Dropwise condensation on superhydrophobic nanostructured surfaces
It is well established that the dropwise condensation (DWC) mode can lead up to several hundred percent enhancement in heat transfer coefficients as compared to the filmwise mode (FWC). Typically, hydrophobic surfaces are expected to promote DWC, while hydrophilic ones induce FWC.
An attractive feature of using superhydrophobic surfaces is to facilitate easy roll-off of the droplets as they form during condensation, thus leading to a significant improvement in the heat transfer associated with the condensation process.
|Θeq = Static Contact Angle
Θad = Advancing Contact Angle
Θre = Receding Contact Angle
ΔΘ = Θad – Θre = Contact Angle Hysteresis
|Hydrophilic surfaces: Θeq < 90°
Hydrophobic surfaces: Θeq > 90°
Superhydrophobic surfaces: Θeq > 150°
Cassie-Bexter wetting regime: high droplet mobility with very low hysteresis. High hysteresis means high adhesion of the drop on the substrate, which decreases the effective superhydrophobic properties even with very high Θeq.
Wenzel wetting regime: vapor could condense between the surface textures. The droplet mobility can be favored by vapor velocity.
|TREATED SAMPLES: Θeq = 159 ± 2° (SUPERHYDROPHOBIC)|
|Flow visualization||Increasing the vapor velocity the size of departing droplets decreases and the frequency increases.
Reducing the droplets departing diameter leads to thermal performance enhancement, due to the lower thermal resistance of the smaller drops.
Heat transfer and pressure drop in single microchannels
|Micro- and minichannels are increasingly being used to achieve high heat transfer rates with compact heat exchangers.||Square channel cross section|
|View of the experimental apparatus with square minichannel inclined of 60°||Condensation inside small diameter channels finds applications in heat pipes and compact heat exchangers for electronic equipment or spacecraft thermal control, in automotive condensers, in residential air conditioning and in refrigeration applications.|
|The experimental apparatus consists of a single circular minichannel with an internal diameter of 0.96 mm and square minichannel with an hydraulic diameter of 1.23 mm. In these two section the heat transfer coefficients during condensation and evaporation are measured and also the pressure drops during adiabatic flow.
In the experimental apparatus there are other two test sections, consisting of two circular minichannel with internal diameter of 0.96 and 2 mm.
|View of the test rig with square minichannel in vertical position|
|View of the whole test rig||The experimental apparatus allow to perform measurements of local heat transfer and pressure drop at different inclinations both in upflow and downflow configurations and so to investigate the effects of shear stress, gravity and surface tension in non horizontal positions.|
|Circular minichannel with internal diameter of 0.96 mm to measured the pressure drop|
|Picture of the circular minichannel test section|
|(Left) Enlarged image of the cross-section of the .n sample where the wall temperature is measured. Junction of the TC is electrically insulated and is glued in its position with high thermally conductive glue. (Right) Contours of wall temperature around the thermocouple in the copper tube.|
|View of the circular minichannel with the coolant .ow passage geometry|
|Experimental test rig||The test rig used for the experimental tests is depicted in the figure. It consists of the primary (refrigerant) loop and of three auxiliary loops: the two cooling water loops and the hot water loop. The subcooled refrigerant from the post-condenser is sent through a mechanical filter and a dehumidifier into an independently controlled gear pump, which is magnetically coupled to a variable speed electric motor.
When the test apparatus is operated for condensation measurements, the fluid is pumped through the Coriolis-effect mass flow meter into the evaporator where the refrigerant is vaporized and superheated in a tube-in-tube heat exchanger.
Depending on the experimental section that is used heat transfer coefficients during condensation or vaporization, or pressure drop in adiabatic flow are evaluated. The fluid then returns to post-condenser where it is completely condensed and from which comes out subcooled. The experimental apparatus has two baths. The first provides the necessary thermal power in the pre-section and in the measuring section, the second one, set at a constant temperature, provides the thermal power to post-condenser. The plant is also connected to an expansion vessel externally pressurized with nitrogen which allows to control the pressure. As regards the materials, the experimental and the evaporator sections are made of copper while, the rest of the system is made of stainless steel.
- bath THERMOHAAKE CT 50W
- bath LAUDA PROLINE RP 1845
- gear pump ISMATEC MCP-Z
- voltage stabilizer
- temperature controller EUROTHERM 3216
- pressure vessel
- electric heater
- n. 2 tension transformers
- differential pressure transducer ROSEMOUNT 3051S
- N°2 relative pressure transducers ROSEMOUNT 3051S
- differential pressure transducer DRUCK
- relative pressure transducer DRUCK
- ice point reference KAYE K170
- data acquisition system NATIONAL INSTRUMENTS NI SCXI-1000
- acquisition software NATIONAL INSTRUMENTS LabVIEW 6
- n. 2 Coriolis effect mass flow meters ENDRESS+HAUSER
- Coriolis effect mass flow meter MICRO MOTION
Heat transfer and pressure drop in microfinned tubes
Air-cooled and water-cooled condensers with in-tube condensation and vaporization of refrigerants are widely used in water chillers, air-conditioners and heat pumps.
Since the end of 1970s, it has become common to enhance heat transfer inside horizontal tubes in heat exchangers for air conditioners by employing finned tubes.
Enlarged image of the fins in the tested microfin tube
|Microfin tubes are typically made of copper and have an outside diameter from 4 to 15 mm, a single set of 50-70 fins with helix angle from 6 to 30°, fin height from 0.1 to 0.25 mm, triangular or trapezoidal fin shapes with an apex angle from 25 to 90°.
Other enhanced surfaces have also been developed; such as cross-grooved tubes that present an additional set of grooves with opposite helical orientation, and herringbone tubes which are characterised by V-shaped grooves.
|The heat transfer enhancement in microfin tubes is partly due to the increase in the effective exchange area, and additionally to the turbulence induced in the liquid film by the microfin.|
Since 2002, experimental heat transfer coefficients during condensation and evaporation in microfin tubes were measured for R22, R134a, R407C and R410A. The heat transfer measurements were compared against performance of an equivalent smooth tube under the same operating conditions, to show the advantages of the microfin tube as compared to the plain tube.
The experimental tests were carried out in a broad range of operating temperatures to enlighten the influence of vapour quality, mass velocity and saturation temperature on the heat transfer coefficient. Comparisons with several models available in the literature were performed.
Pressure drop data have also been taken by means of a differential pressure transducer during two-phase adiabatic flow inside the same microfin tube.
A dedicated test section is set up for flow patterns visualization inside the test tube. The chamber for flow visualization has three circular tempered glass windows (thickness 20 mm). Flow regimes are recorded using a digital video camera equipped with a special macro-lens that allows high resolution and the magnification of very small areas.