# Impact of Using Chevrons Nozzle on the Acoustics and Performances of a Micro Turbojet Engine
## 5 Micro Turbojet Engine Experimental Performances Evaluation  
The measured parameters from Table 2 were used in performance evaluations of the chevron nozzles. It measured the temperature in front of the micro turbojet engine, which was considered as being the temperature at intake level T\({}_{1}\). Using these parameters, the specific consumption and engine efficiency were computed. The relations used in the calculation of specific consumption and compressor efficiency are further presented [30]. The specific consumption S is defined in Equation (2):  
\[\text{S}=3600\cdot\frac{\text{F}_{\text{f}}}{\text{F}}\left[\frac{\text{kg}}{ \text{N}\cdot\text{h}}\right] \tag{2}\]  
Figure 7: Spectral analysis of axial vibration for each regime and tested nozzle.  
The recorded parameters highlight that the temperature in front of the turbine drops by increasing the number of chevrons in idle regime 35,000 RPM, 100,100 RPM, 116,000 RPM, but it has a tendency of remaining constant with a slight increase at 55,000 RPM. The compression ratio of the engine at low regimes (idle and cruise) doesn't present variations, but at high regimes, the compression ration drops when the chevrons' number increases. For the all tested regimes, the fuel flow decreases by increasing the chevrons' numbers. One of the most important observations is related to the generated force, which decreases by increasing the number of chevrons, and the same downward trend is observed for the intake air flow.  
The specific fuel consumption for the chevron nozzle cases present decreases comparing with the reference nozzle.  
The following Figures 11 and 12 present the most important parameters that dictate the micro turbojet engine performances for the three tested chevrons.  
Figure 10: Variation of the compressor efficiency depending on the regime and nozzle type.  
Figure 9: Variation of the specific fuel consumption depending on the regime and nozzle type.  
Assessing the variation of efficiency characteristics of the compressor, depending on the air flow, it can be observed that, at the same regime, the compressor efficiency drops by increasing the number of chevrons.  
For the entire engine round, its operation did not present any problem for all tested nozzles; the only drawback is the force loss for chevron cases. Modification of the exhaust section, by manufacturing the chevrons, has led to growth of the internal resistances in  
Figure 11: Variation of the compressor ratio depending on the intake air flow at constant speed.  
Figure 12: Compressor efficiency characteristic depending on the air flow.  
the engine. This was reflected in the fact that the compressor had to generate greater mechanical work, which led to an increase of the temperature of air after the compressor.  
Because the control law of the engine is based on constant speed, during the nozzle testing, the engine tried to maintain the same power for the compressor, but because the mechanical work on the compressor increased, results of the mechanical work on turbine increased too.  
Growing mechanical work on the turbine can be produced by increasing the temperature in front of the turbine, which is produced by increasing the fuel flow. A bigger fuel flow will lead to a higher speed, which is contrary with the control law, so the only possibility to increase the mechanical work on the turbine, without increasing the speed, was to reduce the fuel flow and implicitly reduce the temperature in front of the turbine. Decreasing the temperature in front of the turbine leads to a temperature reduction after the turbine, which will lead to a decrease in the gas speed out of the engine. The reduction in the intake airflow, fuel flow, and the gas speed explains the reduction of the thrust force.