Leakage Mass Flow Research Articles

A Study for Rotordynamic and Leakage Characteristics of a Long-Honeycomb Seal With Two-Phase, Mainly Air Mixtures

Abstract This paper investigates the impact of the oil (silicone oil PSF-5cSt) presence in the air on the leakage and rotordynamic characteristics of a long-honeycomb seal with length-to-diameter ratio L/D = 0.748 and diameter D = 114.656 mm. Tests are carried out with inlet pressure Pi = 70.7 bars, pressure ratio (PR) = 0.35 and 0.25, inlet liquid volume fraction (LVF) = 0%, 3.5%, and 7%, and shaft speed ω = 10, 15, and 20 krpm. During the tests, the seal is centered. Test results show that leakage mass flow rate m˙ increases (as expected) as inlet LVF increases. Increasing inlet LVF makes direct stiffness K increase more rapidly with increasing excitation frequency Ω. Increasing inlet LVF has a negligible effect on K at low Ω values, but increases K at high Ω values. The value of effective damping Ceff at about 0.5ω is an indicator to the system stability since an unstable centrifugal compressor rotor can precess at about 0.5ω. Increasing inlet LVF increases the value of Ceff at about 0.5ω, reducing the possibility of subsynchronous vibrations (SSVs) at about 0.5ω. San Andrés's model is used to produce predictions. The model assumes that the test fluid in the seal clearance is an isothermal-homogenous mixture. The model adequately predicts m˙, K, and the value of Ceff at about 0.5ω.

Effect of a ring type barrier and rotational speed on leakage flow of gas turbine brush seal

This paper investigates the effect of inserting a ring type barrier on leakage flow of brush seals with different bristles clearances (the distance between bristle pack tip and rotor surface). The ring is placed on both upstream and downstream sides of the bristles. An axisymmetric CFD model is employed to calculate radial pressure distribution along backing plate, axial pressure variation on rotor surface, and leakage mass flow rate of the brush seal. Reynolds-Averaged-Navier-Stokes (RANS) together with non-Darcian porous medium approach is performed to solve the flow field. The accuracy and reliability of the model are evaluated through comparison of the numerical results and experimental data. The results show that inserting the ring is not effective for the brush seal with zero clearance, neither at upstream nor at downstream. In other cases, the downstream ring is considerably more effective than the upstream one, when the ring is tangent to the back of bristles. The greater the distance between the bristles and the ring, the less reduction in leakage flow. Also, the best performance is obtained for the ring height equal to clearance size. Moreover, the effect of rotor rotation on leakage flow is investigated. The results show a negligible decrease in brush seal leakage flow with increasing the rotational speed.

Tip leakage flow analysis of an axial turbine under the effect of separation at low Reynolds number

The tip clearance flow could lead to work reduction and loss generation in turbomachines. However, the effect of separation at low Reynolds number on leakage flow is seldom studied. The previous method for evaluating tip leakage characteristics should also be further researched. Thus, numerical investigations on the tip clearance flow in an unshrouded axial-inflow turbine are conducted at low Reynolds number (3.5 × 104 of the rotor outlet at the designed condition) in the present study. The flow patterns and leakage mass flow rate of the clearance have been analyzed in detail. It is found that the tip clearance flow is greatly affected by the flow separation caused by low Reynolds number. The scraping ratio adopted in previous references does not accord with the clearance flow characteristics at low Reynolds number, especially in the front part of the clearance. A coefficient by −0.70 power of the Reynolds number is proposed to modify the scraping ratio in the present study. The synergy between the velocity and the pressure gradient is innovatively employed to research the tip clearance flow characteristics, and it gives a reliable criterion of indicating the flow patterns in the tip clearance.

