Flow In Minichannels Research Articles

Lattice Boltzmann simulation of near/supercritical CO2 flow featuring a crossover formulation of the equation of state

In this work, we have incorporated a crossover equation of state into the pseudopotential multiphase Lattice Boltzmann Model (LBM) to improve the prediction of thermodynamic properties of fluids and their flow in near-critical and supercritical regions. Modeling carbon dioxide (CO2) properties in these regions is of increasing interest for industrial processes such as CO2 storage and heat transfer where CO2 is used as a working fluid. Despite the importance of accurately modeling near-critical and supercritical fluids, popular classical cubic equations of state (EoS) are not accurate. The proposed crossover EoS is a proven hybrid equation which uses the original classical EoS far from the critical point where it is valid, and near the critical point, it asymptotically switches to using non-analytic scaling laws. It also transforms into the ideal gas EoS as the density approaches zero. In order to demonstrate the validity and versatility of the crossover approach in the prediction accuracy of LBM in near-critical flows, this formulation was incorporated into the Peng-Robinson (P-R) EoS. First, 2D static droplets of CO2 and water at vapor-liquid equilibrium were modeled using both P-R EoS and its crossover formulation, and the numerically predicted results were then compared against the experimental data. The results demonstrate that the model accuracy in representing the thermodynamic behavior of the fluid in near-critical region is improved after adopting the crossover formulation with respect to the classical analytical EoS. The crossover P-R EoS was further applied to study several two-phase CO2 flows in mini-channels at near-critical temperatures. The flow patterns showed a good qualitative agreement with the experimental data in the near-critical region. The study is further extended to multi-component multiphase systems, specifically to water droplet penetration into porous media filled with CO2.

Pore-scale simulation of flow in minichannels with porous fins

<p indent="0mm">Because of its high specific surface area, metal foam is expected to further enhance the flow and thermal performance of a minichannel heatsink. In this study, fully developed flow in minichannels with porous fins was studied. The porous geometry was constructed based on Laguerre-Voronoi tessellation and simulated in a pore scale, which eliminated the influence of parameter selection in a macroscale method. The influence of porosity, filling ratio, and Reynolds number on velocity field and pressure drop was analyzed. The general relationship between the Poiseuille number and filling ratio was determined, and a correlation was obtained using the study results. The results of this study were compared with those of the macroscale method, and finally, the influence of effective viscosity was discussed.

Heat and mass transfer analysis on multiport mini channel shelf heat exchanger for freeze-drying application

An experimental study on heat transfer characteristics of shelf heat exchangers, which are used in small-scale freeze dryers, is presented in this paper. The proposed heat exchanger consists of the multiport mini-channel flow paths, and a comparison with a conventional serpentine is carried out based on uniformity of the product temperature achieved. The experimental results indicate that the mini-channel shelf has better temperature uniformity than the latter. Skimmed milk is used as the test fluid. In the freezing and drying process, the product temperature variation is minimal in the mini-channel heat exchanger, with a variation of 12.5 % and 25%, respectively, and the heat transfer coefficient is found to be from 140 to 267 Wm−2 K−1. The moisture content in the product reduces to 50 % in 2 hours, and the drying rate is higher at 0.032 kg hr−1 after 1 hour of the drying process. The redesign of the heat exchanger will be an essential tool to improve the performance with uniform temperature distribution on the product and to improve the product quality.

Experimental study and CFD modelling on the thermal and flow behavior of EG/water ZnO nanofluid in multiport mini channels

Nanofluid and mini channel are two promising approaches to enhance the heat transfer performance. The present study combined these two techniques together and investigated the heat transfer and flow behavior of ZnO ethylene glycol (EG)/water nanofluid inside two multiport mini channels with the hydraulic diameters of 1.22 mm and 1.42 mm. ZnO nanoparticles with an average diameter of 30 nm were dispersed into 40%/60% EG/water solution to get nanofluids with 0.75% and 1.5% volume concentration. The heat transfer and flow performance under various operational conditions were identified by both experiments and CFD simulation. Experimentally, it reveals that the Nusselt numbers are averagely improved by 6.7% and 9.8% for nanofluids with 0.75% and 1.5% concentration respectively. However, the friction factors are also enlarged by 4.6% and 8.6% correspondingly, which results to a various thermal performance factor ranging from 0.94 to 1.31 with the average values of 1.05 and 1.07. The decrement of channel diameter leads to greater heat transfer coefficients, pressure drop, friction factor but nearly the same Nusselt number. A new correlation is developed to predict the Nusselt number for ZnO nanofluid flow in multiport mini channels with a MARD of 9.2%. Numerically, single-phase model gives better predictions for both Nusselt number and friction factor with the average relative deviations of nearly 16% and 19.1% correspondingly. Mixture model severely overestimates the Nusselt number but shows a reasonable friction factor prediction. The present study is expected to give some guidance for the study of nanofluid in mini channels.

