Performance Evaluation of a Pump-Assisted, Capillary Two-Phase Cooling Loop

2009 ◽  
Vol 1 (2) ◽  
Author(s):  
Chanwoo Park ◽  
Aparna Vallury ◽  
Jon Zuo
Keyword(s):  
Heat Input ◽  
Heat Load ◽  
Fluid Transport ◽  
Active Flow Control ◽  
Working Fluid ◽  
Heat Sources ◽  
Two Phase ◽  
Relative Level ◽  
Maximum Heat ◽  
Two Phase Cooling

A hybrid (pump-assisted and capillary) two-phase loop (HTPL) is experimentally investigated to characterize its thermal performance under stepwise heat input conditions. An integration of mechanical pumping with capillary pumping is achieved by using planar evaporator(s) and a two-loop design separating liquid and vapor flows. The evaporator(s) use a sintered copper grooved wick bonded with a liquid screen artery. No active flow control of the mechanical pumping is required because of the autonomous capillary pumping due to the self-adjusting liquid menisci to variable heat inputs of the evaporators. Unlike other active two-phase cooling systems using liquid spray and microchannels, the HTPL facilitates a passive phase separation of liquid from vapor in the evaporator using capillary action, which results in a lower flow resistance of the single-phase flows than two-phase mixed flows in fluid transport lines. In this work, a newly developed planar form-factor evaporator with a boiling heat transfer area of 135.3 cm2 is used aiming for the power electronics with large rectangular-shaped heat sources. This paper presents the experimental results of the HTPLs with a single evaporator handling a single heat source and dual evaporators handling two separate heat sources, while using distilled water as the working fluid for both cases. For the single evaporator system, the temperature results show that the HTPL does not create a big temperature upset under a stepwise heat load with sudden power increases and decreases. The evaporator thermal resistance is measured to be as low as 0.5 K cm2/W for the maximum heat load of 4.0 kW. A cold-start behavior characterized by a big temperature fluctuation was observed at the low heat inputs around 500 W. The HTPL with dual evaporators shows a strong interaction between the evaporators under an asymmetric heat load of the total maximum heat input of 6.5 kW, where each evaporator follows a different heat input schedule. The temperatures of the dual-evaporator system follow the profile of the total heat input, while the individual heat inputs determine the relative level of the temperatures of the evaporators.

Experimental and Numerical Simulation for Thermal Investigation of Oscillating Heat Pipe Using VOF Model

2020 ◽  
Vol 38 (1A) ◽  
pp. 88-104
Author(s):  
Anwar S. Barrak ◽  
Ahmed A. M. Saleh ◽  
Zainab H. Naji
Keyword(s):  
Thermal Resistance ◽  
Heat Pipe ◽  
Heat Input ◽  
Cfd Simulation ◽  
Working Fluid ◽  
Maximum Deviation ◽  
Inlet Temperature ◽  
Maximum Heat ◽  
Oscillating Heat Pipe ◽  
Vof Model

This study is investigated the thermal performance of seven turns of the oscillating heat pipe (OHP) by an experimental investigation and CFD simulation. The OHP is designed and made from a copper tube with an inner diameter 3.5 mm and thickness 0.6 mm and the condenser, evaporator, and adiabatic lengths are 300, 300, and 210 mm respectively.  Water is used as a working fluid with a filling ratio of 50% of the total volume. The evaporator part is heated by hot air (35, 40, 45, and 50) oC with various face velocity (0.5, 1, and 1.5) m/s. The condenser section is cold by air at temperature 15 oC. The CFD simulation is done by using the volume of fluid (VOF) method to model two-phase flow by conjugating a user-defined function code (UDF) to the FLUENT code. Results showed that the maximum heat input is 107.75 W while the minimum heat is 13.75 W at air inlet temperature 35 oC with air velocity 0.5m/s. The thermal resistance decreased with increasing of heat input. The results were recorded minimum thermal resistance 0.2312 oC/W at 107.75 W and maximum thermal resistance 1.036 oC/W at 13.75W. In addition, the effective thermal conductivity increased due to increasing heat input.  The numerical results showed a good agreement with experimental results with a maximum deviation of 15%.

