RU2387067C1 - Magnetic gas dynamic channel - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to technology of magnetic gas dynamic channels operation, such as MGD-generators and MGD-accelerators and may be used in electrotechnical and aerospace industry, and also in other fields of engineering. In proposed device protection of electrodes and interelectrode insulators of magnetic gas dynamic (MGD) channels includes replacement of interelectrode insulators for grooves (caverns) with arrangement of eddy currents in them. Magnetic gas dynamic channel consists of insulation and electrode walls that represent alternating electrodes and interelectrode insulators. Interelectrode insulators are arranged in the form of caverns between near electrodes with width L that is more than the one, when gas gap is broken down at gas dynamic parametres of border layer and depth h, which relates to groove width

. Surface of groove bottom is made of heat resistant insulator in the form of cylinder or cylindrical surface so that electrode walls are tangential to its surface.

EFFECT: improved protection of electrodes.

6 dwg

Description

The device relates to technical physics, in particular to the technology for organizing the interaction of high-temperature gas and plasma flows with electric current, to the operation technology of magnetogasdynamic channels, both MHD generators and MHD accelerators, and can be used in the electrical and aerospace industries, as well as in other areas of technology.

One of the main drawbacks inherent in MHD devices and hindering their use for technical purposes is the destruction of electrodes and especially interelectrode insulators during their operation.

The main physical mechanism responsible for this destruction is the appearance of microarc discharges on the electrodes, which are peculiar current bridges through the relatively cold boundary layer between the electrodes and the high-temperature flow core that conducts current well. Microarcs are carried by the flow to the trailing edges of the electrodes and exist there for a long time, destroying both the electrode itself and the region of the interelectrode insulator especially adjacent to the electrode.

здесь u - скорость потока, В - индукция, l - расстояние между противоположно расположенными электродами. При течении высокотемпературного газа у стенок (как на изоляционной, так и на электродной стенке) образуется относительно холодный пограничный слой с температурой у стенки, равной температуре стенки T_w. Поскольку температура стенок T_w и прилегающего слоя газа значительно ниже температуры ядра потока T_ст, т.е. T_ст>T_w, то соответственно и электропроводность газа в нем также будет значительно ниже. При достижении разности потенциалов между хорошо проводящим ток ядром потока и поверхностью электрода, достаточной для электрического пробоя холодного участка пограничного слоя, происходит его искровой пробой с образованием электрической дуги между электродом и хорошо проводящим электрический ток ядром потока.There is a method of organizing a discharge in the channels of MHD devices, which consists in the fact that the electrodes either supply high voltage in the case of using the MHD channel in the accelerator version, or an induced voltage arises on the electrodes when used as MHD generators

here u is the flow velocity, B is the induction, l is the distance between the opposite electrodes. When high-temperature gas flows near the walls (both on the insulating and on the electrode wall), a relatively cold boundary layer is formed with a wall temperature equal to the wall temperature T _w . Since the temperature of the walls T _w and the adjacent gas layer is much lower than the temperature of the core of the flow T _st , i.e. T _article > T _w , then, respectively, the electrical conductivity of the gas in it will also be significantly lower. When the potential difference between the well-conducting current core of the stream and the electrode surface, sufficient for electrical breakdown of a cold portion of the boundary layer, is reached, it breaks down with the formation of an electric arc between the electrode and the well-conducting electric current flow core.

Due to its small size (of the order of magnitude of the boundary layer), such an electric discharge is called a microarc.

Two forces act on a microarc: gas-dynamic, displacing it downstream and electro-dynamic, the direction of which depends on the ratio of the electrode polarity vector and the magnetic induction vector. So, for example, in the MHD generator mode at the cathode, the microarcs are displaced under the action of electrodynamic forces to the lower end along the electrode flow, and to the upper end at the anode. In the MHD accelerator mode, on the contrary, microarcs are shifted at the anode to the lower end, and at the cathode, to its upper end in the flow. In the first and second cases, respectively, at the cathode and at the anode, the action of gas-dynamic and electrodynamic forces is directed in one direction. The microarc stops at the junction (edge) of the electrode-insulator and destroys the latter. Typically, the current in a microarc is 5-10 A. If the current through the electrodes is greater than the specified value, then a greater number of discharge microarcs corresponding to the current appear on the electrodes. A detailed description of the process of the occurrence of a microarc discharge in the MHD channel is given in the work of V.I. Kavbasyuk, N.M. Baranov, A.D. Iserov, etc. 6, p. 1294.