Performance analysis of linear compressor using R290 for commercial refrigerator

Linear compressor is a promising alternative for commercial refrigerator using R290. In this paper, a design program of linear compressor is set up. The leakage mass flow rate is modeled based on small hole leakage principle. The errors of the model are within 10%. The cooling performance of linear compressor with different piston diameter for commercial referigerator using R290 is analyzed. As the linear compressor is set to have the same swept volume, small piston diameter is beneficial to the COP improvement, but has a few decrease of cooling capacity. As the linear compressor is set to have the same cooling capacity by adjusting power frequency, COP of the linear compressor with piston diameter of 28 mm and 30 mm gets a little decrease because the power frequency deviates from the inherent frequency. The optimal piston diameter of 12 CC swept volume for the commercial refrigeration is suggested to be from 30 mm to 32 mm.

Impact of hub gap leakage on stator endwall flow in an axial compressor stage with casing treatment

Abstract In an axial compressor stage equipped with slot-type casing treatment, simulation without hub gap between rotors and stators achieves much lower stall margin enhancement than experiment, which results from the over-predicted corner stall in stators. Aiming at clarifying the combined effect of hub leakage flow and casing treatment on operating range and the flow mechanism of leakage flow interacting with stator flow, a detailed numerical research is performed in this compressor stage. The effects of hub leakage mass flow are elaborated in terms of compressor performance and flow fields. Unsteady simulations with appropriate leakage mass flow condition are able to predict the stall margin improvement and total pressure characteristics preferably. Casing treatment slots tend to re-distribute the loading of downstream stator by decreasing loading above 70% span and increasing that below 70% span, which result in the exaggerated corner stall in stators. Hub leakage flow from cavity resists the tendency brought by casing treatment and succeeds in relieving the heavy loading near the hub. The flow mechanism for the effect of cavity leakage flow on stall margin is elucidated. Cavity hub leakage gives rise to a thickened boundary layer upstream stator inlet by producing a separation bubble and aggravating the boundary layer skewing. The thickened boundary layer produces a horseshoe vortex in the stator passage, which induces main flow to rolling up as the pressure-side branch and avoids the emergence of corner stall.

Effect of the injection orientation and position on the leakage flow in a honeycomb-tip turbine cascade

The effect of the cooling injection orientation and position on the tip leakage flow has been studied numerically in a honeycomb-tip turbine cascade. The injected fluid enters each honeycomb cavity through the bottom pipe at a fixed incident angle. The effect of the relative casing motion with six casing speeds is considered. The Reynolds-averaged Navier-Stokes (RANS) method and k-ω turbulence model were used in the three dimensional computations. Then the leakage mass flow rate and the averaged total pressure loss are evaluated. The static pressure and limiting streamlines on the casing are discussed and the flow direction around the gap are extracted to precisely analyze the leakage feature. Besides, the thermal conditions on the blade tip and suction surface are described by the Nusselt number and adiabatic film cooling effectiveness. Finally, the dimensionless temperature contours and velocity vectors inside the gap are plotted to explore the effect of the injection position. The results indicates that the injection feature mainly depends on the injection orientation and finally determines the aerodynamic and thermodynamic performance.

Prediction of state property during hydrogen leaks from high-pressure hydrogen storage systems

In an actual hydrogen leakage process, heat exchange occurs between the gas in a high-pressure storage system and its surroundings. Therefore, the predictions of models that are established based on the isentropic process assumption have large errors. In this study, a high-pressure gas leakage process model including heat exchange (the HEC model) is established. It includes two parts; one is a high-pressure gas storage model without the isentropic process assumption, and the other is a leakage orifice model with the isentropic process assumption. Formulas for the pressure variation and temperature variation based on the mass and energy conservation equations, the Abel–Noble equation of state, and an enthalpy formula based on the Helmholtz free energy are used to establish the gas storage model. The leakage orifice model is built using a mass flow rate formula based on the van der Waals equation of state and Perry's flow coefficient formula. Finally, the proposed high-pressure gas leakage process model including heat exchange is compared with high-pressure hydrogen leakage process models with the isentropic process assumption based on the Abel–Noble and van der Waals equations of state. The results show that the proposed model can yield more promising results in predicting the gas leakage mass flow rate, gas pressure, and temperature inside hydrogen storage systems compared with previous models.