Active Hot Spot Cooling Controlled by Single-Sided Electrowetting-on-Dielectric (SEWOD)

2012 ◽  
Author(s):  
Sung-Yong Park ◽  
Jiangtao Cheng ◽  
Chung-Lung (C.-L. ) Chen
Keyword(s):  
Heat Transfer ◽  
Fluid Transport ◽  
Hot Spot ◽  
Two Phase ◽  
Electrowetting On Dielectric ◽  
Transfer Capability ◽  
Heat Transfer Capability ◽  
Spot Cooling ◽  
On Chip ◽  
Two Phase Cooling

Electrowetting-on-dielectric (EWOD) has attracted as one of the effective on-chip cooling technologies. It enables rapid transport of coolant droplets and heat transfer from target heat sources, while consuming extremely low power for fluid transport. However, a sandwiched configuration in conventional EWOD devices only allows sensible heat transfer, which very limits heat transfer capability of the device. In this paper, we report a novel single-sided EWOD (SEWOD) technology that enables two-phase cooling on a single-sided plate. As a result, heat transfer capability of the SEWOD device can be significantly enhanced. A complete set of droplet manipulation functions necessary for active hot spot cooling has been achieved on SEWOD. Hot spot surface modification to hydrophilic makes a droplet stick on a hot spot and maximize its contact area, greatly improving thermal rejection capability of the device. We have demonstrated two-phase cooling on SEWOD. With successive transportation of four droplets with a volume of 30 μL, the hot spot temperature that was initially heated up to 172°C was able to be stably maintained below 100 °C for 475s. This novel SEWOD-driven cooling technique promises to potentially function as a wickless vapor chamber with enhanced thermal managing capabilities.

Pool Boiling Heat Transfer With Binary Mixture on Open Microchannel Surface

2013 ◽  
Author(s):  
Ankit Kalani ◽  
Satish G. Kandlikar
Keyword(s):  
Binary Mixture ◽  
Heat Dissipation ◽  
Surface Effect ◽  
Pure Water ◽  
Pool Boiling ◽  
Electronics Cooling ◽  
Working Fluid ◽  
Two Phase ◽  
Maximum Heat ◽  
Fluid Medium

Two-phase cooling is considered an attractive option for electronics cooling due to its ability to dissipate large quantities of heat. In recent years, pool boiling has shown tremendous ability in high heat dissipation applications. Researchers have used various fluid medium for pool boiling including water, alcohol, refrigerants, nanofluids and binary mixture. In the current work, binary mixture of water with ethanol was chosen as the working fluid. Plain copper chip was used as the boiling surface. Effect of various concentrations of binary mixture was investigated. A maximum heat flux of 1720 kW/m2 at a wall superheat of 28°C was recorded for 15% ethanol in water. It showed a 1.5 fold increase in CHF over pure water.

Competitive Designs of Evaporator Top Cap of the Micro Loop Heat Pipe

2004 ◽  
Author(s):  
Praveen Kumar Arragattu ◽  
Frank M. Gerner ◽  
Priyanka Ponugoti ◽  
H. T. Henderson
Keyword(s):  
Finite Element ◽  
Pressure Drop ◽  
Finite Element Modeling ◽  
Heat Pipe ◽  
Temperature Drop ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Two Phase ◽  
Maximum Heat ◽  
Element Modeling

