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carbon composites using a low-velocity, high-particle-concentration two-phase jet in a solid rocket motor
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Available at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/carbon
Erosion of carbon/carbon composites using a low-velocity, high-particle-concentration two-phase jet in a solid rocket motor Qiang Li *, Jiang Li, Guo-qiang He, Pei-jin Liu Science and Technology on Combustion, Internal Flow and Thermal-Structure Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
The erosion of a four-direction carbon/carbon composite test piece using a low-velocity,
Received 24 June 2013
high-particle-concentration two-phase jet was studied by the hot firing test of a small solid
Accepted 25 September 2013
rocket motor with an elaborately designed flow path. The linear ablation rates were mea-
Available online 1 October 2013
sured. The ablation surface and microstructure of the carbon/carbon composites were studied by scanning electron microscopy, and the ablation mechanism was investigated. Within the parameter range studied in this paper, mechanical erosion is found to play a dominant role in the ablation of carbon/carbon composites in the impact region. Moreover, the effect of chemical ablation is weak. The erosion of carbon/carbon composites results in blunt fracture tip fibers and a lamellar matrix. The particle impact mass flux is more dominant than the particle impact velocity in the erosion of carbon/carbon composites. At mesoscale, the carbon rods are more resistant to mechanical erosion than fiber bundles. At microscale, the carbon fibers are more susceptible to mechanical erosion than the carbon matrix, whereas the carbon fibers are more resistant to chemical ablation than the matrix and interface. Ó 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon/carbon (C/C) composites are one of the most ideal thermal-protecting materials in aerospace applications because of the extraordinary and unique characteristics, such as low density, high strength, remarkable thermal stability, low thermal conductivity, and extremely low ablation rate [1,2]. The ablation of C/C composites is complex. The process involves the ablation environment, chemical ablation, mechanical erosion, and structures of the composites [3,4]. In the last few years, the ablation performance of C/C composites has been experimentally studied using plasma ablation, oxyacetylene torch, arc heating and motor firing tests [5,6]. Furthermore, the effects of combustion chamber
pressure and temperature, solid propellant type, combustion product composition, as well as density and graphitization degree of C/C composites on the ablation characterizations of C/C composites have been examined. The ablation mechanisms of C/C composites have also been investigated through their morphology [7–10]. However, most of these studies have focused on the ablation of the C/C composite nozzle throat because of its importance. Few studies have been conducted on the erosion of C/C composites by a low-velocity, high-particle-concentration two-phase jet, such as that the erosion of the entrance and head cap of the submerged nozzle, the nose tip of the submerged entry nozzle, and the entrance of the convergent–divergent nozzle [11], where the gas velocity is low and chemical ablation is weak. The shapes of these
* Corresponding author: Fax: +86 (0)29 88494163. E-mail address: [email protected] (Q. Li). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.09.072
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components significantly influence local gas flow and, consequently, the ablation of the nozzle throat and the overall performance of the solid rocket motor (SRM). Thus, the erosion of C/C composites using a low-velocity, high-particle-concentration two-phase jet must be investigated. In this study, an SRM with a specially designed flow path was used to investigate the erosion behavior of 4D C/C composites using a low-velocity, high-particle-concentration two-phase jet in an SRM hot firing test. Ablation rates were measured, and the ablation performance and dominant factors were interpreted according to theoretical simulations. The ablation morphology and microstructure of the C/C composites were investigated, and the ablation mechanism was discussed.
2.
Experimental
2.1.
Description of the ablative media
The C/C composites comprising the C/C composite test piece studied in this paper are the same as those in Ref. [7]. A schematic of the composite preform is shown in Fig. 1. T300 PAN-based carbon fibers and coal-tar pitch were used as the reinforcement and the matrix, respectively. The diameter of the carbon rod was controlled to be 1.5 mm, and the spacing distance between carbon rods was fixed at 4.0 mm. The width and thickness of the fiber bundles were 2.0 and 0.6 mm, respectively. The fiber bundles were preformed into hexagonal shapes with three directional axes (W, X, and Y) aligned along the same plane at 120°. The carbon rods were fixed perpendicular to the bundles. The apparent density of the as-prepared composites was 1.93–1.94 g/cm3. Details of the heat treatment of C/C composites can be found in Ref. [7] and the references therein.
2.2.
141
circular jet controller with an angle joint. The C/C composite test piece was mounted onto the bottom (inner) surface of the test section. The angle formed by the axis of the jet controller and the center line of the test section was adjustable. In the hot firing test, the two-phase flow released from the propellant burning surface flowed through the jet controller, in which the particle number density of the two-phase jet was increased and the spatial distribution was homogenized. Upon leaving the exit of the jet controller, the combustion gases simultaneously changed in flow direction. Under the action of inertia, most condensed particles formed a low-velocity, high-particle-concentration two-phase jet that impacts the surface of the C/C composite test piece. Subsequently, erosion occurred. The Z-direction of the preform was perpendicular to the top surface of the C/C composite test piece, and the WXYplane was parallel to the top surface of the C/C composite test piece. The SRM was loaded with a composite propellant comprising 66.2 wt.% ammonium perchlorate, 15.4 wt.% hydroxyl-terminated polybutadiene, 17.5 wt.% aluminum powder, and other additives. In the hot firing test, the endburning grain was used in the SRM to maintain constant chamber pressure. The angle formed by the axis of the jet controller and the center line of the test section was fixed at 30° in the hot firing test.