Research on dynamic leakage flow in the labyrinth seal of a labyrinth piston compressor

To study the labyrinth leakage flow characteristics of a labyrinth piston compressor, firstly, a transient flow model was built to simulate the fluid flow through labyrinth seals. The dynamic pressures in the labyrinth chambers and the leakage flow rate were obtained by solving the transient flow model. Then, an experimental rig based on a labyrinth piston air compressor with rotational speed of 400 to 900 rpm, and discharge pressure of 0.2 to 0.5 MPa was built to validate the simulation results. In the experiments, dynamic pressures in the cylinder and in the labyrinth chambers were measured at different discharge pressures and different rotational speeds. The simulation results of pressures showed a good agreement with the experimental ones with the maximum relative error of 20%. Later, the transient flow model was used to study the dynamic pressure distribution and leakage flow rate through the labyrinth chambers. The results show that the pressure drop across each labyrinth chamber increases along the leakage flow path from the compression chamber side to the environment side. The pressure drop of the last third chambers accounts for 50% of the total pressure drop of all the chambers. The leakage flow rate increases quickly with the increase of the discharge pressure at low rotational speed. At the speed of 500 rpm, the leakage mass flow doubles when discharge pressure increases from 0.2 MPa to 0.5 MPa. However the growth rate slows down at a high speed of 800 rpm. The leakage characteristics indicate that the labyrinth piston compressor should work at a high rotational speed and a low discharge pressure.

Effect of arbitrary blade tip design on tip leakage flow

Tip geometry modification is frequently used to suppress the tip leakage flow in the turbine cascade however a universally beneficial tip geometry modification design has not been fully discovered. In this paper, the two-surface coupling arbitrary blade tip design method in three-dimensional physical space which satisfies the simple trigonometric function law is proposed and the mathematical parametric description is presented. The effects of different arbitrary blade tips on tip leakage flow have been studied numerically in a highly loaded axial turbine cascade. The aerodynamic performance of different tips is assessed by the tip leakage mass flow rate and the total pressure loss coefficient at the exit section. The Kriging model and genetic optimization algorithm are used to optimize the arbitrary blade tips to obtain the optimal arbitrary blade tip. Compared with the flat tip, the tip leakage mass flow rate is decreased by 10.57% and the area-average total pressure loss coefficient at the exit section is reduced by 8.91% in the optimal arbitrary blade tip.

Modification and optimization strategies for turbine arbitrary blade tips

The influence of different arbitrary blade tip shapes on restraining the tip leakage flow in a highly loaded turbine cascade has been numerically studied. A combined method of establishing and optimizing the arbitrary blade tip shape is proposed by using B-spline surface modeling, Kriging model and genetic optimization algorithm. The results show that the Kriging model established by the B-spline surface modeling method can accurately fit the relationship between the arbitrary blade tip shape and the relevant aerodynamic parameters. The optimal leakage mass flow tip and the optimal total pressure loss tip obtained by genetic algorithm both have strong inhibitory effects on tip leakage flow. Compare to the flat tip at 1%H gap height, the tip leakage mass flow of the optimal leakage mass flow case and the optimal total pressure loss case decrease by 11.14% and 10.23%, respectively, the area-average total pressure loss at exit section is reduced by 8.08% and 7.41%, respectively.

