Takeharu Sakai

Experimental Study of Graphite Ablation in Nitrogen Flow

The speed of nitridation reaction at a graphite surface is evaluated in an inductively coupled plasma heated wind tunnel. A high-temperature nitrogen plasma flow is generated and a graphite rod is heated by the high-temperature nitrogen flows in the test section of the wind tunnel. The amount of mass loss and the surface temperature of the graphite rod are measured during the experiments. The value of the atomic nitrogen number density striking onto the graphite rod is estimated by calculating the flowfield around the graphite rod without accounting for the nitridation at the graphite surface. The speed of nitridation is deduced from the amount of mass loss rate, the surface temperature, and the atomic nitrogen number density. The results show that the speed of nitridation reaction is about 0.003 for the surface temperature of about 1900 K. The uncertainties in the results are discussed and improvements are proposed.

Supersonic Drag Reduction with Repetitive Laser Pulses through a Blunt Body

A drag over a flat-nosed cylinder with repetitive laser pulse irradiations ahead of it were experimentally measured in a Mach-1.92, in-draft wind tunnel. Laser pulses were focused using a plane-convex lens fabricated on the nose of the cylinder at a repetition frequency of up to 10 kHz and power of 70 W at a maximum. The drag was measured using a low-friction piston which was backed by a load cell in a cavity at a controlled pressure. Under the experimentally available operation conditions, an up-to-3 % drag reduction and an efficiency of energy deposition of about 10 were obtained. Mechanisms of the drag reduction are analyzed based on experimental flow visualization and numerical simulation.

Comparison of Enthalpy Determination Methods for Arc-Jet Facility

Four experimental methods of determining the enthalpy of the flow in an arc-jet facility that is, the heat balance method, the sonic throat method, the heat transfer method, and the emission-spectroscopic method, are compared with a computational fluid dynamics (CFD) solution. The comparison is made for the Interaction Heating Facility of NASA Ames Research Center for one operating condition. The mass-averaged enthalpy values determined by the heat-balance method and the sonic throat method are 28.7 and 28.8 MJ/kg, respectively. The lower bound of the centerline enthalpy value determined by the heat transfer rate method is 30.5 MJ/kg. The spectrometric method resulted in the centerline enthalpy value of 40.6 MJ/kg. The CFD solution yields the centerline and the average enthalpy values at the nozzle throat of 41.0 and 27.0 MJ/kg, respectively.

Computational Simulation of High Enthalpy Arc Heater Flows

The flowfields in segmented constrictor-type arc heaters are simulated using a new Navier-Stokes code named ARCFLO3. The validity of the radiation and turbulence modeling employed in ARCFLO3 is assessed by comparing the calculated results with the existing experimental data obtained in the 20 and 60 MW arcjet facilities at NASA Ames Research Center. Comparison is made between the calculated and the measured data for arc voltage, heater efficiency, mass-averaged enthalpy, chamber pressure, heat flux at wall, and total enthalpy in the centerline region of an arcjet flow

Calculation of thermal response of ablator under arcjet flow condition

An integrated computational method is developed to calculate thermal response of ablator under an arcjet flow condition. In the method, the arcjet freestream condition in the test section is evaluated by calculating the flows in the arcjet wind tunnel fully theoretically. The thermal response of the ablator is calculated by loosely coupling the shock layer computational fluid dynamics code and the 2-D version of ablation code using the arcjet freestream condition so evaluated. The method is applied to the heating tests conducted in the 1 MW arcjet wind tunnel for one operating condition. The influence of catalytic conditions of ablating surface and the effect of nitridation reaction and surface roughness on the thermal response of the ablator are investigated. Comparison of the temperature profile at the ablating surface between calculation and measurement suggests that the measured temperature profile can be reproduced with a low catalytic efficiency of the surface. It is found that the nitridation reaction increase the surface temperature moderately, and that the effect of the roughness on the surface were small for the present operating condition.

Supersonic Drag Performance of Truncated Cones with Repetitive Energy Depositions

The drag performance of truncated cones in a supersonic flow of Mach number 2 with repetitive energy depositions is evaluated by using computational fluid dynamics. The calculated result shows a quasisteady flowfield: a virtual spike, which is supported by an axi-symmetric recirculation, is formed in front of the truncated cone. The recirculation is generated due to baroclinic interaction between a bow shock wave and a heated bubble produced by energy depositions. The reduction of the drag over the truncated cone is attributed to the virtual spike so formed. The time averaged drag of the truncated cone depends on the amount of deposited energy, repetition frequency and the area of a truncation surface. The averaged drag can be smaller than that of a sharp cone with the same apex angle, maintaining the energy savings due to drag reduction.