2.3.
Characterization
Before and after the hot firing test, the profiles of the C/C composite test piece were obtained using a surface profile measuring instrument (Proscan 2100, Scantron). The microstructure and morphology of the ablation surface of the C/C composites were characterized by scanning electron microscopy (SEM; JEOL, JSM-6390A) and energy-dispersive spectroscopy (EDS).
Ablation test
An SRM hot firing system was utilized to investigate the erosion of C/C composites using a low-velocity, high-particle-concentration two-phase jet. The hot firing test system primarily consisted of combustion chamber, solid propellant coated by inhibitor layer, steel case, pressure gauge, jet controller, test section, C/C composite test piece and nozzle, and so on, as shown in Fig. 2. The jet controller was connected to the combustion chamber with an integral type flange, and the rectangular test section was linked to the
Fig. 1 – Schematic of the composite preform. (A color version of this figure can be viewed online.)
3.
Results and discussion
3.1.
Ablation environment
In an SRM, the combustion products of the metalized propellant provide an ablation environment for the C/C composites and determine ablation behavior. In this paper, the equilibrium compositions of the propellant combustion products were calculated by the Chemical Equilibrium with Application Code, a free software for calculating chemical equilibrium products based on the
Fig. 2 – Schematic of the hot firing test motor.
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Table 1 – Chemical compositions of SRM combustion products in the combustion chamber. Species
Mole fraction (mol%)
H2 (g) CO (g) HCl (g) H2O (g) CO2 (g) N2 (g) Al2O3 (g)
33.158 25.682 12.729 8.583 8.360 7.204 7.219
particle impact mass flux has the maximum value, where x = 0.3638 m, whereas both gas and particle impact velocities have the maximum value, where x = 0.3671 m. The maximum values of velocities lag behind those of impact mass flux in space. Fig. 4 shows the recorded time history of pressure in the combustion chamber. The duration of the hot firing test is about 6.2 s, and the maximum chamber pressure is about 6.8 MPa and lasts for approximately 4.2 s.
3.2.
free-energy-minimization principle [12,13]. The estimated chamber pressure (7.0 MPa) was used to calculate chemical equilibrium. Results show that the combustion products in the combustion chamber comprise various species, as shown in Table 1. Other parameters such as stagnation pressure, stagnation temperature, and mass fraction of aluminum oxide in the combustion products are shown in Table 2. A three-dimensional model of the viscous compressible two-phase flow in the rocket motor was established. Fluent Software (version 6.3) was used for the computation using Reynolds-averaged Navier–Stokes equations with extra source terms representing the interactions between the two phases. The Lagrangian method was used to track the condensed particles. The Reynolds number was evaluated to be approximately 57 000. Spalart–Allmaras one-equation turbulence model, which was satisfactorily accurate for transonic compressible flow, was used. Gambit was used to generate a mesh containing 682 000 tetrahedral cells. The mass fluxes and temperatures of both phases were specified at the inlet and diameter distributions of the condensed particles. No boundary condition was necessary at the exit of the submerged nozzle for supersonic flow. The measured sizes of the condensed particles from the chamber of SRM [14] were used as a boundary condition during simulation. Collisions between particles were neglected, and the impact of condensed particles on the surface of C/C composite test piece was regarded as perfectly elastic and specular reflection. The parameters of condensed particles impacting on the surface of the C/C composite test piece were counted and post-processed with user-defined functions. In this paper, the particle impact mass flux is defined as the total mass of particles impacting the surface of the C/C composite test piece per unit area per unit time. The particle impact velocity is defined as the number-averaged velocities of particles impacting the same boundary surface triangular mesh attached onto the top surface of the C/C composite test piece. The computed results are plotted in Fig. 3, along with the gas velocity extracted from the symmetry plane of the test section, where x is the distance in the X-direction from the forefront of the C/C composite test piece (x = 0.330 m). The
Macro-scale ablation rate
Fig. 5(a) shows the photograph of the C/C composite test piece before the hot firing test. Before the test, the top surface of the C/C composite test piece is rather rough. Although most of the fiber bundles are damaged because of machining, the array of carbon rods and bundles are visible and in good order, the tips of the carbon rods are lower, and small shallow pores are formed. Fig. 5(b) shows the photograph of the C/C composite test piece after the hot firing test. The C/C composite test piece is still rough. In the impact region of the two-phase jet, the fiber bundles on the top layer are damaged and a small cone-shaped pit is formed. Downstream of the impact region, some large particles about 1.0 mm in diameter can be seen sticking to the tips of the carbon rods. The surface profile of the C/C composite test piece was measured before and after the hot firing test. The spatial resolution of the measured data is 0.02 mm and then the data
Fig. 3 – Distributions of two-phase flow parameters.
Table 2 – Combustion product parameters in the combustion chamber of the test SRM. Stagnation pressure (MPa) Stagnation temperature (K) Al2O3(l) (wt.%)
7.0 3530.0 31.1
Fig. 4 – Time history of combustion chamber pressure.
