Experimental Investigation on Pressure Rise Characteristics in an

Aug 11, 2017 - School of National Defense Engineering, PLA University of Science and Technology, Nanjing ...... ABB Technical Report Paper ... Orlando...
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Experimental Investigation on Pressure Rise Characteristics in an Ethylene Fuelled Wave Rotor Combustor Jianzhong Li,*,† Erlei Gong,‡ Li Yuan,§ Wei Li,† and Kaichen Zhang† †

Key Laboratory of Aero-Engine Thermal Environment and Structure, Ministry of Industry and Information Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ‡ Simulation Technology Research Center, Aero Engine Academy of China, Beijing 101300, China § School of National Defense Engineering, PLA University of Science and Technology, Nanjing 210007, China ABSTRACT: Pressure rise is one of the key operation characteristics of a wave rotor combustor (WRC), which would affect the power performance of the propulsion system based on WRC. To find out the pressure rise characteristics of WRC, a simplified test system of WRC was established, which is a single-channel multiple-cycle WRC with a rotating inlet/outlet port. The effects of the filling speed of air-fuel mixture, the rotating speed of inlet/outlet port, and the equivalence ratio of combustible air-fuel mixture on pressure rise characteristics of WRC are presented. The pressure rise is derived from the flow stagnation of filling combustible air-fuel mixture and the constant volume combustion in channel. With the rotating speed of inlet/outlet port increasing, the pressure rise significantly improves and the maximum average pressure is 6.3 atm. When the rotating speed of inlet/outlet port is constant, there is a suitable filling speed of air-fuel mixture and an optimum equivalence ratio for combustible air-fuel mixture. With the increasing of the rotating speed of inlet/outlet port, the suitable filling speed of combustible air-fuel mixture also increases and the operating range of the equivalence ratio of combustible air-fuel mixture becomes narrow.

1. INTRODUCTION A wave rotor combustor (WRC) is a novel combustion device with high-speed rotation. As shown in Figure 1, the complex waves include shock waves, compression waves, expansion waves, and contact discontinuity in channel of WRC, where the shock waves are generated to achieve the pressure rise of WRC. The transition of deflagration to detonation is achieved, and the detonation combustion improves the thermal efficiency of WRC. The pressure rise and the high thermal efficiency combustion in channel of WRC are associated with the interdisciplinary fundamental scientific problems of aerodynamics and chemical reaction kinetics, such as the formation and propagation of shock wave, formation and development of initial flame, and interaction and coupling mechanism of shock wave and flame. The outstanding merits include high thermal efficiency, high pressure rise, low emission, low fuel consumption, and self-cooling. Therefore, WRC would be one of an important technology to improve sharply the thermal efficiency of gas turbine engine.1−6 The significant potential of WRC which could improve substantially performance of the propulsion system is extensively focused in the world. Asea Brown Boveri (ABB) in Switzerland tested wave rotors for power-generation gas turbine applications with particular attention to wave rotor combustors.7,8 A rotaryvalved single-channel wave rotor combustor was built and tested. The 16.5 cm long, 15 × 15 mm cross section, nonrotating channel facilitated optical observation for ignition and constant-volume combustion. The viability of the constant-volume combustion concept using wave rotor principles was demonstrated, as well as the possibility of meeting emissions limits in applying this benefit to commercial power plants and gas turbine engine. Based on testing of the single-channel stationary combustor, a full wave © XXXX American Chemical Society

rotor with 36 combustor channels was designed. Using sparkplug electric ignition and hot gas injection ignition methods, the air-fuel mixtures in the single-channel and 36-channel wave rotor were ignited successfully. Self-sustaining ignition of ABB test rig and rotor subassembly were then accomplished by employing axial jet injection of burned gas from a neighboring channel. Various fuels were tested, and the air-fuel mixtures were stratified using four injection nozzles. The prototype engine operated successfully until the project was concluded. During its operation, a number of shortcomings were revealed. They included (1) inhomogeneous air-fuel mixture in the cell, resulting in a slow diffusion flame, (2) maximum pressure of 9 bar, relative to expected 10−15 bar, presumably due to leakage, which also caused premature ignition and misfiring at higher chamber pressures, (3) excess thermal stresses on the ignition ring, (4) inadequate cantilever single-bearing rotor support, and (5) complicated and sensitive electromechanical device for controlling leakage gap. Major remedies were recommended to make the system better, which included (1) lead away duct for leakage gas removal, (2) rotor cooling by air, (3) two-sided rotor support, and (4) mechanical control for thermal expansion. These recommendations would necessitate a new rig with major improvements, which was not pursued at the time. The venerable Allison Model 501 engine has an architecture that is easily adaptable in terms of its potential for use as a demonstrator of an ORC/WR.9 Using a bearing in front (in the inlet housing) and a thrust bearing aft of the last stage supported by the compressor diffuser housing, the 14stage compressor rotor is straddle mounted. Between the Received: June 21, 2017 Revised: August 11, 2017 Published: August 11, 2017 A

DOI: 10.1021/acs.energyfuels.7b01769 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. A simplified schematic of WRC.

