Visualization and analysis of the characteristics of transitional underexpanded jets

Highlights

• Transitional underexpanded jets related to direct injection CNG engine are experimentally studied.

• Analyzing the underexpanded jet process with the help of shock wave structures.

• Visualizing axial and various radial cross-sections of underexpanded jets under different inlet conditions.

• LES is used to further explore underexpanded jet characteristics.

• Jet turbulence and mixing process can be enhanced by higher injection pressure.

Abstract

Underexpanded jets can be formed when high-pressure gaseous fuel is injected directly into an engine cylinder. In such conditions, shock waves are formed immediately near the nozzle exit. In the present study, the flow structure and turbulent mixing of pulsed jets issuing from a circular nozzle is investigated using acetone planar laser-induced fluorescence (PLIF). By monitoring axial and various radial cross-sections under different injection pressure conditions, different features of gaseous jets are visualized and interpreted. The temporal development of the axial cross-sections reveals three typical jet flow patterns (subsonic, moderately underexpanded, and highly underexpanded) during the injection. These stages are (1) well described with the observed shock structures and (2) noted to take a considerably long portion of the full injection process. The visualizations from the radial cross sections show how the nozzle inflow conditions may influence the primary and the azimuthal (secondary) instability of the jet which influences the turbulence transition process and the mixing process. The results indicate the importance of inner nozzle flow on the flow behavior. For example, systematic asymmetries in the mean concentration fields are observed. In addition to PLIF data, numerical simulations can be used to support the experimental picture of the jet behavior. We give examples of large-eddy simulations (LESs) in order to further explore the behavior of the underexpanded jets. Results show that LES is able to reproduce the basic physics of underexpanded jets. LES and PLIF compare favorably in terms of the barrel shock structures and the description of the normal shocks. LES also provides detailed flow field information including temperature, Mach number, concentration and scalar dissipation rate (SDR).

Introduction

Reducing air pollution and dependence on fossil fuels is crucial for the sustainable development of the conventional internal combustion engines (ICEs). Natural gas (NG), a fuel abundant in nature, is considered as one of the most promising alternative fuels for ICEs (Korakianitis et al., 2011). Among fossil fuels, NG combustion has the lowest level of greenhouse gas emissions along with negligible amounts of suspended particles and photochemical smog promoters (Baratta et al., 2009). Recently, compressed natural gas (CNG) port-injected spark-ignition (CNG-SI) engines have already reached the commercial production stage (e.g., city buses and taxi cars). However, the power output and the emissions of unburned hydrocarbons are limited by the low volumetric efficiency and a fuel short-circuit from inlet to exhaust. To further improve the performance of CNG engine, many automotive engine researchers believe that CNG direct-injection compression-ignition (CNG-DICI) engine has a great potential to improve the thermal efficiency and to meet the stringent emission regulation limits in the near future (Kalam and Masjuki, 2011, Li et al., 2011, Yu et al., 2013).

A major challenge of CNG-DICI engine is the poor ignition performance of NG due to its low cetane number. This problem could be solved by the dual-fuel (DF) concept (Korakianitis et al., 2011, McTaggart-Cowan et al., 2006). Conceptually, NG would be directly injected into the cylinder as a primary fuel and mixed with air. Subsequently, the fuel–air mixture would be ignited by a small amount of pilot fuel with a high cetane number (e.g. diesel fuel). In order to prevent methane slip and to extend the operation range of the engine with high efficiency, it is necessary for CNG-DICI engine to operate with stratified charge by means of late injection strategy. McTaggart-Cowan et al. (2006) reported that CNG-DICI-DF engines not only maintain power output and thermal efficiency levels compared to conventional pure diesel fueled engine, but they also reach lower NO_x and particle matter (PM) emissions. In addition, it was stated that the improvement in CNG-DICI-DF engines can be obtained by varying the injection pressure of the CNG fuel and diesel pilot fuel. They further investigated the effects of injection pressure on the performance and emissions of a heavy-duty dual-fuel engine using a diesel pilot ignition with late-cycle direct-injection CNG from 210 to 300 bar (McTaggart-Cowan et al., 2007). Based on the combustion parameters, such as the profiles of in-cylinder pressure and the rate of heat release, they concluded that the combustion process at all operation conditions is restricted by the rate at which the fuel and oxidizer are mixing. Increasing the injection pressure increases both the mass flux of fuel into combustion chamber and the in-cylinder turbulence, resulting in enhanced mixing, reducing combustion duration and increasing peak combustion intensities. Moreover, they pointed out that the effects of injection pressure may vary substantially with the in-cylinder conditions because of the variation of the injection pressure ratio (injection pressure to in-cylinder pressure).

