Evaporator Wick Research Articles

Superhydrophilic catenoidal aluminum micropost evaporator wicks

We introduce the superhydrophilic catenoidal aluminum (Al) wicks fabricated by a multi-step wet etching process followed by wet chemical oxidation. The unique three-dimensional sidewall morphology of the developed wick provides the re-pinning of the liquid meniscus during the receding, which substantially increases the thin evaporative film area and the resulting heat transfer performance. The nanostructured aluminum oxide layer (AlO(OH)) was incorporated to enhance both the corrosion resistance as well as the wettability of the wick with water. The experiment shows that the heat transfer coefficient of the developed wick rapidly increases as the heat flux increases up to ~60 W/cm2. The maximum heat transfer coefficient of the catenoidal wick is measured to be ~117% higher than that of the previously-reported cylindrical ones. The numerical simulation clarifies that the increase in the heat transfer coefficient was due to the ~75% increase in the thin evaporative film area. The polarization scanning test shows that the corrosion resistance of the superhydrophilic catenoidal wicks was increased by ~82% compared to the unstructured ones, which clarifies the incorporated aluminum oxide layer acts as an effective corrosion barrier. This work introduces the Al evaporator wicks with substantially improved heat transfer as well as anti-corrosion performances, which will help develop ultra-light, high-performance heat spreaders.

Experimental investigation of a vapor chamber featuring wettability-patterned surfaces

Vapor-chamber heat spreaders are hermetically-sealed systems that rely on metal wicks to circulate a phase-changing liquid and spread heat more efficiently than solid-metal heat sinks. But metal wicks also impose capillary limitations due to their high pressure drop. Surface-wettability patterning does not pose the same limitations and, as shown in this work, could replace metal wicks in vapor chambers to transport the condensate faster and more efficiently. Combining the fast condensate transport characteristic of wettability patterning with the added advantage of domain-regulated dropwise condensation (DWC) and filmwise condensation (FWC) towards enhanced condensation performance, an intriguing hypothesis is explored here by incorporating surface-wettability patterning inside vapor chambers made of copper with and without wicking posts. Wettability patterns with different ratios of superhydrophilic (FWC-promoting) to hydrophobic (DWC-promoting) domains are explored and compared with a control case of unpatterned mirror-finish copper, all located on the condenser side. The heat-spreading capability of the devices from a heated spot of ~0.9 cm2 to an effective vapor chamber area of ~25.8 cm2 is tested at a horizontal orientation where gravity acts perpendicular to the vapor chamber plane and at a vertical orientation. The lowest thermal resistance achieved is 0.24 K/W at 87 W heat load (heat flux of 97 W/cm2) with a horizontally-placed vapor chamber that has a 0.5 mm-thick evaporator wick, a wettability-patterned condenser plate and no wicking posts. The work offers quantitative arguments for incorporating wettability patterning in vapor-chamber technology, and motivates further optimization towards achieving the full potential of this approach.

Improving heat transfer and eliminating Geyser boiling in loop thermosyphons: Model and experimentation

This work presents theoretical and experimental analysis of a new loop thermosyphon, which evaporator has a flat box shape, manufactured by diffusion bonding, designed to promote passive thermal control of avionics. Transient start-up and steady state conditions were considered in this study. The thermal performance of the device was experimentally accessed, with special attention to the Geyser Boiling phenomenon, which is undesirable for aeronautics applications. The phenomenon was eliminated by covering one of the evaporator internal walls by a wick structure. An electrical circuit analogy model was proposed to estimate the overall thermal resistance and the temperature distribution, which depends of the thermal load position in the evaporator external wall. Theoretical predictions were compared favorably with wicked evaporator.

