Initial Pressure Ratio Research Articles

The experimental research and mechanism analysis on the influence of wave rotor rotational speed on the wave system and flow losses of gas wave ejector

Gas wave ejector (GWE) is an efficient injection and supercharging device. The rotational speed of wave rotor is an important parameter affecting the equipment performance. The influence of rotational speed on device performance was intensively studied by experiments and numerical simulations. The results suggest that there is an optimal speed to optimize the equipment performance with fixed pressure ports. Redundant compression waves are generated destroying the ideal wave system when the speed deviates from the optimal value. And the performance reduction induced by these redundant waves is different since the pressure, initial pressure ratio and reversed pressure ratio in the passages of stable pressure region are different in different working conditions. In addition, increasing rotational speed to a certain extent can reduce various types of loss in the passages. However, the viscous dissipation increases sharply if the speed is high enough. The propagation speed of shock wave increases with the increase of rotational speed, which should be taken for granted when designing the geometries of pressure ports. In the experimental conditions, the highest isentropic efficiency and ejection rate GWE can reach about 60% and 80% respectively in the experiments of this paper.

Diaphragmless single-pulse shock tube for high-temperature chemical kinetics studies.

Single-pulse shock tubes are effective tools for measuring chemical kinetics at high temperatures, typically (900-1400) K. However, the use of a diaphragm for shock generation leads to significant shock-to-shock inconsistencies in temperature for a constant initial pressure ratio across the discontinuity. Diaphragms also require replacement after each shock and demand care in cleaning to ensure that the fragments do not contaminate the apparatus. A piston-driven valve design is presented that leads to a highly reproducible postreflected shock temperatures (0.41% at 1147 K and 0.61% at 967 K) in a single pulse varying from (500 to 1200) µs in width over the temperature range of interest. Characterization of the valve was accomplished using both shock-speed measurements and independent measurements of the pulse temperature using reference thermal decomposition reactions.

Explosion characteristics of a pyrolysis biofuel derived from rice husk.

Fast pyrolysis biofuel is considered as one of the promising alternative fuels for future transportation. The authors' research group has improved a pyrolysis biofuel production method for obtaining a biofuel from rice husk. Preliminary research published in Fuel reveals that it has the potential of being used as a gasoline blending component. Before this biofuel can be widely used in real combustors or engines at a large scale, its explosion characteristics should be investigated for the safety consideration during production, transport and storage. This paper quantitatively evaluates explosion pressure, deflagration index, maximum pressure rise rate and combustion duration of this biofuel in the constant volume combustion chamber (CVCC). At the same time, the effects of initial pressure, initial temperature and equivalence ratio on explosion characteristics of this biofuel was also qualitatively analyzed. Linear relationships between explosion pressure, deflagration index, maximum pressure rise rate and combustion duration versus initial pressure and explosion pressure versus initial temperature are given. Results show that explosion pressure is a linear relationship with the initial temperature. However, deflagration index, maximum pressure rise rate and combustion duration are insensitive to the temperature variation.

Understanding strong knocking mechanism through high-strength optical rapid compression machines

Strong knocking combustion has become the greatest challenge for advanced internal combustion engines to pursue thermal efficiency limits at high power density conditions. Arising from enclosed space and extreme combustion situations, the fundamental mechanism for strong knocking combustion has still not been fully understood. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed direct photography, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical rapid compression machine (RCM) with flat piston design. Strong knocking phenomena were reproduced through varying initial thermodynamic conditions, and localized autoignition (AI) initiation and reaction wave evolutions were visualized, companied by synchronous pressure and temperature trajectories. The results show that compared with initial temperature, initial pressure and equivalence ratio exhibit greater influence on the variations of knocking severity. The weighting of different contributors can be further quantified by an effective energy density that shows positive but nonlinear correlations with knocking severity. However, the distinctions between CI and SI knocking characteristics at identical effective energy density also reflect the essential role of the interplay between primary flame propagation and end-gas AI progress. Visualized combustion images show that through improving end-gas thermodynamic state and reactivity sensitivity, the primary flame propagation can enhance localized AI initiation and secondary intensive AI evolutions, facilitating combustion mode transitions into developing detonation. The significant influence of primary flame propagation is diminished until ignition delay time becomes sufficiently short. Finally, with estimated thermal heterogeneities in flat-piston RCM configurations, the ignition modes of strong knocking cycles are quantified by a non-dimensional ignition regime diagram, and favorable scaling agreements with strong and mixed ignition regimes are observed.

