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Experimental investigation of spray characteristics of alternative aviation fuels
Energy Conversion and Management 88 (2014) 1060–1069
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Experimental investigation of spray characteristics of alternative aviation fuels Kumaran Kannaiyan, Reza Sadr ⇑ Micro Scale Thermo Fluids Laboratory, Mechanical Engineering Program, Texas A&M University at Qatar, Education City, Doha 23874, Qatar
a r t i c l e
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Article history: Received 24 July 2014 Accepted 12 September 2014 Available online 13 October 2014 Keywords: Alternate jet fuels Gas-to-liquid Spray characteristics Phase Doppler anemometry
a b s t r a c t Synthetic fuels derived from non-oil feedstock are gaining importance due to their cleaner combustion characteristics. This work investigates spray characteristics of two Gas-to-Liquid (GTL) synthetic jet fuels from a pilot-scale pressure swirl nozzle and compares them with those of the conventional Jet A-1 fuel. The microscopic spray parameters are measured at 0.3 and 0.9 MPa injection pressures at several points in the spray using phase Doppler anemometry. The results show that the effect of fuel physical properties on the spray characteristics is predominantly evident in the regions close to the nozzle exit at the higher injection pressure. The lower viscosity and surface tension of GTL fuel seems to lead to faster disintegration and dispersion of the droplets when compared to those of Jet A-1 fuel under atmospheric conditions. Although the global characteristics of the fuels are similar, the effects of fuel properties are evident on the local spray characteristics at the higher injection pressure. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Combined, aviation and ground transportation consume approximately half of all global petroleum production and contribute to about 60% of worldwide greenhouse gas emissions [1]. The aviation industry is expected to grow rapidly in the near future and use petroleum-based liquid fuels as its major source of energy [1]. However, dwindling oil resources, increasing environmental concerns, and the need for supply security have increased the interest in alternative (synthetic) fuels obtained from sources such as biomass, natural gas, and coal. These synthetic fuels are gaining importance as viable fuel alternatives for gas turbine engines as they do not require major modifications to existing fuel injection/combustion systems [2]. This advantage is further strengthened by the recent inclusion of synthetic hydrocarbons into jet fuel formulations [3]. As a result, the consumption of synthetic hydrocarbon fuels is expected to increase in the near future despite the current limitation on commercial production due to their high cost [4,5]. Gasto-Liquid (GTL) fuel, a liquid fuel derived from natural gas through the Fischer–Tropsch (F–T) process, has been the center of global interest due to its cleaner combustion characteristics compared to those of other synthetic fuels [6–11]. Recent study has investigated the feasibility and energy content of alternative synthetic jet fuels for aviation engines and shown that the use of F–T based fuels ⇑ Corresponding author. Tel.: +974 4423 0149. E-mail address: [email protected] (R. Sadr). http://dx.doi.org/10.1016/j.enconman.2014.09.037 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
can improve the energy efficiency of jet engines [12]. Although the term GTL indicates a chemical process, in this study the term is used to refer to kerosene (jet) fuel that is obtained from natural gas through the F–T process. The above-cited studies highlight the interest in GTL fuels and their role in powering gas turbine engines. However, it is important to note that both the chemical and physical properties of GTL fuels are different from those of conventional jet fuels due to differences in the methodologies by which the fuels are produced. These differences in fuel properties could potentially affect the spray and combustion performance in combustors. It is well recognized that the combustion process in gas turbine engines depends on the evaporation rate of the liquid fuel, which in turn is dependent on the fuel’s atomization characteristics. Furthermore, the combustion process ultimately affects the level of pollutant formation in combustors. Therefore, it is essential to thoroughly understand the atomization characteristics of GTL fuel to better understand the later processes of combustion and pollutant formation in combustors. Several researchers have previously investigated the spray characteristics of different ‘‘alternative’’ fuels [13–17]. Considering the vast nature of this research topic, a brief review of the literature pertaining to GTL aviation fuels is presented below.
1.1. Research background DeWitt et al. [18] conducted an evaluation study of F–T based fuels for use in aviation engines. The authors blended conventional
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JP-8 (military version jet fuel) and GTL fuels and compared their atomization performance in a T63 turbo-shaft engine. They reported a dramatic reduction in mean droplet size for the JP-8/ synthetic fuel blend relative to that of pure JP-8. Pucher et al. [19] also tested the combustion performance of JP-8, fatty acid methyl esters, and GTL fuels in a gas turbine combustor rig. To gain additional insight, they also measured the fuels’ spray characteristics at one location close to the nozzle exit at an injection pressure of 0.69 MPa by using the phase Doppler anemometry (PDA) technique. They reported that the central region of the GTL spray contained smaller droplets compared to the droplets of the JP-8 spray. This difference in droplet size was attributed to the lower density and viscosity of GTL fuel compared to those of JP-8 fuel. In both of these studies, the spray characteristics were measured under ambient atmospheric conditions for one GTL blend and at only a few locations downstream of the nozzle. Kook and Pickett [20] investigated macroscopic spray characteristics such as penetration length and cone angle for diesel, JP-8, JetA, GTL, and coal-based liquid fuels in a constant-volume chamber. Experiments were carried out under operating conditions similar to those encountered in diesel engines. The authors reported that the GTL fuel exhibited a shorter penetration length than the penetration lengths of conventional diesel and jet fuels, which could be attributed to the faster evaporation characteristics of the GTL fuel. Mondragon et al. [21] studied the spray and combustion behavior of alternative jet fuels such as Bio-jet, Jet-A, JP-8, and F–T based synthetic fuels; however, their study did not include the GTL fuel. The authors examined spray characteristics such as the Sauter mean diameter (SMD), dispersion, and spray cone angle by using a pressure swirl nozzle in a quiescent environment. They reported that the measured and predicted SMD values were within 5% of each other for the fuels studied, and that this small difference was attributed to the differences in the fuels’ physical properties. The authors emphasized that additional characterization was needed to quantify the spray features of the fuels beyond the SMD measurement. Recently, Kumaran and Sadr [22] compared the global spray characteristics of three GTL fuels with those of conventional Jet A-1 fuel under atmospheric conditions using the planar laser diagnostic technique of global sizing velocimetry (GSV). A comparison of the droplet size distribution and mean diameters between the GTL and Jet A-1 fuels showed that the spray characteristics of the fuels under atmospheric conditions were similar. Later, the authors reported on the spray characteristics of GTL fuels at two axial locations (60 mm and 100 mm) downstream of the nozzle exit measured using PDA and compared these characteristics with the results of GSV measurements [23]. It is worth mentioning that both of these studies were inconclusive because the spray characteristics were only measured in regions far downstream of the nozzle exit.
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type is typically used as pilot nozzle for ignition purpose in commercial jet engines [25]. The injection pressures used here correspond to the pressure differentials encountered at different stages of an aircraft engine cycle as suggested by RR. The ambient conditions considered in this work are different from those encountered in actual gas turbine combustors, however the results of this study can help to decouple the effect of ambient-assisted evaporation from that of atomization.
2. Experimental details 2.1. Experimental facility and operating conditions An optically accessible experimental facility was designed and built to study jet fuel sprays in a controlled environment under atmospheric conditions. Fig. 1 shows a schematic and photograph of the experimental facility. Fuel is pressurized in the pump module to the desired injection pressure and delivered to the nozzle located inside the spray module. The nozzle injection pressure is monitored using a pressure transducer installed immediately upstream of the nozzle exit. The spray chamber is made of transparent acrylic glass and is vertically positioned, with a height of 1.2 m and a crosssectional area measuring 0.6 m by 0.6 m. A pressure swirl nozzle (proprietary of Rolls-Royce Plc., UK) with an exit diameter of 1 mm is positioned vertically downwards in the middle of the spray chamber by means of a 2-D traversing mechanism. This type of pilot nozzle is typically located at the center of the main nozzle in commercial jet engines for high-altitude relighting purpose. The nozzle flow rates for different fuels are about 0.77 ± 0.03 and 1.54 ± 0.05 L/min at 0.3 and 0.9 MPa injection pressures, respectively. The 2-D traversing setup facilitates the movement of the
1.2. Objective of this study All the above-cited studies highlight the interest in GTL fuel as an alternative to conventional jet fuels. More importantly, although a few studies have investigated the spray characteristics of GTL fuel, none have presented a thorough study of near-nozzle microscopic spray characteristics of these fuels. The objective of this work is to investigate the spray characteristics of GTL jet fuels and compare them with those of a conventional jet fuel. To this end, one commercial GTL fuel and one GTL blend are tested and their spray characteristics are compared with those of the conventional Jet A-1 fuel under atmospheric conditions. The droplet size and velocity in the spray field downstream of a pressure swirl nozzle are measured using phase Doppler anemometry at 0.3 and 0.9 MPa injection pressures. The nozzle used in this work was provided by Rolls-Royce (RR) as part of a research consortium framework [24]. Nozzle of this
Fig. 1. Schematic (a) and photograph (b) of the experimental setup: (1) supply tank, (2) value, (3) high pressure fuel pump, (4) pulsation dampener, (5) pressure gauge, (6) pressure-relief valve, (7) fuel filter, (8) three-way solenoid valve, (9) pressure transducer, (10) nozzle fuel supply pipe, (11) nitrogen gas cylinder, (12) hood, (13) nozzle, (14) spray chamber, (15) PDA transmitter and receiver probes, (16) CCD camera, (17) photomultiplier tube, (18) argon-ion laser and transmitting optics, (19) signal processor, (20) fuel vapor exhaust duct, and (21) liquid fuel drain duct.
