Small Pressure Ratio Research Articles

Analysis of cyclic thermodynamic system combining ammonia gas turbine and transcritical carbon dioxide

Ammonia has attracted significant research attention due to its high hydrogen content and lack of elemental carbon. Unfortunately, the thermal efficiency of ammonia gas turbines is relatively low. To address this problem, this paper proposes a cyclic system that combines an ammonia gas turbine with the T-CO2 power cycle. The cycle, based on comprehensive energy distribution and utilization, makes full use of the cooling capacity of liquid ammonia fuel to cool CO2 below the critical point and uses the waste heat of the ammonia gas turbine in stages. The paper begins by establishing and validating a thermodynamic model of the combined cycle. Numerical simulations show that, under the basic operating conditions, the energy efficiency of the combined cycle is 53.44%. Next, an analysis of the regenerator of the bottom cycle shows that it introduces a low heat source, which avoids a pinch point in the regenerator. Finally, the study investigates how different key parameters affect the performance of the combined cycle. A calculation indicates that changing the temperature and pressure of the ammonia turbine inlet increases the efficiency of the combined cycle by up to 32.75%. The maximum efficiency of the combined thermal cycle is 62.42%. For a gas turbine inlet pressure not exceeding 5 bar, a small pressure ratio improves the cycle efficiency.

Analysis of low frequency pulsation and profile modification of turbine control valve

During the operation of a nuclear power steam turbine's main control valve, the valve frequently operates at a small opening and pressure ratio, causing intense alternating loads and oscillation. To address this, a transient numerical simulation studied low-frequency pulsation and optimized steam. Results showed high amplitude and wide frequency range flow pulsations on the spool and tube wall due to repeated flow field separation. By improving the valve core profile, control over the valve throat jet state reduced pressure pulsation amplitudes significantly, as well as mitigating flow-induced vibration in the valve core cavity and pipeline.

Sensitivity of Laidback Fan-Shaped Hole Discharge Coefficient Under Internal Coolant Crossflow Conditions

Abstract An experimental study was performed on the discharge coefficients of laidback fan-shaped holes under different internal coolant crossflow orientations. The influence of the geometric and flow parameters on the discharge coefficient was investigated under flat plate conditions, where the pressure ratio ranged from 1 to 1.6. The results show that the film hole discharge coefficient is more sensitive to variations in the coolant crossflow under small pressure ratios. In comparison, the discharge coefficient is much less sensitive to the change of coolant crossflow under high pressure. Meanwhile, the length of the cylindrical section varied over the range of 1D–4D, and the length of the expansion section varied from 2D to 6D, where D represents the diameter of the film hole. The results show that the discharge coefficient is much more sensitive to the length of the cylindrical section than to the length of the expansion section. To quantify the sensitivity of the internal crossflow effects on the discharge coefficient, a low-ordered reduced model is proposed for the discharge coefficient of laidback fan-shaped holes. Both the geometric and flow parameters are considered in the model and give prediction errors within 5%.

Nonuniform Clearance Effects on Pressure Distribution and Leakage Flow in the Straight-through Labyrinth Seals

Straight-through labyrinth seal is a simple and reliable noncontact seal structure widely used in aeroengines. During the actual operation of aeroengines, the labyrinth seal clearance might experience a nonuniform variation in the flow direction due to asymmetric structure and uneven temperature. In order to characterize the degree of nonuniformity, the nonuniformity coefficient is defined in current paper. The effect of nonuniform causes (rotor deformation or stator deformation), nonuniform type (convergent clearance or divergent clearance), and nonuniformity coefficient (nonuniformity degree) is carefully studied by numerical simulation. Comparative analysis has shown that there is no obvious difference in flow coefficient (less than 0.8% in current studies) between two nonuniform causes. As for nonuniform type, the nonuniformity impact of divergent type on the flow coefficient is more significant than that of convergent type, as a result of stronger axial inertia. Pressure distributions of teeth cavities indicate that the total pressure drop of the divergent type is more obvious than that of the convergent type when the pressure ratio is the same. With the same dimensionless minimum tip clearance, clearance nonuniformity would result in the increase of leakage flow of the straight-through labyrinth, particularly for the condition with small pressure ratio, large circumferential Mach number, and small dimensionless minimum tip clearance. When the nonuniformity coefficient is within the range of -0.1 to 0.1, the variation curves of nonuniformity impact factor versus the nonuniformity coefficient almost coincide (the maximum deviation is no more than 1.2%) under different operation conditions (Reynolds number, pressure ratio, circumferential Mach number, and dimensionless minimum tip clearance). Current work proves it that the effect of seal clearance nonuniformity on the leakage flow requires special attention for the refined design of aeroengines.

