Recent Advances in Loop Heat Pipes with Flat Evaporator
Abstract
:1. Introduction
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- Sensitivity to internal fluid saturation pressure that can potentially cause stress, deformation and consequently the ballooning of the evaporator wall and wick, which can lead to the deterioration of the heat input surface contact with the heat source and loss of thermal connection between the heat input wall and the wick. The high positive saturation pressure created by certain working fluids in the evaporator may distort the evaporator casing shape or wick structure. Such a circumstance requires a more conscientious design of evaporator casing which might result in an increase in the wall thickness, increase mass or limit the choice of the working fluid, which may restrict the choice of casing materials [4,6].
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- Increased heat leakage (i.e., “parasitic heating”) from the evaporator heating zone and sidewall into the compensation chamber (CC), which results in the increase of the CC temperature and consequently the LHP resistance and frequent failures in the start-up, especially at low heat loads. The construction of a flat-shaped evaporator requires installation of the heating zone very close to CC, which promotes parasitic heating from the evaporator to CC, therefore is a challenge to overcome. Furthermore, the flat-shaped evaporators have a larger sidewall area which facilitates conduction, resulting in a rise in CC temperature. This reduces the overall thermal performance of the LHP and may also cause a failure in the LHP start-up at a low heat load. A novel mechanical and thermal design of the evaporator can be considered to overcome this challenge [6]. For example, this effect can be reduced by: (1) increasing heat exchange intensity in the evaporation zone; (2) decreasing thermal resistance of the evaporator wall through which the heat load is supplied; and/or (3) by enhancing heat exchange to the working fluid at the wall-wick boundary.
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- −
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- Difficult start-up at: (1) low operating temperature (due to low vapor pressure) [1]; (2) high g-loads or restarting after the high-g load period. High g-load conditions might cause a reverse flow of working fluid that influence LHP start-up and restart after start-up or situations where the working fluid stalls in the condenser, causing the onset of evaporator dry-out; (3) when LHP is orientated against gravity, that affects the liquid charge and CC size.
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- Customizing of new wick properties and construction of new wick profiles to build ultra-performance LHP designs, understanding the manufacturing process;
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- Maximizing the distance of the liquid motion in the wick;
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- Organization of effective heat exchange during the evaporation and condensation of the working fluid;
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- Maximizing the heat transport distance.
2. Novel Wick Materials, Wick Properties and Wick Construction
2.1. Bi-Porous Wicks
2.2. Additive Manufactured Wicks and LHPs
2.3. Wick Surface Treatment
2.4. Non-Metallic and Composite Wicks
3. Working Fluid
Nanofluids
4. Modification in Construction of LHP
5. Miniature Flat LHP
6. Conclusions
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- Sensitivity to internal pressure the internal pressure causes stress, deformation and consequently ballooning of the evaporator wall and wick and deterioration of the heating surface contact and loss of thermal connection between the heat input wall and the wick;
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- Increased heat leakage from the evaporator heating zone and sidewall into the compensation chamber (CC), which results in the increase of the CC temperature and consequently the LHP resistance and frequent failures in the start-ups;
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- Increased heat leakage through the wick into the liquid bore, causing the increased temperature of the liquid being supplied to the evaporator and consequently failure of start-ups;
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- Increased possibility of reverse flow of vapor through the joints between the wick and casing into the compensation chamber and/or through the wick in applications where the wick thickness is reduced;
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- The difficulty of sealing the casing/wick structure with long edges needs a special mechanical treatment. This causes leakage of the installation in long term maintenance and consequently the failure of the flat evaporator LHP’s operation and limits the use in space and terrestrial applications.