Figure 2. Schematic of the simplified WRC model.

Combustion characteristics such as the ignition capability of hot-jet flow on combustible mixture and ignition delay time were studied. Chemical kinetics and aerodynamic mixing simultaneously affected the ignition delay time. Karimi15 investigated the characteristics of transverse jet ignition in a wave rotor channel. Using ethylene instead of methane as the fuel can shorten the ignition delay time. An air-fuel mixture of methane and hydrogen significantly shortened the ignition delay time and accelerated the flame propagation. The optimal operating time of jet was 6.1 ms. Baronia16 used numerical simulation method and a four-step detailed chemical reaction mechanism to numerically simulate the flame propagation in a wave rotor channel with propane as the fuel. The development of a swirling flow in the ignition of transversely moving jet is clearly different from that of a central jet. Kilchyk17 numerically investigated the burning rate of premixed combustible air-fuel mixture and flame propagation in the combustion chamber affected by the rotation of WRC and hot-jet ignition position. Without considering the disturbance of complex waves in WRC on the shock wave induced by hot jet, the effects of the centrifugal field on turbulent combustion in the channel of WRC were simulated and the combustion was barely affected when the centrifugal force was less than 4000g. 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 the inlet/outlet port, filling velocity, and equivalence ratio of combustible air-fuel mixture, the pressure rise characteristics in channel and operating range of WRC with the length of 200 mm are presented and discussed.

compressor and turbine, the six cans in a can-annular space are used to burn the combustible air-fuel mixture. The cans are long, which could provide an unusual expanse for the location of the ORC/WR rotor. Engine rotor dynamics are fully developed and known to be completely acceptable with this relatively long rotor. The most feasible option is to hold the turbine inlet conditions at the design level of 2033 °F for applying of pressure rise combustion. 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 is 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 is a second- and third-stage reduction in stages, which increases output power by 20% and reduces SFC 12%. A collaborative work of Rolls-Royce, Indiana UniversityPurdue University Indianapolis (IUPUI), and Purdue University10−12 implemented computational and experimental studies on the combustion process and the performance of WRC. The fuel is ethylene, the oxidant is air, and the WRC has 20 channels with dimensions of 63.5 mm × 63.5 m × 787.4 mm. 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. Indika U. Perera13,14 established a test rig for a single-cycle WRC, in which the unsteady hot-jet ignition in WRC was simulated by rotating a precombustion chamber producing hot jet and stationary single-channel combustion chamber. The effects of rotation condition on the formation of combustible air-fuel mixture and air-fuel distribution in the combustion chamber were ignored. The premixed mixture was ignited in the WRC, and the generated hot-jet flow ignited the premixed mixture in channel. B

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Figure 3. Experimental rig of a single-channel WRC with rotating inlet/outlet port. cross section in size as that of the channel of complete WRC. During the rotating of the inlet port, the combustible air-fuel mixture in the intake passage flows through the opening of the seal plate and enters into the channel to simulate the filling. The hot jet or burned hot gas enters or exits the channel through the jet port on the seal plate or outlet port, respectively. The scheme of all the port positions and opening angle spans on seal plates are the same as that of a complete WRC to ensure that the time sequence of the simplified WRC is identical to practical cases. The ethylene as fuel is mixed with air in the fanned passage. Using support arms with fanned graphite sheets installed on both the ends contacting the seal plates for sealing, the channel of WRC is fixed on the experimental rig. One side of the channel has a sight window for observing the ignition and flame propagation in the channel of WRC. Three dynamic pressure transducers (KD2004GA, sampling frequency of 180 K) are evenly distributed on the channel to measure the pressure. To verify the experimental accuracy, the KD2004GA pressure transducer and PCB pressure transducer were installed at the same cross section of channel to measure the pressure. The pressure uncertainty was about ±500 Pa, corresponding to ±0.5% pressure measured with PCB pressure sensor. The rise time is 2μs. The reliability of the dynamic pressure measurement system was verified in detonation combustor.18 They are tagged as PT1, PT2, and PT3 from the left to the right. To the length of channel and the installation space of pressure transducer, the distance of 50 mm between of pressure transducers was selected. X is the distance from the pressure transducer to the outlet port, where XPT1 = 40 mm, XPT2 = 100 mm, and XPT3 = 160 mm. The spindle motor of a high-precision machine tool is utilized to ensure the concentricity of the entire system. The seal plates are first connected to the revolution shaft to ensure that the inner sides of the seal plates at both the inlet and outlet are parallel to each other and vertical to the revolution shaft. Then, the shaft is fixed in the bearings and the position of the motor is adjusted so that the motor and shaft are concentric. The channel of WRC is fixed on the support arm mounted in the locating grooves of the test platform. The midspan surfaces of the channel and port openings are at the same radial location, and it is also the center of the hot-jet igniter. Moreover, the centerlines of the jet nozzle and channel are aligned to each other, and the jet nozzle exit is kept 5 mm away from the seal plate to accommodate the thermal expansion of jet nozzle. By adjusting the rotating speed of the motor by adjusting its inverter, the rotating speed of WRC is controlled. In this paper, the WRC with the rotating speeds of 900, 1050, 1200, and 1500 rpm were tested. While the mass rate of airflow is kept constant,

