Anti-gravity Condition Research Articles

A novel flat heat pipe for anti-gravity orientations: Leaf fractal evaporator and bi-directional transport capillary channel

Flat heat pipes (FHPs) could be an innovative solution for thermal management in aerospace applications; without the aid of gravity, FHPs could still transport high density energy fluxes with superior temperature control and almost zero energy consumption. Inspired by the powerful transpiration and liquid transport ability of leaf vein structure of plants, a novel FHP design with bionic grading evaporator structured surface was proposed. In addition, a bi-directional transport capillary structure was built inside this FHP, reducing the vapor–liquid flow path and enhancing condensed liquid return simultaneously, and thereafter promoting phase change intensity of FHPs. In the present research, heat transfer performance of FHP with self-wetting fluid as a coolant has been experimentally investigated under anti-gravity, concerning on heat inputs, filling ratios, diameter ratios, porosities of hybrid capillary wick, and two representative tree-like evaporator plate structures (H type and Y type). Our experimental results indicated that heat transfer capability of FHP showed a minimal temperature difference, which was suitable for multi-gravity working conditions. Interestingly, minimal thermal resistance value of 0.45 ℃/W and average enhancement ratio in heat transfer coefficient of 56.39% were achieved simultaneously at anti-gravity orientations for a novel FHP with the 70 PPI and αMF = 0.2 of hybrid capillary wick. Superior vapor flow and diffusion and permeability capability of FHP were further obtained as filling ratio of 30%. Moreover, due to the Marangoni effect, the addition of SRWFs could reduce the axial and radial thermal resistance of the FHP by 5–7 % under anti-gravity condition. Compared with traditional heat sink, the novelty of this passive heat transfer application was capable for realizing high heat transport performance for aerospace flights and applications.

A novel ultra-thin vapor chamber with composite wick for portable electronics cooling

With the development of integrated and ultra-thin portable electronics, the ultimate heat dissipation power in extremely narrow spaces (<1 mm) is increasing year by year. However, the current critical power of ultra-thin vapor chamber (UTVC) is still at a low level, which hinders the development of high-power portable electronic products. In this study, novel UTVCs with a thickness of only 0.5 mm are prepared by utilizing the composite capillary wick. Different from the traditional multi-layer 2D copper mesh wick, the composite (CA/CB) wick is composed of multi-layer 2D copper mesh and 3D spiral woven mesh. UTVCs prepared from three different wicks, referred as to 2D-UTVC, CA-UTVC, CB-UTVC, were compared. Under different working conditions, the CA/CB-UTVC has better heat transfer performance than 2D-UTVC. In the horizontal state, the maximum effective thermal conductivity (Keff) of CA-UTVC and CB-UTVC is significantly improved by 86.8% and 60.2% compared to that of reference 2D-UTVC. The CA design is recommended for low heating power within 40 W, while the CB design is the optimal choice for high heating power above 40 W. The maximum critical power of CB-UTVC is up to 60 W with gravity support. Even at 50 W under anti-gravity condition, the thermal conductivity of CB-UTVC is still up to 11817.1 W/(m K), which is 29.5 times that of copper. The improved heat dissipation is due to the enhanced capillary force of the composite wick. This positive effect is facilitated by the combination of multiple 3D spiral woven meshes in the CB design. The optimized UTVC effectively reduces the maximum temperature by 14.3 °C compared to copper sheet under natural convection at 10 W heating power. The novel UTVC could provide a guarantee for the heat dissipation of high-power integrated portable electronic devices in the future.

Experimental study on long-distance anti-gravity loop heat pipe with submicron-scale porous structure

