High Pressure Side Research Articles

Thermal-Gas-Elastic Coupling Modeling and Performance Analysis of Foil Cylindrical gas Film Seal Considering Speed Slip, Temperature Jump and Shaft Misalignment

Abstract The Compliant foil cylindrical gas film seal is an advanced sealing technology designed for high-speed and high-temperature conditions in a jet engine. Its design involves using a compliant foil to create a cylindrical sealing surface, achieving adaptive sealing in environments with high pressure differences and rotational speeds. For high-speed and high-temperature operation, a speed slip flow model was established by considering film slip flow, wall temperature jump, foil deformation, and gas viscosity-temperature thermal effects. The variations in the lubrication performance of the foil cylindrical gas film seal with changes in shaft misalignment directions, misalignment angles, and operating parameters were investigated. The speed slip leads to a decrease in gas film lift and an increase in leakage. The wall temperature jump phenomenon caused by speed slip leads to a 2.56% decrease in film temperature. The shaft misalignment angles on the high-pressure and low-pressure sides have different impacts on the flow field, with film pressure and temperature differing by 66.78% and 11.19%, respectively.

Numerical study of the effects of unmatched pressure on the supersonic particle-laden mixing layer

The dispersion of monodisperse, inertial particles in a supersonic mixing layer consisting of two sheared flows with differing pressures (P1 for the particle-laden jet flow and P2 for the airflow) is numerically investigated using large Eddy simulation and Euler–Lagrange methods. The calculations reveal the following insights: The pressure disparity between the two flows induces a transverse gas flow effect, which swiftly deflects the mixing layer from the high-pressure side to the low-pressure side. The growth rate of mixing layer increases with the ratio of P2/P1 and while the deflected displacement correlates with the pressure difference |P2-P1|. However, the particles exhibit delayed tracking characteristics to the deflected mixing layer because of their relative relaxation to the transverse gas velocity, particularly in the upstream region of the mixing layer (also known as the Kelvin–Helmholtz instability developing zone or KH zone). Notably, when the P2 exceeds that of the P1, particles can more easily penetrate into the vortices of KH zone, significantly enhancing the downstream gas–particle mixing. This mixing enhancement is particularly pronounced for larger particles due to their increased inertia, which allows them to advance into the vortices of KH zone more effectively than smaller ones.

Thermal safety characteristics analysis of helium xenon gas cooled small reactor system under LOCA accidents

The thermal safety characteristics of the system after a small break loss of coolant accident (SB-LOCA) in a helium xenon gas-cooled small reactor will determine whether the reactor will experience a meltdown and the duration of the meltdown. At present, research on SB-LOCA both domestically and internationally is mostly focused on water-cooled reactors, lacking relevant research on helium xenon gas-cooled reactors (He-Xe GCR). Therefore, it is of great significance to research the thermal safety characteristics of He-Xe GCR after a breach loss of coolant accident. This article improves the gas turbine model based on the original SB-LOCA analysis program for pressurized water reactors. And added a calculation model for the characteristics of helium xenon mixed gas, a heat transfer model for helium xenon mixed airflow, and a compressor model. Finally, a Brayton Cycle Reactor System Analysis program (BRESA) was developed for SB-LOCA analysis of He-Xe GCR. Explore the impact of the size and location of fractures on the thermal safety characteristics of the system using BRESA. Research has found that as the size of the rupture gradually increases, the decrease in reactor core power is greater, and the reactor power is shut down earlier. Comparative analysis of the impact of different breach positions on the residual heat removal characteristics reveals that a breach accident on the high-pressure side of the system poses a greater threat to system safety. Therefore, when setting the safety threshold for the Brayton cycle system, the focus should be on the hazards of high-voltage side breaches. This research achievement assists with the safe operation strategy of He-Xe GCR in the future.

Analysis and Modeling Study on the Effect of Working Parameters of Portable Neutron Tubes on Target Current

Abstract As an important parameter index of sealed neutron tubes, their high-pressure side target current directly affects neutron yield. The working parameters of a neutron tube affect the target current. Using self-developed neutron tubes, the influence of operating parameters of neutron tubes on target current was studied. The effect curves of gas pressure, anode high pressure, and target high pressure on neutron yield were obtained, and the secondary electron suppression characteristics of the target current were obtained. Further, we analyze and fit a multi-parameter coupling model for the target current, to provide a theoretical basis for establishing the working state and improving the performance of sealed neutron tubes.