2. EXPERIMENTAL RIG As shown in Figure 2a, the WRC is composed of 24 channels with the length of 200 mm and the rotor channels are rotating at a constant rotating speed. The two ends of the rotor channels are fixed with inlet and outlet ports of certain opening angle spans. By adjusting the opening angle span and position of the inlet/outlet port, the operating time of WRC can be controlled, as shown in Figure 2b. Considering the measurement difficulties of the parameters related to the pressure in a channel, the flame propagation process, and speed in a rotating wave rotor combustor, based on the principles of relative motion, this study ensures an unchanged relative motion relationship between the inlet/outlet port and rotor channel. Before the simplification, the rotor channel is subjected to a counterclockwise rotation, while the inlet/ outlet port is stationary. The rotor channel is filled by the combustible air-fuel mixture, when it is rotated to the opening of the inlet port, as in time t2−t6 shown in Figure 2b. The rotor channel is filled by the combustible air-fuel mixture, when it is rotated to the outlet port, as in time t1−t4 shown in Figure 2b. An overlap is observed between the filling and no filling times, and their angle span is estimated using the aerodynamic parameters of the combustible air-fuel mixture and shock wave in the channel of WRC. For the simplified WRC, the channel is kept stationary, while the inlet/outlet port is rotated in the clockwise direction. When the opening of the inlet port is rotated to the location of the channel, the combustible air-fuel mixture flows into the channel through the opening of the inlet port, as in time t2−t6 shown in Figure 2b. When the opening of the outlet port is rotated clockwise to the channel, the burned hot gas is discharged through the opening of the outlet port, as in time t1−t4 shown in Figure 2b. Thus, the simplification based on the principles of relative motion ensures the consistency of operating time between a single-channel WRC with rotating inlet/outlet port and the practical complete WRC, which could simulate the dynamic process of the gradual opening or closing for channels of WRC. For the WRC with the rotating speed of 900, 1050, 1200, and 1500 rpm, the centrifugal acceleration of WRC are 81, 110.25, 144, and 225g, respectively. In reference 17, the combustion was barely affected when the centrifugal force was less than 4000g. Therefore, the simplification for WRC is reasonable and feasible. A simplified experimental rig of WRC is shown in Figure 3. It consists of a hot-jet igniter, a single-channel, an intake system, and a driving system, where the seal plate of the inlet/outlet port is connected to the drive shaft and is rotated with respect to the rotor channel by a driving motor because only a single channel of WRC is installed in the experimental rig and the channel is stationary. The entire intake section is designed as a fanned passage with the same C

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inlet and outlet ports are fixed. The wave rotor channels are smooth and straight passages without any combustion intensification apparatus. The air-filling speed and equivalence ratio of filling air are adjusted to explore the filling speed range for a stable operation of WRC at each rotating speed as well as the equivalence ratio range corresponding to various rotating and filling speeds. In the simplified WRC, with respect to the jet igniter, the seal plate would rotate with a high-speed, and thus the jet nozzle cannot be inserted into the ignition port on the seal plate. The thickness of the outlet seal plate is 30 mm. The hot jet is fixed at the same state during the entire test procedures and used as the ignition source. On the basis of the exploration of a stable operating range of WRC, the effects of pressure rise in channel of WRC with each operating condition are also recorded to analyze the influencing factors such as the rotating speed of inlet/outlet port, filling speed of combustible air-fuel mixture, and equivalence ratio on the pressure rise characteristics of WRC.

the equivalence ratio of the combustible air-fuel mixture is controlled by changing the flow rate of fuel. Air flow and fuel are mixed to be a combustible air-fuel mixture in the intake. The volume flow rate of air is measured by the vortex flow meter. The volume flow rate of ethylene is measured by float flow meter. Because the cross-section area of intake is the same with the channel of WRC, the filling speed of combustible air-fuel mixture could be calculated. During the combustible air-fuel mixture flows into the channel of WRC, there are several typical relative positions between the combustible air-fuel mixture and inlet seal plate, as shown in Figure 4. At position a, the combustible air-fuel mixture blows on