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spray, which enables measurement at different locations within the spray without disturbing the optical arrangement. Further details regarding the experimental facility and its operation are reported elsewhere [22,23]. Upon verification of the symmetrical nature of the spray about the nozzle axis, measurements are carried out on one side of the spray at several axial and radial locations downstream of the nozzle exit. The fuel-injection pressures utilized in this work are suggested by Rolls-Royce Plc. and represent a range of pressure differentials encountered in gas turbine engines operating at different stages of an aircraft engine cycle. 2.2. Fuel composition details The chemical compositions of synthetic GTL fuels are less complex than those of conventional jet fuels, however, the fuel properties of GTL fuels are different from conventional jet fuels due to the differences in their production process. The chemical and physical properties of commercial GTL, GTL blend, and conventional Jet A-1 fuels are summarized in Table 1. The commercial GTL fuel is labeled as Commercial Synthetic Paraffinic Kerosene (CSPK) by Shell Inc., and the GTL blend (B-2) is a blend of CSPK and ‘‘Shellsol’’ solvents. Both the GTL fuels were supplied by Shell Inc., to study the effect of three major fuel composition parameters-namely, boiling (carbon cut) range, iso-to-normal paraffin ratio, and cyclic carbon content, on the atomization and combustion characteristics of the fuels. In this work, only the atomization of the fuels investigated, as discussed in later sections. Further details regarding the fuel properties and their preparation procedures are explained elsewhere [26]. 2.3. Optical diagnostics A 2-D phase Doppler anemometry (PDA) system from Dantec Dynamics [27] was employed to perform point-wise, simultaneous measurements of droplet size and velocity. The PDA system consists of a stand-alone argon-ion multi-line laser (from Ion Laser Technologies), a transmitter probe, a receiver probe, a photo-detector unit, and signal-processing hardware and software. A 290-mW multiline laser beam is split into four beams, two green (k = 532 nm) and two blue (k = 488 nm), to form the measurement volume. The resultant power for each beam emitted from the transmitter is approximately 40 mW. All four beams (two green and two blue) emitted from the transmitter converge at the focal length of the front lens and intersect to create the fringe patterns inside the measurement volume. The PDA system is integrated to the experimental setup, the schematic and photograph of the system is shown in Fig. 1. At each wavelength, the frequency of one of the two beams is shifted inside the transmitter to resolve the directional ambiguity in the flow [27]. The green and blue wavelengths are used to measure the axial and radial components of the velocity, respectively.
Table 1 Chemical and physical properties of jet fuels measured at 20 °C and 1 atm [26]. Properties
CSPK
B-2
Jet A-1
Density (kg/m ) Kinematic viscosity (mm2/s) Surface tension (mN/m) Hydrogen-to-carbon ratio Iso-paraffins (wt%) Normal paraffins (wt%) Iso-to-normal paraffin ratio Naphthenes (wt%) Carbon cut
737.5 1.37 23.5 2.27 55.70 43.40 1.55 0.5 Narrow (C7–C13)
762.7 1.60 24.4 2.152 58.13 26.68 2.44 15.6 Wide (C7–C16)
788.1 1.66 26.9 1.92 NA NA NA 0.18 (%v/v) NA
Distillation characteristics T50–T10 (°C) T90–T10 (°C)
6.6 22.5
10 27.5
22.2 67.1
3
The operating parameters for the PDA system are summarized in Table 2. In the PDA measurement technique, the droplet diameter is determined from the difference in the phase shift between two Doppler burst signals emitted by the droplet crossing the measurement volume, whereas the droplet velocity is determined from the Doppler burst frequency. The receiver probe is positioned at an angle of 42° to collect the first-order refraction Doppler signal in forward-scatter mode. The signals are then processed using a signal-processing unit to determine the droplet size and velocity. Further details regarding the operating principle and methodology of the PDA technique are described elsewhere [28]. The measurements performed in this study are carried out at different axial and radial locations downstream of the nozzle exit as shown in Fig. 2. The measurement locations are superimposed on a spray image to facilitate interpretation; however, the dimensions are not to scale. Measurements in the axial direction (x) were carried out in steps of Dx = 10 mm up to 40 mm from the nozzle exit, beyond which (x > 40 mm) the measurements were carried out in steps of Dx = 20 mm. In the region closest to the nozzle exit (x 6 40 mm), measurements in the radial direction were carried out in steps of Dr = 2.5 mm, and from x = 60 to 100 mm, measurements were performed in steps of Dr = 5 mm. However, the axial locations of x = 10 mm and 30 mm are not shown in Fig. 2 and the data for these axial locations are not presented in this work for the sake of brevity. At a given axial location, measurements were carried out in the radial direction until the PDA data sampling rate decreased by 97% relative to the maximum data sampling rate at that axial location. Details regarding the data sampling rate and trends are discussed in Section 3.1. At each radial location, measurements were conducted to collect either a maximum of 10,000 droplet samples or 15 s to obtain statistically independent droplet measurements. Five trials were conducted at each location. 2.4. Methodology verification and uncertainty Phase Doppler anemometry (PDA) is an absolute measurement technique in which the droplet size is determined from the phase difference measured by more than one detector and thus does not require calibration [28]. However, the PDA user settings, including the photomultiplier tube voltage (PMV), signal-to-noise ratio (SNR), and signal gain (SG), play a crucial role in determining the droplet size and number density [28]. A parametric study was performed in this work to evaluate the effects of the PMV and SG settings on the measured droplet diameter. The PMV was varied from 800 V to 1400 V in steps of 200 V, and SG was varied from 10 dB to 30 dB in steps of 4 dB. Based on the outcome of this study, the PMV and SG were selected in the region where the droplet diameter and number density variations showed an asymptotic trend, as suggested by Albrecht et al. [28]. The effect of SNR in this study was observed to have a negligible effect on the measured droplet size, as reported previously in the literature [29]. For all of the subsequent results, the PMV, SG, and SNR were taken to be 1200 V, 24 dB, and -2 dB, respectively. To analyze the uncertainty in the PDA measurements, an integral time scale (Ti) is typically used to ensure that the obtained data are statistically independent from each other. In this case, the integral time was calculated from the auto-covariance of the PDA measurements [27]. The integral time was evaluated at radial locations r = 0, 20 mm, and 40 mm at an axial location of x = 40 mm. The maximum integral time was observed to be 3 ms for this study. The data will be statistically independent when the sampling frequency is equivalent to 2Ti (in this case, 2Ti = 6 ms 167 Hz). However, in this study, the mean sampling frequency derived from data rate was approximately 1000 Hz, which is much higher than the
K. Kannaiyan, R. Sadr / Energy Conversion and Management 88 (2014) 1060–1069 Table 2 Operating parameters of 2-D PDA system. Operating parameters Transmitting optics Focal length Beam spacing Beam waist
400 mm 38 mm 1.35 mm
Receiving optics Focal length Scattering angle Aperture mask
500 mm 42° Mask-B
Measurement volume Diameter (dx) Length (dz) Fringe spacing No. of fringes
189 lm 3.97 mm 5.27 lm 36
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this error. In view of the high repeatability of the measurements, a combination of defect in the operation of the MDG and PDA user settings could have possibly contributed to the bias error. Further details regarding methodology verification and analysis have been reported elsewhere [23]. The experimental results reported in later sections have not been corrected for this bias trend. 3. Results and discussion This section presents and discusses the effect of fuel properties on local spray characteristics such as the mean droplet diameter, the axial droplet velocity, and the Weber number for GTL fuel CSPK, GTL blend B-2, and Jet A-1 fuel. The spray characteristics were measured at two nozzle injection pressures and at different spatial locations downstream of the nozzle exit. Furthermore, global parameters such as the effective spray cone angle and global SMD are also presented and discussed. 3.1. Local spray characteristics
Fig. 2. Details of the axial and radial measurement locations superimposed on a spray image. The dimensions are not to scale.
