Experimental study on spray and atomization characteristics under subcritical, transcritical and supercritical conditions of marine diesel engine

Energy Conversion and Management 195 (2019) 958–971

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Experimental study on spray and atomization characteristics under subcritical, transcritical and supercritical conditions of marine diesel engine

T

Jin Xia, Zhong Huang, Leilei Xu, Dehao Ju , Xingcai Lu ⁎

⁎

Key Lab. for Power Machinery and Engineering of M. O. E., Shanghai Jiao Tong University, 200240 Shanghai, PR China

ARTICLE INFO

ABSTRACT

Keywords: Diesel spray Marine engine Supercritical condition Jet length

In this research, detailed characteristics of diesel spray and atomization based on marine engine scale have been studied under subcritical, transcritical and supercritical conditions. A large constant volume chamber with an inner diameter of 300 mm and a 300 μm single-nozzle injector are used to mimic the in-cylinder thermodynamic condition of marine diesel engine. Sub/trans/supercritical conditions are composed of nine cases by means of different diesel phase transition regions. Optical diagnostic measurements are conducted through backlight illumination technology and schlieren imaging technology in an inert atmosphere. The images are processed with multi-threshold technique to obtain quantitative spray parameters, such as jet and liquid penetrations, averaged lengths, liquid steady time, spray cone angles and R-parameter. Moreover, some new finding about the structure of spray under supercritical condition is also discussed. The results show that the spray liquid periphery is quite smooth and stable with shorter penetration under supercritical condition, meanwhile, the jet of spray behaves as thinner periphery near the injector nozzle region and shows lower density at downstream region. Higher ambient pressure produces a negative effect on the increase of the tip velocity and development of spray. With the transition from transcritical condition to supercritical condition, the liquid length decreases by 32 mm and there exists an obvious decrease of 35 mm from jet length to liquid length. A trade-off relationship between the jet cone angle and jet penetration is also found. Generally, this work is helpful to understand the detailed process of diesel atomization under transcritical and supercritical conditions.

1. Introduction With the air pollution and the stringent emission regulation, large ships in ocean are facing with severe challenge to change the state of dilemma [1]. In spite of this, marine diesel engine will still be one of the most important power generation machines of cruise ship in the future [2]. In order to reduce the pollutant emissions and improve the thermal efficiency of internal combustion engine, some advanced technologies, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) and low temperature combustion (LTC) etc. have been studied widely [3–5]. As a key factor, the process of fuel atomization is significant to the research for future engines. According to these technologies, the injection pressure will increase to 300 MPa and the turbocharged cylinder pressure will also be increased greatly [6]. With such high pressure and temperature in-cylinder environment of engines, the in-cylinder environment has reached critical points of most fuels [7], and the spray will go through transcritical or supercritical conditions. The critical point is defined by the critical pressure and

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temperature. Generally speaking, gas-liquid phases will become indistinguishable when both the pressure and temperature exceed the critical pressure and temperature and this critical point is also the end point of a phase transition curve [8]. At present, the supercritical injection is not commonly defined, but it is not a problem to study the detailed principle in this work. Generally speaking, if both of the temperature and pressure of the ambient are below the critical point of fuel, the ambient condition can be called subcritical condition. If either the temperature or the pressure of the ambient is beyond the critical point of fuel, it can be called transcritical condition [9,10]. If both of the temperature and pressure of the ambient are beyond the critical point of fuel, it can be called supercritical condition [11]. Only the injected fuel and ambient are both above the critical point of the fuel, it can be called supercritical injection [12]. As the fuel is difficult to be heated above its critical temperature before injection, it is practical to name it as transcritical injection for current engine condition. According to previous work [13–15], it has been found that fluids show different properties under supercritical condition in contrast to those under subcritical condition. When the fluid is injected into

Corresponding authors. E-mail addresses: [email protected] (D. Ju), [email protected] (X. Lu).

https://doi.org/10.1016/j.enconman.2019.05.080 Received 7 April 2019; Received in revised form 21 May 2019; Accepted 24 May 2019 Available online 30 May 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature CVC C1, C2 ECU ECN HCCI GTL KE LIF/MIE LTC LES PDA PCCI PIV RANS RME R1 SMD TTL

TH UV-LAS VMS-LES WDF Ca Cv D0 P S1, S2 SL t tr T T100

constant volume chamber the collimating mirror electronic control unit Engine Combustion Network homogeneous charge compression ignition gas-to-liquid the knife-edge laser-induced fluorescence/Mie technology low temperature combustion large eddy simulation Phase Doppler Anemometry premixed charge compression ignition Particle Image Velocimetry Reynolds Average Navier-Stokes rapeseed methyl ester the reflector mirror sauter mean diameter transistor-transistor logic

a f

P

the threshold Ultraviolet/Visible Laser absorption-scattering variational multiscale large eddy simulation wide distillation fuel area coefficient velocity coefficient the diameter of injector nozzle, mm ambient pressure, MPa the jet penetration, mm the liquid penetration, mm the time after start of injection, ms the break up time, ms ambient temperature, K temperature at 100% distilled volume, K the jet cone angle, ° the density of ambient, kg/m3 the density of the fuel, kg/m3 the pressure difference between the injection pressure and the ambient pressure, MPa

