Vertical Minichannel Research Articles

Influence of Subcooling and pressure on flow boiling in a vertical mini-channel: Heat transfer analysis using inverse method

The background of this study is the problem of thermal runaway in the battery cells of electric vehicles. Increasingly powerful and compact battery packs require highly efficient thermal management. One potential solution is the direct liquid cooling of lithium-ion battery cells using a dielectric liquid which boils over a safety limit temperature. This approach intends to prevent in a first step the cell thermal runaway and if it fails in a second step its propagation to its neighbors: the liquid boiling helps to mitigate the cell temperature rise and to transport and spread the heat. The arrangement of the cells in the pack and their close spacing induce a scientific problem of convective boiling in a mini-channel. To enhance designs using this solution, a comprehensive understanding of flow boiling and a precise local heat transfer quantification is essential. The HFE-7100 has been selected as a working fluid for its attractive thermophysical properties and a suitable saturation temperature.The present paper introduces new experimental results of flow boiling dielectric fluid, the HFE-7100, in a vertical rectangular mini-channel. Measurements are carried out from start of boiling to dry-out. Boiling curves are obtained for a mass flux of 391 kg/(m².s), at different pressures (0.7, 1, 1.5 bar) and at different subcoolings (ΔTsub=0;15;30;45∘C). The test section is an aluminum block instrumented with three lines of five K-type thermocouples located all along the flow. The visualization of the flow is achieved through a glass pane with a high-speed camera to identify different flow patterns. The local heat transfer coefficient is calculated by a resolution of a 2D inverse heat conduction method.The experiment mainly aims at determining the heat transfer coefficient, the critical heat flux (CHF) and the pressure drop and at characterizing both the flow regimes and the dry-out phenomenon. The influence of subcooling and pressure on flow boiling is analyzed. An increase in subcooling enhances the CHF and the heat transfer coefficient, and delays the dry-out. Subcooling has an impact on the flow pattern and modifies the type of CHF. In the operating conditions tested, a decrease in pressure enhances CHF and delays dry-out. On the other hand, the heat transfer coefficient increases with rising pressure. Similarly, pressure drops are lower with higher pressure.

Dynamics of Taylor bubble under chemical reaction enhanced mass transfer in minichannel

Taylor bubble dynamic characteristics of CO2 chemical absorption into MEA aqueous solution in a vertical minichannel were systematically investigated in this study. The generation, movement, and shrinkage of bubbles in the minichannel were visualized and monitored using a high-speed camera, and their dynamic behaviors were characterized by image analysis method. The effects of gas and liquid Re numbers and absorbent concentration on two-phase flow patterns, bubble generation frequency, initial bubble length, bubble length decrease rate, and bubble velocity were examined and analyzed. Results showed that chemical reaction-enhanced mass transfer hindered the bubble cap penetration into the main channel, and alleviated the bubble neck thinning, both effects inhibiting the bubble generation. A new Damköhler number (Da)-based correlation has been proposed to predict the initial Taylor bubble length, showing good prediction accuracy for experimental data in the literature using different absorbents. Results also showed that before the formation of sphere bubble at the end of absorption, an approximate linear relationship exists between bubble length decrease rate and bubble velocity, with the slop decided by chemical reaction rate that could be characterized by the Da number. Finally, a simple model has been developed to determine the effective channel length at a given operating condition, providing design guidelines for microchannel-based miniaturized CO2 absorbers.

Intensifying the extraction of rare earth elements by a mini-channel counter-current extractor

Developing new extraction processes is essential for the effective separation of rare earths. Liquid-liquid counter-current extraction of rare earth elements was conducted in innovatively developed mini-channel extractors with a diameter of 4 ∼ 6 mm. 30 % saponified P507/sulfonated kerosene was used as extractant to form monodispersed droplets and flowed upward in the vertical mini-channel by buoyancy. LaCl3 solution (La2O3 + HCl solution) was used as the continuous phase and flowed downward by gravity. The influence of the superficial velocity of organic phase, superficial velocity of aqueous phase, the diameter and length of mini-channel on the extraction yield, volumetric mass transfer coefficient and theoretical stages were studied. The volumetric mass transfer coefficients were in the range of 0.07 × 10−3 ∼ 5.46 × 10−3 s−1. It is much higher than that in co-current mini-channel and traditional extractors. An empirical correlation of volumetric mass transfer coefficients was established with a prediction error of ± 20 %. The strong relative movement of two phases and the maximal driving force of mass transfer give the counter-current mini-channel extractor an excellent mass transfer performance. When the mini-channel length was greater than 250 mm, the number of extraction stages was larger than one. The research results provide guidance for the design of novel extractors and the improvement of rare earth elements extraction.

