Large Pressure Ratio Research Articles

Effect of Hub Gap on Expanding Stability of Centrifugal Compressor

In this paper, the possibility of using hub gap (“ HG ”for short) to suppress gas stall and expand the range of stable working condition of compressor is verified by numerical method. The research shows that when designing flow rate, HG increases, the efficiency and pressure ratio of model level decrease. When there is a large flow, the efficiency and pressure ratio of HG model are higher than those without HG. The larger HG is, the higher the efficiency and pressure ratio is. From the analysis of the expanding stability capacity, the model-level flow margin of small HG and large HG is 5% and 15% higher than that of models without HG, and the comprehensive stall margin is 6.87% and 23.59% higher on average. When operating at unstable working conditions, the action of HG causes the airflow in the high pressure zone to flow into the gap and out of the low pressure zone, which improves the flow distribution in the blade passage and makes the flow field stable. However, when operating near the design condition, the leakage flow and its mixing with the momentum of the mainstream are the main sources of aerodynamic loss. The larger HG is, the greater the pressure loss caused by the leakage flow is. Generally speaking, HG can play the role of compressor expansion stability, but at the cost of efficiency; The bigger HG is, the stronger the role of expanding stability is and the greater the efficiency loss is.

Atomistic investigation on the kinetic behavior of vapour adsorption and cluster evolution using a statistical rate theory approach.

The kinetic behavior of vapor adsorption on a solid surface in an isobaric-isothermal system is investigated by means of molecular dynamics simulations combined with theoretical studies through a statistical rate theory approach. The molecular insights into the formation and evolution of clusters in the adsorbate are presented. Results show that the argon vapor is adsorbed on the silicon surface as different types of clusters. In the initial stage of adsorption, the empty adsorption sites on the surface decrease, and the adsorbed single-molecule-cluster grows rapidly and dominates the interface. The increasing rate of the adsorbed cluster and the declining rate of the empty adsorption site are dependent on the pressure ratio. For a large pressure ratio, the single-molecule-clusters are aggregated to incubate large clusters, and the fraction of a single-molecule-cluster is decreased with time. When the adsorption isotherm is determined, the chemical potential of the adsorbed cluster is expressed from the zeta isotherm model. Then the adsorption kinetics are analyzed through the statistical rate theory. The molecular exchange rate and the instantaneous driving force are calculated. The higher pressure ratio induces the larger chemical potential difference and accelerates the net adsorption rate. The adsorption kinetics derived from MD simulations are in close agreement with the theoretical analysis of the statistical rate theory.

A Quasi-Conservative Discontinuous Galerkin Method for Solving Five Equation Model of Compressible Two-Medium Flows

In this work, we develop a quasi-conservative discontinuous Galerkin method for the simulation of compressible gas-gas and gas-water two-medium flows by solving the five-equation transport model. This spatial discretization is a direct extension of the quasi-conservative finite volume discretization to the discontinuous Galerkin framework, thus, preserves uniform velocity and pressure fields at an isolated material interface. Furthermore, for discontinuities with a large pressure ratio, low density, and a dramatic change of material property where nonphysical values may occur, a strategy for imposing the bound-preserving limiting for volume fraction and a positivity-preserving limiting for density of each fluid and internal energy is developed and analyzed based on the quasi-conservative DG( $$p_1$$ ) discretization. Typical test cases for both one- and two-dimensional problems are provided to demonstrate the performance of the proposed method.

3D transient CFD simulation of an in-vessel loss-of-coolant accident in the EU DEMO fusion reactor

Accidental transients in fusion reactors, such as the in-vessel loss-of-coolant accident (LOCA) considered here, are generally analyzed using system-level codes. However, because of their lumped nature, such tools cannot predict local pressure and temperature values, which are instead of interest to assess the integrity of the vacuum vessel (VV) structures. It is then fundamental to prove that the system-level tools’ predictions are at least conservative. In this work, we analyze the helium flow inside the VV following a LOCA in the helium-cooled blanket of the EU DEMO, using a 3D transient computational fluid-dynamics (CFD) model implemented in the commercial STAR-CCM+ code. In view of the large pressure ratio, a hypersonic flow regime (Ma > 5) develops, with the formation of shock fronts requiring a high time and space resolution. The model is applied to compute the evolution of the pressure distribution inside the VV and then to compare it with the information provided by the system-level GETTHEM model. The predictions by the two models of the intervention time of the VV pressure suppression system are compared, showing that the 0D model is conservative.

