scholarly journals Investigation of Loop Heat Pipe Survival and Restart After Extreme Cold Environment Exposure

2009 ◽  
Author(s):  
Eric Golliher ◽  
Jentung Ku ◽  
Anthony Licari ◽  
James Sanzi
Keyword(s):  
Heat Pipe ◽  
Freezing Point ◽  
Thermal Control ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
The Moon ◽  
Novel Approach ◽  
Condenser Temperature ◽  
Extreme Cold ◽  
Compensation Chamber

NASA plans human exploration near the South Pole of the Moon, and other locations where the environment is extremely cold. This paper reports on the heat transfer performance of a loop heat pipe exposed to extreme cold under the simulated reduced gravitational environment of the Moon. A common method of spacecraft thermal control is to use a loop heat pipe with ammonia working fluid. Typically, a small amount of heat is provided either by electrical heaters or by environmental design, such that the loop heat pipe condenser temperature never drops below the freezing point of ammonia. The concern is that a liquid-filled, frozen condenser would not re-start, or that a thawing condenser would damage the tubing due to the expansion of ammonia upon thawing. This paper reports the results of an experimental investigation of a novel approach to avoid these problems. The loop heat pipe compensation chamber is conditioned such that all the ammonia liquid is removed from the condenser and the loop heat pipe is non-operating. The condenser temperature is then reduced to below that of the ammonia freezing point. The loop heat pipe is then successfully re-started.

Thermal Performance of a Loop Heat Pipe Having Polypropylene Wick in a Flat Evaporator

2005 ◽  
Author(s):  
Joon Hong Boo ◽  
Won Bok Chung
Keyword(s):  
Heat Pipe ◽  
Thermal Performance ◽  
Thermal Load ◽  
Thermal Control ◽  
Working Fluid ◽  
Small Scale ◽  
Loop Heat Pipe ◽  
Horizontal Position ◽  
Condenser Temperature ◽  
Flat Evaporator

A small-scale loop heat pipe (LHP) with polypropylene (PP) wick was fabricated and tested for its thermal performance. The container and tubing of the system were made of stainless steel and several working fluids were used including methanol, ethanol, acetone, and ammonia. The heater and the evaporator were sized so that the system can be applied to a local thermal control including electronics cooling. The heating area was 35 mm × 35 mm and there were nine axial grooves in the flat evaporator (40 by 50 mm) to provide a vapor passage. The pore size of the polypropylene wick inside the evaporator was varied from 0.5 μm to 25 μm. The size of condenser was 40 mm (W) × 50 mm (L) in which ten coolant paths were provided. The inner diameters of liquid and vapor transport lines were 2.0 mm and 4.0 mm, respectively and the length of the two lines was 0.5 m each. The start-up transient as well as steady-state operation was investigated with maximum system operating temperature of 90°C, which was imposed to protect the PP from permanent deformation. The minimum thermal load of 10 W (0.8 W/cm2) and maximum thermal load of 80 W (6.5 W/cm2) were achieved using methanol as working fluid with the condenser temperature of 20°C at horizontal position. For a LHP with ammonia as working fluid, the minimum thermal load of 1 W and maximum thermal load of 87 W (7.1 W/cm2) were achieved for condenser temperature of 0°C at horizontal position. The minimum system thermal resistance was 0.65 K/W.

Investigation of Loop Heat Pipe Startup Using Liquid Core Visualization

2008 ◽  
Author(s):  
B. P. d’Entremont ◽  
J. M. Ochterbeck
Keyword(s):  
Heat Pipe ◽  
Liquid Core ◽  
High Heat ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Two Phase ◽  
Buoyancy Driven Flows ◽  
Compensation Chamber ◽  
Fill Level ◽  
Heat Loads

In this investigation, a Loop Heat Pipe (LHP) evaporator has been studied using a borescope inserted through the compensation chamber into the liquid core. This minimally intrusive technique allows liquid/vapor interactions to be observed throughout the liquid core and compensation chamber. A low conductivity ceramic was used for the wick and ammonia as the working fluid. Results indicate that buoyancy driven flows, both two-phase and single-phase, play essential roles in evacuating excess heat from the core, which explains the several differences in performance between horizontal and vertical orientations of the evaporator. This study also found no discernable effect of the pre-start fill level of the compensation chamber on thermal performance during startup at moderate and high heat loads.

