Previous Research Activities

This page contains descriptions of some of the research projects that have been carried out in the Combustion and Fluid Dynamics Lab 1999-2005. Many of these experiments constitute the foundation of the Current Research projects.

Supersonic Combustion of Hydrogen and Hydrocarbon fuels

Flame Stability

Lean blowout stability limits for hydrogen and hydrocarbon flames are studied for a non-premixed, high-speed flow with fuel injection in the base of a bluff body. These conditions are of practical importance for the stable operation of vehicles employing supersonic combustion during transition from subsonic to supersonic combustion. The airflow upstream of the flameholder is maintained at Mach 1.56 with experimental stagnation temperatures at 600K and 1000K representing the flight regime at the transition to scramjet operation. The results indicated that correlations for flame stability obtained for premixed flames do not apply for this case and the dominant effects are the development of mixing layers at the boundaries of the fuel jets inside the recirculation region.
The supersonic combustion facility at UF uses a vitiated, hydrogen-based heater with a fuzzy controller that maintains the required stagnation temperature demanded for the experiment and a constant 0.21 oxygen mole fraction by oxygen replenishment. Air stagnation pressure and temperature range from 1 - 10 atm. and 300 - 1200 K, respectively. Test section Mach numbers can range from 1.6 - 3.6. The air supply is sufficient to ensure continuous operation. Six interchangeable, supersonic nozzles allow the entrance Mach number to the test section to be varied form 1.6 to 3.6. Each nozzle has a fixed 2.54-cm x 2.54-cm exit area. The isolator is a constant area duct with optical access and additional injection capability.
JP-10 Combustion

References:

M. G. Owens, S. Mullagilli, C. Segal, P. J. Ortwerth and A. B. Mathur, "Thermal Choking Analyses In A Supersonic Combustor", Journal of Propulsion and Power, vol. 17, no. 3, pp. 611-616, May-June 2001.

P. J. Ortwerth, A. B. Mathur, C. Segal, S. Mullagiri, "Combustion Stability Limits of Hydrogen in a Non-premixed Supersonic Flow",
Proceedings of the ISABE Conference, ISABE 99-143, 1999.

Kerosene Flameholding

The study determines the stability of flame in the recirculation region formed in a wall cavity with ignition provided by hydrogen injected in the base of cavity and kerosene injected upstream in the boundary layer formed along the wall leading to the cavity. The experimental conditions of Mach 1.8 and air stagnation temperature 1000 K correspond to the beginning of hypersonic flight regime also referred as cold start conditions. The hydrogen and preinjected kerosene flow rates are modulated during the tests and temperature probes placed in the cavity indicate their effect on the local equivalence ratio. Preinjection of kerosene reduces the local equivalence ratio at low air stagnation temperature by increasing the entrainment of fresh air into the recirculation region. At high temperatures, the additional fuel brought by the presence of kerosene has a detrimental effect on the stability of flame in the cavity.

Reference:

M. G. Owens, S. Tehranian, C. Segal and V.A. Vinogradov, "Flameholding Configurations for Kerosene Combustion in a Mach 1.8 Airflow",
Journal of Propulsion and Power, vol. 14, no. 4, pp.456-461, July-Aug. 1998.

Heat Release Efficiency

The heat release efficiency of gaseous fuels in a Mach 1.6 air stream is investigated at conditions of supersonic flight speed of combined cycle engines.  Stagnation conditions represent a flight enthalpy of Mach 3.7 with flight dynamic pressure of 47,000 Pa (1,000 psf). Direct fuel injection from the base of flameholder is being used to generate a baseline for flameholding with additional injection locations.   The geometry consists of a constant area isolator and a constant-area combustion duct following a sudden expansion.  In addition to the base injection, the fuel is distributed in three zones; (i) upstream of the sudden expansion, (ii) parallel to the main airflow in the core of streamwise vortices formed by the presence of ramps and, (iii) downstream of the flameholder via transverse injection.  The upstream injection has the role to achieve a partial premixing of the fuel prior to arrival to the recirculation region, the ramp injection is an option with the potential to provide enhanced mixing with minimal pressure loss generation and the downstream injection achieves high fuel penetration.  The equivalence ratio from the base injection has been fixed to an equivalence ratio of 0.2 while injection from the other regions was varied up to 0.8.  Combustor entrance Reynolds number was one million based on the test section entrance height.