• An anti-gravity type loop heat pipe (LHP) which can transport heat for a few meters against gravity was proposed. • To generate a capillary force of several hundred kPa, a submicron-scale porous material made of glass was fabricated as a primary wick. • To reduce the heat leak, hydrophilic polytetrafluoroethylene wick was installed at the rear side of the primary wick. • The LHP with the vapor and liquid heat transport length of 4 m was fabricated, and the LHP operation under 4 m anti-gravity condition was accomplished. Although a loop heat pipe (LHP) is capable of transporting heat over a long distance under anti-gravity conditions in principle, it has not been demonstrated yet. This paper reports an experimental study of an anti-gravity type LHP that can transport heat for a few meters against gravity. For a LHP to generate a capillary force of several hundred kPa, a submicron-scale porous material made of glass was selected as the primary wick. The wick properties such as pore radius, permeability, and wettability with the working fluid were evaluated. A flat evaporator-type LHP with a few meters of vapor and liquid transport lines was designed, fabricated, and tested. Firstly, a preliminary experiment was conducted with water as a working fluid under horizontal conditions. The length of the vapor and liquid lines were both 10 m. It was found that there was a large heat leak from the evaporator to the compensation chamber. In addition, the vapor did not reach the condenser because the vapor was condensed in the vapor line. Secondly, experiments were conducted under the horizontal (0 m), 2 m, and 4 m anti-gravity conditions. In order to reduce the heat leak, a hydrophilic polytetrafluoroethylene wick was installed at the rear side of the primary wick. The vapor and liquid transport length were 6.5 m, and the ethanol was selected as a working fluid. These experimental results demonstrated that the LHP could transport heat under horizontal and 2 m anti-gravity conditions, whereas the LHP cannot transport heat under 4 m anti-gravity conditions. Finally, a LHP with a vapor and liquid transport line length of 4 m was fabricated, which was able to operate under 4 m anti-gravity conditions.

Heat transport limitations and performance enhancement of anodized grooved heat pipes charged with ammonia under gravity and anti-gravity condition

Grooved Heat Pipes (GHPs) are the essential heat transfer devices for space applications, as they can effectively work with and without the aid of gravity. In the present study, the heat transport limitations of anodized grooved heat pipes were modelled by incorporating an anodised surface coating parameters under gravity and anti-gravity conditions. Variations in the heat transport limitations such as capillary limit, boiling limit, and entrainment limits due to the anodization over non-anodized grooved heat pipes are presented. The heat transfer performance of an internally anodized and non-anodized grooved heat pipe charged with ammonia as a coolant is experimentally investigated and compared. Two sets of grooved heat pipes are fabricated with 40 numbers of rectangular grooves. Semi-spherical pores are formed by carefully adjusting the voltage and current supply between the electrodes during the anodization process. Experiments in grooved heat pipes are conducted at different inclinations (0-90°) under gravity and anti-gravity for various heat inputs of both heat pipes. Due to the anodization, thermal resistance reduction of 51.1% and enhancement in heat transfer coefficient of 36.6% was obtained at an optimum inclination of 45° for the grooved heat pipes with 0.4 mm groove width. Also, a maximum of 35 W heat input was transferred due to the anodization under anti-gravity with an inclination of 30°. Results suggest that the grooved heat pipes with semi-spherical pores charged with ammonia as a coolant was the best combination for enhanced heat transfer, preferably for space applications.

Evidence of inverted gravity-driven variation in predictive sensorimotor function.

We move our eyes to place the fovea into the part of a viewed scene currently of interest. Recent evidence suggests that each human has signature patterns of eye movements like handwriting which depend on their sensitivity, allocation of attention and experience. Use of implicit knowledge of how earth's gravity influences object motion has been shown to aid dynamic perception. We used a projected ball-tracking task with a plain background offering no context cues to probe the effect of acquired experience about physical laws of gravitation on performance differences of 44 participants under a simulated gravity and an atypical (upward) antigravity condition. Performance measured by the unsigned difference between instantaneous eye and stimulus positions (RMSE) was consistently worse in the antigravity condition. In the vertical RMSE, participants took about 200ms longer to improve to the best performance for antigravity compared to gravity trials. The antigravity condition produced a divergence of individual performance which was correlated with levels of questionnaire-based quantified traits of schizotypy but not control traits. Grouping participants by high or low traits revealed a negative relationship between schizotypy trait level and both initiation and maintenance of tracking, a result consistent with trait-related impoverished sensory prediction. The findings confirm for the first time that where cues enabling exact estimation of acceleration are unavailable, knowledge of gravity contributes to dynamic prediction improving motion processing. With acceleration expectations violated, we demonstrate that antigravity tracking could act as a multivariate diagnostic window into predictive brain function.