Damage Creep Analysis and Instability Delay Time of Ultrahigh pillar considering Initial Transverse Deformation

According to the ultrahigh pillar structure and mechanical characteristics, considering the initial geometric deviation caused by mining disturbance, the simplified model of the pressure bar is established. Based on elastic-plastic theory, the deflection and internal forces of ultrahigh pillar with initial geometric imperfections are analyzed and discussed. Based on the Kachanov theory of damage, the damage of ultrahigh pillar is analyzed, the damage failure time is calculated. The results show that under the influence of the initial deflection, pillar will appear different degree of additional deflection, which grows with the increase of initial deflection and pressure ratio. Analysis of engineering examples show that the start time of damage and instability of delay time of pillar has close relationship with initial deflection, pressure ratio and compressive strength. With the increase of pressure ratio and initial deflection, the damage incubation time and the instability of delay time quickly become small, the greater the compressive strength, the greater the instability of the delay time, which means that the pillar is more stable. The results of this study provide a theoretical basis for stability analysis and control of ultrahigh pillar.

A magnetorheological hydrostatic guideway system for machining vibration control

A novel magnetorheological (MR) hydrostatic guideway system for control of machining vibration is proposed. After the analysis of the relationship between correlation parameters and working variables, a computational fluid dynamics (CFD) model is presented. With this model incorporated into the commercial code FLUENT, a numerical study on performance parameters of MR hydrostatic guideway system is carried out efficiently. Static stiffness and damping coefficients are calculated by using dynamic mesh technique based on perturbation theory. Analysis on dynamic behaviors of the MR hydrostatic guideway under the action of machining force is performed. An experimental study has been undertaken in order to validate the accuracy of the numerical model. It is observed that working variables (magnetic flux density, initial pressure ratio and load ratio) have significant effects on the performance characteristics (flow rate, frictional force, stiffness and damping) of the system. Optimal static stiffness and high damping can be expected simultaneously. Dynamic stiffness of the system can be improved and vibration caused by machining force can be reduced remarkably by increasing magnetic flux density. It is effective to control machining vibration by applying MR hydrostatic guideway system.

Parameter extraction from spherically expanding flames propagated in hydrogen/air mixtures

The characteristics of hydrogen/air flame were studied by using the spherical expanding flame propagated in a constant volume chamber. The influence of ignition induced blast wave and the flame instability on flame propagation was investigated. The nonlinear evaluation method for laminar flame parameter evaluation was established. By using the nonlinear evaluation method and the experimental results of flame propagation, the laminar flame speed and Markstein length were extracted and the difference between the nonlinearly evaluated laminar flame speed and the linearly evaluated one was analyzed. The influence of initial pressure and equivalence ratio on laminar flame speed and flame thickness was investigated. The laminar flame speed varies with equivalence ratio and initial pressure. There exists an equivalence ratio at which the laminar flame speed gets its maximum value. And there also exists an initial pressure at which the laminar flame speed gets its maximum value. The critical radius, Markstein length and flame instability of hydrogen/air flame with different equivalence ratio at different initial pressure had been studied. In hydrogen/air flame the flame stability decreases with the increase of initial pressure, while it increases with the increase of equivalence ratio. The global stability of flame is determined by the combination of the stabilizing effect of stretch effect, thermodiffusive instability mechanism and hydrodynamic instability mechanism.

