Visualization and thermal resistance measurement for the sintered mesh-wick evaporator in operating flat-plate heat pipes

This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26 mm to 0.8 mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–100 W/cm². Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dryout appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance re-rose in response to the appearance and expansion of partial dryout. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.     

Operational characteristics of two biporous wicks used in loop heat pipe with flat evaporator

Two special biporous wicks are adopted in stainless-steel–ammonia loop heat pipes (LHPs) with flat evaporator to enhance their heat transfer performances. The experimental results demonstrate that thermal and hydraulic characteristics of the wick with porosity of 69% (in LHP 2) are better than that of the wick with porosity of 65% (in LHP 1). The maximum heat loads of LHP 1 and LHP 2 could, respectively, reach 120 W (heat flux 11.8 W/cm²) and 130 W (12.8 W/cm²) at the allowable evaporator temperature below 60 °C. Meanwhile, they can start up at heat load as low as 2.5 W. The LHPs show very fast and smooth response to heat load and operate stably without obvious temperature oscillation. The total thermal resistances of the LHPs vary between 1.47 and 0.33 °C/W at heat load ranging from 10 to 130 W.     

A vapor chamber using extended condenser concept for ultra-high heat flux and large heater area

We proposed an extended vapor chamber (EVC), consisting of an evaporator part and an extended condenser part. A layer of compressed copper foam was sintered on the inner evaporator surface. The extended condenser includes a circular-straight groove network and a fin region. The groove network distributes generated vapor everywhere in the internal volume of EVC. A set of capillary holes are machined within fins. A sliced copper foam bar is inserted in each of capillary hole. The peaks of copper foam bar are tightly contacted with the evaporator copper foam piece. Water is used as the working fluid with a heater area of 0.785 cm². A minimum thermal resistance of 0.03 K/W is reached for the bottom heating. The heat flux is up to 450 W/cm² without reaching dryout. The transition point of thermal resistances versus heat fluxes is significantly delayed with the heat flux exceeding 300 W/cm², beyond which thermal resistances are only slightly increased. EVC not only improves temperature uniformity on the evaporator and fin base surfaces, but also evens the temperature distribution along the fin height direction to increase the fin efficiency. Inclination angles and charge ratios are combined to affect the thermal performance of EVC. An optimal charge ratio of 0.3 was recommended. EVC can be used for ultra-high heat flux and larger heater area conditions.     

Evaporation resistance measurement with visualization for sintered copper-powder evaporator in operating flat-plate heat pipes

The evaporation resistances of loosely-sintered copper-powder evaporators were measured in operating flat-plate heat pipes. The evaporation processes was also visualized through a top glass plate. Irregular or spherical powders of different size distributions were investigated. Uniform heating of 16–170 W/cm² was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. The evaporation performance was first examined with the effect of liquid flow resistance minimized, i.e., the copper powders covered only the heated area with the remaining region covered with sintered copper wire screens. Similar to multi-layer mesh wicks, quiescent surface evaporation without nucleate boiling was observed for all test conditions, in spite of the abundant nucleation sites. The water film receded and the evaporation resistance reduced with increasing heat flux. Once partial dryout occurred, the evaporation resistance re-rose. The minimum evaporation resistances were about 0.08–0.09 W cm²/K for wicks containing fine powders. These values are similar with those for multi-layer-mesh wicks having a fine bottom screen. In the absence of fine powders, the minimum evaporation resistances were significantly larger. In the second part for homogeneous sintered-powder wicks, the large flow resistance tended to retard the condensed water from returning to the evaporator. However, this can be compensated by a larger charge and/or a thicker wick.     

Characterization of evaporation and boiling from sintered powder wicks fed by capillary action

The thermal resistance to heat transfer into the evaporator section of heat pipes and vapor chambers plays a dominant role in governing their overall performance. It is therefore critical to quantify this resistance for commonly used sintered copper powder wick surfaces, both under evaporation and boiling conditions. The objective of the current study is to measure the dependence of thermal resistance on the thickness and particle size of such surfaces. A novel test facility is developed which feeds the test fluid, water, to the wick by capillary action. This simulates the feeding mechanism within an actual heat pipe, referred to as wicked evaporation or boiling. Experiments with multiple samples, with thicknesses ranging from 600 to 1200 μm and particle sizes from 45 to 355 μm, demonstrate that for a given wick thickness, an optimum particle size exists which maximizes the boiling heat transfer coefficient. The tests also show that monoporous sintered wicks are able to support local heat fluxes of greater than 500 W cm^?2 without the occurrence of dryout. Additionally, in situ visualization of the wick surfaces during evaporation and boiling allows the thermal performance to be correlated with the observed regimes. It is seen that nucleate boiling from the wick substrate leads to substantially increased performance as compared to evaporation from the liquid free surface at the top of the wick layer. The sharp reduction in overall thermal resistance upon transition to a boiling regime is primarily attributable to the conductive resistance through the saturated wick material being bypassed.     

