Scramjets

Fuel Mixing and Injection Systems

Mostafa Barzegar Gerdroodbary

• Publisher: Butterworth-Heinemann
• Year: 2020
• Print ISBN: 9780128211380
• eBook ISBN: 9780128211403
• Topic: Technology & Engineering
• Subtopic: Aeronautic & Astronautic Engineering
• Index: Technology & Engineering

Scramjet engines are a type of jet engine and rely on the combustion of fuel and an oxidizer to produce thrust. While scramjets are conceptually simple, actual implementation is limited by extreme technical challenges. Hypersonic flight within the atmosphere generates immense drag, and temperatures found on the aircraft and within the engine can be much greater than that of the surrounding air. Maintaining combustion in the supersonic flow presents additional challenges, as the fuel must be injected, mixed, ignited, and burned within milliseconds. Fuel mixing, along with the configuration and positioning of the injectors and the boundary conditions, play a key role in combustion efficiency.Scramjets: Fuel Mixing and Injection Systems discusses how fuel mixing efficiency and the advantage of injection systems can enhance the performance of the scramjets. The book begins with the introduction of the supersonic combustion chamber and explains the main parameters on the mixing rate. The configuration of scramjets is then introduced with special emphasis on the main effective parameters on the mixing of fuel inside the scramjets. In addition, basic concepts and principles on the mixing rate and fuel distribution within scramjets are presented. Main effective parameters such as range of fuel concentration for the efficient combustion, pressure of fuel jet and various arrangement of jet injections are also explained. This book is for aeronautical and mechanical engineers as well as those working in supersonic combustion who need to know the effects of compressibility on combustion, of shocks on mixing and on chemical reactions, and vorticity on the flame anchoring.- Explains the main applicable approaches for enhancement of supersonic combustion engines and the new techniques of fuel injection- Shows how the interaction of main air stream with fuel injections can develop the mixing inside the scramjets- Presents results of numerical simulations and how they can be used for the development of the combustion engines

Chapter 1: Introduction

Abstract

In this chapter, the main challenges for the invention of the scramjet engine is fully presented. The primary configurations for achieving the recent scramjet engine are also reviewed. The non-dimensional parameters are presented and the main advance centers for the development of the scramjet engine are introduced. Besides, the main parts of the scramjet engine is introduced and their roles on the scramjet engine is determined. In addition, required fuel types for different flight ranges are described.

Keywords

Fuel type; History of scramjet; Ramjet; Shock interaction; Supersonic jet

1.1. Main structure of scramjets (Ramjet to supersonic combustion ramjet)

In 1913, Ren'e Lorin introduced the concept of ramjet as the first efficient air-breathing system with a simple operating mechanism [1]. To develop the gas turbine engine for the high-speed range, the internal geometry and design of this engine are developed to obtain high air compression, which enables the engine to operate in high Mach number. Since the performance of the ramjet is not good enough for subsonic flight due to low dynamic pressure, it is not a reliable engine for takeoff conditions.

Prior to the focus on details of combustion systems, however, the types of specific engines, along with their limits, should be initially introduced. Fig. 1.1 illustrates the schematic of four different types of ramjet. The subsonic combustion ramjet works can operate at a subsonic speed as well as supersonic speed within Mach 2–4.5. However, the supersonic combustion ramjet mainly works at a velocity more than Mach 4, and it cannot operate at subsonic speeds.

A conventional can type of liquid-fueled ramjet (LFRJ) or gaseous-fueled ramjet (GFRJ) is illustrated in Fig. 1.1A with a tandem booster. To produce static and low speed thrust, a tandem booster is obligatory. In this model, M₀ > M₁ > 1, and a normal shock system before reaching station 4 allows the air to reach a subsonic speed. Prior to reaccelerate through a geometric throat (M5 = 1) and exit nozzle (M6 >1), fuel is released and burned with the air at low subsonic velocity.

A common combustion chamber for both the boost and sustained phases of flight is a more recent technique for the advance of the previous one, and it is usually denoted as an integral rocket ramjet (IRR). This mostly involves a dump-type rather than a can-type combustor; however, the cycle operation of the ramjet is identical. Fig. 1.1B exemplifies this model for a liquid-fueled IRR, and a solid-fueled version (SFIRR) is demonstrated in Fig. 1.1C. Since the fuel system of solid-fueled ramjets is simpler, they are commonly desired over LFRJs or GFRJs when fuel-throttling necessities are insignificant. Fig. 1.1D demonstrates that the air-ducted rocket (ADR), revealed in its IRR form, is an additional ramjet model. In this type, a fuel-rich propellant is applied to produce a low-to-adequate pressure gaseous fuel supply for the subsonic combustor. The selection of an ADR is mostly based on a compromise between the fuel supply simplicity of the SFIRR and the unlimited throttle ability of the LFRJ or GFRJ.

