For combustion-driven deuterium fluoride/hydrogen fluoride (DF/HF) chemical lasers, fluorine atoms produced in the combustion chamber are the source of the laser energy. The production efficiency of fluorine atoms in the combustion chamber directly determines the upper limit of the chemical efficiency of combustion-driven DF/HF chemical lasers. The atomic fluorine production efficiency limits several characteristic parameters of combustion-driven DF/HF chemical lasers, such as the amplification scale, volume efficiency, and weight efficiency. Therefore, it is necessary to investigate the combustion process of combustion-driven DF/HF chemical lasers thoroughly. In the past, most studies on HF/DF laser combustors were conducted theoretically using thermodynamic equilibrium methods rather than experimentally because of the extremely high temperature of the combustion production gases and the strong corrosivity of the combustion products F, F2, etc. To date, only a few indirect experimental studies have been conducted on HF/DF laser combustors. These experimental studies considered the laser output power as the research object to investigate the working performance of the combustion chamber indirectly without direct observation of the combustion process. In this study, a small combustion chamber platform was built. The ultraviolet–visible and near-infrared spectra of the H2/NF3 combustion flame were directly observed by flame fluorescence spectroscopy. The combustion process of the H2/NF3 mixture in the combustion chamber was analyzed using spontaneous emission spectroscopy. The gas temperature in the combustion chamber was measured using the rotational structure strength distribution of the HF (v=2→v=0) band. The flame temperature distribution along the gas flow direction is provided in combination with an electric translation platform. The influence of flow ratio of oxidant NF3 to fuel H2 on the flame gas temperature distribution was examined.
In this study, a small combustion chamber test platform was built (Fig. 1). The flame shape and temperature distribution of the gas products of H2/NF3 combustion in the combustion chamber were measured and analyzed by fluorescence spectroscopy. The entire combustion chamber is composed of corrosion-resistant stainless steel. The injector panel adopts a three-gas-jet layout, with a row of H2 fuel injection holes in the middle and injection along the horizontal direction. There are two rows of NF3 oxidizer injection holes, one at the top and one at the bottom, and the injection direction is 55° from the horizontal direction. Quartz observation windows are set on both sides of the gas flow passage in the combustion chamber to observe the combustion flame, and a large amount of N2 curtain gas is injected on both sides to protect the quartz observation window. A pressure tap to monitor the pressure of the combustion chamber and an electric spark device for igniting the H2/NF3 mixture are set on the upper side of the combustion chamber. The electric spark device is located 35 mm downstream from the injector panel. A reflective optical fiber collimator coated with a silver film is used to collect the luminescence of the combustion flame transmitted from the quartz viewing window. The optical fiber collimator is fixed on an electric translation platform that can move at a uniform speed to measure the distribution of the fluorescence spectrum of the combustion flame along the gas flow direction. The ultraviolet-visible and near-infrared spectra of the combustion flame are measured using a grating spectrometer, and the wavelength of the spectrometer is calibrated using a pen-shaped low-pressure mercury argon lamp. A mass-flow controller is used to control and measure the flow rate of each gas.
In the ultraviolet–visible spectral region, the luminescence of the H2/NF3 combustion flame mainly includes the radiative transitions of electronically excited molecules, such as N2(B), NF(b), and NH(A) (Fig. 3). In the near-infrared spectral region, the spectrum of the H2/NF3 combustion flame is relatively pure and simple and is mainly composed of the first overtone vibrational rotational transition band (Δv=2) of HF(v) vibrationally excited molecules (Fig. 4). At extremely high temperatures in the combustion chamber, the spectral lines at the high rotation quantum (J) overlap significantly, showing an apparent non-Boltzmann equilibrium. With the increase in the flow rate of oxidant NF3, the start and end positions of the flame move upstream, but the overall flame length changes little (Fig. 7). The gas temperature in the combustion chamber rises rapidly at the beginning upstream, immediately reaches the highest temperature point, and then decreases slowly and linearly (Fig. 8). The highest temperature point is where the flame brightness is the highest and the combustion reaction is the most intense. When the flow ratio of NF3 to H2 gas is small, the gas temperature in the combustion chamber decreases more gently. When the flow ratio of NF3 to H2 gas increases, the gas temperature in the combustion chamber decreases more sharply.
A fluorescence spectrum analysis and temperature distribution measurements of the combustion flame of H2/NF3 mixture were performed. The results show that the fluorescence spectrum in the ultraviolet-visible spectral region is mainly produced by the radiative transitions of electronically excited molecules, such as N2(B), NF(b), and NH(A). The fluorescence spectrum in the near-infrared spectral region is mainly composed of the first overtone vibrational rotational transition band of HF(v) vibrationally excited molecules. The effect of flow ratio of oxidant NF3 to fuel H2 on the H2/NF3 flame length was investigated. The experimental results show that, under the experimental conditions of the three gas jets, the flame length decreases when the flow ratio of oxidant NF3 to fuel H2 increases and increases when the flow ratio of oxidant NF3 to fuel H2 decreases. The effect of flow ratio of oxidant NF3 to fuel H2 on the combustion flame temperature distribution was also investigated. When the flow ratio of oxidant NF3 to fuel H2 is small, the gas temperature in the combustion chamber decreases gently along the flow direction. When the flow ratio of oxidant NF3 to fuel H2 gradually increases, the gas temperature decreases sharply along the flow direction.
