Research on the application of aviation kerosene in a direct injection rotary engine – Part 2: Spray combustion characteristics and combustion process under optimized injection strategies

Energy Conversion and Management 203 (2020) 112217

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Research on the application of aviation kerosene in a direct injection rotary engine – Part 2: Spray combustion characteristics and combustion process under optimized injection strategies

T

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Yao Lu, Jianfeng Pan , Baowei Fan, Peter Otchere, Wei Chen, Biao Cheng School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013 Jiangsu, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Aviation kerosene Rotary engine Spray combustion characteristic Injection strategy Experiment and simulation

Investigating the adaptability of aviation kerosene in the direct injection rotary engine is of great significance and practical value. In this paper, an optically visible constant volume combustion chamber platform with various optical diagnostic techniques was firstly set up to study the spray combustion characteristics of the fuel. Secondly, a reliable three-dimensional dynamic simulation model of direct injection rotary engine was established to investigate the combustion process of aviation kerosene under optimized injection strategies. The experimental results show that the ambient temperature of 800 K and ambient pressure of 20 bar is the ignition limit of aviation kerosene under the condition of rotary engine. With the increase of ambient conditions, the ignition delay and the flame lift-off length shorten continuously. At 850 K ambient temperature, when the ambient pressure reaches 25 bar, the lift-off length will no longer shorten and remains at about 6 mm. The simulation results indicate that under the Injection position A-Injection angle 90° (Case A3), higher combustion efficiency before top dead center can be achieved due to more appropriate distribution, concentration and atomization quality of fuel at ignition timing. The peak pressure in cylinder reaches 38.89 bar, which means a better engine dynamic performance. Meanwhile, acceptable CO and Soot emissions are obtained by the complete combustion of fuel in Case A3, however, the higher combustion temperature makes the production of pollutant NO slightly higher than other injection strategies.

1. Introduction The research on application of aviation kerosene in traditional engines is in full swing [1], and as a rotary-type internal combustion engine, the Wankel rotary engine possesses better dynamic performance than reciprocating engines [2]. Therefore, it is imaginative and creative to apply aviation kerosene in rotary engine and study its operation characteristics. This paper is the second part of the research on adaptability of aviation kerosene in rotary engine. The spray characteristics of aviation kerosene under rotary engine operation conditions and the fuel-air mixing process under different injection strategies have been studied and discussed in the first part of the research [3]. In addition, the advantages of aviation kerosene and rotary engine along with the necessity of incorporating them together are all introduced in Part 1. This paper mainly discusses the spray combustion characteristics of aviation kerosene under rotary engine operation conditions and the fuel combustion process and engine performance under different injection strategies. After the completion of this part, the research of

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aviation kerosene in rotary engine will be more systematic. With these two papers, the spray characteristics and spray combustion characteristics of aviation kerosene under inert and reacting conditions as well as the fuel-air mixing process, the ignition process and the combustion process of the fuel in the chamber under the optimized injection strategy will be presented. As the motivation of this research, the necessity of applying aviation kerosene in rotary engines is mainly induced in the following aspects. Firstly, with the development of single fuel concept (SFC), aviation kerosene has been more and more applied in traditional reciprocating engines [4]. For the last decades, approximately 65% of military vehicles were fueled with diesel oil, while military aircrafts, including helicopters were fueled with aviation kerosene [5]. In the other aspect, about 38.6% of troops were equipped with bulk fuel [6], which is quite inconvenient for the coordination of fuel between armaments and requires more cost in logistical support. In order to solve this problem, North Atlantic Treaty Organization (NATO) proposed the idea of unifying the fuel in the battlefield, specifically using aviation kerosene

Corresponding author at: School of Energy and Power Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China. E-mail address: [email protected] (J. Pan).

https://doi.org/10.1016/j.enconman.2019.112217 Received 14 August 2019; Received in revised form 22 October 2019; Accepted 23 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature DI DI-RE SFC NATO TDC BTDC ATDC LOL

CCD CVCC RNG DPM KH-RT EDC JP-8 UAV ECU ICCD

Direct injection Direct injection rotary engine Single fuel concept North Atlantic Treaty Organization Top dead center Before top dead center After top dead center Lift-off length

Charge coupled device Constant volume combustion chamber Re-Normalisation Group Discrete phase model Kelvin-Helmholtz and Rayleigh-Taylor Eddy-dissipation concept Jet petrol-8 Unmanned aerial vehicle Electronic control unit Intensified CCD

