Supercritical Combustion Research Articles

Supercritical burning of liquid oxygen (LOX) droplet with detailed chemistry

A numerical study of the supercritical combustion of a liquid oxygen (LOX) droplet in a stagnant environment of hot hydrogen is carried out with a detailed chemistry model. Special attention is devoted to ignition process and diffusion flame structure. Ignition consists typically of the propagation of a premixed flame which is initiated in the H 2-rich hot side. Propagation takes place in a nonhomogeneous hot environment (say 1500 K) with a considerable velocity (typically 50 ms −1). Despite the high temperature of the ambiance, the medium ahead of the flame can be considered as frozen during the transit time. In addition, it is found that droplets with diameter less than 1 μm vaporize before burning. A quasi-steady-like diffusion flame is then established. In this regime we observe that the D 2 law is approximately valid. In contrast to the case of a single irreversible reaction, a full chemistry model leads to a very thick flame where chemical consumption and production cover a surrounding zone about four times the instantaneous droplet radius. Reversibility of the reactions plays a determinant role in the flame structure by inducing a large near-equilibrium zone which is separated from a frozen region by a thin nonequilibrium zone. The length scale of the latter region is found to behave as the square root of the instantaneous droplet radius and a detailed analysis shows that just two elementary reactions are involved in this zone. Furthermore, the influence of several parameters is considered; temperature and pressure in the combustion chamber have a weak influence on the burning time. Influence of initial droplet radius confirms that droplet combustion is a diffusion controlled process. Chamber composition is also considered. Finally, it is shown that a precise description of the transport properties in the dense phase is not required.

Supercritical and subcritical combustion of lox with hydrogen in an axisymmetric laminar diffusion flame

Combustion of liquid oxygen (LOX) with H 2 under super- and subcritical conditions was studied both theoretically and experimentally. In the theoretical work, a comprehensive model was formulated and solved numerically to simulate combustion processes of LOX, supplied from a circular tube at its rate of consumption, with ambient hydrogen gas flowing parallel to the axial direction of LOX-supplying tube. The model was based on the conservation equations for a multicomponent system, with consideration of gravitational body force, solubility of ambient gases in liquid, and variable thermophysical properties. A fugacity-based multicomponent thermodynamic analysis with quantum-gas mixing rules was carried out to characterize the solubility of ambient gas at the LOX surface. A stable onion-shaped LOX/H 2 /He diffusion flame was achieved at a pressure of 30 atm. The calculated flame shape agreed well with the shape of the observed luminous flame. Under a supercritical combustion situation at 68 atm, the calculated flame shape exhibited a pearlike contour. From the simulation results, it was found that two counter-rotating vortex structures existed in the flame region due mainly to the combustion-induced baroclinic torque as well as buoyancy-induced flow acceleration and entrainment of the surrounding H 2 /He mixture. From the comparison of calculated results using the flame-sheet model and those based upon equilibrium calculations, the flame-sheet model was found to be able to predict the location of the flame front as well as distributions of temperature, velocity, and major constiuent species with reasonable accuracy.

Combustion of Miscible Binary-Fuel Droplets at High Pressure Under Microgravity

Abstract The objective of this research is to study near-critical and super-critical combustion of droplets consisting of binary fuel mixtures. Experimental results are reported on the burning of fiber-supported droplets of mixtures of n-heptane and n-hexadecane, initially about 1 mm in diameter, under free-fall microgravity conditions. The ambient pressures range up to 3.0 MPa, extending above the critical pressure of both fuels, in room-temperature nitrogen-oxygen atmospheres having oxygen mole fractions of 0.12 and 0.13. Three-stage burning of the binary fuel droplets is observed, and the onset lime of the second stage is compared with the predictions of an existing theory. Experimental evidence of thermo-capillary and/or diffuso-capillary convection during the droplet burning is obtained. The results contribute to improving understanding of binary-fuel droplet-combustion processes at high pressures.

Numerical study of the transient vaporization of an oxygen droplet at sub- and super-critical conditions

Unsteady vaporization of a droplet in a high-pressure quiescent environment has been studied. With spherical symmetry and constant pressure, the process is diffusion controlled. At low pressures droplet heating is significant for most of the droplet lifetime, and an unsteady analysis of the gas phase is required. Then, the vaporization of a liquid oxygen droplet in gaseous hydrogen at moderate and high pressures is considered. At super-critical pressures, the surface temperature reaches the computed critical mixture value where a model for super-critical combustion is needed. The details of the diffusion layer of dissolved hydrogen do not significantly affect the results. Various methods are explored to compute the gas-phase density. Under the model's assumptions, homogeneous nucleation is not likely to occur.

Nonadiabaticity, stoichiometry and mass diffusion effects on supercritical combustion in a tubular reactor

The high-pressure oxidation of dense organic fluids, which are completely miscible in water at supercritical temperatures can be achieved by burning a supercritical combustible mixture in a flow-through tubular reactor. We extend a previous model of this process by specifically accounting for the effects of stoichiometry, mass diffusion and radiative, heat losses on the multiplicity and stability of supercritical premixed flames within the reactor. Our model for the supercritical tubular reactor differs from those for conventional chemical reactors in that the Lewis numbers are assumed to be large and conductive/radiative heat losses occur at the inlet, which behaves much like a flameholder. In general, we show that for sufficiently small heat losses, there are both high-speed, high-temperature and low-speed, low-temperature solution branches for a given steady, planar flame location within the reactor. In addition, the otherwise stable high-temperature flame loses stability to disturbances corresponding to nonsteady, nonplanar modes of supercritical combustion for sufficiently large values of the activation energy and/or sufficiently small or large values of the flow velocity. In the limit of weak stoichiometry and small mass diffusivity, the flame speeds are shown to decrease as the former decreases further and the latter increases, whereas local radiative heat losses, which cause the flame to be located closer to the inlet, are destabilizing with respect to steady, planar burning.