Study on heat transfer performance for loop heat pipe with circular flat evaporator

A loop heat pipe (LHP) with a circular flat evaporator was designed for cooling electronic devices. The flat evaporator with an outside diameter of 41 mm and a thickness of 15 mm was developed with a copper powder wick. The developed evaporator was examined to improve insufficient subcooling of liquid in a compensation chamber, which decreases an operating limitation of the LHP. Many different orientations of the elevation and direction of the evaporator were also considered during all of the experiments for this system. The active heating area was 3 cm × 3 cm, and water was used in the tests. This LHP generated a heat load in excess of 140 W with a total thermal resistance of 0.39 °C/W.     

Visualization of vapor formation regimes during capillary-fed boiling in sintered-powder heat pipe wicks

The current study investigates capillary-fed boiling of water from porous sintered powder wicks used in emerging high-effective-conductivity vapor chamber heat spreaders intended for management of hot spots with heat fluxes exceeding 500 W cm^?2. Characterization of 1 mm thick wicks composed of 100 μm sintered copper particles is performed in a test facility which replicates the capillary feeding conditions that occur in such devices. Boiling curves are obtained for a 5 mm × 5 mm heated input area, along with high-speed in-situ visualization of the evaporation/boiling processes. Understanding the vapor formation regimes is essential to predictive modeling of the observed characteristics. Schematic representations of such regimes along the boiling curves are presented for homogeneous and modified wick structures. In general, incipience of boiling in sintered-powder wicks reduces the effective thermal resistance and, for small heat input areas, does not cause liquid starvation due to a capillary limitation. The thermal performance enhancement provided by two different augmentation methods is quantified and explained in terms of the observed vapor formation characteristics. Patterns fabricated within the sintered powder create multi-scale wicks with regions of different pore size. These patterns reduce thermal resistance throughout the boiling regime by increasing the permeability to vapor exiting the wick, as confirmed by visualization of the preferential vapor venting from the surface. At the highest heat fluxes investigated prior to dryout, a thin liquid film is observed to form in the recessed patterned areas at the base of the wick. Integration of copper-coated carbon nanotubes on to the sintered powder reduces the required superheat for boiling incipience, thus reducing the overall thermal resistance at low heat fluxes. Evaporation and boiling regime heat transfer predictions from several available correlations are compared to the current results, and are shown to corroborate the conclusions regarding vapor permeability.     

Simulation of thermal processes in a flat evaporator of a copper–water loop heat pipe under uniform and concentrated heating

A 3D model has been developed for investigating heat and mass transfer in a flat evaporator of a copper–water loop heat pipe. It takes into account heat-transfer processes in the active zone, the barrier layer of the wick, the wall and the compensation chamber. The problem was solved by the finite difference method with the use of a nonuniform grid adapted to the configuration of the flat evaporator and its geometric peculiarities. Investigations have been carried out for understanding the effect of the heating zone size on heat distribution in the evaporator. The heating area was 9 cm² with a uniform heat supply and 1 cm² with a concentrated one. Numerical simulation has been performed for a heat load range from 20 to 1100 W. Data have shown that a decrease in the heating area at a fixed heat load results in both increasing temperature on the evaporator wall under the heater and local wick draining in the active zone. The results of the model have been verified using results of experimental tests.     

Visualization and performance measurement of operating mesh-wicked heat pipes

This work presents visualization of the evaporation/boiling process and thermal measurements of operating horizontal transparent heat pipes. The heat pipes consisted of a two-layered copper mesh wick consisting of 100 and/or 200 mesh screens, a glass tube and water as the working fluid. Experimental results indicated that nucleate boiling was prompted for a wick having a fine 200-mesh bottom layer. When the fluid charge approximately equaled the pore volume in the wick, the water–vapor interface receded into more curved menisci with increasing heat load Q. Thus, larger capillary forces and evaporation areas were attained to meet the increasing need of liquid supply and evaporation rate at the water–vapor interface. At Q = 40 and 45 W, the water film became less than 100 μm and the nucleate boiling observed at lower heat loads disappeared. Optimal thermal characteristics with smallest thermal resistances across the evaporator and lowest overall temperature distributions were found for such a wick/charge combination. Under a smaller charge, partial dry-out was observed in the evaporator. Under a larger charge, liquid recession with increasing heat load was limited and bubbles grew and burst violently at high heat loads. The effects of different wicks and fluid charges on the evaporation/boiling characteristics were discussed.     

