Gelled propellants offer good storability without sacrificing high energy performance and operability for rocket applications, and are being considered for certain future rocket and atmospheric propulsion applications such as tactical missiles, ramjets, etc
]. The study of the combustion mechanisms of single droplets is necessary for characterizing and understanding the phenomena of spray vaporization and combustion. Liquid propellant droplets have been investigated extensively and comprehensive data have been published [2
]. However, the ignition and combustion characteristics might be different, because of the influence of the gelling agent on the physical and chemical behavior of gelled fuel, so it is essential to conduct experimental and theoretical studies on the combustion of gelled fuel droplets.
Gels are multi-component propellants, and it will burn like a multi-component fuel. Extensive theoretical and experimental work has been carried out in the field of multi-component droplet evaporation/burning to understand the combustion mechanisms. Solomon and Natan presented the multicomponent droplet combustion [3
]. Then, Ghassemi et al
. studied the single droplet evaporation at elevated pressure and temperature [5
]. The results showed a three-staged evaporation: (1) a period of more volatile component evaporation; (2) the period of increasing droplet temperature and almost no evaporation; (3) a quasi linear evaporation period. They also observed microexplosions at low pressure, but no microexplosions at high pressure.
The first study of combustion of an inorganic gellant fuel was conducted by Nachmoni and Natan [6
]. Then, Arnold and Anderson studied the combustion process of inorganic silica gallant, JP-8-based gelled fuels droplets [8
]. The experiments showed that the gelling agent had a considerable influence on the typical d2
-law; a higher amount of gellant agent led to a reduced burning rate, and the droplet size as well as the droplet temperature during combustion depended strongly on the consistency of the gels.
Compared to the inorganic silica gellant, an organic gellant can burn with the base fuel. Nachmoni et al
. were the first to conduct an experimental study on the combustion characteristics of organic gellant, non-metallized, JP-5-based gelled fuels (the droplet was suspended on the Pyrex wire) [6
]. The experimental results indicated that gelled fuels had lower burning rate than pure liquid fuel of the same type, that radius changes obey the d2
-law, and burning rate increased with increasing oxygen mass fraction and pressure. Recently, Solomon and Natan have carried out experimental work on the combustion mechanisms of organic gelled propellant droplets [4
], where the phenomena during the combustion process were similar to those seen with multi-component droplets, and showed two stages: (1) a period of gellant formation; (2) a period of fuel vapor bubble formation and rupture. The process repeated itself until complete consumption of the base fuel and gellant. The results confirmed that the phenomenon of phase separation is not unique to gels and can also appear in multi-component fuels. They also compared the combustion behavior between the sub-critical and the super-critical pressure regimes, and the results showed that these combustion behaviors were different. Based on the experimental evidence of the combustion characteristics of organic gelled droplet, Kunin et al
. developed a time-dependent, two-stage combustion model for organic gellant gelled droplets [11
]. According to this model, the evaporation rate of the liquid fuel from the gelled droplet surface depends on the droplet size and significantly affects the thickness of the gellant layer. The stage during which the gellant layer is formed is almost three orders of magnitude longer than the stage of bubble formation and layer rupture. Another study conducted by Mishra et al
] investigated the effects of gellant concentration, initial droplet diameter, and environmental pressure on the burning characteristics and flame structure.
Another type of gelled propellant are the so-called slurry fuels, which are mixtures of liquid fuel and solid particles, that were under serious consideration as high-energy fuels a few decades ago and can be advantageous, mainly in atmospheric propulsion systems [11
]. Antaki presented the first theoretical model for the transient processes in the rigid slurry droplet during liquid vaporization and combustion [16
]. He derived that the motion of the regressing surface constitutes a “d3
-law” (the diameter of the inner sphere decreases cubically with time). Recent advances in nanoscience and nanotechnology has enabled production, control, and characterization of nanoscale energetic materials, which have shown tremendous advantages over micron-sized materials. Yanan and Qiao conducted a series of experimental studies on the combustion characteristics of liquid fuel droplets with additional nano- and micro-sized aluminum particles [17
]; the results indicated five distinctive stages (preheating and ignition, classical combustion, microexplosion, surfactant flame and aluminum droplet flame) for the n
-decane/nano-Al droplet, they also carried out theoretical analysis to understand the effects of particle size on particle collision mechanisms and aggregation rate.
