CN113676827A - Direct-blowing type variable frequency oscillation experimental device for measuring frequency response function of solid propellant - Google Patents
Direct-blowing type variable frequency oscillation experimental device for measuring frequency response function of solid propellant Download PDFInfo
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- 230000010355 oscillation Effects 0.000 title claims abstract description 182
- 238000005316 response function Methods 0.000 title claims abstract description 48
- 239000004449 solid propellant Substances 0.000 title claims abstract description 38
- 238000007664 blowing Methods 0.000 title claims abstract description 26
- 239000003380 propellant Substances 0.000 claims abstract description 180
- 230000007704 transition Effects 0.000 claims abstract description 10
- 230000002146 bilateral effect Effects 0.000 claims abstract description 5
- 239000003814 drug Substances 0.000 claims description 99
- 238000002485 combustion reaction Methods 0.000 claims description 98
- 229940079593 drug Drugs 0.000 claims description 76
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- 238000002474 experimental method Methods 0.000 description 10
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- 238000010586 diagram Methods 0.000 description 4
- 239000002360 explosive Substances 0.000 description 4
- 239000000843 powder Substances 0.000 description 4
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- 238000011160 research Methods 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- 230000009286 beneficial effect Effects 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000004880 explosion Methods 0.000 description 2
- 239000002737 fuel gas Substances 0.000 description 2
- 230000003534 oscillatory effect Effects 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 230000000007 visual effect Effects 0.000 description 2
- 229920002121 Hydroxyl-terminated polybutadiene Polymers 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
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Abstract
The invention discloses a direct-blowing type oscillation experimental device with continuously variable frequency for measuring a frequency response function of a solid propellant, which comprises a pressure oscillation source, a propellant clamp and a signal source; the two pressure oscillation sources are arranged in bilateral symmetry, and each pressure oscillation source comprises a loudspeaker and a loudspeaker convergence section. The two loudspeakers are horizontal and are arranged symmetrically left and right, and the bell mouths are arranged oppositely; the horn mouth end of each loudspeaker is detachably connected and communicated with the loudspeaker convergence section along the axial direction; the loudspeaker convergence section is in a shell shape with two open ends, the loudspeaker convergence section is in a tubular shape from the loudspeaker connecting end to the far end by outward expansion smooth contraction transition, the loudspeaker convergence section is in a conical tubular shape from the tubular shape to the far end, and the outlet of the conical tubular shape is in a flat shape; there is a phase difference between the two loudspeakers. The experimental device can generate pressure oscillation with large amplitude and long-time stability.
Description
Technical Field
The invention belongs to the technical field of solid rocket engine combustion tests, and particularly relates to a direct-blowing type variable frequency oscillation experimental device for measuring a frequency response function of a solid propellant.
Background
Combustion instability is typically due to coupling of the acoustic field of the combustion chamber with the periodic heat release of the propellant, causing pressure oscillations in the combustion chamber. Combustion instability has a very severe impact on the operation of rocket engines and turbomachines, with light engines not operating at a predetermined thrust and pressure and heavy combustion chamber casing damage and explosion.
Thermoacoustic instability is one of the most prominent forms of combustion instability. According to the Rayleigh criterion, if the unstable heat release is in phase with the oscillating acoustic field of the combustion chamber, coupling will occur, resulting in thermo-acoustic instability. A common form of instability in rocket engines is nonlinear combustion instability, where the primary factor of influence is the nonlinear combustion response of solid propellants.
In order to research the nonlinear combustion response of the solid propellant, a pressure coupling response function of the propellant is required to be obtained, and is defined as the ratio of the relative change rate of pressure to the relative change rate of burning rate. Therefore, it is necessary to simulate the pressure oscillation when the combustion instability of the solid rocket engine occurs by a certain method. The conventional method includes: by introducing external excitation, such as a pulse triggering method, a small rocket engine is used for ignition to generate fuel gas with certain pressure and temperature, and the fuel gas is blown off an explosion aluminum sheet and then enters a closed tester cavity to obtain required pressure oscillation; pressure oscillation with large amplitude and long duration can be generated by reciprocating motion of the motion assembly, such as a rotary valve method and a piston method, but the operation is complicated, the motion assembly needs to be accurately controlled, the pressure oscillation frequency is low, and the maintenance cost is high.
Disclosure of Invention
The invention aims to provide a direct-blowing type oscillation experimental device with continuously variable frequency for measuring a frequency response function of a solid propellant, which can generate pressure oscillation with large amplitude and long-term stability.
The invention adopts the following technical scheme: a direct-blowing type oscillation experimental device with continuously variable frequency for measuring a frequency response function of a solid propellant comprises a pressure oscillation source, a propellant clamp and a signal source; the two pressure oscillation sources are arranged in bilateral symmetry, and each pressure oscillation source comprises a loudspeaker and a loudspeaker convergence section which are connected.
The two loudspeakers are arranged in bilateral symmetry, and the horn mouths are arranged oppositely; the horn mouth end of each loudspeaker is detachably connected and communicated with the loudspeaker convergence section along the axial direction; the phase difference between the two loudspeakers is 180 deg.. The loudspeaker convergence section is in a shell shape with two open ends, and consists of a first convergence section, an equal-straight transition section and a second convergence section which are sequentially connected from the loudspeaker connecting end to the far end, the inner cavity of the first convergence section is in a bell-shaped structure, and the connecting end of the first convergence section and the loudspeaker is in an outward expansion shape. The second convergent section is gradually transited from a large diameter to a small diameter from a connecting end of the second convergent section and the equal straight transition section to a far end, and an inner cavity of the second convergent section is contracted into a flat rectangle at an outlet of the far end of the second convergent section.
