Experimental Investigation on Flame Formation and Propagation

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Experimental Investigation on Flame Formation and Propagation Characteristics in an Ethylene Fuelled Wave Rotor Combustor Jianzhong Li, Erlei Gong, Li Yuan, Wei Li, and Kaichen Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03055 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Experimental Investigation on Flame Formation and Propagation Characteristics in an Ethylene Fuelled Wave Rotor Combustor Jianzhong Li 1,*, Erlei Gong 2, Li Yuan 3, Wei Li 1 and Kaichen Zhang 1 1

Key Laboratory of Aero-engine Thermal Environment and Structure, Ministry of Industry and Information Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao St., Nanjing, 210016, China

2

Simulation technology research center, Aero engine academy of China, Beijing, 101300, China

3

School of National Defense Engineering, The Army Engineering University of PLA, 88 Biaoying Rd., Nanjing, 210007, China

* Correspondence: [email protected]; Tel./Fax: +86-025-84895927

Abstract: To find out the flame formation and propagation characteristics in an ethylene fuelled wave rotor combustor (WRC), a simplified WRC test system was established. It is a single-channel

multiple-cycle WRC with rotating inlet/outlet port. Through varying the parameters such as filling speed of air-fuel mixture, rotating speed of inlet/outlet port and equivalence ratio of combustive air-fuel mixture, the flame formation and propagation characteristics in an ethylene fuelled WRC are presented and discussed. While the rotating speed of inlet/outlet port increases, the propagation speed of flame also gradually increases. When the rotating speed of inlet/outlet port is 1500 rpm, the propagation speed of flame reaches 45 m/s. A higher rotating speed of inlet/outlet port leads to a faster expansion of the reaction zone. Furthermore, the inclination angle and corrugation of flame front in channel of WRC with different rotating speed and different equivalence ratio are presented and discussed.

Keywords: Gas turbine engine; Wave rotor combustor; Flame propagation characteristic

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1. Introduction A wave rotor combustor (WRC) is an unsteady supercharging combustion device with high-speed rotation, which is a novel unsteady combustion technology [1–4]. A typical WRC is mainly composed of inlet duct, seal plates, channel, and outlet duct, shown in Figure 1. The operating process is described in reference [5]. A WRC has multiple channels evenly distributed along the circumferential direction, and the combustion occurs in these channels. A WRC usually adopts a rapid combustion mode like detonation or deflagration that can shorten the duration of a single cycle and increase the operating frequency. When WRC is integrated with other power equipment, owing to its unique supercharging function, a WRC has the potential to significantly enhance the performance of a gas turbine engine [6-7].

Figure 1. A wave rotor combustor. Nalim [8] designed and tested a WRC at near-atmospheric inlet conditions. The stable combustion was achieved and the potential of pressure-gain combustion was demonstrated. Through varying ethylene fuel distribution in the channels, fast deflagrative combustion was observed, but not recorded. Remarkably high flame speeds and a net pressure gain were indirectly indicated from measurements. In this paper, we use high-speed camera to record the flame propagation process in the channel of WRC. Nalim also used the pressure-exchange nonreacting wave-rotor experiments to calibrate the aerothermodynamic design and applying loss models. The viability of wave rotors for realizing a pressure-gain combustor is discussed. Akbari [9-10] used a quasi-one dimensional numerical model to make a systematic comparison between performance of pulse detonation engines (PDE) and detonative wave rotors. Instantaneous detonation and deflagration to detonation transition (DDT) for each engine were considered and flow field of these cases were