Hydrodynamics and Heat Transfer for a Two-Phase Flow in a Heated Vertical Minichannel at High Reduced Pressures

Hydrodynamics and Heat Transfer for a Two-Phase Flow in a Heated Vertical Minichannel at High Reduced Pressures

Numerical Analysis of Heat Transfer Characteristics to Supercritical CO2 in a Vertical Mini-Channel: Transition and Pseudo-Boiling

Numerical Analysis of Heat Transfer Characteristics to Supercritical CO2 in a Vertical Mini-Channel: Transition and Pseudo-Boiling

Computational Analysis of Subcooled Flow Boiling in a Vertical Minichannel with Two Different Shapes under Various Mass Fluxes

In the current research project, two-dimensional numerical simulations are conducted to analyze the effects of geometrical configuration on flow structures and the thermal performances of subcooled flow boiling. The CFD simulations are carried out in two different configurations (straight and periodic constriction expansion) in a minichannel mounted vertically at four mass fluxes (500 kg/m2s; 836.64 kg/m2s; 1170 kg/m2s; and 2535 kg/m2s). The present predicted results exhibit excellent accordance with the previous experiments, with mean errors of 6.39% and 9.78%, demonstrating the efficiency of the present numerical study. The simulation results show that the periodic constriction expansion design provides good mixing between the layers, leading to a 43.11% mean enhancement of the thermal transfer, which is more important than the slight pressure drop penalty of 4.32 for a mass flux of 500 kg/m2s due to the combined pressure drop along the minichannel that resulted from the periodic constriction and expansion regions. Furthermore, the visualization of flow patterns shows that the bubbly flow is the dominant flow regime in the periodic constriction-expansion configuration.

Thermo-hydrodynamic investigation into the effects of minichannel configuration on the thermal performance of subcooled flow boiling

The current research focuses on the development of a numerical approach to forecast strongly subcooled flow boiling of FC-72 as the refrigerant in various vertical minichannel shapes for high-heat-flux cooling applications. The simulations are carried out using the Volume of Fluid method with the Lee phase change model, which revealed some inherent flaws in multiphase flows that are primarily due to an insufficient interpretation of shear-lift force on bubbles and conjugate heat transfer against the walls. A user-defined function (UDF) is used to provide specific information about this noticeable effect. The influence of shape and the inlet mass fluxes on the flow patterns, heat transfer, and pressure drop characteristics are discussed. The computational results are validated with experimental measurements, where excellent agreements are found that prove the efficiency of the present numerical model. The findings demonstrate that the heat transfer coefficient decreases as the mass flux increases and that the constriction design improves the thermal performance by 24.68% and 10.45% compared to the straight and expansion shapes, respectively. The periodic constriction sections ensure good mixing between the core and near-wall layers. In addition, a slight pressure drop penalty versus the thermal transfer benefits for the two configurations proposed is reported.

Measurements of heat transfer coefficients from supercritical fluid flowing in vertical mini channels with constant wall temperature

Currently, efficient heat transmission for compact electronic elements is an essential matter. It needs a heat sink with a liquid cooling scheme that meets these demands as much as feasible. The dimensions of 50.8 × 40.6 × 5.5 mm were adopted for features of heat transfer as well as the fluid flow of supercritical CO2 in the heat sink in this study. The adopted pressures, tem-peratures, and mass velocity ranges were 7.5 to 12 MPa, 35 to 50oC, and 100 to 500 Kg/m2s, respectively, wherein the CO2 cooled under these conditions. The factors of heat transfer, the pressure at levels of local as well as medium degree were determined under these conditions. The medium temperature of CO2 in the adjacent significant point area increased, the pressure decreased and the medium temperature movement factor augmented dramatically. It was also noted that the medium temperature movement factor peaked at the pseudo-critical tempera-ture. However, the maximum temperature movement factor declined increased pressure. Fur-thermore, in contrast to the pressure factor, it was revealed that mass velocity and temperature movement factor had a direct relationship. Using the obtained data, a novel correlation mech- anism for limited convection of super-critical CO2 in regular multi-port micro tubes based on chilling conditions was constructed using the obtained coefficients in this study.