under inert and reactive atmosphere. The schlieren imaging, OH* chemiluminescence and diffused backlight illumination techniques have been used to analyze the influence of nozzle geometry, fuel properties, temperature and pressure on the variation of vapor penetration, spray angle and liquid length. The pressure and temperature of the experiment condition can reach the pressure and the temperature of 9 MPa and 900 K, and it has exceeded the critical point of diesel. Wensing et al. [20] explored the phase behavior of three single-component liquids and their binary mixtures inside a high-pressure, high-temperature constant volume chamber. Therefore, it is easy to identify that the fuel has been injected into supercritical environment. The results revealed the presence of a supercritical state and a transcritical phase change, which showed the transition to a supercritical state with a disappearing phase border rather than evaporating and boiling. Many scholars have also investigated the atomization, combustion and emission characteristics of marine engines. Zhang et al. [21,22] investigated the effects of fatty acid methyl esters and low-level water addition on combustion and emission characteristics of marine diesel engine, which was fueled with biodiesel and pure diesel fuel. The water addition level with weight fraction of 4% in biodiesel emulsion had the optimal fuel-air mixing and emission performance. The kinematic viscosity and ignition delay time were found as important factors for biodiesel to reduce main emissions (CO, HC and NOx) by 40% than conventional diesel at medium and high load, while increase emissions by 10% at low load. Hult and Mayer et al. [23,24] conducted Mie scattering technique to study the in-cylinder liquid structure and droplets in a 500 mm diameter optical two-stroke marine engine. This optical engine was designed with 24 ports for holding optical paths or a marine injector. Moreover, the near-nozzle flow velocity field of a marine engine injector was measured by particle image velocimetry and the interior cavitation was observed by shadowgraphs. Zhang et al. [25] conducted laser-induced fluorescence/Mie technology (LIF/MIE) to study the sauter mean diameter (SMD) and conducted Ultraviolet/ Visible Laser absorption-scattering (UV-LAS) technique to study the distribution of fuel vapor mass in the marine diesel engine condition. The results showed that SMD decreased obviously from transcritical to supercritical condition. Moreover, the decrease of SMD every increase of 100 K was 0.61 μm under supercritical condition but was only 0.21 μm under transcritical condition. The vapor mass under supercritical condition was higher than that under other conditions, which resulted in better atomization process. The experimental from Nomura et al. [26] explained that when the droplet was heated to supercritical condition, the surface tension and latent heat of vaporization would

supercritical condition, the boundary surface between liquid and gas will be obscured, and then it will form an inhomogeneous mixture layer of liquid and gas. Near this inhomogeneous mixture layer, the density of fluid is similar to that of liquid and the transport property of fluid is similar to that of gas, which is called supercritical fluid [13,14]. Compared with the subcritical condition, the physical and chemical properties of fuel are obviously different and the conventional numerical simulation model is also mismatched [15]. For the traditional point of views, the gas-liquid two phase transition phenomenon coupled with droplets breakup and evaporation dominate the spray and combustion process in engines. But many combustion engines, such as internal combustion engine, gas turbine and rocket engines, operate in a wide range of ambient pressures and temperatures during the fuel injection. When the injection environment exceeds the critical point of fuels, it has already not been in the gas-liquid two phase region. In order to understand the different spray characteristics between gas-liquid two phase region and supercritical region so as to improve the accuracy of simulation, it will be helpful to obtain the detailed knowledge of fuel atomization process under transcritical and supercritical conditions. The research about the fuel spray and atomization has been performed widely using optical techniques. Moreover, analysis on the fuel injected into supercritical condition is becoming an important and useful research field. Li et al. [16] conducted a comparative experimental study on microscopic spray characteristics of rapeseed methyl ester (RME), gas-to-liquid (GTL) and diesel. The velocity development and size distributions of fuel droplets in different positions have been studied using Phase Doppler Anemometry (PDA) technique. The experiment was conducted under atmospheric condition, where the fuel was injected from 80 MPa to 120 MPa. As mentioned above, the fuel was injected from transcritical condition into subcritical condition. Yu et al. [17] conducted an experiment to study the macroscopic spray characteristics of wide distillation fuel (WDF), such as kerosene and gasoline/diesel blend fuel. The spray characteristics, such as spray penetration, spray velocity, spray cone angle and air entrainment of WDF have been compared with those of diesel. The constant volume chamber can reach the maximum ambient pressure of 6 MPa and injection pressure of 200 MPa under room temperature condition. The results showed the clear trade-off relationship between the density and the injection pressure on spray penetration and velocity. According to these conditions, the fuels were injected into transcritical condition with only the ambient pressure exceeding the critical point. Gimeno and Paryi et al. [18,19] studied the influence of injection condition on n-dodecane and diesel sprays with Engine Combustion Network (ECN) injectors 959

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become zero, and a large amount of ambient gas would dissolve into the interior of droplets, which behaved as excellent atomization characteristics. With these kinds of specialized experiments about fuel atomization characteristics, the detail of atomization knowledge about diesel in scale of marine engines under supercritical condition is still limited. According to a variety of experiments conducted under different ambient conditions, theoretical analysis and numerical simulations will be a necessary and useful method to explore the detailed characteristics of fuel atomization. Huang et al. [27] formulated a six-component surrogate of commercial diesel based on its critical properties to understand the behavior of transcritical and supercritical injection. The whole process of phase change was recorded under supercritical environment in order to observe the phase transition. Dahms et al. [15] presented a theory to understand the two-phase spray atomization phenomena which involved diffusion-dominated mixing. A new model applying 32-term Benedict-Webb-Rubin equation with Linear Gradient Theory explained how the transition occurred in multicomponent fluids. It has been found that the two-phase interface broke down due to the thickened interface and a reduction of the mean free molecular path at high subcritical temperature condition, which would lead to a continuous phase transition from compressed liquid to supercritical mixture state. At the same time, Oefelein and Dahms et al. [28,29] made an analysis to compare the supercritical spray with classical spray theory, coupled with a new model framework based on large eddy simulation (LES) technique. When the n-heptane was injected into a high-pressure combustion vessel, it treated the fuel entering the chamber as a compressed liquid, and the traditional model of fuel atomization was not completely suitable. This new framework was verified and extended by many alkane-oxidizer combinations to find the variations of interfacial structures and free molecular paths. Zong et al. [30] studied the dynamics of a nitrogen fluid jet under different ambient pressures using LES. It has been found that there existed strong density-gradient regimes around the jet surface which can stabilize the flow development.

