Bubble Length Research Articles

A study of the acoustic effect inside cylindrical bubble produced by underwater electrical discharge

The evolution of a cylindrical gaseous bubble produced by an underwater electrical discharge is considered in the present study. Both the gas flow inside and the water flow around the bubble are theoretically analyzed in a cylindrical coordinate system. By using the potential flow theory and multiple scale expansion method, governing equations of both flows and the bubble surface are formulated. The radial oscillation of the bubble surface is composed of a slow-changing equilibrium part and a fast-changing displacement. The former corresponds to a quiescent water domain and a uniform gas column, and the latter corresponds to acoustic waves in gas and water flows. The axial gas wave can evolve into a stable standing wave if the bubble length is multiples of half a wavelength. The internal acoustic standing wave then causes a synchronous small-amplitude oscillation of the bubble surface when the frequency of the acoustic wave is close to the natural frequency of the bubble surface. An underwater discharge experiment is implemented to validate our theory. Finally, a novel method to estimate the plasma pressure is proposed based on our theory.

Investigation of pressure feedback technique to control ramp based SWBLI

Shock wave boundary layer interaction (SWBLI) and associated boundary layer separation create undesirable phenomena such as enhanced aero-thermodynamic loads, localized turbulent spots, unsteadiness, drag enhancement, pressure recovery losses in high-speed flow applications. It necessitates an investigation on the control of shock-induced boundary layer separation. Therefore, a pressure feedback technique (PFT) to control the boundary layer separation has been studied numerically on a two-dimensional flat plate and ramp configuration. PFT consists of a channel that drains fluid from the separated region and reintroduces it in the core flow under the influence of pressure difference. The effect of various freestream, wall, and geometric conditions on the separation controllability of the PFT is examined. It has been observed that PFT with injection located near the upstream influence point and suction at the ramp foot reduces the length of the separation bubble by 12.15%. Variation of injection location marginally affects the separation reduction capability of PFT, while the performance of PFT is greatly dependent on suction location. The parametric study noted that the optimum suction location, for which the separation size is minimum, shifts downstream with increasing freestream Mach number, wall temperature, and ramp angle. Again the optimum suction location shifts upstream as freestream stagnation temperature to wall temperature ratio increases. Reynolds number is seen to have no impact on the optimum suction location. Moreover, freestream stagnation temperature to wall temperature ratio, which is a governing parameter in SWBLI studies, is also a relevant factor in the mechanism of PFT. Finally, the present limited investigation revealed that the suction end of the PFT positioned between Xsu/L = 1.12 to 1.194 provides a minimum size of shock-induced separation for varying freestream and wall conditions.

Quantification of Reynolds-averaged-Navier–Stokes model-form uncertainty in transitional boundary layer and airfoil flows

It is well known that the Boussinesq turbulent-viscosity hypothesis can introduce uncertainty in predictions for complex flow features such as separation, reattachment, and laminar-turbulent transition. This study adopts a recent physics-based uncertainty quantification (UQ) approach to address such model-form uncertainty in Reynolds-averaged Naiver–Stokes (RANS) simulations. Thus far, almost all UQ studies have focused on quantifying the model-form uncertainty in turbulent flow scenarios. The focus of the study is to advance our understanding of the performance of the UQ approach on two different transitional flow scenarios: a flat plate and a SD7003 airfoil, to close this gap. For the T3A (flat-plate) flow, most of the model-form uncertainty is concentrated in the laminar-turbulent transition region. For the SD7003 airfoil flow, the eigenvalue perturbations reveal a decrease as well as an increase in the length of the separation bubble. As a consequence, the uncertainty bounds successfully encompass the reattachment point. Likewise, the region of reverse flow that appears in the separation bubble is either suppressed or bolstered by the eigenvalue perturbations. This is the first successful RANS UQ study for transitional flows.

