ХИМИЧЕСКАЯ ФИЗИКА, 2014, том 33, № 8, с. 30-36
ГОРЕНИЕ, ВЗРЫВ И УДАРНЫЕ ВОЛНЫ
UDC 541.126
ENHANCED Al-H2O-BASED FUELS COMBUSTION CHARACTERISTICS WITH POLYACRYLAMIDE AT LOW PRESSURES
© 2014 Baozhong Zhu1, Yunlan Sun1*, Huajian Sun2
1School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui 243 002, China 2Jigang Group Heavy Machinery CO., LTD, Jinan, Shandong 250101, China *E-mail: yunlan@mail.ustc.edu.cn Received 18.09.2012; revised 24.10.2013
The combustion behavior of nano-aluminum—water (n-Al—H2O) mixture with addition of polyacrylamide (PAM) was investigated in argon at 0.1—1.5 MPa using a constant-pressure strand burner. The burning rates of n-Al—H2O mixture were measured. The results show that PAM addition can not only help improve the burning rate of n-Al—H2O mixture, but also decrease the pressure index of burning rate. The mixture of n-Al powder and H2O cannot be ignited in argon at 0.1 MPa, but the mixture of n-Al powder and H2O with the 3 wt % PAM can be ignited, and the mixture can support the self-sustaining combustion. The burning rate is 7.64 mm/s. Moreover, the burning rate increases with increasing the pressure. In addition, the combustion process and flame image characteristics were obtained by a high-speed photography technique, and the element composition and surface morphology of the condensed combustion products were evaluated using a scanning electron microscopy combined with energy dispersive X-ray system.
Keywords: combustion characteristics, nano-aluminum powder, water, PAM.
DOI: 10.7868/S0207401X14080123
1. INTRODUCTION
Al-H2O-based fuels are the new high energy density fuels, so the Al-H2O reaction has been proposed for use as a fuel in space [1], underwater explosives and propulsion applications [2] and hydrogen generation for fuel cells [3]. However, all aluminum particles have a passivating oxide layer, which prevents oxidation of the Al particles at ambient conditions, so the reaction activity of Al and H2O is low. Many fundamental studies have been concentrated on aluminum particles burning in different environments such as O2, N2, air and CO2 in the literature [4-8], but few have been devoted to the Al-H2O reaction [9-13] and even less using nano-sized Al [14-16]. Moreover, a number of studies have been done on the Al-H2O reaction and most have utilized thickening agents due to ignition difficulties [3, 17].
Ivanov et al. [17] studied the effect of pressure on ultrafine metal Al powders in a mixture of water combined with a thickening agent, PAM (3 wt %). It was reported that the mixture would not self-deflagrate without the addition of the PAM into the mixture. It can be seen that thickening agent plays an important role in Al-H2O combustion. In a previous study [2], we have found that polyoxyethylene addition can improve the ignition and combustion performance of an
Al-H2O mixture. However, polyoxyethylene is more expensive than PAM. Polyacrylamide is similar some properties such as excellent water-soluble characteristic and relative high viscosity of the solution to polyoxyethylene, and they are also used as flocculant, thickener and cross-linker. In the present research, the effect of PAM on the combustion characteristics ofAl-H2O mixture was studied. In previous literature [17], the effect of pressure on n-Al powders in a mixture of water combined with a thickening agent; PAM (3 wt %) was not investigated at low pressure. Therefore, combustion performance could not be determined. The focus of this study was to examine the combustion behavior of n-Al systems, which use liquid water or liquid water with 3 wt % PAM as the oxidizing ingredient.
Nano-Al particles have several significant advantages over micro- and larger-sized particles, such as reduced combustion times, ignition temperatures, and ignition delays. These differences are attributed to the fact that small particle size increases the surface-to-bulk-atom ratio and the specific surface area of the Al particles, affecting thermodynamic and heat transfer characteristics. Therefore, the uses of n-Al particles in the Al-H2O reaction can exhibit significant advantages over larger size particles. In the present research, the effect of PAM thickening agent on combustion characteristics of n-Al-H2O mixture was investigated at
ENHANCED Al-H2O-BASED FUELS COMBUSTION CHARACTERISTICS
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different pressures. It is hoped that this work would be useful in providing a clearer understanding of combustion performance of Al-H2O-based fuels.
2. EXPERIMENTAL 2.1. Materials and sample preparation
Nano-Al particles were mixed manually in small batches with distilled water (oxidizer) in a sealed container. For tests including PAM, the oxidizer preparation was slightly different, 3 wt % PAM was added to the distilled water and well mixed prior to introducing n-Al powder. After sufficient mixing, water became gel-like. In order to improve the combustion efficiency of Al powders, the mass ratio of H2O (or with 3 wt % PAM) and n-Al powder was 1.1 : 1. Average particle size of nano-sized Al particle is 40 nm and the active aluminum content is 75% (by weight). The main compositions of the tested samples are shown in Table 1. The sample used in the experiment is PAM, its molecular weight is 3000000.0 g/mol. Polyacrylamide was purchased from Shanghai Chemical Reagent Inc. of Medicine Group of China.
