Dry-out Limit Research Articles

Performance-Dryout Limits of Oscillating Heat Pipes: A Comprehensive Theoretical and Experimental Determination

The performance limits of oscillating heat pipes (OHPs) have been the subject of research in recent years in order to understand their limitations and allow their broad application in the aerospace field. The present work describes the experimental determination of the performance dryout limits for an additively manufactured OHP geometry filled with R-134a and R-123, as well as the performance limits theoretical predictions using a recently developed analytical approach. Thermal experiments were carried out using a constant-temperature cold-plate approach at three cold plate temperatures, allowing the OHP device to reach steady state for each power step. Results show good agreement for the performance limits predictions with experimental data for both working fluids R-134a and R-123, opening the way for the analytical framework to be used in the design and development of OHPs.

Dry-out limit enhancement strategy in sodium heat pipes: Overfilling effect and modified capillary limit model

Despite the significant impact of the filling ratio on dry-out of heat pipe, current operating limit models do not adequately address this factor. This study proposes an advanced capillary limit model that incorporates the effect of the filling ratio. An experimental study was conducted to compare the operating limits of sodium heat pipes with filling ratios of 107 % and 246 %. The results showed that the existing capillary model is difficult to predict the operating limits based on filling ratios, leading to significant deviations between theoretical predictions and actual outcomes. Specifically, a heat pipe filled to 107 % experienced dry-out at a point 31.1 % lower than expected, while a heat pipe filled to 246 % reached dry-out at 20.2 % higher than expected. Notably, the overfilled heat pipe maintained a steady state even at 10.5 % above the expected dry out point. The key point of the proposed model, based on optical imaging and temperature data at dry-out, is that the excess liquid in vapor region travels and forms a liquid plug at the end of the condenser, which shortens the length required for capillary transport. Based on this insight, the Chi model was modified to predict the operating limit of overfilled heat pipes more accurately within a 5 % margin of error. The study highlights the critical role of filling ratio in heat pipe design, noting that overfilling can shorten the effective capillary length through liquid plug formation, thereby increasing the dry-out limit. However, these pools also limit the reach of sodium vapor and potentially reduce the effective area of the condenser.

Experimental investigation on the heat transfer performance of pulsating heat pipe with self-rewetting Fe3O4-nanofluid

This paper conducted an experimental study on the heat transfer performance of pulsating heat pipe (PHP) under three different working fluids: ultrapure water, n-butanol self-rewetting fluid (SRWF), and self-rewetting Fe3O4-nanofluid (SRNF). The effects of filling ratio (FR), heating power, and magnetic field are studied. The results mainly show that the PHP with SRNF under a magnetic field, the start-up time increases, the evaporation section operating temperature increases, and the thermal performance decreases as the filling ratio increases. Increasing the FR, applying SRWF and SRNF can effectively increase the dry-out limit of PHP. At FR = 50 %, the overall thermal performance of PHP is best. In 10–130W, the thermal resistance of the SRWF and the SRNF with/without magnetic field is reduced by 14.5 %–27.8 %, 27.6 %–47.9 %, and 4.2 %–24.8 %, respectively, compared with that of ultrapure water. Similarly, compared without a magnetic field, the thermal resistance of SRNF with a magnetic field is reduced by 14.0 %–35.1 %. At FR = 50 %, the heat transfer performance of SRNF-PHP is best with a magnetic field. When the input power is 130W, the effective heat transfer coefficient keff is 5967.6 W/m∙ K, approximately 13.92 times greater than copper.