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were averaged every 0.2 mm. Fig. 6 shows the 2D recession map of the cross section, which lies along the X–Y plane and passes through the bottom of the pit. The top diameter of the pit is approximately 5.2 mm, and its maximum depth approximately 0.53 mm. A close comparison shows that the position of the bottom of the cone-shaped pit (x = 0.361– 0.364 m) is almost the same as the position where the particle impact mass flux has the maximum value (x = 0.3638 m). This finding indicates that, within the parameter range studied, the particle impact mass flux is more dominant than the particle impact velocity in the erosion of the C/C composites. The maximum ablation amount is found at the bottom of the cone-shaped pit. A local magnification of the 2D recession map shows that at the bottom of the pit, the tip of the carbon rod is higher than the surrounding bundles at mesoscale, with a height difference of about 0.122 mm. The measured ablation rate of the carbon rod is about 0.067 mm/s, whereas that of the bundles is approximately 0.089 mm/s. The ablation rate is immeasurable at macroscale 20.0 mm away from the cone-shaped pit.
3.3.
143
Fig. 6 – 2D recession map of the C/C composite surface.
Micro-scale ablation morphology
Fig. 7(a) shows the morphology of the post-test C/C composite test piece. Near the impact region of the two-phase flow jet, the ablated surface of the C/C composite test piece is rougher at mesoscale. Numerous spherical Al2O3 particles with diameters ranging from 100 to 300 lm are deposited onto the ablated surface of the fiber bundles and the tips of carbon rods. The fiber bundles are denudated and burned into broom-like shapes. The tips of the carbon rods at the bottom
Fig. 5 – Photograph of the C/C composite test piece before and after the hot firing test: (a) pre-test, and (b) post-test.
Fig. 7 – Morphology of the C/C composite test piece: (a) pit, (b) carbon rod, and (c) morphology of the carbon rod downstream of the pit.
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Table 3 – EDS characterization of residuals in pore. Element
(keV)
Mass (%)
Error (%)
Atom (%)
C O Al Si Total
0.277 0.525 1.486 1.735
21.63 37.14 40.78 0.49 100.00
0.12 0.14 0.10 0.16
31.88 41.09 26.75 0.28 100.00
of the pit are coarser and higher than the surrounding fiber bundles, indicating that the carbon rods are more resistant to mechanical erosion than the bundles. The carbon rods and fiber bundles outside the pit show no obvious damage or ablation. The velocity of the two-phase flow is relatively smaller than the velocity of the two-phase flow in the nozzle throat. Consequently, the convective heat flux transferring from combustion products to the solid is small, and the surface temperature of the C/C composites is low. As a result, the rate of heterogeneous reactions between the oxidizing species present in the combustion products of the propellant and C/C composites is lower [1,10], and chemical ablation is weakened. Nevertheless, the maximum measured ablation rate of the C/C composite test piece is comparable with that of laboratory SRM experimental data [7–9]. This finding indicates that near the impact region, mechanical erosion from the impact of condensed particles on the top surface of the C/C composite test piece plays a fully predominant role. Moreover, the carbon rods are damaged and the fiber bundles are denudated. Fig. 7(b) shows the high-magnification morphology of the carbon rod at the bottom of the pit shown in Fig. 7(a). A pore is present at the tip of the carbon rod. EDS shows that the residuals in the pore are mainly Al2O3 (Table 3), and it is deduced that pore formation is due to the impact and damage of larger condensed particles. The fibers are peeled off from the carbon matrix, and blunt fracture tips are formed. The tips of the carbon fibers are lower than the matrix, indicating that the carbon matrix is more resistant to mechanical erosion than the carbon fibers. In design and manufacturing, fine-weaving C/C composites with low-fiber-density carbon rods should be used as constituents for the thermal protection components subjected to severe erosion of two-phase flow. Fig. 7(c) shows the high-magnification SEM image of the carbon rod 40 mm downstream of the impact region where the ablation rate is immeasurable at macroscale. The carbon fibers are arranged in good order, and the tips of the fibers are almost flat. These features show that the material has only started its chemical ablation and has not had sufficient time to develop a steady morphology. These results indicate that the C/C composites are less ablated because of low gas velocity, low particle impact velocity and small impact mass flux in this local area. The irregularity of the surface is due to the machining before the test. The interface, which can be well resolved prior to testing, is burned out although the surface temperature of the C/C composites may be lower than that of the nozzle throat, ring-like gaps between the carbon fibers and matrix are formed, suggesting that the interface
can be more prone to oxidization than the carbon matrix and fibers. A lamellar matrix can be seen in the low-fiber-density area, whereas the matrix in the high-fiber-density area is burned out and gaps are formed. This phenomenon indicates that carbon fibers are more resistant to chemical ablation than the matrix and interface. For the thermal protection components subjected to severe chemical ablation, rods with high carbon fiber density should be used.
3.4.
