Heat Transfer in Aerospace Applications

Bengt Sundén, Juan Fu

• Publisher: Academic Press
• Year: 2016
• ISBN:
  9780128097618
• Topic: Technology & Engineering
• Subtopic: Aeronautic & Astronautic Engineering
• Index: Technology & Engineering

Heat Transfer in Aerospace Applications is the first book to provide an overall description of various heat transfer issues of relevance for aerospace applications. The book contains chapters relating to convection cooling, heat pipes, ablation, heat transfer at high velocity, low pressure and microgravity, aircraft heat exchangers, fuel cells, and cryogenic cooling systems.

Chapters specific to low density heat transfer (4) and microgravity heat transfer (9) are newer subjects which have not been previously covered. The book takes a basic engineering approach by including correlations and examples that an engineer needs during the initial phases of vehicle design or to quickly analyze and solve a specific problem. Designed for mechanical, chemical, and aerospace engineers in research institutes, companies, and consulting firms, this book is an invaluable resource for the latest on aerospace heat transfer engineering and research.

• Provides an overall description of heat transfer issues of relevance for aerospace applications
• Discusses why thermal problems arise and introduces the various heat transfer modes
• Helps solve the problem of selecting and calculating the cooling system, the heat exchanger, and heat protection
• Features a collection of problems in which the methods presented in the book can be used to solve these problems

Chapter 1

Introduction

Abstract

This chapter introduces heat transfer in general and then presents specific topics of significance for aerospace applications. Issues on thermal management, cryogenic matters, low-density heat transfer, microgravity, heat pipes, auxiliary equipment, computational methods, and certain measurement methods are then briefly introduced. Later chapters give a more thorough analysis of these various topics.

Keywords

Boundary layer interaction; Heat exchange; Heat transfer modes; High heat flux; Shock wave; Thermal management

1.1. Heat Transfer in General

Heat is a form of energy that is always transferred from the hot part to the cold part in a substance or from a body at a high temperature to another body at a lower temperature. The bodies do not need to be in contact but a difference in temperature must exist.

In some cases the amount of heat transferred can be determined simply by applying basic relations or the laws of thermodynamics and fluid mechanics. In other cases in which the mechanisms of heat transport are not completely known, methods of analogy or empirical methods based on experiments are applied.

Heat can be transferred by three different means, namely, heat conduction, convection, and thermal radiation, as illustrated in Fig. 1.1. Many textbooks are available on general heat transfer, see, e.g., Refs. [1–3].

Heat conduction is a process in which energy transfer from a high temperature region to a low-temperature region is governed by the molecular motion, as in solid bodies and fluids (gases and liquids) at rest, and by the movement of electrons, as for metals.

Figure 1.1 Heat transfer by (a) heat conduction, (b) convection, and (c) thermal radiation.

For heat conduction across the wall in Fig. 1.1 the heat flux is calculated by

q=k(T1−T2)/b

(1.1)

Q=q·A

(1.2)

where q is the heat flux in W/m²; k, the thermal conductivity of the wall material in W/mK; b, the wall thickness; T₁ and T₂, the temperatures on the wall surfaces; and Q, the total amount of heat transferred in W.

When a fluid is flowing along an exterior surface or inside a duct and if the temperatures of the fluid and the solid surface are different, the amount of heat being exchanged is affected by the macroscopic fluid motion. This type of heat transfer is called convection. Depending on how the macroscopic fluid movement is created, forced convection or free (natural) convection prevails. In some cases, both forced and free convection occur simultaneously. The process is then called mixed convection or combined forced and free convection. For this heat transfer mode a heat transfer coefficient is introduced according to

q=α(TS−T∞)=h(TS−T∞)

(1.3)

where α and h is the heat transfer coefficient in W/m²K and T_S and T_∞ are the temperatures of the surface and fluid, respectively. The heat transfer coefficient is, in general, a function of flow velocity, fluid type, temperature, and geometry.