Another main challenge of the CNG-DICI-DF engine is that there is only a very short time for the mixture formation. Thereby, the period of fuel injection can only take 5–10 ms. One of the key questions concerns improving and understanding the mixing efficiency in a CNG engine. In general, when a gaseous fuel is directly injected into the cylinder of an engine, it forms transient turbulent jets that are typically underexpanded jets with strong shock waves near the nozzle exit (Hill and Ouellette, 1999, Ouellette and Hill, 2000). These shock waves can significantly influence the downstream flow field.

In the context of DI gas engines, high-resolution imaging of pulsed underexpanded jets has not, to our best knowledge, been previously carried out using the PLIF technique. Thereby, the objective of this work is to gain an in-depth understanding on the characteristics of pulsed turbulent jets in CNG-engines, and to explore the effects of shock waves on the downstream flow structures and the turbulent mixing. This is accomplished by using a planar laser-induced fluorescence (PLIF) technique. Hiller and Hanson, 1988, Hiller and Hanson, 1990, Lemoine and Leporcq, 1995 have demonstrated the measurement of density and pressure fields of underexpanded flow using the PLIF technique. However, they only focused on the steady flow, and did not give high-resolution flow field images including shock waves phenomena. In the case of high pressure pulsed turbulent jets related to DI gas engine, previous studies have used PLIF technique to study the characteristics of high pressure gas jet in a constant chamber and in optical engines, but they focused more on the macroscopic structures of the jet (e.g., jet penetration), and they did not refer to the shock wave phenomena (Bruneaux, 2002, Bruneaux et al., 2011, Rubas et al., 1998, Salazar and Kaiser, 2009, Salazar and Kaiser, 2010). In addition to PLIF, we end the paper by showing high-resolution images from gas jet simulations in order to complement the experimentally obtained picture. In particular, we demonstrate the power of large-eddy simulation (LES) as a numerical diagnostic tool to give information on e.g. the jet temperature and concentration fields as well as the scalar dissipation rates.

Theoretically, when a gas jet is injected through a circular converging nozzle into another gaseous medium, the maximum mass flow occurs when the velocity at the nozzle exit equals the speed of sound. Under such conditions, the inner nozzle exit pressure (P_e) is always higher than the back pressure (P_∞), the flow is choked and it becomes underexpanded. Assuming a choked flow and an ideal gas that flows isentropically through the nozzle exit, the pressure ratio between upstream fuel-supply pressure of nozzle exit (P₀) and the in-nozzle pressure (P_e), can be defined as (Heywood, 1988):

$$\frac{P_e}{P_0}={\left(\frac{2}{\gamma +1}\right)}^{\frac{\gamma }{\gamma -1}}$$

For a polytropic gas, the ratio of specific heats (γ) is constant (γ ≈ 1.4). The critical pressure ratio (P_e/P₀) is approximately 0.528. For a subsonic jet P_e always equals to P_∞, so P_e/P_∞ ≈ 1.89. Thereby, at high injection pressures the flow can become choked very easily during the injection. At underexpanded conditions, the shock waves can be formed immediately near the nozzle exit.

A vast amount of work has been done to investigate the characteristics of the underexpanded free jets in the past years. Most of these studies are related to the aerospace applications, in particular jet aircraft and rocket propulsion systems (Crist et al., 1966, Donaldson and Snedeker, 1971, Ewan and Moodie, 1986, Otobe et al., 2008). According to the previous studies, a free jet usually has three major variations of the flow pattern (subsonic, moderately underexpanded and highly underexpanded), mainly depending on P₀/P_∞. In aerospace applications supersonic jets are usually considered to be steady state jets since the injection periods are very long (typically hours). In contrast, the high-pressure pulsed jets in the engine related field are much more transient since the injection period can be of the order of 1–10 ms. For pulsed jets in the direct-injection (DI) gaseous fuel engine, due to the high compressibility of the gas and the variation of the in-cylinder pressure (or back pressure P_∞), P₀/P_∞ may also vary significantly during the gas injection. Hence, the jet behavior is more complicated and transient in engine applications than under steady state conditions.

