Nucleate Pool Boiling eXperiment (NPBX) - 01.09.14
Science Objectives for Everyone Nucleate boiling is bubble growth from a heated surface and the subsequent detachment of the bubble to a cooler surrounding liquid (bubbles in microgravity grow to different sizes than on Earth). As a result, these bubbles can transfer energy through fluid flow; the Nucleate Pool Boiling Experiment (NPBX) investigation provides an understanding of heat transfer and vapor removal processes that take place during nucleate boiling in microgravity. This understanding is needed for optimum design and safe operation of heat exchange equipment that uses nucleate boiling as a way to transfer heat in extreme environments of the deep ocean (submarines) and microgravity.
Science Results for Everyone
Bubbles are cool. Really. Boiling cools down engineering components and systems through transfer of energy as bubbles form and detach from a heated surface to cooler surrounding liquid. This investigation looked at the heat transfer process of boiling in microgravity. Results show that a single bubble grows to the size of the chamber without departing from the heater surface, very different behavior from that in gravity. Researchers observed heat flow lower than that on Earth or from previous Shuttle data, but higher than predicted by hydrodynamic theory. The data will help calibrate numerical simulations and optimize design and operation of heat exchange equipment in extreme environments of the deep ocean and space.
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration
March 2011 - September 2011
Previous ISS Missions
NPBX is a unique investigation, nothing like this has flown in space before.
- NPBX experiments aim to observe the effect of microgravity on cooling phenomena without the dominance of earth?s gravity.
- NPBX boiling experiments on the International Space Station (ISS) enable an understanding of bubble growth, detachment and subsequent motion of single and large merged bubbles in microgravity.
- Boiling is a very efficient cooling process for engineering components and systems used in space exploration and on Earth. The experiments will give engineers the capability to achieve cooling of various components and systems used in space in an efficient manner and thereby lead to smaller and lighter space exploration systems. The results from the experiments will also allow the development of more realistic models for the design and operation of cooling equipment utilized in many engineering systems on earth, including thermal power plants.
Nucleate Pool Boiling eXperiment (NPBX) is to experimentally validates varying-gravity models being developed for the bubble dynamics and associated heat transfer (heat transfer coefficients and pool boiling curves). Applications of boiling heat transfer in space can be found in the following areas:
- thermal management
- fluid handling and control
- power systems
- on-orbit storage and supply systems for cryogenic propellants and life support fluids
- for cooling of electronic packages for power systems associated with various instrumentation and control systems
NPBX determines the effect of gravity on bubble nucleation, growth, merger and departure from prescribed cavities during the boiling processes observed in heat exchangers. Based on the experimental data and the numerical simulations, the investigators determine relative contributions of various heat transfer mechanisms to the overall heat flux. The crewmember discerns the scaling of the bubble length and time scales with gravity.
The experiments use a polished aluminum wafer (85 mm diameter) with prescribed sites for steam bubbles to grow during boiling (these sites are known as nucleation sites). The investigators etch five nucleation sites onto the wafer using standard microfabrication techniques. The backside of the aluminum wafers is equipped with the appropriate sensors such that each of the cavities can be controlled independently. This building block type approach requires that the number of cavities that are nucleated be systematically varied from one to five.
The experiments are conducted for various wall temperatures, liquid temperatures, and number of active cavities. The crew record visual data of the bubble dynamics using a camera. The system pressure, gravity levels, and the temperatures of the heater and the liquid are measured. The crew member uses this data along with the visual data to validate the numerical simulations. Based on the results of the numerical simulations performed for various parameters, the investigators determine the scaling of the bubble growth and departure process and the associated heat transfer mechanisms. These models provide a design tool for the development of phase-changes processes in microgravity environments.
Working fluids for thermal management systems, especially for manned spacecraft must have low freezing temperatures and low toxicity. Current manned systems rely on a dual loop system whereby low pressure water is the internal or crew compartment working fluid and high pressure ammonia or refrigerant is the external working fluid. A thin plate separates the two working fluids in the heat exchanger. In the event of a leak across that plate, the results would be catastrophic. Identification of a new working fluid is critical for removal of this risk.
The experiments will give engineers the capability to achieve cooling of various components and systems used in space in an efficient manner and thereby lead to smaller and lighter space exploration systems.
The proposed research provides a fundamental understanding of bubble dynamics and heat transfer during nucleate boiling in extreme conditions. Such an understanding optimizes the design and safe operation of heat exchange equipment employing phase change for transfer of heat in the environments of the deep ocean, extreme cold and high altitudes.