Ablation analysis
The ablation of C/C composites is very complex and involves both chemical ablation and mechanical erosion, which determine the ablation behavior of C/C composites. Chemical ablation is widely known as the primary cause of ablation and is referred to as the heterogeneous reactions between the oxidizing species present in the combustion products of the solid propellant and the C/C composites. According to the classical Arrhenius law, the oxidation rate varies severely with temperature and the oxidant partial pressure [4,15]. Mechanical erosion from condensed particles is regarded as another main factor contributing to the ablation of C/C composites [6,7,9,10]. The role of the condensed particles in solid propellant combustion products can be classified into two categories: mechanical force and mechanocaloric effect. In the hot firing test in this paper, the chemical ablation is weakened due to the low velocity of combustion gases flowing over the C/C composite test piece. Nevertheless, the erosion of particles is predominant in the ablation of the C/ C composites. A multiscale erosion model is necessary for the C/C composites. Fig. 7(b) shows that the size of the particle impact damage is comparable with the microstructural size scale of the C/C composites. Consequently, the microstructural constraints must be considered and an ‘‘averaging law’’ is not applicable at microscale, particle impact events occur in one of the two phases, carbon fiber and matrix. At mesoscale, the particle impact damage to carbon rod and bundle must also be differentiated, as evidently shown in Fig. 7(a). To model the erosion process, a control volume or minimal periodic unit should first be determined at both microscale and mesoscale for the C/C composites, and a multiscale erosion model is essential. At microscale, when one particle impacts on the C/C composites surface, the mechanical force between the particle and the C/C composites can be calculated by the Hertzian equation for a pure elastic collision [16]. By assuming that the contact region to be circular, the maximum contact area and the duration of the impact have been derived by
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Hertzian and the maximum mechanical stress Fp can be calculated. The probability that the particle impacts on the carbon fiber or the matrix can be evaluated depending on the structure of the microscale minimal periodic unit. When the impact event occurs in the carbon fiber, the maximum mechanical stress Fp can be decomposed into normal stress Fn and tangential stress Ft, which are parallel and perpendicular to the fiber orientation, respectively. When the normal stress Fn or the tangential stress Ft exceeds the impact yielding stress of the carbon fiber, erosion occurs. The erosion can be classified into two different modes: brittle mode and ductile mode, which are dominated by the normal stress and the tangential stress, respectively. For the carbon fiber, both modes would take place simultaneously and should be considered. On erosion, the mass loss or volume loss of the carbon fiber should be counted and the effect of particle impact damage on fiber morphology must be determined and transferred to that at mesoscale. If the impact event occurs in the matrix, the maximum mechanical stress Fp can be decomposed into normal stress Fn and tangential stress Ft, which are perpendicular and parallel to the matrix surface, respectively. In the similar way, particle impact damage can be evaluated. At mesoscale, the microscale model is of great directive significance. At macroscale, the linear or inverse rules of mixture may be used to evaluate the erosion rate of the C/C composites. In the preceding description, many correlations must be established and validated, such as the accurate evaluation of the mechanical stress, the measurement of the intrinsic parameters of the C/C composites at different scales, the effect of particle impact damage on morphologies of the two phases, the mass loss or volume loss of the C/C composites at different scales, the way the impact damage is transferred from microscale to mesoscale and from mesoscale to macroscale and so on. Further investigations on these correlations should be thoroughly carried out.
4.
Conclusions
The erosion of a 4D C/C composite test piece by low-velocity, high-particle-concentration two-phase jet was investigated using an SRM. The average ablation rates were measured, and the ablation performance, mechanism, and microstructure of the C/C composites were investigated. The effect of chemical ablation is found to be weak due to low gas velocity. The comparatively high ablation rates are mainly attributed to mechanical erosion. At microscale, the carbon fibers are more susceptible to mechanical erosion than the carbon matrix, whereas carbon fibers are more resistant to chemical ablation than the matrix and interface. At mesoscale, the carbon rods are more resistant to mechanical erosion than fiber bundles. In applications, fine-weaving C/C composites with low-fiber-density carbon rods should be used as the constituents for the thermal protection components subjected to
145
severe erosion of two-phase flow, and carbon rods with high carbon fiber density should be used as the constituents for thermal protection parts subjected to severe chemical ablation.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Contract No. 50876091).
R E F E R E N C E S
[1] Fitzer E, Manocha LM. Carbon reinforcements and carbon/ carbon composites. Berlin: Springer; 1998. p. 190–234. [2] Savage G. Carbon–carbon composites. London: Chapman &Hall; 1993. p. 31–357. [3] Buckley JD, Edie DD. Carbon/carbon materials and composites. New Jersey: Noyes Publications; 1993. p. 267–79. [4] Boury D, Filipuzzi LS. Materials for solid rocket booster nozzle components. 37th AIAA/ASME/SAE/ASEE joint propulsion conference. Salt Lake City (Utah, USA): American Institute of Aeronautics and Astronautics, AIAA; 2001. p. 2001–3438. [5] Liu JJ, Su JM, Chen CL. Study on factors affecting ablative performance of C/C composites. Carbon 2003;2:15–9. [6] Yin J, Xiang X, Zhang HB, Huang BY. Microstructure and ablation performances of dual-matrix carbon/carbon composites. Carbon 2006;44(3):1690–4. [7] Peng LN, He GQ, Li J, Wang L, Qin F. Effect of combustion gas mass flow rate on carbon/carbon composite nozzle ablation in a solid rocket motor. Carbon 2012;50(2):1554–62. [8] Li KZ, Shen XT, Li HJ, Zhang SY, Feng T, Zhang LL. Ablation of the carbon/carbon composite nozzle-throat in a small solid rocket motor. Carbon 2011;49(4):1208–15. [9] Chen B, Zhang LT, Cheng LF, Luan XG. Ablation of pierced C/C composite nozzles in an oxygen/ethanol combustion gas generator. Carbon 2009;47(3):545–50. [10] Kuo KK, Keswani ST. A comprehensive theoretical model for carbon–carbon composite nozzle recession. Combust Sci Technol 1985;42(3–4):145–64. [11] Ellis RA. Solid rocket motor nozzles. Washington, DC: NASA; 1975. SP-8115, p. 1–41. [12] Gordon S, McBride BJ. Computer program for calculation of complex chemical equilibrium compositions and applications I: analysis. NASA Lewis Research Center: NASA Reference Publication; 1994. p. 25–32. [13] Gordon S, McBride BJ. Computer program for calculation of complex chemical equilibrium compositions and applications II: user’s manual and program description. NASA Lewis Research Center: NASA Reference Publication; 1996. p. 65–71. [14] Liu PJ, Bai JH, Yang XM, Zhao ZB. Collection and analysis on the condensed-phase particles in the chamber of SRM. J Solid Rocket Technol 2008;31(5):461–4. [15] Lachaud J, Aspa Y, Vignoles GL. Analytical modeling of the steady state ablation of a 3D C/C composite. Int J Heat Mass Transfer 2008;51:2614–27. [16] Timoshenko SP, Goodier JM. Theory of elasticity. New York: McGraw Hill; 1969. p. 109–64.
