Strain relief devices are crucial for a supercritical CO2 (sCO2) power installation to cope with the wide range of variations in temperature, pressure, mass flow rate, mechanical impact, and vibration in the system. However, the conventional thick-walled strain relief devices have huge spatial extension, limiting their application in vehicle or ship carried mobile power installations, where the spatial size is stringently constrained. In this study, a compact strain relief device with a helical microtube bundle replacing the traditional thick-walled pipe is proposed. Two parameters, i.e., the equivalent elastic modulus (Ex) and the maximum absorbable strain (epsilon mx), are introduced to characterize the load-relief capability and the deformation tolerance, respectively. Threedimensional finite element models (3D FEM) are developed for mechanical analyses under representative operating conditions up to 20.8 MPa and 600 degrees C. The numerical model is validated with analytical solutions for the deformation of a helical spring and the experiment of pressure drop for the fluid flow in a helical tube, with discrepancies less than 2%. Parametric sensitivity studies reveal that both increasing helical diameter and reducing cycle length are effective means to significantly decrease Ex, while maintaining acceptable pressure drops, generally on the order of 10-30 kPa for typical configurations. Based on these findings, a design-driven procedure for configuration selection is established. Comparative analyses with a conventional expansion loop and a bellows expansion joint under the same axial deformation demonstrate that the proposed strain relief device reduces the overall height from 1.065 m to 0.490 m and the total length from 6.088 m to 1.210 m while maintaining the same axial deformation characteristics. The peak von Mises stress is reduced from 124.0 MPa to 84.2 MPa, resulting in an increase of predicted fatigue life from approximately 7100 to 80,000 cycles. The results show that the proposed strain relief device with helical microtube bundles could achieve improved compactness, lower thrust loads on connected equipment, and enhanced structural reliability, therefore is well suited for sCO2 power installations, especially in critical space-constrained circumstances.

The supercritical carbon dioxide Brayton cycle is a closed system with a turbine that generates CO2 gas leaks when operating at high temperatures, pressures, and rotational speeds. Currently, dry gas seals are primarily used to mitigate these issues at the shaft end of rotating components. However, due to their limited heat resistance in high-temperature turbines and the operational challenges they face in such environments, it becomes necessary to apply active cooling measures to the turbine shaft. However, introducing cooling gas affects turbine performance and alters the state of the working fluid, which significantly impacts the overall system performance. This study evaluates the influence of turbine shaft cooling on real operational performance by using an actual system as the basis for analysis. The results show that as the turbine inlet temperature increases, the efficiency loss due to turbine shaft cooling also rises. When the compressor outlet pressure remains constant, an increase in the mass flow rate of working fluid reduces the efficiency loss caused by turbine shaft cooling. When the mass flow rate of the working fluid in the system remains constant, the efficiency loss due to turbine shaft cooling initially increases and then decreases as the compressor outlet pressure rises. Finally, when the cooling gas inlet temperature rises, the system efficiency loss due to turbine shaft cooling increases.

The thermal-hydraulic and structural performance of heat exchangers critically governs the compactness and safety of recuperators in supercritical CO2 (scCO2) power cycles. While the Printed Circuit Heat Exchanger (PCHE) with zigzag channels has been widely studied for its excellent heat transfer, a systematic understanding of how key geometric parameters affect thermal-hydraulic performance and mechanical integrity remain lacking. This study systematically investigates the effects of key geometric parameters on both the thermal-hydraulic and structural performance of a zigzag channel PCHE over a wide range of conditions using 3D simulation. The results indicate that, relative to the straight channel design, the Performance Evaluation Criteria (PEC) varies with both the inclination angle and pitch of zigzag channel, and there are different extreme values in different intervals. This trend is driven by the competition between heat transfer enhancement from secondary flows and the pressure loss due to flow separation. An inclination angle of 40 degrees and a pitch of 10 mm yielded the optimal thermal-hydraulic performance within the tested range. The bending of the channel induces significant stress concentration, with stress on the cold side increasing markedly with the inclination angle, while the effect of pitch is negligible. Optimizing the bend into a rounded transition segment effectively reduced both pressure drop and peak stress. Implementing this optimized design in a 3 MWe pilot-scale scCO2 cycle plant is projected to reduce the weight and volume of recuperator by about 25% and 14%, respectively.