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compared with each other. Because of its fast rotation and large number of channels, the outlet flow of the detonative wave rotor is shown to be more uniform than of the PDE. This is more acceptable to turbine blades for gas turbine applications and may provide the combustor for new generation of aircraft engines. Higher pressure gain is produced in the detonative wave rotor configuration due to the mixture pre-compression by a hammer shock. The venerable Allison Model 501 engine had an architecture which was easily adaptable in terms of its potential for use as a demonstrator of an ORC/WR [11]. The accommodation of the pressure rise could be made while retaining the existing turbomachinery to the highest degree possible through reducing the number of working stages in the compressor. To this end, a second demonstrator engine directed cycle effort was undertaken. The shock reflection cycle was used to achieve pressure rise characteristic. The original overall pressure ratio (OPR) of the engine is 10.3 and that of the demonstrator engine is 8.0. There was a second- and third-stage reduction in stages, which increased output power by 20% and reduces SFC 12%. A collaborative work of Rolls-Royce, Indiana University-Purdue University Indianapolis (IUPUI), and Purdue University [12-14] implemented computational and experimental studies on the combustion process and the performance of WRC. The WRC with ethylene/air has 20 channels. Using hot-jet ignition, several tests were carried out. At the rotational speed of 2100 rpm, the periodical ignition combustion was achieved in the WRC, and a sequential steady operation was achieved. The flame propagation was not discussed. Elharis [15] used ignition gas injector to ignite rich ethylene mixture in the channel of WRC, a rapid deflagrative combustion was achieved. Using a transient gas dynamics and combustion simulation code, test data was analyzed to understand ignition and combustion behavior. Flow of hot gases from passages of the WRC and the ignition gas injector through the occasional source of ignition. A representative test case was examined for a comprehensive discussion on ignition initiation location, ignition delay time, and flame propagation speed through the passages. Karimi [16] studied the effect of different nozzle motion modes on the ignition delay and flame propagation speed in wave rotor channel by numerical simulation and analyzed the causes of different chemical reaction rates from the perspective of intermediate products such as OH. Chinnathambi [17] studied the ignition characteristics of a transverse jet flow in wave rotor channels by performing experiments. When ethylene was used as the fuel, a shorter ignition delay was obtained compared to methane. When a


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mixture of ethylene and hydrogen was used as the fuel, a shorter ignition delay and acceleration of flame propagation were achieved, while the optimal jet flow motion time was 6.1 ms. Elharis [18] developed a calculation method for the ignition and combustion of a two-dimensional wave rotor channel, where a two-step four-component chemical reaction mechanism was used, and the calculation results were consistent with the experimental results. Currently, the multiple-cycle intermittent operation of WRC has been rarely experimentally investigated. Only IUPUI achieved a multiple-cycle intermittent operation at 2100 rpm in the channel of WRC with the length of 787.4 mm. The numerical simulation and experimental investigation on the ignition and flame propagation in a single-cycle single channel were focused. In this paper, changing the rotating speed of inlet/outlet port, filling velocity and equivalence ratio of combustive air-fuel mixture, the initial flame formation and propagation characteristics in an ethylene fuelled WRC with the length of 200mm are presented and discussed. The effects of operating parameters on the flame propagation speed, flame front shape, and the highest distance of flame propagation were evaluated.

2. Experimental Rig of WRC Based on a simplified WRC test system, the initial flame formation and flame propagation characteristics in an ethylene fuelled WRC were studied. The simplified test WRC system mainly includes a hot-jet flow ignition system, simplified WRC, fuel-air combustive mixture filling system, and measurement system, shown in Figure 1.

Figure 1. Experimental Rig of WRC


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Hot-jet igniter was used in the test system. Using propane as the fuel and air as the oxidant, the structure of the hot-jet igniter is shown in Figure 2. The four air branches deliver air into the combustor at an angle of 60° between the axial and inflow directions, forming a swirling flow in the combustor. The fuel inlet delivers fuel to the ring cavity above the combustor walls, and the fuel enters the combustor through evenly distributed tangential holes in the cavity along the tangential direction, interacting with the swirling flow caused by air flow to mix the fuel. The propane-air mixture in the combustor is ignited using the electric ignition plug, and the high-temperature burned gas after combustion induces a jet flow after being ejected by the nozzle for ignition in the test system. The right part of Figure 2 shows the physical object of hot-jet igniter and the induced jet flow flame structure.

