Combustion Characteristics And Mechanisms Of High-Energy Solid Fuel And Al/H₂O Based Fuel

Combustion is the most important method for human to gain energy,and it provides the overwhelming majority of energy consumption in the world.As the developing of human society,not only the requirement of energy is increased but also more and higher demands about the performances of fuels are raised.High-energy solid fuel(NEPE propellant) possesses the advantages of double-base and composite propellants.High-energy solid fuel not only has high energy and large elongation in low temperature,but also has good technical and security property;thereby,it represents the development trend of modern solid fuels. However,there also are a series of problems about the combustion performances of high-energy solid fuel,such as contradiction between energy performance and combustion characteristic,contradiction between burning rate and pressure exponent, and so on.Therefore,it is necessary and important to carry out in-depth investigations on the thermal decomposition and combustion of high-energy solid fuel and its main components.Based on the experimental results,the combustion mechanism and effective means of solving the above problems would be discussed,aiming at improving the comprehensive performances of high-energy solid fuel.Al/H₂O based fuel is one kind of new high-energy fuels,which adopts Al as the fuel and H₂O as the oxidant.The energy density of Al/H₂O based fuel is quite large, about four times larger than that of solid rocket fuels,and it avoids some disadvantages of solid rocket fuels like short working time and poor controllability.In addition,Al/H₂O based fuel is one kind of green fuels,whose combustion products have low pollution and are very friendly to environment.Nowadays,it is special attractive to investigate Al/H₂O based fuel,as energy storage and environmental pollution are becoming more and more serious.High-energy solid fuel and Al/H₂O based fuel are chosen as the research projects in this dissertation;and a variety of experimental techniques are adopted in the researches.Based on the systematical studies of these two fuels,their thermal decomposition and combustion mechanisms are discussed.The contents and findings of the researches in this dissertation are summarized as following.(â… ) 10%nano-Al and nano-Ni are added respectively into PEG(binder of high-energy solid fuel).TG/DTA and in-situ FTIR are adopted to study the effects of nano metals on the thermal degradation of PEG both in inert and air atmosphere, while GC/MS is employed to identify the volatile products from thermal degradation of PEG.The experimental results for PEG thermal degradation in inert atmosphere show that,nano-Al can absorb large numbers of products from PEG thermal degradation due to its large specific surface area,and the heat release is evidently increased in the PEG thermal degradation process.Nano-Ni addition remarkable changes the relative contents of the products from PEG thermal degradation in inert atmosphere,as nano-Ni can promote the cleavage of C-C bond and hydrogenation of radicals,due to its excellent catalysis capabilities.The experimental results in air atmosphere show that,with nano-Al addition the heat release is slowed down in the early period but is accelerated in the late period of the PEG thermal degradation. Nano-Ni addition not only makes the PEG degradation begin as soon as melted,but also makes the PEG thermooxidative degradation process complete much earlier than pure PEG in air atmosphere.(â…¡) 10%nano-Co₃O₄ is added into PEG,and a variety of experimental techniques such as TG/DTA,in-situ FTIR and online Py-GC/MS are adopted to study the effects of nano-Co₃O₄ on the thermal degradation of PEG both in inert and air atmosphere. The experimental results for PEG thermal degradation in inert atmosphere show that, nano-Co₃O₄ can absorb products from PEG thermal degradation and make them decompose on its surface;as a result,PEG degradation absorbs more heat than that for pure PEG in inert atmosphere.Nano-Co₃O₄ addition changes the mechanism of PEG degradation in inert atmosphere,not only leads to the forming of products with cycloether end group,but also changes the relative contents of the products from PEG degradation.The experimental results in air atmosphere show that,the mass loss of PEG thermal degradation undergoes three steps both for pure PEG and with Nano-Co₃O₄ addition;and these three steps are corresponding to the oxidation of PEG end groups,cleavage of PEG backbone and decomposition of large products from PEG degradation.Nano-Co₃O₄ addition makes PEG thermal degradation release more heat,because nano-Co₃O₄ can make large products from PEG degradation decompose to small ones and the reaction between these small products and oxygen in air atmosphere can release additional heat.The experimental results also show that the cleavage of PEG backbone begins at 300℃and the whole thermal degradation is completed at 350℃in air atmosphere;and this process are not evidently changed by nano-Co₃O₄ addition.