Flow In Minichannels Research Articles

Prediction of two-phase mass split in mini tubes

Two-phase flow in single mini tubes and parallel mini channels find a lot of practical applications. Although it is possible to measure pressure drop and void fraction, using methods like visualization, quantifying the mass split of the two phases is a hard task. This is especially the case when dealing with parallel channels where there is a need to correct the maldistribution between the channels, which is a common issue. In the present study, a semi-analytical model based on the separated flow model is proposed to estimate the mass split between air and water, one of the most commonly handled situations, for slug flow regime in horizontal mini channels, with diameters ranging from 1.0 to 2.0mm, where the competing magnitudes of inertia and surface forces makes the flow distribution complicated.

Mini-channel flow condensation enhancement through hydrophobicity in the presence of noncondensable gas

Steam condensation is important for a broad range of industrial applications, including power generation and nuclear containment systems. The presence of noncondensable gases in these systems significantly reduces heat transfer, prompting the need for condensation heat transfer enhancement. Steam was condensed in the presence of nitrogen in hydrophilic and hydrophobic 1.82-mm rectangular mini-channels for a range of experimental conditions: steam mass flux (i.e., 35–75kg/m2s), steam quality (i.e., 0.3<x<0.9), and nitrogen mass fraction (i.e., 0–30%). In the hydrophilic channel, nitrogen mass fractions of 10–30% reduced condensation heat transfer coefficients by 24–55%. Experimental results were well predicted by the Caruso et al. (2013) correlation. Dropwise condensation was observed in the hydrophobic channel, although the addition of nitrogen suppressed nucleation. In the hydrophobic channel, heat transfer was enhanced by 34–205% over the hydrophilic channel in presence of 10–30% nitrogen, particularly at low vapor mass fractions. Heat transfer coefficients in the hydrophobic channel with 30% nitrogen were identical or higher than those of pure steam in the hydrophilic channel at the same mass flux and quality. Heat transfer coefficients strongly depended on vapor mass fraction, defined as the vapor mass flow rate divided by the three-phase (vapor, liquid and nitrogen) mass flow rate.

Experimental research on pressure drop fluctuation of two-phase flow in single horizontal mini-channels

The pressure drop fluctuation of two-phase flow in single horizontal mini-channels with diameters of 1.2mm, 1.6mm and 2.0mm was experimentally investigated. Degassed water was tested in circular horizontal mini-channels with diameters of 1.2mm, 1.6mm and 2.0mm at liquid mass fluxes of 21.2, 42.4 and 84.8kgm−2s−1 as well as heat fluxes of 0–216kWm−2. The pressure drop and the mini-channel wall temperature profiles as well as the visualization results were recorded. Three basic types of the two-phase flow pressure drop fluctuation were identified based on the characteristic frequency, i.e. the low-frequency fluctuation (f<2.0Hz), the medium-frequency fluctuation (2.0Hz<f<10.0Hz) and the high-frequency fluctuation (f>10.0Hz). Besides, it was found that there existed three types of superimposed-frequency fluctuation. Pressure drop fluctuation maps were presented for three diameter channels, showing the single-phase region, the superimposed-frequency region and the low-frequency fluctuation region. The pressure drop fluctuation region depends on the ratio of the heat flux and the mass flux and the channel diameter. The periodic vapour catapult phenomenon was found during the low-frequency fluctuation, which was inferred to be caused by the bubbly/annular alternating flow pattern transition. It was found that the pressure drop and the wall temperature had the approximate fluctuation period with the periodic vapour catapult process. The medium-frequency fluctuation and the high-frequency fluctuation are related to the passage of the elongated bubbles with different periods. In superimposed medium and high frequency fluctuation, the amplitude of the pressure drop fluctuation is relatively low and no synchronous fluctuation was observed between wall temperature and pressure drop profiles.

Calculation Method for Flow Boiling and Flow Condensation of R134a and R1234yf in Conventional and Small Diameter Channels

Abstract The paper presents the results of calculations performed using the authors′ model to predict the heat transfer coefficient during flow boiling for two refrigerants R134a and R1234yf. The experimental data from various past studies have been collected and the calculations have been conducted for the full range of quality variation and a wide range of mass velocity. The aim of the study was to test the sensitivity of the in-house flow boiling and flow condensation model. The importance of taking into account the surface tension as the parameter exhibiting its importance in case of the flow in minichannels, as well as the influence of the reduced pressure were analysed. The obtained numerical results show good consistency with those recorded in the experiments.

