Vapor Core Research Articles

A parametric study on the performance of vapor chamber in association with pillar distribution

The performance of vapor chamber is significantly affected by the wick structure, height of the vapor cavity, pillar structure, working fluid, filling ratio, and heat source area. Besides, it can also be influenced by the pillar distribution, which has not been extensively studied in prior literature. Therefore, the present study investigates the influence of pillar distribution on the vapor chamber performance. Totally 11 different vapor chamber models are fabricated and tested, and water is selected as a working fluid for all the samples. The examined geometric features include different mesh layers, mesh sizes, wick structure in condenser surface, pillar distributions, and vapor core space. The results showed that the evaporator with a single-layered mesh provided the thermal resistance of 0.19 K W−1, while triple-layered mesh yielded a thermal resistance of 0.24 K W−1. Moreover, the 2 × 200 mesh offered a maximum heating load of 161 W with a thermal resistance of 0.18 K W−1 against the maximum heating load of 120 W and 75.6 W by the 2 × 150 and 2 × 100 mesh, respectively. The experimental results showed that increasing the pillar numbers alone may not always reduce the thermal resistance, a better pillar distribution can help to direct the vapor flow and improve the performance of the vapor chamber. The proposed vapor chamber with a novel X-wing patterned pillar distribution can offer a thermal resistance of 0.19 K W−1 at a heating load of 146.6 W.

Real-Time Defects Analyses Using High-Speed Imaging during Aluminum Magnesium Alloy Laser Welding

In this study a continuous wave Ytterbium-doped Yttrium Aluminum Garnet (Yb: YAG) disk laser has been used for welding of AlMg3 casted alloy. A high-speed imaging camera has been employed to record hot vapor plume features during the process. The purpose was to identify a mechanism of pores detection in real-time based on correlations between metallographic analyses and area/intensity of the hot vapor in various locations of the samples. The pores formation and especially the position of these pores had to be kept under control in order to weld thick samples. Based on the characterization of the hot vapor, it has been found that the increase of the vapor area that exceeded a threshold value (18.5 ± 1 mm2) was a sign of pores formation within the weld seam. For identification of the pores’ locations during welding, the monitored element was the hot vapor intensity. The hot vapor core spots having a grayscale level reaching 255 was associated with the formation of a local pore. These findings have been devised based on correlation between pores placement in welds cross-section microscopy images and the hot vapor plume features in those respective positions.

Flow Regimes and Transitions for Two-Phase Flow of R152a During Condensation in a Circular Minichannel

Two-phase flow regimes were experimentally investigated during the entire condensation process of refrigerant R152a in a circular glass minichannel. The inner and outer diameters of the test minichannel were 0.75 and 1.50 mm. The channel was 500 mm long to allow observation of all the two-phase flow regimes during the condensation process. The experiments used saturation temperatures from 30 to 50°C, a mass flux of 150 kg/(m2·s) and vapor qualities from 0 to 1. The annular, intermittent and bubbly flow regimes were observed for the experimental conditions in the study. The absence of the stratified flow regime shows that the gravitational effect is no longer dominant in the minichannel for these conditions. Vapor-liquid interfacial waves, liquid bridge formation and vapor core breakage were observed in the minichannel. Quantitative measurements of flow regime transition locations were carried out in the present study. The experiments also showed the effects of the saturation temperature and the cooling water mass flow rate on flow regime transitions. The results show that the annular flow range decreases and the intermittent and bubbly flow ranges change little with increasing saturation temperature. The cooling water mass flow rate ranging from 38.3 kg/h to 113.8 kg/h had little effect on the flow regime transitions.

Mechanism of transition from elongated bubbles to wavy-annular regime in flow boiling through microchannels

Transition from elongated bubbles to annular flow regime is one of the most intricate phenomena in flow boiling in microchannels. To identify this transition, flow regime maps have been introduced primarily based on visualizations and characterized using average flow properties. However, understanding the mechanism responsible for this transition demands accurate determination of the hydrodynamic characteristics of liquid films around the vapor core. But, the challenges associated with measurements of the thickness and velocity of the liquid films have made it difficult to determine the flow hydrodynamics. Here, we utilize a measurement technique to determine the liquid film thickness along with velocities of the two phases. These measurements have enabled accurate calculation of the cross-sectional void fraction, vapor quality, interfacial shear stress and superficial velocities, which form the coordinates of the existing flow regime maps. The results show that upon transition to the annular flow regime, a remarkable increase in the liquid film thickness occurs due to a substantial increase in the liquid-vapor interfacial shear stress. This shear stress, which is a function of the liquid film thickness, vapor velocity, and channel aspect ratio, is introduced as a criterion for transition from elongated bubbles (EB) to wavy-annular (WA) flow regime.

