Special Topic: Tubes and Pipes
Increased Energy Production Through the Use of Hydrophobic Surface Enhanced Tubes
To sustain world growth, energy use must increase. However, if increased energy use is not accompanied by energy conservation, the increased energy use may create environmental problems.
Part Two of this two-part article examines experimental results of condensation heat transfer enhancement using hydrophobic stainless steel tubes. It includes an evaluation of the effect of hydrophobicity for tube-side condensation heat transfer. This is accomplished by comparing the performance of the enhanced hydrophobic( HYD) tube to a smooth tube.
By David Kukulka, Director of Engineering Development, Rigidized Metals Corporation
Experimental Details
Figure 1 details the experimental apparatus used in this study. An image of the hydrophobic enhanced surface studied here is presented in Figure 2 with surface scans of the hydrophobic surface shown in Figure 3. When tube-side condensation is studied, the test section( as can be seen in Figure 1) of the apparatus is composed of a horizontal counter-flow heat exchanger. Refrigerant flows in the enhanced tube being evaluated, and cooling water is flowing in the external annulus around the enhanced tube. After the test section, the flowrate of the cooling water is measured using a mass flowmeter and the water is returned to the constant temperature water tank. The temperature here is measured using a Pt100 platinum resistance temperature sensor. The refrigerant is heated to a predetermined temperature and quality before it enters the test section. In the condenser, the refrigerant flowing from the test section is completely condensed and subcooled. Additional details regarding the experimental setup and procedure are found in Li et al.( 2021). 1
The purpose of this study was to investigate the impact of hydrophobicity on heat transfer tubes. This study utilized R32 refrigerant to study an enhanced hydrophobic heat transfer tube( HYD) and compared performance to a smooth tube( ST). This lays the foundation for the development of enhanced hydrophobic surfaces that are used in heat transfer applications.
Figure 2: Surface image of the hydrophobic enhanced surface tube.
Tube Details
This study examines the impact of hydrophobicity on the surface of a condensation heat transfer tube and includes a comparison of a smooth tube( ST) and a hydrophobic( HYD) tube. It is important to note that the HYD surface used in this study is not a coating, but it is mechanically produced on the surface of the tube. The sessile drop method is used to optically determine the contact angle between the liquid and a solid surface; contact angles( see Figure 4) of 107.35 ° are found for the HYD tubes and 79.06 °
False color view and height parameters
3D view – Amplified 10 %
ISO 25178 Height Parameters
Sq( μm) 8.024
Ssk-0.9065
Sku 6.986 Sp( μm) 71.17 Sv( μm) 119.9 Sz( μm) 191.1 Sa(µ m) 6.058 Aev( mm 2) 100.0
Figure 3: Surface Scans of the Enhanced Hydrophobic Heat Transfer Surface –( a) Height Parameters,( b) Three-Dimensional View.
Smooth Tube 2B Smooth Tube B / A Hydrophobic Surface
81.32O ° 79.06O ° 107.35O °
Figure 4: Comparison of Contact Angle Measurements( °).
Figure 1: Schematic of the experimental setup.
6 Stainless Steel World Americas- April 2025 | www. ssw-americas. com for the smooth tube. All of the tubes are constructed from stainless steel, with an outer diameter of 12.7 mm and inner diameter of 11.5 mm.
Effect of Subcooling on Hydrophobicity
Figure 5 illustrates the variation of the heat transfer coefficient with flowrate( G) for the ST and HYD tubes. In general, as the subcooling temperature difference increases, the growth trend of the heat transfer coefficient gradually slows down. This trend is more pronounced at the saturation temperature of 45 °℃. This demonstrates that an increase in subcooling temperature( i) impacts the detachment rate of the liquid,( ii) increases droplet accumulation,( iii) leads to thicker liquid film,( iv) increases thermal resistance of the upper wall surface in the microchannel, and( v) weakens the heat transfer effect. The saturation temperature of the ST at 35 °℃ shows the opposite trend. This is due to the smooth surface of the ST, which lacks an enhanced surface and is less affected by the supercooling temperature.
SSWAM april 2025. indd 6 23-04-2025 17:26