Verification experiment of maximum hydraulic diameter in a novel three-dimensional asymmetric oscillating heat pipe

A novel three-dimensional asymmetric oscillating heat pipe (3DAOHP) with unequal curvature radius of upper and lower bends has been designed. Setting the basic factor of diameter from 1.9 mm to 6.2 mm, a verification experiment on the criterion of conventional maximum hydraulic diameter (MHD) has been conducted. The results show that the 3DAOHP can still perform normally and transfer heat effectively, even if the diameter exceeds the MHD by 98.1 %. And the temperature profiles at each end does not show obvious abnormalities. The thermal performance also does not show any deterioration, which still follows the rule that the larger the diameter, the stronger the heat transfer capability. The thermal resistance of the 6.2 mm 3DAOHP can reach 0.07 °C/W at 1300 W without touching the dry-out limit. Moreover, the frequency and amplitude of hot end temperature fluctuation show suppression as the diameter increases. Therefore, the conventional MHD is not a diameter limitation of oscillating heat pipe (OHP) in enhancing heat transfer. Thus, the availability of OHP in high heat loads can be improved by further increasing the tube diameter. These findings are important for the OHP application in higher heat flux.

Experimental investigation of post and vented vapor chamber designs for high heat flux dissipation

To enhance the dryout heat flux of vapor chambers over a 1 cm × 1 cm area, this study describes the testing of four different vapor chamber design architectures (each having an overall size of 50 mm × 50 mm × 5.5 mm). The designs leverage a combination of internal liquid feeding posts and evaporator vapor venting features to improve the capillary-fed boiling performance. The study provides systematic insight into increasing the dryout limit of a vapor chamber and lowering the thermal resistance by assessing the effect of each geometric feature independently. A jet impingement air cooling test facility is used to measure the total thermal resistance of the package in a representative application, while a liquid cooling test facility is used to measure the thermal resistance of the vapor chamber itself. The results show that a vapor chamber with a combination of machined posts coated with sintered wick and vapor vent features has superior performance over vapor chambers with independent features. It dissipated the highest heat flux of 589 W/cm2 and provided the lowest total package thermal resistance of 0.28 K/W for a 1 cm2 heat input area. The performance improvements are attributed to an improved liquid supply utilizing the porous-wick-coated machined posts and a reduction vapor egress pressure utilizing the vent features above the heater area.

Surface evaporation enhancement using porous metasurfaces: 3-D multiscale, open-system wick evaporators

In open-system water-vapor production by direct heat supply, wick evaporators are promising for efficient use of the supplied heat. However, designing a suitable wick structure for achieving high evaporator performance had proven challenging. Here, novel, high-performance porous metasurfaces (unit-cell based capillary structures) are designed, fabricated and tested, using sintered copper powder. In addition, the wicks optimized for yet higher performance are presented. Vertically placed wicks are partially submerged in a pool of water, while heated through a copper substrate by Joule heating. The baseline wick is a monolayer wick, and three different particle diameters (78, 100, and 130 μm) are used. The next variations are bilayer wicks which improve the particle packing and the maximum capillary pressure. The most improvement is achieved by adding strips of capillary arteries (1 mm thick and 1 mm wide, with an interartery gap of 1 mm) on the monolayer. These unit-cell based 3-D porous metasurfaces allow for tailoring the surface design to achieve an optimal thermal-hydraulic performance. Analyses of the wick capillary-viscous dryout limit and the extended-surface heat transfer predict the performance of the fabricated wicks under the test conditions. Overall, an evaporation efficiency of unity and an evaporation enhancement by 50% are recorded.

Thermal management at microscale level: Detailed study on the development of a micro loop heat pipe

• A novel design of micro loop heat pipe is demonstrated. • Temperature reduction of more than 28 ° C is attained. • A new methodology for selection of working fluid is proposed. This study aims to address the problem of removing excessive heat generated in electronic and microfluidic systems. For this purpose, a new design of a micro loop heat pipe is proposed. It is fabricated on a 2-inch silicon wafer, having a cross-section of 35 mm × 17 mm and an overall device depth of 150 μm. Experimental investigations showed that the proposed design has a low heat leak (undesirable heat conduction from the evaporator to the compensation chamber) compared to the various micro heat pipe designs presented in the literature. A temperature reduction of 28.6 °C at 3.78 W heat load is recorded using methanol as a working fluid. This study also reveals that the present methodologies to select working fluids for conventional loop heat pipes cannot be applied to micro loop heat pipes. Hence, a new methodology is proposed for the same, based on four parameters of the working fluid: cooling capacity, dry-out limit, operation in anti-gravity regimes, and force exerted on the evaporator.