Effect of nanoparticle shapes on nanofluid mixed forced and thermocapillary convection in minichannel

To reveal the influence of nanoparticle shapes on thermal transport performance of nanofluid convective boiling flow in minichannel, nanofluid forced flow coupled with gas bubble thermocapillary convection in minichannel is numerically investigated by two-phase mixture model. Firstly, the flow characteristics and heat transfer enhancement mechanism of mixed forced and thermocapillary convection in minichannel with different shaped nanoparticles (sphere, brick, blade, cylinder and platelet shaped particles) is disclosed. Secondly, the contribution of thermocapillary and forced flow to heat transfer is analyzed, and the relation of Nusselt number and total entropy production with nanoparticle concentration are given. Finally, the heat transfer performance of nanofluid flow with different shaped nanoparticles in minichannel is evaluated. The results show that, nanoparticle shape can effectively influence thermocapillary jet structure, and then affect the thermal transport at the vicinity of bubble. For nanofluid with spherical nanoparticle the thermocapillary jet structure is the largest, which is followed by brick, blade, cylinder and platelet shaped nanoparticles. The thermal transport performance of nanofluid with blade-shaped nanoparticle is the best, and the average Nusselt number is improved by 8.3% compared with that of nanofluid with spherical nanoaprticle as nanoparticle volume fraction is 0.05.

Numerical Study on the Flow and Heat Transfer Coupled in a Rectangular Mini-Channel by Finite Element Method for Industrial Micro-Cooling Technologies

Nowadays, cooling high thermal flows in compact volumes continues to be one of the crucial problems in the industry. With the advent of advanced technologies, much more attention has been paid to how to improve the performance of cooling systems in the area of micro-technologies. Rectangular mini-channels are typical representatives which commonly used for cooling applications. However, micro-technologies still face the problem of low performance due to the low productivity of cooling related to unbefitting physical parameter values. Here, this work studies the applicability of the heat transfer scheme of convective flow and flow boiling in a rectangular mini-channel for satisfying the cooling requirement of industrial micro-technologies, through a simulation model governed by the coupled mechanism from Navier-Stokes (N-S) equation and heat transfer equations with phase change effect. In this work, various hydraulic diameters and different inlet fluid speed are used to calculate the different velocity profiles, pressure drops, coefficients of friction and finally, the distribution of the temperatures and dissipated heat flux. The simulation results show the applicability of the rectangular mini-channel in diverse applications such as engine cooling, electronic components, automotive on-board electronics and aerospace engineering. Flow boiling simulation results reveal that the obtained patterns were smooth mixture flow and discrete flow. The dissipated heat flux can reach 1.02–5.34 MW/m2 for a hydraulic diameter of 0.5 mm. We show that the system with the gradient temperature that evolves increasingly along the top and bottom walls of the channels presents the highest heat flux dissipated in flow boiling. Additionally, the fin efficiency of the system is 0.88 and the coefficient value of convective heat transfer is in the range between 5000 &lt; h &lt; 100,000, which indicates the flow boiling heat transfer is effective in the mini-channel when the Reynolds number is less than 400. It provides a significant heat exchange for cooling in these application areas.

Hydrodynamics of gas/shear-thinning liquid two-phase flow in a co-flow mini-channel: Flow pattern and bubble length

The flow patterns and bubble characteristics formed during gas–liquid flows in a circular co-flow mini-channel with carboxymethyl cellulose (CMC) aqueous solutions are investigated experimentally. The pattern transition and bubble length are elucidated by systematically analyzing the influences of the various factors of the ratio of gas–liquid flow rates, CMC solution mass fraction, and surfactant [sodium dodecyl sulfate (SDS)] mass fraction. Five kinds of flow regimes, namely, bubbly flow, Taylor flow, Taylor-annular flow, annular flow, and churn flow, are identified visually in the fully developed region of the inlet side of the channel, and a universal flow-regime map in terms of the gas and liquid inlet flow rates is constructed using water, CMC solution, and polyacrylamide solution to cover a broad range of material properties. It is found that the ratio of gas–liquid flow rates has a remarkable influence on the flow pattern transitions. The CMC solution mass fraction and SDS mass fraction can also affect the flow-regime map by varying the flow drag force and surface tension acting on the bubble in the mini-channel. The bubble length increases with the ratio of gas–liquid flow rates but decreases with the increase in the CMC fraction and SDS fraction. Based on consideration of the rheological properties of the liquid, a scaling law of bubble length in a co-flow mini-channel with shear-thinning liquids is developed, and the results predicted by it can agree with the measurement data very well under present conditions.

