CN106124798A - A kind of based on the method for convection current mixed process velocity field in NMR (Nuclear Magnetic Resonance)-imaging measurement porous media - Google Patents

Abstract

The present invention relates to a kind of based on the method for convection current mixed process velocity field in NMR (Nuclear Magnetic Resonance)-imaging measurement porous media, belong to convection current mixed process velocity field measurement technical field.Its operating procedure is: first the solution filling two kinds of different densities in porous media is carried out upper lower leaf saturated, insert rapidly in nuclear-magnetism probe after the fill out sand tube of saturated two kinds of different densities solution is inverted, the free echo sequence method adding phase encoding gradient pulses is used to obtain the proton density image of fluid and be converted to phase image, difference is done with resting state phase diagram under same pulsed gradient, obtain Phase shift image, use phase method measurement to obtain porous media convection current mixed process velocity field.The method achieve unconventional flow distribution that convection current mixing requires；The method application nmr imaging technique, carries out visual inspection with noncontact, non-interfering means to the velocity field of convection current mixed process, fluid flow will not produce disturbance.

Description

A kind of based on convection current mixed process velocity field in NMR (Nuclear Magnetic Resonance)-imaging measurement porous media Method

Technical field

The invention belongs to convection current mixed process velocity field measurement technical field, relate to a kind of based on NMR (Nuclear Magnetic Resonance)-imaging measurement The method of convection current mixed process velocity field in porous media.

Background technology

The dynamic evolution process of fluid boundary is the importance of hydrodynamics research.Fluid boundary stability is ground Study carefully in basic scientific research and industrial processes, all played pivotal role.We focus in gravitational field due to upper stream Body density is more than lower floor's fluid, and the big density fluid on upper strata, two-phase fluid interface migrates downward under gravity, boundary simultaneously The little density fluid of Mian Chu lower floor moves upward, and causes the convection process in vertical direction, and this phenomenon is referred to as " convection current mixing ". Convection current mixed process frequently occurs in sea ice freezing process, carbon dioxide geologic sequestration process, underground water pollution process etc..

NMR (Nuclear Magnetic Resonance)-imaging (MRI) technology is a kind of new method for visualizing, applies in Chemical Engineering, Spectroscopy Study, doctor Study picture etc..MRI can carry out imaging to containing Hydrogen Proton fluid, and measure and have porous media fluid flow inside solely Special technical advantage.Compared to other velocity field measurement technology, such as LDV etc., MRI can obtain flow process speed Degree field and the short resolution of imaging time are high, can measure container interior flow field through nontransparent external container.

Convection current mixed process needs top fluid density more than lower floor's fluid, this unconventional fluid distrbution bar Part carries out experiment difficulty in making laboratory.For the velocity field measurement of convection current mixed process, existing method exists interferes fluid Flowing or the limitation of spot speed can only be obtained.With regard to numerical simulation aspect, complex disposal process is loaded down with trivial details, needs to arrange complicated limit Boundary's condition also carries out large-scale grid computing.Therefore, non-interfering fluid velocity field measurement has very important engineering significance.

Summary of the invention

The present invention solves that prior art is difficult to carry out asking of velocity field measurement for convection current mixed process in porous media Topic, it is provided that a kind of based on the method for convection current mixed process velocity field in NMR (Nuclear Magnetic Resonance)-imaging measurement porous media.It is right to be applicable to The follow-up Journal of Sex Research of stream mixed process fluid boundary stability.

A kind of based on the method for convection current mixed process velocity field, concrete technical side in NMR (Nuclear Magnetic Resonance)-imaging measurement porous media Case is as follows:

The first step, by the fill out sand tube of the porous media of the saturated big density fluid of saturated for upper strata little density fluid lower floor, inserts Nuclear magnetic resonance imaging system, uses the spin-echo sequence method adding phase encoding gradient pulses, measures fill out sand tube a certain disconnected The resting state proton density image of position, face；

Second step, by fill out sand tube turned upside down, starts convection current mixed process, uses addition phase encoding gradient pulses oneself Cycle wave train method, measures the proton density image at cross section place identical with the first step in convection current mixed process；

3rd step, the resting state proton density image convert to static state phase image first step obtained, by second step The proton density image obtained is converted to convection current mixed process phase image；

4th step, the convection current mixed process phase image obtained by the 3rd step deducts resting state phase image, obtains phase place Shift image；

5th step, the Phase shift image obtaining the 4th step, uses phase method to measure, obtains eventually through formula (1) Convection current mixed process velocity field

${\stackrel{\→}{v}}_t=\frac{{\mathrm{\Φ}}_V}{\stackrel{\→}{G}\mathrm{\Δ}t\mathrm{\γ}}---\left(1\right)$

For t convection current mixed process velocity field, Φ_VFor Phase shift image,It it is gradient pulse added by the first step Intensity, Δ is gradient pulse interval time, and t is the gradient pulse persistent period, and γ is gyromagnetic ratio.