High heat flux phase change on porous carbon nanotube structures

Carbon nanotube (CNT) forests are investigated as porous wick structures for chip-scale heat pipe cooling systems. An analytical model is developed to demonstrate the merits of phase change heat transfer on nanoscale porous structures, compared to that on microscale porous wick. Results indicate that nanoscale porous structures increase the thin-film evaporation surface area by one order of magnitude, which can significantly increase phase change heat transfer efficiency. The pertinent wick structure properties of the CNT forest are experimentally measured. Results show that the CNT forest is highly porous (～95% porosity), and possesses large variations in effective thermal conductivity ranging from 0.8 to 180 W/m K. Effective pore size of the CNT wick structure varies between 50 and 180 nm, which can generate capillary pressure up to two orders of magnitude higher than the microscale wick structure. However, its low permeability, about three to four orders of magnitude lower than the traditional wicks, underscores the necessity of bi-porous CNT wick structures. The bi-porous CNT wick structures are composed of nanoscale porous CNT clusters, separated by microscale (～50 μm wide) passages. Experimental results show a maximum heat flux of 770 W/cm² over a 2 mm × 2 mm heating area. With enhanced thin-film evaporation, heat transfer coefficients are improved by up to 100%, compared to the microscale wick. In contrast, the low CHF ～140 W/cm² over a 10 × 10 mm² heating area is caused by vapor occupation of the microscale pores and the reduction of wick permeability.     

Performance of LHPs with a novel design evaporator

In order to realize an excellent heat transfer performance of the LHPs, including the fast start-up and high heat transfer capacity, a new connection design between the evaporator envelope and the wick surface without the clearance was proposed. The LHPs with a cylindrical evaporator, 22 mm diameter and 80 mm long, were fabricated with water as the working fluid and an 70% inventory.Copper wicks made of different particle sizes were used in both the start-up and heat transfer capacity tests. It was experimentally observed that the sintered wick with 139 μm diameter particles had the best heat transfer performance. It achieved a start-up time of only 150 s under 30 W heat load, a heat transfer capacity of 500 W under the allowable evaporator temperature of 85 °C, and a low thermal resistance of 0.070–0.165 °C/W. Internal temperature measurements were also conducted to determine the mechanism of the heat leak, to identify the heat pipe effect, and to compare the heat leak with different wicks corresponding to the change of the heat load during the operation     

Optimal design of a micro evaporator with lateral gaps

This paper presents an optimal design of a micro evaporator, to maximize the heat transfer coefficient (HTC) and it forms the starting point in developing miniaturized vapor–compression refrigeration system. The experimental design is adopted to determine the optimal parameters of the evaporator for realizing the inlet–outlet conditions of the refrigerating cycle, and for increasing the HTC. The number of lateral gaps, channel width, and lateral gap size were optimized to maximize HTCs of 2062, 2029, and 1895 W/m²K for heating powers of 40, 60, and 80 W, respectively. The refrigerant and the mass flow rate were fixed as R-123 and 0.72 g/s, respectively. Among the three design parameters, the channel width is the most sensitive parameter influencing the HTC. A periodic change of flow pattern was observed in the evaporator with high HTCs, and a dryout was observed in the evaporator with low HTCs.     