Effect of pipe flattening on pressure drop in vapor core and thermal characteristic of miniature round and flat-shape heat pipe with sintered fiber wick

The effect of pipe flattening on pressure drop in vapor core and also heat transfer characteristic of the sintered fiber heat pipes has been investigated. The original diameter of heat pipes were 2 mm and 3 mm, respectively, with the effective length of 100 mm. Thermal performance of the flat-shaped heat pipe with the sintered fiber bundle wick was experimentally investigated under a horizontal orientation in this study. The proper fiber bundle wicks were fabricated at porosity of 0.54, and firmly filled within internal heat pipe wall by the sintering process. The heat pipes were pressed into two final thickness by the flattened process which results in vapor core minimizing. The numerical method was established in modified forms to represent the related wick-heat pipe design parameters, for example, wick thickness, wick porosity, wall thickness, flattened width and height, corresponding to thermal performance of fiber wick heat pipes. The numerical analysis for pressure drop was only focused on vapor core region. The vapor flow was assumed to be laminar and incompressible. It is found that when the heat pipe was flattened, the pressure drop increased. The flattening of heat pipe caused the smaller space of vapor core, therefore the working fluid in vapor phase could not easily transfer heat. In this case, the critical final thickness of 0.45 mm (for original diameter of 2 mm) which the vapor core area nearly approaches to zero, has an effect on the high value of thermal resistance considered in vapor core (z5). In addition, the overall thermal resistance estimated from ESDU79012 agrees well with that obtained from the experiment with the standard deviation (STD) of ±15 %, and it shows that the thermal resistance model can provides a reasonable prediction for the thermal resistance of flat-shaped heat pipes with sintered fiber wicks. Moreover, the new normalized parameter h′v/hv is proposed to represent the appearance of vapor core after pipe flattening which can be used to suggest a desirable normal region of flattened heat pipe with the sintered fiber wick. The critical remaining vapor core height after pipe flattening has been further analyzed to be 0.06 (about 74% from original diameter).

An advanced conduction based heat pipe model accounting for vapor pressure drop

Heat pipes are passive heat transfer devices which play an increasingly important role in state-of-the-art cooling solutions for various technologies like consumer electronics or electric machines. During design phase, reliable simulation results are the key for optimal performance and efficiency. In system level simulations, heat conduction models, characterized by low computational effort, are widely used to predict heat pipe thermal resistance. In order to accurately calculate overall thermal characteristics, a precise prediction of the vapor core’s temperature drop is crucial. In this study, a novel conduction-based method for calculating heat pipe performance is presented. Taking velocity components perpendicular to the main vapor flow into account, an analytic expression for calculating the axial pressure gradient is developed and validated through detailed CFD simulations. Assuming saturation conditions, results are used to determine the vapor core’s local effective thermal conductivity. Parameters needed for calculation are provided for both circular and flat heat pipes under various operating conditions. The final model accounts for phase change thermal resistance and temperature dependent fluid properties. The presented method yields significantly different results compared to state-of-the-art approaches in which thermal vapor resistance is either assumed to be zero or calculated assuming (Hagen-)Poiseuille flow. Simulation results suggest a strong dependence of vapor thermal resistance on evaporation and condensation mass fluxes, thermophysical fluid properties and local vapor temperature.

