To reveal the transient evolution mechanism of cavitation structures in a mixed‑flow pump during the startup process under strongly induced cavitation conditions，experimental investigations on cavitation turbulent structures were conducted during the rapid startup of a mixed‑flow pump at an initial inlet vacuum degree of -30 kPa.High‑speed photography，Voronoi cavitation structure identification，and grayscale quantitative analysis methods were employed to obtain the spatiotemporal evolution characteristics of the cavitation flow field inside the mixed‑flow pump impeller during the 1 s startup process.The results indicate that under the 1 s rapid startup condition，the low‑pressure region inside the impeller forms drastically within a short time，inducing synchronous cavitation excitation and rapid development into large‑scale cavitation clouds，exhibiting transient characteristics of concentrated burst and intense collapse.The cavitation cloud covers multiple flow passages within an extremely short time and undergoes violent collapse，with its evolution process experiencing stages of inception，growth，collapse，and restructuring.Grayscale analysis shows that the cavitation intensity reaches its peak at approximately 0.60T，but significantly weakens as the impeller acceleration is completed.The research findings reveal the spatiotemporal organization laws of cavitation structures during the rapid startup of mixed‑flow pumps，providing references for optimizing pump start‑stop control strategies，mitigating transient cavitation impacts，and improving unsteady cavitation prediction models.

Aiming at the disadvantages of traditional crude oil treatment processes，including large floor area，low efficiency and long treatment duration of gravity settling tanks，a vertical separator equipped with built‑in frustum electrodes was developed in this study.A high‑frequency and high‑voltage pulsed alternating current（AC） power supply was adopted，and the water content at the oil outlet was selected as the evaluation index for separation performance.Demulsification and dehydration experiments of the novel vertical separator with built‑in frustum electrodes were carried out to investigate the influences of split ratio，inlet water content，inlet flow rate，and electric field parameters（voltage，frequency，and electric field action time） on the separation performance.The experimental results indicate that there exists an optimal combination of electric field parameters for the vertical separator.Under the operating conditions of an inlet water content of 30%，an inlet flow rate of 100 mL/min and a split ratio of 0.3，the optimal electric field action time，electric field voltage and electric field frequency are 16.5 min，1.85 kV and 3.2 kHz，respectively，achieving a minimum water content of 0.79% at the oil outlet.The influencing degrees of various factors on the separator performance follow the order of electric field voltage>electric field frequency>electric field action time.This study provides a theoretical reference and technical guidance for the research，development and engineering application of high‑efficiency vertical separators for the dehydration of crude oil emulsions.

Aiming at the problems of low pre‑separation efficiency and unsatisfactory oil-water separation performance for high water‑cut produced fluid，a grid‑type coalescing component was designed based on theoretical analysis of oil‑droplet motion and coalescence behavior to increase the rising velocity of oil droplets and enhance the overall separation performance.Comparative experiments were carried out to systematically evaluate the separation performance of four configurations：separator without coalescing inserts，parallel perforated corrugated‑plate，parallel serpentine‑plate，and the proposed grid‑type coalescing component，under various operating temperatures and residence times.The experimental results demonstrate that at an inlet temperature of 35 ℃ and a residence time of 20 min，the oil content at the aqueous‑phase outlet decreases by 79%~88.4% after installing coalescing inserts.Specifically，the grid‑type coalescing component reduces the aqueous‑phase outlet oil content to 174 mg/L，which considerably outperforms conventional components and satisfies the requirements for subsequent water treatment.As the temperature rises from 25 ℃ to 35 ℃，the separation performance of all separators is improved，and the grid‑type coalescing component achieves the largest reduction （76.8%） in outlet oil content.Balancing separation performance and energy consumption，30  ℃ is identified as the optimal operating temperature.Prolonging the residence time can further boost separation efficiency；the improvement is most pronounced for the grid‑type coalescing component.During 60  min continuous operation，the aqueous‑phase outlet oil content stabilizes at approximately 257 mg/L，the water content at the oil‑phase outlet remains at 29%，and the overall separation efficiency reaches up to 99.8%，exhibiting favorable stability for continuous operation.This novel coalescing component can substantially improve the separation efficiency and operational stability for high water‑cut produced fluid，offering a new technical route and theoretical support for the efficient treatment of oilfield produced fluid.