A novel time strip flow visualisation technique for investigation of intermittent dewetting and dryout in elongated bubble flow in a microchannel evaporator

A flow visualisation study of flow boiling of R245fa in silicon multi-microchannels at low mass flux and moderate heat flux has been carried out with a high speed digital camera. The micro-evaporator had 67 channels of length 20 mm, width 223 μm, and height 680 μm while the fin width between adjacent channels was 80 μm. The base heat flux ranged from 2 to 26 W cm^?2 for a mass velocity of 100 kg s^?1 m^?2, resulting in exit vapour qualities ranging from 10% to 70%. In particular, a novel time strip technique was developed to analyse the recorded image sequences and significantly highlight the various phenomena occurring along given channels. Notably, this technique was able to reveal profound details regarding the intermittent dryout mechanism of liquid films trapped between the elongated bubbles and the heated channel walls. The results show that the intermittent dryout of the evaporating liquid film is comprised of four stages with distinct time scales and dynamics: (i) the growth of liquid film thinning perturbations to a critical amplitude causing the rupture of the metastable liquid film, (ii) a dewetting stage involving expanding dry spots leading to a rivulet flow regime, (iii) evaporation of the rivulets leading to full dryout, and (iv) a rewetting stage. This intermittent dryout mechanism appears to explain the many seemingly contradictory heat transfer coefficient trends observed with changes in vapour quality in microchannels, thus resolving an important heat transfer dilemma. Furthermore, since dryout is an undesirable event during the practical application of a microchannel evaporator, it is important to delay or even suppress the initial rupture of the liquid film that leads to dryout. This can be achieved by manufacturing or treating the channel surfaces to be highly wettable with the chosen refrigerant.     

Effect of copper surface wettability on the evaporation performance: Tests in a flat-plate heat pipe with visualization

The effects of copper surface wettability on the evaporation performance of a copper mesh wick were experimentally studied in an operating flat-plate heat pipe. Different degrees of wettability were obtained by varying the exposure times in air after the wicked plates were taken out of the sintering furnace. Three different working fluids: water, methanol and acetone, which possess different figures of merit, were investigated at the same volumetric liquid charge. The surface wettability was quantified by the static contact angle of sessile water drops on a flat copper surface. While the static contact angles of water drops varied from 10° to 40° for different degrees of wettability, the methanol and acetone drops still fully wetted the copper surface. A two-layer 100 + 200 mesh copper wick, 0.26 mm in thickness, was sintered on a 3 mm-thick copper base plate. A glass plate was adopted as the top wall of the heat pipe for visualization. Uniform heating was applied to the base plate near one end, and a cooling water jacket was connected at the other end. With increasing heat load, the evaporative resistance decreased with liquid film recession until a critical heat load showing the minimum evaporative resistance. Afterwards, partial dryout began from the front end of the evaporator. With decreasing wettability, the evaporating water film receded faster with increasing heat load and the critical heat loads were significantly reduced. In contrast, the critical heat loads for methanol and acetone seemed hardly affected by different wettability conditions. The minimum evaporative resistances, however, remained unaffected by surface wettability for all the three working fluids.     

An experimental investigation of a three-dimensional flat-plate oscillating heat pipe with staggered microchannels

Using water or acetone as the working fluid, the thermal performance of a three-dimensional flat-plate oscillating heat pipe (3D FP-OHP) with staggered microchannels was experimentally investigated by varying heating area, cooling temperature and operating orientation. It was found that when the heating area is larger at the same input power, the heat pipe is less orientation-dependent. When the heating area was decreased, to form a localized heating condition and higher heat flux, the thermal resistance and peak-to-peak amplitudes of temperature oscillations in the evaporator increased. The utilization of water as the working fluid generally provided the lowest thermal resistance for all experimental conditions investigated, but – unlike acetone – resulted in more severe temperature fluctuations in the evaporator during localized heating. The 3D FP-OHP, with overall dimensions of 130.18 × 38.10 × 2.86 mm³, demonstrated to efficiently manage heat fluxes as high as approximately 300 W/cm² at a total heat load of 300 W.     

Experimental studies on a high performance compact loop heat pipe with a square flat evaporator

A thorough experimental investigation was carried out on a copper–water compact loop heat pipe (LHP) with a unique flat, square evaporator with dimension of 30 mm (L)×30 mm (W)×15 mm (H) and a connecting tube having an inner diameter of 5 mm. Using a carefully designed experimental system, the startup process of the LHP when subjected to different heat loads was studied and the possible mechanisms behind the observed phenomena were explored. Two main modes, boiling trigger startup and evaporation trigger startup, were proposed to explain the varying startup behavior for different heat loads. In addition, an expression was developed to describe the radius of the receding meniscus inside the wick, to balance the increased pressure drop along the LHP with increasing heat loads. Finally, insight into how the compact LHP can transfer heat loads of more than 600 W (with a heat flux in excess of 100 W/cm²) with no occurrence of evaporator dry-out was provided.     