Figure 2. Structure of hot-jet igniter The test system in this study was simplified from a 24-channel WRC, as shown in Figure 3. Before the simplification, both the inlet and outlet seal plates of system are stationary, and the 24-channel wave rotor fixes with a shaft rotated relative to the seal plates. When a wave rotor channel passed by the ports on seal plates during the rotating process, it underwent processes including filling, ignition, combustion and exhaust. To observe the flame propagation in a channel of WRC, based on the principle of relative motion, the simplified test system of WRC includes a stationary channel with a sight window on one side. The inlet and outlet seal plates are connected to the shaft and rotate in the opposite direction relative to the wave rotor channel. By rotating the ports relative to the wave rotor channel [5], the operating time sequence of WRC was achieved. Kilchyk [19] simulated the effect of centrifugal force field on

turbulent combustion in a channel of WRC. The effect on combustion of rotating channel was not important when the centrifugal force was less than 4000 g. For the maximum design rotating speed of our WRC is 4000rpm, the centrifugal force is 1600g. Therefore, the simplified test system of WRC in this paper is reasonable. The fundamental dimensions of the WRC are mainly affected by

the installation dimensions of engine. The channel dimensions of WRC are dou=0.2308m, din=0.1752m, Lw=0.2m, Where, dou, din and Lw are the outer diameter, inner diameter, and channel length of WRC, respectively.


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The hot-jet flow would affect the exhaust of WRC. The high-pressure burned gas exhausted from the channel would also affect the operating stability of the hot-jet igniter. To solve this problem, the simplified outlet seal plate consists of Plates A and B. During the exhaust process, the air stream flows through the outlet port on Plate A and exhausted along the radial direction through the hole on Plate B corresponding to the outlet port. Plate B separates the hot-jet flow and outlet port in this process.

Figure 3. Schematic diagram of the simplified WRC In a practical situation, the ethylene-air mixture is filled through the intake section of WRC to 24 channels. Therefore, the intake section is designed as an annular sector as shown in Figure 4. The fuel is distributed to each fuel branch through a fuel manifold. It is injected into the intake section through fuel branches to form a combustive mixture with air. And then, the combustive mixture would be filled into channels of WRC. Because only a single stationary channel of WRC is included in the simplified WRC system, the intake section should be simplified accordingly. The cross-section of the simplified intake section is consistent with that of the channel of WRC in dimensions with only one fuel injection tube. Because the seal plate is in the rotating state, the intake section and seal plate are connected by the friction of the graphite sheet, while the original flange connection is discarded. Optical measurement system is composed of high-speed camera (OLYMPUS i-SPEED) and other


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auxiliary equipment, shown in Figure 5. The length of flame resolved is 156 mm. The frame rate of the high-speed camera was set at 2,000fps (time between each frame was 0.5ms). The velocity measurement using the high speed images depended on the image resolution for the specific experiment.

Figure 4. Combustive mixture filling system

Figure 5. Measurement system 3 Results and discussion

3.1 Initial flame formation and propagation The rotating speed of WRC in the simplified test system refers to the rotating speed of seal plates which is relative to the stationary channel. Four rotating speeds of the inlet/outlet port are used in the tests, namely, Rrs =900rpm , 1050 rpm, 1200 rpm, and 1500 rpm. The flame propagation processes in channel of WRC with the specified air flow filling speed ( V in ) and rotating speed of the inlet/outlet port are shown in Figure 5. Using a high-speed camera, the flame development of a series of high-brightness region was captured, which represents the chemical reaction zone. In the


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images, the reaction zone is marked by high-brightness curves during the flame propagation at Rrs =900rpm , Vin = 6.7 m / s , and Φ=1.44 . The reaction zone in the channel sequentially

undergoes varying processes, which are initial formation, rapid expansion, gradual shrink and even disappearance. The connection of flame fronts at different times does not make a plane, as shown in the images at Rrs =1200rpm . This indicates that the flame does not propagate at an even speed in the channel and the fuel-air distribution is not uniform. By comparing the flame propagation of different operating conditions, the speed of flame propagation slightly varies, especially, affected by the rotating speed of the inlet/outlet port. Varied degrees of corrugated inclination are shown by the flame fronts in channel of WRC under different operating conditions. This indicates the significant turbulent combustion characteristics. The degree and direction of corrugation inclination of flame front varied to some extent in different operating conditions. As the equivalence ratio of ethylene-air combustive mixture increases, the brightness of reaction zone significantly increases, which indicates more intense chemical reaction. With a relatively low equivalence ratio of ethylene-air combustive mixture, chemical reaction occurs only within a part of the length of WRC. For example, if Rrs = 900rpm , Vin = 6.7 m / s , and Φ=0.65 , at t =10 ms, the combustion of WRC is mostly completed, but the farthest flame front reached no more than half of the length of channel. This means the channel of WRC is not fully used and the ability of flame propagation is weak.