Cascaded Multicore Vapor Chambers for Intrapackage Spreading of High-Power, Heterogeneous Heat Loads

A cascaded multicore vapor chamber (CMVC) is designed for dissipating heat from high-flux hotspots simultaneously with a high-total-power background. Current thermal management strategies rely on spreading high local heat fluxes by conduction in the lid of electronics packages. Embedding vapor chambers (VCs) within the lid is an attractive option to directly address intrapackage hotspots. We investigate the design of intralid VCs, for a generic device having a total heat load of 476 W having a background heat flux of 0.75 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , with hotspots of 8 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a 1-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area. A conventional VC design, having a single vapor core, will require a thick evaporator wick to avoid the capillary limit for large total power. The necessity for a thick wick then imposes a large thermal conduction resistance when the VC is exposed to high heat flux hotspots. The proposed CMVC architecture aims to address this limitation. The cascaded architecture comprises a bottom-tier VC having an array of multiple small vapor cores for spreading heat from the small hotspots. These small vapor cores have short paths of liquid return to the evaporator, such that they can handle their footprint heat load while using thin wicks, resulting in a low hotspot thermal resistance. Furthermore, local dampening of the hotspots by the bottom tier then reduces the thermal conduction resistance across the necessarily thick wick in the top tier. Hence, the cascaded architecture has the potential to significantly reduce the overall thermal resistance, relative to a single tier. To substantiate this design rationale, experiments are performed to illustrate that the resistance of a commercial VC can be significantly reduced by interfacing the heat source with an intermediate heat spreader. Reduced-order models are then used to understand the effect of the wick properties (porosity and particle size) and geometric parameters on the thermal performance of the CMVC for the representative power map. The optimal CMVC design offers a thermal resistance (0.66 K/W) that is significantly lower compared to a conventional single-core VC (1.76 K/W) owing to a reduction in the conduction resistances across the internal wicks. That parametric optimization results demonstrate that the thermal resistance of the CMVC is more sensitive to the wick porosity compared to the particle diameter. Furthermore, there exists a wide range of wick properties and vapor core sizes for which near-optimum thermal performance can be attained, which is particularly attractive from the standpoint of flexibility in design and manufacturing.

Dryout characteristics of low-GWP working fluids at low mass and heat fluxes in a vertical 4 mm diameter tube

The dryout phenomena in flow boiling of low-GWP working fluids at low mass and heat fluxes in a vertical 4 mm diameter tube were examined, and a new correlation of the dryout incipience quality has been proposed. The working fluids examined in the present study included R1233zd(E), R1224yd, R245fa, and HFE347pc-f, under the mass flux, the heat flux, and the reduced pressure ranging in 20–40 kg/(m2⋅s), 6–20 kW/m2, and 0.061–0.138 respectively. The dryout incipience quality was quantified based on the measured local heat transfer coefficients, and the flow boiling behavior near the dryout incipience was visualized through a transparent quartz glass tube with ALD-AZO electrodes on its inner wall. It is found that the dryout incipience quality decreases with increasing the heat flux, decreasing the mass flux, and increasing the reduced pressure. Under the same operating condition, R1233zd(E) shows the smallest value of the dryout incipience quality. From the flow boiling visualization, the dependence of the operating condition and the working fluids upon the dryout incipience is found to be associated with features of the entrained droplets in the vapor core. A new correlation of the dryout incipience quality based on the boiling number, the Weber number, the Reynolds number and the density ratio has been proposed. The new model predicts all the experimental data to within ±10%.

Microstructured wettability pattern for enhancing thermal performance in an ultrathin vapor chamber

We investigated the thermal performance of a novel wettability patterned evaporator for an ultrathin vapor chamber. Because the evaporator integrates a wettability patterned substrate underneath the nanostructured mesh wick which can pin the three-phase contact lines on the hydrophilic/hydrophobic boundaries, it enlarges the area of thin-film evaporation. Microstructured wettability pattern is fabricated on the evaporator surface and the wick is pressed onto the evaporator by a micropillar array to make an intimate contact with each other. The micropillar array electroplated on the inner side of the condenser also supports a vapor core as a vapor flow path. The thermal resistance of the ultrathin vapor chamber is experimentally evaluated, and the measurement results show that the wettability pattern underneath the nanostructured mesh wick can greatly reduce the horizontal thermal resistance, giving a better temperature uniformity across the condenser side, though the vertical thermal resistance may be slightly larger than that without a wettability pattern. The highest in-plane effective thermal conductivity of a 200 μm-thick vapor chamber can reach 11914.9 W/(m·K) at 23.91 W/cm2 heat flux, which shows a 210.7% further improvement in comparison with that of the ultrathin vapor chamber with the nanostructured mesh wick only.

