In this study, building upon previous research on fouling, ultrasonic equipment was integrated with a self-developed pool-boiling experimental setup to investigate the scale inhibition and anti-fouling performance of ultrasonic waves in high-concentration CaCO3 solutions under various process conditions. The experimental setup included a pool boiling device (comprising a heating section and a boiling section), an ultrasonic system, and a data acquisition system.
The pool boiling device featured a copper rod (diameter 0.07 m, height 0.1 m) enclosed in a stainless steel electric heating jacket, connected to a test copper column (diameter 0.03 m, height 0.06 m). A Teflon insulation pad was placed on the surface of the copper column to prevent the experimental medium from seeping into the edges. The heater's surface area was 9×10ⴠm². Experimental results focused on the heat transfer behavior of silica-free distilled water, where the heat transfer coefficient was measured at a specific heat flux. The timing began once the solution reached 100°C, with a heating power of 150W and an experimental temperature of 100°C.
Due to the intermittent nature of ultrasound application, the heat transfer coefficient fluctuated over time. Initially, the coefficient increased rapidly as more bubble nucleation sites formed on the heater surface due to rising heat flux. Overall, the application of ultrasound significantly enhanced the heat transfer coefficient. Mechanistically, power ultrasound introduces vibrational energy that alters physical, chemical, and biological properties of substances, accelerating processes. This energy weakens molecular bonds in the liquid and between molecules and the metal surface, increasing molecular mobility and enhancing heat transfer. The heat transfer coefficient curve clearly shows the anti-fouling effect of ultrasound.
From a theoretical perspective, ultrasonic radiation has three key effects on liquids: it creates shear stress at interfaces due to velocity differences, weakening molecular and metal surface bonding; it generates strong pressure peaks through cavitation, which accelerates Ca²+ precipitation and breaks down carbonate scale and impurities into fine particles; and it enhances chemical reactions under the high-temperature, high-pressure environment created by cavitation, altering scaling conditions.
The influence of fouling solution concentration on the heat transfer coefficient was also analyzed. Higher concentrations generally hinder heat transfer, but ultrasound improved performance in both high and low concentration scenarios. At 300 and 1200 mg/L, the heat transfer coefficient was higher, while at 600 and 900 mg/L, it was lower. High concentration increases supersaturation, and ultrasound boosts nucleation, promoting small particle formation and reducing surface scaling. Additionally, longer-lived H radicals enhance the peeling of scale deposits.
Ultrasonic radiation induces cavities and bubbles in the solution. When these collapse or interact, they generate strong pressure peaks that break down scale layers, making them easier to remove. Higher concentrations mean more supersaturation and scale, and stronger ultrasonic effects lead to greater removal. Beyond a critical point, the heat transfer enhancement from ultrasound outweighs the negative impact of scale, leading to increased heat transfer coefficients at high concentrations.
The effect of heat flux on the heat transfer coefficient was studied for a 1200 mg/L solution under different heating powers. At 180W, the coefficient increased by about 1.3 times compared to 150W. This is because higher heat flux increases active nucleation sites, creating more bubbles and boosting heat transfer. The effect is most pronounced in the early stages of heat transfer, with the coefficient eventually stabilizing at a lower value.
In conclusion, regardless of whether the solution contained scale, ultrasound significantly improved heat transfer. Increased heat flux raised the coefficient, especially during the initial phase. In the pool boiling setup, ultrasound effectively enhanced heat transfer in both low and high concentration fouling solutions. A higher heat transfer coefficient means less fouling. At high concentrations, the intense cavitation from ultrasound helps break down carbonate scale into suspended particles, preventing adhesion to the heater surface and reducing scale buildup.
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