The propellant clamp is arranged below the outlet between the convergence sections of the two loudspeakers and used for clamping a vertical propellant medicine strip and driving the propellant medicine strip to ascend or descend in the vertical direction between the two pressure oscillation sources;
the signal source is respectively connected with the two loudspeakers to form a loop and is used for transmitting the electric signals with given frequency and amplitude to the loudspeakers;
the loudspeaker is used for receiving the electric signal input by the signal source, and a vibrating diaphragm of the loudspeaker periodically oscillates at a frequency consistent with the input frequency of the signal source to generate periodic pressure oscillation and transmit the periodic pressure oscillation; the loudspeaker convergence section is used for converging the periodic pressure oscillation generated by the loudspeaker to the far-end outlet, and the periodic pressure oscillation is coupled near the combustion surface of the propellant drug strip so as to generate larger pressure oscillation near the combustion surface and act on the combustion flame of the propellant drug strip in a combustion state.
And a collecting device is arranged at the outlet of the convergence section of the loudspeaker and close to the upper end surface of the propellant drug strip, and the collecting device is used for collecting the pressure oscillation value near the propellant drug strip.
Furthermore, a loudspeaker protection cover covers the outer circumference of the loudspeaker shell, and a closed cavity is formed between the loudspeaker protection cover and the outer wall of the loudspeaker and used for preventing pressure oscillation generated by the loudspeaker from dissipating to the atmospheric environment.
Further, the far-end outlets of the tube bodies of the two loudspeaker convergence sections are on the same axis, wherein the sectional area of the outer flared end of the loudspeaker convergence section is consistent with the bottom area of the diaphragm of the loudspeaker.
Furthermore, the distance between the outlets of the two speaker convergence sections and the propellant drug strip is equal, and the central axis of the speaker convergence sections is flush with the upper surface of the propellant drug strip.
Furthermore, the acquisition device is a dynamic pressure sensor, and the number of the dynamic pressure sensors is two, and the two dynamic pressure sensors are connected with the data acquisition system.
Furthermore, a lifting platform is arranged below the propellant clamp and used for driving the propellant clamp to ascend or descend in the vertical direction so as to enable pressure oscillation generated by the pressure oscillation source to act on different positions of the propellant drug strips.
Furthermore, the signal source comprises a signal generator and a power amplifier which are connected, wherein the power amplifier is also connected with the loudspeaker; the signal generator generates an electric signal with specific frequency and amplitude; the power amplifier is used for receiving the electric signal, amplifying the power of the electric signal and transmitting the power to the loudspeaker.
The invention also discloses a working mode of the direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant, which comprises the following steps:
and 7, deriving pressure data of the dynamic pressure sensor obtained by the data acquisition system in the whole combustion process, and calculating pressure oscillation amplitude, frequency and phase information near the propellant drug strip.
The invention also discloses a calculation method of the pressure coupling response function of the solid propellant, the direct-blowing type oscillation experiment device with continuously variable frequency for measuring the frequency response function of the solid propellant is adopted, and the high-speed camera is arranged in front of or behind the propellant stick, and the calculation method comprises the following steps:
step a, recording images of propellant drug strips burning for a period of time under pressure oscillation by using a high-speed camera, and selecting a moment t0And t0+ delta t, obtaining combustion surfaces at two different moments, and equidistantly selecting a plurality of sampling points on the combustion surfaces; then respectively calculating the vertical displacement delta x corresponding to each sampling point at two different timesi(i＝1,2,...,10)
Δxi＝niα (1)；
Wherein: n isiThe number of vertical displacement pixel points corresponding to the sampling points at two different moments;
alpha the actual single pixel element size of a high speed camera,
the burning rate of the propellant drug strip is calculated by the following formula:
selecting propellant drug strip combustion images at a plurality of moments by taking delta t as a time interval, calculating the combustion speed at different moments, obtaining the oscillation combustion speed change of the propellant within a period of time, and obtaining the disturbance quantity of the oscillation combustion speed of the propellant drug strips
And average amount
Step b, selecting time t0And t0Calculating the mass flow rate from the pressure oscillations in the dynamic pressure sensor over + Δ t time
Wherein: rho is the density of the propellant, r is the burning speed of the propellant, and A is the burning surface area of the propellant;
and respectively expressing the density and the burning rate of the propellant as the addition form of the average quantity and the disturbance quantity, and obtaining the following result:
neglecting the second order small amount ρ 'r', it can be obtained
Then:
wherein: gamma is a correction coefficient;
the resulting pressure coupling response function is:
the invention has the beneficial effects that: 1. the method adopts a loudspeaker as an oscillation source for generating pressure oscillation, is used for generating pressure oscillation which can be stable for a long time and has larger amplitude near the propellant drug strip, can better simulate the pressure oscillation environment when an actual engine generates unstable combustion, researches the combustion process of the propellant drug strip under the pressure oscillation, and calculates to obtain the pressure coupling response function of the propellant. Meanwhile, the method has the advantages of low cost, simplicity in operation and the like. 2. The propellant medicine strip integral lifting mechanism can flexibly adjust the vertical positions of the propellant combustion flame acted by the pressure oscillation, such as the top, the middle and the root of the flame, and can flexibly adjust the horizontal distance between the outlet of the pressure oscillation source convergence section and the propellant medicine strip and the position of the propellant medicine strip in the vertical direction, thereby researching the influence of different pressure oscillation action positions on the propellant medicine strip combustion. 3. The pressure oscillation amplitude generated by the loudspeaker can be continuously adjusted, so that the frequency response characteristic of the solid propellant under the pressure oscillation of different frequencies can be conveniently researched.
