Horizontal Microchannel Research Articles

Hydrodynamics of shear thinning fluid in a square microchannel: a numerical approach

Abstract In this present work, a numerical study was conducted for the formation of a slug bubble for shear thinning non-Newtonian fluid in a cross-junction 2-D square horizontal microchannel. Carboxymethyl cellulose (CMC) of concentration 0.2 (w/w%) percent was used as a continuous phase that shows the shear thinning behavior of non-Newtonian fluid and Nitrogen (N2) was used as the discrete phase. The pressure-based double precision solver was used in ANSYS FLUENT 2021 R2 with the volume of fluid (VOF) method. The finite volume method is applied for the discretization of the continuity and momentum equation. This article also focuses on the fluctuation of static pressure, mechanism of slug, annular, and churn annular flow i.e., obtained by the variation in the inlet velocities. On the other hand, a concept that was applied in this work was also validated with the prior literature data.

An experimental investigation of flow boiling instability of R245fa in a horizontal tube

Two-phase pressure drop oscillations during flow boiling of pure fluid R245fa in a horizontal microchannel with a diameter of 0.9 mm were experimentally investigated. Three mass fluxes G = 400, 630, and 1100 kg/m2 s with three different heat fluxes q = 40, 50, and 55 kW/m2 were tested under the inlet pressure of 211.72 kPa and inlet subcooling of 17 K. Flow instabilities were analyzed based on images and temporal oscillation results of two-phase mass flux (G), pressure drop (ΔP), and inlet pressure (Pin). Four types of high-amplitude/low-frequency are identified, the main reasons for which are found to be flow patterns in the test section and upstream compressible volume of vapor. The inlet pressure oscillation is attributed to the mixture between expansion vapor and subcooled liquid in the inlet plenum. Effects of mass flux and heat flux on the pressure drop oscillation are also discussed through the variation trends on ΔP vs q/G and the fast Fourier transform.

Mechanistic model to predict oscillating frequency of flow boiling in large length to diameter ratio micro-channel heat sinks

This study presents a new mechanistic model for density wave oscillation (DWO) of flow boiling in horizontal micro-channel heat sinks that enables prediction of oscillation frequency, utilizing both flow visualization results and transient experimental data. The proposed model is based on the hypothesis that the period of oscillation consists of two sub-periods, flow surge and reversal, and establishes a set of 1D transient conservation equations. The flow surge period is associated with liquid rapidly entering the channels, momentarily establishing high pressure gradient between inlet and outlet plenums before the incoming flow is decelerated by a combination of increased flow resistance and bubble expansion within the channels. The initial location of the bubble nucleation is estimated using a criterion for onset of nucleate boiling (ONB) that governs bubble formation from cavities in superheated near-wall liquid. The flow reversal period is associated with back flow into the inlet and estimated by time elapsed before the reversed flow returns to its original location where bubble expansion and onset of annular flow are initiated. Unlike prior models, the proposed model simulates a full cycle of flow oscillation. In this model, a new mass conservation criterion is introduced to ensure agreement between predicted and experimental mass outflows during one cycle. Also developed is a detailed spatial and temporal description of the high density wave front (HDWF) largely responsible for both pressure drop and mass velocity fluctuations during the oscillation period. Finally, a periodic flow boundary condition is imposed on flow velocities and locations to achieve same flow behavior at the start and end of cycle. The proposed model shows good agreement with measured oscillation frequencies.

Collaborative removal of fine catalysts particles and insoluble organics from methanol-to-olefins reaction wastewater using horizontal swirl regeneration micro-channel separation (HSRMS)

The methanol-to-olefins (MTO) process is regarded as a crucial approach for ensuring the sustainable growth of China's chemical industry. However, the fine catalyst particles and insoluble organics in the water system poses a serious threat to the stability of the MTO process. A novel horizontal swirl regeneration micro-channel separation (HSRMS) method for purifying the MTO reaction wastewater was developed in this study. On-site experiments were conducted using a device with a capacity of 1 t/h to investigate the separation and regeneration effects under various parameters and to verify the feasibility of this method through a long-term observation. The results showed that this method could effectively remove the fine solids and insoluble organics in the reaction wastewater, and the device operated stably with fluctuations in both the quality and quantity of wastewater for 24 consecutive days. The suspended solids in the effluent was maintained below 5 mg/L, and the average removal efficiency for insoluble organics exceeded 75%. Furthermore, the excellent regeneration performance was achieved, with solid particles and oil in the media reduced to less than 0.2% after regeneration. This method could ensure long-term stable operation of the water system and outperform other commonly used reaction wastewater treatment methods in terms of investment, maintenance cost, freshwater supplementation, and energy consumption.