VRDE [17] manufactured a UAV based on the rotary engine. The test results show that the UAV can meet the airworthiness certification both in output power and fuel economy. In addition, the rotary engine is also directly used as an aircraft engine, such as 2034R aircraft engine produced by John Deere Technologies International [18]. Previously, most of the aviation internal combustion engines were fueled with gasoline [19]. However, recently aviation kerosene is increasingly replacing the gasoline fuel in the aviation internal combustion engines due to its better stability and low temperature combustion ability [20]. At present, the application of aviation kerosene in reciprocating piston engines has been carried out one step ahead. For example, Pan et al. [21] carried out periodic fatigue analysis of the oil pan of a piston aviation engine, and the research findings provide dependable guidance for the design of piston aviation engines. Wang et al. [22] explored the suppression effect of knocking by the method of water injection in an aviation kerosene fueled piston aero engine, and the results of the study also provides a method which will effectively reduce the knock of spark ignition piston engines fuelled with aviation kerosene. As stated above, it can be seen that the research on the application of aviation kerosene in reciprocating piston engines has reached a further stage, but the exploration of the application of aviation kerosene in rotary engines has not yet been carried out, so it is particularly important and necessary to take the first step in this field. For both rotary type and reciprocating piston type engines, the fuel injection strategy is very important, as it will affect the performance of the engine, and is worth to be taken into consideration at the beginning of engine design [23]. The research of injection strategy in different kinds of reciprocating engines has been widely carried out. Lee et al. [24] investigated the effects of injection timing and injection angle on the flow field and the wall attachment of liquid fuel films in a port dual injection engine. The results showed that compared with the routine injection method, the amount of fuel film decreases by 73.04% when the engine runs at the best operation condition designed in their study. Gong et al. [25] studied how the strategy of secondary injection and intake valve open timing affect cylinder knocking and engine performance. Their research is meaningful in terms of figuring out the influence of injection strategy on the occurrence frequency of super-knock and engine performance under low-speed high-load operating condition in gasoline direct injection engines. In terms of rotary engine, the injection strategy is more critical due to the significant flow field in cylinder. Fan et al. [26] numerically studied the influence of hydrogen injection strategy on mixture formation and combustion process in the natural gas/hydrogen dual-fuel rotary engine which promotes the development of natural gas fueled rotary engine. Chen et al. [27] carried out research on the effects of diesel injection strategy on engine performance and pollutant emissions in a direct injection (DI) diesel rotary engine. Therefore, besides exploring the spray and spray combustion characteristics of aviation kerosene under the operation condition of rotary engine, it is also necessary to find an appropriate injection strategy by which the engine can obtain optimized combustion process. From the research background summarized above, it can be concluded that the application of aviation kerosene in the rotary engine is meaningful. However, there are few existing relevant research results

instead of diesel oil simultaneously in the piston engines and aero engines [7]. Based on this concept, considerable related studies have been carried out. Lee [8] et al. systematically studied the spray, combustion and emission characteristics of Jet petrol-8 (JP-8) aviation kerosene in a high-power diesel engine. The research results showed that under the same operating conditions, aviation kerosene has shorter spray penetration length and larger spray cone angle than diesel fuel. In the combustion stage, the faster fuel-air mixing rate enables aviation kerosene to achieve higher combustion efficiency. Chen [9] et al. studied the heat release rate, thermal efficiency, ignition delay and cyclic variation of RP-3 aviation kerosene in a piston aero engine. It is found in their research that compared with diesel, the engine efficiency is higher when it is fueled with RP-3 aviation kerosene. So it can be concluded that the application of aviation kerosene in traditional land engines has good prospects because of its excellent combustion performance. Meanwhile, the rotary engine is widely used in the battlefield due to its powerful dynamic performance. It is applied as the power source of various types of military Unmanned aerial vehicle (UAV) [10], transport vehicle [11] and naval ships [12]. Therefore, to comply with the development of SFC, it is imperative to study the application of aviation kerosene in the rotary engine. Besides in the military field, the rotary engines are generally employed as power generation equipment on commercial basis due to its lightweight structure and high power to weight ratio. In the automotive field, AVL List GmbH [13] designed an electric vehicle extender based on a rotary engine. The research shows that the system has the advantages of light weight, compact structure and low noise. In addition, as the power source of the generator, the rotary engine can work in several fixed optimal operation conditions, so the fuel economy and pollutant emission are also guaranteed. Scoot et al. [14] used Advanced Vehicle Simulator to compare the fuel economy and greenhouse gases emissions between electric vehicles based on rotary engine extender and piston engine extender. The results show that, whether fuel consumption rate or pollutant emissions, the electric vehicle equipped with rotary engine performs better, which means the rotary engine has more advantages than the traditional internal combustion engine in the case of power generation. In the civil field, the rotary engine is also used as the power source of small portable power generation devices. Richard et al. [15] designed a small power generation system based on the natural-gas fueled rotary engine. The system not only has the advantages of lightness, portability and power generation stability, but also has low pollutant emission level, which has wide commercial prospects. From the research stated above, it can be concluded that the rotary engine has unique advantages as the power source of the power generation device. If it is used as the power generation device on the aircraft, it will play a role in reducing the body mass and the volume of the generator set. Therefore, it is promising to study the application of rotary engines in aircraft using aviation kerosene as fuel. Thirdly, the information available suggests that not only reciprocating engines but also rotary engines have been used as aero engines or UAV engines directly. Hooper [16] designed a UAV based on spv580 stepped piston engine, which has an output power of 30 KW35.4 KW at operation speed of 5000 rpm. As for the rotary engine, 2

