## Vol 15, No 4 (2016)

**Year:**2016**Articles:**25**URL:**https://journals.ssau.ru/vestnik/issue/view/169

### Full Issue

###### Abstract

One of the promising trends in the improvement of stand test data management and control systems and emergency protection systems involves the development of intellectual peripheral devices that provide communication with test articles. Due to the incorporation of their own processors in such devices they can solve problems of receiving data from sensors and initiating lower level control actions (independently from the master controller) on the actuators of controlled objects, which enhances the reliability and quality of control. The paper presents the latest developments directly related to the control of liquid-propellant rocket engine (LRE) parameters during stand tests, i.e. control of electro-hydraulic tilt actuators of LRE combustion chambers; control of stepping motors for the regulation of mixture ratio and control of vernier engines; receiving, processing and recording of signals from turbine flow meters and turbopump RPM sensors.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):9-19

###### Abstract

The article presents the results of calculations and experimental studies of a vacuum system with an aft diffuser for high-altitude firing tests of low-thrust liquid rocket engines. The aim of the study was to determine the geometric dimensions and setting parameters of aft diffusers. The diffusers ensure operation of the vacuum system that implements the regime of unseparated flow of the combustion products in the nozzle of the engine being tested. We carried out the experiments using 50 N liquid rocket engines with the geometric area ratio equal to 50. We investigated the impact of the setting parameters of the aft diffuser relative to the engine’s nozzle exit: the position of the inlet section of the diffuser relative to the nozzle exit varied in the range from -2 to +10 mm in the axial direction. We analyzed the influence of geometrical parameters of aft diffusers that differed in the inner diameter of the channel. The relative channel length was equal to 10. We determined the structural variables of an aft diffuser and its position relative to the engine’s nozzle exit and the pressure ranges after the exit section of the diffuser that ensure the mode of unseparated flow of the combustion products in the engine nozzle. The efficiency of the vacuum system of the test bench increases as the internal diameter of the aft diffuser approaches the diameter of the nozzle. Other things being equal, the vacuum system provides longer periods of engine tests maintaining the design mode of nozzle operation.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):20-32

###### Abstract

The paper presents a calculation procedure based on the mathematical model of a pilot operated gas pressure regulator. The model takes into account the parameters of the feedback channel, the gas damper, the flow force at the main valve poppet and the dynamics of the fuel tank pressurization system of a launch vehicle. We analyzed the influence of the gas damper incorporated in the pressure relief valve on the pressure control system performance. Stability domains in the space of the regulator parameters are calculated. We assessed the control system stability margin for varying gas damper parameters. The proposed procedure of regulator design ensures the required stability margin of the gas pressure control system. The static accuracy of the system remains unchanged.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):33-46

###### Abstract

The paper presents the mathematical tools and «virtual engine» software used in digital automatic control systems of modern gas turbine engines (GTE). The software is based on a real-time fully variable thermogasdynamic engine model operating in onboard computers. The new approach makes it possible to control the engine by the parameters inaccessible for measurement due to the «virtual engine» software in the gas turbine engine control system. The engine fault tolerance improves. We synthesized the control loops of a bypass turbojet engine with a high bypass ratio according to critical parameters (engine thrust, gas temperature in the combustor, stall margin) determined by calculations. We propose a methodology of restoring information on inlet airflow data and values of control factors in case of failure of information channels. The quality of engine regulation improved considerably. The lifetime of the engine increased essentially in the context of varying its characteristics in operation. We developed a methodology of real-time identification of an onboard engine thermogasdynamic simulation model based on the reverse feedback approach.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):47-56