2.2. The flame image and burning rate measurements
The experimental setup used for flame image is shown in Fig. 1. It mainly consists of a combustion chamber, fitted with two optical viewing windows. The desired initial pressure of the combustion chamber was
Table 1. The sample compositions studied
Sample Composition
A-l n-Al/H2O
A-2 n-Al/H2O (З% PAM + H2O)
attained by regulating the flow of purge gas through inlet and exhaust valves before ignition. The optical viewports of the combustion chamber were used for realtime recording of the combustion process by a digital video camera. The experimental setup was also used to measure the burning rate. The luminous combustion wave was recorded using the digital video camera, allowing burning rate to be determined by tracking the position of the reaction front over time and the pressure index was calculated according to Vieile
n
r = ap .
The obtained mixture was placed in a quartz cylinder (height 40 mm, inner diameter 8 mm) and ignited by a hot Nichrome coil embedded in the top layer of loaded sample. The ends of the Nichrome wire were attached to wire leads, allowing a power supply to be connected via electrode feedthroughs after the strand assembly was loaded into the pressure vessel. Once the strand assembly was loaded, and all wire and gas connections were made, pressurization of the chamber was initiated. In this investigation, argon was used as
Computer
Combustion Chamber
Window
Ignition Wiri Sample
CCD Camera
i
■'ù
Argon
Inlet Valve
Manometer
В
■ Off-gas
Exhaust Valve
о , о
Variable Voltage Power Supply
\
Argon Cylinder
Fig. 1. Schematic of the strand burner experimental setup. ХИМИЧЕСКАЯ ФИЗИКА том 33 № 8 2014
35 r
-2 0 2 4 6 8 10 12 14
t, s
Fig. 2. The burning rate of A-1 sample at different pressures: □ - 0.5 MPa (r = 2.37 mm/s), A - 1.0 MPa (r = 2.72 mm/s), O - 1.5 MPa (r = 5.38 mm/s).
the purge gas. The sample was loaded into a quartz tube with a 10 mm OD (8 mm ID) x 40 mm length. The experiments were conducted at 0.1-1.5 MPa.
2.3. SEM/SED analysis
The formation of the condensed phase is of great importance to many applications of Al-H2O combustion. In order to study the condensed combustion products of different samples, the condensed combustion products could be analyzed by using scanning electron microscopy (SEM)/energy dispersive X-ray analysis (EDS).
3. RESULTS AND DISCUSSIONS 3.1. Burning rate and pressure index
The burning rates and pressure indexes ofA-1 sample and A-2 sample are shown in Figs. 2, 3. As shown in these figures, the burn rate is profoundly affected by chamber pressure. At 0.1 MPa, A-1 sample cannot be self-sustaining combustion. However, A-2 sample can be ignition and the burning rate is 7.64 mm/s. The burning rates increase with increasing the pressure, and the burning rate of A-2 sample is higher than that of A-1 sample at different pressures. It is clear that PAM has an important impact on the burning rate of Al-H2O-based fuels. It could be predicted that two mechanisms would favor PAM promoting effects on combustion performance ofAl-H2O-based fuels [18]. The flash pyrolysis products of PAM such as aromatic compounds and 4-hydroxy-4-methyl-2-pentanone can protect Al from Al oxides coating by forming metal chelates. Moreover, the combustion of nitrogenous compounds such as Acetaldazine which is energy material, enhance the igniting and combustion performance ofAl-H2O-based fuels. However, it can also be found from Figs. 2, 3 that the increasing percentage of burning rate ofA-2 sample compared with A-1 sample present reducing trends with increasing the pressure. This shows that the effect of PAM on the burning rate of Al-H2O-based fuels decreases with increasing the pressure.
Compared the burning rate of A-1 sample with the result of Risha et al. [15], the burning rate of n-Al with H2O is ten times lower. This can be caused the packing density. The packing density of the samples varied with the equivalence ratio. The linear burning rate varied
ENHANCED Al-H2O-BASED FUELS COMBUSTION CHARACTERISTICS
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with packing density [14]. As the reference [14] shown, the linear burning rate decreased by nearly 51% for a packing density increase of ~32%. In addition, some n-Al powder has been oxidized, and the active aluminum content of n-Al powder is only 75%. This also results in the decrease of burning velocity.
The burning rate of energetic materials increases linearly as pressure increases in a ln r versus ln p plot
represented by r = apn at constant initial temperature T0, where r is burni
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