There also existed some transition of the turbulent kinetic energy form axial to radial direction. When the ambient pressure increased, the transition into that regimes would be earlier and the spatial growth rate of the surface instability wave would increase. Moussaed et al. [31] proposed a novel strategy to blend the variational multiscale large eddy simulation (VMS-LES) model and RANS model based on a blending parameter. The simulation was conducted with a flow past a circular cylinder in the supercritical regime at different Reynolds numbers. According to these restrictive experiments and developing theories about supercritical fluid, the spray characteristic of marine diesel engine combined with the supercritical theories and optical diagnostic technology are rarely studied, therefore it is necessary to explore more supercritical characteristics of fuel atomization in marine engine-like conditions. As the detailed knowledge of fuel spray and atomization characteristics of marine diesel engine under supercritical condition is still limited, the aim of this work is to experimentally analyze it in a large constant volume chamber based on marine engine scale with optical diagnostic measurements under subcritical, transcritical and supercritical conditions. These conditions are composed of a wide range of pressures and temperatures. The images acquired are processed with multi-threshold technique to obtain quantitative spray parameters, such as jet and liquid penetration, averaged length, spray cone angle and Rparameter [32]. Additionally, some new finding about the structure of spray under supercritical condition will also be discussed. 2. Experimental setup and methodology The detailed description of the experimental setup will be provided in this section. Additionally, experimental conditions, optical techniques, image acquisition and process will also be introduced. The schematic layout of experimental setup is shown in Fig. 1. The test chamber of this paper is based on a large constant volume chamber (CVC) with optical access and capable of mimicking internal

Fig. 1. Schematic layout of experimental setup coupled with backlight illumination technology and schlieren imaging technology. 960

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thermodynamic environment during the fuel injection of a marine engine cylinder. In this work, optical measurement through backlight illumination technology and schlieren imaging technology is designed to perform for the diesel spray under subcritical, transcritical and supercritical conditions, and these optical methods are also widely adopted by many scholars [18,33,34]. The acquired images are processed with a multi-threshold technique, which is important to obtain the quantitative experimental results.

2.2. Experimental conditions As the aim of this work is to explore the detailed spray characteristics in sub/trans/supercritical conditions, nine different cases are selected in this work, which is listed in Table 3 in detail. Lin and Huang et al. [27,37] have studied the thermal stability of diesel with a wide range of temperatures, and the critical point of real diesel has also been estimated using their own empirical correlations, which is 713–743 K, 1.9–2.2 MPa. As mentioned earlier, subcritical condition means both of the temperature and pressure are below the critical point of the fuel, transcritical condition means either the temperature or the pressure is beyond the critical point, and supercritical condition means both of the temperature and pressure are beyond the critical point. As listed in Table 3, the ambient temperatures are from 600 to 800 K, and the ambient pressures are 1, 3 and 5 MPa. Among these cases, they are composed of subcritical, transcritical (either the temperature or the pressure exceeds the critical point) and supercritical conditions. As there exists uncertainty for experimental apparatus, five repetitive injections are performed for each case, which are all under the same condition and will minimize the error caused by the fluctuation and randomness of the spray development. The measurement errors, including the pressure and temperature acquisition of ambient and fuel, schlieren background brightness as well as the calibration analyzation for spray images, are presented in Table 4.

2.1. Constant volume chamber and setup The whole CVC setup is composed of six sections: in-cylinder bore, optical path, gas intake vessel, gas heater device, electronic control system and injection system. Unlike the automotive diesel engine, the marine diesel engine has much larger cylinders and injector nozzles in order to satisfy the greater demand of torque and power. The CVC employed in this work has an inner diameter of 300 mm and a total incylinder volume of 43 L to mimic the marine engine’s cylinder, which can reach the maximum ambient pressure and temperature of 6 MPa and 900 K in an inert atmosphere. Although the ambient pressure during injection is usually higher than the maximum pressure of CVC (above 7 MPa) under 100% load of most marine engines, this study mainly focuses on the medium load condition (maximum 5 MPa) due to the safety issue of quartz window restriction [35]. Three optical glass windows are placed around the CVC to form a T-shaped layout, and only two in-line windows are used to hold optical path in this work. The three optical windows around the CVC are made of quartz with a diameter of 140 mm to visualize the process of fuel atomization, whose penetration is longer in marine engines than that in automotive engines [33]. On the fourth window surface, there are gas intake port and data acquisition ports. The pure nitrogen is used in this work to provide an inert environment. A nitrogen storage vessel with a maximum pressure of 15 MPa is connected to the CVC with a solenoid valve, which can fill the CVC to the required conditions within 30 s. A 22 kW pre-heating device connects the nitrogen storage vessel and the CVC, which can preheat the gas during the process of transportation and is coupled with an electrical heating wire inside the CVC. The inner electrical heating wire with a power of 10 kW is placed at the bottom of the CVC as a primary heating method, as the heating interface is directly in touch with the internal ambient gas. The pre-heating device can output a stable and homogeneous supercritical environment, while the inner electrical heating wire can provide a high-temperature environment quickly. The data from the sensors of temperature and pressure is recorded by a data acquisition card (NI PCI-6251). The data acquisition system is embedded in a LabVIEW-based control system, which are created to control the file save, solenoid valve and the synchronization among the start of injection, trigger of high-speed camera and the beginning of recording data. The injector used in this work is located at the top of the CVC, which is connected with the high-pressure common rail and the fuel pump control system. The whole injection system is driven by an electric motor with a power of 20 kW. The high-pressure common rail can reach the maximum pressure of 260 MPa controlled by the electronic control unit (ECU) of fuel pump control system. The injector has a single-hole nozzle with a diameter of 300 μm, which can represent the nozzle for the marine diesel engine, and the spray characteristic of this type of nozzle is rarely studied by ECN [36]. The detailed specifications are listed in Table 1. In order to avoid the influence of fuel temperature before injection, it is fixed at 303 K. So around the injector tip, the water with a constant temperature is continuously circulating to maintain the temperature of fuel stable before start of injection. The fuel used in this work is No. 0 diesel, whose properties are listed in Table 2.

