Horizontal Rectangular Microchannel Research Articles

Heat transfer characteristics of supercritical CO2 in horizontal rectangular microchannels

Due to the micro-channel size effect and thermophysical property variations, the heat transfer characteristics of S-CO2 are unclear which are crucial for the design of printed circuit heat exchanger (PCHE). This study experimentally investigated the heat-transfer phenomenon and heat-transfer coefficient of supercritical carbon dioxide (S-CO2) heated in horizontal rectangular microchannels. The effects of different parameters, including an operating pressure of 9–12 MPa, mass flow rate of 290–1200 kg/m2·s, and heat flux of 25–50 kW/m2, on the S-CO2 heat-transfer characteristics were analysed. The experimental results show that the peak value of the heat-transfer coefficient appears near the pseudo-critical point, and the peak value moves in the direction of high temperature and decreases as the operating pressure increases. Compared with the 2-mm-diameter channel, the buoyancy has a smaller effect on heat transfer in the 1-mm-diameter channel. Consequently, a new heat transfer correlation was developed wherein the microchannel size effect and thermophysical property variations are fully considered.

Experimental study on the pressure drop characteristics of supercritical CO2 in horizontal rectangular microchannels

The pressure drop characteristics of supercritical carbon dioxide (S-CO2) are experimentally studied in horizontal rectangular microchannels with hydraulic diameters of 1 and 2 mm, which are used in printed circuit heat exchangers. The operating conditions include an operating pressure of 9–12 MPa, a mass flux of 290–1200 kg/m2 s, and a heat flux of 25–50 kW/m2. The four contributors to pressure drop in the S-CO2 microchannels were investigated. The results indicate that the total pressure drop decreases for lower operating pressures and mass fluxes, whereas the heat flux has little impact. The most significant contributor to the total pressure drop is the frictional pressure drop. The accelerational pressure drop increases with the heat flux. Eventually, a new frictional resistance correlation is proposed where the microchannel size and thermophysical property variations are considered.

Pressure Change for Single- and Two-Phase Non-Newtonian Flows through Sudden Contraction in Rectangular Microchannel

The flow characteristics of the single-phase liquid and the gas–liquid two-phase flows including the Newtonian and non-Newtonian liquids were experimentally investigated in a horizontal rectangular micro-channel with a sudden contraction—specifically the pressure change across the contraction. The rectangular cross-sectional dimension has Wu × Hu (width × height) = 0.99 × 0.50 mm2 on the upstream side of the contraction and Wd × Hd = 0.49 × 0.50 mm2 on the downstream side. The resulting contraction ratio, σA (=Wd/Wu), was 0.5. Air was used as the test gas (in the case of the gas–liquid two-phase flow experiment), distilled water and three kinds of aqueous solution, i.e., glycerin 25 wt%, xanthangum 0.1 wt% and polyacrylamide 0.11 wt% were used as the test liquid. The pressure distribution in the flow direction upstream and downstream of the channel was measured. The pressure change and loss at the sudden contraction were determined from the pressure distribution. In addition, the pressure change data were compared with the calculation by several correlations proposed by various researchers as well as a newly developed correlation in this study. From the comparisons, it was found that calculations by the newly developed correlations agreed well with the measured values within the error of 30%.

Experimental Investigation and Prediction on Pressure Drop during Flow Boiling in Horizontal Microchannels.

In this paper, two-phase pressure drop data were obtained for boiling in horizontal rectangular microchannels with a hydraulic diameter of 0.55 mm for R-134a over mass velocities from 790 to 1122 kg/(m2·s), heat fluxes from 0 to 31.08 kW/m2 and vapor qualities from 0 to 0.25. The experimental results show that the Chisholm parameter in the separated flow model relies heavily on the vapor quality, especially in the low vapor quality region (from 0 to 0.1), where the two-phase flow pattern is mainly bubbly and slug flow. Then, the measured pressure drop data are compared with those from six separated flow models. Based on the comparison result, the superficial gas flux jg is introduced in this paper to consider the comprehensive influence of mass velocity and vapor quality on two-phase flow pressure drop, and a new equation for the Chisholm parameter in the separated flow model is proposed as a function of the superficial gas flux jg. The mean absolute error (MAE) of the new flow correlation is 16.82%, which is significantly lower than the other correlations. Moreover, the applicability of the new expression has been verified by the experimental data in other literatures.