statistically independent data rate. Consequently, only one-sixth of the sampled data that are statistically independent contribute to the uncertainty estimation, as suggested by Benedict and Gould [30]. Therefore, the estimator of the variance of the mean diameter is calculated as r 2 ¼ r2 =N (where N = measurement sample ðdÞ d d size/6) and the uncertainty in the mean diameter will be equal to 1:95rd at a 95% confidence interval [28]. For example, the uncertainty in the mean droplet diameter (d10) of a Jet A-1 spray operating at an injection pressure of 0.3 MPa measured at an axial location of x = 40 mm along the nozzle axis (r = 0) is 14 ± 0.44 lm. In all of the presented results, the error bars represent the 95% confidence interval. A mono-disperse droplet generator (MDG) that produces a uniform droplet diameter stream was used to establish the overall uncertainty in the PDA measurements. The MDG produces a train of constant-diameter droplets when the excitation frequency of the piezo-electric transducers in the reservoir matches its resonant frequency [31]. The droplet diameter was altered by varying the liquid flow rate and excitation frequency. Water was used in the MDG to generate droplets along the nozzle axis inside the spray chamber. The PDA setup was then used to measure the droplet information, and the obtained results were compared with the diameter predicted by using the correlation provided by the MDG manufacturer. The droplet diameters measured by PDA exhibited a bias shift (10%) from the predicted diameter values. After a detailed analysis and discussion with TSI engineers, it was concluded that the MDG may have produced very small droplets (called satellite droplets) along with the parent droplets to cause
Although the spray characteristics were measured at more axial locations, the local spray characteristics are presented only for the axial locations shown in Fig. 2, as discussed in Section 2.3. The measurements along the radial direction were continued until the data rate decreased below 3% of the maximum value at a given axial location. The measurement data rate (number of droplets measured per second) can be affected by the PDA user settings and does not represent any physical parameter of interest. However, the PDA settings were identically maintained across different experimental conditions. The measurement data rate may be used as a metric to determine the farthest measurement location from the nozzle axis in the radial direction where the presence of droplets becomes insignificant. As a result, the differences in the data rate profiles could be related to the differences in the fuel spray characteristics. The radial profiles of the measurement data rate at different axial locations (x) and injection pressures for CSPK and Jet A-1 fuels are presented in Fig. 3. The data rates were normalized for each radial location (r) using the maximum data rate at that axial location to highlight the differences in trends at different axial locations. Only the CSPK and Jet A-1 fuel data rate profiles are presented because the B-2 fuel data rate profiles are similar to those of the CSPK fuel. The data rate trends are shown in 3-D with colored lines obtained using a fifth-order polynomial fit to facilitate interpretation and discussion. At an injection pressure of 0.3 MPa, the radial data rate profiles show an initial drop in the outward radial direction from the nozzle axis (r = 0) before reaching a second peak for both fuels at axial locations closer to the nozzle exit (x = 20 and 40 mm). However, the magnitude of the initial drop in Jet A-1 is higher than that of CSPK. This initial drop may be attributed to the swirl imparted to the fluid by the nozzle. Beyond x > 40 mm, the second peak is not distinctly observed as the swirl intensity decreases with axial distance. The difference in data rate profiles between the fuels could be due to the low kinematic viscosity and surface tension of the CSPK compared with those of the Jet A-1 fuel, thus resulting in faster disintegration and dispersion of droplets in the CSPK. These fluid properties affect the flow dynamics inside the nozzle geometry, which in turn affect the primary atomization of the fuel. With an increase in injection pressure, the effect of these fuel properties on primary atomization (in terms of data rate) can be observed over a larger axial distance due to a swirl enhancement proportional to the increase in injection pressure. In all cases, the maximum data rate is observed in the spray core region, i.e., within 0 < r < 20 mm at
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Fig. 3. Variation of normalized data rate (frequency) at different axial locations for CSPK (left) and Jet A-1 (right) fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
0.3 MPa and within 0 < r < 40 mm at 0.9 MPa. The increase in the spread of the peak region with an increase in injection pressure could be related to the increase in the spray cone angle. Furthermore, an estimate of the spray boundary can also be obtained from the data rate profiles using the locus of the outermost radial measurement location (i.e., the maximum radial measurement location R) at different axial locations. Here, R is taken as the radial distance where the data rate reaches 3% of the maximum data rate at that axial location. The variation in R as a function of axial distance is shown in Fig. 4. At an injection pressure of 0.3 MPa, the spray boundary is observed to be similar between the fuels, whereas at 0.9 MPa, a small difference between the fuels can be detected. Fig. 5 shows the radial variation in the arithmetic mean droplet diameter (d10) measured at different axial locations downstream of the nozzle exit for the CSPK, B-2, and Jet A-1 fuels. The difference in d10 between the fuels is within the measurement uncertainty at most of the axial locations. Therefore, the d10 data are compared across different axial locations for a given fuel instead of comparing different fuels at a given axial location. The effect of injection pressure and fuel type on the radial distribution of the mean droplet size is shown along the vertical and horizontal directions, respectively. Because it is difficult to distinguish the radial profiles in the region 0 < r < 20, a closer view of that region is also shown for each case. The error bars representing the 95% confidence interval are shown at only two axial locations (x = 60 mm and 100 mm) to avoid data congestion. Fig. 5 shows the radial variation in the mean droplet diameter at each axial location up to the previously determined radial distance R. For all fuels, d10 decreases with an increase in injection pressure at all axial locations except at x = 20 mm, where the effect is observed to be minimal. As expected for a pressure swirl nozzle, the mean droplet size at the nozzle axis (r = 0) increases with axial distance at both injection pressures. At 0.9 MPa, the values of d10 for CSPK are lower than those of the B-2 and Jet A-1 fuels at all axial locations. This difference could be related to the combined
Fig. 4. Variation in the maximum radial measurement location (R) as a function of axial distance for CSPK and Jet A-1 fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom).
effect of low kinematic viscosity and surface tension of CSPK compared to those of B-2 and Jet A-1, as reported by Lee and Reitz [32]. This reduction in droplet size could further be promoted by the low distillation characteristics of CSPK relative to those of Jet A-1; however, the effect of this factor is minimal in this case under ambient atmospheric conditions. The radial profiles of B-2 and Jet A-1 fuels are similar because the physical properties of the fuels are similar to one another (see Table 1). At both pressures, closer to the nozzle
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Fig. 5. Comparison of mean droplet diameter (d10) radial profiles measured at different axial locations for CSPK, B-2, and Jet A-1 fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
exit (at x = 20 and 40 mm), the mean droplet diameter near the spray boundary (near R) shows a continuously increasing trend. It should be noted that beyond the last data point shown, the data rate near the spray boundary declined by more than 97% (see Fig. 3), indicating a large negative radial gradient for droplet distribution. Beyond this point is the region outside of the spray boundary, where no droplets are expected to be detected by the probe in the radial direction. Moving further downstream from the nozzle exit (at x = 60 and 80 mm), a region with a smoother negative radial gradient for droplet distribution is observed near the spray boundary before the droplet counts fall below 3%. Ultimately, at x = 100 mm, the mean droplet diameter exhibits a plateau trend before decreasing from the peak value in that region. This shift in the trend is due to the increase in the spread of the spray boundary region with an increase in distance along the axial direction. For example, larger droplets are contained within a narrow spray boundary region (large spatial gradient) closer to the nozzle exit, whereas this region spreads out radially with an increase in axial distance. Therefore, more measurement points are located within the spray boundary at farther axial distances, enabling better detection of the plateau and the decreasing trend of the mean droplet diameter. The overall shape and trends of the d10 axial profiles are similar across the different fuels. Fig. 6 shows a comparison of radial profiles for the local Sauter mean diameter (SMD), which is of interest for combustion applications, at different axial locations downstream of the nozzle exit for the CSPK, B-2, and Jet A-1 fuels at injection pressures of 0.3 MPa and 0.9 MPa. The local SMD is independently determined at each radial location based on the droplet sizes measured at that location. For all axial locations, the local SMD is observed to initially decrease in the radial direction from the nozzle axis, increasing thereafter until reaching the spray periphery. This trend is similar for all fuels and may be attributed to the swirl motion imparted to the fluid in the pressure swirl nozzle [33,34]. The decrease in SMD with axial distance at a given radial location is more evident at 0.9 MPa than at 0.3 MPa. In contrast to the d10 profiles, Fig. 7 shows no clear trend that could be inferred from the radial profiles of the SMD between fuels. The comparison is only shown at two axial locations, x = 40 mm
and 80 mm, because the trends at other axial locations are similar. In addition to the droplet diameter profiles, the droplet velocity profiles are also compared between the fuels. Fig. 8 shows the radial variation of droplet mean axial velocity for CSPK, B-2, and Jet A-1 at different injection pressures. It should be noted that because the radial profiles of the mean axial velocity of the droplets are significantly different, the data are compared between the fuels at a given axial location, in contrast to the comparisons made in Figs. 5 and 6. The radial profiles are only presented at x = 40, 60, and 80 mm because the trends are similar at other axial locations. In addition, different y-axis ranges are considered for 0.3 MPa and 0.9 MPa cases to better highlight the differences thereof. At 0.3 MPa, the fuels exhibit only a slight difference in droplet mean axial velocity profiles at all axial locations, which is in line with the trend shown in Fig. 3. With the increase in injection pressure, the droplet velocities of the GTL fuels show a much higher magnitude than those of Jet A-1. It must be noted that the significant difference in droplet mean axial velocities is seen only at 0.9 MPa injection pressure which may be associated to the dominance of inertial force. In addition to the inertial force, the surface tension of Jet A-1 is higher than CSPK and B-2 by about 12% and 9%, respectively. This will have an influence on the droplet disintegration and dispersion at such high flow field. This corroborates well with the data rate and its distribution presented in Fig. 3. Therefore, it would be reasonable to say that this difference could be a combined effect of high inertial force and the difference in surface tension between the fuels. At an axial location closer to the nozzle exit, x = 40 mm, the radial profiles show a continuously increasing trend from the nozzle axis towards the spray periphery because larger droplets with higher momentum are drawn towards the spray periphery. Further downstream, at x = 60 mm, the large entrainment of ambient air reduces the droplet velocity in the spray periphery region due to higher drag, resulting in a plateau in the velocity profile for the GTL fuels. As mentioned earlier, the velocity profile of Jet A-1 fuel shows an increasing trend because droplet disintegration and dispersion are delayed in this case. At x = 80 mm, the radial velocity profile shows a plateau for Jet A-1 fuel, whereas it is continuously decreasing for the other fuels due to the large entrainment of ambient air.
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Fig. 6. Comparison of radial profiles of local Sauter mean diameter (SMD) measured at different axial locations for CSPK, B-2, and Jet A-1 fuels at 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
Fig. 7. Comparison of SMD profiles between fuels at axial locations, x = 40 mm (left) and 80 mm (right) measured at 0.3 MPa (top) and 0.9 MPa (bottom).
In addition to the radial variation in droplet mean axial velocity, the Weber number ðWe ¼ qg U 2R d32 =rÞ, a non-dimensional parameter, was also determined to identify the significance of the forces involved in the droplet characteristics of the different fuels. In this case, qg is the ambient gas density and UR is the relative velocity between the gas environment and the droplet. Because the ambient velocity was negligible in this study, the relative velocity was assumed to be equal to the droplet velocity (UR Ud). The SMD (d32) was considered to be the characteristic dimension, and r represents the surface tension of the liquid fuel. The local Weber number was calculated at all radial locations by using the physical properties of the fuels (Table 1), the local SMD, and the mean axial droplet velocity from the PDA measurements. The radial variations
of the We number for all fuels at different injection pressures are shown in Fig. 9. It should be noted that the radial variation in the Weber number is presented in different y-axis ranges for the injection pressures of 0.3 MPa and 0.9 MPa to facilitate data interpretation and more clearly highlight the trends observed. The effect of fuel properties on the We number was observed to be dominant in the near nozzle axial locations at 0.9 MPa when compared to those observed at 0.3 MPa. At 0.9 MPa, the radial profiles of We for Jet A-1 were lower than those of the GTL fuels owing to a lower droplet velocity and higher surface tension, although the density in this case was higher than that of the GTL fuels. However, the steep variation became gradual with the increase in axial distance from the nozzle exit, as observed in the trends of the droplet
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Fig. 8. Radial variation in the mean axial droplet velocity at several axial locations downstream of the nozzle exit for CSPK, B-2, and Jet A-1 at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom).
velocity radial profiles (see Fig. 8). Near the spray periphery, We was close to zero because the droplet velocity decreased in the spray periphery region. In addition to the radial variation, the axial variation in the local SMD and the droplet mean axial velocity along the nozzle axis are shown in Fig. 10. As expected for a pressure swirl nozzle, the local SMD is shown to increase and the axial velocity is found to decrease with axial distance for all fuels. Although the GTL fuels exhibit similar droplet velocity decay at 0.9 MPa, their SMD axial decay trends are not similar. The reason behind this anomalous behavior is not clear and needs to be further investigated in the future. In summary, all the above results, both radial and axial variations of droplet characteristics clearly indicate that the fuel properties, viscosity and surface tension, have larger effects in the nearnozzle region. This effect is more pronounced under high inertial force (i.e., higher injection pressure).