Experiments on vapour-liquid equilibrium of CO2-ionic liquid under flow conditions and influence on its refrigeration cycle

CO2 is a potential alternative refrigerant because of its excellent thermodynamic properties and environmental friendliness, but its high operating pressure in the refrigeration cycle limits its wide application. To reduce the operational high pressure of the conventional CO2 compression refrigeration system, a novel CO2 compression-absorption refrigeration system with ionic liquid (IL) as absorbent was proposed. The IL [emim][Tf2N] was selected and combined with CO2 as a working pair. Experimental investigations on the dynamic vapour-liquid equilibrium of CO2-[emim][Tf2N] under flow conditions and the influence on its refrigeration cycle were conducted. The effects of solution pump frequency, CO2 inlet temperature and pressure, and system pressure ratio on the key parameters such as depressurization amplitude, absorption completion and desorber temperature were investigated. Within the working conditions of this study, the system operational high pressure could be reduced by 29% at most, and the absorption completion of CO2-IL mixture at the absorber outlet could reach about 70%. It was found that both CO2 inlet pressure and temperature had important effects on the key parameters, and a larger pressure ratio could obtain larger depressurization amplitude and absorption completion and lower desorption temperature. These provide a reference for further optimizing system design and improving system performance.

Series view method based thermodynamic modeling and analysis for innovative precooled aeroengines with different turbine-compressor coupling schemes

Fuel direct precooled engines are proposed to propel hypersonic cruise and acceleration vehicles. The engine branch includes a variety of cycle configurations. It is realized that based on a series view modeling concept, the different configurations can be parameterized via a unified thermodynamic model through which the issues concerning cycle synthesis and design can be converted to a parameter/configuration optimization problem. With the proposed model, analysis for representative cycles characterized by different turbine-compressor coupling schemes is carried out to reveal how the cycle performances are determined by its arrangements. The results indicate that the engine using fuel turbine possesses a high comprehensive performance, and the one using gas turbine can achieve larger air pressure ratio — which is beneficial for component level synthesis with rocket engines. Besides, the optimum performances for the cycles evaluated are obtained when the fuel flow required for precooling is equal to that permitted for stoichiometric combustion. Moreover, the engine performance seems more sensitive to air side pressure loss of precooler for the design with smaller air pressure ratio, and for engines with larger pressure ratio, the efficiencies of air compressor and gas turbine are the dominant factors.

Experimental study of the cavitation noise and vibration induced by the choked flow in a Venturi reactor.

In this paper, the cavitation performance and corresponding pressure pulsation, noise and vibration induced by the choked cavitating flow in a Venturi reactor are investigated experimentally under different cavitation conditions by using high-speed camera and high frequency sensors. Based on the instantaneous continuous cavitation images, the Proper Orthogonal Decomposition (POD), a tool to analyze the large-scale cavitation flow structure, is applied to investigate the choked cavitating flow dynamics. The POD results show that two mechanisms, re-entrant jet flow mechanism and shock wave mechanism, govern the shedding and collapse of cavitation cloud at different pressure ratios. These mechanisms contribute to the variation of pressure pulsation, noise and vibration at different pressure ratios. The pressure pulsation spectrum behaves differently in various cavitation regions induced by the choked cavitating flow. Due to the existence of low pressure in re-entrant region, the influence of high frequency fluctuation on pressure pulsation caused by re-entrant flow is small. Moreover, with the increase of pressure ratio, the induced noise and vibration intensity decreases gradually, then increases and reaches a maximum value. Finally, it drops to a low and stable level. Despite different inlet pressures, the intensity of cavitation noise and vibration reaches the maximum value at the same pressure ratio. Specifically, the FFT analysis of noise and vibration signals indicates that low frequency component prevails at small pressure ratio owing to the re-entrant jet mechanism, while high frequency component prevails at large pressure ratio owing to the shock wave mechanism. The relationship between the choked cavitation dynamics and the induced pressure pulsation, noise and vibration in the Venturi reactor is highlighted. The results can provide guidance for the optimal operation condition of the Venturi reactor for cavitation applications such as water treatment.

