**1. Introduction**

The concept of hypersonic flight has attracted worldwide attention since it was proposed in the 1940s [1–3]. Since there, different hypersonic vehicles were developed as well as hypersonic missiles, aircraft and re-entry vehicles. However, the significant number of technical challenges have surfaced which are critical to the successful development of these high-speed flight vehicles. Understanding, analyzing and predicting high-speed flow around blunt bodies pose a practical and important engineering problem; faster and better design of new flight vehicles depends on it. The high heat of a hypersonic aircraft during flight imposes severe demands on the materials and structures, so the reduction of heat transfer rate plays an important role during the conceptual design of re-entry vehicles. Classical thermal protection systems such as ablatives [4] are less adapted to the rapid growth of spacecraft technology: ablatives are related to the coating thickness, and are not convenient for shape change. To improve the flow field in front of a vehicle nose, additional solutions such as active cooling approach aero-spikes [5] and opposing jets [6] have been developed. Different strategies exhibit varying characteristics in a hypersonic flow field.

The concept of a spiked blunt body was first proposed by Bogdonoff [7]. Flow separation in front of blunt bodies at supersonic speeds, have been made since the early 1950s [8–11]. Spikes have been shown to create a separation zone over blunt bodies, lowering the aerodynamic heat rate and pressure distribution, which is beneficial for thermal protection and drag reduction. In the 1960s, separation characteristics and the resulting flow instability have been the subject of extensive research. Maull explored the effects of spike length and shape on the flow field properties of blunt bodies, concluding that flow oscillation was generated by two factors: shock wave-induced separation and flow reattachment [12]. Using a mix of spike lengths and cone angles, wood explored spiked cone cylinders flying at Mach 10 and established five possible flow patterns with related scopes [13]. Reding et al. examined unstable aerodynamics for a spiky drag reduction device based on structure deflection coupled with thermal expansion generated by aerodynamic heating. Many studies on numerical simulations and solutions have allowed for a great level of insight with regards to spiked blunt bodies since the rapid rise of computer technology began in the 1990s. Many of these researches were validated by collaboration between experiments and numerical simulations. Mehta investigated the relationship between the aerodynamic heat flux and pressure distribution over spiked bodies at Mach 6.8 [14].

The opposing jet technique was introduced initially by Lophtoff [15] and Warren [16], and its obvious effects on drag and heat reduction for aircraft were realized. The interaction between the opposing jet and the free stream determines the flow pattern. Hayashi et al. [17, 18] investigated the opposing jet using both experimental and numerical methods in several investigations. Their tests were carried out in a traditional blow-down type wind tunnel with a free stream of Mach 3.98, and the axisymmetric Navier-Stokes equations were solved using the implicit finite difference method. Their research revealed that the ratio of the opposing jet's stagnation pressure to that of the free stream had a significant impact on the flow mode. Isao Tamada et al. [19] investigated the heating reduction of the ogive body and hemispherical nose cylinder body experimentally and numerically, at M = 3.98 and M = 8.0. They found that local Reynolds number management and recompressed shock monument are critical to reducing aerodynamic heating, and that the ogive body was more effective at reducing heating with the same mass flow rate because of its large enough recirculation region to cover the entire nose tip.

*Aero Heating Optimization of a Hypersonic Thermochemical Non-Equilibrium Flow… DOI: http://dx.doi.org/10.5772/intechopen.101659*

Huang et al. [20, 21] investigated some opposing jet configurations with other cooling approaches, such as spike, aerodisk, and forward-facing cavity, and discovered combined promising drag and heat flux reduction effects, as well as the coupling mechanism between the self-sustained oscillations induced by the jet and the unsteady modes induced by the other configurations. An experiment was conducted by Jiang et al. [22] which a new concept of the non-ablative thermal protection system for hypersonic vehicles was first proposed, and the blunt spike was combined with lateral jets for developing a shock reconstruction system at the front side of hypersonic vehicles, to achieve effective wave for reducing drag under non-zero attack angles and also to avoid severe aero heating (rocket).

In this work, a blunt re-entry vehicle is modeled and analyzed in ANSYS Fluent 19 which represents the distribution of heat flux on the surface of a representative Lobb sphere blunt body and the coupled effects of thermochemical non-equilibrium and chemical reactions on the hypersonic air flows. Validation is performed with the obtained CFD results which are in good agreement with the experimental values of Liu and Jiang [23] for blunt spike body and Hayashi et al. [24] for opposing jet. Next, optimization is carried out by placing a jet at the front of the blunt spike body. The main simulation results are discussed by comparing the spike and jet heat reduction configurations.
