Effect of oxygen rich environment on detonation characteristics of pulse detonation engine

The detonation tests of propane/air pulse detonation engine at normal temperature were carried out in this paper. The effects of chemical ratios on the detonation performance of propane/air pulse detonation engine in oxygen-rich environment were studied. The propagation modes and characteristics of detonation waves and the detonation characteristics of the engine under different conditions were analyzed. Detonation tests were conducted on propane/air mixtures with oxygen mass fraction of 20%, 25%, 30% and 35% respectively. The results showed that: In the detonation experiment of the propane/air mixture with oxygen content of 20%, 25% and 30%, the flame wave propagation velocity in the detonation tube is low, which fails to form a stable detonation wave. The detonation effect of the fuel mixture with oxygen content of 20% is the worst, and the propagation velocity of flame wave is 384.00 m/s. In the experiment of propane/air mixture with oxygen content of 35%, the detonation velocity and pressure obtained by pressure signal analysis are both greater than the theoretical CJ detonation velocity, and the detonation state is successfully reached.


Introduction
Pulse detonation engine is a new type of engine that generates high temperature and high pressure gas through pulse detonation wave to obtain thrust.It has the advantages of high thermal cycle efficiency, simple structure and so on.It has broad application prospects in aerospace, weapons and civil fields.
Propane is a typical fuel with advantages such as high activity and easy storage [1].Its detonation characteristics are similar to aviation gasoline, and it has been taken as a research object by many scholars in the field of detonation combustion [2][3][4].Ting J M [5] respectively studied the single detonation properties of C3H8/O2 and C3H8/air mixtures, and used the detonation waves generated by C3H8/O2 to ignite C3H8/air mixtures.The results show that the mixture of C3H8/O2 generates detonation wave at a very short distance and can make the mixture of C3H8/air detonate.The distance between dedetonation and detonation (DDT) increased with the increase of pipe diameter.Li C M [6] explored the influence of pipe diameter conditions and nitrogen content on detonation characteristics of C3H8/O2.The study showed that DDT distance was shortened with the decrease of pipe diameter for the chemically appropriate ratio and slightly rich C3H8/O2 mixture.For the lean oil mixture, DDT distance does not decrease in the diameter of the tubule.Yu H [7] simulated the detonation process of hydrogen and air mixture from a one-dimensional enclosed space, and the experiment showed that the initial temperature, pressure and enclosed space size all had an impact on the generation of detonation waves, and the back and forth reflection of pressure waves could further shorten the DDT process.Lu F K [8] directly used propane as fuel to conduct a multi-cycle detonation test in a pulsed detonation engine.
2 In summary, many scholars have studied the single detonation and multi-cycle detonation of propane, which shows that propane has a great application prospect in detonation combustion.However, current researches on the characteristics of DDT under different initial parameters mainly focus on hydrogen and ethylene, and there is still a lot of work to be done on the initiation characteristics of propane and DDT.On this basis, we will conduct a study on the detonation characteristics of propane/air pulse detonation engine at normal temperature, carry out the pulse detonation experiment in different oxygen rich environment under the chemical proper ratio, and analyze the detonation wave propagation mode, propagation characteristics and the detonation characteristics of the engine.