Noninvasive platinum thin-film microheater/temperature sensor array for predicting and controlling flow boiling in microchannels

Thermal sensors and actuators are usually introduced at/in micro-channels in two ways: first, in an invasive manner through drilled holes which can lead to leakage problems and a change in channel dimensions and second, mounted in a non-invasive manner outside the channel which reduces heat transfer, accuracy, precision and response time. These problems are multiplied as soon as microchannel boiling with associated rapid thermal effects is investigated. Therefore, in this work, a platinum microheater/temperature sensor array has been developed to provide non-invasive, but nevertheless direct heating and temperature measurement at several locations along microchannels. This array comes without the mentioned disadvantages, resulting in ultra-fast response times that are limited only by the measurement speed of the sensor resistance and linear temperature coefficient of resistance. The working platinum area of each element in the array is only 238 nm thick, 0.1 mm wide and 0.5–2 mm long. The structures can be operated in heating and temperature measurement mode simultaneously. They are integrated into microreactors with the working surface as a side wall of a microchannel and in direct contact with the flowing fluid. Since the structures are deposited on transparent Pyrex glass in a cleanroom process, optical observation of two-phase flow boiling processes is possible for flow regime identification. The maximum operating temperature of the Pt microstructures is 450 °C and the linear temperature coefficient of resistance is about 2.98 × 10−3 °C−1. The relative measurement error of temperature measurements is less than 0.05% due to 2.9 µm thick highly conductive electroplated gold pads which form the electrical contact to the Pt microheaters/T-sensors. The platinum heater structures are capable of generating stationary vapor bubbles with a defined size proportional to the applied heating power. They can also predict two-phase flow boiling regimes (e.g. slug flow), vapor bubble length and frequency via the periodic signal of the measured temperature. Furthermore, they allow accurate calculation of heat transfer coefficients using the exact fluid temperatures measured with the Pt structures at the inlet and outlet of the microchannel, rather than analytically calculated values from temperature measurements outside the channel.

Investigation of bubble structure in a microchannel under microgravity conditions: Effects of discontinuous wettability with dynamic contact angle

A thorough understanding of the dynamics of bubble/liquid two-phase flows under microgravity conditions is essential for achieving efficient heat transfer between spacecraft and space stations. This paper examined the influence of the discontinuous wettability of a microchannel's wall on bubble structure movement under microgravity conditions. The volume-of-fluid methodology involving a dynamic contact angle code was employed by considering wall-adhesion equations to predict the form of the bubble/liquid interface at and near the wall. The flow field's governing equations were numerically solved utilizing the finite volume method. Initially, the results were compared to previously reported data for validation. Subsequently, the effects of discontinuous wettability with different configurations on the structure of bubbles were investigated; new insights were obtained into the size and shape of generated bubbles, as well as the slug length as a function of the wall wettability. The results showed that for a uniformly hydrophilic surface, the bubble had a concave shape under microgravity conditions, while for a uniformly superhydrophobic surface, the bubble had a convex shape. A microchannel wall surface with discontinuous wettability led to the generation of an asymmetric bubble, a variation in slug length, changes in the bubble's length and shape, and a continuous phase structure. Furthermore, variations in the wall wettability caused the flow upstream to vary and affected all four stages of bubble generation, namely lag, filling, necking, and the starting point of detachment.

