Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: A novel induced draft hybrid (dry/wet) dephlegmator (HDWD) is introduced which can enhance the performance of dry air-cooled condenser systems and a model is developed to simulate its performance. The HDWD consists of two stages of cooling with the steam flow in series and the air flow in parallel through both stages. The first stage consists of downwardly inclined finned tube bundles, similar to conventional air-cooled condenser bundles, and the second stage comprises horizontal bare tube bundles of which the outer surface can selectively be operated dry or wet by spraying it with deluge water. A comparison of the HDWD with other existing and new concepts reveals the significant advantages that this technology has over other technologies. This thesis is a continuation of the work done by Heyns (2008), who introduced a forced draft HDWD and investigated its performance, and Owen (2013), who conducted a parametric investigation on the induced draft HDWD to improve the basic design proposed by Heyns (2008). In his thermo-flow model, Heyns (2008) conducted a thermal analysis assuming equal air flow through both stages and a constant vapor temperature inside the bundles, while Owen (2013) solved the thermal, steam-side pressure drop and draft equations. Both their models make use of empirical correlations by Gaddis & Gnielinski (1985), Zukauskas (1987), Mizushina et al. (1967) and Niitsu et al. (1967) for the performance characteristics. The current model solves the thermal, steam-side pressure drop and draft equations of the HDWD with better accuracy in the steam-side pressure drop calculation. The airside heat and mass transfer and loss coefficient correlations found in literature were found to deviate significantly from each other resulting in uncertainty regarding their suitability. Based on the recommendations by Owen (2013), a bare tube test bundle with 19 mm outer diameter tubes arranged with a triangular pitch of 38 mm is therefore designed, manufactured and tested to investigate the performance characteristics of the bundle experimentally under dry and wet operating conditions to evaluate the correlations from literature. The experimental data confirmed the applicability of the correlations of Gaddis & Gnielinski (1985) and Zukauskas (1987) to predict the dry HDWD performance, since they deviated by 7% and 5% respectively from the experimental data in die applicable range. The heat transfer during wet operation is slightly over estimated within 5% with the correlations of Mizushina et al. (1967) in the range of application. The air-side pressure drop during wet operation is underestimated by 29 to 39% by Niitsu et al. (1967) and the correlation obtained from the experimental results is therefore used in the current model. Using the current HDWD model indicates significant performance enhancement using tubes with a smaller diameter in the delugeable bundle compared to the larger tubes of Heyns (2008) and agrees very well with the model of Owen (2013). / AFRIKAANSE OPSOMMING: ‘n Hibriede geïnduseerde trek deflegmator is voorgestel wat die werksverrigting van huidige lugverkoelde kondenserstelsels kan verbeter en ‘n model is ontwikkel wat die werksverrigting simuleer. Die deflegmator bestaan uit twee stadiums van verkoeling met die stoomvloei in serie en die lugvloei in parallel deur beide stadia. Die eerste stadium bestaan uit skuins, afwaartse finbuisbundels, soortgelyk aan konvensionele lugverkoelde kondenser-finbuisbundels, en die tweede stadium bestaan uit horisontale kaalbuisbundels waarvan die buite-oppervlak selektief droog of nat bedryf kan word deur dit met verkoelingswater te benat. ‘n Vergelyking tussen die deflegmator en ander huidige en nuwe konsepte toon merkbare voordele wat die tegnologie bo die ander tegnologië het. Die tesis is ‘n voortsetting van die werk van Heyns (2008), wie ‘n hibriede geforseerde trek deflegmator voorgestel en die werksverrigting daarvan ondersoek het, en Owen (2013) wat ‘n parametriese studie op die hibriede geïnduseerde trek deflegmator gedoen het om die basiese ontwerp voorgestel deur Heyns (2008) te verbeter. In sy termovloei-model, het Heyns (2008) die termiese analise ondersoek met die aanname van gelyke hoeveelhede lugvloei deur beide stadia met ‘n konstante damp-temperatuur binne die bundels, terwyl Owen (2013) die termiese, stoomkant-drukval and trekvergelykings opgelos het. Beide hul modelle maak van die empiriese korrelasies van Gaddis & Gnielinski (1985), Zukauskas (1987), Mizushina et al. (1967) en Niitsu et al. (1967) gebruik vir die werkverrigtingskarakteristieke. Die huidige model los die termiese, die stoomkant-drukval asook die trekvergelykings van die deflegmator op met verbeterde akkuraatheid van die stoomkant-drukval berekeninge. Die lugkant warmte- en massaoordrag en verlieskoëffisiënt korrelasies wyk baie van mekaar af wat lei tot onsekerhede rakende hul toepasbaarheid. Gebaseer op die voorstelle van Owen (2013) is ‘n kaalbuisbundel met 19 mm buite-diameter buise met ‘n driehoekige steek van 38 mm dus ontwerp, vervaardig en getoets om die werksverrigtingskarakteristieke van die bundel eskperimenteel te ondersoek onder droë en nat toestande om die korrelasies van die literatuur te evalueer. Die eksperimentele data bevestig die toepaslikheid van die korrelasies van Gaddis & Gnielinski (1985) en Zukauskas (1987) om die droë werksverrigting van die deflegmator te voorspel, aangesien hul met 7% en 5% onderskeidelik afwyk van die eksperimentele data in die toepaslike bestek. Die warmteoordrag tydens natwerking is ietwat oorskat binne 5% met die korrelasies van Mizushina et al. (1967) in die bestek van toepassing. Die lugkant-drukval tydens natwerking is onderskat met 29 tot 39% deur Niitsu et al. (1967) en die korrelasie verkry vanaf die eksperimentele resultate is dus gebruik in die huidige model. Deur gebruik te maak van die huidige model van die deflegmator dui merkbare werksverrigting verbetering aan deur van buise met ‘n kleiner diameter in die benatbare bundel te gebruik in vergelyking met die groter buise van Heyns (2008) en stem baie goed ooreen met die model van Owen (2013).

