Investigations of Biporous Wick Structure Dryout

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Qingjun Cai ◽  
Ya-Chi Chen
Keyword(s):  
Heat Flux ◽  
Heat Pipe ◽  
High Speed ◽  
Heat Fluxes ◽  
Large Temperature ◽  
Vapor Flow ◽  
High Heat Flux ◽  
Temperature Hysteresis ◽  
Liquid Transport ◽  
Wick Structure

Dryout in a heat pipe evaporator is caused by insufficient condensate supply through the wick structure. Dryout is generally considered a failure of the heat pipe operation. However, traditional dryout theory may not fully explain the heat and mass transport limitations in the biporous (biwick) wick structure due to new mass transfer mechanisms, such as liquid splash at high heat flux, and vapor bubble/jet occupation of liquid transport passages. This article investigates the dryout phenomenon in carbon nanotube (CNT) based biwick structure. The incipience and expansion of the dryout zone on the CNT biwick structure are visualized. Variation of the evaporator temperatures at various heat fluxes is measured to characterize the temperature responses on the biwick dryout. Results based on both visualization and measurement show that dryout of CNT biwick structures is affected by vapor flow induced droplet splash and vapor occupation of liquid transport passages, which reduces the liquid supply to the hottest region and creates a local dry zone. On the curves of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point during the heating period, and associated with the existence of a large temperature hysteresis in a heating and cooling cycle. Experimental measurement also shows that over 12% of the liquid by volume is lost without being phase changed, due to high-speed vapor flow induced liquid splash. Liquid splash and interactions between vapor and liquid flows also increase the pressure drop weight in the evaporator over the system loop and result in more notable heating area effect on biwick structures when compared with traditional monowick structures.

Numerical Investigation of a Transient Operation of a Heat Pipe With Experimental Validation

2004 ◽  
Author(s):  
Gustavo Gutierrez ◽  
Juan Catan˜o ◽  
Tien-Chien Jen
Keyword(s):  
Heat Flux ◽  
Heat Pipe ◽  
Stokes Equations ◽  
Solid Wall ◽  
Control Volume ◽  
Navier Stokes ◽  
Interface Temperature ◽  
Vapor Flow ◽  
Navier Stokes Equation ◽  
Wick Structure

In this paper, a full transient analysis of the performance of a heat pipe with a wick structure is performed. For the vapor flow, the conventional Navier-Stokes equations are used. For the liquid flow in the wick structure, which is modeled as a porous media, volume averaged Navier-Stokes equation are adopted. The energy equation is solved for the solid wall and wick structure of the heat pipe. The energy and momentum equations are coupled through the heat flux at the liquid-vapor interface that defines the suction and blowing velocities for the liquid and vapor flow. The evolution of the vapor-liquid interface temperature is coupled through the heat flux at this interface that defines the mass flux to the vapor and the new saturation conditions to maintain a fully saturation vapor all the time. A control volume approach is used in the development of the numerical scheme. A parametric study is conducted to study the effect of different parameters that affect the thermal performance of the heat pipe. And experimental setup is developed and numerical results are validated with experimental data. The results of this study will be useful for the heat pipe design and implementation in processes that are essentially transient and steady state conditions are not reached like for example drilling applications.

Pool Boiling Using Thin Enhanced Structures Under Top-Confined Conditions

2006 ◽  
Vol 128 (12) ◽  
pp. 1302-1311 ◽  
Author(s):  
Camil-Daniel Ghiu ◽  
Yogendra K. Joshi
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Pool Boiling ◽  
Single Layer ◽  
Temperature Limit ◽  
Heat Fluxes ◽  
The Other ◽  
Vapor Flow ◽  
Maximum Heat ◽  
Maximum Heat Flux

An experimental study of pool boiling using enhanced structures under top-confined conditions was conducted with a dielectric fluorocarbon liquid (PF 5060). The single layer enhanced structures studied were fabricated in copper and quartz, had an overall size of 10×10mm2, and were 1mm thick. The parameters investigated in this study were the heat flux (0.8-34W∕cm2) and the top space S(0-13mm). High-speed visualizations were performed to elucidate the liquid/vapor flow in the space above the structure. The enhancement observed for plain surfaces in the low heat fluxes regime is not present for the present enhanced structure. On the other hand, the maximum heat flux for a prescribed 85°C surface temperature limit increased with the increase of the top spacing, similar to the plain surfaces case. Two characteristic regimes of pool boiling have been identified and described: isolated flattened bubbles regime and coalesced bubbles regime.

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.

