66th ESA’s Parabolic Flight Campaign (May 2017)

SCIENTIFIC OBJECTIVE

The aim of this project is to study the influence of different porous materials on the heat transfer processes during boiling in porous media. Specifically we used three different porous materials:

• ceramic porous material (pores size 16μm) saturated with water
• metallic porous material (pores size 10μm) saturated with tap water
• potato

The fundamental objective is to examine the contribution of gravity in boiling models over porous surfaces. The technological objective is to understand how gravity level affect phenomena such as bubble formation and detachment, and how one can manipulate these parameters to obtain a better control of the process.

EXPERIMENT

In an effort to investigate the influence of gravity on heat and mass transfer phenomena over and below the surface of a porous medium, we performed experiments in the 66^th ESA PFC (May 2017). Several experiments were conducted by immersing different porous in hot oil (extra virgin olive oil at 130 ^oC). The hot liquid triggers boiling over the surface of the porous medium. To our knowledge this is the first time that a natural product (i.e. potato) is employed as a medium to study boiling phenomena at microgravity conditions.

RESULTS

Based on the data analysis of the conducted experiments, it appears that:

• Every tested artificial porous materials (i.e. ceramic and metallic) efficiently simulates the boiling behavior of the natural porous material (i.e. potato)
• Boiling continues regularly and systematically in every porous substrate tested even at microgravity conditions
• The time when the boiling initiates, depends on the porous material.
• The total production of vapor is not seriously affected (if at all; data analysis in progress) by gravity level.
• Bubble dynamics (size, population, generation frequency, growth) is affected by gravity level.
• Bubble production during microgravity may be due to intermittent local pressure increase inside the pores (due to the enormous density difference between liquid and vapor during phase change) leading to spontaneous steam ejection to outside of the pores and consequent pressure relief. Ongoing data analysis will confirm or decline the above hypothesis.

Click to see the Pictures Gallery here.
Click to see the Video Gallery here. (Internet Explorer/Microsoft Edge is recommended for videos playback)

64th ESA’s Parabolic Flight Campaign (April 2016)

SCIENTIFIC OBJECTIVE

The aim of the project is to better understand the heat and mass transfer processes in porous media. The fundamental objective is to examine the contribution of gravity in boiling models over porous surfaces. The technological objective is to understand how gravity level and orientation affect phenomena such as bubble formation and detachment, and how one can manipulate these parameters to obtain a better control of the process. Phase I of the project included a series of hypergravity experiments performed successfully in LDC/ESTEC where a significant non-linear influence of increased gravity levels (≥1g) was found on the examined phenomena. The present Phase II of the project aims at identifying the respective influence of reduced gravity levels (≤1g, not only zero gravity).

EXPERIMENT

In an effort to investigate the influence of gravity on heat and mass transfer phenomena over and below the surface of a porous medium, we performed experiments in the 64th ESA PFC (April 2016). To our knowledge this is the first time that boiling in porous substrates is examined at microgravity conditions. Several experiments were conducted by immersing a porous material initially fully saturated with water in hot oil. The hot liquid triggers boiling over the surface of the porous medium. This particular heating approach cancels the effect of natural convection in the surrounding liquid layers and circumvents the problem of non-uniform heating of the exposed porous surface.

RESULTS

Based on the data analysis of the conducted experiments, it appears that:

• Boiling continues regularly and systematically in porous substrates even at microgravity conditions
• The examined two average pores sizes do not affect the boiling process
  There are indications that:
• The total production of vapor is not seriously affected (if at all; data analysis in progress) by gravity level.
• Bubble dynamics (size, population, generation frequency, growth) is affected by gravity level.
• Bubble production during microgravity may be due to intermittent local pressure increase inside the pores (due to the enormous density difference between liquid and vapor during phase change) leading to spontaneous steam ejection to outside of the pores and consequent pressure relief. Ongoing data analysis will confirm or decline the above hypothesis.

Click to see the Pictures Gallery here.
Click to see the Video Gallery here.