A preliminary model of water and salt transport in a laboratory scale pressure-retarded osmosis (PRO) module

Nowadays, more and more attention is being paid to the production of green energy. Recently, energy from salinity gradients has also attracted interest. One such process that can generate useful work is the pressure-retarded osmosis (PRO), which uses a semi-permeable membrane to separate the feed and pressurized draw solutions. The semi-permeable membrane allows the transport of solvent (water) from the feed solution (diluted low pressure) side to the draw solution (concentrated high pressure) side, and thus, the excess of pressurized water on the draw side can be used to generate power, i.e., by expansion in the turbine. This work presents a preliminary numerical model developed to study the water and NaCl salt transport through the semi-preamble membrane and in the PRO module designed to perform experimental studies. The model was first verified for simple 2D module geometry. It was then used to study the flow through a simplified 3D module, mimicking the real one used in laboratory-scale experiments. The influence of the hydrodynamic pressure and the NaCl draw solution’s mass flow rate in the module on the energy generation efficiency was examined. The maximum power density obtained for half the osmotic pressure of the NaCl draw solution (i.e., for 28 bar) was found to be equal to approximately 4 W/m2. An increase in the flow rate of the draw solution causes a decrease in the thickness of the boundary layer at the membrane, which reduces the effect of the external concentration polarization. This results in an increase in the concentration difference on either side of the membrane, contributing to a non-linear increase in power density depending on this mass flow rate. Increasing the average velocity from 0.02 m/s to 0.1 m/s increases the power density by up to 80%, from approximately 2.6 to 4.7 W/m2.

Effect of the refrigerant charge on transcritical CO2 direct evaporation ice-making system

The low global warming potential and zero ozone depletion potential of CO2 make it highly suitable for application in ice rinks. The refrigerant charge prediction model of the transcritical CO2 direct evaporation ice-making system is proposed and compared. According to the predictive results, the performance of the ice-making system was experimentally studied at three predictive refrigerant charges. The results show that the ice-making system with the refrigerant charge predicted by the comprehensive model with GC + ECH (gas cooler refrigerant charge model and evaporator refrigerant charge model with Hughmark correlations) presents the optimum performance. The refrigerant charge in the pipeline accounts for 46.6 %–57.2 % of the total refrigerant charge, so proper pipeline arrangements are crucial to reduce the overall refrigerant charge, especially before and after the throttle valve. The experiment data shows that the optimal high-side pressure of the ice-making system is between 7.9 MPa and 8.3 MPa, and the optimal COP (coefficient of performance) is between 1.34 and 1.75. Excessive or insufficient refrigerant charge leads to overall superheating or overall unsaturation of the refrigerant at the outlet of the evaporator. At the proposed refrigerant charge, both cooling capacity and COP increase by 8.3 %–38.3 % and 21.5 %–30.6 % respectively compared to other charges tested. In addition, the ice-making time at the proposed refrigerant charge can be saved by 10.0 %–15.6 %.

Study on characteristics of wet compression-absorption refrigeration cycle with CO2-ionic liquid mixture

The novel wet compression-absorption (WCA) refrigeration cycle with CO2-ionic liquid mixture can bring a new solution for the development and promotion of CO2 in the refrigeration field, but the depressurization mechanism of this cycle still needs further research. Based on the vapor-liquid equilibrium data of CO2-[emim][Tf2N] mixture, the p-h diagrams of CO2-[emim][Tf2N] mixture with the specific global CO2 mole fraction were first calculated and constructed. The cycle flow of WCA refrigeration cycle with CO2-[emim][Tf2N] mixture was plotted on the diagram, the cycle characteristics and depressurization mechanisms of this cycle were analyzed. The results show that there is a temperature slip in the CO2-ionic liquid mixture during the absorption/desorption process. Both strengthening the effectiveness of cooling measures and increasing the proportion of ionic liquid in the mixture can further reduce the operating pressure at the high-pressure side, but the latter may cause a reduction in the cycle refrigeration performance and energy efficiency. For the WCA refrigeration cycle with CO2-ionic liquid mixture, it is crucial to equip an internal heat exchanger, which can significantly improve the corresponding refrigeration capacity and energy efficiency. This study can provide important references for the design and optimization of WCA refrigeration system using a CO2-ionic liquid mixture.

