Modified Regression Rate Formula of PMMA Combustion by a Single Plane Impinging Jet

A modified regression rate formula for the uppermost stage of CAMUI-type hybrid rocket motor is proposed in this study. Assuming a quasi-steady, one-dimensional, an energy balance against a control volume near the fuel surface is considered. Accordingly, the regression rate formula which can calculate the local regression rate by the quenching distance between the flame and the regression surface is derived. An experimental setup which simulates the combustion phenomenon involved in the uppermost stage of a CAMUI-type hybrid rocket motor was constructed and the burning tests with various flow velocities and impinging distances were performed. A PMMA slab of 20 mm height, 60 mm width, and 20 mm thickness was chosen as a sample specimen and pure oxygen and O2/N2 mixture (50/50 vol.%) were employed as the oxidizers. The time-averaged regression rate along the fuel surface was measured by a laser displacement sensor. The quenching distance during the combustion event was also identified from the observation. The comparison between the purely experimental and calculated values showed good agreement, although a large systematic error was expected due to the difficulty in accurately identifying the quenching distance

Interactive combustion of two-dimensionally arranged quasi-droplet clusters under microgravity

To investigate the mutual interactions between droplets in the spray combustion, combustion of 2-dimensionally arranged quasi-droplet clusters is studied under microgravity. Quasi-droplet samples, which are solid in room temperature and change into liquid just after the ignition, consist of alcohol (propanol, butanol, pentanol, or hexanol) and polyethylene glycol with a volumetric ratio of 2:1. Seven samples sustained by glass rods form a 2-dimensional quasi-droplet cluster. Electrically heated nichrome wires ignite all samples in the cluster simultaneously. Single envelope flames that surround the clusters appeared. The results show that the sample spacing has a strong effect on the shape and movement of the flame. Sample clusters with large sample spacings come to the external group combustion through the scavenging combustion mode, whereas the small spacing clusters start directly with the external group combustion. At large sample spacings, the distance from the edge of the sample cluster to the flame (flame distance) increases to a maximum value and then decreases with time. The period of flame growth is prolonged with decreasing sample spacing and finally, at a small enough sample spacing, the flame distance keeps increasing until the flame disappears. This flame movement is attributed to the fuel vapor accumulation effect, which becomes more dominant with decreasing sample spacing. The burning lifetime decreases monotonically and approaches the value of the single flame with increasing sample spacing. The flame distance decreases monotonically and approaches the single flame radius with increasing sample spacing also. These results render important confirmations of the external group combustion phenomena and prove the importance of the two kinds of unsteadiness, i.e., the scavenging combustion with large droplet interval and the fuel vapor accumulation effect with small droplet interval, in group combustion.Copyright © 2002 The Combustion Institute

Stabilized combustion of circular fuel duct with liquid oxygen

This research is an investigation of the flame spread opposed to a liquid oxidizer flow in a solid fuel duct. Several firing tests were conducted using liquid oxygen as the oxidizer and solid poly methyl methacrylate (PMMA) as the fuel. The results indicate that the flame spread rate decreased with increasing oxidizer port velocity and decreasing port diameter. This study reveals through visual confirmations and empirical correlations of the flame spread rate that the flame spread opposed to liquid oxygen in a solid fuel duct can be classified as stabilized combustion. Extinction and abnormal regression were observed when oxidizer port velocity was high and port diameter was small. Furthermore, the cooling of the solid fuel by the liquid oxygen flow had a strong effect on the transition between normal regression and extinction, or abnormal regression. A model of the flame spread rate which considers the heat balance at the fuel surface assuming a fully developed thermal boundary layer is introduced and shown to agree well with the experimental results. Lastly, it is revealed that the difference in kinematic viscosity between liquid oxygen and gaseous oxygen is the main reason dependency of port diameter on flame spread rates differs between the liquid oxygen tests in this study and gaseous oxygen tests in previous studies