6 7 ( 2 0 1 4 ) 1 4 0 –1 4 5
Available at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/carbon
Erosion of carbon/carbon composites using a low-velocity, high-particle-concentration two-phase jet in a solid rocket motor Qiang Li *, Jiang Li, Guo-qiang He, Pei-jin Liu Science and Technology on Combustion, Internal Flow and Thermal-Structure Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
The erosion of a four-direction carbon/carbon composite test piece using a low-velocity,
Received 24 June 2013
high-particle-concentration two-phase jet was studied by the hot firing test of a small solid
Accepted 25 September 2013
rocket motor with an elaborately designed flow path. The linear ablation rates were mea-
Available online 1 October 2013
sured. The ablation surface and microstructure of the carbon/carbon composites were studied by scanning electron microscopy, and the ablation mechanism was investigated. Within the parameter range studied in this paper, mechanical erosion is found to play a dominant role in the ablation of carbon/carbon composites in the impact region. Moreover, the effect of chemical ablation is weak. The erosion of carbon/carbon composites results in blunt fracture tip fibers and a lamellar matrix. The particle impact mass flux is more dominant than the particle impact velocity in the erosion of carbon/carbon composites. At mesoscale, the carbon rods are more resistant to mechanical erosion than fiber bundles. At microscale, the carbon fibers are more susceptible to mechanical erosion than the carbon matrix, whereas the carbon fibers are more resistant to chemical ablation than the matrix and interface. Ó 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon/carbon (C/C) composites are one of the most ideal thermal-protecting materials in aerospace applications because of the extraordinary and unique characteristics, such as low density, high strength, remarkable thermal stability, low thermal conductivity, and extremely low ablation rate [1,2]. The ablation of C/C composites is complex. The process involves the ablation environment, chemical ablation, mechanical erosion, and structures of the composites [3,4]. In the last few years, the ablation performance of C/C composites has been experimentally studied using plasma ablation, oxyacetylene torch, arc heating and motor firing tests [5,6]. Furthermore, the effects of combustion chamber
pressure and temperature, solid propellant type, combustion product composition, as well as density and graphitization degree of C/C composites on the ablation characterizations of C/C composites have been examined. The ablation mechanisms of C/C composites have also been investigated through their morphology [7–10]. However, most of these studies have focused on the ablation of the C/C composite nozzle throat because of its importance. Few studies have been conducted on the erosion of C/C composites by a low-velocity, high-particle-concentration two-phase jet, such as that the erosion of the entrance and head cap of the submerged nozzle, the nose tip of the submerged entry nozzle, and the entrance of the convergent–divergent nozzle [11], where the gas velocity is low and chemical ablation is weak. The shapes of these
* Corresponding author: Fax: +86 (0)29 88494163. E-mail address: [email protected] (Q. Li). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.09.072
CARBON
6 7 (2 0 1 4) 1 4 0–14 5
components significantly influence local gas flow and, consequently, the ablation of the nozzle throat and the overall performance of the solid rocket motor (SRM). Thus, the erosion of C/C composites using a low-velocity, high-particle-concentration two-phase jet must be investigated. In this study, an SRM with a specially designed flow path was used to investigate the erosion behavior of 4D C/C composites using a low-velocity, high-particle-concentration two-phase jet in an SRM hot firing test. Ablation rates were measured, and the ablation performance and dominant factors were interpreted according to theoretical simulations. The ablation morphology and microstructure of the C/C composites were investigated, and the ablation mechanism was discussed.
2.
Experimental
2.1.