Direct numerical simulation of the turbulent flow around a Flettner rotor

The three-dimensional turbulent flow around a Flettner rotor, i.e. an engine-driven rotating cylinder in an atmospheric boundary layer, is studied via direct numerical simulations (DNS) for three different rotation speeds (α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document}). This technology offers a sustainable alternative mainly for marine propulsion, underscoring the critical importance of comprehending the characteristics of such flow. In this study, we evaluate the aerodynamic loads produced by the rotor of height h, with a specific focus on the changes in lift and drag force along the vertical axis of the cylinder. Correspondingly, we observe that vortex shedding is inhibited at the highest α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document} values investigated. However, in the case of intermediate α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document}, vortices continue to be shed in the upper section of the cylinder (y/h>0.3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$y/h>0.3$$\\end{document}). As the cylinder begins to rotate, a large-scale motion becomes apparent on the high-pressure side, close to the bottom wall. We offer both a qualitative and quantitative description of this motion, outlining its impact on the wake deflection. This finding is significant as it influences the rotor wake to an extent of approximately one hundred diameters downstream. In practical applications, this phenomenon could influence the performance of subsequent boats and have an impact on the cylinder drag, affecting its fuel consumption. This fundamental study, which investigates a limited yet significant (for DNS) Reynolds number and explores various spinning ratios, provides valuable insights into the complex flow around a Flettner rotor. The simulations were performed using a modern GPU-based spectral element method, leveraging the power of modern supercomputers towards fundamental engineering problems.

Numerical research on the hydrodynamic effect on the leakage of two-phase refrigerant-oil mixture in the flank gaps of compressors

This paper investigates the two-phase leakage of the flank gap in rotary compressors by using the coupled models of compressible Reynolds equation and refrigerant-oil pair solubility. The models are verified by both the classical analytical model and the published results in the static and dynamic situations. Moreover, results also firstly show how the hydrodynamic pressure, cavitation, and leakage mass flow rate can be influenced by the motion of the roller in the rotary compressor. Compared to the static situation, the pressurized zone in the high-pressure side due to the hydrodynamic effect can potentially increase the leakage mass flow rate of the mixture.

Bacterial cellulose as a promising material for pulmonary valve prostheses: In vivo study in a sheep model.

Currently, no consensus exists regarding the most durable prosthesis for pulmonary valve replacement. Bacterial cellulose is a resistant, nonbiodegradable, nonpyrogenic bioimplant with low hemolysis and clotting properties. We hypothesized that bacterial cellulose heart valve prostheses could be an attractive alternative for pulmonary valve replacement. We conducted a large animal model experiment in three adult sheep. The animals underwent open-heart surgery and cardiopulmonary bypass for bacterial cellulose conduit implantation in the pulmonary position. The sheep were followed for seven months, and clinical and laboratory parameters were analyzed. Echocardiographic evaluations were performed at 3 and 7 months. After seven months, the sheep were sacrificed and an autopsy was performed. The explanted conduits were radiologically and histopathologically analyzed. All sheep survived the operation, showing good recovery and normal health status; no adverse events were noted during the 7-month postoperative follow-up. Interval laboratory findings were normal with no signs of hemolysis or infection. Echocardiographic analysis after 7 months revealed a normal mean pressure gradient with excellent cusp motion and coaptation; a trace of regurgitation was found in two sheep. X-ray analysis of the explanted conduits revealed no structural defects in the leaflets with minimal calcification. Histological examination showed slight thickening of the conduit by pannus formation. No material failure, no calcification inside the material, and only minor calcification extrinsic to the matrix were observed. This pilot study provides evidence that bacterial cellulose may be suitable for pulmonary valve prostheses and surgical pulmonary artery plasty. Further studies on the high pressure side of the left heart are needed.

The effect of a dual condensing system on the driving range for battery electric vehicles