Heat transfer by radiation does not require any medium to propagate. The heat transfer between two surfaces by radiation is in fact maximum when no media is present between the surfaces. Radiation may occur between surfaces, as well as between a surface and a participating medium, such as gas. Heat exchange by radiation is governed by electromagnetic waves according to Maxwell's theory or in the form of discrete photons according to Planck's hypothesis. For the solid body radiative heat exchange illustrated in Fig. 1.1, the heat flow rate is given by

Qnet=A1F12εeff(T14−T24)

(1.4)

where F₁₂ is the view factor; ε_eff, the effective emissivity; and A₁, the area of surface 1.

If a medium participating in radiation is present between the surfaces, the calculation of the radiative heat exchange becomes more complicated [4,5].

In many branches of engineering and technology, it is of great interest to be able to calculate temperature distributions and heat fluxes. In order to design, size, and rate heat exchangers, e.g., condensers, evaporators, and radiators, analysis of heat transfer is needed. Huge applications of this type of equipment appear frequently in heat and power generation, process industries, etc. Design and sizing of air-conditioning equipment, electronics cooling, and thermal insulation of buildings require knowledge in heat transfer. For vehicles, many heat transfer problems are present.

To enable stress and strain analysis in equipment exposed to high temperature and/or gradients, analysis of the temperature field and heat loads is needed. In manufacturing, production, and treatment of materials, heat transfer is also important. Cooling of electronics and other equipment carrying electric currents is an important application area of heat transfer. Also in combustion devices, heat transfer is of significance because of thermal radiation and convection. Processing and treatment of food require analysis of heat and mass transfer.

1.2. Specifics for Aerospace Heat Transfer

Thermal management requirements for aerospace applications grow continuously, whereas the allotments on weight and volume remain constant or shrink. To meet the high heat flux removal requirements, compact, high-performance, and lightweight heat transfer equipment are needed. Heat transfer systems in military aircraft are increasingly using fuel as a heat sink. Heat transfer loops involving several fuel-to-liquid heat exchangers are used to cool electronics, engine oil, hydraulic oil, and elements of the thermal management system. Heat exchangers based on microchannels are very suitable, as they offer opportunities for high heat flux removal because of their good thermal performance and extremely compact size.

Aerospace challenges include reduced gravity, low or no atmospheric pressure, extreme temperatures, aerodynamic heating, dynamic vibration, shock loads, and extended duration operations. Also alternative power sources are needed for aerospace vehicles. One of the possible alternatives is fuel cells. Providing reliability, compactness, and high-energy power sources for aerospace applications is important, and fuel cells might be a good candidate. As hydrogen is a common fuel, a lot of effort has been spent on its production, transportation, storage, system design, and safe and effective handling.

Heat transfer issues are also demanding challenges for aerospace propulsion. In general, the chemical energy in the fuel is transformed into useful work of propulsive thrust at maximum effectiveness. The propulsion system must then operate at a very high temperature and pressure. It is important to protect the propulsion surfaces from the hostile thermal environment. One way to achieve this is to develop materials capable of withstanding the hostile environment and offer an adiabatic surface that will not melt or lose its structural integrity. Another approach is to immediately cool the exposed surfaces.

For subsonic and supersonic flights, the turbine engine is the backbone. The hot sections include the combustor, the turbine, the exhaust valves, and some other components. The turbine is deemed to be the most demand...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Nomenclature
• Chapter 1. Introduction
• Chapter 2. Ablation
• Chapter 3. Aerodynamic Heating: Heat Transfer at High Speeds
• Chapter 4. Low-Density Heat Transfer: Rarefied Gas Heat Transfer
• Chapter 5. Cryogenics
• Chapter 6. Aerospace Heat Exchangers
• Chapter 7. Heat Pipes for Aerospace Application
• Chapter 8. Fuel Cells
• Chapter 9. Microgravity Heat Transfer
• Chapter 10. Computational Methods for the Investigations of Heat Transfer Phenomena in Aerospace Applications
• Chapter 11. Measuring Techniques
• Appendix 1. Governing Equations for Momentum, Mass, and Energy Transport
• Appendix 2. Dimensionless Numbers of Relevance in Aerospace Heat Transfer
• Index