Variable Gravity Conditions Research Articles

Numerical investigation of natural circulation loop with supercritical CO2 on the thermal control system of micro spacecrafts

Aiming at the heat dissipation problem under the strict constraints of mass, volume, and power consumption in micro spacecrafts, the feasibility of applying a natural circulation loop (NCL) to the thermal control system is performed for the CubeSats (Cube satellites) of orbiting operations or the micro-rovers of extraterrestrial explorations. A 3-dimensional NCL model under variable gravity conditions corresponding the environment of aerospace is established, and the supercritical CO2 as the coolant. The heat transfer and thermal performance of NCL under gravity range of 10−6 g to 10 g are analyzed numerically, including the Earth orbit, the surface of the Moon and the Mars etc. The calculation results present that the gravity has a great influence on the flow circulation characteristics and thermal performance of the NCL. The applicability of employing NCL on the thermal control system of micro spacecrafts is validated with the gravity no less than 0.05 g. It is worth noting that, by designing the range of temperature from inlet to outlet of the heated tube covering the large specific heat region, the superior thermal performance can be obtained on account of the strong heat-carrying capacity of supercritical fluid, even under the low gravity with a small mass flow rate. Finally, a flow correlation is proposed considering the effect of gravity, flow regime, buoyancy force, and geometric factors, which is suitable on the NCL in the thermal control system of micro spacecrafts, and the R2 is 99.77%.

Dynamics of phase change of gallium under magnetic field and thermocapillary effects under variable gravity conditions

We have numerically investigated the melting and solidification process of a low melting temperature metallic phase change material, gallium, in a semi-circular enclosure with a hot circumferential wall under the influence of thermocapillarity and externally applied uniform magnetic field for different gravity conditions. The governing equations coupled with the enthalpy porosity method are solved to predict the effects of applied magnetic field direction (0 ≤ θ ≤ 90°), and strength (0 ≤ Ha ≤ 125), Rayleigh number (Ra = 4.656 × 102, 4.656 × 104, 1 × 105, 1 × 10⁶), Marangoni number (Ma = -1.44 × 103, −5 × 103 and −1 × 104) and gravity conditions (0.05g, 0.25g, 0.5g and 1g) on the velocity, temperature and liquid fraction profiles. The predictions of the developed numerical models are compared with the experimental and numerical data reported in the literature and found to be in good agreement. The results reveal that the surface tension gradient driven convection significantly enhances the melting process under the microgravity condition without the magnetic field. Fluctuating behaviour of transient average Nusselt number under normal gravity conditions observed during melting of gallium in the presence of thermocapillary and natural convection is reduced by applying the magnetic field. The complete gallium melting time is increased by 5.48% for the change in Hartmann number from 0 to 25 applied at θ = 45°. The direction of the applied magnetic field marginally increases the complete melting time for θ ≥ 75° and insignificantly affects the complete solidification time. The increase in Rayleigh and Marangoni numbers considerably alters fluid flow patterns and the temperature field.

Analysis of heat transfer coefficients for simulation of the heat exchange between oil and the internal cavity faces of the isolated piston at variable gravity conditions

The paper deals with the problems related to the development of a new piston design for replacement during repair in 2-stroke D-100 engines for locomotives. The paper addresses the design, the mass of which is 35 % less than the mass of the prototype. The results of thermal calculation, which served the initial data for the temperature and heat transfer coefficients evaluation over the heat-receiving surface of the combustion chamber of the piston, coincide with the experiment of prof. G. Rosenblit. The integral boundary conditions were refined by calculating five successive engine cycles, αP = 1830 W m–2 K–1 and Teq p = 970 K were selected for further research. Next, heat transfer from the piston internal cavity to the oil, which is supplied through a nozzle in the small head of the connecting rod, was studied. The study included a 3D simulation of the cooling oil movement from the nozzle to the inner surface, both in the stationary mode (Earth’s gravity field) and taking into account the variable acceleration of the piston at the design mode. In this case, the location of the holes in the nozzle, the flow parameters and the properties of the oil were simulated. The study showed that taking piston acceleration into account leads to a slight difference in heat transfer coefficient in the center of the piston (830–800 W m–2 K–1) and rather large at the periphery (600 … 520 W m–2 K–1 taking into account the acceleration of the piston against 460 … 420 W m–2 K–1). It is shown that the nozzle with 4 holes and the angle of inclination of the side holes φ = 55° gives a more efficient cooling.