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- The creation of novel wick properties or construction techniques that improves the heat transfer capability of the overall LHP, decrease the effect of heat leak through the wick to CC, improve LHP operation reliability and stability, improve the start-up time at low operating temperatures or low operating power, overcome deformation of the evaporator and maximize heat transfer distance. Furthermore, innovative wicks can strongly enhance the LHP heat transfer performance, thermal conductivity as the wicks have greater wettability. The wick treatment improves hardness and hence prevent deformation of flat evaporator LHPs;
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- Utilization of novel LHP manufacturing techniques (i.e., AM) allows the development of efficient devices with complex geometry and high surface area to volume ratio (A/V) in order to maximize the interaction between the heat source and heat sink or to maximize the surface area for evaporation/condensation processes and fabrication of products with a lower cost-to-complexity ratio and quicker production time compared to other manufacturing processes and gives the possibility of producing customized and complex freeform shapes, which are in LHPs. Furthermore, the utilization of novel LHP manufacturing techniques overcomes the above-presented challenge of sealing casing/wick that causes leakage and consequently the failure of the LHP;
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- The creation of novel wick materials helps to reduce the parasitic heating from the evaporator heating zone and sidewall into CC and hence improves the LHP start up-time;
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- The selection of novel working fluids (i.e., nanofluids) significantly improves the heat transfer performance of the LHP;
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- The modification of the construction of a flat evaporator LHP may overcome start-up difficulties caused by the temperature overshoot at the start-up period, especially at low heat loads and might reduce or even eliminate a parasitic heal leakage in the evaporator;
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- The utilization of novel manufacturing techniques increases the potential of LHP miniaturization and the possibility for dissipating high heat fluxes to take advantage of the passive cooling systems for electronic devices in multiple applications.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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Research Group | Working Fluid | Evaporator Casing Material | Evaporator Dimensions | Power | Maximum Heat Flux | Thermal Resistance | Wick | Heat Transport Distance | Effect |
---|---|---|---|---|---|---|---|---|---|
Li et al. 2012 [18] | Methanol | Copper | Ø74 mm × H28 mm | 40 W–100 W | 10.4 W/cm2 | N/A | Nickel powder; Three kinds of pores: the first one is made by pore former, and mean diameter is about 120 µm; The second one is the gap between nickel powders, mean diameter is less than 2 µm; The third one is pore generated by numbers of nickel powder agglomeration, mean diameter is about 10 µm; | 300 mm |
|
Chen et al. 2012 [17] | Ammonia | Stainless-steel | Ø43 mm × H15 mm | 2.5 W–130 W | 12.8 W/cm2 | 0.33 °C/W | Nickel powder; Two kinds of pores: the first one is made by a pore former, and mean diameter is about 106 µm; The second one is the gap between nickel powders, mean diameter is 4.4 µm to 5.6 µm; | 335 mm |
|
Liu et al. 2012 [14] | Methanol | Brass | Ø74 mm × H28 mm | 20 W–160 W | 16.8 W/cm2 | 0.46 °C/W | Primary wick: Sintered nickel powder; Secondary wick: Several hundred layers of stainless steel mesh; | 300 mm |
|
Wu et al. 2015 [19] | Ammonia | N/A | Ø16 mm × L65 mm | 50 W–800 W | N/A | 0.094 °C/W | Sinter nickel powder; Small pore diameters were around 7 µm with a range of around 1–10 µm; PMMA diameter 250 µm to 297 µm; | 470 mm |
|
Kumar et al. 2018 [20] | Ethanol | Copper | 25 mm × 30 mm × 10 mm | 7 W–17 W | 11 W/cm2 | N/A | Naphthalene as the pore former. The average particle diameter of the copper and naphthalene powder is determined using the imaging method which is found≈10 ± 6μm and 7.5 ± 4μm, respectively; | N/A |
