Static Pressure Ratio Research Articles

Large Eddy Simulations of Strongly Non-Ideal Compressible Flows through a Transonic Cascade

Transonic flows of a molecularly complex organic fluid through a stator cascade were investigated by means of large eddy simulations (LESs). The selected configuration was considered as representative of the high-pressure stages of high-temperature Organic Rankine Cycle (ORC) axial turbines, which may exhibit significant non-ideal gas effects. A heavy fluorocarbon, perhydrophenanthrene (PP11), was selected as the working fluid to exacerbate deviations from the ideal flow behavior. The LESs were carried out at various operating conditions (pressure ratio and total conditions at inlet), and their influence on compressibility and viscous effects is discussed. The complex thermodynamic behavior of the fluid generates highly non-ideal shock systems at the blade trailing edge. These are shown to undergo complex interactions with the transitional viscous boundary layers and wakes, with an impact on the loss mechanisms and predicted loss coefficients compared to lower-fidelity models relying on the Reynolds-averaged Navier–Stokes (RANS) equations.

Multi-point design optimization of a high-performance intake for scramjet-powered ascent flight

Scramjet engines are one of the most promising hypersonic airbreathing propulsion techniques for efficient and economic access to space. Besides capturing and compressing the air to desirable flow conditions, the intake plays a vital role in determining the intake drag, flow compression efficiency, started flow condition that governs the overall scramjet engine performance. For access to space, the flight trajectory practically comprises varying freestream conditions and altitude to enable ascent flight, constituting an optimization problem with off-design conditions as well as design and physical constraints to be considered. This paper presents the results and insights obtained from a first ever multi-point multi-objective optimization study of an axisymmetric scramjet intake conducted by means of surrogate-assisted evolutionary algorithms coupled with high-fidelity computational fluid dynamics. It aims to minimize the intake drag and maximize the compression efficiency at two differing conditions, i.e., Mach 7.7 at an altitude of 30 km and Mach 10 at 33.5 km. Key design factors for high-performance intakes that can achieve desirable flow compression for the combustor are identified by global sensitivity analysis and examining the trade-off characteristics. Underlying flow physics is elucidated by probing into the representative flowfields at two design conditions with adaptive mesh refinement so as to gain insights into phenomena and characteristics to realize scramjet-powered ascent flight. It has been found that the non-dominated solutions constitute clusters with respect to length and exit radius, with each cluster characterized by distinct tendencies in terms of compression efficiency loss, total drag, static pressure ratio and mean exit temperature. Shorter scramjet intakes have been found to be prone to flow separation due to impingement of a reflected shock off the centerline on the third ramp that causes an adverse pressure gradient.

Flow Control Effect of Spanwise Distributed Pulsed Arc Discharge Plasma Actuation on Supersonic Compressor Cascade Flow

To achieve efficient control of supersonic compressor cascade flow, a type of spanwise distributed pulsed arc discharge plasma actuation (PADPA) was designed. To simulate the influences of PADPA on the flow field, a phenomenological model was established. Then, the flow control effects of PADPA on supersonic compressor cascade flow were researched numerically. The results show that under low static pressure ratio condition, the compressive wave induced by PADPA reduced the intensity of the passage shock wave, which eventually reduced shock wave loss. It was also found that PADPA produced an adverse pressure gradient (pre-compression effect) around the actuation location, which reduced the strength of the high adverse pressure gradient induced by the passage shock wave. The airflow on both sides of the actuation location was accelerated by PADPA owing to the spanwise distributed layout. Thus, it improved the ability of the boundary layer to resist the effect of the adverse pressure gradient and reduced the separation zone. Consequently, the total pressure loss was reduced by 6.8%. Under high pressure ratio condition, the effect of PADPA on the suction side controlling the large separation of the boundary layer was insignificant. The total pressure loss also increased slightly.