Effects of variable gravity conditions on additive manufacture by fused filament fabrication using polylactic acid thermoplastic filament

The capability to manufacture items in space is an exploration enabling advancement, and will be crucial for sustainable human exploration as we progress beyond Earth orbit. The extrusion based Fused Filament Fabrication (FFF) method using thermoplastics represents a robust and simple methodology applicable to printing parts for both current and future human spaceflight exploration missions. Understanding the performance and behaviour of the FFF process under varying gravity loads is therefore an important knowledge gap that needs to be addressed in order to fully appreciate the characteristics of space manufactured elements. At present, it is not well understood how gravity can influence the characteristics of such elements fabricated in variable gravity environments. In this study, we detail an experiment conducted on a parabolic flight campaign (PFC) wherein we produced a number of FFF polylactic acid (PLA) polymer test articles and compared them to terrestrially fabricated articles. We report on the methodology and the operational parameters used, as well as presenting an analysis of the samples via optical microscopy and tomography. Compressive, tensile and other technical properties are reported herein. A number of explanations are presented to explain the variance in specimens relative to terrestrial reference samples.

Effects of gravity level on bubble detachment, rise, and bouncing with a free surface

Bubble detachment, rise, and bouncing upon impact with a free surface is studied experimentally in variable gravity conditions. Previous investigations focused on the effects of fluid properties such as viscosity or surface tension on the rise and bouncing dynamics. Gravity force is a crucial factor in the detachment, rise and bouncing processes. However, the effect of different gravity levels has never been studied experimentally. In this paper we analyze the role of gravity in the detachment, rise velocity and bouncing motion of millimetric bubbles colliding with a free surface. Single air bubbles in ethanol are detached from a nozzle by the buoyancy force. After reaching a terminal velocity, the rising bubble interacts with the free surface in a bouncing process prior to coalescence. The equivalent bubble diameter at detachment decreases as the gravity level increases, in agreement with the theoretical prediction. An expression for the terminal velocity as a function of gravity is proposed. The terminal velocity is found to increase with the gravity level, although bubbles are smaller at higher values of gravity. The bouncing process has been modelled by a damped oscillator, in which the free surface acts as an elastic membrane. An expression for the frequency of bouncing as a function of gravity has been obtained, showing a good agreement with the experimental results. The motion of the bubble during the bouncing process can be approximated by an underdamped oscillator even if viscosity is negligible. Therefore, viscosity is not the main responsible for damping, which is probably due to energy transfer from the bubble to the fluid in the form of vortex and surface waves generation.

Experimental analysis of a Flat Plate Pulsating Heat Pipe with Self-ReWetting Fluids during a parabolic flight campaign

A Flat Plate Pulsating Heat Pipe (FPPHP) filled with an ordinary liquid (water) and a self-rewetting mixture (dilutes aqueous solutions of long-chain alcohols with unusual surface tension behavior) is investigated under variable gravity conditions on board a ‘Zero-g’ plane during the 65th Parabolic Flight Campaign of the European Space Agency. The FPPHP thermal performance in terms of evaporator and condenser temperatures, start-up levels and flow regimes is characterized for the two working fluids and a power input ranging from 0 to 200 W (up to 17 W/cm2 at the heater/evaporator wall interface). The experimental set-up also includes a transparent plate enabling the visualization of the oscillating flow patterns during the experiments. For a low power input (4 W/cm2), the pulsating heat pipe filled with pure water is not able to work under low-g conditions, because the evaporator immediately exhibits dry-out conditions and the fluid oscillations stops, preventing heat transfer between the hot and cold side and resulting in a global increase of the temperatures. On the other hand, the FPPHP filled with the self-rewetting fluid runs also during the microgravity phase. The liquid rewets several times the evaporator zone triggering the oscillatory regime. The self-rewetting fluid helps both the start-up and the thermal performance of the FPPHP in microgravity conditions.