Drawings
Fig. 1 is a schematic structural diagram of a direct-blowing variable frequency oscillation experimental device for measuring a frequency response function of a solid propellant.
Fig. 2 is a graph showing the fluctuation of the combustion flame of the bi-component propellant stick over time under pressure oscillation.
Fig. 3 is a graph showing the oscillation of combustion flame of bi-component propellant strips under different power amplifier amplitudes.
Fig. 4 is a schematic diagram of the configuration of the inner cavity of the convergent section of the loudspeaker.
Fig. 5 is a schematic diagram of the calculation of the burning rate of the propellant powder strip.
Wherein: 1. a speaker protective cover; 2. a speaker convergence section; 3. a speaker; 4. a propellant charge; 5. a propellant clamp; 6. a lifting platform; 7. a power amplifier; 8. a signal generator; 9. a pressure oscillation source; 10. a signal source; 11. a dynamic pressure sensor; 12. a data acquisition system.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
The invention relates to a direct-blowing type oscillation experimental device with continuously variable frequency for measuring a frequency response function of a solid propellant, which comprises a pressure oscillation source 9, a propellant clamp 5 and a signal source 10, as shown in figure 1; the two pressure oscillation sources 9 are arranged in bilateral symmetry, and each pressure oscillation source 9 comprises a loudspeaker 3 and a loudspeaker convergence section 2 which are connected with each other.
The two loudspeakers 3 are horizontal and symmetrically arranged from left to right, and the bell mouths are oppositely arranged; the horn mouth end of each loudspeaker 3 is detachably connected and communicated with the loudspeaker convergence section 2 along the axial direction; as shown in fig. 4, the loudspeaker convergence section 2 is a casing with two open ends, and is composed of a first convergence section 2-3, an equal straight transition section 2-2 and a second convergence section 2-1 which are connected in sequence from the connection end of the loudspeaker 3 to the far end, the inner cavity of the first convergence section 2-3 is a bell-shaped structure, the longitudinal section of the inner cavity of the first convergence section 2-3 is composed of two tangent arcs, and the connection end of the first convergence section and the loudspeaker 3 is in an outward expansion shape; with the bell-type configuration, the pressure oscillations generated by the loudspeaker 5 have minimal reflection losses in the convergent section 2 of the loudspeaker.
The second convergent section 2-1 is gradually transited from a large diameter to a small diameter from the connecting end of the second convergent section 2-1 and the straight transition section 2-2 to the far end, and the inner cavity of the second convergent section 2-1 is contracted into a flat rectangle at the outlet of the far end so as to be consistent with the shape of the propellant drug strip 4. The second convergent section 2-1 is provided to further converge the pressure oscillations generated by the loudspeaker 5 and to transition the inner cavity of the convergent section, which is circular in cross-section, to the outlet of the convergent section, which is rectangular in cross-section and similar in size to the propellant stick. The function of the equal straight transition section 2-2 is that the transition between the first convergence section 2-3 and the second convergence section 2-1 is smoother, and the reflection loss is reduced. The length of the segment is short to reduce losses during the transfer of pressure oscillations. The structural design of the loudspeaker convergence section 2 enables the pressure oscillation generated by the loudspeaker 3 to act on the propellant drug strip to the maximum extent, thereby being more beneficial to researching the combustion characteristic of the propellant drug strip under the large-amplitude pressure oscillation.
In operation, a phase difference exists between the two loudspeakers 3. Preferably, the phase difference between the two loudspeakers 3 is 180 °, when the loudspeaker 3 at one end generates the maximum amplitude at the same position, the loudspeaker 3 at the other end is reversed, and the maximum amplitude can also be generated at the same position, and at this time, the oscillation amplitudes of the two loudspeakers 3 are superposed to the maximum.
A loudspeaker protection cover 1 is arranged outside the loudspeaker 5, and a closed cavity is formed between the loudspeaker protection cover 1 and the outer wall of the loudspeaker 5 and used for preventing pressure oscillation from dissipating to the atmospheric environment.
The far-end outlets of the tube bodies of the two loudspeaker convergence sections 2 are on the same axis, wherein the sectional area of the flaring ends of the loudspeaker convergence sections 2 is consistent with that of the diaphragm of the loudspeaker 3. The shape of the cross-section of the outlet of the convergent section 2 of the loudspeaker can be varied to study the effect of different outlet shapes on pressure oscillations. The loudspeaker 3 and the loudspeaker convergence section 2 are fixedly arranged on the vertical frame, and the horizontal distance between the outlet of the loudspeaker convergence section 2 and the propellant explosive strip 4 can be adjusted by adjusting the position of the vertical frame.
The loudspeaker 3 is used for receiving an electric signal input by a signal source 10, and a vibrating diaphragm of the loudspeaker 3 periodically oscillates at a frequency consistent with the input frequency of the signal source 10 to generate periodic pressure oscillation and transmit the pressure oscillation; the loudspeaker convergence section 2 is used for converging the periodic pressure oscillation generated by the loudspeaker 3 to a far-end outlet, and is coupled near the combustion surface of the propellant drug strip 4 so as to generate larger pressure oscillation near the combustion surface and act on the combustion flame of the propellant drug strip 4 in a combustion state.
And a collecting device is arranged at the outlet of the loudspeaker convergence section 2 and is positioned close to the upper end surface of the propellant drug strip 4, and the collecting device is used for collecting the pressure oscillation value near the propellant drug strip 4.
