Search query: ultra high heat flux

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abstract: A two phase heat spreader has been developed for cooling high heat flux sources in high power lasers, high intensity light emitting diodes, and semiconductor power devices. The heat spreader targets the passive cooling of heat sources with fluxes greater than 5 W/mm2 without requiring any active power consumption for the thermal solution. The prototype vapor chamber consists of an evaporator plate, a condenser plate and an adiabatic section, with water as the phase change fluid. The custom designed high heat flux source is composed of a platinum resistive heating pattern and a temperature sensor on an aluminum nitride substrate which is soldered to the outside of the evaporator. Experiments were performed with several different microstructures as evaporator surfaces under varying heat loads. The first microstructure investigated, a screen mesh, dissipated 2 W/mm2 of heat load but with an unacceptably high evaporator temperature. A sintered copper powder microstructure with particles of 50 mm mean diameter supported 8.5 W/mm2 without dryout. Four sets of particle diameters and different thicknesses for the sintered copper powder evaporators were tested. Additionally, some of the sintered structures were coated with multi walled carbon nanotubes (CNT) that were rendered hydrophilic. Such nano structured evaporators successfully showed a further reduction in thermal resistance of the vapor chamber.Copyright (c) 2010 by ASME
author_names: Mitsuo Hashimoto

Hiroto Kasai

Kazuma Usami

Hiroyuki Ryoson

Kazuaki Yazawa

Justin A Weibel

Suresh V Garimella
title: Nano Structured Two Phase Heat Spreader for Cooling Ultra High Heat Flux Sources
year: 2010

abstract: A new welding method, flux bands constricting arc (FBCA) welding, is proposed to compensate for the shortage of insufficient weld width of laser welding T joints in high steel sandwich panels. The arc behavior (arc burning position, arc shape, arc heat, and arc stability) before and after sticking the flux bands (GMAW and FBCA welding) to the ultra narrow gap groove was tested. Results indicate that flux bands have solid wall constricting effect (SWCE) and thermo compression effect (TCE) on the arc and self producing slag and gas function in FBCA welding. In ultra narrow gap groove, the arc burning position climbing up phenomenon (APCP) occurs without flux bands. The SWCE of flux bands on the arc effectively suppresses the APCP because of the insulation of flux bands. In the FBCA welding process, the effective heating area of the arc is increased by at least 5 mm2 compared with that in GMAW. When the groove gap decreases, flux bands not only compress the arc from an inverted bell shape to a rectangular shape, but also make the 660 degC isotherm on the core plate to increase from 3 mm to 8 mm. In the end, the proportion of unstable arc burning time is reduced by 86.85% the fluctuation of arc voltage and welding current are also significantly reduced by the flux bands because of their SWCE on the arc.
author_names: Lei Wang

Ji Sen Qiao

Jianhong Chen
title: The Mechanism of Effect of Flux Bands on The Arc Behavior in Flux Bands Constricting Arc Welding Process
venue: Materials
year: 2020

abstract: The Pierre Auger Observatory is the most sensitive air shower detector for primary photons with energies above \sim 0.2$ EeV. Using the Surface Detector array (SD) photons in the zenith angle range from 30$\circ$ to 60$\circ$ can be identified through the broad time structure of the signals expected to be induced in the SD stations. An additional signature for photon induced air showers is the steeper lateral distribution of secondary particles at the ground with respect to nucleonic showers. Stringent limits are set to the diffuse flux of ultra high energy (UHE) photons above 10 EeV, using SD data collected between 2004 and mid 2018 with an exposure of $40\,000$ km$^2\$sr$$yr. Below 1 EeV, unprecedented separation power between primary photons and hadrons can be achieved by combining observables from the low energy enhancements of the Pierre Auger Observatory, namely three upwards pointing fluorescence telescopes, which can directly measure the atmospheric depth of the shower maximum, and the 750 m SD array, which again accesses the steeper lateral distribution of photon induced air showers. For the first time, limits on the integral photon flux below 1 EeV are presented using data collected by the Pierre Auger Observatory between mid 2010 and 2015. This extends the range of photon searches at the Pierre Auger Observatory to about three decades in energy.
author_names: Julian Rautenberg
title: Limits on ultra high energy photons with the Pierre Auger Observatory
venue: Proceedings of 36th International Cosmic Ray Conference PoS(ICRC2019)
year: 2019

