Heat Transfer In Microgravity Research Articles

Enhanced heat transfer in microgravity from asymmetric sawtooth microstructure with engineered cavities

Nucleate pool boiling in microgravity is influenced by stagnant vapor bubble dynamics, leading to early dryout of the heated surface compared to terrestrial conditions. The Asymmetric Sawtooth and Cavity-Enhanced Nucleation-driven Transport (ASCENT) investigation studied an engineered microstructure in a nucleate pool boiling setup aboard the International Space Station (ISS) from November 2022-January 2023. The experiment was housed in the Microgravity Science Glovebox (MSG) and used degassed FC-72 as the test liquid. The engineered microstructure consisted of repeating mm-sized 60°-30° sawtooth structures located within a hermetically sealed square ampoule as the test chamber. Experimental investigations were conducted to explore a range of input heat fluxes, ranging from 0.5 to 2.3 W/cm². The resulting vapor dynamics and heat transfer were compared against a flat baseline surface. Both surfaces contained 250 µm square cavities spaced 1-mm apart. The constrained dimensions of the square chamber severely influenced vapor bubble dynamics across the heated surface. The presence of a liquid layer beneath vapor slugs on the microstructure was observed, in contrast to the baseline surface, where no liquid layer was visually detected. The transient heat transfer coefficient from the microstructure was higher than the baseline surface as the vapor bubbles grew larger at constant heat flux due to liquid film access provided by the asymmetric ridges on the microstructure. A significant increase in the heat transfer coefficient was observed at 1.8 W/cm2, potentially due to vigorous nucleation in the observed liquid pockets along the microstructure, increasing the heat transfer coefficient from 2900 to 6200 W/m2K. Nucleation was observed at heat fluxes as low as 0.5 W/cm2 from the engineered cavities, and a quasi-steady heat transfer coefficient of 2200 W/m2K was obtained at 2.3 W/cm2 from the microstructure surface. The quasi-steady analysis also indicated that both surfaces performed similarly in microgravity and terrestrial gravity in the same experimental setup. The results demonstrate that the liquid film dynamics underneath vapor slugs influence heat transfer to a large extent in microgravity conditions.

The effect of gravity on cryogenic transfer line chilldown performance using pulse flow and low thermally conductive coatings

Future cryogenic propulsion systems will require efficient methods to chill down transfer lines and propellant tanks prior to transfer of propellant while in space. When chilling down the transfer line, there is an inherent tradeoff between propellant mass consumed and chilldown time to steady state, depending on mission constraints and desires. This paper examines an efficiency parameter based on a simplified energy balance that can be used to compare the performance of different chilldown methods and to determine the optimal chilldown method. The parameter, along with chilldown time and mass, is then applied to recent liquid nitrogen line chilldown experiments conducted in both terrestrial and microgravity to compare efficiencies between numerous continuous and pulse chilldown tests on bare as well as coated tubes across a range of thermodynamic conditions. Despite a well-known reduction in heat transfer in microgravity due to the absence of buoyancy-assisted cooling, results indicate no appreciable degradation in chilldown efficiency at moderate-to-high Reynolds numbers in microgravity relative to terrestrial gravity.

Nucleate Pool Boiling eXperiment (NPBX) in microgravity: International Space Station

A series of nucleate pool boiling experiments were conducted in the Boiling Experimental Facility (BXF) located in the Microgravity Science Glovebox (MSG) on board the International Space Station (ISS) during the period March–May, 2011. Nucleate Pool Boiling eXperiment (NPBX) was one of the two experiments housed in the BXF. Results of experiments on natural convection, nucleate pool boiling heat transfer and critical heat flux are described. Perfluoro-n-hexane was used as the test liquid. The test liquid contained dissolved gas. The test surface was a polished aluminum disc (89.5mmdia.) heated from below with strain gage heaters. Five cylindrical cavities were formed on the surface with four cavities located at the corners of a square and one in the middle, to study bubble dynamics and initiate nucleate boiling. During experiments the magnitude of mean gravity level normal to the heater surface varied from 1.7×10−7ge to 6×10−7ge.The results of the experiments show that at low superheats, bubbles generated on the heater surface slide and merge to yield a large bubble located in the middle of the heater. At high superheats, the large bubble may lift off from the heater but continue to hover near the surface. In both these scenarios, the large bubble serves as a vapor sink. Natural convection heat transfer in microgravity was found to be consistent with that predicted by available correlations. Steady state nucleate boiling and maximum heat fluxes are found to be lower than those obtained under earth normal gravity conditions. The heat transfer coefficients for nucleate pool boiling are found to be weakly dependent on the level of gravity (h/hge∝(g/ge)1/8). Maximum heat flux also shows a weaker dependence on gravity than that given by the hydrodynamic theory of boiling. The data are useful for calibration of results of numerical simulations. Any correlations that are developed for nucleate boiling heat transfer under microgravity condition must account for the existence of vapor escape path (large vapor bubble acting as a sink) from the heater, relative size of the large bubble and heater, and the size and geometry of the chamber used.

