Modeling and Performance Simulation of a Micro Turbojet Engine Using Flownex

Objectives: The study presents the results of modelling and analysis of a micro turbojet engine that can develop thrust in the range of 4 kN, that is sufficient to propel small UAVs and Drones. The modelling and simulation is carried out in Flownex simulation environment which is a powerful one-dimensional tool commercially available to analyze the performance of engine components by establishing network linkages between the components. Methods/Findings: This study also uses the “Turbo power match” module for matching of power produced between the turbine and compressor components in the engine assembly as well as “Adiabatic flame element” module to estimate the combustion parameters in the engine. The results show that the analyzed system produces a thrust of 3.79 kN at an inlet air pressure of 1.01 bar. The burnt gases in the combustor are found to reach a maximum of 556.17 K which permits the use of existing materials in the construction of this engine. The one-dimensional eliminates the time needed for analysis and hence enables quick implementation of design parameters resulting in cost reduction and time delay in manufacturing of engines. Application: This performance analysis provides vital information needed for micro turbine engine manufacturers and drone designers and hence plays a vital role in the future propulsion of UAVs and Drones.


Introduction
Modelling and simulation of the dynamic behaviour of aircraft engine to optimize its performance, even manufacturing a prototype for actual testing leads to better results, cost cutting and time saving. In this process it is essential to consider the complex dynamic behaviour of the interactions between engine stages like and low pressure compressors, turbines, nozzle exit etc.,. In addition, this analysis results in obtaining first-hand information on the parameters that affects the optimal working of the engine and enables precise modelling of materials and complex blade shapes. The dynamic behaviour analysis of these engine mechanisms enables increase in jet engine performance through detailed analysis of systems in the engine and fine-tuning, resulting in more power with less fuel. It also helps to enhance the design by taking more phenomena into account, which can be important to assure safety as well as simulate the global jet engine system coupled to detailed component analysis. The comprehensive modelling and detailed outcomes on the performance of a turbojet engine suitable to propel future UAVs and Drones that necessitates thrust in the range of 4 kN is presented in this study.
For more than a decade, the manufacturing and assembling of complex systems like engines has been made more simple by the use of modelling and simulation software that gives enormous freedom to the designer to make changes and analyze the performance beforehand. Similarly, the use of software in Micro turbine engine design and analysis applied to drone propulsion has been extensively used by the researchers. However, there is a handful of reported work available in the open literature on the modeling and analysis of turbojet engine assembly. Some of the relevant works carried out by researchers related to the present work is given below.
In 1 carried out research on the analysis of the performance of a Titanium alloy-based turbine and compressor in a jet engine by estimating the maximum stress experienced in the components and the total deformation undergone under the test conditions. From the study, it is seen that the maximum compressive and tensile strengths of the turbine and compressor were found to be 930 MPa and 1070 MPa. In 2 carried out the performance analysis of a single-spool and two-spool engine and compared their performances. From the study, it is evident that the cooling of turbine causes huge impact on the net load which is found to be highly nonlinear. It is also observed that the overall efficiency is nearly 5% for a Turbine Inlet Temperature (TIT) of 1600K while the optimum efficiency is obtained when the reheat combustor is placed at nearly 10%-20% of the expansion section. He also configured various types of engines and proposed numerical models for the analysis of Jet engines. In 3 performed study on a turbocharger with a newly developed combustion chamber obtained from an automobile. It is evident from the study that equivalence ratio and the quality of mixing of primary and secondary air influence the combustion quality and the speed of the engine. In 4 used Siemens NX 8.0 to design and analyze an annular combustion chamber with low bypass flow in a turbofan engine while simulating the aerodynamic flow characteristics using ANSYS 14.5. The simulation study shows that the Specific Fuel Consumption (SFC) can be reduced with a marginal reduction in the size of the combustor and regulation of temperature and pressure loss in the combustor. In 5 developed a cost-effective micro jet engine that is capable of generating 60 kN to 70 kN needed for powering helicopters. The speed of the engine in the analysis was assumed to be around 50,000 rpm to 70,000 rpm. The novelty of the engine was the use of a bigger compressor and turbine than the normal engines of this category. In 6 used CATIA and ANSYS to design and analyze a turbojet engine. The impeller of the compressor was designed to develop 2.9 bar at 8000 rpm with a combustion chamber of 320 mm outer diameter and 240 mm inner diameter.
It is seen from the above literatures that the modelling and simulation of a complete engine as a whole is not available. Hence, this study aims at filling the gap by carrying out the performance analysis of a micro turbojet engine assembly using Flownex software. The outcomes of this study will be very much useful for drone manufacturers and engine assemblers for drone propulsion

Modelling of Turbojet Engine Subsystems
Flownex SE is developed in order to facilitate design and one-dimensional analysis of aerospace propulsion engines. This modelling and analysis tool can be used to perform modelling as well as on-design and off-design and simulation of gas turbine engine and its networks. The main parts of a gas turbine engine are the air inlet, impeller and Compressor, the combustion chamber, single spool turbine, exit nozzle, pipes, and heat transfer elements are chosen based on the analysis from a library in the software. The model developed using the various engine components from the library and the network developed to analyze the engine performance is shown in Figure 1. The boundary conditions for all the subsystems in the inlet and exit conditions are connected through nodes to ensure smooth and essential connectivity and flow between the subsystems.

