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Abstract

The organic Rankine cycle is a suitable technology for heat-to-power conversion from thermal sources at temperatures between 80 and 380 °C. However, the simple organic Rankine cycle poses some limitations when it comes to the effective utilisation of open sensible heat sources, which is mainly related to isothermal evaporation. Two-phase expansion has been proposed as a means of improving cycle performance, but, efficient expanders that can tolerate wet conditions are required. Using certain molecularly complex fluids it is theoretically possible to design a so-called wet-to-dry cycle, in which the fluid transitions from a two-phase state to a superheated vapour regime in a process known as flash boiling. Previous studies indicate that the wet-to-dry cycle can potentially generate 30 % more power from heat sources at temperatures between 150 and 250 °C. Since the fluid can hypothetically escape the two-phase dome during expansion, existing turboexpander architectures can be employed so long as the wet portion of the expansion is confined to the
turbine nozzle to avoid erosion of the rotor blades.

The key characteristic of a two-phase turboexpander designed for the wet-to-dry cycle is the nozzle, which has to facilitate complete mixture vapourisation in an efficient manner. Due to the lack of relevant experimental data, the concept of the proposed two-phase turboexpander must be assessed with the aid of numerical tools. To this day, numerical studies focused on the wet-to-dry expansion have either neglected non-equilibrium effects or oversimplified the expansion by assuming an inviscid one-dimensional flow. The main objective of this work is to numerically investigate flash boiling in a passage representative of a turboexpander nozzle designed for the wet-to-dry cycle and to assess the feasibility of designing a nozzle that can achieve complete mixture vapourisation under relevant operating conditions. To this end, a suitable non-equilibrium flash boiling model is implemented to conduct numerical simulations that account for non-equilibrium effects, two-dimensional flow variations, effects of turbulence and viscosity, as well as the influence of drag, lift and turbulent dispersion forces. The model is applied to simulate the wet-to-dry expansion of the siloxane MM in a converging-diverging nozzle for a range of inlet pressures from 500 to 1250 kPa, inlet vapour qualities from 0.1 to 0.5, and for a range of droplet sizes.

The research presented in this thesis give insight into the underlying physics of wet-to-dry expansion under a range of operating conditions relevant to the wet-to-dry cycle. The phase-change process, extent of non-equilibrium effects, spatial distribution of phases as well as flow uniformity are assessed. Nozzle geometries designed with a non-equilibrium tool are investigated to evaluate the importance of incorporating non-equilibrium effects in the nozzle design routine. Lastly, an optimisation study is carried out, which aimed at maximising the vapourisation rate in the nozzle, while maintaining an efficient expansion.

The results indicate that the droplet diameter has the greatest impact on the two-phase expansion, and it is generally difficult to achieve complete mixture vapourisation when droplets are too large. The wet-to-dry transition can be more readily accomplished at high operating pressures but these conditions are associated with reduced cycle performance. Droplet breakup has been found to be significant and suitable methods for accounting for these effects have been proposed. Including non-equilibrium effects in the nozzle design tool is important and can potentially help generate better-performing nozzles with an enhanced vapourisation rate. Finally, the vapourisation rate can be enhanced by means of optimisation techniques but simultaneous consideration of expansion efficiency is essential.

This thesis contributes to a better understanding of flash boiling of organic fluids in passages designed for high-speed two-phase flows. This research also highlights the important challenges that are likely to accompany the design of a two-phase turboexpander for the wet-to-dry cycle, providing a step forward towards developing a turbine prototype.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QC Physics T Technology > TJ Mechanical engineering and machinery T Technology > TP Chemical technology
Departments:	School of Science & Technology > Department of Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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