Dryout Studies of Carbon Nanotube Bi-Porous Structure

2011 ◽  
Author(s):  
Qingjun Cai ◽  
Ya-Chi Chen ◽  
Chung-lung Chen
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat Flux ◽  
Carbon Nanotube ◽  
Heat Pipe ◽  
Maximum Temperature ◽  
High Heat Flux ◽  
Evaporator Temperature ◽  
Maximum Heat ◽  
Wick Structure

Dryout occurring in a heat pipe evaporator section is caused by insufficient coolant supply of wick structure, and generally considered as a failure mode of heat pipe operation. However, traditional dryout theory does not fit the bi-porous (bi-wick) wick structure research on the new mass transfer mechanism, such as liquid splash at high heat flux. The reported maximum heat flux on the similar wick structure may show a large deviation. Accordingly, dryout studies of bi-wick structures become critical and necessary to understand the limitation of the heat and mass transfer. In this article, carbon nanotube (CNT) clusters are used to investigate dryout of bi-wick structures. Within a closed system, evaporation and boiling phase change on CNT bi-wick structures is visualized to provide direct views on the occurrence and expansion of dryout zone. At the same time, the evaporator temperature variations versus heat flux are measured to characterize the temperature responses upon the bi-wick dryout. Investigations based on both visualization and measurement results show that dryout of CNT bi-wick structures are caused by insufficient liquid supply to create temperature elevation and in-plane heat transfer increase of the evaporator substrate. On the curvatures of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point on the heating section, and associated by the existence of large hysteresis of heat transfer performance. Numerical modeling of the temperature distribution on dried wick structure further indicates that traditional temperature measurement approaches are hardly to detect the occurrence of dryout and to provide the maximum temperature. High temperature hotspot on dried wick structure can be more destructive than temperature sensor measured.

High Heat Flux Phase Change on Silicon Wick Structures

2012 ◽  
Author(s):  
Qingjun Cai ◽  
Avijit Bhunia ◽  
Yuan Zhao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Test Chamber ◽  
Nucleate Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Maximum Heat ◽  
Wick Structure ◽  
Silicon Pillars

Silicon is the major material in IC manufacture. It has high thermal conductivity and is compatible with precision micro-fabrication. It also has decent thermal expansion coefficient to most semiconductor materials. These characteristics make it an ideally underlying material for fabricating micro/mini heat pipes and their wick structures. In this paper, we focus our research investigations on high heat flux phase change capacity of the silicon wick structures. The experimental wick sample is composed of silicon pillars 320μm in height and 30 ∼ 100μm in diameter. In a stainless steel test chamber, synchronized visualizations and measurements are performed to crosscheck experimental phenomena and data. Using the mono-wick structure with large silicon pillar of 100μm in diameter, the phase change on the silicon wick structure reaches its maximum heat flux at 1,130W/cm2 over a 2mm×2mm heating area. The wick structure can fully utilize the wick pump capability to supply liquid from all 360° directions to the center heating area. In contrast, the large heating area and fine silicon pillars 10μm in diameter significantly reduces liquid transport capability and suppresses generation of nucleate boiling. As a result, phase change completely relies on evaporation, and the CHF of the wick structure is reduced to 180W/cm2. An analytical model based on high heat flux phase change of mono-porous wick structures indicates that heat transfer capability is subjected to the ratio between the wick particle radius and the heater dimensions, as well as vapor occupation ratio of the porous volume. In contrast, phase change heat transfer coefficients of the wick structures essentially reflect material properties of wick structure and mechanism of two-phase interactions within wick structures.

Explorations of Carbon Nanotube Wick Structure for High Heat Flux Cooling

2008 ◽  
Author(s):  
Qingjun Cai ◽  
Chung-Lung Chen ◽  
Guangyong Xiong ◽  
Zhifeng Ren
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Carbon Nanotube ◽  
Microelectromechanical Systems ◽  
High Heat ◽  
Structure Design ◽  
Experimental Investigations ◽  
High Heat Flux ◽  
Multiwall Carbon Nanotube ◽  
Wick Structure