The propellant clamp 5 is arranged below the outlet between the two loudspeaker convergence sections 2 and used for clamping the propellant drug strips 4 in the vertical direction and driving the propellant drug strips 4 to ascend or descend in the vertical direction between the two pressure oscillation sources 9; the propellant clamp 5 can be selected from a wide variety of types, such as a cylindrical housing with an open top. A lifting platform 6 is arranged below the propellant clamp 5, and the lifting platform 6 is used for driving the propellant clamp 5 to ascend or descend in the vertical direction.
The device adopts a signal source 10 to provide electric signals, the signal source 10 is respectively connected with two loudspeakers 3 to form a loop, and the loop is used for transmitting the electric signals with given frequency and amplitude to the loudspeakers 3. Specifically, the signal source 10 includes a signal generator 8 and a power amplifier 7 connected to each other, wherein the power amplifier 7 is further connected to the speaker 3; the signal generator 8 generates an electrical signal of a specific frequency and amplitude; the power amplifier 7 is used for receiving the electric signal, amplifying the power of the electric signal and transmitting the amplified power to the loudspeaker 3.
Dynamic pressure sensors 11 are arranged at the outlets close to the two loudspeaker convergence sections 2, each dynamic pressure sensor 11 is connected with a data acquisition system 12, and the dynamic pressure sensors 11 are used for measuring and acquiring pressure oscillation values near the propellant drug strips 4. Two dynamic pressure sensors 11 are arranged, and one dynamic pressure sensor is arranged at each outlet of the left and right loudspeaker convergence sections 2.
The outlets of the two loudspeaker convergence sections 2 are equal in distance to the propellant drug strip 4, if the distances are both 5cm, the propellant drug strip 4 is used as a center and is symmetrically distributed. This distance also can adjust according to the experiment demand, and is nearer apart from the export of speaker convergence section 2, and pressure oscillation amplitude is bigger, and the axis of speaker convergence section 2 flushes with the upper surface of propellant medicine strip 4. To ensure that the pressure oscillations act on the combustion flame.
The invention also discloses a working mode of the direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant, which comprises the following steps:
And 3, arranging the dynamic pressure sensor 11 at a symmetrical position of the propellant drug strip 4 at a distance of 2cm and near an outlet of the loudspeaker convergence section 2.
The electrical signal output by the signal generator 8 has multiple forms according to the specific requirements of the experiment, and the main forms are as follows:
firstly, signals with different waveforms, such as sine waves, sawtooth waves, square waves and the like, are used for researching the influence of pressure oscillation with different waveforms on the combustion of propellant medicine strips;
signals of different frequencies comprise:
2.1 when the influence of pressure oscillation under single frequency on propellant stick combustion is researched, the output frequency of the signal generator 8 is set to be the required single frequency;
2.2 when the frequency corresponding to the peak of the combustion response function of the propellant is unknown, the signal generator 8 needs to be set to a frequency sweep over a range of frequencies to determine the peak frequency of the combustion response function: the schematic diagram of the frequency sweeping scheme is shown in fig. 2, where an initial frequency f of the frequency sweep is set, and the input frequency is increased to 2f, 3f, and the like step by step, that is, the frequency multiplication corresponding to the initial frequency. The scanning time corresponding to each scanning frequency is set to be 10 times of the corresponding scanning period, namely, the data of the combustion speed fluctuation of the propellant drug strip under the corresponding frequency is obtained. The total time of the sweep is the minimum of the time of propellant combustion and the recording time of the high speed camera recording system. For example, when the initial scanning frequency is set to 50Hz, the frequency multiplication is 100Hz, 150Hz … respectively, and the corresponding scanning time is 0.2s,0.1s,1/15s … respectively. And calculating the pressure coupling response function of the propellant under different frequencies, reducing the frequency sweep range near the frequency of the larger pressure coupling response function, repeating the process, and further determining the corresponding frequency when the coupling response function is maximum through the frequency sweep.
2.3 when the frequency corresponding to the peak value of the propellant combustion response function is known, if the influence of the peak value frequency on the propellant medicine strip combustion needs to be researched, adopting the method in 2.1; if it is desired to investigate the peak frequency and its frequency multiplication to promote oscillatory combustion of the propellant charge, a frequency sweep strategy similar to that of 2.2 is employed, in which case the initial frequency f is set to the frequency corresponding to the peak of the propellant combustion response function.
and 6, igniting the propellant drug strip (4) when the pressure oscillation data peak value near the propellant drug strip (4) does not change, and continuously measuring the pressure data and the combustion surface retreating image data near the propellant drug strip (6).
The following requirements are made on the relevant parameters of the high-speed camera recording system:
firstly, resolution ratio: the resolution of the high speed camera should be high enough to provide a sufficient field of view, i.e., to photograph a sufficient range of propellant firing surfaces. Typical camera resolutions are typically 1024 pixels by 1024 pixels or 2048 pixels by 2048 pixels.
Secondly, pixel size: i.e. the size of a single pixel of the camera light sensitive element. When the areas of the photosensitive elements are the same, the smaller the pixel size is, the larger the number of pixels is, and the higher the resolution is. A typical high speed camera pixel size is 10 μm.
