In this work, on the basis of the above-mentioned study of fouling, the ultrasonic equipment was used in conjunction with a self-developed pool-boiling experimental device to perform experiments on the scale inhibition and anti-fouling effects of ultrasonic waves in high-concentration CaCO3 fouling solutions and under different process conditions. the study. Experimental Experimental Devices Experimental devices include pool boiling equipment (heating section and boiling section), ultrasonic equipment, and data acquisition system.
The pool boiling device heat source consists of a copper rod (diameter 0.07m, height 0.1m) jacket stainless steel electric heating jacket and is connected to the test copper column, wherein the test copper column is a copper column (diameter 0.03m, height 0.06m), on which A Teflon insulation pad was added on the surface to prevent the experimental medium from penetrating the edge of the copper column. The surface area of ​​the heater is 9×104 m2. Experimental results and analysis of the effect of ultrasonic on the heat transfer of the silica-free distilled water The heat transfer coefficient of the non-scaling distilled water at a certain heat flux was experimentally determined. The results are shown in The timing starts when the temperature of the solution reaches 100°C, the same below. Heating power 150W, experimental temperature 100 °C.
Due to the interval of opening ultrasound, the heat transfer coefficient fluctuates up and down. The rapid increase of heat transfer coefficient at the initial stage is due to the increase of the number of bubble nucleation on the heater surface due to the gradually increased heat flux, and thus the heat transfer coefficient increases. In general, the addition of ultrasound can significantly increase the heat transfer coefficient. From a mechanistic perspective, power ultrasound is a technique that uses the energy in the form of vibrations to change some of the physical, chemical, and biological characteristics of a substance or to speed up the process. The vibration energy provided by the ultrasonic wave is large, so that the binding force between the molecules of the liquid and between the molecules and the metal surface is weakened, thereby increasing the moving speed of the molecules and facilitating the transfer of heat between the molecules. From the curve of the heat transfer coefficient with time, the ultrasonic wave has obvious anti-fouling performance.
From theoretical analysis, ultrasonic radiation has three effects on the liquid and has anti-fouling ability: the ultrasonic propagation velocity produces a velocity difference with the change of the medium, thereby forming shear stress at the interface, resulting in molecular and molecular, molecular The weakening of the bond strength with the metal surface prevents the deposition of dirt crystals on the metal surface; the strong pressure peak generated by the cavitation of the ultrasonic wave in the fluid medium accelerates the precipitation of Ca2+, and precipitates the carbonate scale and particles. Impurities are broken into fine particles and suspended in the medium; Ultrasonic waves accelerate the chemical reaction in the special physical environment of high temperature and high pressure created by the fluid cavitation, changing the scaling conditions of the scale.
The influence of the concentration of the fouling solution on the heat transfer coefficient The higher the concentration of the fouling solution is, the higher the concentration of the fouling solution is, and the more adverse the heat transfer is, but after the ultrasonic wave is applied, the heat transfer effect is different. In this study, the anti-fouling effect of the ultrasonic wave in the higher and lower concentration of the fouling solution is better.
Heating power 150W, experimental temperature 100 °C. The influence of the concentration of the fouling solution on the heat transfer coefficient can be seen from the figure. When the concentration of CaCO3 fouling solution is 300 and 1200 mg/L, the heat transfer coefficient is higher, and the heat transfer coefficient is lower when the concentration is 600 and 900 mg/L. . According to the analysis, when the solution concentration is high, the ultrasonic wave greatly increases the nucleation rate of the supersaturated solution, which is favorable for forming a large number of small precipitated particles in the solution, thereby eliminating the supersaturation of the solution and alleviating the scaling of the solid surface. pressure. At the same time, the lifetime of H radicals produced by hydrolysis is relatively long, and it produces a reducing effect that can peel off the generated scale.
Ultrasound radiation is directly generated by the treatment solution to generate a large number of cavities and bubbles. When these cavities and bubbles collapse or squeeze each other, a certain range of strong pressure peaks are generated to destroy the generated scale layer and make it easy to fall off. The higher the solution concentration, the higher the degree of supersaturation and the more scale. At the same time, the greater the enhancement of the nucleation rate by the ultrasonic wave, the stronger the radiation effect to the solution and the more the scale layer falls off.
When a certain critical point is reached, the effect of the ultrasonic wave increasing the heat transfer coefficient exceeds the effect of reducing the heat transfer coefficient by the CaCO3 scale. Therefore, when the solution concentration is high, the heat transfer coefficient tends to increase. Effect of Heat Flux on Heat Transfer Coefficient Under the same conditions of ultrasonic loading, the heat transfer coefficient of the solution with a concentration of 1200 mg/L at different heat fluxes (heating power) was examined with time. The experimental temperature is 100 °C. It can be seen that at the same concentration of dirt, when the heating power is increased from 150W to 180W, the heat transfer coefficient can be increased by about 1.3 times.
This is due to the fact that under high heat fluxes, the active nucleation sites increase, resulting in more active gas bubbles and a significant increase in the heat transfer coefficient. Experiments also show that the change in heat flux is more significant in the initial stage of heat transfer. At higher heat fluxes, the heat transfer coefficient shows a higher value, but will eventually stabilize at a lower value. Effect of Heat Flux on Heat Transfer Coefficient of Scale-Containing Solutions Conclusion Regardless of whether there is an inscale solution, the use of ultrasonic waves significantly enhances the heat transfer effect on the surface of the heater. The increase in heat flux increases the heat transfer coefficient, but The effect on the initial stage of heat transfer is more pronounced.
In the pool boiling device, it is advantageous to increase the heat transfer coefficient in the lower and higher concentration of the fouling solution. An increase in the heat transfer coefficient means a reduction in the amount of dirt. When the concentration of the fouling solution is high, the enormous cavitation effect of the ultrasonic wave in the liquid medium facilitates the breaking of the precipitated carbonate scale into fine particles suspended in the medium to prevent the scale crystal from adhering to the surface of the heater, thereby relieving the solids. Surface scale pressure reduces the formation of dirt.