Three-dimensional numerical study of the directional heat transfer in an L-shaped carbon/carbon composite thermal protection system

Highlights

• 3D C/C L-fiber directional heat transfer thermal protection system is proposed.

• LBM coupled FVM method considering the radiant heat transfer is adopted.

• Failure temperature for directional heat transfer system is obtained.

• Relationships between heat flux of radiation and heat conduction is studied.

Abstract

Designing an accurate and efficient heat protection system for the area around the high-temperature stagnation point on supersonic aircraft is still an important topic of research. In the present study, a three-dimensional high thermal conductivity carbon/carbon (C/C) composite thermal protection system embedded with L-shaped carbon fiber bundles for directional heat transfer is proposed as a high-efficiency thermal protection design. A multi-scale method, which couples the finite volume method (FVM) and lattice Boltzmann method (LBM), is developed to investigate the directional heat transfer in the proposed structure. The FVM is used to calculate the heat radiation information, which is needed to solve the energy equation with the LBM. Further, the failure temperature of the proposed thermal protection structure is defined. The effects of the porosity, carbon fiber bundle and pore diameter on the directional heat transfer of the L-shaped C/C composite thermal protection system are investigated in detail. The results show that the effective thermal conductivity of the proposed thermal protection system increases with increasing temperature and carbon fiber bundle diameter. It decreases with increasing porosity when the temperature is below 1000 °C. There exists the competitive relationship between the pore heat radiation and carbon fiber bundle thermal conductivity. An increased porosity results in a decrease in the failure temperature of the proposed thermal protection structure, while increasing the carbon fiber bundle diameter can increase the failure temperature. These findings can provide some new insights for designing a high-performance C/C composite thermal protection system.

Introduction

In recent years, aircraft have developed toward enabling higher speeds and longer flights, resulting in an increased influence of aerodynamic heating [1]. Aerodynamic heating can change the shape, aerodynamic layout, and performance of the aircraft, and it can also affect the usage of electronic equipment in aircraft. Therefore, a thermal protection system is essential for the safety of the payload inside aircraft [2,3]. Thus, an efficient thermal protection system is necessary to protect the aircraft. Currently, the thermal protection system can be divided into many types, including heat sink [4], ablation [5,6], heat insulation [[7], [8], [9]], heat pipe cooling [10], convection cooling [11], transpiration cooling [12], and liquid film cooling [13,14], among which, the multilayer or integrated thermal protection system are widely used for thermal protection of aircraft [15,16], and these generally consist of an ablation-resistant layer and a thermal insulation layer for non-ablative thermal protection system [17,18]. The ablation-resistant layer is critical and determines the overall performance of the thermal protection system. The layer for directional heat transfer, which is typically treated as the ablation-resistant layer, can reduce the heat from the head or leading edge of an aircraft.

A well-designed heat-guiding structure can greatly reduce the temperature of the area near the stagnation point to alleviate the thermal load of the ablation-resistant layer near this point. It also can make the entire thermal protection layer approximately isothermal, thus reducing the thermal stress of the thermal protection structure. Many models have been developed to study the heat transfer in above structures. For example, Wu [19] proposed a method combining heat protection and heat control based on the basic principles and rules of the thermal protection system and thermal control system to reduce the weight of the structure. However, their model ignored the radiative heat transfer. Li et al. [20] designed several types of directional heat conduction structures, including panels composed of only carbon fibers or carbon tubes, directional heat conduction fibers contained within heat insulation materials, and directional transportation of the heat flow enhanced by adding fluids based on the heat insulation materials. They designed structures that conformed to the material characteristics to account for the anisotropy of the material, but no performance analysis was conducted and the pores were not considered. Later, Sun et al. [21] proposed a structure combing an outer ablation layer, middle thermal guiding layer, and an inner thermal insulation layer close to the outer surface of aircraft. The thermal conduction differential equation and law of energy conservation were then used to obtain the temperature field of the structure. They found that the inclusion of a directional high thermal conductivity layer in the thermal protection system could reduce the maximum temperatures of the outer and inner wall surfaces by 9.1% and 31.5%, respectively. However, the calculation carried out was two-dimensional (2D), and the pores in the structure were not considered. Although the radiative heat from the surface was considered, the effect of internal radiation was neglected. Ai et al. [22] studied the effect of the acceleration overload of the aircraft on the internal liquid backflow of the heat-guiding structure containing the liquid working medium. They found that the dredging structure had an effective heat redistribution property when the overload was less than 4g, which indicates that heat redistribution structures containing liquid are not suitable for high-mobility aircraft. The use of directional heat transfer can improve the properties of the thermal protection system. Thus, a new directional heat transfer structure that is highly accurate and efficient should be needed.

In addition, it is inevitable that there will be pores in the thermal protection system. The influence of pores on the thermal protection system should be considered during the heat transfer process. For example, Zhou et al. [23] studied the influence of pores in the coatings of carbon–carbon materials on the thermal protection structure and noted that the pores in the coating act as a channel for oxygen to reach the underlying carbon–carbon material. They found that the local thermal protection could be invalidated in severe cases. The calculation was just based on a one-dimensional equation, and thus the influence of space effects cannot be excluded. Gusarov et al. [24] studied the radiative heat transfer process of carbon fiber materials with high porosity in an atmosphere reentry thermal protection system. A model was built using a multi-phase method to describe the heat transfer and radiation reflection in porous carbon fiber materials. The results showed that 90% of the incident radiation at a wavelength of approximately 1 μm could be absorbed by an outermost surface of approximately 200–300 μm. However, the calculation was only based on the assumption of a gray isotropic medium, which may cause errors near the stagnation point or leading edge where the temperature gradient is high. Gulli et al. [25] built a 2D reconstruction of the sample to perform a permeability test and confirm the velocity on the control surface. However, there are certain errors in this 2D image method. These errors arise from the image resolution of the 2D porosity calculation, which prevents the shape of the gray histogram from being captured accurately. It can be found that the pores have an obvious influence on the properties of thermal protection system. However, the performance of a directional heat transfer thermal protection system considering the effect of pores has not yet been reported.

As reviewed above, the performance of a new directional heat transfer structure, the carbon/carbon (C/C) composite thermal protection system embedded with L-shaped carbon fiber bundles owing a high thermal conductivity, has not been reported. The relationship between the high thermal conductivity and heat radiation has also not been clearly represented. Therefore, this study considers the effect of a heat-guiding layer containing L-shaped carbon fiber bundles on the heat transfer performance while also considering the influence of pores. The heat transfer considered includes the internal radiant heat transfer, and the radiative heat transfer equation is used to obtain the relevant radiation information. The radiation results are then introduced to the traditional lattice Boltzmann governing equation to obtain the temperature field. Thus, a multi-scale method is proposed. The effects of the pores and carbon fiber bundle on the directional heat transfer in the L-shaped C/C composite thermal protection system are investigated in detail.