3.2. Global spray characteristics This section presents and discusses global spray parameters such as the effective spray cone angle and global SMD. The global parameters were determined using the data measured at all axial locations. Based on the radial profiles of the mean diameters, an approximate spray boundary can be determined based on the radial location at which the maximum of those parameters occurs (referred to as ‘‘rmax’’) [35]. Fig. 11 shows rmax as a function of axial distance for the mean diameters at 0.9 MPa. In Fig. 11, only the CSPK and Jet A-1 fuels are shown because they exhibit the maximum difference in fuel properties. The radial locations of maxima between the fuels overlap at certain locations; to resolve this feature, hollow symbols (for CSPK) are plotted at a larger scale than the filled symbols (for Jet A-1 fuel). The spray boundary previously obtained (see Fig. 4) is also shown to facilitate the comparison. The
Fig. 9. Comparison of radial profiles of local Weber number (We) calculated for CSPK, B-2, and Jet A-1 fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
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Fig. 10. Axial decay of Sauter mean diameter (SMD) and mean axial droplet velocity measured along the spray centerline (r = 0) for CSPK, B-2, and Jet A-1 at injection pressures of 0.3 MPa and 0.9 MPa.
spray boundary and rmax are consistent with the trends shown in Figs. 5 and 6, where the maximum mean diameter is observed near the spray periphery. The trends observed at an injection pressure of 0.3 MPa are similar and therefore not presented. The effective cone angle can be determined using the following relation [35]:
he ¼ 90 tan1 ðdr max =dxÞ
ð1Þ
The effective spray cone angles at 0.3 MPa and 0.9 MPa were determined to be 96° and 104°, respectively, with a maximum change of 4% and 2% between the fuels at 0.3 MPa and 0.9 MPa, respectively. Although the injection pressure increased threefold, the change in the effective spray cone was only 8%. As Lefebvre reported, this variation is due to the insensitive nature of the spray cone angle (after an initial rise) with respect to the injection pressure when kerosene fuel is injected into ambient atmospheric conditions using a pressure swirl nozzle [36]. It should also be noted that the effective spray cone angle estimated in this study for Jet A-1 fuel is within 6% of that reported for the same fuel and similar type of nozzle by Marchione et al. [35]. The marginal difference in effective spray cone angle could be due to the differences in operating conditions and nozzle dimensions.
The measured (global) SMD is also compared with those estimated using an empirical relation proposed for pressure swirl nozzles. This empirical relation was developed by Lefebvre [37] based on the results of several experiments performed to study the effects of liquid properties, nozzle dimensions, and ambient conditions on SMD:
_ 0:25 SMD ¼ 2:25 r0:25 l0:25 m DP0:5 q0:25 L L g L
ð2Þ
where mL is the mass flow rate, DPL is the pressure difference, and
lL is the liquid-phase dynamic viscosity. In this case, the global SMD values are calculated from the individual PDA measurements at each radial location based on a common time-period determination. The PDA measurement provides the arrival and transit time of the droplets in the measurement volume. Because the sampling rate varies with the radial location of the spray, a common time period (based on the arrival time of the droplets) is selected when postprocessing the data. From all the radial locations, the droplet diameters that fall within the common time period are included in the global SMD calculation. Therefore, the collection of droplet diameters from all the radial locations resembles the global size distribution of the spray at a given operating pressure. The trends of the measured SMD among the fuels are in line with those of the expected SMD, as reported in Table 3. At 0.3 MPa, the measured SMD values are observed to be lower than the estimated SMD by a maximum of 29%, whereas at 0.9 MPa, the difference between the measured and estimated SMD values is approximately 12%.
Table 3 Comparison of measured and estimated SMD values for CSPK, B-2, and Jet A-1 at different injection pressures. Fuel
SMD (lm) 0.3 MPa
Fig. 11. Comparison of radial locations of rmax of mean diameters (d10, SMD) for CSPK (open symbols) and Jet A-1 (filled symbols) at an injection pressure of 0.9 MPa. The spray boundary obtained using R is also shown as a dashed line.
CSPK B-2 Jet A-1
0.9 MPa
Measured
Estimated
Measured
Estimated
81 79 81
105 111 114
58 60 65
64 68 70
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The difference between the measured and estimated SMD values could be due to a bias shift in measurements, as discussed in Section 2.4. 4. Conclusions The results of this work show that the GTL and Jet A-1 fuels have similar global spray parameters, such as the effective spray cone angle and global SMD. Also, the differences between the radial profiles of the local SMD for the GTL and Jet A-1 fuels were minor. However, the droplet mean axial velocities of the Jet A-1 fuel were considerably lower than those of the GTL fuels. The difference was more prominent at the higher injection pressure which could be attributed to the difference in the inertial force and to the surface tension of the fuels. The spray characteristics of the CSPK-GTL fuel suggest that its lower kinematic viscosity and surface tension cause a faster disintegration and dispersion of the droplets in the core region of the spray compared to that of the Jet A-1 fuel. These results must be interpreted with caution as they do not fully take into account the difference in the volatilization characteristics of the tested fuels. Therefore, it is essential to investigate the spray characteristics of the fuels under gas turbine combustor conditions to understand the true benefits of the GTL fuel for such applications. In spite of the differences in the ambient conditions of these experiments and the actual combustor condition, the results of this study can help in decoupling the effect of ambient-assisted evaporation from that of atomization. Acknowledgements This publication was made possible by funding from the Qatar Science and Technology Park (QSTP) and NPRP Grant # 5-671-2278. We would also like to thank all the project partner team members at Rolls-Royce Plc UK, Shell Global Solutions UK, and DLR Germany for their continued support. In addition, we would like to thank Dr. Mahesh Panchagnula at the Indian Institute of Technology Madras for his valuable suggestions. References [1] Agarwal RK. Environmentally responsible air and ground transportation. In: Proceedings of 49th AIAA aerospace sciences meeting. Orlando, Florida; 4–7 January, 2011, AIAA-2011-0965. [2] Blakey S, Rye L, Wilson CW. Aviation gas turbine alternative fuels: a review. Proc Combust Inst 2011;33(2):2863–85. [3] Enright C. Aviation fuel standard takes flight. ASTM standardization news in magazines and newsletters. [accessed July 2014]. [4] Wood DA, Nwaoha C, Towler BF. Gas-to-liquids (GTL): a review of an industry offering several routes for monetizing natural gas. J Nat Gas Sci Eng 2012;9:196–208. [5] Rajab K, Karimi IA. Evaluation of utilization alternatives for stranded natural gas. Energy 2012;40(1):317–28. [6] Corporan E, DeWitt MJ, Belovich V, Pawlik R, Lynch AC, Gord JR, et al. Emission characteristics of a turbine engine and research combustor burning a Fischer– Tropsch jet fuel. Energy Fuels 2007;21(5):2615–26. [7] Le Clercq P, Domenico MK, Rachner M, Ivanova E, Aigner M. Impact of Fischer– Tropsch fuels on aero-engine combustion performance. In: Proceedings of 48th AIAA aerospace sciences meeting. Orlando, Florida; 4–7 January, 2010, AIAA2010-0613. [8] Saffaripour M, Zabeti P, Kholghy M, Thomson MJ. An experimental comparison of the sooting behavior of synthetic jet fuels. Energy Fuels 2011;25:5584–93. [9] Fyffe D, Moran J, Kumaran K, Sadr R, Al-Sharshani A. Effect of GTL-like jet fuel composition on GT engine altitude ignition performance Part I: Combustor operability. In: Proceedings of ASME Turbo Expo 2011: power for land, sea and air. Vancouver, Canada; 6–10 June, 2011, GT2011-45487.