Experimental Investigation of the Miniature Loop Heat Pipe With Flat Evaporator

2005 ◽  
Author(s):  
Randeep Singh ◽  
Aliakbar Akbarzadeh ◽  
Masataka Mochizuki ◽  
Thang Nguyen ◽  
Vijit Wuttijumnong
Keyword(s):  
Thermal Resistance ◽  
Heat Pipe ◽  
Thermal Control ◽  
Capillary Forces ◽  
High Heat ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Wick Structure ◽  
Flat Evaporator ◽  
Heat Loads

Loop heat pipe (LHP) is a very versatile heat transfer device that uses capillary forces developed in the wick structure and latent heat of evaporation of the working fluid to carry high heat loads over considerable distances. Robust behaviour and temperature control capabilities of this device has enable it to score an edge over the traditional heat pipes. In the past, LHPs has been invariably assessed for electronic cooling at large scale. As the size of the thermal footprint and available space is going down drastically, miniature size of the LHP has to be developed. In this paper, results of the investigation on the miniature LHP (mLHP) for thermal control of electronic devices with heat dissipation capacity of up to 70 W have been discussed. Copper mLHP with disk-shaped flat evaporator 30 mm in diameter and 10 mm thickness was developed. Flat evaporators are easy to attach to the heat source without any need of cylinder-plane-reducer saddle that creates additional thermal resistance in the case of cylindrical evaporators. Wick structure made from sintered nickel powder with pore size of 3–5 μm was able to provide adequate capillary forces for the continuos circulation of the working fluid, and successfully transport heat load at the required distance of 60 mm. Heat was transferred using 3 mm ID copper tube with vapour and liquid lines of 60 mm and 200 mm length respectively. mLHP showed very reliable start up at different heat loads and was able to achieve steady state without any symptoms of wick dry-out. Tests were conducted on the mLHP with evaporator and condenser at the same level. Total thermal resistance, R total of the mLHP came out to be in the range of 1–4°C/W. It is concluded from the outcomes of the investigation that mLHP with flat evaporator can be effectively used for the thermal control of the electronic equipments with restricted space and high heat flux chipsets.

Steady-State Operation of a Loop Heat Pipe: Network Thermofluid Model and Results

2003 ◽  
Author(s):  
Nima Atabaki ◽  
B. Rabi Baliga
Keyword(s):  
Steady State ◽  
Heat Pipe ◽  
Horizontal Tube ◽  
Operating Conditions ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Sintered Metal ◽  
Steady State Operation ◽  
Transport Line ◽  
Compensation Chamber

A network thermofluid model of a loop heat pipe (LHP) operating under steady-state conditions is presented. Attention is focused on a simple LHP, with one evaporator, a vapor transport line, a single condenser, a liquid transport line, and a compensation chamber. The evaporator is an internally grooved circular pipe, with a cylindrical wick installed on its inner surface. The wick is made of a sintered metal. The condenser is a horizontal tube covered with a high-thermal-conductivity sleeve, and the outer temperature of the sleeve is maintained at a constant sink temperature. Quasi one-dimensional mathematical models of the fluid flow and heat transfer in each of the elements of the LHP, and collectively of the entire LHP, are proposed and discussed. The working fluid considered in this work is ammonia, but the proposed model can work with any suitable fluid. Results pertaining to the LHP performance for a range of operating conditions are presented, compared (qualitatively) to corresponding results of an earlier experimental investigation in the literature, and discussed.

scholarly journals Development of a Compensation Chamber for Use in a Multiple Condenser Loop Heat Pipe

2013 ◽  
Author(s):  
Nicholas A. Roche ◽  
Martin Cleary ◽  
Teresa B. Peters ◽  
Evelyn N. Wang ◽  
John G. Brisson
Keyword(s):  
Heat Pipe ◽  
Heat Sink ◽  
High Performance ◽  
Computational Simulation ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Operating Characteristics ◽  
Side Pressure ◽  
Compensation Chamber ◽  
Dry Out

