Methane Vapor Bubble Growth due to Depressurization and Boiling

Abstract

Cryogens such as liquid methane enable highly efficient propulsion systems in spacecraft. The low saturation temperature causes the liquid to boil in most technical systems. The resulting vapor bubble growth in a superheated liquid can quickly pass through multiple length scales. Under the conditions of reduced gravity in spaceflight applications, bubbles usually remain close to their point of nucleation. During the bubble growth, caused either by a change in saturation conditions due to a reduction of tank pressure or by heat flowing into the cryogenic tank, the displacement of liquid can pose issues for the operation of the spacecraft. This can manifest in the drying up of liquid reservoirs or the loss of a controlled liquid position in the tank. Therefore, predictions of the growth dynamics of vapor bubbles are required.

The bubble growth caused by depressurization is of special interest for two reasons. Firstly, the depressurization of a saturated liquid causes a spatially uniform superheat. This changes the bubble growth dynamics compared to the common assumption of a thermal boundary layer near a solid body for boiling models. Secondly, depressurization of a cryogenic tank is an important step in a potential procedure to introduce a subcooling inside the liquid in order to allow for transfer without cavitation. In a single species system the decrease in pressure lowers the saturation temperature and causes evaporation and thus a transfer of thermal energy from the liquid phase to the vapor phase. A following pressurization raises the saturation temperature again and thus subcools the liquid, which allows for transfer without cavitation. This is of special relevance for spacecraft which have to rely on cryogenic fuels for extended mission durations. There, thermal conditioning of the propellant after extended coasting phases becomes mandatory because heat is constantly flowing into the propellant from the environment. Only if a subcooling is introduced prior to any pumping of propellant can it be ensured that cavitation will not occur in the pumps.

In this thesis the results of twelve drop tower experiments are presented. These experiments were carried out with the aim of investigating bubble dynamics and providing reference data for numerical simulations. Experimental observations focused on the evaluation of vapor bubble growth caused by a depressurization or by the application of a heat flow. Repeatability was investigated and found to be good. Bubble growth during three sets of depressurization parameters as well as three different magnitudes of heat flow were evaluated. The growth behavior was correlated with the applied stimuli. Based on this, numerical investigations into the bubble growth caused by a depressurization have been performed and compared to the experimental results. The established experimental and numerical data points can be used as a basis for numerical models to perform simulations on the technical scale.