Finding an optimal blade-profile to improving the performance of partially admitted turbines

Purpose This paper aims to present the designing and investigating various types of impulse blade profiles to find the optimal profile that has better performance than the first or original blade. The studied model is a turbine with an output power below 1 MW and a large pressure ratio up to 20, which is used to gain relatively high specific work output. As a result of its low mass flow rate, the turbine is used under partial-admission conditions. The turbine’s stator is a group of convergence–divergence nozzles that provide supersonic flow. Design/methodology/approach More than 10 types of two-dimensional blade profiles were designed using the developed preliminary design calculations and numerical analysis. The numerical results are validated using the existing experimental results. Finally, the case with improved performance is introduced as the final optimum case. Findings It was found that the performance parameters such as efficiency, power and torque are increased by more than 8% in the selected best model, in comparison with the original model. Moreover, the total pressure loss is 12% decreased for the selected model. Finally, the selected profile with superior performance is proposed. Originality/value Simultaneous numerical tests are conducted to examine the interaction of different supersonic blade profiles with the partially injected flow to the rotor.

CFD study of the 2D gas piston in pulse tube cryocoolers

There are three parts of the gas moving through the pulse tube of the pulse tube cryocooler, part I is the gas moving between the cold heat exchanger and the pulse tube, part II is the gas oscillating in the pulse tube all the time, part III is the gas moving between the pulse tube and the warm heat exchanger. The part of the gas that always moves in the pulse tube performs the same function as a solid piston; this part of the gas is called the gas piston. The shape and the position of the gas piston changes with time and the operating conditions rather than remaining fixed. A 1D model cannot capture the change of the gas piston near the wall, but a 2D model capturing changes in the axial and radial directions can. Further, the uniformity of the fluid flow field in the pulse tube can be depicted by the shape of the gas piston which can be simply envisioned as a 2D gas piston in the pulse tube. In this study a CFD method is used to obtain the details of the 2D gas piston. The velocity field in the pulse tube is obtained by the commercial code ANSYS Fluent, and a LaGrange particle tracing method is introduced to process the velocity data in order to obtain the boundary of the 2D gas piston. Additionally, this work investigates the influence of various parameters including pressure ratio, pulse tube aspect ratio and frequency on the shape of the gas piston. It reveals that a larger pressure ratio, larger aspect ratio and lower frequency cause a larger deformation of the gas piston in one cycle. These effects are especially noticeable at the warm end of pulse tube, which displays a larger deformation than other positions in the pulse tube under the influence of changes in those parameters.

Performance evaluation of novel double internal auto-cascade two-stage compression system using refrigerant mixtures

Heat pumps have been widely applied owing to their high energy efficiency. Two-stage and quasi-two-stage systems are effective in enhancing the performance of heat pumps under large pressure ratio. Meanwhile, refrigerant mixtures are commonly regarded as potential substitutes for heat pumps because they can provide more flexibility in searching for new alternatives. In contrast to previous conclusions regarding gas-injected heat pumps with pure or azeotropic refrigerants, the type of economizer has an important effect on the system performance. In this study, a novel double internal auto-cascade two-stage compression system suitable for refrigerant mixtures is proposed, which can effectively modulate the concentration of the refrigerant going to the evaporator and the injection port, drive the refrigerant rich in high-pressure composition to enter the evaporator, thereby improving the system performance. The results indicate that under the operational working conditions, the flashing pressure is the only independent parameter and the optimal value exists in this novel system. Compared with the flash tank system, the novel system exhibits a 9.6% higher heating capacity and a 6.1% COP higher. The largest increases in the heating capacity and COP relative to the intermediate heat exchanger system are 2.1% and 2.5%, respectively.