###### Abstract

The article discusses ways of utilizing exhaust gas heat to improve the power efficiency of heat engines. The concept of an air turbine engine is chosen to make a generator drive with the capacity of 150 kW intended to produce energy for a gas-pumping unit. In an air turbine engine atmospheric air heated by the exhaust gases of the gas turbine drive of a gas pumping unit serves as the working fluid. Three thermodynamic cycle schematics for an air turbine engine (ATE) are analyzed. The optimal design of an ATE comprising a two-stage axial compressor, a recuperative air heater (RAH), an ejector and a single-stage axial turbine is determined on the basis of thermodynamic calculations. Compressed air from the ATE compressor unit is used in the ejector as the active flow, while the combustion products of the NK-16ST engine are used as the passive flow. The recuperative air heater and ejector are located in the exhaust line of the gas pumping unit. The chosen design concept of the air turbine engine corresponds to the technical requirements of gas transmission enterprises.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):57-63

###### Abstract

The article describes a method of simulating combustion processes in a low-thrust rocket engine. The engine operates on gaseous propellants: oxygen and hydrogen. Transient simulation was performed using ANSYS CFX software. Three well-known mechanisms of oxygen and hydrogen combustion reactions for the stationary mode are considered and described in detail. A method of converting data on gas properties specified by coefficients of state equations into the NASA format was developed as one of the results of research. It was found that the initial component composition can be obtained fast by stationary simulation using an EDM combustion model. The difficulties connected with the application of the FRC combustion model, associated with a large scatter of reference data are revealed and described. A way of generation of a Flamelet-library with an ANSYS CFX – integrated CFX-RIF generator is described. A method of simulation of transient combustion processes in a low-thrust rocket engine based on the Flamelet-library is proposed. Cyclical motion of the temperature field in the chamber resembling the precession of a vortex flow core was detected in the course of testing the method. The proposed method can be used to study this process and other transient processes in rocket engines.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):64-74

###### Abstract

The first airplane working on cryogenic fuel was developed in the USSR in early 1980s on the basis of the passenger aircraft Tu-154. It was then modified using a hydrogen engine installed instead of one of the three kerosene engines. That bypass turbo-jet engine was developed in Russia (Samara) under the guidance of General Designer academician N.D. Kuznetsov. The engine working on liquid hydrogen was designated NK-88 and the one working on liquid methane – NK-89. A unique hydrogen turbopump unit was created for the supply of cryogenic fuel. The unit can also be used for liquid methane due to the possibility of readjusting the rotor frequency. To minimize the financial expenditures it is proposed to use the turbo-pump in low-thrust rocket engines that use liquid methane as fuel.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):75-80

###### Abstract

The use of innovative techniques in the production of parts and assembly units is one of the promising trends connected with increasing mass-energy characteristics of liquid- propellant jet engines. The article presents a concept of development of a liquid-propellant jet engine combustion chamber with the use of modern technologies and materials. We propose making the mixing head according to the additive technology. The cylindrical part of the combustion chamber integrated with the nozzle is to be made of carbon–carbon composite material. The article presents a configuration of innovative combustion chamber design and indicates its advantages. The results of calculating geometrical parameters are given for certain initial data. The formulas and results of calculations of the specific heat flow rate along the length of the combustion chamber and the nozzle are also given in the article. The existing methods of over-heating protection of the wall of the chamber are described in the article. The results of testing all these methods of over-heating protection show that they are not efficient enough for the innovative combustion chamber. The new method suggested by the author gives the opportunity to solve the problem by means of calculating the wall thickness taking into account the influence of inner pressure and high temperatures. The method includes both solving the task of transient heat conduction by the finite-deference method and strength calculation using the momentless theory of shells.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):81-90

###### Abstract

The article describes a thermo-gas-dynamic model of small gas turbine engines. The model takes into account the influence of the engine size on the efficiency of work processes in the crucial components. Gas turbine engines are classified according to their size depending on the value of the gas generator mass flow rate corrected by the compressor exit parameters. An important feature of the working process in small gas turbine engines is that hydraulic losses in the flow section increase with the decrease in the engine’s size due to the increase of the boundary layer relative thickness. The efficiency of the compressor and turbine also decrease because of the increase in relative radial clearances. These factors are taken into account in computer modeling by making allowances for the initial values of compressor efficiency, fuel combustion efficiency, the total pressure loss coefficient and turbine efficiency. The suggested approaches were used to improve computer models of gas turbine engines. It is shown that reducing the engine size results in considerable decrease of the work process optimal parameters and specific parameters. Taking into account the influence of the engine size on the efficiency of its components widens the range of its applicability and improves the adequacy. Thus, the models provide a more adequate solution for the optimization of working process parameters and can be used for conceptual designing of small gas turbine engines.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):91-101