The supercritical carbon dioxide Brayton cycle is a closed system with a turbine that generates CO2 gas leaks when operating at high temperatures, pressures, and rotational speeds. Currently, dry gas seals are primarily used to mitigate these issues at the shaft end of rotating components. However, due to their limited heat resistance in high-temperature turbines and the operational challenges they face in such environments, it becomes necessary to apply active cooling measures to the turbine shaft. However, introducing cooling gas affects turbine performance and alters the state of the working fluid, which significantly impacts the overall system performance. This study evaluates the influence of turbine shaft cooling on real operational performance by using an actual system as the basis for analysis. The results show that as the turbine inlet temperature increases, the efficiency loss due to turbine shaft cooling also rises. When the compressor outlet pressure remains constant, an increase in the mass flow rate of working fluid reduces the efficiency loss caused by turbine shaft cooling. When the mass flow rate of the working fluid in the system remains constant, the efficiency loss due to turbine shaft cooling initially increases and then decreases as the compressor outlet pressure rises. Finally, when the cooling gas inlet temperature rises, the system efficiency loss due to turbine shaft cooling increases.

The supercritical carbon dioxide Brayton cycle, as a promising next-generation power cycle, has the advantages of high efficiency and small size, while now exhibits actual operating efficiencies significantly lower than those predicted by theoretical models. In this study, a steady-state analysis model of the supercritical carbon dioxide Brayton cycle is developed to investigate the impact of internal losses, including pipeline pressure loss, leakage reinjection, and turbine cooling, on the overall system performance. Through evaluations under both design-point and off-design operating conditions, it is found that internal losses markedly degrade system efficiency. Specifically, the efficiency loss due to internal losses increases with rising turbine inlet temperature and working medium mass flow rate, whereas it decreases with increasing compressor outlet pressure. Exergy analysis further reveals that internal losses reduce the net power output and increase system exergy losses, primarily due to enhanced exergy losses in components such as pipelines, auxiliary heaters, compression pumps, and coolers. Quantitative assessment indicates that the influence of internal losses varies under different operating conditions. Among them, pipeline pressure loss has the most significant impact on system performance at design conditions, followed by turbine cooling, while the effect of leakage reinjection is relatively minor. This study provides insights that can support more accurate modeling, improved system design, and performance optimization of supercritical carbon dioxide Brayton cycle systems.

Rotating machinery in supercritical power cycles typically employs dry gas seals or labyrinth seals, and the leakage gas needs to be reinjected to maintain long-term operation. This paper focuses on a MW-scale supercritical CO2 power generation system. State of leakage gas after passing through the dry gas seals and labyrinth seals were estimated. The impact of reinjecting the leakage gas on system performance was evaluated through simulations. The effects of different leakage reinjection locations on the performance of the supercritical CO2 Brayton cycle were analyzed, and the performance impacts of using dry gas seals versus labyrinth seals were compared. The results demonstrated that using a dry gas seal and reinjecting the leakage gas back into the hotend inlet of the high-temperature regenerator led to the smallest reduction in system efficiency, with a decrease of 1.013 %. For the labyrinth seal, reinjecting the leakage gas from the compressor into the turbine inlet, and from the turbine into the hot-end inlet of the high-temperature regenerator minimized the reduction in system efficiency. When the outlet pressure of leakage gas after the labyrinth seal ranged from 5 to 9 MPa, system efficiency improved as the outlet pressure increased, and the heat exchanger area per unit of power output decreased, enhancing the overall compactness of the power plant. These findings provide valuable insights for the design and optimization of supercritical CO2 Brayton cycle leakage reinjection systems.

The supercritical Carbon Dioxide (sCO2) 2 ) closed Brayton cycle is a promising power generation technology, while the efficient cooling of CO2 2 and precise control of Compressor Inlet Temperature (CIT) is crucial for the high cycle efficiency and stable compressor operation due to the acute variation of thermophysical properties in the near-critical region. In conventional indirect cooling scheme, an intermediate single-phase water circuit is used to transfer heat from CO2 2 to water at the precooler, and then from water to the environment at the cooling tower, having the disadvantage of large pumping work consumption and high thermal resistance. In this work, a self- driven two-phase looped thermosyphon that significantly enhances the heat transfer by internal evaporation and condensation, is proposed to replace the water circuit. An experimental ultra-compact cooling system, consisting of a looped thermosyphon combined with micro-channel evaporator and condenser, filled with R134a coolant, is designed, fabricated, and tested. Visualized observation of the two-phase flow pattern and simultaneous measurement of the temperature, pressure and mass flow rates are conducted. A nodal analysis method is adopted, and a MATLAB code is developed for analyzing the internal fluid flow and the coupled sCO2-R134a-Air 2-R134a-Air heat transfer, which is validated by experiment data. The results show that, the CO2 2 temperature could be accurately maintained at a specified near-critical point with a fluctuation of less than 1 K, and the average heat-releasing temperature can be reduced, while considerable pumping work, usually accounting for 2-5 % of the rated power output can be saved, thus contributing to increased cycle efficiency and system compactness.