Thirdly, sampling rate: the number of pictures recorded by the high speed camera per unit time. According to the Nyquist sampling theorem, the sampling frequency needs to be more than 2 times greater than the highest frequency of the signal. Therefore, the sampling rate of the high-speed camera needs to be set to 10 times or more of the highest oscillation frequency to obtain an oscillation burning rate result with sufficient accuracy. For example, when the maximum oscillation frequency of the speaker 3 is 500Hz, the sampling rate of the high-speed camera should be set to 5000Hz or more;
fourthly, sampling time: depending on the memory size of the memory module of the high speed camera itself and the sampling rate. The image recorded by the high-speed camera recording system is directly stored in a storage module in the high-speed camera and then is exported to other storage media. Under the condition that the memory sizes of the storage modules are the same, the higher the sampling rate is, the shorter the sampling time is. Typical sampling time of the high-speed camera is about 5-20 s;
fifthly, magnification: after the high-speed camera is externally connected with a lens, a shot object, namely the propellant medicine strip 4, can be amplified, and the pixel size of the unit area recorded by the high-speed camera is the actual pixel size at the moment. The magnification of the camera imaging system is equal to the camera pixel size/actual pixel size. The appropriate magnification needs to be chosen to meet the measurement requirements of the experiment. The selection method of the magnification is as follows:
for a typical three-component composite propellant (AP/HTPB/Al), the combustion speed r at normal pressure is approximately equal to 5-20 mm/s. According to the output frequency of the signal generator 8, in order to accurately obtain the variation condition of the burning rate in an oscillation period, n counting points are required to be selected in each period for calculating the burning rate (n is more than or equal to 5). And if the maximum output frequency of the signal generator 8 is fm and the corresponding minimum pressure oscillation period is 1/fm, the minimum sampling time interval delta t is 1/nfm, namely, an image recorded by a high-speed camera is selected every delta t to be used for calculating the burning rate. A typical image of a fire face recorded by an actual high-speed camera recording system is shown in fig. 3, where the grid represents the pixel points and the filled area is the actual fire face. It can be seen that for each column of pixels, the actual fire plane may occupy one pixel (e.g., column 1), two pixels (e.g., column 3), or more pixels. Meanwhile, it should be considered that the actual combustion surface is not completely moved back and forth, and the combustion surface perpendicular to the field direction of the camera, i.e. the combustion surface in front of and behind the focal plane of the camera, may also affect the selection of the actual combustion surface. Therefore, there is a certain error in determining the position of the combustion face. The error in selecting the fire surface is defined as the positive and negative pixels of the visual fire surface. For example, for column 1, where the visual face position is in row 0, the error is selected to be +1, +2, -1, -2.
Within the sampling time interval Δ t, if the distance between two burning surface images in the same row at two moments obtained by the high-speed camera recording system is too close, the error of the burning rate calculation is large, and even the burning rate obtained by calculation is possibly 0. This is due to the fact that the actual pixel size is too large, resulting in too small a distance traveled by the same row of combustion faces during the sampling interval Δ t. Therefore, within the sampling time interval Δ t, there is a minimum distance between two burning planes, and considering the error in the burning plane selection, the minimum distance that can identify the two burning planes is defined as 4 pixels, i.e., Δ x ═ 4xp, where xp is the actual unit pixel size. Only if the minimum distance is larger than the minimum distance, the accurate burning speed of the propellant can be obtained through calculation, the requirement that the delta x/delta t is less than or equal to r is met, and the solution is carried out to obtain the value that the xp is less than or equal to r/(4 nfm). Thus, the actual magnification is ultimately expressed as M ═ xreal/xp ≧ (4nfmxreal)/r, where xreal is the camera pixel size. Selecting a typical value: the pixel size xreal of the camera is 10 mu M, the burning speed r is 10mm/s, the number n of counting points is 10, the maximum loudspeaker pressure oscillation frequency fm is 100Hz, the actual pixel size xp obtained through calculation is 2.5 mu M, and the magnification factor M is 4 times. For a high speed camera recording system with a resolution of 2048 pixels by 2048 pixels, the recording can record a length of 2048 × xp-5.12 mm. The time required for the propellant stick to burn for 5.12mm is 0.512s calculated by the average burning rate of 10mm/s, the sampling time of the high-speed camera recording system is greater than the value (and the higher sampling time is required according to actual conditions considering that the high-speed camera recording system needs to start recording before the propellant stick is ignited), and the total time output by the signal generator is less than the value so as to record the burning condition of the propellant stick to the maximum extent. Typically, the high-speed camera magnification is selected to be 5-10 times according to actual requirements.
And 7, deriving pressure data of the dynamic pressure sensor 11 obtained by the data acquisition system 12 in the whole combustion process, and calculating pressure oscillation amplitude, frequency and phase information near the propellant drug strip 4 by utilizing postprocessing software such as Origin and the like. And deriving data of propellant burning surface retreating within a period of time acquired by a high-speed camera, and calculating the burning speed change of the propellant.
In order to verify the practical effect of the direct-blowing type frequency continuously variable pressure oscillation experimental device for measuring the frequency response function of the solid propellant and develop the feasibility of the propellant drug strip oscillation combustion research, the direct-blowing type frequency continuously variable pressure oscillation experimental device for measuring the frequency response function of the solid propellant is adopted to measure the combustion flame fluctuation condition of the propellant drug strip in a period of time under the oscillation condition, and the experimental device is concretely as follows:
selecting a double-component propellant medicine strip for experiment, wherein the adopted propellant medicine strip 4 is a slender strip, and the size is 5mm multiplied by 20 mm; the distance between the dynamic pressure sensor 11 and the propellant medicine strip 4 is 2 cm; the outlet of the convergent section 2 of the loudspeaker in the pressure oscillation source 9 is 5cm away from the propellant powder strip.