Test system and test
The propane/oxygen-rich air detonation test system is shown in Figure 1, which mainly includes detonation tube, gas supply system, pressure signal data acquisition system, ignition control system and high-speed camera.The gas supply system uses standard propane/air/oxygen cylinders to supply gas to the test system.The diameter of the detonation tube is 30mm, the length of the tube is 600mm, and the spoiler with 150mm length is installed inside the tube, which is composed of 7 spoilers.The blocking ratio is 0.25, and the tail of the tube is sealed with PET film.The pressure signal data acquisition system uses PCB sensors, and the spacing between pairs is 90mm.The ignition control system uses high-energy igniter discharge, and the discharge energy is 1J.The detonation test system was carried out at normal temperature, and the gas temperature in the detonation tube was 302K and the pressure in the detonation tube was 0.1013MPa.Then, the hot state test of the pulse detonation engine was carried out in different oxygen-rich environments under the chemical proper ratio of propane/oxygen-rich air, and the oxygen mass fraction of the fuel in different oxygen-rich environments was 20%, 25%, 30% and 35%, respectively.Propane, oxygen and air are filled into the premix chamber through the mass flow controller in accordance with the prescribed flow rate respectively.After the mixed gas is evenly mixed, fuel is filled into the head of the detonation tube, wherein the ratio of fuel and oxidizer complies with the appropriate chemical ratio of the complete combustion of propane.In the process of the experiment, the detonation tube was pumped by vacuum pump first, stopped when the pressure number was -0.098 MPa.According to the ratio of experimental conditions, the mass flow controller of propane, oxygen and air was turned on successively, and the mixed gas was fully mixed in the mixing chamber and charged into the detonation tube through the intake section of the detonation tube.Close the mass flow controller until the pressure indication number is 0 MPa, and then close the mixing chamber outlet valve.At this moment, the process of filling gas into the detonation tube is completed.A 5Hz ignition signal is sent out by a PC, and the mixed gas inside the detonation tube is ignited by a high-energy igniter.When the ignition head releases an electric spark, the mixed gas inside the detonation tube is ignited to form an initial flame wave, and the flame wave and leading shock wave rapidly propagate downstream to accelerate the chemical reaction of the mixed gas at the back end of the detonation tube, and DDT process is quickly realized under the action of the baffle.It's a transition from combustion to detonation.When the detonation wave propagates in the detonation tube, the PCB high-frequency dynamic sensor generates signals under the action of the dynamic pressure in the tube, which are fed back to the PC through the PCB signal conditioning instrument and high-speed data acquisition system.The D-10 channel signal acquisition and analysis system software is written by LabView software.During the experiment, the frequency of PCB pressure signal sensor data was 1000K Samples/s.The high-speed camera is placed 2 m in front of the detonation tube, and its height is consistent with that of the detonation tube.The frame number of the high-speed camera is set to 20000, and the exposure time depends on the experimental conditions.

