變密度炭化復(fù)合材料的熱防護(hù)模型及其數(shù)值模擬
發(fā)布時(shí)間：2018-08-25 08:21
【摘要】：航天器返回艙穿越地球大氣層飛行時(shí)受到嚴(yán)重的氣動(dòng)加熱,為了克服由于氣動(dòng)加熱造成的"熱障",對(duì)返回艙進(jìn)行熱防護(hù)是必不可少的。根據(jù)返回艙再入大氣層時(shí)的環(huán)境特點(diǎn),即高比焓、低熱流密度、低壓、低剪力和較長(zhǎng)的再入時(shí)間,通常選用炭化復(fù)合材料作為熱防護(hù)材料。針對(duì)急劇變化的航天器服役環(huán)境,均勻密度炭化復(fù)合材料的熱防護(hù)效率比較低,因此,變密度炭化復(fù)合材料的設(shè)計(jì)是熱防護(hù)系統(tǒng)發(fā)展方向。本文基于傳熱學(xué)、物理化學(xué)、氣動(dòng)熱力學(xué)、燃燒學(xué)、數(shù)值傳熱學(xué)、數(shù)值分析等理論,開(kāi)展變密度炭化復(fù)合材料的熱防護(hù)模型及其數(shù)值模擬研究。基于炭化復(fù)合材料燒蝕機(jī)理,假設(shè)熱解反應(yīng)均發(fā)生在熱解面上,發(fā)展了變密度炭化復(fù)合材料的一維熱解面模型。根據(jù)傳熱學(xué)理論,對(duì)原始材料層與炭化層分別建立一維瞬態(tài)傅里葉熱傳導(dǎo)方程,方程中熱物理性能參數(shù)是密度或密度與溫度的函數(shù);在移動(dòng)的熱解面處建立溫度連續(xù)條件與熱解能量守恒條件;在燒蝕表面處建立能量守恒關(guān)系,它與表面燒蝕率相關(guān),而碳的氧化燒蝕率是壁面溫度的函數(shù)。基于數(shù)值傳熱學(xué)方法,利用二階中心差分格式和一階向前差分格式分別對(duì)靜態(tài)坐標(biāo)系下變密度炭化復(fù)合材料導(dǎo)熱偏微分方程中的空間項(xiàng)和時(shí)間項(xiàng)進(jìn)行離散,獲得隱式的離散格式。針對(duì)帶有移動(dòng)邊界與移動(dòng)界面的離散格式,提出一種新的非線性計(jì)算方法:利用上一時(shí)刻的結(jié)果確定材料總厚度、原始材料層厚度與炭化層厚度,更新空間節(jié)點(diǎn)及熱解面節(jié)點(diǎn),利用三對(duì)角陣算法和牛頓迭代法對(duì)當(dāng)前時(shí)刻的隱式離散格式進(jìn)行求解;利用當(dāng)前時(shí)刻的溫度,由燒蝕表面溫度及燒蝕率的函數(shù)求得燒蝕表面移動(dòng)距離,由不動(dòng)點(diǎn)迭代法求得滿足熱解面能量守恒條件的熱解面移動(dòng)距離。將上述計(jì)算方法通過(guò)MATLAB編程實(shí)現(xiàn),進(jìn)一步分析了在常熱流作用下均勻材料的熱響應(yīng)以及在變熱流作用下均勻與變密度材料的熱響應(yīng)。數(shù)值結(jié)果表明:變密度炭化復(fù)合材料熱解面模型可以應(yīng)用于求解均勻密度炭化復(fù)合材料的燒蝕及熱響應(yīng);變密度炭化復(fù)合材料具有更高的有效熱熔,能夠提高熱防護(hù)系統(tǒng)的防熱效率。為了更能精確地反映炭化復(fù)合材料的熱響應(yīng),建立變密度炭化復(fù)合材料的一維熱解層模型,其特點(diǎn):在炭化層與原始材料層之間的熱解層中既有熱解反應(yīng)又有熱解氣體流動(dòng),熱解反應(yīng)導(dǎo)致熱解層的密度不斷變化,熱解層的熱物理性能是密度與溫度的函數(shù)。在該模型中,除了原始材料層和炭化層的控制方程、燒蝕表面邊界條件分別與熱解面模型的相同之外,增加熱解層的瞬態(tài)熱傳導(dǎo)方程,兩個(gè)內(nèi)部移動(dòng)界面的溫度與熱流連續(xù)條件。為了簡(jiǎn)化計(jì)算,熱解層的密度與熱物理性能參數(shù)做線性處理。利用熱解面模型的離散方法,對(duì)熱解層數(shù)學(xué)模型構(gòu)造其隱式的離散格式。針對(duì)帶有移動(dòng)邊界和雙移動(dòng)界面的非線性離散方程組,發(fā)展新的求解方法:利用上一時(shí)刻的結(jié)果確定材料總厚度、原始材料層厚度、熱解層厚度與炭化層厚度,劃分空間節(jié)點(diǎn),更新移動(dòng)界面節(jié)點(diǎn),采用三對(duì)角陣算法和牛頓迭代法對(duì)當(dāng)前時(shí)刻的隱式離散格式進(jìn)行求解;由燒蝕表面溫度及燒蝕率的函數(shù)確定燒蝕表面移動(dòng)距離,由牛頓弦截法確定滿足熱流連續(xù)條件的內(nèi)部雙界面移動(dòng)距離�；贛ATLAB平臺(tái),利用上述求解方法對(duì)隱式的離散格式進(jìn)行編程,計(jì)算分析了均勻材料在常熱流作用下的熱響應(yīng)、均勻材料和變密度材料在變熱流作用下的熱響應(yīng),以及對(duì)比分析了均勻材料在常熱流作用下熱解面模型與熱解層模型的計(jì)算結(jié)果。數(shù)值結(jié)果表明:通過(guò)對(duì)均勻材料熱解層模型的數(shù)值計(jì)算結(jié)果與前人的試驗(yàn)結(jié)果對(duì)比,驗(yàn)證了所建立的熱解層模型可以應(yīng)用于求解均勻材料的熱響應(yīng);在服役過(guò)程中,變密度熱防護(hù)層燒蝕表面上的有關(guān)參數(shù)(溫度、燒蝕率、熱解氣體質(zhì)量流率)及各層厚度不僅與氣動(dòng)熱流有關(guān),還與材料密度分布息息相關(guān);變密度材料具有較高的有效熱熔,能夠提高熱防護(hù)系統(tǒng)的防熱效率;經(jīng)過(guò)兩個(gè)模型的對(duì)比,發(fā)現(xiàn)熱解面溫度的選取對(duì)熱解面模型的計(jì)算精度至關(guān)重要。上述兩個(gè)模型中均假設(shè)表面燒蝕率為溫度的函數(shù),未考慮熱解氣體在激波層內(nèi)的燃燒反應(yīng)對(duì)材料表面燒蝕的影響。為了精確分析材料表面的燒蝕率,基于熱解氣體燃燒的層流流動(dòng)假設(shè),利用氣動(dòng)熱力學(xué)、傳熱學(xué)、燃燒學(xué)、物理化學(xué)等理論,建立炭化復(fù)合材料的熱-流-化學(xué)-燒蝕多場(chǎng)耦合模型,該模型包含:正激波方程組、熱解層數(shù)學(xué)模型、熱解氣體的對(duì)沖擴(kuò)散燃燒模型以及材料表面氧化燒蝕模型,并提出了"開(kāi)始反應(yīng)面"與"臨界速度"概念。利用擬牛頓法通過(guò)編寫(xiě)FORTRAN代碼求解非線性正激波方程組獲得正激波后氣體溫度和流速;利用熱解層模型的計(jì)算方法,求得燒蝕表面上的溫度和熱解氣體流速;將求得的結(jié)果作為對(duì)沖擴(kuò)散燃燒模型的邊界條件,利用OPPDIF程序求解熱解氣體的對(duì)沖擴(kuò)散燃燒模型,獲得燒蝕表面附近的氧氣質(zhì)量分?jǐn)?shù);把氧氣質(zhì)量分?jǐn)?shù)和燒蝕表面溫度等參數(shù)代入材料表面氧化燒蝕模型,利用MATLAB平臺(tái)編程計(jì)算獲得表面燒蝕率;再把求得的燒蝕率代入熱解層模型中,重復(fù)上述計(jì)算步驟,直至表面燒蝕率的迭代誤差滿足精度要求,便可確定當(dāng)前時(shí)刻的表面燒蝕率�；贑++、MATLAB及ACCESS等計(jì)算機(jī)語(yǔ)言,開(kāi)發(fā)出一套高超音速氣動(dòng)熱環(huán)境下炭化復(fù)合材料熱防護(hù)仿真軟件。借助該軟件平臺(tái),分析激波層內(nèi)熱解氣體燃燒反應(yīng)對(duì)材料表面燒蝕的抑制作用。數(shù)值結(jié)果表明:熱解氣體的燃燒反應(yīng)在一定程度上抑制了炭化復(fù)合材料表面的燒蝕速率,但對(duì)材料內(nèi)部溫度場(chǎng)影響不大。