abstract: Efficient RNA extraction methods are needed to study transcript regulation. Such methods must lyse the cell without degrading the genetic material. For cyanobacteria this can be particularly challenging because of the presence of the cyanobacteria cell envelope. The great breath of cyanobacterial shape and size (unicellular, colonial, or filamentous multicellular) created a variety of cell lysis methods. However, there is still a lack of reliable techniques for nucleic acid extraction for several types of cyanobacteria. Here we designed and tested 15 extraction methods using physical, thermic or chemical stress on the filamentous cyanobacteria Planktothrix agardhii. Techniques based on the use of beads, sonication, and heat shock appeared to be too soft to break the Planktothrix agardhii cell envelope, whereas techniques based on the use of detergents degraded the cell envelope but also the RNA. Two protocols allowed to successfully obtain good quality RNA. The first protocol consisted to manually crush the frozen cell pellet with a pestle and the second was based on the use of high intensity ultra sonication. When comparing these two, the high intensity ultra sonication protocol was less laborious, faster and allowed to extract 3.5 times more RNA compared to the liquid nitrogen pestle protocol. The high intensity ultra sonication protocol was then tested on five Planktothrix strains, this protocol allowed to obtain >8.5 mg of RNA for approximatively 3.5 x 108 cells. The extracted RNA were characterized by 260/280 and 260/230 ratio to 2, indicating that the samples were devoid of contaminant, and RNA Quality Number to 7, meaning that the integrity of RNA was preserved with this extraction method. In conclusion, the method we developed based on high intensity ultra sonication proved its efficacy in the extraction of Planktothrix RNA and could be helpful for other types of samples.
author_names: Sandra Kim Tiam

Katia Comte

Caroline Dalle

Charlotte Duval

Claire Pancrace

Muriel Gugger

Benjamin Marie

Claude Yepremian

Cecile Bernard
title: Development of a new extraction method based on high intensity ultra sonication to study RNA regulation of the filamentous cyanobacteria Planktothrix
venue: PloS one
year: 2019

abstract: We experimentally study high pressure pool boiling heat transfer enhancement using a copper sintered particle wick structure in water. The wicks are fabricated using the multi step sintering process using 200 mm sintered copper powder. Thermophysical properties of water change at elevated pressure, therefore changing the bubble dynamics. For the single layer wick, Critical Heat Fluxes (CHF) of 179.63, 182.42, and 198.16 W/cm2 are found at 0, 103.4, and 206.8 kPa, respectively. The maximum CHF is found at 206.8 kPa with the wick superheat of 9.9 K. A 110% enhancement is found in the CHF for 206.8 kPa, compared to 0 kPa. We observed a CHF 1.8 times higher compared to the plain surface at 0 kPa. The maximum Heat Transfer Coefficient (HTC) of 252.46 W/cm2 K is found at a heat flux of 100 W/cm2 and a pressure of 206.8 kPa. The high pressure pool boiling result for the single layer wick shows that the heat transfer coefficient is enhanced by 100% compared to 0 kPa. We suggest the reasons for enhancement of the pool boiling performance is primarily due to high rate of bubble generation, high bubble release frequency, and reduced thermal hydraulic length modulation, and enhanced thermal conductivity due to the sintered wick layer. Our analysis suggests that the Rayleigh critical wavelength decreases by 4.67% with varying pressure, which may cause the bubble pinning between the sintered particles and prevents bubble coalescence. Similarly, the role of pressure in enhancing heat transfer compared to the effect of the wicking layer is also analyzed and found that the critical flow length, lu reduces by three times for 200 mm diameter particles. We suggest that the porous wick layer provides a capillary assist to liquid flow effect, and delays the surface dry out. The pressure and surface modification amplify the boiling heat transfer performance. All these reasons may contribute to the CHF and HTC enhancement in the wicking layer at high pressure.
author_names: Smreeti Dahariya

