Multiphase Flows

Methods

In the bath cryostat at the center of the experimental setup is a single artificial nucleation site. The vapor bubble growth in the cryogenic fluid is initiated there. Tests were performed with hydrogen at a temperature of 20 K and methane with a temperature of 111 K as test fluids.

During the microgravity vapor bubble growth was initiated by heating to investigate the bubble dynamics during a boiling process at constant pressure. In this case the bubble grows because heat transfer from the solid to the liquid superheats the liquid above the saturation temperature and thus causes evaporation.

In addition to the boiling results bubble growth during a depressurization of a saturated single species system was investigated as well. This process is of special interest during the preparation of an engine restart of a rocket upper stage in space. During the depressurization the saturation temperature decreases and a uniform superheat is introduced in the liquid. This is different to the commonly modeled process of nucleate boiling at an immersed body and thus requires a different modelling approach. The bulk of the energy required for the evaporation of liquid is supplied by the uniformly superheated bulk liquid and not a thin thermal boundary layer around a wall.

Results

Experimental data in cryogenic hydrogen and cryogenic methane was optically evaluated and the bubble growth at the given thermodynamic state was correlated to the applied stimuli. This data could then be used to construct numerical models of the physical heat and mass transfer processes involved in the bubble growth process.

The project KASIMOFF 2 under the grand number 50RL2290 is funded by financial means of the German Federal Ministry for Economic Affairs and Climate Action (BMWK) through the German Aerospace Center (DLR e.V.).

The projects ZBOT-FT and ZBOT-FT 2 are part of a series of four experiments on the International Space Station (ISS): ZBOT-1 (successfully completed in 2017), ZBOT-NC, ZBOT-DP, and ZBOT-FT (planned for 2029, currently in Phase A). ZBOT-FT is the first project with German involvement. Within the scope of ZBOT-FT and ZBOT-FT 2, in addition to laboratory, parabolic flight, and drop tower experiments, an experiment for the International Space Station is being developed in cooperation with Airbus Defence and Space, as well as the project partner Prof. Kassemi from Case Western Reserve University and NASA Glenn Research Center, USA.

Methods
To investigate the fluid mechancial and thermodynamic processes during the filling process of a tank with liquid, the liquid removal process, and the fluid transfer, both experimental and theoretical methods are employed. The experimental methods consist of four different approaches, distinguished by the residual acceleration acting on the test cell.

1. Laboratory experiments under Earth's gravity (1g).
2. Drop tower experiments in Bremen under microgravity (10⁻⁶g) with a free-fall duration of up to 9 seconds.
3. Parabolic flight experiments under reduced gravity (±10⁻²g) lasting approximately 22 seconds per parabola.
4. Experiments on the International Space Station, conducted in a permanent microgravity environment.

The experiments are continuously accompanied and assisted by theoretical studies. The theoretical work primarily involves numerical simulations using appropriate computational fluid dynamics (CFD) programs such as FLOW3D and ANSYS Fluent.

Results
The conducted experiments provide test data under both reduced gravity and Earth's gravity. The data includes local pressure and temperature measurements, volumetric flow rate measurements, as well as high-resolution black-and-white image recordings using high-speed cameras. These data offer insights into the behavior and stability of the free surface during tank filling, the flow dynamics and phase separation within and at a screen channel during liquid removal, and the thermodynamic states during fluid transfer.

The project ZBOT-FT 2 under the grand number 50WM2248 is funded by financial means of the German Federal Ministry for Economic Affairs and Climate Action (BMWK) through the German Aerospace Center (DLR e.V.).

The contribution of the University of Bremen, ZARM, lies in the processing of physical principles of the behavior of liquid hydrogen in a closed container, the tank, taking into account all the influences during the aircraft operation (temperature, pressure, acceleration profiles, etc.). As a result, the technical realization of the storage and distribution system of liquid hydrogen can be optimally adapted to the environmental conditions. The intended work represents an expansion of knowledge for aeronautical applications.

Objectives and methods

The focus of the work, done by the University of Bremen, ZARM, with regard to the project WAKOS is on the theoretical modeling of the fluid mechanical and thermodynamic processes that are to be expected during the operation of a hydrogen aircraft in the fuel tanks. These processes are modelled with the aid of three different theoretical approaches, which differ in terms of the spatial resolution of the problem being computed:

1. Numerical simulations using CFD software, such as FLOW-3D and Star CCM +, to predict instationary fluid mechanical and thermodynamic processes in one, two, or three spatial directions,
2. Layer models for predicting physical processes in one spatial direction,
3. Node models for predicting physical processes without spatial resolution.

In order to validate the tools so-called benchmark calculations are performed. Hereby, published cryogenic experiments and theoretical models with known analytical or numerical solutions are implemented in and solved by the CFD tools before a comparison of the results with the known solutions is performed. The comparison leads to an evaluation of the modeling capacities of the tool with regard to the considered physical processes.

Results

On the left-hand-side of this website entry, a numerical simulation of a published cryogenic experiment is shown. The software FLOW3D has been used in order to numerically simulate the phase change process during a liquid nitrogen experiment under Earth's gravity conditions. The experiment is described in the paper Ludwig and Dreyer 2014 from the publication list. The simulation uses the so-called Volume of Fluid Method in order to numerically solve the mass, linear momentum and energy conservation equations for the liquid and the gaseous phase with respect to boundary conditions and initial values. The domain is defined in a Cartesian coordinate system with a resolution of the three spatial directions: x, y, z. Symmetry planes are used in order to simplify the domain to a quarter of the full setup. The simulation has been performed by Dr.-Ing. André Pingel.

The project WAKOS under the grand number 20M2106H is funded by financial means of the German Federal Ministry for Economic Affairs and Climate Action (BMWK) through the German Aerospace Center (DLR e.V.).