Leakage Characteristics of a Novel Hole-Pattern Damping Seal With Dovetail-Like Diversion Grooves: Numerical Simulation and Experiment

Abstract Reasonable design of the cavity shape of the damping hole effectively reduces the leakage rate of the hole-pattern damping seal (HPDS). A novel HPDS with dovetail-like diversion grooves was proposed, and the geometry outline of the dovetail-like diversion groove was described by using the hyperelliptic curve. Computational fluid dynamics commercial software was used to simulate the three-dimensional flow field of the dovetail-like hole-pattern damping seal (D-HPDS). The leakage characteristics of the damping seals with a typical circular hole, leaf-like hole, and dovetail-like hole were compared numerically and experimentally. The three-velocity zone model of HPDS was proposed to reveal the mechanism of leakage reduction effect of the D-HPDS compared with typical circular hole-pattern damping seal (C-HPDS). The influence of operating parameters and key geometric parameters on leakage characteristics of the D-HPDS was studied. The results show that the leakage rate of the D-HPDS can be reduced significantly in a wide range of operating conditions and seal clearance compared with the typical C-HPDS, and the maximum reduction amplitude of leakage rate reaches up to 25% at a relatively small pressure ratio. The main reasons for the leakage reduction of the D-HPDS can be explained by the fact that the change of outflow direction from the cavity induced by the interaction effect of adjacent cavities, further resulting in the formation of turbulent vortex in the cavity and the low-speed flow zone in the sealing gap. For the design goal of low leakage rate, the inclined angle of the dovetail-like diversion groove is appropriate between 45 deg and 55 deg, the tip extension ratio is appropriate between 0.8 and 0.95.

Study of thrust vector control for the rotating detonation model engine

The detonation wave in a rotating detonation engine is highly adaptable to the incoming flow, making the wave easier to control. In this study, a numerical simulation method is used to analyze the working process and flow field structure of a rotating detonation model engine with dual cavity injection of an H2/air mixture by controlling the injection pressure ratio of the dual cavity and the number of detonation wave heads. It is found that the rotating detonation engine offers the possibility to control the thrust vector with two different modes. The first is a one-cycle alternate control mode with a small injection pressure ratio. Here two deflections occur in different directions occur across one detonation wave propagation cycle, but the overall deflection direction is in the low-pressure region. The second is a one-way control mode, with a large injection pressure ratio, and the deflection direction towards the low-pressure region. For the multi wave-mode, it belongs to one-way control mode because of constant deflection direction in the low-pressure area. From the perspective of thrust distribution along the circumference, the one-way control strategy satisfies the ability of a rotating detonation thrust vector control.

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.

Molecular insight into the formation of adsorption clusters based on the zeta isotherm.

This work presents a series of molecular dynamics simulations of argon adsorption on a silicon substrate with different lattice orientations. From the simulation results, the density profiles are discussed and the amount of adsorbed particles is obtained at different pressures. It is found that the solid surface orientation has a great influence on the density distributions and atomic arrangements near the surface. With the collected data, the thermal constants derived from the expression of zeta adsorption isotherms are determined. The calculated isotherms agree well with the simulation results. Also, from a microscopic point of view, the molecular insights show that the structures of the adsorbates are present as clusters with different numbers of particles. The size of the clusters changes with pressure. At a relatively small pressure ratio, most of the clusters consist of a single molecule. As the pressure ratio increases, larger sized clusters appear, forming various cluster-types. The molecular cluster distributions are closely consistent with the basic approximation of the zeta adsorption isotherm. Furthermore, the surface adsorption sites determined from molecular dynamics simulation show good agreement with that predicted by the zeta isotherm model, which reaffirms the effectiveness of the theoretical model. When the isotherm is extended to a pressure ratio greater than unity, a finite amount of adsorption is predicted and the wetting conditions are obtained. Affected by the solid surface orientations, the pressure ratio at wetting for the silicon substrate with the (111) surface plane is larger than those of the (100) and (110) surfaces, indicating that a higher subcooling is required for the wetting transition.

