Experimental And Numerical Simulation Of Fouling Growth Characteristics Of CaCO₃ In Circular Tube

The deposition of fouling in heat exchanger is not only a complex processes combined with energy transfer, mass transfer, and momentum transfer, but also a physical chemistry processes affected by many factors. Fouling is the most common and difficult problem to be solved in industrial circulating water system. In China the main crystallization fouling is calcium carbonate, so the study of calcium carbonate fouling has highly relevant and practical significance. For this regard, the study is taken calcium carbonate fouling as the research object, the growth characteristic of calcium carbonate fouling on the test tubes wall is studied by experimental study and numerical simulation, and at the same time the morphology of calcium carbonate crystallization is investigated by scanning electron microscopy(SEM) in order to find out its distribution and analyze its growth patterns and characteristics.In the experiment, supersaturated calcium carbonate solution with concentrations of 50, 150 and 200 mg/L is used as circulating cooling water, the experimental conditions is controlled as following: fluid velocity in the test tube is 0.2 m/s(Re=2854), water bath temperature is 85±0.5 oC, and inlet temperature of test tube is 63±0.5 oC. Under these conditions, the experiment is focused on studying the effects on heat transfer characteristics caused by growth of calcium carbonate crystallization fouling on AISI 316 L stainless steel test tube wall, furthermore, getting the heat transfer correlation and the thermal resistance correlation. The results show that the overall coefficient of heat transfer is decreasing firstly, followed by increasing, and finally decreasing with increase in the experimental time, but the change of fouling resistance is reverse with time. The study also found that the fouling process can be divided into induction period and the fouling period, the induction period can also be divided into positive resistance region(initial stage) and negative resistance region, the higher the concentration calcium carbonate of solution, the shorter the induction period, and the faster in the fouling growth. Through observed by scanning electron microscopy(SEM) image, it found that CaCO3 fouling appears as hexahedral calcite when the concentration is 50 mg/L, and when the concentration is 150 mg/L the crystal form of CaCO3 fouling displays both hexahedral calcite and needle- like aragonite while, when the concentration is 200 mg/L the crystal of CaCO3 fouling becomes needle-like aragonite.Based on the experimental results above, supersaturated calcium carbonate solution with concentrations of 50 mg/L is used as circulating cooling water, the growth characteristics of fouling is investigated at different stage under the same experiment conditions as shown above, the purpose of study is to obtain the heat transfer correlation and the thermal resistance correlation, and the crystal morphology and distribution in two stages are observed with a scanning electron microscope(SEM). Experimental results show that the growth of fouling is different in thermal resistance region, negative thermal resistance region and fouling growing region, which is closely related to the interaction between the heat resistance and turbulence caused by fouling. Observed by SEM found that the d istribution of fouling turn gradually from loose to dense along the flow direction. In initial phase, crystallization fouling on the wall just is shown as sporadic distribution while, with time increasing, the fouling deposition as well as its density on heating surface is gradually increasing.Mechanism of CaCO3 fouling growth at induction period is numerically studied by software programming of FORTRAN. The results show that there is a negative and a positive thermal resistance region in the process of CaCO3 fouling growth. In the negative region, the effect of fouling resistance on heat transfer is less than that caused by turbulence of fouling on heat transfer, Nu/Nu0>1; in the positive region, the effect of fouling resistance on heat transfer is more than that caused by turbulence of fouling on heat transfer, Nu/Nu0<1.