Only a few studies have provided detailed information on high pressure pulsed jets in gas engines. Hill and Ouellette (1999) investigated the effects of injection pressure ratio on the penetration in a fixed volume chamber using the Schlieren technique, and they developed an analytical relationship for jet tip penetration based on self-similar characteristics of transient turbulent jets. Rubas et al. (1998) examined the natural gas direct injection and mixing in an optical engine using planar laser-induced fluorescence (PLIF) technique. The injection pressure was 180 bar and cylinder pressure as high as 20 bar was used to match the in-cylinder density during the injection in a firing engine. Recently Salazar and Kaiser (2010) also used PLIF technique to study the mixing process in an optically accessible DI hydrogen engine with high pressure injection (80–116 bar). They found that both the tumble flow and nozzle designs can influence the mixture formation and distribution. Baert et al. (2010) visualized the transient jets using planar laser sheet Mie scattering (PLMS) and measured the jet flow field using particle image velocimetry (PIV). The jet penetration and jet angle were investigated under different injection pressure ratios (15–40 bar). More recently, the effects of the injection pressure on the mixture formation were also reported by Roy et al., 2011, Bruneaux et al., 2011 using different optical techniques. Nearly all of the previous studies focused only on the macroscopic characteristics of the jets (e.g., jet penetration and jet cone angle). In contrast, only little attention has been put on detailed investigations on the shock wave patterns and their connections to the mixture formation. Actually, the flow in the near-field region can significantly influence the downstream flow structures and turbulent mixing, in particular for the underexpanded jet, since the jet angle can be increased by the expansion shock waves, but also the jet turbulence can be enhanced by the shock-induced instability. Therefore, the detailed information of the near-field region is crucial for the complete understanding of jet mixing in a DI gas engine.

With the development of laser and imaging technology, a number of non-intrusive optical techniques, such as laser Doppler velocimetry (LDV) and particle image velocimetry (PIV), planar laser sheet Mie scattering (PLMS) and phase Doppler particle anemometry (PDPA), have been used to investigate the flow field (Eckbreth, 1996). These optical techniques rely on seeding the flow with small particles (or droplets) and observing the motion of those particles. Thereby, they are very suitable for liquid fuel sprays which naturally contain small droplets in the flow field. With particle seeding, these techniques can be suitable for investigating the low speed gas flows (for example, in-cylinder flow and low pressure gas jets) as the seeding particles can follow the low speed flows very well and represent the true fluid physics. However, the particle-based optical techniques are not efficient for the high speed gas flow, especially for the highly underexpanded jets. First, the particles may not follow such high speed and high frequency containing flow because of the limitation of the response of the seeding particles to the rapid changes of velocities across shock waves. Second, particles cannot be used to observe small-scale structures, since the particle seeding density is limited by secondary scattering, sampling ambiguities, and coherent scattering effects (Miles and Lempert, 1997).

In fact, there are two well-known non-intrusive and non-particle seeding flow-field image techniques, the Schlieren technique and the shadowgraphy technique, to visualize the structure of underexpanded jets (Settles, 2001). Based on the index-of-refraction effects, the general profile of shock waves can be observed with these two optical techniques. However, the main limitation of these two techniques is that the obtained two-dimensional images are integrated across the whole flow field, which means that the inner flow structure cannot be observed. As a non-intrusive non-particle seeding laser-sheet based optical technique, PLIF has the ability to capture the characteristics of the high speed turbulent flow (Eckbreth, 1996). The basic principle of PLIF is based on the excitation of tracer molecules within the laser sheet. Consequently, electromagnetic radiation is emitted when the molecule returns back to a lower energy state. Almost all aliphatic hydrocarbons, forming also the major part of combustion fuels, are transparent within the spectral range of interest and therefore do not give any fluorescence signal. Thereby, the tracer used in PLIF usually plays an important role in PLIF measurement system, in particular in the mixing process of non-reacting flows (Schulz and Sick, 2005). Acetone is a well-known and widely used fluorescent tracer. Lozano et al. (1992) confirmed the linearity of acetone fluorescence emission as a function of both incident laser intensity and acetone partial pressure in the constant temperature and pressure non-reacting flow field. A detailed account of the photo-physics of laser exited acetone fluorescence can be found in the reference (Thurber, 1999).

Here, we describe results from a 3-year project concentrating on gaining an in-depth understanding on physics of fuel jets in natural gas engines. In particular, the project aimed at using the standard acetone PLIF technique and numerical simulations together as complementary techniques supporting each other. Thereby, the present paper has several objectives. First, we aim at analyzing gaseous fuel jet characteristics in gas engine applications. In particular, this task is to be completed using a measurement system based on the acetone PLIF technique. Second, we aim at providing information on the transient stages and shock development times of the gas jets. Third, the aim is to provide information to jet modelers on the practical matters (e.g. asymmetric features) present in real jets. Fourth, the aim is to demonstrate the usage of LES as a promising diagnostic tool to complement any experimental data on underexpanded jets.