Flow and Heat Flux Behavior of Micro-bubble Jet Flows Observed in Thin, Twisted-Wire, Subcooled Boiling in Microgravity

Thin wire, subcooled boiling experiments were performed onboard an aircraft flying a parabolic trajectory to provide microgravity conditions for improved observation of jet flow phenomena and their behavior in the absence of buoyant forces. A new type of nucleation jet flow was observed in microgravity. This new micro-bubble jet flow is seen at medium to high heat fluxes and is characterized by a region of the wire that forms multiple jet columns which contain micro-bubbles. These columns flow together and penetrate tens of millimeters into the bulk fluid. Bubble behavior on the wire was observed to progress from a dominance of larger isolated bubbles on the wire to a dominance of micro-bubble jet flows on the wire as heat flux was increased. There was also a transient transition from a few large isolated bubbles to micro-bubble jet flow dominance for a set heat flux. A cross correlation calculation provided velocities of micro-bubbles in the flow, which were in the range of 4-14 mm/s. These velocities were used with convection correlations to show that fluid flows induced by jet flows are a significant contributor to the subcooled boiling heat transfer in microgravity, but are not the primary contributor. Additionally, a relative bubble area analysis approximates the direct contribution of these jet flows to the overall heat dissipation. These micro-bubble jet flows, which are only observed on thin wires (not flat surfaces), and the convection currents they induce, have the potential to allow for sustained fluid motion to occur in microgravity.

Study on pool boiling heat transfer in microgravity

摘要: 对利用中国返回式卫星搭载开展的两次微重力池沸腾空间实验及地基常重力和落塔短时微重力实验的结果进行了评述. 研究发现微重力时丝状加热器沸腾传热会略有强化, 而平板加热器则在高热流条件下明显恶化. 微重力时, 气泡脱落前存在沿加热面的横向运动, 加剧了相邻气泡间的合并, 合并气泡会在其表面振荡作用下从加热面脱落. Marangoni 效应对于微重力气泡行为有重要影响. 关键词: 微重力 / 池沸腾 / 传热 / 气泡行为 / 空间实验

FLOW BOILING HEAT TRANSFER IN MICROGRAVITY: RECENT PROGRESS

FLOW BOILING HEAT TRANSFER IN MICROGRAVITY: RECENT PROGRESS

POOL BOILING IN MICROGRAVITY AND IN THE PRESENCE OF ELECTRIC FIELD: EVALUATION OF THE VOID FRACTION IN THE ARIEL EXPERIMENT

The ARIEL experiment was flown aboard a Foton-M2 satellite and hosted in a FLUIDPAC facility, sharing its optical diagnostics and power systems. Its aim was to investigate boiling heat transfer in microgravity on a surface of industrial relevance, at high heat rates. The effect of an externally applied electrostatic field on boiling performance was also tested. The void fraction in microgravity was much larger than in normal gravity condition: the application of electric field was very effective in reducing it. In the absence of electric field, bubbles coalesced giving origin to a large mass of vapor residing near the surface. The application of reduced bubble coalescence and suppressed boiling heat transfer oscillations thus enhanced boiling performance in microgravity. In this paper, a quantitative evaluation of overall and near-wall void fraction was performed, based on the digital processing of the video images. The results showed that the cyclic oscillation of the near-wall void fraction was well correlated with temperature oscillations of the heated wall. Besides, the electric field was very effective in reducing overall void fraction even at low values of the applied voltage.

Pool Boiling Heat Transfer in Microgravity

Pool Boiling Heat Transfer in Microgravity

Flow boiling heat transfer in microgravity: Recent results

Flow boiling heat transfer (FBHT) allows high performance heat transfer due to latent heat transportation and its use is therefore important to reduce size and weight of space platforms and satellites. Its knowledge is also important for the safe operation of existing single-phase systems in case of accidental increase of heat generation rate. The number of existing researches on flow boiling in reduced gravity is very small due to both larger heat loads and available room in a 0-g apparatus for experiments. Consequently, coherence in existing data is somewhat missing. This lecture will summarize the results of the few research carried out on FBHT in microgravity, with special emphasis to the recent research carried out at ENEA, in the frame of an ESA (European Space Agency) project.