Thermal properties measurement and performance evaluation of water/ZnO nanofluid in a mini channel with offset fins

The present study experimentally and numerically investigated the heat transfer and flow behavior of ZnO/water nanofluid in a mini channel with offset fins with a 1.59mm hydraulic diameter. The thermal properties including density, viscosity and thermal conductivity with 0.75% and 1.5% volume concentration were measured and compared with correlations firstly. Then, the thermal and flow characteristics under different operational conditions were evaluated. Results indicate that the existing correlations can desirably predict the density and thermal conductivity but heavily underestimate the viscosity. The existence of offset fins eliminates the obvious transition zone from laminar flow to turbulent flow. Even though the Nusselt number of nanofluid can be improved by 7.9% and 10.5% averagely for concentrations of 0.75% and 1.5%, the friction factor is also significantly enlarged. The heat load shows negligible impact on the flow and heat transfer performance. For the thermal performance factor, its value is around 1 under different concentration and Reynolds number, which means that no benefit will be brought about by nanofluid considering both thermal and hydrodynamic performance. As the existing correlation fails to accurately predict the Nusselt number, a new correlation is put forward with a MARD of 1.7% for ZnO nanofluid flow in mini channel with offset fins. Numerically, the simulation results of single-phase, mixture and Eulerian model were compared with experimental ones. It is found that the single-phase model gives the best prediction of Nusselt number with a MARD of 8.64%. In addition, the mixture model has the smallest prediction derivation of friction factor with a MARD of 18.8%.

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.

The applicability of heat transfer correlations to flows in minichannels and new correlation for subcooled flow boiling

This paper focuses on the applicability of heat transfer correlations to flows in minichannels. The data were taken from own research conducted at the experimental stand for subcooled flow boiling heat transfer in an asymmetrically heated rectangular minichannel oriented with seven inclination angles (0°, 30°, 60°, 90°, 120°, 150°, and 180°) relative to the horizontal plane. Various cooling liquids (FC-72, HFE-649, HFE-7100 or HFE-7000) flowing along a minichannel were heated by a thin plate. The plate had a smooth or enhanced surface on the side that was in contact with the fluid. The enhanced surfaces were obtained by vibration-assisted laser surface texturing or electromachining. Porous surfaces produced by soldering or sintering iron powder to the heated surface were also tested. Infrared thermography was used to determine changes in temperature on the outer smooth surface of the plate. Local heat transfer coefficients in the contact surface between the heated plate and the cooling fluid were determined using the one-dimensional mathematical method. A dozen or so heat transfer correlations selected from the literature have been used in calculations. It was found that the majority of correlations enabled the predicting of Nusselt number within error bands of ±25%. The smallest mean absolute percentage error in determining the Nusselt number was obtained using Mikielewicz J. correlation. A new correlation for subcooled flow boiling heat transfer in minichannels was proposed. The mean absolute percentage error of the Nusselt number, amount to 14%, was achieved according to own correlation.

Experimental investigation of laminar flow and heat transfer characteristics in square minichannels with twisted tapes

The flow and heat transfer characteristics of distilled water in a minichannel heat sink (MCHS) fitted with twisted tapes under laminar flow are investigated experimentally in present study. Nine twisted tapes with lengths of Lt=50mm, 100 mm and 150 mm, and twist ratios of Y = 3, 4, and 5 are employed as inserts. The results show that average heat transfer coefficient (have), pressure drop per unit length (ΔPpul) and thermal enhancement factor (η) increase with the decrease in twist ratio or/and the increase in twisted tape length. The heat transfer enhancement factor (have/have,0, where subscript 0 denotes plain MCHS) and the pressure drop increasing factor (ΔPpul/ΔPpul, 0) are in the ranges of 1.29–2.49 and 1.68–4.72, respectively. The average values of η for each MCHS with twisted tape are in a range of 1.09–1.46. The full-length (Lt=150mm) twisted tape with Y = 3 yields the best heat transfer rate, the highest flow resistance and the best overall thermal performance compared to other eight twisted tapes. In addition, the empirical correlations for have and ΔPpul are developed based on the experimental results. The predicted have and ΔPpul are within ±5% and ±6% deviation, respectively, compared to the experimental data.