Description of the ablative media
The C/C composites comprising the C/C composite test piece studied in this paper are the same as those in Ref. [7]. A schematic of the composite preform is shown in Fig. 1. T300 PAN-based carbon fibers and coal-tar pitch were used as the reinforcement and the matrix, respectively. The diameter of the carbon rod was controlled to be 1.5 mm, and the spacing distance between carbon rods was fixed at 4.0 mm. The width and thickness of the fiber bundles were 2.0 and 0.6 mm, respectively. The fiber bundles were preformed into hexagonal shapes with three directional axes (W, X, and Y) aligned along the same plane at 120°. The carbon rods were fixed perpendicular to the bundles. The apparent density of the as-prepared composites was 1.93–1.94 g/cm3. Details of the heat treatment of C/C composites can be found in Ref. [7] and the references therein.
2.2.
141
circular jet controller with an angle joint. The C/C composite test piece was mounted onto the bottom (inner) surface of the test section. The angle formed by the axis of the jet controller and the center line of the test section was adjustable. In the hot firing test, the two-phase flow released from the propellant burning surface flowed through the jet controller, in which the particle number density of the two-phase jet was increased and the spatial distribution was homogenized. Upon leaving the exit of the jet controller, the combustion gases simultaneously changed in flow direction. Under the action of inertia, most condensed particles formed a low-velocity, high-particle-concentration two-phase jet that impacts the surface of the C/C composite test piece. Subsequently, erosion occurred. The Z-direction of the preform was perpendicular to the top surface of the C/C composite test piece, and the WXYplane was parallel to the top surface of the C/C composite test piece. The SRM was loaded with a composite propellant comprising 66.2 wt.% ammonium perchlorate, 15.4 wt.% hydroxyl-terminated polybutadiene, 17.5 wt.% aluminum powder, and other additives. In the hot firing test, the endburning grain was used in the SRM to maintain constant chamber pressure. The angle formed by the axis of the jet controller and the center line of the test section was fixed at 30° in the hot firing test.
2.3.
Characterization
Before and after the hot firing test, the profiles of the C/C composite test piece were obtained using a surface profile measuring instrument (Proscan 2100, Scantron). The microstructure and morphology of the ablation surface of the C/C composites were characterized by scanning electron microscopy (SEM; JEOL, JSM-6390A) and energy-dispersive spectroscopy (EDS).
Ablation test
An SRM hot firing system was utilized to investigate the erosion of C/C composites using a low-velocity, high-particle-concentration two-phase jet. The hot firing test system primarily consisted of combustion chamber, solid propellant coated by inhibitor layer, steel case, pressure gauge, jet controller, test section, C/C composite test piece and nozzle, and so on, as shown in Fig. 2. The jet controller was connected to the combustion chamber with an integral type flange, and the rectangular test section was linked to the
Fig. 1 – Schematic of the composite preform. (A color version of this figure can be viewed online.)
3.
Results and discussion
3.1.
Ablation environment
In an SRM, the combustion products of the metalized propellant provide an ablation environment for the C/C composites and determine ablation behavior. In this paper, the equilibrium compositions of the propellant combustion products were calculated by the Chemical Equilibrium with Application Code, a free software for calculating chemical equilibrium products based on the
Fig. 2 – Schematic of the hot firing test motor.
142
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Table 1 – Chemical compositions of SRM combustion products in the combustion chamber. Species
Mole fraction (mol%)
H2 (g) CO (g) HCl (g) H2O (g) CO2 (g) N2 (g) Al2O3 (g)
33.158 25.682 12.729 8.583 8.360 7.204 7.219
particle impact mass flux has the maximum value, where x = 0.3638 m, whereas both gas and particle impact velocities have the maximum value, where x = 0.3671 m. The maximum values of velocities lag behind those of impact mass flux in space. Fig. 4 shows the recorded time history of pressure in the combustion chamber. The duration of the hot firing test is about 6.2 s, and the maximum chamber pressure is about 6.8 MPa and lasts for approximately 4.2 s.
3.2.
free-energy-minimization principle [12,13]. The estimated chamber pressure (7.0 MPa) was used to calculate chemical equilibrium. Results show that the combustion products in the combustion chamber comprise various species, as shown in Table 1. Other parameters such as stagnation pressure, stagnation temperature, and mass fraction of aluminum oxide in the combustion products are shown in Table 2. A three-dimensional model of the viscous compressible two-phase flow in the rocket motor was established. Fluent Software (version 6.3) was used for the computation using Reynolds-averaged Navier–Stokes equations with extra source terms representing the interactions between the two phases. The Lagrangian method was used to track the condensed particles. The Reynolds number was evaluated to be approximately 57 000. Spalart–Allmaras one-equation turbulence model, which was satisfactorily accurate for transonic compressible flow, was used. Gambit was used to generate a mesh containing 682 000 tetrahedral cells. The mass fluxes and temperatures of both phases were specified at the inlet and diameter distributions of the condensed particles. No boundary condition was necessary at the exit of the submerged nozzle for supersonic flow. The measured sizes of the condensed particles from the chamber of SRM [14] were used as a boundary condition during simulation. Collisions between particles were neglected, and the impact of condensed particles on the surface of C/C composite test piece was regarded as perfectly elastic and specular reflection. The parameters of condensed particles impacting on the surface of the C/C composite test piece were counted and post-processed with user-defined functions. In this paper, the particle impact mass flux is defined as the total mass of particles impacting the surface of the C/C composite test piece per unit area per unit time. The particle impact velocity is defined as the number-averaged velocities of particles impacting the same boundary surface triangular mesh attached onto the top surface of the C/C composite test piece. The computed results are plotted in Fig. 3, along with the gas velocity extracted from the symmetry plane of the test section, where x is the distance in the X-direction from the forefront of the C/C composite test piece (x = 0.330 m). The
Macro-scale ablation rate
Fig. 5(a) shows the photograph of the C/C composite test piece before the hot firing test. Before the test, the top surface of the C/C composite test piece is rather rough. Although most of the fiber bundles are damaged because of machining, the array of carbon rods and bundles are visible and in good order, the tips of the carbon rods are lower, and small shallow pores are formed. Fig. 5(b) shows the photograph of the C/C composite test piece after the hot firing test. The C/C composite test piece is still rough. In the impact region of the two-phase jet, the fiber bundles on the top layer are damaged and a small cone-shaped pit is formed. Downstream of the impact region, some large particles about 1.0 mm in diameter can be seen sticking to the tips of the carbon rods. The surface profile of the C/C composite test piece was measured before and after the hot firing test. The spatial resolution of the measured data is 0.02 mm and then the data
Fig. 3 – Distributions of two-phase flow parameters.