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[10] Mosbach T, Gebel GC, Le Clercq P, Sadr R, Kumaran K, Al-Sharshani A. Investigation of GTL-like jet fuel composition on GT engine altitude ignition and combustion performance Part II: Detailed diagnostics. In: Proceedings of ASME Turbo Expo 2011: power for land, sea and air. Vancouver, Canada; 6–10 June, 2011, GT2011-45510. [11] Badami M, Nuccio P, Pastrone D, Signoretto A. Performance of a small-scale turbojet engine fed with traditional and alternative fuels. Energy Convers Manage 2014;82:219–28. [12] Hileman JI, Stratton RW, Donohoo PE. Energy content and alternative jet fuel viability. J Propul Power 2010;26(6):1184–95. [13] Bolszo CD, McDonell VG. Evaluation of plain-jet air blast atomization and development of alternative fuels in small gas turbine engine application. Atomization Sprays 2009;19(8):771–85. [14] Nguyen D, Ishida H, Shioji M. Gas-to-liquid sprays at different injection and ambient conditions. J Eng Gas Turbines Power 2011;133(32804):1–10. [15] Park SH, Lee CS. Applicability of dimethyl ether (DME) in a compression ignition engine as an alternative fuel. Energy Convers Manage 2014;86:848–63. [16] Agathou MS, Kyritsis DC. Electrostatic atomization of hydrocarbon fuels and bio-alcohols for engine applications. Energy Convers Manage 2012;60:10–7. [17] Yoon SH, Park SH, Suh HK, Lee CS. Effect of biodiesel–ethanol blended fuel spray characteristics on the reduction of exhaust emissions in a common-rail diesel engine. J Energy Resour Technol 2011;132(4). 042201: 1–7. [18] DeWitt MJ, Striebich R, Shafer L, Zabarnick S, Harrison WE, Minus DE, Edwards T. Evaluation of fuel produced via the Fischer–Tropsch process for use in aviation applications. In: Proceedings of AIChE spring national meeting. Houston, Texas; 22–26 April, 2007, Paper 58b. [19] Pucher G, Allan W, LaViolette M, Piotras P. Emission from a gas turbine sector rig operated with synthetic aviation and biodiesel fuel. J Eng Gas Turbines Power 2011;133(111502):1–8. [20] Kook S, Pickett LM. Liquid length and vapor penetration of conventional, Fischer–Tropsch, coal-derived, and surrogate fuel sprays at high-temperature and high-pressure ambient conditions. Fuel 2012;93:539–48. [21] Mondragon UM, Brown CT, McDonell VG. Evaluation of spray and combustion behavior of alternative fuels for JP-8. In: Proceedings of 50th AIAA aerospace sciences meeting. Nashville, Tennessee; 9–12 January, 2012, AIAA-2012-0348. [22] Kumaran K, Sadr R. Effect of fuel properties on spray characteristics of alternative jet fuels using Global Sizing Velocimetry. Atomization Sprays 2014;24(7):575–97. [23] Kumaran K, Sadr R. Spray characteristics of Fisher–Trophsch alternate jet fuels. In: Proceedings of ASME Turbo Expo 2013. San Antonio, Texas; 3–7 June, 2013, GT2013-95761. [24] Shell News and Media release.; 2009 [accessed September 2014]. [25] Lefebvre AH. Fifty years of gas turbine fuel injection. Atomization Sprays 2000;10(3–5):251–76. [26] Bauldreay JM, Bogers PF, Al-Sharshani A. Use of surrogate blends to explore combustion-composition links for synthetic paraffinic kerosines. In: Proceedings of 12th international conference on stability, handling and use of liquid fuels. Sarasota, Florida; 16–20 October, 2011. [27] PDA reference manual. Dantec dynamics. ; 2012. [28] Albrecht HE, Borys M, Damaschke N, Tropea C. Laser Doppler and phase Doppler measurement techniques. Berlin: Springer-Verlag; 2003. [29] Kapulla R, Najera SB. Operation conditions of a phase Doppler anemometer: droplet size measurements with laser beam power, photomultiplier voltage, signal gain, and signal-to-noise ratio as parameters. Meas Sci Technol 2006;17:221–7. [30] Benedict LH, Gould RD. Towards better uncertainty estimates for turbulence statistics. Exp Fluids 1996;22:129–36. [31] MDG-100. Monosize droplet generator, operation and service manual. USA: TSI Inc.; 2010. [32] Lee K, Reitz RD. Investigation of spray characteristics from a low-pressure common rail injector for use in a homogenous charge compression ignition engine. Meas Sci Technol 2004;15(3):509–19. [33] Santolaya JL, Aisa LA, Calvo E, Garcia I, Cerecedo LM. Experimental study of near-field flow structure in hollow cone pressure swirl sprays. J Propul Power 2007;23(2):382–9. [34] Li T, Nishida K, Hiroyasu H. Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer. Fuel 2011;90:2367–76. [35] Marchione T, Allouis C, Amoresano A, Beretta F. Experimental investigation of a pressure swirl atomizer spray. J Propul Power 2007;23(5):1096–101. [36] Lefebvre AH. Atomization and sprays. New York: CRC Press; 2000. [37] Lefebvre AH. The prediction of Sauter mean diameter for simplex pressureswirl atomizers. Atomization Sprays 1987;3(1):37–51.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Experimental investigation of spray characteristics of alternative aviation fuels Kumaran Kannaiyan, Reza Sadr ⇑ Micro Scale Thermo Fluids Laboratory, Mechanical Engineering Program, Texas A&M University at Qatar, Education City, Doha 23874, Qatar
a r t i c l e
i n f o
Article history: Received 24 July 2014 Accepted 12 September 2014 Available online 13 October 2014 Keywords: Alternate jet fuels Gas-to-liquid Spray characteristics Phase Doppler anemometry
a b s t r a c t Synthetic fuels derived from non-oil feedstock are gaining importance due to their cleaner combustion characteristics. This work investigates spray characteristics of two Gas-to-Liquid (GTL) synthetic jet fuels from a pilot-scale pressure swirl nozzle and compares them with those of the conventional Jet A-1 fuel. The microscopic spray parameters are measured at 0.3 and 0.9 MPa injection pressures at several points in the spray using phase Doppler anemometry. The results show that the effect of fuel physical properties on the spray characteristics is predominantly evident in the regions close to the nozzle exit at the higher injection pressure. The lower viscosity and surface tension of GTL fuel seems to lead to faster disintegration and dispersion of the droplets when compared to those of Jet A-1 fuel under atmospheric conditions. Although the global characteristics of the fuels are similar, the effects of fuel properties are evident on the local spray characteristics at the higher injection pressure. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Combined, aviation and ground transportation consume approximately half of all global petroleum production and contribute to about 60% of worldwide greenhouse gas emissions [1]. The aviation industry is expected to grow rapidly in the near future and use petroleum-based liquid fuels as its major source of energy [1]. However, dwindling oil resources, increasing environmental concerns, and the need for supply security have increased the interest in alternative (synthetic) fuels obtained from sources such as biomass, natural gas, and coal. These synthetic fuels are gaining importance as viable fuel alternatives for gas turbine engines as they do not require major modifications to existing fuel injection/combustion systems [2]. This advantage is further strengthened by the recent inclusion of synthetic hydrocarbons into jet fuel formulations [3]. As a result, the consumption of synthetic hydrocarbon fuels is expected to increase in the near future despite the current limitation on commercial production due to their high cost [4,5]. Gasto-Liquid (GTL) fuel, a liquid fuel derived from natural gas through the Fischer–Tropsch (F–T) process, has been the center of global interest due to its cleaner combustion characteristics compared to those of other synthetic fuels [6–11]. Recent study has investigated the feasibility and energy content of alternative synthetic jet fuels for aviation engines and shown that the use of F–T based fuels ⇑ Corresponding author. Tel.: +974 4423 0149. E-mail address: [email protected] (R. Sadr). http://dx.doi.org/10.1016/j.enconman.2014.09.037 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
can improve the energy efficiency of jet engines [12]. Although the term GTL indicates a chemical process, in this study the term is used to refer to kerosene (jet) fuel that is obtained from natural gas through the F–T process. The above-cited studies highlight the interest in GTL fuels and their role in powering gas turbine engines. However, it is important to note that both the chemical and physical properties of GTL fuels are different from those of conventional jet fuels due to differences in the methodologies by which the fuels are produced. These differences in fuel properties could potentially affect the spray and combustion performance in combustors. It is well recognized that the combustion process in gas turbine engines depends on the evaporation rate of the liquid fuel, which in turn is dependent on the fuel’s atomization characteristics. Furthermore, the combustion process ultimately affects the level of pollutant formation in combustors. Therefore, it is essential to thoroughly understand the atomization characteristics of GTL fuel to better understand the later processes of combustion and pollutant formation in combustors. Several researchers have previously investigated the spray characteristics of different ‘‘alternative’’ fuels [13–17]. Considering the vast nature of this research topic, a brief review of the literature pertaining to GTL aviation fuels is presented below.
1.1. Research background DeWitt et al. [18] conducted an evaluation study of F–T based fuels for use in aviation engines. The authors blended conventional
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JP-8 (military version jet fuel) and GTL fuels and compared their atomization performance in a T63 turbo-shaft engine. They reported a dramatic reduction in mean droplet size for the JP-8/ synthetic fuel blend relative to that of pure JP-8. Pucher et al. [19] also tested the combustion performance of JP-8, fatty acid methyl esters, and GTL fuels in a gas turbine combustor rig. To gain additional insight, they also measured the fuels’ spray characteristics at one location close to the nozzle exit at an injection pressure of 0.69 MPa by using the phase Doppler anemometry (PDA) technique. They reported that the central region of the GTL spray contained smaller droplets compared to the droplets of the JP-8 spray. This difference in droplet size was attributed to the lower density and viscosity of GTL fuel compared to those of JP-8 fuel. In both of these studies, the spray characteristics were measured under ambient atmospheric conditions for one GTL blend and at only a few locations downstream of the nozzle. Kook and Pickett [20] investigated macroscopic spray characteristics such as penetration length and cone angle for diesel, JP-8, JetA, GTL, and coal-based liquid fuels in a constant-volume chamber. Experiments were carried out under operating conditions similar to those encountered in diesel engines. The authors reported that the GTL fuel exhibited a shorter penetration length than the penetration lengths of conventional diesel and jet fuels, which could be attributed to the faster evaporation characteristics of the GTL fuel. Mondragon et al. [21] studied the spray and combustion behavior of alternative jet fuels such as Bio-jet, Jet-A, JP-8, and F–T based synthetic fuels; however, their study did not include the GTL fuel. The authors examined spray characteristics such as the Sauter mean diameter (SMD), dispersion, and spray cone angle by using a pressure swirl nozzle in a quiescent environment. They reported that the measured and predicted SMD values were within 5% of each other for the fuels studied, and that this small difference was attributed to the differences in the fuels’ physical properties. The authors emphasized that additional characterization was needed to quantify the spray features of the fuels beyond the SMD measurement. Recently, Kumaran and Sadr [22] compared the global spray characteristics of three GTL fuels with those of conventional Jet A-1 fuel under atmospheric conditions using the planar laser diagnostic technique of global sizing velocimetry (GSV). A comparison of the droplet size distribution and mean diameters between the GTL and Jet A-1 fuels showed that the spray characteristics of the fuels under atmospheric conditions were similar. Later, the authors reported on the spray characteristics of GTL fuels at two axial locations (60 mm and 100 mm) downstream of the nozzle exit measured using PDA and compared these characteristics with the results of GSV measurements [23]. It is worth mentioning that both of these studies were inconclusive because the spray characteristics were only measured in regions far downstream of the nozzle exit.
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type is typically used as pilot nozzle for ignition purpose in commercial jet engines [25]. The injection pressures used here correspond to the pressure differentials encountered at different stages of an aircraft engine cycle as suggested by RR. The ambient conditions considered in this work are different from those encountered in actual gas turbine combustors, however the results of this study can help to decouple the effect of ambient-assisted evaporation from that of atomization.