50th ESA’s Parabolic Flight Campaign (May 2009)

SCIENTIFIC OBJECTIVE

Heat transfer over sub-millimeter spheroidal solids submerged in fluids is of interest in many engineering processes such as manufacturing systems, packed beds and many electronic components of nearly spherical shape. Apart from heat conduction (molecular thermal diffusion) another important mechanism of heat transfer in the above processes is natural convection which calls for the macroscopic buoyant transport of fluid due to local density differences: hotter regions move against gravity whereas colder regions move along gravity. The presence of natural convection can lead to heat transfer rates many times larger than that of pure heat conduction.

Despite the huge literature devoted to natural convection heat transfer rates over spheres (and to a smaller extent over spheroids) there is not a generally accepted correlation especially for small Rayleigh numbers. Existing correlations for external (open domain) geometries predict a continuous progressively increasing contribution of natural convection to heat transfer with respect to gravity starting from zero gravity. This means that at the common residual gravity level during parabolas (g-jitters) it is possible to have a measurable effect of natural convection.
Based on the experiments conducted on the 49th PFC, evidence was provided that during parabolas there was no natural convection in water and glycerol. Natural convection appeared in water during normal gravity and, more distinctly, during hypergravity periods.

EXPERIMENT

In an effort to discriminate between the effects of gravity level and liquid properties, we performed experiments in the 50th ESA PFC (May 2009) with different test liquids. Several experiments were conducted in water, FC-72 and solid microparticles dispersed in water, where the temperature evolution of a sub millimeter size spheroid heater with a temperature dependent heat source was recorded at low gravity.

First, we did tests with water and then, we did tests with FC-72 (a liquid refrigerant) because of its much lower kinematic viscosity than water which allows more profound appearance of natural convection during different g-levels. Finally, we did tests with microparticles dispersed in water in two forms: as a packed bed and as a dense suspension. We used two separate size classes of particles in order to introduce different degrees of heterogeneity in the liquid phase and therefore examine in another way whether liquid conditions are strong enough to create natural convection currents.

RESULTS

Based on the data analysis of the conducted experiments, it appears that the different thermophysical properties of the examined liquids appeared to influence heat transfer from the heater to an extent comparable to the effect of different gravity levels. Moreover, the experiments in water and FC-72 showed that there was a threshold before natural convection appears and above that threshold, the natural convection is more profound in FC-72 than water. In addition, closely packed particles suppress entirely natural convection but this is not so for dense particle suspensions. Although the analysis of the data is still in progress, it appears that heat transfer during parabolas is rather governed by heat conduction.

Click to see the Pictures Gallery here.

49th ESA’s Parabolic Flight Campaign (3-7 November 2008)

SCIENTIFIC OBJECTIVE

Heat transfer over a sub-millimeter spheroidal solid is of interest in many engineering processes, such as manufacturing systems, heat transfer in packed beds and for many electronic components of nearly spherical shape. One important mechanism of heat transfer in the above processes is the natural convection which leads to heat transfer rates many times larger than that of pure conduction. Despite the huge literature devoted to natural convection heat transfer rates over spheres (and to a smaller extent over spheroids) there is not a generally accepted correlation especially for small Rayleigh numbers (Rayleigh number expresses the ratio of convective to conductive transport). Existing correlations for external geometries predict a progressively increasing contribution of natural convection to heat transfer with respect to gravity (starting from zero gravity). According to these correlations natural convection emerges even in the case of g-jitters (small accelerations in reduced gravity environments due to crew motions, mechanical vibrations, atmospheric drag, earth gravity gradient and other sources).

EXPERIMENT

To test the validity of these correlations, experiments were performed during the 49th ESA PFC (November 2008) for the estimation of heat transfer rates at low gravity. Heat pulses were given to a miniature thermistor with a nearly spheroidal shape immersed in a liquid (water and glycerol) and its thermal response was registered during heating in parabolic flights. The gravity value fluctuated randomly (g-jitters) within ± 2.6×10-2 g during the low gravity phases, whereas reached a peak value of about 1.6-1.8 g during the high gravity phases. The contribution of natural convection was undoubtedly estimated from runs in which acceleration varied from 0 g to 1.8 g.