Aiming at the problem of poor signal performance of the high‑pressure water jet target detection due to the interference of media such as soil layers and sand layers covering the surface of deeply buried targets，a method of using high‑pressure rotating jet to strip the surface soil layer was proposed to improve the detection efficiency of subsequent high‑pressure water jets.The standard k-ε turbulence model and VOF multiphase flow model were applied to study the influence of different nozzle types，rotating elements，and nozzle contraction angles on the pressure loss characteristics of rotating spray head.Numerical simulations and experimental verifications of the soil‑breaking efficiency of rotating spray head under different incident angles and spray distances were conducted.The results show that the optimal geometric parameters inside the spray head are as follows：the nozzle type is a conical long straight shape，the rotating element is a single‑head with a 12 mm pitch，and the nozzle contraction angle is 30°.On this basis，when the incident angle of the rotating spray head is 60° and the spray distance is 15 cm，the area of stripped soil layer is the largest.The research can provide a reference for the application of high‑pressure rotating jets in the detection of large‑scale special targets.

Based on the issues of low separation accuracy and high pressure drop in existing deep purification processes for produced compressed air，this study proposes an integrated wellhead sand and liquid removal device for compressed air energy storage applications，which couples multiple separation mechanisms including sedimentation，wet capture，and filtration.The effects of gas flow rate，inlet sand concentration，inlet liquid concentration，and sand particle size on separation performance were systematically investigated.The results indicate that as gas flow rate and inlet sand concentration increase，both sand and liquid removal efficiencies first rise and then decline，while pressure drop increases；the maximum sand and liquid removal efficiencies reach 99.92% and 99.96%，respectively，with a minimum pressure drop of 0.46 kPa.As inlet liquid concentration increases，sand removal efficiency first increases and then decreases，liquid removal efficiency increases，and pressure drop rises；the maximum sand and liquid removal efficiencies are 99.90% and 99.98%，respectively，with a minimum pressure drop of 1.03 kPa.When treating sand particles with larger diameters，the device achieves higher sand and liquid removal efficiencies and a lower pressure drop.Under all tested conditions，liquid removal efficiency exceeds 99.53%，sand removal efficiency exceeds 99.74%，and pressure drop remains below 6.72 kPa.The separation efficiencies for particles of 5 μm and 10 μm are higher than 97.27% and 99.01%，respectively.The recommended operating conditions are gas flow rates of 165~240 m³/h，inlet sand concentrations of 0.45~0.9 kg/m³，and inlet liquid concentrations of 1.3~7 kg/m³，within which the device maintains high sand and liquid removal efficiencies and relatively low pressure loss.The proposed integrated wellhead sand and liquid removal device can effectively remove sand particles and liquid droplets entrained in compressed air，thereby ensuring operational safety for downstream energy conversion equipment and providing a technical reference for air purification in compressed air energy storage systems utilizing abandoned oil and gas reservoirs.

Addressing the high‑energy‑consumption issue caused by turbulence in fluid pipeline transportation，this study introduces polyacrylamide drag‑reducing agents into the flow field.By combining numerical simulation and particle image velocimetry （PIV） experiments，the influence of the drag‑reducing agent on the turbulent drag reduction characteristics in a circular pipe is investigated，and the intrinsic relationship between the drag reduction performance and the flow field structure is revealed.After comparing three low‑Reynolds‑number k-ε models，the Lam-Bremhorst model was selected for its simulation accuracy above 96.56%.Results indicate that DRA addition thins the near‑wall boundary layer，enhancing flow performance and suppressing turbulence.In the flow rate range of 5.7~13.7 m³/h，drag reduction reaches 30%~40.9%，centerline flow velocity increased by 2.97%~2.24%，turbulence intensity decreases by 3.86%~3.81%，and near‑wall turbulent dissipation drops by 45.2%~28.6%.Mechanistic analysis shows that polyacrylamide molecules generate elastic stress in the near‑wall region.This both suppresses turbulent coherent structures and kinetic energy fluctuations，and thins the boundary layer while improving the velocity profile，thereby reducing flow resistance.The study provides a theoretical basis for understanding the near‑wall action mechanism of DRAs and for improving energy efficiency in pipeline flow.

To address the erosion wear issue of bends in ice-water two‑phase flow pipelines for polar ships，a study on buoyancy-flow coupled erosion characteristics under different relative orientations was carried out.Numerical simulations were performed on a marine DN100 bend using the Euler-Lagrange bidirectional coupling method，and the erosion wear characteristics under three flow directions （along the X，Y，and Z axes of a 3D coordinate system） were analyzed for different particle sizes and bend angles.The results show that the spatial distribution of erosion hot spots varies significantly under different flow directions.The erosion intensity is the strongest in the X direction，forming an asymmetric erosion hot spot on the outer arc and lower side of the bend，while the erosion intensity is lower and the spatial distribution is more symmetric in other directions.The region near the 60° bend angle is the most severely eroded area.As the particle size increases，the average erosion rate reaches a valley at d_p=0.5 mm，and the influence of buoyancy on particle aggregation position and erosion asymmetry is further amplified.In engineering applications，the layout of pipelines in the X direction should be avoided as much as possible，and enhanced protection should be provided at the 60° bend position.The research results can provide a reference for the layout optimization and local corrosion‑resistant design of bends in ice-water two‑phase flow pipelines for polar ships.