###### Abstract

This article describes a method of CFD-modeling of a two-stage high-pressure pump. The main feature of the pump is the hydraulic drive of the low-pressure stage that takes energy from a high-pressure flow. The speed of the turbine is determined by the power balance of a low-pressure rotor. The modeling technique presented in this paper includes two major advantages over previous studies. The first feature is the determination of the speed of rotational velocity during the CFD-calculation by a special methodology. The second feature is cavitation simulation to assess its impact on the pre-pump workflow at a relatively low inlet pressure. Recommendations for the use of software (ANSYS CFX, NUMECA AutoGrid5, ANSYS ICEM CFD) are an important part of the simulation technology described. These recommendations concern the choice of the modeling area, mesh generation, choice of turbulence models, verification of convergence, post-processing of the results. The adequacy of the CFD-model was evaluated by comparing the calculated and experimental performance obtained on a test rig. The use of the resulting methodology of pump simulation improves the productivity and increases the efficiency of pumps with a hydro-drive of the low-pressure stage.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):102-113

###### Abstract

The article presents the results of theoretical and experimental studies of the peculiarities of the working process of low-thrust hypergolic- propellant liquid rocket engines (LTLRE) with the thrust of less than 1 N. We also discuss ways of improving their efficiency, reliability and stability of parameters. To study the stability of the hydraulic characteristics of LTLRE capillary nozzle elements we experimentally investigated the hydrodynamic characteristics of the capillaries under conditions of isothermal flow of water and heat supply and proposed a method of their calculation. Using the proposed calculation method we studied the changes of the capillary hydraulic resistance for the engine injector head and showed that the effect of thermal factors can lead to significant changes in the hydraulic characteristics of nozzle elements of engines with the thrust of less than 1 N and, consequently, to the instability of its parameters and off-design engine operation. A theoretical and simulation study was carried out to determine the region of total liquid-phase mixing depending on the capillary diameter and the thrust of the engine. It is shown that low efficiency of liquid-phase mixing of the components is the main reason for the low efficiency of LTLRE with the thrust of less than 1 N. We investigated experimentally the variation of energy parameters of a jet-mixing engine with the thrust of less than 1 N depending on the thrust level. We propose to use a precombustion chamber as a possible way of intensifying the intrachamber workflow. The influence of prechamber on the energy parameters of a LTLRE with the thrust of less than 1 N is analyzed. The possibility of intensifying the intrachamber workflow with the use of a prechamber is shown. Recommendations on the choice of the geometry of the prechamber are given.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):114-125

###### Abstract

The paper analyzes aeroelastic (flutter) and aerodynamic (rotating stall) oscillations in gas turbine engine compressors. We review the external factors that influence these oscillations. We propose a new method of diagnosing hazardous oscillations. The method is based on the analysis of frequency characteristics. The novelty of the method is elements of forecasting introduced in it. We infer the presence of dangerous oscillations by a certain threshold level of the amplitude of the dynamic signal. We forecast this threshold depending on the operation conditions of a gas turbine engine. If the amplitude of the signal reaches the threshold level the engine speed is decreased by changing the rate of fuel consumption. This measure prevents an emergency. The method was implemented during bench tests of an aircraft gas turbine engine.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):126-132