Flow boiling heat transfer enhancement in vertical minichannel heat sink with non-uniform microcavity arrays under electric field

Effects of electrohydrodynamics (EHD) on the bubble behavior and enhanced heat dissipation mechanism for four surface configurations and various heat fluxes in an upward flowing minichannel heat sink were studied experimentally. Three different minichannel configurations with microcavities array were fabricated using direct metal laser sintering. Flow boiling experiments of nanofluid (SiO2-R141b) under different nonuniform electric fields were performed. Homogeneity test of Variance Analysis was introduced to analyze the effect of design and operation parameters on heat transfer. Coupled effect of microcavity and electric field was further found to improve boiling heat transfer efficiency. Visualization shows that the diameter of small bubbles leaving microcavity increased under electric field. The bubbles wiggled between wire electrode and heated wall by action of electric field in bubbly flow district. Furthermore, the mechanism of perturbation of small bubbles flowing along the heating wall under electric field was discussed. It was revealed that the micro-concave cavity array distributed dense downstream and sparse upstream had a large impact on heat transfer coefficient (HTC), especially under electric field. The maximum synthetic enhancement factor of 2.11 was obtained for given flow conditions. Furthermore, a new predictive equation was proposed by a nonlinear regression equation to consider the effects of micro-concave cavity distribution and electric field factors on thermal transfer performance, and this correlation can predict the HTC affected by non-uniform micro-concave arrays well within the ± 10% error band.

Study of flow boiling in a vertical mini-channel with surface structuring: Heat transfer analysis using inverse method

Using a boiling fluid to cool components is a very efficient process, which is often used in micro/mini channel flows. A proposed application is the direct contact liquid cooling of lithium-ion battery cells, where a dielectric fluid is required and where the effects a cell thermal runaway could be mitigated by a convective boiling flow between the cells. For such applications, better designs require both to understand confined convective boiling with coupled flow phenomena and to quantify the local heat transfer accurately. The present paper introduces new experimental results of flow boiling heat transfer of a dielectric fluid, the HFE-7100, in a vertical rectangular 1 mm deep, 30 mm wide and 120 mm long mini-channel. The test section is an aluminum block instrumented with three lines of five K-type thermocouples located all along the flow. The experiment mainly aims at determining the heat transfer coefficient and CHF and at characterizing both the flow regimes and the dry-out phenomenon. Measurements are carried out from start of boiling to dry-out, at different mass fluxes (140, 390 and 648 kg/(m2.s)) and at a pressure of 1.1 bar. These tests were performed on a reference unmodified surface and compared to two biphilic surfaces including coating techniques. The visualization of the flow is achieved through a glass pane with a high-speed camera to identify different flow patterns. The local heat transfer coefficient is calculated by a resolution of a 2D inverse heat conduction method. This approach uses a numerical Finite Difference Method (FDM) for the spatial discretization and a Tikhonov's method for regularization. The inverse 2D method permits to follow the evolution of the heat transfer coefficient along the channel with the visualized flow pattern. The experimental results show a small sensitivity of the mass flux on the heat transfer coefficient and a predominance of nucleate boiling. On the other hand, there is a significant sensitivity of the mass flux on the dry-out occurrence. Finally, the sensitivity of two techniques of biphilic surface coating on boiling and on the dry-out occurrence is quantified and discussed.

Numerical investigation of electrohydrodynamically enhanced flow boiling inside minichannels: The seeding model

In the present study, three-dimensional numerical simulations are performed to examine the effect of applied direct current electric fields on the subcooled flow boiling heat transfer in a vertical minichannel. The volume of fluid model is used to capture the liquid–vapor interfaces, and the conservation laws, together with the electrohydrodynamic (EHD) equations, are solved using the finite volume method. Two mass transfer models of Lee and Fourier are evaluated, and a new seeding algorithm is developed based on the physics of the bubble formation and departure on the heated walls, integrated with the Fourier model. The early transition from slug to churn/annular flow regimes, due to EHD forces, is observed in the numerical solutions with both models, in agreement with the available experiment. Nevertheless, the Lee model tuned for the electric-free condition fails to predict the EHD-induced heat transfer enhancement, while the Fourier model with the new seeding algorithm captures this phenomenon with reasonable accuracy. Based on the present numerical results, this effect can be attributed to the migration of small bubbles toward the walls under the effect of electric fields, forming recirculating flow regions near the walls, augmenting the flow mixing, and mitigating hot spots.