Experimental study of gas-liquid flow in a rectangular microchannel using LIF method

This paper aims at experimental study of gas-liquid flow in a horizontal rectangular hydrophilic microchannel with a cross-section of 200×400 μm. The ethanol and nitrogen are used as working liquids and gas accordingly in the microchannel with crossflow T-mixer at the channel inlet. Experimental data are obtained using high-speed visualization, laser flow scanning, and LIF method. The elongated bubble flow and transition flow are observed in the hydrophilic rectangular microchannel depending on gas and liquid flow rates. Using the LIF method, the non-uniform liquid film distribution is observed in the channel cross-section. The liquid film thickness is measured for elongated bubble flow and transition flow and compared with the Taylor law.

An experimental investigation of gas-liquid flowcharacteristics in the microchannel

The two-phase gas-liquid flow regimes in the horizontal rectangular microchannel using the high-speed video camera and dual-laser scanning were studied. Experiments were carried out in the microchannel with cross section 217x370 μm for the water-nitrogen mixture. The T-shaped mixer was used for gas-liquid flow formation. The influence of T-shaped entrance on the length of a gas and liquid bridges in the microchannel was established. The total pressure drop was measured in the microchannel taking into account the distribution of the gas and liquid bridges.

An Examination of the Validity of MRPS Method for the Detection of Label-free E. Coli and Enterococci in Ships Ballast Water

To test for compliance with the D-2 standard for ballast water management convention of the International Maritime Organization, the bacteriological quality of ship’s ballast water is determined to evaluate the risk of invasion of non-indigenous species both onboard ship and in Port. In this article, hydrodynamic flow focusing particles viewed through a microfluidic resistive pulse sensor (MRPS) was used to detect the pathogenic bacteria from ship’s ballast water. Escherichia coli and Enterococci were addressed as bacterial indicators to determine of their characteristic and concentration. The system provided the individual particle-by-particle readout in rather large 20 μm x 10 μm x 8 μm (length x width x height) horizontal rectangular microchannel duct. The average volume flow rate was 4.5 μl/min. However, it returned online and real time results for a complete of 30 μl sample volume during 15 min. The system was low maintenance, high in sensitivity, good accuracy and reliable. Targeted bacteria needed neither labelling nor extracting DNA.

Effect of wall surface wettability on pressure drop of gas-liquid two-phase flow in horizontal rectangular micro-channel

Effect of wall surface wettability on pressure drop of gas-liquid two-phase flow in horizontal rectangular micro-channel

Numerical investigation of the bubble growth in horizontal rectangular microchannels

ABSTRACTTo explore the mechanism of flow boiling in microchannels, the processes of a single-vapor bubble evaporating and two lateral bubbles merging in a 2D microchannel are investigated. The temperature recovery model based on volume of fluid method is adopted to perform the flow boiling phenomena. The effects of wall superheat, Reynolds number, contact angle, surface tension, and two-bubble merger on heat transfer are discussed. Wall superheat dominates the bubble growth and is roughly proportional to wall heat flux. The update of velocity and temperature fields’ distribution in the channel increases with increasing inflow Reynolds number, which improves the wall heat flux markedly. Besides, the area of thin liquid film between the wall and the bubble is enlarged by reducing the contact angle, thus, expanding the wall heat flux several times compared with the single-phase cross section. However, variation of surface tension (0.0589, 0.1 N/m) is found to be insignificant.

Features of two-phase flow patterns in horizontal rectangular microchannels of height 50 μm

The horizontal microchannel with the height of 50 micrometres and width of 40 mm of a rectangular cross-section has been used to study two-phase flow. The classical patterns of two-phase flow in the channel (bubble, stratified, churn, jet, and annular) have been detected. Experimental information allows us to define the characteristics of the regimes and to determine precisely the boundaries between the patterns of the two-phase flows.