Influence of leading edge with real manufacturing error on aerodynamic performance of high subsonic compressor cascades

To investigate the influence of real leading-edge manufacturing error on aerodynamic performance of high subsonic compressor blades, a family of leading-edge manufacturing error data were obtained from measured compressor cascades. Considering the limited samples, the leading-edge angle and leading-edge radius distribution forms were evaluated by Shapiro-Wilk test and quantile–quantile plot. Their statistical characteristics provided can be introduced to later related researches. The parameterization design method B-spline and Bezier are adopted to create geometry models with manufacturing error based on leading-edge angle and leading-edge radius. The influence of real manufacturing error is quantified and analyzed by self-developed non-intrusive polynomial chaos and Sobol’ indices. The mechanism of leading-edge manufacturing error on aerodynamic performance is discussed. The results show that the total pressure loss coefficient is sensitive to the leading-edge manufacturing error compared with the static pressure ratio, especially at high incidence. Specifically, manufacturing error of the leading edge will influence the local flow acceleration and subsequently cause fluctuation of the downstream flow. The aerodynamic performance is sensitive to the manufacturing error of leading-edge radius at the design and negative incidences, while it is sensitive to the manufacturing error of leading-edge angle under the operation conditions with high incidences.

Regenerative effect of heat exchanger in pulse tube refrigerator

Abstract The heat exchanger is an important component of pulse tube refrigerators (PTRs). The heat transfer in a cold heat exchanger under periodic unsteady flow is studied with the CFD method. The basic heat transfer mechanism has been explained, including the local heat transfer coefficient, heat flux, the temperature distribution of the narrow tube, and the relation between the Nusselt number and the Reynolds number. The regenerative effect of the cold heat exchanger of PTRs has been presented. The cold heat exchanger is cooled down by the low temperature gas due to the oscillating flow, and a temperature gradient is formed inside gradually. The regenerative effect will decrease the expansion work of the PTRs and lower the efficiency of the system, especially under bigger pressure ratio conditions.

DEM simulation of the size effect on the wetting deformation of rockfill materials based on single-particle crushing tests

Abstract Rockfill materials undergo additional deformation when immersed in water under constant-stress conditions. This phenomenon caused by flooding is called wetting deformation. Researchers have investigated wetting deformation via many factors through laboratory tests on scaled materials. However, the size effect on wetting deformation has seldom been studied due to the limited size of test equipment. To investigate this topic, a set of single-particle crushing tests was conducted on Gushui rockfill materials under wet and dry conditions, and triaxial wetting tests of samples with different grain sizes under constant confining pressure and stress ratio conditions were simulated by discrete element method (DEM). The single-particle crushing tests showed that wetting significantly reduced particle strength and stiffness and that particle strength decreased with increasing particle size. The stress-strain curves obtained by the DEM simulations were in good agreement with the experimental results. The wetting deformation of the large-sized sample was larger than that of the small-sized sample, especially under a high stress ratio and confining pressure. The wetting volumetric and axial strains had a linear relationship during flooding, and their ratio was greatly affected by the stress ratio. The size effect on wetting behavior should not be ignored, especially for high-rockfill dams.

Static intra-access pressure ratio and cardiovascular events in patients undergoing haemodialysis

Static intra-access pressure ratio (SIAPR) measurement, using haemodialysis machine transducers, is the vascular access surveillance method in patients undergoing haemodialysis. However, little is known about the relationship between the SIAPR and arterial stiffness, and the clinical usefulness of the SIAPR in predicting cardiovascular events. A total of 209 patients undergoing maintenance haemodialysis were evaluated. The SIAPRs ranged from 0.01 to 0.52 (median: 0.23). When the patients were divided into two groups according to their median of SIAPR, the incidence of previous cardiovascular disease, E/E′ ratio, and brachial-ankle pulse wave velocity were significantly higher in the patients with SIAPRs of ≤0.23 than in those with SIAPRs of >0.23. Conversely, patients with worse comorbid status had a lower SIAPR than patients without it. In the Kaplan-Meier analysis, the cumulative incidence of cardiovascular events was significantly higher in the patients with SIAPRs of ≤0.23 than in those with SIAPRs of >0.23 (P < 0.001). In the multiple Cox regression analysis, an increase in the SIAPR was associated with a reduced risk for cardiovascular events [hazard ratio: 0.36, 95% confidence interval: 0.21–0.60, P = 0.001]. Therefore, a low SIAPR related with arterial stiffness was a predictor for cardiovascular events.