Influence of groove parameters on the thermal hydraulic performance of a composite porous vapor chamber: A numerical study

Composite porous vapor chamber (CPVC) with uniform radial grooves in the evaporator wick demonstrated good thermal hydraulic performance at high heat flux. To reveal the effects of groove parameters on the performance of CPVCs, the temperature, velocity and pressure distributions of CPVCs were analyzed based on a simplified numerical model. Qualitative and quantitative results indicated that: (1) the influence of groove parameters were contradictory; larger parameter values could enhance the thermal performance and benefit the vapor flow, while smaller ones would facilitate the liquid circulation and reduce the maximum pressure drop; (2) the groove width and the number of grooves had more influence on the temperature and velocity distributions, while the effect of the diameter of the circular cavity on the maximum pressure drop was larger; (3) the performance of CPVCs could be obviously improved by adding a wick column at the center of the evaporator wick, which eliminated the risk of insufficient capillary pressure. The study potentially provides a practical engineering approach for the analysis of wicks with multi-artery channels, and a qualitative basis for determining appropriate groove parameters.

Thin hybrid capillary two-phase cooling system

A novel hybrid two-phase cooling system was developed that integrated a mechanically pumped two-phase loop with a capillary-driven two-phase cooling device. The latter cooling mechanism was based on evaporation/boiling from wick structures made by sintering copper particles on the interior surfaces of a copper cold plate. The cold plate provided cooling to two surfaces and each of them included four heaters in series. The novelty of the developed technology was preventing flooding of the evaporator wicks by isolating the evaporation surface from the pumped liquid flow that fed them. This arrangement allowed for a high liquid feed flow rate much greater than would be allowed by a capillary pumped system while maintaining a low thermal resistance at the evaporation surface. Using this approach, the cooling system removed over 850 W with a low pumping power below 1.0 W while using R245fa as the working fluid. The equivalent heat fluxes exceeded 970 W/cm2 over areas less than 0.12 cm2. The measured thermal resistance was as low as 0.09 K-cm2/W. The presented thermal management solution enables an increase in the power of high heat flux electronic devices beyond the state-of-the-art.

Heat pipe dryout and temperature hysteresis in response to transient heat pulses exceeding the capillary limit

The balance between the capillary pressure provided by the wick in a heat pipe or vapor chamber and the flow resistance to liquid resupply at the evaporator determines the maximum heat load that can be sustained at steady state. This maximum heat load is termed as the capillary limit; operation at steady heat loads above the capillary limit will result in dryout at the evaporator wick. However, different user needs and device workloads can lead to highly transient heat loads in applications ranging from consumer electronic devices to server processors. In these instances, the operation of heat pipes must be assessed in response to brief transient heat loads which could be higher than the notional capillary limit that governs dryout at steady state. In the current study, experiments are performed to characterize the transient thermal response of a heat pipe subjected to heat input pulses of varying duration that exceed the capillary limit. Transient dryout events due to a wick pressure drop exceeding the maximum available capillary pressure can be detected from an analysis of the measured temperature signatures. It is demonstrated that under such transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. If the heating pulse is sufficiently long as to induce transient dryout, the heat pipe may experience an elevated steady-state temperature even after the heat load is reduced back to a level lower than the capillary limit. The steady-state heat load must then be reduced to a level much below the capillary limit to fully recover the original thermal resistance of the heat pipe. This characteristic temperature hysteresis following transient dryout has significant implications for the use of heat pipes for pulsed power dissipation. Further experiments are performed to bound the range of heat loads over which the temperature hysteresis is present following a transient dryout event.

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.

Optimization design of grooved evaporator wick structures in vapor chamber heat spreaders

This article proposes a novel optimization method for designing the layout of grooved evaporator wick structures in vapor chamber heat spreaders. Firstly, the design is transformed into a general ‘Area-to-Line’ (AL) constructal problem with conductive heat transfer in a disc configuration. Then, constructal optimization of the first order high conductivity channels (HCCs) with strict length scale is carried out based on entransy dissipation rate (EDR) minimization. According to the optimal constructs, the uniformly heated disc is divided into a series of identical sectors whose perimeter is set as the heat sink and other boundaries are all adiabatic. Based on this, a new growth simulator is developed from the inspiration of natural branching phenomena, and applied to obtain the full constructal layouts of the HCCs. In such simulation, the HCCs are described by parameterized level-set surfaces, which start at the heat sink line and grow toward the disc center according to the design sensitivities. The evaporator wick structure derived by growth simulation shows a unique topology exhibiting dichotomous, hierarchical and branching characteristics, which make the capillary pressure improve, and the temperature gradient more homogeneous. The proposed method contributes to the vapor chamber heat spreaders with better thermal performance.