2.3. Optical techniques and image acquisition 2.3.1. Backlight illumination technology Backlight illumination technology is applied for liquid penetration and liquid cone angle measurement, which is shown in Fig. 1. The LED matrix with a total of 960 LED beads has been used as the back light source with the power of 12 V and 12 W. These 960 LED beads are uniformly distributed on a plane of 200 mm * 200 mm with a diffuser plate to illuminate in order to ensure the backlight intensity constant. A high-speed camera (FASTCAM Photron Mini AX) records the fuel atomization at a constant back light intensity with the scattering of light by droplets at inert environment. The photos are taken at a frame rate of 13,600 fps and a shutter speed of 1/50,000 s. Each frame has a resolution of 512 pixels * 512 pixels. Five continuous injections are performed with the interval of 0.2 s and the duration of 1500 μs for each one. 2.3.2. Schlieren imaging technology Schlieren imaging technology is designed to capture the change of density in a flow field, where slight disturbance will be visualized by a high-speed camera [38]. Based on the well-known principle, the optical setup using schlieren of this work has been shown in Fig. 1. The light from a point light source, which is located at the focal point of collimating mirror C1, is collimated by mirror C1. Then the light becomes parallel through the region of interest and reaches the other collimating mirror C2. Both the diameters of mirror C1 and C2 are 200 mm, and they are bigger than the diameter of optical windows (140 mm), which ensure the whole testing region can be visualized and recorded. Finally, Table 1 Injector specifications and capabilities.

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Injector properties

Specifications and capabilities

Units

Applications Injector model Injector type Nozzle style Orifice diameter Maximum pressure Injection duration Fuel mass per injection

BOSCH System Diesel Injectors CRIN-20-6DL Solenoid-driven, common-rail Single-hole, on-axis 300 200 1500 45

– – – – μm MPa μs mg

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gradient grayscales are recorded for better imaging. The same highspeed camera mentioned above is implemented with schlieren imaging technology. The density gradients between the spray and ambient gas can be captured in order to analyze the whole jet contour.

Table 2 Thermophysical properties of No. 0 diesel. Fuel properties

No. 0 diesel

Units

Cetane number Lower heat value (LHV) Fuel density* [27] Kinematic Viscosity* Surface tension* Thermal conductivity* T100 [27] Critical temperature [37] Critical pressure [37]

52.4 42.76 829 3.2 28.4 0.124 631 713–743 1.9–2.2

– MJ/kg Kg/m3 mm2/s mN/m W/mK K K MPa

2.3.3. Image acquisition and analysis Original grayscale images of the spray were recorded by the highspeed camera, but the images are not convenient to analyze macroscopic characteristics quantitatively. Therefore the image processing and analysis are important to obtain the accurate experimental results, and a multi-threshold technique is used to capture the quantitative spray parameters [39]. As mentioned above, the macroscopic parameters such as liquid penetration and liquid cone angle can be obtained by backlight illumination technology. The developed multi-threshold algorithm is implemented in the image processing of raw images, which is shown in Fig. 2. Actually, the first raw image is composed of a pixel matrix, where each value corresponding to a pixel is the local signal intensity. Initially, the last image before start of injection is adopted as the background image, which is subtracted by each following image during the process of injection. The subtracted image A is composed of grayscales with a range from black of 0 to strongest white of 255. Then a suitable threshold (TH) value is selected to distinguish the spray as strongest white from the ambient gas as weakest black as a binary image [40]. The spray regions and noise levels both depend on TH, so the selection of TH is important in spray image analysis [41]. However, some blurry margin and small particles must be optimized for latter analysis. Marginal closing (image B) and removing noise particles (image C) are performed in steps. Finally, the raw image is contoured by the boundary detection of image D. As the spray may not only behave as gaseous phase under supercritical condition, the classical vapor penetration is replaced by the definition called jet penetration in this work. The jet and liquid penetrations mean the vertical length from the location of injector tip to the bottom location of spray contour as labeled by SL in Fig. 3. The spray cone angle, whether liquid cone angle or jet cone angle, is defined as the angle of an imaginary isosceles triangle. The area of this isosceles triangle is equal to the area above 50% of liquid or jet penetration [42]. The image processing procedure for schlieren imaging technology is shown in Fig. 4. Because of the variable ambient density, the background image before start of injection is quite different from the background of following images. However, it can be considered as similar between two consecutive images [18], and the iteration method is adopted as shown in Fig. 4. Taking the ninth image after start of injection for example, previous eight images have been processed and combined to an integral image E (1–8). Then, the ninth raw image is subtracted by the eighth raw image and is shown as image A (9), but some large particles are visible because of the moving background. Next, the same methods with Fig. 2, such as binarization (image B (9)),

* Thermophysical properties at the fuel temperature of 303 K. Table 3 Experimental cases classified by subcritical, transcritical and supercritical conditions. Ambient temperature (K)

Injected oil temperature (K)

Rail pressure (MPa)

Ambient pressure (MPa)

Ambient density (kg/m3)

Ambient conditions

600

303 303 303 303 303 303 303

180 180 180 180 180 180 180

1 3 5 1 3 5 1

5.6 16.8 28.1 4.8 14.4 24.1 4.2

303 303

180 180

3 5

12.6 21.0

Subcritical Transcritical (P)* Subcritical Transcritical (P)* Transcritical (T)* Supercritical

700 800

* Transcritical (T or P) condition means either the temperature or the pressure of fuel exceeds the critical point. Table 4 Measurement errors and uncertainty analysis. Measurements

Measuring range

Accuracy

Ambient temperature Ambient pressure Fuel temperature Fuel injection pressure Schlieren light source voltage Calibration (mm per pixel)

600–800 K 1–5 MPa 303 K 180 MPa 0–250 V 70 mm/image pixels

± 0.5% ± 0.5% ± 1.6% ± 1.1% ± 2.0% ± 2 pixels

a knife-edge (KE) is located at the focal point of mirror C2. With the change of density caused by spray development, some part of refracted light is blocked by the KE, while the non-refracted light will fully go through the KE. As a result, there exists a gradient change of light intensity. As the KE can slide to block the amount of light ray, different

Fig. 2. The image processing of raw images obtained by backlight illumination technology with multi-threshold technique.