Pool boiling heat transfer in microgravity

A temperature-controlled pool boiling (TCPB) device has been developed to study the bubble behaviors and heat transfer in pool boiling phenomenon both in normal gravity and in microgravity. The results on heat transfer and bubble dynamic behavior in the experiments aboard the 22nd Chinese recoverable satellite and those in normal gravity before and after the flight experiment are reported and discussed in the present paper. The onset-boiling temperature is independent, or at least, dependent much weakly on gravity. Heat transfer of nucleate boiling in microgravity is slightly enhanced, while the scale of CHF with gravity is contrary to the traditional viewpoint and can be predicted by LD-Zuber correlation. A forward-and-backward lateral motion of vapor bubbles is observed along the wire before their departure from the wire in microgravity, while three critical bubble diameters divide the observed vapor bubbles into four regions in microgravity. These distinctive bubble behavior can be interpreted by Marangoni effects.

High heat flux cooling by microbubble emission boiling.

In subcooled flow boiling of water in a horizontal rectangular channel, microbubble emission boiling occurred at higher subcooling of liquid in transition boiling, and the heat flux increased more than the critical heat flux. The maximum heat flux reached 10 MW/m(2) for a channel with 12 mm x 14mm cross-section at 40K liquid subcooling and 0.5 m/sec liquid velocity. For smaller rectangular channels with 14 mm x 5mm, 14mm x 3mm, and 14mm x 1mm cross-sections, the maximum heat flux was 7 MW/m(2)-more than 20 times the cooling limit of a present day CPU. Microbubble emission boiling is expected to realize high heat flux cooling for electronic devices. In convection boiling with subcooled water jet, the same boiling regime and heat flux were obtained for a downward heating surface and an upward heating surface. In subcooled flow boiling with strong convection, the hydrodynamic force is predominant for vapor-liquid exchange. Accordingly, microbubble emission boiling is expected for high heat flux cooling or high heat flux heat transfer in microgravity.

Dielectrophoresis with application to boiling heat transfer in microgravity. I. Numerical analysis

This article presents a numerical analysis for the use of a dielectrophoretic (DEP) force on vapor bubbles to sustain nucleate boiling heat transfer in space where the gravity-driven buoyancy force is absent. The analytic and numerical solution procedures for the DEP force are outlined for an infinite-plate and a finite-plate electrode geometry, respectively. A simple analysis is presented that describes the limits on the magnitude of the DEP force. A comparison is made between the DEP forces produced near the edges of the finite-plate electrodes (where there is a relatively high-gradient electric field) to the forces produced between the electrodes and away from the edges (where there is a relatively low-gradient electric field). The predicted DEP forces are then used to study the effectiveness of bubble transport for both high and low electric-field gradients in microgravity. To effectively maintain boiling in space with the DEP force, a knowledge of the electric field is required which addresses both the DEP force strength and the DEP force spatial distribution in order to appropriately balance between the short-distance vapor bubble detachment needs and the long-distance bubble transport requirements.

Dielectrophoresis with application to boiling heat transfer in microgravity. II. Experimental investigation

The objective of this paper is to analyze experimentally the feasibility of utilizing the dielectrophoretic (DEP) force to sustain boiling in space where the gravity-driven buoyancy force is absent. First, a bubble trajectory experiment is present to determine the magnitude of the DEP force produced at the edge of two diverging-plate electrodes to the highly nonuniform electric field. The results showed that the DEP forces measured experimentally are equal to those predicted in Part I to within 10%–15%. Second, the bubble detachment diameter and frequency of bubbles generated with a gold-film, single-bubble heater were measured in microgravity so that the masking effect of gravity could be eliminated. In particular, a comparison is made between the DEP forces produced with a relatively high-electric-field gradient at the edge of two flat-plate electrodes to the DEP forces produced with the relatively low electric-field gradient between two diverging-plate electrodes. It was discovered that the bubble detachment diameter and frequency could be reduced to a single curve by plotting the data against effective gravity ratio [g(b,e)] due to the dielectrophoretic forces. In this way, the normalized bubble diameter at the time of breakoff from the heated surface was found to vary inversely as the effective gravity ratio to the 1/2 power.

Pool Boiling Heat Transfer in Microgravity. Experiments with CFC-113 and Water Utilizing a Drop Shaft Facility.

Microgravity pool boiling experiments were carried out utilizing the drop shaft at the Japan Microgravity Center, which enables control of microgravity to within one-ten-thousandth of terrestrial gravity for a period of 10 seconds. Vapor bubbles were generated on a Joule-heated stainless steel or ITO film backed with a thin alumina or glass substrate, which was immersed in a pool of subcooled or saturated liquid at atmospheric pressure. The CHF (critical heat flux) to CFC-113 was lowered, in microgravity, to one-fifth of that achieved under the normal terrestrial condition. In contrast, the CHF to water remained more than one-half that achieved under the terrestrial condition. This difference in the gravity dependence of CHF is presumably ascribable to the difference in the frequency of bubble detachment from the heater surface between CFC-113 and water in microgravity, which is dependent on the difference in thermophysical properties, particularly the surface tension and the heat of vaporization, between the two fluids.