Effective Anisotropic Properties-Based Representation of Vapor Chambers

An easy-to-use representation of vapor chambers is developed in terms of effective anisotropic properties. This approach enables accurate simulation of the vapor chamber represented as a solid conduction block by assigning appropriate values to its effective density, specific heat, in-plane thermal conductivity, and through-plane thermal conductance. These effective properties are formulated such that the vapor chamber operation in terms of steady-state and transient thermal responses matches a full, physical simulation of phase change and energy transport in the vapor core; they are intrinsic properties that can be applied independently of the boundary conditions and heat input.

A Simple Model to Identify Dryout Location: Validation and Computational Fluid Dynamics Predictions

Prediction of dryout location is essential to identify reliable heat flux margins for the safe operation of different thermal systems. The present study proposes a simple model to predict the dryout type critical heat flux location in a circular tube and an annulus. Based on the flow topology, the annular flow boiling regime is demarcated into two zones viz., vapor flow in the core region and liquid film flow in the annular gap between the vapor core and the wall surface. An expression is derived for the dryout location which is purely a function of operating conditions and fluid properties. The dryout location and quality predictions of the present model are found to be in very good agreement and within 10% error margin against the experimental data for tube and annulus geometries. The present study is extended to investigate the effect of eccentricity on the dryout location and the inner wall temperature. It is observed that the eccentricity results in early occurrence of dryout in the narrow gap region of an eccentric annulus. Furthermore, the predictions from the present model are compared with the look-up table data, subchannel analysis and computational fluid dynamics simulation predictions.

Development of ultrathin thermal ground plane with multiscale micro/nanostructured wicks

In this paper, TGPs with thickness of around 0.53–0.6 mm are developed with copper plate as the casing material, nanostructured copper foam as the wick and electroplated copper pillar as the support of vapor core. The effects of charge amount of water, nanostructured wick, the number of copper foam layers and vapor core thickness on the thermal performance of the developed TGP are experimentally studied. It is found that increasing the number of copper foam layers or vapor core thickness can greatly lower the TGP thermal resistance. Specifically, a TGP with a double layers copper foam wick has a much better thermal performance than that with single layer even though the total wick thickness and porosity remain unchanged. Experimental results also show that the multiscale micro/nanostructured wick is beneficial for improving the TGP thermal performance. Small change of vapor core thickness can lead to great change of TGP thermal performance. The ultrathin TGP can reach an effective thermal conductivity of 2207 W/(m·K) at an input power of 22.66 W over a heating area of 8 mm × 8 mm.

Discussion of the Effect of Shielding Gas and Conductivity of Vapor Core on Metal Transfer Phenomena in Gas Metal Arc Welding by Numerical Simulation

Gas metal arc welding is indispensable in many fields of industry. In this process, various kinds of shielding gas are used, and they significantly affect the behaviors of the arc plasma and metal transfer. In this study, these behaviors with various kinds of shielding gas are numerically investigated. In addition, the influence of the electrical conductivity of the metal vapor is discussed. Simulation results show that with Ar gas, spray transfer occurs at an arc current of more than 240 A, and with CO2 gas, the transfer mode is globular, even at an arc current of 300 A. The calculation results show that the current path near the wire tip critically determines droplet behavior. With Ar gas, the current path is spread out, covering the molten wire, whereas with CO2 gas, the current path is concentrated at the bottom of the molten wire. Therefore, to achieve spray transfer, the current path needs to be spread at the wire tip; however, if the spreading is excessive, the transfer mode becomes streaming transfer. To investigate the influence of the metal vapor, a numerical experiment using pseudo metal vapor was carried out. Even with CO2 gas, the electrical conductivity of the metal vapor was low, and thus the current path was not concentrated at the bottom of the molten wire, allowing spray transfer. The numerical results show that metal transfer phenomena can be regulated by controlling the electrical conductivity of the metal vapor.