Table 2 – Combustion product parameters in the combustion chamber of the test SRM. Stagnation pressure (MPa) Stagnation temperature (K) Al2O3(l) (wt.%)
7.0 3530.0 31.1
Fig. 4 – Time history of combustion chamber pressure.
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were averaged every 0.2 mm. Fig. 6 shows the 2D recession map of the cross section, which lies along the X–Y plane and passes through the bottom of the pit. The top diameter of the pit is approximately 5.2 mm, and its maximum depth approximately 0.53 mm. A close comparison shows that the position of the bottom of the cone-shaped pit (x = 0.361– 0.364 m) is almost the same as the position where the particle impact mass flux has the maximum value (x = 0.3638 m). This finding indicates that, within the parameter range studied, the particle impact mass flux is more dominant than the particle impact velocity in the erosion of the C/C composites. The maximum ablation amount is found at the bottom of the cone-shaped pit. A local magnification of the 2D recession map shows that at the bottom of the pit, the tip of the carbon rod is higher than the surrounding bundles at mesoscale, with a height difference of about 0.122 mm. The measured ablation rate of the carbon rod is about 0.067 mm/s, whereas that of the bundles is approximately 0.089 mm/s. The ablation rate is immeasurable at macroscale 20.0 mm away from the cone-shaped pit.
3.3.
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Fig. 6 – 2D recession map of the C/C composite surface.
Micro-scale ablation morphology
Fig. 7(a) shows the morphology of the post-test C/C composite test piece. Near the impact region of the two-phase flow jet, the ablated surface of the C/C composite test piece is rougher at mesoscale. Numerous spherical Al2O3 particles with diameters ranging from 100 to 300 lm are deposited onto the ablated surface of the fiber bundles and the tips of carbon rods. The fiber bundles are denudated and burned into broom-like shapes. The tips of the carbon rods at the bottom
Fig. 5 – Photograph of the C/C composite test piece before and after the hot firing test: (a) pre-test, and (b) post-test.
Fig. 7 – Morphology of the C/C composite test piece: (a) pit, (b) carbon rod, and (c) morphology of the carbon rod downstream of the pit.
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Table 3 – EDS characterization of residuals in pore. Element
(keV)
Mass (%)
Error (%)
Atom (%)
C O Al Si Total
0.277 0.525 1.486 1.735
21.63 37.14 40.78 0.49 100.00
0.12 0.14 0.10 0.16
31.88 41.09 26.75 0.28 100.00
of the pit are coarser and higher than the surrounding fiber bundles, indicating that the carbon rods are more resistant to mechanical erosion than the bundles. The carbon rods and fiber bundles outside the pit show no obvious damage or ablation. The velocity of the two-phase flow is relatively smaller than the velocity of the two-phase flow in the nozzle throat. Consequently, the convective heat flux transferring from combustion products to the solid is small, and the surface temperature of the C/C composites is low. As a result, the rate of heterogeneous reactions between the oxidizing species present in the combustion products of the propellant and C/C composites is lower [1,10], and chemical ablation is weakened. Nevertheless, the maximum measured ablation rate of the C/C composite test piece is comparable with that of laboratory SRM experimental data [7–9]. This finding indicates that near the impact region, mechanical erosion from the impact of condensed particles on the top surface of the C/C composite test piece plays a fully predominant role. Moreover, the carbon rods are damaged and the fiber bundles are denudated. Fig. 7(b) shows the high-magnification morphology of the carbon rod at the bottom of the pit shown in Fig. 7(a). A pore is present at the tip of the carbon rod. EDS shows that the residuals in the pore are mainly Al2O3 (Table 3), and it is deduced that pore formation is due to the impact and damage of larger condensed particles. The fibers are peeled off from the carbon matrix, and blunt fracture tips are formed. The tips of the carbon fibers are lower than the matrix, indicating that the carbon matrix is more resistant to mechanical erosion than the carbon fibers. In design and manufacturing, fine-weaving C/C composites with low-fiber-density carbon rods should be used as constituents for the thermal protection components subjected to severe erosion of two-phase flow. Fig. 7(c) shows the high-magnification SEM image of the carbon rod 40 mm downstream of the impact region where the ablation rate is immeasurable at macroscale. The carbon fibers are arranged in good order, and the tips of the fibers are almost flat. These features show that the material has only started its chemical ablation and has not had sufficient time to develop a steady morphology. These results indicate that the C/C composites are less ablated because of low gas velocity, low particle impact velocity and small impact mass flux in this local area. The irregularity of the surface is due to the machining before the test. The interface, which can be well resolved prior to testing, is burned out although the surface temperature of the C/C composites may be lower than that of the nozzle throat, ring-like gaps between the carbon fibers and matrix are formed, suggesting that the interface
can be more prone to oxidization than the carbon matrix and fibers. A lamellar matrix can be seen in the low-fiber-density area, whereas the matrix in the high-fiber-density area is burned out and gaps are formed. This phenomenon indicates that carbon fibers are more resistant to chemical ablation than the matrix and interface. For the thermal protection components subjected to severe chemical ablation, rods with high carbon fiber density should be used.
