Research Proposal on Combat Aircraft Nozzle Performance

1. Abstract

This research proposal discusses the working of the combat aircraft nozzle in the first part of the report. A nozzle is a propelling device that propels the aircraft in the desired direction. An aircraft nozzle converts chemical energy to propulsion. NASA had conducted a CFD analysis of the nozzle and had found that an increase in the nozzle geometry results in an increase in the Mach number of the aircraft. The experimental plan revolves around proving that various performance factors (Static Pressure, Static Temperature and Mach Number) changes while travelling from the converging side of the nozzle to the diverging side of the nozzle. Theoretical calculations as well ANSYS simulations will be carried out to prove the theory. After the simulations, small scale 3D printed nozzles would be constructed to carry out real world simulations. Data received from the simulations carried out on these 3D printed nozzles would not only help us to prove the theory but would also help us identify the optimum nozzle design for in-flight performance.

2. Literature Review

A nozzle is a device which is premeditated to regulate the characteristics or direction of a fluid stream as it exits an enclosed tube or chamber. A nozzle can be used to adjust or direct the flow of a fluid. These devices are used to control the rate of flow, path, shape, speed, mass and the compression of the steam that emerges from them.  The velocity of fluid may vary according to the energy of its pressure. (Gamble, Terrell and DeFrancesco, 2004)

A propelling nozzle is a device that converts the internal energy of a working gas into propulsive force. These nozzles may have a variable geometry or a fixed geometry to vary the exit area in order to provide the ideal condition required for a particular mission or flight. (Gamble, Terrell and DeFrancesco, 2004)

There are three main types of Nozzles, Converging Nozzle, Diverging Nozzle and Converging-Diverging Nozzle (Straight and Cullom, n.d.). Combat Aircrafts mainly use converging-diverging nozzle as flight performance can be varied with altitude. In a C-D nozzle, the convergent section of the nozzle is connected to a divergent section of the nozzle.

Figure 1: Converging-Diverging Nozzle (Converging Diverging Nozzle, n.d.)

In this nozzle the hot fluid passes through the combustion chamber which narrows down to a minimum area (Throat) and then passes to the divergent section of the nozzle. Choking the flow of the fluid is dependent on the size of the throat (Oswatitsch and Rothstein, n.d.). Generating thrust is the most basic function, but in addition to producing thrust, there are variety of important functions which are in effect because of these nozzles.

TVC or Thrust Vector control is the ability of an aircraft to modify its angular velocity. An aircraft can perform maneuvers or can achieve extraordinary climb rates which are impossible to achieve with conventional control surfaces by the help of TVC. Not only aircrafts but many missiles also use these techniques. Aerodynamic control surfaces become futile if the aircraft or missiles flies outside the atmosphere. Therefore, TVC becomes essential at this point as it would be the only primary source of maneuverability. Thrust Reversal is another useful technique used by aircrafts. By using reverse thrust, an aircraft creates a temporary diversion which diverts the engine’s thrust forward rather than backward. This would help in slowing down an aircraft after touchdown. Nozzles also help in the reduction of sound generated due to the reaction of various chemicals inside the nozzle causing thrust. Another function of the nozzle is the suppression of infrared radiation. Infrared radiation signature emitted by the chemical reaction inside the nozzle can be detected by enemy radar. Therefore, enclosing the nozzle in a shroud helps in the absorption of such signatures which enables the aircraft undetectable to enemy radar.

A CFD Analysis was conducted on a C-D Nozzle by varying the throat area. The following conditions of flight are given below:

Three conditions were considered: - Standard Converging-Diverging nozzle, Velocity plot for an increment of 20% area of throat and Velocity Plot for a decrement of 20% area of throat. It was observed that an increase in the nozzle geometry results in an increase in the velocity value (Mach number) and vice versa (Karthikeyan, P. and Masthiraj, N.V., n.d.).

In the current state of affairs, it has been theoretically established on ANSYS Work Bench (Fluent) that in the case of a converging-diverging nozzle, the parameters of flight such as Static Pressure decreases, Temperature inside the nozzle decreases and Mach number increases while travelling from a converging section to a diverging section of the nozzle. However, this has not been practically proven. 3D printing of a small-scale converging-diverging nozzle would have to take place in order to prove or disprove the mentioned hypothesis. Research on the variation of performance levels of the C-D Nozzle with varying altitudes (over-expansion and under-expansion) needs to be undertaken in order to study the effect of atmosphere on the efficiency of nozzles. Theoretical simulations on ANSYS as well as small scale experiments need to be conducted.

3. Experimental Plan

This research will help in understanding the various performance variations of the C-D nozzle at various flight conditions. The various research objectives are given below:

1. Various performance levels need to be found by conducting simulations (ANSYS).
2. Small scale 3-D models of the C-D nozzle need to be constructed.
3. Real world simulation of 3D models needs to be conducted.

All theoretical simulations will be carried out using the ANSYS Work Bench (Fluent). The nozzle will be divided into 4 sections for finer meshing. Structured meshing will be used in order to get accurate results. Standard initialization will be used as the initializing method.

The research will be based on theoretical calculations as well as real world simulations to verify the obtained theoretical results. Using these results, an optimum nozzle design could also be achieved. The ANSYS simulations would be the first stage in conducting the research. In the ANSYS, various parameters such as Static pressure, Static Temperature and Mach Number will be found at different altitudes. This research will deal with overexpansion and under expansion cases. This would give an idea on the ideal conditions as well as an idea on the real-world scenario. The next stage would be to construct 3D printed small scale models in order to test the theoretical hypothesis. If successful, then real scale testing can take place.

4. Data Analysis Plan

There are two ways in which data will be accumulated, firstly theoretical simulations will be carried out and secondly physical small-scale testing data will be carried out. The data received from the theoretical simulations will include Static Pressure, Static Temperature and Mach number at various flight conditions. The theoretical simulations will provide results which would not take efficiency into account. Hence, to take the said factor into account, as mentioned previously, small-scale testing would need to be considered. After successful completion, large-scale testing will need to be carried forward. For each flight condition, four to five runs would be required in order to get accurate results. Averages would then be calculated.

Analyzing data will help to find the appropriate choke area required for optimum performance at various levels of altitudes. It is important to find the optimum choke area as the aircraft can adjust the choke area in-flight to have an ideal performance with ideal efficiency of combustion. After the analysis we would be able to find the optimum nozzle design to facilitate best performance in the case of over-expansion and under-expansion.

5. Conclusion

This research proposal would be essential to understand the performance factors of a combat aircraft nozzle at various performance scenarios such as under-expansion and over-expansion. This would help us to identify the optimum converging-diverging nozzle design required for optimum performance and efficiency.

References

Gamble, E., Terrell, D. and DeFrancesco, R., 2004. Nozzle Selection And Design Criteria | Joint Propulsion Conferences. [online] Arc.aiaa.org. Available at: .

Straight, D. and Cullom, R., n.d. Performance Of A 2D-CD Nonaxisymmetric Exhaust Nozzle On A Turbojet Engine At Altitude. [online] Ntrs.nasa.gov. Available at: .

Oswatitsch, K. and Rothstein, W., n.d. FLOW PATTERN IN A CONVERGING-DIVERGING NOZZLE. [online] Ntrs.nasa.gov. Available at: .

Karthikeyan, P. and Masthiraj, N.V., Optimization of CD nozzle using variable geometry in CFD analysis.

Engapplets.vt.edu. n.d. Converging Diverging Nozzle. [online] Available at: .