Numerical simulation of deposition characteristics of microorganisms and particles mixed fouling in sewage heat exchange tube

Abstract

Sewage source heat pump technology is an innovative way of energy recovery. It can use the heat in all kinds of sewage to heat and cool buildings. This technology not only saves energy, but also has good economic benefits. At present, the main problem of this technology is the fouling problem. Microbial fouling and particle fouling are the main components of fouling in sewage heat exchangers. In this paper, the mixed dirt composed of microbial dirt and particulate dirt is taken as the research object. The deposition characteristics of mixed fouling in sewage were studied by numerical simulation. The mathematical model of mixed fouling deposition was established. The effects of various factors on mixed fouling deposition were simulated and analyzed by using the model. The results show that when the flow rate increases from u = 0.5m/s to u = 1.5m/s, the thermal resistance value of mixed dirt equilibrium in winter condition decreases from 27.97×10− 4 m2·K·W− 1 to 9.82×10− 4 m2·K·W− 1, a decrease of 64.9%. At this time, the thermal resistance in summer condition is reduced from 36.41×10− 4 m2·K·W− 1 to 11.15×10− 4 m2·K·W− 1, a decrease of 69.4%. The higher the inlet temperature, the higher the deposition rate of mixed dirt, and the shorter the time required for thermal resistance to reach equilibrium. However, the inlet temperature has little influence on the thermal resistance of mixed dirt. When the particle size of microbial particles increases from 0.1mm to 0.5mm, the thermal resistance value of mixed dirt when it reaches equilibrium in winter condition increases from 17.34×10− 4 m2·K·W− 1 to 21.55×10− 4 m2·K·W− 1, increasing by 24.3%. At this time, the thermal resistance in summer condition increased from 18.94×10− 4 m2·K·W− 1 to 23.49×10− 4 m2·K·W− 1, an increase of 24.0%. When the size of hard particles increases from 1µm to 40µm, the thermal resistance in winter conditions increases from 14.52×10− 4 m2·K·W− 1 to 19.56×10− 4 m2·K·W− 1, an increase of 34.6%. At this time, the thermal resistance in summer condition increased from 16.68×10− 4 m2·K·W− 1 to 20.91×10− 4 m2·K·W− 1, an increase of 25.4%. The thermal resistance of mixed dirt increased by 0.97×10− 4 m2·K·W− 1 in winter condition and 1.06×10− 4 m2·K·W− 1 in summer condition with an increase of 200 mg/L microbial particle concentration. The thermal resistance increases by 7.3×10− 4 m2·K·W− 1 in winter and 8.1×10− 4 m2·K·W− 1 in summer for every 200 mg/L increase in hard particle concentration. The research results of this paper can provide scientific basis for the design of sewage heat transfer system and have important engineering application value.