For low-pressure dual-fuel marine engines, the NG is injected into the cylinder after the scavenging ports are closed, then the mixing time of NG and air is as long as 110 °CA to the ignition. Therefore, the formation and distribution of the mixture are mainly influenced by the flow field in the cylinder. The flow-field changes and influencing factors of the scavenging and compression process of the X92DF engine are analyzed in detail below.
3.2.1. Influence of Flow on the Distribution of NG
Figure 8 displays the variation of swirl ratio (SR) and tumble ratio (TR) and turbulent kinetic energy (TKE) with crankshaft angles during the scavenging process and compression process from 90 °CA to 350 °CA. It can be observed that the formation and change from the in-cylinder flow field are mainly affected before the diesel injection by the scavenging process and NG injection process.
Figure 8a shows the result of comparing the calculated SR under two different initial flow fields, in the cylinder and without the NG injection process. On one hand, it shows that for the calculated result of no flow field in the cylinder at the initial moment (black line), the SR begins to rise rapidly after the scavenging port is opened. However, when the flow field after combustion is adopted as the in-cylinder initial condition (red line), the SR basically shows a downward trend during the scavenging process. The main reason is that the angular momentum of the overall gas in the cylinder decays faster than the swirl provided by the scavenging process. On the other hand, it can be observed that by comparing it with the SR calculated without NG injection process (blue dotted line), the injection of NG has great influence on the level of swirl in the cylinder. This is mainly due to the transverse flow movement in the cylinder being enhanced, resulting in NG being injected into the cylinder at a certain angle along the direction of the original flow field in the cylinder.
Figure 9 shows the variation of velocity distribution on a cross section of NG injection valves during the injection process of nature gas. It shows that the process of NG injection clearly impacts the flow field originally formed by the scavenging gas and promotes the swirl movement in the cylinder.
Figure 8b presents the evolution of TR in the cylinder. Compared to
Figure 8a, it is observed that the change degree of the SR is obviously higher than that of the TR during the process of scavenging and NG injection. It indicates that the level of in-cylinder swirl has a great impact on the flow field in the cylinder.
Figure 8c presents the variation of the TKE with crankshaft angles during the process of scavenging and compression. It illustrates that the influence of the NG injection process on the in-cylinder turbulence is obviously higher than that of the scavenging process. The main reason is that during the injection process of NG, the large speed of the injection makes the swirl continuously generate in the cylinder and form a strong turbulence.
The above numerical results indicate that the level of swirl directly effects the flow field and can be used as an evaluation index of airflow movement in the process of ventilation and compression. Moreover, the injection process of NG has a significant influence on the level of swirl, which will directly affect the mixing process and the formation quality and distribution of premixed gas.
3.2.2. Effects of NG Injection Condition on NG Distribution
In order to further analyze the effect of NG injection on the in-cylinder flow field and mixture distribution in the cylinder, by changing the NG injection valve structure and NG injection conditions to analyze the effects of NG injection pressure, NG injection angle and the number of gas valves on the swirl and the turbulence, the effect of NG injection conditions on NG distribution is obtained.
Figure 10 displays the variation of SR for different NG injection conditions.
Figure 10a,c shows that with the increase in the number of NG nozzles and injection pressure, the SR increases slightly during the NG injection process. However, the SR is not significantly improved after the end of the natural gas injection, and the swirl movement level in the cylinder is basically the same as before fuel injection. It is explained that increasing the injection pressure and the number of injection nozzles leads to the speed of the injection outlet being improved and the promotion effect of the NG injection on the swirl in-cylinder being enhanced in the process of NG injection. In order to ensure the same gas injection quality, the injection duration is shortened when the injection pressure is enhanced and the number of injection nozzles is increased. Therefore, the change of pressure and number of injection valves have little influence on the promoting the swirl motion of the original flow field. Additionally, as displayed in
Figure 10b, with the increase in injection angle, the strengthening effect of the NG injection on the SR becomes more obvious. This is mainly caused by the interference between the two NG flows becoming stronger with the decreasing injection angle, resulting in the strengthening effect of the NG injection on swirl being relatively reduced.