3. PRESSURE RISE CHARACTERISTICS OF WRC The four rotating speeds of the inlet/outlet port (900, 1050, 1200, and 1500 rpm) were chosen for WRC. At each rotating speed, the filling speed and equivalence of combustible air-fuel mixture were explored. The pressure in channel of WRC were measured and discussed. As shown in Figure 6, at various rotating speeds, there is an optimal pressure rise and stable operation and the corresponding filling speeds and equivalence ratio are specific. Pressure rise combustion in channel of WRC with each rotating speed is achieved. When the rotating speeds are 900 and 1050 rpm, the pressure rise capabilities are poor. The peak pressures are 0.1 MPa. When the rotating speed is 1200 rpm, the average of peak pressures is 0.35 MPa. When the rotating speed is 1500 rpm, the average of peak pressures is 0.5 MPa. At lower rotating speed, the pressure rise is lower owing to the leakage and the lower filling mass ratio of air-fuel mixture (the heat release is smaller).19 With the increasing of rotating speed, the pressure rise capability increased significantly. While the rotating speed is increased, the time of pressure rise is shorter and the time of inlet/out opening and closing is the same at each rotating speed of WRC. At higher rotating speed, the leakage is owing to the gap between the end surface of channel and the graphite plate. At lower rotating speed, the leakage is also owing to the unreasonable time of inlet/out opening and closing. For the rotating speed of 1500 rpm, the optimal pressure rise profiles are presented, which has corresponding specific filling speed of combustible air-fuel mixture and equivalence, as shown in Figure 7. Using three pressure transducers PT1, PT2, and PT3 at the three locations, the averages of peak pressures are recorded and more than 0.4 MPa. In particular, at the location of PT2, it is higher than 0.5 MPa. The magnified graph of the pressure profiles in a single cycle shown in Figure 7d indicates that the pressure rise at the location of PT2 is highest and that at the location of PT1 it is lower slightly. The pressure rise at the location of PT3 is least. The locations of PT1 and PT3 are approach to the outlet and inlet ports, respectively. The static and dynamic sealing and leaking problems are unavoidable between the inlet/outlet port and the channel of WRC, and the leaking will decrease the pressure.19 The time of precompresssion is about 11.364 ms, and the time of combustion is 8.56 ms. The ignition delay time of hot jet ignition is about 2.31 ms. When the high-temperature burned gas of hot jet igniter is injected into the channel of WRC, the pressure rises immediately and keeps constant value. When the outlet port opens, the pressure decreases rapidly and the

Figure 4. Positions of intake section and seal plate during filling process. the seal plate and stagnates to produced compression wave, which induces that the combustible air-fuel mixture becomes a discontinuous flow. At position b, the combustible air-fuel mixture smoothly flows into channel of WRC, but it will be affected by the complex waves produced during last operating cycle. The position c is the transition of flow regime at position b to position a. The position d is the transition of flow regime of position a to position b. Therefore, the filling process of combustible air-fuel mixture is intermittence. During the test, the simplified WRC first reaches the preset rotating speed and the air filling speed is adjusted to a certain value. At this moment, the air supply system of the jet igniter is turned on and propane is provided as a fuel. After the hot jet becomes steady, the fuel supply system of WRC is turned on to form a combustible air-fuel mixture with a certain equivalence ratio in the intake section. The combustible air-fuel mixture enters the channel of WRC through the rotating seal plate and would be ignited by a hot jet after precompressing. Then, the burned gas is discharged from the outlet port. Using pressure transducers, the pressure rise of combustion in the channel of WRC is recorded. The combustion in the channel of WRC is a complex process and completely affected by aerodynamics, thermodynamics, and structural factors. The pressure rise characteristics are affected by the rotating speed, filling speed, operating time sequence, ignition condition, and formation of combustible air-fuel mixture. To isolate some effect factor, one or several variables are kept constant and one or certain variables are changed to determine the effects of that or certain variables on the pressure rise characteristics of WRC. The test procedures and relevant approaches are shown in Figure 5. In the established test system, the rotating speed of WRC is controlled at a fixed value. For each rotating speed, the opening angle spans of the

Figure 5. Test procedures and approaches. D

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Figure 6. Pressure profiles in channel of WRC with four different rotating speeds.

Figure 7. Pressure profiles in channel of WRC with rotating speed of 1500 rpm.

E

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Figure 8. Pressure profiles in channel of WRC with the rotating speed of 1200 rpm and the filling speed of 29.5m/s.

burned gas of WRC is expanded quickly to exhaust for output power. Changing the equivalence ratio of combustible air-fuel mixture, the pressure profiles in the channel of WRC with the rotating speed of 1200 rpm and the filling speed of 29.5m/s are shown in Figure 8. When the equivalence ratio is 0.42, the average of peak pressure is only 0.06 MPa. With the equivalence ratio increases, the average of peak pressures also significantly increase. While the equivalence ratio is 0.62, the peak pressures at the corresponding locations of the three pressure transducers PT1, PT2, and PT3 are 0.4, 0.45, and 0.3 MPa, respectively. When the equivalence ratio is increased, the WRC could not operate stably, which means that the supercharged combustion is not implemented at each cycle and indicates that there is a range of equivalence ratios for WRC with a specific rotating speed and filling speed of air-fuel mixture. To illustrate the pressure rise process in the channel of WRC, the experimental data of some operating conditions are analyzed in detailed. Figure 9 shows the pressure profiles in the channel of WRC with the rotating speed of 1200 rpm, the filling speed of 29.5 m/s and the equivalence ratio of 0.56. The signal sampling duration in the graph is 0.4 s, which includes 8 operating cycles of WRC. By observing the pressure curve of one of the cycles magnified in the graph, the partial magnified curves indicate that the stagnation of the filling air flow on seal plate would cause a compression wave opposite to the air flow direction in the channel of WRC. This compression wave increases the pressure in the channel to achieve the procompression of combustible air-fuel mixture. The stagnation compression wave is then reflected in the channel. Because of leakage, the pressure in the channel slightly decreases before

