CN105424564A - Tuning method for microwave measurement parameters of pulverized-coal concentration - Google Patents

Abstract

The invention discloses a tuning method for microwave measurement parameters of the pulverized-coal concentration. The tuning method is characterized by including the steps of selecting optimal microwave transmitting frequency and selecting a microwave probe installing mode. Site verification proves that the microwave frequency and the probe installing mode determined by the tuning method for the microwave measurement parameters of the pulverized-coal concentration meet the experiment condition, and therefore the effectiveness and the practicability of the tuning method for the microwave measurement parameters of the pulverized-coal concentration are verified; compared with a traditional tuning method for the site microwave frequency and the probe installing mode, the tuning method for the microwave measurement parameters of the pulverized-coal concentration has the advantages of being scientific, accurate, easy to achieve, high in applicability and application value, good in effect, free of influences of extra factors and the like.

Description

The setting method of breeze concentration microwave measurement parameter

Technical field

The invention belongs to field of measuring technique, is a kind of setting method of breeze concentration microwave measurement parameter.

Background technology

Power plant's pulverized coal density of primary air refers to the coal powder density before boiler-burner nozzle in powder feeding pipe, and it is an important parameter, will directly have influence on the coal powder density sprayed in burner hearth, thus affects the combustion position of boiler.Therefore, the homogeneity controlling coal powder density in primary air piping and powder delivering speed is very important to the safe and economical operation of boiler, for controlling the pulverized coal flow of each burner and concentration, first must can carry out on-line measurement accurately to coal powder density, but, because breeze airflow belongs to Dual-Phrase Distribution of Gas olid, it is very very difficult for will carrying out online Measurement accuracy to coal powder density.There is the method for multiple measurement coal powder density at present, as heat balance method of, supercritical ultrasonics technology, capacitance method etc., but all can not reach gratifying effect.

In recent years, microwave measurement coal powder density is a kind of new method, high with its distinctive precision, measure convenient, change the plurality of advantages such as simple, real-time, more and more by people are paid attention to, and as coal powder density measure main method.The concrete principle of microwave measurement coal powder density is duff pipe Lu Zhongyong flange attaching one section of measuring tube, press certain angle corresponding tilt along coal dust flow direction and arrange microwave generator and microwave receiver, microwave can cause beam attenuation with during coal pipe particles collision in measuring tube, can react the concentration of coal dust by measuring its beam attenuation value.

In the method for microwave measurement coal powder density, Microwave emission frequency and probe mounting means are two very important parameters, and they directly determine the precision measuring coal powder density, but at present to the conclusion how selecting this two parameters ununified all the time.And due to power plant's service condition restriction, directly test very difficult at power plant scene, build experiment table and also have many restrictions, therefore the present invention is based on above-mentioned reason, by the method for theory deduction and electromagnetic simulation software HFSS simulation modeling to Microwave emission frequency, the selection of probe antenna and the selection of installation site provide a kind of effective solution.Up to now, have no about bibliographical information related to the present invention and practical application.

Summary of the invention

The object of the invention is, for solving microwave frequency in prior art concentration of pulverized coal, probe antenna mounting means is difficult to the problem determined, and providing a kind of science can determine the setting method of the breeze concentration microwave measurement parameter of the mounting means of microwave frequency in concentration of pulverized coal, probe antenna accurately.

The technical scheme realizing the object of the invention employing is: a kind of setting method of breeze concentration microwave measurement parameter, is characterized in that, it has in comprising:

1) selection of best Microwave emission frequency

Microwave transmits and can be equivalent to microwave and transmit in circular waveguide in boiler First air pipeline, and in circular waveguide, microwave transmission may also exist various modes simultaneously, according to the relation of electromagnetic field and the direction of propagation, generally be divided into two large classes: transverse electric wave (TE) ripple, transverse electric wave (TE) ripple refers to that electric field is 0 at the component in transmission z direction, transverse magnetic wave (TM) ripple, transverse magnetic wave (TM) ripple refers to that magnetic field is 0 in transmission z durection component, and TE ripple and TM ripple are difference also divided into various higher mode TE according to cut-off wave number _nmand TM _nm, they have different cutoff frequencys, and the mould that wherein cutoff frequency is minimum is TE ₁₁mould, it is the basic mode of circular waveguide, the energy transferring between can effectively avoiding when only having basic mode to transmit in waveguide because of different mode and cause microwave signal distortion, and known cutoff frequency formula is

