Mohammad Kassemi

On the validity of purely thermodynamic descriptions of two-phase cryogenic fluid storage

This paper presents a comprehensive analysis of the transport processes that control the self-pressurization of a cryogenic storage tank in normal gravity. A lumped thermodynamic model of the vapour region is coupled with the Navier-Stokes and energy equations governing heat, mass and momentum transport in the liquid. These equations are discretized using a Galerkin finite-element method with implicit time integration. Three case studies are considered based on three different heating configurations imposed on the tank wall: liquid heating, vapour heating and uniform heating. For each case, the pressure and temperature rise in the vapour and the flow and temperature distributions in the liquid are determined. Results are compared to a lumped thermodynamic model of the entire tank. It is shown that the final rate of pressure rise is about the same in each case and close to that predicted by thermodynamics even though the actual pressures are different because of varying degrees of thermal stratification. Finally, a subcooled liquid jet is used to mix the liquid and limit the pressure rise. Even so, there is still some thermal stratification in the liquid, and as a result the final vapour pressure depends on the particular heat distribution.

Self-Pressurization of Large Spherical Cryogenic Tanks in Space

The pressurization of large cryogenic storage tanks under microgravity conditions is investigated by coupling a lumped thermodynamic model of the vapor region with a complete solution of the flow and temperature fields in the liquid. Numerical results indicate that in microgravity both buoyancy and natural convection are still important and play a significant role in phase distribution and tank pressurization. A spherical vapor region initially placed at the center of the tank deforms and moves to one side of the tank before any significant pressure rise. Long-term results obtained with the vapor region near the tank wall show that, even in microgravity, natural convection leads to thermal stratification in the liquid and significantly alters the initial pressure rise. The final rate of pressure rise agrees with a lumped thermodynamic model of the entire system, but the final pressure levels depart from thermodynamic predictions because of initial transients. The history of the maximum liquid superheat and subcooling is also determined for each configuration.

Modeling Interfacial Turbulent Heat Transfer during Ventless Pressurization of a Large Scale Cryogenic Storage Tank in Microgravity

A two-phase CFD model for pressurization of cryogenic storage tanks is presented using both the Sharp Interface and VOF approaches for representing the phase boundary and the associated interfacial heat, mass and momentum transfer between the liquid and the vapor regions. Both models were validated against the microgravity pressurization data provided by the Saturn S-IVB AS-203 experiment, with the VOF model producing better agreement. Since proper representation of turbulence effects is crucial for predicting interfacial heat and mass transfer with fidelity, two different engineering models for turbulence, namely, the k- and the Shear-Stress Transport (SST) k- are considered. The fidelity of the two turbulence models for storage tank problems is assessed. The impacts of different turbulent parameters associated with the models, such as initial distributions of the respective turbulent quantities and different interfacial boundary conditions, are also studied. The results of our study underscore the fact that accurate modeling of turbulent interfacial heat transfer is crucial for predicting correct self-pressurization and thermal stratification in the cryogenic storage tank. In this context all the important aspects of turbulent modeling at the interface need to be properly addressed. This includes: 1) initial turbulence level; 2) interfacial turbulence B.C.; 3) the contribution of interface deformations to the enhancement of the interfacial turbulent heat transfer.

Investigation of Tank Pressurization and Pressure Control—Part II: Numerical Modeling

A multizone model is used to predict both the self-pressurization and pressure control behavior of a ground-based experiment. The multizone model couples a finite element heat conduction model of the tank wall to the bulk conservation equations in the ullage and the liquid. Comparisons are made to the experimental data presented in a companion paper. Results suggest that the multizone model can predict self-pressurization behavior over a variety of test conditions. The model is also used to predict the pressure control behavior when a subcooled axial mixing jet is used to thermally destratify and cool the bulk liquid. For fast jet speeds, the multizone model does a reasonably predict the pressure collapse behavior. Comparisons were also made between the data and a homogeneous thermodynamic model. These comparisons highlight the deficiencies of the homogeneous modeling approach.

Modeling Active Pressure Control in a Large Scale Cryogenic Storage Tank in Normal Gravity

In the present paper, two different approaches for predicting turbulent heat transfer at the liquid-vapor interface are developed and implemented into a CFD model for a large scale liquid hydrogen storage tank. In the first approach, the interface is captured using the VOF method. Here, the interfacial energy, mass, and momentum balances are incorporated using source terms at the diffuse interface region. The second approach considers a sharp interface at the boundaries of the liquid and vapor computational cells, where the interfacial mass, momentum, and energy balances are rigorously enforced. In both models, the kineticsbased Schrage equation is used to account for the evaporative and condensing interfacial mass flows [25]. The flow, temperature, and turbulence fields, as well as the interfacial mass fluxes predicted by the two models during active tank depressurization via liquid mixing, are compared against each other. The ullage pressure and vapor temperature evolutions are also compared against experimental data obtained from the K-Site jet mixing experiment [2]. The Shear-Stress Transport (SST) k- turbulence model was utilized in this study. Since proper modeling of turbulence effects at the interface is crucial for predicting interfacial heat and mass transfer with fidelity, different interfacial boundary conditions for turbulent quantities were thoroughly studied.