Thermo-electrohydrodynamic convection in a differentially heated vertical slot

International audienceApplication of a high-frequency a.c. electric field to a non-isothermal layer of dielectric fluid generates convective flow though the Thermo-Electrohydrodynamic (TEHD) instability. The driving force is dielectrophoretic (DEP) one, which arises from the thermal variation of dielectric permittivity $\epsilon$. This convection has attracted geophysicists' interest, as the DEP force can be regarded as thermal buoyancy force in an electric effective gravity $\mathbf{g}_e \propto \boldsymbol{\nabla}\mathbf{E}^2$, where $\mathbf{E}$ is the applied electric field. One can simulate thermal convection under different gravity conditions through this analogy by using different geometrical configurations of electrodes. In particular, the analogy enables us to examine global scale geophysical flows in a laboratory experiment with concentric spherical electrodes, e.g., Mantle convection considered in the GeoFlow experiments.A number of theoretical, numerical, and experimental investigations have been done recently in different electrode configurations. In the present talk, we report a theoretical study on the TEHD convection for moderate Prandtl number fluids ($Pr \lesssim 10$) in a tall vertical slot, where the lateral walls serve as planar electrodes imposing a temperature gradient as well as an electric field to a fluid layer. If both Grashof and electric Rayleigh number, $Gr=\alpha\Delta T g d^3/\nu^2$ and $L=\alpha\Delta T {\rm g}_e d^3/\nu\kappa$, respectively, are small, the flow system is in conduction regime, where natural convection develops to form a vertical shear flow except in the regions near the top and bottom walls. Either $Gr$ or $L$ exceeds a critical value, instabilities arise and a cellular secondary flow develops. The flow state in this convection regime is determined by seeking numerically an exact solution of governing nonlinear equations in the Fourier-Chebyshev spectral space. The flow behavior in the vicinity of the critical state is determined by following the path of solution in the $Gr$-$L$ phase space. The Nusselt number $Nu$ is computed to show that the Earth's gravity affects significantly the heat transfer in the TEHD convection

Thermo-electrohydrodynamic convection in a differentially heated vertical slot

International audienceApplication of a high-frequency a.c. electric field to a non-isothermal layer of dielectric fluid generates convective flow though the Thermo-Electrohydrodynamic (TEHD) instability. The driving force is dielectrophoretic (DEP) one, which arises from the thermal variation of dielectric permittivity $\epsilon$. This convection has attracted geophysicists' interest, as the DEP force can be regarded as thermal buoyancy force in an electric effective gravity $\mathbf{g}_e \propto \boldsymbol{\nabla}\mathbf{E}^2$, where $\mathbf{E}$ is the applied electric field. One can simulate thermal convection under different gravity conditions through this analogy by using different geometrical configurations of electrodes. In particular, the analogy enables us to examine global scale geophysical flows in a laboratory experiment with concentric spherical electrodes, e.g., Mantle convection considered in the GeoFlow experiments.A number of theoretical, numerical, and experimental investigations have been done recently in different electrode configurations. In the present talk, we report a theoretical study on the TEHD convection for moderate Prandtl number fluids ($Pr \lesssim 10$) in a tall vertical slot, where the lateral walls serve as planar electrodes imposing a temperature gradient as well as an electric field to a fluid layer. If both Grashof and electric Rayleigh number, $Gr=\alpha\Delta T g d^3/\nu^2$ and $L=\alpha\Delta T {\rm g}_e d^3/\nu\kappa$, respectively, are small, the flow system is in conduction regime, where natural convection develops to form a vertical shear flow except in the regions near the top and bottom walls. Either $Gr$ or $L$ exceeds a critical value, instabilities arise and a cellular secondary flow develops. The flow state in this convection regime is determined by seeking numerically an exact solution of governing nonlinear equations in the Fourier-Chebyshev spectral space. The flow behavior in the vicinity of the critical state is determined by following the path of solution in the $Gr$-$L$ phase space. The Nusselt number $Nu$ is computed to show that the Earth's gravity affects significantly the heat transfer in the TEHD convection