$f_{c_{mn}}=\frac{p_{mn}}{2\mathrm{\π}a\sqrt{\mathrm{\μ}\mathrm{\ϵ}}}$

Wherein f is cutoff frequency, p _nmfor matching the root of Baire function, a is pipe radius, and μ is magnetic permeability in pipeline, and ε is specific inductive capacity in pipeline, and power plant's pipe radius is about 0.3m, and other value is known, draws TE ₁₁the cutoff frequency of mould is minimum, is about 292.83MHZ, TM ₀₁the cutoff frequency of mould is only second to TE ₁₁mould, is about 453.21MHZ, and therefore selecting frequency is between 292.83MHZ to 453.21MHZ, for the ease of calculating, chooses the peak low band 300MHZ of microwave;

2) selection of microwave probe mounting means

The mounting means of microwave probe specifically comprises probe setting angle and mounting distance, first with electromagnetic simulation software hfss, modeling is carried out to microwave probe, the probe antenna used is quarter-wave monopole antenna, coaxial feed mode is taked to carry out feed, it is emulated, known by the antenna pattern observing probe antenna, probe has maximum gain in the horizontal direction, therefore when placing microwave probe, transmitting probe should be parallel with the angle of receiving transducer, ensure the transmission of microwave signal, then the probe designed is inserted pipeline, obtain the specific inductive capacity in the block mold pipeline of microwave measurement coal powder density, specific inductive capacity size is relevant with coal powder density in pipeline again, coal powder density is larger, the specific inductive capacity of potpourri is larger, by changing the specific inductive capacity of pipeline, just can simulate the change of First air pipeline coal powder density, known, the relative dielectric constant of air is about 1, the relative dielectric constant of pure coal dust is about 2.7, be ε=[m for mixture dielectric constant formula ₁(ε ₁) ^1/3+ m ₂(ε ₂) ^1/3] ³, wherein m ₁for the mass concentration of air, m ₂for the mass concentration of coal dust, ε ₁for dielectric constant of air, ε ₂for coal dust specific inductive capacity, in power plant, coal powder density is all generally within 0.2kg/kg ~ 1.0kg/kg scope, according to mixture dielectric constant formula ε=[m ₁(ε ₁) ^1/3+ m ₂(ε ₂) ^1/3] ³, choosing two groups of mixture dielectric constant is 1.4 and 1.7, changes the spacing of launch and accept probe, observes probe transmission attenuation, finally chooses 0.5m for probe mounting distance.

The microwave frequency determined according to the setting method of breeze concentration microwave measurement parameter of the present invention and probe mounting means, through on-site verification, meet experiment condition, thus demonstrate validity and the practicality of the setting method of breeze concentration microwave measurement parameter of the present invention, and compared with traditional on-site parameters (microwave frequency, mounting means of popping one's head in) setting method, it is accurate that the setting method of breeze concentration microwave measurement parameter of the present invention has science, be easy to realization, applicability is strong, using value is high, effect is good, not by advantages such as extra factor affect.

Accompanying drawing explanation

Fig. 1 is that microwave passes ripple mode schematic diagram in circular waveguide;

Fig. 2 is microwave probe structural drawing in the present invention;

The return loss plot of microwave probe in the present invention of Fig. 3 position;

Fig. 4 is the antenna pattern of microwave probe in the present invention;

Fig. 5 is the one-piece construction figure of microwave measurement concentration experiment in the present invention;

Fig. 6 is transmission attenuation schematic diagram.

Embodiment

Below in conjunction with accompanying drawing, the present invention is further detailed.