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electric heater strip are operated on the control platform. The injection pressure of the nozzle and the injection pulse width of the injector are controlled by the control computer through the ECU (Electronic control unit). In the experiment, the method of obtaining the spray combustion flame form is the natural flame luminescence imaging. The natural light radiated from the flame is captured by the high-speed digital camera to obtain the flame form and the ignition delay. The model of the highspeed camera used in this research is Photron SA-Z, The speed of frame rate is 60,000/s, the size of the image is 896 * 368, and the exposure time of the machine is 8.4 μs. In the spray combustion process, the combustion intermediate product OH radical is often used to mark liftoff length (LOL) of the flame, and at the same time it can reflect the exact range of the diffusion combustion. In this paper, OH-chemical fluorescence is used to catch the distribution of OH radicals during the combustion process of fuel [28]. Specifically, the OH radical is captured by distributed ICCD (Intensified CCD) system, which includes the filter, the signal intensifier and charge coupled device (CCD) camera. The installation sequence and function of each part are as follows: Firstly, the 310 nm filter is installed at the front of the whole system to ensure that the chemical fluorescence of OH radical is captured. Secondly, since the fluorescence signal is very weak, the signal intensifier is installed in front of the CCD camera so as to strengthen the signal. Lastly, the CCD camera used for the experiment is HiSense MkII camera, which has a resolution of 1344 * 1024 and a minimum exposure time of 200 ns. The high-speed camera and ICCD system take the flame through two adjacent windows and is arranged 90° with each other to ensure no interference exist. The fuel used in this research is Chinese standard RP3 aviation kerosene, which has been introduced in Part 1 in detail [3]. Some of the important fuel properties are shown in Table 1.

that can be found in this direction. So the adaptability of aviation kerosene as the fuel in the rotary engine is urgently worth investigating. 2. Objectives The Part 1 of this research focused on the basic spray characteristics of aviation kerosene under rotary engine operation conditions and the fuel-air mixing process in the combustion chamber under 10 different injection strategies. In the experimental work, the influence of ambient temperature and pressure on the penetration length, spray cone angle and atomization quality of the fuel was investigated. In terms of simulation work, the simulation results suggested that the flow field in cylinder and the direction of fuel injection determined the atomization and distribution of aviation kerosene in the engine. Several satisfactory injection strategies with appropriate fuel distribution at the ignition timing were found. As the second part of the research, this paper will continue the unfinished work in Part 1. Firstly, a visible constant volume combustion (CVCC) platform for aviation kerosene spray combustion was set up and the spray combustion characteristics of aviation kerosene were studied under different rotary engine operation conditions. Secondly, the reliable EDC combustion model is coupled in the three-dimensional dynamic simulation model of a direct injection rotary engine (DI-RE), with which the combustion process of aviation kerosene in the combustion chamber under different injection strategies is investigated. The optimal operating condition is obtained by analyzing the engine performance and emissions. With this part of the work, a complete understanding of the adaptability of aviation kerosene in rotary engine can be comprehended. 3. Experimental setup and model generation

3.2. Ambient conditions and samples of experimental results 3.1. Optical diagnostic equipment for spray combustion characteristics The ignition limit of aviation kerosene in rotary engine is the primary problem to be studied. The determination of the ignition limit will provide guidance for the ignition mode of aviation kerosene in different types of rotary engines. In addition, it is also important to study the effects of ambient pressure and ambient temperature on the spray combustion process of aviation kerosene under rotary engine operation conditions [29]. Based on the purposes stated above, a total of 12 operation conditions, including the ambient temperature range of 700–850 K and the ambient pressure range of 20–30 bar, were selected according to the temperature and pressure in the real rotary engine combustion chamber near the top dead center (TDC). The specific

Fig. 1 shows the schematic and photograph of the optical setup for aviation kerosene spray combustion experiment. The spray combustion experiment is also carried out in a CVCC which is same with the experimental work in Part 1. In the experiment, the fuel is supplied by a high pressure common rail system. Different from the previous experiment, air is supplied as the ambient gas to fill up the CVCC since the spray combustion experiment needs to be carried out under reacting condition. The target ambient temperature of rotary engine combustion chamber conditions is obtained by the electric heater strip installed in the CVCC. The switch of the inflatable valve and the opening of the

Fig. 1. Schematic and photograph of optical setup for spray combustion. 3

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as nonslip condition and the temperature of the wall is set to 400 K. 300 K is selected as the temperature of the fuel and air, which is in accordance with the normal atmospheric temperature. In order to ensure that the simulation results can correctly reflect the physicochemical reaction processes such as spray and combustion of aviation kerosene, a diesel-aviation kerosene 48 component and 152 step chemical reaction mechanism simplified by Yang [32] is used for the numerical calculation. In addition, the 10 different fuel injection strategies designed in this study are also shown in Fig. 3. The injection angle is defined as the angle between the fuel injection direction and the horizontal line. The specific value of injection position and the injection angle are given in Table 3. Moreover, the main operating parameters of the engine, including engine speed, fuel injection parameters, ignition parameters and the phase of the intake and exhaust ports are also listed in Table 3.

