Experimental Study of Graphite Ablation in Nitrogen Flow, Part II: Further Numerical Analysis

Abstract

The ablative behavior of graphite heated in an inductively coupled plasma wind tunnel is analyzed using an integrated computational method to examine the probability value of nitridation reaction occurring at graphite surface. In this method, the plasma torch freestream condition at the entrance of the test chamber is evaluated by calculating the flows in the plasma torch. The thermal response of the graphite test piece is calculated by loosely coupling the thermochemical nonequilibrium fluid dynamics code and two-dimensional heat conduction equation solver using the freestream condition so valuated. Using a computational tool, surface contours of the graphite test piece at several time steps are calculated by varying the probability value of nitridation reaction and are compared with those measured in the heating test. The effect of impurities remaining in the test chamber is also taken into account. As a result of the study, it is suggested that a probability value of 0.003 for nitridation reaction is reasonable to explain the amount of mass loss of the graphite test piece in the heating test. Nomenclature c p = specific heat at constant pressure, J=kg K J s = mass flux of species, kg=m 2 s k = surface reaction velocity, m=s M s = molecular weight of species s, kg=mol R = universal gas constant, 8:314 J=mol K r = mass-loss rate, kg=s S = recession, m s = mass-loss rate, m=s T = temperature, K = reaction probability = density, kg=m 3 Subscripts g = graphite nit = nitridation reaction oxi = oxidation reaction

Laboratory Investigation of the Active Nitridation of Graphite by Atomic Nitrogen

“…The majority of our measurements were obtained at 1273 K; the average value for these is about 3 10 3 . This average is very similar to the reaction efficiencies reported by Suzuki et al [18,20] for their ICP graphite ablation experiments and is the value that produces the best agreement between experiment and modeling of those experiments [21]. Compared with our test conditions, most of their ICP experiments were conducted at higher temperatures (1822-2184 K), higher total pressure (10 kPa), and much higher N-atom concentrations (by 2-3 orders of magnitude).…”

“…The N-atom concentration at the stagnation point of the test specimens was computed for each test case using a thermochemical nonequilibrium computational fluid dynamics code [6]. From the computed N-atom densities and the measured sample mass losses, Suzuki et al [18] reported reaction efficiencies of about 2:5 10 3 to 3:2 10 3 over a temperature range of 1822-2184 K. More recent measurements by Suzuki et al [20] have extended the data to lower temperatures where they find a reaction efficiency of about 1:4 10 3 at 1400 K. Suzuki et al [21] have also performed further numerical modeling of their experiments and reported the best agreement between computed and measured carbon ablation when the nitridation efficiency is set to 3:2 10 3 . The reaction efficiencies determined by Suzuki et al [18][19][20][21] are similar to those of Goldstein [15] and are about 2 orders of magnitude lower than those obtained by Park and Bogdanoff [16] at lower temperatures.…”

Thermochemical Nonequilibrium 2D Modeling of Nitrogen Inductively Coupled Plasma Flow

“…Because there is no contact between the heating element and working gas, the produced ICP flow is highly pure. This advantage and the high-enthalpy property make ICP flows the preferred sources to develop the thermal protection systems (TPSs) of reentry vehicles, study nitridation reactions on the ablator surfaces [1] , investigate variations of electron dynamics [2] , prepare nanoparticles of intermetallic compounds [3] , etc.…”

Numerical Investigation of Flow Fields in Inductively Coupled Plasma Wind Tunnels

“…On one hand, because there is sufficient space and a stable environment in the vacuum chamber, precision measuring instruments can easily be set up in it and reliable experimental data can be acquired there for validation of numerical methods. On the other hand, if detailed flow properties inside the vacuum chamber can be obtained, they will be very useful for tentative analysis and determining correct flow parameters for further studies such as investigation of nitridation reactions occurring at a graphite surface [12] and simulation of the thermal response of an ablator [13] .…”