We report the design and analysis of a novel compensation chamber for use in PHUMP, a multiple condenser loop heat pipe (LHP) capable of dissipating 1000 W. The LHP is designed for integration into a high performance air-cooled heat sink to address thermal management challenges in advanced electronic systems. The compensation chamber is integrated into the evaporator of the device and provides a region for volumetric expansion of the working fluid over a range of operating temperatures. Additionally, the compensation chamber serves to set the liquid side pressure of the device, preventing both flooding of the condensers and dry out of the evaporator. The compensation chamber design was achieved through a combination of computational simulation using COMSOL Multiphysics and models developed based on experimental work of previous designs. The compensation chamber was fabricated as part of the evaporator using Copper and Monel sintered wicks with various particle sizes to achieve the desired operating characteristics. Currently, the compensation chamber is being incorporated into a multiple condenser LHP for a high performance air-cooled heat sink.

Minimizing the Wick Thickness in a Planar Microscale Loop Heat Pipe Using Efficient Thermodynamic Design

2011 ◽  
Author(s):  
Navdeep S. Dhillon ◽  
Jim C. Cheng ◽  
Albert P. Pisano
Keyword(s):  
Heat Flow ◽  
Heat Pipe ◽  
Thermal Characteristics ◽  
Heat Pipes ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Two Phase ◽  
Loop Heat Pipes ◽  
Evaporator Section ◽  
Compensation Chamber

Theoretical and numerical thermodynamic analysis of the evaporator section of a planar microscale loop heat pipe is presented, to minimize the permissible wick thickness in such a device. In conventional cylindrical loop heat pipes, a minimum wick thickness is required in order to reduce parasitic heat flow, and prevent vapor leakage, into the compensation chamber. By taking advantage of the possibilities allowed by microfabrication techniques, a planar evaporator/compensation chamber design topology is proposed to overcome this limitation, which will enable wafer-based loop heat pipes with device thicknesses on the order of a millimeter or less. Thermodynamic principles governing two-phase flow of the working fluid in a loop heat pipe are analyzed to elucidate the fundamental requirements that would characterize the startup and steady state operation of a planar phase-change device. A three dimensional finite element thermal-fluid solver is implemented to study the thermal characteristics of the evaporator section and compensation chamber regions of a planar vertically wicking micro-columnated loop heat pipe. The use of in-plane thermal conduction barriers to reduce parasitic heat flow into the compensation chamber is demonstrated.

A Flow Visualization Study on the Temperature Oscillations Inside a Loop Heat Pipe With Flat Evaporator

2013 ◽  
Author(s):  
Dongchuan Mo ◽  
Guansheng Zou ◽  
Shushen Lu ◽  
L. Winston Zhang
Keyword(s):  
Flow Visualization ◽  
Heat Pipe ◽  
Temperature Oscillation ◽  
Heating Power ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Interface Motion ◽  
Temperature Oscillations ◽  
Flat Evaporator ◽  
Compensation Chamber

This paper presents a flow visualization study on the temperature oscillations inside a loop heat pipe in order to gain a better understanding of its heat transfer characteristics. A flat loop heat pipe (FLHP) with a flat evaporator instead of a typical cylindrical evaporator was built using copper as the shell and water as the working fluid. An experimental setup was designed by using the transparent material instead of copper in some parts of the FLHP. The experiment results showed that there were at least three different flow patterns in the vapor line as the heating power increased. The temperatures in different locations of the loop oscillated even when the heating power was kept constant. The largest amplitude of the temperature oscillation in the loop was located at the condenser outlet. It was found that the temperature oscillation at the condenser outlet could be divided into two types, one with smaller amplitudes and the other with larger amplitudes. The smaller amplitude temperature oscillations were always there when the heating power was increased step by step, while the larger amplitude temperature oscillations would disappear initially and show up later. Finally, the location of the vapor/liquid interface inside the condenser varied with the temperature oscillations, resulting in liquid/vapor interface motion in the compensation chamber.