Performance of a linear compressor driven JT cryocooler at liquid helium temperature

The JT cryocooler at liquid helium temperature driven by large pressure ratio linear compressor(s) is one of the core components in the deep space detectors. Also, future quantum detection technology needs to work at or below the liquid helium temperature zones. The JT cryogenics system for obtaining the liquid helium temperature zone is mainly composed of a regenerative precooling unit and a JT cooling unit. The relative technology of the regenerative precooling unit is more mature than that of the JT unit, and related research reports are also numerous. A valved linear compressor (VLC) capable of providing a large pressure ratio is a key technology in the JT cryocooler. At present, the deep space exploration projects such as JWST and SPICA have adopted the VLC. In order to study the working characteristics of VLC, a test system of a closed JT cryocooler operating at liquid helium temperature is built. The cryocooler is precooled by a GM cooler and the precooling temperature of 10 K is selected. A single-stage VLC is used in the study and the output performance is tested independently. The experimental results show that when the charging pressure is 0.15 MPa, a pressure ratio of 3.7 with the mass flow of 4.5 mg/s can be obtained. While raising the charging pressure to a value of 0.21 MPa, the pressure ratio can increase to 4.5. Based on this performance of the single-stage VLC, the theoretical cooling capacity of JT cycle is analyzed. The coupling performance test of the VLC with a JT unit is developed. After about 30 h later, the evaporator temperature can be cooled down to the liquid helium temperature zone. By increasing the piston displacement of the VLC, the dead volume of the compression chamber can be reduced, thereby improving the output performance. With the displacement increased from 6.2 to 7.5 mm by supplementing the input power of 15 W, the cooling temperature can be reduced from about 4.4 to 3.94 K. A minimum temperature of 3.91 K is achieved by further adjusting the piston displacement and the VLC operating frequency. The cooling performance at liquid helium temperature is mentioned in the paper. A maximum cooling capacity of 10.8 mW at 4.09 K can be obtained. The study can provide a reliable research basis for multi-stage compression and lower temperature zones systems.

A discontinuous Galerkin method for the simulation of compressible gas-gas and gas-water two-medium flows

In this paper, we develop a new discontinuous Galerkin method for the simulation of shocks and interfaces in compressible gas-gas and gas-water two-medium flows by solving the γ-based model. The spatial discretization is carefully designed to possess the following features: discrete conservation in terms of the total mass, total momentum and total energy; high-order accuracy and consistency for smooth flows; free of oscillations at an isolated material interface. In order to handle potential discontinuities arising in the simulation, a nonlinear limiter based on the weighted essentially non-oscillatory (WENO) strategy is employed to suppress numerical oscillations and to preserve high order of accuracy in regions of smooth flows. The WENO reconstruction is imposed on suitably selected quantities, rather than the conserved ones. In case for discontinuities with large pressure ratio, low density and dramatic change of material property where unphysical variables may be encountered, a posteriori solution correction on the subcell level is locally adopted to enhance the robustness. A series of typical test cases for both one- and two-dimensional problems are provided to demonstrate the performance of the proposed method.