The objectives of the present study were: (1) to investigate the morphological changes of polymer gelled UDMH droplets during the overall combustion process; and (2) to explore the effects of ambient pressure and oxygen fraction on burning rate and microexplosions. The droplet combustion experiment and diagnostic methods are described. Several distinctive combustion stages are identified for freely falling droplets and droplets on a hot plate. Furthermore, theoretical results of burning rate are provided and compared with experimental results, and the differences between theoretical and experimental results are analyzed.
2. Experimental Setup
A schematic diagram of the experimental setup is shown in Figure 1
. It consists of a high pressure chamber with a maximum pressure capacity of 2.5 MPa. The heating subsystem utilizes a temperature controller to control the chamber temperature; a 220 V AC power supply is used as the power for the heater coil. A plunger micropump which supplies a minimum volume 0.623 μL every time is used for generating single droplets. The diameter range of droplets is about 1–3 mm, and the maximum pressure of the pump outlet is 3.8 MPa. Burning processes are recorded by a high-speed (up to 120,000 frame/s) digital camera (Photron FASTCAM-ultima APX camera) with a speed of 4000 fps and a resolution of 512 × 1024 pixels. The height of the optical windows is 100 mm, and the images are analyzed by Image Lab software.
UDMH is used as the base fluid, and hydroxypropyl cellulose (HPC) is used as the gellant, whose maximum concentration is 3.0 wt%. The test chamber is purged firstly by injecting nitrogen gas, which replaces the air inside the chamber, then oxygen gas is injected. By changing the pressure of the test chamber before injection of oxygen gas, and after the injection, different experimental ambient conditions can be obtained. The heating subsystem can heat the gas in the test chamber up to 600 K (the thermocouple is located at the outlet of the droplet generator), and the droplets can be ignited automatically for every experimental condition. Around 5–6 tests are conducted to improve the accuracy.
Schematic of the experimental apparatus.
Schematic of the experimental apparatus.
For liquid fuels, plunger micropumps are widely used for generating single droplet, however, due to the high viscosity of gelled fuel, the droplet generating system may be blocked. Since gelled fuel is a shear-thinning non-Newtonian fluid, a spinning impeller is placed inside the tank to ensure that the gelled fuel fed will be from the plunger micropump to the test chamber. If the supplied volume of gelled fuel is constant every time, and the gelled fuel is stirred vigorously, the diameter of droplets can be seen as a constant.
The uncertainty within the ambient pressure measurement is ±0.02 MPa. Due to the characteristics of the gelled fuel and normal gravity conditions, the droplets are not always spherical. A mean droplet diameter is determined from surface area and then converted to an equivalent droplet diameter. Uncertainty in the determination of droplet diameter comes from two sources. One is the scale factor, which is used to convert the droplet diameter size in terms of pixels to real units. A metal needle with a known diameter (2 mm) is used as reference scale. At the worst case, the measurements over a wide range of light intensities show that error is less than ± 0.02 mm. Another source for error in measuring droplet diameter is the optical effect of the hot and dense environment. Considering the density gradient around droplet and droplet falling, the range of errors for droplets is well within ± 5% for the measuring technique used. Each of the measurements for the burning rates is obtained after an average of around 5–6 tests; the errors obtained in the values of the burning rate constant are in the range of ±4%.
In this paper, the burning characteristics of organic gallant-based gelled UDMH droplets is studied experimentally utilizing single-isolated free falling droplets instead of the conventional suspended droplet approach. Morphological transformations of the gelled UDMH droplet involved in the combustion processes are obtained by employing a fast-speed camera, whilst the effects of ambient pressure and oxygen fraction on burning rate are also analyzed. The results can be summarized as follows:
(1) The experimental observations indicate three distinct phases (gellant layer formation, bubble formation and growth, vapor jetting and microexplosion), and four main phenomena (droplet deformation, bubble formation and growth, vapor jetting and luminous jetting flame with “horn” shape) in the combustion process. Bubbles can reach almost the same order of magnitude as the droplets. The high yield stress (compared with Thixotrol ST) and polymer chain structure of polymer gellant are put forward to explain the results.
(2) The burning rate constant is found to increase with both pressure and oxygen fraction. The microexplosion time is seen to increase as the pressure increases, while it decreases as the oxygen fraction increases. The intensity of microexplosions decreases with pressure due to increased resistance, and increases with the oxygen fraction due to increased flame temperature.