Ethylene Combustion Test

Hydrogen Combustion Test

Reference:

D. Cuesta, C. Segal, "Effects of Hydrogen and Ethylene Injection Schemes in a Supersonic Airstream", 12th International Space Planes and Hypersonic Systems and Technologies Conference, Norfolk, VA, AIAA-2003-6913, Dec 2003.

Fuel Mixing

For supersonic combustion ramjets to become practical, fuel must be thoroughly mixed and burned in the air stream in a minimum length combustor, to minimize drag losses as well as powerplant size and weight. This has traditionally proved difficult due to the tendency for higher speed fluid flows to maintain greater segregation than low speed flows. Some means of inducing random turbulence or regular vortices are typically employed to encourage mixing, though at a drag loss.
The focus of this study is to determine the physical principles that govern the mixing of fuel in the compressible air stream so that more intelligent injectors and combustors may be designed by industry. The fuel may be liquid, gas or combinations of both. The underlying assumption is that there exist consistent rules that will accurately describe the mixing process for a range of flow conditions, fuels and fuel injection rates. Some aspects of the flow that are subject to examination are temperature, velocity, pressure and boundary layer thickness. Density of the fuel and flow rate affect how easily the fuel molecules penetrate the air stream, while fuel and air temperature affect the rate of diffusion of fuel into air.
The technique used to investigate these processes will be Schlieren photography. Injected fuels will be doped with acetone, which fluoresces under laser stimulation, to show the pattern of fuel spread in the air stream. The known time lag between laser stimulation and radiation emission also allows determination of local fuel velocities while shock, Mach and expansion wave patterns show the local air Mach numbers, from which the local air velocity may be determined by measuring temperature in the well-instrumented test section.

This research has been continued through the application of laser-induced fluorescence and mass spectrometry used to measure the mixture fractions in the region behind the rear-ward facing step.

Reference:

M. G. Owens, S. Mullagiri, C. Segal and V. A. Vinogradov, "Effects of Fuel Pre-injection on Mixing in a Mach 1.6 Airflow", Journal of Propulsion and Power, vol. 17, no. 3, pp. 605-610, May-June 2001.

High Energy Density Fuels

A new class of pentacyclic hydrocarbon-compounds with high-energy/high density has been synthesized to be used as fuels or fuel additives, with the expectation that their inclusion in the fuel mixture will result in a net increase in volumetric efficiency of the current generation of powerplants. These compounds exhibit high density and contain a moderate amount of strain energy, which contributes to the energy output during combustion. Through direct measurements, melting and boiling points, specific heats and latent heats of fusion were determined. Droplet combustion experiments with binary solutions of solid high-energy fuels in kerosene indicated increased effervescence and higher heat output. The liquid high-energy-density formulations exhibited microexplosive combustion behavior.
Identification of combustion products of Methylated Pentacycloundecane Alkene Dimer (MPCU) via Fourier Transform Infrared Spectrometry (FTIR) and Gas Chromatography/Mass Spectroscopy (GC/MS) indicated that major components are CO2, H2O, CH4, C2H2, toluene, benzene and pentagonal ring structures such as cyclopentadiene and methyl-cyclopentadiene, along with small concentrations of ethylbenzene, dimethyl benzene and phenylethyne. These aromatic compounds are suspected to lead to the formation of PAH and the soot that was observed in the combustion products. Specific heat capacities, viscosities, surface tensions and flammability limits of the mixtures of MPCU with JP-10 and their burning rates have been quantified.  This research was performed in conjunction with Dr. Alan P. Marchand of the University of North Texas.

Reference:

C. Segal, S. Pethe and K. R. Williams, "Combustion of High-Energy-High-Density Fuels", Combustion Science and Technology, vol.163, pp. 229-244, Feb. 2001.

Detonation Enhancement Using Energetic Additives

For pulse detonation engines (PDE) to become practical propulsion devices over a broad regime from start to supersonic flight condensed phase fuels will be required. High frequency is required for cycle efficiency, hence very repeatable and rapid ignition times are necessary. These two requirements strongly suggest that direct initiation of a detonation wave in mixtures of fuel droplets in air will be the preferred mode of ignition for PDE applications. High Energy Density (HED) additives, which have shown in the past to accelerate the burning rates of liquid fuel formulations, can have a substantial impact on the detonation properties of heavier hydrocarbon fuels.