There are various methods and design options to prevent the destruction of the MHD channel by micro-arcs. So, in patent application RU 97117707, IPC ⁶ Н02К 44/08, 1997, it was proposed to make a channel of a linear magnetohydrodynamic generator with electrode walls in the form of hollow bodies of revolution, their outer surfaces being fire, the axis of rotation are located in the plane of symmetry of the channel, and the electrodes themselves are rotated around a specified axis.

The disadvantage of this device is the need for a mechanism by which the electrode walls must be driven, the presence of gaps between the walls, which leads to their clogging with an alkaline additive, as well as products of electrode erosion.

Another solution aimed at increasing the service life of the electrodes is to interrupt the current between the electrodes during microarc formation (see copyright certificate SU 1414278A, IPC Н02К 44/08, 1984).

Interruption of the current during the occurrence of a microarc leads to an interruption of the force acting on the Lorentz force flux, which in the case of the MHD generator will lead to the appearance of its pulsating non-stationary regime, which significantly worsens its characteristics, and if the MHD channel is used to accelerate the gas flow

In the patent RU 2111601, IPC Н02К 44/08, 1996, “MHD device”, each electrode is divided into segments, with one of the segments of the anode connected directly to the power source, and the rest through a chopper, with the first segment of the anode downstream and the last the cathode flow segment is connected directly to the load or power sources, the inputs of the remaining electrode segments are connected through a switch with a frequency chopper. The latter should be connected to an adjustable frequency generator that controls the operation of the switch.

This invention has the same disadvantages as above. The presence of switches connected to a controlled frequency control generator will also lead to the appearance of a pulsating mode that impedes the operation and removal of electric energy from the electrodes when using the MHD channel in the generator mode and is unacceptable when using it in the MHD accelerator mode, especially when used as element of a hypersonic wind tunnel.

In the copyright certificate SU 1376898, IPC Н02К 44/10, 1985, the design of the combined electrode of the MHD generator in the form of a cooled metal frame with fins made of heat-conducting insulating material is proposed, so that the fins form a cell in which the conductive insulating material is located . In the frame of the electrode, water cooling channels are made. In this invention, the objective is to increase the surface temperature of the electrode to reduce the intensity of its erosion by reducing the magnitude of the current microarc.

However, increasing the surface temperature of the electrode and the use of a highly conductive ceramic material does not solve the problem of eliminating erosion of the electrode-insulator interface.

The closest to the claimed in its technical essence and the achieved result is the channel design of the MHD device, proposed in the copyright certificate SU 1741589, IPC Н02К 44/08, 1990

MHD channel contains electrode walls made of electrodes and interelectrode insulators, side walls made of electrical insulating material. The engagement between the electrodes and interelectrode insulators is made in the form of a groove in the insulator and a corresponding protrusion in the electrode.

The disadvantage of this design is the insecurity of the interelectrode insulator from destruction by its microarc.

The objective of the invention is to increase the wear resistance of the MHD channel, increase its service life and realize the possibility of working at high thermal loads.

The technical result of the invention is to increase the wear resistance of magnetogasdynamic (MHD) channels of both MHD generators and MHD accelerators, as well as other electrical devices, in which the high-temperature gas flow interacts with electric current through electrode systems.

