CN103228073A - Resonant cavity used for improving microwave pretreatment effect of timbers - Google Patents

Abstract

A resonant cavity used for improving the microwave pretreatment effect of timbers comprises a horizontal cuboid microwave cavity A and two vertical cuboid microwave cavities B and C, wherein the B and C are respectively connected with the upper surface and the lower surface of the A; the contact surfaces of the B and C with the A are hollow rectangles; the left-and-right width of each rectangle is 0.24765 m, and the anteroposterior length of each rectangle is 0.12382 m; an upper opening of the B and a lower opening of the C are open; a front opening and a rear opening of the A are open; the left-and-right width of the A equals to that of the B and that of the C; and the up-and-down height of the A ranges from 0.08 to 0.10 m. The resonant cavity can improve the microwave pretreatment effect of the timbers, and is of great importance for the later immersion treatment of the timbers and the preparation for new material with high performance.

Description

A kind of resonant cavity that is used to improve timber Microwave Pretreatment effect

Technical field

The present invention relates to timber Microwave Pretreatment technical field, be used for directly providing the resonant cavity of microwave energy specifically to timber.

Background technology

Microwave drying timber is to make the whole heat of giving birth to of timber self with uniform microwave field, can eliminate temperature gradient and moisture gradient in the dry run, wherein moisture is discharged the expansion constriction coefficient basically identical that makes timber from inside to outside simultaneously, timber can be avoided losses such as distortion, cracking, withered, carbonization to greatest extent in dry run like this, can improve the lumber recovery of timber.Compare with the traditional drying mode, have that rate of drying is big, energy-conservation, production efficiency is high, uniform drying, cleaner production, easily realize automation control and advantage such as improve the quality of products being easy to control in the microwave drying process; And do not have noise and noxious gas emission, belong to the environmental protection dry technology.The timber internal temperature that micro-wave drying makes is higher than the surface.Make the outside pressure differential of the inner formation of timber, quickened the surface migration of moisture, so the speed of microwave drying timber is much larger than 30 times of fast 20 – of convective drying speed.Reduce more than 25% than far infrared and steam drying energy consumption.The microwave drying material can make the timber temperature inside be higher than 60 ℃, uses worm's ovum parasitic in the timber to kill, reach effect mothproof, mildew-resistant very soon.To get suitable microwave heating speed earlier for various timber of different nature, reach best process conditions, can obtain better benefit.

China's quick-growing plantation area and storage capacity all occupy first place in the world, but the fast growing wood ubiquity defectives such as poor permeability, dry and later stage be anticorrosion, fire-retardant, makes its solid wood high value added utilization not make a breakthrough always.The high-intensity microwave preliminary treatment is the new technology that appears at the Wooden modifying field in recent years, its basic principle is to utilize high-intensity microwave that unseasoned lumber is carried out instantaneous processing, make the interior moisture of timber obtain abundant energy at short notice, produce the gentle body heat of phase transformation and press effect, under steam expanded power drives, destroy the timber internal structure, improve the fluid migration ability, for the drying in timber later stage, impregnate processing, even very favourable precondition is created in the new material preparation.At present, countries such as the U.S., Australia and China have all begun to manufacture experimently high-strength timber Microwave Pretreatment special equipment, and have studied the influence rule of Microwave Pretreatment to timber measurement expansion rate, permeability and rate of drying.Its Primary Study is the result show, the high-intensity microwave preliminary treatment of optimization can make timber measurement increase more than 10%, and the anticorrisive agent infiltration capacity increases 10-14 doubly, and rate of drying improves 5-10 doubly, and dry mass significantly improves.Univ Melbourne Australia even also timber is disassembled and reconstruction processing by this technology, manufactured experimently out have high osmosis, low-density novel timber and novel wooden composite material with high strength, high surface hardness.This research is that a brand-new road has been opened up in the high materialization utilization of quick-growing plantation timber resources.But in timber Microwave Pretreatment research process, the researcher finds: timber is through after the Microwave Pretreatment, and microcosmic that it is inner and macroporosity distribute even inadequately, and the preparation of impregnating processing and high-performance new material to the later stage of timber has brought very big difficulty.These inhomogeneous and character and dielectric constant skewness timber itself have certain relation, yet we can't change the attribute of timber itself, can only improve it and heat external condition to reduce its drying loss.