Study and Analysis of Fuel Injection at Busemann Inlet at Mach 7

The incomplete mixing of fuel and the drag induced in the engine are the main problems encountered in scramjets. Proper mixing of fuel can be achieved if the fuel is injected from the inlet of the engine. The fluid domain of Busemann Inlet at Mach 7 with boundary layer fuel injection using slot injectors along the wall was simulated with hydrogen fuel. Static pressure ratio at throat and inlet, along with Mach number at the throat were the design parameters of the Busemann inlet. Hydrogen fuel injection at two different equivalence ratios (0.3 and 0.7) was simulated successively at a temperature of 100 K and Mach 2.5. The state of mixing was studied for three different injection angles (30°, 45°, and 60°) for different equivalence ratios. The injection angles do not have a substantial effect on the hydrogen fuel mixing with the mainstream flow for equivalence ratio 0.3 Whereas, the dispersion of fuel increases as the increase in injection angles for equivalence ratio 0.7. But the total pressure recovery is maintained to the design value for equivalence ratio 0.3 rather than 0.7. Similarly, 45° fuel injection maintains a higher total pressure recovery than in other injection angles of 30° and 60°. Injection technique other than slot injection can be used to study further the effect of fuel mixing in Busemann Inlet.

Performance of a solar thermal power plant with direct air-cooled supercritical carbon dioxide Brayton cycle under off-design conditions

The use of an efficient and compact supercritical carbon dioxide (sCO2) Brayton cycle in concentrated solar thermal power plants has the potential to reduce costs of electricity generation. Heat rejection in the hot-arid climate is of great concern to the power cycle, especially by natural draft dry cooling technologies. For this purpose, a comprehensive design and rating analyses of the sCO2-air cooling process was conducted based on short natural draft dry cooling towers. This approach is featured with the capture of non-linear characteristics in physical properties of CO2 and geometry of fin-tube air-cooled heat exchangers. It is found that the proposed methodology successfully predicted the experimentally observed outlet temperatures of an existing cooling tower. By utilizing off-design models of heat exchanger and turbomachinery, a direct air-cooled recompression sCO2 cycle was investigated for a parabolic trough solar plant with thermal energy storage (TES). The impacts of pressure ratio, recompression fraction, shaft speed and boundary conditions, i.e., ambient air temperature and solar intensity, were investigated on the power output and key parameters of the power plant under quasi steady state conditions. The results show that the recompression fraction significantly affects the pitch point in the recuperators, the optimum value of which decreases with an increase in compressor inlet pressure and in shaft speed. In addition, the direct air-cooled power system depends strongly on ambient environments, and is able to handle lower solar intensities without deterioration in electricity generation by the buffering of TES. The cooling tower approach decreases non-linearly as the ambient temperature increased, indicating that a fixed approach of typical 15 °C results in a conservative electricity production at hot climatic conditions.

Analysis of Important Chemical Pathways of n-Heptane Combustion in Small Skeletal Mechanisms

Reduced mechanisms for n-heptane combustion have been constructed using the novel ACR method, with the complex mechanism from CRECK, including low-temperature chemistry, as a starting point. Tailored mechanisms for each part of the combustion process, ignition, flame propagation, and extinction, were created to identify differences and common trends in mechanism composition. The simulations were carried out for n-heptane/air mixtures for zero-dimensional homogenous reactors, one-dimensional freely propagating flames, and counter-flow diffusion flames at temperature, pressure, and equivalence ratio conditions relevant to engine combustion. The smallest mechanism, 28 species, only targets laminar burning velocity, while the largest single-property mechanism is for ignition over a wide temperature range, with 48 species. A compound mechanism covering all conditions consists of 65 species, this mechanism includes reaction paths describing important subchemistries like the peroxyl radical chemistry governing low-temperature ignition and the C/H/O chemistry of importance to flame propagation. Compared to the CRECK mechanism, all the reduced mechanisms have fewer than 2% of the number of reactions. (Less)

Investigation of Differential Evolution Algorithm for a Tandem Cascade Optimization

