Internal Combustion Processes of Liquid Rocket Engines

Modeling and Numerical Simulations

Zhen-Guo Wang

• Publisher: Wiley
• Year: 2016
• ISBN:
  9781118890059
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Luft- & Raumfahrttechnik

This book concentrates on modeling and numerical simulations of combustion in liquid rocket engines, covering liquid propellant atomization, evaporation of liquid droplets, turbulent flows, turbulent combustion, heat transfer, and combustion instability. It presents some state of the art models and numerical methodologies in this area. The book can be categorized into two parts. Part 1 describes the modeling for each subtopic of the combustion process in the liquid rocket engines. Part 2 presents detailed numerical methodology and several representative applications in simulations of rocket engine combustion.

1
Introduction

A liquid rocket engine, which is also called a liquid propellant rocket engine, is a chemical rocket engine using liquid chemicals (liquid propellant) as the energy source and the working fluid. Liquid rocket engine technology has drawn researchers’ attention and been quite a hot topic in aerospace and aeronautic research during the last 70 years. In the short long history of human aviation, i.e., from the A-4 engine of the German V2 missile, to the F-1 engine of the U.S. lunar landing rocket “Saturn 5” and further to reusable space shuttle main engines, every milestone event is closely linked with the progress made in liquid rocket engine technology. Because liquid rocket engines have the characteristics of high specific impulse, repeatable starting, arbitrary working hours setting, multiple usage, adjustable thrust, etc., they are bound to occupy the dominant position in the area of aerospace propulsion long into the future.

The liquid rocket engine uses liquid fuels as the propellant. In a liquid rocket engine, the liquid chemical propellants combust in the combustion chamber and produce very high pressure gas. The gas is accelerated when it flows downstream through the nozzle and produces impulse, i.e., thrust, for the aircraft. There are several types of liquid propellants. The scheme, structure, ignition and thermal protection, etc. of the liquid rocket engine have a close relationship with the characteristics of the propellants used by the engine system.

The expansion of liquid rocket application requires more in-depth studies on the basic theory and design method of the liquid rocket engine. Numerical simulation of the combustion process in a liquid rocket engine is also an important research direction. This chapter introduces the basic configuration and working process of liquid rocket engines, and then discusses the main objective and research method of the numerical simulation of the combustion process in a liquid rocket engine.

1.1 Basic Configuration of Liquid Rocket Engines

A liquid rocket engine consists of a thrust chamber (which consists of an injector, a combustor, and a nozzle), a propellant feed system, propellant tanks and various automatic regulators, etc. This section mainly introduces the propellant feed system and the thrust chambers, which are closely associated with the combustion process.

1.1.1 Propellant Feed System

The propellant feed system is employed to deliver the propellants from the containing tanks to the thrust chamber and can be divided into two categories according to the working mode, namely, the pressure feed system and the turbo-pump feed system.

1.1.1.1 Pressure Feed System

The pressure feed system pushes the propellants to the thrust chamber or the propellant gas generator by the high pressure gas in the tanks of the propellants. The high pressure gas, i.e., the pressed gas, can be pre-stored in cylinders as the storage gas and can also be generated by a liquid or solid gas generator during the working process of the liquid rocket engine. The main requirements for the pressed gas are as follows: (i) high density while under the pressed state, (ii) low relative molecular mass under the pressed state, (iii) minor solubility with propellant, (iv) no or minor chemical reaction with the propellants, and (v) no solid and liquid impurities.

The pressure feed system can employ inert gases as the pressed gas. This kind of pressure feed system has two type of working mode, namely, the regulated pressure mode and the blow-down mode. The former employs a pressure regulator to maintain the pressure in the propellant tank, and also maintains the thrust at a constant value. The latter stores the propellant and the pressed gas in one tank. The pressure drops during the adiabatic expansion of gas, fewer propellants are injected into the combustor and therefore the pressure in the combustion chamber also drops. Typical pressure feed systems are (i) those with high-pressure gas cylinders and (ii) those with gas generators. The former can employ air, nitrogen, helium, and some other inert gas as the pressed gas. The main drawback of air is that the contained oxygen has a relatively high boiling point, and therefore it cannot be used to press cryogenic propellants. Helium can be used to press all existing liquid propellants. Although such a pressure feed system has a relatively large size and heavy mass, it has the characteristics of a simple structure and high reliability. It is also simple to employ and ensures repeatable starting of the engine.

In pressure feed systems with a gas generator, a single-component liquid fuel gas generator using a monopropellant as the source of the driven pressure and the propellant decomposition can be realized by catalysis or heating according to the kind of propellant. In dual-component liquid fuel gas generators, the high pressure gas can be obtained from the two propellant components by burning under oxygen-rich or fuel-rich conditions. The temperature of the gas is determined by the propellant component mixed ratio in the gas generator.

The structure of the pressure feed system is simple and reliable. However, as the propellant tanks must withstand high internal pressure, the pressure feed system is relatively bulky and it is often employed by spacecraft-attitude-control engines. Sometimes, to ensure the reliability of manned flight, although the engine thrust is large, a pressure feed system is also employed, such as the service module engine, drop class, and upgraded engines of Apollo spacecraft.

1.1.1.2 Turbo-Pump Feed System

A turbo-pump feed system employs pumps to deliver propellants, and the pump acquires the driven force from a turbo. In the turbo-pump feed system, a turbo-pump assembly is necessary. The basic requirements for a liquid rocket engine turbo-pump are as follows:

1. If the mass flow rate of a given propellant is given, we need to ensure the pressure at the engine outlet matches the requirement of the engine system.
2. The turbo-pump should be as small and light as possible.
3. The turbo-pump is to have as high an efficiency as possible.
4. The turbo-pump should ensure stable operation at all engine operating conditions and the pressure pulsation and mechanical vibration must be minor.
5. The turbo-pump is to be compatible with corrosive liquid and cryogenic liquids. Friction is not allowed between the components of the oxidizer pump because the heat created by the friction may produce a local high temperature, even an explosion.
6. The turbo-pump is to be capable of sucking propellants that contain a small amount of gas or steam.

There are three common types of cycle program for the turbo-pump feed system, namely, gas generator cycle, expansion cycle, and staged combustion cycle. The gas generator cycle and the staged combustion cycle can employ most of the commonly used liquid propellants. The expansion cycle engine is commonly used in an engine that employs liquid hydrogen as thrust chamber coolant, because liquid hydrogen is a good absorbing-heat medium and it does not decompose.

In the gas generator cycle, the turb...