The invention relates to a direct-blowing type oscillation experimental device with continuously variable frequency for measuring a frequency response function of a solid propellant and a working mode, wherein the parameters are set as follows: the horizontal distance between the outlets of the speaker convergence sections 2 at the left end and the right end and the propellant medicine strip 4 is 5cm, and the top end of the propellant medicine strip 4 is flush with the central axis of the speaker convergence section 2; the oscillation frequency of the loudspeaker 5 is chosen to be 100Hz and the amplitude of the power amplifier 7 is set to 20 for a total amplitude of 60. The high-speed camera is arranged on the front side or the rear side of the propellant drug strip 4, the experiment is completed according to the working mode of the invention, the high-speed camera is adopted to shoot the fluctuation situation of the combustion flame of the bipropellant in a period of time under the oscillation condition, as shown in figure 2, (a) - (f) are the combustion flame of the bipropellant in a period of time recorded by the high-speed camera, and the sampling time interval delta t is 1/400 s. It can be seen that under pressure oscillations the flame exhibits a periodic oscillation. The change of the flame gray value in the image acquired by the high-speed camera in the period of time is extracted, FFT analysis is carried out on the gray value, the periodic swinging frequency of the flame is calculated to be 100.2Hz and is consistent with the pressure oscillation frequency generated by the loudspeaker, and the forced vibration phenomenon of the flame caused by the pressure oscillation generated by the loudspeaker is explained, namely the flame periodically swings at the frequency consistent with the input frequency of the loudspeaker 3. Therefore, the oscillation frequency of the propellant drug strip 4 can be changed by adjusting the oscillation frequency of the loudspeaker 3, so that the combustion characteristics of the propellant under different pressure oscillation frequencies can be researched.
Meanwhile, in the experiment, the influence of different power amplifier amplitudes on the flame fluctuation of the bipropellant under the oscillatory combustion is also considered. The amplitudes of the power amplifiers 7 used are 0, 14, 30 and 46, respectively; typical results of combustion of the bipropellant at different power amplifier amplitudes are shown in figure 3. As can be seen from fig. 3, as the amplitude of the power amplifier is increased, the oscillation of the flame of the two-component propellant is gradually increased, the height of the flame is reduced, and the oscillation angle of the flame is increased. Experiments show that when the amplitude of the power amplifier reaches more than 55, the flame of the bi-component propellant is blown out. Therefore, it is necessary to control the amplitude of the power amplifier 7 to be less than 50 to obtain an effective propellant oscillation combustion flame oscillation process to prevent the flame from being blown out due to too large amplitude of pressure oscillation and thus failing to obtain relevant data. At present, the input power of the existing power amplifier 7 is sufficient for researching the combustion characteristics of the solid propellant under different amplitudes and pressure oscillation frequencies.
The experiment proves that the direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant can be used for researching the flame fluctuation condition of the solid propellant under pressure oscillation, and the pressure coupling response function of the solid propellant can be obtained through further data processing and analysis. The method comprises the following specific steps:
step A, extracting disturbance quantity and average quantity of pressure oscillation by using pressure oscillation data measured by a dynamic pressure sensor 11;
and B, recording an image of the combustion of the propellant drug strip 4 in a period of time by using a high-speed camera.
And step C, determining the actual single pixel size of the camera by using the resolution test target.
D, selecting 1/10f as a time interval, and calculating the vertical displacement of a certain number of sampling points in each time interval on the initial combustion surface of the propellant explosive strip 4; wherein: f is the frequency of the pressure oscillations (i.e. 10 sampling points per oscillation period)
And E, dividing the vertical displacement by the time interval, calculating the burning rate in each time interval to obtain the burning rate change in a period of time, and extracting the disturbance quantity and the average quantity of the burning rate.
Step F, using formula
And calculating a pressure coupling response function value.
The propellant combustion response refers to the periodical change generated by the influence of the periodically changed sound pressure and sound vibration speed on the combustion speed of the propellant when the combustion surface is subjected to the pressure oscillation. The pressure response, i.e., the response of the rate of combustion to the acoustic pressure, can be generally defined as:
equation (a) represents the propellant mass flow rate
And the relative disturbance amount of the pressure oscillation p'.
In order to study the combustion instability occurring in an engine, the relevant combustion characteristics of the propellant in an oscillating environment need to be studied. In this process, the key step is the acquisition of the pressure coupling response function.
The pressure oscillation disturbance p' and the average in the formula (a)
Directly measurable from dynamic pressure sensorsAnd extracting pressure oscillation data.
In the invention, a calculation method of a pressure coupling response function of a solid propellant adopts the direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant, and a high-speed camera is arranged in front of or behind the propellant stick 4, and the calculation method comprises the following steps:
step a, recording an image of the propellant drug strip 4 burning for a period of time under pressure oscillation by using a high-speed camera, as shown in fig. 5, wherein the sampling rate of the high-speed camera requires the maximum pressure oscillation frequency to be more than or equal to 10 times, so as to obtain burning rate data with sufficient precision. Assuming that the propellant powder strip moves backwards and forwards in a plane during combustion, selecting a time t0And t0+ Δ t, two combustion surfaces at different moments are obtained, and a plurality of sampling points (10 are taken as an example) are equidistantly selected on the combustion surfaces, so that the combustion speed calculation result is ensured to be more accurate. Determining the actual single pixel size alpha (millimeter/pixel) of the camera by using the resolution test target, and then respectively calculating the vertical displacement delta x corresponding to each sampling point at two different timesi(i＝1,2,...,10)，
Δxi＝niα (1)；
Wherein: n isiThe number of vertical displacement pixel points corresponding to the sampling points at two different moments;
alpha the actual single pixel element size of a high speed camera,
the burning rate of the propellant stick 4 is calculated by the following formula:
selecting the combustion images of the propellant drug strips 4 at a plurality of moments by taking delta t as a time interval, calculating the combustion speeds at different moments to obtain the variation of the oscillation combustion speed of the propellant in a period of time, and obtaining the disturbance quantity of the oscillation combustion speed of the propellant drug strips 4
And average amount
The average value refers to an average value of the sum of the maximum amplitude and the minimum amplitude of the oscillation, and the disturbance amount is an absolute value of the difference between the maximum amplitude or the minimum amplitude and the average value.