Relevant with heating condition microwave frequency and the microwave heating cavity of mainly containing, the Temperature Distribution of timber during the two common decision heating.The microwave frequency that is used for microwave drying timber has two kinds of 0.915Ghz and 2.45Ghz, finds that in practice frequency is that the microwave of 0.915Ghz is comparatively favourable to timber heating-up temperature distributing homogeneity, can consider to use this frequency more in design.After frequency is selected, the design of microwave cavity has just been transferred in the center of problem, therefore, how the microwave cavity that is evenly distributed by the development microwave energy is developed the efficient microwave pre-processing device, has become the core and the key issue of the high-strength Microwave Pretreatment technical research of restriction timber.

At present, a kind of resonant cavity that is used for the timber Microwave Pretreatment is arranged, be single port oblique incidence structure, high-temperature area off-center zone was unfavorable for forming the low inside and high outside temperature difference when problem of existence was heating, caused pretreating effect not good.

Summary of the invention

Problem to be solved by this invention is that high-temperature area is distributed in positive center and guarantees the uniformity of Temperature Distribution when how to make the timber Microwave Pretreatment.

In conjunction with microwave timber drying theory, we have simulated multiple situation, are the temperature distribution evenness design of a heating resonant cavity preferably below.

A kind of resonant cavity that is used to improve timber Microwave Pretreatment effect, the cuboid microwave cavity B and the C that comprise a horizontal cuboid microwave cavity A and two vertical directions, B and C join with the upper and lower surface of A respectively, the contact-making surface of B and C and A all is the rectangle of a hollow out, and this rectangle left and right sides is wide to be 0.24765m, and front and back are long to be 0.12382m, the end opening of the suitable for reading and C of B is an opening, two mouthfuls of openings before and after the A, A left and right sides width and B, C equate that the altitude range up and down of A is at 0.08-0.10m.

The front and back length of A is greater than 0.20m.

The height of B and C is all greater than 0.02m.

The front and back position of B and C is at the mid point of A.

The timber upper and lower surface has synchronous symmetrical microwave energy feed-in, this feed-in mode makes electromagnetic energy concentrate on timber more equably, select the optimum height of microwave cavity A can elevate the temperature more even, simultaneously because microwave energy concentrates on the timber middle part relatively, cause the middle part temperature higher, help the drying of timber.Improving timber Microwave Pretreatment effect impregnates the later stage of timber and handles and the preparation of high-performance new material all has important meaning.

Description of drawings

Fig. 1 is a twoport feed-in cavity resonator structure schematic diagram

Fig. 2 is a single port feed-in cavity resonator structure schematic diagram

Fig. 3 is the timber multi-section temperature profile on the parallel yz of twoport feed-in plane

Fig. 4 is the timber multi-section temperature profile on the parallel yz of single port feed-in plane

Fig. 5 is the temperature profile on the twoport feed-in timber central cross-section

Fig. 6 is the temperature profile on the single port feed-in timber central cross-section

Fig. 7 is the timber multi-section temperature profile on the parallel yz of h=0.07m plane

Fig. 8 is the timber multi-section temperature profile on the parallel yz of h=0.08m plane

Fig. 9 is the timber multi-section temperature profile on the parallel yz of h=0.09m plane

Figure 10 is the timber multi-section temperature profile on the parallel yz of h=0.10m plane

Figure 11 is the timber multi-section temperature profile on the parallel yz of h=0.11m plane

Figure 12 is the timber multi-section temperature profile on the parallel yz of h=0.12m plane

Figure 13 is a timber temperature profile on the h=0.07m central cross-section

Figure 14 is a timber temperature profile on the h=0.08m central cross-section

Figure 15 is a timber temperature profile on the h=0.09m central cross-section

Figure 16 is a timber temperature profile on the h=0.10m central cross-section

Figure 17 is a timber temperature profile on the h=0.11m central cross-section

Figure 18 is a timber temperature profile on the h=0.12m central cross-section

Figure 19 is the variation relation figure of Temperature Distribution variance and A height h

Embodiment

The concrete key factor of implementing this invention is the structure and the dimensional parameters in chamber, and we will determine dimensional parameters more accurately, so that this resonant cavity reaches its best effort effect.