Due to the fact that there are numerous factors and parameters associated to tandem cascade, and upstream and downstream blades have intensive interference and unknown matching correlation, it is difficult to create tandem blades with high performance based on conventional optimization design method. Therefore, developing an optimization strategy that can take into account key parameters of tandem blades is a feasible way to get optimal design of tandem cascade. In the present work, the Sphere and Rastrigin test functions were adopted to compare and analyze five differential evolution modes, and the DE/r2b/2 evolution mode was then obtained, which can be applied to engineering practice. Based on the present evolutionary pattern, the combination the effects of axial overlap (AO), percent pitch (PP), camber ratio (TR), chord ratio (CR) and approximate incidence angle of the rear blade (Kb-b) are considered comprehensively. Thus, the optimization process and strategy are designed, and the optimization objective function is determined. The optimization results show that under the condition of designed Mach number and large angle of attack (AOA), comparing with the original blades, the optimization makes the static pressure ratio of the tandem blades increased by 1.14%, and the total pressure loss coefficient decreased by 6.57%.

Study on the combined effect of various injection conditions on the drag reduction by a counter-flow jet in supersonic flow

Parametric investigations of drag reduction by counter-flow jet in supersonic flow have been conducted through numerical simulations. An axisymmetric RANS equation was solved to analyze the drag reduction effect of each parameter. This study revealed a general tendency of the drag reduction effect on various shapes and counter-flow jet conditions and confirmed the conditions for obtaining the maximum drag reduction. In addition, the counter-flow jet structure and flow transition were also analyzed. Five parameters, jet mass flow rate, nozzle size, nozzle exit Mach number, gas total temperature, and gas types, were applied to analyze the drag reduction and flow structure. The results showed that the jet mass flow rate is the dominant factor of drag reduction and flow transition by counter-flow jet and the nozzle size has little effect on drag reduction. Regarding the individual effects of each parameter, a high nozzle exit Mach number, high total temperature, and low molecular weight of the gas are desirable for a higher drag reduction ratio with a low mass flow rate. On the other hand, because a thrust reversal phenomenon tends to occur easily at high temperatures, proper trade-off between each parameter is essential for obtaining the maximum drag reduction ratio. In addition, the nozzle flow conditions, particularly the static pressure ratio at the nozzle exit, have a large effect on the flow structure of the counter-flow jet.

Flutter in Started and Unstarted Transonic Linear Cascades: Simultaneous Measurements of Unsteady Loads and Shock Dynamics

AbstractIn the present work, we report on an experimental study of bending/flap mode flutter of a blade within a linear cascade at transonic conditions. Driven by the motivation to understand the contribution of shock location/dynamics to flutter characteristics, we have performed simultaneous measurements of shock dynamics using high-speed shadowgraphy combined with unsteady load measurements on an oscillating blade within the cascade. The flutter characteristics in terms of energy transfer from the fluid to the blade and shock dynamics have been mapped out over a range of blade oscillation frequencies and static pressure ratios (SPRs) across the cascade, the latter being important as they decide the mean location of the passage shocks. SPR values studied include both conditions where the shock is within the passage (started cascade) and where the passage shock is pushed ahead of the leading edge of the blades (unstarted cascade). These measurements show characteristically different flutter behavior for an unstarted cascade compared with a started cascade, the former having received very little attention in the literature. While both these cases show small excitation levels at low reduced frequencies, the unstarted cascade case exhibits an additional relative narrow region of excitation at higher reduced frequencies with approximately an order of magnitude higher excitation energies. Comparison of the shock dynamics between the two excitation regimes shows significant differences in the phase of the leading edge shock in addition to changes in the suction side shock phase indicating that the two excitation regimes are of different origin.