Multiwall carbon nanotube (MCNT) has high thermal conductivity, nano size pores and high capillary pressure. All these physical properties make it an ideal candidate as a wick structure in a micro sized heat pipe/spreader. In this paper, experimental investigations evaluate heat transfer performance of the carbon nanotube (CNT) wick and demonstrate its ability to handle high heat flux cooling. The CNT wick structure used for high heat flux experiments employs the bi-wick structure design to overcome high flow resistance in CNT clusters. The wick fabrication technique integrates both microelectromechanical systems (MEMS) patterning and thermal chemical vapor deposition (CVD) CNT growth processes. In high heat flux experiments, the CNT cluster functions as the first order wick structure and provides a large capillary force. The spacing among CNT clusters acts as the second order wick structure thus setting up low resistance liquid supply channels and vapor ventilation paths. Preliminary experiments are conducted in an open chamber system with vertical CNT bi-wick sample setup. Heat flux, as high as 400W/cm2, is demonstrated over 0.16mm2 heating area. Dryout was not observed, whereas the heater soft-bonding material fails at the higher testing heat flux. The experimental results indicate that the CNT bi-wick structure is capable of high heat flux cooling and promises to be the heat transfer element in new generation microelectronics cooling systems.

A Mathematical Modeling Method on Micro Heat Pipe with a Trapezium-Grooved Wick Structure

2010 ◽  
Vol 29-32 ◽  
pp. 1686-1694
Author(s):  
Xi Bing Li ◽  
Shi Gang Wang ◽  
Jian Hua Guo ◽  
Dong Sheng Li
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Chemical Engineering ◽  
High Heat ◽  
High Heat Flux ◽  
Micro Heat Pipe ◽  
Flow Limit ◽  
Wick Structure ◽  
Design And Manufacture

With heat flux increasing and cooling space decreasing in the products in microelectronics and chemical engineering, micro heat pipe has become an ideal heat radiator for products with high heat flux. Through analyzing the factors influencing the structure, strength and heat transfer limits of circular micro heat pipe with trapezium-grooved wick structure, the heat transfer models are established in this paper, including the models of viscous limit, sonic limit, entrainment limit, capillary limit, condensing limit, boiling limit, continuous flow limit and frozen startup limit. The study lays a powerful theoretical foundation for the design and manufacture of circular micro heat pipe with a trapezium-grooved wick structure.

Investigations of High Heat Flux Cooling Using Carbon Nanotube Bi-Wick Structure and Integrated Platinum Thermometer/Heater

2009 ◽  
Author(s):  
Qingjun Cai ◽  
Yuan Zhao ◽  
Chialun Tsai ◽  
Chung-lung Chen
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Carbon Nanotube ◽  
High Performance ◽  
Empty Space ◽  
High Heat ◽  
High Heat Flux ◽  
Resistance Variation ◽  
Wick Structure ◽  
Reliable Temperature

With the increase of power consumption in compact electronic devices, passive heat transfer cooling technologies with high heat flux characteristics are highly desired in microelectronics industries. Carbon nanotube (CNT) cluster/forest has high effective thermal conductivity, nano pore size and large porosity, which can be used as wick structure in a heat pipe heatspreader and provides high capillary force for high heat flux thermal management. In this research, investigations of high heat flux cooling of the CNT bi-wick structure are associated with the development of a reliable thermometer and high performance/interface free heater. A 100nm thick and 600μm wide Z-shaped platinum wire resistor is fabricated on the backside of a CNT sample substrate to heat a 2×2mm2 wick area. As a heater, it provides direct heating effect without thermal interface and is capable of over 800°C high temperature operation. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation with temperature after the annealing process is applied. The CNT sample substrate is silicon. The backside of the silicon substrate is thermally oxidized to create a 2μm thick and pinhole-free SiO2 layer so that the platinum heater and thermometer can survive from the server CNT growth environments and without any electrical leakage. For high heat flux cooling, the CNT bi-wick structure is composed of 250μm tall, 100μm wide stripe-like CNT clusters and 50μm empty space. Using 1×1cm2 CNT bi-wick samples, experiments are completed in both the open and saturated environments. Testing results of CNT bi-wick structure demonstrate 600W/cm2 heat transfer capacity and good thermal & mass transport characteristics in the nano level porous media.