To meet the lightweight design requirements of nuclear power pneumatic actuator brackets，a topology optimization study was conducted on the bracket using HyperWorks software.An optimization method based on the variable density method was adopted，with the objective function of minimizing structural weight and the design variable being the material density of elements within the design domain.The optimal material distribution was solved under displacement constraints （±0.29 mm） and stress constraints （≤248 MPa）.Based on the solution results，the bracket model was reconstructed，and static analysis validation and modal validation were performed.The results show that after topology optimization，the bracket weight reduction rate reached 24.22%，with the main weight reduction area being the side walls of the bracket.The maximum displacements of the bracket in the X，Y，and Z directions are all less than ±0.29 mm，and the stresses were reduced by 46.96%，53.58%，and 58.29%，respectively，compared with those before optimization.The resonance frequencies in the X，Y，and Z directions （123.3，88.3，139.28 Hz） were all much higher than the frequency requirement for nuclear power operating conditions （33 Hz），indicating that the bracket has good vibration resistance.This lightweight design method effectively improves the design efficiency of the bracket and reduces its manufacturing cost，providing a reference for the optimization of pneumatic actuator brackets with similar structures.

To address the problems of insufficient opening force and excessive leakage in external spiral‑groove mechanical seals under high‑parameter operating conditions，a three‑dimensional flow‑field model of the end‑face liquid film was established.Computational fluid dynamics （CFD） simulations were combined with experimental testing to systematically investigate the coupled influence mechanisms of structural and operating parameters on the liquid-film flow‑field characteristics.The performance differences between the single spiral groove and derivative stepped spiral grooves were comparatively analyzed in terms of opening force and leakage rate.The results show that groove depth has a significant non‑monotonic effect on sealing performance.Relatively favorable overall liquid‑film performance is achieved when the groove depth is 15 μm，the spiral angle is 18°，and the groove diameter ratio is 0.5.Both the opening force and leakage rate increase approximately linearly with increasing medium pressure and rotational speed，while medium pressure has a more pronounced effect on the opening force.Compared with the single spiral groove，the derivative stepped spiral grooves effectively enhance the hydrodynamic effect through secondary pressure peaks generated by the steps.Under a high‑pressure condition of 5.5 MPa，the opening force of the three‑step groove is 8.03% higher than that of the single spiral groove，demonstrating superior load‑carrying capacity.At a high‑speed condition of 13 000 r/min，its leakage rate is reduced by approximately 6.3%.The stepped structure therefore offers advantages in both high load‑carrying capacity and low leakage under high‑parameter operating conditions，providing a theoretical basis and data support for the optimal design of high‑performance mechanical seal structures.

To address the issues of insufficient air volume and abnormal sound quality in the dual centrifugal fan system with reversing scroll，based on the single‑factor analysis method，the influence laws of blade profile，volute tongue structure，and collector form on the aerodynamic performance of the fan system were investigated，and structural optimization design of the air‑conditioning scroll reversing centrifugal fan was carried out to obtain the fan structure scheme with the best comprehensive performance.Orthogonal experiments were adopted to clarify the influence weights of structural parameters such as volute tongue inclination angle，anti‑swirl ring height，and guide ring outlet diameter on the performance of the optimized fan.The results show that the order of influence weights of each parameter on air volume is guide ring outlet diameter>volute tongue inclination angle>anti‑swirl ring height；the order of influence weights on sound quality is volute tongue inclination angle>guide ring outlet diameter>anti‑swirl ring height.Considering both air volume and sound quality indicators，the preferred scheme achieves an air volume of 1 223 m³/h，which is about 5% higher than the original scheme，and the noise peak is reduced by about 5 dB.This study can provide a reference for the design of air‑conditioning scroll reversing centrifugal fans.

