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.     

Spreading dynamics of droplet on an inclined surface

A three-dimensional unsteady theoretical model of droplet spreading process on an inclined surface is developed and numerically analyzed to investigate the droplet spreading dynamics via the lattice Boltzmann simulation. The contact line motion and morphology evolution for the droplet spreading on an inclined surface, which are, respectively, represented by the advancing/receding spreading factor and droplet wetted length, are evaluated and analyzed. The effects of surface wettability and inclination on the droplet spreading behaviors are examined. The results indicate that, dominated by gravity and capillarity, the droplet experiences a complex asymmetric deformation and sliding motion after the droplet comes into contact with the inclined surfaces. The droplet firstly deforms near the solid surface and mainly exhibits a radial expansion flow in the start-up stage. An evident sliding-down motion along the inclination is observed in the middle stage. And the surface-tension-driven retraction occurs during the retract stage. Increases in inclination angle and equilibrium contact angle lead to a faster droplet motion and a smaller wetted area. In addition, increases in equilibrium contact angle lead to a shorter duration time of the middle stage and an earlier entry into the retract stage.     

Free convection in tilted rectangular enclosures heated at the bottom wall and vented by different slots-venting arrangements

Free convection from a tilted rectangular enclosure heated at the bottom wall and vented by uniform slots opening at different walls of the enclosure was experimentally investigated. The experiments were carried out to study the effects of venting arrangement, opening ratio and enclosure's tilt angle on the passive cooling of the enclosure. The experiments were carried out at a constant heat flux of 250 W/m² and for enclosure tilt angles ranging from 0° to 180°. Three different venting arrangements of the air from the enclosure were studied: (1) top-venting arrangement, (2) side-venting arrangement, and (3) top and side-venting arrangement. Each venting arrangement was studied at different opening ratios of 1, 0.75, 0.5 and 0.25. The results showed that: (1) for top-venting arrangement, the Nusselt number decreases as the tilt angle of the enclosure increases, (2) for side-venting and side and top-venting arrangements, the Nusselt number increases as the tilt angle increases in the range [0°, 90°], then it decreases with the increase of the tilt angle, (3) for the three venting arrangements and at any tilt angle, the Nusselt number increases with the increase of the opening ratio of the slots, (4) for any tilt angle and at any opening ratio, the top and side-venting arrangement has the highest rate of cooling of the enclosure, and (5) for small tilt angles, the rate of cooling of the enclosure for top-venting arrangement was higher than that for side-venting arrangement, but with increasing tilt angle, the rate of cooling for side-venting arrangement becomes higher than that for top-venting arrangement. Correlations were developed for the three venting arrangements to predict the average Nusselt number of the enclosure in terms of the opening ratio and the enclosure tilt angle.     

New printed circuit heat exchanger with S-shaped fins for hot water supplier

A new PCHE with an S-shaped fin configuration was applied to a hot water supplier in which cold water of 7 °C is warmed to 90 °C through heat-exchange with supercritical CO₂ of 118 °C and 11.5 MPa pressure. The fin and plate configurations were determined using 3D CFD simulations for the CO₂ side and H₂O side and the thermal–hydraulic performance of hot water supplier was evaluated. Compared with a hot water supplier that is currently used in a residential heat pump, the new PCHE provides about 3.3 times less volume; and lower pressure drop by 37% in the CO₂ side and by 10 times in H₂O side.     

梯度润湿表面上液滴定向迁移及合并行为的格子Boltzmann模拟

采用改进的格子Boltzmann方法，对梯度润湿性表面上液滴的定向迁移及合并行为进行了数值模拟，该模型在精度和稳定性上都有很大改善，同时，研究了梯度润湿性表面上液滴定向迁移和合并的动力学特性，并对液滴尺寸及润湿梯度对液滴动力学特性的影响规律进行了分析。数值结果表明，液滴在梯度润湿性表面运动时会发生形变，且动态接触角逐渐减小。润湿梯度对液滴定向迁移行为有显著影响，润湿梯度越大，液滴左右侧接触线位移越大，润湿长度增加越快。但是液滴尺寸对接触线位移影响较小。润湿梯度对液桥宽度基本无影响，但对液滴初始合并时间有显著影响。     