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（a） Rrs = 900 rpm , Vin = 6.7 m / s

(b) Rrs = 1050 rpm , Vin = 12 m / s


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(c) Rrs = 1200 rpm , Vin = 23.7 m / s

(d) Rrs = 1500 rpm , Vin = 34.1m / s Figure 5. Flame propagation under different rotating speeds


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3.2 Trend of flame propagation speed The flame propagation speed in this paper describes the mean flame propagation speed at a certain distance. As shown in Figure 6, assuming that the flame is observed to reach position X1 at time t1 and after a short while position X2 at time t2, the mean flame propagation speed within this duration is:

Vflame =

X 2 -X 1 t2 − t1

Furthermore, for the convenience of analysis, if the speed obtained by the above formula is the transient propagation speed of flame at ( X 2 +X 1 ) 2 , then the smaller

t2 − t1 induces the less

error of the results obtained based on this assumption. The development of flame front was captured using a high-speed camera. Therefore, the minimum value of

t2 − t1 is determined by

the number of frames through the high-speed camera. In the tests, the time interval is specified as 0.5 ms to 1.5 ms to obtain a relatively high precision.

Figure 6. Definition of flame propagation speed Based on the definition of flame propagation speed in Figure 6 and corresponding to the operating conditions in Figure 5, the flame propagation speed distribution are shown, where the horizontal axis X is the longitudinal direction of channel of WRC and position X = 0 is located at the left edge of the sight window. A dashed auxiliary line is shown in each graph. On the left side of the auxiliary line, the flame propagation speed rapidly increases. When the jet port is opened, hot-jet flow enters the channel and exerts a significant impact on the ethylene-air combustive mixture. Thus, the flame rapidly propagates under the effect of hot-jet flow. On the right side of the auxiliary line, the jet port is closed, and the effect of hot-jet flow disappeared. Therefore, the variation amplitude of the flame propagation speed decreases. Despite all this, on the right side of the auxiliary line, the flame propagation speed still oscillates significantly. This distribution law of


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flame propagation speed causes difficulty in comparison among different operating conditions. For convenience in comparison, the trend of flame propagation speed is analyzed, and the flame propagation speeds are calculated using the statistical approach. Focusing on one operating condition, the flame propagation speeds V flame on the left side of the auxiliary line are filtered out, while those on the right side with small oscillation are processed by arithmetic averaging. The mean value represents the flame propagation speed of this operating condition. Unfortunately, in this paper, a plot showing the flame propagation at fixed equivalence ratio and over a range of rotational speed for ease of comparison couldn’t be provided. While the rotational speed is increased, the operating cycle time becomes short. For the fixed length of WRC, the filling speed of combustive mixture must be increased through increasing the velocity of air and the mass flow of air is increased. If the same equivalence ratio is kept, the mass flow of fuel must be increased. Because the fuel injection is a single direct injection hole in this paper, the injection pressure of fuel must be increased to add mass flow of fuel. Then, the jet trajectory would be changed to deteriorate the quality of combustive mixture and fuel distribution. For different rotational speed, if the same equivalence ratio is kept, the WRC could not work steadily sometimes. Now, a new fuel injection would be designed to keep the quality of combustive mixture and fuel distribution at different mass flow of fuel. Then, the further research will be conducted.

