For a micro-scale (< 10 kW) transcritical CO2 waste heat recovery power system, the scroll-type expander is a potential candidate. However, the scroll expander suffers from leakage and pressure imbalance issues because of the high-pressure working conditions. The current study designs twelve different sealing cavities based on the reference of labyrinth seals and presents a transient CFD analysis to investigate the flow behaviors. The results show that the sealing cavity has a positive impact on the machinery performance, where the isentropic efficiency improves from 0.907 % to 0.952 % for the single group. Increasing the height and cavity number of single-group sealing can improve the performance while enlarging the cavity spacing shows the opposite. There is no significant difference between the three different shapes of RST, ITST and RTST. However, the improvement in the instantaneous leakage ratio is remarkable, the leakage reduces from 55.3 % to 70.2 %. For the multi-group sealing cavity, the isentropic efficiency slightly improves to 0.982 %, and the pressure imbalance gets partially optimized. The locations of the sealing cavity are important to solve the pressure imbalance between two symmetrical working chambers. The paper suggests designing the upstream sealing cavity for a lower-pressure working process and downstream for a higher-pressure working process, which can ideally achieve the maximum pressure balance.
The scroll-type expander can be the promising candidate for micro-scale (< 10kW) transcritical CO2 waste heat recovery power system. The performance of the expander significantly depends on the flank clearance design. The current paper provides a transient CFD analysis for transcritical CO2 scroll expander, which includes 19 flank clearance sizes from 20 to 200μm. The results showed that increasing the flank clearance led to a significant exergy efficiency drop from 89.13% to 46.72%. Although large flank clearance is normally perceived as negative, it indeed eliminates the possibility of over-expansion. The "turning point" for the selected expander geometry is roughly around 40μm. Excessive flank clearance enlargement resulted in a severe under-expansion issue, and the average expansion ratio was reduced from 3.19 to 1.8. The velocity of leakages also increased from 90 to 370 m/s due to the increased pressure differences. The pressure imbalance issue between two symmetrical working processes got continuously optimised by increasing the flank clearance. The optimal flank clearance range of 100 to 150μm has been established with appropriate pressure balance and efficiency. This paper provides useful insights and design guidelines in the sizing of operational flank clearances for transcritical CO2 scroll expander. The micro-scale (grid) power plant has the potential to change the energy structure considering grid stability and load balancing, and the performance of micro-scale expanders is the key to achieving the objective.
To explore the feasibility of the application of steel slag powder (SSP) in steam-cured precast concrete, 0% and 20% SSP were used to replace cement and prepare cement paste, and the early age performance of steam-cured (80 °C for 7 h and 7 d) SSP-blended cement paste, including different types and amounts of hydrates, the microstructure and mechanical properties were investigated and compared with those of 28 d standard-cured SSP sample. The results show that SSP addition promotes the generation of laminar C-S-H gels and granular C-S-H gels after an initial 7 h steam curing. Further extending the lasting time of 80 °C steam curing to 7 days favors the production of hydrogarnet and crystalline C-S-H, of which the amount of formation of hydrogarnet in SSP composite cement paste is less and the particle size is smaller than those in the control sample. However, steam curing increases the gap between the number of hydrates formed in SSP-blended cement paste and the control paste. The delayed hydration effect of SSP on cement offsets the promoting effect of steam curing on the hydration of cement; in consequence, the incorporation of SSP seems to be detrimental to the hydration of steam-cured cement paste.
Recent developments in the field of renewable energy have led to a renewed interest in low-grade heat (< 500 K). The low-grade heat is widely wasted by the lack of efficient heat recovery technologies. It is also limited by the system size, which defines as the micro to small-scale (< 50 kW). Although ORC based unit has been implemented in this field, the CO2 based waste heat recovery units can be more capable in the size construction. The performance of the expander plays a vital role in the system's efficiency. Thus, the current paper provides thermodynamic and CFD analysis of a scroll expander regarding a micro-scale T-CO2 recovery system (< 10 kW) with a 400 K low-grade heat source. In the current CFD model, all the fluid domains were constructed by structural mesh. It also successfully integrated with the thermodynamic table to simulate two-phase T-CO2. This model can be the first scroll expander model for T-CO2 power system and gap the bridge of utilising the scroll machinery in this field. The CFD methodology was successfully validated by the new-built testing platform and previous data. The energy performance of T-CO2 and ORC (R123) based scroll expanders are compared by isentropic and exergy efficiency. The results showed that isentropic and exergy efficiencies of T-CO2 were 7% and 14% higher than the R123. It also identified higher irreversibilities of T-CO2 by the exergy of the working fluids. The pressure and temperature distributions identified the over-expansion and reversed flow characteristics, and the pressure imbalance of the initial expansion chambers denoted the reversed flow.
Abstract The control of the clearances in a scroll compressor is crucial to ensure the reliability and efficiency of compressor, especially for the compressors without radial compliance mechanism. In this study, the fluid-solid-thermal simulation is conducted to get the temperature field, the pressure load distribution on scroll wraps. Then, the influences of the temperature, pressure and the inertial force on the deformation of scroll wraps are investigated. On these bases, the variation of gap size and potential contact between scroll wraps is analysed under various orbiting radii. It is shown that the deformation caused by temperature is more significant than the pressure and inertial force. The contact occurs even though orbiting follows the theoretical orbiting radius due to the positive radial deformation of scroll wrap and negative radial deformation of inner wall of fixed scroll.
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