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Fig. 5. Schematic diagram of diesel’s phase transition, where ‘a’ represents subcritical condition; ‘b’ represents transcritical (T) condition; ‘c, d’ represents transcritical (P) condition; ‘e, f’ represents supercritical condition.

optimization, the boundary is easy to be detected for the final optimized image. 3. Results and discussion In this section, diesel spray and atomization characteristics under different ambient conditions using backlight illumination and schlieren imaging technology will be discussed. The spray structure, jet and liquid penetrations as well as the cone angles will be used to describe the spray behavior. Moreover, the jet and liquid averaged lengths, liquid steady time and R-parameter [32], which represents the tip velocity of spray, will also be taken into discussion. As mentioned in Section 2.2, four ambient conditions, called subcritical condition, transcritical (T or P) conditions and supercritical condition have been defined. As shown in Fig. 5, six cases from ‘a’ to ‘b’ have been classified by means of different diesel phase transition regions and will be analyzed in detail. Since the diesel is a mixture including many kinds of compositions, the phase transition boundary is only a schematic diagram. The critical point of diesel is not constant and this ‘point’ in Fig. 5 is replaced with a critical region (713–743 K, 1.9–2.2 MPa). Test parameters of six cases under four ambient conditions including the ambient pressures and

Fig. 3. The definition of spray cone angle and penetration adopted in this work.

marginal closing (image C (9)) and removing noise particles (image D (9)), are performed in steps. Because of the in-homogeneities of schlieren image, extra dilatation method (image E (9)) needs to be performed. Finally, the processed ninth image E (9) is combined to previous image E (1–8) and forms a combination image. According to

Fig. 4. The image processing of raw images obtained by schlieren imaging technology with multi-threshold technique. 963

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temperature of 700 K. Higher luminousness and lower scattering can be found in the downstream region for spray ‘d’ than spray ‘a’, and both of them behave narrower tip periphery than spray ‘c’ discussed above. At the same time, the result shows that spray ‘d’ decreases liquid penetration by almost 34% relative to spray ‘a’ from 91 mm to 65 mm at 1.0 ms after start of injection. The reason is that denser ambient gas is drawn into the spray tip plume during the break up time with higher ambient pressure. When the ambient pressure increases, more gas is entrained into the spray, which decreases the penetration velocity because of the conservation of momentum, then decreases the penetration length finally. For the comparison between the sprays under transcritical (T, P) and supercritical conditions, the transition from spray ‘b’ and ‘d’ to spray ‘f’ is understandable. With only the temperature or pressure exceeding the critical point, the structure of spray ‘b’ and ‘d’ tends to be more similar with spray ‘f’, which behaves as shorter penetration and quite smooth and stable periphery under supercritical condition. Different from the classical two-phase spray phenomenon that droplets breakup and vaporize coupled with ligaments are detected near the jet interface such as spray ‘c’ and ‘d’ [47], the smooth and stable periphery is developed with interfacial diffusion layers [15], as a result of the reduced gas-liquid interface and vanished surface tension forces under supercritical condition. It can be thought that spray ‘f’ is a composition of spray ‘b’ and ‘d’, because on the one hand, spray ‘b’ behaves like smoother boundary under transcritical (T) condition, on the other hand, spray ‘d’ behaves like shorter penetration under transcritical (P) condition. For the sprays under the same ambient pressure of 5 MPa, the ambient pressure of spray ‘c’, ‘d’ and ‘f’ has exceeded the critical point of diesel. With the ambient temperature increasing for spray ‘c’, ‘d’ and ‘f’, it means the downstream of the spray transfers from unevaporated state to supercritical phase. In addition, the liquid penetration length from spray ‘c’ to ‘f’ has decreased by almost 28% from 70 mm to 50 mm as the difference between transcritical and supercritical condition. Moreover, as spray ‘e’ and ‘f’ are both under supercritical condition, their structure is almost same except the liquid penetration length, which decreases by around 16% from spray ‘e’ (60 mm) to spray ‘f’ (50 mm). The reason is when the ambient pressure increases, the

Table 5 Test plan under different ambient conditions from ‘a’ to ‘f’ Ambient conditions

Spray codes

Ambient pressure (MPa)

Ambient temperature (K)

Subcritical Transcritical (T) Transcritical (P)

a b c d e f

1 1 5 5 3 5

700 800 600 700 800 800

Supercritical

temperatures are listed in Table 5. 3.1. Structure of spray under different ambient conditions 3.1.1. The structure of liquid spray It is believed that the discharged fuel is more influenced by the highpressure injection than the ambient interaction initially [43], existing as the visual droplets with certain diameter before fully evaporation and phase transition. These droplets result in the scattering of light and then be captured by high-speed camera. Fig. 6 shows a comparison of the spray structure using backlight illumination technology under four different ambient conditions, which are all at 1.0 ms after start of injection. The drawn contour of sprays in Fig. 6 describes the boundary of liquid droplets. For the spray under transcritical (P) condition, it is clear to observe that spray ‘c’ has the widest tip periphery as the only spray under the lowest ambient temperature of 600 K, and the evaporation is hardly obvious than others. At the same time, spray ‘c’ is under the highest ambient pressure of 5 MPa, so the unevaporated droplets at the downstream section firstly lose momentum and are pushed aside by the subsequent droplets [44]. About the influence of the ambient pressure and temperature on the penetration and dispersion of spray, Siebers et al. from Sandia National Laboratory have also performed amount of groundbreaking spray work [42,45,46]. For the comparison between the sprays under subcritical and transcritical (P) conditions, spray ‘a’ and ‘d’ are under the same ambient

