Heat pipes in modern heat exchangers Vasiliev, Leonard L.
Applied Thermal Engineering,
2005, 2005-1-00, 20050101, Volume:
25, Issue:
1
Book Review, Journal Article
Peer reviewed
Heat pipes are very flexible systems with regard to effective thermal control. They can easily be implemented as heat exchangers inside sorption and vapour-compression heat pumps, refrigerators and ...other types of heat transfer devices. Their heat transfer coefficient in the evaporator and condenser zones is 10
3–10
5 W/m
2
K, heat pipe thermal resistance is 0.01–0.03 K/W, therefore leading to smaller area and mass of heat exchangers. Miniature and micro heat pipes are welcomed for electronic components cooling and space two-phase thermal control systems. Loop heat pipes, pulsating heat pipes and sorption heat pipes are the novelty for modern heat exchangers. Heat pipe air preheaters are used in thermal power plants to preheat the secondary–primary air required for combustion of fuel in the boiler using the energy available in exhaust gases. Heat pipe solar collectors are promising for domestic use. This paper reviews mainly heat pipe developments in the Former Soviet Union Countries. Some new results obtained in USA and Europe are also included.
Abstract On the basis of many theoretical studies about rotating heat pipes, a simplified mathematical model is established based on the lumped parameter method, ignoring the temperature difference ...inside each section of the heat pipe is ignored, and comparing the calculated results with the experimental data. It is identified that this model has a good applicability at high speeds and low liquid filling rates, and effectively reduces the amount of calculation, which is of practical significance to the engineering application of rotating heat pipes.
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•A novel loop heat pipe module was developed for server CPU cooling.•Wicks connecting evaporator and condenser were implemented.•The 4 mm thick module dissipates 150 W with chip ...temperature below 85 °C.•Tested directions impact the heat transfer performance weakly.
The rapid development of distributed edge computing system demands urgently that the thermal management solution for server CPUs in micro-data centers have the characteristics of compact structure, high performance, maintenance-free, and energy saving. This paper focuses on a new cooling concept enabled by a compact loop heat pipe. Through removing compensation chamber, selecting a spindle-shaped evaporator structure with aid of an auxiliary gradient sintered capillary wick connecting the condenser and the evaporator, this new proposed loop heat pipe module can provide high reliability operation, meanwhile maintaining high thermal performance. Under forced air cooling, it is demonstrated that the 4 mm thick loop heat pipe module with finned tube condenser could dissipate a heat load of 150 W under a wide fan voltage range from 12 V to 24 V at tested directions when the chip junction temperature was below 85 °C, and could manage a heat load up to 275 W without dry-out. The proposed new loop heat pipe module with finned tube condenser is a potential solution to cool edge-computing server CPUs.
•New battery thermal management proposed with Loop Heat Pipes and graphite sheets.•Loop Heat Pipe is a passive technology so no need for parasitic power or maintenance.•Feasibility verified through ...numerical model validated by experimental demonstrator.•Proposed design reduces maximum temperature by 3.6 °C compared to liquid cold plate.•Numerical model proved useful as design tool for working fluid and materials.
Aiming to improve on key Electric Vehicles issues, such as maximum temperature during fast charging, parasitic power and cost reduction, an innovative thermal management system for a 3-cell battery module, including a flat plate Loop Heat Pipe and graphite sheet inserts, is presented. The proposed Loop Heat Pipe, located at the bottom of the module, can transfer efficiently up to 150 W from the battery module to a remote heat exchanger connected to the HVAC chiller of the vehicle. Graphite sheets have the twofold function of promoting heat transfer along the cell plane direction, while hindering it in the transverse direction. Since Loop Heat Pipes need no electrical power, this design allows for a reduction of parasitic power consumption, as well as a reduction in the need of maintenance, with respect to standard forced air or liquid-based thermal management systems. The design feasibility was verified thanks to a Lumped Parameter Model, which was validated against in-house experimental data, using a copper/copper flat plate Loop Heat Pipe and two different working fluids, ethanol and water. Results showed that this design complies with the battery thermal requirements both at pack and cell level, with maximum temperature during fast charge and temperature spread across the cell being 31.5 °C and 2 °C, respectively, during ambient temperature tests. When compared against a liquid cold plate design, this new design lowered the maximum temperature after fast charge by 3.6 °C. Finally, the developed model proved able to predict the effects of design parameters on the thermal performance of the system.
