Thermodynamic Vent System Research Articles

Active Insulation Technique Applied to the Experimental Analysis of a Thermodynamic Control System for Cryogenic Propellant Storage

A technological barrier for long-duration space missions using cryogenic propulsion is the control of the propellant tank self-pressurization (SP). Since the cryogenic propellant submitted to undesired heat load tends to vaporize, the resulting pressure rise must be controlled to prevent storage failure. The thermodynamic vent system (TVS) is one of the possible control strategies. A TVS system has been investigated using on-ground experiments with simulant fluid. Previous experiments performed in the literature have reported difficulties to manage the thermal boundary condition at the tank wall; spurious thermal effects induced by the tank environment spoiled the tank power balance accuracy. This paper proposes to improve the experimental tank power balance, thanks to the combined use of an active insulation technique, a double envelope thermalized by a water loop which yields a net zero heat flux boundary condition and an electrical heating coil delivering a thermal power Pc∈[0−360] W, which accurately sets the tank thermal input. The simulant fluid is the NOVEC1230 fluoroketone, allowing experiments at room temperature T ∈ [40–60] °C. Various SP and TVS experiments are performed with this new and improved apparatus. The proposed active tank insulation technique yields quasi-adiabatic wall condition for all experiments. For TVS control at a given injection temperature, the final equilibrium state depends on heat load and the injection mass flow rate. The cooling dynamics is determined by the tank filling and the injection mass flow rate but does not depend on the heat load Pc.

Numerical modeling of self-pressurization and pressure control by a thermodynamic vent system in a cryogenic tank

This paper presents a numerical model of a system-level test bed—the multipurpose hydrogen test bed (MHTB) using the Generalized Fluid System Simulation Program (GFSSP). MHTB is representative in size and shape of a space transportation vehicle liquid hydrogen propellant tank, and ground-based testing was performed at NASA Marshall Space Flight Center (MSFC) to generate data for cryogenic storage. GFSSP is a finite volume-based network flow analysis software developed at MSFC and used for thermofluid analysis of propulsion systems. GFSSP has been used to model the self-pressurization and ullage pressure control by the Thermodynamic Vent System (TVS). A TVS typically includes a Joule–Thompson (J–T) expansion device, a two-phase heat exchanger (HEX), and a mixing pump and liquid injector to extract thermal energy from the tank without significant loss of liquid propellant. For the MHTB tank, the HEX and liquid injector are combined into a vertical spray bar assembly. Two GFSSP models (Self-Pressurization and TVS) were separately developed and tested and then integrated to simulate the entire system. The Self-Pressurization model consists of multiple ullage nodes, a propellant node, and solid nodes; it computes the heat transfer through multilayer insulation blankets and calculates heat and mass transfer between the ullage and liquid propellant and the ullage and tank wall. A TVS model calculates the flow through a J–T valve, HEX, and spray and vent systems. Two models are integrated by exchanging data through User Subroutines of both models. Results of the integrated models have been compared with MHTB test data at a 50% fill level. Satisfactory comparison was observed between tests and numerical predictions.

Screen channel liquid acquisition device outflow tests in liquid hydrogen

This paper presents experimental design and test results of the recently concluded 1-g inverted vertical outflow testing of two 325×2300 full scale liquid acquisition device (LAD) channels in liquid hydrogen (LH2). One of the channels had a perforated plate and internal cooling from a thermodynamic vent system (TVS) to enhance performance. The LADs were mounted in a tank to simulate 1-g outflow over a wide range of LH2 temperatures (20.3–24.2K), pressures (100–350kPa), and flow rates (0.010–0.055kg/s). Results indicate that the breakdown point is dominated by liquid temperature, with a second order dependence on mass flow rate through the LAD. The best performance is always achieved in the coldest liquid states for both channels, consistent with bubble point theory. Higher flow rates cause the standard channel to break down relatively earlier than the TVS cooled channel. Both the internal TVS heat exchanger and subcooling the liquid in the propellant tank are shown to significantly improve LAD performance.

Thermodynamic vent system performance testing with subcooled liquid methane and gaseous helium pressurant

LCH 4 testing was conducted at the Marshall Space Flight Center using the multipurpose hydrogen test bed (MHTB) to evaluate the performance of a spray-bar thermodynamic vent system (TVS) with subcooled LCH 4 and gaseous helium (GHe) pressurant. Thirteen days of testing were performed in November 2006, with total tank heat leak conditions of about 715 W and 420 W at a fill level of approximately 90%. A total of 23 TVS cycles were completed. The TVS successfully controlled the ullage pressure within a prescribed control band but did not maintain a stable liquid saturation pressure. This was likely due to a TVS design not optimized for this particular propellant and test conditions, and possibly due to a large artificially induced heat input directly into the liquid.

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.

Coupled thermofluid analysis method with application to thermodynamic vent systems

Many practical thermofluid systems involve multiple solid materials and fluid media with vastly differing thermal properties. Thermal and fluid flow analysis in such systems requires a numerical method capable of coupling all participating solid materials and fluid media together. In this article, such a numerical method is presented. The method is based on the multidomain finite volume formulation. Different materials or fluids are represented with different computational domains. A novel implicit coupling procedure at regular and conjugate heat transfer interfaces is designed to obtain the solutions simultaneously and efficiently. The numerical method is first validated against a natural convection between thick-walled eccentric tubes. It is then applied to analyze a zero-G thermodynamic vent system. The preliminary three-dimensional analysis results are presented and the thermodynamic characteristics are discussed. 11 refs.

Electric orbit transfer vehicle cryogenic propellant system

An electric orbit transfer vehicle (EOTV) is intended to transfer payloads from low Earth orbit (LEO) to higher orbits using low-thrust solar-electric propulsion and hydrogen propellant. Because of its high specific impulse and synergistic sharing of power supply, attitude control and communication systems with the payload, the highly efficient EOTV transfer stage permits use of a smaller, less costly launch vehicle than if orbit transfer were accomplished using chemical propulsion. Study of the propellant storage and supply system for an EOTV intended to fly a 168 day spiral trajectory from LEO to geosynchronous orbit (GEO) reveals that the low propellant flow rate needed by the thrusters can be supplied by the boil-off from the storage tank, eliminating the need for any overboard venting. The tank can be fabricated under the same pressure-stabilized, thin, stainless steel monocoque construction as the current Centaur upper stage, and insulated with Centaur fixed foam and MLI. The tank contains a thermodynamic vent system (TVS) for control of tank pressure in zero and low gravity and for supply of propellant to the thrusters. An external compressor, accumulator and regulator condition the hydrogen boil-off provided by the TVS and provide for start-up and shut-down transients. The resulting system is simple, has a very low structural mass fraction and builds on the Centaur cryogenic upper stage technology, which has been operational for over 25 years.

Prediction of self-pressurization rate of cryogenic propellant tankage

The SOLA-ECLIPSE code is being developed to enable prediction of the behavior of cryogenic propellants in spacecraft tankage. A brief description of the formulations used for modeling heat transfer and for determining the thermodynamic state is presented. Code performance is verified through comparison to experimental data for the self-pressurization of scale-model liquid hydrogen tanks. SOLA-ECLIPSE is used to examine the effect of initial subcooling of the liquid phase on the self-pressurization rate of an on-orbit full-scale liquid hydrogen tank typical for a chemical-propulsion orbit transfer vehicle. The computational predictions show that even small amounts of subcooling will significantly decrease the self-pressurization rate. Further, if the cooling is provided by a thermodynamic vent system, it is concluded that small levels of subcooling will maximize propellant conservation.