Air-side performance of louver-finned flat aluminum heat exchangers at a low velocity region

The heat transfer and pressure drop characteristics of heat exchangers having louver fins were experimentally investigated. The samples had small fin pitches (1.0–1.4 mm), and experiments were conducted up to a very low frontal air velocity (as low as 0.3 m/s). Below a certain Reynolds number (critical Reynolds number), the fall-off of the heat transfer coefficient curve was observed. The critical Reynolds number was insensitive to the louver angle, and decreased as the louver pitch to fin pitch ratio (L _p /F _p ) decreased. Existing correlations on the critical Reynolds number did not adequately predict the data. The heat transfer coefficient curves crossed over as the Reynolds number decreased. Possible explanation is provided considering the louver pattern between neighboring rows. Different from the heat transfer coefficient, the friction factor did not show the fall-off characteristic. The reason was attributed to the form drag by louvers, which offsets the decreased skin friction at low Reynolds numbers. The friction factor increased as the fin pitch decreased and the louver angle increased. A new correlation predicted 92% of the heat transfer coefficient and 94% of the friction factor within ±10%.     

Experimental investigation of inclined liquid water jet flow onto vertically located superhydrophobic surfaces

In this study, the behaviour of an inclined water jet, which is impinged onto hydrophobic and superhydrophobic surfaces, has been investigated experimentally. Water jet was impinged with different inclination angles (15°–45°) onto five different hydrophobic surfaces made of rough polymer, which were held vertically. The water contact angles on these surfaces were measured as 102°, 112°, 123°, 145° and 167° showing that the last surface was superhydrophobic. Two different nozzles with 1.75 and 4 mm in diameters were used to create the water jet. Water jet velocity was within the range of 0.5–5 m/s, thus the Weber number varied from 5 to 650 and Reynolds number from 500 to 8,000 during the experiments. Hydrophobic surfaces reflected the liquid jet depending on the surface contact angle, jet inclination angle and the Weber number. The variation of the reflection angle with the Weber number showed a maximum value for a constant jet angle. The maximum value of the reflection angle was nearly equal to half of the jet angle. It was determined that the viscous drag decreases as the contact angle of the hydrophobic surface increases. The drag force on the wall is reduced dramatically with superhydrophobic surfaces. The amount of reduction of the average shear stress on the wall was about 40%, when the contact angle of the surface was increased from 145° to 167°. The area of the spreading water layer decreased as the contact angle of the surface increased and as the jet inclination angle, Weber number and Reynolds number decreased.     

Infiltration of liquid droplets into porous media: Effects of dynamic contact angle and contact angle hysteresis

We present a detailed theory for infiltration, which accounts for a general model for the dynamic contact angle between the droplet and the porous medium as well as contact angle hysteresis, and analyze the resulting equations of motion. The theory shows that infiltration of droplets into dry porous media involves three phases due to contact angle hysteresis: (1) An increasing drawing area (IDA) phase during which the interface between the droplet and the porous medium increases, (2) a constant drawing area (CDA) phase during which the contact line of the droplet remains pinned, and (3) a decreasing drawing area (DDA) phase. The theory is based on the following assumptions: (1) The droplet has the shape of a spherical cap, (2) the porous medium consists of a bundle of vertical tubes of same size, and (3) the pressure within the droplet is uniform. We find that infiltration always consists of a cascade process formed by the IDA, CDA, and DDA phases, where the entire process may begin or end in any of the three phases. The entire process is formulated with four nondimensional parameters: Three contact angles (initial, advancing, and receding) and a porous permeability parameter. A comparison of our theory to experimental data suggests that one should use different parameterizations for the dynamic contact angle models of the IDA and DDA phases. In general, the IDA and DDA phases are described by integro-differential equations. A numerical-solution approach is presented for solving the dynamic equations for infiltration. The total time of infiltration and the time dependence of drawing area are critically affected by the occurrence of the IDA, CDA, and DDA phases as well as by the permeability. With ordinary differential equations (ODEs), we are able to approximate the IDA phase and to describe exactly infiltration processes that start out with the CDA or DDA phase.     