NPBX utilizes a single heater array, consisting of 5 independently controlled nucleation sites (heaters). The NPBX heater heats up during the test to the temperature at which a bubble starts to nucleate on it. This bubble then grows to a point at which it detaches from the heater. Depending on the test, one to five bubbles are created. During this process, various data are acquired and stored. The investigators have designed NPBX to develop a mechanistic model of nucleate boiling heat transfer in reduced gravity for pool boiling using a predefined boiling surface.
NPBX requires crew time to set up the hardware to perform eight test sessions. The crew captures the data from the test sessions on videotapes and hard drives. If Ku band is available video data is down-linked from ISS to the Glenn Research Center TeleScience Center in Cleveland, OH. The crew stows the drive and videotapes for later return to Earth for analysis by the investigator.
This investigation requires the crew to setup the BXF hardware in the Microgravity Science Glovebox. The crew installs the NPBX samples and activates the BXF, at this point the BXF is automated to perform the test sessions for NPBX. NPBX runs for five hours and performs a total of eight test sessions. The crew performs a hard drive and a videotape change out half way through the NPBX session. After the session is completed the crew deactivates the BXF hardware. The hard drive, videotape and NPBX samples are labeled and stowed for return to Earth.
In the Nucleate Pool Boiling Experiment (NPBX) single bubble dynamics (e.g., inception and growth), multiple bubble dynamics (lateral merger and departure, if any), nucleate pool boiling heat transfer, and critical heat flux using Perfluoro-n-hexane as the test liquid are investigated.
The results of the experiments show that a single bubble continues to grow to occupy the size of the chamber without departing from the heater surface. During lateral merger of bubbles, at high superheats (heating the surface beyond the boiling point) a large bubble may lift off from the surface but continue to hover near the surface while pulling smaller neighboring bubbles into it and growing in size consistent with predictions from numerical simulations. At low superheats, bubbles at neighboring sites simply merge to yield a larger bubble. The larger bubble mostly locates in the middle of the heated surface and serves as a sink for vapor generated on the heated surface. The latter mode continues to persist when boiling is occurring all over the heater surface. This behavior of vapor removal is very different from that at earth normal gravity where single or merged bubbles rapidly lift off from the surface. Heat fluxes for steady state nucleate boiling and critical heat fluxes are found to be much lower than those obtained under Earth normal gravity conditions and also lower than previous data obtained on space shuttle, but higher than that predicted by the hydrodynamic theory extrapolated to microgravity. Aside from experimental conditions, rate of nucleate boiling heat transfer will be dependent on relative heater size and fluid confinement. These data are useful for calibration of results of numerical simulations with the condition that correlations which are developed for nucleate boiling heat transfer under microgravity condition must account for the existence of vapor escape path (sink) from the heater, size of the heater, and the size and geometry of the chamber (Dhir et al. 2012).
Dhir VK, Warrier GR, Aktinol E, Chao DF, Eggers JC, Sheredy WA, Booth W. Nucleate Pool Boiling Experiments (NPBX) on the International Space Station. Microgravity Science and Technology. 2012; 24(5): 307-325.
Ground Based Results Publications
Abarajith HS, Dhir VK, Warrier GR, Son HH. Numerical simulation and experimental validation of the dynamics of multiple bubble merger during pool boiling under microgravity conditions.Annals of the New York Academy of Sciences. 2004; 1027: 235-258.
Jiang S, Dhir VK. Spray cooling in a closed system with different fractions of non-condensibles in the environment. International Journal of Heat and Mass Transfer. 2004; 47: 5391-5406.
Warrier GR, Dhir VK. Visualization of Flow Boiling in Narrow Rectangular Channels. Journal of Heat Transfer. 2004; 126: 495.
In Earth's gravity (image on the left) the action of buoyancy allows the bubbles to overcome surface tension forces. The bubbles rise upward away from the heater surface. In microgravity (image on the right) the buoyancy force is very weak. Consequently, the bubbles often remain attached to the heater because of surface tension and become large as more vapor is produced due to the continuous input of energy from the heater. Courtesy of University of California.
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The figure above shows a comparison of the experimental and numerical bubble shapes during bubble growth, merger and departure for low gravity conditions. The numerical simulations match very well with the experimental data. Water was the test fluid used.
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NBPX heater wafer. Polished aluminum wafer (reflecting a black background) that was manufactured with five nucleation cavities (~20 microns in diameter) to initiate boiling at specific locations. Courtesy of Glenn Research Center.
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