The Temperature Distribution feature of twoport feed-in and single port feed-in is described with the method for analog computation below.

We have adopted finite element analysis software COMSOL Multiphysics to simulate, and in this analog computation, selecting water content for use is 60% timber,

Its conductive coefficient is

(0.23* 1-0.72* (25-T)/100) [w/ (m ℃)] (temperature when T is this)

Specific heat is

2650*(1+T/100)^0.2（?J/kg?℃）

Density is 570 (kg/m3)

Absorption coefficient is 9.97 (1/m)

Dielectric constant is 7.40-1.42j

Initial temperature is 25 ℃

The microwave of feed-in is the TE10 ripple, and frequency is 0.915GHz, and the power of every mouthful of feed-in is 10kw.

Calculate the temperature on the timber every bit, need be divided into a lot of small parts to timber, energy and the temperature relation of studying each part respectively are (certain, need the heat transmission of consideration and remainder), and then the result in each step comprehensively to together, thereby finished whole calculating, the basic thought of finite element analysis that Here it is.

In our analog computation, the specific heat of timber, with conductive coefficient along with variations in temperature, because temperature is time dependent, therefore, specific heat, conductive coefficient, temperature all are the functions of time, if guarantee this three all is the value of synchronization, and that just must be quite little dividing ground the computing time in each step, and this is obviously very loaded down with trivial details and there is no need.Can take such computational methods, supposition timber temperature is constant in very short a period of time earlier, therefore specific heat and conductive coefficient also are constants, just can be with the energy of these two constant calculations timber absorption during this period of time, obtain the variations in temperature of timber simultaneously, new temperature value substitution specific heat and conductive coefficient are continued cycle calculations like this, calculate preset time always.

Be that example is illustrated with the twoport feed-in below, the single port feed-in is that port number is different with feed-in power.

Following expression formula is the origin of coordinates with limit, bottom right, microwave cavity A rear surface intersection point, and parameter f is a microwave frequency, and w is a waveguide length, and d is a duct width, and ε, μ are the dielectric constant and the magnetic permeability of air.

Propagation constant

$\mathrm{\β}=\frac{2\mathrm{\π}}{c}\sqrt{f^2-{\left(\frac{c}{2w}\right)}^2}$

The feed-in port one

Input power

$P=\frac{1}{2}{\left(\frac{\mathrm{\ϵ}}{\mathrm{\μ}}\right)}^{\frac{1}{2}}{\∫}_0^w{\∫}_0^d{\left|E_x\right|}^2{d}_y{d}_x$

The input electric field

$\left(\begin{array}{c}{E}_x=\sin (\mathrm{\π}\·y/w)\\ E_y=0\\ E_z=0\end{array}\right.$

Feed-in port 2

$P=\frac{1}{2}{\left(\frac{\mathrm{\ϵ}}{\mathrm{\μ}}\right)}^{\frac{1}{2}}{\∫}_0^w{\∫}_0^d{\left|E_x\right|}^2{d}_y{d}_x$

$\left(\begin{array}{c}{E}_x=\sin (\mathrm{\π}\·y/w)\\ E_y=0\\ E_z=0\end{array}\right.$

During simulation, the material that heating chamber is selected for use is a copper, and the boundary condition of copper is

$\sqrt{\frac{{\mathrm{\μ}}_0{\mathrm{\μ}}_r}{{\mathrm{\ϵ}}_0{\mathrm{\ϵ}}_r-j\frac{\mathrm{\σ}}{\mathrm{\ω}}}}n\×H+E-(n\·E)n=(n\·E_s)n-E_s$