Heat Transfer Enhancement in Compact Phase Change Microchannel Heat Exchangers for High Flux Laser Diodes

2018 ◽  
Author(s):  
Jensen Hoke ◽  
Todd Bandhauer ◽  
Jack Kotovsky ◽  
Julie Hamilton ◽  
Paul Fontejon
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Flow Boiling ◽  
Laser Diodes ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Maximum Heat ◽  
Average Maximum

Liquid-vapor phase change heat transfer in microchannels offers a number of significant advantages for thermal management of high heat flux laser diodes, including reduced flow rates and near constant temperature heat rejection. Modern laser diode bars can produce waste heat loads >1 kW cm−2, and prior studies show that microchannel flow boiling heat transfer at these heat fluxes is possible in very compact heat exchanger geometries. This paper describes further performance improvements through area enhancement of microchannels using a pyramid etching scheme that increases heat transfer area by ∼40% over straight walled channels, which works to promote heat spreading and suppress dry-out phenomenon when exposed to high heat fluxes. The device is constructed from a reactive ion etched silicon wafer bonded to borosilicate to allow flow visualization. The silicon layer is etched to contain an inlet and outlet manifold and a plurality of 40μm wide, 200μm deep, 2mm long channels separated by 40μm wide fins. 15μm wide 150μm long restrictions are placed at the inlet of each channel to promote uniform flow rate in each channel as well as flow stability in each channel. In the area enhanced parts either a 3μm or 6μm sawtooth pattern was etched vertically into the walls, which were also scalloped along the flow path with the a 3μm periodicity. The experimental results showed that the 6μm area-enhanced device increased the average maximum heat flux at the heater to 1.26 kW cm2 using R134a, which compares favorably to a maximum of 0.95 kw cm2 dissipated by the plain walled test section. The 3μm area enhanced test sections, which dissipated a maximum of 1.02 kW cm2 showed only a modest increase in performance over the plain walled test sections. Both area enhancement schemes delayed the onset of critical heat flux to higher heat inputs.

Thermal Analysis of Cold Plate for Direct Liquid Cooling of High Performance Servers

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
Bharath Ramakrishnan ◽  
Yaser Hadad ◽  
Sami Alkharabsheh ◽  
Paul R. Chiarot ◽  
Bahgat Sammakia
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Thermal Resistance ◽  
High Heat ◽  
High Heat Flux ◽  
Cold Plate ◽  
Liquid Cooling ◽  
Maximum Heat ◽  
Effective Heat ◽  
Cold Plates

Data center energy usage keeps growing every year and will continue to increase with rising demand for ecommerce, scientific research, social networking, and use of streaming video services. The miniaturization of microelectronic devices and an increasing demand for clock speed result in high heat flux systems. By adopting direct liquid cooling, the high heat flux and high power demands can be met, while the reliability of the electronic devices is greatly improved. Cold plates which are mounted directly on to the chips facilitate a lower thermal resistance path originating from the chip to the incoming coolant. An attempt was made in the current study to characterize a commercially available cold plate which uses warm water in carrying the heat away from the chip. A mock package mimicking a processor chip with an effective heat transfer area of 6.45 cm2 was developed for this study using a copper block heater arrangement. The thermo-hydraulic performance of the cold plates was investigated by conducting experiments at varying chip power, coolant flow rates, and coolant temperature. The pressure drop (ΔP) and the temperature rise (ΔT) across the cold plates were measured, and the results were presented as flow resistance and thermal resistance curves. A maximum heat flux of 31 W/cm2 was dissipated at a flow rate of 13 cm3/s. A resistance network model was used to calculate an effective heat transfer coefficient by revealing different elements contributing to the total resistance. The study extended to different coolant temperatures ranging from 25 °C to 45 °C addresses the effect of coolant viscosity on the overall performance of the cold plate, and the results were presented as coefficient of performance (COP) curves. A numerical model developed using 6SigmaET was validated against the experimental findings for the flow and thermal performance with minimal percentage difference.