A Hybrid CFD-Mathematical Model for Simulation of a MEMS Loop Heat Pipe

2004 ◽  
Author(s):  
M. Ghajar ◽  
J. Darabi ◽  
N. Crews
Keyword(s):  
Pressure Drop ◽  
Heat Pipe ◽  
Heat Transport ◽  
Capillary Tube ◽  
Heat Rate ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Two Phase ◽  
Transport Distance ◽  
Compensation Chamber

A Hybrid CFD-Mathematical (HyCoM) model was developed to predict the performance of a Micro Loop Heat Pipe (MLHP) as a function of input heat rate. A micro loop heat pipe is a passive two-phase heat transport device, consisting of microevaporator, microcondenser, micro-compensation chamber (CC), and liquid and vapor lines. A CFD model was incorporated into a loop solver code to identify heat leak to the CC. Two-phase pressure drop in the condenser was calculated by several two phase correlations and results were compared [2]. Capillary tube correlations [3] were used for pressure drop calculations in fluid lines. Effects of working fluid and change in geometry were studied. For a heat transport distance of 10 mm, the base model MLHP was 50mm long, 16mm wide and 1mm thick. In the base model, widths of the grooves, liquid and vapor lines, evaporator, and condenser were 55μm, 200μm, 750μm, 2mm, and 4mm respectively.

Experimental Investigation of Heat Transfer Performance of a Miniature Loop Heat Pipe with Flat Evaporator

2011 ◽  
Vol 71-78 ◽  
pp. 3806-3809
Author(s):  
Xian Feng Zhang ◽  
Shuang Feng Wang
Keyword(s):  
Heat Pipe ◽  
Heat Load ◽  
Temperature Oscillation ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Operating Characteristics ◽  
Maximum Heat ◽  
Power Cycle ◽  
Flat Evaporator ◽  
Compensation Chamber

The present work experimentally investigated the operating characteristics of a miniature loop heat pipe (LHP) under different power cycle. The miniature LHP with flat evaporator of 8mm thick is made of copper. The evaporator with sintered copper power wick is in series structure with compensation chamber. Water is working fluid. It is found that the LHP can start up at heat load of 15W with temperature oscillation and the maximum heat load is 160W with Rl=0.068°C/W. The LHP operates unstably under low heat load. The oscillating frequency of temperature rises with heat load increased. The operating performance of the LHP is affected by the power cycle.

Operating Ranges of the Planar Loop Heat Pipe Under Non-Vacuum Conditions

2005 ◽  
Author(s):  
Junwoo Suh ◽  
Ahmed Shuja ◽  
Praveen Medis ◽  
Srinivas Parimi ◽  
Frank M. Gerner ◽  
...  
Keyword(s):  
Heat Pipe ◽  
Heat Dissipation ◽  
Energy Transport ◽  
Technical Conference ◽  
Open Loop ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Two Phase ◽  
Vapor Line ◽  
Compensation Chamber

As the trend of high throughput in small packages continues, the heat dissipation becomes a very critical design issue in electronic devices and spacecrafts. The two phase loop heat pipe utilizes the latent heat of working fluid. It consists of an evaporator, compensation chamber, condenser, and liquid and vapor line. The primary wick used as a core part to circulate the working fluid is located in the evaporator. The planar loop heat pipe uses coherent porous silicon (CPS) wick as opposed to the conventional cylindrical configuration, which uses a sintered amorphous metal wick. The clear evaporator machined from Pyrex glass and transparent silicone tubes were utilized to monitor the complex phenomena which occur in the evaporator. Tests were conducted under the non-vacuum condition without a secondary wick. DI-water was used as a working fluid. Like an open loop test previously conducted, there was an operating range in which the liquid could be properly pumped from the compensation chamber to the vapor line under the pumping motion. In this device, more than 6 Watts could be convected from the evaporator to the ambient. Therefore circulation was not observed until powers greater than 6 Watts. There was a circulation of working fluid occurring due to energy transport within the loop when the input power was from 7.94 Watts to 17.6 Watts. The quantity of heat transportation to the loop was calculated by acquiring the empirical heat transfer coefficient. From this calculation it was found that, roughly, 12.1 Watts was transported to the loop and 5.51 Watts was convected to the ambient from the evaporator itself when the applied power was 15.27 Watts.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.

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