Vapor Core Research Articles

Numerical analysis on thermal hydraulic performance of a flat plate heat pipe with wick column

A simplified thermal hydraulic model is developed to investigate the influence of wick column on the performance of a flat plate heat pipe (FPHP). The governing equations of the FPHP are solved by using the computational fluid dynamics package FLUENT. The temperature, velocity and pressure fields are obtained. The validity of the model is confirmed by comparing the present solutions with the open literature data. The numerical results show that with the increase of the wick column size, the maximum velocity of the liquid and vapor decreases while the total thermal resistance and capillary heat transfer limit of the FPHP increases gradually. The performance of the FPHP may degrade if the wick column is placed inside the vapor core asymmetrically.

Numerical analysis of a screen mesh wick heat pipe with Cu/water nanofluid

A two-dimensional transient numerical model is developed to predict the vapor core, wall temperatures, vapor pressure, vapor velocity and liquid velocity in the screen mesh wick heat pipe. The mass, momentum and energy equations are solved numerically for liquid and vapor regions. The effect of Cu–water nanofluid on the heat transfer performance of heat pipe is studied using the developed model and the same is compared with that of the DI water. The transient profiles of vapor and liquid velocities in the wick region and the location of dry-out in the evaporator section of the heat pipe are also obtained using the developed model. It is found that the addition of nanoparticles leads to the reduction in wall temperature, operating pressure, vapor temperature and total resistance of the heat pipe; thereby, increasing the heat transfer of the heat pipe at the same heat load. Interestingly, the liquid and vapor velocities of the heat pipe charged with 0.1wt% of Cu-water nanofluid is found to be 20% higher when compared with that of the heat pipe with DI water at the same operating conditions. It is also observed that the decreased pore size of the wick due to the addition of nanoparticles increases the effective thermal conductivity of the wick structure which acts as a coating layer and enhances the heat transfer capability of the heat pipe.

The phase separation concept condensation heat transfer in horizontal tubes for low-grade energy utilization

This paper reports condensation heat transfer in a horizontal tube with a 14.81 mm inside diameter and a 1200 mm heat transfer length, by using the phase separation concept. The hollow mesh cylinder was formed by packaging two layers of mesh screen surface. The outer layer had the pore size of 76 μm. This invention constructs a multiscale condenser tube. The modulation of stratified flow and annular flow was focused on. Liquid was held within the mesh cylinder to increase the vapor covered tube wall surface to enhance the heat transfer for stratified flows. For annular flows, liquid droplets in the vapor core were captured by the mesh screen surface to increase the vapor void fractions near the tube wall, enhancing the hat transfer. The measurements showed that the heat transfer coefficients could be more than two times of those in the bare tube. The total thermal resistance was decreased by 45.6%, maximally. The heat transfer enhancement ratios were increased with the vapor mass fluxes, Gxin, for the condensation flow along the whole tube length. A pressure analysis explained the increased heat transfer trend. The increment of heat transfer enhancement ratios was suppressed with a liquid flow part in the tube.

Theoretical model for local heat transfer coefficient for annular flow boiling in circular mini/micro-channels

This study examines two-phase heat transfer characteristics associated with annular flow boiling in circular mini/micro-channels with circumferentially uniform heat flux. A theoretical control-volume-based model is developed based on the assumptions of smooth interface between the annular liquid film and vapor core, and uniform film thickness around the channel’s circumference. Droplet entrainment and deposition effects are incorporated in the model with the aid of new correlations for initial liquid droplet quality at the onset of annular flow, and deposition mass transfer coefficient, respectively. The model also accounts for interfacial suppression of turbulent eddies due to surface tension with the aid of an eddy diffusivity model specifically tailored to shear-driven turbulent films. The model shows excellent predictive capability against 149 convective boiling dominant data points for saturated flow boiling in circular mini/micro-channels, evidenced by an overall mean absolute error of 16.5%, with 91.3% and 98.0% of the data falling within ±30% and ±50% error bands, respectively.