CO2 Absorption by Liquid Films under Taylor Flow in Serpentine Minichannels

The mass transfer in CO2 absorption can be intensified in the miniaturized devices. In this paper, the weak CO2 absorption in aqueous glycerol solutions in a serpentine minichannel is studied, with...

Effect of cross-sectional shape on void fraction characteristics of vertically upward gas-liquid two-phase flows in mini-channels

Effect of cross-sectional shape on void fraction characteristics of vertically upward gas-liquid two-phase flows in mini-channels

Enhancing thermal-hydraulic performance of counter flow mini-channel heat sinks utilizing secondary flow: Numerical study with experimental validation

Continual growth of hydraulic and thermal boundary layers along stream wise direction in conventional straight fin mini-channel heat sink (MCHS) causes gradual deterioration of their thermal performance. To enhance thermal-hydraulic performance by breaking and re-development of the boundary layers, this research aims to introduce a novel water cooled inter-connected counter flow mini-channel sink (ICMCHS). Two inter-connectors (ICs) were positioned transversely between two counter flow mini-channels (CMCs) which segmented the flow domain into three zones (zone 1–3). Secondary flow was generated through the ICs utilizing the pressure difference of the adjacent CMCs resulting in disruption of the hydraulic and thermal boundary layers. To examine the effect of the ICs location and width on the thermal-hydraulic characteristics of the counter flow mini-channel heat sink (CMCHS), the present numerical studies were carried out for nine different cases (case 1–9) by varying ICs width from 1 mm to 1.5 mm and ICs location from 4 mm to 9 mm. A corresponding conventional CMCHS was chosen as the base case in contrast to the newly proposed ICMCHS. Experiments were also carried out for CMCHS to validate numerical results, and excellent agreement was found between measured values and the corresponding numerical results. At the lowest considered Re (Re = 150), a maximum value of Performance Evaluation Criterion (PEC) was achieved to ~1.22 for the highest length of zone 1 and 3 and the lowest ICs width (case 7), whereas at the highest Re (Re = 1044), the maximum PEC value (~1.42) was recorded for the intermediate length of zone 1 and 3 and the highest ICs width (case 6).

미니채널 내 물-에탄올 혼합물로 구성된 단일 액체 슬러그 계면 깨짐 현상에 대한 실험적 연구

A two-phase flow in mini-channels is consist with various flow regimes (such as bubble slug, annular, churn flow) according to gas and liquid phase flow rates. A previous researches revealed that in a case of slug flow in hydrophobic mini-channels, gas and liquid phase are perfectly separated by interfaces and triple contact line. In this study, the single dry slug flow experiments in circular mini-channel (D = 1.018 mm) are conducted to observe interfacial break-up phenomena in high capillary number range (Ca < 0.02). The slug is consist with D.I. water or D.I. water-ethanol binary mixtures (5% and 10%, mole fraction). On the base of previous researches, we calculate the pressure drop at moving triple contact line. In an addition, a single dry slug flow is visualized by using high-speed camera. Through the experiment, three regimes of pressure drop are observed; steady, loss, separation. As a result, criteria between steady and loss regimes is closely related to capillary number, and criteria between loss and separation regimes is related to surface tension.

Comprehensive heat transfer performance analysis of nanofluid mixed forced and thermocapillary convection around a gas bubble in minichannel

For exploration of the comprehensive heat transfer performance of nanofluid convective boiling flow in minichannel, nanofluid forced flow coupled with thermocapillary convection induced by bubble in minichannel is investigated. Firstly, the heat transfer enhancement mechanism of thermocapillary jet driven by gas bubble in minichannel is discussed. Secondly, the contribution of thermocapillary and forced flow to heat transfer is analyzed. Finally, the relation of Nusselt number with nanoparticle concentration and diameter and thermal transport performance evaluation are given. Thermocapillary jet in minichannel can effectively enhance the heat transport near gas bubble, and local heat transfer can be altered by adjusting the concentration and size of nanoparticle. As nanoparticle concentration increases the average Nusselt number (Nuave) of lower wall and total entropy generation (Sgen) increase, and as αp increases from 0 to 0.05 the Nuave is increased by 7.5%. However, with nanoparticle diameter increasing the Nuave and Sgen decrease, and the Nuave as dp = 100 nm is decreased by 2.4% relative to that of dp = 25 nm.

3D Reconstruction of Slug Flow in Mini-Channels with a Simple and Low-Cost Optical Sensor.