Experimental investigation of liquid retention in a cyclone evaporator under variable gravity conditions

For the first time a cyclone evaporator has been designed and tested with perfectly wetting liquids in two separate experiments on condensation, at variable gravity conditions during parabolic flights. Two liquids (FC-72 and HFE-7100), with surface tension coefficient of 12–15 mN/m, were used in the experiments. The concept of the liquid retention by the centrifugal force and the specific design of the walls were validated in the first series of experiments in isothermal conditions. The liquid behaviour inside the experimental cell in weightlessness is reported and discussed for the variable liquid volume (100–200 ml) inside the cell and the rotating velocity of the magnetic stirrer inside the evaporator (0–900 rpm). The data obtained during the isothermal case were used to improve the design of the evaporation cell for the non-isothermal case, tested during a second parabolic flight campaign. In this second experiment, both the power (1.8–16 W) and rotational velocity of the magnetic stirrer (0–250 rpm) were varied. For a fluid charge of 100 ml the same fluid never left the evaporator in all tests performed with rotation of the magnetic stirrer. The vapour pressure in the cell increased during microgravity period. It was found that the stabilization of vapour pressure after transition to microgravity decreases with an increase in the rotational speed of the magnetic stirrer. We conclude that the cyclone evaporator is also an ideal candidate for those systems that have to work in microgravity and rely on pool boiling, because the cyclone chops large bubbles that would otherwise occupy the whole evaporator space and reduce the heat-transfer rate.

High Throughput Fluorescent Screening of Membrane Potential and Intracellular Calcium Concentration Under Variable Gravity Conditions

In addition to the presence of specific gravity receptors in living organisms, biological membranes were found to directly respond to gravity changes. Among others, changes in membrane permeability and as a consequence in membrane potential and intracellular ion concentrations have been demonstrated mainly by using electrophysiological techniques. However, the acquired amount of data up to now is low due to technical limitations of electrophysiology in microgravity platforms. Optical techniques will be able to deliver much higher amounts of data here, especially in case high throughput techniques based on 96 well plate (or higher numbers of wells) readers can be used. In this manuscript we present a new set-up for parabolic flight campaigns based on a multi-purpose plate reader for photometric, luminescent and fluorometric measurements. In a first series of experiments during a parabolic flight campaign the system was verified for membrane potential and intracellular calcium concentration measurements of neuronal cells using fluorescent dyes.

Mobility performance of a rigid wheel in low gravity environments

To investigate influences of gravity on mobility of wheeled rovers for future lunar/planetary exploration missions, model experiments of a soil–wheel system were performed on an aircraft during variable gravity maneuvers. The experimental set-up consists of a single rigid wheel and a soil bed with two kinds of dry sands: lunar soil simulant and Toyoura sand. The experimental results revealed that a lower gravity environment yields higher wheel slippage in variable gravity conditions. In addition to the partial gravity experiments, the same experiments with variable wheel load levels were also performed on ground (1 g conditions). The on-ground experiments produced opposite results to those obtained in the partial gravity experiments, where a lower wheel load yields lower slippage in a constant gravity environment. In low gravity environments, fluidity (flowability) of soil increases due to the confining stress reduction in the soil, while the effect of the wheel load on sinkage decreases. As a result, both of these effects are canceled out, and gravity seemingly has no effect on the wheel sinkage. In the meantime, in addition to the effect of wheel load reduction, the increase of the soil flowability lessens the shear resistance to the wheel rotation, as a result of which the wheel is unable to hold sufficient traction in low gravity environments. This suggests that the mobility of the wheel is governed concurrently by two mechanisms: the bearing characteristics to the wheel load, and the shearing characteristics to the wheel rotation. It appears that, in low gravity, the wheel mobility deteriorates due to the relative decrease in the driving force while the wheel sinkage remains constant. Thus, it can be concluded that the lunar and/or Mars’ gravity environments will be unfavorable in terms of the mobility performance of wheels as compared to the earth’s gravity condition.