Receding liquid level in evaporator wick and capillary limit of loop thermosyphon

Macro/micro-scale structures of the evaporator wick of two-phase cooling systems have huge impacts on the flow resistance, thermal resistance, and capillary limit of heat transfer of the two-phase systems. In this study, various evaporator designs such as multilayer and monolayer evaporator wick bases, and porous and tubular wick posts were tested for the thermal performance in a loop thermosyphon (LTS) serving as a test platform. The monolayer wick built with mono-size sintered copper particles embedded in a single layer of a wire mesh was used to reduce the evaporator thermal resistance. The tubular wick posts were used to decrease the flow resistance of the liquid supply to the evaporator wick and eventually increase the capillary limit. The thermophysical and hydrodynamic properties (e.g. thermal resistance and permeability) of the monolayer wick were numerically determined for different liquid levels in the wick base, which was related to various boiling conditions (e.g. flooded, thin-film evaporation, and dryout). The measured thermal resistances were used to predict the liquid level in the wick and boiling conditions. The thermal performance and capillary limit of the LTS were predicted using a thermal-hydraulic network model. It was found from the numerical and experimental results that the evaporator with the monolayer wick base and tubular wick posts provides superior thermal performance such as low thermal resistances (0.097 K-cm2/W) and high capillary limit (309.8 W/cm2 for an elevation difference of 47 cm) as compared to the baseline evaporator using the multilayer wick base and porous wick posts.

Numerical analysis on the thermal hydraulic performance of a composite porous vapor chamber with uniform radial grooves

Composite porous vapor chamber (CPVC) with good thermal performance in temperature uniformity and high heat-flux limit was recently developed, and its wick structure consisted of a condenser wick and an evaporator wick with uniform radial grooves. However, the underlying mass and heat transfer mechanisms of the CPVC are unclear, hindering its further development. A simplified numerical model is presented in this paper to study the thermal hydraulic performance of the CPVC. By analyzing of the thermal performance, liquid/vapor velocity and pressure distributions of the CPVC with the input heat flux of 6 × 105–28 × 105 W/m2, the influence of the wick structures, the wick porosity configuration and the powder size on the thermal hydraulic performance of the CPVC is investigated. Results show that the wicks can provide radial multi-artery channels for liquid backflow and heat conductive passages for heat transfer. The wick porosity affects the performance of the CPVC more than the powder size does. To obtain better performance, the configuration of the evaporator wick porosity and the condenser wick porosity should make the maximum pressure drop in the wicks slightly less than the maximum capillary pressure. However, the optimal porosity configuration varies with the powder size. Larger powder size results into smaller optimal porosity, and vice versa. In addition, the wick porosities should be as close as possible to the lower bound of their range. To facilitate the fluid flowing through the multi-artery channels, the evaporator wick porosity should be slightly larger than the condenser wick porosity. The work is useful for optimizing CPVC.

Numerical study of flat plate solar collector performance with square shape wicked evaporator

In this work, a system of a heat pipe is implemented to improve the performance of flat plate solar collector. The model is represented by square shape portion of the evaporator section of wicked heat pipe with a constant total length of 510 mm, and the evaporator section inclined by an angle of 30o. In this models the evaporator, adiabatic and condenser lengths are 140mm, 140mm, and 230mm respectively. The omitted energies from sunlight simulator are 200, 400, 600, 800 and 1000 W/m2 which is close to the normal solar energy in Iraq. The working fluid for all models is water with fill charge ratio of 240%. The efficiency of the solar collector is investigated with three values of condenser inlet water temperatures, namely (12, 16 and 20o C). The numerical result showed an optimum volume flow rate of cooling water in condenser at which the efficiency of collector is a maximum. This optimum agree well with the ASHRAE standard volume of flow rate for conventional tasting for flat plate solar collector. When the radiation incident increases the thermal resistance of wicked heat pipe is decreases, where the heat transfer from the evaporator to condenser increases. The numerical results showed the performance of solar collector with square shape evaporator greater than other types of evaporator as a ratio 15 %.

