Marcos Lema

Assessment of Slag Accumulation in Solid Rocket Boosters: Part II, Two-Phase Flow Experiments

In solid propellant rocket motors incorporating a submerged nozzle, the entrapment of liquid residues of the combustion in the cavity formed in the surrounding of the nozzle integration part can lead to the accumulation of slag with a considerable mass. The long-term goal of the present study is to characterise experimentally the driving parameters of the slag accumulation in a stagnant area modelling the nozzle cavity. The experimental database will be used for validation of a numerical tool. In the present article measurements are shown in two-phase flow condition using a coldgas simplified model to determine effectively the main parameters that are influencing the droplet entrapment. Furthermore, the deformation of the accumulated liquid surface is analysed and presented as well.

Experimental and Numerical Multiphase-Front Fluid Hammer

The aim of this work is to study the fluid hammer and related multiphase phenomena taking place in a confined environment, where pressurized liquid faces vacuum conditions after opening an isolation valve. This paper describes the experimental facility built for this purpose, the tests performed, and shows the comparison with the numerical results obtained with a transient one-dimensional software based upon conservation equations. Because the multiphase phenomena taking place in the liquid front moving along the pipe are complex and difficult to interpret, a graphical tool by means of X-t diagrams was adapted by the authors to characterize the physical phenomena involved. Contrary to the classical water hammer analysis published in the literature so far, the X-t diagrams allow a simultaneous study in time and space for each of the magnitudes involved in the evolution of the fluid hammer.

Multiphase Investigation of Water Hammer Phenomenon Using the Full Cavitation Model

The aim of this work is to clarify some issues raised recently on the characterization of the water hammer, by presenting a numerical analysis on the multiphysics taking place. For this purpose, the commercial code CFD-ACE+ is used with the so-called full cavitation model, which takes into account the effect of absorption and desorption of a noncondensable gas. A comparison between two different compressibility models is conducted, making use of an inverse engineering technique, which allows the tracking of the medium speed of sound. Finally, a numerical reproduction of a valve opening is proposed, leading to a sudden acceleration of the fluid and consequent vaporization of the liquid. The numerical results confirm high sensitivity of the water hammer prediction regarding the amount of noncondensable gas present in the liquid.

Assessment of Slag Accumulation in Solid Rocket Boosters: Summary of the VKI research

In solid propellant rocket motors incorporating a submerged nozzle, the entrapment of liquid residues of the combustion in the cavity formed in the surrounding of the nozzle integration part can lead to the accumulation of slag with a considerable mass. The goal of the present study is to characterize experimentally the driving parameters of the slag accumulation in a stagnant area modelling the nozzle cavity. Furthermore, the experimental database is used for validating a numerical tool as well. In the present article a summary of the VKI (von Karman Institute) research carried out during the last four years is shown. Single-phase and two-phase measurements are performed using a cold-gas simplified model. Simultaneously, the experimental configuration is simulated numerically with the help of a commercial solver (CFD-ACE+). The final aim of the article is to point out the driving forces of the droplet entrapment process.

Single-Phase Internal Flowfield Validation with an Experimental Solid Rocket Motor Model

Incorporating a submerged nozzle for solid propellant rocket motors, the resulting cavity formed in the vicinity of the nozzle integration part enhances the entrapment of liquid residues of the combustion and leads to the accumulation of slag with a considerable mass. As a first step for the slag accumulation assessment, an experimental and numerical investigation of the single-phase internal flowfield is performed and the numerical tool is validated. The measurements are carried out in a cold-gas simplified model in which the flow is injected from the forward end (no wall injection). Simultaneously, the experimental configuration is simulated numerically with the help of a commercial solver (CFD-ACE+). The experimental results allowed the full characterization of the complex recirculation zone downstream of the inhibitor model and in the cavity. It was also demonstrated that the airflow in the cavity is slightly altered in case the gas-slag interface is represented either as a rigid or a liquid. The time-averaged statistical results of the three-dimensional simulation in the midspan plane compare very well to the experimental database, ensuring a satisfactory validation. The mean streamlines close to the walls, plotted from the computational fluid dynamics field, allow better understanding of the flow structures. The analysis reveals a two-dimensional flowfield for the studied geometry with locally some three-dimensional behaviors, especially along the main recirculation bubble interface.