Nill Patel

Munonyedi Kelvin Egbo

Gisuk Hwang

Amy Rachel Betz
title: High Pressure Pool Boiling Heat Transfer Enhancement Mechanism on Sintered Particle Wick Surface
venue: Frontiers in Mechanical Engineering
year: 2019

abstract: Recently, the use of electrical vehicles has abruptly increased due to environmental crises. The high energy density of lithium ion batteries is their main advantage for use in electric vehicles (EVs) However, the thermal management of Li ion batteries is a challenge due to the poor heat resistance of Lithium ions. The performance and lifetime of lithium ion batteries are strongly affected by the internal operating temperature. Thermal characterization of battery cells is very important to ensure the consistent operation of a Li ion battery for its application. In the present study, the OHP (Oscillating Heat Pipe) system is proposed as a battery cooling module, and experimental verification was carried out. OHP is characterized by a long evaporator section, an extremely short condenser section, and almost no adiabatic section. Experimental investigations were conducted using various parameters such as the filling ratio, orientation, coolant temperature, and heat flux. Average temperature of the heater's surface was maintained at 56.4 degC using 14 W with 25 degC coolant water. The experimental results show that the present cooling technology basically meets the design goal of consistent operation.
author_names: Ri-Guang Chi

Seok-Ho Rhi
title: Oscillating Heat Pipe Cooling System of Electric Vehicle's Li Ion Batteries with Direct Contact Bottom Cooling Mode
venue: Energies
year: 2019

abstract: Process temperature is an important process parameter for hot rolled strips, directly affecting microstructure and property. Recently, ultra fast cooling (UFC) technology is widely used in development and production of hot rolled strips, because of its high cooling rate, which can reduce alloy element and improve microstructure and property.1,2) In order to get ideal microstructure of hot rolled strips, UFC and laminar cooling on run out table are jointly utilized to cool hot strips from FDT (Finishing Delivery Temperature) down to UFC T (UFC delivery temperature) and CT (coil temperature) by setting process path. In addition, UFC can be used individually, by which cools strip to UFC T, and then to CT in air condition. Therefore, precisely and highly flexible control of UFC T and CT in cooling section is extremely important. To achieve precise control of UFC T and CT on run out table, many models and strategies are adopted. Physical model can achieve accuracy temperature control by describing heat transfer process. But for convenience industrial applications, the physical model is usually simplified.3,4) Statistical models have been developed to solve the situation that closely reflects reality as much accurately as possible.5) And there are also many artificial intelligence algorithm models,6 10) such as fuzzy control model, neural network model and instant base learning by k NN algorithm are utiResearch and Application of Model and Control Strategies for Hot Rolled Strip Cooling Process Based on Ultra Fast Cooling System
author_names: Zhen-lei Li

Yun-jie Li

G Yuan
title: Research and Application of Model and Control Strategies for Hot Rolled Strip Cooling Process Based on Ultra Fast Cooling System
year: 2020

abstract: Abstract. We investigated the interactions between the air pollutants and the structure of the urban boundary layer (UBL) over Beijing by using the data mainly obtained from the 325 m meteorological tower and a Doppler wind lidar during 1 4 December 2016. Results showed that the pollution episodes in this period could be characterized by low surface pressure, high relative humidity, weak wind, and temperature inversion. Compared with a clean daytime episode that took place on 1 December, results also showed that the attenuation ratio of downward shortwave radiation was about 5 24 and 63 in afternoon hours (from 12:00 to 14:00 local standard time, LST) on 2 4 December, respectively, while for the net radiation (Rn) attenuation ratio at the 140 m level of the 325 m tower was 3 27 and 68 The large reduction in Rn on 4 December was not only the result of the aerosols, but also clouds. Based on analysis of the surface energy balance at the 140 m level, we found that the sensible heat flux was remarkably diminished during daytime on polluted days and even negative after sunrise (about 07:20 LST) till 14:00 LST on 4 December. We also found that heat storage in the urban surface layer played an important role in the exchange of the sensible heat flux. Owing to the advantages of the wind lidar having superior spatial and temporal resolution, the vertical velocity variance could capture the evolution of the UBL well. It clearly showed that vertical mixing was negatively related to the concentrating of pollutants, and that vertical mixing would also be weakened by a certain quantity of pollutants, and then in turn worsened the pollution further. Compared to the clean daytime on 1 December, the maximums of the boundary layer height (BLH) decreased about 44 and 56 on 2 3 December, when the average PM2.5 (PM1) concentrations in afternoon hours (from 12:00 to 14:00 LST) were 44 (48) ug m 3 and 150 (120) ug m 3. Part of these reductions of the BLH was also contributed by the effect of the heat storage in the urban canopy.
author_names: Linlin Wang