The two-phase transcritical carbon dioxide (tCO2) vortex tube serves as a critical component in tCO2 refrigeration and power cycles, making experimental investigation of its performance imperative. A test rig for evaluating the performance of the vortex tube in a tCO2 power cycle was developed. A compact, high-pressure resistant, and hermetic vortex tube was designed and fabricated, providing reference for future structural optimization of highpressure tCO2 vortex tubes. The inlet conditions were classified into supercritical liquid-like and supercritical gas-like regions, and experiments were conducted to investigate the effects of mass flow rate, cold mass fraction, inlet temperature, and pressure on performance of the vortex tube. Notably, this research presents the first experimental data for a tCO2 vortex tube expanding from the liquid-like region into either the liquid phase or two-phase region. Results show the tCO2 vortex tube exhibits weak temperature separation effects. When the inlet condition lies in the gas-like region, the temperature separation performance of the vortex tube is higher than in the liquid-like region; the performance of vortex tube demonstrates greater sensitivity to inlet mass flow rate variations compared to liquid-like region conditions. Both cold and hot outlet temperatures remain below the inlet temperature.

With the trend toward miniaturization of functional devices, material preparation and thermal management processes are also limited to small spaces. Microchannels have emerged as an optimal solution for these challenges. Microchannel-based reactors can generate hybrid materials, and the integration of microchannel heat sinks and substrates can control the temperature of high-power devices. The microstructure within microchannels significantly influences fluid flow and heat transfer, impacting the efficiency of both reaction and heat dissipation processes. Pin-fins are widely used microstructures due to their ability to increase heat transfer area and enhance fluid mixing. In order to find the optimal structure of the fins, it is essential to explore a vast parameter space. In this paper, artificial neural network and genetic algorithm are combined to optimize the copper irregular pin–fin microchannels. Initially, a large number of numerical simulations are performed, focusing on adjustable parameters such as fin radii in various directions, while monitoring the heating surface temperature and the pressure drop of the fin section. Then, nearly 2000 sets of accumulated data are used to train the neural network, establishing the relationship between structural and performance parameters. Finally, a genetic algorithm is employed for multi-objective optimization, yielding a Pareto front. The findings reveal that the newly obtained optimized microchannels exhibit superior thermal–hydraulic performance compared to traditional microchannels. The mechanism of heat transfer enhancement in the optimized microchannel has been revealed: the arrangement of asymmetric fins allows for more thorough contact between the fluid and the fins. Based on this rule, the newly designed multi-fin microchannels exhibit better performance under both fixed heat flux and fixed temperature conditions. In addition, doping high thermal conductivity materials into the substrate to form composite materials can significantly improve the heat transfer performance of microchannels, and using materials with different doping ratios in different parts of the microchannel can effectively improve the temperature uniformity of the heating surface. Thus, uniform-temperature microchannels are designed by combining metal materials (such as copper and aluminum) with non-metal materials (like diamond and graphite).

This study develops a system simulation model based on the megawatt-class supercritical carbon dioxide Brayton cycle unit in Hengshui, China, to examine the effect of pipe section resistance on system performance. The results indicate that increased pressure loss in pipe sections significantly reduces power generation efficiency and power supply efficiency. Specifically, a 100% increase in pressure loss results in 3.290% and 4.377% decreases in power generation efficiency and power supply efficiency, respectively. Moreover, higher pressure loss leads to increased CO2 mass flow rate at design power generation load. When the pressure loss is doubled, it requires a 17.593% increase in CO2 mass flow rate to achieve the design power. Pressure loss in high-pressure section has minimal impact on system performance, while resistance in low-pressure section significantly affects it. For instance, a 100% increase in pressure loss from the compressor outlet to the low temperature regenerator inlet causes a 0.37% decrease in system power supply efficiency, and a similar increase from the high temperature regenerator outlet to the low temperature regenerator inlet leads to a 0.76% decrease. These findings provide meaningful guidance for component design and piping layout optimization for similar systems.