###### Abstract

The paper discusses improving the weight and size characteristics of propulsion systems with liquid thrusters due to the replacement of conventional compressed-gas systems by hot gas systems where a gas generator operating on propellants is used for hot gas production. Possible algorithms of the operation of such feed systems are discussed. A mathematical model has been created for the purpose of computational and theoretical study of operating processes in the considered feed system. The model represents system units as lumped volumes. Differential equations of weight, internal energy, and combustion gases concentration are written for every lumped volume. The differential equation of unit structure temperature is also added. The differential equations of mass and internal energy are converted into equations of pressure and temperature. The model is expressed through mass flow ratios and heat exchanges between the system units, dependences of thermo-physical properties on the temperature, combustion gas concentrations and the ratio of fuel components in the gas generator. The temperature, pressure, concentration of combustion gases in the system units and the unit structure temperature are specified as input parameters. The simulation results showed satisfactory agreement with the experimental data, which makes it possible to use the developed model for computational and theoretical studies of the hot gas pressure feed systems.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):133-142

###### Abstract

The paper presents numerical calculations of a fuel spray downstream a swirl-type fuel injector carried out for various pressures in the combustion chamber using the model of liquid film motion. The effect of the chamber pressure, or, to be more exact, the air density in the pressure chamber on the fuel spray characteristics is investigated. The mathematical model was constructed on the assumption of one- dimensional and steady swirling flow. The liquid is considered to be incompressible and have zero pressure gradient in the direction of the film motion and in the tangential direction. The influence of viscous forces on the motion of liquid is neglected, but the viscous interaction at the interface between liquid and gas is taken into account. The change of velocity in the circumferential and normal directions can be neglected, because in practice the film thickness is considerably smaller than the spray radius. It is shown that the pressure increase in the combustion chamber makes spray characteristics significantly different from those observed at atmospheric pressure. An increase of pressure results in increasing the thickness of the fuel film and decreasing the spray-cone angle. It leads to an increase in the average Sauter diameter in the spray of fuel atomized by the pressure atomizer. The air flow downstream the swirl nozzle has the opposite influence on the size of drops in case of increased pressure in the chamber.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):143-149

###### Abstract

The paper justifies the possibility of using cold-flow test data to predict the combustion efficiency for a 13.34 N thruster combustion chamber. The results of modeling working liquid (water) flowing through hydraulic passages of single injection elements and coaxial swirl injectors are presented. We performed the simulation using the Reynolds-averaged Navier-Stokes equations that describe a turbulent flow of a two-phase non-compressible liquid. We used both one-velocity and two-velocity flow models. We modeled the turbulence by the BSL isotopic turbulence model on a computational block-structure grid. The computational grid consisted mostly of HEXA elements. The orientation of the elements coincided with the flow direction in the injector hydraulic channels, which improved the flow resolution accuracy in the boundary layer near the solid wall. We compared the results of calculation with the cold-flow test data. The use of the one-velocity flow model in the design area of a single injector and the two-velocity model in the design area of the mixing element yielded the results that agreed satisfactorily with the cold-flow data.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):150-161

###### Abstract

A model of non-stationary heat and mass transfer of fuel droplets with a swirling air flow is developed. The model includes the definition of the air flow structure, identification of configurations of liquid jets flowing into the air flow, the calculation of characteristics of their breakup taking into account the processes of drop breaking and coalescence, the calculation of droplet and fuel vapor concentration distribution in the work volume. The study was performed as applied to a combustion chamber with a front gas-dynamic flame holder. The optimal combination of major gas-dynamic parameters that determine the mode of operation of the device in question – the value of the air flow swirl and the intensity of the air traverse feed through the flame holder – is determined. The flow structure and the distribution of liquid-droplet and vaporous fuel concentrations downstream the flame holder are specified for this combination. The calculation results are compared with the experimental data.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):162-173

