高溫高壓CO2狀態(tài)方程研究

韓 勇, 郭向利, 龍新平

(1. 中國工程物理研究院化工材料研究所, 四川 綿陽 621999; 2. 中國工程物理研究院, 四川 綿陽 621999)

1 引 言

炸藥爆轟時,爆轟產(chǎn)物處于高溫高壓狀態(tài),爆轟產(chǎn)物壓力由數(shù)十吉帕逐漸衰減至兆帕量級,溫度也由數(shù)千開爾文逐漸衰減至1000K甚至更低,建立能夠準(zhǔn)確描述爆轟產(chǎn)物氣體高溫、高壓的狀態(tài)方程對有效表征爆轟產(chǎn)物狀態(tài)變化過程具有十分重要的意義。

目前,實現(xiàn)爆轟產(chǎn)物氣體所處的高溫、高壓試驗條件十分困難,關(guān)于高溫、高壓氣體狀態(tài)的實驗數(shù)據(jù)很少。分子動力學(xué)(MD)/蒙特卡洛方法(MC)是研究爆轟產(chǎn)物氣體高溫高壓熱力學(xué)性質(zhì)的有力工具,它們通過有限的分子數(shù)目,結(jié)合邊界的約束處理,能夠準(zhǔn)確計算氣體的宏觀熱力學(xué)性質(zhì)。然而MD方法和MC方法計算耗時,且無法直接應(yīng)用。解析形式的狀態(tài)方程則可以直接應(yīng)用于實際問題,形成子程序直接納入相關(guān)程序中應(yīng)用。在描述爆轟產(chǎn)物氣體方面,已有BKW[1]、VLW[2]、LJD[3]、JCZ[4]等解析狀態(tài)方程形式。其中,VLW狀態(tài)方程為我國吳雄教授借鑒相似理論,認(rèn)為各階維里系數(shù)在高溫下是相似的,高階維里系數(shù)可以通過二階維里系數(shù)求得,進而將維里物態(tài)方程以一種簡化形式寫出,形成了VLW爆轟產(chǎn)物狀態(tài)方程[5-6],其成功應(yīng)用于炸藥爆轟性能的理論計算。與BKW狀態(tài)方程相比,VLW狀態(tài)方程在描述爆轟環(huán)境下氣體高溫高壓熱力學(xué)狀態(tài)時有了一定改善。然而VLW狀態(tài)方程形式仍然存在一些不足,VLW狀態(tài)方程不能有效描述爆轟產(chǎn)物氣體組分的高溫高壓熱力學(xué)狀態(tài),從而使其所采用的爆轟產(chǎn)物勢參數(shù)缺乏必要的物理基礎(chǔ)支撐。作者[7]前期應(yīng)用VLW狀態(tài)方程計算H2O沖擊Hugoniot曲線的結(jié)果表明,即使在優(yōu)化后的勢參數(shù)條件下,VLW狀態(tài)方程仍然不能在寬的壓力范圍有效描述H2O的高溫高壓狀態(tài)。

針對VLW狀態(tài)方程存在的上述不足,本研究提出了一種描述爆轟環(huán)境下高溫、高壓氣體的對比態(tài)維里型狀態(tài)方程VHL(Viral-Han-Long),該狀態(tài)方程基于LJ勢能函數(shù),通過對炸藥爆轟產(chǎn)物中的重要氣體組分CO2高溫高壓熱力學(xué)狀態(tài)的描述,表明對比態(tài)VHL狀態(tài)方程能夠有效地描述爆轟環(huán)境下CO2氣體的壓力、體積和溫度(pVT)熱力學(xué)關(guān)系。