Experimental investigation on flow boiling bubble motion under ultrasonic field in vertical minichannel by using bubble tracking algorithm

Bubble dynamics is important in flow boiling of minichannel, and ultrasonic field effects bubble behaviors. However, flow boiling bubble movements in minichannels under ultrasonic field have received little research attention and are still poorly understood. In this paper, the effects of ultrasonic field on bubble dynamics are experimentally studied by capturing the bubble motion behaviors of the flow boiling bubbles. The ultrasonic frequencies are set to 23, 28, 32, and 40 kHz. Bubble tracking algorithm, which studies the growth, trajectories, velocities, and traveled distances for bubbles, is created to qualitatively describe bubble motion behavior of flow boiling in minichannel. It is found that after the application of ultrasound, the detachment frequency, velocity, and travel distance of the bubbles significantly increases, and the growth behavior and trajectory are extremely complex, the two-phase gas-liquid flow is extremely unstable. The bubbles gain kinetic energy as the ultrasound frequency increases. Finally, numerical simulations are used to quantitatively investigate the mechanism of bubble motion in microchannels under ultrasonic fields.

Interface Capturing Flow Boiling Simulations in a Compact Heat Exchanger

Abstract High-fidelity flow boiling simulations are conducted in a vertical mini channel with offset strip fins (OSF) using R113 as a working fluid. Finite-element code PHASTA coupled with level set method for interface capturing is employed to model multiple sequential bubble nucleation using transient three-dimensional approach. The code performance is validated against experiments for a single nucleation site in a vertical rectangular channel. To assess code performance, a study on the bubble departure from the wall in a mini channel with OSF is carried out first. Contributions from the microlayer are not considered due to low heat flux values applied to the channel (1 kW/m2). The influence of surface characteristics, such as contact angle and liquid superheat on bubble dynamics, is also analyzed as well as the local two-phase heat transfer coefficient. For higher void fractions, two conical nucleation cavities are introduced in the same channel with OSF. Observed bubble characteristics (departure diameter, bubble departure frequency) are evaluated and bubble trajectories are presented and analyzed. The local heat transfer coefficient is then evaluated for each simulation. The results show approximately a 2.5 time increase in the local heat transfer coefficient when the individual bubbles approach the wall. With smaller bubble nucleation diameters, the heat transfer coefficient can increase by up to a factor of two. Thus, the current work demonstrates the flow modeling capability of the boiling phenomena in complex geometry with OSF.

Flow Boiling Heat Transfer Intensification Due to Inner Surface Modification in Circular Mini-Channel

This work aimed to study the intensification of flow boiling heat transfer and critical heat flux (CHF) under conditions of highly reduced pressures due to a modification of the inner wall surface of a mini-channel. Such research is relevant to the growing need of high-tech industries in the development of compact and energy-efficient heat exchange devices. We present experimental results of the surface modification effect on hydrodynamics and flow boiling heat transfer, including data on the CHF. A description of the experimental stand and method for modifying the test mini-channel is also presented. The studies were carried out with freon R-125 in a vertical mini-channel with a diameter of 1.1 mm and a length of 50 mm, in the range of mass flow rates from G = 200 to 1400 kg/(m2s) and reduced pressures between pr = p/pcr = 0.43 and 0.56. The maximum surface modification effect was achieved at a reduced pressure of pr = 0.43, the heat transfer coefficient increased up to 110%, and the CHF increased up to 22%.

Boiling Heat Transfer during Flow in Vertical Mini-Channels with a Modified Heated Surface

The process with change of phase during flow in mini-channels plays a significant role in many industrial applications, such as microelectronics. Furthermore, methods for heat transfer intensification during flow in channels of small cross-section are still being sought. In this work, studies of the effect of using a modified heated surface on intensification boiling heat transfer in rectangular mini-channels during upward and downward flow are performed. The test section of a group of seven parallel mini-channels 1 mm deep was investigated during the subcooled and saturated flow boiling of FC-72. The temperature of the outer heated wall surface was measured using an infrared camera. During the experiments, two-phase flow structures were captured with a quick camera. Local heat transfer coefficients at the contact surface between the working fluid and the heated surface were determined with the use of a one-dimensional calculation method. To present the results, local temperature measurements and heat transfer coefficients, boiling curves and two-phase flow patterns are shown and analyzed. The results concerning two directions of vertical flow along mini-channels are discussed. Several modified heated surfaces and one smooth were tested for comparison. The main objective was to find out how the modified surface of the heated wall can intensify boiling heat transfer with upward and downward refrigerant flow in mini-channels of rectangular cross-section.

