High Pressure Ratio Research Articles

Roles and mechanisms of casing treatment on different scales of flow instability in high pressure ratio centrifugal compressors

The development of single-stage high pressure ratio centrifugal compressors for small gas turbine engines is hindered by compressor flow instabilities with various temporal and spatial scales. In this paper, the effect of a self-recirculation casing treatment device for suppressing various scales of flow instability has been discussed. Rig tests of a high pressure ratio centrifugal compressor with and without a casing treatment device are performed, and transient pressure signals are measured at endwalls by fast response pressure transducers. By analyzing measured pressure signals in both time and frequency domain, the casing treatment device is found effective in eliminating rotating instability and stall at the impeller inlet at low speeds, as well as suppressing surge of the compression system at middle and high speeds. The flow recirculation process generated inside the casing treatment device removes the vortical structures at the impeller inlet tip section, therefore eliminating the regional rotating instability and stall. Besides, the casing treatment device turns the pressure rise characteristics of the impeller more negative by enhancing the impeller work input, but turns that of the vaned diffuser more positive by increasing the incidence at the vaned diffuser inlet. Combining both effects, mild surge and the deep surge are suppressed.

Effects of Transient Heat Transfer on Compressor Stability

The effects of heat transfer between the compressor structure and the primary gas path flow on compressor stability are investigated during hot engine re-acceleration transients. A mean line analysis of an advanced, high-pressure ratio compressor is extended to include the effects of heat transfer on both stage matching and blade row flow angle deviation. A lumped capacitance model is used to compute the heat transfer of the compressor blades, hub, and casing to the primary gas path. The inputs to the compressor model with heat transfer are based on a combination of full engine data, compressor test rig measurements, and detailed heat transfer computations. Nonadiabatic transient calculations show a 8.0 point reduction in stall margin from the adiabatic case, with heat transfer predominantly altering the transient stall line. 3.4 points of the total stall margin reduction are attributed to the effect of heat transfer on blade row deviation, with the remainder attributed to stage rematching. Heat transfer increases loading in the front stages and destabilizes the front block. Sensitivity studies show a strong dependence of stall margin to heat transfer magnitude and flow angle deviation at low speed, due to the effects of compressibility. Computations for the same transient using current cycle models with bulk heat transfer effects only capture 1.2 points of the 8.0 point stall margin reduction. Based on this new capability, opportunities exist early in the design process to address potential stability issues due to transient heat transfer.

Multipoint shape optimisation of an automotive radial compressor using a coupled computational fluid dynamics and genetic algorithm approach

Automotive turbochargers operate over a wide range and require high efficiencies and pressure ratios. These conflicting requirements and a myriad of design parameters render iterative design techniques unfeasible. However, over the last decades the combination of numerical flow solvers and evolutionary algorithms has established itself as a viable option in the pursuit of reaching desired performance characteristics.This study seeks to perform a three-dimensional, multipoint and multiobjective optimisation of an automotive radial compressor by modifying blade shape as well as the meridional contour of the flow path. The method couples steady-state computational fluid dynamics (CFD) with a genetic algorithm (GA) to maximise isentropic efficiency in the region close to surge, while ensuring no significant reduction in choke margin.The results of two optimisation studies are presented and a flow-field analysis based on entropy generation rate is carried out revealing regions of flow improvement. The results are further compared against experimental data, indicating good agreement between the numerical and test data. The experiments however imply a detrimental impact on the surge margin for larger impeller speeds, which is attributed to unfavourable blade loading. Two additional optimisation runs are presented mitigating the effect of loading unbalance between main blade and splitter.

