A modelling study is performed to compare the plasma flow and heat transfer characteristicsof low-power arc-heated thrusters (arcjets) for three different propellants: hydrogen,nitrogen and argon.The all-speed SIMPLE algorithm is employed to solve the governing equations,which take into account the effects of compressibility,Lorentz force and Joule heating,aswell as the temperature- and pressure-dependence of the gas properties.The temperature,velocityand Mach number distributions calculated within the thruster nozzle obtained with differentpropellant gases are compared for the same thruster structure,dimensions,inlet-gas stagnantpressure and arc currents.The temperature distributions in the solid region of the anode-nozzlewall are also given.It is found that the flow and energy conversion processes in the thrusternozzle show many similar features for all three propellants.For example,the propellant is heatedmainly in the near-cathode and constrictor region,with the highest plasma temperature appearingnear the cathode tip; the flow transition from the subsonic to supersonic regime occurs withinthe constrictor region; the highest axial velocity appears inside the nozzle; and most of the inputpropellant flows towards the thruster exit through the cooler gas region near the anode-nozzlewall.However,since the properties of hydrogen,nitrogen and argon,especially their molecularweights,specific enthalpies and thermal conductivities,are different,there are appreciable differencesin arcjet performance.For example,compared to the other two propellants,the hydrogenarcjet thruster shows a higher plasma temperature in the arc region,and higher axial velocitybut lower temperature at the thruster exit.Correspondingly,the hydrogen arcjet thruster has thehighest specific impulse and arc voltage for the same inlet stagnant pressure and arc current.Thepredictions of the modelling are compared favourably with available experimental results.
The flow in a low-powered arc gas heater combined with a supersonic nozzle of throat diameter less than1 mm is quite complicated and difficul to describe in quantitative detail. Experiments on arc-heated supersonic jet thrusters of monatomic gases argon and helium have been carriedoutandtheirperformancemeasured.Theflowcharacteristics are analyzed with the help of numerical simulation.Results show that the viscous effect is the most important factor causing the large difference between ideal and real performance. A large outer section of the exit flow is slowmoving. This is especially pronounced in helium, where 70 %of the exit area of the nozzle might be in subsonic flow. Friction forces can be much larger than the net thrust, reaching several times higher in helium, resulting in very low efficien cies. Other factors causing the differences between ideal and real flow include: complex flow in the throat region, electric arc extending to the nozzle expansion section, heat transfer to the inlet gas and from the hot plasma, and environmental pressure in the vacuum chamber. It is recognized that the ordinary concepts of supersonic nozzle flow must be greatly modifie when dealing with such complicated situations. The general concepts presented in this paper could be helpful in guiding the design and operation of this equipment.
A 1 kW-class arcjet thruster was fired in a vacuum chamber at a pressure of 18 Pa.A gas mixture of H_2 : N_2 = 2.8 : 1.5 in volume at a total flow rate of 4.3 slm was used as thepropellant with an input power fixed at 860 W. The time-dependent thrust, nozzle temperatureand inlet pressure of the propellant were measured simultaneously. Results showed that with theincrease in nozzle temperature the thrust decreased and various losses increased. The physicalmechanisms involved in these effects are discussed.
