Research On Characteristics Of Aero-engine Precooler With Medium/Low Temperature Chemical Reaction

Precooling technology using fuel as coolant is considered to be the main way to broaden the flight Mach upper limit of turbojet engines.For the long-term flight and reusable aviation kerosene precooling turbojet engine,during Ma₀=3-4 precooling stage,the aviation kerosene needs to work in the 300-700 K medium/low temperature region to avoid coking in the precooler tubes.Therefore,the physical heat sink of aviation kerosene is seriously insufficient,resulting in a large amount of fuel consumption for precooling.Based on the traditional precooling technology,this dissertation proposes a new type of chemical precooling technology in the medium/low temperature region,that is,using the low-temperature heat absorption characteristics of fuel to improve the heat sink of fuel in the medium/low temperature region,to save fuel and improve engine performance.The existence of the medium/low temperature region chemical reaction brings new characteristics and design challenges to the precooler,and will ultimately affect the engine performance.This dissertation focuses on the above problems:Through the research on the chemical precooling scheme in the medium/low temperature region for aero-engine,the chemical precooling scheme in the medium/low temperature region based on aviation kerosene/methanol dual fuel and its benefits to fuel saving and engine performance improvement are obtained.Firstly,through the precooling demand analysis and heat sink evaluation,methanol is selected as the precooling fuel,and its low-temperature catalytic cracking endothermic characteristics are used to improve the fuel heat sink.To meet the dual needs of engine precooling and combustion,a chemical precooling scheme in the medium/low temperature region based on aviation kerosene/methanol dual fuel was constructed.The thermal performance analysis results of the chemical precooling aero-engine in the medium/low temperature region under the design condition of Ma₀=4 show that under the calculation conditions in this dissertation,when the precooling temperature is 575 K,the compressor pressure ratio is 5,and the cracking rate of methanol is 80%,the total fuel gas ratio is 0.0979,and the mass fraction of methanol is 84.3%.Compared with the precooling scheme of pure aviation kerosene,the fuel is saved by 68%.The specific thrust and specific impulse can reach more than1000 N·s/kg and more than 1000 s respectively.To obtain the influence mechanism of surface cracking reaction on methanol flow and heat transfer characteristics in micro tubes and the influence law of key parameters,the numerical model of methanol surface cracking flow and heat transfer in micro tubes of precooler was established and analyzed.When the mass flux is constant,reducing the diameter of the tube can increase the contact area to promote the reaction.Reducing the inlet methanol flow rate is conducive to the reaction;The increase of pressure is not conducive to the reaction and heat transfer but will increase the physical heat sink of the fuel.The Dittus-Boelter formula and Gnielinski formula cannot be used to predict the heat transfer performance with a methanol surface cracking reaction.It is revealed that the competition mechanism between the weakened heat transfer of methanol pyrolysis and the enhanced heat transfer of surface catalytic cracking reaction under the condition of low heat mass ratio,and the increase of heat mass ratio will weaken the enhanced heat transfer effect of surface cracking reaction.The influence mechanism of methanol surface cracking reaction on the performance of precooler and the influence law of structural parameters on the performance of precooler were obtained by studying the influence of structural parameters on the performance of precooler.When the precooler adopts the staggered tube bank structure,reducing the circumferential tube spacing and increasing the axial tube length can improve the methanol cracking rate and air cooling effect.When the number of radial tube rows is the same,the air cooling effect will increase with the increase of the number of tube passes,but it will not affect the methanol cracking rate.The precooler with micro tube banks has the advantages of compact structure and high surface reaction cracking rate,which can improve the heat transfer temperature difference and total heat transfer coefficient.Finally,through the impact assessment of the chemical precooling in the medium/low temperature region on the performance of the aero-engine,the benefits of the specific thrust of the engine and fuel saving during Ma₀=3-4 are obtained.Under the same arrangement of tube bank,the axial length of the chemical precooler in the medium/low temperature region is 0.4 m,and that of the traditional aviation kerosene precooler is 0.615 m;Under the same air mass flow,the chemical precooler in the medium/low temperature region has better cooling performance,and the work weight ratio can reach 0.85 MW/kg under the design working condition.At the same time,it also brings greater air pressure loss,and the average total pressure recovery coefficient is about 0.89.When considering the real performance of the precooler,the better cooling effect of the chemical precooler in the medium/low temperature region brings a higher compressor pressure ratio.Compared with the pure aviation kerosene precooling scheme,when Ma₀=4,the fuel is saved by 68%,the specific thrust is increased by 16%,and the specific impulse is increased by more than 700 s.During Ma₀=3-4,the fuel is saved by43%on average,the specific thrust is increased by 8%on average,and the specific impulse is increased by more than 500 s on average.