Experimental investigation on enhancement in thermal characteristics of sintered wick heat pipe using CuO nanofluids

The enhancement in heat transfer characteristics of a copper sintered wick heat pipe with surfactant free CuO nanoparticles dispersed in DI water is experimentally studied. The effect of heat input, tilt angle and weight fractions of nanoparticles on the heat pipe thermal resistance, heat transfer coefficient in evaporator and condenser sections, thermal conductivity and thermal efficiency are investigated. The experimental results are evaluated for the vapor temperatures directly measured at the center core of heat pipe. Interestingly, a temperature difference of 5.1°C is observed between the heat pipe surface and the vapor core in the evaporator section. Results showed a reduction in the thermal resistance of 66.1% and enhancement in the heat transfer coefficient and thermal conductivity of 29.4% and 63.5% is respectively, observed for 1.0wt.% of CuO/DI water nanofluid at 45° tilt angle compared with heat pipe kept at horizontal position. Similarly, the thermal efficiency is also improved by 24.9% for the same tilt angle and weight fraction of CuO nanofluid. The use of nanoparticles and tilt angle enhances the operating range and thermal performance of heat pipe when compared with that of the heat pipe with DI water.

Transient analysis of heat transfer and fluid flow in a polymer-based Micro Flat Heat Pipe with hybrid wicks

A transient analysis of a polymer-based Micro Flat Heat Pipe with hybrid wicks was performed via a solid numerical procedure. Strongly coupled velocity, temperature, and pressure fields together with phase change and significant change of vapor density are the major challenges during a heat pipe modeling. Finite volume method was used to discretize the two-dimensional unsteady Navier–Stokes governing equations while kinetic theory was used to address the condensation/evaporation at the interface. Implicit approach was adopted to discretize time gradient terms. The density of the vapor core was calculated by the delay method while a new under-relaxation approach was successfully used to secure the convergence. To validate the accuracy of the present numerical model, the consistency of the numerical method was examined by different parameters and the numerical results were compared to experimental data at steady state conditions. The transformation point and transformation line criteria were introduced in the analysis for the first time to track the finalization of the evaporation and the beginning of the condensation process.

Condensation in a Square Minichannel: Application of the VOF Method

A number of steady-state numerical simulations of condensation of R134a at mass fluxes of 400 kg m−2 s−1 and 800 kg m−2 s−1 inside a 1-mm square cross section minichannel are proposed here and compared against simulations in a circular cross section channel with the same hydraulic diameter. The volume of fluid (VOF) method is used to track the vapor–liquid interface, and the effects of interfacial shear stress, surface tension, and gravity are taken into account. A uniform wall temperature is fixed as a boundary condition. At both mass velocities the liquid film and the vapor core are treated as turbulent; a low-Re form of the SST k-ω model has been used for the modeling of turbulence through both the liquid and vapor phases. Numerical simulations are validated against experimental data. The influence of the surface tension on the shape of the vapor–liquid interface may provide some heat transfer enhancement in a square cross section minichannel, but this depends on the mass flux and it may be not significant at high mass velocity, as confirmed by experimental data and by the present numerical work. The gravity force is shown to be responsible for the liquid film thickness increase at the bottom of the channel in the circular cross section, but the gravity force has a minor effect in the square minichannel; at these mass velocities, the heat transfer mechanism is dominated by shear stress and surface tension.

Investigation of localized dryout versus CHF in saturated flow boiling

Determining flow boiling critical heat flux (CHF) using mechanistic models or empirical correlations requires careful validation with the aid of reliable databases. But, while many new databases are being made available in the literature, the methods used to detect CHF vary greatly, producing different CHF estimates for the same fluid and operating conditions. The variations in detection method are the result of both heated wall design and criteria used to terminate an experiment in response to wall temperature excursions. This study investigates the interfacial phenomena preceding the occurrence of CHF for flow boiling with a finite inlet vapor void. Experiments are conducted with FC-72 in a rectangular channel that is heated along one side. Temporal records of the heated wall temperatures are used to track the complex transient response of the heated wall, and identify differences between temperature excursions associated with momentary localized dryout and those with true CHF. It is shown that the flow enters the channel fully separated, with a liquid layer sheathing all four channel walls surrounding a central vapor core. At high heat fluxes, a wavy vapor layer begins to form beneath the liquid layer adjacent to the heated wall, and cooling is provided mostly through wetting fronts associated with the wave troughs in accordance with the Interfacial Lift-off Model. However, depending on mass velocity, inlet quality and flow orientation, conditions may arise that cause breakup of the heated wall liquid layer into ligaments that are entrained in the vapor core. This phenomenon causes localized dryout and wall temperature excursions at heat fluxes well below CHF, but the wall is able to recover from these excursions by a combination of reattachment of ligaments with the heated wall and lateral heat conduction within the wall itself. Recommendations are made concerning construction of the heated wall and CHF detection in pursuit of reliable CHF data.