Flow resistance of low-frequency pulsatile turbulent flow in mini-channels

Plate-type nuclear reactor fuel is currently getting increasing attentions as it features excellent heat transfer ability and compact structure. To gain an insight into the influences of channel size and pulsatile parameters on flow resistance characteristics, steady and pulsatile turbulent experimental investigation were performed. Three channels with varying height were used. The covered ranges of time-averaged Re Reta=(0.5–2) ×104, dimensionless frequency sqrtω’= 0.3–3.2, and pulsatile velocity amplitude Av= 0.04–0.93. A normalized parameter, friction factor ratio C =λta/λta, was proposed to denote the effects of flow fluctuation on time-averaged friction factor, where λta and λta are time-averaged and steady friction factor. The results show that in channel I (40 × 2 mm2), the time-averaged friction factor is remarkably larger than the steady values in the ranges of 1.5<sqrtω’<3.2.. The friction factor ratio C increases with the increasing sqrtω’ and Av, decreases with the increasing Reta. Further analysis indicated the friction factor ratio C is a function of dimensionless acceleration α*=4Avω′2/Reta. As the channel height increasing, the time-averaged friction factor decreases gradually to the same values as that of steady flows in the present ranges of sqrtω’. Finally, correlations to predict time-averaged friction factor were proposed.The effect of channel size is inferred to associate with the fractions of turbulent Stokes layer thickness to half-height of channels. In mini-channels, the turbulence Stokes layer is almost filled the entire flow area. As channel height increasing, the turbulence, which generated in near wall layer, cannot timely propagates into pipe core region before decaying-which cause the impact of superimposed unsteadiness fades rapidly. The effect of dimensionless frequency is interpreted as the response time of fluid to the rapid changes of pulsatile pressure gradient.

微細流路内における気液二相流動特性に関する研究

微細流路内における気液二相流動特性に関する研究

The interaction between fluid flow and ultra-hydrophobic surface in mini channel

Interaction of liquid with ultra-hydrophobic surface is accompanied by creation of layer of air. The effect of the air film has a potential of use in industry in many applications. The quality of the surface is influenced by matrix roughness, the character of physical or chemical cover. There was developed a method for analysis of the liquid flow and the air film using the lighting in volume, visualization with CCD camera and long distance microscope, and optical filters. There were prepared four stainless steel samples of inner channel of dimensions (80 × 8 × 8) mm and initial surface roughness Ra 0.33, Ra 1.0, Ra 2.0, and Ra 2.2. The inner channel was treated with plasma and commercial hydrophobic coating Greblon (WEILBURGER Coatings GmbH). There was realized study focused on the liquid flow velocity profile close to the air film. There are present results for laminar, transient and turbulent flows. The study also estimated the air film thickness depending on the Re number. The knowledge of the air film behaviour helps applied suitable degree of processing and cover for the target application.

Evaporation Rate of a Liquid Layer Streamlined by Gas Flow in Minichannel

Paper presents the experimental results of mass evaporation rate from ethanol layer surface into a moving in parallel way air flow. Dependences of the mass evaporation rate on the gas velocity, temperature difference between gas and liquid layer, and liquid layer thickness were obtained. The assumption about the formation of the concentration boundary layer at the interface was made. It is found that with increasing the gas flow velocity the gradient of vapor concentration in the gas phase increases; as a result the evaporation rate rises. When changing the liquid layer thickness, the local maximum of evaporation rate was observed, but for 2 mm thickness layer two maxima were found. Probably, it is due the appearance of unstable structures on the liquid surface.

Laser-Based Measurement of Gas–Liquid Two-Phase Flows in Micro and Mini Channels using Multiple Photodiode Arrangement

Two-phase flow measurement is important in process and chemical industries. In the present work, a measurement technique for the characterization of two-phase flows using a laser and an arrangement of the photodiodes is proposed and tested. Two-phase flow experiments were carried out in 1.80, 3.14, and 4.68-mm-diameter glass tubes. A high-speed camera was used to analyze the various optical patterns formed by a stationary laser beam on a screen during bubbly, slug, and stratified flows, which have been discussed in detail in Navisa et al. (2015). Photodiodes are arranged on the screen based on high-speed camera recordings. A procedure is demonstrated to calculate the flow regime velocity and length based on the photodiode outputs. The designed nonintrusive instrumentation determines the two-phase flow parameters in real time for flow in optically transparent mini channels. The accuracy of the measurement technique and the limitations are also discussed.