Development of a High Performance Vapor Chamber for High Heat Flux Applications

2008 ◽  
Author(s):  
Yuan Zhao ◽  
Chung-Lung Chen
Keyword(s):  
Heat Flux ◽  
High Performance ◽  
Experimental Tests ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Vapor Chamber ◽  
Porous Network ◽  
Heat Spreader ◽  
Wick Structure

This paper introduces a high performance vapor chamber heat spreader with a novel bi-dispersed wick structure. The main wick structure is a sintered porous network in a latticed pattern, which contains not only small pores to transport liquid by capillary forces, but also many slots to provide large passages to vent vapor from heated surfaces. The copper particles have a diameter of approximately 50 μm; they produce an effective pore radius of approximately 13 μm after sintering. The slots have a typical width of approximately 500 μm. Unlike traditional bi-dispersed wick structures, the latticed wick structures provide undisrupted liquid delivery passages and vapor escape channels and thus greatly improve the heat transfer performance. Preliminary experimental tests were conducted and the results were analyzed. It was shown by the experiments that vapor chamber heat spreaders with the latticed wicks present three times improvement on heat spreading performance, comparing with a solid copper heat spreader, and much improved capacity to handle hot spots with local heat fluxes exceeding 300 W/cm2, which will have great impacts on extending heat pipe technology from traditional low to medium heat fluxes to high heat flux applications.

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.

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.

Heat Flux Determination From Ultrasonic Pulse Measurements

2008 ◽  
Author(s):  
M. R. Myers ◽  
D. G. Walker ◽  
D. E. Yuhas ◽  
M. J. Mutton
Keyword(s):  
Heat Flux ◽  
Data Reduction ◽  
High Speed ◽  
Acoustic Pulse ◽  
Time Of Flight ◽  
Ultrasonic Pulse ◽  
Heat Fluxes ◽  
Measured Signal ◽  
Suitable Model ◽  
Ultrasonic Time

Ultrasonic time of flight measurements have been used to estimate the interior temperature of propulsion systems remotely. All that is needed is acoustic access to the boundary in question and a suitable model for the heat transfer along the path of the pulse train. The interior temperature is then deduced from a change in the time of flight and the temperature dependent velocity factor, which is obtained for various materials as a calibration step. Because the acoustic pulse samples the entire temperature distribution, inverse data reduction routines have been shown to provide stable and accurate estimates of the unknown temperature boundary. However, this technique is even more interesting when applied to unknown heat flux boundaries. Normally, the estimation of heat fluxes is even more susceptible to uncertainty in the measurement compared to temperature estimates. However, ultrasonic sensors can be treated as extremely high-speed calorimeters where the heat flux is directly proportional to the measured signal. Through some simple one-dimensional analyses, this work will show that heat flux is a more natural and stable quantity to estimate from ultrasonic time of flight. We have also introduced an approach for data reduction that makes use of a composite velocity factor, which is easier to measure.

Development of High Speed and High Temperature Atomic Layer Thermopiles

2019 ◽  
Author(s):  
Lakshya Bhatnagar ◽  
Guillermo Paniagua
Keyword(s):  
Heat Flux ◽  
High Temperature ◽  
High Speed ◽  
Cooling System ◽  
Numerical Study ◽  
Measurement Data ◽  
Atomic Layer ◽  
Heat Fluxes ◽  
Heat Transfer Analysis ◽  
Uncertainty Estimates

Abstract This work aims to provide a technique with which high frequency heat flux measurement data can be acquired in systems with high operational temperatures and high-speed flows with quantifiable and accurate uncertainty estimates. This manuscript presents the detailed calibration and application of an atomic layer thermopile, for heat fluxes with a frequency bandwidth of 0 to 1MHz. Two calibration procedures with a detailed uncertainty analysis. The first procedure consists using a laser to deliver radiation heat flux, while the second consists of a convective heat blowdown experiment. The use of this probe is demonstrated in a high-speed environment at Mach 2. The sensor effectively captures the passage of the normal shock wave and the values are compared with those computed using surface temperature measurement. Finally, a numerical study is carried out to design a cooling system that will allow the sensor to survive in high temperature conditions of 1273K while the sensor film is maintained at 323K. A two-dimensional axisymmetric conjugate heat transfer analysis is carried out to obtain the desired geometry.

Optimization of Refrigeration Systems for High-Heat-Flux Microelectronics

2008 ◽  
Author(s):  
P. E. Phelan ◽  
Y. Gupta ◽  
H. Tyagi ◽  
R. Prasher ◽  
J. Cattano ◽  
...  
Keyword(s):  
Heat Flux ◽  
Energy Consumption ◽  
System Design ◽  
High Heat ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
High Heat Flux ◽  
Vapor Compression ◽  
Refrigeration System ◽  
Compressor Inlet

Increasingly, military and civilian applications of electronics require extremely high heat fluxes, on the order of 1000 W/cm2. Thermal management solutions for these severe operating conditions are subject to a number of constraints, including energy consumption, controllability, and the volume or size of the package. Calculations indicate that the only possible approach to meeting this heat flux condition, while maintaining the chip temperature below 50 °C, is to utilize refrigeration. Here we report an initial optimization of the refrigeration system design. Because the outlet quality of the fluid leaving the evaporator must be held to approximately less than 20%, in order to avoid reaching critical heat flux, the refrigeration system design is dramatically different from typical configurations for household applications. In short, a simple vapor-compression cycle will require excessive energy consumption, largely because of the superheat required to return the refrigerant to its vapor state before the compressor inlet. A better design is determined to be a “two-loop” cycle, in which the vapor-compression loop is coupled thermally to a primary loop that directly cools the high-heat-flux chip.

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