Fig. 6. Instantaneous spray images at 1.0 ms after start of injection with backlight illumination technology, where ‘a’ represents subcritical condition with yellow contour; ‘b’ represents transcritical (T) condition with green contour; ‘c, d’ represents transcritical (P) condition with blue contour; ‘e, f’ represents supercritical condition with red contour. 964

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For the further comparison between the sprays under the subcritical and transcritical (T) conditions, spray ‘a’ and ‘b’ are both under the ambient pressure of 1 MPa. It can be found the penetration length of spray ‘a’ and ‘b’ is almost similar no matter for liquid penetration in Fig. 6 or jet penetration in Fig. 7. In addition, combined with the previous analysis between spray ‘d’ under transcritical condition and spray ‘f’ under supercritical condition, in Fig. 6 the liquid penetration of spray ‘d’ is about 20 mm longer than that of spray ‘f’, although jet penetrations are almost same in Fig. 7. As the biggest difference of liquid and jet penetration is 26 mm and happens under supercritical condition, we can believe that the spray ‘f’ has some more obvious change from the location of liquid penetration length to jet penetration length. According to the previous researches [11,50–53], classical boundary between fluid and ambient gas is not clear any more, which explains the special periphery under supercritical condition. Moreover, it is widely accepted that there is not classical breakup and atomization process in supercritical environment, and that is also why the difference between liquid and jet penetration is more obvious.

resistance for spray development becomes stronger. This phenomenon is also in accordance with the conclusion by Higgins and Siebers in their early work [46]. 3.1.2. The structure of jet As the backlight illumination depends on the scattering of visual light, some invisible phenomena, such as evaporation and possible supercritical phase transition, will not be observed in detail. Hence, the images of schlieren under the same conditions with that of backlight illumination are shown in Fig. 7, where the drawn contour of sprays describes the structure of overall jet. For the spray under the transcritical (T) condition, the contour of spray ‘b’ tip is much narrower than others in capturing section [48], where the spray penetrates downstream with higher speed to capture the ambient gas. As its ambient temperature is 800 K, which exceeds the critical temperature of diesel, once the ambient pressure increases, it means that it transfers from gas phase region to supercritical fluid region as shown in Fig. 5. For the comparison between the sprays under the same ambient pressure of 5 MPa but different temperatures, the penetration length of spray ‘c’, ‘d’ and ‘f’ is almost same, since the influence of the ambient temperature on jet penetration is negligible [49]. As the lower luminousness represents higher scattering in schlieren imaging, spray ‘c’ shows lower luminousness at downstream region because of more unevaporated droplets, and spray ‘e’, ‘f’ shows the highest luminousness under supercritical condition. Moreover, as the ambient temperature of spray ‘d’ is between the spray ‘c’ and ‘f’, the upstream section shows lower luminousness like spray ‘c’ while the downstream section shows higher luminousness like spray ‘f’. It means there exists a transition near the downstream section of spray ‘d’ when the ambient temperature exceeds the critical point. The visualization of transition from the transcritical to supercritical condition has been recorded by Huang et al with the ambient temperature increasing from the transcritical to supercritical condition then descending [27]. It is widely accepted that under supercritical condition, the boundary between liquid and gas would be not visible any more, and thermophysical properties were between both of liquid and gas [10]. Moreover, the periphery of spray ‘f’ near the injector nozzle region under supercritical condition is narrower than spray ‘c’ and ‘d’. That means the density near the periphery of this region is lower. The similar phenomenon has also been captured by Chehroudi et al. [11], who studied the initial growth process of jet under supercritical conditions. It was found that there existed a twophase mixture with the density descending evidently and the boundary would be difficult to identify.

3.2. Jet length and R-parameter 3.2.1. Jet penetration and length The results of jet penetration, acquired using multi-threshold technique, are shown in Fig. 8. It can be found that the ambient pressure plays a more important role in jet penetration, as the nine cases collapse to isolated three parts classified by three different colors, where different intensities with the same color represent different ambient temperatures with the same ambient pressure. It also means the jet penetration behaves not obvious difference under different ambient temperatures but the same ambient pressure. This is because the penetration resistance is stronger with the higher ambient pressure, and the jet overall structure is the same with different ambient temperatures according to the principle of schlieren imaging. Moreover, it is noticeable to observe that the spray tip develops rapidly in a linear function at the beginning period of injection, then the tip velocity decreases gradually. The whole process can be separated as two periods. For the sake of clarity, the uncertainty analysis of jet penetration is shown in Fig. 9. It can be found the uncertainty is higher before 0.4 ms, as the spray develops quickly with the tip velocity of about 140 m/s. When the spray develops stably after 0.4 ms, the uncertainty percentage is limited within 5%. In order to analyze the influence of different ambient conditions on the jet penetration, an averaged jet penetration length is calculated and named as jet length. The jet length is defined as the averaged value

Fig. 7. Instantaneous spray images at 1.0 ms after start of injection with schlieren imaging technology. 965

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condition transfers from transcritical condition to supercritical condition under the same ambient pressure, such as from the spray ‘c’ to spray ‘d’ and then to spray ‘f’, the jet length decreases slightly and tends to be constant. The reason is that the jet downstream behaves as higher luminousness under supercritical condition, which influences the boundary detection, and the difference between spray ‘c’ and ‘f’ is only 5 mm and can be negligible. In Fig. 10(b), the jet length decreases 20 mm rapidly when the ambient condition transfers from subcritical condition to supercritical condition, such as from spray ‘a’ to spray ‘f’. 3.2.2. R-parameter R-parameter is the derivative of the jet penetration relative to the square root of time [32], which represents the external flow and the tip velocity of spray. As discussed above, a two-period development of jet penetration has been observed with an initially linear increasing then a slow deceleration. Detailed empirical correlations to describe the spray tip penetration have been proposed by many scholars. Naber and Siebers [42] proposed two-period correlations of jet penetration, which were later modified by Payri et al. [34]. The two-period correlations (S1 and S2) are described as follows: Before the breakup time (t < tr), the jet penetration is:

Fig. 8. Jet penetrations under different ambient conditions, where different intensities with the same color represent different temperatures.