A methodology for the preliminary design and performance prediction of high-pressure ratio radial-inflow turbines

Modern power generation technologies, such as organic Rankine cycle power systems, require turboexpanders operating with high-efficiency and high power density. These features often lead to high-pressure ratios machines, characterised by the presence of choking and supersonic flow conditions. This paper proposes a comprehensive methodology for the preliminary design and performance prediction of radial-inflow turbines operating at high-pressure ratios. A steady-state, mean-line model of a radial-inflow turbine is developed including real-gas effects and a detailed modelling strategy for the treatment of choking flow conditions. In addition, a set of loss models tailored to high-pressure ratio radial-inflow turbines is developed. After a global sensitivity analysis, the model is calibrated by means of a multi-objective optimisation with a Genetic Algorithm and using the data of six high-pressure ratio turbines with total-to-total pressure ratios up to 5.8. The calibration method allows a significant reduction in the overall predicted deviation of the turbine isentropic efficiency and mass flow rate. The design model yields predicted deviations in isentropic efficiency within ± 3 %-points and the off-design model within 5%. The methodology and the results are intended to be used as a benchmark for the future development of radial-inflow turbines in high-pressure ratio applications.

Experimental study on R-22, R-427A, R-161 and R-290 in air-source heat pump for space heating at low ambient temperatures

To help reduce the warming impacts, this study evaluates the drop-in performances of R-427A, R-161 and R-290 in R-22 air source heat pump with EVI (Economized Vapor Injection). The results show that the IPLV(H) of R-161 and R-290 arises 5∼8% relative to R-22 with EVI on. A reversal occurs between R-290 and the baseline with EVI on, making the IPLV(H) of R-290 superior to that of R-22 by 3.6∼5.2% from worse performance with EVI off. At -20/41 °C condition, the capacity and COP of R-290 increase by 38% and 19%, respectively. Considering COP and discharge temperatures, R-290 is better-performed to replace R-22 in high pressure ratio conditions. Based on LCCP (Life Cycle Climate Performance) model, 11% of the equivalent CO2 emission can be reduced by using R-161 and R-290. However, the discharge temperature of R-161 is high in the high pressure ratio condition. R-290 is the most synthetically beneficial alternative.

Analyses of Long-Term Off-Design Performance Strategy and Operation of a High-Pressure Ratio Intercooled Brayton Helium Gas Turbine Cycle for Generation IV Nuclear Power Plants

The intercooled cycle (IC) is a simplified novel proposal for generation IV nuclear power plants (NPP) based on studies demonstrating efficiencies of over 45%. As an alternative to the simple cycle recuperated (SCR) and the intercooled cycle recuperated (ICR), the main difference in configuration is no recuperator, which reduces its size. It is expected that the components of the IC will not operate at optimum part power due to seasonal changes in ambient temperature and grid prioritization for renewable sources. Thus, the ability to demonstrate viable part load performance becomes an important requirement. The main objective of this study is to derive off-design points (ODPs) for a temperature range of −35 °C to 50 °C and core outlet temperatures (COTs) between 750 °C and 1000 °C. The ODPs have been calculated using a tool designed for this study. Based on the results, the intercooler changes the mass flow rate and compressor pressure ratio (PR). However, a drop of ∼9% in plant efficiency, in comparison to the ICR (6%) was observed for pressure losses of up to 5%. The reactor pressure losses for IC have the lowest effect on plant cycle efficiency in comparison to the SCR and ICR. Characteristic maps are created to support first-order calculations. It is also proposed to consider the intercooler pressure loss as a handle for ODP performance. The analyses brings attention to the IC an alternative cycle and aids development of cycles for generation IV NPPs specifically gas-cooled fast reactors (GFRs) and very-high-temperature reactors (VHTRs), using helium.