Junkai Liu

Zhiqiu Gao

Yubin Li

Meng Huang

Sihui Fan

Xiaoye Zhang

Yuanjian Yang

Shiguang Miao

Han Zou

Yele Sun

Yong Chen

Ting Yang
title: Vertical observations of the atmospheric boundary layer structure over Beijing urban area during air pollution episodes
venue: Atmospheric Chemistry and Physics
year: 2019

abstract: Thermal interface materials (TIMs) used for bonding components are important for creating a thermally conductive path which improves heat dissipation. Low density, porous carbon foams are commonly used for thermal management applications and devices. Their high surface area to volume ratio enables cooling more effectively via different heat transfer methods. Many studies have adopted different methods to analytically or computationally analyze the effective thermal conductivity of carbon foams. Others have studied the participation of TIMs used in composite materials. However, very few studies have analyzed the microscale effects in heat transfer of the interaction between TIM and carbon foams. The amount of contact between a carbon foam and a bonded surface has hardly been reported in the literature. In this study, the carbon foam is deposited with thin layers of graphene until reaching the desired foam density; this type of foam is known as the graphitic foam. Graphene's highly anisotropic thermal properties result in high thermal conductivity in the planar direction but low in the normal direction. With these anisotropic thermal characteristics, the objective of this study is to determine the effect of TIM thickness on thermal conductivity of the graphitic foam. It was hypothesized that the direction which heat enters the graphitic foam and the size of the cross sectional area normal to the heat flux direction would affect the overall effective thermal conductivity. As commonly known, a gap created between ligands (foam structure) and the bonded surface would likely reduce the overall effective thermal conductivity. At the gap, heat is transferred via the TIMs or the graphitic foam through conduction, depending on if a direct contact exists between the graphitic foam and the bonded surface. The filler types used for the TIMs are hypothesized to play a critical role in the heat portion transferred via the TIMs. The heat transfer in 2 D becomes extremely complicated while anisotropic materials (graphene coating) and isotropic materials (TIMs) interact. Furthermore, the non uniform structure of the carbon foam introduces more complexity to the heat transfer at the interface. A computational model using ANSYS finite element program was developed in this study to help the analysis. The results demonstrate that the parameters at the interface can be optimized to improve the overall effective thermal conductivity of the interface.
author_names: Fangbing Lin

Eric C Anderssen

Raymond K Yee
title: Heat Transfer Interface to Graphitic Foam
venue: Volume 8: Heat Transfer and Thermal Engineering
year: 2019