Skeletal mechanism construction for heavy saturated methyl esters in real biodiesel fuels

Methyl decanoate, methyl-5-decenoate and methyl-9-decenoate have been widely adopted as biodiesel surrogates for engine simulation. However, these surrogates have relatively short chain length and very different physical properties compared with biodiesel molecules. Additionally, it is difficult to distinguish the unsaturation degree of different biodiesel fuels by using these surrogates. The direct use of the heavy methyl esters in real biodiesel fuels as surrogates is essential to accurately simulate biodiesel fuels. In this work, a four-part scheme to formulate skeletal mechanism for heavy saturated methyl esters has been proposed. Within the scheme, the oxidation mechanism is divided into four parts: low temperature oxidation, high temperature decomposition, ester group reactions and detailed C4-C0 chemistry. A skeletal mechanism for two of the five main methyl esters in real biodiesel fuels, i.e. saturated methyl palmitate and methyl stearate, has been constructed. The obtained skeletal mechanism contains only 6 fuel-dependent species and 13 fuel-dependent reactions for each heavy saturated methyl ester. Extensive validations were performed against shock tube experimental data for ignition delay timing under different initial pressure, temperature and equivalence ratio. The ignition delay behavior at high temperature has been well captured by the developed skeletal mechanism. As for ignition at low to medium temperature (from 650 K to 900 K), there is no experimental data due to the low vapor pressure and high melting point of heavy methyl esters. The comparison of ignition behavior at low temperature has been made between several models. Furthermore, the oxidation of n-decane/methyl palmitate and benzene/methyl stearate in a jet-stirred reactor has been utilized to validate the important species concentration. Good agreements have been observed through the validations. The results indicate that the developed skeletal mechanism is capable of predicting the combustion characteristics of methyl palmitate and methyl stearate.

The characteristics of flame propagation in ammonia/oxygen mixtures.

As a carbon free fuel and a hydrogen-energy carrier, ammonia is expected to be one of the promising energy carriers and to be widely used in industries. In this study, the parameters of ammonia/oxygen flame, such as laminar flame speed, the Markstein length, laminar flame thickness, and critical radius of flame instability onset have been investigated experimentally and numerically. A spherical ammonia /oxygen flame propagated in a constant volume chamber and a high speed digital Schilieren photograph system were used in the study. The influences of initial pressure and equivalence ratio on flame propagation have been investigated. It was found that the maximum laminar flame speed in ammonia/oxygen mixture is 1.09 m/s. The flame thickness decreases with the initial pressure increase. Generally, Markstein length increases with the increasing of equivalence ratio while it decreases with the increasing of initial pressure. The minimum critical radius in ammonia/oxygen is 1.8 cm. And the critical radius decreases with the increase of initial pressure. The mechanisms of flame instability in ammonia/oxygen mixture were analyzed, and the stabilize effect of flame stretch in ammonia/oxygen have been observed.

Experimental study on flame stability of biogas / hydrogen combustion

The flame stability of biogas blended with hydrogen combustion was experimentally studied in the constant volume combustion bomb. The variations of characteristic parameters of flame instability and effect of pressure and fuel component proportion on flame shape were analyzed. The experimental results show that the flame instability increases with the decrease of equivalence ratio, and the global flame stability decreases with increase of CO2 fractions. With increase of initial pressure of biogas and hydrogen mixture, Markstein length decreases, hydrodynamic instability decreases, but the thermal mass diffusion instability has no effect. The effect of increase of the hydrogen ratio on flame stability is more obvious, with the increase of initial pressure and hydrogen ratio together, both hydrodynamic instability and thermal mass diffusion instability increase. This research can provide experimental basis for the design and development of biogas blended with hydrogen engines.