S1 = Cv

2 P

t

(1)

f

After the breakup time (t > tr), the jet penetration is:

S2 =

Cv0.5 (2Ca )0.25 P 0.25D00.5 0.5 t tan ( /2)0.5 a0.25

(2)

Where tr is defined as the breakup time to divide two periods:

tr =

(2Ca)0.5 Cv tan ( /2) (

f a

D0 (3)

P )0.5

According to the equations from Eq. (1) to Eq. (3), the jet penetration is proportional to t during the first period, and then proportional to t0.5. Based on Eq. (2), a novel parameter was defined and used to represent the tip velocity of spray, which is explained in Eq. (4). When the spray cone angle, ambient pressure and spray momentum are constant, this parameter is considered as constant. Aldaraví [32] and Payri et al. [18] named it as R-parameter in their studies, and it showed a significant advantage to describe the development of the spray. For a verification and comparison, an example is shown in Fig. 11 under supercritical condition.

Fig. 9. Uncertainty analysis of jet penetration from schlieren images.

from when the tip velocity decreases to 10 m/s to the end time of injection (1.5 ms), and three averaging calculation periods are drawn in Fig. 8. The variations of jet length under different ambient temperatures and pressures are shown in Fig. 10. In Fig. 10 (a), when the ambient

R=

(S2) (t 0.5)

Cv0.5 (2Ca )0.25 P 0.25D00.5 = M 0.25 tan ( /2)0.5 a0.25

a

0.25

tan ( /2)

0.5

(4)

Fig. 10. Variations of jet lengths under different ambient temperatures and pressures, where sprays from ‘a’ to ‘f’ are defined in Table 5, and four different colors of symbol accord with that drawn in Fig. 7. 966

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diesel engine usually has larger cylinders (about 300 mm and larger), but also it is equipped with larger injector nozzles for higher flow rates. It can be found that the development of liquid and jet penetrations is consistent until the time when the liquid penetration stops developing and tends to be constant. The duration from when the liquid penetration tends to be constant to the end time of injection (1.5 ms) is called steady time in this work. During the steady time, the liquid penetration fluctuates but always in a certain range. This phenomenon is more noticeable under supercritical conditions, such as the circled parts drawn in Fig. 14. It can be explained that more ambient hot gases recirculate from downstream to the liquid tip section. Moreover, the fluctuation also verifies the well-known knowledge that the density is between both of liquid and gas, and the two-phase boundary is unstable under supercritical condition [13,14]. In addition, with the ambient pressure increasing, the start of steady time all happens later. It is more obvious for the lower ambient temperature, as the ambient pressure dominates the breakup and vaporization. For example, in Fig. 14(a) the start time for spray ‘b’ is around 0.7 ms, but in Fig. 14(c), the start time for spray ‘d’ is 1.0 ms. Moreover, the start of steady time is constant for supercritical condition, such as spray ‘e’ and ‘f’, which is both stable at 0.7 ms. For the sake of clarity, the uncertainty analysis of liquid penetration is shown in Fig. 15, and the error range is limited within 7%. In order to analyze the influence of various parameters on the liquid penetration, an average value is calculated during the steady time and named as liquid length. As shown in Fig. 16, the variations of liquid length are noticeable especially under higher ambient temperature and pressure. In Fig. 16(a), the liquid length is continuously descending from subcritical (yellow symbol) to supercritical condition (red symbol). When the ambient pressure is above the critical pressure (red and purple dashed lines), the decrease of liquid length is even more obvious, for example the decrease is 32 mm from spray ‘c’ (86 mm) to spray ‘f’ (54 mm). It is because once the spray is injected into supercritical condition, classical atomization and vaporization phenomenon is replaced by supercritical fluid properties, such as reduced gas-liquid interface and vanished surface tension forces [15]. This agrees with previous conclusions about liquid spray structure. As the ambient temperature hardly have obvious effect on the jet length, there exists a difference between jet and liquid length. As shown in Fig. 17, the biggest difference of 35 mm happens under supercritical condition and it also accords with the analysis about jet structure. In Fig. 16 (b), the jet and liquid length both decrease with the ambient pressure increasing. When the ambient temperature is above the critical temperature, the decrease of liquid length is also more obvious (purple dashed line), for example the decrease is 20 mm from spray ‘b’ (86 mm) to spray ‘e’ (66 mm).

Fig. 11. An example and comparison between jet penetration and R-parameter.

As shown in Fig. 12(a), R-parameter is not constant at the beginning and end parts during the injection process, and the fluctuation is more evident under lower ambient pressure (blue line). According to Fig. 12(b), the error range is limited within 0.15 m/s0.5 with the percentage of 5% after 0.4 ms, which can be ignored and so error bars are not shown in Fig. 13. In order to analyze the influence of different ambient conditions on the R-parameter, average values of 9 cases are calculated in a steady time from 0.4 ms to 1.0 ms after start of injection. The variations of R-parameters under different ambient temperatures and pressures are shown in Fig. 13. It is easy to find R-parameter is less affected by the ambient temperature than the ambient pressure. According to Eq. (4), it can be seen that the R-parameter is related to the spray cone angle tan ( /2) 0.5 and ambient density a 0.25 , but not related to the ambient temperature. This result is also similar with the previous conclusions about jet length. The R-parameter under supercritical condition becomes smaller than that under subcritical condition. The tendency of both R-parameter and jet length are nearly same in Fig. 13 (a), which also means the second period of spray penetration dominates the eventual jet development. 3.3. Liquid length The liquid penetration is acquired using multi-threshold technique based on backlight illumination images. As shown in Fig. 14, dashed lines are depicted to represent liquid penetration. It is evident to find that liquid penetration (around 80 mm) is longer than that of automotive diesel engine (around 15 mm [33]) when the ambient pressure and temperature is 5 MPa and 800 K. The reason is not only the marine

Fig. 12. (a) R-parameter calculated under different ambient conditions. (b) Uncertainty analysis of R-parameter. 967

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Fig. 13. Variations of R-parameter under different ambient temperatures and pressures, where dashed lines represent jet lengths and solid lines represent Rparameters.