2. Experimental details 2.1. Experimental facility and operating conditions An optically accessible experimental facility was designed and built to study jet fuel sprays in a controlled environment under atmospheric conditions. Fig. 1 shows a schematic and photograph of the experimental facility. Fuel is pressurized in the pump module to the desired injection pressure and delivered to the nozzle located inside the spray module. The nozzle injection pressure is monitored using a pressure transducer installed immediately upstream of the nozzle exit. The spray chamber is made of transparent acrylic glass and is vertically positioned, with a height of 1.2 m and a crosssectional area measuring 0.6 m by 0.6 m. A pressure swirl nozzle (proprietary of Rolls-Royce Plc., UK) with an exit diameter of 1 mm is positioned vertically downwards in the middle of the spray chamber by means of a 2-D traversing mechanism. This type of pilot nozzle is typically located at the center of the main nozzle in commercial jet engines for high-altitude relighting purpose. The nozzle flow rates for different fuels are about 0.77 ± 0.03 and 1.54 ± 0.05 L/min at 0.3 and 0.9 MPa injection pressures, respectively. The 2-D traversing setup facilitates the movement of the
1.2. Objective of this study All the above-cited studies highlight the interest in GTL fuel as an alternative to conventional jet fuels. More importantly, although a few studies have investigated the spray characteristics of GTL fuel, none have presented a thorough study of near-nozzle microscopic spray characteristics of these fuels. The objective of this work is to investigate the spray characteristics of GTL jet fuels and compare them with those of a conventional jet fuel. To this end, one commercial GTL fuel and one GTL blend are tested and their spray characteristics are compared with those of the conventional Jet A-1 fuel under atmospheric conditions. The droplet size and velocity in the spray field downstream of a pressure swirl nozzle are measured using phase Doppler anemometry at 0.3 and 0.9 MPa injection pressures. The nozzle used in this work was provided by Rolls-Royce (RR) as part of a research consortium framework [24]. Nozzle of this
Fig. 1. Schematic (a) and photograph (b) of the experimental setup: (1) supply tank, (2) value, (3) high pressure fuel pump, (4) pulsation dampener, (5) pressure gauge, (6) pressure-relief valve, (7) fuel filter, (8) three-way solenoid valve, (9) pressure transducer, (10) nozzle fuel supply pipe, (11) nitrogen gas cylinder, (12) hood, (13) nozzle, (14) spray chamber, (15) PDA transmitter and receiver probes, (16) CCD camera, (17) photomultiplier tube, (18) argon-ion laser and transmitting optics, (19) signal processor, (20) fuel vapor exhaust duct, and (21) liquid fuel drain duct.
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spray, which enables measurement at different locations within the spray without disturbing the optical arrangement. Further details regarding the experimental facility and its operation are reported elsewhere [22,23]. Upon verification of the symmetrical nature of the spray about the nozzle axis, measurements are carried out on one side of the spray at several axial and radial locations downstream of the nozzle exit. The fuel-injection pressures utilized in this work are suggested by Rolls-Royce Plc. and represent a range of pressure differentials encountered in gas turbine engines operating at different stages of an aircraft engine cycle. 2.2. Fuel composition details The chemical compositions of synthetic GTL fuels are less complex than those of conventional jet fuels, however, the fuel properties of GTL fuels are different from conventional jet fuels due to the differences in their production process. The chemical and physical properties of commercial GTL, GTL blend, and conventional Jet A-1 fuels are summarized in Table 1. The commercial GTL fuel is labeled as Commercial Synthetic Paraffinic Kerosene (CSPK) by Shell Inc., and the GTL blend (B-2) is a blend of CSPK and ‘‘Shellsol’’ solvents. Both the GTL fuels were supplied by Shell Inc., to study the effect of three major fuel composition parameters-namely, boiling (carbon cut) range, iso-to-normal paraffin ratio, and cyclic carbon content, on the atomization and combustion characteristics of the fuels. In this work, only the atomization of the fuels investigated, as discussed in later sections. Further details regarding the fuel properties and their preparation procedures are explained elsewhere [26]. 2.3. Optical diagnostics A 2-D phase Doppler anemometry (PDA) system from Dantec Dynamics [27] was employed to perform point-wise, simultaneous measurements of droplet size and velocity. The PDA system consists of a stand-alone argon-ion multi-line laser (from Ion Laser Technologies), a transmitter probe, a receiver probe, a photo-detector unit, and signal-processing hardware and software. A 290-mW multiline laser beam is split into four beams, two green (k = 532 nm) and two blue (k = 488 nm), to form the measurement volume. The resultant power for each beam emitted from the transmitter is approximately 40 mW. All four beams (two green and two blue) emitted from the transmitter converge at the focal length of the front lens and intersect to create the fringe patterns inside the measurement volume. The PDA system is integrated to the experimental setup, the schematic and photograph of the system is shown in Fig. 1. At each wavelength, the frequency of one of the two beams is shifted inside the transmitter to resolve the directional ambiguity in the flow [27]. The green and blue wavelengths are used to measure the axial and radial components of the velocity, respectively.
Table 1 Chemical and physical properties of jet fuels measured at 20 °C and 1 atm [26]. Properties
CSPK
B-2
Jet A-1
Density (kg/m ) Kinematic viscosity (mm2/s) Surface tension (mN/m) Hydrogen-to-carbon ratio Iso-paraffins (wt%) Normal paraffins (wt%) Iso-to-normal paraffin ratio Naphthenes (wt%) Carbon cut
737.5 1.37 23.5 2.27 55.70 43.40 1.55 0.5 Narrow (C7–C13)
762.7 1.60 24.4 2.152 58.13 26.68 2.44 15.6 Wide (C7–C16)
788.1 1.66 26.9 1.92 NA NA NA 0.18 (%v/v) NA
Distillation characteristics T50–T10 (°C) T90–T10 (°C)
6.6 22.5
10 27.5
22.2 67.1
3
The operating parameters for the PDA system are summarized in Table 2. In the PDA measurement technique, the droplet diameter is determined from the difference in the phase shift between two Doppler burst signals emitted by the droplet crossing the measurement volume, whereas the droplet velocity is determined from the Doppler burst frequency. The receiver probe is positioned at an angle of 42° to collect the first-order refraction Doppler signal in forward-scatter mode. The signals are then processed using a signal-processing unit to determine the droplet size and velocity. Further details regarding the operating principle and methodology of the PDA technique are described elsewhere [28]. The measurements performed in this study are carried out at different axial and radial locations downstream of the nozzle exit as shown in Fig. 2. The measurement locations are superimposed on a spray image to facilitate interpretation; however, the dimensions are not to scale. Measurements in the axial direction (x) were carried out in steps of Dx = 10 mm up to 40 mm from the nozzle exit, beyond which (x > 40 mm) the measurements were carried out in steps of Dx = 20 mm. In the region closest to the nozzle exit (x 6 40 mm), measurements in the radial direction were carried out in steps of Dr = 2.5 mm, and from x = 60 to 100 mm, measurements were performed in steps of Dr = 5 mm. However, the axial locations of x = 10 mm and 30 mm are not shown in Fig. 2 and the data for these axial locations are not presented in this work for the sake of brevity. At a given axial location, measurements were carried out in the radial direction until the PDA data sampling rate decreased by 97% relative to the maximum data sampling rate at that axial location. Details regarding the data sampling rate and trends are discussed in Section 3.1. At each radial location, measurements were conducted to collect either a maximum of 10,000 droplet samples or 15 s to obtain statistically independent droplet measurements. Five trials were conducted at each location. 2.4. Methodology verification and uncertainty Phase Doppler anemometry (PDA) is an absolute measurement technique in which the droplet size is determined from the phase difference measured by more than one detector and thus does not require calibration [28]. However, the PDA user settings, including the photomultiplier tube voltage (PMV), signal-to-noise ratio (SNR), and signal gain (SG), play a crucial role in determining the droplet size and number density [28]. A parametric study was performed in this work to evaluate the effects of the PMV and SG settings on the measured droplet diameter. The PMV was varied from 800 V to 1400 V in steps of 200 V, and SG was varied from 10 dB to 30 dB in steps of 4 dB. Based on the outcome of this study, the PMV and SG were selected in the region where the droplet diameter and number density variations showed an asymptotic trend, as suggested by Albrecht et al. [28]. The effect of SNR in this study was observed to have a negligible effect on the measured droplet size, as reported previously in the literature [29]. For all of the subsequent results, the PMV, SG, and SNR were taken to be 1200 V, 24 dB, and -2 dB, respectively. To analyze the uncertainty in the PDA measurements, an integral time scale (Ti) is typically used to ensure that the obtained data are statistically independent from each other. In this case, the integral time was calculated from the auto-covariance of the PDA measurements [27]. The integral time was evaluated at radial locations r = 0, 20 mm, and 40 mm at an axial location of x = 40 mm. The maximum integral time was observed to be 3 ms for this study. The data will be statistically independent when the sampling frequency is equivalent to 2Ti (in this case, 2Ti = 6 ms 167 Hz). However, in this study, the mean sampling frequency derived from data rate was approximately 1000 Hz, which is much higher than the
K. Kannaiyan, R. Sadr / Energy Conversion and Management 88 (2014) 1060–1069 Table 2 Operating parameters of 2-D PDA system. Operating parameters Transmitting optics Focal length Beam spacing Beam waist
400 mm 38 mm 1.35 mm
Receiving optics Focal length Scattering angle Aperture mask
500 mm 42° Mask-B
Measurement volume Diameter (dx) Length (dz) Fringe spacing No. of fringes
189 lm 3.97 mm 5.27 lm 36
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this error. In view of the high repeatability of the measurements, a combination of defect in the operation of the MDG and PDA user settings could have possibly contributed to the bias error. Further details regarding methodology verification and analysis have been reported elsewhere [23]. The experimental results reported in later sections have not been corrected for this bias trend. 3. Results and discussion This section presents and discusses the effect of fuel properties on local spray characteristics such as the mean droplet diameter, the axial droplet velocity, and the Weber number for GTL fuel CSPK, GTL blend B-2, and Jet A-1 fuel. The spray characteristics were measured at two nozzle injection pressures and at different spatial locations downstream of the nozzle exit. Furthermore, global parameters such as the effective spray cone angle and global SMD are also presented and discussed. 3.1. Local spray characteristics
Fig. 2. Details of the axial and radial measurement locations superimposed on a spray image. The dimensions are not to scale.