•A 0.6-mm-thick LHP with a novel wick structure fabricated by a new manufacturing process was proposed.•Thermal resistance of 0.11 K/W between the evaporator and the condenser was achieved at 20 W.•A ...three-dimensional heat conduction model was constructed to analyze the temperature distribution and the heat leakage.•The proposed ultra-thin LHP is a promising option for the thermal management of mobile electronics.
This paper presents a new cooling solution for thermal management using an ultra-thin loop heat pipe (LHP) with an evaporator of 0.6-mm thickness designed to be embedded in thin electronic devices. A prototype model of thin LHP was fabricated using chemical etching of thin copper plates and a diffusion-bonding process. The experimental investigation was conducted by measuring the temperature of each part of the thin LHP against the heat input to evaluate the heat transport performance of the thin LHP. Thermal resistance between the evaporator and the condenser of 0.11 K/W for a horizontal orientation, 0.03 K/W for a bottom heat orientation, 0.28 K/W for a top heat orientation was obtained at 20 W. It was confirmed that there is little dependence of the operating orientation on the thin LHP, which is capable of transferring a heat load of 20 W even in different orientations. A three-dimensional heat conduction analysis for the evaporator was carried out. From these calculations, the heat distribution in the evaporator and the quantity of heat leakage from the evaporator to the liquid line during steady operation was ascertained. The result of this investigation shows that the proposed LHP is a promising option for the thermal management of mobile electronics.
•A novel flat ELR evaporator LHP has been proposed and tested.•The novel LHP can overcome the shortcoming of traditional flat ELR evaporator LHP.•The LHP was investigated at different orientations ...and heat sink conditions.•The LHP can effectively dissipate a maximum heat load of 750 W (83.33 W/cm2).
A novel flat evaporator loop heat pipe (LHP) has been designed and proposed to overcome the shortcoming of the traditional flat evaporator LHP that cannot operate normally in some positive gravity direction (especially +90º), and to solve the heat dissipation challenges of high heat flux electronic chips. The vapor channels of the capillary wick for the novel LHP were arranged in double rows. And a 1 mm thick smaller particle size copper powder embedded with two 0.5 mm thick stainless steel rings was sintered on the internal surface of the compensation chamber. The thermal characteristics of the novel LHP were investigated at nine different LHP orientations at heat sink temperature from 10 to 40 °C and flow rate of the cooling water of 1 to 4 L/min. The experimental study indicated that the novel flat evaporator LHP can operate normally in some positive gravity directions (especially +90º). And the LHP orientations have a significant influence on the junction temperature and the thermal performance of LHP. Compared with other LHP orientations, the junction temperature at the most favorable LHP orientation was only of 83.60 °C under the highest heat load of 750 W (83.33 W/cm2). The junction temperature increased from 86.67 to 88.64 °C at the maximum heat load of 750 W when the heat sink temperature was increased from 10 to 40 °C. The effect of flow rate of cooling water on the thermal characteristics of the novel LHP was less considerable in the range from 1 to 4 L/min
•Large pores and small pores reduced flow resistance and sustained capillary force.•Driving force was composed by capillary force, phase-change force, gravity force.•Formation of vapor phase in ...compensation chamber caused temperature pulsation.•Two operation modes were concluded and minimum thermal resistance was 0.382 °C/W.•Gravity assistance trebled the thermal performance than horizontal condition.