3.4. Liquid and jet cone angles The liquid and jet cone angles are measured and defined in Fig. 3 using two optical diagnostic measurements described in Section 2.3. In Fig. 18, three graphs have the same tendency that the liquid cone angle decreases at the beginning of injection and then tends to be stable till the end of injection. The reason is that the initial fuel temperature is set at engine conditions (303 K) in this work, it requires a certain time for heat transfer to heat up the discharged fuel spray. As a result, there exists a steady time from when the cone angle tends to be stable (0.5 ms) to the end of injection (1.5 ms). During the period of steady time, an average value is calculated to analyze the influence of various parameters on the liquid and jet cone angles. As shown in Fig. 19, it is clear to find there exists a trade-off relationship between the ambient temperature and pressure on the liquid cone angle. In addition, in Fig. 19(a) the jet cone angle is bigger than liquid cone angle, which means more hot gas entrainment, evaporation and phase transition happen near the injector outlet. It is also found that the liquid cone angle decreases with the ambient temperature increasing, while the jet cone angle behave as almost constant. This can be explained according to previous conclusion about liquid and jet length, as the spray angle and penetration have a strong relationship. Moreover, for better comparison and analysis, Fig. 20 shows the cone angle difference between the liquid and jet cone angles for six selected cases. The general cone angle difference is around 14° except the spray ‘c’ is around 10°. The reason is that the ambient temperature of 600 K is only for the spray ‘c’, while others are above or near the critical region (730 K). In Fig. 19(b), the ambient pressure has an increasing influence on liquid and jet cone angles. This tendency of the ambient pressure on

Fig. 15. Uncertainty analysis of liquid penetration from backlight illumination images.

the cone angles in Fig. 19(b) is contrary to that on the penetration lengths in Fig. 16(b). The trade-off relationship between jet cone angle and jet penetration can also be explained by previous Eq. (2), where the jet penetration S2 is relative to the tan ( /2) 0.5, so when the jet cone angle increases, the jet penetration behaves as a descending tendency. 4. Conclusions In this work, diesel spray and atomization characteristics under

Fig. 14. Liquid penetrations compared with jet penetrations under different ambient conditions, where dashed lines represent liquid penetrations and others are same with Fig. 8. 968

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Fig. 16. Variations of liquid lengths compared with jet lengths under different ambient temperatures and pressures, where dashed lines represent liquid lengths and solid lines represent jet lengths.

time, spray cone angle and R-parameter have been discussed. The following conclusions are drawn from the results obtained: 1. Under supercritical condition, because of the reduced gas-liquid interface and vanished surface tension forces, the spray liquid periphery is quite smooth and stable with shorter penetration than other ambient conditions. The jet of spray behaves as narrower periphery near the injector nozzle region and shows lower density at downstream region. 2. The jet penetration behaves as similar under the same ambient pressure but different temperatures, and higher ambient pressure produces a negative effect on the increase of the tip velocity and development of spray. When the condition transfers from subcritical to supercritical under the same ambient temperature, the jet length decreases by 20 mm rapidly. As the tendency of both R-parameter and jet length are same, the second period of spray penetration dominates the eventual jet development. 3. With the transition from transcritical condition to supercritical condition, the liquid length decreases by 32 mm and there exists an obvious decrease of 35 mm from jet length to liquid length. During the steady time, the liquid penetration fluctuates but always in a certain range especially under supercritical condition. 4. A trade-off relationship is found between the ambient temperature and pressure on the liquid cone angle, while only the ambient pressure has an increasing effect on the jet cone angle. In addition, when the jet cone angle increases, the jet penetration behaves as a descending tendency.

Fig. 17. The length difference between liquid and jet lengths, where sprays from ‘a’ to ‘f’ are defined in Table 5.

subcritical, transcritical and supercritical conditions have been analyzed in a large constant volume chamber based on marine engine scale. Optical diagnostic measurements are conducted through backlight illumination technology and schlieren imaging technology in an inert atmosphere. Nine cases are classified by means of different diesel phase transition regions as sub/trans/supercritical conditions. The detailed characteristics of spray structure, spray penetration and length, steady

Fig. 18. Liquid cone angles under different ambient conditions, where different intensities with the same color represent different temperatures. 969

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Fig. 19. Variations of liquid cone angles compared with jet cone angles under different ambient temperatures and pressures, where dashed lines represent liquid cone angles and solid lines represent jet cone angles.

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Fig. 20. The cone angle difference between liquid and jet cone angles, where sprays from ‘a’ to ‘f’ are defined in Table 5.

As the high turbocharge and high injection pressure are becoming a tendency for future marine engines, the study of this work provides the experimental and theoretical base for the injection strategy under supercritical condition. For the future work, the spray combustion characteristics will be discussed in various reactive ambient conditions to optimize the emission control and combustion efficiency. Moreover, the spray and combustion characteristics about surrogates of marine light and heavy fuel oil will also be studied to promote the formulation of combustion kinetics and reaction mechanism. Declaration of Competing Interest None. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51425602), the Natural Science Foundation of Shanghai (Grant No. 18ZR1418700). We appreciated the technical support from 711 Heavy Duty Automobile Co. Ltd. 970

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