Direct simulation of flow-boiling crisis and its porous-metasurface control for very large dryout limit

The Zuber pool-boiling hydrodynamic instability theory establishes that the critical heat flux qCHF,Z is based on rising surface vapor columns with liquid counterflow in a unit cell the of size of the critical wavelength λc determined by the Kelvin-Helmholtz and Rayleigh-Taylor interfacial instabilities. In the flow boiling, the forced velocity ul,o creates a leading-edge region complementing these phase-buoyant flows and breaking the periodicity of the Zuber unit cell. Here, with the direct numerical simulations (DNS), i.e., CFD-VOF-LES, of the Zuber unit cell for saturated water (one atm), the effect of ul,o on the hydrodynamic dryout qCHF,h is examined. The results show that the upstream liquid flow penetrates the boiling region, forming surface liquid tracks meandering between the vapor columns, and upon qCHF,h these tracks become unstable causing surface dryout. For ul,o as small as 5 cm/s, the axial liquid inertia deflects the vapor track and establishes the surface liquid track, raising qCHF,h noticeably over qCHF,Z and this is in good agreement with the available experiments. From the available theories, DNS-results and dimensional analysis, a relation is found between ul,o and λc establishing the wavelength modulation enhancement of the qCHF,h by increasing ul,o which decreases λc. In this new relation qCHF,h is proportional to ul,o1/6. The limit on this modulation enhancement is the capillarity limit qCHF,c= 3.3 MW/m2 compared to qCHF,Z= 1.1 MW/m2Next, the DNS results show that anisotropic arrangement of the vapor sites stabilizes the surface liquid track, thus increasing the qCHF,h. This geometric control of the vapor sites is achieved with a 3-D perforated porous coating, e.g., the flow-boiling canopy wick (FBCW). This geometric-modulated CHF enhancement is much larger than the wavelength-modulation enhancement and its vapor shear instability limit is overcome with the use of levees (geometric confinement) allowing for q> 10 MW/m2. However, the FBCW has an internal capillary-viscous limit qCHF,c−v (about 5.2 MW/m2 using sintered-powder wick) which favors small λc and currently is the bottleneck for achieving extremely large qCHF,h (which favors large λc).

Analytic characterization and operational limits of a hybrid two-phase mechanically pumped fluid loop based on the capillary pumped loop

Two-phase mechanically pumped fluid loops have the potential to provide numerous benefits for spacecraft based applications. Here a hybrid two-phase capillary and mechanically pumped fluid loop that shows promise for spacecraft thermal control applications is analyzed and its operational limits are characterized. Data from an experimental system that incorporates a 3D printed evaporator is correlated to several models that together provide a comprehensive characterization of the system behavior. The loop performance is analyzed using pointwise as well volume-averaged governing equations and resistance network models, to predict flooding and dryout limits. Also, system-level numerical (including 3-D CFD simulations) transient and linear-response analyses are performed predicting the changing loop phase distributions and the observed time-dependent behavior. With the 3D-printed evaporator, a notably high evaporator conductance of 30 W/cm2-K and heat flux over 10 W/cm2 over large area are achieved with negligible temperature non-uniformity.

Flow-boiling canopy wick capillary-viscous limit

It was predicted that the perforated, leveed flow-boiling canopy wick (FBCW) can significantly increase the hydrodynamic instability limit of the flow boiling [1,2]. The perforations represent and modulate the vapor columns in the boiling crises hydrodynamic stability theory. The lower capillary-viscous limit, governed by the evaporator wick maximum capillary pressure and the viscous drag within the three-dimensional composite wick, may control the actual dryout performance. Saturated-water flow-boiling (one atm) experiment results are presented employing an FBCW, created from sintered-copper particles, with and without the hydrodynamic-stabilizing levees. The bimodal bilayer evaporator wick allows for sufficiently large maximum capillary pressure and permeability, compared to the monolayer wick. The experimental results indicate that the levees can increase the critical heat flux (CHF) over that without levees, whereas the existing maximum capillary pressure controls disallow achieving this larger hydrodynamic heat flux limit. This capillary-viscous dryout limit is predicted by employing the minimum surface energy principles, network models, and computational fluid dynamics (CFD), in good agreement with experiments. By employing both CFD and flow visualizations, good agreement is also established between the predicted and observed two-phase hydrodynamics above the canopy wick. They indicate that although the local dryout commences downstream; however, dryout is experimentally observed when a critical fraction of the liquid supply tracks dry downstream. The measured large thermal conductance (heat transfer coefficient) is beneficial for the predictions, verifying the vapor-occupied space between the canopy and evaporator wicks.