Kriging surrogate model applied in the mechanism study of tip leakage flow control in turbine cascade by multiple DBD plasma actuators

The dielectric barrier discharge (DBD) plasma actuator, in which electrodes are asymmetric arranged, has already demonstrated its ability in flow control. In the present work, the configuration of multiple plasma actuators is placed at the suction side of the cascade top to realize the tip leakage control. However, massive configurations appear when the number of plasma actuators increases, resulting in the investigation of actuator configuration for tip leakage flow control becomes a challenge. The surrogate modeling approach provides a cheap and efficient method for investigating the effect of multiple plasma actuators on the tip leakage flow control. By constructing an approximation model, tip leakage mass flow rates of all configurations are obtained in the present work. What's more, the flow structures in the tip clearance controlled by the plasma actuators are explained in the process of topological analysis. The results show that the tip leakage mass flow rate is decreasing with the number of active plasma actuators increasing. However, the decreasing would reach its limits in the process of adding plasma actuators. In the analysis of flow topology, single actuator would generate a small vortex at the suction side to cause an obstacle in the tip leakage flow. While the continuous arrangements of plasma actuator is beneficial to generate a larger obstacle in the tip clearance, which resulting in a significant reduction of m˙CFD and the promotion of Cp.

Investigation on tip leakage flow characteristics of radial inflow micro turbine used in helium liquefying system

Tip clearance leakage flow with different axial and radial clearance in a radial inflow helium micro turbine impeller used in helium liquefying system under the stage environment is investigated using a three-dimensional transient viscous flow simulation. The comparison of the numerical results with the experimental results is proved to verify the accuracy of the numerical method. The results indicate the formation and development mechanism of the tip leakage flow. The leakage flow effects in the helium turbine passage have slightly transient fluctuations. Then leakage vortex develops in the impeller passage and causes large leakage flow loss. Rotational speed and expansion ratio have great impact on leakage flow characteristics. Furthermore, there is a notable difference of the performance affected by axial clearance and radial clearance. The leakage mass flow and leakage flow loss are more affected by the radial clearance than the axial clearance. In addition, empirical formulas are obtained to describe the leakage mass flow and flow loss caused by the axial clearance and radial clearance. Finally, a modified semi-empirical formula is obtained based on the numerical results to calculate leakage flow loss correctly. The results in this paper can give a further understanding of leakage flow in radial inflow micro turbine and provide a method to estimate the tip clearance effect on performance and flow loss quantitatively.

Numerical Investigations of an Axial Exhaust Diffuser Coupling the Last Stage of a Generic Gas Turbine

It is well known that the last stage of a turbine and the subsequent diffuser should be viewed at and designed as a coupled system rather than as single standalone components. The turbine outlet flow imposes the inlet conditions to the diffuser, whereas the recovered dynamic pressure in the diffuser directly controls the turbine back pressure. With changing operating point, the turbine outflow can vary significantly. This results consequently in large variations of the diffuser performance. A major role in the coupled system of turbine and diffuser can be attributed to the tip leakage flow. While it is desirable to minimize the tip leakage with regard to the turbine, a higher leakage mass flow can often be beneficial for the diffuser performance. As there is currently a trend toward aggressive and hence shorter diffusers which are particularly prone to separation, the question arises where the optimum for this tradeoff problem lies. To investigate the performance in the coupled turbine/diffuser system, a generic last stage with shrouded rotor and axial exhaust diffuser has been designed. The components are representative for heavy duty stationary gas turbine applications. Results are presented for three different operating points representing part-load (PL), design-load (DL), and over-load (OL) condition. Three different seal gap widths are taken into account to control the leakage flow. The results indicate that an operating point-dependent optimum gap width can be found for the coupled system efficiency, whereas the maximum turbine performance is always achieved with a minimum gap width.

Variable-Speed-Drive-Based Estimation of the Leakage Rate in Compressed Air Systems

Leakage is a significant cause of energy losses in compressed air systems (CASs). This paper presents a novel method for estimation of the leakage flow rate of a screw compressor-powered CAS using variable-speed-drive-based soft sensing. The proposed method utilizes an identification run sequence applied in the control scheme of the variable-speed drive at a time of no intended compressed air consumption. Data from a mass flow soft sensor of the screw compressor and pressure measurement are employed to produce estimates for the CAS volume and leakage mass flow rate as a function of system pressure. The feasibility of the approach is verified with laboratory measurements. The proposed method offers a practical and effective solution for monitoring of leakage in CASs.

