Bianca R. Capra

Radiative and total heat transfer measurements to a Titan Explorer Model

This paper reports on the design and testing of a heat transfer gauge suitable for the spectrum of radiation encountered during entry to the atmosphere of Titan. Experiments were performed in the X3 expansion tube at a velocity of 6 km/s, and conditions corresponding to aerocapture with a 70 degree sphere-cone at 16 degrees angle of attack. The radiation gauges used thin film sensing elements mounted behind a borosilicate window on the windward surface, and were therefore shielded from the convective heat transfer. The sensors are uniformly sensitive for radiation in the wavelength range from 0.2 to 2μm, and give an integrated measurement of the total radiant flux in this band. Externally mounted thermocouple gauges were used to measure the total heat transfer, and hence the ratio of radiant to convective heat flux could be found. The Titan atmosphere was simulated by a mixture of 5% methane (by volume) in nitrogen. Comparative tests using pure nitrogen confirmed that without the methane content, negligible radiation was produced, consistent with CN being the primary radiator. The issues of scaling radiation from flight to laboratory model are discussed, and it is shown that for weakly coupled flows, the absolute level of surface radiant heat flux stays the same to a first order if binary scaling is used. The gauges, and the super-orbital expansion tube facility are shown to be useful resources for the study of nonequilibrium radiating flows.

Subscale testing of the Fire II vehicle in a superorbital expansion tube

Testing of the first heatshield of the Fire II reentry vehicle was performed in the X1 superorbital expansion tube at The University of Queensland. The test model was a 1:28 scale replica of the flight vehicle, and incorporated three thermocouples: stagnation and two radial. A trajectory point towards the end of the first experimental testing period, at a total flight time of 1639.5s, an altitude of 61.5Km and velocity 11.1km/s was simulated in the expansion tube. Stagnation point heat transfer was obtained using a fast response coaxial type E thermocouple. In the current analysis the convective and radiative heating components were treated independently, where the convective component was scaled with the length scale and the absolute value of the radiative heat transfer was held constant. From this, the overall contribution of the radiative heat transfer to the total heat rate is decreased in the expansion tubes from an 18% contribution in flight to less than 1%, whereas the convective component was increased by a factor of 28. This results in the convective heat transfer being the major mode of heat transfer in the experimental models. From the Fay and Riddell empirical convective heat transfer correlation it was shown that the parameter Ch√Re should remain constant between the flight and experimental tests provided ρL scaling is maintained. Results from the current study show good agreement with the convective heating component of the flight vehicle and the Ch√Re values are in agreement to within 20% of the flight results. The results obtained in this study give a strong indication that the relative radiative heat transfer contribution in the expansion tube tests is less than that in flight, supporting the analysis that the absolute value remains constant with ρL scaling.

Radiative and total heat transfer measurements to a Titan Explorer model

This paper reports on the design and testing of a heat transfer gauge suitable for the spectrum of radiation encountered during entry to the atmosphere of Titan. Experiments were performed on a 70° sphere-cone in the X3 expansion tube at average freestream velocity, pitot pressure, static pressure, and density of 5:83 km=s, 76 kPa, 1.15 kPa, and 2:43 × 10 -3 kg=m 3, respectively. The radiation gauges used thin-film-sensing elements mounted behind a borosilicate window on the windward surface and were therefore shielded from the convective heat transfer. The assembled gauges are uniformly sensitive for radiation in the wavelength range from 0.347 to 2 mand give an integrated measurement of the total radiant flux in this band. Externally mounted thermocouple gauges were used to measure the total heat transfer and the ratio of radiant to the total heat flux found. The Titan atmosphere was simulated by a mixture of 5% methane (by volume) in nitrogen. Comparative tests using pure nitrogen confirmed that, without the methane content, negligible radiation was produced, which is consistent with cyanogen being the primary radiator. Results from this study demonstrate that the gauges and the superorbital expansion tube facility are useful resources for the study of nonequilibrium radiating flows.

Porous Versus Porthole Fuel Injection in a Radical Farming Scramjet: Numerical Analysis

Numerically computed engine performance of a nominally two-dimensional radical farming scramjet with porous (permeable C/C ceramic) and porthole fuel injection is presented. Inflow conditions with Mach number, stagnation pressure, and enthalpy of 6.44, 40.2MPa, and 4.31 MJ/kg respectively, and fuel/air equivalence ratio of 0.44 were maintained, along with engine geometry. Hydrogen fuel was injected at an axial location of 92.33mm downstream of the leading edge for each investigated injection method. Results from this study show that porous fuel injection results in enhanced mixing and combustion compared to porthole fuel injection. This is particularly evident within the first half of the combustion chamber where porous fuel injection resulted in mixing and combustion efficiencies of 76% and 63% respectively. At the same location, porthole fuel injection resulted in efficiencies respectively of 58% and 46%. Key mechanisms contributing to the observed improved performance were the formation of an attached oblique fuel injection shock and associated stronger shock-expansion train ingested by the engine, enhanced spreading of the fuel in all directions and a more rapidly growing mixing layer.

A Detailed Investigation of Nominally 2-D Radical-Farming Scramjet Combustion

This paper reports on an investigation of the flow/chemistry coupling inside a nominally two-dimensional inlet-fuelled scramjet configuration. The experiments were conducted at a freestream Mach number of 7.3 and a total flow enthalpy of 4.3 MJ/kg corresponding to a Mach 9.7 flight condition. The phenomenon of radical-farming has been studied in detail using two-dimensional OH* chemiluminescence imaging and emission spectroscopy. High signal levels of excited OH (OH*) were detected behind the first shock reflections inside the combustion chamber upstream of any measurable pressure rise from combustion, which occurred towards the rear of the combustor. The production of OH in the first hot pocket initiates the ignition process and then accelerates the combustion process in the next downstream hot pocket. This was confirmed by numerical simulations of premixed hydrogen/air flow through the scramjet. Chemical kinetics analyses reveal that the ignition process is governed by the interaction between various reaction groups leading to a chainbranching explosion for low mean temperature and pressure combustion flowfields.