причем дно выемки выполнено из жаростойкого изоляционного материала, а поверхность дна выемки выполнена цилиндрической так, что стенки электрода являются касательными к ней.The solution of the problem and the indicated technical result are achieved by the fact that in the claimed magnetogasdynamic channel, consisting of insulating and electrode walls, including electrodes and interelectrode insulators, interelectrode insulators are made in the form of grooves between adjacent electrodes with a width L greater than the one at which breakdown of the gas gap with gas-dynamic parameters of the boundary layer h, and depth h, which is in relation to the width of the recess

moreover, the bottom of the recess is made of heat-resistant insulating material, and the surface of the bottom of the recess is made cylindrical so that the walls of the electrode are tangent to it.

Figure 1 shows the cross section of the MHD channel.

Figure 2 presents the design of the electrode wall of the MHD channel

Figure 3 shows a fragment of an electrode wall consisting of two electrodes and an interelectrode insulator.

Figure 4 shows the distribution of static pressure along the length of the MHD channel when testing it in idle mode, when using recesses (caverns) as interelectrode insulators.

Figure 5 shows the distribution of static pressure along the length of the MHD channel in the operating mode when recesses (cavities) are used as interelectrode insulators.

Figure 6 shows the distribution of the total pressure behind the shock wave p ' ₀ across the output section of the nozzle, respectively, when the MHD channel is idle and in acceleration mode.

In figures 4-6 are shown for comparison, similar results obtained when testing the MHD channel with smooth walls (prototype).

причем поверхность дна выемки выполнена из жаростойкого изоляционного материала в виде цилиндра или цилиндрической поверхности таким образом, что стенки электрода являются касательными к его поверхности. Радиус цилиндрической выемки равен L/2.MHD channel (figure 1) contains two electrode 1 and two insulating 2 walls located opposite each other, adjacent to the poles of the electromagnet 3. The electrode wall (figure 2) contains an insulating block 4 with copper electrodes 5 mounted on it, between which are the interelectrode insulators 6. The interelectrode insulators 6 are made in the form of recesses 7 (Fig. 3) between adjacent electrodes with a width L greater than that at which the breakdown of the gas gap occurs with gas-dynamic parameters of the boundary layer and bina h, in proportion to the width of the recess

moreover, the surface of the bottom of the recess is made of heat-resistant insulating material in the form of a cylinder or a cylindrical surface so that the walls of the electrode are tangent to its surface. The radius of the cylindrical recess is L / 2.

The device operates as follows. Under the action of the flow in the recess 7 located between the electrodes 5, a vortex gas movement is formed in such a way that the velocities of the boundary layer and the vortex motion on the outer side of the recess practically coincide. Since the bottom of the recess is made in the form of a cylindrical surface, the walls of which are tangent to the walls of the electrodes 5, then in the case of vortex movement in the recess 7 there are no additional vortices that would introduce additional aerodynamic drag and thereby increase the speed difference between the rotating gas region and the boundary layer. The region of gas vortex motion from opposite sides is bounded by insulating walls 2. Since the static pressure on the axis of the vortex is much lower than at its boundaries, gas flows from the outer region of the vortex to its axis on the insulating walls 2. This phenomenon, widely known in the art, is called secondary gas flow. Secondary flows form two jets of gas flowing along the axis of the vortex from the insulating walls 2 and directed towards each other. During their interaction, reverse flows are formed already along the outer surface of the vortex from the place of their interaction, located almost in the center of the interelectrode insulator 6 back to the walls. Under the influence of this flow, the microarcs arising on the electrodes 5 will be displaced to the walls, fall into the cold parietal boundary layer and die out there. As a result of the effect of an electric discharge, arcs will arise again and again, drift to the walls, etc. Due to their movement, the surface of the electrode 5 will not collapse as intensely as if the arcs were in one place.