Regard the border outside copper bound circle and the feed-in port as desired electrical conductor border

n×E=0

The electric field wave equation

$\&dtri;\&times;{{\mathrm{\&mu;}}_r}^{-1}(\&dtri;\&times;E)-{{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="111809DEST\_PATH\_130531145147.png,126535DEST\_PATH\_130531145202.png"\; state="\{\{state\}\}">k</figure-callout>}}_0}^2({\mathrm{\&epsiv;}}_r-\frac{\mathrm{j\&sigma;}}{\mathrm{\&omega;}{\mathrm{\&epsiv;}}_0})E=0$

The electric field that changes can produce magnetic field, and the gross energy feed-in is made up of electric field energy and magnetic field energy two parts

Q=Q _rh+Q _ml

$Q_{\mathrm{rh}}=\frac{1}{2}\mathrm{Rc}(J\&CenterDot;E^{*})$

$Q_{\mathrm{ml}}=\frac{1}{2}\mathrm{Rc}(\mathrm{j\&omega;B}\&CenterDot;H^{*})$

Do not consider the exchange heat of timber border and air, promptly wood surface is regarded as heat-insulating

$-n\&CenterDot;(-k\&dtri;T)=0$

Assumed temperature is a constant before this step, calculates the heat and the variation of temperature of absorption with this, again with the next circulation of new temperature computation, and so forth.

Heat temperature equation

${\mathrm{\&rho;C}}_P=\frac{\&PartialD;T}{\&PartialD;t}+\mathrm{\&rho;}{C}_{P}u\&CenterDot;\&dtri;T=\&dtri;\&CenterDot;(k\&dtri;T)+\underset{...}{Q}$

Twoport feed-in cavity resonator structure is introduced

As shown in Figure 1, to be the place ahead towards the X-axis positive direction, then the Y-axis positive direction is a left, and Z axle positive direction is the top.

This resonant cavity comprises the cuboid microwave cavity B and the C of a horizontal cuboid microwave cavity A and two vertical directions, B and C join with the upper and lower surface of A respectively, the contact-making surface of B and C and A all is the rectangle of a hollow out, the wide w=0.24765m in this rectangle left and right sides, the long d=0.12382m in front and back, the end opening of the suitable for reading and C of B is an opening, two mouthfuls of openings before and after the A, A left and right sides width and B, C equates, that A highly is up and down h=0.08m, the front and back length of A is 2w, and the height of B and C all is 0.05m, and the front and back position of B and C is at the mid point of A, cuboid timber center overlaps with the A center, timber highly is up and down 0.06m, and left and right sides width is 0.22m, and front and back length is 3w.Microwave is to distinguish feed-in from the end opening of the suitable for reading and C of B.

Single port feed-in cavity resonator structure is introduced

As shown in Figure 2, the difference of single port feed-in and twoport feed-in is to have lacked C, and microwave cavity B tilts, and the contact-making surface of B and A is at the middle part of A, and microwave becomes 60 degree angle single port feed-ins from the suitable for reading of B with the xy plane.

Temperature profile

After software simulation calculating, we have obtained the timber multi-section temperature profile on the two kinds of parallel yz of feed-in situation planes, as Fig. 3 Fig. 4, from two figure, all can draw, multi-section is similar to the Temperature Distribution of central cross-section, and the central cross-section that can select both is object as a comparison, simultaneously, microwave energy is absorbed by the central area mostly, and selecting central cross-section is that the meaning represented is arranged very much as research object.

Fig. 5 Fig. 6 is respectively the temperature profile on twoport feed-in and the single port feed-in timber central cross-section, as can be seen from the figure twoport feed-in high temperature distributed area is positioned at the center, single port feed-in high-temperature area then below, illustrate that the twoport feed-in can guarantee that energy is distributed in the timber core.

Thickness is the above-mentioned timber microwave heating temperature distribution situation of 0.06m to the height h that we have simulated A respectively 0.07,0.08,0.09,0.10,0.11, during 0.12m, to determine the optimum height of A.