Two-phase flow patterns in short horizontal rectangular microchannels

The two-phase flow in a short horizontal channel of a rectangular cross-section with the height of 100–500 µm and width of 9–40 mm was studied experimentally. The use of the Schliren and fluorescent methods made it possible to reveal the flow of liquid in the channel and to determine its characteristics quantitatively. The features of the churn, jet and drop flow patterns were studied in details. Two particular regimes that can be distinguished represent formation of immobile drops on the channel walls because of the liquid film or liquid bridges breakage and appearance of mobile drops due to the two-phase flow instabilities. It is found out that formation of various two-phase flow patterns and transitions between them are determined by instabilities of the liquid–gas flow in the side parts of a channel. Frontal instability has been observed during the liquid–gas interaction in the region of liquid output from the nozzle. It is shown that a change in the height and width of the horizontal channels has a substantial effect on the boundaries between the flow regimes. One of the results is that the region of the churn regime increases significantly with decreasing thickness of the channel.

Two-phase flow in short horizontal rectangular microchannels with a height of 300 μm

The two-phase flow in a narrow short horizontal channel with a rectangular cross section is studied experimentally. The channel has a width of 10, 20, or 30 mm and a height of 300 μm. The specifics of formation of such two-phase flows are investigated. It is demonstrated that the regions of bubble and churn flow regimes grow and constrain the region of jet flow as the channel gets wider. The boundaries of the regions of annular and stratified flow regimes remain almost unaltered.

Experimental investigation of gas–non-Newtonian liquid two-phase flows from T-junction mixer in rectangular microchannel

In this study, gas–non Newtonian liquid two-phase flows in a horizontal rectangular microchannel have been investigated. The rectangular microchannel has a hydraulic diameter of 0.235mm, and a width and depth of 0.24mm and 0.23mm, respectively. A T-junction-type gas–liquid mixer was used to introduce gas and liquid in the channel. Polyacrylamide aqueous solutions with different mass concentration, which exhibits pseudo plastic with viscoelasticity, were used as non-Newtonian liquids, while the nitrogen gas was used as the test gas. The flow pattern, the bubble length, the liquid slug length, the bubble velocity and frictional pressure drop in two-phase flow were measured. Each experimental data are compared to those for nitrogen gas–water two-phase flow. From the comparison, rheological properties of polyacrylamide solutions are found to significantly affect flow parameters measured. In addition, two-phase frictional pressure drop could be correlated by using Lockhart–Martinelli method with a newly developed two-phase friction multiplier as function of void fraction and rheological flow index.

F221 マイクロ・ミニ流路内の曲がり部を通過する二相流の圧力損失(OS9 熱・流動(5))

Two-phase gas-liquid pressure drop data through 180° return bends have been obtained for horizontal rectangular micro-channel and mini-channel. To know the effects of channel sizes, the hydraulic diameters were 0.25 mm and 3 mm respectively as the micro- and the mini-channels. The curvature radii of the bends were 0.500 mm and 0.875 mm for the micro-channel, while 6 mm for the mini-channel. Water, surfactant and glycerin aqueous solutions, ethanol and hydrofluoroether-7200 were used as the test liquid, while nitrogen gas and air as the test gas. The bend pressure drop data were correlated by using an approach of Padilla et al., in which the pressure drop is the sum of two components, i.e., frictional and singular pressure drops. The approach was applicable to the present data if the frictional pressure drop was calculated by Lockhart-Martinelli method and the singular pressure drop was calculated by a newly developed correlation.

Two-phase flow in a horizontal rectangular microchannel

Two-phase flow in a rectangular short horizontal channel 200 μm high was studied experimentally. The use of the fluorescent method made it possible to reveal flow of liquid in the channel and to determine its characteristics quantitatively. The existence of the regime of separate (stratified) flow is established. Based on analysis of previous investigations and newly obtained data, it is shown that a change in the height of the horizontal channel has a substantial effect on the boundaries between the regimes. The region of the churn regime increases with decreasing thickness of the channel.

