Study Of Gain Generators With Fuel Preheating And Film Cooling For Combustion Driven DF/HF Lasers

In order to make combustion-driven DF/HF chemical lasers more efficient and compact,the study of gain generators with fuel preheating and film cooling for combustion driven DF/HF lasers has been carried out in this PhD thesis.The main purposes are as follows:by fuel preheating,to improve combustion-driven DF/HF lasers’operating efficiency,and greatly reduce lasers’fuel consumption and volume/weight;by the optimization of cooling film structure,to minimize the effective flow rate of cooling film.Theoretical analysis and numerical simulation of fuel preheating based on different fuel systems of DF/HF lasers has been carried out.Results of theoretical analysis show that,for DF/HF lasers with fuel systems of（C₂H₄+NF₃+He^p）+（D₂+He^s）,（H₂+NF₃+He^p）+（D₂+He^s）,（H₂+F₂+He^p）+（D₂+He^s）,（D₂+NF₃+He^p）+（H₂+He^s）and（D₂+F₂+He^p）+（H₂+He^s）,when fuels in the combustor are all preheated,and the preheating temperature is 1100K,the total fuel consumption in the combustor to produce the same flow rate of F atoms is reduced by about 38.9%,32%,55.3%,32%and 54.6%,respectively;And when only He^p is preheated to 1100K,the total fuel consumption in the combustor to produce the same flow rate of F atoms is reduced by about 32%,25%,53.6%,25%and 52%,respectively.Results of numerical simulation show that,under the condition of same flow rate of F atoms,for DF/HF lasers with the fuel systems of（H₂+NF₃+He^p）+（D₂+He^s）and（D₂+NF₃+He^p）+（H₂+He^s）,when fuels in the combustor are all preheated,and the preheating temperature is 1100K,the output power of lasers is improved by about18.2%and 13.6%,and the specific power of lasers is improved by about 46.4%and43.8%,respectively.For DF lasers with the fuel system of（H₂+NF₃+He^p）+（D₂+He^s）,when only He^p is preheated to 1100K,the output power and specific power of lasers are improved by about 13%and 34%,respectively.The structure of nozzles with cooling film has been optimized by numerical simulations.Smulation results show that moving the position of cooling film slits close to nozzle throat,reducing the width of cooling film slits,shortening the length of nozzle throats,and increasing the angle of contraction section wall,are all useful methods to reduce the effective flow rate of He cooling film.When the position of cooling film slit is moved to AR≈2,the width of cooling film slits is reduced to about 0.1mm,the length of nozzle throat is shortened to 0.1mm,and the angle of the contraction section wall is increased to 45°（half angle）,the minimum effective flow rate of He cooling film is decreased to about 8%,which is reduced by about 69%compared to that in the original nozzles with cooling film.For nozzles with cooling film,a small flow rate of cooling film may cause the generation of vortices in cooling film slits.Once a vortex is generated,F atoms in the main flow may be entangled in the vortex,causing the recombination of F atoms to F₂and the decrease of F atoms’flow rate entering optical cavity,which may reduce lasers’output power.At the same time,a small flow rate of cooling film cannot provide sufficient protection for nozzle throats,which may lead to the expansion of nozzle throat wall and the increase of combustor’s balanced total pressure.Based on the two rules above,the method to test the minimum effective flow rate of cooling film by experiments was designed.The engineering design and validation tests of a fuel heater have been carried out.Results of validation tests show that,when the fuel heater is heated to 600℃electrically,it can heat N₂ with a flow rate of 11.016-36.72g/s from room temperature to529-490℃,and the heat transfer power and coefficient are about 5.82-17.91kW and58.6-154.5W?m^-2?K^-1,respectively.The overall design of a electrically-driven fuel heater that can be used in high power lasers has been carried out.Heat energy is stored in the heater by phase change of filling materials.This fuel heater has a compact structure,large heat transfer area,strong mobility,and can heat He with a flow rate of 39 mol/s（t=30s）up byΔT=800K theoretically.When fuels are preheated to 1100K,the fuel consumption of DF/HF lasers is reduced by about 1/3;and the optimized cooling film structure can reduce the effective flow rate of cooling film by about 2/3,both of which can reduce lasers’fuel consumption and volume/weight by about 1/3,showing great significance for the compact and efficient development of combustion driven DF/HF lasers in future.