Reciprocating compressor valve damage estimation under varying speeds through the acoustic emission technique

Purpose The purpose of this paper is to examine the effect of reciprocating compressor speeds and valve conditions on the roor-mean-square (RMS) value of burst acoustic emission (AE) signals associated with the physical motion of valves. The study attempts to explore the potential of AE signal in the estimation of valve damage under varying compressor speeds. Design/methodology/approach This study involves the acquisition of AE signal, valve flow rate, pressure and temperature at the suction valve of an air compressor with speed varrying from 450 to 800 rpm. The AE signals correspond to one compressor cycle obtained from two simulated valve damage conditions, namely, the single leak and double leak conditions are compared to those of the normal valve plate. To examine the effects of valve conditions and speeds on AE RMS values, two-way analysis of variance (ANOVA) is conducted. Finally, regression analysis is performed to investigate the relationship of AE RMS with the speed and valve flow rate for different valve conditions. Findings The results showed that AE RMS values computed from suction valve opening (SVO), suction valve closing (SVC) and discharge valve opening (DVO) events are significantly affected by both valve conditions and speeds. The AE RMS value computed from SVO event showed high linear correlation with speed compared to SVC and DVO events for all valve damage conditions. As this study is conducted at a compressor running at freeload, increasing speed of compressor also results in the increment of flow rate. Thus, the valve flow rate can also be empirically derived from the AE RMS value through the regression method, enabling a better estimation of valve damages. Research limitations/implications The experimental test rig of this study is confined to a small pressure ratio range of 1.38–2.03 (free-loading condition). Besides, the air compressor is assumed to be operated at a constant speed. Originality/value This study employed the statistical methods namely the ANOVA and regression analysis for valve damage estimation at varying compressor speeds. It can enable a plant personnel to make a better prediction on the loss of compressor efficiency and help them to justify the time for valve replacement in future.

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.

Numerical investigation of the flow characteristics of underexpanded methane jets

In the present work, numerical simulations are carried out to investigate underexpanded methane jets with phase separation effects. In order to predict the fuel injection and the mixture formation in the constant volume chamber, a hybrid, pressure-based solver is combined with a vapor-liquid equilibrium model and a moving mesh methodology. The thermodynamic models are based on the cubic equation of state of Soave, Redlich, and Kwong. A compressibility correction for the widely known kωSST turbulence model is implemented additionally. Application-relevant simulations with a total fuel pressure of 300 bars and five different chamber pressures ranging from 12 to 60 bars were defined. Furthermore, the influence of two fuel and chamber temperatures, 294 and 363 K, is analyzed. Depending on the chamber pressure, two different flow structures of the potential core can be distinguished: (1) A series of typical shock barrels for small pressure ratios and moderately underexpanded jets and (2) a shear layer consisting of a two-phase mixture which enfolds the potential core for high pressure ratios and highly underexpanded jets. Increasing the fuel temperature leads to less significant phase separations, while an increase in the chamber pressure does not affect the structure of the potential core. A comparison with experimental measurements shows a very good agreement of the simulated structure of the potential core, providing evidence that the underlying phenomena are predicted correctly and suggesting that a moving mesh strategy and consistent two-phase thermodynamics implementation are necessary for a physical representation of high-pressure injections.

Numerical CFD simulations on a small-scale ORC expander using a customized grid generation methodology

Positive displacement machines are the most suitable devices for small-scale waste heat to power conversion units based on an Organic Rankine Cycle (ORC) due to their capabilities of handling small mass flow rates and high pressure ratios. Among the technologies, sliding vane machines provide unique features such as low operating revolution speed, geometrical flexibility and uncomplicated manufacturing. Nonetheless, research and product development in this field have been constrained by the lack of interfaces between deforming and moving fluid domains that characterize sliding vane devices and the design tools at the state of the art. This research work tackles this challenge and presents the development of an analytical grid generation for sliding vane machines that is based on user defined nodal displacement. Through this approach, the numerical methodology discretizes the fluid domains enclosed between the cells and ensures conservation of intrinsic quantities by maintaining the cell connectivity and structure. Transient 3D single phase simulations on a small scale ORC expander were further set up in the ANSYS CFX solver and provided insights on the main flow field as well as in the leakage paths between rotor blade tips and casing. The numerical results were eventually validated with reference to an experimental dataset related to a waste heat to power conversion application in compressed air systems where the sliding vane ORC expander worked with R236fa, at a pressure ratio of 2.65 and at 1551 RPM.