For this purpose ignition delays of JP-10 and blends of JP-10 are measured from the shock-wave ignition of fuel/air mixtures. A Mach 5 shock-tube is used with temperature regime studied from 1200K to 2500K and pressure from 10atm to 30atm. The mixtures used were JP-10 with Methylated Pentacyclic Alkene Dimer (MPCU), Ethyl-Hexyl Nitrate, Nitro-Norbornane, Dinitro-Norbornane and Dicyclopentadiene (DCPD) in proportions of 10-50% on a mass-basis. CH and OH emissions are monitored for all the mixtures to indicate ignition times. The results showed a branching of the CH signal at temperatures over 1400 K, unnoticed in the OH signal. This branching is expected to result from the competition of two reactions, each dominating the low and the high temperature regime, respectively.

Figure

Reference:

D. W. Mikolaitis, C. Segal, and A. Chandy, "Ignition Delay for JP-10/Air and JP-10/High Energy Density Fuel/Air Mixtures", Journal of Propulsion and Power, vol. 19, no. 4, pp. 601-606, July-August 2003.

Inlets

Oscillatory Flows in PDE Inlets

Severe effects on the backpressure, induced by the valving system of the detonation tubes, will affect the operation of the inlet including the potential of     hammershock and unstarting of the inlet. If a single inlet is used as a plenum for multiple detonation tubes, the backpressure is then expected to have a reduced effect on the inlet flowfield. However, the spillage from a closing valve into an adjacent opening one may affect the combustion chamber operation by affecting the fuel-air mixture ratio. Among the proposed PDE configurations that have been considered are multi-tube detonation devices connected to a common inlet. These designs allow for the generation of continuous thrust by initiating the detonation and recharging of the detonation ducts at controlled frequencies. Such configurations raise the issues of inlet-combustion chamber interactions resulting in unsteady inlet flow fields since the inlet exit plane experiences non-uniform pressure fields arising from the operation of the PDE detonation tube valves. Two-dimensional inlets, with back-pressure exited by translational mechanisms and axisymmetric inlets with rotating valves were studied in the M=2-4 range.
Effects of Blockage on Shock Train System

References:

J. Gustavsson, V. Nori and C. Segal, "Inlet/Engine Interactions in an Axisymmetric Pulse Detonation Engine System", Journal of Propulsion and Power, vol. 19, no. 2, pp. 282-286, March 2003.

V. Nori, N. Lerma, J. Gustavsson, C. Segal, "Forced Oscillations in a Mixed-Compression Inlet at Mach 3.5 for Pulse Detonation Engine Systems", Journal of Fluids Engineering , 128 (3), pp. 494-506, 2006.

Hypersonic Inlets

Injection of liquid fuel in the inlet of a vehicle flying at hypersonic speed is related to the development of liquid fuel-based supersonic-combustion ramjets as a means to increase the residence time and achieve partial fuel pre-mixing prior to arrival at the combustion chamber. The strong liquid interaction with the inlet high-momentum airflow and shock wave system offers a mechanism for rapid jet and droplet breakup, hence improved mixing. This study evaluated the penetration and spreading of liquid jets in a two-dimensional external-internal compression inlet at Mach 3.5. Schlieren imaging have been used as the visualization technique for penetration studies and light scattering for jet spreading. By using thin pylons to create a low-pressure region at the liquid injection station the penetration increased compared with a non-pylon configuration while reducing the pressure losses associated with transverse jet injection. The pylons contributed to lift the liquid from the injection surface with less lateral spreading than the non-pylon-injection case thus avoiding the presence of a low-speed combustible mixture in the inlet/isolator boundary layers and providing a mechanism to eliminate potential flashback.
Several practical issues arise in the case of pre-injection of liquid fuel in the engine duct: a) mixing efficiency, flow deceleration and inlet performance, and b) the ability to avoid flashback by eliminating the residence of the fuel on the inlet/isolator walls. This study shows the effects of the presence of pylons on the liquid fuel penetration and spreading in a two-dimensional, external-internal compression inlet in a Mach 3.5 airflow. Liquid jets with a diameter equal to the width of the pylons have been injected transverse to the flow in the base of the pylons. For all the dynamic pressure ratios the liquid penetrated immediately to the height of the pylons, then the jet column was abruptly broken and the liquid was carried into the inlet airflow. The pylons showed the capability to facilitate rapid liquid penetration into the airflow core eliminating liquid arrival at the inlet's walls.