[Abstract]:In order to overcome the "thermal barrier" caused by aerodynamic heating, it is necessary to provide thermal protection for the spacecraft's reentry capsule. According to the environmental characteristics of the reentry capsule, such as high specific enthalpy, low heat flux, low pressure, low shear force and long reentry time, it is usually selected. Carbonized composites are used as thermal protection materials. The thermal protection efficiency of uniform density carbonized composites is relatively low in the rapidly changing spacecraft environment. Therefore, the design of variable density carbonized composites is the development direction of thermal protection systems. Based on the ablation mechanism of carbonized composites, assuming that all the pyrolysis reactions take place on the pyrolysis surface, a one-dimensional pyrolysis surface model of variable density carbonized composites is developed. According to the theory of heat transfer, the original material layer and carbonized layer are built separately. A one-dimensional transient Fourier heat conduction equation is established in which the thermophysical parameters are a function of density or density and temperature; a temperature continuity condition and a pyrolysis energy conservation condition are established at the moving pyrolysis surface; and an energy conservation relation is established at the ablated surface, which is related to the ablation rate of the surface, while the ablation rate of carbon is a function of the wall temperature. Based on the numerical heat transfer method, the space and time terms of the partial differential equation of heat conduction for variable density carbonized composites in static coordinate system are discretized by using the second-order central difference scheme and the first-order forward difference scheme respectively, and an implicit discrete scheme is obtained. A new non-linear calculation method is proposed, which uses the results of the previous time to determine the total thickness of material, the thickness of original material and the thickness of carbonization layer, updates the spatial nodes and pyrolysis surface nodes, and uses the tridiagonal matrix algorithm and Newton iterative method to solve the implicit discrete scheme of the current time. The moving distance of the ablation surface is obtained by the function of ablation rate, and the moving distance of the pyrolysis surface satisfying the energy conservation condition of the pyrolysis surface is obtained by the fixed point iteration method. The numerical results show that the pyrolysis surface model of variable density carbonized composites can be used to solve the ablation and thermal response of uniform density carbonized composites; the variable density carbonized composites have higher effective hot melting and can improve the thermal protection efficiency of the thermal protection system. In order to reflect the thermal response of the carbonized composites more accurately. A one-dimensional pyrolysis layer model of variable density carbonized composites was established. The pyrolysis reaction and gas flow were found in the pyrolysis layer between the carbonized layer and the raw material layer. In order to simplify the calculation, the density and thermophysical parameters of the pyrolysis layer are linearly treated. The dissociation of the pyrolysis surface model is used. An implicit discrete scheme is constructed for the mathematical model of pyrolysis layer. A new method is developed for solving the nonlinear discrete equations with moving boundary and double moving interface. The total thickness of material, the thickness of raw material, the thickness of pyrolysis layer and the thickness of carbonization layer are determined by the results of the previous time, and the spatial nodes are divided and the moving boundary is