Cooling Performance of a 16-Nozzle Array in Variable Gravity

The objective ofthis paper was to investigate the cooling performance of 16-nozzle spray array using FC-72 as the working fluid in variable-gravity conditions. A flight-test experiment was modified to accommodate a 16-nozzle spray array, which was then tested in the parabolic flight trajectory environment of NASA's C-9 reduced-gravity aircraft. The 16-nozzle array was designed to cool a 25.4 x 25.4 mm 2 area on a thick-film resistive heater used to simulate an electronic component. Flight tests were conducted over the course of two flight weeks (each week consisting of four flights and each flight consisting of 40 to 60 parabolas). The mass flow rate through the 16-nozzle spray array ranged from 13.1 ≤ m ≤ 21.3 g/s. The heat flux at the thick-film resistor ranged from 2.9 ≤ q ≤ 25 w / 2 , the subcooling of the working fluid ranged from 1.6 ≤ ΔT sc ≤ 18.4°C, the saturation temperature ranged from 37.4 ≤ T sat ≤ 47.2°C, and the absorbed air content in the working fluid was C = 10.1,14.3 and 16.8% by volume. The spray chamber pressure ranged from 42 ≤ P ≤ 78 kPa and the acceleration ranged from -0.02 ≤ a ≤ -2.02 g. Two-phase cooling was emphasized, but some single-phase data were also collected. A one-dimensional model was used to predict the heater surface temperature from the heat input and mean heater base temperature. It was found that the cooling performance was enhanced in microgravity over terrestrial and elevated gravity. In addition, a sudden degradation in performance was found at high mass flow rates in microgravity, possibly due to liquid buildup on the surface between the nozzle impact zones. A high degree of subcooling was found to be beneficial, but the dissolved air content had little effect on the heat transfer performance in either microgravity or elevated gravity.

Measurements and Modeling of Variable Gravity Effects on Water Distribution and Flow in Unsaturated Porous Media

Liquid behavior under reduced gravity conditions is of considerable interest for various components of life‐support systems required for manned space missions. High costs and limited opportunities for spaceflight experiments hinder advances in reliable design and operation of elements involving fluids in unsaturated porous media such as plant growth facilities. We used parabolic flight experiments to characterize hydraulic properties under variable gravity conditions deduced from variations in matric potential over a range of water contents. We designed and tested novel measurement cells that allowed dynamic control of water content. Embedded time domain reflectometry probes and fast‐responding tensiometers measured changes in water content and matric potential. For near‐saturated conditions, we observed rapid establishment of equilibrium matric potentials during the recurring 20‐s periods of microgravity. As media water content decreased, the concurrent decrease in hydraulic diffusivity resulted in limited attainment of equilibrium distributions of water content and matric potential in microgravity, and water content heterogeneity within the sample was influenced by the preceding hypergravity phase. For steady fluxes through saturated columns, we observed linear and constant hydraulic gradients during variable gravity, yielding saturated hydraulic conductivities similar to values measured under terrestrial gravity. Our results suggest that water distribution and retention behavior are sensitive to varied gravitational forces, whereas saturated hydraulic conductivity appears to be unaffected. Comparisons between measurements and simulations based on the Richards equation were in reasonable agreement, suggesting that fundamental laws of fluid flow and distribution for macroscopic transport derived on Earth are also applicable in microgravity.

Effect of simulated microgravity on oxidation-sensitive gene expression in PC12 cells

Oxygen utilization by and oxygen dependence of cellular processes may be different in biological systems that are exposed to microgravity (micro-g). A baseline in which cellular changes in oxygen sensitive molecular processes occur during micro-g conditions would be important to pursue this question. The objective of this research is to analyze oxidation-sensitive gene expression in a model cell line [rat pheochromocytoma (PC12)] under simulated micro-g conditions. The PC12 cell line is well characterized in its response to oxygen, and is widely recognized as a sensitive model for studying the responses of oxygen-sensitive molecular and cellular processes. This study uses the rotating wall vessel bioreactor (RWV) designed at NASA to simulate micro-g. Gene expression in PC12 cells in response to micro-g was analyzed by DNA microarray technology. The microarray analysis of PC12 cells cultured for 4 days under simulated micro-g under standardized oxygen environment conditions revealed more than 100 genes whose expression levels were changed at least twofold (up-regulation of 65 genes and down-regulation of 39 genes) compared with those from cells in the unit gravity (unit-g) control. This study observed that genes involved in the oxidoreductase activity category were most significantly differentially expressed under micro-g conditions. Also, known oxidation-sensitive transcription factors such as hypoxia-inducible factor-2α, c-myc, and the peroxisome proliferator-activated receptor-γ were changed significantly. Our initial results from the gene expression microarray studies may provide a context in which to evaluate the effect of varying oxygen environments on the background of differential gene regulation of biological processes under variable gravity conditions.

