TYPiCAllY transient heat transfer in hypersonic flow is measured by using surface mounted instrumenta-tion such as thermocouples, thin film gauges and calorimeters. 1–4 However, such instrumentation have several limitations such as low survivability in harsh experimental conditions, maximum operating temperature, a limit on the number of gauges that can be placed on a given model and disrupting the surface flow, catalycity and thermal properties. To overcome such limitations, it is proposed to use infrared thermography to measure heat transfer in expansion tubes.

Infrared thermography is an optical technique that is based on thermal radiation. The radiation emitted by an object in the infrared region is measured by a camera and converted to a surface temperature map or a point measurement. To facilitate such studies in hypervelocity wind tunnels, knowledge of the shock layer radiation is required, as it will be superimposed on top of the thermal signal of interest. Rees5 successfully demonstrated a single channel proof-of-concept system for infrared thermography in expansion tubes. Cullen et al. 6 extended this technique to air using a 2D IR camera array. Cullen has developed a heated model thermography system for air (personal communication with Timothy Cullen); the same concept is being followed here with the appropriate adaption for CO2/N2 mixtures. While computational tools such as NEQAIR can be used to predict the shock layer radiation, preliminary thermography experiments for Earth re-entry conditions by Cullen et al. 6 indicate higher than predicted radiation, attributed to test gas contamination. This necessitated experiments to identify source of the contaminants. The aim of this study is to experimentally quantify the shock layer radiation in a Martian atmosphere by performing emission spectroscopy at an MSL trajectory point. Radiative heating is usually ignored for flows at speeds lower than about 10 km/s in air. 7 Historically, radiative heating has been assumed to be negligible for previous Mars missions such as Viking, Pathfinder, Phoenix and even MSL due to the low entry speeds. 8 The ExoMars mission9 and the upcoming Mars2020 mission8 however, considered the contribution of radiative heating. The majority of Martian atmosphere is composed of CO2 (96%) with trace components of N2 (2%) and Ar (2%). Radiative heating cannot be neglected at lower velocities (3-5 km/s) due to CO2 and CO being major radiators in the IR region. 9 CN is a strong radiator and also contributes to radiative heating in CO2/N2 mixtures. 10 Several tests have been conducted in EAST to measure radiance at Mars entry conditions. 7, 11–15 Majority of the work at lower velocities has focused on characterising the infrared region, specifically, 2.7 µm and 4.3 µm CO2 bands. 14, 15 Cruden7 characterised the mid-infrared radiation in CO2 at velocities from 3-7 km/s and found CO2 to be a significant radiator at lower velocities. The magnitude of heating was found to decrease at higher velocities as CO2 dissociates. Low velocity measurements (3 km/s) of CO2 mid-infrared radiation were also performed in CO2/N2 mixtures. 11 The current paper presents experimental shock layer radiation measurements in the infrared region from 0.78 to 5.1 µm at a MSL trajectory point. The measured infrared radiation will allow an appropriate window to be identified for performing infrared thermography. The final goal is to study turbulent heat flux using infrared thermography at high Reynolds number conditions that have not been achieved before in expansion tubes.