(a)

(b)


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(c)

(b)

Figure 7. Distribution of flame propagation speed Figure 8 shows the trend of flame propagation speed after the processing. Among the studied impact factors, the effect of rotating speed of inlet/outlet port on the flame propagation speed of combustion of WRC is the most significant. Both the value and variation range of flame propagation speed gradually increases as the rotating speed of inlet/outlet port increasing. In particular, at 1500 rpm and filling speed of 25.78 m/s, the maximum flame propagation speed reached 45 m/s, achieving an intense supercharging rapid combustion. The effect of ethylene-air mixture filling speed on flame propagation speed shows a small regularity. As the equivalence ratio of filling ethylene-air mixture increases, at the rotating speed of 900 rpm, the flame propagation speed first increases and then decreases. At a rotating speed of 1050 rpm, the flame propagation speed gradually increases as the equivalence ratio increases. At a rotating speed of 1200 rpm, while the filling speed is 23.72m/s or 33.97m/s, the flame propagation speed increases as the equivalence ratio increases. While the filling speed is 29.54m/s, except the equivalence ratio of 0.6279, the flame propagation speed increases as the equivalence ratio increases. As the mass flow of fuel increases, the injection trajectory of fuel would be changed and the fuel distribution would prejudice the flame propagation. While the filling speed is 18.59m/s and the equivalence ratio is 0.499, the flame propagation speed is highest. This indicates that the filling speed of 18.59m/s is not avail for the WRC with rotating speed of 1200 rpm [5]. At a rotating speed of 1500 rpm, the trends of flame propagation speed with equivalence ratio of fuel-air combustive mixture are disorderly and unsystematic. From the pressure tested [5], the pressure is higher than that of WRC with other rotating speed. The high pressure would affect the filling process of next cycle to reduce the mass flow of fuel-air combustive into channel. The interaction between cycles


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is next work .

(a)

(b)

(c)

(d)

Figure 8. Trend of flame propagation speed To explain the significant impact of rotating speed of inlet/outlet port on flame propagation speed, an analysis is performed based on the development of reaction zone in combustion. Figure 9 shows the variation of proportion of reaction zone with time. The proportion of reaction zone is defined as η = Areaction Atotal , where Areaction is the area with brightness higher than the set critical value, and Atotal is the total cross-section area of wave rotor channel. Notably, both Areaction and

Atotal are calculated from the cross-section captured by high-speed camera. To ensure the universal applicability of the discussion and better understand the analysis for each rotating speed, shown in Figure 9. A case of relatively good combustion performance is selected as the representative to analyze the development of the range of reaction zone. At most combinations of rotating speed of inlet/outlet port and equivalence ratio of filling ethylene-air mixture, the trends of development in the proportion of reaction zone with time are similar. At the initial stage of combustion, the hot-jet ignites the ethylene-air mixture in the channel of WRC to form the


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initial flame, and the reaction zone becomes small. As time proceeds, the energy releases by the reaction zone continuously diffuses to the surrounding fresh ethylene-air mixture and ignites the ethylene-air mixture in the vicinity. Thus, the proportion of reaction zone rapidly increases (within the region of dashed box), and finally near the end of combustion. The reaction zone gradually decreases and even disappears. This phenomenon is caused by the combined consequence of gradual burnout of fuel in gas mixture, leaking of leakage gap, and expansion wave induced by exhaust. The disappearance of reaction zone indicates the start of the preparation of a new cycle of combustion in channel of WRC. The developments of reaction zone under different working conditions show similarity. The unique features are observed when the conditions are varied, especially the effect of rotational speed of inlet/outlet port. As shown in Figure 9, at a fixed rotational speed, while the equivalence ratio of ethylene-air mixture changes, the expanding sections of reaction zone shows a certain similarity. It means the slopes of the curve of

η

in the dashed box varying with

t

are similar.

The slope of the long edge of the dashed box is assumed to represent the expansion rate of reaction zone. The graphs show that the slope increased from 0.05 at 900 rpm to 0.195 at 1500 rpm as the rotating speed increases. This shows that the increasing of the rotating speed of inlet/outlet port could help to improve the expansion rate of reaction zone. The rapid development from the initial flame would intense combustion in channel of WRC. This is beneficial in enhancing the combustion performance of WRC. This can be verified by the above analysis of flame propagation speed.