updated. The implicit discrete scheme is solved by using tridiagonal matrix algorithm and Newton iteration method at the current moment. The moving distance of the ablation surface is determined by the function of the ablation surface temperature and ablation rate, and the moving distance of the internal two interfaces satisfying the condition of continuous heat flow is determined by Newton chord cut method. The implicit discrete scheme is programmed to calculate and analyze the thermal response of homogeneous materials under constant heat flux, homogeneous materials and variable density materials under variable heat flux, and the results of the pyrolysis surface model and the pyrolysis layer model of homogeneous materials under constant heat flux are compared and analyzed. Comparing the numerical results of the pyrolysis layer model with the experimental results, it is verified that the pyrolysis layer model can be used to solve the thermal response of homogeneous materials; in the course of service, the parameters (temperature, ablation rate, mass flow rate of pyrolysis gas) and the thickness of each layer are not only related to the aerodynamic heat. It is found that the selection of pyrolysis surface temperature is very important to the calculation accuracy of the pyrolysis surface model. Both models assume that the ablation rate is a function of temperature. In order to accurately analyze the ablation rate of the material surface, based on the laminar flow hypothesis of pyrolytic gas combustion, the thermo-hydro-chemical-ablative multi-field coupling of carbonized composites was established by using the theories of aerothermodynamics, heat transfer, combustion and physical chemistry. The model consists of forward shock equations, pyrolysis layer model, pyrolysis gas combustion model and material surface oxidation and ablation model. The concepts of "starting reaction surface" and "critical velocity" are proposed. The gas temperature after forward shock is obtained by writing FORTRAN code to solve nonlinear forward shock equations. The temperature on the ablated surface and the velocity of pyrolytic gas were calculated by using the pyrolytic layer model, and the results were taken as the boundary conditions of the contra-diffusion combustion model. The contra-diffusion combustion model of pyrolytic gas was solved by OPPDIF program, and the oxygen mass fraction near the ablated surface was obtained. The ablation surface temperature and other parameters are substituted into the material surface oxidation ablation model, and the ablation rate is calculated by MATLAB platform programming. Then the ablation rate is substituted into the pyrolysis layer model, and the above calculation steps are repeated until the iterative error of the ablation rate meets the accuracy requirement. The current ablation rate can be determined based on C++, M. A set of simulation software for thermal protection of carbonized composites in hypersonic aerothermal environment was developed by using computer languages such as ATLAB and ACCESS. The inhibition effect of combustion reaction of pyrolytic gases in shock layer on the surface ablation of materials was analyzed by using the software platform. The numerical results show that the combustion reaction of pyrolytic gases inhibits carbonization to a certain extent. The ablation rate of the composite surface has little effect on the temperature field inside the composite.
【學(xué)位授予單位】：北京交通大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類號(hào)】：V445.1;V25
本文編號(hào)：2202312