A numerical study of heat transfer and pressure drop of hydrocarbon mixture refrigerant during boiling in vertical rectangular minichannel

Plate-fin heat exchangers (PFHE) are probably the most common type of heat exchangers. However, studies on the heat transfer performance and pressure drop of hydrocarbon mixture refrigerant in a PFHE have rarely been conducted. In this paper, boiling heat transfer and friction pressure drop characteristics of hydrocarbon mixture refrigerant in PFHE were investigated numerically. A model was established on boiling flow in vertical rectangular minichannel, and also validated by the experiment data from literature. Results indicated that the boiling heat transfer coefficient and pressure drop increased with the increase of quality and mass flux. However, they were slightly impacted by the heat flux. This was because that the main boiling mechanism was forced convective boiling, while the effect of nucleation boiling is slight on the heat transfer. The simulation data were compared with some well-known heat transfer and pressure drop correlations. The Liu and Winterton’s correlation showed the best agreement with a mean absolute deviation mostly less than ±15% for heat transfer. The calculation on Mishima and Hibiki’s correlation was less than the simulated results because that the influence of heat flux was ignored for friction pressure drop in that. Meanwhile, a new correlation for pressure drop was developed with deviation less than ±15%. The presented research is helpful in designing more effective PFHE.

Experimental investigation of MWCNT–water nanofluids flow and convective heat transfer characteristics in multiport minichannels with smooth/micro-fin surface

This paper reports on the results of an experimental study on the heat transfer and flow performance of multiwalled carbon nanotubes (MWCNT) in aqueous suspensions flowing in multiport minichannel flat tubes with smooth/micro-fin surface under a constant heat flux condition. The MWCNT–water nanofluids volume concentrations are 0.001%, 0.005%, 0.01%, and 0.1%. Results indicate that the friction factor and the Nusselt number of the nanofluids are higher than those of water. The Nusselt number does not increase with volume concentration. The heat transfer of nanofluids in the micro-fin tubes is higher than that of nanofluids in the smooth tube. However, the heat transfer enhancement of nanofluids in the micro-fin tubes is lower than that of nanofluids in the smooth tube. The effect of micro-fin spacing is more evident in nanofluids than in water. Nanofluids with a concentration of 0.01% flowing in the tube with closely spaced micro-fin tube exhibit the optimal thermal performance in accordance with the performance evaluation criterion.

Characterisation of gas-liquid two-phase flow in minichannels with co-flowing fluid injection inside the channel, part II: gas bubble and liquid slug lengths, film thickness, and void fraction within Taylor flow

Current research proofs the potential of apparatuses containing minichannel flow structures to intensify gas-liquid-solid contacting processes. The excellent heat and mass transfer in these devices as well as a sharp RTD mainly result from the Taylor flow regime. A proper design of corresponding contactors requires precise information on the provided interfacial areas. However, the characterisation of gas-liquid Taylor flow with industrially relevant fluids at elevated pressure and created by capillary injection devices gained little attention so far.This work analyses adiabatic gas-liquid Taylor flow in a square minichannel of 1.0mm hydraulic diameter using water, water-glycerol, or water-ethanol mixtures as liquid phase and hydrogen or nitrogen as gas phase to cover a broad range of material parameters. In the mixing zone located within the flow channel, gas was injected into the co-flowing liquid by so-called capillary injectors with variable inner diameter (0.184, 0.317, 0.490mm).Two different bubble forming mechanisms were identified leading to a complex interaction between physical properties of the fluids, geometrical parameters and the observed gas bubble and liquid slug lengths. According to the Pi-theorem, these lengths were affected by 6 dimensionless groups, namely (uG,s/ uL,s), ReL, WeL, (dIn,CI/ dh), (dOu,CI / dh), and Θ*. Based on more than 370 experimental data, novel correlations to predict gas bubble and liquid slug lengths were developed.