Slip and convective flow of Williamson nanofluid influenced by Brownian motion and thermophoresis mechanism in a horizontal microchannel

The current work is an attempt to explicate the repercussions caused by the transmission of heat as well as mass along with the enhancement of heat transmission for a steady MHD Williamson nano-substance flowing through the microchannel with slip and convective boundary conditions. The consequence of magnetic field and viscous dissipation on the flow is recorded. The simultaneous impact of the two well-known slip-mechanisms Brownian movement and thermophoresis is elaborated explicitly. The governed non-linear systems obtained were illustrated numerically via Runge-Kutta Fehlberg 4–5th order method based on shooting scheme. The pertinent features of assorted parameters have been scrutinized with the aid of graphs. Results reveal that magnetic parameter along with buoyancy ratio parameter was observed to decline the velocity whereas Weissenberg number displays both rising and depleting conduct on velocity. Temperature was reported to boost with Brownian motion and thermophoresis parameter whereas concentration profile declines with the same. Also drag across the enclosures of the microchannel with wall heat flux are computed and studied through graphs.

Characterization of Bubble Transport in Porous Media Using a Microfluidic Channel

This study investigates the effect on varying flow rates and bubble sizes on gas–liquid flow through porous media in a horizontal microchannel. A simple bubble generation system was set up to generate bubbles with controllable sizes and frequencies, which directly flowed into microfluidic channels packed with different sizes of glass beads. Bubble flow was visualized using a high-speed camera and analyzed to obtain the change in liquid holdup. Pressure data were measured for estimation of hydraulic conductivity. The bubble displacement pattern in the porous media was viscous fingering based on capillary numbers and visual observation. Larger bubbles resulted in lower normalized frequency of the bubble breakthrough by 20 to 60 percent. Increasing the flow rate increased the change in apparent liquid holdup during bubble breakthrough. Larger bubbles and lower flow rate reduced the relative permeability of each channel by 50 to 57 percent and 30 to 64 percent, respectively.

Quantification of flow distribution and heat capacity potential of a microchannel evaporator

Maldistribution of fluid among the ports reduces the heat capacity of a microchannel heat exchanger. It is essential to understand the maldistribution mechanism to optimize the design of the heat exchanger. Quantification of the distribution helps analyze the maldistribution issue. This study proposes a method to quantify the flow distribution and capacity potential in the microchannel heat exchangers by integrating a microchannel heat exchanger model and infrared thermography. The method calculates the liquid and vapor mass flow rate at the inlet of each microchannel tube by comparing the wall temperature of a heat exchanger from simulation and the infrared image. The method is validated with tests of a 49-tube horizontal microchannel evaporator with R134a. The exit region of the top tubes has an extremely high surface temperature, which forms a superheat region. More liquid flows to the tubes located at the center of the heat exchanger. The total mass flow rate is low when the tube has a long superheat region. The heat capacity of the tested evaporator has 9.6–21.3% improvement potential when uniformly distributed. When the refrigerant outlet superheats of the evaporator is lower, the improvement potential is higher. In the superheat region, the heat flux is much lower than the average value. This quantification method provides a tool for a more profound investigation of the maldistribution issue of a microchannel evaporator, and it can be expanded to heat exchangers with different structures.

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.

Buongiorno model analysis on Carreau fluid flow in a microchannel with Non-linear thermal radiation impact and irreversibility

ABSTRACT The main focus of the current work is to explore the thermal energy and mass transfer process in conducting non-Newtonian fluid flows through a horizontal microchannel along with irreversibility analysis and to investigate the upshot of nonlinear radiation, convective boundary conditions, and magnetic field. Slip regime and temperature jump conditions are used at the boundaries of the microchannel. The rheological characteristics of the Carreau fluid model are also considered. The Buongiorno model was used in this study, which emphasizes the light on Brownian motion and thermophoresis phenomena that occur during the fluid flow. Runge-Kutta Fehlberg’s fourth-fifth order was used to solve governing equations numerically. The significant influence of effective parameters on entropy generation, Bejan number, velocity, temperature and concentration profile have been discussed with the aid of graphs. Entropy generation rises by 40% at the left wall of the channel and is depleted by 36% at the right wall when the thermophoresis parameter augments by 200%. Amplified concentration profile with an escalation of Brownian parameter. The temperature field diminishes with inflation of the radiation parameter.

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.

Flow boiling instability and pressure drop characteristics based on micro-pin-finned surfaces in a microchannel heat sink

In this work, flow boiling pressure drop characteristics based on the hydrodynamic instability with deionized water on the micro-pin-finned surfaces in a horizontal narrow microchannel with a hydraulic diameter of 952 µm were investigated. The micro-pin-finned surfaces can be divided into three types based on the size and arrangement of the fins (A30-120/S30-120/A50-120). The experiments were conducted under the mass flux from 200 to 500 kg/m2s and the inlet temperature of 30 to 50 ℃. The two-phase flow patterns on the micro-pin-finned surfaces were captured by a high-speed camera. The flow boiling instabilities in the microchannel are studied, and the two-phase flow pattern, wall temperature, heat fluxes and mass fluxes as well as pressure drop are observed and analyzed. The experiment results show the mass flux has a significant effect on the pressure drop, while it has almost no change with the increasing heat flux in the microchannel heat sink during the nucleate boiling. Besides, the pressure drop of the A30-120 and S30-120 surfaces is less than that of smooth surface due to the effect of hydrophobic surface and bubble drag reduction. The maximum pressure drop of the A30-120 surface is only 0.970 kPa corresponding to the mass flux and critical heat flux (CHF) of 500 kg/m2s and 341.4 W/cm2 in this study, respectively. And the overall performance of the micro-pin-finned surfaces is also evaluated in the microchannel heat sink, which can achieve more than a 30% reduction in pressure drop and thermal resistance compared with the smooth surface.

Flow boiling heat transfer characteristics on micro-pin-finned surfaces in a horizontal narrow microchannel

In this paper, an experimental investigation of flow boiling in a horizontal narrow microchannel with hydraulic diameter of Dh = 952 μm was conducted. Flow boiling heat transfer characteristics of micro-pin-finned surfaces were evaluated over the mass fluxes range from 200 - 500 kg/m2s (corresponding flow velocity: 0.2 - 0.5 m/s), using deionized water as the working fluid with different inlet temperatures Tin = 30 - 50 °C. The outlet pressure of the microchannel was about 1 atm. The boiling curves, heat transfer coefficients, bubble behaviors and heat transfer enhancement mechanism were discussed with the variation of heat flux, mass flux and inlet temperature. The results indicated that the micro-pin-finned surfaces could improve the critical heat flux (CHF) and heat transfer coefficient (HTC) greatly due to numerous nucleate sites and large heat transfer area enhancement ratio. The CHF up to 360 W/cm2 has been demonstrated at a mass flux of 500 kg/m2s. Compared with a smooth surface, a 240% higher heat transfer coefficient (18.4 W/cm2K) on the S30-120 surface has been achieved at a mass flux of 500 kg/m2s with an inlet temperature of 30 °C. The boiling curves of the micro-pin-finned surfaces presented an obvious “hook back” phenomenon after reaching the onset of the nucleate boiling (ONB) and then the wall temperature had a slight rise with the increase of the heat flux. Moreover, the intensive micro-pin-finned arrangements showed a significant wicking effect and promoted liquid replenishment, which could destroy the liquid boundary and enhance turbulence. The bubble behaviors were also captured to explain the heat transfer enhancement mechanism in a horizontal narrow microchannel.