Experimental analysis of thin vapor chamber with composite wick structure under different cooling conditions

In this work, two types of thin vapor chambers, namely, copper foam vapor chamber (CFVC) and copper mesh vapor chamber (CMVC), are developed to enhance the thermal performance of vapor chambers for cooling high heat flux electronic devices. The evaporator wick is a composite wick of sintered copper powder-mesh with multi-artery structures made of several sintered powder bars on a thin evaporator zone. The condenser wicks for CFVC and CMVC are copper foam wick and 3-layer 200 mesh wick, respectively. A water-cooled testing system is set up to investigate the thermal performance of the vapor chamber. The characteristic parameters, including the temperature distribution, thermal resistance, filling ratio and critical heat flux (CHF) of vapor chambers are studied. The results indicate that the thermal performance of the vapor chamber is significantly affected by the cooling water temperature. At cooling water temperatures of 50 and 30 °C, the CHF of the CFVC can reach 180 W/cm2, which is increased by 100–200% compared to that of the CMVC. The CFVC exhibits a better heat transfer performance and adaptability to the cooling water temperature, given that the copper foam wick has a larger capillary performance parameter and a smaller contact thermal resistance.

Experimental analysis of operation failure for a neon cryogenic loop heat pipe

As a highly efficient cryogenic heat transfer device, cryogenic loop heat pipes (CLHP) hold significant application potential in the thermal control of future generation space infrared detection system. However, operation failure or anomaly occurs more easily for CLHPs, no matter in the supercritical startup or steady-state operation, due to very low operating temperature combined with narrow operating temperature range. In this work, a neon CLHP operating at 30–40 K was developed, and its operating characteristics were investigated experimentally mainly about the temperature fluctuation and operation failure phenomena. To be specific, the influences of charged pressure of working fluid, primary heat load and impurity air on the supercritical startup, and the effects of heat sink temperature, charged pressure of working fluid and auxiliary heat load on the steady-state operation were studied. Experimental results showed that the application of small primary and secondary heat loads may lead to a supercritical startup failure. Insufficient working fluid under relatively low charged pressure would cause temperature fluctuation of the secondary evaporator during the supercritical startup. There existed a maximum percentage of impurity gas, i.e., about 0.35%, allowing the CLHP to achieve a successful supercritical startup. The secondary evaporator would dry out if the CLHP was subject to improper heat sink temperature, charged pressure of working fluid or auxiliary heat load, due to insufficient liquid supply to the secondary evaporator wick or the capillary limit.

Simultaneous wick and fluid selection for the design of minimized-thermal-resistance vapor chambers under different operating conditions

The thermal resistance of a vapor chamber is primarily governed by conduction across the evaporator wick and the saturation temperature gradient in the vapor core. The relative contributions of these two predominant resistances can vary dramatically with vapor chamber operating conditions and geometry. In the limit of very thin form factors, the contribution from the vapor core thermal resistance dominates the overall thermal resistance of the vapor chamber; recent work has focused on working fluid selection to minimize overall thermal resistance in this limit. However, the wick thermal resistance becomes increasingly significant as its thickness increases to support higher heat inputs while avoiding the capillary limit. It therefore becomes critical to simultaneously consider the contributions of the wick and vapor core thermal resistances in the development of a generalized methodology for vapor chamber working fluid selection. The current work uses a simplified thermal-resistance-network-based vapor chamber model to explore selection of working fluids and wick structures that offer the minimum overall thermal resistance as a function of the vapor chamber thickness and heat input. An illustrative example of working fluid selection, for cases with and without the contribution of wick thermal resistance, is first used to demonstrate the potential significance of the wick thermal resistance on fluid choice. This influence of the wick on working fluid selection is further explained based on the wick properties (effective pore radius, permeability, and effective thermal conductivity). The ratio of effective pore radius to wick permeability is found to be the most critical wick parameter governing the overall vapor chamber resistance at thin form factors where minimizing the wick thickness is paramount; the wick conductivity becomes an equally important parameter only at thicker form factors. Based on this insight, a new approach for vapor chamber design is demonstrated, which allows simultaneous selection of the working fluid and wick that provides minimum overall thermal resistance for a given geometry and operating condition.