It is as follows that phase method measures convection current mixed process velocity field principle:

During NMR (Nuclear Magnetic Resonance)-imaging, if proton density is distributed asWhereinFor locus, nuclear magneton place.Profit It is calculated by formula (2) with signal intensity S (t) of NMR system shooting proton density image:

$S\left(t\right)={\∫}_V\mathrm{\ρ}\left(\stackrel{\→}{r}\right){\exp }^{-{\mathrm{\Φ}}_V}d\stackrel{\→}{r}---\left(2\right)$

Add phase place Φ of the spin-echo sequence shooting picture of phase encoding gradient pulses_VChange is due to main field strength Cause with additional gradient pulse, use integrating factor t' the time (0, t) take indefinite integral:

${\mathrm{\Φ}}_V=\mathrm{\γ}{\∫}_0^t(B_0+\stackrel{\→}{G}(t^{\′}\left)\stackrel{\→}{r}\right){\mathrm{dt}}^{\′}---\left(3\right)$

Wherein B₀For main field strength.In formula (3)It it is the precession phase under the Larmor frequency that causes of main field Position, can be by removing in detector receiver.It is that the phase place that additional gradient pulse causes changes.Through Fourier After conversion, nuclear magneton position becomes phase spaceProton density can represent by following form:

$\mathrm{\ρ}\left(\stackrel{\→}{r}\right)=\∫\left(s\right(\stackrel{\→}{k}){\exp }^{2\mathrm{\π}\stackrel{\→}{k}\stackrel{\→}{r}}d\stackrel{\→}{k}---\left(4\right)$

Doing the displacement formula containing Hydrogen Proton fluid moved along a straight line in convection current mixed process can discrete be:

$\stackrel{\→}{r}\left(t\right)={\stackrel{\→}{r}}_0+{\stackrel{\→}{v}}_t\×t+\frac{1}{2}{\stackrel{\→}{a}}_0\×t^2+\mathrm{...}---\left(5\right)$

For initial coordinate,For acceleration.

By formula (5) band applying aspect formula (3), can obtain:

${\mathrm{\Φ}}_V={\mathrm{\Φ}}_0+{\stackrel{\→}{r}}_0\mathrm{\γ}{\∫}_0^t\stackrel{\→}{G}\left(t^{\′}\right){\mathrm{dt}}^{\′}++{\stackrel{\→}{v}}_t\mathrm{\γ}{\∫}_0^t{t}^{\′}\stackrel{\→}{G}\left(t^{\′}\right){\mathrm{dt}}^{\′}+\frac{1}{2}{\stackrel{\→}{a}}_0\mathrm{\γ}{\∫}_0^t{t}^{\′2}\stackrel{\→}{G}\left(t^{\′}\right){\mathrm{dt}}^{\′}+\mathrm{...}---\left(6\right)$

Wherein Φ₀It it is the initial phase in 0 moment；

OrderIn representation formula (6) i-th, then formula (6) is reduced to:

${\mathrm{\Φ}}_V={\mathrm{\Φ}}_0+{\stackrel{\→}{m}}_0\mathrm{\γ}{\stackrel{\→}{r}}_0+{\stackrel{\→}{m}}_1\mathrm{\γ}{\stackrel{\→}{v}}_t+\frac{1}{2}{\stackrel{\→}{m}}_2\mathrm{\γ}{\stackrel{\→}{a}}_0+\mathrm{...}---\left(7\right)$

Add the spin-echo sequence of phase encoding gradient pulses, add a pair contrary bipolar field of intensity equidirectional Gradient (-G, G),Integral area in time domain is 0, then integral termEqual to 0, owing to convection current mixes Conjunction process velocity variations is slow,Ignore the impact of higher order term, then formula (7) is reduced to:

${\mathrm{\Φ}}_V={\mathrm{\Φ}}_0+\stackrel{\→}{m}\mathrm{\γ}{\stackrel{\→}{v}}_t---\left(8\right)$

What formula (8) reacted is that phase place change exists linear relationship with speed, can obtain phase place and flow velocity by demarcation Relation.By Phase shift imageWith resting state image Φ_V=Φ₀Do difference to remove non-speed factor and draw The Φ risen₀Change, the relation between speed and the Phase shift of the fluid finally flowed in magnetic field is rewritten asIts InRefer to the gradient of velocity encoded cine initial time, obtain t convection current mixed process velocity fieldI.e.

