOF方法将微液滴在粗糙壁面上的接触现象转化为不可压缩两相流动问题,并对其进行三维数值模拟.选择具有柱形突起和槽道两种微结构的壁面进行模拟.计算了不同粗糙系数时液滴在突起结构表面的静止形态和接触角,计算结果和实验数据吻合得较好.和理论模型进行比较,分析了经典模型的适用范围.对于微槽道结构的壁面,计算给出不同方向测量得到的液滴接触角.实现了液滴在倾斜壁面上滑落过程的模拟.液滴沿斜面下滑时,前进角和后退角的变化存在周期性,这一周期性变化和表面粗糙结构密切相关.     

Effects of contact angle on condensate topology, drainage and efficiency of a condenser with minichannels

Epoxy coatings with good adhesion characteristics have been developed that are suitable for large-scale manufacturing and application in compact heat exchangers. Two of them, with a static contact angle for water of 105° and 79° have been tested in a finned-tube condenser comparing flat plate minichannels on the gas-side. Contrary to the expectation, the thermal efficiency of the 105° condenser is slightly less than that of the 79° one. This is due to a reduction of condensate drop size at detachment, resulting in relatively small dry paths after drainage. In the present study, geometry and coating thickness of the two coated exchangers used have not been optimized, since emphasis has been on the effect of contact angle. The finned-tube geometry is shown to be less favourable, for a condenser, than the compact cross-flow plate geometry with minichannels on both the gas- and coolant side that was tested previously.     

Thermal behavior of spiral fin-and-tube heat exchanger having fly ash deposit

This research investigates the effect of fly-ash deposit on thermal performance of a cross-flow heat exchanger having a set of spiral finned-tubes as a heat transfer surface. A stream of warm air having high content of fly-ash is exchanging heat with a cool water stream in the tubes. In this study, the temperature of the heat exchanger surface is lower than the dew point temperature of air, thus there is condensation of moisture in the air stream on the heat exchanger surface. The affecting parameters such as the fin spacing, the air mass flow rate, the fly-ash mass flow rate and the inlet temperature of warm air are varied while the volume flow rate and the inlet temperature of the cold water stream are kept constant at 10 l/min and 5 °C, respectively.

From the experiment, it is found that as the testing period is shorter than 8 h the thermal resistance due to the fouling increases with time. Moreover, the deposit of fly-ash on the heat transfer surface is directly proportional to the dust–air ratio and the amount of condensate on heat exchange surface. However, the deposit of fly-ash is inversely proportional to the fin spacing. The empirical model for evaluating the thermal resistance is also developed in this work and the simulated results agree well with those of the measured data.     

Mesoscale investigation of frost formation on a cold surface

Frost formation on a horizontal flat copper surface was experimentally investigated using microscopic observations. The experiments were carried out on −20 to 0 °C copper surfaces with 22 °C air and 15–85% relative humidities. The experiments showed that the frost formation on a cold surface generally begins with the formation and growth of condensate droplets, freezing of the super-cooled condensate droplets, formation and growth of initial frost crystals on the frozen droplets, growth of frost crystals accompanied by the collapse of some of the crystals, and finally frost layer growth. The freezing onset time and diameter of the super-cooled condensate droplets were characterized. The initial frost crystals can be classified into four groups according to their appearance and shape, with the variations of the frost crystal shape as a function of the cold surface temperature and air humidity.     

Effects of oxidation and surface roughness on contact angle

Contact angle is known to be a parameter that effects boiling. This study was undertaken to measure contact angle of high and low surface tension fluids on copper and aluminum surfaces.Data were taken for polished, oxidized, and rough surfaces. A simple, yet fairly accurate method of measuring the static equilibrium contact angle of a solid/liquid interface is presented. The principles of a line light source and tilting plate were modified and then combined in the design of this apparatus. The angles obtained and their variation with the solid surface properties were in good agreement with previously published data. The contact angle of distilled water o of the organic fluids and refrigerants tested were in the range of 2–5°. Roughness and oxidation reduce the contact angle. If the depth of the roughness is less than 0.5 μm contact angle. The apparatus is fairly simple in construction, is inexpensive, and has good reproductibity. The measured angles were then compared to those measured with the sessile drop method.     

Investigation of the possibility of frost release from a cold surface

An experimental study has been conducted to investigate the possibility of causing frost to release from a cold surface. Both hydrophilic and hydrophobic surfaces were examined. Mechanical vibrations were attempted to release the frost from the hydrophobic surface. A thermoelectric cooler (TEC) was used to provide a cooling source for the frosting surface. A 40 mm square test sample of aluminum fin stock, 0.25 mm thickness, was attached on the cold side of the TEC with thermal grease. A uniform frost pattern was observed on the hydrophilic surface. However, the frost pattern on the hydrophobic surface was non-uniform and “pock-marked”. The frost on the hydrophobic surface could not be released by surface vibration. The melted frost (condensate) on the hydrophilic surface wetted the surface. However, the condensate existed as droplets on the hydrophobic surface. It appears that hydrophilic coatings should not be applied to the surfaces of evaporators that experience freezing condition, because when the frost melts, the fully wetted hydrophilic surface may act to retain condensate, which immediately re-freezes after defrosting. If the evaporator must operate under either freezing or non-freezing conditions, a hydrophilic coating is probably preferable.     

Retraction and Adhesion of a Single Droplet Normal Impact on the Solid Surface

The impact behavior of individual biomass oil droplets was investigated on solid surfaces having different structures (flat, cylindrical, and spherical) using the high-speed video technique. This makes it possible to compare the evolution of the droplet impact on various surface structures. The impact behaviors of retraction–oscillation and adhesion are analyzed for different hydrophobic surfaces. The influence of the Weber number (We), the surface structure, and the surface curvature is further examined by focusing on the retraction and stable adhesion (thickness, adhesion, and contact angle) for different biomass oil droplets. The results show that the retraction factor gradually increases as We increases to some critical value, beyond which the increase rate slows down or the retraction factor begins to decrease. The largest retraction factor is observed on the flat surface and the smallest one appears on the spherical surface. The adhesion thickness of the liquid film oscillates periodically over time, and its oscillation amplitude gradually decreases with a constant frequency, which is smaller for the more hydrophobic surfaces. The curvatures of the cylinder and sphere have little influence on the stable adhesion behavior. For the different droplet types, the adhesion diameter on the flat surface gradually increases as We rises, whereas the adhesion thickness gradually decreases with increase in We. These results are helpful for understanding the impact behaviors of biomass oil droplets with high viscosity and small surface tension on solid surfaces.

     

Compositive effects of orientation and contact angle on critical heat flux in pool boiling of water

An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact angle into account is established. The predicting correlation agrees reasonably well with the experimental data.