Effect of burst disk parameters on the release of high-pressure hydrogen

Hydrogen is regarded as an alternative energy carrier in the next decades and high-pressure hydrogen storage is treated as the best option. However, unexpected spontaneous ignition would occur during high-pressure hydrogen sudden release, which induces a severe safety issue. For improving the safety application of hydrogen, an experimental investigation has been conducted. Different diameter ring gaskets are employed to change the opening ratio χ. Pressure transducers and light sensors are used to record the pressure variation and possible light signals inside the tube, respectively. It is found that the burst disk is unable to fully open during high-pressure hydrogen release when χ < 1, resulting in forming a convergent nozzle. This structure leads to the speed reduction for supersonic flow. Consequently, the speed of shock and shock overpressure inside the tube reduce significantly. The spontaneous ignition cannot be initiated even though the initial pressure ratio is as high as 90 when χ ≤ 1/2. The minimum initial pressure ratio required for spontaneous ignition increases to 64.1 when χ = 2/3. The flame is dimmer for small opening ratio cases. The shock overpressure outside the tube is reduced significantly, which decreases the damage to the facilities and humans to a large extent.

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.

Relationship of the combustion characteristics of natural gas-hydrogen/carbon dioxide mixtures with the ion current and pressure parameters

The characteristics of the ionization current, pressure, and flame schlieren photos during the combustion process of natural gas-hydrogen/CO2 blends were studied using high-speed schlieren data acquisition technology and flame ionization analysis methods. According to the ion current waveform, pressure waveform, and flame propagation photos, the relationship of the characteristic parameters between the ion current and pressure were analysed using available experimental data. The ionization current waveform can reflect not only the entire combustion process (including ignition phase, the front flame phase and the post flame phase) but also the effect of the initial pressure, equivalence ratios, hydrogen fraction and CO2 dilution ratios on the combustion process.This results of this study show that the amplitude and characteristic timing of the ion current can be used to predict the change of the chamber pressure. The ion current and pressure will increase with the increase of the initial pressure, equivalence ratio and hydrogen fractions, but the parameters will decrease with the increase of the CO2 dilution ratio. The characteristic timing of the ion current and pressure peaks is advanced with the increase of the equivalence ratio and hydrogen fractions; however, the peaks are postponed by the increase in the initial pressure and CO2 dilution ratio. The corresponding relationship of the characteristic timing between the ion current and pressure is basically unchanged. The results of this study provide a new approach for exactly controlling the combustion process.

Asymptotic scaling laws and semi-similarity solutions for a finite-source spherical blast wave

A spherical blast wave generated by a sudden release of a sphere of compressed gas is an important model problem to understand blast phenomena such as volcanic eruptions and explosive detonations. The resulting explosion flow physics, such as the instability at the gas contact discontinuity and the interaction between the shock wave and the gas contact, are dictated by the initial pressure and sound-speed ratios between the compressed gas and the ambience. Since the initial pressure and sound-speed ratios vary over a wide range in practical applications, it is of interest to investigate the scaling laws and similarity solutions for the spherical symmetric explosion flow. In the present study, numerical simulation of a spherical blast wave is performed. A long-term length scale that incorporates the initial charge radius and the initial pressure ratio is introduced. The trajectories of the main shock normalized by the long-term length scale for a wide range of parameters collapse after a short transition time, indicating an asymptotic similarity solution exists for the far field in the long term. With the assistance of this similarity solution, the full evolution of the main shock can be obtained semi-analytically. For near-field features, i.e. the gas contact and the secondary shock wave, only semi-similarity solutions are observed, which depend on the initial sound-speed ratio but not the initial pressure ratio. The gas contact and the secondary shock share the same scaling relations. Asymptotic analysis is performed to obtain the short-term dynamics of the gas contact, including the gas contact acceleration and the Atwood number, which are the key parameters determining the Rayleigh–Taylor instability development at the gas contact. The asymptotic contact radius as$t\rightarrow \infty$is also obtained, which is found to be well represented by the long-term length scale and thus only depends on the initial pressure ratio. A simple model of an oscillating bubble is employed to explain the scaling relation of the asymptotic gas contact radius.