Numerical and experimental study of turbulent impinging twin-jet flow

The two dimensional impinging circular twin-jet flow with no-cross flow is studied numerically and experimentally. The theoretical predications are carried out through numerical procedure based on finite volume method to solve the governing mass, momentum, turbulent kinetic energy and turbulent kinetic energy dissipation rate. The parameters studied were jet Reynolds number (9.5 × 10⁴  Re  22.4 × 10⁴), nozzle to plate spacing (3  h/d  12), nozzle to nozzle centerline spacing (l/d = 3, 5 and 8) and jet angle (0°  θ  20°). It is concluded that the stagnation primary point moves away in the radial main flow direction by increasing the jet angle. This shift becomes stronger by increasing the nozzle to nozzle centerline spacing (l/d). A secondary stagnation point is set up between two jets. The value of pressure at this point decreases by decreasing Reynolds number and/or increasing the jet angle.

The sub atmospheric region occurs on the impingement plate. It increases strongly by increasing Reynolds number and decreases as the jet angle and/or a nozzle to plate spacing increases. The spreading of jet decreases by increasing nozzle to plate spacing. The intensity of re-circulation zone between two jets decreases by increasing of h/d and jet angle. The increase of turbulence kinetic energy occurs within high gradient velocity.     

Numerical analysis of secondary droplets characteristics due to drop impacting on 3D cylinders considering dynamic contact angle

This study is investigating three-dimensional numerical simulation of a Newtonian droplet impact and break on two square cylinders based on dynamic contact angle of droplet at the spatial interface between two solid–fluid phases. The droplet impact details and morphology studied in the present work could provide ideas for the spray wall impingement modeling in the simulation of many industrial applications, such as spray painting and liquid cooling of surfaces. The droplet impact is investigated on two square cylinders in 9 different modes with different droplet diameters and physical conditions such as different positions of droplet. The volume of fluid (VOF) method was used with open-source software. The results have been compared and validated quantitatively and qualitatively with the experimental results. Results represent droplet diameter into cylinder dimension and velocity profiles are affected on number of broken droplets, break times and droplet deformation. Also, mean velocities of droplet after impact on two square cylinders at first break time were 0, 0.025, 0.12, 0.47, 0.11, 0.08, 0.2, 0.012, 0.19 m/s for cases 1–9, respectively. Moreover, in case 7 that droplet diameter into cylinder dimension was 2, the maximum number of break-up into secondary droplets was 10 drops that occurred for 4 times.

     

Effect of fin thickness on air-side performance of herringbone wavy fin-and-tube heat exchangers

In the present study, the effects of fin thickness on the heat transfer and friction characteristics of fin-and-tube heat exchangers having herringbone wavy fin configuration are experimentally investigated. The experimental apparatus consists essentially of a well insulated open wind tunnel and herringbone wavy fin-and-tube heat exchangers made from aluminium plate finned, copper tube. Air and water are used to be working fluids in air-side and tube-side, respectively. A total of 10 samples of the fin-and-tube heat exchangers are tested. The experimental procedures are conducted by keeping the inlet water temperature at a pre-selected value, adjusting the water volumetric flow rate at a specific value and varying the air velocity. The results are presented as plots of the Colburn factor and friction factor against the Reynolds number based on the fin collar outside diameter (Re_Dc). From the results, it is found that for number of tube rows (N) = 2, the Colburn factor increases with increasing fin thickness. For N 4, the Colburn factor decreases with increasing fin thickness when Re_Dc < 1800, and increases with increasing fin thickness when Re_Dc > 2500. The friction factor increases with increasing fin thickness when fin pitch (F_p) 1.81 mm.