Dynamics of the Internal Flow in Swirl Atomizers by CFD Simulations

Introduction

The bi-propellant core injectors inject the components of the propellant into the combustion chamber in such a way as to optimize the combustion process by generating greater thrust of the engine depending on the bi-propellant torque employed. The proper mixing ratio between fuel and oxidant is done by means of thermodynamic calculations. During the operation of the engine the combustion process takes place in distinct regions of the chamber and the central part of the chamber is the one with the highest temperature. The peripheral injectors are regularly distributed close to the wall of the combustion chamber and have the function of thermally insulating the wall by the column of liquid represented by the fuel.

Figure 1.

Schematic of injection of rocket propellant liquid engine developing 400 N of thrust

In these (peripheral) injectors only fuel is injected in such a way that in the region near the walls the richest fuel mixture forms. The combustion of this mixture occurs with reduced temperatures when compared to the burning in the central region of the chamber. Therefore, the temperature developed in the central region of the chamber is approximately 3000 - 3200 K and in the periphery around 1000 - 1200 K. The constructive material of these walls of the chamber must withstand such temperatures and are usually inconel and niobium. In some cases of bi-propellant pairs, when the thermal properties of the oxidant are higher than that of the fuel, the oxidant can be used to protect the walls of the combustion chamber. But, generally for this purpose is used the fuel because the oxidant is the active component, for this reason, to avoid the oxidation of the walls, the application of the fuel is justified. In most liquid propellant-based propellant designs, the inner shell of the combustion chamber walls is formed by jet-type injectors, also known as “impinging jet”. This is due to the fact that this type of injector has a smaller spray angle than the core, for example, and therefore the injection of the propellant can be directed directly into the walls of the combustion chamber, which provides a better contact of the fuel with the wall.

In this case the heat exchange between the fuel and the wall of the chamber is favored, making the cooling of the walls more efficient.

Injector Types

The main functions of the injectors are described below:

• 1.

  Ensure continuous injection of the propellant into the combustion chamber without interruption.

• 2.

  Inject exactly the determined mass quantity of propellant to ensure the mixing ratio between the fuel and oxidant.

• 3.

  Ensure the immediate defragmentation of the liquid sheet.

• 4.

  Properly atomize the bi-propellant pair ensuring a droplet size dispersion leading to an efficient combustion process.

• 5.

  Ensure the homogenous distribution of the propellant in the combustion chamber.

Injectors can be classified in several ways, but according to (Bayvel & Orzechowski, 1993), the best way to classify them is by the type of energy that is used in the atomization of the liquid. Table 1 shows this classification of injectors.