statistically independent data rate. Consequently, only one-sixth of the sampled data that are statistically independent contribute to the uncertainty estimation, as suggested by Benedict and Gould [30]. Therefore, the estimator of the variance of the mean diameter is calculated as r 2 ¼ r2 =N (where N = measurement sample ðdÞ d d size/6) and the uncertainty in the mean diameter will be equal to 1:95rd at a 95% confidence interval [28]. For example, the uncertainty in the mean droplet diameter (d10) of a Jet A-1 spray operating at an injection pressure of 0.3 MPa measured at an axial location of x = 40 mm along the nozzle axis (r = 0) is 14 ± 0.44 lm. In all of the presented results, the error bars represent the 95% confidence interval. A mono-disperse droplet generator (MDG) that produces a uniform droplet diameter stream was used to establish the overall uncertainty in the PDA measurements. The MDG produces a train of constant-diameter droplets when the excitation frequency of the piezo-electric transducers in the reservoir matches its resonant frequency [31]. The droplet diameter was altered by varying the liquid flow rate and excitation frequency. Water was used in the MDG to generate droplets along the nozzle axis inside the spray chamber. The PDA setup was then used to measure the droplet information, and the obtained results were compared with the diameter predicted by using the correlation provided by the MDG manufacturer. The droplet diameters measured by PDA exhibited a bias shift (10%) from the predicted diameter values. After a detailed analysis and discussion with TSI engineers, it was concluded that the MDG may have produced very small droplets (called satellite droplets) along with the parent droplets to cause
Although the spray characteristics were measured at more axial locations, the local spray characteristics are presented only for the axial locations shown in Fig. 2, as discussed in Section 2.3. The measurements along the radial direction were continued until the data rate decreased below 3% of the maximum value at a given axial location. The measurement data rate (number of droplets measured per second) can be affected by the PDA user settings and does not represent any physical parameter of interest. However, the PDA settings were identically maintained across different experimental conditions. The measurement data rate may be used as a metric to determine the farthest measurement location from the nozzle axis in the radial direction where the presence of droplets becomes insignificant. As a result, the differences in the data rate profiles could be related to the differences in the fuel spray characteristics. The radial profiles of the measurement data rate at different axial locations (x) and injection pressures for CSPK and Jet A-1 fuels are presented in Fig. 3. The data rates were normalized for each radial location (r) using the maximum data rate at that axial location to highlight the differences in trends at different axial locations. Only the CSPK and Jet A-1 fuel data rate profiles are presented because the B-2 fuel data rate profiles are similar to those of the CSPK fuel. The data rate trends are shown in 3-D with colored lines obtained using a fifth-order polynomial fit to facilitate interpretation and discussion. At an injection pressure of 0.3 MPa, the radial data rate profiles show an initial drop in the outward radial direction from the nozzle axis (r = 0) before reaching a second peak for both fuels at axial locations closer to the nozzle exit (x = 20 and 40 mm). However, the magnitude of the initial drop in Jet A-1 is higher than that of CSPK. This initial drop may be attributed to the swirl imparted to the fluid by the nozzle. Beyond x > 40 mm, the second peak is not distinctly observed as the swirl intensity decreases with axial distance. The difference in data rate profiles between the fuels could be due to the low kinematic viscosity and surface tension of the CSPK compared with those of the Jet A-1 fuel, thus resulting in faster disintegration and dispersion of droplets in the CSPK. These fluid properties affect the flow dynamics inside the nozzle geometry, which in turn affect the primary atomization of the fuel. With an increase in injection pressure, the effect of these fuel properties on primary atomization (in terms of data rate) can be observed over a larger axial distance due to a swirl enhancement proportional to the increase in injection pressure. In all cases, the maximum data rate is observed in the spray core region, i.e., within 0 < r < 20 mm at
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Fig. 3. Variation of normalized data rate (frequency) at different axial locations for CSPK (left) and Jet A-1 (right) fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
0.3 MPa and within 0 < r < 40 mm at 0.9 MPa. The increase in the spread of the peak region with an increase in injection pressure could be related to the increase in the spray cone angle. Furthermore, an estimate of the spray boundary can also be obtained from the data rate profiles using the locus of the outermost radial measurement location (i.e., the maximum radial measurement location R) at different axial locations. Here, R is taken as the radial distance where the data rate reaches 3% of the maximum data rate at that axial location. The variation in R as a function of axial distance is shown in Fig. 4. At an injection pressure of 0.3 MPa, the spray boundary is observed to be similar between the fuels, whereas at 0.9 MPa, a small difference between the fuels can be detected. Fig. 5 shows the radial variation in the arithmetic mean droplet diameter (d10) measured at different axial locations downstream of the nozzle exit for the CSPK, B-2, and Jet A-1 fuels. The difference in d10 between the fuels is within the measurement uncertainty at most of the axial locations. Therefore, the d10 data are compared across different axial locations for a given fuel instead of comparing different fuels at a given axial location. The effect of injection pressure and fuel type on the radial distribution of the mean droplet size is shown along the vertical and horizontal directions, respectively. Because it is difficult to distinguish the radial profiles in the region 0 < r < 20, a closer view of that region is also shown for each case. The error bars representing the 95% confidence interval are shown at only two axial locations (x = 60 mm and 100 mm) to avoid data congestion. Fig. 5 shows the radial variation in the mean droplet diameter at each axial location up to the previously determined radial distance R. For all fuels, d10 decreases with an increase in injection pressure at all axial locations except at x = 20 mm, where the effect is observed to be minimal. As expected for a pressure swirl nozzle, the mean droplet size at the nozzle axis (r = 0) increases with axial distance at both injection pressures. At 0.9 MPa, the values of d10 for CSPK are lower than those of the B-2 and Jet A-1 fuels at all axial locations. This difference could be related to the combined
Fig. 4. Variation in the maximum radial measurement location (R) as a function of axial distance for CSPK and Jet A-1 fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom).
effect of low kinematic viscosity and surface tension of CSPK compared to those of B-2 and Jet A-1, as reported by Lee and Reitz [32]. This reduction in droplet size could further be promoted by the low distillation characteristics of CSPK relative to those of Jet A-1; however, the effect of this factor is minimal in this case under ambient atmospheric conditions. The radial profiles of B-2 and Jet A-1 fuels are similar because the physical properties of the fuels are similar to one another (see Table 1). At both pressures, closer to the nozzle
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Fig. 5. Comparison of mean droplet diameter (d10) radial profiles measured at different axial locations for CSPK, B-2, and Jet A-1 fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
exit (at x = 20 and 40 mm), the mean droplet diameter near the spray boundary (near R) shows a continuously increasing trend. It should be noted that beyond the last data point shown, the data rate near the spray boundary declined by more than 97% (see Fig. 3), indicating a large negative radial gradient for droplet distribution. Beyond this point is the region outside of the spray boundary, where no droplets are expected to be detected by the probe in the radial direction. Moving further downstream from the nozzle exit (at x = 60 and 80 mm), a region with a smoother negative radial gradient for droplet distribution is observed near the spray boundary before the droplet counts fall below 3%. Ultimately, at x = 100 mm, the mean droplet diameter exhibits a plateau trend before decreasing from the peak value in that region. This shift in the trend is due to the increase in the spread of the spray boundary region with an increase in distance along the axial direction. For example, larger droplets are contained within a narrow spray boundary region (large spatial gradient) closer to the nozzle exit, whereas this region spreads out radially with an increase in axial distance. Therefore, more measurement points are located within the spray boundary at farther axial distances, enabling better detection of the plateau and the decreasing trend of the mean droplet diameter. The overall shape and trends of the d10 axial profiles are similar across the different fuels. Fig. 6 shows a comparison of radial profiles for the local Sauter mean diameter (SMD), which is of interest for combustion applications, at different axial locations downstream of the nozzle exit for the CSPK, B-2, and Jet A-1 fuels at injection pressures of 0.3 MPa and 0.9 MPa. The local SMD is independently determined at each radial location based on the droplet sizes measured at that location. For all axial locations, the local SMD is observed to initially decrease in the radial direction from the nozzle axis, increasing thereafter until reaching the spray periphery. This trend is similar for all fuels and may be attributed to the swirl motion imparted to the fluid in the pressure swirl nozzle [33,34]. The decrease in SMD with axial distance at a given radial location is more evident at 0.9 MPa than at 0.3 MPa. In contrast to the d10 profiles, Fig. 7 shows no clear trend that could be inferred from the radial profiles of the SMD between fuels. The comparison is only shown at two axial locations, x = 40 mm
and 80 mm, because the trends at other axial locations are similar. In addition to the droplet diameter profiles, the droplet velocity profiles are also compared between the fuels. Fig. 8 shows the radial variation of droplet mean axial velocity for CSPK, B-2, and Jet A-1 at different injection pressures. It should be noted that because the radial profiles of the mean axial velocity of the droplets are significantly different, the data are compared between the fuels at a given axial location, in contrast to the comparisons made in Figs. 5 and 6. The radial profiles are only presented at x = 40, 60, and 80 mm because the trends are similar at other axial locations. In addition, different y-axis ranges are considered for 0.3 MPa and 0.9 MPa cases to better highlight the differences thereof. At 0.3 MPa, the fuels exhibit only a slight difference in droplet mean axial velocity profiles at all axial locations, which is in line with the trend shown in Fig. 3. With the increase in injection pressure, the droplet velocities of the GTL fuels show a much higher magnitude than those of Jet A-1. It must be noted that the significant difference in droplet mean axial velocities is seen only at 0.9 MPa injection pressure which may be associated to the dominance of inertial force. In addition to the inertial force, the surface tension of Jet A-1 is higher than CSPK and B-2 by about 12% and 9%, respectively. This will have an influence on the droplet disintegration and dispersion at such high flow field. This corroborates well with the data rate and its distribution presented in Fig. 3. Therefore, it would be reasonable to say that this difference could be a combined effect of high inertial force and the difference in surface tension between the fuels. At an axial location closer to the nozzle exit, x = 40 mm, the radial profiles show a continuously increasing trend from the nozzle axis towards the spray periphery because larger droplets with higher momentum are drawn towards the spray periphery. Further downstream, at x = 60 mm, the large entrainment of ambient air reduces the droplet velocity in the spray periphery region due to higher drag, resulting in a plateau in the velocity profile for the GTL fuels. As mentioned earlier, the velocity profile of Jet A-1 fuel shows an increasing trend because droplet disintegration and dispersion are delayed in this case. At x = 80 mm, the radial velocity profile shows a plateau for Jet A-1 fuel, whereas it is continuously decreasing for the other fuels due to the large entrainment of ambient air.