FLATTENING EFFECT ON HEAT TRANSFER CHARACTERISTICS OF A SINTERED-WICK HEAT PIPE

The effect of pipe flattening on heat transfer characteristics and the internal phenomena of a sintered-wick heat pipe has been investigated by using three-dimensional Finite Element Method. The calculation domains were focused at three important regions, i.e., vapor core, wick and wall. The Cartesian coordinates and the three-dimensional tetrahedral elements were applied in this model. The selected total elements were 638,400 to ensure the accuracy. The original diameter and total length of heat pipe were 6 mm and 200 mm, respectively. The composite wick made from sintered copper powder and grooved copper pipe was applied with water as working fluid. The vapor flow was assumed to be laminar and incompressible. The predicted results from the program were validated with the experimental results conducted with all similar controlled parameters. It was found that the predicted wall temperature and thermal resistance agreed well with the experimental data with the standard deviations of ±5.95 and ±32.85%, respectively. Furthermore, the overall thermal resistances of the tubular heat pipes (original diameter of 6 mm), which were flattened into the final thickness of 4.0 and 3.0 mm, decreased from 0.91 to 0.83°C/W due to an increase of the contacted surface for heat transfer surface. However, the overall thermal resistance of a flattened heat pipe with the final thickness of 2.5 mm increased to 0.88°C/W, resulting from drastic increase of pressure drop in narrower vapor core. The pivotal final thickness of flattened heat pipe, which is the minimum thickness of pipe to be flattened, has been analysed to be 2.75 mm (about 45% from original diameter).

Climbing film, flooding and falling film behavior in upflow condensation in tubes

Upflow condensation in vertical tubes is complicated by the relative magnitude of the opposing vapor shear and gravity. This study examines the different flow regimes for condensation of FC-72 in a vertical tube using both high-speed video imaging and detailed heat transfer measurements. Four regimes are identified, falling film, where the condensing film drains downwards by gravity opposite to low velocity vapor flow, oscillating film, corresponding to film flow oscillating between upwards and downwards, flooding, where film begins to be sheared upwards by the vapor core, and climbing film, where high vapor velocity causes the film to be sheared upwards. The four flow regimes are well segregated in a flow regime map based on dimensionless superficial velocities of the vapor and liquid. The condensation heat transfer coefficient is shown to decrease axially because of gradual thickening of the film, except for high mass velocities, where turbulence and intensified interfacial waviness cause downstream heat transfer enhancement. An annular flow model is constructed, which shows fair predictions for the climbing film regime. The predictive accuracy of the model is influenced by flow oscillations occurring downstream of the climbing film region and inability of the model to account for interfacial waves.

3D Heat Transfer Analysis of a Miniature Copper-Water Vapor Chamber with Wicked Pillars Array

A three-dimensional analysis of the heat and mass transfer phenomena inside a vapor chamber is essential for correctly understanding its thermal performance limitations and structural optimization. This paper presents a complete three-dimensional numerical analysis and comparative study of two different miniature vapor chambers designs with identical external geometry and dimensions but different internal structures: one having a wicked pillar array and the other one without the wicked pillars array. The distribution of the wicked pillar array in the vapor core was aligned. Detailed comparative experimental results are also reported, which were performed to verify the calculations from the numerical simulations. It was found that the numerical and experimental results agree quite well, especially at high heat flux values. It is also observed that the vapor chamber with wicked pillars had a better thermal performance than the conventional design, with a 5% decrease in terms of total thermal resistance due to the added extra channels that allow a better flow of the working fluid to the evaporator surface. An insight into how improving the thermal performance of a vapor chamber is provided through the detailed three-dimensional numerical simulations.

Numerical simulation of annular two-phase flow considering the four involved mass transfers

In the present study, a numerical model is developed for simulation of annular two-phase flow considering bubbly flow regime in the liquid film along with the four involved mechanisms of mass transfer those are evaporation, entrainment, deposition and condensation. In the numerical approach, liquid film accompanied by fine nucleated bubbles are simulated with innovative model named suction model, the whole domain containing liquid film and the vapor core is simulated by volume of fluid model. While the vapor and the entrained droplets are treated as homogeneous flow. The interface between the liquid and the vapor is traced by level set formulation. The model is then validated by experimental models of Lee & Lee and Stevanovic et al. and shows a good precision such that it predicts the experimental results of Stevanivic et al. Better than their own numerical model. This issue is due to the least possible simplifying assumptions along with considering the effect of boiling in liquid film and all mechanisms of mass transfer in the fluid flow.