Hybrid Rockets as Post-Boost Stages and Kick Motors

Hybrid rockets are attractive as post-boost stages and kick motors due to their inherent safety and low cost, but it is not clear from previous research which oxidizer is most suitable for maximizing Delta V within a fixed envelope size, or what impact O/F shift and nozzle erosion will have on Delta V. A standard hybrid rocket design is proposed and used to clarify the impact of component masses on Delta V within three 1 m(3) envelopes of varying height-to-base ratios. Theoretical maximum Delta V are evaluated first, assuming constant O/F and no nozzle erosion. Of the four common liquid oxidizers: H2O2 85 wt%, N2O, N2O4, and LOX, H2O2 85 wt% is shown to result in the highest Delta V, and N2O is shown to result in the highest density Delta V, which is the Delta V normalized for motor density. When O/F shift is considered, the Delta V decreases by 9% for the N2O motor and 12% for the H2O2 85 wt% motor. When nozzle erosion is also considered, the Delta V decreases by another 7% for the H2O2 85 wt% motor and 4% for the N2O motor. Even with O/F shift and nozzle erosion, the H2O2 85 wt% motor can accelerate itself (916 kg) upwards of 4000 m/s, and the N2O motor (456 kg) 3550 m/s

Instability of the vertical annular flow with a radial heating and rotating inner cylinder

International audienceA linear stability analysis of the flow confined in a differentially rotating cylindrical annulus with a radial temperature gradient has been performed. Depending on values of control parameters (the Taylor number, the Grashof number, and the Froude number), it has shown flow destabilization to axisymmetric or non-axisymmetric modes. Analysis of different terms involved in the evolution rate of the perturbation kinetic energy has allowed us to isolate the dominant terms (centrifugal force or buoyancy force) in the destabilization process. We have shown that the centrifugal buoyancy can induce the asymmetry of the temperature gradient on critical states

Instability of the vertical annular flow with a radial heating and rotating inner cylinder

International audienceA linear stability analysis of the flow confined in a differentially rotating cylindrical annulus with a radial temperature gradient has been performed. Depending on values of control parameters (the Taylor number, the Grashof number, and the Froude number), it has shown flow destabilization to axisymmetric or non-axisymmetric modes. Analysis of different terms involved in the evolution rate of the perturbation kinetic energy has allowed us to isolate the dominant terms (centrifugal force or buoyancy force) in the destabilization process. We have shown that the centrifugal buoyancy can induce the asymmetry of the temperature gradient on critical states

Computational Analysis and Regression Laws for Nozzle Erosion Prediction in Hybrid Rockets

The erosion of the nozzle throat can represent one of the major limitations against the future widespread use of hybrid rocket engines (HREs) in the space industry. In fact, nozzle erosion in HREs can be more severe and harder to predict than in solid rockets due to the higher concentration of oxidizing species in the combustion products and to mixture ratio shifts and/or throttling. Therefore, an accurate understanding of the erosion phenomenon is of fundamental importance for the technological advancement of HREs. This work is focused on the investigation of graphite nozzle erosion in HREs burning high-density polyethylene with two different oxidizers, oxygen and nitrous oxide. First, the results of a computational fluid dynamics parametric analysis are used to derive closed-form regression laws for the rapid estimation of nozzle throat erosion and wall temperature depending on chamber pressure and mixture ratio. Then, a one-dimensional transient heat conduction solver is loosely coupled with the aforementioned regression laws, allowing to reconstruct the transient heating process within the solid. The obtained numerical results are validated against experimental data. Finally, the effect of gas-phase reactions on the heterogeneous reactions occurring at the nozzle surface is highlighted when moving from fuel-rich to oxidizer-rich conditions