Numerical Simulation and Experimental Validation of the Dynamics of Multiple Bubble Merger During Pool Boiling Under Microgravity Conditions

Numerical simulation and experimental validation of the growth and departure of multiple merging bubbles and associated heat transfer on a horizontal heated surface during pool boiling under variable gravity conditions have been performed. A finite difference scheme is used to solve the equations governing mass, momentum, and energy in the vapor liquid phases. The vapor-liquid interface is captured by a level set method that is modified to include the influence of phase change at the liquid-vapor interface. Water is used as test liquid. The effects of reduced gravity condition and orientation of the bubbles on the bubble diameter, interfacial structure, bubble merger time, and departure time, as well as local heat fluxes, are studied. In the experiments, multiple vapor bubbles are produced on artificial cavities in the 2-10 micrometer diameter range, microfabricated on the polished silicon wafer with given spacing. The wafer was heated electrically from the back with miniature strain gage type heating elements in order to control the nucleation superheat. The experiments conducted in normal Earth gravity and in the low gravity environment of KC-135 aircraft are used to validate the numerical simulations.

Growth Monitoring with Lettuce Grown under Variable Gravity Conditions

Growth Monitoring with Lettuce Grown under Variable Gravity Conditions

Thermal-Hydraulic Analysis of Bulk Evaporation and Condensation in a Multiphase Nuclear Fuel Cell

A thermal-hydraulic model is developed to simulate and study the dynamic behavior of bulk evaporation and condensation processes in a multiphase nuclear fuel cell. The phase-change process is driven and controlled by internal heat generation and wall heat removal under constant volume condition. The modeling involves variable gravity conditions that allow for performance analysis of the multiphase nuclear fuel for terrestrial and space applications. A complete set of governing equations for both liquid and vapor phases is developed and numerically solved. The model is used to simulate the operation of a multiphase nuclear fuel cell at zero-gravity and microgravity levels. The temperature and phase distribution, the flow field, and the evolution of the liquid-vapor interface are computed and demonstrated.

Numerical Simulation of Unidirectional Unsteady Solidification under Variable Gravity Conditions

Numerical Simulation of Unidirectional Unsteady Solidification under Variable Gravity Conditions

Laser drilling into an absorbing liquid

The interaction of continuous CO2 laser radiation with water is described. Tightly focused radiation at powers of up to 120 W is used to generate and maintain a conical depression in the water surface similar to the keyhole created during laser penetration welding. The observed penetration depth of up to 7 mm is explained in terms of a hydrodynamic model and observations on the interaction induced liquid flows are described. The momentum reaction to the flow of steam up out of the keyhole generates a downward flow in the liquid around the keyhole with a velocity of ∼20 cm/s under the conditions of these experiments. The experiment has also been performed under variable gravity conditions provided by the NASA KC-135 Microgravity Aircraft to observe effects associated with changes in hydrostatic pressure and buoyancy on the system.

Dynamic shear of granular material under variable gravity conditions

This paper describes some experiments with granular materials which recently have been conducted aboard the NASA KC-135 aircraft during variable gravity maneuvers. The main experimental apparatus consisted of a small drum containing granular material which was rotated slowly while the angle assumed by the slip surface with respect to the horizontal was observed and recorded photographically. Conventional wisdom has held that this 'dynamic angle of response' was a material constant, independent of (among other things) gravitational level. The results presented here are quite contrary, suggesting instead an angle that varies with the reciprocal of the square root of gravity. This finding may have important consequences on the understanding of many active processes in Planetary Geology involving granular materials and may provide qualitative confirmation of some of the theoretical predictions of modern models of granular shear flows.