Selection of working fluid for low enthalpy heat source Organic Rankine Cycle in Dholera, Gujarat, India

Organic Rankine Cycle (ORC) is a technique which can generate electricity by using low to medium enthalpy geothermal water. This paper narrates selection of working fluid which is the most important factor in designing of ORC. In this paper four fluids, R600, R600a, R134a and R245fa were chosen for screening. The screening is done with respect to hot water temperature at generator inlet side and pressure ratio condition. Analysis of working fluid in this paper is done for six parameters namely ORC efficiency, Volumetric Flow Ratio (VFR), Expander Size Parameter (SP), Total heat exchanger utilised area (UA), Network output and Turbine outlet volume flow rate. This analysis were done in order to select the suitable working fluid for ORC integrated with space heating and cooling plant at Dholera. This ORC uses the output water of 75–80 °C obtained from heating side of the space heating and cooling system. As the water temperature which is used in ORC is a low enthalpy it needed a working fluid which can increase the efficiency and exergy of ORC. After screening the results shows that R245fa is a suitable working fluid for ORC integrated with space heating and cooling plant in Dholera, Gujarat, India.

Axial Impact Load of a Concrete-Filled Steel Tubular Member with Axial Compression Considering the Creep Effect.

The dynamic loads acting on concrete-filled steel tubular members under axial impacts by rigid bodies were studied herein by FEM. The whole impact process was simulated and the time history of the impact load was obtained. The effects of eight factors on the axial impact load were studied; these factors were the impact speed, mass ratio, axial pressure ratio, steel ratio, slenderness ratio, concrete strength, impact position, and boundary conditions. Besides this, the effects of concrete creep on the impact load were also considered by changing the material parameters of the concrete. The results show that axial impact load changes with time as a triangle. The peak value of impact load increases and the impact resistance improves with the growth of the axial pressure ratio, steel ratio, slenderness ratio, and concrete strength after creep occurs. As the eccentricity of the axial impact acting on a concrete-filled steel tubular member increases, the peak value of the impact load decreases. The enhancement of constraints at both ends of the member can improve the impact resistance. The creep reduction coefficients for the peak axial impact load of a concrete-filled steel tubular member under axial compression and considering the creep effect over 6 months and 30 years are 0.60 and 0.55, respectively. A calculation formula for the peak value of impact load was suggested based on the existing formula, and its accuracy was proved by finite element calculation in this study.

Cu-K/Al2O3 based catalysts for conversion of carbon dioxide to methane and carbon monoxide

A series of Cu-K/Al2O3 catalysts were synthesized by wet impregnation technique. The reduced catalysts were further used for conversion of carbon dioxide to methane and carbon monoxide. Moreover, the fresh and used catalysts were characterized to investigate the changes in the surface morphology, metal dispersion, surface area, crystalline phases, and functional groups of studied catalysts. The SEM analysis of fresh and spent catalysts showed no remarkable difference in surface morphology with irregular shaped agglomerated particles. Furthermore, TEM micrographs presented the well distribution of metal catalyst over alumina support. The decrease in surface area from 115 to 77 m2/g for Cu1.62-K0.5/Al2O3 after reaction was related to sintering and oxidation of catalyst during reaction. XRD revealed the disappearance of some minor peaks which can be associated with the sintering of spent catalyst. FTIR also presented some new peak for spent catalyst which can be linked with metal oxides. Moreover, various reaction conditions of temperature (230, 400, and 600 °C), pressure (1 and 7 bar), and feed molar ratio of H2/CO2 (2:1 and 4:1) were investigated using different Cu loading (0, 1, 1.25, 1.62, and 4 weight percent). A maximum CO2 conversion of 63% with 39% CH4 selectivity was achieved by using Cu1.62-K0.5/Al2O3 at 600 °C, molar ratio of H2/CO2 4 under 7 bar. The presence of K on the surface of synthesized catalyst increased the CO2 conversion from 48% (Cu1/Al2O3) to 55% (Cu1-K0.5/Al2O3) at above mentioned reaction conditions which suggested the promoter effect of K during conversion of carbon dioxide.

Experimental investigation of flow separation in a planar convergent-divergent nozzle

Under the conditions of nozzle pressure ratio (NPR) of 2.79, 2.82 and 2.95, the flow field was observed by dynamic pressure measurement and focusing schlieren technique, and the flow separation characteristics of a planar convergent-divergent nozzle were studied. The results show that as the NPR increases, the nozzle flow structure changes from asymmetric to a symmetrical configuration. Under the condition of NPR=2.79, the flow in the nozzle goes obviously downward to the bottom wall of the nozzle, and the restricted shock separation (RSS) occurs on the bottom wall; under the condition of NPR=2.82 and NPR=2.95, the flow in the nozzle tends to be symmetrical, RSS appears on both the top and bottom walls. Based on spectrum analysis, it is found that there is obvious low-frequency oscillation characteristics near the separation point.