Characterisation of gas-liquid two-phase flow in minichannels with co-flowing fluid injection inside the channel, part I: unified mapping of flow regimes

Present research highlights the potential of apparatuses with integrated minichannel packings to intensify gas-liquid-solid contacting. Especially an operation of these devices within the Taylor flow regime gained extraordinary attention due to its excellent heat and mass transfer and the segmented flow characteristics. However, criteria for flow regime transitions are mainly developed from water-similar fluids and are contradictory which hinders uniform flow regime prediction.This work presents a systematic analysis of adiabatic gas-liquid downflow in a square minichannel of 1.0mm hydraulic diameter. In the mixing zone located within the flow channel, gas was injected into the co-flowing liquid by so-called capillary injectors with variable inner diameter (0.184, 0.317, 0.490mm). Experiments were conducted using water, water-glycerol, and water-ethanol mixtures to cover a broad range of material properties. The gas and liquid superficial velocities were varied between 9.81·10-4…2.72m/s and 1.7·10−4…0.80m/s, respectively. Taylor flow, Taylor-annular flow, annular flow, churn flow, and bubbly flow were observed. Using the Pi-theorem, 8 significant dimensionless groups dictating the flow transition were identified, namely uG, s/uL, s, ReG, ReL, WeG, WeL, Θ*, dIn, CI/dh, and dOu, CI/dh. Based on more than 1500 experimental data, criteria for the regime transitions of Taylor flow are provided. The derived flow regime map shows good agreement for all applied liquids and for the two larger injector geometries.

Development of a model for entropy generation of water-TiO2 nanofluid flow considering nanoparticle migration within a minichannel

Thermal, frictional and total entropy generation rates are numerically investigated for the water-TiO2 nanofluid flow in a circular minichannel considering particle migration. Then, an Artificial Neural Network (ANN) model is developed for prediction of the entropy generation rates in terms of Reynolds number, particle concentration and particle size using the obtained numerical data. The entropy generation rates are examined both locally and globally. The particle migration causes non-uniform distribution of the concentration and thermophysical properties of the nanofluid. This non-uniformity becomes more intense by increasing each of the parameters of concentration, particle size and Reynolds number, such that for mean concentration of 4% and particle size of 80nm, the values of concentration increment from the wall to the pipe center are about 32% and 66% for Reynolds numbers of 1000 and 2000, respectively. The results indicate that the particle migration can have significant effects on the entropy generation rates, especially at great particle sizes and high concentrations. To address the contribution of each of the factors in the total entropy generation, Bejan number is calculated. The Bejan number reduces by increasing the concentration and Reynolds number, but increases by the particles enlargement. At all concentrations, the Bejan number is greater than 0.8 which indicates that more than 80% of the generated entropy comes from heat transfer. The developed ANN model can predict the outputs with an acceptable accuracy, such that the values of MAE for rates of thermal, frictional and total entropy generation are respectively 4.265×10−6, 1.606×10−6 and 4.305×10−6 based on the test data.

Prediction of flow boiling heat transfer coefficient for carbon dioxide in minichannels and conventional channels

AbstractIn the paper presented are the results of calculations using authors own model to predict heat transfer coefficient during flow boiling of carbon dioxide. The experimental data from various researches were collected. Calculations were conducted for a full range of quality variation and a wide range of mass velocity. The aim of the study was to test the sensitivity of the in-house model. The results show the importance of taking into account the surface tension as the parameter exhibiting its importance in case of the flow in minichannels as well as the influence of reduced pressure. The calculations were accomplished to test the sensitivity of the heat transfer model with respect to selection of the appropriate two-phase flow multiplier, which is one of the elements of the heat transfer model. For that purpose correlations due to Müller-Steinhagen and Heck as well as the one due to Friedel were considered. Obtained results show a good consistency with experimental results, however the selection of two-phase flow multiplier does not significantly influence the consistency of calculations.

Pressure drop of HFE7000 and HFE7100 during flow condensation in minichannels

Attention is focused on the issues of flow resistance and pressure drop of perspective refrigeration fluids during condensation in the two-phase flow in minichannels. The case of the flow through a cylindrical single channel of 2.3 mm inner diameter was carefully analyzed. Pressure drop tests have been performed in diabatic flow conditions. The effect of heat flux, mass flux, vapour quality and saturation temperature on the two-phase pressure drop have been scrutinised. In the course of investigations, the new fluids, namely HFE 7100 and HFE 7000, have been tested. Calibration of the measurement system for the case of single phase flow and comparison of the results with the Blasius friction correlation show the accuracy of ±15%. Additionally, the data were compared with some well established correlations for two-phase flow exhibiting that the correlation due to Fronk and Garimella (2010) and the in-house model are found in best agreement with the experimental data.