Heat transfer deterioration (HTD) is one of the important issues in the study of supercritical fluid (SCF) heat transfer. However, when the SCF crosses the pseudo-critical point, the none-quilibrium process occurs in liquid, so SCF is very complicated. Recently, the existence of SCF pseudo-boiling on a macro scale has sparked controversy. There is still no unified understanding of the mechanism of gas-like and liquid-like transition affecting heat transfer. In this work, it is assumed that SCF has a macroscopic phenomenon similar to subcritical flow boiling. By analogy with subcritical boiling heat transfer, a boiling critical point model is proposed to describe the HTD in supercritical CO2. Our study reveals that the HTD caused by pseudo-boiling only occurs under large temperature gradient, which makes the superheated liquid-like layer cover the wall, and the gas-like and liquid-like may present different distribution forms, thus changing the heat transfer characteristics. When the wall temperature is higher than the pseudo-critical temperature and the enthalpy of the fluid layer covering the wall exceeds a certain value, the HTD may occur. The proposed theoretical model can explain the experimental results well, and the prediction accuracy of heat transfer correlation considering pseudo-boiling is greatly improved. In this work, the connection between supercritical heat transfer and subcritical heat transfer is established theoretically, which provides a new idea for studying the deterioration of SCF heat transfer, thus enriching the theory of supercritical heat transfer.

The startup process is the crucial transition phase of the supercritical carbon-dioxide Brayton cycle, so it is essential to focus on and investigate the transient performance for the system’s safety and stability. The pressure in the buffer tank approaches the safety upper limit with different startup schemes during the joint commissioning of the compressor and heater in a MWe-scale experiment system, while the maximum temperature is 309 °C. Hence, dynamic simulations are carried out to explore the dynamic startup characteristics from a cold state or a warm state to the turbine pre-start condition, in which 60% of the rated mass flow rate and 67% of the rated compressor speed are reached in the end. The results show that, when starting from a cold state, the startup scheme of simultaneously heating and speeding up has a limited effective application scope. Two venting operations during the above process help the system establish heat regeneration and promote temperature uniformity in the system. Furthermore, when starting from a warm state with an existing temperature gradient in the system, the startup scheme of simultaneously heating and speeding up is more effective and has a more extensive range of control.

SUMMARY The airtightness of heated tobacco product (HTP) packs is a very important indicator for the product quality and is also of great importance during the conditioning process. A method for evaluation of the airtightness was developed based on the air pressure difference in a constant pumping configuration. The essential feature of this method is that the pressure difference between the inside and the outside of the HTP packs during the deflation process is used to characterize the sealing quality of HTP packs. The detailed setup, the principle as well as the determination procedure are described. The accuracy and the repeatability of the method were assessed, and the effect of airtightness on the conditioning process was also investigated. The developed method is proven to be reliable with a standard deviation less than 0.09 kPa and repeatability less than 0.30 kPa. In addition, it was found that, although the transmission of moisture between HTPs and atmosphere could not be entirely prevented by the packs, airtightness still plays a significant role during the conditioning process, especially if the airtightness was at a relatively low level (e.g., lower than 1.5 kPa under a pumping flow rate of 200 mL/min). The method provides a promising way to assess and monitor the sealing quality of HTP packs, and it is suggested that the airtightness of the pack should not be lower than 2 kPa under a pumping flow rate of 200 mL/min. [Contrib. Tob. Nicotine Res. 32 (2023) 140–145]

Conjugate fluid-solid heat transfer in a pin-fin microchannel heat sink is an effective way to dissipate heat from the heating surface with high heat flux. The introduction of fins increases the heat exchange area and enhances flow turbulence, while it increases the flow resistance at the mean time. The thermal-hydraulic performance of heat sink is affected by fin shape, density and flow parameters. In this paper, contrived numerical simulations of the flow and heat transfer process in elliptical pin-fin microchannel heat sink are carried out, including 2033 cases with different fin sizes, numbers and flow velocities. The simulation results show that the flow velocity and fin transverse width are the main factors affecting heat transfer and fluid flow. Three artificial neural networks are established to predict the average tem-perature, the temperature non-uniformity of heating surface and the pressure drop of microchannel. The predicted results show that the pump power and heating surface temperature are contradictory objec-tives. A microchannel with the optimal thermal-hydraulic performance is selected. It has numerous fins which are longer in the flow direction. The empirical correlations for Nusselt number and friction coeffi-cient of the optimal microchannel are proposed.(c) 2023 Elsevier Ltd. All rights reserved.