(â…¢) Two typical products(1,4-dioxane and 1,2-dimethoxyethane) of PEG thermal degradation are chosen as the fuels in the combustion experiments,based on the above experimental results about the products identifying from PEG thermal degradation.Molecular beam mass spectrometry combined with tunable synchrotron vacuum ultraviolet photoionization is adopted to identified combustion products and intermediates in 1,4-dioxane and 1,2-dimethoxyethane flames.About 20 and 30 intermediate species are identified in 1,4-dioxane and 1,2-dimethoxyethane flames respectively,and their mole fraction profiles are evaluated.Compared with reported oxygenated hydrocarbons,the numbers of intermediates species in these two flames are rather small,and these intermediates always have rather small molecular weight. The experimental results show that the formations of intermediates,both hydrocarbons and oxygenated hydrocarbons,are closely linked to the structure of fuels.Species produced from H atom abstraction and beta scission of fuels usually have much higher concentrations than others,therefore the concentrations of C₁ and C₂ intermediates are much larger than those of C₃ and C₄ intermediates.The cyclic structure of 1,4-dioxane leads to the prominent difference in combustion characteristics between 1,4-dioxane and the non-cyclic oxygenated hydrocarbons.(â…£) High-energy solid fuel(PEG/NG/BTTN/AP/HMX) samples with different ratios of AP/HMX are designed and prepared,and a variety of experimental techniques are adopted to study the effects of AP/HMX ratio on the thermal decomposition and combustion performances of high-energy solid fuel.The experimental results show that the thermal decomposition of AP and HMX can promotes each other because of the "linkage-mutualism" mechanism between them, and this makes the thermal decomposition of both AP and HMX begin and complete much earlier.AP addition makes the dark zone above the burning surface to become thinner or even disappear.With the increase of AP content,the reaction in the flame area becomes fiercer and the flame brightness becomes stronger.The burning rate of high-energy solid fuel increases as the pressure increases,and the burning rate increases as the AP content increases under fixed pressure,which shows that changing AP/HMX ratio is an effective method to adjust the burning rate of high-energy solid fuel.The analysis of quenched surface shows that HMX is easier to decompose than AP and PEG in the burning surface,and when AP and HMX are both existent,the reaction in the burning surface is much fiercer than that when one of them is absent.(â…¤) 5%and 15%Al powders are added into high-energy solid fuel (PEG/NG/BTTN/AP/HMX,AP/HMX=1/3) respectively,and the effects of Al addition on the thermal decomposition and combustion performances of high-energy solid fuel are studied.The experimental results show that,only when the added Al reaches a certain ratio the thermal decomposition of high-energy solid fuel can be markedly influenced.Structural features of flame are changed evidently and flame temperature is increased markedly,after adding Al powders.With pressure increasing, the Al powders burn more sufficiently and the Al₂O₃ smog becomes thicker.With the Al content increasing,the burning rate of high-energy solid fuel decreases slightly under low pressure while the heat in the burning surface increases.The increasing of the heat on the burning surface not only leads to oxidants decomposition and higher burning rate faster,but also makes the reaction layer thicker.(â…¥) Al/H₂O based fuel samples without additive and with 10%oxidant(HMX or CL-20) are designed and prepared,and a series of experiment are carry out to study the effects of the oxidants on the combustion performances of Al/H₂O based fuel.The experimental results show that compared with CL-20 addition,HMX addition not only makes Al powders burn much thoroughly but also has better effect in increasing the temperature of burning surface.The experimental results also show that,CL-20 addition makes the heat transfer faster and increases of the burning rate are more markedly than that with HMX addition.(â…¦) Methylmethacrylate(MMA) is chosen as the organic addition of Al/H₂O based fuel,for the sake of improving the combustion performances such as burning rate.First,molecular beam mass spectrometry combined with tunable synchrotron vacuum ultraviolet photoionization is adopted to identify combustion products and intermediates in fuel-rich MMA flame,and about 30 combustion intermediates including isomeric species are observed.Mole fraction profiles of the intermediates in the flame are obtained,and the combustion characteristics of MMA are analyzed and discussed.The experimental results show that the combustion of MMA not only reduces soot emission but also has low concentration of some potential toxic by-products.Second,the effects of MMA addition(10%) on the combustion performances of Al/H₂O based fuel in air atmosphere are studied.The experimental results show that the flame of Al/H₂O based fuel becomes diffused and the burning rate increases markedly after adding MMA.These changes are not only because MMA is inflammable and volatile,but also related to the large numbers of intermediates in MMA combustion.The experimental results also show that,the content of activated Al in the combustion residue of Al/H₂O based fuel increases after adding MMA,which shows that MMA addition is bad for the efficiency of Al powders combustion.This is because the combustion of MMA consumes lots of oxygen,leading to the restrain of Al powders combustion.