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.

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.

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.

Heat Transfer Characteristics of a Loop Heat Pipe With Flat Evaporator Having a Bypass Line

2006 ◽  
Author(s):  
J. H. Boo ◽  
E. G. Jung
Keyword(s):  
Heat Pipe ◽  
Thermal Load ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Thermal Loads ◽  
Load Range ◽  
Loop Heat Pipes ◽  
Start Up ◽  
Bypass Line ◽  
Flat Evaporator

It is common for loop heat pipes to experience start-up problems under low thermal loads. The start-up characteristic of a loop heat pipe with flat evaporator was investigated experimentally in this study. A bypass line was attached to the evaporator to alleviate the difficulties associated with startup. Axial grooves were provided in the flat evaporator to serve as a vapor passage. The evaporator and condenser had dimensions of 40 mm (W) by 50 mm (L). Coolant paths were machined inside the condenser. 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 thermal load range was up to 80 W using methanol as a working fluid with the condenser temperature of 10°C at horizontal position. The results showed that a minimum thermal load for the startup was lowered by 37% when the bypass line was employed. However, a periodical performance variation was observed once the startup was achieved under very low thermal loads.

scholarly journals A Novel Analytical Modeling of a Loop Heat Pipe Employing Thin-Film Theory: Part II—Experimental Validation

Energies ◽  
2019 ◽  
Vol 12 (12) ◽  
pp. 2403 ◽  
Author(s):  
Eui Guk Jung ◽  
Joon Hong Boo
Keyword(s):  
Steady State ◽  
Heat Pipe ◽  
Film Theory ◽  
Design Parameters ◽  
Working Fluid ◽  
Coolant Temperature ◽  
Loop Heat Pipe ◽  
Analysis Model ◽  
Proposed Model ◽  
Flat Evaporator

Part I of this study introduced a mathematical model capable of predicting the steady-state performance of a loop heat pipe (LHP) with enhanced rationality and accuracy. Additionally, investigation of the effect of design parameters on the LHP thermal performance was also reported in Part I. The objective of Part II is to experimentally verify the utility of the steady-state analytical model proposed in Part I. To this end, an experimental device comprising a flat-evaporator LHP (FLHP) was designed and fabricated. Methanol was used as the working fluid, and stainless steel as the wall and tubing-system material. The capillary structure in the evaporator was made of polypropylene wick of porosity 47%. To provide vapor removal passages, axial grooves with inverted trapezoidal cross-section were machined at the inner wall of the flat evaporator. Both the evaporator and condenser components measure 40 × 50 mm (W × L). The inner diameters of the tubes constituting the liquid- and vapor-transport lines measure 2 mm and 4 mm, respectively, and the lengths of these lines are 0.5 m. The maximum input thermal load was 90 W in the horizontal alignment with a coolant temperature of 10 °C. Validity of the said steady-state analysis model was verified for both the flat and cylindrical evaporator LHP (CLHP) models in the light of experimental results. The observed difference in temperature values between the proposed model and experiment was less than 4% based on the absolute temperature. Correspondingly, a maximum error of 6% was observed with regard to thermal resistance. The proposed model is considered capable of providing more accurate performance prediction of an LHP.

Effect of Wick Characteristics on the Thermal Performance of the Miniature Loop Heat Pipe

2009 ◽  
Vol 131 (8) ◽  
Author(s):  
Randeep Singh ◽  
Aliakbar Akbarzadeh ◽  
Masataka Mochizuki
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Thermal Performance ◽  
Working Fluid ◽  
Vapor Flow ◽  
Loop Heat Pipe ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics

Two phase heat transfer devices based on the miniature version of loop heat pipe (LHP) can provide very promising cooling solutions for the compact electronic devices due to their high heat flux management capability and long distance heat transfer with minimal temperature losses. This paper discusses the effect of the wick properties on the heat transfer characteristics of the miniature LHP. The miniature model of the LHP with disk-shaped evaporator, 10 mm thick and 30 mm disk diameter, was designed using copper containment vessel and water as the working fluid, which is the most acceptable combination in electronic cooling applications. In the investigation, wick structures with different physical properties including thermal conductivity, pore radius, porosity, and permeability and with different structural topology including monoporous or biporous evaporating face were used. It was experimentally observed that copper wicks are able to provide superior thermal performance than nickel wicks, particularly for low to moderate heat loads due to their low heat conducting resistance. With monoporous copper wick, maximum evaporator heat transfer coefficient (hev) of 26,270 W/m2 K and evaporator thermal resistance (Rev) of 0.06–0.10°C/W were achieved. For monoporous nickel wick, the corresponding values were 20,700 W/m2 K for hev and 0.08–0.21°C/W for Rev. Capillary structure with smaller pore size, high porosity, and high permeability showed better heat transfer characteristics due to sufficient capillary pumping capability, low heat leaks from evaporator to compensation chamber and larger surface area to volume ratio for heat exchange. In addition to this, biporous copper wick structure showed much higher heat transfer coefficient of 83,787 W/m2 K than monoporous copper wick due to improved evaporative heat transfer at wick wall interface and separated liquid and vapor flow pores. The present work was able to classify the importance of the wick properties in the improvement of the thermal characteristics for miniature loop heat pipes.

Thermal Performance of the Mini-Loop Heat Pipe (LHP)

2009 ◽  
Author(s):  
K. Tanaka ◽  
M. Katsuta ◽  
Y. Ohuchi ◽  
K. Saitho
Keyword(s):  
Heat Transfer ◽  
Heat Pipe ◽  
Thermal Performance ◽  
Physical Development ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Transfer Capability ◽  
Heat Transfer Capability ◽  
Basic Equations ◽  
Test Result

In this paper, we describe the outline of experimental manufacture of the LHP. This LHP is made of a copper. The evaporator is φ19×95mm, the vapor tube is φ5×300mm, the condenser is φ3.5×600mm and the liquid tube is φ 3.5×300mm. The wick is made of the sintering copper. The working fluid is methanol. Second, we describe the test result of heat transfer capability of this LHP. Finally, I describe the basic equations that resolve the physical development of the LHP and how to estimate the each temperatures of the LHP.

Experimental Study on Heat Transfer Capability of a Miniature Loop Heat Pipe

2016 ◽  
Author(s):  
Guohui Zhou ◽  
Ji Li ◽  
Lucang Lv
Keyword(s):  
Heat Pipe ◽  
Heat Load ◽  
Electronics Cooling ◽  
High Heat ◽  
Working Fluid ◽  
Loop Heat Pipe ◽  
Evaporator Temperature ◽  
Liquid Line ◽  
Horizontal Orientation ◽  
Flat Evaporator

In this paper, a miniature loop heat pipe (mLHP) with a flat evaporator is illustrated and investigated experimentally, with water as the working fluid. The mLHP can be applied for the mobile electronics cooling, such as tablet computers and laptop computers, with a 1.2 mm thick ultra-thin flat evaporator and a thickness of 1.0 mm for the vapor line, liquid line and condenser. A narrow sintered copper mesh in the liquid line and a part of the condenser as the secondary wick can promote the flow of the condensed working fluid back to the evaporator. The experimental results showed that the mLHP could start up successfully and operate stably at low heat load of 3 W in the horizontal orientation, and transport a high heat load of 12 W (the heat flux of 4 W/cm2) with the evaporator temperature below 100 °C in different test orientations by natural convection, showing good operational performance against gravity field. The minimum mLHP thermal resistance of 0.32 K/W was achieved at the input heat load of 12 W in the horizontal orientation.

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.

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