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Sayma, Abdulnaser
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Sayma
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Abdulnaser
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Abdulnaser Sayma
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Pressure profile optimisation of a nozzle for wet-to-dry expansionOgrodniczak, Pawel; Sayma, Abdulnaser; T White, MartinWet-to-dry expansion within the nozzle guide vane of an ORC turbine has been proposed as a means to improve the power output of ORC systems for waste-heat recovery (< 250 °C). However, given the rapid fluid acceleration in the stator, the phases can develop significant velocity and temperature disparity due to the density difference and finite rate of interphase heat transfer. Since these factors can significantly affect the phase-change process, wet-to-dry nozzle design techniques must account for non-equilibrium effects. The first part of this paper aims to further validate a previously developed quasi-1D inviscid nozzle design tool that accounts for non-equilibrium effects. The interphase mass, momentum and energy exchange models have been updated using correlations better tailored to evaporating droplet flows, while the drag equation and vapour mass fraction definition have been updated. The validation is performed using nonequilibrium CFD simulations, which, unlike the design model, account for lateral flow variations, viscous and turbulence effects, and secondary momentum forces. The results showed that these effects delay the evaporation, with CFD-predicted outlet vapour mass fraction being about 10 to 15% smaller than predicted by the quasi-1D tool. However, the overall flow behaviour and phase-change pattern were in satisfactory agreement, justifying the use of the design tool for 1D optimisation. In the second part of the paper, the quasi-1D tool is coupled to a gradient-based optimiser to optimise the nozzle pressure profile and enhance evaporation of siloxane MM for expansions with inlet pressure ranging from 450 to 650 kPa, and inlet quality of 0.3. The design variables are the control points of a Bezier curve that defines the shape of the intended nozzle pressure profile. CFD simulations of the optimised geometries indicate a 5.5 to 6.6% increase in the outlet vapour mass fraction, which was raised from 84.9, 88.2 and 90.7% to 90.4, 94.8 and 97.2% for the 450, 550 and 650 kPa inlet pressures respectively. More abrupt expansion in the optimised nozzles resulted in the development of a shock and led to nozzle efficiency deterioration compared to the baseline. This emphasises the need for multi-objective optimisation, which will be conducted in future studies.- EVALUATION OF THE PERFORMANCE OF MULTIPLE SUPERCRITICAL CO2 POWER CYCLES IN WASTE HEAT RECOVERY APPLICATIONSChibli, Hicham A.; Read, Matthew; Sayma, AbdulnaserThis paper aims to systematically evaluate the common supercritical carbon dioxide power cycles that are suitable for low to high grade waste heat recovery applications, with source temperatures ranging from 450 to 850 °C. A number of cycles varying from the simple to the fully cascaded split cycles are investigated against a predefined waste heat source for multiple temperatures and flow rates with a target power net yield of the order of 1 MWe. The conditions at the inlet to the main compressor in the sCO2 loop are also varied to quantify the role of the environmental heat sink in effective heat recovery. The sCO2 cycle parameters including the mass flowrate and split ratio as well as the effectiveness of the individual heat exchangers are optimized for multiple pressure ratios. Additionally, a detailed exergy analysis is conducted to identify the sources of entropy generation in the various components and enable a detailed comparison of the various cycle configurations reviewed. Improving the quality of the heat source from 450 to 850 °C yielded ~30% increase in cycle performance, while degrading the heat sink quality from 15 to 35 °C reduced the net output power by ~30%. The cascaded and split cycles were determined to achieve the highest performance across all conditions considered.