<p>We study the effect of minute perturbations by using blowing jets at upstream and bio-inspired micro denticles on turbulence large-scale motions which are observed to be crucial in controlling heat transfer, noise and drag reduction. This work is divided into two phases. In first phase, we studied the effect of blowing perturbations at upstream on large-scale motions and associated co?herent vortical structures which are crucial in enhancing heat transfer by promoting mixing. The second phase is focused on impact of flow dynamics in preventing the biofouling using micro bioinspired structures and the importance of flow regime in designing the antifouling coating us?ing bioinspired structures is demonstrated, and subsequently, separation bubble dynamics and its characterization is carried out for a transonic channel imposed with pressure gradient to further expand our thesis outcomes to utilize micro bioinspired structures in aerospace applications, noise reduction, and to delay separation.</p> <p><br></p> <p>Extensive studies were focused on the importance of large-scale motions (LSM) and their con?tribution to TKE and turbulence mixing. Although there are studies focusing on the λ2 coherent vortical structures and large-scale motions separately, there are no studies addressing the control?ling using upstream perturbations on the large-scale motions and their associated λ2 vortices. In the first phase of our studies, we used the DNS data of channel flow for Reτ = 394 generated using in-house code. In these simulations, we created blowing perturbations using spanwise jets of low blowing ratio, 0.2, placed at upstream. The spatial large-scale motions are extracted using a a novel 3D adaptive Gaussian filtering technique developed based on Lee and Sung [1] for turbulent pipe flows. POD is used to extract the energetic large-scale motions and coherent vortical structures are extracted using λ2-criterion for its efficiency in educing coherent structures in cross flow jets. The results show that the upstream perturbations enhance streamwise heat flux via energetic LSM and also create a secondary peak of scalar production in the log-layer showing that the perturbations alter LSMs to enhance the heat transfer. Filtered large-scale field from Gaussian filtering technique have an integral length scale greater than 2h (where h is channel half-height) are used to obtain λ2 vortices. The resulted λ2 vortices are of ring-type and have higher signature of temperature than their counterpart. The pre-multiplied spectra shows that the upstream perturbations can excite the large-scale wave-numbers which are in the same order as the jet diameter and spacing between them. Simulations show the presence of secondary peak in the log-layer and increased turbulence production which are eminent of large-scales. Furthermore, our results suggest that jet spacing and diameter are crucial in exciting large-scale field to control turbulent flows.</p> <p><br></p> <p>Evans, Hamed, Gorumlu, et al. [2] modeled the denticles present on Mako shark skin into a diverging micro-pillars. They conducted experimental studies in a water tunnel using these on the back of airfoil exposed to an adverse pressure gradient flow. They observed that presence of these pillars reduced the re-circulation bubble (form drag) by 50%. They proposed a blowing and suction type mechanism by which the micro pillars interact with the boundary layer. However, the details of underlying interfacial mechanism is not completely understood. The unique impact of flow conditions on anti-biofouling and the corresponding mechanisms for the first time is illustrated. We employed commercially available bioinspired structures as micro-diverging pillars making it feasible to apply in real life. We demonstrated the underlying mechanism by which bio?inspired structures are responsible for anti-biofouling. To study the pressure gradient effects on the separation under transonic conditions, we performed direct numerical simulations (DNS) in a non?equilibrium flow created by a sinsuoidal contraction and also, we quantified the separation length,</p> <p>detachment, and attachment points of separation bubble imposed with various pressure gradients and their variation in the transonic and subsonic regimes. We noticed that the resultant shear at the attachement led to the enhancement of coherent structures which are extended into the outer layer under transonic flow which is quite different than the subsonic flow.</p>