A semi-empirical model for thermal resistance and dryout during boiling in thin porous evaporators fed by capillary action

Two-phase passive heat transport devices such as vapor chambers, loop heat pipes, and capillary pumped loops utilize porous evaporators for phase change and to drive fluid transport. Nucleate boiling can occur within such capillary-fed porous evaporators, especially under high-heat-flux operation, as has been visually observed in various experimental studies in the literature. However, prior modeling efforts have typically only considered single-phase flow of liquid through a completely saturated porous medium for characterizing the dryout limit and thermal performance. The present work offers a new semi-empirical model for prediction of thermal resistance and dryout during boiling in capillary-fed evaporators. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer periphery of the evaporator to its center, with vapor flow across the thickness, is considered to obtain the local liquid and vapor pressures. The capillary pressure and the relative permeabilities (fraction of single-phase permeabilities) for two-phase flow in the porous medium are modeled as a function of the local liquid saturation. The heat flux at which the liquid saturation at the center of the evaporator becomes zero is defined as the dryout limit of the evaporator. Experiments are conducted on sintered copper particle evaporators of different particle sizes and heater areas to collect data for model calibration. To demonstrate the wider applicability of the model for other types of porous evaporators, the model is further calibrated against a variety of dryout limit and thermal resistance data collected from the literature. The model is shown to predict the experimentally observed trends in the dryout limit with mean particle/pore size, heater size, and evaporator thicknesses.

Thermodynamic Analysis of the Dryout Limit of Oscillating Heat Pipes

The operating limits of oscillating heat pipes (OHP) are crucial for the optimal design of cooling systems. In particular, the dryout limit is a key factor in optimizing the functionality of an OHP. As shown in previous studies, experimental approaches to determine the dryout limit lead to contradictory results. This work proposes a compact theory to predict a dryout threshold that unifies the experimental and analytical data. The theory is based on the influence of vapor quality on the flow pattern. When the vapor quality exceeds a certain limit (x = 0.006), the flow pattern changes from slug flow to annular flow and the heat transfer decreases abruptly. The results indicate a uniform threshold value, which has been validated experimentally and by the literature. With that approach, it becomes possible to design an OHP with an optimized filling ratio and, hence, substantially improve its cooling abilities.

Effect of geometries on the heat transfer characteristics of flat-plate micro heat pipes

The geometries of wicking structure of flat-plate micro heat pipes (FPMHPs) determine the capillary force and quantity of working fluid, which remarkably affect thermal performance. In this study, flat-plate micro heat pipes (FPMHPs) with different geometries were designed and fabricated, and their heat transfer characteristics were compared. Results showed that the thermal performance of the FPMHPs was relatively weak at α = 0° because the condensate backflow can only be driven by capillary force. At α = 90°, the A-4-type FPMHP presented the best performance under the common joint effect of capillary force, gravitational force, and the quantity of working fluid. The minimum thermal resistance and the maximum thermal conductivity of this FPMHP were 0.89 K/W and 1739.98 W/(m·K), respectively. A visualization study was also conducted to validate and investigate the heat transfer process of the vapor–liquid phase–change and the heat transfer limit in the FPMHP. We found that a slug-train unit formed when the heating power was sufficiently large, causing a local dryout phenomenon. The evaporation section presented a complete dryout with the further increase in heating power. Therefore, the heat transfer limit in the mini/micro FPMHP was dependent on the dryout limit.