abstract: Nucleate boiling is an efficient heat transfer mechanism that enables the dissipation of high heat fluxes at low temperature differences. Heat transfer phenomena during nucleate boiling are closely linked to the two phase flow morphology that evolves in time and based on the operating conditions. In particular, the critical heat flux, which is the upper limit for the nucleate boiling regime, can be triggered by hydrodynamic mechanisms resulting from interactions between the liquid and vapor phases. The aim of this thesis is to characterize the two phase flow morphology evolution during nucleate boiling at high heat fluxes in two configurations: pool boiling, and confined and submerged two phase jet impingement. The characterization is performed via non invasive, high speed optical based diagnostic tools. Experimental characterization of liquid vapor interfaces during boiling is often challenging because the rapidly evolving vapor structures are sensitive to invasive probes and multiple interfaces can occlude one another along a line of sight. In this thesis, a liquid vapor interface reconstruction technique based on high speed stereo imaging is developed. Images are filtered for feature enhancement and template matching is used for determining the correspondence of local features of the liquid vapor interfaces between the two camera views. A sampling grid is overlaid on the reference image and windows centered at each sampled pixel are compared with windows centered along the epipolar line in the target image to obtain a correlation signal. To enhance the signatures of true matches, the correlation signals for each sampled pixel are averaged over a short time ensemble correlation. The three dimensional coordinates of each matched pixel are determined via triangulation, which yields a set of points in the physical world representing the liquid vapor interface. The developed liquid vapor interface reconstruction technique is a high speed, flexible and non invasive alternative to the various existing methods for phase distribution mapping. This technique also has the potential to be combined with other optical based diagnostic tools, such as tomographic particle image velocimetry, to further understand the phase interactions.The liquid vapor interface reconstruction technique is used to characterize liquid vapor interfaces above the heated surface during nucleate pool boiling, where the textured interface resulting from the boiling phenomena and flow interactions near the heated surface is particularly suited for reconstruction. Application of the reconstruction technique to pool boiling at high heat fluxes produces a unique quantitative characterization of the liquid vapor interface morphology near heated surface. Analysis of temporal signals extracted from reconstructions indicate a clear transition in the nature of the vapor flow dynamics from a plume like vapor flow to a release mode dominated by vapor burst events. Further investigation of the vapor burst events allows identification of a characteristic morphology of the vapor structures that form above the surface that is associated to the square shape of the heat source. Vapor flow morphology characterization during pool boiling at high heat fluxes can be used to inform vapor removal strategies that delay the occurrence of the critical heat flux during pool boiling.As compared to pool boiling, nucleate boiling can be sustained up to significantly higher heat fluxes during two phase jet impingement. The increases in critical heat flux are explained via hydrodynamic mechanisms that have been debated in the literature. The connection between two phase flow morphology and the extension of nucleate boiling regime is investigated for a single subcooled jet of water that impinges on a circular heat source via high speed visualization from two synchronized top and side views of the confinement gap. When boiling occurs under subcooled exit flow conditions and at moderate heat fluxes, the regular formation and collapse of vapor structures that bridge the heated surface and the orifice plate is observed, which causes significant oscillations in the pressure drop across. Under saturated exit flow conditions, the vapor agglomerates in the confinement gap into a bowl like vapor structure that recurrently shrinks, due to vapor break off at the edge of the orifice plate, and replenishes due to vapor generation. The optical visualizations from the top of the confinement gap provide a unique perspective and indicate that the liquid jet flows downwards through the vapor structure, impinges on the heated surface, and then flows underneath the vapor structure, as a fluid wall jet the keeps the heated surface wetted such that discrete bubbles continue to nucleate. At high heat fluxes, intense vapor generation causes the fluid wall jet to transition from a bubbly to a churn like regime, and some liquid droplets are sheared off into the vapor structure. The origin of critical heat flux appears to result from a significant portion of the liquid in the wall jet being deflected off the surface, and the remaining liquid film on the surface drying out before reaching the edge of the heater.The flow morphology characterizations presented in this dissertation further the understanding of flow and heat transfer phenomena during nucleate boiling. In the pool boiling configuration, the vapor release process was quantitatively described; during two phase jet impingement, a possible mechanism for critical heat flux was identified. Opportunities for future work include the utilization of image processing techniques to extract quantitative measurements from two phase jet impingement visualizations. Also, the developed liquid vapor interface reconstruction technique can be applied to a boiling situation with a simpler liquid vapor interface geometry, such as film boiling, to generate benchmark data for validation and development of numerical models.
author_names: C Hernandez
title: Characterization of Two Phase Flow Morphology Evolution during Boiling via High Speed Visualization
year: 2019