Test results and analysist
In order to explore the influence of oxygen-rich environment on the detonation performance of propane/air pulse detonation engine, this paper carried out experimental studies on propane in air and oxygen-rich environment, and calculated the CJ detonation velocity vCJ and theoretical detonation pressure PCJ of propane in air and oxygen-rich environment combined with CEA software.The oxygen mass fraction of fuel in group A1, A2, A3 and A4 mixture is 20%, 25%, 30% and 35% respectively.As shown in Table 1.The four groups of gas mixtures in Table 1 are all proportioned in a certain proportion, and all conform to the appropriate chemical ratios of propane/hydrocarbon fuel in air and oxidizer in air.Under normal temperature and pressure, the above four groups of mixed gases were filled into the detonation tube for detonation experiment, and the experiment was repeated for 3 times to ensure the accuracy of the experimental results.In the experiment, the detonation tube was pumped to -0.098MPa, and the propane/air/oxygen mass flow controller was opened at the same time to fill gas into the gas mixing chamber, then the valve of the gas mixing chamber was opened to fill gas into the detonation tube, and the air inlet valve was closed when the pressure representation number was 0 MPa.The mixture ratio was in accordance with the volume fraction ratio in the above table.To ensure the accuracy of chemical equivalent ratio; Through the ignition control system, the ignition signal is sent to the high-energy igniter to ignite the mixed gas, and the flame wave accelerates to form a stable detonation wave propagating downstream under the action of the spoiler.The wave front quickly sweeps the unburned mixed gas, improves the gas temperature and pressure in the detonation tube, accelerates the chemical reaction of the mixed gas, and then forms a reflected shock wave propagating to the tube out of the tube port.A stable detonation is achieved.By analyzing the pressure signal of the PCB dynamic pressure sensor at the end face of the detonation tube, the signal is processed and analyzed by the high-speed data acquisition system, and the propagation velocity of flame wave in the tube and the change of pressure in the tube are obtained.Figure .2 shows the pressure signal curve measured by the PCB dynamic pressure sensor after the ignition of propane/air/oxygen mixed gas with different ratios in the detonation tube under the above experimental conditions.In the figure, the four sensors (T1, T2, T3 and T4) are evenly distributed on the tube wall, and the sensor spacing (d) is 9 cm.Four pressure sensors record the time when the flame wave reaches the sensor respectively, and the flame wave velocity v and pressure p can be obtained by analyzing the interval time Δt-1 and the peak pressure between the sensor signals, in which the flame wave velocity formula v=d•Δt -1 .Figure 2 (a), (b), (c) and (d) respectively show the pressure signal sensor curves in the detonation tube after ignition with four groups of mixing ratio A1, A2, A3 and A4. Figure 2 (a) shows the signal curve of the pressure sensor after the detonation test of propane in pure air.It can be seen from the figure that when the ignition control system releases the ignition signal through the high-energy igniter, the mixed gas in the detonation tube is ignited, forming a detonation wave, and the flame surface moves downstream in the tube.The pressure in the tube is 0.35MPa, and the velocity v of the flame wave reaching the T4 sensor is 389.61m/s.The flame wave reaches the exit end face of the detonation tube wall after 1ms after ignition.At this time, a reflected shock wave is formed and propagates into the tube.When the reflected shock wave reaches T4 sensor, the peak pressure measured by T4 sensor is 0.74MPa, which is twice the pressure of the flame wave.Figure 2  (b), (c) and (d) all show the pressure signal curves in the detonation tube after propane combustion in oxygen-rich air.Under the condition of equal chemical equivalent ratio, the increase of oxygen content in the detonation tube greatly improves the detonation characteristics of propane.After the ignition head discharge, the pressure curves of T1, T2, T3 and T4 sensors rapidly rise to the maximum within 1-2μs after the arrival of the flame wave.The rise curves all have a very steep rise edge, which can be approximately seen as a straight line.By analyzing the difference of the time interval between the rise edge of different sensors, combined with the tube spacing between the sensor pairs, The propagation velocity v of the flame wave in the detonation tube can be obtained.In Figure 2 (b), (c) and (d), the propagation velocity v of the flame wave reaching the T4 sensor is 703.13m/s,910.81m/s and 1800.85m/ssuccessively.The pressure p in the pipe is 0.60 MPa, 0.82 MPa, 4.30 MPa, respectively.By comparing the above flame wave propagation velocity with the theoretical values of detonation performance parameters of the mixed gas in Table 1, it can be seen that the flame wave propagation velocity v and the pressure p in the tube of the three groups A1, A2 and A3 after the ignition of the mixed gas through the detonation tube are significantly different from the theoretical values, and neither of them reaches the detonation velocity of CJ.It can be concluded that the three groups of mixed gas only form a deflation flame surface in the detonation tube.There was no detonation wave; After the mixed gas of group A4 is ignited in the detonation tube, the flame wave velocity v and the pressure p in the tube reach the detonation velocity vCJ of CJ, which can be regarded as forming a stable detonation state in the tube.