Step b, selecting time t0And t0The pressure oscillation value in the dynamic pressure sensor 11 during the + Δ t time, the mass flow rate is calculated
Wherein: rho is the density of the propellant, r is the burning speed of the propellant, and A is the burning surface area of the propellant;
and respectively expressing the density and the burning rate of the propellant as the addition form of the average quantity and the disturbance quantity, and obtaining the following result:
neglecting the second order small amount ρ 'r', it can be obtained
Then:
wherein: gamma is a correction factor, i.e. without taking density variations into account
And the error caused when the combustion surface change A' is not considered, and the value range of the error is between 0.8 and 1.2. For a bipropellant with an AP content of 80%, γ is 1.08. Therefore, only the relative variation of the burning rate needs to be calculated again
And then multiplying by the correction coefficient gamma of the response function to obtain the corresponding combustion response function value.
The resulting pressure coupling response function is:
by changing the pressure oscillation frequency, the pressure coupling response function under different pressure oscillation frequencies can be measured, so that the relation between the pressure coupling response function and the frequency can be obtained
Eventually, the combustion instability of the solid propellant is predicted.
By adopting the structure of the experimental device, a pressure oscillation environment which is stable for a long time and controllable in frequency can be provided. The method for generating pressure oscillation adopted by the invention generates an input signal with a specified waveform through the signal generator 8, amplifies the power by using the power amplifier 7, and finally inputs the signal to the loudspeaker 3 to enable the vibrating diaphragm of the loudspeaker 3 to oscillate, thereby generating pressure oscillation with a certain amplitude at the outlet of the loudspeaker convergence section 2. Experiments prove that the oscillation amplitude can meet the requirement of researching the oscillation combustion flame fluctuation of the solid propellant. By adding the loudspeaker protection cover 1 and specially optimizing the design of the inner cavity of the loudspeaker convergence section 2, the pressure oscillation generated by the loudspeaker 3 can be better conveyed to the vicinity of the propellant powder strip. The vertical position and the horizontal distance of the outlet of the loudspeaker convergence section 2 relative to the propellant explosive strip 4 can be adjusted, so that the influence of the pressure oscillation position on the flame fluctuation of the propellant explosive strip can be conveniently researched. In addition, the pressure oscillation frequency generated by the loudspeaker 3 can be continuously adjusted, so that the combustion response characteristic of the solid propellant under different oscillation frequencies can be conveniently researched.
Claims (10)
1. A direct-blowing type oscillation experimental device with continuously variable frequency for measuring a frequency response function of a solid propellant is characterized by comprising a pressure oscillation source (9), a propellant clamp (5) and a signal source (10);
the two pressure oscillation sources (9) are arranged in bilateral symmetry, and each pressure oscillation source (9) comprises a loudspeaker (3) and a loudspeaker convergence section (2) which are connected;
the two loudspeakers (3) are horizontal and are arranged symmetrically left and right, and the bell mouths are arranged oppositely; the horn mouth end of each loudspeaker (3) is detachably connected and communicated with the loudspeaker convergence section (2) along the axial direction; the phase difference between the two loudspeakers (3) is 180 degrees;
the propellant clamp (5) is arranged below the outlet between the two loudspeaker convergence sections (2) and used for clamping a vertical propellant drug strip (4) and driving the propellant drug strip (4) to ascend or descend in the vertical direction between the two pressure oscillation sources (9);
the signal source (10) is respectively connected with the two loudspeakers (3) to form a loop and is used for transmitting the electric signals with given frequency and amplitude to the loudspeakers (3);
the loudspeaker (3) is used for receiving an electric signal input by the signal source (10), and a diaphragm of the loudspeaker (3) periodically oscillates at a frequency consistent with the input frequency of the signal source (10) to generate periodic pressure oscillation and transmit the periodic pressure oscillation; the loudspeaker convergence section (2) is used for converging the periodic pressure oscillation generated by the loudspeaker (3) to a far-end outlet, and is coupled near the combustion surface of the propellant drug strip (4) so as to generate larger pressure oscillation near the combustion surface and act on the combustion flame of the propellant drug strip (4) in a combustion state;
and collecting devices are arranged at the outlets of the two loudspeaker convergence sections (2) and at the positions close to the upper end surfaces of the propellant drug strips (4), and are used for collecting and monitoring pressure oscillation values near the propellant drug strips (4).
2. The direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant is characterized in that a loudspeaker protective cover (1) is arranged on the periphery of the outer shell of the loudspeaker (5), and a closed cavity is formed between the loudspeaker protective cover (1) and the outer wall of the loudspeaker (5) and is used for preventing pressure oscillation generated by the loudspeaker (5) from dissipating to the atmospheric environment.
3. The direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant is characterized in that the convergent section (2) of the loudspeaker is in a shell shape with two open ends, the shell shape consists of a first convergent section (2-3), an equal-straight transition section (2-2) and a second convergent section (2-1) which are sequentially connected from the connecting end of the loudspeaker (3) to the far end, the inner cavity of the first convergent section (2-3) is in a bell-shaped structure, and the connecting end of the first convergent section and the loudspeaker (3) is in an outward-expanding shape;
the second convergent section (2-1) is gradually transited from a large diameter to a small diameter from the connecting end of the second convergent section (2-1) and the straight transition section (2-2) to the far end, and the inner cavity of the second convergent section (2-1) is contracted into a flat rectangle at the far end outlet.