Geometric-confinement suppression of flow-boiling instability using perforated wick: Part I CHF and conductance enhancement

Bubbles formed in flow boiling merge and pickup momentum downstream and inhibit liquid supply to the heated surface. To investigate these competitions between the phases adjacent to the heated surface (hydrodynamic instabilities) and to establish leading-edge liquid supply tracks, the flow-boiling canopy wick (FBCW) was previously introduced. The FBCW is a periodic, 3-D porous (sintered, metallic powder) and perforated structure enabling capillary suction to optimally separate and direct the liquid and vapor paths adjacent to the heated surface, while the liquid track is formed between the periodic perforations.Here, addition of levees (geometric confinement) on the edge of the perforations ensures the hydrodynamic stability (by diverting the vapor stream and wall-stabilizing the liquid track), thus extending the liquid track between the levees. The irrigated canopy is connected to porous posts and monolayer-evaporator for liquid supply and the vapor generated over the monolayer wick escapes through the perforations. This thin evaporation wick results in large thermal conductance. The critical heat flux (CHF) or dryout limits (e.g., liquid-vapor hydrodynamic instability, capillary-viscous) of this leveed FBCW are examined using analytical and numerical (CFD, including the VOF technique) simulations. The CFD results for saturated water at 1 atm and for a horizontal, rectangular channel heated at the lower surface show the surface liquid track is stabilized by the levees and the flow-boiling CHF and thermal conductance are enhanced significantly beyond the plain surface, reaching the capillary-viscous dryout limit. The leading-edge liquid track irrigation of the wick allows for use of smaller liquid velocities. The results for the FC-72 fluid are also presented.

Influence of filling ratio on the thermal performance and efficiency of a thermosyphon operating with Al[formula omitted]O[formula omitted]-water based nanofluids

Two-phase closed thermosyphons (TPCTs) are excellent heat transfer devices, widely used in the industry for heat transfer applications. This work examines the effect of employing Al2O3-water based nanofluids at different concentrations and the influence of different factors, such as filling ratio (FR) and power inputs, on the thermal performance and operation limits of a glass TPCT. The results show that the efficiency of the TPCT can be increased by up to 14.8% by using an alumina nanofluid at 0.1 wt% (instead of water), a 45% FR, and 60 W of power. However, similar results were also found when the FR of water was increased. Finally, different operating limits such as dry-out limit, geyser boiling as well as nanoparticle barrier were observed as a result of modifications of experimental conditions and their effects on thermal performance, it was concluded that operating limits involves a negative impact on the thermal performance of the TPCT, nevertheless, the enhancement of the TPCT performance with the nanofluid at 0.1 wt% and 60 W was achieved due to geyser boiling.

The role of vapor venting and liquid feeding on the dryout limit of two-layer evaporator wicks

Vapor chambers developed for high-heat-flux operation require advanced evaporator wick designs that can sustain capillary flow when boiling occurs over the heater region. A two-layer evaporator wick integrates a thin base wick layer that is supplied with liquid from a thick cap layer through an array of vertical feeding posts distributed over the heated area. This design allows boiling to occur within the thin base layer, while separating the incoming liquid feeding and outgoing vapor venting pathways. In our prior work, boiling in two-layer wicks was experimentally demonstrated to provide high-heat-flux dissipation over larger heater areas and at low thermal resistance. The current study experimentally explores the effect of two-layer wick design parameters, specifically the dimensions that alter the area available for liquid feeding and vapor venting, on the thermal performance and dryout limit of the wick, using water as the working fluid. Four different two-layer wick designs are fabricated over a 1 cm2 evaporator area by sintering 180–212 μm copper particles. Increasing the vapor-venting area from 7% to 16% of the total evaporator area yielded a significant increase in the dryout limit, from 315 W/cm2 to 405 W/cm2. Increasing the liquid-feeding area using wider posts increased the dryout limit further. Finally, a parametrically optimized design with fewer but larger posts and vents resulted in better performance compared to a design with denser features. With this two-layer wick design, we demonstrate an extremely high dryout limit of 512 W/cm2 over the large 1 cm2 heated area at a thermal resistance of 0.08 K/W.