Area-scalable high-heat-flux dissipation at low thermal resistance using a capillary-fed two-layer evaporator wick

A two-layer sintered porous evaporator wick for use in vapor chambers is shown to offer very high performance in passive high-heat-flux dissipation over large areas at a low thermal resistance. The two-layer wick has an upper cap layer dedicated to capillary liquid feeding of a thin base layer below that supports boiling. An array of vertical posts bridges these two layers for liquid feeding, while vents in the cap layer provide an unimpeded pathway for vapor removal from the base wick. The two-layer wick is fabricated using a combination of sintering and laser machining processes. The thermal resistance of the wicks during boiling is characterized in a saturated environment that replicates the capillary-fed working conditions of a vapor chamber evaporator. Thermal characterization tests are first performed using conventional single-layer evaporator wicks to analyze the effect of sintered particle size on capillary-fed boiling of water. Of the particle size ranges tested, wicks sintered from 180 to 212 μm-diameter particles provided the best combination of high dryout heat flux and a low boiling resistance. A two-layer evaporator wick comprising particles of this optimal size and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm2 over a 1 cm2 heat input area while also maintaining a low thermal resistance of only ∼0.052 K/W. The thermal performance of the two-layer wick is compared against various hybrid and biporous evaporator wicks previously investigated in the literature. While previous wick designs are typically restricted to small areas and low power levels or high surface superheats when dissipating such heat fluxes, the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than were previously possible.

Experimental investigation of boiling regimes in a capillary-fed two-layer evaporator wick

Vapor chambers with transformative evaporator wick designs capable of passively dissipating high heat fluxes over large areas, while maintaining low thermal resistances, can meet the thermal management needs of next-generation power semiconductor devices. Our prior work proposed a two-layer evaporator wick structure to enhance the performance of vapor chambers operating at high heat fluxes. The current study experimentally characterizes the capillary-fed boiling heat transfer behavior in such a two-layer evaporator wick, compared to a homogeneous (single-layer) wick, over a 1 cm2 evaporator area. The two-layer design comprises a thin base wick layer that is fed with liquid from a thick cap wick layer above using an array of vertical posts. The two-layer wick is fabricated using a sequence of sintering and laser-machining steps to form the base wick layer (200 µm), array of liquid-feeding posts, and cap wick layer (800 µm) using 90–106 µm copper particles. A test facility is constructed to replicate the conditions that exist at the evaporator of a vapor chamber; the novel facility design uses a physical restriction to prevent flooding of the wicks during testing. Two-layer wicks having 5 × 5 and 10 × 10 arrays of liquid feeding posts are characterized, along with a 200 µm-thick single-layer evaporator wick. The 10 × 10 array provides a >400% enhancement in the dryout heat flux compared to the single-layer wick. High-speed visualizations are used to identify the characteristic regimes of boiling operations for the wicks. The single-layer wick exhibits a partial dryout mode of operation, where a dry spot formed in the center of the heated evaporator area causes an increase in the thermal resistance with heat flux. In contrast, the distributed feeding provided by the two-layer wicks mitigates the development of this partial dryout regime and maintains a constant low resistance (∼0.1 K/W) during capillary-fed boiling until a complete dryout event occurs. This study demonstrates the significant enhancement in dryout heat flux offered by the liquid-feeding approach realized in the two-layer evaporator wicks characterized here.

Design of an Area-Scalable Two-Layer Evaporator Wick for High-Heat-Flux Vapor Chambers

A hybrid two-layer evaporator wick is proposed for passive, high-heat-flux dissipation over large areas using a vapor chamber heat spreader. For such applications, the evaporator wick layer must be designed to simultaneously minimize the device temperature rise and minimize the flow resistance to a capillary feeding of the wick. This requires a strategy that exploits the benefits of a thin wick for reduced thermal resistance and a thick wick for liquid feeding. In the present design, a thick cap layer of wick material evenly routes liquid to a thin, low-thermal-resistance base layer through an array of vertical liquid-feeding posts. This two-layer structure decouples the functions of liquid resupply (cap layer) and capillary-fed boiling heat transfer (base layer), making the design scalable to heat input areas of ~1 cm2 for operation at 1 kW/cm2. A reduced-order model is developed to demonstrate the potential performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The two-layer wick design is found to sustain liquid feeding at higher heat fluxes, without reaching the capillary limit, compared to single-layer evaporator wick designs.