(a)

(b)


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(c)

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(d)

Figure 9. Trend of range development of each reaction zone Many factors could affect the increasing rate of the reaction zone range. An important factor is the characteristics of the operating process of WRC. Figure 10 shows the schematic diagram of the position relationship of the hot-jet flow igniter. For example, when the seal plate is at the outlet, the channel of WRC is during the ignition process. The channel of WRC in this paper is stationary, and the seal plate rotates with a shaft in the tests. When the ignition port on the outlet port rotates at the igniter, the jet flow enters the channel through the ignition port to achieve ignition. The end of channel close to the igniter is exposed to the port and jet flow igniter within time of initial flame is formed by the burned gas in the channel within the time of

τ

τ . If the

(shown as case (a)

on the right side of Figure 10), the flame would propagate along the channel and the pressure increases. The high-temperature and high-pressure burned gas would flow out from the channel through the ignition port and induce a significant leakage. When the opening angle of the ignition port is fixed (fixed at 15° in this study), a higher rotational speed of WRC leads to a smaller

τ

and shorter time impacted by leakage. It makes for forcing the combustion to develop along the channel in one direction as early as possible. The results shown in Figure 9 are consistent with this case. Case (b) on the right side of Figure 10 shows the approach to solve the performance loss induced by leakage during the ignition. When the ignition port leaves the position of channel, the initial flame is formed in the channel of WRC. It can be achieved by relatively increasing the ignition delay. The increasing of the rotational speed of WRC would decrease the overlapping time of ignition port and channel and change the distribution of fuel-air mixture at the ignition location.


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Figure 10. Relative positions of igniter, seal plate, and wave rotor channel 3.3 Trend of flame front shape Figure 11 shows the distribution of flame front shape under different operating conditions. In each operating condition, a corrugated deformation occurs at the flame front with significant turbulent combustion characteristics. When the rotating speed of inlet/outlet port is 900 rpm, at each filling speed, the inclination trend of flame front is consistent. When the equivalence ratio of fuel-air combustive mixture in channel of WRC is lean, the flame propagation speed is relatively high on the internal arc surface in the fan-shape passage in channel. The flame front inclined downwards. When the equivalence ratio of fuel-air combustive mixture in channel of WRC is rich, the flame propagated relatively fast on the external arc surface of channel of WRC, and the flame front up-tilted. When the rotational speed of inlet/outlet port is 1050 rpm, the stable operating states are lean state. Therefore, the flame fronts are substantially downward inclined with the only exception that the flame front became up-tilted when the filling speed is 14.59 m/s and the equivalence ratio of ethylene-air mixture increases to Φ = 0.996. By summarizing the cases with rotating speeds of 900 rpm, it can be regarded that under low rotating speeds (900 rpm and 1050 rpm), the critical point of flame front upward/downward inclines in channel of WRC is approximately located at the stoichiometric ratio. The effect of equivalence ratio of ethylene-air mixture on flame front shape is more significant than that of rotational speed or filling speed. When the rotating speed of inlet/outlet port increases relatively high speeds (1200 rpm and 1500 rpm), the effects of filling speeds of ethylene-air mixture on flame front shape become more significant. At a rotating speed of 1200 rpm and filling speed of 18.59 m/s, the transition point of flame front inclination direction decreases to Φ = 0.667 and Φ = 0.756. When the filling speed increases to 23.72 m/s and 29.54


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m/s, the transitional critical point decreases to below Φ = 0.535 and Φ = 0.419. When the filling speed increases to 33.97 m/s, the transitional critical point decreases to the location between Φ = 0.424 and Φ = 0.494. Obviously, while the flame front inclination direction change significantly, the critical point of equivalence ratio decreases compared to that in the low rotating speed state. When the rotating speed increases to 1500 rpm, not only the equivalence ratio of transitional critical point further decreases, but the inclination of flame front also changes to a certain degree. For example, at the filling speed of 34.17 m/s, the flame inclines downwards under a relatively lean state. But, when the equivalence ratio is increased to Φ = 0.484 , the flame inclines upwards, unlike all other operating conditions.

