CFD Modelling and Analysis of Micro-scale Transcritical CO2 Scroll Expander
Creators: name: Du, Yuheng
Publisher: name: University of Surrey
Year: 2023

description: There is an increasingly demanding for the development of renewable energy. The majority
description: of renewable energy sources are classified as low-grade heat, which needs an advanced power
description: cycle to recover energy. The transcritical carbon dioxide power cycle (T-CO₂) can provide
description: the advantages of the components’ feasibility and high Carnot efficiency with low-grade heat
description: sources. The performance of the expander plays a vital effect in the cycles, while the common
description: turbines are suffering extremely high rotational speed for micro-scale size. Therefore, it gives
description: the scroll expander (volumetric type) a chance for the micro-scale transcritical carbon dioxide
description: power cycle with the potential benefits of lower rotational speed, simpler geometry design,
description: and lower cost. However, the leakages of scroll type are reported in the previous organic
description: Rankine cycle (ORC) applications. The flank leakage is unavoidable because of the working
description: fundamentals between fixed and orbiting scrolls and it is caused by the pressure difference
description: between upstream high-pressure chambers and downstream low-pressure chambers. This
description: leakage can be more serious with high-pressure working conditions of the transcritical carbon
description: dioxide power cycle. Therefore, the development of scroll expanders applying to micro-scale
description: transcritical carbon dioxide power systems is still at the very early stage and limited literature
description: focuses on this field. The micro-scale scroll expander can potentially fill the gap if the
description: leakage issues get solved.
description: The main objective of this Ph.D. research is to construct a preliminary micro-scale scroll
description: expander prototype for transcritical carbon dioxide power cycle using computational fluid
description: dynamics (CFD) methodology. The transient three-dimensional CFD model is expected
description: to analyse the thermodynamic and aerodynamic performances of different scroll expander
description: designs. The model is also intended to assess the effectiveness of extra sealing designs for
description: flank clearance leakage as well as the optimal flank clearance size.
description: The current thesis uses the publication style and the major findings are related from
description: chapter 4 to chapter 7. Therefore, there are four major contributions of the current: 1) The
description: transcritical carbon dioxide scroll expander demonstrates better performance than the R123.
description: The average isentropic and exergy efficiency benefits are around 14 % and 7 %, respectively.
description: The specific power output of transcritical carbon dioxide scroll expander is about four times
description: higher than R123. Over-expansion and reversed flow are observed in the standard flank
description: clearance design (20 µm) and the flow characteristics are more sensitive to scroll geometry
description: rather than working fluids. 2) Increasing the size of flank clearance from 20 to 200 µm can
description: drop down the isentropic efficiency from 87.4 % to 44.8 % and the exergy efficiency shows a
description: similar trend. At the same time, increasing the size is beneficial to lower the manufacturing
description: cost and there is a trade-off relation between manufacturing cost and isentropic efficiency.
description: The surrogate-assisted multi-objective optimisation algorithm (SAMO) successfully solves
description: the trade-off relation and the final Pareto front indicates that the flank clearance of 127 to
description: 200 µm has the most potential for the sealing design. In addition, the SAMO significantly
description: reduces the CFD computational time from 90 hours to 15 seconds. 3) Increasing the size of
description: flank clearance can effectively reduce the possibilities of over-expansion pheromones while
description: excessively enlarging the size results in under-expansion. The "turning point" is 40 µm for
description: the current design. The pressure imbalance between two symmetry expansion chambers is
description: gradually reduced when enlarging the flank clearance. The most efficient flank clearance to
description: balance the pressure was about 100 to 150 µm for this particular expander geometry under
description: investigation. 4) The function of the sealing cavity in the transcritical CO2 scroll expander is
description: achieved by the throttling process, where the jet divergence occurs in flank clearance and
description: then the dissipation process works in the seal cavity. The seal cavity is beneficial to reduce
description: leakage and increase the expansion ratio. The fundamentals of this design are similar to the
description: labyrinth seals for the turbine. The different geometrical sealing cavity designs of singlegroup
description: contribute a limited improvement to the isentropic efficiency, ranging from 0.91 %
description: to 0.95 %. The height and cavity number indicate the linear positive trend to the isentropic
description: efficiency, while the cavity spacing shows the opposite. There is no clear difference between
description: the three sealing cavity groups of rectangular-shaped teeth (RST), isosceles trapezoid-shaped
description: teeth (ITST), and right-angled trapezoid-shaped teeth (RTST). The transient leakage ratio
description: keeps a consistent trend with the isentropic efficiency. The transient leakage got massively
description: reduced by the sealing cavity from 55.3 % to 70.2 %. However, the transient leakage
description: reduction for one rotating angle has a very limited function in the overall performance. The
description: final decision of sealing cavity for multi-groups is based on the trade-off relation between
description: sealing performance and manufacturing. The locations of the sealing cavity are important
description: to solve the pressure imbalance between two symmetrical working chambers. The paper
description: suggests designing the upstream sealing cavity for a lower-pressure working process and
description: downstream for a higher-pressure working process, which can ideally achieve the maximum
description: pressure balance.