Enhancement of loop heat pipe heat transfer performance with superhydrophilic porous wick

A loop heat pipe (LHP) can effectively improve the heat dissipation capacity for a variety of equipment and instruments with high heat flux. A superhydrophilic porous wick was fabricated in a LHP to investigate the effect of wettability and nanostructures on heat transfer performance. These wicks can be achieved by copper powder sintering and surface modification through hydrogen peroxide (H2O2) oxidation. For the LHP with a superhydrophilic wick (SH-LHP), the heat transfer performance was significantly enhanced in comparison with the LHP with a hydrophilic wick (H-LHP). SH-LHP can startup at a lower heat load, and it has smaller temperature fluctuations. At a high heat load, it first enters the normal working mode, and it has better temperature uniformity. When the heat load is 240 W, the temperature in the center of the evaporator is 64.3 °C, which is nearly 8.7 °C below that of H-LHP. Under anti-gravity condition, SH-LHP shows its superior heat transfer performance at a high heat load; the maximum heat transfer coefficient increased by 50% in comparison with the H-LHP. The main mechanism has three aspects. First, superhydrophilic wicks have a large capillary force that provides a sizable driving force and an adequate liquid supply for the circulation and phase transformation of the working fluid. Second, the superhydrophilic wick has multi-scale structures in the nanometer to millimeter range, which effectively regulates the vapor-liquid distribution and bubble formation. Third, the nanostructures on the superhydrophilic wick's surface provide more nucleation sites and a large surface area for phase change heat transfer.

Flat-evaporator-type loop heat pipe with hydrophilic polytetrafluoroethylene porous membranes

This paper describes an experimental study of a flat-evaporator-type loop heat pipe (LHP) with wicks made from hydrophilic polytetrafluoroethylene (PTFE) porous membranes, which have small pore sizes but high porosity and permeability. To demonstrate the applicability of these membranes, the LHP was designed completely and fabricated, after which the performance was experimentally investigated under a 0.52 m anti-gravity condition at a constant heat sink temperature of 80 °C. Two types of membranes were used, possessing different pore diameters and permeabilities. The pore diameter and permeability of wick 1 were 0.44 µm and 2 × 10−14 m2, respectively, while wick 2 had a pore diameter and permeability of 1.40 µm and 5 × 10−14 m2, respectively. A special wick support was designed and fabricated to ensure contact between the wick and the groove fins and to prevent the shrinkage of the PTFE membranes. Pure water was used as the working fluid. The effect of the PTFE wick characteristics on the LHP thermal performance was investigated by measuring the temperature at each point and the compensation chamber pressure. The LHP achieved steady-state operation at heat loads up to 1000 W, with a minimum thermal resistance of 0.052 K/W. Wick 2, which had a larger pore size and higher permeability, exhibited better performance than wick 1. The LHP operating temperature decreased by 10 °C, and the thermal resistance decreased by approximately 20% between wick 1 and wick 2.

Experimental investigation on ethane pulsating heat pipe based on Stirling cooler

An experimental system was built to investigate heat transfer characteristics of an ethane pulsating heat pipe (EPHP) based a Stirling cooler. The EPHP is made of copper pipe with an inner diameter of 2 mm, consisting of 20 tubes. The effects of filling ratios (FRs), heat inputs (HIs) and inclination angles (IAs) on the performance of the EPHP were studied under different temperature regions. During the cooling process, the EPHP shows a high heat transfer efficiency. The optimal FR corresponding to the lowest thermal resistance is about 30%. For the same FR, the best heat transfer performance of EPHP is not obtained at IA of 90°, but a specific IA between vertical and horizontal orientation. The thermal resistance increases slowly when the IA increases under anti-gravity condition, but when the IA is close to −90°, the thermal resistance increases rapidly.

Study on a loop heat pipe for a long-distance heat transport under anti-gravity condition

This paper reports the test results and evaluation of gravity effects on long-distance loop heat pipe (LLHP) with 10m distances for heat transport. First, the LLHP was designed based on the one-dimensional steady-state model, fabricated and tested. Test results showed the LLHP could transport heat up to 340W for 10m and the thermal resistance between the evaporator and the condenser was 0.12K/W under horizontal condition. Next, the LLHP was tested in top-heat mode. The maximum heat transports were 310, 270, 220W and the thermal resistances were 0.15, 0.17, 0.22K/W under 30, 60, 100cm anti-gravity condition respectively. The heat transfer efficiency of the LLHP was discussed in detail. The evaluation results showed 71.4% of the 340W heat load was dissipated at the condenser under the horizontal condition. On the other hand, 62.8, 61.8, 57.1% of the maximum heat load was dissipated at the condenser under 30, 60, 100cm anti-gravity condition respectively. The calculation model was in good agreement with the experimental results by considering the existence of a vapor pocket between the evaporator case and the vapor-liquid interface. This analysis indicated the vapor blanket generates easily and becomes thicker as anti-gravity effect increases.