The setting method of breeze concentration microwave measurement parameter of the present invention, it has in comprising:

1) selection of Microwave emission frequency

As shown in Figure 1, it is typical circular waveguide transmission problem that microwave transmits in First air pipeline, the transmission of microwave is the function of three dimensions and time, according to magnetic field, the relation of electric field and the direction of propagation can be divided into various modes, microwave also also exists various modes in transmitting procedure in circular waveguide, according to the relation of electromagnetic field and the direction of propagation, generally be divided into two large classes: transverse electric wave (TE) ripple, transverse electric wave (TE) ripple refers to that electric field is 0 at the component in transmission z direction, transverse magnetic wave (TM) ripple, transverse magnetic wave (TM) ripple refers to that magnetic field is 0 in transmission z durection component.And transverse electric wave (TE) ripple and transverse magnetic wave (TM) ripple can also be divided into various higher mode TE according to the difference of cut-off wave number _nmand TM _nm, and generally, waveguide is single mode transport in transmitting procedure, is so not only easy to excitation and coupling, the energy transferring between can also effectively avoiding because of different mode and cause microwave signal distortion.Therefore by selecting circular waveguide size and frequency of operation, can make in circular waveguide, to only have basic mode to transmit.TE in circular waveguide ₁₁mould is the basic mode of circular waveguide, has minimum cutoff frequency, is secondly TM ₀₁mould, the radius a choosing power plant's First air pipeline is 0.3m, known cutoff frequency wherein f is cutoff frequency, p _nmfor matching the root of Baire function, a is pipe radius, and μ is magnetic permeability in pipeline, and ε is specific inductive capacity in pipeline, can draw TE ₁₁the cutoff frequency of mould is about 292.83MHZ, TM ₀₁the cutoff frequency of mould is about 453.21MHZ, and therefore selecting frequency is between 292.83MHZ ~ 453.21MHZ, ensures to only have basic mode to transmit in circular waveguide, for the ease of calculating, chooses the peak low band 300MHZ of microwave.

2) selection of microwave probe mounting means

As shown in Figure 2, adopt electromagnetic simulation software hfss to carry out modeling to the concentration of pulverized coal experiment of microwave measurement power plant, first carry out modeling to microwave transmitting probe, probe takes coaxial feed mode, kind is quarter-wave monopole antenna, is the one of element antenna.Electricity size is called quarter-wave monopole antenna close to quarter-wave monopole antenna.Now antenna does not have secondary lobe, and the input impedance of monopole antenna is close to 50 ohm, can directly and characteristic impedance be that the coaxial cable of 50 ohm matches.

As shown in Figure 3, in Antenna Design, return loss is an important parameter, and return loss is the parameter representing signal reflex performance.Return loss illustrates that a part for incident power is reflected back to signal source.Usually require that incident power is the bigger the better, reflective power is little as far as possible, so just has more power to be sent to load.Target is in typical case the return loss of at least 10dB.Can find out in the drawings, antenna has larger return loss (RL) about 300MHZ frequency band, is about-25db, meets general Antenna Design requirement.

As shown in Figure 4, from the field intensity direction of the antenna pattern of microwave probe, unipole antenna has maximum gain in the horizontal direction, go out directly over oscillator and almost there is no gain, thus it is known when placing microwave probe, transmitting probe should be parallel with the angle of receiving transducer, ensures the transmission of microwave signal.

As shown in Figure 5, pipe radius is 0.3m, pipe wall material is iron, select the long 2m of pipeline, experiment microwave frequency is 300MHZ, and the specific inductive capacity of pipeline internal medium is the key factor affecting microwave experiment, it is contacted directly with the decay size in microwave transmission process, and specific inductive capacity size in pipeline is relevant with coal powder density in pipeline, coal powder density is larger, and the specific inductive capacity of potpourri is larger.By changing the specific inductive capacity of pipeline, the change of First air pipeline coal powder density just can be simulated.Known, the relative dielectric constant of air is about 1, and the relative dielectric constant of pure coal dust is about 2.7, and the formula for mixture dielectric constant is ε=[m ₁(ε ₁) ^1/3+ m ₂(ε ₂) ^1/3] ³, wherein m ₁for the mass concentration of air, m ₂for the mass concentration of coal dust, ε ₁for dielectric constant of air, ε ₂for coal dust specific inductive capacity, draw the relation between coal powder density and specific inductive capacity thus only need given pipeline mixture dielectric constant just can obtain coal powder density.In actual power plant runs, coal powder density is all generally within 0.2kg/kg ~ 1.0kg/kg scope, therefore according to mixture dielectric constant formula ε=[m ₁(ε ₁) ^1/3+ m ₂(ε ₂) ^1/3] ³choosing two groups of mixture dielectric constant is 1.4 and 1.7, respectively corresponding coal powder density 30% and coal powder density 50%, changes the spacing of launch and accept probe, observes probe transmission attenuation.

As shown in Figure 6, as can be seen from transmission attenuation schematic diagram, when probe distance gets 0.1m, loss change is not obvious, and this shows that the spacing of probe is too near, and decay has significant change when being greater than 0.3m, and in general, the distance between probe no more than 2 times of pipe diameter, otherwise can cause the signal degree of association between probe to decline, therefore select 0.3m-1.2m as probe mounting distance, choose 0.5m in the present invention as probe mounting distance.

Claims (1)

1. a setting method for breeze concentration microwave measurement parameter, is characterized in that, it has in comprising:

1) selection of best Microwave emission frequency

Microwave transmits and can be equivalent to microwave and transmit in circular waveguide in boiler First air pipeline, and in circular waveguide, microwave transmission may also exist various modes simultaneously, according to the relation of electromagnetic field and the direction of propagation, generally be divided into two large classes: transverse electric wave (TE) ripple, transverse electric wave (TE) ripple refers to that electric field is 0 at the component in transmission z direction, transverse magnetic wave (TM) ripple, transverse magnetic wave (TM) ripple refers to that magnetic field is 0 in transmission z durection component, and TE ripple and TM ripple are difference also divided into various higher mode TE according to cut-off wave number _nmand TM _nm, they have different cutoff frequencys, and the mould that wherein cutoff frequency is minimum is TE ₁₁mould, it is the basic mode of circular waveguide, the energy transferring between can effectively avoiding when only having basic mode to transmit in waveguide because of different mode and cause microwave signal distortion, and known cutoff frequency formula is

$f_{c_{mn}}=\frac{p_{mn}}{2\mathrm{\π}a\sqrt{\mathrm{\μ}\mathrm{\ϵ}}}$

Wherein f is cutoff frequency, p _nmfor matching the root of Baire function, a is pipe radius, and μ is magnetic permeability in pipeline, and ε is specific inductive capacity in pipeline, and power plant's pipe radius is about 0.3m, and other value is known, draws TE ₁₁the cutoff frequency of mould is minimum, is about 292.83MHZ, TM ₀₁the cutoff frequency of mould is only second to TE ₁₁mould, is about 453.21MHZ, and therefore selecting frequency is between 292.83MHZ to 453.21MHZ, for the ease of calculating, chooses the peak low band 300MHZ of microwave;

2) selection of microwave probe mounting means

The mounting means of microwave probe specifically comprises probe setting angle and mounting distance, first with electromagnetic simulation software hfss, modeling is carried out to microwave probe, the probe antenna used is quarter-wave monopole antenna, coaxial feed mode is taked to carry out feed, it is emulated, known by the antenna pattern observing probe antenna, probe has maximum gain in the horizontal direction, therefore when placing microwave probe, transmitting probe should be parallel with the angle of receiving transducer, ensure the transmission of microwave signal, then the probe designed is inserted pipeline, obtain the specific inductive capacity in the block mold pipeline of microwave measurement coal powder density, specific inductive capacity size is relevant with coal powder density in pipeline again, coal powder density is larger, the specific inductive capacity of potpourri is larger, by changing the specific inductive capacity of pipeline, just can simulate the change of First air pipeline coal powder density, known, the relative dielectric constant of air is about 1, the relative dielectric constant of pure coal dust is about 2.7, be ε=[m for mixture dielectric constant formula ₁(ε ₁) ^1/3+ m ₂(ε ₂) ^1/3] ³, wherein m ₁for the mass concentration of air, m ₂for the mass concentration of coal dust, ε ₁for dielectric constant of air, ε ₂for coal dust specific inductive capacity, in power plant, coal powder density is all generally within 0.2kg/kg ~ 1.0kg/kg scope, according to mixture dielectric constant formula ε=[m ₁(ε ₁) ^1/3+ m ₂(ε ₂) ^1/3] ³, choosing two groups of mixture dielectric constant is 1.4 and 1.7, changes the spacing of launch and accept probe, observes probe transmission attenuation, finally chooses 0.5m for probe mounting distance.