The air-conditioning system of a battery electric vehicle (BEV) serves a dual purpose as a cabin space cooling and thermal management of the battery. However, this operation requires high electrical energy. Consequently, the efficiency of air-conditioning (AC) systems is a critical factor in the energy consumption of BEVs. This study conducted a comprehensive model-based analysis on both the cabin and AC systems to investigate the influence of the condenser design on the energy consumption of BEVs. The two different types of condensing system were considered, a full sized air-cooled condenser and dual condensers (a water-cooled condenser with a minimized air-cooled condenser). The performance of the BEV AC system was investigated under diverse external and driving conditions. We discovered that the AC system utilizing the dual condensers reduces the high-side pressure compared with the air-cooled condensing system alone. In addition, the coefficient of performance (COP) was improved from 2.7% to 9.6% depending on airflows passing through the cooling module, particularly under urban driving conditions (such as idling and low-speed driving). Furthermore, when comparing the power consumption of the AC system to the overall energy consumption required by the vehicle within the North American SC03 fuel economy and emission measurement driving mode, it was found that the compressor's power consumption in the dual condensing system was reduced by 13%, compared to the air-cooled condenser system. In addition, the power consumption of the cooling fan was decreased by 17%. In conclusion, the dual condensing system is an effective condensing methodology capable of enhancing the overall driving distance per unit energy (km/kWh) by up to 7.9%.

Safety aspects of Organic Rankine Cycles (ORC) with combustible working fluid and sub-ambient condenser pressure

ORCs are often designed with the lower pressure limited by the ambient pressure, to avoid air leaking into the working fluid. Safely lowering this limit will be beneficial for power-production efficiency. First, cycle operations with air inleak are investigated. Exemplary cases with n-pentane and benzene are used. The investigation combines tools from thermodynamics of cycle working fluids, pump operation and analysis and combustion science. It appears that pumps customarily used in ORC will forward only liquid, and air mixing into the working fluid at the high-pressure side will not be sufficient for flammable conditions. Next, combustion properties of the air/working fluid mixture in the condenser are evaluated. Transient zero-dimensional and one-dimensional steady state laminar premixed flames are applied for the modelling. Detailed chemical mechanisms from the CRECK and TDTVT groups are used for benzene and n-pentane. It is found that some substances, like pentane, will not make flammable mixtures within the conditions of the power cycle. However, fluids that at the relevant temperatures have saturation pressure much lower than the ambient, might give flammable mixtures with air inleak. For potential sub-ambient use of these, ignition energy sources and quenching geometries have to be considered carefully.

Using Charge Determination Design of Experiments to Develop A Refrigerant Charge Health Status Model for Heat Pump Systems

Refrigerant based heat pump systems are becoming an integral system in electric vehicle architectures due to their high efficiencies in providing heating and cooling to people and components within the car. An important component in heat pump systems that determines optimal efficiency is the amount of refrigerant. As such, the capability to model refrigerant charge helps quantify the health status of the heat pump system, whereby the lack or over abundance of refrigerant in a heat pump refrigerant system leads to various other component failures, e.g., liquid slugging, compressor overheating, material fatigue in heat exchanger, and degraded/stuck expansion valves. In designing a heat pump system, engineers need to perform a set of design of experiments to determine an optimal refrigerant charge based on a set of performance metrics in the presence of certain noise factors. This optimal refrigerant charge provides conditions where the heat pump system operates efficiently in both heating and cooling, in addition to facilitating operational conditions that will not lead to secondary component degradation or damage. The search for optimal refrigerant charge is classified as refrigerant charge determination, whereby engineers incrementally increase the refrigerant in the heat pump system in operation of heating/cooling and collect data about performance metrics. Some of the key performance metrics used to determine efficiency of a heat pump system include i) compressor inlet superheat temperature, ii) condenser outlet subcool temperature, iii) compressor high side pressure, iv) compressor low side pressure, v) condenser outlet pressure, and vi) condenser quality estimate. Furthermore, this process follows design of experiments concepts and is performed for both heating and cooling modes of operation. In this paper, we leverage refrigerant charge determination as a training data source to develop refrigerant charge models, where several performance metrics are health indicators used as model inputs and the amount of refrigerant added to the heat pump system are ground truth refrigerant charges used as model outputs. In this paper we develop regression models to estimate the total refrigerant charge, which is used to classify different health states of refrigerant based on levels of performance degradation corresponding to specific refrigerant charge thresholds. We trained a robust linear regression model using this charge determination data and found that the worst case estimation error was less than 10% with respect to the refrigerant charge grouth truth.