Working Fluid Inventory Effect on Heat Transfer Performance of a Grooved Micro Heat Pipe

2013 ◽  
Vol 589-590 ◽  
pp. 559-564
Author(s):  
Xi Bing Li ◽  
Yun Shi Ma ◽  
Xun Wang ◽  
Ming Li
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Heat Transfer Performance ◽  
Mathematic Model ◽  
High Heat ◽  
Working Fluid ◽  
High Heat Flux ◽  
Transfer Performance ◽  
Flux Concentration

As a highly efficient heat transfer component, a micro heat pipe (MHP) has been widely applied to the situations with high heat flux concentration. However, a MHPs heat transfer performance is affected by many factors, among which, working fluid inventory has great influence on the security, reliability and frost resistance of its heat transfer performance. In order to determine the appropriate working fluid inventory for grooved MHPs, this paper first analyzed the working principle, major heat transfer limits and heat flux distribution law of grooved MHPs in electronic chips with high heat flux concentration, then established a mathematic model for the working fluid inventory in grooved MHPs. Finally, with distilled water being the working fluid, a series of experimental investigations were conducted at different temperatures to test the heat transfer performances of grooved MHPs, which were perfused with different inventories and with different adiabatic section lengths. The experimental results show that when the value of α is roughly within 0.40±0.05, a grooved MHP can acquire its best heat transfer performance, and the working fluid inventory can be determined by the proposed mathematic model. Therefore this study solves the complicated problem of determining appropriate working fluid inventory for grooved MHPs.

A Mathematical Modeling Method for Capillary Limit of Micro Heat Pipe with Sintered Wick

2011 ◽  
Vol 175 ◽  
pp. 335-341
Author(s):  
Xi Bing Li ◽  
Chang Long Yang ◽  
Gong Di Xu ◽  
Wen Yuan ◽  
Shi Gang Wang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Heat Dissipation ◽  
High Heat ◽  
Experimental Investigations ◽  
High Heat Flux ◽  
Micro Heat Pipe ◽  
Copper Powders ◽  
Capillary Limit

With heat flux increasing and cooling space decreasing in microelectronic and chemical products, micro heat pipe has become an ideal heat dissipation device in high heat-flux products. Through the analysis of its working principle, the factors that affect its heat transfer limits and the patterns in which copper powders are arrayed in circular cavity, this paper first established a mathematical model for the crucial factors in affecting heat transfer limits in a circular micro heat pipe with a sintered wick, i.e. a theoretical model for capillary limit, and then verified its validity through experimental investigations. The study lays a powerful theoretical foundation for designing and manufacturing circular micro heat pipes with sintered wicks.

Enhanced Pool Boiling Performance on Micro-, Nano-, and Hybrid-Structured Surfaces

2011 ◽  
Author(s):  
Qian Li ◽  
Wei Wang ◽  
Chris Oshman ◽  
Benoit Latour ◽  
Chen Li ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Thermal Management ◽  
Pool Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Maximum Heat ◽  
Structured Surfaces ◽  
Maximum Heat Transfer ◽  
Functional Components

Thermal management plays an important role in both high power electronics and energy conversion systems. A key issue in thermal management is the dissipation of the high heat flux generated by functional components. In this paper, various microstructures, nanostructures and hybrid micro/nano-structures were successfully fabricated on copper (Cu) surfaces, and the corresponding pool boiling heat transfer performance was systematically studied. It is found that the critical heat flux (CHF) of hybrid structured surfaces is about 15% higher than that of the surfaces with nanowires only and micro-pillars only. More importantly, the superheat at CHF for the hybrid structured surface is much smaller than that of the micro-pillared surface (about 35%), and a maximum heat transfer coefficient (HTC) of about 90,000W/m2K is obtained. Compared with the known best pool boiling performance on biporous media, a much larger HTC and much lower superheat at a heat flux of 250W/cm2 have been obtained on the novel hybrid-structured surfaces.

Mechanisms of Coupled Heat Transfer and Flow of High Heat Flux Pulsating Heat Pipe

2004 ◽  
Author(s):  
Wei Qu ◽  
Yantao Qu ◽  
Tongze Ma
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Latent Heat ◽  
High Heat ◽  
High Heat Flux ◽  
Sensible Heat ◽  
Pulsating Heat Pipe ◽  
Coupled Heat Transfer ◽  
Heating Section

The mechanisms of coupled heat transfer and flow are modeled to describe the looped pulsating heat pipe of high heat flux. The latent heat transfer produces the pressure difference between the heating section and cooling section. This can provide the operational driving force to overcome the total flow resistances. While the sensible heat transfer contributes more to the transferred power. The results demonstrate that the circulation flow velocity can balance the heat and mass transfers automatically. And the ratio of latent heat transfer to sensible heat transfer is within 30 percent.

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