scholarly journals Recent Advances in Loop Heat Pipes with Flat Evaporator

Entropy ◽  
2021 ◽  
Vol 23 (11) ◽  
pp. 1374
Author(s):  
Pawel Szymanski
Keyword(s):  
Reverse Flow ◽  
Construction Materials ◽  
Poor Performance ◽  
Heat Pipes ◽  
Manufacturing Cost ◽  
Heating Zone ◽  
Loop Heat Pipes ◽  
Start Up ◽  
Flat Evaporator ◽  
Compensation Chamber

The focus of this review is to present the current advances in Loop Heat Pipes (LHP) with flat evaporators, which address the current challenges to the wide implementation of the technology. A recent advance in LHP is the design of flat-shaped evaporators, which is better suited to the geometry of discretely mounted electronics components (microprocessors) and therefore negate the need for an additional transfer surface (saddle) between component and evaporator. However, various challenges exist in the implementation of flat-evaporator, including (1) deformation of the evaporator due to high internal pressure and uneven stress distribution in the non-circular casing; (2) heat leak from evaporator heating zone and sidewall into the compensation chamber; (3) poor performance at start-up; (4) reverse flow through the wick; or (5) difficulties in sealing, and hence frequent leakage. This paper presents and reviews state-of-the-art LHP technologies; this includes an (a) review of novel manufacturing methods; (b) LHP evaporator designs; (c) working fluids; and (d) construction materials. The work presents solutions that are used to develop or improve the LHP construction, overall thermal performance, heat transfer distance, start-up time (especially at low heat loads), manufacturing cost, weight, possibilities of miniaturization and how they affect the solution on the above-presented problems and challenges in flat shape LHP development to take advantage in the passive cooling systems for electronic devices in multiple applications.

Noncondensible Gas, Mass, and Adverse Tilt Effects on the Start-up of Loop Heat Pipes

1999 ◽  
Author(s):  
Jane Baumann ◽  
Brent Cullimore ◽  
Boris Yendler ◽  
Eva Buchan
Keyword(s):  
Heat Pipes ◽  
Loop Heat Pipes ◽  
Start Up

Loop Heat Pipe Design, Manufacturing, and Testing: An Industrial Perspective

2009 ◽  
Author(s):  
W. G. Anderson ◽  
P. M. Dussinger ◽  
S. D. Garner ◽  
J. R. Hartenstine ◽  
D. B. Saraff
Keyword(s):  
Heat Pipes ◽  
Two Phase ◽  
Temperature Changes ◽  
Loop Heat Pipes ◽  
Flow Balance ◽  
Power Cycling ◽  
Condenser Temperature ◽  
Phase Systems ◽  
Compensation Chamber ◽  
Design And Testing

Loop Heat Pipes (LHPs) are two-phase devices that can passively transport heat over long distances relative to other passive two phase systems such as heat pipes. Most of the art of LHP fabrication is in the primary and secondary wick. The manufacturing steps for an LHP are described, including the tests to validate the LHP during manufacture. The tests include wick property testing (pore size, permeability, and thermal conductivity), secondary wick testing, and parallel flow balance design and testing. The required tests after the LHP is fabricated include low power starts, shutdown through compensation chamber heating, unbalanced condenser temperature tests, transient testing — both power cycling and condenser temperature changes, and maximum power tests.

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.

The Study of PTFE Wicks Application to Loop Heat Pipes with Flat Evaporator

2015 ◽  
Vol 775 ◽  
pp. 54-58
Author(s):  
Shen Chun Wu ◽  
Shih Syuan Yan ◽  
Chen Yu Chung ◽  
Shen Jwu Su
Keyword(s):  
Heat Flux ◽  
Flat Plate ◽  
Pore Radius ◽  
Operating Temperature ◽  
Heat Pipes ◽  
Loop Heat Pipes ◽  
Heat Leakage ◽  
Flat Evaporator ◽  
Temperature And Pressure ◽  
Effective Pore Radius

This study investigates the application of PTFE wicks to flat-plate loop heat pipes (FLHPs). PTFE’s low heat transfer coefficient effectively prevents heat-leakage, which is a problem with using metal wicks, lowering the operating temperature and pressure. This paper uses PTFE particles to form wicks, and the effect of PTFE on flat-plate LHP performance is investigated. Experimental results shows that the highest heat load reached was 100W, with lowest thermal resistance of 0.61°C/W, and heat flux of about 10W/cm2, For the wick properties, the wick had an effective pore radius of the wick was around 9.2μm, porosity of 47%, and permeability of 1.0 x 10-12m2. Compared to the highest heat flux reported in literature thus far for PTFE flat-plate LHPs, the heat flux in this study was enhanced by around 50%.

Derivation and Validation of a Figure of Merit for Loop Heat Pipes With Medium Temperature Working Fluids

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Wukchul Joung ◽  
Jinho Lee ◽  
Sanghyun Lee ◽  
Joohyun Lee
Keyword(s):  
Steady State ◽  
Thermal Performance ◽  
Figure Of Merit ◽  
Saturation Pressure ◽  
Heat Pipes ◽  
Working Fluid ◽  
Medium Temperature ◽  
Working Fluids ◽  
Loop Heat Pipes ◽  
Compensation Chamber

The working fluids of loop heat pipes (LHPs) play an important role in the operation of the LHPs by influencing the operating temperatures and the heat transfer limits. Therefore, the proper selection of a working fluid is a key practice in LHP fabrication, and there has been a high demand for an appropriate index that enables the quantitative comparison of the steady-state thermal performance of the working fluids. In this work, a figure of merit for LHPs was theoretically derived and experimentally verified. In particular, the pressure losses in the LHP operation were balanced with the saturation pressure difference between the evaporator and the compensation chamber to derive the figure of merit. This derived figure of merit for LHPs successfully predicted the steady-state thermal performance of the tested working fluids within the variable conductance regime. In the constant conductance regime, the differences in the condenser cooling capacity and in the liquid subcooling for different working fluids determined the thermal performance of each working fluid. The limitations and prospects of the proposed figure of merit were discussed in detail.

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