Speaker
Description
Spacecraft using cryogenic fuels are currently in development to extend the duration of missions from hours to weeks and months. To mitigate the pressure rise caused by the transfer of thermal energy into the tank from the environment, venting maneuvers are a possible operation. Depressurization maneuvers will introduce a superheat in the bulk liquid, that was previously at the saturation temperature of the initial pressure, and thus might lead to large-scale phase change occurring in the tank both at the free surface and at favorable nucleation sites at the tank wall. The results shown here aim to investigate the depressurization process on the scale of a single nucleation site.
A series of three experiments was performed using liquid methane in the Drop Tower at the University of Bremen which offers 4.74 s of compensated gravity. The experimental setup consists of a single species system of liquid and gaseous methane. Inside the glass cylinder, a polished stainless steel structure is used to introduce a single, artificial nucleation site into the liquid. The setup is equipped with temperature and pressure sensors, and optical access was provided by two endoscopes. The liquid was brought into an isothermal state at the starting pressure. After that, the experimental setup was released into free fall and the system experienced microgravity. Once the initial sloshing motion of the free surface was damped, a valve was opened for 140 ms. The decrease in ullage pressure uniformly superheated the liquid. This led to an immediate generation of vapor at the artificial nucleation site. During the remaining time of the experiment, the generated vapor formed a single bubble above the cavity. The superheat of the liquid decreased due to evaporation at the interface of bulk liquid and the ullage, which led to rising ullage pressure.
The image data was evaluated for the radius of the vapor bubble. This experiment was replicated four additional times and the results indicated good reproducibility. The spatially uniform superheat makes the experimental results well suited for comparison against analytical models. A variety of analytical models that are valid for spherical symmetry and a constant superheat. The absence of buoyancy under compensated gravity conditions allows increased observation time and for the establishment of a temperature distribution in the liquid which is close to the analytical solution. Deviations from the analytical model are introduced because a constant superheat is not given in the presented experiments. Therefore, models were applied using both the highest and lowest measured superheat of 3.5 K and 1.5 K. The analytical solutions thus should envelop the experimental data and act as upper and lower bounds for the expected bubble size, which they do favorably. The use of a cryogenic fluid allows for a range of scaling parameters that is close spaceflight applications.
| Keywords | Microgravity, Methane, Phase change, Bubble Dynamics |
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