Lattice Boltzmann investigation of flow boiling in a microchannel

A hybrid multiphase lattice Boltzmann model is adopted to investigate the flow boiling heat transfer process in a horizontal microchannel with consideration of bubble dynamics. The flow pattern transition in a microchannel, involving single-phase flow, bubbly flow, slug flow, and contact-slug flow, is reproduced by varying the heat flux. The influencing parameters, including the liquid mass flux, the heating wall wettability, and the confinement height of channels, on dynamic bubble behaviors and heat transfer performance are discussed. The results indicate that a sequence of single-phase flow, bubbly flow, slug flow, and contact-slug flow occurs in a microchannel with increasing heat flux. Correspondingly, the dominant heat transfer mechanism experiences the liquid convection (single-phase flow), nucleate boiling (bubbly flow), the microlayer evaporation (slug flow), and the hybrid gas convection and microlayer evaporation (contact-slug flow). The increase of mass flux or confinement height extends the range of heat flux for high-efficiency bubbly and slug flow. The hydrophilic heating wall is preferred for better heat transfer performance in a microchannel due to the strengthened microlayer evaporation and less vapor occupation of the effective nucleation area.

Improved curved-boundary scheme for lattice Boltzmann simulation of microscale gas flow with second-order slip condition

An improved curved-boundary scheme with second-order velocity slip condition for multiple-relaxation-time-lattice Boltzmann (MRT-LB) simulation of microgas flow is proposed. The proposed interpolation bounce-back (IBB)-explicit counter-extrapolation (ECE) scheme adopts the IBB method to describe the curved boundary, while the ECE method is employed to predict the slip velocity on gas-solid interface. To incorporate the effect of second-order velocity slip term and the influence of boundary curvature, a slip velocity model is also derived, from which the gas slip velocity is captured by the ECE discretization method. The influence of fictitious slip velocity can be eliminated by adopting the present ECE method, and the influence of actual offset between the lattice node and the physical boundary can be well considered by the IBB method. The proposed IBB-ECE boundary scheme is then implemented with the MRT-LB model and tested by simulations of force-driven gas flow in horizontal (inclined) microchannel, gas flow around a micro-cylinder, and Couette flow between two micro-cylinders. Numerical results show that the proposed IBB-ECE scheme improves the computational accuracy of gas slip flow (0.001<Kn≤0.1) when compared with other boundary schemes reported in the literature, and provides a precise and easy implementing scheme for curved boundary with second-order slip condition.

Effect of NP shapes on Fe3O4 – Ag/kerosene and Fe3O4 – Ag/water hybrid nanofluid flow in suction/injection process with nonlinear-thermal-radiation and slip condition; Hamilton and Crosser's model

This study examines the flow and thermal properties of water and kerosene hybrid nanofluids in a horizontal microchannel. The effects of nonlinear thermal radiation, heat source coefficient, convective and slip boundary conditions on nanofluids in a microchannel have been studied. The impact of different nanoparticle shape factors on the temperature of nanofluids has also been analysed. The governing partial differential equations have been transformed in to ordinary differential equations using dimensionless terms. In the next step, the corresponding ordinary differential equations have been cracked using the RKF45 numerical approach, and the particles shape factor on dimensionless velocity and energy fields for hybrid nanofluid then reported. The results revealed that the spherical and lamina shape factors for both water and kerosene hybrid nanofluid differ the most, the water-based hybrid nanofluid has a higher velocity than the kerosene-based hybrid nanofluid, the thermal energy of both water and kerosene-based hybrid nanofluid was higher in the lower channel plate than the upper channel plate, flow rate falls near the lower channel wall and increases towards the upper channel wall as Reynolds number increases. Thermal energy and entropy generation decrease as the temperature parameter rises.

Experimental investigation on flow boiling characteristics of HFO-1234yf in a 0.5 mm microchannel

The flow boiling heat transfer of environmentally friendly refrigerant HFO-1234yf in a horizontal circular microchannel is experimentally investigated. The inner diameter and heating length of the test channel are 0.5 mm and 300 mm, respectively. Experiments are conducted at relatively high mass flux ranging from 750 to 1500 kg m−2s−1, heat flux from 25 to 85 kW m−2 and inlet pressure between 0.604 and 0.634 MPa. The heat flux is stepwise imposed to the test channel until dryout occurs. The flow boiling heat transfer characteristics are analyzed in detail. In the pre-dryout region, the heat transfer coefficient (HTC) is found to be mainly determined by the heat flux, while the effect of mass flux is weak. Two different HTC behaviors are observed at low and high mass fluxes, which can be attributed to the prominent effect of thin liquid film evaporation on heat transfer and the occurrence of partial dryout. Moreover, the oscillation of mass fluxes and wall temperatures are observed at high mass flux conditions, resulting in heat transfer deterioration. In the post-dryout region, the critical heat flux (CHF) is found to increase with mass flux, while the critical vapor quality would decrease. Finally, the experimental data of HTC and CHF are compared with correlations from the literature and good agreements are presented.

