Development of a small-sized turbofan engine with a thrust of 50 kgf

Engine Design and Thermodynamic Analysis

At the initial stage, the task was to design the engine configuration and define its main parameters. A thermo–gas–dynamic cycle analysis was performed to determine:

• Engine size and mass flow rates
• Pressure and temperature distribution along the gas path
• Parameter variation across different operating conditions

A velocity–altitude performance analysis was conducted to evaluate changes in thrust and specific fuel consumption as a function of flight speed and altitude.

Due to the small scale of the engine, lower component efficiencies were assumed compared to large commercial turbojet engines. As a result, a reduced compressor pressure ratio was selected to better reflect realistic performance for a micro turbojet engine.

A reference engine with similar dimensions and operating characteristics (TJ100 class) was used as a close prototype to validate the selected parameters.

Overall Engine Configuration

Air enters the engine through the inlet, passes through the compressor, and is delivered to the combustion chamber where fuel combustion occurs. The high-temperature gases then expand through the turbine, which provides the power required to drive the compressor. The remaining energy is converted into thrust through the exhaust nozzle.

The working fluid of the engine is air.

Combustion Chamber and Fuel System

An analytical combustion chamber design was performed using established methodologies. The resulting combustion chamber geometry was defined based on airflow distribution, residence time, and stability requirements.

The fuel system consists of:

• A fuel tank and pump
• Fuel composed of kerosene with approximately 5% oil for lubrication

The evaporative tube was selected as the key element of the combustion system and became the focus of the detailed investigation.

Evaporative Tube Concept and Mixture Formation

The special part of this work is dedicated to the fuel–air mixing process and preparation of the fuel–air mixture before it enters the primary combustion zone.

Fuel is supplied to the evaporative tube (which also functions as a mixing chamber) through a plain-jet injector. This injector type was selected due to its simpler design and higher flow coefficient compared to centrifugal injectors. As a result, a lighter fuel pump can be used, since lower fuel supply pressure is required.

The evaporative tube operates as an ejector system. A high-velocity kerosene jet entrains air delivered from the compressor due to pressure differences. As the mixture travels downstream:

• Kerosene droplets evaporate due to heat transfer with the airflow
• The airflow itself is heated by the tube wall
• A phase transition occurs from liquid droplets to kerosene vapor

As evaporation progresses, the vapor concentration increases along the tube length, enriching the mixture.

Experimental and Parametric Study

One of the key objectives of the study was to intensify the kerosene evaporation process. To achieve this, several parameters were varied:

• Fuel nozzle diameter: 0.5 mm to 0.8 mm
• Evaporative tube length: 100 mm (based on literature review)
• Tube diameter: 8 mm
• Fuel mass flow rate: 2.6 g/s (kept constant, derived from combustion chamber calculations)

The airflow rate was determined by the total inlet pressure and varied depending on internal flow processes within the tube.

The excess air coefficient (α) at the outlet of the evaporative tube was maintained between 0.2 and 0.3, ensuring a non-flammable mixture to prevent ignition inside the tube.

Combustion Zone Air Distribution

After leaving the evaporative tube, the fuel–air mixture enters the primary combustion zone, where it is progressively diluted with air:

• Primary zone: α ≈ 0.6
• First row of dilution holes: α ≈ 1.2
• Second row of dilution holes: α ≈ 1.8
• Mixing zone (overall): α ≈ 3.5

This staged air distribution ensures stable combustion, controlled temperature rise, and protection of downstream components.

Numerical simulations were also conducted to study the influence of evaporative tube wall temperature on fuel evaporation and mixture quality.

Conclusion

This project demonstrates the feasibility of using an evaporative tube. Based fuel preparation system in a small turbojet engine. The design ensures safe mixture formation upstream of the combustion chamber, improves evaporation efficiency, and supports stable combustion across operating conditions.

The work integrates thermodynamic analysis, combustion design, and practical fuel system considerations relevant to small gas turbine applications.

This project was conducted under the supervision of Professor Khaliulin R.R .