The Micro Loop Heat Pipe (LHP) is a two phase device that may be used to cool electronics, solar collectors and other devices in space applications. A LHP is a two-phase device with extremely high effective thermal conductivity that utilizes the thermodynamic pressure difference developed between the evaporator and condenser and capillary forces developed inside its wicked evaporator to circulate a working fluid through a closed loop. While previous experiments have shown reduction in chip temperature, maximum heat flux was less than theoretically predicted. This paper addresses the main problem with the past designs of top cap which has been the conduction of heat from the heat source to the primary wick. The new top cap design provides conduction pathways which enables the uniform distribution of heat to the wick. The provision of conduction pathways in the top cap increases the pressure losses and decreases the temperature drop. The feasible competitive designs of the top cap with conduction pathways from the fabrication point of view were discussed in detail. Calculation of pressure drop and temperature drop is essential for the determination of optimal solutions of the top cap. Approximate pressure drop was calculated for the top cap designs using simple 2-D microchannel principles. Finite element modeling was performed to determine the temperature drop in the conduction pathways. The conditions used for arriving at the optimal design solutions are discussed. A trapezoidal slot top cap design was chosen for fabrication as it was relatively easy to fabricate with available MEMS fabrication technologies. The exact pressure drop calculation was performed on the fabricated top cap using commercial flow solver FLUENT 6.1 with appropriate boundary conditions. The temperature drop calculation was performed by finite element modeling in ANSYS 6.1. Obtained values of pressure drop and temperature drop for fabricated trapezoidal slot top cap was found to be within the optimal limits.

Thermal Characteristics of a Circular Finned Thermosyphon Using Different Working Fluids

2014 ◽  
Vol 575 ◽  
pp. 322-328 ◽  
Author(s):  
Narayanan Alagappan ◽  
Narayanan Karunakaran
Keyword(s):  
Experimental Study ◽  
Ethylene Glycol ◽  
Thermal Resistance ◽  
Thermal Performance ◽  
Heat Input ◽  
Base Fluid ◽  
Thermal Characteristics ◽  
Working Fluid ◽  
Two Phase ◽  
Evaporation And Condensation

The two-phase closed thermosyphon (TPCT), which is essentially a gravity-assisted wickless heat pipe, utilizes the evaporation and condensation of the working fluid inside the TPCT to transport heat. This experimental study was carried out to understand the thermal performance of circular finned thermosyphon using nanofluid with alcohol and was analyzed, compared with alcohol and base fluid DI water. The concentration of nanoparticle used in this setup was 110mg/lit of TiO2combined with 0.2 ml of ethylene glycol. The heat input (Q) were 10W, 12W, 14 W and 16 W and the orientation 30°, 45°, 60° and 90°.The results demonstrate that TiO2nanofluid with 0.2 ml of ethylene glycol improves the performance through reduction in thermal resistance by 85.86%.

Design and Analysis of Ejector Powerplant System

2013 ◽  
Author(s):  
Menandro S. Berana ◽  
Edward T. Bermido
Keyword(s):  
High Pressure ◽  
Ambient Temperature ◽  
Waste Heat ◽  
Temperature Drop ◽  
Working Fluid ◽  
Low Grade ◽  
Heat Sources ◽  
Two Phase ◽  
Evaporator Temperature ◽  
Working Fluids

