| In recent years, with the increasing concerns about energy crisis and environmental issues, a cleaner, more efficient and environmentally adaptable energy system has become a pressing need for us. Acting as an ecologically sensitive and efficient fuel, natural gas is often recognized as a promising option by governments and used for various power systems. Among them, the side-ported natural gas-fueled rotary engine not only inherits the advantages of the side-ported rotary engine, such as large specific power output, simple and compact design, multi-fuel capability, low noise and favorable performance at low speeds, but also brings the excellent combustion characteristics of natural gas. Therefore, the side-ported natural gas-fueled rotary engine has an advantage over the reciprocating engine in many respects like the range-extender system of electric vehicle. However, comparing with the reciprocating engine, the movement and combustion process of the mixture in the rotary engine present different characteristics for its special structural characteristics and special operation mode. The flattened combustion chamber of the traditional rotary engine will result in high unburned hydrocarbon emissions. When natural gas is used in the rotary engine, its low burning speed could also exacerbate the above problem. In addition, the basic research on the working process of the natural gas rotary engine is still rare. Therefore, the study on flow field, combustion process and the optimization of the technical parameters have important scientific and practical value.This article first discusses the current status and development trend. On the basis of the analysis of technical difficulties, the optical rotary engine test-bed is built and a Particle Image Velocimetry (PIV) is used to visualize the 2-D flow field in the rotor housing central plane under low rotate speed. Meanwhile, on the basis of the FLUENT simulation software, a three-dimensional dynamic simulation model based on the chemical reaction kinetics is established by writing dynamic mesh programs, choosing the RNG k-ε turbulent model, the eddy-dissipation concept (EDC) combustion model and a reduced reaction mechanism. The three-dimensional dynamic simulation model is validated by the experimental results. Some of the important information is reflected through simulation, which is difficult to obtain through experiment, in terms of the 3-D flow field, the temperature field and the concentration fields of some intermediate in cylinder under normal rotate speed. Furthermore, the effect of the structural parameters, operating parameters and adding hydrogen on the flow field, combustion process and emissions is simulated and analyzed. Some achievements with scientific significance and values have been acquired:(1) The optical rotary engine test-bed is completed. On the base of the side-ported Z160F rotary engine, the test-bed used to visualize the 2-D flow field in the rotor housing central plane, is set up through completing the optical modification of the rotary engine and installing a frequency converter and a motor, a PIV system, a shaft encoder and a smoke generator.(2) The 2-D flow field in the rotor housing central plane under low rotate speed is tested and analyzed by PIV experiment. In the PIV experiment, the smoking oil particles from a smoke generator are chosen as a tracer, and the 2-D flow field is successful acquired. It can be seen from the experimental results, that there exists a counterclockwise flow motion in the test area during the intake and compression strokes, and a counterclockwise swirl is caught at a certain crank angle. Furthermore, the flow speed decreases gradually from the intake stroke to the compression stroke. This is mainly because the flow in the combustion chamber is accelerated by the high intake flow during the intake stroke. However, in the compression stroke, the high intake flow disappears. The flow in the combustion chamber is only pushed by the rotor. Therefore, the flow velocity in the combustion chamber is relatively low.(3) The simulation model on flow field and combustion process in cylinder is established and validated. On the basis of the software FLUENT 14.0, a three-dimensional dynamic simulation model is established by writing dynamic mesh programs and choosing the RNG k-ε turbulent model, the eddy-dissipation concept (EDC) combustion model and a reduced reaction mechanism. The three-dimensional dynamic simulation model based on the chemical reaction kinetics is also validated by the experimental data.(4) The 3-D flow field under normal engine operating conditions is studied by numerical simulation. At the intake stroke, after the intake flow impinges on the lower cylinder head, two vortexes are formed at the front and the rear of combustion chamber, respectively. The radius of the two vortexes is changing with the motion of the rotor. In the compression stroke, the two vortexes gradually break into a unidirectional flow because of sharp decrease in the combustion chamber volume. It is also found that the main formation reason of the swirl detected in the central cross-section of cylinder block though PIV experiment, is that the flow which came from the back of combustion chamber and bypassed the inlet to the front of the combustion chamber, turns counterclockwise to form the swirl.(5) The flow field, volume coefficient and average turbulence kinetic energy in the combustion chamber are simulated in detail by varying rotating speed, intake pressure and intake angle. In the intake stroke, with the increase of the rotating speed, the intake pressure and the intake angle, the flow pattern formed in the combustion chamber changes from the large-scale swirl to the small-scale tumble. With the increase of the intake pressure, the volumetric coefficient increases continuously. However, with the increase of rotating speed and the intake angle, the volumetric coefficient increases initially and then decreases. The average turbulence kinetic energy in the combustion chamber increases with the increase of the rotating speed and the intake pressure. Under the computational conditions, the average turbulence kinetic reaches the maximum value at the intake angle of 15°.(6) The ignition and combustion process of the rotary engine fueled with natural gas is simulated and analyzed by coupling the reduced mechanism. During the combustion process, the flame surface near the leading spark plug develops in a symmetrical way, because the leading spark plug is located at the unidirectional flow zone at ignition timing. However, the development of the flame surface near the trialing spark plug is biased toward the lower cylinder head, because the trailing spark plug is located at the transition region between the unidirectional flow located at the upper cylinder head and the tumble located at the lower cylinder head at ignition timing, and the tumble makes the flame develop rapidly towards the upper cylinder head. The mass fraction of intermediates CH2O, OH, CO all increases initially and then decreases in the combustion process. The positions of peak mass fraction value of those intermediates are all after top dead center. The generation speed of NO is slower than the combustion speed of CH4.(7) The combustion process in the combustion chamber of the rotary engine fueled with natural gas is studied in detail by varying the structural parameters and operating parameters, for example, ignition position, ignition advance angle, pocket position, pocket shape and ignition slot. In terms of ignition position and ignition advance angle, when the trailing spark plug is located at the rear of the tumble zone, the flame propagation speed can be increased by making full use of the tumble. Furthermore, the fuel in the middle and rear of combustion chamber can also be burnt without delay. With advanced ignition timing, the time between ignition timing and top dead center (TDC) increases. Furthermore, the time between ignition timing and the timing of the tumble disappearance also increases. The overall combustion rate increases continually. In terms of pocket position, when the rotor pocket is located at the front end of rotor surface along length direction and the center of the rotor surface along width direction, a high oblique speed flow in the middle of combustion chamber are made full use to increase flame speed. The flame propagation speed can be improved. In terms of pocket shape and ignition slot, when the combustion chamber configuration has a middling pocket coupled with a ignition slot located at the middle of the width direction of rotor surface, the area of the high speed oblique flow in the middle of combustion chamber is increased, and the gross combustion rate can be further increases. However, the drawback is a certain increase in NO emissions.(8) The efficiency of the rotary engine fueled with natural gas can be increased by adding hydrogen. Compared with the other hydrogen blending modes, the overall combustion rate for hydrogen low-pressure early injection is the fastest. This is mainly because the low-pressure early injection, not only allows the hydrogen in the combustion chamber to be distributed evenly, but also results in high concentration areas of hydrogen located at the front of the trialing spark plug, which can be used to increase the combustion rate.This paper provides experiment and simulation basis for the study of rotary engine and a theoretical foundation for improving the performance of the side-ported rotary engine fueled with natural gas. Meanwhile, the methods and conclusions presented in this paper can also be applied to the rotary engine systems using other fuels. |
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