Experimental investigation on the sintered wick of the anti-gravity loop-shaped heat pipe

The Anti-Gravity Loop-Shaped Heat Pipe (AGLSHP), designed with a special sintered wick structure, is a novel device used in electronic heat dissipation under the anti-gravity condition. Two kinds of wick structures, single-powder (SP) and continuous step-graded (CSG) structures were evaluated for design optimization. The copper powder utilized in the experiment were divided into six particle sizes (<25, 25–50, 50–75, 75–100, 100–125, and >125μm) to investigate their influence. The results show that the heat transfer performance of the sintered wick samples changes significantly with the variation of the copper powder size. SP75–100 is the optimal particle size for single-powder sintered wick heat transfer, while CSG25–50 is the optimal particle size for the continuous step-graded sintered wick. Furthermore, the CSG sintered wick possesses better overall performance than the SP sintered wick, obtaining higher maximum operating power and lower start-up temperature.

Cellular Migration to Electrospun Poly(Lactic Acid) Fibermats

Nonwoven fabrics prepared via an electrospinning method, so-called electrospun fibermats, are expected to be promising scaffold materials for bone tissue engineering. In the present work, poly(L-lactic acid) (PLLA) fibermats, consisting of fibers with diameters ranging from 1 to 10 μm, were prepared by electrospinning. Mouse osteoblast-like cells (MC3T3-E1) were seeded on the fibermats with various fiber diameters (10, 5 and 2 μm; they are denoted by samples A, B and C, respectively) and cultured in two different directions in order to compare the migration behaviours into the scaffold of the normal condition and the anti-gravity condition. The cells in/on the fibermats were observed by laser confocal microscopy to estimate the cellular migration ability into them. When the MC3T3-E1 cells were cultured in the normal direction, the thickness of their layer increased to approx. 90 μm in the sample A, consisting of 10-μm fibers after 13 days of culture, while that in the sample C, consisting of 2-μm fibers, did not increase. When the MC3T3-E1 cells were cultured in the anti-gravity condition, the thickness of the cell layer in the sample A increased to approx. 60 μm. These results mean that the MC3T3-E1 cells migrated into the inside of sample A in either the normal direction or the anti-gravity one. The cellular proliferation showed no significant difference among the fibermats with three different fiber diameters; MC3T3-E1 cells on the fibermat with 2 μm fiber diameter grew two-dimensionally, while they grew three-dimensionally in the fibermat with 10 μm fiber diameter.

Experimental investigation on anti-gravity loop heat pipe based on bubbling mode

Traditional loop heat pipes (LHPs) have disadvantages including high operating temperature, large heat resistance and low heat transfer efficiency under anti-gravity condition. An anti-gravity loop heat pipe (AG-LHP), in which a bubbler is added between the condenser and the compensation chamber (CC), is studied and analyzed in this paper. The bubbler is designed to enforce circumfluence of the working fluid. Integral outside fins (IOF) are used as inner cylinder surface structure of the bubbler. Bubbling experiment was conducted to evaluate the performance of the bubbler with IOF surface structure, compared with those with smooth and knurled surface structures. The infrared ray (IR) thermal imaging method was used to investigate the working process of the bubbler. Furthermore, start-up experiment was carried out to investigate the start-up phenomenon and the performance at steady state of the AG-LHP. The results of the bubbling experiment under different heating power showed that the bubbler with IOF inner cylinder surface structure had the highest bubbling height and the shortest steady time, which demonstrated that the IOF surface structure effectively improved the thermal and steady performance of the bubbler. The start-up experiment confirmed that the AG-LHP could achieve steady state in 20min and could easily adapt to different heating loads under anti-gravity condition. The thermal resistance of the AG-LHP can approach to a value as high as 150W/K with the heating load of 80W.