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Fig. 6. Comparison of radial profiles of local Sauter mean diameter (SMD) measured at different axial locations for CSPK, B-2, and Jet A-1 fuels at 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
Fig. 7. Comparison of SMD profiles between fuels at axial locations, x = 40 mm (left) and 80 mm (right) measured at 0.3 MPa (top) and 0.9 MPa (bottom).
In addition to the radial variation in droplet mean axial velocity, the Weber number ðWe ¼ qg U 2R d32 =rÞ, a non-dimensional parameter, was also determined to identify the significance of the forces involved in the droplet characteristics of the different fuels. In this case, qg is the ambient gas density and UR is the relative velocity between the gas environment and the droplet. Because the ambient velocity was negligible in this study, the relative velocity was assumed to be equal to the droplet velocity (UR Ud). The SMD (d32) was considered to be the characteristic dimension, and r represents the surface tension of the liquid fuel. The local Weber number was calculated at all radial locations by using the physical properties of the fuels (Table 1), the local SMD, and the mean axial droplet velocity from the PDA measurements. The radial variations
of the We number for all fuels at different injection pressures are shown in Fig. 9. It should be noted that the radial variation in the Weber number is presented in different y-axis ranges for the injection pressures of 0.3 MPa and 0.9 MPa to facilitate data interpretation and more clearly highlight the trends observed. The effect of fuel properties on the We number was observed to be dominant in the near nozzle axial locations at 0.9 MPa when compared to those observed at 0.3 MPa. At 0.9 MPa, the radial profiles of We for Jet A-1 were lower than those of the GTL fuels owing to a lower droplet velocity and higher surface tension, although the density in this case was higher than that of the GTL fuels. However, the steep variation became gradual with the increase in axial distance from the nozzle exit, as observed in the trends of the droplet
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Fig. 8. Radial variation in the mean axial droplet velocity at several axial locations downstream of the nozzle exit for CSPK, B-2, and Jet A-1 at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom).
velocity radial profiles (see Fig. 8). Near the spray periphery, We was close to zero because the droplet velocity decreased in the spray periphery region. In addition to the radial variation, the axial variation in the local SMD and the droplet mean axial velocity along the nozzle axis are shown in Fig. 10. As expected for a pressure swirl nozzle, the local SMD is shown to increase and the axial velocity is found to decrease with axial distance for all fuels. Although the GTL fuels exhibit similar droplet velocity decay at 0.9 MPa, their SMD axial decay trends are not similar. The reason behind this anomalous behavior is not clear and needs to be further investigated in the future. In summary, all the above results, both radial and axial variations of droplet characteristics clearly indicate that the fuel properties, viscosity and surface tension, have larger effects in the nearnozzle region. This effect is more pronounced under high inertial force (i.e., higher injection pressure).
3.2. Global spray characteristics This section presents and discusses global spray parameters such as the effective spray cone angle and global SMD. The global parameters were determined using the data measured at all axial locations. Based on the radial profiles of the mean diameters, an approximate spray boundary can be determined based on the radial location at which the maximum of those parameters occurs (referred to as ‘‘rmax’’) [35]. Fig. 11 shows rmax as a function of axial distance for the mean diameters at 0.9 MPa. In Fig. 11, only the CSPK and Jet A-1 fuels are shown because they exhibit the maximum difference in fuel properties. The radial locations of maxima between the fuels overlap at certain locations; to resolve this feature, hollow symbols (for CSPK) are plotted at a larger scale than the filled symbols (for Jet A-1 fuel). The spray boundary previously obtained (see Fig. 4) is also shown to facilitate the comparison. The
Fig. 9. Comparison of radial profiles of local Weber number (We) calculated for CSPK, B-2, and Jet A-1 fuels at injection pressures of 0.3 MPa (top) and 0.9 MPa (bottom). For line color code please refer to the web version.
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Fig. 10. Axial decay of Sauter mean diameter (SMD) and mean axial droplet velocity measured along the spray centerline (r = 0) for CSPK, B-2, and Jet A-1 at injection pressures of 0.3 MPa and 0.9 MPa.
spray boundary and rmax are consistent with the trends shown in Figs. 5 and 6, where the maximum mean diameter is observed near the spray periphery. The trends observed at an injection pressure of 0.3 MPa are similar and therefore not presented. The effective cone angle can be determined using the following relation [35]:
he ¼ 90 tan1 ðdr max =dxÞ
ð1Þ
The effective spray cone angles at 0.3 MPa and 0.9 MPa were determined to be 96° and 104°, respectively, with a maximum change of 4% and 2% between the fuels at 0.3 MPa and 0.9 MPa, respectively. Although the injection pressure increased threefold, the change in the effective spray cone was only 8%. As Lefebvre reported, this variation is due to the insensitive nature of the spray cone angle (after an initial rise) with respect to the injection pressure when kerosene fuel is injected into ambient atmospheric conditions using a pressure swirl nozzle [36]. It should also be noted that the effective spray cone angle estimated in this study for Jet A-1 fuel is within 6% of that reported for the same fuel and similar type of nozzle by Marchione et al. [35]. The marginal difference in effective spray cone angle could be due to the differences in operating conditions and nozzle dimensions.
The measured (global) SMD is also compared with those estimated using an empirical relation proposed for pressure swirl nozzles. This empirical relation was developed by Lefebvre [37] based on the results of several experiments performed to study the effects of liquid properties, nozzle dimensions, and ambient conditions on SMD:
_ 0:25 SMD ¼ 2:25 r0:25 l0:25 m DP0:5 q0:25 L L g L
ð2Þ
where mL is the mass flow rate, DPL is the pressure difference, and
lL is the liquid-phase dynamic viscosity. In this case, the global SMD values are calculated from the individual PDA measurements at each radial location based on a common time-period determination. The PDA measurement provides the arrival and transit time of the droplets in the measurement volume. Because the sampling rate varies with the radial location of the spray, a common time period (based on the arrival time of the droplets) is selected when postprocessing the data. From all the radial locations, the droplet diameters that fall within the common time period are included in the global SMD calculation. Therefore, the collection of droplet diameters from all the radial locations resembles the global size distribution of the spray at a given operating pressure. The trends of the measured SMD among the fuels are in line with those of the expected SMD, as reported in Table 3. At 0.3 MPa, the measured SMD values are observed to be lower than the estimated SMD by a maximum of 29%, whereas at 0.9 MPa, the difference between the measured and estimated SMD values is approximately 12%.
Table 3 Comparison of measured and estimated SMD values for CSPK, B-2, and Jet A-1 at different injection pressures. Fuel
SMD (lm) 0.3 MPa
Fig. 11. Comparison of radial locations of rmax of mean diameters (d10, SMD) for CSPK (open symbols) and Jet A-1 (filled symbols) at an injection pressure of 0.9 MPa. The spray boundary obtained using R is also shown as a dashed line.
CSPK B-2 Jet A-1
0.9 MPa
Measured
Estimated
Measured
Estimated
81 79 81
105 111 114
58 60 65
64 68 70
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The difference between the measured and estimated SMD values could be due to a bias shift in measurements, as discussed in Section 2.4. 4. Conclusions The results of this work show that the GTL and Jet A-1 fuels have similar global spray parameters, such as the effective spray cone angle and global SMD. Also, the differences between the radial profiles of the local SMD for the GTL and Jet A-1 fuels were minor. However, the droplet mean axial velocities of the Jet A-1 fuel were considerably lower than those of the GTL fuels. The difference was more prominent at the higher injection pressure which could be attributed to the difference in the inertial force and to the surface tension of the fuels. The spray characteristics of the CSPK-GTL fuel suggest that its lower kinematic viscosity and surface tension cause a faster disintegration and dispersion of the droplets in the core region of the spray compared to that of the Jet A-1 fuel. These results must be interpreted with caution as they do not fully take into account the difference in the volatilization characteristics of the tested fuels. Therefore, it is essential to investigate the spray characteristics of the fuels under gas turbine combustor conditions to understand the true benefits of the GTL fuel for such applications. In spite of the differences in the ambient conditions of these experiments and the actual combustor condition, the results of this study can help in decoupling the effect of ambient-assisted evaporation from that of atomization. Acknowledgements This publication was made possible by funding from the Qatar Science and Technology Park (QSTP) and NPRP Grant # 5-671-2278. We would also like to thank all the project partner team members at Rolls-Royce Plc UK, Shell Global Solutions UK, and DLR Germany for their continued support. In addition, we would like to thank Dr. Mahesh Panchagnula at the Indian Institute of Technology Madras for his valuable suggestions. References [1] Agarwal RK. Environmentally responsible air and ground transportation. In: Proceedings of 49th AIAA aerospace sciences meeting. Orlando, Florida; 4–7 January, 2011, AIAA-2011-0965. [2] Blakey S, Rye L, Wilson CW. Aviation gas turbine alternative fuels: a review. Proc Combust Inst 2011;33(2):2863–85. [3] Enright C. Aviation fuel standard takes flight. ASTM standardization news in magazines and newsletters.
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