Channel Of Constant Cross Section Research Articles

HYDRODYNAMIC MEANS OF HEAT EXCHANGE INTENSIFICATION IN CONSTANT CROSS-SECTION ROUND CHANNELS

The purpose of research is the analysis of innovative designs of hydrodynamic means of intensification of heat exchange in channels of constant cross-section of technological equipment and pipelines, intended for use in the thermal power, chemical, food and related industries. The study of the state of the issue and trends in the development of structural and technological design of hydrodynamic elements of pipes and tubular elements for the intensification of heat exchange are based on a critical analysis of scientific and technical sources of information and, first of all, patent information of the leading countries of the world. The classification of these means for intensification of heat exchange in equipment and pipelines of thermal energy, chemical, food and related industries is proposed. The most interesting designs of these elements proposed by scientists, designers and inventors of the leading countries of the world are considered. A critical assessment of most of the above constructions is provided, their advantages and disadvantages are discussed. Designs of means of intensification of heat exchange in channels are analyzed depending on the change in flow continuity, the degree of mobility of the working element, the nature of the effect on the flow over time, relative length, assembly/disassembly possibilities, the degree of curvature of the channel, material, level of assembly, the degree of rigidity, the degree of mobility of the working element, the possibility of changing the geometry of the working channel , the possibility of temperature regulation, as well as the degree of uniformity of the heat flow in the circular direction of the channel. Means according to the change of flow continuity, namely turbulizers and cavitators, were analyzed in more detail. It is shown that, despite the significant variety of structural and technological design of hydrodynamic means of heat exchange intensification in round channels of constant cross-section, the most demanded by the industry are technologically diverse removable turbulizers that can be replaced or completely removed from the channel if necessary. Less common are cavitators, because they are usually characterized by less versatility and manufacturability, as well as greater sensitivity to the flow rate of the transported flow and its physical properties. Bibl. 44, Fig. 19.

Numerical simulation of diverging plasma magneto-hydrodynamic acceleration channels

This paper presents a numerical study of plasma flow in a magneto-hydrodynamic acceleration channel. The results show that in the channel with equal electrode pair spacing, increasing the magnetic field strength reduces the Joule heat dissipation and the temperature. Increasing the applied voltage increases the external energy input, increasing both the exit Mach number and the internal temperature. Reducing the electrical conductivity achieves a balance between acceleration and temperature rise. For acceleration channels of different shapes, a constant cross section channel can only accelerate the fluid to near sonic speeds, whereas a diverging channel can achieve supersonic speeds. In addition, maintaining constant electrode spacing in the diverging channel effectively prevents fluid deceleration at the rear of the acceleration channel caused by the reverse Lorentz force under high magnetic field strength.

Газодинамическая стабилизация и интенсификация макрокинетических процессов окисления метана в высокоэнтальпийном кислородсодержащем потоке

The paper presents experimental results of studying the macrokinetic intensification and stabilization processes of methane oxidation (combustion) in the high-enthalpy oxygen-containing flow inside the constant cross section channel being finite along its length. Calculation and experimental data are presented on the methane oxidation gas-dynamic intensification and stabilization in the recirculation zone of the high-enthalpy oxygen-containing flow formed behind the wedge-shaped bluff body. Computational and experimental studies enabled to consider various configurations of the bluff bodies differing in their number and degree of the constant cross section channel obstruction. Dependence of the gas relative residence time in the recirculation zone behind the bluff bodies was determined for various configurations. Range of the initial enthalpy values of the high-enthalpy oxygen-containing flow of 350--700 kJ/kg was considered. Regularities were established for the influence of the flow obstruction degree on the physical and chemical processes completion in the channel under study. Methane oxidation intensity in the high-enthalpy oxygen-containing flow was compared with and without the gas-dynamic stabilization. The level of lower limit value of the fuel excess coefficient corresponding to the stable methane ignition and combustion was determined. The data obtained indicate intensification in the methane oxidation diffusion-kinetic regimes and make it possible to evaluate the factors that are limiting completion of the physical and chemical processes