###### Abstract

The aim of the present work was to develop a methodology for assessing reliability on the basis of physical characteristics of an element. To this goal, we used the structure-energy power failure theory. The use of the theory makes it possible to determine the probability of failure using the information on the internal damage of the material. The gas turbine engine blades of the first, second and third turbine stages, new and reconstructed high-pressure compressor blades of stage 5 were the object of the analysis. We used an industrial computer tomograph XTH 450 LC to carry out non-destructive testing. We carried out destructive tests on stands of the electrodynamic type. The probabilities of failure calculated by using the structure-energy failure theory correspond to the values of reliability indices obtained by the destructive experiments. We developed a computer program based on a statistical method for estimating the reliability of an aircraft engine. The program takes into account the reliability index of rotor blades calculated with the use of the structure-energy failure theory.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):174-183

###### Abstract

Creation of a low-noise water flow control systems apparatus is the main objective of the research. The technique of carrying out the tests is based on the principles of acoustics, flow mechanics and the control theory. The main schemes of regulating the water flow rate are presented in the paper. It is shown that the scheme with the use of a flowmeter is the most promising in terms of improvement of vibronoise characteristics. A modular principle of constructing feed water rate control systems is presented. The design features of application of multistage rotary units of the apparatus meeting the hydrodynamic noise requirements for low rates without application of dissipative elements (e.g. mufflers) are formulated for the first time. Energy comparison of various control system schemes is carried out. It is shown that further improvement of operating devices in terms of vibronoise characteristics is possible with the use of the principle of serial-parallel water flow regulation. The results help to improve the mass-dimensional characteristics of systems and to increase their energy efficiency, to provide a low level of noise and to reduce the cost of production. The modularity of the apparatus ensures a high level of standardization and interchangeability.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):184-194

###### Abstract

A mechatronic sample conditioning system requires a high degree of process automation. Special attention is to be paid to the cooling of a sample and maintaining its temperature and pressure. Therefore, the study of thermal performance of the sample cooler (heat exchanger) is an important step in the development of a sample conditioning system. The helically coiled sample cooler, highly efficient, compact and simple in its design, is the most perfect heat exchanger of the double-pipe type. The article discusses the problem of determining the efficiency of a countercurrent sample cooler. The experimental apparatus for testing the cooler is described. On the basis of processing the experimental data obtained by the least-square method a semi-empirical dependence for determining the heat transfer coefficient with an error of less than 8% was obtained. The semi-empirical dependence for the cooler under consideration is applicable for temperature ranges and flow rates that occur in sample conditioning systems for thermal power plants. The results obtained make it possible not only to verify the CFD calculations aimed at improving the cooler design but also to create a high-quality mathematical model of the cooler incorporated in a mechatronic sample conditioning system, to develop an algorithm for maintaining the desired temperature and diagnosing the amount of deposits on coil surface.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):195-203

###### Abstract

Ultralow power turbine drives are used in aerospace and other transport equipment as energy for various units, mostly supplementary ones. The main components of the drives are an input device, a very low power turbine and an output device. Improving the efficiency of low-power turbine drives is a vital task. Energy indicators, such as power efficiency and specific working medium consumption of the working fluid are some of the most important performance indicators. The article presents configurations of axial and centripetal turbine ultra-low power, describes their basic operating and geometrical parameters that influence the energy efficiency of the drive. We present mathematical models of the efficiency of the turbines of two types, obtained from the results of gas-dynamic computational experiments. The results of statistical correlation and regression analysis of computational experiments presented in the article show the adequacy of the mathematical models. We analyzed the degree of influence of the model factors and their interactions on the changes in the efficiency of using the influence coefficients and graphical analysis of the results of the optimization parameters of the turbine.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):204-215

###### Abstract

The paper discusses the main stages of the automatic quantitative analysis of parameters of the γ'-dispersed phase in high-temperature polycrystalline nickel alloys and indicates their main difference from the standard methods of analyzing disconnected phases. We present 4 classes of characteristics of dispersed phase particles. These characteristics include the volume fraction, the dimensional parameters of particles and the spatial distribution parameters. For example, we calculated the parameters of the γ'-phase of a blade made of a heat resisting nickel alloy In792. The volume fraction and the equivalent diameter of the γ'-phase disperse particles increase in operation while their concentration decreases. These parameters of the γ'-phase change with changes in the mechanical properties of the alloy. The shape of the particles and the scatter of γ'-phase concentration change little in the process of operation.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):216-223