The comparison of average TKE for different injection conditions of NG is presented in
Figure 11. As depicted in
Figure 11, the turbulence intensity is enhanced during the NG injection process with the increasing injection pressure and number of injection nozzles. However, the TKE of each scheme gradually decreases after the end of the NG injection and tends to be consistent at near TDC, which indicates that the conditions of NG injections have little influence on the in-cylinder TKE due to the injection mass remaining unchanged for different injection conditions; this means that there is no extra energy to compensate for the dissipation of TKE, leading to the interference intensity of the NG injection process on the original flow field in the cylinder not changing significantly.
Figure 12 illustrates the distribution of CH
4 for different NG injection conditions at −10° ATDC. The position of sections in
Figure 12 is consistent with
Figure 6.
Figure 12a shows the distribution of in-cylinder equivalence ratios calculated under the condition of original gas injection. Compared with the case of
Figure 12a, changing the NG injection conditions has no obvious improvement in the quality of the mixture. On the contrary,
Figure 12b,c show that the mass of CH
4 escaping from the exhaust valve increases with the increasing injection pressure. Moreover,
Figure 12d–g show that the areas of high and low concentration of CH
4 expand with increasing injection angle and the number of injection nozzles. The reason is mainly that although the different conditions of NG injection could cause different degrees of disturbance to the in-cylinder flow field during the NG injection process, the overall average TKE in the cylinder has not been significantly improved after the end of the NG injection. Thus, changing the injection conditions does not significantly enhance the mixing process.
As mentioned above, it can be concluded that the increase in swirl intensity during the process of natural gas injection has little influence on the distribution of the equivalence ratio before TDC. To further reveal the effect of swirl on the flow field and the distribution of equivalence ratio in cylinder, the influence of the swirl motion in the formation and distribution of the in-cylinder mixture was studied in by removing the component of the swirl motion. In the simulation calculation process, the X/Y movement speed of the in-cylinder gas at the end of the NG injection (260 °CA) was removed by applying the MAP function of CONVERGE, which realized the calculation of no swirl motion in the cylinder at the initial moment.
Figure 13 displays the CH
4 distribution of the Y-direction in the cylinder at the conditions of swirl motion and no swirl motion. The result shows that the distribution of CH
4 is basically the same from the end of the NG injection (260 °CA) to the exhaust valve being closed (290 °CA), and is distributed near the cylinder wall and mainly moves upward along the wall surface. However, when the exhaust valve is closed (290 °CA), there is an obvious difference between the distribution of CH
4 under the condition of no swirl motion compared to that in the presence of swirl motion; the distribution of CH
4 appears to be a stratified phenomenon in the compression process. The reason is that the velocity of gas radial diffusion is lower, due to the cylinder swirl movement being removed from the cylinder at the initial moment.
Figure 14 shows the density probability distribution of equivalence ratios under different swirl states after NG injection. The density probability distribution of equivalence ratios under the condition of no swirl motion at the initial time is not obviously different compared with that of the condition of swirl motion in the cylinder during a period of time after the NG injection (270 °CA–290 °CA). When the exhaust valve is closed (290 °CA), the overall equivalence ratio distribution in the cylinder is obviously different, with the piston running upward. This implies that the influence of the swirl movement of the late compression process on the transportation of NG is higher than that before the exhaust valve is closed. Meanwhile, the distribution of CH
4 and the velocity vector of the Z-direction section in-cylinder under the condition of no swirl at the initial moment are depicted in
Figure 15. The red horizontal line in
Figure 13 is the selected slice location of
Figure 15. As seen in
Figure 15, at the late stage of the compression process, a swirl motion is formed in the cylinder as the piston moves upward, which promotes the radial diffusion of CH
4 from the cylinder wall. Therefore, it further illustrates that the level of swirl of the late compression stage has an important influence on the uniformity of gas distribution.
From the above discussion, it can be deduced that the main factors affecting the transmission process of NG and distribution of CH4 concentration are the level of swirl at the late compression stage and the turbulence intensity in-cylinder. Therefore, it is essential to improve the level of swirl in the cylinder at an appropriate time and increase the turbulence intensity in the cylinder without affecting the gas injection quality and pressure, so as to compensate for the attenuation of turbulent motion.