Multiplicity and Stability of Supercritical Combustion in a Nonadiabatic Tubular Reactor

Abstract Combustible mixtures under supercritical pressures and temperatures are capable of supporting a self-propagating flame under certain conditions. We consider the multiplicity and stability of such flames in a nonadiabatic tubular reactor as a function of inflow velocity, inlet and volumetric heat loss parameters, reactor length, and overall activation energy. We find that a steady, planar flame can only exist within certain parameter ranges, and that there are both high- and low-temperature solutions which correspond to a given flame position within the reactor. In addition, a linear stability analysis shows that for sufficiently large values of the overall activation energy, the high-temperature solution is unstable to disturbances which correspond to nonsteady, nonplanar supercritical flames. Consequently, we show that for typical values of this parameter, the basic solution branch representing high-temperature, steady combustion is stable when the flame lies well within the reactor, but it loses stability to spinning and/or pulsating modes of burning when the inflow velocity is varied so that the flame approaches either end of the reactor.

Combustion of a fuel droplet in supercritical gaseous environments

An experimental study was made of the combustion phenomena of single suspended fuel droplets in supercritical gaseous environments. The temperatures, combustion lifetimes and burning rate constants were measured for various kinds of fuel. The final droplet temperature was observed to be nearly equal to its critical temperature and almost independent of ambient pressure under supercritical conditions. For all the fuels tested in the present experiments, the combustion lifetime correlated well with the reduced pressure, which was defined as the ratio of the ambient pressure to the critical pressure of the fuel. It was concluded that the critical point of the fuel was an important criterion in the supercritical combustion of a fuel droplet. In the supercritical gaseous environments tested here, increasing the pressure resulted in a very slight decrease in the combustion lifetime and a fairly large increase in the burning rate constant.

Evaluation of a locally homogeneous flow model of spray combustion

A model of spray combustion which employs a second-order turbulence model was developed. The assumption of locally homogeneous flow is made, implying infinitely fast transport rates between the phase. Measurements to test the model were completed for a gaseous n-propane flame and an air atomized n-pentane spray flame, burning in stagnant air at atmospheric pressure. Profiles of mean velocity and temperature, as well as velocity fluctuations and Reynolds stress, were measured in the flames. The predictions for the gas flame were in excellent agreement with the measurements. The predictions for the spray were qualitatively correct, but effects of finite rate interphase transport were evident, resulting in a overstimation of the rate development of the flow. Predictions of spray penetration length at high pressures, including supercritical combustion conditions, were also completed for comparison with earlier measurements. Test conditions involved a pressure atomized n-pentane spray, burning in stagnant air at pressures of 3, 5, and 9 MPa. The comparison between predictions and measurements was fair. This is not a very sensitive test of the model, however, and further high pressure experimental and theoretical results are needed before a satisfactory assessment of the locally homogeneous flow approximation can be made.

Design and development of a high pressure facility for droplet combustion experiments

In order to provide information to evaluate critically the quasi-steady droplet combustion theories at elevated pressures and the super-critical droplet combustion theories, a high pressure facility has been designed and developed for investigating photographically the combustion of freely falling n-pentane drops at pressures up to 615 lb in-2 absolute (corresponding to n-pentane reduced pressure of 1.26). The good quality photographs enable quantitative determination of mass consumption rates, flame geometry and drag coefficients. They also allow a qualitative assessment of predicted high pressure effects to be made and provide details of new observations at high pressures.

Bipropellant droplet combustion in the vicinity of the critical point

Droplet conditions during high-pressure combustion were examined both experimentally and theoretically. Measurements were made of steady burning temperatures and the pressures required for the supercritical combustion of n -octane and n -decane droplets burning in air. The measurements were made under zero gravity conditions in order to eliminate convection and to prevent the droplet from falling due to reduced surface tension near the critical point. Theoretical results were obtained with the conventional low-pressure treatment of phase equilibrium at the droplet surface as well as with high-pressure models which allowed for real gas effects as well as finite ambient gas solubility. A variable-property gas-phase model was employed in the analysis which allowed for the diffusion of dissolved gases along with the fuel. The high-pressure models yielded reasonably good predictions of the measured conditions required for supercritical burning (on the order of 2-2.5 times the critical pressure of the pure fuel for n -octane and n -decane droplets burning in air). The low-pressure model was not satisfactory, predicting supercritical burning pressures roughly one-half the measured values. The concentration of dissolved gas becomes quite high as the supercritical burning condition is approached (20–40% for paraffins heavier than n -octane). The calculations also indicated conditions where water should condense on the droplet surface, both during heat-up and steady burning, for the n -paraffin hydrocarbons.

Supercritical bipropellant droplet combustion

Observations were made of the burning of ndividual fuel droplets at pressures sufficiently high for the droplet to reach its critical temperature during combustion. The droplet was supported from a thermocouple junction within a small pressurized chamber and ignited with a hot wire. To prevent the droplet from falling from the thermocouple, due to reduced surface tension near the critical point, the experiment was carried out under zero-gravity conditions in a free-fall apparatus. Decane droplets were employed in the experiments at pressures in the range of 100–2000 psia, and ambient gas compositions varying from air to pure oxygen. In air, little evaporation occurred prior to the time when the droplet reached its critical temperature at pressures greater than 2 1/2 times the critical pressure. At these high pressures, available theories of supercritical combustion were in reasonable agreement with the experimental measurements of combustion lifetime, both with regard to the influence of pressure and ambient oxygen concentration. The theories also gave a good prediction of flame-zone position during combustion.