Clearance Effects on Rotordynamic Performance of a Long Smooth Seal With Two-Phase, Mainly-Air Mixtures

This paper experimentally studies the effects of changing radial clearance Cr on the performance of a long (length-to-diameter ratio L/D = 0.65) smooth seal under mainly-air (wet-gas) conditions. The test fluid is a mixture of air and silicone oil. Tests are conducted with Cr = 0.188, 0.163, and 0.140 mm, inlet pressure Pi = 62.1 bars, exit pressure Pe = 31 bars, inlet liquid volume fraction LVF = 0%, 2%, 5%, and 8%, and shaft speed ω = 10, 15, and 20 krpm. The seal's complex dynamic stiffness coefficients Hij are measured. The real parts of Hij cannot be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, frequency-dependent direct KΩ and cross-coupled kΩ stiffness coefficients are used. The imaginary parts of direct Hij produce frequency-independent direct damping C. Test results show that, for all pure- and mainly-air conditions, decreasing Cr decreases (as expected) the leakage mass flow rate m˙. Under mainly-air conditions, decreasing Cr decreases KΩ. This outcome is contrary to the test results at pure-air conditions, where KΩ increases as Cr decreases. Since an unstable centrifugal compressor rotor may precess at approximately 0.5ω, the effective damping Ceff at about 0.5ω is used as an indicator of the impact a seal would have on its associated compressor. For pure-air conditions, when Ω ≈ 0.5ω, decreasing Cr increases Ceff and makes the seal more stabilizing. This trend continues after the oil is added. A bulk-flow model developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) produces predictions to compare with test results. m˙ predictions correlate with measurements. Under pure-air conditions, the model correctly predicts the effects of changing Cr on KΩ and the Ceff value near 0.5ω. After the oil is added, as Cr decreases, predicted KΩ increases while measured KΩ decreases. Also, for mainly-air cases and Ω ≈ 0.5ω, decreasing Cr does not discernibly change predicted Ceff but increases the measured value.

Experimental Analysis of Small Diameter Brush Seals and Comparisons With Theoretical Predictions

The paper presents the experimental results obtained for brush seals of 38 mm diameter operating with air at pressure differences up to 7 bars and rotation frequencies up to 500 Hz. The seals had bristles of 70 μm diameter, made of Haynes 25. Seals with two radial interferences (0 and 100 μm) between the brush and the rotor were tested. The presented running in procedure underlines the influence of the initial wear on the brush temperatures. The test results consisted of leakage mass flow rates. The temperatures of a limited number of points on the brush and on the rotor were also recorded. The results confirmed the important impact of the radial interference on the leakage. The test data were further confronted with theoretical predictions obtained with an original model. The model considers the brush as a deformable porous medium. Its local porosity and permeability are obtained from a fluid–structure interaction between the bristle pack and the leakage flow. The comparisons showed nearly close values of the mass flow rates. The differences between experimental and theoretical predictions are considered to be due to an underestimation of the porosity because the model neglects the friction forces between bristles and between the bristles and the rotor.

Experimental and Numerical Investigations of Closed Radial Inflow Turbine With Labyrinth Seals

It is a common practice to use closed impeller in radial inflow turbine against the flow leakage from tip clearance of impellers, especially in small volume flow condition. It utilizes labyrinths between the shroud and the case to abate the higher pressure leakage. Experimental and computational investigations of shroud clearance flow in a radial inflow turbine with labyrinth seals are presented in this paper. Compared with the result without leakage, numerical computation result including the leakage of labyrinth seals agrees better with that of the experiment result, which indicates that the leakage of labyrinth seals cannot be neglected. Several geometrical arrangements with a series of different clearance of labyrinth seals are investigated experimentally and numerically, and the dimensionless shroud clearance is of 0%, 0.6%, 1.2%, 1.8%, 2.7%, 3.6%. Finally, the character of flow and loss is analyzed by computational fluid dynamics (CFD) tools. The results indicate that the labyrinth seal flow has no effect on the main flow passage and mainly causes different leakage mass flow.