Figure 9. Pressure profiles and magnified curve of one cycle for WRC.

the ignition. However, the reciprocation of the stagnation compression wave favors further mixing of fuel and air in the channel and rapidly exchanges energy. When the hot jet flow of igniter enters the channel through the ignition port, the hot-jet flow itself as a disturbance would induce a new compression wave in the channel, slightly weaker than the stagnation compression wave. After this compression wave is caused by jet, the pressure in the channel increases rapidly, and a shock wave of combustion is formed.20 Owing to drastic chemical reaction, the shock wave of combustion is stronger than the stagnation compression wave and compression wave induced by the hot jet. The pressure in the channel has high values within a certain F

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Figure 10. Peak pressure distribution along the channel of WRC.

period after the combustion and then decreases rapidly under the effect of the expansion wave. In the magnified curves shown in Figure 10, time parameters corresponding to each variation on curve are labeled, where the combustion time tcom = 9.95 ms. The ignition delay time is defined as the duration between jet disturbance and generation of a distinct combustion wave, and then tdelay = 1.83 ms. The traveling time of the precompression wave tpre = 10.79 ms. The precompression time is close to the theoretical design shown in the lower left corner in Figure 10, and the sum of the combustion time and ignition delay time is close to the theoretical design of combustion time. Two auxiliary lines are drawn on the measured pressure curve of measuring point PT1 in Figure 9 to show the variation of peak pressure between cycles, where the red line denotes the peak pressure induced by combustion and the blue line denotes that induced by filling air-fuel mixture stagnation precompression. Therefore, the two dashed lines, as a basis of comparison, correspond to the averages of peak pressures. Figure 9 shows that the peak pressures induced by combustion are located around the red dashed line, some above it while some below it, indicating certain fluctuations during combustion in channel of WRC with the same structure, rotating speed, and boundary conditions. One of reason is the unsteadiness of the turbulent combustion itself, and the other is the variation of filling air-fuel mixture induced by the pressure of the high-pressure burned gas of the previous cycle in channel of WRC at the initial phase of filling process of each cycle. However, the precompression process induced by the stagnation compression is relatively stable, and the peak pressures of precompression almost remain at the same level.

Figure 10 shows the statistical averages of the test data for WRC with four representative rotating speeds. The value of each point is the average of all the cycles in the sampling duration of 0.4 s, where the red curve represents the pressure of shock produced by combustion,20 while the black curve represents the pressure of precompression. As shown in the graphs, the trend of first increasing and then decreasing is common in the peak pressures of compression induced by combustion for WRC with each rotating speed. The peak pressure of measuring point PT2 is the highest, whereas the peak pressure of the measuring point PT3 is the lowest. This phenomenon can also be verified in Figure 7d. The measuring point PT1 is close to the ignition end. At the initial phase of combustion, the chemical reaction is not drastic and the pressure is low. As the combustion proceeds, the flame propagation speed increases and the pressure produced by combustion constantly follows the compression produced by the combustion of the previous period and the waves eventually overlap, thus strengthening the pressure produced by combustion. At the location of PT2, the pressure rise increase by 10%. Ideally, at the measuring point of PT3, the pressure would further increase. However, the location of PT3 is close to the inlet port. When the flame propagates to the intake section, the inlet port is opened for the filling of the next cycle. Thus, an expansion is formed in the channel of WRC. Thus, the pressure of that point significantly decreases, and a pressure disturbance is caused in the intake section owing to a part of the highpressure burned gas entering the intake section. The inherent leakage between the rotor and stator of WRC is the cause of low pressure at measuring points PT1 and PT3. Unlike the pressure produced by combustion, the precompression became G

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Figure 11. Variation of the precompression effect in channel of WRC.

weaker from PT1 to PT3 because there is no energy to be added during the precompression process. The leakage factor leads to the reduction of pressure at the measuring point.19 Similar to the pressure produced by combustion, the precompression pressure at the location of PT3 also rapidly decreased, which is caused by the expansion wave generated at the inlet port. As a part of the operating procedures of WRC, the effect of precompression on pressure rise is analyzed first. The pressure ratio of precompression is defined as δp = (P2 + P0)/(P1 + P0), where P0, P1, and P2 are the ambient pressure, inlet pressure, and pressure behind precompression wave. Figure 11 shows the effects of various factors on precompression effect. The graphs show that δp gradually increases with the filling speed at a low rotating speed (900 and 1050 rpm). When the rotating speed of WRC increases to 1200 rpm, the pressure ratio of precompression first increases with filling speed and then rapidly decreases. When the rotating speed is further increased to 1500 rpm, the effect of filling speed on precompression is completely irregular. Theoretically, a higher air filling speed leads to a higher kinetic energy converted to static pressure. The pressure ratio of precompression should be higher, as well exhibited at a low rotating speed. Concerning the irregularity at a high rotating speed, it should be associated with the corresponding combustion characteristics. Figure 11 also shows that the pressure ratio of precompression decreases substantially with the equivalence ratio increasing, except the rare points on the boundary limit of a stable operating range of

