Analyses of Cryogenic Propellant Tank Pressurization based upon Experiments and Numerical Simulations
The active-pressurization of cryogenic propellant tanks for the launcher application was analyzed in this study. Therefore, ground experiments, numerical simulations and analytical studies were performed with the following important results: In order to minimize the required pressurant gas mass, a higher pressurant gas temperature is advantageous or the application of helium as pressurant gas. Numerical simulations of the pressurant gas mass using Flow-3D show good agreement to the experimental results. The dominating heat transfer during pressurization appears from the injected pressurant gas to the axial tank walls and the predominant way of phase change during the active-pressurization phase depends on the type of the pressurant gas. After the end of the pressurization phase, a significant pressure drop occurs. A theoretical model is presented for the analytical determination of this pressure drop.
Thermodynamics and emission modeling of liquefied natural gas (LNG) tanks and fueling stations
THERMODYNAMICS AND EMISSION MODELING OF LIQUEFIED NATURAL GAS (LNG) TANKS AND FUELING STATIONS César Augusto Sandoval León In this work a thermodynamics and emissions model of liquefied natural gas (LNG) tanks and fueling stations was developed, allowing for the calculation of methane (CH4) emitted from tanks and the prediction of methane emissions in future scenarios. The detailed dynamic thermodynamic model determined the thermodynamic state (pressure, temperature, specific volume) and properties (enthalpy, internal energy, specific heat, etc.) of the liquid and vapor phase methane in the storage tank in order to determine the rate of LNG boil off and venting. The model employed differential forms of the energy balance and mass balance and thermodynamic property relations and data to describe the evolution of liquid and vapor quantity, state and properties with time as a function of fueling station activity. In addition, the temperature inside the storage tank was determined by two approaches: a homogeneous or a stratified distribution, where the homogeneous model assumed a uniform saturated temperature throughout the tank and the stratified model determined a temperature profile in the tank. The model accounted for varying ambient conditions, varying mass flow of LNG into and out of the tank as a result of refueling the tank, fuel dispensing, recirculation to chill dispensing equipment, return of vapor from vehicle tanks for vapor balancing, and release of boil off gas (BOG) to maintain safe tank operating pressure. Furthermore, this work could be adapted to develop a comprehensive model for LNG vehicle fuel tanks. The model was validated with experimental data acquired by the Center for Alternative Fuels, Engines, and Emissions (CAFEE) of West Virginia University (WVU) from vehicle tanks and at LNG fueling stations in the United States. The complete LNG Fueling Station Model achieved an average error of -0.36 psia/day (1.13%) in the rate of pressure change with respect to time using the stratified approach and an average error of -1.67 psia/day (-10.43%) using the homogeneous approach. Two LNG fueling station tanks of 15,000 gallons and 25,000 gallons capacity were used. For fueling stations both approaches presented cases of over prediction and under prediction. The stratified approach had an error in dP dt ⁄ between -8.00 psia/day (-39.03%) and 2.75 psia/day (28.23%). The homogeneous approach had an error in dP dt ⁄ between -7.49 psia/day (-36.55%) and 2.15 psia/day (17.38%). Validation of pressure rise in vehicle tanks achieved an average error of -1.01 psia/day (-2.64%) using the stratified approach and 14.56 psia/day (204.25%) using the homogeneous approach. Two LNG vehicle tanks of 120 gallons and 150 gallons capacity were used. For vehicle tanks the homogenous approaches always over predicted and the stratified approach presented cases of over prediction and of under prediction. The stratified approach had an error in dP dt ⁄ between -6.31 psia/day (-44.90%) and 4.13 psia/day (21.40%). The homogeneous approach had an error in dP dt ⁄ between 2.35 psia/day (12.16%) and 39.48 psia/day (883.96%).
Parametrically forced gravity waves in a circular cylinder and finite-time singularity
In this paper we present results on parametrically forced gravity waves in a circular cylinder in the limit of large fluid-depth approximation. The phase diagram that shows the stability-forcing-amplitude threshold and the wave-breaking threshold has been determined in the frequency range of existence of the lowest axisymmetric wave mode. The instability is shown to be supercritical for forcing frequencies at and above the natural frequency and subcritical below in a frequency range where the instability and breaking thresholds do not coincide. Above the instability threshold, the growth in wave amplitude is exponential, but with an initial time delay. The wave-amplitude response curve of stationary wave motions exhibits steady-state wave motion, amplitude modulations and bifurcations to other wave modes at frequencies where the parametric instability boundary of the axisymmetric mode overlaps with the neighbouring modes. The amplitude modulations are either on a slow time scale or exhibit period tripling and intermittent period tripling, without wave breaking. In the wave-breaking regime, a finite-time singularity may occur with intense jet formation, a phenomenon demonstrated by others in fluids of high viscosity and large surface tension. Here, this singular behaviour with jet formation is demonstrated for a low viscosity and low kinematic surface tension liquid. The results indicate that the jet is driven by inertial collapse of the cavity created at the wave trough. Therefore, the jet velocity is determined by the wave fluid velocity but depends, in addition, on kinematic surface tension and viscosity as these affect the last stable wave crest shape and the cavity size.