Effects of the Back Plate Inner Diameter on the Frictional Heat Input and General Performance of Brush Seals

Reducing losses in the secondary air system of gas and steam turbines can significantly increase the efficiency of such machines. Meanwhile, brush seals are a widely used alternative to labyrinth seals. Their most valuable advantage over other sealing concepts is the very small gap between the sealing package and the rotor and thus reduced leakage mass flow. This small gap can be achieved due to the great radial flexibility without running the risk of severe detrimental deterioration in case of rubbing. Rubbing between rotor and seal during operation might occur as a result of e.g., an unequal thermal expansion of the rotor and stator or a rotor elongation due to centrifugal forces or manoeuvre forces. Thanks to the flexible structure of the brush seal, the contact forces during a rubbing event are reduced; however, the frictional heat input can still be considerable. Particularly in aircraft engines with their thin and lightweight rotor structures, the permissible material stresses can easily be exceeded by an increased heat input and thus harm the engine’s integrity. The geometry of the seal has a decisive influence on the resulting contact forces and consequently the heat input. This paper is a contribution to further understand the influence of the geometrical parameters of the brush seal on the heat input and the leakage during the rubbing of the seal on the rotor. In this paper, a total of three seals with varied back plate inner diameter are examined in more detail. The experimental tests were carried out on the brush seal test rig of the Institute of Thermal Turbomachinery (ITS) under machine-relevant conditions. These are represented by pressure differences of 1 to 7 bar, surface speeds of 30 to 180 m/s and radial interferences of 0.1 to 0.4 mm. For a better interpretation, the results were compared with those obtained at the static test rig of the Institute of Jet Propulsion and Turbomachinery (IFAS) at the Technical University of Braunschweig. The stiffness, the blow-down and the axial behaviour of the seals as a function of the differential pressure can be examined at this test rig. It could be shown that the back plate inner diameter has a decisive influence on the overall operating behaviour of a brush seal.

Experimental performance evaluation on leakage characteristics of R32 in rolling piston type rotary compressor

The rolling piston compressor has occupied the compressor market for room air conditioners for years. Compressor efficiency is directly related to the energy efficiency ratio of room air conditioners. As an important replacement of HCFCs, R32 is widely used in the field of refrigeration and air-condition. In this paper, the mass flow rate of refrigerant-lubricant oil mixture in R32 compressor is measured both experimentally and theoretically. In the experiment, influences of minimal clearance, length, geometry of radial leakage channel and minimal clearance of piston face leakage channel on leakage mass flow rate of refrigerant-lubricant mixture, pure lubrication and refrigerant have been investigated in this study. Leakage characteristics and volumetric efficiency of R32 and R410A are respectively compared. Calculation results of the theoretical formulas and the semi-empirical formulas are compared with the experimental results and the semi-empirical formulas have satisfactory reliability and practicality.

Numerical study of the effect of the cavity depth on the leakage control in a cooled honeycomb-tip turbine cascade

The effect of the cavity depth on the leakage flow and tip cooling has been numerically investigated in a honeycomb-tip turbine cascade with cooling injection. Coolant is ejected through the center holes on the honeycomb cavity bottoms. Three dimensional flow fields were simulated using the Reynolds-averaged Navier-Stokes (RANS) method and the k-ω turbulence model. Then the tip configurations are evaluated according to several performance parameters, such as leakage mass flow rate, total pressure loss and film cooling effectiveness, in the upper passage, the gap and the honeycomb cavities. Furthermore, the secondary velocity streamlines are plotted at the cascade exit to characterize the upper passage vortices. The isothermal surface and contours of dimensional temperature are presented to explore the mixing between the coolant and the cavity vortices. The numerical results show that the aerodynamic and thermodynamic performance differs significantly with the cavity depth.