Experimental investigation on the cavitation performance in a venturi reactor with special emphasis on the choking flow

Experiments were conducted to investigate the performance of choked cavitating flow in a transparent venturi reactor at different pressure ratios by high speed camera technique. Cavitation images of various flow conditions and corresponding pressure variations were analyzed to study the development performance of the cavitating flow. This work provides a thorough understanding of the evolution of cavitation regions, collapse mechanism, averaged wall pressure and pressure pulsation at different pressure ratios. The cavitation regions in venturi under choking flow condition can be divided into inception and developing region, fusion region and collapse region. Further analyses on the processed images reveal that cavitation performance can be approximately divided into two sections by a transition pressure ratio of 0.71. At smaller pressure ratios (pr < 0.71), the re-entrant jet governs the cavitation dynamics and induces complex flow regions. And the core position of collapse region moves upstream fast with the increase of pressure ratio. At larger pressure ratios (pr > 0.71), cavitation dynamics is governed by the shock wave. And the core position of collapse region moves upstream slowly with the increase of pressure ratio. The spectrum distribution of cavitation cloud in collapse region is concentrated and the peak frequency increases slowly at small pressure ratios. However, the spectrum distribution of cavitation cloud is relatively decentralized at large pressure ratios. The measured pressure data indicates that averaged pressure in diffuser goes through three stages at different slopes with the increase of pressure ratio under choking flow condition. In addition, the variations of pressure pulsation intensity (PPI) influenced by cavitation regions, collapse mechanisms, back pressure and collapse distance are exhibited in the present work.

RANS/ILES Analysis of the Flow Pattern and the Acoustic Characteristics of a Supersonic Off-Design Jet at Large Nozzle Pressure Ratios

The outflow of a cold supersonic off-design jet with the nozzle pressure ratio of 21.8 is calculated using the high-resolution RANS/ILES method. For the nozzle under consideration and at the given nozzle pressure ratio the exit-to-ambient pressure ratio is 0.6. The jet is characterized by a long supersonic region with many “barrels.” The calculations are validated against the experimental data. The fluctuating and acoustic characteristics of the jet flow under consideration are compared with the characteristics of other supersonic jets. The geometric dimensions of the barrels and the acoustic properties of the jet are estimated from the known empirical relations for supersonic jets.

Theoretical study of two-stage water vapor compression systems

Water vapor compression is a key technology that greatly affects system performance in heat pump and mechanical vapor compression/recompression applications. In this paper, two-stage water vapor compression methods (cascaded centrifugal compressors and combined centrifugal and twin-screw compressors) were proposed and studied to deal with the water compression process with large suction volume flow rates and high compression pressure ratios. A mathematical model was developed, and the thermal characteristics, achievable volume flow rate and saturation temperature rise of the two water compression systems were presented. The two-stage compression process using cascaded centrifugal compressors was found to be better for applications with total saturation temperature rise lower than 40 °C. The two-stage compression process using combined centrifugal and twin-screw compressors was found to satisfy applications with a saturation temperature rise as high as 80 °C. Further investigation showed that the second method could deliver a suction volume flow rate of 1418 m3/min with a suction vapor temperature of 50 °C using a current market-available twin-screw compressor with a capacity of 600 m3/min. The performance of the combined systems was largely affected by the pressure ratio and the compressor efficiency, system suction vapor temperature, and total vapor saturation temperature rise. The analyses showed that the combined system had high energy efficiency above 3.4 under all studied working conditions.

Simulation of a shock tube with a small exit nozzle

Shock tubes are frequently used to rapidly heat up reaction mixtures to study chemical reaction mechanisms and kinetics in the field of combustion chemistry [1]. In the present work, the flow field inside a shock tube with a small nozzle in the end plate has been investigated to support the analysis of reacting chemical mixtures with an attached mass spectrometer and to clarify whether the usual assumptions for the flow field and the related thermodynamics are fulfilled. In the present work, the details of the flow physics inside the tube and the flow out of the nozzle in the end plate have been investigated. Due to the large differences in the typical length scales and the large pressure ratios of this special device, a very strong numerical stiffness prevails during the simulation process. Second-order ROE numerical schemes have been employed to simulate the flow field inside the shock tube. The simulations were performed with the commercial code ANSYS Fluent [2]. Axial-symmetric boundary conditions are employed to reduce the consumption of CPU time. A density-based transient scheme has been used and validated in terms of accuracy and efficiency. The simulation results for pressure and density are compared with analytical solutions. Numerical results show that a density-based numerical scheme performs better when dealing with shock-tube problems [5]. The flow field near the nozzle is studied in detail, and the effects of the nozzle to pressure and temperature variations inside the tube are investigated. The results show that this special shock-tube setup can be used to study high-temperature gas-phase chemical reactions with reasonable accuracy.