The present work provides a new approach for 3D image reconstruction of gas-liquid two-phase flow (GLF) in mini-channels based on a new optical sensor. The sensor consists of a vertical and a horizontal photodiode array. Firstly, with the optical signals obtained by the vertical array, a measurement model developed by Support Vector Regression (SVR) was used to determine the cross-sectional information. The determined information was further used to reconstruct cross-sectional 2D images. Then, the gas velocity was calculated according to the signals obtained by the horizontal array, and the spatial interval of the 2D images was determined. Finally, 3D images were reconstructed by piling up the 2D images. In this work, the cross-sectional gas-liquid interface was considered as circular, and high-speed visualization was utilized to provide the reference values. The image deformation caused by channel wall was also considered. Experiments of slug flow in a channel with an inner diameter of 4.0 mm were carried out. The results verify the feasibility of the proposed 3D reconstruction method. The proposed method has the advantages of simple construct, low cost, and easily multipliable. The reconstructed 3D images can provide detailed and undistorted information of flow structure, which could further improve the measurement accuracy of other important parameters of gas-liquid two-phase flow, such as void fraction, pressure drop, and flow pattern.

Thermally Developing Flow and Heat Transfer in Elliptical Minichannels with Constant Wall Temperature.

Laminar convective heat transfer of elliptical minichannels is investigated for hydrodynamically fully developed but thermal developing flow with no-slip condition. A three-dimensional numerical model is developed in different elliptical geometries with the aspect ratio varying from 0.2 to 1. The effect of Reynolds number (25 ≤ Re ≤ 2000) on the local Nusselt number is examined in detail. The results indicate that the local Nusselt number is a decreasing function of Reynolds number and it is sensitive to Reynolds number especially for Re less than 250. The effect of aspect ratio on local Nusselt number is small when compared with the effect of Reynolds number on local Nusselt number. The local Nusselt number is independent of cross-section geometry at the inlet. The maximum effect of aspect ratio on local Nusselt number arises at the transition section rather than the fully developed region. However, the non-dimensional thermal entrance length is a monotonic decreasing concave function of aspect ratio but a weak function of Reynolds number. Correlations for the local Nusselt number and the thermal developing length for elliptical channels are developed with good accuracy, which may provide guidance for design and optimization of elliptical minichannel heat sinks.

Characteristics of frictional pressure drop of two-phase nitrogen flow in horizontal smooth mini channels in diabatic/adiabatic conditions

The two-phase frictional pressure drop of nitrogen under diabatic and adiabatic conditions is investigated experimentally in two horizontal mini channels. The effects of factors in a variety, including the channel diameter, mass flux, heat flux, inlet pressure and vapor quality are examined on the frictional pressure drop and liquid-phase multiplier of the two-phase flow. The two-phase frictional pressure drop predicted by previous correlations from other studies are compared with the experimental results of this study, and the lowest mean absolute deviation is 18.3% for adiabatic conditions and 10.9% for adiabatic conditions. Based on the experimental results of nitrogen flow, two new correlations in the form of the Chisholm-C correlation are developed to predict the two-phase frictional pressure dropby involving the effect of surface tension, and the mean absolute deviation of which is 12.3% and 10.4% for the adiabatic and diabatic conditions respectively. The new correlations are also shown able to well predict the experimental frictional pressure drop of other studies under different experimental conditions.

Improvement of two-phase refrigerant distribution for upward flow of a parallel flow minichannel heat exchanger using insertion devices

In a parallel flow heat exchanger, significant mal-distribution of flow occurs due to phase separation. In this study, various insert devices (perforated tube, perforated tube with perforated plate, orifice and perforated tube, concentric perforated tube) were investigated to obtain an improved flow distribution in a 36 channel parallel flow heat exchanger. The test section was made to closely simulate an actual heat exchanger. Tests were conducted for upward flow for the mass flux from 57 to 241 kg m−2 s−1 and quality from 0.2 to 0.4 using R-410A. Of the investigated insert devices, concentric perforated tube yielded the best flow distribution. Insertion of the concentric perforated tube reduced the thermal degradation from 61% to 14%. Furthermore, the preferred number of holes of the concentric perforated tube was dependent on the mass flux. At a low mass flux, an insert having small number of holes was preferred, whereas the reverse was true at a high mass flux. At a low mass flux, the effect of inlet vapor quality on flow distribution was significant. At a high mass flux, however, the effect of vapor quality on flow distribution was minimal. Possible explanations on the flow distribution behavior were provided through flow visualization in the header.