${\stackrel{\→}{v}}_t=\frac{{\mathrm{\Φ}}_V}{\stackrel{\→}{m}\mathrm{\γ}}=\frac{{\mathrm{\Φ}}_V}{\stackrel{\→}{G}\mathrm{\Δ}t\mathrm{\γ}}---\left(9\right)$

ThereforeWithLinear.

The invention has the beneficial effects as follows: the method achieve unconventional flow distribution that convection current mixing requires；Should Method application nmr imaging technique, visualizes the velocity field of convection current mixed process with noncontact, non-interfering means Observation, fluid flow will not produce disturbance, by obtaining the velocity field of convection current mixed process after quantitative analysis.

Accompanying drawing explanation

Fig. 1 is fill out sand tube and the convection current mixing of two kinds of different densities fluids of the upper lower leaf saturated porous media of the present invention Velocity field measurement system structure schematic diagram.

Fig. 2 is convection current mixed process velocity field image in porous media.

Detailed description of the invention

Below in conjunction with the accompanying drawings and with specific embodiment the present invention is elaborated:

Water/heavy water two kinds of different densities fluids convection current mixed process velocity field measurement method in porous media, its operation Step is as follows:

1) saturated porous media, will be equipped with the fill out sand tube of porous media, to be immersed in dyeing (being easy to observe interface location) water-soluble In liquid, insert evacuation in vacuum chamber, it is ensured that the fully saturated porous media of aqueous solution.By the fill out sand tube of fully saturated aqueous solution from Vacuum chamber takes out, is slowly injected into big density heavy aqueous solution from fill out sand tube lower end with syringe pump, it is ensured that two kinds of fluid boundarys are stable, Big density heavy aqueous solution injects from fill out sand tube lower end until interface reaches specific bit and postpones stopping, as shown in Figure 1.

2) fill out sand tube of the porous media of the saturated heavy water of upper strata saturation water lower floor is inserted in nuclear magnetic resonance imaging system, make By the spin-echo sequence method of the addition phase encoding gradient pulses resting state hydrogen matter to fill out sand tube two-phase fluid interface location Sub-density image measures；

3) fill out sand tube is taken out from nuclear magnetic resonance imaging system, and turned upside down (upper strata be saturated heavy water lower floor be full And water), the most carefully move in nuclear magnetic resonance imaging system, start convection current mixed process, use and add phase encoding gradient The free echo sequence method of pulse, measures the Hydrogen Proton density image of convection current mixed process at two-phase fluid interface location.

4) by resting state Hydrogen Proton density image convert to static state phase image, by close for the Hydrogen Proton of convection current mixed process Degree image is converted to convection current mixed process phase image；

5) deduct resting state phase image with convection current mixed process phase image, obtain Phase shift image.

6) to Phase shift image, phase method measurement is used to obtain two kinds of different densities fluids of water/heavy water at porous media Middle convection current mixed process velocity field, as shown in Figure 2.

Claims (2)

1. measure the method for convection current mixed process velocity field in porous media based on NMR (Nuclear Magnetic Resonance)-imaging for one kind, it is characterised in that step Rapid as follows:

The first step, by the fill out sand tube of the porous media of the saturated big density fluid of saturated for upper strata little density fluid lower floor, inserts nuclear-magnetism Resonance imaging system, uses the spin-echo sequence method adding phase encoding gradient pulses, measures fill out sand tube a certain section position The resting state proton density image put；

Second step, by fill out sand tube turned upside down, starts convection current mixed process, use add phase encoding gradient pulses from cycle Wave train method, measures the proton density image at cross section place identical with the first step in convection current mixed process；

3rd step, the resting state proton density image convert to static state phase image that the first step is obtained, second step is obtained Proton density image be converted to convection current mixed process phase image；

4th step, the convection current mixed process phase image obtained by the 3rd step deducts resting state phase image, obtains Phase shift Image；

5th step, the Phase shift image obtaining the 4th step, uses phase method to measure, obtains convection current eventually through formula (1) Mixed process velocity field

${\stackrel{\→}{v}}_t=\frac{{\mathrm{\Φ}}_V}{\stackrel{\→}{G}\mathrm{\Δ}t\mathrm{\γ}}---\left(1\right)$

For t convection current mixed process velocity field, Φ_VFor Phase shift image,It is the intensity of gradient pulse added by the first step, Δ is gradient pulse interval time, and t is the gradient pulse persistent period, and γ is gyromagnetic ratio.

One the most according to claim 1 is based on convection current mixed process velocity field in NMR (Nuclear Magnetic Resonance)-imaging measurement porous media Method, it is characterised in that described proton density image is Hydrogen Proton density image.