2 對比態(tài)VHL狀態(tài)方程

理論上,任何氣體的狀態(tài)方程,都可以用維里形式描述。然而,實踐中隨著維里系數(shù)階數(shù)的提高,計算的復(fù)雜性迅速增大。Barker等[8]基于LJ勢能函數(shù),針對無量綱第三階、第四階、第五階系數(shù),進行了精確的理論計算,其以表格形式列出了特定溫度時的系數(shù)值,不適宜實際使用。在前期研究中,作者等[9]提出用一種簡化維里型狀態(tài)方程形式描述高溫甲烷氣體的熱力學(xué)pVT狀態(tài)。為有效描述爆轟環(huán)境下其它氣體產(chǎn)物的pVT熱力學(xué)關(guān)系,本研究提出了一種基于對比態(tài)原理的維里型狀態(tài)方程形式VHL,表達(dá)式見(1)。

(1)

C*=c1T*c2+c3T*c4+c5T*c6+c7T*c8+c9T*c10

D*=d1T*d2+d3T*d4+d5T*d6+d7T*d8+d9T*d10

E*=e1T*e2+e3T*e4

b0=2πNσ3/3

式中,p為氣體實際壓力,GPa;V為摩爾體積,cm3·mol-1;T為溫度,K;R為摩爾氣體常數(shù)8.3145,J·mol-1·K-1;N為阿佛加德羅常數(shù)6.022045×1023mol-1,第二階無量綱維里系數(shù)B*采用變步長辛卜生求積法近似計算獲得[10];C*、D*分別為第三階和第四階無量綱維里系數(shù),c1～c10、d1～d10為常數(shù),通過該表達(dá)式所得第三階和第四階無量綱維里系數(shù)值與理論值十分吻合; 對于第五階以上的無量綱維里系數(shù),則采用組合函數(shù)表示,其系數(shù)值e1～e4、a、b、f由無極性分子CH4的熱力學(xué)數(shù)據(jù)[11-12](溫度1000 K以上的112組pVT數(shù)據(jù))確定; 各階維里系數(shù)的擬合常數(shù)獲得方式見文獻[9]所述,具體數(shù)值見表1。T*為無量綱溫度,bCH4為67.21,w為對比態(tài)參量,VHL狀態(tài)方程通過參量w實現(xiàn)其它氣體與甲烷狀態(tài)方程的對比。ε、σ為LJ勢參數(shù)。

表1VHL狀態(tài)方程中各階維里系數(shù)擬合常數(shù)值

Table1Fitting constant values of each order viral coefficient in VHL Equation of State(EOS)

coefficientvaluecoefficientvaluecoefficientvaluec1-0.96665 d1-2.31154 e12.15701c2-4.51039 d2-7.49608 e2-2.52736c3-2.58733 d3-2.56864 e30.27080c4-0.85487 d4-1.14947 e4-1.00190c52.08033 d52.09534 a-0.00243c6-0.51631 d6-0.84757 b0.00018c72.02825 d72.81486 f3.60800c8-2.15543 d8-4.44480c9-0.12489 d9-0.30669c10-7.65161 d10-11.36945

3 CO2氣體高溫高壓熱力學(xué)狀態(tài)的計算

由于液態(tài)CO2樣品制備條件苛刻、難度大,其高溫、高壓的基礎(chǔ)實驗數(shù)據(jù)較少。1990年初,Schott等[13]才發(fā)表了用化爆技術(shù)在5～30 GPa區(qū)域獲得的一組試驗點。Nellis等[14]用二級輕氣炮技術(shù)在25～70 GPa區(qū)域獲得了一組試驗點。劉福生等[15]利用二級輕氣炮作沖擊加載手段,獲得了CO2在20～60 GPa區(qū)域六個Hugoniot數(shù)據(jù)點。實驗數(shù)據(jù)點偏少,且缺乏溫度的直接測量數(shù)據(jù)。因此,本研究以Belonoshko等[12]的分子動力學(xué)計算數(shù)據(jù)作為CO2的基礎(chǔ)數(shù)據(jù),溫度范圍為718～4978 K,壓力范圍為0.5116～111.078 GPa,基于VHL狀態(tài)方程,應(yīng)用復(fù)形調(diào)優(yōu)法[16]優(yōu)化了CO2的LJ勢參數(shù),勢參數(shù)值與文獻值比較見表2所示。采用VHL、VLW狀態(tài)方程計算結(jié)果和分子動力學(xué)計算值比較見表3所示,不同溫度或壓力下CO2體積計算偏差如圖1、圖2所示。由表3、圖1和圖2可得,采用VHL狀態(tài)方程計算得CO2體積平均絕對偏差為0.971%,最大偏差為4.04%, VHL狀態(tài)方程計算所得體積偏差與壓力和溫度參量無明顯的相關(guān)性。VLW狀態(tài)方程計算所得體積偏差則與溫度具有明顯的相關(guān)性,在較低溫度下,計算所得體積偏差較大，最大偏差87.149%,隨著溫度的升高,計算體積偏差逐漸減小,但仍普遍高于VHL狀態(tài)方程計算結(jié)果,采用VLW狀態(tài)方程計算所得平均絕對偏差20.2%。