|
Zhang et al. 2020 [21] | Ammonia | Stainless steel | Ø60 mm × H25 mm | 2.5 W–180 W | 10.8 W/cm2 | 0.252 °C/W | Nickel powder; Pore diameter was about 5.8μm. The small pores were formed by the nickel particles and the large pores were formed by dissolving the pore formers. | 1600 mm |
|
Research Group | Evaporator Casing Material | Evaporator Dimensions | Power | Thermal Resistance | Wick | Heat Transport Distance | Effect |
---|---|---|---|---|---|---|---|
Esarte et al. 2017 [26] | Copper | Volume 2827 mm3 Active length 23.2 mm | 57 W, 120 W | 0.15 °C/W | Stainless steel Pore radius 80 µm | 100 mm |
|
Anderson et al. 2017–2021 [11,12,27,28] | Stainless steel | Ø25.4 mm × L10.16 mm | 5 W– 350 W | 0.13 °C/W | Stainless steel Pore radius 4.9 µm | N/A |
|
Hu et al. 2020 [29] | Stainless steel | Flat dish Ø42 mm × H2 mm | 20 W– 160 W | 0.031 °C/W | Stainless steel Pore radius 100 µm | 150 mm |
|
Research Group | Wick | Working Fluid | Casing Material | Evaporator Dimensions | Power | Maximum Heat Flux | Thermal Resistance | Heat Transport Distance | Effect |
---|---|---|---|---|---|---|---|---|---|
Wu et al. (2015) [51] | Sintered PTFE (polytetrafluoroethylene) Pore radius of 1.7 µm, the porosity of 50%, and permeability of 6.2 × 10−12 m2 | Ammonia | Aluminium | L65 mm × Ø12.5 mm | 600 W | 0.145 °C/W | 470 mm |
| |
Wu et al. (2017) [46] | Sintered PTFE (polytetrafluoroethylene) Pore radius of 1.8 µm, porosity of 49%, and permeability of 5.3 × 10−12 m2 | Water + Butanol aqueous solution to form self-rewetting fluid | n/a | L65 mm × Ø15.5 mm | 400 W | 0.32 °C/W | 470 mm |
| |
Santos et.al. (2010) [43,44] | Ceramic porous wick Pore radius 1–3 µm, porosity of 50%, and permeability of 35 × 10−15 m2 | Acetone and Water | Stainless steel | L25 mm × Ø10 mm | 25 W | 5.3 °C/W | 260 mm and245 mm |
| |
He et al. (2020) [49,50] | Sintered nickel wick Pore radius 3–10 µm, the porosity of 70% and permeability of 2.39 × 1013 | R245fa | Composite copper and stainless steel | L80 mm × W80 mm × H21 mm | 150 W | n/a | 270 mm |
| |
Xin et al. (2018) [48] | Composite wick having different effective thermal conductivities—higher thermal conductivity on the side close to the vapour Channels and lower thermal conductivity on the side close to the liquid in the compensation chamber The outer layer (pure nickel) pore radius 5 µm (85.6%), porosity 51.3% The inner layer (Ni–10 wt% Cu) Pore radius 5 µm (68.3%), porosity 51.3% | Ammonia | n/a | L40 mm × Ø20.5 mm | 10 W | n/a | 260 mm |
|
Research Group | Nanofluid | Evaporator Casing Material | Evaporator Dimensions | Power | Maximum Heat Flux | Thermal Resistance | Wick | Heat Transport Distance | Effect |
---|---|---|---|---|---|---|---|---|---|
Gunnasegaran et al. 2013 [53] | Silica nanofluid (SiO2–H2O) | Copper | L50 mm × W50 mm × H4 mm | 20 W–100 W | 1.304 °C/W | Mesh Size—n/a | 830 mm |
| |
Putra et al. 2014 [54] | Al2O3-Water | Copper | Tube Ø 20 mm and 100 mm in length | 10 W–30 W | 0.68 °C/W | Porous biomaterial (Collaria) mean pore diameters a 83 µm, 56 µm, 170 µm, 124 µm | 270 mm |
| |
Wan et al. 2015 [55] | Cu-Water | Copper | L55 × W50 × H18 | 25 W–150 W | 0.065 °C/W | Porous copper wick mean pore diameter 65 µm | 350 mm |
| |
Tharayil et al. 2016 [56] | Graphene–water | Copper | L20 mm × W20 mm × H7.5 mm | 20– 380 W | 0.083 °C/W | Screen mesh wick (100 mesh) 0.25 mm 7 layers | 127 mm |
|
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Szymanski, P.; Law, R.; MᶜGlen, R.J.; Reay, D.A. Recent Advances in Loop Heat Pipes with Flat Evaporator. Entropy 2021, 23, 1374. https://doi.org/10.3390/e23111374
Szymanski P, Law R, MᶜGlen RJ, Reay DA. Recent Advances in Loop Heat Pipes with Flat Evaporator. Entropy. 2021; 23(11):1374. https://doi.org/10.3390/e23111374
Chicago/Turabian StyleSzymanski, Pawel, Richard Law, Ryan J. MᶜGlen, and David A. Reay. 2021. "Recent Advances in Loop Heat Pipes with Flat Evaporator" Entropy 23, no. 11: 1374. https://doi.org/10.3390/e23111374
APA StyleSzymanski, P., Law, R., MᶜGlen, R. J., & Reay, D. A. (2021). Recent Advances in Loop Heat Pipes with Flat Evaporator. Entropy, 23(11), 1374. https://doi.org/10.3390/e23111374