Figure 1.1 Schematic of ramjet: (A) conventional can combustor ramjet (CRJ); (B) integral rocket/dump combustor ramjet (LFIRR); (C) solid-fueled ramjet (SFIRR); (D) air-ducted rocket (ADR) [2].

As the velocity of the vehicles increases beyond M = 4, the supersonic combustion engine is the only choice for efficient flight. Fig. 1.2 demonstrates a standard scramjet engine with two hybrid alternatives. As shown in Fig. 1.2A, air at supersonic or hypersonic velocity is entered to a lower, although still supersonic, speed at station 4 in the traditional scramjet engine. Fuel is then released from the walls (holes, slots, cavities, pilots, etc.) and/or in-stream protrusions (struts, tubes, pylons, etc.), where it combines and burns with the air in a largely diverging area combustor. Different from terminal normal shock system in the subsonic combustion ramjet, the hybrid impacts of heat accumulation and diverging area in the combustor of scramjet as well as the lack of a geometric exit nozzle throat and create a shock train placed at and upstream of the combustor entry. Its strength varies between the equivalent of a normal shock and no shock.

Figure 1.2 Schematic of scramjet: (A) supersonic combustion ramjet, (B) dual combustor ramjet (DCR), (C) ejector scramjet (EJS) [2].

Fig. 1.2B presented the schematic of the most advanced of these ideas as the dual combustor ramjet (DCR). All of the features of the scramjet remained in the DCR except a section of the air is diverted to a small. By more uniform fuel distribution, a near-stoichiometric, flame can be preserved, and the released energy preserves fuel into combustible products when they arrive the supersonic combustor, even at Mach 3 flight velocity.

The last supersonic combustion cycle, which is a development of the gas-generator-type cycle, is the ejector scramjet revealed in Fig. 1.2C. Dissimilar to other types of supersonic combustion engine cycles, it can provide static thrust using axial injectors fed by high-pressure supersonic gaseous fuel or fuel/oxidizer. This high-pressure/velocity fuel-rich ejector propels air into the supersonic combustor where combustion occurs, creating static thrust. Since nozzle throat does not exist in this model, combustion occurs at the lower speeds in a mixed subsonic/supersonic flow. Before exiting of the flow from the combustor, it moves along a thermal throat, that is, it works as a dual-mode scramjet.

As the flight speed goes beyond Mach 2, the compressor as the central part of a turbojet is not required to increase the pressure. Since the velocity of the inlet flow is high enough, the application of the inlet and diffuser could manage the pressure and velocity of the main inlet stream in the combustion chamber. For higher Mach number (M > 3), due to the formation of the terminal shock resulted from subsonic, the considerable pressure loss as well as high-temperature flow increase the energy loss of the mainstream. To increase the engine performance, it is recommended to preserve the mainstream in the supersonic speed and increase the temperature of the mainstream within the combustion chamber. To compare the operating condition of various available engines for high-speed vehicles, Fig. 1.3 illustrates the range of performance of the different engines for a wide range of velocity. It is clearly demonstrated that another system of propulsion is required for the takeoff condition of the ramjet or scramjet.

Fig. 1.4 schematically compares the structure of the ramjet engine with scramjet one. As shown in the figure, the main difference between these two engines is the subsonic condition in the section of the combustion. Ferri [3] compared the main characteristics of these two engines for the specific flight condition where the Mach is 12 at the altitude of 40 km, and the fuel is hydrogen with a stoichiometric ratio of an inlet air stream. According to his results, the stagnation pressure recovery of the scramjet is more than ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Chapter 1. Introduction
• Chapter 2. Basic principle of supersonic combustion chamber
• Chapter 3. Injection and mixing of single fuel jet in SCRAMJET engine
• Chapter 4. Multi techniques for fuel injections
• Chapter 5. Numerical procedures and simulation
• Chapter 6. Modern techniques for fuel injections
• Index