Cars velocity and temperature measurements in a hydrogen resistojet: Comparison with Monte Carlo calculations

A rarefied flow field that simulates that of a resistojet was studied experimentally and numerically. The experimental studies used the nonpexturbing optical technique coherent anti-Stokes Raman scattering (CARS) to measure velocity and translational temperad ture of molecular hydrogen. The measurements were made at three points along the axis inside the nozzle of the jet and at one axial and three off-axis points outside the nozzle. Measurements at each of these points were made for two flow rates of pure hydrogen at both ambient and elevated (660 K) temperatures. A direct simulation Monte Carlo (DSMC) technique was employed to model this flow under these conditions. Both the experimental and numerical results showed the acceleration and cooling of the gas as it proceeded down the axis of the interior of the nozzle. The experimental and numerical results for the velocity were in good agreement for the unheated flow but the measured velocities for the positions furthest from the nozzle throat were approximately 15% below the computed values for the heated flow. This difference is tentatively attributed to perturbation of the flow caused by the holes in the side of the nozzle used for the laser beam access. The measured and calculated temperatures were in good agreement within the error of the measurements. *Member AIAA Copyright @American Institute of Aeronautics and Astrcnautics, Inc 1995 All Rights Reserved v Introduction The useful life of a satellite in orhit is often limited by the amount of propellant canied onboard for station keeping or drag make-up. Thrusters with higher specific impulse make more effective use of the available fuel and extend the life of these satellites. Accordingly, replacement of hydrazine monopropellant thrusters by resistojets would extend the l i e of these satellites by more than 40%, resulting in a significant reduction in life cycle costs. Resistojets have been used in approximately 40 commercial spacecraft since 1980. As a step toward their wider use (especially on military satellites), a comprehensive understanding of this flow field is required for performance improvement, spacecraft integration, and to provide a basis for anomaly resolution. Review of the technical literature revealed that no comprehensive understanding of the flow field is available. In particular, a theoretical model of the plume that can relate design modifications and operational changes to performance or spacecraft interaction with the plume is lacking. Standard Navier-Stokes calculations have been shown to be inaccurate in describing resistojet flow fields.' For the rarefied flows that are characteristic of resistojets, the Knudsen number becomes so great that the assumptions invoked in applying the Navier-Stokes equations are invalidated. Shock tube studies have shown that the direct simulation Monte Carlo (DSMC) technique offers better correspondence to experimental data under conditions where the Navier-Stokes equations break down. Initial experimental and theoretical studies of low-density supersonic jets of nitrogen and hydrogen have confirmed this conclusion.’,’ The DSMC solutions, however, are quite sensitive to the models employed to simulate the interaction between molecules and the interaction between the gas and the nozzle-wall surface. Thus wider application of this technique to rarefied flows (and in particular to resistojets), requires more detailed experimental measurements to anchor the calculations. To extend the available data base of these low density flows, a diagnostic system to map velocity and translational temperature of the exhaust both inside and outside the nozzle of an operating resistojet was developed.’ This diagnostic uses a nonlinear optical technique called coherent anti-Stokes Raman scattering (CARS). An optical technique is preferred because it will not permrb the flow field and will not change the values of the parameters undergoing measurement. The CARS technique allows a species to be probed with sufficient spatial resolution to generate maps that are useful for comparison with Monte Carlo calculations. Furthermore, it is applicable to the chemical species that are the main momentum carriers of the thrust in a hydrazine resistojet I&, N2, NH,). This applicability allows questions such as the degree of velocity and temperature slippage among the momentum carriers to be studied--topics that cannot be addressed by techniques that probe seeded or minor species. Hydrazine resistojets are of considerable importance because they can replace standard hydrazine monopropellant chemical thrusters with minimal modifcations to satellite systems. For example, existing DSCS 111 (Defense Satellite Communication System) satellites are being retrofitted with resistojets and this system change will increase the life of the satellite 3040%. The direct simulation Monte Carlo method used here has been applied to other low Reynolds number flows and is described elsewhere and, only a short description of the procedure is given in this paper.’,2,4,5 The previous studies have not assessed the DSMC technique for internal nozzle flows nor for heated flows of molecular hydrogen. Both of these issues are addressed in this study. The organization of the paper is conventional. After outlining the characteristics of the CARS technique, a description of the new experimental facility developed for this work is presented. This is followed by a discussion of the experimental procedure which includes the resistojet specifications and operating conditions. After a brief section describing the DSMC techniques, the measured and calculated velocities and temperatures are compared in the main section of the paper. The paper ends with a short summa^^, i/‘ Experimental Investigation CARS Diamostic A detailed discussion of the CARS technique applied to the resistojet was presented previously and the method is only briefly discussed here.) CARS is a four-wave mixing process in which a pump beam at angular frequency op and a Stokes beam at o, interact through the third order susceptibility of a medium, xZs, to create a new beam at anti-Stokes frequency w. When the difference frequency between the two input beams equals a resonance in the (hydrogen) medium the strength of the anti-Stokes signal which exits as a collimated beam becomes large. If the input beams are monochromatic and phasematched (aligned so that photon momentum is conserved), the essentials of the CARS process can be understood from a simple expression for the signal intensity ‘ . where c is the speed of light, and Ip, and IJ, are the intensites of the pump and Stokes beams respectively. In a low pressure gaseous medium, phasematching can be achieved by collinearly aligning the input beams. Using this geometry here, good spatial resolution is attained by tightly focusing the input beams (since the signal intensity depends on the cube of these intensities) and is the spatial resolution. For molecules with no excited vibrational modes, the susceptibility scales linearly with the number density of the ground state of the resonance being probed. Accordingly, CARS measurements in low density high temperature flows are signal strength limited inasmuch as the signal decreases quadratically with this number density. Eq. 1 also indicates that there is a direct trade-off between the spatial resolution and signal strength because the signal strength also scales as e’, At the temperatures encountered in this work, nearly all the hydrogen is in the J = 1 rotational level and only the Q(1) resonance is used to obtain the velocity and temperature information.