3.4.
Ablation analysis
The ablation of C/C composites is very complex and involves both chemical ablation and mechanical erosion, which determine the ablation behavior of C/C composites. Chemical ablation is widely known as the primary cause of ablation and is referred to as the heterogeneous reactions between the oxidizing species present in the combustion products of the solid propellant and the C/C composites. According to the classical Arrhenius law, the oxidation rate varies severely with temperature and the oxidant partial pressure [4,15]. Mechanical erosion from condensed particles is regarded as another main factor contributing to the ablation of C/C composites [6,7,9,10]. The role of the condensed particles in solid propellant combustion products can be classified into two categories: mechanical force and mechanocaloric effect. In the hot firing test in this paper, the chemical ablation is weakened due to the low velocity of combustion gases flowing over the C/C composite test piece. Nevertheless, the erosion of particles is predominant in the ablation of the C/ C composites. A multiscale erosion model is necessary for the C/C composites. Fig. 7(b) shows that the size of the particle impact damage is comparable with the microstructural size scale of the C/C composites. Consequently, the microstructural constraints must be considered and an ‘‘averaging law’’ is not applicable at microscale, particle impact events occur in one of the two phases, carbon fiber and matrix. At mesoscale, the particle impact damage to carbon rod and bundle must also be differentiated, as evidently shown in Fig. 7(a). To model the erosion process, a control volume or minimal periodic unit should first be determined at both microscale and mesoscale for the C/C composites, and a multiscale erosion model is essential. At microscale, when one particle impacts on the C/C composites surface, the mechanical force between the particle and the C/C composites can be calculated by the Hertzian equation for a pure elastic collision [16]. By assuming that the contact region to be circular, the maximum contact area and the duration of the impact have been derived by
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Hertzian and the maximum mechanical stress Fp can be calculated. The probability that the particle impacts on the carbon fiber or the matrix can be evaluated depending on the structure of the microscale minimal periodic unit. When the impact event occurs in the carbon fiber, the maximum mechanical stress Fp can be decomposed into normal stress Fn and tangential stress Ft, which are parallel and perpendicular to the fiber orientation, respectively. When the normal stress Fn or the tangential stress Ft exceeds the impact yielding stress of the carbon fiber, erosion occurs. The erosion can be classified into two different modes: brittle mode and ductile mode, which are dominated by the normal stress and the tangential stress, respectively. For the carbon fiber, both modes would take place simultaneously and should be considered. On erosion, the mass loss or volume loss of the carbon fiber should be counted and the effect of particle impact damage on fiber morphology must be determined and transferred to that at mesoscale. If the impact event occurs in the matrix, the maximum mechanical stress Fp can be decomposed into normal stress Fn and tangential stress Ft, which are perpendicular and parallel to the matrix surface, respectively. In the similar way, particle impact damage can be evaluated. At mesoscale, the microscale model is of great directive significance. At macroscale, the linear or inverse rules of mixture may be used to evaluate the erosion rate of the C/C composites. In the preceding description, many correlations must be established and validated, such as the accurate evaluation of the mechanical stress, the measurement of the intrinsic parameters of the C/C composites at different scales, the effect of particle impact damage on morphologies of the two phases, the mass loss or volume loss of the C/C composites at different scales, the way the impact damage is transferred from microscale to mesoscale and from mesoscale to macroscale and so on. Further investigations on these correlations should be thoroughly carried out.
4.
Conclusions
The erosion of a 4D C/C composite test piece by low-velocity, high-particle-concentration two-phase jet was investigated using an SRM. The average ablation rates were measured, and the ablation performance, mechanism, and microstructure of the C/C composites were investigated. The effect of chemical ablation is found to be weak due to low gas velocity. The comparatively high ablation rates are mainly attributed to mechanical erosion. At microscale, the carbon fibers are more susceptible to mechanical erosion than the carbon matrix, whereas carbon fibers are more resistant to chemical ablation than the matrix and interface. At mesoscale, the carbon rods are more resistant to mechanical erosion than fiber bundles. In applications, fine-weaving C/C composites with low-fiber-density carbon rods should be used as the constituents for the thermal protection components subjected to
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severe erosion of two-phase flow, and carbon rods with high carbon fiber density should be used as the constituents for thermal protection parts subjected to severe chemical ablation.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Contract No. 50876091).
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