Development and Application of a Thin Flat Heat Pipe Design Optimization Tool for Small Satellite Systems

Abstract With easier access to space and the growing integration of power-dense components, small-scale thermal management solutions are increasingly in demand for small satellite systems. Due to the strict mass and volume requirements commanded by such power-dense small spacecraft, heat pipes with thin and flat architectures provide nearly ideal solutions for the efficient transfer and dissipation of heat. Unlike traditional heat pipes, however, the performance of thin heat pipes is heavily dependent on details of the internal heat pipe structure, including the vapor core geometry and structural mechanical characteristics. In this study, the development and testing of a new computational modeling and optimization tool are presented for the design of thin flat heat pipes. The computational model is described in detail and includes parameters that define properties of the liquid wick, vapor core, and structural case. The model is coupled to a gradient-based optimization procedure that minimizes a multi-objective cost function for a range of operating conditions. The cost function is expressed as the weighted sum of the total temperature drop, the liquid/vapor pressure ratio, the total mass of the heat pipe, and the structural deflection of the heat pipe during operation. The combined computational modeling and optimization tool is then used to design a copper-methanol flat heat pipe for a small satellite mission, where the optimization is performed with respect to both cold and hot orbital conditions. Validation of the optimized heat pipe is performed using computational fluid dynamics (CFD) simulations of the initial and final designs.

Heat transfer and pressure drop of film condensation in a horizontal minitube for HFO1234yf refrigerant

This paper applies the computational fluid dynamics (CFD) simulation to study the condensate two-phase thermofluid characteristics of refrigerant HFO-1234yf flowing through horizontal straight and convergent passages for guiding the design of minitube heat exchangers. The theoretical analysis employs the volume-of-fluid (VOF) method to model the progression of film condensation process for determining the distributions of velocity, pressure, temperature, vapor volume fraction and film thickness for liquid-vapor interfacial flows. The Lee model is formulated as a user defined function to treat the phase change effect at the interface. The predictions of heat transfer coefficients and pressure drops agree reasonably well with the measured data from the referencing literature at the mass and heat fluxes of 200–800 kg/m2 s and 9.9–24.3 kW/m2 for validation of the computational model. The simulated results are presented to capture the complex two-phase flow behaviors during the film condensation progression, including the formation of interfacial ripples of annular films, wave-shaped films with elongated necking regions, shedding of liquid ligaments and breakup of these ligaments entrained into the vapor core. The VOF calculations also estimate the average heat transfer coefficient and pressure drop up to 11.3 kW/m2 K and 55.1 kPa at a mass flux of 800 kg/m2 s for the straight tube, showing similar thermal performance to refrigerant HFC-134a reported by open literature. The convergent passage essentially raises the average heat transfer coefficient and pressure drop because of more severe shear stresses over the film surface. The correlations of heat transfer coefficient and pressure drop are compared with CFD predictions for straight and convergent tubes. The performance assessment is thus conducted using the validated correlations to guide the redesign of minitube heat exchangers with HFO1234yf as an alternative refrigerant in various energy systems. Two design changes are presented using HFO1234yf refrigerant to enlarge the total surface area up to 13.49% in straight tubes or broaden the cooling surface area of only 6.4% in convergent tubes for matching the performance of R134a.

Semi-Analytical Modeling of Critical Heat Flux in Convective Boiling of Nanofluids

Using two-phase flows because of their high thermal conductivity coefficients is favorable in various industries such as oil, petro chemistry, and power plants that require significant thermal conductivity coefficients in heat exchangers. On the other hand, the limiting factor in benefiting from these flows is the critical heat flux phenomenon, the onset of which must be delayed by various means. One of the methods to achieve this goal is using nanofluids. Nanofluids, due to their higher thermal conductivity in comparison with base fluids, have demonstrated the capability to delay the onset of critical heat flux phenomenon. In the present Paper, silver, copper, alumina and nickel water-based nanofluidic flows in heat exchangers are analytically studied, and the critical heat flux is assessed under different mass flux and volume fraction conditions. Reports of Look Up Table are employed for calculation of critical heat flux in the present Paper. All four mechanisms of mass transfer are simulated, and pressure variations in both dimensions are considered. Effects of mass transfer on vapor core pressure are comprehensively assessed. The results obtained indicate a directly proportional relation between mass velocity and critical heat flux. In addition, increasing the concentration of nanoparticles also increases the critical heat flux. Of the studied nanofluids, the highest critical heat flux increase was observed for a 5 vol % water–silver nanofluid with a 81.6% increase in the critical heat flux relative to pure water.