Influence of the shape on the hydraulic resistance of bypass channels inside a smart pig for low pressure gas pipeline inspection

The in-line smart pig for gas pipeline diagnostics is driven by the pumped gas and performs quality diagnostics only if its speed is constant, when the information reading is uniform along the length of the pipeline. In a low-pressure gas pipeline, if there are inhomogeneities on its walls, the condition of the pig speed constancy is easily broken, as the force of resistance to the pig movement on the stoppers is comparable with the difference of pressure forces acting on its ends. A central bypass channel of constant cross-section is usually used to control the pig velocity. The paper shows insufficiency of such method of regulation, leading to noticeable flow pulsations at the outlet of the channel. The use of variable cross-section of the bypass channel along its length in the form of a Laval nozzle to reduce flow pulsations is proposed. The numerical simulation of gas flow in Laval nozzles of different configuration is performed; it is shown that for all considered nozzles, the pressure pulsation at the channel outlet is 20-30 times less than that in the cylindrical channel, which simplifies the velocity control of such pig. However, it is not possible to completely avoid detachment of the flow from the walls of the expanding part of the nozzle, so in order to reduce the area of the detachment zone and reduce the flow resistance force, additional peripheral bypass channels connecting the inlet end of the pig with the diffuser section of the central bypass are added. Thus, a 40% reduction of gas flow resistance force through the bypass compared to the cylindrical bypass is obtained.

Heat transfer and flow structure in a plane diverging channel

Heat transfer in diverging channels is essentially different from the one in channels of constant cross section. The mechanism of heat transfer enhancement in diverging channels compared to the channels of constant cross section is based on increased turbulence intensity. The present paper suggests new approach to the analysis and generalization of heat transfer data. Heat transfer on the wall of a diverging channel should be considered similarly to heat transfer from a plate instead of heat transfer in a channel. The data on heat transfer coefficient on the wall of a plane diverging channel at different expansion angles can be generalized by the Stanton number as a function of the Reynolds number and Kays acceleration parameter, St∼f(Re, K). This function has characteristic sections along the wall that are typical of laminar, transitional and turbulent boundary layers. Stanton and Reynolds numbers based on the distance from the inlet and the velocity at the diverging channel inlet can be employed for data generalization in all the considered ranges of geometries and flow regimes. Kinematic structure of flow is examined to reveal hydrodynamic mechanisms behind heat transfer formation. Optical measurements provide the profiles of velocities and turbulence parameters in characteristic sections of the channel. Mechanism of turbulent structure formation in diverging channels is examined. Correlation between hydrodynamics and heat transfer is analyzed. Correlation between local coefficients of heat transfer and near-wall Reynolds stress is revealed.

Experimental Study of Methane Combustion Efficiency in a High-Enthalpy Oxygen-Containing Flow

The article presents the experimental study analysis results of the methane combustion efficiency in a high-enthalpy oxygen-containing flow (HOF) inside of constant cross-section channel, finite along the length. The range of initial HOF enthalpies was considered from 350 to 700 kJ/kg. The regularities of the HOF initial enthalpy influence on the methane combustion efficiency were obtained. The values of the fuel excess coefficient under which the maximum coefficients of methane combustion completeness in the HOF are realized were determined. The data obtained indicate the realization of transitional diffusion-kinetic modes of methane combustion and make it possible to assess the factors limiting the combustion process.

Vibroextrusion flow of concrete mixtures in a right four-corner pyramidal channel