Loop heat pipe, acting as one of the applications of porous media, has been extensively researched for its reliability and robustness. In order to enhance the heat transfer capacity of the loop heat pipe and adapt it to long-distance thermal control system, high-performance capillary wick is urgently required. In this paper, a comprehensive study about the biporous wick fabrication and operating mechanism is introduced. Contrasting to the conventional monoporous wick, the coaction of large and small pores could reduce the flow resistance and sustain sufficient capillary force. The driving force for loop circulation was comprised of capillary force, phase-change driving force, and gravity force. Furthermore, a flat-plate loop heat pipe with long heat transfer distance was constructed to test the performance limit of the biporous wick. Test results under horizontal condition indicated that the loop transferred the maximum heat load of 110 W (heat flux of 6.6 W/cm2) and the minimum thermal resistance was 0.382 °C/W. Besides, the intermittent formation of vapor phase inside compensation chamber would cause the temperature pulsation during low heat load range. While increasing the heat load, the subcooled liquid became sufficient and the pulsation was restrained. By introducing gravity effect at a tilt angle of 4.6°, the working performance was improved for the wick could be quickly infiltrated by the subcooled fluid even under long heat transfer distance. The maximum heat load was 330 W (heat flux of 19.9 W/cm2) and the minimum thermal resistance was 0.161 °C/W at a heat load of 240 W.
The paper contains an analytical review of developments, results of tests and simulation of loop heat pipes (LHPs) with disk-shaped, rectangular and flat-oval evaporators. Two main directions have ...been noted in the development of flat evaporators, which may be arbitrarily separated into evaporators with opposite replenishment (EORs) and evaporators with longitudinal replenishment (ELRs). The bodies of such evaporators are made of stainless steel, copper, aluminum. For making wicks use is made of sintered powders and mesh of stainless steel, nickel, titanium, copper, polytetrafluoroethylene (PTFE) and ceramics. Monoporous and biporous capillary structures are considered. Water, ammonia, methanol, ethanol, and acetone have been tested as working fluids. The best results were shown by the combination “copper–copper–water” at temperatures above 70 °C, when on trials an evaporator thermal resistance of less than 0.01 °C/W and a heat flux close to 1000 W/cm2 were achieved. For temperatures below 70 °C the most efficient combination is “stainless steel–nickel–ammonia”.
•A review of development of loop heat pipes with flat evaporators is presented.•Evaporators with opposite replenishment and longitudinal replenishment are defined.•Stainless steel–nickel–ammonia is the most efficient combination for 40–70 °C.•Copper–copper–water combination is the most efficient for 70–100 °C.
•Ethanol-water mixture was used in a loop heat pipe for aircraft anti-icing.•The loop heat pipe with ethanol-water mixture obtained better performance.•Phase-change inhibition explains the better ...performance of the system with mixture.•60% concentration of mixture reduced response time of the system to 60.16 min.•60% concentration of mixture enabled the highest temperature distribution on wing.
Owing to the increasing demand of an energy-saving aircraft anti-icing technology and cooling of hydraulic system, loop heat pipes are gradually becoming efficient heat transfer mediums that can meet both anti-icing and heat dissipation requirements. To fundamentally investigate the heat transfer performance of a loop heat pipe used for aircraft anti-icing, a stainless steel-nickel one was fabricated and tested at three typical inclination angles under both steady and transient states, with the system performance as the final evaluation index. Additionally, to address the freezing problem under negative temperature flight conditions, ethanol-water mixture with four concentrations was specially used as the working fluid. The steady-state results showed that, 60% concentration of mixture enabled the loop heat pipe to obtain a lower operating temperature as 178.1 °C, and a smaller thermal resistance as 0.26 °C/W at 300 W. In transient tests, the loop heat pipe with 60% concentration of mixture operated robustly and stably, and reduced the total response time by 30.18%, about 26 mins, than in pure ethanol group. It also achieved the highest temperature distributions on the wing at −20°, which were about 31.8 °C higher than those in pure ethanol group. This study aims to effectively guide the utilization of ethanol-water mixture in loop heat pipes, and shows highly practical value in further applications of loop heat pipes in aircraft anti-icing.