An ejector is a device with no moving components and is made up of four main parts: converging-diverging nozzle, suction chamber, mixing section and diffuser. It has become popular in refrigeration system as it gives the advantage of recovering expansion energy from high pressure difference into compression energy. In this study, the potential use of ejector in powerplants that use low-grade or low temperature heat sources was conceptualized and analytically investigated. A novel combination of the ejector and the organic Rankine cycle (ORC) was proposed. The driving fluid in the ejector of the proposed powerplant cycle is the high-pressure liquid in the separator that is just circulated back to the evaporator in the ORC. Further increase in turbine temperature drop (TTD), which can increase the power output and efficiency of the plant, can be achieved through expansion, mixing and recompression processes in the ejector. Ocean thermal energy conversion (OTEC), solar-boosted OTEC (SOTEC), solar-thermal, waste-heat driven, biomass and geothermal powerplants were considered in the analysis. Mathematical models in our previous studies were developed and used to calculate for nozzle and ejector parameters. The geometric profile of the ejector for optimization with categorized heat sources was determined. Isentropic, internally reversible, and irreversible two-phase nozzle expansions were analyzed. Two-phase flow calculations were continued in the mixing section. It was assumed that the constant-pressure mixing of the primary and secondary fluids occur at the hypothetical throat inside the constant-area section. Calculation for shock wave in the mixing section was also done. The diffuser was analyzed in a similar manner with the nozzle. Calculation for other components and plant efficiencies was finally conducted. Ammonia and propane which are both natural working fluids were used in the analysis. Evaporator temperature range from 293.15 K to 393.15 K and condenser and ambient temperatures range from 283.15 K to 308.15 K were used in the analysis. The lowest ambient temperature of 283.15 K was used for the OTEC and SOTEC powerplants. It was shown that ammonia and propane can operate up to 11 K and 12 K below the ambient temperature, respectively. Ejector efficiency ranged from 90 to 95% for both working fluids. The maximum efficiencies of the ejector powerplant were 19.2% for ammonia and 14.9% for propane, compared to 11.7% and 9.8% of the conventional ORC. It was analytically determined that the efficiency of the ejector powerplant is higher than that of the ORC powerplant for the same working fluid and conditions of the evaporator, condenser and the ambient.

Experimental Development and Computational Optimization of Flat Heat Pipes for CubeSat Applications

2017 ◽  
Vol 139 (2) ◽  
Author(s):  
Steven A. Isaacs ◽  
Diego A. Arias ◽  
Derek Hengeveld ◽  
Peter E. Hamlington
Keyword(s):  
Thermal Conductivity ◽  
Pure Copper ◽  
Working Fluid ◽  
Vapor Pressures ◽  
Two Phase ◽  
Experimental Development ◽  
Maximum Heat ◽  
Computational Optimization ◽  
Major Bottleneck ◽  
And Performance

Due to the compact and modular nature of CubeSats, thermal management has become a major bottleneck in system design and performance. In this study, we outline the development, initial testing, and modeling of a flat, conformable, lightweight, and efficient two-phase heat strap called FlexCool, currently being developed at Roccor. Using acetone as the working fluid, the heat strap has an average effective thermal conductivity of 2149 W/m K, which is approximately five times greater than the thermal conductivity of pure copper. Moreover, the heat strap has a total thickness of only 0.86 mm and is able to withstand internal vapor pressures as high as 930 kPa, demonstrating the suitability of the heat strap for orbital environments where pressure differences can be large. A reduced-order, closed-form theoretical model has been developed in order to predict the maximum heat load achieved by the heat strap for different design and operating parameters. The model is validated using experimental measurements and is used here in combination with a genetic algorithm to optimize the design of the heat strap with respect to maximizing heat transport capability.

Development and Experimental Validation of a Micro/Nano Thermal Ground Plane

2011 ◽  
Author(s):  
H. Peter J. de Bock ◽  
Shakti Chauhan ◽  
Pramod Chamarthy ◽  
Chris Eastman ◽  
Stanton Weaver ◽  
...  
Keyword(s):  
Experimental Validation ◽  
Ground Plane ◽  
Electronics Cooling ◽  
Heat Pipes ◽  
Working Fluid ◽  
Heat Sources ◽  
Two Phase ◽  
Evaporation And Condensation ◽  
Two Phase Heat Transfer ◽  
Thermal Ground Plane

Heat pipes are commonly used in electronics cooling applications to spread heat from a concentrated heat source to a larger heat sink. Heat pipes work on the principles of two-phase heat transfer by evaporation and condensation of a working fluid. The amount of heat that can be transported is limited by the capillary and hydrostatic forces in the wicking structure of the device. Thermal ground planes are two-dimensional high conductivity heat pipes that can serve as thermal ground to which heat can be rejected by a multitude of heat sources. As hydrostatic forces are dependent on gravity, it is commonly known that heat pipe and thermal ground plane performance is orientation dependent. The effect of variation of gravity force on performance is discussed and the development of a miniaturized thermal ground plane for high g operation is described. In addition, experimental results are presented from zero to −10g acceleration. The study shows and discusses that minimal orientation or g-force dependence can be achieved if pore dimensions in the wicking structure can be designed at micro/nano-scale dimensions.