More

Reference:

T. Livingston, C. Segal, M. Schindler and V. A. Vinogradov, "Penetration and Spreading of Liquid Jets in an External-Internal Compression Inlet",AIAA Journal, vol. 38, no. 6, pp. 989-994, June 2000.

Filtered Rayleigh Scattering

Filtered Rayleigh scattering is a new, non-intrusive measurement technique, where the Doppler shift produced when narrow-band laser light is scattered against small particles is used to quantify the velocity of the particles. When the scattering particles consist of the fluid molecules themselves, this technique is capable of measuring not only one component of the bulk flow velocity, but also the temperature, density and pressure of the fluid in a complete plane. However, due to the sensitivity of the Rayleigh light signal to noise from entrained particles in the flow, its capability of measuring the flow velocity in a flow with a degree of impurity typical of actual wind tunnel flows was questioned. While previous workers have attained uncertainties of only a few m/s, this required purified gas flow and/or custom-built experimental setups. To quantify the level of uncertainty in a non-purified flow using commercially available systems, experiments were carried out in a free, nearly perfectly-expanded M=2.22 jet were carried out. Through comparison with an earlier investigation and to pressure probe data, it was concluded that the uncertainty in the present setup was 40 m/s. The chief source of uncertainty was laser wavelength drift. Based on a quantitative uncertainty analysis, it was concluded that the laser wavelength was more closely monitored, uncertainties of the order of 10 m/s could be achieved in areas with strong scattered light signal.

More

References:

J. Gustavsson, C. Segal, "Filtered Rayleigh Scattering Velocimetry - Accuracy Investigation in a M=2.22 Axisymmetric Jet", 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, AIAA 2004-0021, Jan 2004.

J. P. R. Gustavsson, C. Segal, "Filtered Rayleigh Scattering Velocimetry - Accuracy Investigation in a M=2.2 Axisymmetric Jet", Experiments in Fluids, 38, pp. 11-20, 2005.

Combustion Chemistry

Fluorinated Polymer Decomposition and Combustion

The time-varying concentrations of gases formed as solid and liquid highly fluorinated polymers containing SF5 groups were investigated using fourier-transform infrared spectrometry. Several different new compounds designed to release HF and F2 upon heating were studied in both inert argon and oxidizing air atmospheres. The experiment took place in a heated nickel-coated stanless steel testchamber fitted in the smaple compartment on an FTIR spectrometer. The test was initiated when a milligram-size sample of the polymer under study was quickly heated (>100 K/s) on electric heater spiral inside the test chamber and numerous IR spectra were acquired during the tests.

More

References:

Gustavsson, J P R, Lerma, N, Segal, C, "Combustion Characterization of Fluorinated Polymers", AIAA Paper 2004-3883, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Ft Lauderdale, Florida, July 11-14, 2004.

Gustavsson, J. P. R., Segal, C., "Combustion and thermal decomposition of fluorinated polymers", Journal of Combustion Science and Technology (accepted June 2006, to be published Dec 2006).

 

 

Catalytic Combustion

 

This study evaluated the catalysed reactions in a rich fuel-air mixture as it passed through the pre-combustor stage of a low-NOx gas turbine combustor. The test facility consisted of two concentric tubes with the outer surface of the inner tube covered with a catalyst. The inner tube contained a cooling air flow while the annular region contained a fuel-air mixture that reacted on the catalytic surface. The inner and outer air flows can be independently controlled by a number of massflow controllers from Alicat, allowing equivalence ratios and fuel composition to be varied over a wide range. The incoming air streams can also be preheated through passing through two independently controlled electric heaters. Measurements are carried out using mass spectrometry of gas samples drawn from above the catalyst surface at different axial locations. In parallel with the experimental tests, CFD simulations using Fluent were carried out.

 

 

References:

S. Pathak and C. Segal, “Numerical simulation of Hydrogen Assisted Methane Combustion on Platinum Catalyst”, 22nd National Convention of Aerospace Engineers, NCAE Paper 103-2008, India, November 2008.

 

S. Pathak and C. Segal, “Surface Coverage Effects on Catalytic Combustion of Syngas on Platinum”, International Conference on Aerospace Science and Technology, INCAST 2--9-018, Banglore, India, June 2008.

 

Current research topics

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