4. A direct-blowing frequency-continuously-variable oscillation experimental device for measuring the frequency response function of a solid propellant, as claimed in claim 3, wherein the distal outlets of the tubular bodies of the two speaker convergence sections (2) are on the same axis, and the cross-sectional area of the outward-expanding inlet end of the speaker convergence section (2) is consistent with the bottom area of the diaphragm of the speaker (3).
5. The direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant is characterized in that the outlets of the two speaker convergence sections (2) are at the same distance from the propellant drug strip (4), and the central axis of the speaker convergence section (2) is flush with the upper surface of the propellant drug strip (4).
6. The direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant is characterized in that the acquisition devices are dynamic pressure sensors (11), and the two dynamic pressure sensors (11) are connected with a data acquisition system (12).
7. The direct-blowing type oscillation experimental device for measuring the frequency response function of the solid propellant is characterized in that a lifting platform (6) is arranged below the propellant clamp (5), and the lifting platform (6) is used for driving the propellant clamp (5) to lift or lower in the vertical direction, so that pressure oscillations generated by a pressure oscillation source (9) act on different positions of the propellant charge (4).
8. The direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant is characterized in that the signal source (10) comprises a signal generator (8) and a power amplifier (7) which are connected, wherein the power amplifier (7) is also connected with a loudspeaker (3); the signal generator (8) generates an electrical signal of a specific frequency and amplitude; the power amplifier (7) is used for receiving the electric signal, amplifying the power of the electric signal and transmitting the power to the loudspeaker (3).
9. The operation mode of the direct-blowing type oscillation experimental device with continuously variable frequency for measuring the frequency response function of the solid propellant is characterized by comprising the following steps of:
step 1, clamping and fixing a propellant medicine strip (4) to be tested on a propellant clamp (5), and fixing the propellant clamp (5) on a lifting platform (6) to form a propellant medicine strip integral lifting mechanism;
step 2, placing the propellant drug strip integral lifting mechanism between two pressure oscillation sources (9), wherein the central axis of the loudspeaker convergence section (2) is flush with the top of the propellant drug strip (4), and the distance between an outlet and the propellant drug strip is 5 cm; a high-speed camera is arranged in front of or behind the propellant drug strip (4);
step 3, arranging a dynamic pressure sensor (11) at a symmetrical position of the propellant drug strip (4) at a distance of 2cm and near an outlet of the loudspeaker convergence section (2);
step 4, setting a signal source (10), adjusting a signal generator (8) to output an electric signal with set frequency and amplitude, transmitting the electric signal to a power amplifier (7), and then transmitting the electric signal to a loudspeaker (3) in a pressure oscillation source (9); the vibrating membranes of the two loudspeakers (3) periodically oscillate at a frequency consistent with the input frequency of the signal source (10), so as to generate periodic pressure oscillation, and the periodic pressure oscillation is transmitted to the loudspeaker convergence section (2); the loudspeaker convergence section (2) is used for converging the periodic pressure oscillation generated by the loudspeaker (3) to a far-end outlet, and is coupled near the upper end face of the propellant drug strip (4) so as to generate larger pressure oscillation near the upper end face;
step 5, opening a data acquisition system (12) and a high-speed camera, and recording pressure oscillation data and combustion surface retreating image data near the propellant drug strip (4);
step 6, igniting the propellant drug strip (4) when the pressure oscillation data peak value near the propellant drug strip (4) does not change, and continuously measuring the pressure data and the combustion surface retreating image data near the propellant drug strip (6);
step 7, deriving pressure data of the dynamic pressure sensor (11) obtained by a data acquisition system (12) in the whole combustion process, and calculating pressure oscillation amplitude, frequency and phase information near the propellant drug strip (4); and deriving data of propellant burning surface retreating within a period of time acquired by a high-speed camera, and calculating the burning speed change of the propellant.
10. A method for calculating a pressure coupling response function of a solid propellant, which is characterized in that a direct-blowing type oscillation experimental device with continuously variable frequency for measuring a frequency response function of the solid propellant, as claimed in any one of claims 1 to 8, is adopted, and the method comprises the following steps:
step a, recording an image of a propellant drug strip (4) burning for a period of time under pressure oscillation by using a high-speed camera, and selecting a moment t0And t0+ delta t, obtaining combustion surfaces at two different moments, and equidistantly selecting a plurality of sampling points on the combustion surfaces; then respectively calculating the vertical displacement delta x corresponding to each sampling point at two different timesi(i＝1,2,...,10)
Δxi＝niα (1)；
Wherein: n isiThe number of vertical displacement pixel points corresponding to the sampling points at two different moments;
alpha the actual single pixel element size of a high speed camera,
the burning speed of the propellant drug strip (4) is calculated by the following formula:
selecting combustion images of the propellant drug strips (4) at a plurality of moments by taking delta t as a time interval, calculating the combustion speeds at different moments to obtain the variation of the oscillation combustion speed of the propellant within a period of time, and obtaining the disturbance quantity of the oscillation combustion speed of the propellant drug strips (4)
And average amount
Step b, selecting time t0And t0Calculating the mass flow rate from the value of the pressure oscillations in the dynamic pressure sensor (11) during the + Deltat time
Wherein: rho is the density of the propellant, r is the burning speed of the propellant, and A is the burning surface area of the propellant;
and respectively expressing the density and the burning rate of the propellant as the addition form of the average quantity and the disturbance quantity, and obtaining the following result:
neglecting the second order small amount ρ 'r', it can be obtained
Then:
wherein: gamma is a correction coefficient;
the resulting pressure coupling response function is:
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