The purpose of the research was to obtain calculation formulas for describing the flow of concrete mixtures and determining their viscosity directly in the process of vibroextrusion in regular quadrangular pyramidal channels.
 In solving the flow problems, it was taken into account that concrete mixtures are non-Newtonian systems in the conditions of a vibration field, and hydrodynamic theories were used to calculate the processes and rheological characteristics.
 Since the calculation formulas for describing the flow of fluids in a regular quadrangular pyramidal channel are absent and the channel has a square cross section, an analytical dependence was used to characterize the process, which describes the flow of a Newtonian fluid in a rectangular channel of constant cross section. The authors proposed for use simplified formulas for determining the maximum flow rate and flow rate of a Newtonian fluid in a channel of rectangular cross-section.
 The degree of decrease in the flow velocity and the flow rates of a Newtonian fluid in a channel of rectangular cross-section in comparison with the flow between flat parallel plates are analyzed.
 To describe the flow of concrete mixtures in regular quadrangular pyramidal channels, the proposed coefficients for reducing the speed and flow rate, as well as the existing formulas for the flow between flat symmetric stationary walls, which converge, were used.
 The possibility of using the obtained formula for the flow rate in a regular quadrangular pyramidal channel for calculating the viscosity of a concrete mixture during vibration extrusion is shown. To simplify the experimental procedure, a new formula for calculating the viscosity based on the expiration time for a certain amount of concrete mixture has been proposed.
 The formulas obtained are convenient for further mathematical processing and have no restrictions in their application. The proposed method for determining the viscosity expands the possibilities of studying the rheological properties of concrete mixtures in the vibroextrusion of fiber-reinforced concrete mixtures.

Gas outflow into vacuum over a forward- and backward-facing step in a wide range of rarefaction

The direct simulation Monte Carlo method has been applied to investigate the outflow of rarefied gas into vacuum over a forward- and backward-facing step. The non-equilibrium effects at the ends of the step channel have been taken into account by considering the geometry of sufficiently large regions directly adjacent to the inlet and outlet of the channel. The mass flow rate through a step channel as well as the gas flow field both inside the step channel and in the upstream and downstream regions have been calculated over a wide range of gas rarefaction. The calculation results have been compared with the corresponding data for a channel of constant cross-section. The system configuration has been found to have a pronounced effect on the simulation findings. The so-called Knudsen minimum has been established in the dependence of the mass flow rate on the gas rarefaction, and a gas circulation zone has been found in the flow field.

Dynamics of initiation of the pseudoshock combustion mode in a constant cross section channel

The results of a study on the dynamics of initiation of a pseudo-shock combustion mode in a channel of constant cross section under pulsed action on the flow are presented. Various conditions are considered (kerosene flow rate, change in channel length and working gas in control pulses). Pulsating combustion mode is implemented. It is shown that the reduction of the length of the constant section leads to a decrease in combustion stability. When applying control pulses, two scenarios of the development of disturbances can be implemented: with intensification of combustion and the formation of wave structures, or with attenuation without intensification of combustion. In case of combustion, the motion of the disturbance along the channel can conditionally be divided into two stages: 0 <t <4 ms and t > 4 ms. The duration of the first stage can be interpreted as a delay in the ignition of kerosene; in the second stage, the perturbation velocity relative to the channel walls is almost constant. The use of kerosene (synthesis gas) conversion products in pulses accelerates the transition from diffusion to pseudo-shock combustion, and the rate of disturbance propagation increases by about 2 times.

Efficient Generator of Low-Temperature Argon Plasma with an Expanding Channel of the Output

To study the thermophysical, electrophysical and optical properties of argon, as well as the implementation of various plasma-chemical reactions, a direct-current generator of a high-enthalpy argon plasma jet with a self-adjusting arc length and an expanding channel of the output electrode has been developed. A comparative analysis of the electrophysical characteristics (current–voltage characteristics—CVC, efficiency) in the expanding and cylindrical channels of constant cross section was carried out. Electrical, calorimetric and spectral studies have shown that the created generator of low-temperature plasma provides the formation of a slightly divergent plasma jet of argon with a diameter of 5–8 × 10–3 m and enthalpy of 5–10 MJ/kg and a mass-average temperature at the outlet of the gas-discharge channel of 5–12 × 103 K with an electron concentration in the axial plasma of 1017 cm–3, the total electric power of the arc discharge 2–10 kW and the plasma-forming gas consumption rate of 1.5–3 × 10–3 kg/s. Depending on the initial conditions at a distance of 0–3 × 10–2 m from the nozzle section of the low-temperature plasma generator, the plasma flow velocity varies from 990 to 300 m/s.