Unsteady Effects of Wake on Downstream Rotor at Low Reynolds Numbers

In a compressor, the periodic wake is an inherently unsteady phenomenon that affects the downstream flow conditions and loading distribution. Thus, understanding the physical mechanisms of these unsteady effects is important for eliminating flow losses and improving compressor performance, particularly at low Reynolds numbers. To understand the influence of the upstream wake on the downstream flow field structure, this paper describes numerical simulations of a one-stage high-pressure compressor at altitudes of 10–20 km. The influence of the wake on rotor flow blockage at different Reynolds numbers is analyzed, and the unsteady interaction between the upstream wake and boundary layer or tip leakage flow is discussed. The results indicate that the wake has a beneficial effect on the efficiency of the rotor at high Reynolds numbers, but this weakens and becomes negative as the Reynolds number decreases. The wake can reduce the flow blockage in the mainflow region. Due to the wake, the length of the laminar separation bubble at high Reynolds numbers decreases and that at low Reynolds numbers increases. In addition, the unsteadiness of the wake causes separation bubbles to appear periodically at high Reynolds numbers and induces an open separation bubble at low Reynolds numbers. The Kelvin–Helmholtz instability can dominate the transition process of the boundary layer, which is also affected by the disturbance vortex induced by the wake. Regarding the tip leakage flow, the wake can reduce the flow blockage at high Reynolds numbers but increase the flow blockage at low Reynolds numbers. The interaction at low Reynolds numbers causes a double-leakage flow, which finally leads to the large-scale separation of the suction surface boundary layer. The large-scale separation causes flow blockage in the tip region and prevents the rotor wake from propagating downstream. On the contrary, the unsteady wake can pass through the tip clearance vortex and inhibit the separation of the suction boundary layer at high Reynolds numbers, which is reflected in a larger amplitude of one blade passage frequency. Therefore, the flow loss in the downstream flow field at high Reynolds numbers is significantly reduced at high Reynolds numbers.

Gradient Heatmetry and PIV Investigation of Heat Transfer and Flow near Circular Cylinders

This paper presents an experimental investigation of convective heat transfer around circular cylinders installed one after another. The experimental approach based on the combined application of gradient heatmetry and particle image velocimetry. The method made it possible to measure velocity fields and heat flux simultaneously. Investigation of the flow characteristics and the heat transfer coefficient distribution over a system of cylinders was carried out for various Reynolds numbers in the range of Re = (4…40) × 103. In addition, the distance between the cylinders varied in the experiment. The results showed the the influence of the re-circulation bubble length and the features of vortex formation on the flow pattern and characteristics for the configuration under study in the entire range of regimes. The results were compared with experimental and numerical data from previous literature and found to be in good agreement. Our innovative method based on gradient heatmetry showed high information content and a fairly high accuracy of measurement.

Steady flow of power-law fluids past a slotted circular cylinder at low Reynolds number

Steady laminar flow past a slotted circular cylinder was investigated for non-Newtonian power-law fluids at the low Reynolds number (Re) range (5 ⩽ Re ⩽ 40). Flow simulation was carried out for shear-thinning fluids with their power-law indices (n) varying from 0.2 to 1 (n = 0.2, 0.4, 0.6, 0.8, and 1). The normal (case A) and the slotted (case B) circular cylindrical geometries were considered, where the slit was placed between the front and the base pressure stagnation points. A finite volume method was used to calculate the flow field. The flow characteristics, such as flow separation angles, wake size, coefficients of pressure (Cp), and drag (CD), were studied for different Re and n values. For all n values, the slotted cylinder effectively delayed the flow separation. It showed much better pressure recovery than the normal cylinder due to the interaction between the self-bleed from the slit exit to the cylinder wake. The vorticity of this bleed influenced the wake's vorticity, and an increase of 3%–26.4% in higher maximum surface vorticity was reported for the slotted cylinder. An increase of 0.7%–6.5% in the bubble length was observed for the normal cylinder due to early flow separation. An enhanced pressure recovery across the slotted cylinder resulted in a significant drop in the pressure drag with 0.2%–4.56% reduction in the overall drag coefficient.

Control of supersonic compression corner flow using a plasma actuator

The control performance of a pulsed nanosecond dielectric barrier discharge (NSDBD) plasma actuator with varying pulse voltages and locations on a supersonic compression corner is studied using experiments and numerical simulations. The compression corner with a flat plate length of 60 mm and a ramp angle of 10° under laminar flow separation is experimentally investigated in a Ludwieg wind tunnel under a unit Reynolds number of 7.8 × 106 m−1 and Mach number of 4. The plasma actuators are placed either upstream or downstream of the separation point, extending in the spanwise direction. The Schlieren technique is used to visualize the shock wave interaction and estimate the propagation speed of the induced shock by the plasma actuator. For the numerical simulations, a one-zone inhomogeneous phenomenological plasma model is adopted to predict key discharge parameters and simulate the fast-heating region. The results show that the reduction of separation bubble length is up to 17% and 45%, respectively, in the cases of upstream and downstream of the separation point under a high applied voltage of 50 kV. The evolution of the flow structures is examined to reveal the underlying control mechanism. The results indicate that the high-speed external fluid is entrained into the original separation region after NSDBD activation upstream of the separation point, resulting in flow reattachment upstream of the corner. The entrained fluid with high momentum compels the main separation to move downstream, accompanied by the fragmentation of the original shear layer. Furthermore, the suppression of the separation region is more effective when the plasma actuator is installed close to the separation region and in the first 200 μs during one pulse, providing a good suggestion for the actuation frequency and installed location.