Setting up Parameters for Analysis
The input parameters used in the Flownex simulation for analyzing the micro turbojet engine is shown in Table  1. The combustor thickness was assumed to be 1 mm as the geometry of the combustor is not considered for estimation of adiabatic flame temperature in this onedimensional analysis. All the simulations were assumed to be conducted in the ground conditions only 7 .

Matching of Components in Flownex Model
The next important step in the analysis is that the components and other systems in the engine need to be matched 8,9 . Flownex direct design function and the turbo power match tools are used for doing the same. The function tool in Flownex with "SE design" performs automatic computation on the sizing and matching of the subsystems to obtain system conditions. The component matching is done by the tool with equal number of constrains and independent variables by using matrix iteration method.
The equality constraints are set as shaft excess power and turbine flow rate whereas the turbine geometry and the rotational speed are chosen as independent variables. Turbo Power Match (TPM) is another tool used for matching the engine components which can be done both in the steady state and transient mode. The execution of matching tool to match the power obtained from the engine in the present study is shown in Figure 2.  The excess shaft power is expressed by the relation 9 PExcess = η m * ΣP positive − ΣP Negative where η m is the mechanical efficiency, ΣP Positive is the power supplied by turbine and ΣP Negative is the power required by the compressor.

Simulation of Engine Assembly Performance
The Adiabatic Flame element in the module, estimates the composition of the fuel-air mixture and the maximum temperature arrived at by the chemical mixture based on chemical reaction. Since the combustion chamber dimensions are not considered in this analysis, the program does not calculate the mass balance or pressure drop across the chamber. However, the mass balance and pressure drop calculation is done by Flownex SE. This can be achieved by placing any Flownex fluid flow component along with the Adiabatic Flame element as shown below in Figure 3.

Results and Discussion
The complete assembly with dimensions, performance chart and mixed fluid is drawn in Flownex SE Drawing page and system performance is analyzed by changing ambient conditions of the system as shown in Figure 4.
The system analysis is carried out for different inlet conditions of pressure and the results are tabulated in Table 2. It is observed that, as ambient pressure decreased below 1 bar, the maximum temperature; mass source, exit Mach and thrust also decreased but increase in ambi-ent pressure above 1 bar the maximum temperature, mass source, exit Mach and thrust also increases. In this study, the analysis is performed for inlet pressure 0.98 to 1.5 bars to check the response of the system. The highest temperature reached during the analysis is found to be within 1000°C for a thrust range of 3.579 kN to 7.372 kN, which indicates that our design requirement is met and the design is safe.  In the single spool turbojet engine, the compressor is run by the power developed in the turbine. Hence, to sustain the compression process and engine operation, the turbine is expected to produce more power than the compressor requirement at all inlet conditions. The power generated in the turbine and the energy used by the compressor for various ambient conditions is shown in Figure 5.
It is observed that as ambient pressure increases, the power generated by the turbine also increases, thereby providing the work required by the compressor. The maximum velocity of flow in the combustor is found to be 23.363 m/s, while the exit velocity is 433.157 m/s which is around 0.847 Mach (Subsonic Exit). The results of power output from Turbine and compressor as computed by the simulation in Flownex is summarized in Table 3.

Conclusions
In this study, Flownex simulation programme has been used for component matching and investigation of the engine. The simulation results show that, Flownex can be effectively used to design and simulate the working of a gas turbine engine. The results from this study will be used to further investigate the successful integration of modified compressor and turbine stages into the turbojet engines.
• The output thrust of the engine analyzed in this study is found to be 3.79 kN at an inlet air pressure of 1.01 bar at an inlet flow rate of 8.71 kg/s. • The maximum temperature experienced during the performance analysis is found to be 556.17 K, which implies that the available materials in market can be used for the construction of these engines.
This study provides a one dimensional analysis of a micro turbojet engine that will be used to propel future UAVs having a thrust requirement in the range of 4 kN.

Acknowledgement
The authors express their heartfelt thanks to DHIO Research & Engineering Pvt Ltd, Bangalore for providing