Об использовании конечно-элементной аппроксимации на неструктурированной сетке для анализа устойчивости течений жидкости в каналах постоянного сечения

Об использовании конечно-элементной аппроксимации на неструктурированной сетке для анализа устойчивости течений жидкости в каналах постоянного сечения

Compressible Fanno flows in micro-channels: An enhanced quasi-2D numerical model for turbulent flows

Fanno theory provides an analytical model for one-dimensional confined viscous compressible flows. The model holds under the assumptions of adiabatic flow and constant cross-section channel. From theory, the differential of every flow-related quantity is expressed as a function of Mach number and friction factor. One-dimensional flow numerical models can be derived by discretizing Fanno equations. However, theory does not assess how to evaluate friction, while the model works properly only if friction is estimated correctly. Compressibility and turbulence act by deforming the velocity profile making it flatter. Assuming the friction factor function of the Reynolds number alone, in line with incompressible flow theory, is thus not correct. Better correlations should include the Mach number to address compressibility effects. Here, the impact of turbulence and compressibility on the velocity profiles in a micro-channel is analysed by means of CFD simulations. Friction factor correlations are deduced for turbulent micro-flows. The impact of the velocity profile on other quantities, such as dynamic pressure and bulk temperature, needed for the numerical model operation, is also evaluated. Additional correlations for these quantities overcome the instrinsic limits of the one-dimensional model, necessarily unaware of local velocity profiles, in a quasi-2D fashion significantly improving its predicting capabilities.

Friction and leakage analysis of the blocker-type valve designed for a revolving vane expander

Friction and leakage are two known issues that contribute to inefficiencies of rotary vane machines, and the Revolving Vane (RV) mechanism was introduced to reduce them. When used as an expander, a mechanism to determine the timing of the suction process in RV machine is required. In this study, a blocker-type valve is proposed for the suction control, and a simulation is carried out to investigate its friction and leakage characteristics. The valve comprises a stationary blocker that blocks high-pressure working fluid from entering the working chambers during expansion. In order to reduce friction, there should be a clearance between the rotary parts and the stationary blocker, which inevitably causes leakage of the working fluid into the working chambers. The leakage was modelled as a compressible flow through a convergent nozzle followed by a Fanno flow through an adiabatic constant cross-section channel. The results showed that the amount of leakage in the absence of lubricants is almost higher than ideal expander flow rate. However, this leakage demonstrated a positive contribution to the output torque as the results of the cylinder wall shear stress. The results suggested the importance of using lubricants in this valve design.

Linear Modes for Channels of Constant Cross-Section and Approximate Dirichlet–Neumann Operators

We study normal modes for the linear water wave problem in infinite straight channels of bounded constant cross-section. Our goal is to compare semi-analytic normal mode solutions known in the literature for special triangular cross-sections, namely isosceles triangles of equal angle of \(45^{\circ }\) and \(60^{\circ }\), see Lamb (Hydrodynamics. Cambridge University Press, Cambridge, 1932) , Macdonald (Proc Lond Math Soc 1:101–113, 1893), Greenhill (Am J Math 97–112, 1887), Packham (Q J Mech Appl Math 33:179–187, 1980), and Groves (Q J Mech Appl Math 47:367–404, 1994), to numerical solutions obtained using approximations of the non-local Dirichlet–Neumann operator for linear waves, specifically an ad-hoc approximation proposed in Vargas-Magana and Panayotaros (Wave Motion 65:156–174, 2016), and a first-order truncation of the systematic depth expansion by Craig et al. (Proc R Soc Lond A: Math, Phys Eng Sci 46:839–873, 2005). We consider cases of transverse (i.e. 2-D) modes and longitudinal modes, i.e. 3-D modes with sinusoidal dependence in the longitudinal direction. The triangular geometries considered have slopping beach boundaries that should in principle limit the applicability of the approximate Dirichlet–Neumann operators. We nevertheless see that the approximate operators give remarkably close results for transverse even modes, while for odd transverse modes we have some discrepancies near the boundary. In the case of longitudinal modes, where the theory only yields even modes, the different approximate operators show more discrepancies for the first two longitudinal modes and better agreement for higher modes. The ad-hoc approximation is generally closer to exact modes away from the boundary.