Experimental Investigation of Capillary-Driven Two-Phase Flow in Water/Butanol under Reduced Gravity Conditions

The capillary flow of water/butanol mixture is investigated in a single groove heat pipe model on board the Airbus A310 ‘Zero-G’ of the European Space Agency. As working fluid for evaporation-based heat transfer devices like conventional or pulsating heat pipes, these kinds of mixtures give more stable behaviour and higher dry-out limit with respect to pure water because of an anomalous behaviour of the surface tension with temperature. The groove is embedded in a semi-transparent test cell that allows for the qualitative visualization of the liquid distribution along the channel and is heated and cooled at two opposite sides with an electrical resistance and a water circulation system. The evaporation/condensation process is regulated changing the power input in a range between 5 and 30 W and the liquid distribution is detected from a top window using a CCD camera and a LED illumination device. The results show that the liquid distribution is affected by the gravity level and this effect is normally masked on ground.

Experimental and theoretical investigations on temperature limitation in solar thermal collectors with heat pipes: Effect of superheating on the maximum temperature

Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry-out limit. With this approach vapour formation in the solar circuit can be completely avoided, which is essential to reduce costs of solar thermal systems by simplified and more reliable solar circuits. The design of “deactivating” collector heat pipes with a desired maximum temperature requires a comprehensive understanding of the heat transfer processes in the heat pipe, in particular when dry-out takes place. We developed a model, which allows calculating the maximum fluid temperature in the collector for various working fluids. Compared to existing approaches, the effect of superheated vapour in the heat pipe during stagnation is additionally considered. The paper describes the theoretical model in detail and its extensive experimental validation. The results show that the model is able to predict the maximum fluid temperature with an accuracy better than 5 K. Based on parametric studies with different working fluids, we analyse and discuss the temperature limitation and its effect on the collector performance.

Investigation on the temperature dependence of filling ratio in cryogenic pulsating heat pipes

The pulsating heat pipe (PHP) is an efficient two-phase heat transfer device. Its heat transfer performance and internal working fluid flow patterns are affected by a variety of parameters. The filling ratio (FR) is one of the most important parameters, and it is usually regarded as a temperature-independent constant during the operation of the PHP. In this study, the fact that the FR is temperature dependent is pointed out, recognizing that the specific volumes of the liquid working fluid and vapor working fluid change with temperature. FR lines on a T-v diagram are used to analyze the trend of the FR with temperature. Also the influence of the variation of the FR on the heat transfer performance is discussed with the experimental data obtained from a hydrogen-filled PHP and a helium-filled PHP. For a cryogenic PHP, the optimal FR is close to the critical FR, and different initial FRs cause the PHP to either reach a dry-out limit or reach a liquid convection limit. Moreover, the temperature dependence of the FR for room temperature fluids can be ignored.

Visualization experiment on boiling heat transfer and flow characteristics in separated heat pipe system

This paper presents an experimental study on boiling heat transfer and flow characteristics in a separated heat pipe system. The flow patterns in liquid and vapor pipes are clearly observed, and the boiling heat transfer coefficient is measured. The main aim of this study is to examine the heat transfer characteristics at different filling ratios using a combination of visualization and measurement methods. First, the relationship between the boiling heat transfer coefficient and the heating capacity is analyzed at different filling ratios when the heat pipe system operates normally. Second, the conditions for reaching the heat transfer limits and the phenomena occurring at these limits are analyzed. The analysis of the heat transfer characteristics shows that for a given heating capacity, the boiling heat transfer coefficient in the evaporator remains nearly constant over a wide range of filling ratios. At a given filling ratio, the heat transfer coefficient increases with an increase in the heating capacity. Moreover, the flow pattern at the evaporator exit changes from slug flow to bubble flow. Two types of heat transfer limits, i.e., the flooding limit and the dry-out limit, are analyzed at sufficiently high and low filling ratios.