Optimisation of screw spindle vacuum pumps with variable rotor pitch regarding load-lock operation

Screw spindle vacuum pumps are characterised by high suction speed and the ability to achieve high pressure ratios in a dry running process. If the dimensions of the pump are comparable to the volume of the system’s receiver, evacuation only takes a few seconds. For this reason, a typical application for screw spindle vacuum pumps is the evacuation of a load-lock chamber in a predefined time during a clocked production process. In order to reduce power consumption, screw spindle vacuum pumps with an internal change of volume are used almost exclusively. These pumps are commonly with variable pitch rotors. In this article, a new approach for optimisation of variable pitch rotors in consideration of the requirements of the load-lock process is presented. A dimensionless energetic efficiency is defined based on an idealised load-lock process, this parameter is maximised by the optimisation algorithm. The rotor pitch progression, an important factor in pump performance, is approximated by segments of constant pitch at the suction and discharge sides with a linear decrease in between. The overall wrap angle, the minimum rotor pitch, the crown circle diameter, the root circle diameter, and the circumferential speed are constant for the optimisation, during which the rotor length and load-lock pressure are varied. Optimisations are carried out for double-lobed cycloid profiles. The optimisations result in selection of internal volume ratios which are significantly below a theoretical calculated value for the idealised lock-lock process. This is explained by the internal clearances and details of compression process. The studies reveal a positive influence of the rotor length on the load-lock efficiency, this is because the proper rotor length leads to an uniform compression. It is also shown that the wrap angle of the segment on the suction side should be kept low.

Shock oscillation in highly underexpanded jets**Project supported by the National Natural Science Foundation of China (Grant No. 11602028) and the Science and Technology Project of General Administration of Quality Supervision Inspection and Quarantine of China (Grant Nos. 2017QK119 and 2017QK188).

The oscillatory motions of shocks in highly underexpanded jets with nozzle pressure ratios of 5.60, 7.47, 9.34, and 11.21 are quantitatively studied by using large eddy simulation. Two types of shock oscillations are observed: one is the Mach disk oscillation in the streamwise direction and the other is the shock oscillation in the radial direction. It is found that the Mach disk moves quickly in the middle of the oscillatory region but slowly at the top or bottom boundaries. The oscillation cycles of Mach disk are the same for different cases, and are all dominated by an axisymmetric mode of 5.298 kHz. For the oscillation in the radial direction, the shocks oscillate more toward the jet centerline but less in the jet shear layer, and the oscillation magnitude is an increasing function of screech amplitude. The cycles of the radial shock oscillation switch randomly between the two screech frequencies for the first two cases. However, the oscillation periodicity is more complex for the jets with high nozzle pressure ratios of 9.34 and 11.21 than for the jets with the low nozzle pressure ratios of 5.6 and 7.47. In addition, the shock oscillation characteristics are also captured by coarse mesh and Smagorinsky model, but the coarse mesh tends to predict a slower and weaker shock oscillation.

The thermodynamic and economic characteristics of the modern combined cycle power plant with gas turbine steam cooling

This paper presents a methodology to clearly identify energy benefits (in terms of power and efficiency) resulting from the use heat from gas turbine steam cooling and heat from cooling air. These potential benefits are evaluated in terms of the characteristics of combined cycle power plants in a wide range of pressure ratios β = 10 ÷ 100. Calculations were made for four variants (optimistic and conservative), differing in terms of the performance characteristics of the compressor and turbine isentropic efficiency as a function of β and the permissible temperature of the expander blades in the gas turbine. The steam part of the power plant in question is composed of a classical steam cycle with a triple-pressure subcritical heat recovery steam generator (HRSG) with reheating, to which gases from a gas turbine are fed at a constant temperature of 630 °C. The power plant also has an additional steam cycle utilizing the heat from cooling the air from the intercooler of the air compressor. The impact of using steam cooling on the electrical efficiency of the gas turbine as well as the entire power plant is presented. This was compared to classical open-air cooling, achieving a gas turbine electrical efficiency seven percentage points higher for high pressure ratios. Using steam and air heat, the combined cycle power plant achieves a net electrical efficiency of more than 65% for β ≥ 95. This paper presents the calculation algorithm of the break-even cost of electricity Celb-e (at the NPV = 0) and the cost of the components as a function of the pressure ratio β. In addition, the efficiency and power degradation of the power plant over time is presented. The lowest cost Celb-e = 64.85 €/MWh (at the fuel price 8.179 €/GJ) was obtained for the compression ratio β = 31. But, the range of compression ratios for which this is close to the optimum is wide and is within the range of β = 18 ÷ 53. Sensitivity analysis of the break-even cost of electricity to changes in microeconomic factors was performed for case O1 (characterized by the highest net electrical efficiency of the unit), such as: investment costs, gas fuel price, power plant operating time and CO2 emission allowance cost. This showed that a change in these factors has little effect on the change in the optimal gas turbine pressure ratio, while significantly affecting the profitability of the combined cycle power plant. Calculations show that the thermodynamic optimum (maximum efficiency) and economic optimum (minimum break-even cost of electricity) do not overlap.