Periodic Phenomena on a Partially Cavitating Hydrofoil

ABTRACT This paper aims at evaluating the sensitivity of the numerical simulations of patch and sheet/cloud cavitation to turbulence modeling in 2D. The periodic phenomena related to cavity shedding and cloud formation on a partially cavitating NACA0015 foil are studied within a wide range of conditions, on relatively coarse meshes. The finite volume CFD package OpenFoam is used. A Homogeneous Equilibrium Approach is employed, in which cavitation modeling is achieved through a transport equation for the vapor volume fraction. The effect of turbulence modeling is analyzed, comparing the results yielded by Implicit Large Eddy Simulation (ILES) and a URANS k-ω SST model in two versions: the conventional one of Menter, and a modified one in which turbulent viscosity is made sensitive to vapor fraction. The cavity dynamics is well captured with ILES, and its dominant frequencies are well validated with experimental data. As for the RANS models, it is found as expected that the k-ω SST in its original formulation leads to an unrealistic stabilization of the cavity. With the modified version, the dynamics of the cavity is triggered in the sheet/cloud regime, and the periodic character of cavity shedding/cloud formation is well captured. However, the frequency of the process is a little lower than with ILES, and close to patch cavitation, the modification is not sufficient to trigger a substantial dynamics. This is due to the smaller cavity volumes predicted by the k-ω SST model, which delays slightly shedding. Through processing of the flow fields, it is demonstrated that the frequency of the travelling and collapsing vapor clouds is related to the frequency of cavity shedding.

A New Approach to Laminar Flowmeters

After studying the performance and characteristics of actual laminar flowmeters a new disposition for this type of sensors is proposed in such a way that the measurement errors introduced by the intrinsic nature of the device can be minimized. The preliminary study shows that the developing entry region introduces non-linearity effects in all these devices. These effects bring about not only errors, but also a change in the slope of the linear calibration respect of the Poiseuille relation. After a subsequent analysis on how these non-linearity errors can be reduced, a new disposition of this type of flowmeters is introduced. This device makes used of flow elements having pressure taps at three locations along its length and connected to three isolated chambers. In this way, the static pressure can be measured at three locations and contributed to by the pressure taps at the level of each chamber. Thus the linearization error is reduced with an additional advantage of producing a reduced pressure drop.

Comparison of Different Passive Control Solutions for Reducing SRM Pressure Oscillations Using Cold Flow Experiments

Cold gas experiments are used to study the pressure oscillations occurring in solid rocket motors (SRM) and the performance of different ways of passive control of these oscillations. Previous studies stated that flow–acoustic coupling is mainly observed for nozzles including cavity. The nozzle geometry has an effect on the pressure oscillations through a coupling between the acoustic fluctuations induced by the cavity volume and the vortices traveling in front of the cavity entrance. The most important reduction of pressure oscillations is obtained by removing the cavity located around the nozzle head. It has been firstly proved that removing the cavity located around the nozzle head is a very good solution both for the axial and radial flow injection configurations, with a reduction factor up to 10. However, the nozzle integration cannot be avoided and this solution can then not be implemented on real flight. A permeable membrane (with holes to allow the combustion gas to pass through) placed in front of the cavity allows a reduction by a factor 1.5. The Helmholtz resonator shows small attenuation of the pressure oscillations; however, its design can be optimized in order to maximize the acoustic damping. The 3D-shaped inhibitors show a good attenuation of the pressure fluctuations, especially when the opening cross-section is increased. This increase results in a shift of the Mach number associated to excitation. For a similar cross-section, the asymmetric inhibitor (crenel-shaped) provides a net reduction of 48% compared to an axisymmetric inhibitor. So, the asymmetry of the inhibitor provides a promising way of reducing the pressure oscillations.