Internal volume ratio optimization and performance analysis for single-screw expander in small-scale middle temperature ORC system

Single-screw expander (SSE) has the potential to meet the high pressure ratio condition in small-scale middle temperature ORC system. In this paper, variable internal volume ratio (from 3.00 to 8.00), new methods for calculating mass flow rate and friction power are integrated into the structure-based SSE thermodynamic model. And the maximum calculation error of mass flow rate, volume efficiency and shaft efficiency is 2.8%, 2.1% and 2.3% respectively compared with the experimental data. Herein, the influence of internal volume ratio on shaft efficiency, shaft power, volume efficiency, intake/exhaust pressure loss and friction loss are studied. The optimal internal volume ratio and shaft efficiency for five working fluids (R123, HFO-1336mzz(Z), R601, Cyclopentane, R245fa) are obtained when evaporation temperature changes from 373 K to 463 K. The results show that the optimal internal volume ratio is not the bigger the better when SSE works at high pressure ratio condition, because intake pressure loss also increases with the increase of internal volume ratio. The optimal shaft efficiency increases with the decrease of friction power, but the optimal internal volume ratio is almost unaffected. Furthermore, reducing intake pressure loss is a powerful means to improve the performance of SSE with large internal volume ratio.

The application of nanofiltration membrane for recovering lithium from salt lake brine

In the current study, the applicability of nanofiltration membrane for recovering lithium from salt lake brine with a high magnesium-to-lithium mass ratio was evaluated under different operating pressure, temperature, multi-ion presence, and concentration ratio conditions. The results showed that the yield of lithium and its separation from magnesium increase with pressure, with the rejection of magnesium reaching 92% at 3.5 MPa. The maximum membrane flux of 50 L/m2 h is achieved at 313 K, while the separation efficiency decreases with increasing temperature. Furthermore, the existing sodium and potassium compete with lithium to pass through the nanofiltration membrane. The yield of lithium reaches 99% when the concentration ratio is 4. However, membrane flux and separation efficiency decrease with increasing concentration ratio. Ionic fractionation is found to be governed by the special properties of the ions, dielectric exclusion, and steric hindrance. Our results indicate that DK membrane shows considerable promise as a means to separate magnesium and lithium and to recover lithium from salt lake brine with high magnesium-to-lithium mass ratios.

Extraction of parboiled rice bran oil with supercritical CO2 and ethanol as co-solvent: Kinetics and characterization

This study aimed to evaluate the behavior of supercritical CO2 as solvent and ethanol as co-solvent for obtaining parboiled rice bran oil. The experiments were carried out at different temperatures (40, 60, 80 °C), pressures (100, 150, 200 bar) and ethanol to rice bran mass ratios (0:1, 0.5:1, 1:1, 2:1). An extraction using pure ethanol was also performed for comparison purposes. At 100 bar the scCO2 acted just as a process carrier (and extractions only occurred with the presence of ethanol) while at 200 bar it acted as extractor agent of the process. The highest extraction yield using scCO2 + Ethanol (obtained on a single run) was 25.48 wt % (mass of extracted oil related to the initial mass of rice bran) at 40 °C, 200 bar and ethanol to bran mass ratio of 1:1, after 250 min and spending 45.94 g CO2 g−1 of bran. A slightly higher yield (26.32 wt %) was obtained around three times faster and using less than half of the CO2 by a sequence of two extractions performed at the same conditions of temperature, pressure and initial ratio of ethanol to rice bran. In general, the ethanol addition at the beginning of the process showed to be effective for reducing the necessary time for almost complete extraction. However, applying higher amounts (ethanol to rice bran mass ratio of 2:1) reduced the initial extraction rate. The extracts obtained in this work were also characterized by physicochemical analysis, total phenolic content, antioxidant activity, and thermal analyses.