On the thermal and fluidic characteristics of steam condensation in an air-cooled condenser
thesisposted on 2022-08-29, 08:46 authored by Alan O'Donovan
With anthropogenic climate change being one of the dominant socio-economic topics of the last decade, renewable energy has benefited from policies and legislation limiting carbon emissions. The technologies to exploit renewable energy sources have experienced significant growth and development as a result. One of the foremost technologies is Concentrated Solar Power (CSP), which has the potential to provide 25% of the world’s electricity by 2050. Successful, wide-scale deployment of CSP, however, will be dependent on the development of enhanced air-cooling strategies for use in the plant’s Rankine cycle. This thesis is focused on the condensate-side performance of a modular air-cooled condenser for use in CSP plants. The overarching objective is to enhance understanding of condensing flows of steam in air-cooled condensers (ACCs) at typical Rankine cycle operating conditions. This was principally achieved through an an experimental programme, consisting of thermal and hydrodynamic measurements, which was predominantly carried-out on circular tube condensers. The experimental programme was divided into two main streams - experimentation relating to full-scale multi-row condensers, and experimentation relating to a reduced-scale equivalent condenser. In both cases, measurements were generated by investigating parameters related to ACCs, such as steam/condensate mass flow rate, air mass flow rate, and condenser inclination angle. Throughout an experimental programme that evolved in response to various condensate side phenomena encountered, a test facility capable of investigating the condensate-side characteristics of a full-scale condenser at realistic Rankine-cycle conditions was developed. The hydrodynamic characteristics were quantified by the condensing pressure loss which, for the experimental conditions examined in this study, was found to be quite small, in the range of 120 Pa - 280 Pa over a vapour Reynolds number range of 1890 - 5150. It was shown that such relatively small magnitudes were due to momentum recovery offsetting the frictional losses in the flow - a phenomenon which appears unique to condensing flows. However, a parametric investigation concluded that this will not always be the case, and that a threshold point around D = 0.02 m and Lt = 4 m exists, after which the frictional losses tend to exceed the momentum recovery. Thermal characteristics were expressed by the condensate-side thermal resistance, which was shown to vary in its contribution to the overall thermal resistance with vapour Reynolds number. The average contribution was quantified as 26% at Rev = 2280 to 13% at Rev = 4420. The reduced-scale condenser allowed for a more robust investigation into the thermal and fluidic mechanisms, which were not possible on the full-scale. Through a novel, non-invasive measurement technique, the predominant two-phase flow regimes were inferred. In general, it was found that annular flow exists nearest the tube inlet, with the flow deviating to stratified-wavy as the flow progresses through the tube. Local heat transfer measurements were related to this flow topology, in that large condensing Nusselt numbers were measured at the tube inlet, progressively decreasing towards the outlet, and ultimately tending to converge as the tube exit was approached. A multi-dimensional element to the heat transfer was observed as the Nusselt number was measured around the inner tube circumference. It was seen that the Nusselt number decreased from the top to the bottom of the tube - suggesting a deterrent to heat transfer, in the form of a condensate pool, resides towards the bottom. Providing context to the overall investigation in this thesis is a thermodynamic model, which incorporates measured results to highlight limitations in current modelling approaches in the literature. It was seen that neglecting to account for the condensateside thermal resistance in modelling approaches can lead to very different results, in terms of net plant output.
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