Speaker
Description
In this work, we examine the use of an in-house developed slug calorimeter in the 350 kW inductively coupled plasma (ICP) wind tunnel at the Plasmatron X facility, developed by the Center for Hypersonics & Entry System Studies (CHESS) at the University of Illinois Urbana-Champaign. Plasmatron X is currently the largest ICP facility in the United States, which allows near-continuous operation, dedicated to aerothermal testing for hypersonic flight and reentry environments. The plasma is generated in a radiofrequency (RF) ICP torch by applying a 2.1 MHz excitation to a three-turns induction coil surrounding a 100 mm diameter ceramic tube. The input plate power ranges from 13.5 kW to 350 kW. Gas is injected into the torch body through central and sheath gas lines, the latter being injected at a 15-degree angle to introduce swirl to the flow. The possible operating gases include air, argon, carbon dioxide, hydrogen, methane, and nitrogen. The RF torch can then be interfaced with different nozzle configurations to get the desired flow field inside the cylindrical stainless steel vacuum chamber (1.2 m internal diameter, 1.8 m length) where the jet is discharged.
The wall heat flux depends on the shape of the sample and the chemical and physical properties of the material, such as surface absoprtivity, catalycity, thermal conductivity, etc. As there is no direct measurement of the wall heat flux on a generic thermal protection material, the duplication of this important effect relies in turn on the matching the flow conditions that generate said flux, primarily the centerline freestream enthalpy of flow. However, enthalpy cannot be directly measured directly either, and its determination must rely on semi-empirical correlations or rebuilding procedures. According to the Local Heat Transfer Simulation (LHTS) methodology, the duplication of the hypersonic flight condition heat flux at stagnation point can be achieved in a ground subsonic facility under the Local Thermochemical Equilibrium (LTE) assumption if the flight total enthalpy, the total pressure, and the velocity gradient are locally matched on the test sample. Key to apply LHTS and determine enthalpy is a reliable experimental value of the stagnation point cold wall heat flux.
A slug calorimeter provides a simple and reliable way to estimate cold wall heat flux by measuring the rate at which a slug of material heats up while subjected to a heat input. It was selected among the different techniques for its simplicity, reliability, and adaptability, allowing for easy adjustments to study various sample shapes. Characterizations of the Plasmatron X cold wall heat fluxes are performed with air, nitrogen, and carbon dioxide plasma, and using two types of nozzles, a straight 100 mm diameter nozzle, and a 21.80 mm throat diameter/86.50 mm exit diameter converging-diverging (CD) nozzle. For the experimental tests of the present work, two types of nozzles were used: a straight nozzle with a 100 mm diameter, and a contoured converging-diverging nozzle with 21.80 mm throat diameter and a 86.50 mm exit diameter. During each test, the rotary arm is equipped with a water-cooled Pitot probe (3.18 mm inner diameter, 9.53 mm outer diameter) to measure the stagnation pressure, and a slug calorimeter probe for the heat flux measurement. The slug calorimeter is mounted on a water-cooled copper holder with an external diameter of 20.32 mm. The water-cooling circuit cools down only the interface between the slug calorimeter and the probe holder.
The slug calorimeter used in this works consists of a cylindrical slug (7.62 mm diameter, 10.16 mm length) of Oxygen-Free High-Conductivity (OFHC) C101 copper held concentrically inside an C101 holder by three equally spaced 316 stainless steel spheres pushed against the slug body by 316 stainless steel set screws. A conical cavity is machined on the back face of the slug in order to connect the two wires of a 30 AWG K-type thermocouple by punching them into the cavity using a C101 copper pin.
The heat flux that is imposed on the front face of the slug by the external environment can be defined in terms of three contributions: convective heat flux, radiative heat flux, and chemical heat flux (also called diffusive heat flux). The radiative term depends on the absoprtivity of the slug material front surface. The chemical term is given mainly by the exothermic recombination reactions of the free species present in the dissociated plasma flow that take place on the material surface. In particular, copper is a material with a high catalycity, and the heat flux on a fully catalytic wall could increase up to 50% with respect to a low catalytic wall. Additionally, this chemical contribution depends on the boundary layer type. In an equilibrium boundary layer, surface catalycity of the sample does not play a role because the particles already recombine beforehand and transfer their energy to the gas mixture, whereas in a frozen boundary layer the recombination time is much longer than the diffusion time through the boundary layer so that all potential reaction partners that enter the boundary layer also reach the front surface of the sample thus increasing the wall heat flux if the surface encourages recombination due to its high catalycity. The electrode-less technology for the plasma discharge used in inductively coupled plasma (ICP) wind tunnel provides a pristine environment which enables the study of gas surface interactions such as the effects of surface catalycity on the wall heat flux.
In order to study the effect of the surface catalycity on the cold wall heat flux of an air plasma, the results of a copper slug are compared with the heat flux measurements of two calorimeters with the same shape but different surface materials. In particular, the copper calorimeters have been coated with a 20 nanometers layer of silver and silicon dioxide using magnetron sputter guns. Silver and silicon dioxide are materials that present respectively one of the higher and lower catalycity, thus providing two boundaries for the flow enthalpy reconstruction from cold wall heat flux measurements.
Slug calorimeters of different shapes are tested at different operating conditions relevant to TPS material testing. In particular, air, nitrogen, and carbon dioxide plasma are examined using a straight nozzle and a converging-diverging nozzle, at different power and pressure conditions. For all tests, a direct proportionality between heat flux and power was observed. In particular, this proportionality assumes a linear dependence for the tests with a straight nozzle configuration. For the CD nozzle configuration, higher powers resulted in increased heat fluxes as well, but it appears that heat flux is more influenced by variations in chamber pressure with respect to the straight nozzle case. The effects of the calorimeter shape and surface material are discussed as well, providing a useful overview of these effects on the heat flux characterization of the Plasmatron X ICP wind tunnel.
Summary
Experimental values of cold wall heat flux are crucial to characterize ground test facilities used to design and study thermal protection materials for reentry vehicles. Slug calorimeters provide a simple and reliable way to estimate cold wall heat flux by measuring the rate at which a slug of material heats up while subjected to a heat input. Heat flux measurements obtained with an in-house developed copper slug calorimeter probe inserted in the plasma jet of the Plasmatron X inductively coupled plasma (ICP) facility are investigated in this paper.
Slug calorimeters of different shapes and different coating materials are tested at different operating conditions relevant to TPS material testing. Characterizations are performed under air, nitrogen, and CO2 plasma, and using two types of nozzles, a straight 100 mm diameter nozzle, and a 21.8 mm throat diameter/86.5 mm exit diameter converging-diverging nozzle. Experimental results at different operating conditions are provided, focusing on the effects of chamber pressure, input power, sample shape, and surface material on the measured cold wall heat flux.