To address the issues of cumbersome design procedures and long design cycles for the throttling profile of straight‑through single‑seat control valves，a high‑efficiency design method for the valve plug profile based on an orifice throttling model combined with variations in flow resistance coefficient is proposed.By establishing a calculation model for the effective flow area of the control valve at arbitrary openings，the analytical expression for the equal‑area curve is derived，and subsequently the mathematical expression for the valve plug profile coordinates is obtained.Based on this valve plug throttling profile design method，a preliminary DN50 straight‑through single‑seat control valve model with equal percentage flow characteristic is established.Simulation conditions are set according to the test requirements of GB/T 30832—2014，numerical simulation analysis is carried out，and flow coefficient tests are conducted on the control valve.The results show that the initial design exhibits flow coefficient simulation deviations beyond the allowable range specified in IEC 60534-2-4：2009 for openings below 60%.After optimizing the valve plug profile using an iterative method that mutually maps the structural characteristics and inherent flow characteristics，the simulation deviations across the full opening range are reduced to 0.30%~10.78%，meeting the standard requirements.The measured deviations of the optimized control valve under the main operating range and full‑open conditions are 2.24%~9.66%，meeting the accuracy requirements of the IEC 60534-2-4：2009 standard，and its flow characteristic approaches the equal percentage inherent flow characteristic.This study provides a reference for simplifying the design process of valve plug profiles and improving design efficiency.

To address the issues of high energy consumption and low efficiency associated with electric heating defrosting in air‑cooled refrigerator evaporators，a dynamic CFD defrosting model was established.This model was used to investigate the melting characteristics of various tube rows operating under a constant power mode.Using the mass of melted frost as a quantitative indicator，the defrosting process was divided into six sub‑stages.Based on the principle of dynamic matching between frost load and heat supply，five stepped power reduction strategies were designed to explore the effects of variable power on defrosting energy consumption，efficiency，and the improvement of heat distribution.The results indicate that all variable power strategies achieve varying degrees of improvement compared to the constant power mode.Among them，the power step reduction strategy 1 based on the entire defrosting process performs the best.It reduces defrosting energy consumption by 14.49%，increases defrosting efficiency by 5.72%，reduces average defrosting power by 35.27%，and reduces defrosting residual heat by 19.09 kJ.This study can provide a reference for the design of variable power control strategy for electric heating defrosting systems in air‑cooled refrigerator evaporators.

To address the issue of abnormal noise at 5 200 Hz occurring in variable refrigerant flow （VRF） indoor units under heating operation，a dedicated test bench for the electronic expansion valve （EEV） was constructed to identify the excitation conditions responsible for this high‑frequency noise.Large eddy simulation （LES），coupled with the Schnerr‑Sauer cavitation model，was employed to compute the temporal evolution of the vapor volume fraction inside the EEV and the corresponding pressure fluctuation spectrum.Additionally，the acoustic finite element method was applied to determine the acoustic cavity modes within the valve chamber.The results indicate that after the refrigerant passes through the throttling region of the EEV，pronounced localized vortices are generated.The pressure at the vortex core drops below the local saturation vapor pressure，triggering localized flash cavitation.The pressure fluctuation frequency induced by cavitation couples with the acoustic cavity mode of the valve chamber at 5 237 Hz，thereby amplifying pressure pulsations near that frequency and radiating audible noise.To mitigate this issue，a chamfered flow‑guiding design on the valve seat and the addition of a buffer chamber were implemented.These modifications effectively suppress cavitation and eliminate the high‑frequency noise during heating operation.

To address the issues in traditional data‑driven fault diagnosis methods for five‑cylinder pumps，such as difficulty in obtaining fault data，insufficient diagnostic capability during the early stage of equipment operation，and poor model generalization，a multi‑domain digital twin model of the five‑cylinder pump was established.The motor system，control system，mechanical system，and hydraulic system of the five‑cylinder pump were modeled，the mathematical models and parameter transfer relationships of each subsystem were clarified，and the overall model was consequently constructed.Based on fault mechanism analysis，the injection and response quantitative analysis of four typical faults，namely piston leakage，inlet blockage，bearing damage，and motor open circuit，were realized.A multi‑domain simulation model coupled with electromechanical‑hydraulic‑control was built using MATLAB/Simscape to obtain simulation data after fault injection.The consistency of the digital twin model was verified using experimental data from the XZQ2200 drilling pump.To demonstrate the application of the model，a drilling rig circulation system model was constructed using the five‑cylinder pump digital twin model，and a TimesNet‑based fault diagnosis method was proposed.The results show that the root mean square error between the twin data and the measured data is 0.039 2.The fault diagnosis model achieves a fault location classification accuracy of 98.33% and a fault classification accuracy of 89.44% on the twin dataset.The established five‑cylinder pump digital twin model has high fidelity and can effectively support typical fault mechanism analysis and fault diagnosis model construction.This study provides a new technical approach for predictive maintenance of complex equipment.