Simulation studies on performance of micro heat pipes used in laptops

In this work, a computational analysis is used to study the thermal performance of a heat pipe utilising conventional fluids. Three of the most common fluids namely, acetone, methanol and water are considered as the working fluid. The analysis was made under different operating conditions with the parameters such as effective length, condenser length, and ratio of outer to vapour core diameter. The influence of these operating parameters on the performance parameters namely mass of the heat pipe, heat transfer rate and number of heat pipe was obtained. A substantial change in performance parameters of the heat pipe is observed when using acetone as working fluid, due to its high evaporation rate and high vapour pressure.

Flat flexible polymer heat pipes

Flat, flexible, lightweight, polymer heat pipes (FPHP) were fabricated. The overall geometry of the heat pipe was 130 mm × 70 mm × 1.31 mm. A commercially available low-cost film composed of laminated sheets of low-density polyethylene terephthalate, aluminum and polyethylene layers was used as the casing. A triple-layer sintered copper woven mesh served as a liquid wicking structure, and water was the working fluid. A coarse nylon woven mesh provided space for vapor transport and mechanical rigidity. Thermal power ranging from 5 to 30 W was supplied to the evaporator while the device was flexed at 0°, 45° and 90°. The thermal resistance of the FPHP ranged from 1.2 to 3.0 K W−1 depending on the operating conditions while the thermal resistance for a similar-sized solid copper reference was a constant at 4.6 K W−1. With 25 W power input, the thermal resistance of the liquid–vapor core of the FPHP was 23% of a copper reference sample with identical laminated polymer material. This work shows a promising combination of technologies that has the potential to usher in a new generation of highly flexible, lightweight, low-cost, high-performance thermal management solutions.

Modeling and Design Optimization of Ultrathin Vapor Chambers for High Heat Flux Applications

Passive phase-change thermal spreaders, such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In this paper, a numerical model for ultrathin vapor chambers has been developed, which is suitable for reliable prediction of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled, and the model predictions are compared with experiments on custom-fabricated vapor chamber devices. The working fluid for the vapor chamber is water and a condenser side temperature range of 293 K-333 K is considered. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive. The vapor space in the ultrathin devices offers significant thermal and flow resistances when the vapor core thickness is in the range of 0.2 mm-0.4 mm. The performance of a 1-mm-thick vapor chamber is optimized by studying the variation of thermal resistance and total flow pressure drop as functions of the wick and vapor core thicknesses. The wick thickness is varied from 0.05 to 0.25 mm. Based on the minimization of a performance cost function comprising the device thermal resistance and flow pressure drop, it is concluded that the thinnest wick structures (0.05 mm) are optimal for applications with heat fluxes below 50 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , while a moderate wick thickness of 0.1 mm performs best at higher heat flux inputs 50 (>;W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).

Behaviour of the iron vapour core in the arc of a controlled short-arc GMAW process with different shielding gases

The controlled metal transfer process (CMT) is a variation of the gas metal arc welding (GMAW) process which periodically varies wire feeding speed. Using a short-arc burning phase to melt the wire tip before the short circuit, heat input to the workpiece is reduced. Using a steel wire and a steel workpiece, iron vapour is produced in the arc, its maximum concentration lying centrally. The interaction of metal vapour and welding gas considerably impacts the arc profile and, consequently, the heat transfer to the weldpool. Optical emission spectroscopy has been applied to determine the radial profiles of the plasma temperature and iron vapour concentration, as well as their temporal behaviour in the arc period for different mixtures of Ar, O2 and CO2 as shielding gases. Both the absolute iron vapour density and the temporal expansion of the iron core differ considerably for the gases Ar + 8%O2, Ar + 18% CO2 and 100% CO2 respectively. Pronounced minimum in the radial temperature profile is found in the arc centre in gas mixtures with high Ar content under the presence of metal vapour. This minimum disappears in pure CO2 gas. Consequently, the temperature and electrical and thermal conductivity in the arc when CO2 is used as a shielding gas are considerably lower.