Heat transfer characteristics of gas–liquid flow in horizontal rectangular micro-channels

An experimental study of heat transfer characteristics of air–water flow in horizontal micro-channels was carried out in this work. The gas–liquid mixture from a y-shaped mixing chamber was forced to pass through a plenum inlet and entered 21 parallel rectangular micro-channels 40mm long in the direction of flow. Each channel had a width and a depth of 0.45 and 0.41mm, respectively. The test runs were done at a heat load of 80W, with superficial Reynolds numbers of gas and liquid ranging between 54–142 and 131–373, respectively. A Stereozoom microscope and camera system were employed to conduct flow visualization. To explore the dependence of a Nusselt number on the flow characteristics, two inlet sections with different designs were used in this work. The experiments revealed that the formation of small gas slugs instead of gas core flow involves an increase in Nusselt numbers. In this work, the gas–liquid flow gave heat transfer enhancement up to 80% over the liquid flow.

An experimental investigation on the confined and elongated bubbles in subcooled flow boiling in a single microchannel

The characteristics of the confined bubble and elongated bubble in subcooled flow boiling in a single horizontal rectangular microchannel with hydraulic diameter Dh=1mm are studied experimentally. The channel with 1 × 1mm cross section is fabricated in a thin copper plate whose confinement number is Co=2.8 and the degassed deionized water is used as the working fluid. Visualization on the confined and elongated bubbles inside the microchannel is carried out by employing a high-speed CCD camera with a microlens. The recorded images are carefully analyzed to illustrate the behaviors of the confinement and elongation processes of the bubble. The boiling number is used as an adjustable parameter to regulate the operating conditions which is eventually found to take a vital role in the bubble elongation process. Two formation patterns of the confined and elongated bubble are identified and the interactions between the neighboring confined and elongated bubbles are elucidated.

Measurement and Correlation of Pressure Drop for Gas-Liquid Two-phase Flow in Rectangular Microchannels

The pressure drop of gas-liquid two-phase flow in microchannel is of fundamental importance in heat and mass transfer processes. In this work, the pressure drop of gas-liquid two-phase flow in horizontal rectangular cross-section microchannels was measured by a pressure differential transducer system. Water, ethanol and n-propanol were used as liquid phase to study the effects of capillary number on pressure drop; air was used as the gas phase. Four microchannels with various dimensions of 100 μm × 200 μm, 100 μm × 400 μm, 100 μm × 800 μm and 100 μm × 2000 μm (depth × width) were used for determining the influence of configuration on the pressure drop. Experimental results showed that in micro-scale, the capillary number also affected the pressure drop remarkably, and in spite of only one-fold difference in aspect ratio, the variation of pressure drop reached up to near three times under the same experimental conditions. Taking the effects of aspect ratio and surface tension into account, a modified correlation for Chisholm parameter C in the Chisholm model was proposed for predicting the frictional multiplier ϕ 2 l , and the predicted values by the proposed correlation showed a satisfactory agreement with experimental data.

Characteristics of Two-Phase Flows in a Rectangular Microchannel with a T-Junction Type Gas-Liquid Mixer

In this study, gas–liquid two-phase flows in a horizontal rectangular microchannel have been investigated. The rectangular microchannel has a hydraulic diameter of 0.235 mm, and a width and depth of 0.24 mm and 0.23 mm, respectively. A T-junction-type gas–liquid mixer was used to introduce gas and liquid in the channel. In order to know the effects of liquid properties, distilled water, ethanol, and HFE7200 were used as the test liquids, with nitrogen gas was used as the test gas. The flow pattern, the bubble length, the liquid slug length, and the bubble velocity in two-phase flow were measured with a high-speed video camera, and the void fraction was determined from the bubble velocity data and the superficial gas velocity data. In addition, the pressure drop was also measured with a calibrated differential pressure transducer. The bubble length data were compared with the calculation by the scaling law proposed by Garstecki et al. [7]. The bubble velocity data and/or the void fraction data were well correlated with the well-known drift flux model [12] with a new distribution parameter correlation developed in this study. The frictional pressure drop data were also well correlated with the Lockhart-Martinelli method with a correlation of the two-phase friction multiplier.

Pressure Drop, Boiling Heat Transfer and Flow Patterns during Flow Boiling in a Single Microchannel

The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m2s and heat fluxes from 200 to 500 kW/m2. The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions.