CN101639453B - Correcting method aiming at CT ray hardening in multi-phase flow system - Google Patents

Abstract

The invention provides a correcting method aiming at CT ray hardening in a multi-phase flow system, which comprises the following steps: 1) using fluid dynamics software to generate a simulated flow field of the multi-phase flow system; 2) selecting one two-dimensional section of the simulated flow field to acquire flow field data of the two-dimensional section; 3) selecting materials and thicknesses of different filter plates, and simulating a process of scanning the two-dimensional section by CT to acquire simulated two-dimensional reconstructed images corresponding to the materials and the thicknesses of the different filter plates respectively; and 4) determining the materials and the thicknesses of the filter plates required during actual CT measurement according to the similarity of the simulated two-dimensional reconstructed images and the simulated flow field data. The correcting method has the advantages that the defect that the precise repeatable flow field cannot be produced in a physical experiment can be overcome by combining computational fluid mechanics and CT simulation; and the hardening correcting parameters of a CT system can be determined through computer simulation so as to save a large amount of manpower, materials and financial resources.

Description

A kind of at the ray hardened bearing calibration of CT in the heterogeneous flow system

Technical field

The invention belongs to heterogeneous mobile field of measuring technique, specifically, the present invention relates to the acquisition methods of the ray hardened correction parameter of a kind of CT.

Background technology

Heterogeneous flowing extensively is present in the commercial production.Heterogeneous flow system is a typical complication system, its inner heterogeneous texture has multiple dimensioned feature, away under the EQUILIBRIUM CONDITION, little change of operating parameter not only affects the structure of micro-scale, and may cause the great change of total system macrostructure, influenced the performance of reactor greatly.Further investigation and control polyphasic flow device need be with reliable experimental as the bases, and wherein, the measurement that device cross section material concentration distributes is an important content.

Polyphasic flow concentration measuring method commonly used at present comprises optical fiber probe method, capacitance probe method, sampling method, CT method etc.Wherein first three methods all needs measurement component is inserted inside reactor, and such drawback has two: one, and measurement component can stream field produces and disturbs, and makes the measurement result distortion; The 2nd, the inside reactor particle rubs mutually and produces the operate as normal of a large amount of static and then meeting interferometry instrument.CT is as a kind of nondestructiving detecting means of non-interfering type, cause the very big concern in multiphase flow measurement field in recent years, at present be applied to the experimental study of solution-air, gas-solid system (referring to list of references 1:Chaouki, J.Larachi, F.and Dudukovic, M.P., Industrial ﹠amp; EngineeringChemistry Research, 1997,36,4476-4503).

The workflow that CT measures is generally: measured object places the central authorities of CT machinery turntable, and the radiographic source of CT machine and detector relative fixed after the ray that radiographic source is launched is decayed through measured object, detect transmitted intensity after the decay by detector on turntable.Along with the rotation of turntable, gather under the different angles measured object to the dampening information of ray, the raw data of gathering is carried out pre-treatment after, carry out image reconstruction according to corresponding algorithm, obtain the two-dimensional section figure of measured object.The reconstruction algorithm of CT mainly can be divided into two big classes: filtered back projection (FBP) and mathematical iterations method (ART), (referring to list of references 2:Kak and M.Slaney, Principles of computerized tomographic imaging, IEEE Press (1988)).The factor of restriction CT measuring accuracy has a lot, as radiogenic energy selection, the spacing of detector and the distance between quantity, source and the detector etc.These factors have determined that the exploitation of CT is pointed, just are divided into variety classeses such as thoracic cavity CT, skull CT as medically CT.The CT that industrial circle is used, specific aim is stronger.And equal property when the principle of work of CT has determined that the measurement result of CT inevitably has.Five generation CT from the generation CT multi-source multidetector up till now of the most original moving-rotary work pattern, Measuring Time shortens to a second level by a hour level, but equal measuring method when the principle of work of CT has determined that it is a kind of, can't obtain the instantaneous sectional view of measured object, this just means that if measured object is not an absolute rest will there be motion artifacts in the cross sectional reconstruction figure that obtains.For the industry CT of measuring static model, the time equal property only determined the cycle of measuring, little to the measurement result influence, but for heterogeneous flow system, the motion of measured object be can not ignore the influence of measurement result.

Because the X ray that the CT radiographic source sends is a continuum, therefore, its decay is non-linear when ray passes measured object, measured object can at first fall low energy ray attenuation partly, thereby make the penetration capacity of ray strengthen, i.e. " hardening " in general sense, this process is called ray hardened.In the experiment measuring, need add filter disc to the hardening characteristics of measured object, the low energy of ray energy spectrum partly be filtered, thereby make ray energy spectrum more approach desirable monoenergetic spectrum, thereby make measurement result genuine and believable at ray.

Before CT, especially the industry CT exploitation, need carry out a large amount of experiments the hardening correcting parameters of CT is optimized.For the measuring object of static state, can demarcate CT with the certain criteria thing, reach this purpose by repeated experiments.But for heterogeneous flow system, may obtain hardly one section accurately repeatably the flow field for the CT optimization hardening correcting parameters that experimentizes, therefore, present research for polyphasic flow, perhaps directly adopt medicine CT, perhaps use static model to replace heterogeneous flow field, in a word, the heterogeneous influence of flowing the CT hardening correcting being produced lacks quantitative research method at present.

Summary of the invention

The present invention is directed to when CT is applied to heterogeneous flow measurement and can't quantitatively determine this difficult problem of hardening correcting parameters, propose a kind of by heterogeneous flow field of computer simulation and CT scan, quantitative definite sweep object is the hardening correcting parameters in heterogeneous flow field, for designing and developing of CT saved the lot of manpower and material resources financial resources.

In order to realize this purpose, provided by the invention at the ray hardened bearing calibration of CT in the heterogeneous flow system, comprise the steps:

1) utilize fluid mechanics software to generate the simulation flow field of heterogeneous flow system;

2) choose a two-dimensional section of simulating the flow field, draw the flow field data of this two-dimensional section;

3) choose the material and the thickness of different filter discs, the process that simulation CT scans described two-dimensional section draws the simulation 2-d reconstruction image corresponding to different filter disc materials and thickness respectively;

4), determine the material and the thickness of filter disc required when actual CT measures according to the degree of approximation of each simulation 2-d reconstruction image and described analog stream field data.

In the technique scheme, described step 2) in, described flow field data are described two-dimensional section material distributed datas in a series of moment.

In the technique scheme, described step 2) in, described material distributed data can be solid volume branch rate, solid phase density or gaseous phase volume branch rate.

In the technique scheme, in the described step 3), described simulation CT comprises the process that described two-dimensional section scans:

31) according to described flow field data, and selected filter disc material and thickness, calculate the data for projection of each detector cells of the CT under each angle;

32) carry out 2-d reconstruction according to described data for projection, draw described 2-d reconstruction image.

In the technique scheme, described step 31) comprising: set the moment that different angles scan in the actual measurement process; During data for projection under calculating a certain angle, described flow field data are exactly the material distributed data of the corresponding described two-dimensional section constantly of current angle.

In the technique scheme, in the described step 4), described analog stream field data is the average of described two-dimensional section material distributed data in the certain hour section; The described time period is the time period corresponding to the start-stop of simulation CT scan.

In the technique scheme, in the described step 4), the degree of approximation of simulation 2-d reconstruction image and described analog stream field data draws according to normalization mean absolute deviation criterion.

The present invention has following technique effect with respect to prior art: can't produce in the Physical Experiment and can repeat the flow field accurately as the hardening correcting sweep object, and the mode in the logical analog computation generation of the present invention simulation flow field has solved this problem well, therefore the present invention can obtain the ray hardened correction parameter based on the CT in heterogeneous flow field, is better than using in the prior art static model to replace the way in heterogeneous flow field; In addition, the present invention draws up the hardening correcting parameters that can determine the CT system by computer mould, for the exploitation of CT system provides guidance, therefore can save the lot of manpower and material resources financial resources.

Description of drawings

Below, describe embodiments of the invention in conjunction with the accompanying drawings in detail, wherein:

Fig. 1 is the invention synoptic diagram, and wherein 1 is the reactor of computer simulation, and 2 are the CT scan face of computer simulation, and 3 is the radiographic source of this CT, and 4 is the detector of this CT;

Fig. 2 is that a two-dimensional section is in different concentration profiles constantly in the reactor of computer simulation;

Fig. 3 simulates the CT scan synoptic diagram, and line 5 is the path of an X ray process of simulation; Real, dotted line is respectively the perspective view of facing two angles mutually;

X-ray energy spectrum when Fig. 4 is added different filter disc by simulation CT;

Fig. 5 is the raw data sinogram figure (sinogram) that the simulation CT scan obtains;

Fig. 6 rebuilds the two-dimentional tomograph that obtains according to the data for projection of analog scanning;

Fig. 7 is the quantitative comparison that loads different filter disc reconstructed results;

Table 1 is under the different operating modes, and CT is to recirculating fluidized bed measurement result table.

Embodiment

Below in conjunction with accompanying drawing and preferred embodiment the present invention is described further.

Embodiment

Circulating fluid bed reactor is widely used in fields such as petroleum refining, boiler, metallurgy, cross section material distribution situation is an important bid of reactor operation conditions in the reactor, CT can not insert glitch-free measurement reactor content distribution sectional view, has caused the great interest of industry in recent years.But for X-ray CT, because its power spectrum is continuous distribution, but not a monodrome, so inevitably produce ray hardened phenomenon during actual measurement, cause between measurement result and actual value to have deviation, need carry out hardening correcting according to the characteristics of measurand.But because the heterogeneous flow system in such reactor belongs to typical complication system, little change of initial operation condition can cause the drastic change of flow field local message, therefore, be difficult to produce experimentally one section accurately flow field repeatably, determine hardening correcting parameters for experiment.With the polyphasic flow reactor of the CT scan computer simulation of simulating, can avoid a top described difficult problem, reach the purpose of determining correction parameter.

1) according to fluid media (medium), solid dielectric, reactor shape, utilize fluid mechanics software, generate the simulation flow field.In the present embodiment, use commercial cfdrc Fluent, simulation one section of riser reactor, as shown in Figure 1, this riser reactor is fluid media (medium) with the air, beaded glass is a solid dielectric;

2) choose 1.505 meters, distribution grid top, this position belongs to riser concentrated phase section, and Gas-solid Two-phase Flow is more violent, flows more representative to the influence of CT imaging for research.Export the different flow field data constantly of this depth of section, carry out linear interpolation, output two-dimensional section material distributed data matrix, as shown in Figure 2, the grayscale bar on right side is being represented solid volume branch rate among the figure; It should be noted that, the flow field data herein and the data of two-dimensional section data can be got material physical parameters such as the volume fraction, solid phase density, gaseous phase volume branch rate of solid phase, in fact each parameter can be changed mutually, and final analog result is not produced materially affect; Data type can be taken as int type, float type and double type, does not do particular determination;

3), after flow parameter reaches convergence, preserve duration and be 1 minute statistics step-length and be 20 milliseconds computational data by statistics;

4) simulation CT scans selected cross section.Wherein, the voltage of X-ray source is 80kVp, and interior filter disk is the Be of 1.2 millimeters thick and the Al of 2 millimeters thick.The CT scan process be with the information mapping of the corresponding point on the fault surface to corresponding probe access, this process is a line integral process.The process of analog scanning is as follows:

Riser reactor according to simulation configures a series of sweep parameters such as detector cells length, source-detector distance, anglec of rotation step-length, rotational speed.

The 0th angle, source-position of detector is shown in solid line among Fig. 3.By the Lambert-beer law as can be known, following rule is observed in the decay that takes place when X ray penetrates uniform dielectric:

I＝I ₀e ^-μL

I ₀Be incident X-rays intensity, I is an outgoing X ray intensity, and μ is the linear attenuation coefficient of this uniform dielectric, and L is the thickness that ray penetrates this medium.

For the such nonhomogeneous media of Dual-Phrase Distribution of Gas olid, outgoing intensity (the detected X ray intensity of detector) then is:

$I=I_0{e}^{-{\∫}_L{\mathrm{\μ}}_l\mathrm{dl}}$

μ in the formula _lFinger is at the linear attenuation coefficient of the medium at dl place, l be on the raypath L a bit.

After ray 1 passed the decay of tested flow field among the figure, ray was detected by s detector, and this process can be expressed from the next:

$p_{(s,0)}=-\ln \left(\frac{I}{I_0}\right)={\∫}_L{\mathrm{\μ}}_l\mathrm{dl}$

P wherein _{(s, 0)}Represent the projection on s detector cells of the 0th angle.

For the continuous X-ray of power spectrum, this process is followed following rule:

$I={\mathrm{<figure-callout\; id="0"\; label="I"\; filenames="S2008101174429E00021.png,S2008101174429E00031.png"\; state="\{\{state\}\}">I</figure-callout>}}_0\&Integral;\mathrm{\&Omega;}\left(E\right)e^{-{\&Integral;\mathrm{\&mu;}}_{E,S}\mathrm{ds}}\mathrm{dE}$

$P=-\ln \left(\frac{I}{I_0}\right)=-\ln (\&Integral;\mathrm{\&Omega;}\left(E\right)e^{-\&Integral;{\mathrm{\&mu;}}_{E,S}\mathrm{ds}}\mathrm{dE})$

$P_{(s,\mathrm{\&theta;})}=-\ln (\&Integral;\mathrm{\&Omega;}\left(E\right)e^{-{\&Integral;}_{-\&infin;}^{\&infin;}{\&Integral;}_{-\&infin;}^{\&infin;}{\mathrm{\&mu;}}_E(x,y)\mathrm{\&delta;}(x\cos \mathrm{\&theta;}+y\sin \mathrm{\&theta;}-s)\mathrm{dxdy}}\mathrm{dE})$

p _{(s, θ)}Represent the projection on s detector cells of θ angle.Ω (E) represents ray energy spectrum, μ _E(x, y) representative is E time point (x, linear attenuation coefficient y) at energy level.

The linear attenuation coefficient μ of measurand is checked in by NIST (National Institute ofStandard technology) database (http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.html) in the above-mentioned formula.

After the projection information of this angle is finished in calculating, the radiographic source of simulation CT and detector are by the rotation step-length rotation of setting (being that the space geometry position changes), the flow field material distributed data of this moment is also calculated in Mo Ni flow field synchronously simultaneously, calculates the data for projection of the 1st all detector cells of angle successively.

After finishing a week scanning, the full angle data for projection that obtains represents with the form of sinogram figure, as shown in Figure 4.

5) according to the CT image reconstruction algorithm, carry out two-dimension image rebuild, used herein is filtered back projection (FBP) reconstruction algorithm.

$\mathrm{\&mu;}(x,y)=\frac{1}{{4\mathrm{\&pi;}}^2}{\&Integral;}_0^{2\mathrm{\&pi;}}\mathrm{d\&theta;}{\&Integral;}_{-\&infin;}^{\&infin;}\frac{1}{x\cos \mathrm{\&theta;}+y\sin \mathrm{\&theta;}-s}\frac{\&PartialD;P_{\mathrm{\&theta;}}\left(s\right)}{\&PartialD;s}\mathrm{ds}$

(x is to give directions (x, y) linear attenuation coefficient of some place medium y) to μ

6) aanalogvoltage is 80kVp, loads the X-ray spectrum of 5 millimeters thick Al, 1 millimeters thick Cu, 2 millimeters thick Cu respectively, repeating step 4-5.It should be noted that Al, Cu are filter disc materials comparatively commonly used, but the filter disc kind is not limited in this two kinds of materials, also can be materials such as Be, W; The thickness of material is thick more, and the ray energy of elimination is many more, and the final detected photon stoichiometric number of detector is few more, and signal intensity is poor more, and concrete material thickness range should be decided according to used ray tube voltage.In addition, should be noted that when analog computation that interior filter disk is as broad as long on computing method with outer filter disc.

7) the simulation CT measurement result that will load different filter disc thickness with calculate the flow field contrast.Here calculate the flow field that average that the flow field refers to the material distributed data of described two-dimensional section in the certain hour interval is formed, described time interval refers to the pairing time interval of simulation CT scan.

As shown in Figure 7, by normalization mean absolute deviation criterion d, the d value that can calculate 5 millimeters Al, 1 millimeter Cu and 2 millimeters Cu is respectively 15.48%, 7.71%, 7.67%.The computing method of criterion d are as follows:

$d=\frac{\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}|t_n-r_n|}{\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}\left|r_n\right|}$

Wherein, t _nBe the reconstructed value of n point, r _nIt is the simulation flow field value of n point.

As can be seen, the Cu filter disc more approaches actual value (calculating flow field) than the reconstructed results of Al filter disc, when filter disc adds to 1 millimeter Cu and 2 millimeters Cu, the reconstructed results difference is not very big, but 2 millimeters Cu have filtered more photon metering, if detect same stoichiometric number, the latter need use than the former bigger electric current.Therefore, to the flow field of computer simulation, the optimum thickness that loads filter disc is 1 millimeter Cu.

The correction hardening parameter that this method is obtained is applied to the measurement of polyphasic flow reactor cycles fluidized bed, record the average material concentration in riser cross section under the different operating modes (superficial gas velocity Ug and solid circulating rate Gs), and its result is measured optical Fiber Method the most commonly used with present polyphasic flow material concentration compare.The results are shown in Table 1, as can be seen, this method can effectively effectively be proofreaied and correct the ray hardened phenomenon of CT generation heterogeneous flowing.

Table 1

Above-described specific embodiment has been described in detail purpose of the present invention, technical scheme and beneficial effect.Institute's foregoing that it should be understood that is not limited to the present invention only for specific embodiments of the invention.All within spirit of the present invention and principle, any modification of being made, be equal to and replace and improvement etc., all should be included within protection scope of the present invention.

Claims (7)

1. one kind at the ray hardened bearing calibration of CT in the heterogeneous flow system, comprises the steps:

1) utilize fluid mechanics software to generate the simulation flow field of heterogeneous flow system;

2) choose a two-dimensional section of simulating the flow field, draw the flow field data of this two-dimensional section;

3) choose the material and the thickness of different filter discs, the process that simulation CT scans described two-dimensional section draws the simulation 2-d reconstruction image corresponding to different filter disc materials and thickness respectively;

4), select the material of suitable filter disc and thickness filter disc material and thickness as actual CT measurement according to the degree of approximation of the flow field data of each simulation 2-d reconstruction image and described two-dimensional section and the photon stoichiometric number that detector detected of CT.

2. according to claim 1 at the ray hardened bearing calibration of CT in the heterogeneous flow system, it is characterized in that described step 2) in, the flow field data of described two-dimensional section are the material distributed data of described two-dimensional section in a series of moment.

3. according to claim 2 at the ray hardened bearing calibration of CT in the heterogeneous flow system, it is characterized in that described step 2) in, described material distributed data can be solid volume branch rate, solid phase density or gaseous phase volume branch rate.

4. according to claim 2ly it is characterized in that at the ray hardened bearing calibration of CT in the heterogeneous flow system in the described step 3), described simulation CT comprises the process that described two-dimensional section scans:

31) according to the flow field data of described two-dimensional section, and selected filter disc material and thickness, calculate the data for projection of each detector cells of the CT under each angle;

32) carry out 2-d reconstruction according to described data for projection, draw described 2-d reconstruction image.

5. according to claim 4 at the ray hardened bearing calibration of CT in the heterogeneous flow system, it is characterized in that described step 31) comprising: set the moment that different angles scan in the actual measurement process; During data for projection under calculating a certain angle, the flow field data of described two-dimensional section are exactly the material distributed data of the corresponding described two-dimensional section constantly of current angle.

6. according to claim 2ly it is characterized in that at the ray hardened bearing calibration of CT in the heterogeneous flow system in the described step 4), the flow field data of described two-dimensional section are the average of described two-dimensional section material distributed data in the certain hour section; The described time period is the time period corresponding to the start-stop of simulation CT scan.

7. according to claim 1 at the ray hardened bearing calibration of CT in the heterogeneous flow system, it is characterized in that, in the described step 4), the degree of approximation of the flow field data of simulation 2-d reconstruction image and described two-dimensional section draws according to normalization mean absolute deviation criterion.

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