Evaluation and Optimization of a Cross-Rib Micro-Channel Heat Sink.

This article presents a novel cross-rib micro-channel (MC-CR) heat sink to make fluid self-rotate. For a thermal test chip (TTC) with 100 w/cm2, the cross-ribs micro-channel were compared with the rectangular (MC-R) and horizontal rib micro-channel (MC-HR) heat sinks. The results show that, with the cross-rib micro-channel, the junction temperature of the thermal test chip was 336.49 K, and the pressure drop was 22 kPa. Compared with the rectangular and horizontal ribs heat sink, the cross-rib micro-channel had improvements of 28.6% and 14.3% in cooling capability, but the pressure drop increased by 10.7-fold and 5.5-fold, respectively. Then, the effects of the aspect ratio (λ) of micro-channel in different flow rates were studied. It was found that the aspect ratio and cooling performance were non-linear. To reduce the pressure drop, the inclination (α) and spacing (S) of the cross-ribs were optimized. When α = 30°, S = 0.1 mm, and λ = 4, the pressure drop was reduced from 22 kPa to 4.5 kPa. In addition, the heat dissipation performance of the rectangular, staggered fin (MC-SF), staggered rib (MC-SR) and cross-rib micro-channels were analyzed in the condition of the same pressure drop, MC-CR still has superior heat dissipation performance.

Thermal performance of insulated gate bipolar transistor module using microchannel cooling base plate

• Microchannels were proposed for Insulated gate bipolar transistor module cooling. • The performance of two counter-flow microchannel base plate designs was studied. • Horizontal counter-flow microchannel showed a better thermal–hydraulic performance. • Longitudinal vortex generators enhanced heat transfer of the microchannel cooling. The insulated gate bipolar transistor (IGBT) module cannot meet industrial requirements under high-power density due to the high junction temperature and non-uniform temperature distribution. To overcome these problems, two novel microchannel designs (i.e. longitudinal counter-flow microchannel and horizontal counter-flow microchannel) were proposed for the base plate cooling of the IGBT module, in this study. A heat transfer model was developed to investigate the heat transfer performance, pressure drop, temperature distribution of a full-size IGBT module with the proposed microchannel cooling designs. The results showed that the horizontal counter-flow microchannel design had better heat transfer performance, lower pressure drop, more uniform temperature distribution and higher energy efficiency in comparison to the longitudinal counter-flow microchannel cooling design. To further improve the heat transfer performance, longitudinal vortex generators was applied in the horizontal counter-flow microchannel design. It was found that the Nusselt number of the horizontal counter-flow microchannel with longitudinal vortex generators reached 10.86, which increased by 61% compared with the basic horizontal counter-flow microchannel design.

A New Assessment Modus for Heat Transfer Characteristics of Supercritical Co2 in Horizontal Heated Micro-Channel

A New Assessment Modus for Heat Transfer Characteristics of Supercritical Co2 in Horizontal Heated Micro-Channel

The flow topology transition of liquid\u2013liquid Taylor flows in square microchannels

This work investigates the change of the flow topology of Taylor flow and qualitatively relates it to the excess velocity. Ensemble-averaged 3D2C-upmu PIV measurements simultaneously resolve the flow field inside and outside the droplets of a liquid–liquid Taylor flow that moves through a rectangular horizontal microchannel. While maintaining a constant Capillary number Ca = 0.005, the Reynolds number (0.52 le {text{Re}} le 2.14), the viscosity ratio (0.24 le lambda le 2.67) and surfactant concentrations of sodium dodecyl sulfate (0–3 CMC) are varied. We experimentally identified the product of the Reynolds number Re and the viscosity ratio lambda to indicate the momentum transport from the continuous phase (slugs) into the droplets (plugs). The position and size of the droplet’s main vortex core as well as the flow topology in the cross section of this vortex core changed with increased momentum transfer. Further, we found that the relative velocity of the Taylor droplet correlates negatively with the evoked topology change. A correlation is proposed to describe the effect quantitatively.Graphical abstract