Experimental and theoretical investigation on cellular instability of methanol/air flames

Cellular instability of spherically expanding methanol/air flames was investigated in a constant-volume single-chamber cylindrical combustion vessel at initial temperatures of 373–423 K, initial pressures of 1–10 atm and equivalence ratios of 0.7–2.1. Flame morphology shows that the methanol/air flames suffer from cellular instability, especially at 5–10 atm. Stability analysis was performed to determine the logarithmic growth rate of disturbance, the critical Peclet number and the critical flame radius. The influences of initial temperature, initial pressure and equivalence ratio on cellular instability were analyzed to identify the influential factors. Both experimental results and stability analysis indicate cellular instability monotonically increases with increasing temperature or pressure and non-monotonically varies with increasing equivalence ratio. The reduced critical Peclet number and flame thickness slightly destabilize the flame surface as temperature increases. The almost unchanged critical Peclet number and the greatly reduced flame thickness with increasing pressure make the critical flame radius smaller and the flame surface more unstable at high pressures. The monotonically decreased critical Peclet number and the non-monotonically varied flame thickness result in the non-monotonic variation of cellular instability in methanol/air flames versus equivalence ratio. The critical flame radius consequently increases with increasing equivalence ratio under very rich conditions, while the corresponding flame surface becomes more smooth. The non-monotonic variation of cellular instability versus equivalence ratio is anticipated to exist widely in combustion of heavy fuels.

Experimental comparison of dynamic responses of CO2 transcritical power cycle systems used for engine waste heat recovery

CO2 transcritical power cycle (CTPC) is attractive to engine waste heat recovery (WHR) due to its advantages of miniaturization and unique thermophysical properties of its working fluid. Since there is no, if any, literature considering dynamic characteristics of CTPC systems, in current work, a series of dynamic tests has been conducted on a kW-scale CTPC test bench for engine WHR. Effects of mass flow rate and pressure ratio on dynamic responses are mainly focused and compared among four CTPC systems, i.e. a basic CTPC (B-CTPC), a CTPC with a recuperator (R-CTPC), a CTPC with a preheater (P-CTPC) and a CTPC with both a recuperator and a preheater (PR-CTPC). Dynamic characteristics are quantifiedby time constant and settling time. Results show that the PR-CTPC system has the fastest dynamic responses among these four layouts and the P-CTPC system responds more quickly than the R-CTPC system thanks to the gas–liquid heat exchange. Moreover, for the same layout, larger initial CO2 mass flow rate brings faster responses while initial system pressure ratio has little impacts on system dynamic responses. Also, dynamic characteristics of a basic CTPC system and a basic organic Rankine cycle (ORC) system are compared. The ORC system adopts R123 as its working fluid. Results indicate the basic CTPC system responds almost four times faster than the basic R123-ORC system.

Thermo-economic analyses on a new conceptual system of waste heat recovery integrated with an S-CO2 cycle for coal-fired power plants

Waste heat recovery from boiler exhaust flue-gas is an effective way to save energy in coal-fired power plants. The integration of a low-pressure economizer (LPE) is a conventional choice for waste heat recovery. In this study, thermodynamic analyses of a coal-fired power plant (CFPP) integrated with an LPE is conducted on a 600 MW CFPP as reference case. Standard coal consumption rate (SCCR) of the power plant could be decreased by 1.76 g·(kW h)−1 by the LPE. Exergetic analysis reveals that significant irreversibility is exhibited by the air pre-heater (APH) of the waste heat recovery system with an LPE. Guided by the exergetic analysis of the conventional waste heat recovery system, a new conceptual system for waste heat recovery integrated with an S-CO2 cycle for CFPPs is designed in this study. In this novel system, the boiler flue-gas is split into two flows: one to heat air in APH, and another to drive an S-CO2 power cycle as the heat source. Parameters of the S-CO2 power cycle are thermodynamically optimized with the help of Genetic Algorithm. Optimal initial pressure and pressure ratio are 9.136 MPa and 5.84, respectively. Maximum cycle efficiency of the S-CO2 power cycle is 17.39%. With the optimal parameters, SCCR of the CFPP integrated with the S-CO2 power cycle decreases by 3.80 g·(kW h)−1. If the LPE is further integrated, the reduction of SCCR can reach 5.19 g·(kW h)−1. The essential reason for the significant energy saving is revealed through exergetic analysis. Exergy losses and destructions also decrease significantly in the novel waste heat recovery system. Finally, the economic performance of the proposed system is evaluated. Results show that the novel waste heat recovery system is economically suitable, and that capital investment could be recycled in 3.067 years.