Effects of nozzle arrangement angle on the performance of partially admitted turbines

This paper presents the experimental and numerical evaluations of the effects of a supersonic Nozzle arrangement angle (NAA) on the performance of an impulse axial turbine. The studied model is a small turbine with a large pressure ratio that is used to gain high specific work output. As a result of its low mass flow rate, the turbine is used under partial admission conditions. The turbine stator is a group of convergence–divergence nozzles that provide supersonic flow. Five stators with different NAAs are modeled and tested. Results show that NAA exerts considerable impact on the performance of the turbine. In this work, suggestions for achieving a decent design process for partly arranged nozzles are offered, and the proposed model and numerical analysis are validated using experimental tests.

Using a Bowed Blade to Improve the Supersonic Flow Performance in the Nozzle of a Supersonic Industrial Steam Turbine

For solar plants, waste-energy recovery, and turbogenerators, there is a considerable amount of waste energy due to low mass flow rate. Owing to the high specific power output and large pressure ratios across the turbine, a supersonic industrial steam turbine (IST) is able to utilize the waste energy associated with low mass flow rate. Supersonic IST has fewer stages than conventional turbines and a compact and modular design, thus avoiding the excessive size and manufacturing cost of conventional IST. Given their flexible operation and ability to function with loads in the range of 50–120% of the design load, supersonic IST offers significant advantages compared to conventional IST. The strong shock-wave loss caused by supersonic flows can be reduced by decreasing the shock intensity and reducing its influence; consequently, a supersonic IST can reach higher efficiency levels. Considering the demonstrated utility of bowed blades in conventional IST, this paper presents a study of the use of bowed blades in a supersonic IST. For this purpose, first, the shock-wave structure in the supersonic flow field was analyzed and compared with experimental results. Then, four different bowed blades were designed and compared with a straight blade to study the influence of bowed blades on the shock-wave structure and wetness. The results indicate that S-shaped bowing can improve the efficiency of supersonic turbines, and the energy-loss coefficient of the stators can be decreased by 2.4% or more under various operating conditions.

Experimental study on a one-stage Vuilleumier cryocooler with large pressure ratio

A detailed experimental results of one-stage VM cryocooler with larger pressure ratio is presented and discussed in this paper. By increasing the stroke distance of hot displacer from 20 mm to 32 mm, the pressure ratio raised to 1.8 at the terminal temperature. Besides, TFL-spring combined with labyrinth sealing was used on cold displacer. Finally, a no-load temperature of 7.55 K was attained and the cooling capacity is 1.98 W@15 K. Although the terminal temperature was not apparently improved, it still showed great potential to reach liquid helium temperature when adding an extra stage.

Compressible simulations of bubble dynamics with central-upwind schemes

ABSTRACTThis paper discusses the implementation of an explicit density-based solver, that utilises the central-upwind schemes for the simulation of cavitating bubble dynamic flows. It is highlighted that, in conjunction with the Monotonic Upstream-Centered Scheme for Conservation Laws (MUSCL) scheme they are of second order in spatial accuracy; essentially they are high-order extensions of the Lax–Friedrichs method and are linked to the Harten Lax and van Leer (HLL) solver family. Basic comparison with the predicted wave pattern of the central-upwind schemes is performed with the exact solution of the Riemann problem, for an equation of state used in cavitating flows, showing excellent agreement. Next, the solver is used to predict a fundamental bubble dynamics case, the Rayleigh collapse, in which results are in accordance to theory. Then several different bubble configurations were tested. The methodology is able to handle the large pressure and density ratios appearing in cavitating flows, giving similar predictions in the evolution of the bubble shape, as the reference.