A Dual-Source Organic Rankine Cycle (DORC) for Improved Efficiency in Conversion of Dual Low- and Mid-Grade Heat Sources

2009 ◽  
Author(s):  
F. David Doty ◽  
Siddarth Shevgoor
Keyword(s):  
Heat Source ◽  
Heat Input ◽  
Temperature Limit ◽  
Working Fluid ◽  
Inlet Temperature ◽  
Low Grade ◽  
Heat Sources ◽  
Reaction Products ◽  
Pinch Point ◽  
Low Grade Heat

Detailed thermodynamic and systems analyses show that a novel hybrid cycle, in which a low-grade (and low-cost) heat source (340 K to 460 K) provides the boiling enthalpy and some of the preheating while a mid-grade source (500 K to 800 K) provides the enthalpy for the final superheating, can achieve dramatic efficiency and cost advantages. Four of the more significant differences from prior bi-level cycles are that (1) only a single expander turbine (the most expensive component) is required, (2) condenser pressures are much higher, (3) the turbine inlet temperature (even with a low-grade geothermal source providing much of the energy) may be over 750 K, and (4) turbine size is reduced. The latent heat of vaporization of the working fluid and the differences in specific heats between the liquid and vapor phases make full optimization (approaching second-law limits) impossible with a single heat source. When two heat sources are utilized, this problem may be effectively solved — by essentially eliminating the pinch point. The final superheater temperature must also be increased, and novel methods have been investigated for increasing the allowable temperature limit of the working fluid by 200 to 350 K. The usable temperature limit of light alkanes may be dramatically increased by (1) accommodating hydrogen evolution from significant dehydrogenation; (2) periodically or continually removing undesired reaction products from the fluid; (3) minimizing the fraction of time the fluid spends at high temperatures. Detailed simulation results are presented for the case where (1) the low-grade heat source (such as geothermal) is 400 K and (2) the mid-grade Concentrated Solar Power (CSP) heat source is assumed to be 720 K. For an assumed condensing temperature of 305 K and working fluid flow rate of 100 kg/s, preliminary simulations give the following: (1) low-grade heat input is 25 MWT; (2) mid-grade heat input is 24 MWT; (3) the electrical output power is 13.5 MWE; and (4) the condenser rejection is only 35 MWT. For comparison, with a typical bi-level ORC generating similar power from this geothermal source alone, the low-grade heat requirement would be ∼100 MWT.

Experimental Analysis of Dual-Evaporators Hybrid Two-Phase Cooling Loop

2012 ◽  
Author(s):  
Chanwoo Park ◽  
Michael Crepinsek
Keyword(s):  
Experimental Analysis ◽  
Heat Input ◽  
Total Heat ◽  
Two Phase ◽  
Mechanical Pump ◽  
The Individual ◽  
Cooling Loop ◽  
Two Phase Cooling

A mechanical pump-assisted and capillary-driven (hybrid) two-phase cooling loop, with dual-evaporators in the same loop placed in parallel and series, was constructed to experimentally investigate the performance of the multi-evaporators cooling loop. This paper discusses various heat input experiments using the dual-evaporators loop that were tested up to 1200 Watts, or 600 Watts (102 W/cm2) for each evaporator. Difficulties and limitations experienced with both parallel and series tests are discussed. It is found from the tests that the total heat inputs in the system determine the system temperatures and pressures and the individual heat input to each evaporator determines the evaporator temperatures. Setting up the evaporators in parallel allows for more cooling than series.

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