The SCRAMSPACE I scramjet flight design and construction

The design activities of the development of the SCRAMSPACE I scramjet-powered free-flight experiment are described in this paper. The objectives of this flight are first described together with the definition of the primary, secondary and tertiary experiments. The Scramjet configuration studied is first discussed together with the rocket motor system selected for this flight. The different flight sequences are then explained, highlighting the SCRAMSPACE I free-flyer separation and re-orientation procedures. A design trade-off study is then described considering vehicle stability, packaging, thermo-structural analysis and trajectory, discussing the alignment of the predicted performance with the mission scientific requirements. The global system architecture and instrumentation of the vehicle are then explained. The conclusions of this design phase are that a vehicle design has been produced which is able to meet the mission scientific goals and the procurement & construction of the vehicle are ongoing.

H2-O2 porous fuel injection in a radical farming scramjet

This paper reports on the experimental testing of oxygen compatible ceramic matrix composite porous injectors in a nominally two-dimensional hydrogen fuelled and oxygen enriched radical farming scramjet in the T4 shock tunnel facility. All experiments were performed at a dynamic pressure of 146 kPa, an equivalent flight Mach number of 9.7, a stagnation pressure and enthalpy of 40 MPa and 4.3 MJ/kg respectively and at a fuelling condition that resulted in an average equivalence ratio of 0.472. Oxygen was pre-mixed with the fuel prior to injection to achieve enrichment percentages of approximately 13%, 15% and 17%. These levels ensured that the hydrogen-oxidiser mix injected into the engine always remained too fuel rich to sustain a flame without any additional mixing with the captured air. Addition of pre-mixed oxygen with the fuel was found to significantly alter the performance of the engine; enhancing both combustion and ignition and converting a previously observed limited combustion condition into one with sustained and noticeable combustion induced pressure rise. Increases in the enrichment percentage lead to further increases in combustion levels and acted to reduce ignition lengths within the engine. Suppressed combustion runs, where a nitrogen test gas was used, confirmed that the pressure rise observed in these experiments as attributed to the oxygen enrichment and not associated with the increased mass injected.

Chemiluminescence imaging in supersonic combustors operating in radical-farming mode

Emission spectroscopy was used to investigate ignition and combustion characteristics of supersonic combustion ramjet engines. Two-dimensional scramjet models with inlet injection, fuelled with hydrogen gas, were used in the study. The scramjet engines were configured to operate in radical farming mode, where combustion radicals are formed behind shock waves reflected at the walls. The chemiluminescence emission signals were recorded in a two-dimensional, time-integrated fashion to give information on the location and distribution of the radical farms in the combustors. High signal levels were detected in localised regions immediately downstream of shock reflections, an indication of localised hydroxyl formation supporting the concept of radical farming. Results are presented for a symmetric as well as an asymmetric scramjet geometry. These data represent the first successful visualisation of radical farms in the hot pockets of a supersonic combustor. Spectrally resolved measurements have been obtained in the ultraviolet wavelength range between 300 and 400 nm. This data shows that the OH* chemiluminescence signal around 306 nm is not the most dominant source of radiation observed in the radical farms.

Total Heat Transfer Measurements on a Flight Investigation of Reentry Environment Model

Stagnation-point total heat transfer was measured on a 1:27.7 model of the Flight Investigation of Reentry Environment II flight vehicle. Experiments were performed in the X1 expansion tube at an equivalent flight velocity and static enthalpy of 11 km/s and 12.7 MJ/kg, respectively. Conditions were chosen to replicate the flight condition at a total flight time of 1639.5 s, where radiation contributed an estimated 17-36% of the total heat transfer. This contribution is theorized to reduce to <2% in the scaled experiments, and the heating environment on the test model was expected to be dominated by convection. A correlation between reported flight heating rates and expected experimental heating, referred to as the reduced flight value, was developed to predict the level of heating expected on the test model. At the given flow conditions, the reduced flight value was calculated to be 150 MW/m2. Average stagnation-point total heat transfer was measured to be 140 ± 7% W/m2, showing good agreement with the predicted value. Experimentally measured heat transfer was found to have good agreement of between 5 and 15% with a number of convective heating correlations, confirming that convection dominates the tunnel heating environment, and that useful experimental measurements could be made in weakly coupled radiating flow

Aerothermal–Structural Analysis of a Rocket-Launched Mach 8 Scramjet Experiment: Ascent

This paper reports on the methodology and results of a weak-coupled aerothermalstructural analysis on the ascent phase of the SCRAMSPACE Mach 8 scramjet flight experiment. This vehicle was essentially un-shrouded during the flight trajectory, relying on the thin, 5mm thick aluminium external shell of the payload to maintain structural integrity and protect the flight experiment. As such, understanding the thermal-structural response of the vehicle was imperative to mission success. Using two- and three-dimensional models, an iterative procedure was employed to compute the flowfield, convective heating, wall temperatures and structural coupling at flight times covering both peak heating and peak surface temperature. Accounting for such coupling resulted in a 150K reduction in wall temperature compared to the more conservative cold wall assumption. Despite this, peak temperatures remained of the order of 550 K. Further, thermally induced stresses within these regions were in excess of four times the material failure limits. Irreversible material failure during ascent was therefore concluded likely to occur on the external shell. Two alternate materials, steel 1006 and copper, were therefore assessed with the results indicating that steel sections on the external shell resulted in the best thermal-structural response of the payload.