Theoretical and experimental investigation on the effects of internal heat exchangers on a reversible automobile R744 air-conditioning system under various operating conditions

This paper presents a theoretical cycle analysis and an experimental study of the effects of internal heat exchangers (IHXs) on reversible automobile R744 air conditioning systems. The cycle analysis shows that the system efficiency benefits most with a high-efficiency IHX in the transcritical mode where the gas cooler exit temperature is high, while the IHX generally has a trivial or even negative effect on the system performance in the subcritical mode. The experimental results reveal that with the IHX, the optimal refrigerant charge is reduced from 1450 g to 1300 g, and the optimal high-side pressure is reduced by 0.24 to 0.67 MPa. Moreover, for both basic and IHX systems, it is experimentally confirmed that one linear control equation can relate the COP-maximizing high-side pressure with the gas-cooler/condenser exit temperature and functions well in both trans- and subcritical modes. That reveals the potential of combing the high-side pressure control in the transcritical mode and the subcooling control in the subcritical mode. Agreed with the theoretical analysis, the IHX has a negligible effect on COP in the subcritical mode in the experiments, while it improves the system efficiency by up to 13.8% in the transcritical mode. The highest benefit is gained in the pull-down condition that has high ambient and cabin temperatures and requires fast cooling.

Modeling and investigation of the performance of a solar-assisted ground-coupled CO2 heat pump for space and water heating

The rise in the popularity of heat pumps should be accompanied by the increase in the utilization of environmentally-safe working fluids. To push for wider uptake of hybrid heat pump systems that use natural working fluids, information about their performance and operating characteristics should be made available. This study models a CO2 solar-assisted ground-coupled heat pump (SAGCHP) system and investigates its performance for space and water heating. Sensitivity analysis, parametric study, and long-term performance simulation were implemented, with the system’s seasonal performance factor (SPF), levelized cost of heating (LCOH), and ground temperature change (GTC) considered as performance indicators. It was seen that ensuring a good combination of design and operating specifications can result in an SPF (∼3.5) and LCOH (0.184 USD/kWh) that are comparable with those of SAGCHP systems that use conventional working fluids. The heat pump’s high-side pressure and output temperature exhibited notable effects on all the performance indicators. Below the optimal operating pressure, a 5% increase in the heat pump’s high-side pressure brought about a ∼7–10% improvement to the SPF, a ∼3–5% reduction to the LCOH, and a ∼6–10% increase to the GTC. Reducing the output temperature by 5% increased the SPF by ∼7–8%, decreased the LCOH by ∼3%, and increased the GTC by ∼6%. Parametric studies identified the presence of optimal heat pump discharge pressure and heat source circulation rate that should be used for operations. Long-term simulation shows that managing the GTC ensures the longevity of the system. Rather than oversizing the borehole heat exchangers (BHEs), it is more practical to reduce ground temperature decline by increasing the BHE spacing or by adding solar collectors.

Experimental Study on the Feasibility of Quick Startup of Instant Heat Pump Water Heaters Based on Active Control of Heat Sink Flow Step

The influence of flow step ratio (FSR) on the startup characteristics of instant heat pump water heaters (IHPWHs) with natural mixture M (R744/R290 (12/88)) under nominal conditions was studied experimentally to verify the feasibility of a new quick startup method. The results show that the FSR had a marked effect on the startup time of system performance parameters. Under the optimal FSR of 0.6, the shortest system startup time and available hot water supply time were 700 s and 250 s, respectively, which were markedly shorter than those in the conventional startup. Therefore, rapid startup of the system and rapid production of usable domestic hot water can be realized by controlling the flow step. The influence of flow step on the variation trend of system performance parameters was obviously different, and there was no slow warming section for the heat sink outlet temperature (HSOT) under three FSRs. The HSOT, heating capacity, and high pressure side pressures had the maximum values in the quick startup, and the maximum values were obviously affected by the FSR. The FSR had no marked effect on the minimum suction pressure. The refrigerant pressures and refrigerant temperatures fluctuated markedly in both rapid and conventional starts.

Numerical Exploration of Transverse Sonic Jet In Hypersonic Cross-flow

At high altitudes, due to low air density, the control response of highly agile and manoeuvrable aerospace vehicles through conventional aerodynamic surfaces is rather slow and inadequate. The control demand of such vehicles can be met very effectively with high-pressure side jets positioned at different locations of the vehicle. Additionally, as none of the components of passive jet control intrudes in the flow path in a passive state, no additional drag is incurred. CFD methods are increasingly being used to characterize jet-controlled aerospace vehicles. Before using in the design exercise, CFD methods need to be validated against reliable experimental results to find its range of applications and error band. In this work, a recent experimental study of transverse sonic injection into Mach 5 hypersonic stream is numerically explored by solving three-dimensional Reynolds Averaged Navier Stokes (RANS) equations using a commercial CFD solver. The computed surface pressures show a reasonable match with experimental results for different momentum ratios of jet and free stream flows. The predictive capabilities of different turbulence models were assessed. The spreading and penetration characteristics of the injectant are analysed from the computed data. The computed penetration and spreading of the injectant depicts nearly linear dependence with the momentum ratio.