Table 1 Fuel properties. Properties

Value

Lower heating value (MJ/kg) Cetane number Density (20 C)/[kg·L−1] Kinematic viscosity (mm2/s) Boiling point (°C) Flash point (°C) Total heat release rate (kW)

43.2 38–46 0.81 1.81 176 38 12.775

experimental scheme is shown in Table 2. Similar to the previous experiment, the diameter of the single-hole nozzle used in the experiment is 0.18 mm, the injection pressure is 60 MPa, and the injection pulse is set as 4.5 ms to obtain a complete spray combustion process. In order to ensure the reliability of the experimental data, nine repetitive experiments were carried out under each working condition and the average value was calculated. A sample of the flame form taken by the high-speed camera and the distribution of the combustion intermediate OH radicals captured by the ICCD system are shown in Fig. 2. The key physical quantities of the spray combustion process are illustrated as follows: The ignition delay refers to the time range from the beginning of fuel injection to the beginning of fuel heat release. It is used to characterize the ignition speed of fuel and it is determined not only by the physical and chemical properties of the fuel, but also by the ambient temperature and pressure [30]. As it is shown in Fig. 2 (a) and (b), in the experiment, the timing of the first photograph of natural light radiated by flame is defined as the ignition timing, and the time interval between the ignition timing and the fuel injection timing is the ignition delay. Fig. 2(c) gives the example of aviation kerosene spray combustion flame captured by the high-speed camera. Fig. 2 (d) shows the distribution of combustion intermediate OH radicals taken by the ICCD system, and it should be noticed that the result shown in Fig. 2(d) is the accumulation result of all the OH radicals captured by the ICCD system during the process of spray combustion, rather than the distribution of OH radicals at a given time. So it can describe the ultimate range of the fuel diffusion combustion, rather than the position of the flame forward at a certain time. As it is shown in Fig. 2(d) and (e), the LOL is defined as the distance from the nozzle to the top end of the fuel exothermic region. The final data is obtained by precisely calculating the proportion between the length of the grid in the calibrated photograph and the LOL intercepted in the photograph with the computational process shown in Eq. (1). The Lgrid in the equation refers to the length of the grid in calibration which is designed and known in advance. The Npixel and npixel refer to the number of pixel points of the LOL and the pixel points per grid respectively.

LOL =

3.4. Key computational models selected and the validation To ensure that the simulation results are reliable, the key computational models used in the simulation process need to be rigorously verified by experiments. The main computational models used in this study and related validation work are shown in Table 4. Firstly, the turbulence model, whose main function is to couple the flow field and the fuel, determines the concentration and final distribution of fuel in the combustion chamber. The Re-Normalisation Group (RNG) κ-ε turbulence model is selected in the study. The reliability of the model has been verified by PIV experiment in our previous work [33]. Secondly, for the direct injection engine, the fuel spray process is the first step of the whole research. The reasonable spray model can not only simulate the trajectory of fuel injected into the combustion chamber, but also reflect the breakup and evaporation process of the fuel. The spray model used in this research is the Discrete Phase Model (DPM) spray model and the Kelvin-Helmholtz and Rayleigh-Taylor (KH-RT) breakup model, the dependability has been verified by experiment in Part 1 [3]. Finally, the combustion model, which directly determines the combustion reaction, the in-cylinder pressure and the fuel consumption rate should be validated as well. Although the complex 48 component 152 step chemical reaction mechanism has been used to describe the combustion process of aviation kerosene, the combustion model still needs further verification. The eddy-dissipation concept (EDC) combustion model is adopted in this research. The rationality of this model has been verified in previous works under different operating conditions and different fuels, including diesel [27], gasoline [34] and natural gas [35]. In order to guarantee the rigorousness of research, as shown in Fig. 4, the simulation results in this study are compared with the data in reference [31]. The comparison shows that the simulation results are in good agreement with the literature data. The error of peak pressure is less than 1.5%, and the average error during the combustion process is less than 5.0%. This adequately illustrates that the selected combustion model is reliable. In summary, the reliability of all the computational

Lgrid × Npixel npixel

(1)

Table 2 Experimental operation conditions.

3.3. Rotary engine model generation and operation conditions The DI-RE used for numerical simulation work is AR741 rotary engine equipped in RQ-7 Shadow UAV [31]. The structure of the engine is shown in Fig. 3. In Part 1 [3], the geometric model and mesh generation process of the engine have been introduced in detail, furthermore, the injection, evaporation and mixing process of aviation kerosene in the rotary engine has also been successfully simulated. Therefore, the parameters of the engine and the modeling process are no longer described in this paper. The simulation work is carried out in the Ansys Fluent software. The boundary conditions applied in the simulation work are as follows [27], the boundary of the inlet port and outlet port are respectively defined as pressure inlet and pressure outlet while the value is set as atmospheric pressure. The rotor wall is defined 4

Experimental number

Ambient temperature

Ambient pressure

1 2 3 4 5 6 7 8 9 10 11 12

700 K 700 K 700 K 750 K 750 K 750 K 800 K 800 K 800 K 850 K 850 K 850 K

20 bar 25 bar 30 bar 20 bar 25 bar 30 bar 20 bar 25 bar 30 bar 20 bar 25 bar 30 bar

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Fig. 2. Samples of flame form and OH distribution acquired by experiment.

models has been verified, and it can be concluded that the three-dimensional dynamic simulation model of the rotary engine can be used to study the application and adaptability of aviation kerosene.