где ω - циклотронная частота, τ - среднее время между двумя электронными столкновениям, n_е - число заряженных частиц, С_е - их средняя скорость, m_е -масса электрона, Q - эффективное сечение передачи импульса при столкновениях. Для величины напряжения потенциал Холла E_H между соседними электродами имеем:The width and depth of the excavation are determined by the magnitude of the Hall voltage between the electrodes of the MHD channel. The distance between the edges of the recess (cavity) formed by adjacent electrodes should be greater than the distance at which, under the action of the Hall potential, an electrical breakdown of this gas gap occurs. The value of the Hall potential is calculated from the known values of the magnetic field applied to the MHD channel and is determined by the value of the Hall parameter β

where ω is the cyclotron frequency, τ is the average time between two electron collisions, n _e is the number of charged particles, C _e is their average speed, m _e is the electron mass, Q is the effective momentum transfer cross section for collisions. For the voltage value, the Hall potential E _H between adjacent electrodes has:

- вектор плотности тока в пространстве между электродами, В - магнитная индукция, L - расстояние между соседними электродами. Используя известные методы, описанные в литературе, см. например Y.K.Messerle "Magnetohydrodynamic Electrical Power Generation" John Wiley and Sons Ltd Unesco Energy Engineering series 1995; R.J. Rosa. Magnetohydrodynamic Energy conversion Hemisphere Publishing Corporation Washington 1987, можно рассчитать значение Е_H применительно к конкретному МГД-каналу или электротехническому устройству.where σ is the gas conductivity in the vicinity of the recess (cavity), β is the Hall parameter in the same place, respectively

is the current density vector in the space between the electrodes, B is the magnetic induction, L is the distance between adjacent electrodes. Using well-known methods described in the literature, see for example YKMesserle "Magnetohydrodynamic Electrical Power Generation" John Wiley and Sons Ltd Unesco Energy Engineering series 1995; RJ Rosa. Magnetohydrodynamic Energy conversion Hemisphere Publishing Corporation Washington 1987, the value of E _H can be calculated for a particular MHD channel or electrical device.

Distinctive features of the claimed invention are:

- implementation of the interelectrode insulator in the form of a recess (cavity) with the bottom wall (bottom) of the insulator;

- giving the bottom wall (bottom) the shape of a cylindrical surface that fits into the recess without a gap;

- the choice of the distance between the electrodes based on the conditions of the absence of breakdown of the gas gap under the action of the Hall potential;

где h - глубина выемки, L - ее ширина.- the choice of the ratio between the depth of the excavation and its transverse size in the ratio

where h is the depth of the recess, L is its width.

The implementation of the interelectrode insulator in the form of a recess (cavity) will avoid contact of the microarc at the end of the electrode with the insulator on the one hand, and on the other hand, the location of the insulator at the bottom of the recess in the region where the heat flux is much smaller, which will increase the MHD channel resource, since it to a certain extent depends on the state of electrical insulating structural elements (insulators). Perturbations in the flow introduced by a recess (cover) on the surface of the MHD device are largely determined by the choice of relations between the characteristic geometrical parameters of the recess: its width, depth, and the ratio of these values to the characteristic size of the boundary layer on the surface at the location of the recess. Minimum perturbations of the flow take place if its geometrical dimensions are chosen in such a way that the type of flow, called the “open recess”, is realized, while the ratio of the thickness of the boundary layer 5 to the depth of the recess h should be δ / h <1. In this case, the boundary layer at the leading edge of the recess is torn off and forms, like a jet, a free viscous mixing layer, usually thin. In the recess itself, there is a circulation flow around the axis of the recess. In order to exclude the formation in the corners of the recess, which is the junction of the electrode-insulator of additional vortex systems, it is proposed to make the bottom of the recess in the form of a cylindrical surface so that the electrode walls are tangent to it. In this case, the total pressure loss in the vortex induced in the recess by an external flow will be minimal. Due to the presence of lateral insulating walls of the MHD channel, restricting a recess (cavity) from opposite sides, secondary vortex flows arise in it, directed along the axis of the recess from the side walls to its center towards each other. When they collide in the center of the recess along the edge of the electrodes, reverse flows arise, directed from the center to the insulating walls, thereby shifting the microarcs from their location to the insulating walls to the region of the cold boundary layer, where they should go out.

The choice of the distance between the electrodes, ensuring the absence of electrical breakdown under the action of the Hall potential, was considered above.