Comprehensive hydraulic performance improvement in a pump-turbine: An experimental investigation

Pumped storage power plants, which is known as only large-scale energy management equipment, plays a vital important role in energy field. Hump characteristic and S characteristic are two classical unsteady hydraulic characteristics when pump-turbine operating at turbine mode and pump mode, respectively. It has been found that these two characteristics are both strongly related to the complex vortex evolution process in vaneless region while only few papers focus on the elimination mechanism of them. In present paper, a scaled runner with optimized high-pressure side (HPS) is designed and manufactured based on multi-objective optimization process arming at eliminating unsteady characteristic, i.e. hump characteristic and S characteristic, while maintaining efficiency characteristic unchanged. Thereafter, hydraulic experiments are conducted to investigate the impact of HPS geometry on hydraulic performance (efficiency, hump margin, S margin and pressure fluctuation) of object pump turbine. The experimental results show that compared with the original runner, the weighted average efficiency for pump mode increases by 0.1% while the weighted average efficiency for turbine mode decreases by 0.1%. Moreover, the S margin increases from 76.7 m to 87.2 m and the S2 unsteady region is largely increased. The hump margin decreases a little while it can recover to the original level through increasing 0.6% of the original ratio scale. Moreover, the runner with optimized HPS can effectively reduce the pressure fluctuation amplitude in vaneless region up to 33.3% and 21.4% for turbine mode and pump mode, respectively.

Simulation of Air Ingestion in a Mechanical Seal With Inward Pumping Spiral Grooves

Abstract Inward pumping spiral groove seals are used in many applications, such as transmission gearboxes or electric vehicles, as they can provide zero leakage and very low operating friction. The spiral grooves are connected to the low pressure side (typically atmosphere) and pump the fluid inward that is to say toward the high pressure side (inlet). If the rotational speed is high enough, the low pressure fluid (air) completely occupies the sealing area and is used as a low friction lubricant. However, for lower operating speeds or higher fluid pressures, the seal can be lubricated by both fluids simultaneously. This occurs particularly during start-up and shutdown. In this work, a numerical simulation of the two phases in the sealing gap of an upstream pumping spiral groove seal during start-up is performed. The establishment of the film is analyzed and the effect of the rotational speed and operating pressure on the fluid composition (air and liquid) in the sealing interface is studied. For the analyzed seal, it is shown that the amount of air pumped in the sealing area is controlled by the duty parameter.

Mechanism of runner high-pressure side on stall characteristics at typical unsteady operating points in both modes of a pump turbine

Stall phenomenon, a classical physical phenomenon which is located in the vaneless region of a pump–turbine and accompanied by a complex vortex evolution process, is strongly related to the formation of hump unsteady region at the pump mode and S unsteady region at the turbine mode. In the present paper, a detached eddy simulation model is employed to numerically investigate the impact of runner high-pressure side (HPS) on stall characteristics at typical unsteady operating points, namely, a valley point in the hump region at the pump mode and a runaway point in the S region at the turbine mode. It is found that the stall characteristics at both investigated points are obviously changed: For the valley point, only three fixed stall cells exist in the original plan, while four additional rotating stall cells appear and rotate at the speed of 0.02nr (nr, runner rotation speed) in the optimized plan (OPT). The distinctive coexistence phenomenon of both fixed stall and rotating stall is reported for the first time and is attributed to the complex vortex evolution controlled by optimized HPS; for the runaway point, both the intensity and frequency of the stall characteristic are slightly increased in OPT. Moreover, for both operating points, the optimized HPS can effectively decrease the backflow at shroud, resulting in a significant decrease in the relative backflow rate within a complete flow period, of which 17.3% is for the valley point and 4.8% is for the runaway point. Finally, a local hydraulic loss rate (LHLR) method is adopted to investigate the hydraulic loss evolution process, and it is found that the high LHLR region in OPT is more concentrated in both circumferential direction and radial direction in the vanless region at both operating points. Based on the runner with optimized HPS proposed in the present paper, many unsteady hydraulic characteristics that is related to the stall phenomenon might be eliminated to some extent.