表2本研究所采用CO2勢參數(shù)值與文獻值比較

Table2Comparison of the potential parameter values of CO2used in this paper and literature ones

(ε/k)/Kb0/mL·mol-1reference247.063.37[17]205.085.05[5]181.867.30thispaper

圖1不同壓力下VHL狀態(tài)方程、VLW狀態(tài)方程預(yù)測CO2的體積誤差百分比

Fig.1Error percentage of volume predicted by VHL EOS and VLW EOS at different pressure

圖2不同溫度下VHL狀態(tài)方程、VLW狀態(tài)方程預(yù)測CO2的體積誤差百分比

Fig.2Error percentage of volume predicted by VHL EOS and VLW EOS at different temperature

為驗證對比態(tài)VHL狀態(tài)方程在更低壓力下的有效性,同時與VLW狀態(tài)方程計算結(jié)果比較,本研究引

用NIST數(shù)據(jù)庫的數(shù)據(jù)[18],對CO2在1000 K,20～800 MPa的熱力學(xué)狀態(tài)進行了計算,結(jié)果見表4所示。采用對比態(tài)VHL狀態(tài)方程計算得CO2體積絕對平均偏差為0.698%,采用VLW狀態(tài)方程計算所得體積絕對平均偏差為11.988%。在固定溫度1000 K條件下,隨著壓力的增加,VLW計算所得體積偏差逐漸增大,在壓力為800 MPa時,體積偏差最大達(dá)-19.771%。其原因可能與VLW狀態(tài)方程形式的高階維里系數(shù)過度簡化有關(guān),隨著壓力增大,描述多個氣體分子同時相互作用的高階維里系數(shù)的準(zhǔn)確性要求提高,而VLW狀態(tài)方程中,除第二階維里系數(shù)與理論值相符合外,隨著維里系數(shù)階級的增大,其與理論值的差距也逐漸增大,故CO2的體積計算結(jié)果偏差隨壓力增大而增大。而本文所提出的對比態(tài)VHL狀態(tài)方程的第三階、第四階維里系數(shù)均與理論值吻合,高階維里系數(shù)則通過甲烷高溫高壓熱力學(xué)狀態(tài)數(shù)據(jù)優(yōu)化獲得,其具有扎實物理基礎(chǔ),因此,該狀態(tài)方程能夠很好描述CO2高溫狀態(tài)下較低壓力范圍內(nèi)的熱力學(xué)狀態(tài)。

表3VHL、VLW狀態(tài)方程計算CO2高溫pVT關(guān)系

Table3ThepVTrelation of CO2at high temperature calculated by the means of VHL EOS and VLW EOS