Figure 11. Flame front shapes under different working conditions

3.4 Trend of farthest distance of flame propagation To enhance the overall performance of the combustion system, the utilization of space in combustor should be maximized to achieve a compact structure, lightweight, and small volume of combustion system. Figure 12 summarizes the farthest distances of flame propagating in channel of WRC under all the tested working conditions. The statistical results are based on the flame propagation captured by high-speed camera. In the graphs, the horizontal axis is the longitudinal direction of sight window. The left edge of sight window is X = 0 mm, and the right edge is X = 156 mm. Considering the error of the flame incandescence spectra, when the flame propagates to farther than X = 150 mm, the flame can be regarded as propagating through the entire wave rotor channel. The green dashed line in the graphs indicates the right edge of sight window, and the


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points present in ellipse belong to the working conditions. The flame propagates through the entire channel of WRC. Because the equivalence ratio of ethylene-air mixture has a higher effect on the maximum position of flame propagation, it can be set as the longitudinal coordinate. The graphs show that at any rotating speed, the flame partly propagating in the channel exists during the combustion in WRC. By defining the farthest distance of flame propagation as X , the filling speed of ethylene-air mixture affects X , but the regularity of this effect is not significant. At 1050 rpm, the effect of filling speed of ethylene-air mixture is the most significant. Under the same equivalence ratio and relatively low filling speed, X of flame propagation is higher, because a low filling speed indicates a weak dispelling capability of next cycle on a partial developed flame. If the cases of rotating speed of 900 rpm are excluded, in the range of stable operating of WRC, the minimum of X gradually increases with the rotating speed. When the rotating speed increases to 1500 rpm, the minimum of X reaches more than 140 mm, because the flame has the condition to propagate farther before the filling of a next cycle with a high flame propagation speed under a high rotational speed condition.

(a)

(b)

(c)

(d)


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Figure 12. Farthest distance of flame front

4 Conclusions

In this study, using a simplified test system of WRC as a platform, the effects of rotating speed of inlet/outlet port, filling speed of ethylene-air mixture, and equivalence ratio of ethylene-air mixture on flame propagation of WRC are investigated. The rotating speed of inlet/outlet port significantly affects the flame propagation speed. As the rotating speed increases, the propagation speed and range of flame gradually increase. When the rotating speed of inlet/outlet port is 1500 rpm, the flame propagation speed reached 45 m/s. The effects of flow speed and equivalence ratio on flame propagation speed are complex. The range of reaction zone first expands, but then decreases as the time proceeds. At a fixed rotating speed and varied equivalence ratio of fuel-air combustive mixture, the development trends of range of reaction zone are similar. But the expansion rate of reaction zone rapidly increases as the rotating speed of inlet/outlet port increases. When the rotating speeds increase from 900 rpm to 1500 rpm, the variation rates of reaction zone range against time increase from 0.05 to 0.195. The flame front inclines towards the inner arc surface of channel under a low equivalence ratio condition. When the equivalence ratio increases to a critical value, the inclination direction of flame is opposed. At a low rotating speed, the critical rotating speed is near the stoichiometric ratio. As the rotating speed gradually increases, the critical value significantly decreases. At 1500 rpm, the inclination of flame also changes. The farthest distance of flame propagation in channel is significantly influenced by the equivalence ratio and rotating speed of inlet/outlet port. With the increasing of equivalence ratio, the flame propagates farther. This relationship lasts until the flame propagates through the entire length of wave rotor. At a high rotating speed, the flame tends to propagate farther.

Acknowledgments This work was supported by “the National Natural Science Foundation of China”, NO.51476077.

References

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Nomenclature WRC

wave rotor combustor

PDE DDT

pulse detonation engine deflagration to detonation transition

Vin

inlet velocity of combustible mixture

Rrs

rotating speed of inlet/outlet port

Φ

equivalence ratio

t1

time while the flame arrives at one position

t2

time while the flame arrives at another position

X1

position of flame at time t1

X2

position of flame at time t2

Vflame

propagation velocity of flame

Areaction

area with brightness higher than the set critical value

Atotal

η

total cross-section area of wave rotor channel proportion of reaction zone

X

horizontal axis

τ

ignition time

dou

outer diameter of WRC

din

inner diameter of WRC

Lw

channel length of WRC


Experimental Investigation on Flame Formation and Propagation

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