Andreas Rees | German Aerospace Center (DLR) (original) (raw)

Papers by Andreas Rees

For a detailed experimental investigation of superheated cryogenic fluids the new cryogenic test ... more For a detailed experimental investigation of superheated cryogenic fluids the new cryogenic test bench M3.3 with a temperature controlled injection system was built at the DLR Institute of Space Propulsion in Lampoldshausen. In a first test campaign flash boiling liquid nitrogen sprays were visualized by means of high-speed shadowgraphy to determine the resulting spray patterns and spray angles. The cryogenic spray patterns are compared with break-up correlations for storable fluids from literature. The dependence of droplet velocities and diameters on the injection conditions was investigated at a constant position in the spray by means of Phase Doppler Anemometry. Nomenclature χ onset criterion [-] ∆T ∗ degree of superheat [-] Φ correction factor [-] ρ density [kg/m3] σ surface tension [N/m] θ spray angle [°] cp specific heat [J/(K kg)] D diameter [m] D10 arithmetical mean diameter [μm] H height [m] hvap heat of vaporization [J/kg] Ja Jakob number [-] L length [m] p pressure [Pa] ...

International Journal of Multiphase Flow

International Conference on Liquid Atomization and Spray Systems (ICLASS)

Technology development for propulsion systems of upper stages and reaction control thrusters is d... more Technology development for propulsion systems of upper stages and reaction control thrusters is driven by green propellants to substitute hydrazine. At high-altitude conditions prior to ignition the liquid propellants are injected into the combustor at near-vacuum. Due to the sudden pressure drop the liquid is in a superheated thermodynamic state resulting in an eruptive evaporation and fast expansion, a process called flash boiling. To know the composition related to phase and atomization is important for both to determine the probability of a successful ignition and to avoid destructive pressure peaks. Furthermore, the experimental results provide a data base for further numerical investigations about superheated cryogenic fluids. Hence, the cryogenic test bench M3.3 with a temperature-controlled injection system for an experimental investigation of cryogenic flash boiling sprays is in operation at DLR. By means of laser-based Phase Doppler diagnostics the velocity and droplet size distributions of this kind of sprays were determined and two different droplet populations were found. The analysis shows that the flashing sprays are axially symmetrical and that both droplet populations are linked to each other due to similar fluctuations in the transient injection phase.

International Conference on Liquid Atomization and Spray Systems (ICLASS)

Technology development for propulsion systems of upper stages and reaction control thrusters is d... more Technology development for propulsion systems of upper stages and reaction control thrusters is driven by green propellants to substitute hydrazine. At high-altitude conditions prior to ignition the liquid propellants are injected into the combustor at near-vacuum. Due to the sudden pressure drop the liquid is in a superheated thermodynamic state resulting in an eruptive evaporation and fast expansion, a process called flash boiling. To know the composition related to phase and atomization is important for both to determine the probability of a successful ignition and to avoid destructive pressure peaks. Hence, we visualized flash boiling liquid nitrogen (LN2) sprays by means of shadowgraphy at the cryogenic test bench M3.3 at DLR. The data analysis of these sprays revealed that the commonly used degree of superheat Rp on its own is not sufficient to describe the flash boiling process. This paper focuses on the description of this issue by comparing shadowgraph images of different LN2 sprays to each other and by the discussion of the kinetics of the sprays in terms of velocity and pressure distributions obtained by numerical simulation. Despite similar degrees of superheat Rp, the spray morphology as well as the spray kinetics show serious deviations from each other.

Atomization and Sprays, 2015

ABSTRACT This paper investigates the morphology of a flash-atomizing jet at conditions representa... more ABSTRACT This paper investigates the morphology of a flash-atomizing jet at conditions representative for rocket engine operations. Due to its relevance for the aerospace industry, both storable and cryogenic rocket engine propellants are considered, namely ethanol [as inert equivalent of monomethyl hydrazine (MMH)] and liquid oxygen (LOx). A comparison between the flashing behavior of these two fluids is conducted. Despite the differences in their physical properties, a close similarity in the spray characteristics is found in terms of spray shape, spreading angles, and evolution of the flashing regimes as a function of the initial superheat. Based on this similarity, the applicability of a novel, nucleationbased onset criterion (Â parameter) for the fully flashing regime is verified for cryogenic propellants, showing a satisfactory agreement. This result has important implications. First, it corroborates that jet disintegration at highly superheated conditions is mainly controlled by the kinetics of phase transitions (i.e., the nucleation rate). Second, it explains the differences in the degree of superheat (Rp) at onset of the fully flashing regime between ethanol and LOx sprays, respectively. The low operating temperatures in cryogenic systems result in a significant increase of the energy barrier to nucleation. Consequently, the inception of nucleate boiling can occur either at significantly higher Rp values (homogeneous process) or may be triggered by heterogeneous effects. The associated increase of the Â parameter at the onset provides a good indication of the transition to heterogeneous nucleate boiling

International Journal of Multiphase Flow

International Conference on Liquid Atomization and Spray Systems (ICLASS)

Technology development for propulsion systems of upper stages and reaction control thrusters is d... more Technology development for propulsion systems of upper stages and reaction control thrusters is driven by green propellants to substitute hydrazine. At high-altitude conditions prior to ignition the liquid propellants are injected into the combustor at near-vacuum. Due to the sudden pressure drop the liquid is in a superheated thermodynamic state resulting in an eruptive evaporation and fast expansion, a process called flash boiling. To know the composition related to phase and atomization is important for both to determine the probability of a successful ignition and to avoid destructive pressure peaks. Hence, we visualized flash boiling liquid nitrogen (LN2) sprays by means of shadowgraphy at the cryogenic test bench M3.3 at DLR. The data analysis of these sprays revealed that the commonly used degree of superheat Rp on its own is not sufficient to describe the flash boiling process. This paper focuses on the description of this issue by comparing shadowgraph images of different LN2 sprays to each other and by the discussion of the kinetics of the sprays in terms of velocity and pressure distributions obtained by numerical simulation. Despite similar degrees of superheat Rp, the spray morphology as well as the spray kinetics show serious deviations from each other.

Atomization and Sprays, 2015