Browsing by Author "Janabi, Abdullah K. O. Al-"

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Convective heat transfer fouling of aqueous solutions on modified surfaces
(2011) Janabi, Abdullah K. O. Al-; Müller-Steinhagen, Hans (Prof. Dr. Dr.-Ing. habil.)
The present research study was part of the European project "MEDESOL" entitled "Seawater desalination by innovative solar-powered membrane-distillation system". The project aimed at developing a stand-alone desalination unit to produce fresh water with a maximum of 50 m3/day. Several components such as suitable membrane and efficient solar collectors had to be developed as well as a plate heat exchanger for a maximum life expectancy with least deposition occurrence on its surfaces. The contribution of ITW to this project was mainly concerned with the development of innovative anti-fouling heat transfer surfaces that could substantially i) reduce the design and operation penalties, and ii) increase the membrane distillation efficiency. To achieve this goal, this study endeavoured to address some outstanding questions such as: 1) The impact of surface texture in terms of roughness and structured grooves on the fouling propensity. 2) The effect of various modified surfaces on adhesion mechanisms between the deposits and the heat transfer surfaces. 3) A criterion that would predict whether a modified surface would foul or not. Accordingly, a comprehensive and rigorous set of fouling runs was performed with calcium sulphate solution as working fluid. The operating conditions followed closely to those expected in the membrane distillation desalination unit i.e. a fluid velocity of up to 0.3 m/s and a maximum surface temperature of 90°C. The experimental results showed that i) increasing surface roughness causes a significant reduction of the nucleation correction factor. As a consequence, a higher fouling rate would be expected due to the reduction in the energy barrier limit of crystallization; ii) in case of grooved surfaces, the generated eddies within the crossed grooves have vortexes which are opposite to those of the mainstream. This would overcome the adhesion forces between the crystals and the heat transfer surfaces and gave rise to longer induction time and lower fouling rate; and iii) the deposition process was strongly affected by altering the surface energy properties, particularly the electron donor component ( ) for the coatings investigated in this study. Lower initial fouling rate and longer induction time were the main features of coatings having higher. A new criterion has also been developed that predicts whether a modified surface will foul or not. In this criterion, the total interaction energy depends strongly on the Lewis acid-base energy while the contribution of the Lifshitz-van der Waals energy is marginal. A validation of the new criterion was carried out by comparison with the experimental results of the present study as well as some previous investigations of precipitation and biofouling. The validation results confirmed the reliability and accuracy of the new criterion. Finally, the coatings that performed best in terms of fouling mitigation in the laboratory tests were selected for field assessment in two successive phases. No sign of fouling was observed in either phase. However, in phase 1, substantial flaking-off of the coatings occurred in particular around the inlet and outlet sections of the heat exchanger as well as at the contact points between plates. In phase 2, a thinner coating was applied with a more roughened surface which resulted in far better stickiness of coatings to the surface.