[Abstract]:In order to overcome the "thermal barrier" caused by aerodynamic heating, it is necessary to provide thermal protection for the spacecraft's reentry capsule. According to the environmental characteristics of the reentry capsule, such as high specific enthalpy, low heat flux, low pressure, low shear force and long reentry time, it is usually selected. Carbonized composites are used as thermal protection materials. The thermal protection efficiency of uniform density carbonized composites is relatively low in the rapidly changing spacecraft environment. Therefore, the design of variable density carbonized composites is the development direction of thermal protection systems. Based on the ablation mechanism of carbonized composites, assuming that all the pyrolysis reactions take place on the pyrolysis surface, a one-dimensional pyrolysis surface model of variable density carbonized composites is developed. According to the theory of heat transfer, the original material layer and carbonized layer are built separately. A one-dimensional transient Fourier heat conduction equation is established in which the thermophysical parameters are a function of density or density and temperature; a temperature continuity condition and a pyrolysis energy conservation condition are established at the moving pyrolysis surface; and an energy conservation relation is established at the ablated surface, which is related to the ablation rate of the surface, while the ablation rate of carbon is a function of the wall temperature. Based on the numerical heat transfer method, the space and time terms of the partial differential equation of heat conduction for variable density carbonized composites in static coordinate system are discretized by using the second-order central difference scheme and the first-order forward difference scheme respectively, and an implicit discrete scheme is obtained. A new non-linear calculation method is proposed, which uses the results of the previous time to determine the total thickness of material, the thickness of original material and the thickness of carbonization layer, updates the spatial nodes and pyrolysis surface nodes, and uses the tridiagonal matrix algorithm and Newton iterative method to solve the implicit discrete scheme of the current time. The moving distance of the ablation surface is obtained by the function of ablation rate, and the moving distance of the pyrolysis surface satisfying the energy conservation condition of the pyrolysis surface is obtained by the fixed point iteration method. The numerical results show that the pyrolysis surface model of variable density carbonized composites can be used to solve the ablation and thermal response of uniform density carbonized composites; the variable density carbonized composites have higher effective hot melting and can improve the thermal protection efficiency of the thermal protection system. In order to reflect the thermal response of the carbonized composites more accurately. A one-dimensional pyrolysis layer model of variable density carbonized composites was established. The pyrolysis reaction and gas flow were found in the pyrolysis layer between the carbonized layer and the raw material layer. In order to simplify the calculation, the density and thermophysical parameters of the pyrolysis layer are linearly treated. The dissociation of the pyrolysis surface model is used. An implicit discrete scheme is constructed for the mathematical model of pyrolysis layer. A new method is developed for solving the nonlinear discrete equations with moving boundary and double moving interface. The total thickness of material, the thickness of raw material, the thickness of pyrolysis layer and the thickness of carbonization layer are determined by the results of