A Numerical Study for Predicting Steam Ejector Performance at Different Pressure Ratios

This paper presents a numerical study for predicting the steam ejector performance for different pressure ratios. The considered steam ejector operates at small pressure ratios of back to motive pressures. Both the suction and motive fluids are regarded to be dry steam. Due to low pressure created by the motive steam nozzle flow, entrainment of steam to be mixed with motive steam where both resume flowing toward the ejector exit. Mass ratio of suction to motive flows is an important element to describe the ejector performance.The present study aims to optimize the steam ejector efficiency that runs at different pressure ratio for each suction pressure. The variation of the mass ratio and ejector efficiency with the ejector back pressure at different values of the suction pressure and constant motive pressure, also a variation of the mass ratio and ejector efficiency with the ejector back pressure at different values of the motive pressure and constant suction pressure are investigated. The numerical results are validated with the available experiments from the literature. The results show that the mass ratio is almost constant at low values of back pressure, depending on the suction and back pressures, then the mass ratio decreases sharply with increasing the back pressure. The suction pressure has the positive consequence on the mass ratio. Moreover, the results show that the ejector efficiency increases with increasing the back pressure to gain its upper limit value, subsequently that the efficiency decreases with raising the back pressure. Also, raising the suction pressure will cause an improvement in the ejector efficiency. The value of back pressure at which the maximum efficiency is achieved and its value increases with magnifying the suction pressure.

CFD study on particle separation performance by shock inception during natural gas flow in supersonic nozzle

Supersonic nozzles with low intensity shock have been reported by oil and gas sectors ideal equipment for eliminating undesired second phases from natural gas during purification process. In this paper, analytical and numerical methods are developed to predict the location of shock for an axisymmetric convergent-divergent nozzle. Six different shapes of nozzle are examined to demonstrate the impact of nozzle shape on shock position. Results are compared to experimental data obtained from subscale testing to demonstrate the limitation of the numerical technique. The computational fluid dynamics simulation analysis predicts shock appearance when nozzle pressure ratio > 1.2 for almost all geometries. Simulation results have also shown that the shockwave predicted by the elliptical nozzle occurs farthest from the throat compared to the other nozzle shapes at relatively small nozzle pressure ratio. However, as nozzle pressure ratio increases, the shockwave using the hexagonal nozzle is shown to occur farthest from the throat.

Experimental Study on Pressure Losses in Circular Orifices for the Application in Internal Cooling Systems

The cooling air flow in a gas turbine is governed by the flow through its internal passages and controlled by restrictors such as circular orifices. If the cooling air flow is incorrectly controlled, the durability and mechanical integrity of the whole turbine may be affected. Consequently, a good understanding of the orifices in the internal passages is important. This study presents experimental results for a range of pressure ratios and length-to-diameter ratios common in gas turbines including even very small pressure ratios. Additionally, the chamfer depth at the inlet was also varied. The results of the chamfer depth variation confirmed its beneficial influence on decreasing pressure losses. Moreover, important effects were noted when varying more than one parameter at a time. Besides earlier mentioned hysteresis at the threshold of choking, new phenomena were observed, e.g., a rise of the discharge coefficient for certain pressure and length-to-diameter ratios. A correlation for the discharge coefficient was attained based on the new experimental data with a generally lower error than previous studies.

Organic Rankine Cycle as Bottoming Cycle to a Combined Brayton and Clausius - Rankine Cycle

A preliminary evaluation has been made of a possibility of bottoming of a conventional Brayton cycle cooperating with the CHP power plant with the organic Rankine cycle installation. Such solution contributes to the possibility of annual operation of that power plant, except of operation only in periods when there is a demand for the heat. Additional benefit would be the fact that an optimized backpressure steam cycle has the advantage of a smaller pressure ratio and therefore a less complex turbine design with smaller final diameter. In addition, a lower superheating temperature is required compared to a condensing steam cycle with the same evaporation pressure. Bottoming ORCs have previously been considered by Chacartegui et al. for combined cycle power plants [ Their main conclusion was that challenges are for the development of this technology in medium and large scale power generation are the development of reliable axial vapour turbines for organic fluids. Another study was made by Angelino et al. to improve the performance of steam power stations [. This paper presents an enhanced approach, as it will be considered here that the ORC installation could be extra-heated with the bleed steam, a concept presented by the authors in [. In such way the efficiency of the bottoming cycle can be increased and an amount of electricity generated increases. A thermodynamic analysis and a comparative study of the cycle efficiency for a simplified steam cycle cooperating with ORC cycle will be presented. The most commonly used organic fluids will be considered, namely R245fa, R134a, toluene, and 2 silicone oils (MM and MDM). Working fluid selection and its application area is being discussed based on fluid properties. The thermal efficiency is mainly determined by the temperature level of the heat source and the condenser conditions. The influence of several process parameters such as turbine inlet and condenser temperature, turbine isentropic efficiency, vapour quality and pressure, use of a regenerator (ORC) will be presented. Finally, some general and economic considerations related to the choice between a steam cycle and ORC are discussed.