The centrifugal compressor is the core component of supercritical CO2 power cycle, and its performance and operation stability are research hotspots. However, there are few experimental studies, especially for compressors used in Mwe-scale power cycles. In this paper, based on a 1 MWe supercritical CO2 power cycle, a single-stage centrifugal supercritical CO2 compressor is designed with speed of 40,000 RPM, a pressure ratio of 2.5 and a mass flow of 16.3 kg/s. In order to carry out the compressor test, a general experimental platform for MWe sCO2 compressors is built. In the test, the mass flow range is 13.5~18 kg/s and the maximum experimental pressure ratio is close to 2.0. The performance curve of the compressor of 31,000 ± 1000 RPM is obtained, and the historical curve of the experiment is given. Then, the experimental curve is compared with the design curve using a dimensionless method. The isentropic head coefficient of the experimental curve is lower than the design value, and the experimental curves shift towards the boundary of small flow coefficient. Finally, the influence of compressor inlet condensation on compressor performance and the change of operating boundary is preliminarily explained.

The present work focuses mainly on the effects of heat input, filling ratio, inclination angle, tube diameter and coolant temperature on the thermal performance of a wraparound heat pipe charged with R134a. Results show that thermal resistance decreases with the increase of heat input when the filling ratio is larger than 40%. An optimal filling ratio for the heat pipe with the best performance exists between 50% and 60%. The pressure of working fluid in the heat pipe exceeds 1.6 MPa in the 70% and 80% filling ratios experiments. For larger inclination angles (theta > 10 degrees), the thermal resistance decreases with increasing the heat input and finally tends to a stable value. For heat loads of 420 W and greater, the values of thermal resistance are 0.056, 0.07, 0.034 and 0.027 K/W for outer diameters of 8, 10, 12 and 16 mm, respectively. No significant difference in thermal resistance at different coolant temperatures is observed for heat inputs greater than 300 W. In all experiments, for a 22 degrees inclination angle, an outer diameter of 16 mm, and a filling ratio of 50%, the best performance of heat pipe is observed and the lowest value of thermal resistance is 0.027 K/W.

针对两相环路热虹吸管中出现的间歇沸腾不稳定现象,分别以R134a、水和无水乙醇作为工质,通过流场可视化实验观测,探究了间歇沸腾出现的条件及其对环路传热特性的影响.实验结果表明,在中等充液率和中等加热热流密度条件下更容易发生间歇沸腾现象;流型的周期性变化引起环路内部压力和温度波动,同时会增加环路的均温性;流型变化和波动特性因工质不同而有所差别,水作为工质时,波动周期更长,流型变化及压力波动更复杂.

The coupled momenta induced by thermal effects during evaporation near liquid-vapor interfaces cause complex three-dimensional flow structures, called thermal buoyant-capillary flows. In this study, we investigated the coupled flow mechanisms, observing the internal flow structure of evaporating micro-mini scale menisci in microtubes (680-1560 mu m) by the micro-particle image velocimetry (PIV) technique. The horizontal and vertical motions of 1-mu m fluorescent particles in volatile ethanol was visualized to obtain the shape of the flow in high-resolution (10-20 mu m resolution). The evaporation rate (2-1800 ng/s) and environmental temperature (20-50 degrees C) were controlled using the Joule heating method. We discuss the effects of varying the thermal conditions and tube size on the position and strength of the vortices. In addition to the experimental work, a simplified numerical simulation was also carried out to estimate the thermal properties that were not measured during experiment. As a result, it was explained that the vortices near the wall of the upper tube are initialized by weak thermal effects from the wall, and higher heating condition cause them to move down the tube and enhance their vorticity by dominant buoyancy effect. In addition, vortices in tubes with large diameters are weaker and their motion is delayed due to the increase in gravitational effects. (C) 2019 Elsevier Ltd. All rights reserved.

This paper presents an interval prediction algorithm based on grey system modelling, which is proposed for the forecasting of strong-oscillation time series with small samples. In the proposed algorithm, the upper and lower envelope of an oscillating sequence is obtained through cubic spline interpolation, and distance between the envelope and the fitted sequence derived from grey system model is dynamically expanded according to the oscillation intensity. After that, prediction value of the envelope distance sequence is calculated, and adjusted adaptively based on the new information priority principle. Finally, the interval prediction result is obtained. To verify the performance of the algorithm, five application cases from different fields were adopted. Compared with five representative algorithms in the recently related field, the proposed algorithm has distinct advantages in the prediction of small-sample strong oscillation time series.