No. T/K p/GPaV/cm3·mol-1MD[12]VHLVLWerror/% VHL VLW1718.80.51164040.9118.132.275-54.6752798.80.58294040.3125.830.775-35.4253920.20.63964040.7331.431.825-21.4254963.90.68354040.3632.260.900-19.350511130.77324040.4335.221.075-11.950612300.84604040.4436.721.100-8.200712960.92084039.8536.74-0.375-8.1508720.60.77453535.9514.892.714-57.4579774.70.84463535.5320.111.514-42.54310902.00.94643535.4625.661.314-26.68611805.71.4803030.3617.631.200-41.23312999.41.7433029.9922.55-0.033-24.8331311901.9913029.8024.87-0.667-17.1001415092.2903030.0427.480.133-8.4001516752.4393030.1728.400.567-5.3331618072.6683029.8128.49-0.633-5.0331719452.8323029.7528.86-0.833-3.80018773.82.1682727.2613.790.368-49.22719887.32.3622727.0217.49-0.515-35.6042010162.4882727.1120.00-0.184-26.3622111683.6992524.7119.42-1.160-22.3202219834.9522524.7723.39-0.920-6.4402327196.0672524.7224.72-1.120-1.1202436377.0652525.0426.030.1604.1202544197.9142525.1826.720.7206.8802650608.0822525.8727.813.48011.240

Table3continued

No. T/K p/GPaV/cm3·mol-1MD[12]VHLVLWerror/% VHL VLW27700.84.4822222.662.7994.040-87.14928812.94.7212222.4511.923.076-45.27129913.44.9552222.3114.032.433-35.5833010245.2902222.1215.431.561-29.1553112187.9302020.1915.350.950-23.25032186410.522019.5617.35-2.200-13.25033262812.182019.6318.86-1.850-5.70034432115.052019.9220.70-0.4003.50035497816.362019.8821.00-0.6005.00036803.211.351818.238.881.278-50.68937122812.471818.0013.310.000-26.05638164113.561818.0015.070.000-16.27839197714.521817.9915.94-0.056-11.44440241415.641818.0216.770.111-6.83341275616.741817.9617.17-0.222-4.61142316117.561818.0417.680.222-1.77843346918.241818.0617.990.333-0.05644783.019.471616.147.190.875-55.08745118620.621615.8811.14-0.750-30.37546164521.921615.8812.91-0.750-19.31347200622.941615.9213.79-0.500-13.81348231824.111615.9014.31-0.625-10.56249277025.071616.0015.010.000-6.18750307226.321615.9415.27-0.375-4.56351338427.041615.9915.59-0.063-2.56252395627.991616.1216.160.7501.00053803.726.221515.097.010.600-53.28054119527.391514.8510.27-1.000-31.53355160828.631514.8311.77-1.133-21.53356206929.971514.8712.81-0.867-14.60057240030.921514.9113.36-0.600-10.93358271131.951514.9213.77-0.533-8.20059307532.891514.9714.19-0.200-5.40060340133.671515.0114.520.067-3.20061393834.741515.1015.010.6670.06762787.235.671414.126.170.857-55.90063121937.021413.849.51-1.143-32.07164165938.361413.8110.91-1.357-22.07165205039.581413.8311.71-1.214-16.35766234040.501413.8512.17-1.071-13.07167279341.971413.8912.74-0.786-9.00068319042.681413.9613.19-0.286-5.78669340144.181413.9013.29-0.714-5.07170789.649.451313.145.701.077-56.12371119250.771312.868.57-1.077-34.05472159452.071312.819.80-1.462-24.64673198453.321312.8210.57-1.385-18.69274231354.381312.8411.07-1.231-14.84675288656.211312.8711.74-1.000-9.69276307957.431312.8511.89-1.154-8.53877355158.001312.9312.34-0.538-5.07778395658.711312.9912.67-0.077-2.53879782.069.771212.215.111.750-57.425

Table3continued

No. T/K p/GPaV/cm3·mol-1MD[12]VHLVLWerror/% VHL VLW80120871.241211.917.86-0.750-34.52581161372.641211.848.94-1.333-25.50882199173.981211.839.61-1.417-19.91783240775.371211.8410.17-1.333-15.25084285476.901211.8610.64-1.167-11.33385323878.241211.8710.97-1.083-8.58386349879.161211.8811.16-1.000-7.00087807.0100.71111.264.962.364-54.882881209102.21110.987.12-0.182-35.264891599103.61110.908.06-0.909-26.755901971104.91110.888.66-1.091-21.273912416106.61110.879.20-1.182-16.400922866108.11110.889.63-1.091-12.500933012109.21110.879.73-1.182-11.545943326109.61110.899.98-1.000-9.236953959111.11110.9210.41-0.727-5.364