Table 3 Injection strategies used in this research. Injection strategies Case

4. Results and discussion 4.1. Spray combustion characteristics

Injection position

Injection angle (°)

Specifications

Value

A1 A2 A3 A4 B1

A A A A B

135 120 90 45 90

4000 0.667 100 20 BTDC 60

Case B2 Case B3 Case C1

B B C

45 30 30

Case C2

C

0

Case C3

C

−30

Engine speed (rpm) Equivalent ratio Throttle opening (%) Ignition timing (°CA) Injection pressure (MPa) Injection timing(° CA) Injection duration (s) Intake port duration (°CA) Exhaust port duration (°CA) Exhaust-Intake Overlap (°CA)

Case Case Case Case Case

4.1.1. Ignition limit and flame development The ignition limit and the flame development process are important characteristics during the spray combustion of the fuel, both of which can be observed in the spray combustion process caught by the high speed camera. The spray combustion process of aviation kerosene under different ambient temperature and pressure is illustrated in Fig. 5. It should be noted that under the ambient temperature of 700 K (experimental number 1–3), the fuel did not reach the ignition condition, and the camera only took the black background like Fig. 2(a), so under these conditions the photographs are not listed in the figure. It can be seen from Fig. 5 that when the ambient temperature is 750 K and the pressure is 20 bar, the camera did not capture the natural light radiated by the flame, which indicates that the fuel still failed to be ignited. This means that the ambient condition does not reach the ignition limit of aviation kerosene. When the ambient temperature was kept at 750 K and the ambient pressure increased to 25 bar, the camera caught weak natural light radiated from the cool-flame 2400 μs after the injection started which is similar to the diesel fuel [7]. This phenomenon

Engine operation conditions

80 BTDC 0.001 404 396 128

illustrates that the aviation kerosene reaches the ignition limit but the combustion is unstable. The reaction process is mainly low temperature combustion, accompanied with weak light radiation and the ignition delay is quite long. The flame does not therefore possess the complete ignition and diffusion process. When the ambient temperature rose up to 800 K, the ignition and

Fig. 3. Structural diagram of the AR741 rotary engine. 5

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Table 4 Computational models applied in the simulation work. Model

Name

Error and literature

Turbulence model Spray model Combustion model

RNG κ-ε model Discrete phase model and KH-RT droplet breakup model EDC combustion model

Less than 5%/[33] Less than 7.51%/[3] Less than 10%/[27,34,35]

Table 5 Ignition limit of aviation kerosene under rotary engine operation conditions.

20 bar 25 bar 30 bar

700 K

750 K

800 K

850 K

● ● ●

● ◎ ◎

○ ○ ○

○ ○ ○

● Ignition failure, ◎Unstable combustion, ○Normally combustion.

environmental density increase, thus the spray penetration length is reduced and the spray cone angle increases, which leads to the improvement of the spray quality of the fuel, and achieving the effect of promoting combustion as well. At the same time, it can be seen from the figure that the sensitivity of spray combustion to ambient temperature is higher than that to ambient pressure. Thus, compared with ambient pressure, increasing ambient temperature can more effectively improve the ignition and spray combustion quality of fuel. The ignition limit of aviation kerosene under these operation conditions can therefore be obtained from experiments. As shown in Table 5, the stable ignition conditions of the fuel require both ambient temperature of 800 K and ambient pressure of 20 bar upwards. The determination of ignition limit is of great significance to the application of aviation kerosene in DI-RE. Because of the generally low compression ratio determined by the engine structure, for most conventional sized rotary engines, the ambient conditions in the compression stage cannot reach so high. In addition, when the engine encounters operation conditions such as cold start stage, the ambient conditions cannot rise up to the ignition limit as well. Under such conditions, the spark plug assisted ignition mode should be applied to ensure the normal combustion of fuel in the rotary engine.

Fig. 4. Comparison of the simulation result and literature data.

4.1.2. Ignition delay and flame lift-off length The ignition delay and flame lift-off length can reflect the quality of the combustion and determine the production of Soot emission [36]. The definition and the method of measurement have been introduced in Fig. 2. Fig. 6 shows the ignition delay and LOL under different ambient conditions. Since the aviation kerosene cannot be fully ignited and the combustion process is inadequate under ambient temperature 750 K, the ignition delay and LOL under this condition is not listed. In general, the ignition delay of aviation kerosene is longer compared with diesel since the cetane number is lower [7]. It can be seen from the figure that

Fig. 5. Spray combustion process of aviation kerosene under different ambient conditions.

diffusion combustion process of fuel can be clearly observed from the figure, which means the increase of ambient temperature and pressure can promote the ignition and diffusion combustion of aviation kerosene. With the increase of ambient parameters, the ignition delay of the fuel shortened, and the combustion duration of the fuel extended. The phenomena stated above can be explained as follows: when the ambient pressure remains unchanged, the effect of increasing temperature on the spray combustion of aviation kerosene is that the fuel can gain more energy from the environmental medium during the spray development process, thus the ignition limit can be reached more quickly. On the other hand, the high temperature promotes the combustion reaction which also leads to longer combustion duration [36]. When the ambient temperature remains unchanged, higher ambient pressure makes the