WRC with rotating speed of no more than 1200 rpm. This fact shows that the effect of precompression is associated with the combustion characteristics when WRC operates. On one hand, different combustion levels lead to different temperature distributions in channel of WRC. Therefore, the propagation speed of precompression wave is affected in the next new cycle, and furthermore the cold-state pressure rise effect is affected. On the other hand, different combustion levels lead to different strengths of pressure wave produced by combustion, and the pressure wave produced by combustion would enter the intake section under improper organization of the time sequence of WRC. Thus, the state of filling air flow would change, and this change would directly affect the precompression of the next cycle in turn. The irregular phenomenon related to the coldstate pressure ratio on the boundary limit of the stable operating range of WRC is also caused by the corresponding combustion characteristics. With the rotating speed of wave rotor increasing, the cold-state pressure ratio gradually increases. For instance, at 900 rpm, the maximum of δp is 24%, while at 1500 rpm, it can reach a peak value of close to 90%, and the minimum is more than 30%. In fact, this is not the only effect of the rotating speed of WRC. At a high rotating speed, the range of filling speed for a stable operation of WRC is broad and mainly located in the region of a relatively high speed. However, a higher filling speed results in a stronger precompression induced by air flow stagnation, resulting in a higher cold-state pressure ratio. The rotating speed of WRC can be regarded as indirectly affecting the pressure rise effect of H

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Figure 12. Variation of pressure rise in channel of WRC.

cold-state precompression by affecting the air filling speed range for a stable operation of WRC. This mechanism also leads to a relatively concentrated distribution of cold-state pressure ratio at a low rotating speed while a larger variation range of cold-state pressure ratio at a relatively high rotating speed. For instance, at 900 and 1050 rpm, the variation of coldstate pressure ratio is no more than 20%, while at 1200 and 1500 rpm, the difference between the maximum and minimum of this pressure ratio is almost 60%. Figure 12 shows the variation of peak pressures produced by combustion in channel of WRC with various filling speeds and different equivalence ratios. Among the three pressures monitoring points along the channel of WRC, the peak pressure produced by combustion at the second monitoring point is the highest, while the remaining two monitoring points are more affected by jet impulse and end surface leakage. Therefore, the middle monitoring point as the representative is used to analyze the response trend of the pressure rise effect of WRC. To decrease the difference between cycles, the values are the corresponding average values of all the cycles within 0.4 ms. At the rotating speed of 900 rpm, the pressure rise of combustion decreases with the filling speed increasing. At the rotating speed of 1050 rpm, the pressure rise is the highest when the filling speed is 12.0 m/s and the least when the filling speed is 14.6 m/s. At the rotating speed of 1200 rpm, the pressure rise is small when the filling speed is 23.7 m/s but at a similar level for other filling speeds. At the rotating speed of 1500 rpm, the pressure rise is the highest when the filling speed is 17.1 m/s. A change in the filling speed leads to a variation in

time sequence of WRC, and its effect on peak pressure of combustion is more intense than that on precompression. The reason is that a pressure wave propagates at a local velocity of sound directly proportional to the square root of temperature. If the temperature of hot gas in the channel of WRC after the combustion temperature is 1500 K, the propagation speed of pressure wave produced by combustion would be at least twice that of the precompression wave speed. Therefore, it is more sensitive to the change in time sequence. The equivalence ratio of air-fuel mixture also significantly affects the peak pressure produced by combustion. In total, except the operating condition of rotating speed of 1500 rpm and filling speed of 22.5 m/s, the pressure ratio of combustion increases with the increasing of equivalence ratio, in contrast to the trend of pressure rise of precompression. The reason is that the fuel per unit volume released more energy as the equivalence ratio increases. Thus, the combustion is more drastic and a higher pressure rise can be achieved. As the equivalence ratio further increases, without changing the filling speed, the fuel jet path and distribution can change and deviate from a proper distribution of air-fuel mixture, decreasing the forming quality of air-fuel mixture in channel of WRC and thus decreasing the pressure rise of WRC. As the rotating speed varies, at a rotating speed of 1050 rpm, the peak pressure produced by combustion is only 0.07 MPa, which is even lower than that at 900 rpm, indicating that the time sequence of 1050 rpm is disadvantageous of organizing combustion for WRC. Nevertheless, overall, as the rotating speed of WRC increases, the pressure rise of combustion also increases. The reason is that a high I

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Figure 13. Fluctuation index σP of peak pressure in channel of WRC with different rotating speed, different filling speed, and different equivalence ratio.

