Influence of sealing cavity geometries on flank clearance leakage and pressure imbalance of micro-scale transcritical CO2 scroll expander by CFD modelling
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Abstract:
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.Keywords:
Leakage (economics)
Overall pressure ratio
Working fluid
Isentropic process
Air entrainment
Conceptual design of scroll expander and scroll compressor for 10 kW-class Stirling engine utilizing solar energy as heat source has been carried out. CO2 was chosen as the working fluid, since it is nontoxic, inflammable and it has relatively higher density among probably usable gases. Gas temperature at the expander inlet was set at 700∂ C, and that at the compressor inlet was at 40°C. System efficiency and engine power output were calculated for the high side pressure ranging from 5 MPa to 8 MPa with the pressure ratio of 1.5~4. System efficiency reached maximum at the pressure ratio of about 2.5, and the peak efficiency increased with increasing high side pressure. Due to safety concern, the pressure condition of 6 MPa / 2.5 MPa was chosen as design condition. Orbiting scroll members for the expander and compressor were designed to have double-sided structure in order to reduce the overall scroll size and to cancel out the axial gas forces acting on the orbiting scroll base plate. Expander and compressor were connected by two common shafts. Timing belt were adopted to link these two shafts, functioning as anti-rotation device for the orbiting scroll members. Gear assembly was used to extract the net power output from the expander. By parametric study on the scroll profile, smaller possible size for the scroll members was obtained. With the shaft speed of 3600 rpm, the shaft output of the designed scroll expander was calculated to be 45.4 kW, while input power for the scroll compressor was 34.5 kW, yielding 10.9 kW for the output power of the Stirling engine. System efficiency was estimated to be about 7.3%, and overall efficiencies of the scroll expander and compressor were around 84.1% and 88.3%, respectively.
Scroll compressor
Overall pressure ratio
Maximum power principle
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Isentropic process
Organic Rankine Cycle
Working fluid
Thermal efficiency
Thermodynamic cycle
Degree Rankine
Diaphragm (acoustics)
Overall pressure ratio
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Energy and exergy analysis of a Brayton cycle with an ideal gas is given. The irreversibility of the adiabatic processes in turbine and compressor is taken into account through their isentropic efficiencies. The net work per cycle, the thermal efficiency and the two exergy efficiencies are expressed as functions of the four dimensionless variables: the isentropic efficiencies of turbine and compressor, the pressure ratio, and the temperature ratio. It is shown that the maximal values of the net work per cycle, the thermal and the exergy efficiency are achieved when the isentropic efficiencies and temperature ratio are as high as possible, while the different values of pressure ratio that maximize the net work per cycle, the thermal and the exergy efficiencies exist. These pressure ratios increase with the increase of the temperature ratio and the isentropic efficiency of compressor and turbine. The increase of the turbine isentropic efficiency has a greater impact on the increase of the net work per cycle and the thermal efficiency of a Brayton cycle than the same increase of compressor isentropic efficiency. Finally, two goal functions are proposed for thermodynamic optimization of a Brayton cycle for given values of the temperature ratio and the compressor and turbine isentropic efficiencies. The first maximizes the sum of the net work per cycle and thermal efficiency while the second the net work per cycle and exergy efficiency. In both cases the optimal pressure ratio is closer to the pressure ratio that maximizes the net work per cycle.
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Brayton cycle
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Exergy efficiency
Thermal efficiency
Thermodynamic cycle
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Scroll compressor
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Isothermal process
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this study describes the development of a micro-radial turbine for organic Rankine cycle powered by low temperature heat source. To achieve the aim, different working fluids with operating conditions were investigated to identify the most efficient turbine for low-grade heat source with temperature less than 85 °C. In previous studies related to organic Rankine cycle analysis, the isentropic efficiency of the turbine was assumed constant, while in this work, the isentropic efficiency is calculated at different operating conditions for each working fluid. The ANSYS R17 - CFX software is used to perform the three-dimensional computational fluid dynamic analysis of the radial-inflow turbine for a number of organic working fluids (R141b, R245fa and n-pentane) and different operating conditions. The real fluid properties using equations of state were employed and results showed that n-pentane has the highest performance for all operating conditions. The maximum total isentropic efficiency of turbine was about 80.15% with 5.119 kW power output and 10.34% cycle thermal efficiency.
Organic Rankine Cycle
Isentropic process
Working fluid
Rankine cycle
Pentane
Degree Rankine
Inflow
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Organic Rankine Cycle
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Rankine cycle
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Organic Rankine Cycle
Carnot cycle
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In order to study the influence of the number of centrifugal impeller blades on the aerodynamic performance of the centrifugal compressor, the influence of the number of blades with and without splitter on the pressure ratio, isentropic efficiency, mass flow, shaft power and outlet airflow angle of the compressor is analysed. The research results show that for transonic miniature centrifugal compressors, increasing the number of blades can improve the compressor's isentropic efficiency and pressure ratio. The compressor's pressure ratio and isentropic efficiency first increase and then decrease with the increase in the number of blades. When the number of blades with splitter blades is 12, the pressure ratio of the centrifugal compressor reaches the maximum of 1.69, and the isentropic efficiency is 68.8%; increasing the splitter blade compressor has little change in pressure ratio, but can improve the compressor Isentropic efficiency and mass flow. The number of blades is an effective method to improve its isentropic efficiency and pressure ratio. This analysis method provides a reference for the judicious selection of the number of blades and provides a reference for the design of miniature centrifugal compressors under high Reynolds number.
Isentropic process
Centrifugal compressor
Overall pressure ratio
Axial Compressor
Splitter
Mass flow
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Organic Rankine Cycle
Isentropic process
Working fluid
Thermal efficiency
Degree Rankine
Rankine cycle
Electrical efficiency
Thermodynamic cycle
Maximum power principle
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Isentropic process
Working fluid
Organic Rankine Cycle
Exergy efficiency
Rankine cycle
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