###### Abstract

The paper presents the results that allowed obtaining the dependence of laminar flame propagation speed *Sl* on the equivalence ratio for a wide range of pressures and temperatures during methane combustion. A literature review was carried out to summarize the experimental data on the measurement of the *Sl*. The *Sl* was calculated using a kinetic mechanism GRI 3.0 within the required pressure and temperature range. The calculation results were generalized in the MATLAB software product to verify the *Sl* power dependencies on pressure and initial temperature. The results of calculation on the basis of the obtained approximating dependence were compared with the experimental data and results obtained by other authors. It was found that the exponents of power for the dependency on pressure and temperature are described not by constants or linear relations, but by second-degree equations on the fuel-air ratio. The results can be used in three-dimensional simulation of combustion processes and in calculations performed using engineering practices.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):224-234

###### Abstract

The article discusses the formation of the surface layer in hydroabrasive jet machining. We propose a mathematical model of surface microrelief formation under the influence of abrasive particles on the work surface. A single act of contact interaction of an abrasive particle with the surface is the basis of the model. The microrelief of the work surface is presented in the form of isotropic functions. We regard contact interaction of abrasive particles with the surface as the intrusion of a rigid sphere in a plastic half-space. We developed an algorithm and software package for the implementation of surface modeling. The article presents the results of calculations of surface roughness on the basis of the proposed model, as well as the results of experimental studies of roughness with the use of full-scale specimens made of a VT9 titanium alloy and a ЖС6Ф heat-resistant alloy. The model makes it possible to select the necessary technological process parameters to provide the desired surface roughness.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):235-242

###### Abstract

The article deals with the task of increasing the efficiency of rotary piston engines by implementing an economical diesel cycle in a module with a three-arc epitrochoid and an external gearing kinematics scheme. The necessity of the diesel cycle is caused by the need of reducing specific fuel consumption in rotary piston engines that are currently widely used in light aviation, mostly for remotely piloted aircraft. The Wankel rotary piston engine with a two-arc epitrochoid, though having a number of advantages over traditional piston engines, is inferior to them in efficiency, particularly in partial load modes. The implementation of the diesel cycle in а cycloidal piston engine with a three-arc epitrochoid makes it possible to reduce specific fuel consumption by 30% while retaining the listed positive qualities, as well as to use diesel fuel which is preferable for high-altitude aircraft due to lesser tendency to gas formation in pipelines. The engine stator inner loop is made up by three arcs of the epitrochoid that form three working chambers equally-spaced along the perimeter, which provides a decrease of the stator thermal deformation during the operation of the engine. A method is developed to determine the geometric parameters of a rotary piston engine with a three-arc epitrochoid, and design work is carried out in the КОМPAS-3D code for a 50 h. p. diesel engine to be used in light aviation and as a drive of electrical generators, pumps and compressors for ground application.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):243-251

###### Abstract

Machining of parts with sculptured surfaces using computer numerical control equipment and the control of their geometry is accomplished by 3D-models describing the master part. The article presents a technique of compensation of manufacturing errors, based on the incorporation of coordinate measuring of the part geometry in the machining process. The proposed technique involves changing of the machining program by restructuring 3D-model of the part according to the information on deviations obtained after the measurement. The technique allows the analysis of the measurement results and on-line correction of control programs for CNC metalworking equipment. The analysis of the measurement results consists in determining whether it is necessary to compensate the errors, and filtering the random error components not taken into account in compensation. Operational adjustment of control programs for metalworking equipment makes it possible to improve the quality of its products and the efficiency of production processes. The developed technique was implemented in the MATLAB application software package.

**VESTNIK of Samara University. Aerospace and Mechanical Engineering**. 2016;15(4):252-264