Temperature profile

After software simulation calculating, we have obtained the timber multi-section temperature profile on parallel yz plane under the highly different situation of h, as Fig. 2-7, from figure, all can draw, multi-section is similar to the Temperature Distribution of central cross-section, and the central cross-section that can select both is object as a comparison, simultaneously, microwave energy is absorbed by the central area mostly, and selecting central cross-section is that the meaning represented is arranged very much as research object.

Fig. 8-13 be respectively height h 0.07,0.08,0.09,0.10,0.11, during 0.12m the temperature profile on the timber central cross-section.

Variance ratio is than uniformity

By the Probability Theory and Math Statistics relevant knowledge as can be known, standard deviation can reflect the dispersion degree of a data set, and standard deviation is more little, and it is more little that data depart from its average, and promptly the data set dispersion degree is low more, and data distribute even more on the whole.We can come the C.T distributing homogeneity with variance.

Evenly get the pairing temperature value of each point on the central cross-section, ask its variance then.

Because x=0.24675 is the front and back symmetric points on the X-axis, gets a plane from 0.21675-0.24675 every 0.01 and can represent overall distribution.With h=0.08m is example, central section temp distribution standard deviation S(0.21675 when process calculates the twoport feed-in)=28.4044, S(0.22675)=29.68396, S(0.23675)=30.61189, S(0.24675)=30.83701, the mean value of standard deviation is 29.88432, this mean value and central cross-section S(0.24675)=30.83701 be more or less the same, in the comparison afterwards, directly adopt the standard deviation of central cross-section to get final product.

Through calculating not central section temp distribution standard deviation S(0.07 simultaneously of h value)=28.77943, S(0.08)=30.83701, S(0.09)=28.00241, S(0.10)=25.67909, S(0.11)=26.66922, S (0.12)=27.46088.

Analyze and conclusion

From Fig. 8-13 as can be seen, high-temperature area shifts to the left and right sides behind h 〉=0.11m, and medium temperature is lower on the contrary, does not form low inside and high outside Temperature Distribution form, is unfavorable for the drying of wood, therefore, and height h＜0.11m of microwave cavity A.

As can be seen from Figure 14, when h≤0.08m, standard deviation is bigger, and promptly temperature distribution evenness is bad, and Temperature Distribution too concentrates on the center, may cause that central temperature is too high to cause damage to timber, so the height h of microwave cavity A〉0.08m.

To sum up, consider that from temperature distribution evenness and drying efficiency two aspects the height of microwave cavity A is preferably in 0.08m＜h in the following span＜0.11m.

Therefore our this twoport feed-in resonant cavity that draws can improve timber microwave drying temperature distribution evenness really, and the center temperature is higher simultaneously, helps the drying of timber, and this resonant cavity has its Practical significance.

Claims (4)

1. resonant cavity that is used to improve timber Microwave Pretreatment effect, it is characterized by the cuboid microwave cavity B and the C that comprise a horizontal cuboid microwave cavity A and two vertical directions, B and C join with the upper and lower surface of A respectively, the contact-making surface of B and C and A all is the rectangle of a hollow out, and this rectangle left and right sides is wide to be 0.24765m, and front and back are long to be 0.12382m, the end opening of the suitable for reading and C of B is an opening, two mouthfuls of openings before and after the A, A left and right sides width and B, C equate that the altitude range up and down of A is at 0.08-0.10m.

2. according to a kind of resonant cavity that is used to improve timber Microwave Pretreatment effect in the claim 1, the front and back length that it is characterized by A is greater than 0.20m.

3. according to a kind of resonant cavity that is used to improve timber Microwave Pretreatment effect in the claim 1, the height that it is characterized by B and C is all greater than 0.02m.

4. according to a kind of resonant cavity that is used to improve timber Microwave Pretreatment effect in claim 1 or 2 or 3 or 4, it is characterized by the mid point of the front and back position of B and C at A.

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