Two-phase refrigerant distribution in a two row/four pass parallel flow minichannel heat exchanger

R-410A distribution was experimentally studied for an evaporator having two row/four pass configuration. The evaporator has inlet, intermediate and top (or bottom) row-crossing headers. Tests were conducted for the mass flux from 70 to 130kgm−2s−1 with inlet quality 0.2 and exit superheat 5°C. Flow distribution at the inlet header generally follows the well-known trend. For downward configuration, more liquid flows into upstream channels. For upward configuration, more liquid flows into downstream channels. At the intermediate header, flow distribution is dependent on the direction of the tubes. When tubes are heading upward, more liquid is supplied into upstream. When tubes are heading downward, on the contrary, more liquid is supplied into downstream channels. Flow distribution at the top row-crossing header is entirely different from that at the bottom row-crossing header. Thermal degradation by flow mal-distribution is much larger for top row-crossing header configuration than for bottom row-crossing header configuration. Correlations are developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number. Header pressure drops are obtained by subtracting flat tube pressure drops and other minor pressure drops from measured pressure drops.

Condensation of refrigerants in a multiport tube with rectangular minichannels

This study investigated the condensation heat transfer and pressure drop characteristics of refrigerants R134a, R32, R1234ze(E), and R410A in a horizontal multiport tube with rectangular minichannels, in the mass velocity range of 100–400 kg m−2 s−1 and saturation temperature set at 40 and 60 °C. The effect of mass velocity, vapor quality, saturation temperature, refrigerant properties, and hydraulic diameter of rectangular channels on condensation characteristics is clarified. A new correlation is proposed for predicting the frictional pressure drop for condensation flow in minichannels. A heat transfer model for condensation heat transfer in rectangular minichannels is developed considering the flow patterns and effects of vapor shear stress and surface tension. Then, based on this model, a new heat transfer correlation is proposed. The proposed correlations successfully predict the experimental frictional pressure drop and heat transfer coefficients of the test refrigerants in horizontal rectangular minichannels.

Coupling relationship analysis between flow and pyrolysis reaction of endothermic hydrocarbon fuel in view of characteristic time correlation in mini-channel

In order to study the coupling relationship between flow and pyrolysis of endothermic hydrocarbon fuel in mini-channel of regeneratively cooled advanced aeroengines, characteristic time was defined to describe the time scales for flow and pyrolysis reaction in pyrolysis reaction flow field. The DamkÖhler number (Da) was used to quantitatively describe the relationship and degree of coupling between flow and pyrolysis reaction in mini-channel in terms of characteristic time. A 1D model was established to describe the coupling relationship of flow and pyrolysis reaction, and validated through experimental data. Simulation and experimental results indicated that: relationship and degree of coupling between flow and pyrolysis reaction could be quantitatively described using Da. The flow field could be divided into three regions according to the distributions of Da and chemical heat sink: frozen flow, non-equilibrium flow and equilibrium flow. Each region exhibited the different coupling characteristics between flow and pyrolysis reaction, which resulted in different distribution of chemical heat sink. There was a maximum rate of chemical heat sink releasing between non-equilibrium region and equilibrium region in particular. And the variation of flow had no influence on the maximum rate of chemical heat sink releasing.

Analytical Model With Nonadiabatic Effects for Pressure Drop and Heat Transfer During Boiling and Condensation Flows in Conventional Channels and Minichannels

In this paper a method developed earlier by the authors is applied to calculations of pressure drop and heat transfer coefficient for boiling flow and condensation flow with account of nonadiabatic effects for some recent data collected from the literature. The first effect, the modification of interface shear stresses in an annular flow pattern, is considered through incorporation of the so-called “blowing parameter.” The mechanism of modification of shear stresses at the vapor–liquid interface for such case is presented in detail in the paper. In the case of annular flow it contributes to thickening and thinning of the liquid film, which correspond to condensation and boiling, respectively. There is also another influence of the wall heat flux, where it is influencing the bubble nucleation in the case of the bubbly flow pattern. As a result, a modified general form of the two-phase flow multiplier, applicable both to boiling flow and condensation flow, is obtained, in which the nonadiabatic effects are clearly pronounced. The obtained model of the two-phase multiplier, incorporating the nonadiabatic effects, is additionally used in predictions of the heat transfer coefficient.