The choice of the ratio between the depth of the recess and its transverse size was due to the analysis of the magnitude of the heat fluxes to the electrodes, to the wall of the recess along the bottom of the recess. Minimum heat fluxes occur in the case of "open excavation". At the point of attachment of the detached flow at the rear electrode, the heat flux is 1.2-1.4 times greater than the heat flux to the smooth wall. Since the electrode is metal with good thermal conductivity, such an increase in heat flux does not harm the structure.

The following is a description of the experimental MHD channel and the results of its tests, made in accordance with the claimed invention.

In the new design of the electrode wall, a part of the interelectrode insulator directly adjacent to the electrodes is replaced by a recess (cavity), the width of which is selected from the condition that there is no electrical breakdown between the electrodes in the gas gap. The value of the breakdown voltage was calculated based on the static pressure in the flow p _st , the gas temperature in the boundary layer near the wall T _st , the presence of additives of easily ionizable elements in it and the magnetic field hindering breakdown B = 2.5 T. The value of the Hall potential was determined from the known relations for the MHD channel with a known magnitude of the magnetic induction B. The depth of the cavity is chosen close to its width, as in this case a single-vortex flow system is realized in it. In this example, the cavity width was chosen equal to 2.5 mm and the same depth. Other solutions are possible depending on the flow characteristics. Below are the results of an experimental check of the operation of the MHD channel with an interelectrode insulator made in the form of a recess, the bottom of which is made of an electrical insulator and compared with the results of tests of a smooth channel under the same conditions. Experiments to study the effect of interelectrode grooves on the gas-dynamic and electrical characteristics of MHD channels were carried out with a channel 555 mm long with 60 pairs of electrodes, an input cross section F _BX = 15 × 5 mm and an exit cross section 15 × 20 mm. At the exit from the MHD channel, nozzles with an exit cross section F _exx = 25 × 25 mm and 80 × 80 mm were docked. The distribution of static pressure along the length of the MHD channel, the total gas pressure behind the shock wave at the exit from the MHD channel and the exit from the nozzle, were measured. An electric arc heater with braking parameters T ₀ = 3500 K, p ₀ = 2 ata was used as a source of conductive gas. The number M at the entrance to the channel was equal to M = 2. The flow rate of working gas - air was 15 g / s. An alkali element additive was introduced into the stream in the form of a eutectic in the amount of 1.5% of the gas mass flow rate. The magnitude of the magnetic field was 2.47 Tesla. The experiments were carried out both in idle mode and in the MHD gas acceleration mode. According to these tests conducted on a model channel with interelectrode recesses, the difference in the distribution of static pressure along the length of the MHD channel when testing it at idle mode, from the corresponding pressure distribution along the length of the MHD channel with smooth walls in the same mode is not more than 8% for example, see figure 5.

Also insignificant was the difference in the distribution of static pressure along the length of the MHD channel when operating in the flow acceleration mode, for example, see Fig. 6. In the MHD flow acceleration mode, the total pressure behind the shock wave for a channel with interelectrode insulators was less than 9% corresponding to a smooth channel (see Fig. 4). A comparison of the pulsation characteristics of the electric discharge between the electrodes of the MHD channel with smooth walls and with recesses instead of the insulator showed their almost complete identity. Inspection of the state of interelectrode insulators and the channel as a whole showed that after 15 tests no damage to its structure was noted. At the same time, when testing a channel with smooth interelectrode insulators, their destruction took place after 5-7 starts.

Claims (1)

Magnetogasdynamic channel, consisting of insulating and electrode walls, including electrodes and interelectrode insulators, characterized in that the interelectrode insulators are made in the form of grooves between adjacent electrodes with a width L greater than that at which a breakdown of the gas gap occurs with gas-dynamic parameters of the boundary layer and depth h in the ratio

moreover, the bottom of the recess is made of heat-resistant insulating material, and the surface of the bottom of the recess is made cylindrical so that the walls of the electrode are tangent to it.

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