Open Access Integrated Therapeutic and Diagnostic Platforms for Personalized Cardiovascular Medicine

It is undeniable that the increasing costs in healthcare are a concern. Although technological advancements have been made in healthcare systems, the return on investment made by governments and payers has been poor. The current model of care is unsustainable and is due for an upgrade. In developed nations, a law of diminishing returns has been noted in population health standards, whilst in the developing world, westernized chronic illnesses, such as diabetes and cardiovascular disease have become emerging problems. The reasons for these trends are complex, multifactorial and not easily reversed. Personalized medicine has the potential to have a significant impact on these issues, but for it to be truly successful, interdisciplinary mass collaboration is required. We propose here a vision for open-access advanced analytics for personalized cardiac diagnostics using imaging, electrocardiography and genomics.

Effect of void location on segregation patterns in microgravity solidification

Three recent microgravity experiments have been hampered by convection caused by unwanted voids and/or bubbles in the melt. In this work, we present a numerical study to describe how thermocapillary convection generated by a void or bubble can affect a typical microgravity solidification process. A detailed numerical model for the Bridgman solidification of a doped single crystal from its dilute binary melt is developed which solves the quasi-steady Navier Stokes equations together with the conservation equations for transport of energy and species. The complicating effects of thermocapillary convection generated by the void and solutal rejection at the melt-solid interface are included. Numerical simulations indicate that void-generated thermocapillary convection can affect segregation patterns drastically, especially, if the thermocapillary vortex penetrates the solutal boundary layer at the growth interface. Two lateral void positions are considered with the void placed either in the center of the ampoule or on the side wall. From a transport point of view, three different segregation regimes are identified for each lateral void location based on the distance between the void and the growth interface. These range from a diffusion-controlled regime where most of the radial nonuniformity in the interfacial composition is due to the interface curvature with minimal convective effects to a fully mixed regime where the penetration of the solutal boundary layer by the thermocapillary vortex tends to homogenize the interfacial compositions drastically. Naturally, the extent of each of these regions will not only depend on the size and lateral position of the void but also on the material and growth properties of the system under consideration.

Ventricular Chamber Sphericity During Spaceflight and Parabolic Flight Intervals of Less Than 1 G

INTRODUCTION Pathology driven alterations in the geometric shape of the heart have been found to result in regional changes in ventricular wall stress and a remodeling of the myocardium. If reductions in the gravitational forces acting on the heart produce similar changes in the overall contour of the ventricles, this modification might also induce adaptations in the cardiac structure during long-term spaceflight. In this study we examined the changes in left ventricle (LV) shape in spaceflight and during parabolic flights. METHODS The diastole dimensions of the human LV were assessed with echocardiography during spaceflight and in parabolic flights which replicated the gravity of the Moon, Mars, and spaceflight and were compared to findings in Earths gravity. LV dimensions were translated into circularity indices and geometric aspect ratios and correlated with their corresponding gravitational conditions. RESULTS During parabolic flight, a linear relationship (r = 0.99) was found between both the circularity index and geometric aspect ratio values and the respective gravitational fields in which they were measured. During spaceflight (N = 4) and parabolic flights (N = 3), there was an average 4.1 and 4.4% higher circularity index and a 5.3 and 8.1% lower geometric aspect ratio, respectively. CONCLUSIONS A correlative trend was found between the degree of LV sphericity and the amount of gravitational force directed caudal to the longitudinal orientation of the body. The importance of this finding is uncertain, but may have implications regarding physiologic adaptations in the myocardial structure secondary to changes in LV wall stress upon prolonged exposure to microgravity.

Comparison of Several Zero-Boil-Off Pressure Control Strategies for Cryogenic Fluid Storage in Microgravity

Four different tank pressure control strategies based on various combinations of active cooling and/or forced mixing are investigated numerically. The first, and most effective, strategy uses a subcooled liquid jet to simultaneously mix and cool the bulk liquid. The second strategy is based on separate mixing and cooling via a forced uncooled liquid jet and an independent cold finger. The third strategy uses a cold finger alone with no forced mixing. Finally, the fourth strategy examines the effect of mixing alone without any active cooling. Detailed numerical solutions are obtained for each case by solving the Navier―Stokes and energy equations in the liquid region coupled to a lumped heat and mass treatment of the vapor region. It is shown that the most rapid and effective means of countering self-pressurization is achieved with a subcooled liquid jet. In the case of separate mixing and cooling, the pressure can still be reduced, but over a much longer period of time. Finally, cooling without any forced mixing is able to limit the pressure rise, but not very effectively, although for long-duration storage in which rapid pressure control is not required, this may still constitute a viable approach. While presenting the results of the various simulation case studies, an in-depth comparative analysis of transport phenomena associated with each case is also performed from which salient engineering recommendations are derived for optimization of the zero-boil-off design.

A Numerical Study of Tank Pressure Control in Reduced Gravity

Recent studies suggest that Zero Boil-O (ZBO) technologies, aimed at controlling the pressure inside cryogenic storage tanks, will play a prominant role in meeting NASA’s future exploration goals. Small-scale experiments combined with validated and veried computational models can be used to optimize and then to scale up any future ZBO design. Since shortcomings in previous experiments make validating comprehensive two-phase o w models dicult at best; the Zero Boil-O Tank (ZBOT) experiment has been proposed to y aboard the International Space Station. In this paper, a numerical model has been developed to examine several test points in the ZBOT test matrix. Specically , a numerical model is developed to evaluate four pressure control strategies after the tank undergoes a period of self-pressurization. The four strategies include axial liquid jet mixing, mixing provided by a sub-cooled liquid jet, bulk liquid cooling provided by a cold-nger and coldnger cooling with axial jet mixing. Results indicate, that over the time scales under consideration, sub-cooled liquid jet mixing is the most eectiv e means to reduce tank pressure.