Experimental analysis of flow and heat transfer in a miniature porous heat sink for high heat flux application

A novel miniature porous heat sink system was presented for dissipating high heat fluxes of electronic device, and its operational principle and characteristics were analyzed. The flow and heat transfer of miniature porous heat sink was experimentally investigated at high heat fluxes. It was observed that the heat load of up to 280 W (heat flux of 140 W/cm²) was removed by the heat sink with the coolant pressure drop of about 34 kPa across the heat sink system and the heater junction temperature of 62.9 °C at the coolant flow rate of 6.2 cm³/s. Nu number of heat sink increased with the increase of Re number, and maximum value of 323 for Nu was achieved at highest Re of 518. The overall heat transfer coefficient of heat sink increased with the increase of coolant flow rate and heat load, and the maximal heat transfer coefficient was 36.8 kW(m² °C)^?1 in the experiment. The minimum value of 0.16 °C/W for the whole thermal resistance of heat sink was achieved at flow rate of 6.2 cm³/s, and increasing coolant flow rate and heat fluxes could lead to the decrease in thermal resistance. The micro heat sink has good performance for electronics cooling at high heat fluxes, and it can improve the reliability and lifetime of electronic device.     

Loop heat pipe for cooling of high-power electronic components

In this paper, we present a new development of loop heat pipe (LHP) technology in its applications to cooling systems for high-power IGBT elements. An advanced method of LHP evaporator wick manufacturing has been proposed. Following this approach, a 16 mm outer diameter and 280 mm-length LHP evaporator was designed and manufactured. Nickel and titanium particles were used as raw material in LHP evaporator wick fabrication. LHP with a nominal capacity as high as 900 W for steady-state condition and more than 900 W for a periodic mode of operation at a temperature level below 100 °C and a heat transfer distance of 1.5 m was designed through the cooling of a high-power electronic module. An experimental program was developed to execute LHP performance tests and monitor its operability over a span of time. An investigation of the effects of LHP performance of parameters such as evaporator and condenser temperatures and LHP orientation in a gravity field was brought about. As regards the results of this initial series of tests, it was found that LHP spatial orientation within the nominal range of heat loads has no drastic effect on overall LHP functioning, whereas condenser temperature does play an important role, especially in the range of heat load close to critical. A 2D nodal model of the evaporator was developed and provides us with confirmation of the suggestion that when high-power dissipation levels are available, low wick conductivity is well adapted for LHP applications.     

Heat transfer analysis of a loop heat pipe with biporous wicks

Because the evaporative heat transfer of a wick structure in a loop heat pipe is exceedingly sensitive to the internal volume fractions of liquid and vapor phases, the purpose of this study was to investigate the evaporative heat transfer of various biporous wick parameters by controlling the particle size of pore former, the pore former content, and the sintering temperature. A statistical experiment was carried out to analyze the evaporative heat transfer of the biporous wicks and to understand the effects of the parameters more effectively. The statistical analysis indicated a clear and strong relationship between the effect of the pore former content and the evaporative heat transfer of a biporous wick. This is because the pore former content had a great influence on the probability of large interconnecting pores and an extended surface area for liquid film evaporation in a biporous wick. Experimental results also showed that, at the sink temperature of 10 °C and the allowable evaporator temperature of 85 °C, the evaporative heat transfer coefficient of the biporous wick, which reached a maximum value of 64,000 W/m² K, was approximately six times higher than that of the monoporous wick.     

Operating characteristics of a miniature cryogenic loop heat pipe

Aiming at future space applications, a miniature cryogenic loop heat pipe (CLHP) with nitrogen as the working fluid was designed, whose condenser could provide the interface with the cold finger of cryocooler, and its operating characteristics were experimentally investigated in this work. Based on the experimental results, important conclusions below have been drawn: (1) with only 2.5 W applied to the secondary evaporator, the CLHP can realize the supercritical startup, and the larger the heat load applied to the secondary evaporator, the sooner the temperature drop process of the primary evaporator; (2) when the heat load applied to the primary evaporator is no less than 3 W, the primary evaporator can operate independently; whereas when it is smaller than 3 W, the secondary evaporator must be kept in operation to assist the normal operation of the primary evaporator; (3) the CLHP has a heat transport capacity of 12 W × 0.56 m, and its thermal resistance decreases with the increase of the heat load applied to the primary evaporator; (4) the CLHP has the ability to operate with a small heat load applied to the primary evaporator for a long time, and manifests good thermal control performance.