RESULTS

The primary scope was to study the influence of residual g-jitters on the heat transfer from the heater to the surrounding liquid. The results showed that a minimum (critical) Rayleigh number is required for the onset of natural convection (contrary to predictions of existing theories and correlations for external geometries). This means that there is no influence of g-jitters on heat transfer and the only heat transfer mechanism for miniature heaters at low gravity conditions is pure conduction. Using only conduction terms, an approximate mathematical model was developed for the transient heat transfer problem in the experimental set up which describes the experimental data sufficiently well. This is an additional confirmation that the only heat transfer mechanism at low gravity is conduction.

Click to see the Pictures Gallery here.
Click to see the Movie Gallery here.

38th ESA’s Parabolic Flight Campaign (26-28 October 2004)

Click to see the Pictures Gallery here.
Click to see the Movie Gallery here.

35th ESA’s Parabolic Flight Campaign (14-16 October 2003)

SCIENTIFIC OBJECTIVE

This work investigates the growth of bubbles emerging from a liquid saturated with a dissolved gas when its temperature is locally and suddenly raised above the saturation value, yet below the boiling temperature. This will be accomplished by fast heating of a bubble-free saturated liquid sample by submerged heaters and registering the bubble growth (due to gas desorption) by video recording for later analysis. By performing runs under variable power and duration, questions can be addressed relating to: the characteristic time of the growth process, the effects of degree of supersaturation and density of nucleation sites and the transient behavior of the system during heating.

Experimenting in a microgravity environment will permit the decoupling of bubble growth from buoyancy effects and thus facilitate understanding of the basic mechanisms governing the observed phenomena. In addition, the low gravity conditions will permit the investigation of large bubbles where inertia, viscosity and surface tension are less significant and heat and mass transfer dominate the process. In this time regime, two different phases of bubble growth will be investigated. The earlier stages where the bubbles are large enough but still far from each other (practically isolated) will be studied using a miniature spherical heater. The later stages where bubbles grow so large that they interact with their neighbors will be examined using a larger flat heater.

Bubble generation and growth in liquids plays a key role in diverse fields of technology such as polymer and glass processing, flotation separations, pumps and hydraulic power recovery systems. It also plays an important role in human physiology, e.g. blood oxygenation, bubbles growing in the tissue of astronauts and divers during a hyperbaric or hypobaric decompression. Also it is of critical value in studying physical phenomena such as cavitation, nucleation and boiling.

TECHNICAL DESCRIPTION OF THE EXPERIMENT

Four test fluids are to be examined in two consecutive parabolic flight campaigns: water, blood serum, n-heptane and a refrigerant (C2F3Cl3). The selection of the test fluids is based mainly on their potential for applications but also on the good knowledge of their physicochemical properties. The test liquids will be saturated with CO2 or N2, gases chosen mainly because of their practical significance. The saturated liquid will be contained in test cells, which will be placed inside a small thermostat and stabilized at constant temperature. Two types of heater geometries (Figure 1), are installed inside the test cells to create local supersaturation: a small axisymmetrical thermistor (0.25 mm) to serve as a point heater and a flat platinum resistance layer (3×7 mm, approx. 1mm thick) to serve as a large plane surface heater.

In the first type of experiments, the temperature of the liquid will be raised locally by the miniature point heater, with a preset heating rate, resulting in a solution of increasing degree of supersaturation. At fixed temperature, the characteristic time of bubble generation and growth from a supersaturated solution is very small, typically of the order of a few seconds. Therefore, 10 to 15 seconds of μ-gravity time is considered sufficient for one complete run.

A second type of experiments calls for heat pulses given to the plate heater resulting in a temperature gradient perpendicular to its surface. Of particular interest then, will be the observation of many bubbles growing in close proximity to each other over the heater surface and their growth rate dependence on heating rate. Again, a time span of 10 to 15 seconds allows the bubbles to grow to a sufficient size for meaningful interactions.

Click to see the Pictures Gallery here.