Thermal Radiation Cooling of External Vehicle Surfaces
External surfaces (the external flow path) of hypersonic flight vehicles primarily are radiation cooled. The radiation cooling effect depends on certain properties of the attached viscous flow, the boundary layer, and on the emis – sivity coefficient of the surface material. Hence the wall temperature and its gradient, respectively the heat flux in the gas at the wall—the thermal state of the surface—are not constant, on the contrary, they are functions of the location on the vehicle surface. The concept of the “thermal state of a surface” was introduced in Section 1.4. It governs both “thermal loads” and “thermal surface effects”. This holds for the external flow path as well as for the internal flow path which is present on airbreathing CAV’s and ARV’s.
In this chapter we discuss in detail the matter of surface radiation cooling before the classical topics of aerothermodynamics are treated in the following chapters. Basics of surface thermal radiation cooling are treated in Section 3.2. With simple approximations we try to obtain a basic understanding of the aerothermodynamics of radiation-cooled surfaces. Treated too are nonconvex effects and scaling approaches. Results of computations with numerical methods are used to illustrate the findings. A case study, Section 3.3, gives a detailed account. Some examples of viscous thermal surface effects are treated in the Chapters 7, 8, and 9. In Chapter 10 a number of further examples is discussed. Thermal loads on structures are not a central topic of this book.