Fig. 6. Ignition delay and lift-off length under different ambient conditions. 6

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temperature, and the relationship between LOL and ambient pressure loses linearity. This is because under the same injection parameters, different conditions share the same mass flow rate of aviation kerosene. When the ambient temperature reaches 850 K, the air entrainment rate and combustion speed of aviation kerosene reach the upper limit and tend to be the same, so the location of ignition stays at 6 mm away from the nozzle. Fig. 7 shows the form of spray combustion flame at a fixed time and the distribution of OH radical in the whole combustion process under various ambient conditions. The changes of LOL discussed above are more clearly presented in this figure. Meanwhile, it can be seen that with the increase of ambient temperature and pressure, the OH radicals distribute in a longer vertical range, which means the flame length increases during the spray combustion process. This is because the fuel is easier to be ignited and the flame propagation speed is strengthened in more ideal ambient conditions. Also the flame diffuses to a further range and the quality of combustion is improved.

when the ambient pressure remains unchanged, and the ambient temperature rises from 800 K to 850 K, the ignition delay of aviation kerosene is obviously shortened. This is because the higher ambient temperature strengthens the heat exchange between the fuel and environmental medium, that makes the molecular motion of fuel become more intense and eventually the reaction of chemical process is improved [36]. The atomized fuel can absorb enough energy to reach the ignition limit in a shorter time. At the same time, keeping the ambient temperature unchanged and increasing the ambient pressure has a similar effect on the ignition delay. When the ambient temperature is constant, the higher ambient pressure will shorten the ignition delay. The reasons for this phenomenon are as follows: From the conclusion of Part1, it is known that when the ambient pressure is increased, the spray cone angle becomes larger, the fuel entrains and mixes with air more quickly, and the quality of the spray is improved, which makes the fuel easier to be ignited. So it can be concluded that for aviation kerosene applied in rotary engines, the higher ambient conditions are more favorable to the ignition process of fuel. The LOL refers to the distance between the nozzle and the top end of the flame. After injection from the nozzle, the fuel absorbs a large amount of air and is ignited in the LOL area. Therefore, the LOL not only represents the air-fuel ratio of premixed combustion, but also has an important influence on the formation of Soot. As it is shown in Fig. 6, the LOL becomes shorter when the ambient pressure remains unchanged and the ambient temperature increases. The cause of this phenomenon is similar with that of ignition delay, it is because the spray can obtain more energy from the environment when the temperature is higher, which not only shortens the ignition delay of fuel, but also makes the location of ignition advance. When the ambient temperature is kept unchanged, under the condition of 800 K, increasing the ambient pressure can continuously shorten the LOL. This is because the increase of ambient pressure leads to higher environmental density, which results in the fuel spray absorbing more air in the same area. Thus the efficiency of fuel-air mixing becomes better and is more conducive to spray combustion. Like diesel, the sensitivity of lift-off length to a change in either temperature is non-linear [37], and under the ambient temperature of 850 K, when ambient pressure reaches 25 bar and 30 bar, the LOL is 6.12 mm and 6.87 mm respectively. It can be seen that the ambient pressure no more affects the LOL under this

4.2. Combustion process in the engine under different injection strategies 4.2.1. Engine performance and fuel consumption rate Since the ambient conditions of the rotary engine used in this study near TDC cannot reach up to the compression ignition limit achieved by the experiment, it is necessary to use spark plugs to assist the ignition and ensure the operation of the engine. The peak pressure in cylinder during combustion process can directly reflect the dynamic performance of the engine. Fig. 8 presents the peak pressure in cylinder of the rotary engine acquired by simulation work under different injection strategies. It can be found that the peak in-cylinder pressure of Case A3, Case B2 and Case C3 during the combustion process is 38.89 MPa, 33.23 MPa and 34.34 MPa respectively, while the peak pressure under other injection strategies is less than 32 MPa. Compared with other conditions, these three injection strategies mentioned above can achieve higher peak pressure, while the peak pressure in Case A3 is significantly higher than that under the other two conditions. The reasons for the discrepancy are as follows: taking the fuel distribution at ignition timing in Part 1 into consideration, the fuel distribution in Case A3, Case B2 and Case C3 is more ideal than that in other working conditions. The fuel is mainly distributed in the middle of the combustion chamber under the combined influence of injection position,

Fig. 7. Flame form and OH radical distribution. 7

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between different cases at different crank angles, the growth process of the flame during the flame developing period, the flame propagation speed in fast combustion period and the flame forward at different times can be more intuitively understood [26,27]. Fig. 10 gives the distribution of HO2 radicals at 15°CA, 0°CA and −30°CA BTDC under four different injection strategies. It can be seen from the figure that the distribution range of HO2 radicals in Case A3 is larger than that in other conditions at any crank angle, which indicates that the flame diffusion range under this injection strategy is larger and the combustion speed of fuel is faster, which is consistent with the MFB listed in Section 4.2.1. Specifically, it can be seen from the figure that at 15°CA BTDC which is shortly after ignition, a high-quality ‘8’ shaped flame forward has been formed around the two spark plugs in Case A3. This is due to the fact that distribution and concentration of fuel around the two spark plugs are all appropriate. After ignition, the ignition nucleus is formed around the spark plugs and then develops around, shortly afterwards, the flame forwards meet in the middle of the combustion chamber. By contrast, the concentration of HO2 radicals in Case B2 is lower, and the flame diffusion range formed by the front spark plug is smaller, while the flame diffusion range formed by the latter spark plug is large, but the radical concentration is still low. The reasons for this phenomenon are as follows: as it is shown in Part 1, in Case B2, the concentration of fuel near the front spark plug is high, the excessive equivalence ratio leads to the combustion of fuel unsatisfactorily and the flame propagation speed is slow in this condition. On the contrary, the fuel concentration around the latter spark plug is low, although the low equivalence ratio results in local lean combustion, which leads to the acceleration of combustion rate, the main part of fuel distribution is not around it, so it is difficult to contribute to the increase of the in-cylinder pressure of the engine. In terms of Case C2, the fuel distribution at ignition timing is similar with that in Case B2. The difference is that the fuel concentration near the front spark plug is higher and the quality of fuel atomization is worse, while the fuel concentration near the latter spark plug is moderate. This leads to the flame ignited by the latter spark plug developing well, but the flame ignited by the front spark plug is of poor quality. Compared with Case C2, the quality of fuel atomization near the front spark plug in Case C3 is better, so the flame propagation speed and the concentration of HO2 radicals formed by the front spark plug is higher than that in Case C2, thus, the peak in-cylinder pressure reached by Case C3 is higher as well. The quality of flame in the flame developing period has a direct impact on the diffusion speed and range of flame in the fast combustion period. This explains why the range of flame forward in Case A3 is broader than other injection strategies at 0°CA and −30°CA BTDC. It also proves that the engine can achieve higher combustion efficiency under this injection strategy. As the combustion process continues, the volume and shape of the

