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. Carbon neutrality is an important goal to consider when evaluating energy use; organizations trying to achieve carbon neutrality are exploring ways to optimize energy use, minimize waste energy, and minimize carbon use. Energy conservation must be addressed in current (retrofit) and future designs. One method of conserving energy and maximizing energy use is with enhanced heat transfer surfaces. Enhancing heat transfer and increasing the efficiency of heat exchangers are essential in trying to maximize heat transfer performance.

Part One of this two-part article explores the enhancement of heat transfer and condensation performance through the establishment of hydrophobic structures.

By David Kukulka, Director of Engineering Development, Rigidized Metals Corporation

Condensing heat exchangers are widely used in various applications (power, HVAC, process industry, etc.), and the methods for improving energy efficiency are objectives in most designs. A variety of passive methods are used to enhance heat transfer, including (i) producing a heat transfer surface that increases heat transfer performance, (ii) increasing heat transfer surface area – extended surfaces and/or enhanced surfaces, (iii) embedded enhancement devices, (iv) coil enhancement, and (v) fluid additives. To enhance condensation performance, it is desired to minimize the resistance that the liquid film at the heat transfer surface produces. One way to do this is to enhance droplet condensation. This can be accomplished through the creation of hydrophobic structures (low free energy surface) on the heat transfer surface. A hydrophobic surface can be produced by creating micro- and/or nanostructured structures on the engineered heat transfer surface/tube.

Results include heat transfer measurements over a variety of conditions. As the mass flow rate increases, the heat transfer coefficient of the hydrophobic tube increases by approximately 20% (when compared to a smooth tube). This enhancement is attributed to the hydrophobic structure that is created. This surface promotes droplet condensation on the inner surface of the tube; it increases the droplet detachment rate and increases the heat transfer efficiency.

Introduction

Increasing energy use and the production of greenhouse gases (the result of using many sources of energy) have created environmental problems that must be addressed in current and future designs. Carbon neutrality is an important goal that needs to be considered when evaluating energy use; organizations and countries are exploring ways to optimize energy and minimize carbon use. To sustain the development of today’s world, energy conservation is needed. As discussed in Kapustenko et al. (2023),¹ developed countries dominate carbon dioxide emissions in the area of energy production, and thermal power generation is responsible for the highest carbon dioxide production. Enhancing heat transfer and increasing the efficiency of heat exchangers are essential in trying to achieve carbon-neutral designs.

Surface Structure

Several previous studies have explored ways of enhancing the surface structure of a heat transfer surface to produce more energy, while investigators are studying the use of hydrophobic surfaces. Water droplets bead up easier on hydrophobic surfaces; a surface that repels water has a low surface energy. Contact angle measurements are used to classify the hydrophobic character of a surface. A surface is said to be hydrophobic when the contact angle of a water droplet exceeds 90 degrees. If the contact angle is greater than 150 degrees, it is considered superhydrophobic. (See Figure 1 for a classification of surfaces using contact angle of a drop of water on a surface).

Hydrophobic surfaces are rough (on the micro and nanometer scale). This prevents water from spreading out or being absorbed into the surface. There are a couple ways of creating a hydrophobic surface: one way is to put a coating on the surface, while a better method is to mechanically alter the surface and create a hydrophobic surface. Hydrophobic coatings are used in many consumer products (i.e., outdoor clothing, Teflon cookware, etc.). However, hydrophobic coatings (applied to process/heat transfer surfaces) can deteriorate over time due to fluid flow, use, wear, erosion, etc. A typical metal surface (2B mill finish) can be altered mechanically to create a hydrophobic surface using a process that mechanically alters the surface by rolling a microtexture on the metal’s surface. Figure 2 shows a comparison (contact angle measurements) of typical (mill) metal surfaces with the mechanical altered hydrophobic surface. As can be seen, the contact angle of the hydrophobic surface is substantially enhanced.

Condensing heat exchangers are widely used in air conditioning systems. Typically, the most effective way to enhance condensation is to thin the liquid film that is on the heat transfer surface. A variety of passive methods are used to enhance heat transfer, including (i) textured engineered heat transfer surfaces, (ii) extended surfaces, (iii) embedded enhancement devices, (iv) enhancement coils, and (v) fluid additives. Use of a hydrophobic surface (a specialized engineered enhanced heat transfer surface) is an important method to consider in order to enhance droplet condensation designs; these hydrophobic designs have a low free energy surface.

Rykaczewski et al. (2012)⁽²⁾ investigated the process of droplet formation on nanostructured superhydrophobic surfaces. They proposed a model that quantitatively describes the growth of droplets. Currently, there have been relatively few studies conducted on hydrophobic-enhanced composite structures. In their study, Yao et al. (2014)⁽³⁾ established a hydrophobic-hydrophilic hybrid surface with micro arrays; this was part of their study on wetting behavior. Experimental results showed that the micropillar spacing ratio was a key factor affecting the results of droplet condensation on the hybrid surface. When the spacing between the micropillars was less than 50 μm, the droplets that had aggregated were dislodged from the surface. However, when the spacing between the micropillars was approximately 50 μm, the agglomerated droplets filled the valleys of the mixed surfaces and formed a thin liquid film. Egab et al. (2020)⁽⁴⁾ compared the effect of hybrid wettability on the enhancement of film and droplet condensation. This thin liquid film reduces thermal resistance and increases the condensation rate.

Modifying the surface structure of a tube is a passive enhancement method that requires additional study. For many areas there is a lack of knowledge regarding the performance of hydrophobic surfaces. To produce efficient designs, an experimental performance analysis must take place to obtain that information. It is impossible to rely solely on a theoretical analysis or a numerical analysis to optimize a design with these novel surfaces.

Part Two of this two-part article will present some of the results for hydrophobic tubes.