pressures of all the cycles in the measured duration, and σP is the deviation of the peak pressure of combustion in channel of WRC. The pressure rise characteristics are closer to each other among the WRC cycles as the value of σP is smaller. However, the differences among the WRC cycles are larger and the repeatability becomes worse at a higher value. Therefore, the value of σP indicates the fluctuation of peak pressure rise of combustion in channel of WRC. Figure 13 shows that the fluctuation index σP of peak pressure in channel of WRC with different rotating speed, different filling speed, and different equivalence ratio. At a low rotating speed, the fluctuation index is low with a maximum of 2%, which indicates a small difference among the peak pressures of combustion cycles and relative stability of peak pressure in the channel of WRC. When the rotating speed of WRC is increased to 1200 or 1500 rpm, the fluctuation index increased substantially with the maximum of more than 20%. The reason is that the air filling is more susceptible to the counter pressure in the channel of WRC at a high rotating speed. The operating model time sequence shown in Figure 2 shows that the discharging duration tout = tout1 + tout2 was achieved. Within tout = t1 − t2, the inlet port does not open. Therefore, the high pressure burned gas in channel of WRC expands freely through the outlet port. However, within tout = t4 − t2, the inlet port opens and both the filling and discharging processes occur simultaneously. If the free expansion of the high pressure burned gas in channel of WRC is insufficient

rotating speed corresponds to a broad range of filling speed for a stable operation, and the speed is also high. A higher filling speed shows more air-fuel mixture per unit time filled in channel of WRC and a higher precompression pressure ratio is achieved at a higher rotating speed. For a fixed volume in channel of WRC, more air-fuel mixture participates in burning and more energy is released in combustion, thus producing a higher pressure rise. This is also an indirect effect of the rotating speed of WRC on pressure rise by affecting the filling speed for a stable operation range of WRC. On the other hand, as the rotating speed of WRC increased, the duration for a complete cycle of WRC became shorter and the duration for the performance loss factors such as leakage is also shorter. Therefore, it is beneficial to maintain a higher pressure rise of WRC. For the effect of the pressure rise produced by combustion on air filling, the working process of the next cycle would be affected to some extent, which would lead to differences among the cycles of WRC and fluctuation of the peak pressure. The differences among the cycles of WRC are presented by defining the index of fluctuation of peak pressure among the cycles of WRC as follows: n

σP =

1 (∑ (Pi − P ̅ ))2 N i=1

(1)

where N is the number of cycles in the measured duration, Pi is the peak pressure of each cycle, P̅ is the average of peak J

DOI: 10.1021/acs.energyfuels.7b01769 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels within time tout1, a counter pressure would occur in the filling process and the extent of this effect is related to the internal pressure in the channel of WRC. tout1 is inversely proportional to the rotating speed of WRC. The rotating speed is smaller, tout1 is higher, and the time of the high pressure burned gas to expand freely is longer. The expansion is more sufficient and the effect on the filling process is smaller. Thus, at a low rotating speed, the unevenness of peak pressure in channel of WRC is small, which means the peak pressure is steady. As the equivalence ratio of the filling air increases, the fluctuation index of peak pressure in channel of WRC substantially increases. The reason is that a high equivalence ratio results in a high pressure after the combustion of air-fuel mixture in channel of WRC. Thus, the extent of free expansion is insufficient and the counter pressure effect on the filling air-fuel mixture is significant within the same period tout1. At the rotating speed of 900 rpm, when the filling speed of combustible air-fuel mixture is 6.7 m/s, the fluctuation index of peak pressure is relatively small, which indicates the operating process is a relative stability. Whereas the fluctuation index is relatively high for Vin = 9.2 m/s and Vin = 12.1 m/s. At a rotating speed of 1050 rpm, when the filling speed of combustible air-fuel mixture is 12.0 m/s, the fluctuation index is the highest, showing a relative instability. At the rotating speed of 1200 rpm, the fluctuation indexes are almost similar. At the rotating speed of 1500 rpm, the fluctuation index of peak pressure and the corresponding peak pressure rise shown in Figure 12 are the same trend. At the same rotating speed and filling speed, as the equivalence ratio of air-fuel mixture increases, the fluctuation index of peak pressure also increases which indicates the intensification of unsteady working cycle of WRC. The reason is mainly the change in the jet path and distribution of fuel during the formation of the air-fuel mixture and reverse propagation of pressure in the channel of WRC.

for isolating, which would result in ignition failure. State b has a moderate filling speed, and a proper combustible air-fuel mixture fills the ignition zone. Thus, the reliability of ignition is guaranteed, and it helps to create efficient combustion in the channel of WRC. In state c, the filling speed is too high and a part of the air-fuel mixture enters the outlet port, which would cause a waste of fuel and leads to an early ignition that means the self-ignition of the air-fuel mixture in the channel would cause an unstable operation of WRC. The state of air-fuel mixture distribution is best for the ignition and combustion of WRC, shown in Figure 14b. The Vin is the filling speed of WRC, and the theoretical speed to form state (b) is Vi = Lw/tfuel = 3LwRrs/60000, where Lw is the channel length of WRC and Rrs is the rotating speed of WRC. Figure 15 shows the ratio of the minimum filling speed of the air-fuel mixture for a stable operation of WRC to Vi changes as

Figure 15. Range of filling speed for the reliable operation of WRC.