Simulation and scaling analysis of a spherical particle-laden blast wave

A spherical particle-laden blast wave, generated by a sudden release of a sphere of compressed gas–particle mixture, is investigated by numerical simulation. The present problem is a multiphase extension of the classic finite-source spherical blast-wave problem. The gas–particle flow can be fully determined by the initial radius of the spherical mixture and the properties of gas and particles. In many applications, the key dimensionless parameters, such as the initial pressure and density ratios between the compressed gas and the ambient air, can vary over a wide range. Parametric studies are thus performed to investigate the effects of these parameters on the characteristic time and spatial scales of the particle-laden blast wave, such as the maximum radius the contact discontinuity can reach and the time when the particle front crosses the contact discontinuity. A scaling analysis is conducted to establish a scaling relation between the characteristic scales and the controlling parameters. A length scale that incorporates the initial pressure ratio is proposed, which is able to approximately collapse the simulation results for the gas flow for a wide range of initial pressure ratios. This indicates that an approximate similarity solution for a spherical blast wave exists, which is independent of the initial pressure ratio. The approximate scaling is also valid for the particle front if the particles are small and closely follow the surrounding gas.

Experimental study of the face stability of shield tunnel in sands under seepage condition

Understanding of the failure mechanism and limit support pressure of a shield tunnel face under seepage condition is important in engineering design and construction. Nine physical model tests, i.e. three tests in dry sands, three tests in submerged sands under undrained condition, and three tests under seepage condition were carried out. A rigid plate was set in front of the tunnel face to maintain stability at the initial state. By moving the plate backward from the soil, the displacement and earth pressure curve of the tunnel face was obtained. The earth pressure dropped sharply to a constant value which corresponds to the limit support pressure required to stabilize the tunnel face. The limit pressure was found to increase with the cover-to-diameter ratio in a shallow tunnel, and it turned to be irrelevant to cover-to-diameter ratio in a deep tunnel. The ratio of limit pressure and initial earth pressure decreases with the cover-to-diameter ratio, indicating the effect of soil arching in stabilizing tunnel face stability. Under the same water level condition, the limit pressure of tunnel face with seepage is larger than the one without seepage. The limit support pressure under seepage condition is about 70% of the earth pressure at rest; it is about 50% under undrained condition and is about 15% in dry sand. The flow line around the tunnel face was traced by infused pigment, and the distribution was shown to be irrelevant with water level. The image captured by a HD camera during the test was analyzed by PIV (particle image velocimetry) analysis; the soil particle movements and the distribution of shear strain showed the failure mechanism of tunnel face. The failure mode is a combination of a wedge with slip arc and a prism. Comparing with the failure mode in dry sands and in saturated sands below water table undrained condition, the inclination angle of the wedge block under seepage condition is much smaller.

A comparative study of biomass integrated gasification combined cycle power systems: Performance analysis

The Biomass Integrated Gasification Combined Cycle (BIGCC) power system is believed to potentially be a highly efficient way to utilize biomass to generate power. However, there is no comparative study of BIGCC systems that examines all the latest improvements for gasification agents, gas turbine combustion methods, and CO2 Capture and Storage options. This study examines the impact of recent advancements on BIGCC performance through exergy analysis using Aspen Plus. Results show that the exergy efficiency of these systems is ranged from 22.3% to 37.1%. Furthermore, exergy analysis indicates that the gas turbine with external combustion has relatively high exergy efficiency, and Selexol CO2 removal method has low exergy destruction. Moreover, the sensitivity analysis shows that the system exergy efficiency is more sensitive to the initial temperature and pressure ratio of the gas turbine, whereas has a relatively weak dependence on the initial temperature and initial pressure of the steam turbine.