Experimental and Numerical Study of Boiling HFE-7100 in a Vertical Mini-channel

Experimental and Numerical Study of Boiling HFE-7100 in a Vertical Mini-channel

Soft-ANN based correlation for air-water two-phase flow pressure drop estimation in a vertical mini-channel

In this paper, an Artificial Neural Network soft matrix correlation to estimate the pressure drop of air-water two-phase flow is developed. The applicability of the model is extended by using dimensionless physical numbers as inputs (Air-Reynolds number, Water-Reynolds number, and the ratio of Air Inertial Forces to Water Inertial Forces), so the model can be implemented for vertical pipes with the proper combination of diameter-velocity-density-viscosity allowing estimations of dimensional numbers within the range of: Air-Reynolds numbers (430–6100), Water-Reynolds number (2400–7200), and Air-Water-Inertial forces ratio (1.6–1834), including the diameter range from 3 to 28 mm. Experimental measurements of frictional pressure drop of water-air mixtures are determined at different conditions. A search of the most suitable density, viscosity, and friction models was conducted and used in the model. The performance of the proposed ANN correlation is compared against published expressions showing good approximation to experimental data; results indicate that the most used correlations are within a mean relative error ( mre) of 23.9–30.7%, while the proposed ANN has a mre = 0.9%. Two additional features are discussed: i) the applicability and generality of the ANN using untrained data, ii) the applicability in laminar, transitional, and turbulent flow regimen. To take the approach beyond a robust performance mapping, the methodology to translate the ANN into a programmable equation is presented.

Heat transfer characteristics during flow along horizontal and vertical minichannels

This work focuses on boiling heat transfer studies during cooling liquids flow (FC-72, HFE-649, HFE-7000 or HFE-7100) along a single minichannel of 1.7 mm depth, vertically or horizontally oriented. The temperature of the outer smooth side of the plate was measured by infrared thermography. The observations of the flow structures were carried out on the enhanced side of the plate contacting fluid. The heat transfer coefficient (HTC) on the enhanced plate–fluid contact surface was computed from the Robin boundary condition. The inverse heat conduction problem in the heated wall was solved by the Finite Element Method with the shape functions based on the Hermite interpolation and the Trefftz functions. The results were presented as relationships between HTC and the spatial variable referred to the flow direction at subcooled and saturated boiling and as boiling curves. Mean relative errors of HTC were determined. Two-phase flow patterns were shown and discussed. Selected correlations for boiling heat transfer have been used in calculations for comparison between experimental and predicted Nusselt numbers. The smallest mean absolute percentage error of the Nusselt number was achieved for Mikielewicz J. correlation at subcooled boiling. At saturated boiling no satisfactory results were achieved. Therefore, own correlation was proposed.

Flow boiling heat transfer characteristics of low global warming potential refrigerants in a vertical mini-channel

Flow boiling heat transfer characteristics in narrow channels have been investigated extensively by researchers due to its wide range of applications in micro-electromechanical systems, however, being a complex transport process the controlling mechanisms still lack clarity in understanding. Refrigerants related environmental hazards also urged to look for alternative environment friendly refrigerants. It has been noticed that relatively less information is available in the literature specifically for environmentally benign mediums. This study is focused on experimental findings for flow boiling heat transfer performance of low global warming potential refrigerants (R152a, R600a, and R1234yf). The test object was a vertical stainless steel tube (1.60 mm inner diameter and heated surface length 245 mm), experiments were done under upward fluid-flow conditions. The tested conditions were: heat flux 5-245 kW/m2, 50-500 kg/m2s mass velocities while saturation temperatures were 27 ?C and 32 ?C. The effects of operating parameters like heat and mass fluxes, saturation temperature, and vapor quality on heat transfer were analyzed in detail. It was noticed that heat transfer coefficients were significantly influenced by heat flux and operating pressure level whereas the same were not significantly affected by mass flux and vapor quality. The experimental data of heat transfer was compared with the prediction from various macro and micro scale correlations from the literature.

Numerical analysis on flow pattern and heat transfer characteristics of flow boiling in the mini-channels

Flow pattern and heat transfer of flow boiling for different flow orientation, mini-channel width and height were presented in this work. The data were obtained by the numerical simulation with the coolant of R141b flow in a vertical mini-channel. Orientation includes upward and downward. A constant heat flux was loaded at the wall of the channel, of which the width ranges from 1 mm to 3 mm, and a length changes from 200 mm to 400 mm. Subsequently the impact of those parameters that referred to heat flux, inlet temperature and inlet temperature of the coolant could be investigated by carrying out the numerical calculation. In addition, a validation for the model was illustrated in comparison with the previously experimental data.