the rotating speed of wave rotor varies. The graph shows that at a relatively high rotating speed, the inlet boundary range for stable operation of WRC is relatively wide, corresponding to a high filling speed of inlet that can reach a Mach of ∼0.1 at the most. Under the experimental operating conditions, when the rotating speed of WRC is 1200 rpm, the operating range of WRC is the widest. The reason is that the duration of filling is short for WRC with a relatively high rotating speed. If the filling speed is less than normal, an insufficient filling of air-fuel mixture occurs as shown in Figure 14a. The fuel in the ignition zone is lean or even no fuel is distributed in the zone. Thus, the WRC cannot work normally. The graph also shows that at a relatively low rotating speed (Rrs = 900 rpm, 1050 rpm), Min(Vin)/Vi < 1, but at a relatively high rotating speed (Rrs = 1200 rpm, 1500 rpm), Min(Vin)/Vi > 1. This indicates that at a relatively high rotating speed, the requirement for the filling speed of the air-fuel mixture in the ignition of WRC is stricter. Owing to the effect of flow state in channel of WRC on the filling, when the actual filling speed of the air-fuel mixture is higher than the theoretical filling speed, the forming state of the air-fuel mixture required by the ignition can be achieved. However, at a low rotating speed, even though Vin < Vi and the state shown in Figure 14a are formed, the hot jet can still ignite the air-fuel mixture in the channel of WRC owing to the low filling speed of air flow, low pressure ratio of precompression, and deeper penetration of jet flow. Figure 16 shows the equivalence range for a stable operation of WRC at various rotating speeds. At a relatively low rotating speed, the filling speed range for a stable operation of WRC is relatively narrow, but the equivalence ratio range is relatively broad. The reason is that the precompression induced by flow

4. OPERATING RANGES OF WRC The tested operating conditions are considered and discussed, which includes the operating ranges of filling speed, equivalence ratio, and rotating speed of WRC. The filling speed of air-fuel mixture affects the precompression in the filling process. A higher air-fuel mixture speed leads to a stronger compression wave caused by the stagnation of air-fuel mixture due to seal plate, which induces a higher pressure rise to increase the overall pressure ratio of WRC. As shown in Figure 14, when the filling speed is too low (state a), prior to the ignition, a certain proportion of the area between the concentrated region of the air-fuel mixture in the channel of WRC and the hot-jet burned gas from the igniter is injected in the lean mixture or pure air

Figure 14. State of air-fuel mixture forming in channel of WRC. K

DOI: 10.1021/acs.energyfuels.7b01769 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

rotating speed of WRC and filling speed of the air-fuel mixture. When the rotating speed of WRC is more than 1000 rpm, a stable operation is possible only in the lean equivalence ratio range.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86-025-84895927. ORCID

Jianzhong Li: 0000-0001-6797-5740 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China Grant 51476077. Figure 16. Range of equivalence ratio for stable operation of WRC.

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stagnation is relatively weak for WRC with a low filling speed, which means that the pressure ratio of precompression of WRC is small. Therefore, the resistance of hot jet entering the channel of WRC for ignition is small, favoring ignition, and a small filling speed helps in the stable development of a flame without extinguishing it. Besides, corresponding to a low-speed filling, the WRC operates at a low rotating speed. In each channel and each cycle of WRC, the maximum duration time of combustion is large, providing sufficient time for flame propagation. Thus, the WRC can operate stably even at a low propagation speed of flame in a rich fuel state. At a high rotating speed, the WRC can operate stably only in a lean equivalence ratio range and the range for a stable operation is relatively narrow. When the rotating speed decreases, the operating range can be extended from a lean equivalence ratio to a rich equivalence ratio.

5. CONCLUSIONS On the basis of the principles of relative motion, a simplified test system for WRC was established and the pressure rise characteristics of WRC were experimentally studied. The effects of variables such as the filling speed of air-fuel mixture, rotating speed of inlet/outlet port, and equivalence ratio of air-fuel mixture on pressure rise characteristics were evaluated. The stable operating ranges of WRC were statistically analyzed. With the rotating speed increases, the peak pressure of combustion significantly increases. The maximum pressure rise reached 6.3 atm when the rotating speed is 1500 rpm and the fluctuation range of peak pressure also increases. Besides, owing to the leakage issue around the ports, the peak pressure is the highest at the middle of the channel of WRC. When the rotating speed of inlet/outlet port varies, the filling speed of the air-fuel mixture and optimal operating ranges of equivalence ratio of air-fuel mixture vary accordingly. As the rotating speed of inlet/outlet port increases, the corresponding filling speed of air-fuel mixture increases and the operating range of equivalence ratio of air-fuel mixture becomes narrower. The range of filling speed for a stable operation of WRC can reach a Mach of 0.1. Except at 1500 rpm, as the rotating speed of wave rotor increases, the range of filling speed for a stable operation increases and the widest operating range is obtained at a rotating speed of 1200 rpm. The range of equivalence ratio for a stable operation of WRC decreases with increasing the L

DOI: 10.1021/acs.energyfuels.7b01769 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b01769 Energy Fuels XXXX, XXX, XXX−XXX