Fig. 8. Peak pressure in cylinder under different injection strategies.

injection angle and in-cylinder flow field. The appropriate concentration and distribution of fuel-air mixture cooperating with the spark plugs are more conducive to the ignition process [38]. The fuel is ignited and then diffused rapidly in the combustion chamber, which makes the combustion and heat release of the fuel near TDC more sufficient, so that a greater peak pressure can be obtained. The combustion process of aviation kerosene in the chamber under different injection strategies will be analyzed specifically in the following part. Among the 10 injection strategies designed, some injection strategies, such as Case A1 (injection position A-injection angle 135°), Case B1 (injection position B-Injection angle 90°), Case A2 (injection position A-injection angle 120°), are claimed to be disadvantageous in Part 1 due to the inappropriate fuel distribution. The results obtained in this paper also prove that the engine performance in these cases is unsatisfactory. Therefore, no further analysis of such conditions is needed. Selectively, the combustion process in Case A3 (injection position A-injection angle 90°), Case B2 (injection position B-injection angle 45°), Case C2 (injection position C-injection angle 0°) and Case C3 (injection position A-injection angle −30°) that possess more reasonable fuel distribution and better dynamic performance are listed and analyzed in this paper. Fig. 9 shows the mass fraction of the fuel burned (MFB) at TDC and 50°CA ATDC which is acquired by the simulation work. From the figure, it can be seen that the trend of MFB is similar with that of the peak pressure in cylinder, and higher peak pressure can be obtained under the condition of faster fuel consumption rate. At TDC, nearly 50% of the fuel has been consumed in Case A3, while the fuel consumption rate under other conditions is about 20%-30%. The non-negligible disparity between the cases indicates that the fuel-air mixture in Case A3 is easily ignited after the spark, and the flame diffuses rapidly. Under this injection strategy, the flame developing period of the engine is shortened, and the fuel in cylinder enters into a fast combustion period ahead of schedule. The MFB before TDC has a direct influence on the peak pressure in the cylinder during the combustion process [39], which is because the volume of the rotary engine combustion chamber near TDC reaches the minimum. Under the combined effect of volume change, the intense combustion and heat release of fuel in this stage can make the in-cylinder pressure rise to a higher peak value. Since the combustion of fuel concentrates at this stage, it can be explained why the peak pressure in Case A3 is higher than that of other injection conditions.

4.2.2. Flame propagation and fuel consumption process The intermediate products of the combustion of fuel, such as HO2, are of great significance for the diagnosis of combustion process in the engine. By analyzing the concentration and distribution of HO2 radicals

Fig. 9. Mass fraction of fuel burnt at TDC and 50°CA ATDC. 8

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Fig. 10. Comparison of HO2 radical distributions at 15, 0 and −30° CA BTDC under different injection strategies.

4.2.3. Peak temperature in cylinder and emissions The pollutant emissions of the engine are closely related to the incylinder temperature during the combustion process. Figs. 12 and 13 respectively show the peak combustion temperature of the fuel and the mass fraction of main pollutants at 100°CA ATDC under four different injection strategies. The peak temperature means the maximum temperature that the in-cylinder medium can reach during the whole combustion process and is monitored by the software. Compared to the diesel fuel, aviation kerosene can achieve higher in-cylinder peak temperature and lower Soot emissions due to its higher heat value as well as H2 mass fraction [27]. It can be seen from Fig. 12 that the variation of peak temperature between different injection strategies is consistent with peak pressure. The peak temperature in Case A3 is higher, and the distribution range of high temperature region at TDC is also wider. The temperature in combustion chamber is mainly affected by the chemical reaction of fuel combustion. Interactively, the rate of chemical reaction is also promoted by the high temperature in cylinder [38]. Higher ignition quality in Case A3 leads to a faster initial combustion reaction rate and causes the chain effect between in-cylinder temperature and reaction rate, which finally results in higher in-cylinder peak temperature. Fig. 13 shows the mass fractions of CO, CO2, NO and Soot in the combustion chamber at 100°CA ATDC under different injection strategies. It can be seen from the figure that the mass fraction of CO and