Experimental study of the combustion efficiency of two-phase gasification products of energetic boron-containing condensed compositions in a high-enthalpy airflow

This paper describes an experimental setup and a method for determining the combustion efficiency of the gas and condensed phases of gasification products of energetic boroncontaining condensed formulations in a high-enthalpy subsonic airflow. The results of investigation of the combustion of two-phase combustible mixtures in a channel of constant cross section at various temperatures, pressures, and component ratios are given. The combustion regularities of boron-containing condensed phases in a high-enthalpy airflow are identified. The obtained data can be useful in computational and experimental studies of propulsion system operation.

Effect of the opening angle of the gas-discharge path on the power efficiency of a plasmatron

To increase the stability of the discharge arc, experimental studies of a plasma generator with a divergent channel of the output electrode are carried out. It is shown that the optimum opening angle of flow of 6° between the inside wall of the channel and the channel axis leads to increased stability of the arc compared to a cylindrical channel of constant cross section.

Анализ влияния типа форсунок и направления впрыска жидкости на эффективность двухфазного смесеобразования в канале постоянного сечения

Анализ влияния типа форсунок и направления впрыска жидкости на эффективность двухфазного смесеобразования в канале постоянного сечения

Development and testing of the ACT-1 experimental facility for hypersonic combustion research

A new pulsed-arc-heated hypersonic wind tunnel facility, designated as ACT-1 (Arc-heated Combustion Test-rig 1), has been developed and built at the University of Notre Dame in collaboration with the University of Illinois at Urbana-Champaign and Alta S.p.A. The aim of the design is to provide a suitable test platform for experimental studies on supersonic and hypersonic turbulent combustion phenomena. ACT-1 is composed of a high temperature gas-generator system and a model scramjet combustor that is installed in an open-type vacuum test section of the wind tunnel facility. The gas-generator is designed to produce high-enthalpy (stagnation temperature = 2000 K–3500 K) hypersonic flows for a run time up to 1 s. The supersonic combustor section is composed of a compression ramp (scramjet inlet), an internal flow channel of constant cross-section, a fuel jet nozzle, and a flame holder (wall cavity). The facility allows three-way optical accesses (top and sides) into the supersonic combustor to enable various advanced optical and laser diagnostics. In particular, planar laser Rayleigh scattering (PLRS), high-speed schlieren imaging and OH-planar laser induced fluorescence (OH-PLIF) have successfully been implemented to visualize the turbulent flows and flame structures at high speed flight conditions.

Periodic microstructures for improved lens-to-waveguide coupling efficiency in microlens array planar solar concentrators

Thin form-factor planar sunlight concentrators fabricated using low-cost materials in conjunction with high-efficiency solar cells may prove economically competitive with silicon photovoltaics. A periodic structure with planar interfaces employing an array of microlenses and coupling structures is amenable to high-volume imprint manufacturing and hence a drastic cost reduction. A numerical study of periodic prismatic void structures aligned with microlenses to efficiently couple incoming light into a constant cross-section channel waveguide is reported. Three different prism-like cut-out structures: a bare prism, a planar connected prism, and a pointed prism are proposed. Numerical results show that the proposed pointed prism design offers a high-efficiency coupling with one-sided output (90.8% and 85.4% peak efficiency at concentration ratios of 320× and 400×, respectively).

Analysis of regimes of magnetogasdynamic interaction between a current layer and an argon flow

A nonstationary three-dimensional magnetogasdynamics (MGD) model is used to study the dynamics of a current layer interacting with a transverse magnetic field in a supersonic argon flow through a channel of constant cross section. The MGD interaction regimes and the features of the current layer formation for various external resistances and channel widths are analyzed as based on numerical results.