Measurement of flow rate characteristics of pneumatic components based on the dynamic regularity of polytropic exponents

To improve the measurement precision of flow rate characteristics of pneumatic components, a new measurement method is proposed, which is based on the dynamic regularity of instantaneous polytropic exponents during discharge. The dynamic regularity of polytropic exponents is obtained by the stop method, and it can be concluded as two change stages based on pressure time constant. According to piecewise polynomial fitting curves of instantaneous polytropic exponents, a new algorithm for the identification of flow rate characteristics is proposed. This method can accurately measure some components with a large sonic conductance and a small critical pressure ratio. Four pneumatic components are measured to compare the identification precision of those different methods. The results of comparison show that the new polytropic exponent method and its new algorithm have the higher precision than other alternative methods for the ISO6358 method. The identification errors of sonic conductance of the new method are less than 3%, and the identification errors of critical pressure ratio of the new algorithm are less than 15%.

Exergy optimisation of a Brayton cycle-based cogeneration plant

An endoreversible Brayton cycle-based cogeneration plant has been optimised based on the dimensionless total exergy output rate criterion with a constraint on power-to-heat ratio. An optimal pressure ratio parameter and the maximum dimensionless total exergy output rate have been numerically determined. The optimisation results have been compared with those based on the total useful energy rate criterion. It shows that a cogeneration plant based on the maximum total exergy output rate criterion has higher total exergy output rate and smaller pressure ratio while lower exergy efficiency than that based on the maximum total useful energy rate criterion.

Small recuperated ceramic microturbine demonstrator concept

It has been about a decade since microturbines first entered service in the distributed generation market, and the efficiencies of these turbogenerators rated in the 30–100 kW power range have remained essentially on the order of 30%. In this time frame the cost of fuel (natural gas and oil) has increased substantially, and efforts are now underway to increase the efficiency of microturbines to 40% or higher. Various near-term means of achieving this are underway by utilizing established gas turbine technology, but now based on more complex thermodynamic cycles. A longer-term approach of improving efficiency is proposed in this paper based on the retention of the basic recuperated Brayton cycle, but now operating at significantly higher levels of turbine inlet temperature. However, in small low pressure ratio recuperated microturbines embodying radial flow turbomachinery this necessitates the use of ceramic components, including the turbine, recuperator and combustor. A development approach is proposed to design, fabricate and test a 7.5 kW ceramic microturbine demonstrator concept, which for the first time would involve the coupling of a ceramic radial flow turbine, a ceramic combustor, and a compact ceramic fixed-boundary high effectiveness recuperator. In a period of some three years, the major objectives of the proposed small ceramic microturbine R&D effort would be to establish a technology base involving thermal and stress analysis, design methodology, ceramic component fabrication techniques, and component development, these culminating in the assembly and testing to demonstrate engine structural integrity, and to verify performance. This would provide a benchmark for more confidently advancing to increased size ceramic-based turbogenerators with the potential for efficiencies of over 40%. In addition, the power size of the tested prototype could possibly emerge as a viable product, namely as a natural gas-fired turbogenerator with the capability of meeting the total energy needs of an average house.

Influence of the PACVD process parameters on the properties of titanium carbide thin films

Wear-resistant titanium carbide (TiC) coatings were deposited by plasma-assisted chemical vapour deposition on high-speed steel. The influence of the partial pressure ratio of methane and titanium tetrachloride ( p CH 4 / p TiCl 4 ), the plasma voltage and the pulse/pause ratio on the composition and properties of the TiC coatings have been investigated. Small partial pressure ratios p CH 4 / p TiCl 4 up to about 15 result in nearly stoichiometric TiC coatings at high plasma voltage. Higher partial pressure ratios cause excess carbon, which is bound as C-C or C-H. An increasing carbon content was found also for both increasing plasma voltage and increasing pulse/pause ratio. The layer hardness was found to decrease with increasing excess carbon. No significant influence of composition on the friction coefficient has been detected.