tailieunhanh - Heat Transfer Theoretical Analysis Experimental Investigations and Industrial Systems part 14

Tham khảo tài liệu 'heat transfer theoretical analysis experimental investigations and industrial systems part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 510 Heat Transfer - Theoretical Analysis Experimental Investigations and Industrial Systems 1- Formation of foulant materials in the bulk of the fluid or initiation of the fouling the first event in the fouling process is preceded by a delay period or induction period td as shown in Fig. 3 the basic mechanism involved during this period is heterogeneous nucleation and td is shorter with a higher nucleation rate. The factors affecting td are temperature fluid velocity composition of the fouling stream and nature and condition of the heat exchanger surface. Low-energy surfaces unwettable exhibit longer induction periods than those of high-energy surfaces wettable . In crystallization fouling td tends to decrease with increasing degree of supersaturation. In chemical reaction fouling td appears to decrease with increasing surface temperature. In all fouling mechanisms td decreases as the surface roughness increases due to available suitable sites for nucleation adsorption and adhesion. 2- Transport of species means transfer of the fouling species itself from the bulk of the fluid to the heat transfer surface. Transport of species is the best understood of all sequential events. Transport of species takes place through the action of one or more of the following mechanisms Diffusion involves mass transfer of the fouling constituents from the flowing fluid toward the heat transfer surface due to the concentration difference between the bulk of the fluid and the fluid adjacent to the surface. Electrophoresis under the action of electric forces fouling particles carrying an electric charge may move toward or away from a charged surface depending on the polarity of the surface and the particles. Deposition due to electrophoresis increases with decreasing electrical conductivity of the fluid increasing fluid temperature and increasing fluid velocity. It also depends on the pH of the solution. Surface forces such as London-van der Waals and electric double layer interaction .