Calculation of High-Enthalpy Aerothermal Environment in an Arcjet Facility

S EGMENTED-CONSTRICTOR type arc-heated wind tunnels are used to test the heat shield materials for spacecraft thermal protection systems. The arc-heated wind tunnel consists of an upstream electrode (anode) chamber, constrictor section, downstream electrode (cathode) chamber, and a diverging–converging nozzle connecting to a test chamber. In the test section, the heat shield materials are exposed to a high-enthalpy flow environment produced by the facility. The high-enthalpy environment is often such that the flowdoes not reach equilibrium condition at the edge of the boundary layer over the tested material. In such a case, we need to calculate the flow properties at the surface of the material using a computational fluid dynamics approach to understand the thermal response of the material [1]. For this purpose, the arcjet freestream conditions must be known accurately. To calculate an arcjet freestream condition, two important physical processes occurring in the arcjet wind tunnel should be accounted for: the heating process in the arc heater region upstream of the nozzle throat and the relaxing process in the expanding nozzle region downstream of the nozzle throat. The ARCFLO3 code has been developed recently to calculate the flowfield in the segmentedconstrictor type of arc heaters [2]. Unlike the arc heater flowfield code named ARCFLO developed in the 1970s [3], which is able to calculate the flow in the constrictor section, this new code calculates the flow from the anode chamber to the nozzle throat [2]. Arcjet freestream conditions can be calculated fully theoretically if a nonequilibrium expanding nozzle calculation is made with the calculated flow properties at the nozzle throat obtained by using the ARCFLO3 code. In addition, because the radial distribution of the flow properties at the nozzle throat is calculated with the ARCFLO3 code, the unified computational method can give the radial flow properties in the arcjet freestream at the test section. We tried to make such a unified calculation very recently for one operating condition in an arcjet facility [1]. However, the question remains as to how well such a computational approach predicts the flow properties in an arcjet freestream. It is the purpose of the present work to test the validity of the unified method. The method is applied to calculate the flowfield in a 0.75-MW arcjet wind-tunnel facility at the Institute of Aerospace Technology of the Japan Aerospace Exploration Agency (IAT/JAXA) in Japan. This facility was chosen for the following reasons: 1) In the recent measurement [4] in the IAT/JAXA arcjet facility, the operational characteristic parameters for a wide range of conditions were obtained. The experimental data offer an opportunity to test the current state of the computational modeling in the proposed method. 2) In our previous work, the ARCFLO3 code was applied for the arc heater flowfield calculation only in a high-power-level arc heater, such as the 20or 60-MW arcjet facility at NASA Ames Research Center [2]. The applicability of the ARCFLO3 code to submegawatt class facilities is unknown.

Application of Planck-Rosseland-Gray Model for High-Enthalpy Arc Heaters

An upgraded ARCFLO computer code using a radiation model named Planck-Rosseland-Gray(PRG) model is developed. The developed method is capable of analyzing both an air and a carbonaceous gas flow in the constrictor of an arcjet wind tunnel. In the first part of this paper, the method is tested against the experimental data for air. The testing is carried out by modifying the turbulence parameters employed in the ARCFLO code. Radiation calculation is made by accounting for electron thermal nonequilibrium. Agreements between experimental data and calculation are obtained by choosing a set of the turbulence parameter appropriately. In the second part, a carbonaceous gas flow is preliminary calculated with the turbulence parameter so chosen. The depth of carbon condensation is evaluated approximately. The characteristic data such as mass averaged enthalpy, etc., are presented for typical operation conditions.

ASSESSMENT OF PLANCK-ROSSEL AND-GRAY MODEL FOR RADIATING SHOCK LAYER

A wavelength-independent absorption coefficient model which consists of a combination of the Planck, Rosseland and Gray-gas approximations (PRG model) is developed. Accuracy of the model is assessed by comparing with a line-by-line calculation and a multiband model. The comparison is made for one-dimensional radiative transport along the stagnation streamline for typical spacecrafts entering Earth, Mars, Venus, and Jupiter. The developed model closely reproduce both the heat flux values reaching wall and the distribution of the radiative heat flux values.

Interaction between laser-induced plasma and shock wave over a blunt body in a supersonic flow

Abstract The interaction of the laser-induced plasma produced by a pulse laser energy deposition with the shock wave over a blunt body in a freestream Mach number of 3 is studied numerically and experimentally. The unsteady flow phenomena during the interaction are analysed by the comparison of the flow structures between experiment and calculation to understand the effect of the energy deposition on the reduction of the pressure on the wall of the blunt body. The stagnation point pressure on the centre-line of the blunt body is compared between experiment and calculation to confirm the qualitative feature of the unsteady flowfields so analysed. The present result shows that the unsteady flow phenomena are consistent with the time variation of the stagnation point pressure, and that the time variation of the pressure is in a good agreement between experiment and calculation. The drag averaged over an interaction time is evaluated based on the calculated data.