rotary engine combustion chamber keeps changing as well. Therefore, the residual fuel moves continuously under the effect of the rotor wall and the flow field in the cylinder. Analyzing the distribution of fuel and oxygen is meaningful to understand the combustion process of fuel and to explain the variation of the in-cylinder pressure. Fig. 11 shows the distribution of fuel and oxygen in combustion chamber under different injection strategies at TDC and 30°CA ATDC. It can be seen from the figure that at TDC, the unburned aviation kerosene in Case A3 and Case B2 mainly distributes in the rear part of the chamber, while the unburned aviation kerosene in Case C2 and Case C3 mainly distributes in the middle and front of the combustion chamber. When the crank angle rotates to 30°CA ATDC, the flow field in the combustion chamber is mainly unidirectional flow, so the intensity and velocity of flow in the rear part of the combustion chamber is low due to no reflux and new flow field supplement. Therefore, in Case A3 and Case B2 the unburnt fuel is still stagnated in the middle and rear of the combustion chamber. In contrast, the fuel in Case C2 and Case C3 moves further to the front of the chamber due to the strong in-cylinder flow field, and eventually converges at the narrow gap near the rotor seal, thus forming a region with a high local equivalence ratio which is negative to combustion. This also explains why Case C3 is still less efficient than Case A3 although with good ignition quality. Correspondingly, as shown in Fig. 11, because of the fastest fuel consumption rate, the oxygen consumption rate in Case A3 is also the fastest. 9

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Fig. 11. Distributions of fuel and O2 at 0 and −30° CA BTDC under different injection strategies.

Case A3, mass fraction of CO2 and NO under Case B2 and Case C2 are relatively low, but a large number of incomplete combustion products such as Soot and CO, are produced. So it can be concluded that the injection strategy in Case A3 can not only achieve higher engine power performance, but also maintain a low level of pollutant emissions except NO. 5. Conclusion This paper is the second part of the research on the adaptability of aviation kerosene in direct injection rotary engine. In this research, firstly, the spray combustion characteristics of aviation kerosene under rotary engine operation conditions were studied in an optically visible constant volume combustion chamber platform. Secondly, the combustion process of aviation kerosene in the engine under different injection strategies was studied on the basis of a reliable three-dimensional dynamic simulation model. The main results obtained are as follows:

Fig. 12. Peak temperature and temperature distribution at TDC under different injection strategies.

1. The ignition limit of aviation kerosene were explored by spray combustion experiments in the range of ambient temperature 700–850 K and ambient pressure 20–30 bar. It is found that when ambient conditions reach 750 K and 25 bar, cool-flame can be observed. The reaction process of the fuel is mainly low temperature combustion, which is insufficient. When the ambient conditions reach 800 K and 20 bar, the ignition of aviation kerosene is reached and the combustion process is stable and mature. 2. The increase of ambient temperature and pressure can promote the ignition and diffusion combustion of aviation kerosene. Compared with ambient pressure, ambient temperature can more effectively affect the ignition and spray combustion quality of fuel. With the increase of ambient conditions, the ignition delay and the flame lift-

CO2, NO and Soot are inversely proportional under each injection condition. This is because CO is the product of incomplete combustion, while CO2 is the product of complete combustion of fuel, so the mass fraction of CO and CO2 in one case is opposite. Similarly, NO is a kind of pollutant formed under the condition of high temperature and rich oxygen, and under this condition most of the Soot can be oxidized, so the production of NO and Soot is inversely proportional as well [40]. Specifically, due to the improvement of combustion efficiency, more fuel is burnt in Case A3, so the CO and Soot emissions are relatively low. As a product of complete combustion, more CO2 production is acceptable, but at the same time, because of the higher temperature in the combustion process, more NO emission is generated. Compared with 10

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Fig. 13. Mass fraction of main emissions at 100° CA ATDC under different injection strategies.

off length are shortened continuously. However, when the temperature reaches 850 K and pressure reaches 25 bar, the lift –off length will no longer shorten and remains at about 6 mm. 3. Compared with other injection strategies, under the injection position A-Injection angle 90° (Case A3), the fuel distribution and concentration around the spark plugs at ignition timing are more appropriate due to the coupling effect of injection angle, injection position and flow field in cylinder. The ignition quality is improved and the flame propagation speed is increased. With the higher combustion efficiency of fuel, the peak pressure in cylinder reaches up to 38.89 bar. The engine can obtain better power performance at this injection strategy. 4. On the premise of better dynamic performance, the injection strategy of Injection position A-Injection angle 90° (Case A3) can achieve lower CO and Soot pollutant emissions. However, due to the higher in-cylinder temperature during the combustion process, the production of pollutant NO will be slightly higher than other injection strategies. So it can be concluded that Case A3 is the more optimized injection strategy for the application of aviation kerosene in direct injection rotary engines.

[7] [8] [9]

[10]

[11] [12] [13]

[14]

[15]

Declaration of Competing Interest

[16]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[17] [18] [19]

Acknowledgements [20]

Thanks from the authors go to the National Natural Science Foundation of China (Nos. 51576093 and 51606089) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJKY19_2554) for their financial support.

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