Thermal design optimization of evaporator micropillar wicks

Heat pipes and vapor chambers act as efficient heat spreaders since they rely on phase change of the coolant. The evaporator design is critical and typically high performance is characterized by the high heat dissipation capability with low thermal resistance. In the past, there have been numerous experimental and modeling studies focused on the design of evaporator wicks of different geometries, but systematic studies to simultaneously optimize both the heat flux and thermal resistance have been limited. In this work, we developed a comprehensive model that considers both aspects to provide design guidelines for evaporator micropillar wicks. We show that capillary limited heat dissipation is best captured with a recently developed numerical model as compared to previous analytical models. We also developed a numerical model to obtain the effective wick thermal conductivity, which is a function of pillar diameter, pitch, and height. Smaller diameters with smaller pitches of the pillars had more thin film area and had larger effective wick thermal conductivities. Our parametric investigations show that trade-offs between lowest thermal resistance and maximum heat carrying load exists, and the actual wick geometry will be dictated by application specific requirements. Finally, we highlight the importance of accurately obtaining the accommodation coefficients to predict the effective wick thermal conductivity. The present work would enable in optimal design of micropillar wicks (with low thermal resistance and high dry-out heat flux) and the same methodology can be extended to other types of wick structures as well.

Mechanical-capillary-driven two-phase loop: Numerical modeling and experimental validation

A mechanical-capillary-driven (Hybrid) Two-Phase Loop (HTPL) is a phase change cooling device that utilizes both mechanical (active) and capillary (passive) pumping. The HTPL consists of a mechanical pump, evaporator, condenser, and liquid reservoir which are connected through two separate loops for liquid and vapor flows. The hybrid (active/passive) pumping and scalable evaporator design of the HTPL make it possible to handle challenging thermal requirements such as high heat flux heat acquisition, large heat transfer area, reliable operation, long distance thermal transport with a small temperature difference. In this paper, the operational characteristics of the HTPL were numerically and experimentally investigated by varying heat inputs, flow rates of mechanical pump, and heat sink temperatures. A pressure relation between liquid and vapor phases in the evaporator was proposed to determine three distinctive boiling modes (flooded, partially flooded, and capillary) and heat transfer limits were discussed. The operating range of the capillary mode, which is a desirable boiling condition with low thermal resistances, is extended by reducing the pressure drop of the liquid supply in the evaporator and increasing a capillary pressure head in the evaporator wick. A reduction of heat sink temperature increases the system thermal resistance of the HTPL, especially in the vapor line which exists from the evaporator to the condenser. A capillary limit of the HTPL, by increasing pump flow rate, approaches a boiling limit which is estimated to be 259.8 W/cm2.

Phase change heat transfer characteristics of porous wick evaporator with bayonet tube and alkali metal as working fluid

Porous wick evaporator is an important power source of alkali metal thermal to electric converter (AMTEC) available for different kinds of energy sources including renewable energy. A bayonet tube structure is adopted in porous wick evaporator which is based on the principle of capillary pumped loop (CPL). The phase change heat transfer characteristics in the evaporator were numerically studied and the impact of the bayonet tube was investigated. The working fluid temperatures on two feature lines were selected to analyze the effect of bayonet tube on temperature of porous wick evaporator. The results show that for the evaporator using alkali metal as working fluid, a stable vapor-liquid interface is maintained at wick-groove interface. The bayonet tube divides the working fluid flow in porous wick into two parts, and the temperature change rule inside the bayonet tube is different from that outside the bayonet tube. The bayonet tube structure in the evaporator causes the temperature distribution in liquid channel to become more uniform and the maximum temperature to be decreased. The fluid temperature in the evaporator, especially in the liquid channel, can effectively be affected by the thermal resistance per unit area, length and diameter of bayonet tube. These parameters for bayonet tube must be limited within acceptable ranges in a specific operating condition, and there are optimum values to guarantee the best performance of the evaporator.