the previous time, and the spatial nodes are divided and the moving boundary is updated. The implicit discrete scheme is solved by using tridiagonal matrix algorithm and Newton iteration method at the current moment. The moving distance of the ablation surface is determined by the function of the ablation surface temperature and ablation rate, and the moving distance of the internal two interfaces satisfying the condition of continuous heat flow is determined by Newton chord cut method. The implicit discrete scheme is programmed to calculate and analyze the thermal response of homogeneous materials under constant heat flux, homogeneous materials and variable density materials under variable heat flux, and the results of the pyrolysis surface model and the pyrolysis layer model of homogeneous materials under constant heat flux are compared and analyzed. Comparing the numerical results of the pyrolysis layer model with the experimental results, it is verified that the pyrolysis layer model can be used to solve the thermal response of homogeneous materials; in the course of service, the parameters (temperature, ablation rate, mass flow rate of pyrolysis gas) and the thickness of each layer are not only related to the aerodynamic heat. It is found that the selection of pyrolysis surface temperature is very important to the calculation accuracy of the pyrolysis surface model. Both models assume that the ablation rate is a function of temperature. In order to accurately analyze the ablation rate of the material surface, based on the laminar flow hypothesis of pyrolytic gas combustion, the thermo-hydro-chemical-ablative multi-field coupling of carbonized composites was established by using the theories of aerothermodynamics, heat transfer, combustion and physical chemistry. The model consists of forward shock equations, pyrolysis layer model, pyrolysis gas combustion model and material surface oxidation and ablation model. The concepts of "starting reaction surface" and "critical velocity" are proposed. The gas temperature after forward shock is obtained by writing FORTRAN code to solve nonlinear forward shock equations. The temperature on the ablated surface and the velocity of pyrolytic gas were calculated by using the pyrolytic layer model, and the results were taken as the boundary conditions of the contra-diffusion combustion model. The contra-diffusion combustion model of pyrolytic gas was solved by OPPDIF program, and the oxygen mass fraction near the ablated surface was obtained. The ablation surface temperature and other parameters are substituted into the material surface oxidation ablation model, and the ablation rate is calculated by MATLAB platform programming. Then the ablation rate is substituted into the pyrolysis layer model, and the above calculation steps are repeated until the iterative error of the ablation rate meets the accuracy requirement. The current ablation rate can be determined based on C++, M. A set of simulation software for thermal protection of carbonized composites in hypersonic aerothermal environment was developed by using computer languages such as ATLAB and ACCESS. The inhibition effect of combustion reaction of pyrolytic gases in shock layer on the surface ablation of materials was analyzed by using the software platform. The numerical results show that the combustion reaction of pyrolytic gases inhibits carbonization to a certain extent. The ablation rate of the composite surface has little effect on the temperature field inside the composite.
【學(xué)位授予單位】：北京交通大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類號(hào)】：V445.1;V25
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