Continued Experimental Study on the Friction Contact between a Labyrinth Seal Fin and a Honeycomb Stator: Slanted Position

Labyrinth seals are a state-of-the-art sealing technology to prevent and control leakage flows at rotor–stator interfaces in turbomachinery. Higher pressure ratios and the economical use of cooling air require small clearances, which lead to potential rubbing events. The use of honeycomb liners allows for minimal leakage by tolerating rub events to a certain extent. A previous study within an EU project investigated the complex contact conditions of honeycomb liners, with the idealized contact of a seal fin and a single parallel metal foil representing the honeycomb double foil section. In the present work, the results for the slanted foil position are shown and compared to the previous results. The variation of rub velocity, incursion speed, incursion rate, and seal geometry in a test rig allows for the identification of the influence on contact forces, temperatures, and wear. For the slanted position, significantly lower friction temperatures are observed, leading to a higher ratio of abrasive wear. Overall, the rub test results demonstrate strong interactions between the contact forces, friction temperatures, and wear.

Conserved schemes with high pressure ratio, high particle density ratio and self-similar method

The paper proposes a numerical investigation for three conserved schemes when applied to the nonlinear ultra-relativistic Euler equations with an initial high pressure ratio and with an initial high particle density ratio. The results show that two of these schemes work efficiently and the third one may present inaccurate results even applied over a very delicate mesh. Several problems of the relativistic Euler system have self-similar solutions which can be solved by more efficient techniques. We also introduce a representation for self-similar ultra-relativistic Euler equations, which can be solved by the proposed schemes in this article and does not need to reconstruct schemes. Numerical tests are given for the one-dimensional shock tube problems to deal with the problem under consideration. These tests display that increasing the order of accuracy does not help much in upgrading the results. It is also indicated that one-dimensional results solved by self-similar ultra-relativistic Euler equations are almost identical to the exact solutions.

Performance evaluation of decomposition methods to diagnose leakage in a reciprocating compressor under limited speed variation

Abstract Reciprocating compressors (RCs) are used in pressure based applications to achieve high-pressure ratio. An efficient detection of a leakage in a RC by vibration based signal processing techniques can avoid high energy losses. Conventional signal processing methods used to examine the faults in non-stationary vibration signals have been proved to be inefficient. This paper aims to detect valve leakage in a RC by evaluating the performance of signal decomposition techniques i.e. empirical mode decomposition (EMD) and recently developed variational mode decomposition (VMD). These non-stationary signal processing techniques have been widely applied for condition monitoring of mechanical systems. In the present study, the vibration signals of RC at different levels of leaks under limited speed variation were acquired, decomposed and compared. To detect the fault features exhibiting leakage, the vibration signals were decomposed by EMD and VMD respectively. Post decomposition, FFT analysis along with RMS and kurtosis were evaluated from decomposed signals. The characteristics frequencies were clearly exhibited by VMD and the responses of kurtosis were also better with VMD. The fault detection performance of VMD was found better than EMD.