Characterization and thermal modeling of a miniature silicon vapor chamber for die-level heat redistribution

Vapor chambers are passive heat spreaders that can improve system level temperature uniformity through efficient heat transport in a high effective thermal conductivity vapor core. Fabricating a vapor chamber out of silicon is highly appealing due to the potential for direct integration schemes with existing semiconductor devices, but may be impractical from a cost perspective if the size of the vapor chamber must be much larger than the die. We investigate the potential benefit to using a miniature silicon vapor chamber with an active vapor transport region of 1 × 1 cm2 for the purpose of die-level heat redistribution. Due to the high amount of liquid charging precision required for working with such small-scale vapor chambers, a reduced order thermo-fluidic model is developed to predict the effect of both heat flux and liquid charge on the overall device thermal performance. The model incorporates wick microstructure effects and is validated against experimental results from a prototype device to agree within ±25%. The thermal performance of the vapor chamber is benchmarked against simulation results for solid silicon spreaders of comparable dimensions and is found to improve the hotspot temperature uniformity at heat fluxes above 60 W/cm2.

A critical heat flux model for unilateral heating rectangular narrow channel

In most designs, the coolant channel is under non-uniform heating, mainly one-sided heating. There is no clear understanding of the problem of unilateral heating. In order to deepen the understanding of the mechanism of CHF in the unilateral heating rectangular narrow channel, a three-fluid transient model of annular flow was developed for the rectangular narrow channel heated on one side. Basic conservation equations were established for the liquid film, droplets, and vapor core respectively, and the constitutive model involved void fraction, interphase friction, wall friction, and the calculation of entrainment rate and deposition rate. The finite difference method was used to solve the three-fluid transient model of annular flow numerically. Within a wide range of experimental parameters (pressure 0.1–15.0 MPa, mass flux 300.0–17100.0 kg/m2s, inlet subcooling 6.5–292.5 K, length of channel section 4.0–50.0 mm, width of channel section 2.0–6.35 mm, length of channel 200–1550 mm, and CHF value 0.63–28.4 MW/m2), most of the calculation error of the model were within 30%, which proved the validity of the three-fluid transient model of annular flow.

Monte Carlo model of annular flow in boiling water reactors

Annular flow regime consists of a liquid film layer, a wave layer attached on liquid film layer and droplets non-uniformly dispersed within vapor core. Each has a great importance from the viewpoint of nuclear reactor safety calculations as annular flow occupies about two-thirds of total length of a BWR fuel rod. For this purpose, the annular flow in unit cell geometry was modeled by Monte Carlo method for various void distributions. Effect of each distribution was investigated to calculate the magnitude of the deviation in infinite multiplication factor due to modeling. Since the phase structures present in annular flow are added step-by-step (starting from homogeneous model approach (HEM)) in this work, δk is calculated by taking the difference from the previous case. The results show that k∞ is strongly dependent on liquid film thickness, existence of wave, shape of wave and droplets. Liquid film thickness leads to increase k∞ whereas wave and droplets have a negative impact on k∞. Wave shape has double-sided effect on k∞. Although maximum deviation is about 400 pcm for liquid film layer, 300 pcm for wave layer, 150 pcm for wave shape and 300 pcm for droplets, the combined effect is at most 400 pcm.

Study of Miniaturization of a Silicon Vapor Chamber for Compact 3D Microelectronics, via a Hybrid Analytical and Finite Element Method

The interest in silicon vapor chambers (SVCs) has increased in the recent years as they have been identified as efficient cooling systems for microelectronics. They present the advantage of higher thermal conductivity and smaller form factor compared to conventional heat spreaders. This work aims to investigate the potential miniaturization of these devices, preliminary to integration on the backside of mobile device chips, located as close as possible to hotspots. While detailed numerical models of vapor chamber operation are developed, an easy modeling with low computational cost is needed for an effective parametric study. Based on the study of the operating limits, this paper shows the thinning potential of a water filled micropillar for a device operating below 10 W and identify the corresponding vapour core height, and wick thickness.