Prediction of dryout and post-dryout wall temperatures using film thickness model

In this paper, a model for calculating dryout and post-dryout wall temperature for two phase flow in a vertical tube using film thickness model is presented. The model is based on the mass and energy balance equations for vapor core, liquid film and corresponding closure laws for interface transfer processes. It is assumed that the dryout is reached when the local film thickness in the annular flow approaches zero. Thermal-hydraulic processes along the whole length of the boiling channel are simulated, from sub-cooled liquid flow at channel inlet, up to the liquid film dryout, gas entrained droplets, mist flow and post dryout region at tube exit. In the post dryout region, thermal non-equilibrium was assumed between vapor and entrained droplets and wall temperature was obtained iteratively by using a wall heat flux partitioning model. Also, the model is verified against several available experimental data and proved viable for prediction of the wall temperature and the dryout location.

Theoretical study on the characteristics of critical heat flux in vertical narrow rectangular channels

Abstract A mathematical separated flow model of annular upward flow has been developed to predict the critical heat flux (CHF) in uniformly heated vertical narrow rectangular channels. The theoretical model is based on fundamental conservation principles: the mass, momentum, and energy conservation equation of the liquid film and the momentum conservation equation of the vapor core together with a set of closure relationships. The comparisons between the theoretically predicted results and the experimental data demonstrate that a good agreement between them has been achieved. With the applications of the present model, the influences of several parameters (such as system pressure, inlet mass flux and channel size) on the CHF in rectangular channels are analyzed, and the CHF is compared with that in annular channels.

Theoretical model for annular flow condensation in rectangular micro-channels

This study examines the pressure drop and heat transfer characteristics of annular condensation in rectangular micro-channels with three-sided cooling walls. A theoretical control-volume-based model is proposed based on the assumptions of smooth interface between the annular liquid film and vapor core, and uniform film thickness around the channel’s circumference. Mass and momentum conservation are applied to control volumes encompassing the liquid film and the vapor core separately. The model accounts for interfacial suppression of turbulent eddies due to surface tension with the aid of a new eddy diffusivity model specifically tailored to shear-driven turbulent films. The model predictions are compared with experimental pressure drop and heat transfer data for annular condensation of FC-72 along 1 × 1 mm 2 parallel channels. The condensation is achieved by rejecting heat to a counterflow of water. The data span FC-72 mass velocities of 248–367 kg/m 2 s, saturation temperatures of 57.8–62.3 °C, qualities of 0.23–1.0, and water mass flow rates of 3–6 g/s. The data are also compared to predictions of previous separated flow mini/micro-channel and macro-channel correlations. While some of the previous correlations do provide good predictions of the average heat transfer coefficient, they fail to capture axial variation of the local heat transfer coefficient along the channel. The new model accurately captures the pressure drop and heat transfer coefficient data in both magnitude and trend, evidenced by mean absolute error values of 3.6% and 9.3%, respectively.

Experimental and theoretical study of critical heat flux in vertical upflow with inlet vapor void

This study explores the mechanism of flow boiling critical heat flux (CHF) for FC-72 in a 2.5 mm × 5 mm vertical upflow channel that is heated along its 2.5 mm sidewall downstream of an adiabatic development section. Unlike most prior CHF studies, where the working fluid enters the channel in liquid state, the present study concerns saturated inlet conditions with finite vapor void. Temperature measurements and high-speed video imaging techniques are used to investigate the influence of the inlet vapor void on interfacial behavior at heat fluxes up to CHF as well during the CHF transient. The flow entering the heated portion of the channel consists of a thin liquid layer covering the entire perimeter surrounding a large central vapor core. Just prior to CHF, a fairly continuous wavy vapor layer begins to develop between the liquid layer covering the heated wall and the heated wall itself, resulting in a complex four-layer flow consisting of the liquid layer covering the insulated walls, the central vapor core, the now separated liquid layer adjacent to the heated wall, and the newly formed wavy vapor layer along the heated wall. This behavior in captured in a new separated control-volume-based model that facilities the determination of axial variations of thicknesses and mean velocities of the four layers. Incorporating the results of this model in a modified form of the Interfacial Lift-off CHF Model is shown to provide fairly good predictions of CHF data for mass velocities between 185 and 1600 kg/m 2 s, evidenced by a mean absolute error of 24.52%.