Mitigation of impinging tones using central protrusion

This article demonstrates that a simple modification such as installing a protrusion on an impingement plate can lead to reduction of jet impingement tones. This experimental work compares the acoustic variations in jets impinging on a flat plate, and plates with a central protrusion. The key strategy here is the modification of the stagnation flow region brought about by the introduction of the central protrusion. It is observed that the toneless acoustic power is almost the same for jet impinging on the plate with and without the central protrusion. It is also found that the protrusion is more effective in reducing tones in supersonic impinging jets than the tones in subsonic impinging jets. However, there is a slight increase in acoustic power when the jet impinges on a larger protrusion (greater than jet diameter) at high nozzle pressure ratios. Further, high speed flow visualization is carried out to relate the shock oscillations with the tonal frequencies. The noise source location is estimated by performing FFT on the flow visualization images. The acoustic and optical measurements are compared in terms of tonal frequency and found to be in agreement with each other.

Flow instability evolution in high pressure ratio centrifugal compressor with vaned diffuser

Compressor stability is an essential issue for developing single-stage high pressure ratio centrifugal compressors. In this paper, a comprehensive stability map of a centrifugal compressor stage with the peak pressure ratio of 6.2 has been illustrated. Fast response pressure transducers are mounted inside the casing or the back-plate wall to measure internal transient behaviors. From Fourier analysis of pressure signals, various instabilities across time and length scales are identified. At low speeds, the impeller inlet rotating instability develops in scale and evolves into the inducer stall, which eventually induces the mild surge and deep surge of the compression system. At middle speeds, the compressor successively experiences stable state, mild surge, rotating instability, and deep surge. The mild surge region coincides with the dip region of the compressor S-shape pressure rise curve, and deep surge occurs when the compressor pressure rise raises to the second peak. At high speeds, mild surge and deep surge abruptly occur without preceding stall or rotating instability. To explain the complex surge behavior, the mechanical analogy between the compression system and the mass-spring-damper system is applied. Both mild surge and deep surge transients are found to belong to the dynamic instability of the mass-spring-damper system, and the exact form of the surge state will be determined by eigenvalues of the system governing equation.

Effect of Load Distribution at the Roots among Stages on Performance of a High Bypass-ratio Fan/Booster

The modern high-performance aero-engines have raised higher requirements on the fan/booster, such as high through-flow, high pressure ratio, high efficiency and a wide range of stable operation. In this paper, a method called arbitrary camber lines blading design method for both axial and centrifugal compressors and three dimensional CFD (computational fluid dynamics) calculation are used to design and analyse the results of the aerodynamic design of a high bypass-ratio fan/booster. By changing load distribution on the roots among the rotors, the flow of the fan/booster can be affected. The results of the study show that increasing the load on upstream stages roots can help relieve the separation at the roots of the downstream stages, which leads to the increase of the total pressure, adiabatic efficiency and stall margin of the core duct of the fan/booster.

Nonlinear dynamic analysis of a rotor-bearing system with porous tilting pad bearing support

Porous tilting pad bearings (PTPBs) show a potential for using in high-speed, high precision rotating machinery that requires high bearing stiffness and good stability. In this study, the nonlinear characteristics of a rotor supported on PTPBs are investigated. The Darcy equations and the modified Reynolds equations are adopted to establish the gas flow model in porous materials and gas film region, respectively. The pad motions and rotor motions are also included in the numerical nonlinear model. The advantages of PTPBs over tilting pad journal bearings are discussed. PTPBs with externally pressurized gas can avoid friction during startup and shutdown, improve the load capacity at low rotational speeds, and effectively suppress subsynchronous vibrations at high rotational speeds. The nonlinear characteristics of the rotor-bearing system with various bearing parameters, such as supply pressure ratio, nominal bearing clearance, pad tilting and radial stiffness, are analyzed. Poincare maps, predicted rotor orbit and bifurcation diagrams are used. High supply pressure ratio and small nominal bearing clearance can significantly decrease the orbit size and increase the critical speed. Low tilting and high radial stiffness can improve system stability. This study reveals the complex nonlinear characteristics of the system and provides guidance for designing PTPBs for using in high-speed rotating machinery.