Note: error=100×(VEOS-VMD)/VMD

表41000 K時CO2pVT關(guān)系的VHL、VLW狀態(tài)方程計算值及與NIST數(shù)據(jù)庫數(shù)據(jù)的比較

Table4Comparison of the calculated values of CO2pVTrelation at 1000 K by VHL EOS and VLW EOS and the data of CO2in NIST database

No.p/GPa V/cm3·mol-1 NIST[18] VLW VHLerror/% VLW VHL10.02434.60436.9437.400.5290.64820.04228.77230.1231.500.5811.18730.06161.37161.6163.700.1431.47240.08128.35127.5130.30-0.6621.52750.10108.89107.0110.50-1.7361.43860.1296.07393.3597.31-2.8341.28970.1486.96983.5787.96-3.9081.13580.1680.15376.1980.94-4.9440.98590.1874.84270.4175.48-5.9220.847100.2070.57865.7671.08-6.8260.716110.2267.06961.9167.47-7.6920.595120.2464.12458.6864.43-8.4900.482130.2661.61355.9261.84-9.2400.374140.2859.44253.5359.60-9.9460.270150.3057.54251.4457.64-10.6040.172160.3255.86249.5855.91-11.2460.079170.3454.36447.9354.36-11.835-0.009180.3653.01746.4552.97-12.387-0.092190.3851.79845.1151.71-12.912-0.170200.4050.68743.8950.56-13.410-0.242210.4249.66942.7749.52-13.890-0.309220.4448.73241.7548.55-14.327-0.371230.4647.86640.8047.66-14.762-0.430240.4847.06239.9246.83-15.176-0.484250.5046.31239.1146.07-15.551-0.533260.5245.61138.3545.35-15.919-0.578270.5444.95437.6444.67-16.270-0.621280.5644.33636.9744.04-16.614-0.660290.5843.75436.3543.45-16.922-0.697300.6043.20435.7642.89-17.230-0.732310.6242.68335.2042.36-17.532-0.764

Table4Continued

No.p/GPaV/cm3·mol-1NIST[18]VLWVHLerror/%VLWVHL320.6442.18934.6741.85-17.822-0.794330.6641.71934.1741.38-18.095-0.822340.6841.27233.7040.92-18.347-0.848350.7040.84533.2440.49-18.619-0.872360.7240.43832.8140.08-18.863-0.896370.7440.04932.4039.68-19.099-0.920380.7639.67632.0139.30-19.322-0.941390.7839.31931.6338.94-19.555-0.963400.8038.97631.2738.59-19.771-0.983

Note: error=100×(VEOS-VNIST)/VNIST.

4 結(jié) 論

本研究提出了一種基于LJ勢能函數(shù)的對比態(tài)維里型狀態(tài)方程VHL用于描述爆轟環(huán)境下CO2的高溫高壓熱力學(xué)狀態(tài)。采用VHL狀態(tài)方程計算得CO2體積平均絕對偏差為0.971%,最大偏差為4.04%,采用VLW狀態(tài)方程計算所得平均絕對偏差20.2%,最大偏差87.149%。因此,在計算CO2高溫、中高壓熱力學(xué)狀態(tài)時,VHL狀態(tài)方程的計算準(zhǔn)確性得到了大幅度提高。VHL狀態(tài)方程計算所得體積偏差與壓力和溫度參量無明顯的相關(guān)性; VLW狀態(tài)方程計算所得體積偏差則與溫度具有明顯的相關(guān)性,隨著溫度的升高,計算體積偏差逐漸減小。

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