It is common practice to design water chiller units and heat exchangers in such a way that they do not operate within the transition region. This is mainly due to the perceived chaotic behaviour as well as the paucity of information in this region. Due to design constraints or change of operating conditions, however, exchangers are often forced to operate in this region. This is even worse for enhanced tubes as much less information within this region is available. It is also well known that the entrance has an influence on where transition occurs, adding to the woes of available information. The purpose of this study is thus to obtain heat transfer and friction factor data in the transition region of fully developed and developing flows inside smooth and enhanced tubes, using water as the working fluid, and to develop correlations from these results. The use of different inlets, tube diameters and enhanced tubes was also investigated with regards to the commencement of transition. Heat transfer and pressure drop data were obtained from six different types of tubes with diameters of 15.88 mm (5/8′′) and 19.02 mm (3/4′′). Low fin enhanced tubes with a fin height to diameter ratio of 0.4 and helix angles of 18◦ and 27◦ were investigated. Heat transfer was obtained by means of an in-tube heat exchanger with the cooling of water being used as the test fluid. Reynolds numbers ranged between 1 000 and 20 000 while Prandtl numbers were in the order of 4 to 6. Uncertainties in heat transfer coefficient and friction factors were on average below 2.5% and 10% respectively. Adiabatic friction factor results showed that the use of different inlets influenced the commencement of transition. The smoother the inlet profile the more transition was delayed, confirming previous work done. The effect of increasing tube diameters had a slight delay in transition. Enhanced tubes caused transition to occur at lower Reynolds numbers which was accounted for by the fin height and not the helix angle. Heat transfer results showed that transition occurred at approximately the same Reynolds number for all the different inlets and enhanced tubes. This was attributed to the secondary flow forces influencing the growing hydrodynamic boundary layer. These secondary flow forces also influenced the laminar heat transfer and diabatic friction factors with both these parameters being higher. Turbulent enhanced tube heat transfer results were higher than those of the smooth tube, with the tube with the greatest helix angle showing the greatest increase. Correlations were developed for all the tubes and their inlets and predicted all the data on average to within 3%. / Thesis (PhD)--University of Pretoria, 2010. / Mechanical and Aeronautical Engineering / unrestricted

L’amélioration de la récupération d’énergie dans les procédés industriels passe par une meilleure compréhension des phénomènes d’encrassement dans les échangeurs de chaleur. L’encrassement se caractérise par la formation de dépôts non désirés sur les surfaces d’échange de l’échangeur. Cette étude porte sur l’amélioration de l’efficacité énergétique des raffineries de pétrole par réduction de l’encrassement dans les échangeurs de chaleur du train de préchauffe.Pour cela, une boucle d’essai reproduisant les niveaux d’échange et les écoulements rencontrés en raffinerie a été employée pour tester un échangeur de type tubes et calandre. Les fluides traités sont du pétrole brut et de résidu de distillation atmosphérique (coupe lourde du pétrole) comme dans le procédé industriel. On s’intéresse à l’influence de la géométrie de l’échangeur. Des faisceaux de tubes avec des corrugations internes hélicoïdales et des ailettes externes sont successivement testés. Pour chacune des technologies, les paramètres opératoires optimums permettant de limiter la formation d’un dépôt encrassant sont identifiés sur une gamme de température de film et de cisaillement s’échelonnant respectivement de 230 à 300°C et de 1,3 à 8,8 N/m2. L’efficacité des tubes optimisés pour lutter contre l’encrassement est évaluée par comparaison avec l’encrassement obtenu sur le faisceau de tubes lisses pris comme référence. Les deux technologies se sont révélées efficaces pour réduire l’encrassement. Par rapport aux tubes lisses, l’encrassement a été réduit d’un ordre de grandeur à cisaillement et température de film équivalentes aussi bien avec les tubes structurés qu’ailetés.En parallèle, une simulation d’encrassement numérique (CFD) a été élaborée pour mieux comprendre le développement de l’encrassement dans l’échangeur équipé de tubes lisses. Les phénomènes dominants à l’origine de l’encrassement de l’échangeur ont été déterminés : la vitesse de formation du dépôt, peu affectée par les variations spatiales de la thermo-hydraulique, pilote l’encrassement. Les hétérogénéités d’encrassement sont causées par les disparités locales de la vitesse d’arrachement du dépôt qui varie en grande proportion mais dont l’influence sur la vitesse d’encrassement est faible. Elles sont dues à une inhomogénéité spatiale du cisaillement qui est la conséquence de l’établissement de l’écoulement dans les tubes. Une tentative d’amélioration de la précision de la prédiction de l’encrassement a été également entreprise en optimisant les paramètres d’un modèle d’encrassement existants et en tenant compte du vieillissement du dépôt. / Improving energy recovery in industrial processes requires a better understanding of fouling phenomena in heat exchangers. Fouling is the grow up of unwanted materials on heat transfer surfaces. This study focuses on improving the energy efficiency of petroleum refineries by reducing fouling in heat exchangers of the pre-heat train.For this purpose, a test rig reproducing thermal and flow characteristics encountered in the last heat exchanger of the pre-heat train was used to test a pilot-scale shell-and-tube heat exchanger. Fluids are the same as the ones processed in refineries: crude oil and atmospheric tower bottom, a heavy residue of oil. This study focuses on the influence of the heat exchanger geometry. Internally helically-finned tubes and externally low-finned tube bundles are successively tested. For each one, optimum operating parameters which reduce fouling are identified over a 230-300°C film temperature range and a 1.3-8.8 N/m2 wall shear stress range. The fouling mitigation efficiency of the enhanced tubes is compared with smooth tubes taken as a reference. Compared to smooth tubes, fouling on helically-finned and low-finned tubes is reduced by an order of magnitude when they are operated at equivalent wall shear stress and film temperature.In addition, a numerical (CFD) fouling simulation has been developed to provide a better understanding of the fouling in the heat exchanger with smooth tubes. Dominant phenomena driving fouling in the heat exchanger were determined. The deposition rate is weakly impacted by the spatial variations of the thermo-hydraulic and controls the overall fouling rate. Heterogeneities in fouling rate are caused by the local scattering in the removal rate, which varies in a broad range but whose impact on the overall fouling rate is low. The broad range of the shear stress is the consequence of the fluid flow entrance effects. An attempt to improve accuracy of the fouling model has been undertaken by optimizing the model parameters and by considering aging of the deposit.