Effects of tunneling-induced soil disturbance on the post-construction settlement in structured soft soils

Earth pressure balanced shield machine (EPBS) is widely adopted for metro tunnel construction in urban areas. Support pressure on excavation face and grouting at the EPBS tail are the main measures to control the ground disturbance and settlement. The post-construction settlement is of significance to the serviceability of metro tunnel. By weakening the compression parameters of underlying soils related to soil disturbance degree (SDD) and using layer-wise summation method, this study developed a method to predict the post-construction ground and tunnel settlement. The SDD is quantitatively evaluated by the shear-strain state obtained from a series of isotropically consolidated undrained triaxial compression (CIU) tests. Based on a case history of metro tunnel constructed in structured soft soils, a suite of three dimensional finite-element analyses are performed to investigate soil disturbance during tunneling construction. The numerical simulation and the applicability of the proposed method are validated by comparing the numerical results with the field measurements. In addition, influences of support pressure and grout ratio on the tunneling-induced soil disturbance, ground and tunnel settlement are revealed. The SDD shows remarkable falloff within 0.32D above the tunnel crown, while 0.24D below the tunnel invert. Furthermore, within the above distance range, the linear relation between the SDD and distance from the tunnel crown or invert can be built. The analysis results show that the excessively high or low support pressure and grout ratio can both cause severe SDD which results in the significant post-construction settlement. Grout ratio of about 200% and support pressure of 2.2–2.4P0 are effective for reducing both the post-construction ground and tunnel settlement. The research presented in this paper provides a reference for controlling the post-construction ground and tunnel settlement constructed in structured soft soils.

A near-isothermal expander for isothermal compressed air energy storage system

Compressed air energy storage technology is considered as a promising method to improve the reliability and efficiency of the electricity transmission and distribution, especially with high penetration of renewable energy. Being a vital component, the expander takes an important role in compressed air energy storage operation. The specific work of an expander can be improved through an isothermal expansion compared with the adiabatic expansion process due to a nearly constant temperature which enables the expander to operate with a high pressure ratio. In this study, a specific reciprocating expander with a high pressure ratio was developed and its adiabatic expansion characteristics were measured. Numerical modelling was performed to simulate adiabatic expansion. This model was also validated by experimental results. Based on these findings, we propose a quasi-isothermal expansion process using water injection into the expander cylinder. Modelling was also extended to simulate the quasi-isothermal process by introducing water–air direct heat transfer equations. Simulation results showed that when spraying tiny water droplets into the cylinder, the specific work generated was improved by 15.7% compared with that of the adiabatic expansion under the same air mass flowrate, whilst the temperature difference was only about 10% of that of the adiabatic process, and cylinder height was decreased by 8.7%. The influence of water/air mass flowrate ratio and the inlet temperature on the expander performance was also studied.

Experimental study on properties of magnesium oxychloride cement

Magnesium oxychloride cement is an air-hardening cementitious material formed by the reaction of magnesium oxide and magnesium chloride solution. This kind of cement has high strength and high folding pressure ratio, which is the unmatched performance of ordinary cement concrete. This article tests the modification status of magnesium oxychloride cement after adding organic solution, and provides reference for the processing and design of magnesium oxychloride cement components in the future.

Estimation of Frictional Coefficient between Punch and Billet at Back-Stroke in Backward Can Extrusion Test of Steel

A backward can extrusion test provides severe tribological conditions because high pressure, high temperature, and large surface expansion ratio affect the lubricant. During the forward stroke these conditions intensify with increasing cup depth of the extruded workpiece; additionally, the back-stroke force during retraction of the punch rises to a significant level under a poor-lubricated condition. This study estimates the coefficient of friction μp between punch and workpiece during the back-stroke by combining experiments using conventional soap-phosphate coated steel and numerical analysis by FEM. The values of μp were estimated to be 0.09 and 0.03 in case of small and large workpiece depth, respectively. Friction decreased with elevating temperature.