The ignition signal is input to the high-energy power supply through the ignition control system.It can be seen from the PCB sensor pressure signal curve that at 0.25s, the ignition head will release the electric spark to ignite the unignited gas in the detonation tube.The time for the flame wave to reach the sensor is calculated, combined with the position of each sensor in the distance from the ignition head to the detonation tube.The velocity v when the flame wave reaches the sensor position in the tube can be calculated, as shown in Figure 3.The curves A1, A2, A3 and A4 respectively correspond to the combustion velocity curves of mixed gas with different numbers in detonation tubes as a function of tube spacing.As can be seen from the figure, A1, A2 and A3 mixed gases do not reach the detonation velocity vCJ-C3H8 after ignition, while A4 mixed gases reach the detonation velocity vCJ-C3H8, indicating that a stable detonation state is formed.The flame propagates downstream along the detonation tube, and the velocity of flame wave gradually rises, reaching the maximum velocity of 436.89m/s, 720.00m/s, 1022.36m/s and 1851.85m/srespectively in the middle part of the detonation tube.Then the flame speed was reduced to 389.61m/s, 703.13m/s, 910.81m/s and 1800.85m/s.This is because the flame wave reaches over divergence after accelerating through the spoiler, at which time the flame propagation speed reaches the maximum, and then the flame velocity decreases to reach a stable flame propagation state.Figure 4 shows the flame propagation process of propane/air mixed gas in the detonation tube.The whole process is recorded by high-speed camera, in which the ratio of A1 mixed gas is propane/air equivalent ratio, and A2、A3 and A4 are propane/oxygen-rich air equivalent ratio.As can be seen from Figure 4 (a) (b)and (c), when propane is mixed with pure air or a small amount of oxygen-rich air for ignition, the ignition head of the detonation tube head releases low-energy electric spark to ignite the mixed gas at the front end of the intake section, generating flame wave and detonation pressure wave.The flame wave accelerates after passing through the turbulence plate and propagates to the rear section of the detonation tube, at which time detonation pressure wave appears blue.As the pressure wave propagates to the outlet of the detonation tube, the flame wave is still propagating in the middle of the tube.At this time, the pressure wave and the flame wave are decoupled, forming a yellow and blue reaction area.When the flame wave propagates to the tube port, the air moves from the tube port to the head of the detonation tube, and the flame gradually goes out.As can be seen from Figure 4 (d), when propane and a certain amount of oxygen-rich air are mixed and ignited, a bright flame wave is formed at the head.The flame wave quickly moves to the back end of the detonation tube after the acceleration action of the spoiler.The deuteration pressure wave and flame wave are decoupled at the back end of the spoiler, and the deuteration pressure wave quickly triggers the chemical reaction of unburned fuel in the back part of the tube, showing the reaction zone in blue.The flame wave follows the deflagration pressure wave to the rear section of the detonation tube.When the deflagration pressure wave reaches the end face of the detonation tube, a reflected shock wave is formed and propagates back to the tube, and coincides with the flame wave at a certain time, and the overlap section is clearly defined.When the reflected wave reaches the PCB dynamic pressure sensor, the pressure signal curve of the sensor rises abruptly, and the pressure size is twice that of the first flame wave.As the reflected shock wave propagates to the tube, the detonation tube presents a state of negative pressure, air enters the detonation tube, the flame in the tube gradually extinguishes, and the combustion pressure in the tube gradually decreases.

Conclusion
Based on the propane/air detonation characteristics, detonation tests of propane/air pulse detonation engine under different fuel oxygen content were carried out in this paper.In the experiment, detonation tests of propane/air/oxygen mixture gas under the appropriate chemical ratio were carried out to explore the effects of different oxygen-rich environment on detonation characteristics of propane/air pulse detonation engine.Analysis of experimental results in this chapter shows that: (1) In the detonation experiment of propane/air/oxygen mixture with oxygen content of 20%, 25% and 30% in the fuel at the appropriate chemical ratio, the flame wave propagation velocity in the detonation tube is low, which fails to form a stable detonation wave, among which the fuel mixture with oxygen content of 20% has the worst detonation effect.The propagation velocity of the flame wave is 384.00 m/s.

Figure 2 .
Figure 2. Pressure signal curves of mixed gas detonation tubes for each group.

Figure 3 .
Figure 3. Flame propagation velocity variation curve with sensor position.

4 .
(a) A1 Group gas mixture (b) A2 Group gas mixture (c) A3 Group gas mixture (d) A4 Group gas mixture Figure Propane/air mixed gas flame wave propagation process.

Table 1 .
Theoretical values of detonation performance parameters of mixed gas.