Structure and dynamics of a laminar separation bubble near a wing root: towards reconstructing the complete LSB topology on a finite wing

The influence of the wing root junction on the laminar separation bubble forming on the suction surface of a semispan NACA 0018 wing cantilevered from the wind tunnel test section wall is studied using surface flow visualisations, particle image velocimetry and surface pressure measurements at a chord Reynolds number of 125 000 and an angle of attack of 6 $^\circ$ . The test section wall boundary layer upstream of the wing is turbulent, and the spanwise influence of the junction on the separation bubble extends well beyond the test section wall boundary layer thickness. Substantial three-dimensionality is seen in the separation bubble flowfield near the wing root, where earlier transition and a reduction in separation bubble thickness is observed. In contrast with the wing tip, earlier transition and a reduction in separation bubble length occurs near the wing root. Outside of the junction affected region, the separation bubble is similar to separation bubbles forming on two-dimensional geometries, and displays mild spanwise waviness. The transition process away from the end affected regions is characterised by the formation of spanwise roll-up vortices that are shed in a nearly two-dimensional manner across the span. The analysis of the results shows that, near the wing root, the increased level of perturbations leads to earlier vortex roll-up and spanwise flow contributes to more rapid vortex breakdown. The results in the wing root region are complemented by the analysis of data from Toppings and Yarusevych (J. Fluid Mech., vol. 929, 2021, A39) in the wing tip region to provide a more holistic outlook on the laminar separation bubble topology and dynamics on the entire finite wing.

Fluid–Solid Interaction Modeling of Elastohydrodynamic Lubrication Point Contacts

Abstract Elastohydrodynamically lubricated point contact was investigated using a two-way partitioned fluid–solid interaction (FSI) model. ansys Mechanical finite element modeling software was used to compute elastic (and plastic) deformation of the solid bodies, while ansys fluent computational fluid dynamics software was used to model the fluid with the Navier–Stokes equations. The current model is not limited by Reynolds equation assumptions, allowing for the investigation of pressure, viscosity, and temperature variation across point contact elastohydrodynamic lubrication (EHL) films. Solid body material stress distribution and fluid behavior such as cavitation were also investigated. The details of model development are described. Validation of the model is presented across a range of loads and speeds for cases when the Reynolds equation is applicable. The results are in excellent agreement. Various slide-to-roll ratios were investigated considering a non-Newtonian fluid with thermal effects to characterize lubricant properties within the EHL film. Results demonstrate notable lubricant viscosity and temperature variations within the EHL film thickness both along and perpendicular to the rolling direction for cases with high slide-to-roll ratio. Cavitation was also considered, and cavitation bubble lengths were found to agree well with results found in open literature. Finally, the effects of material plasticity on solid body response were investigated. The FSI model developed in this research provides new insights on a classical EHL problem.

Flow over a confined mounted fence at low and moderate Reynolds number: A numerical study

Direct numerical simulations of a flow over a wall mounted fence confined by two parallel walls were conducted at low and transitional Reynolds number (50⩽ReD⩽2000) for blockage ratios ranging from 0.25 (low blockage) to 0.9 (high blockage). A primary recirculation bubble forms downstream of the fence and the length of this recirculation region increased with Reynolds numbers. A secondary recirculation bubble appeared on the opposite wall to the mounted fence for the laminar cases with higher blockage ratios at sufficiently large Reynolds number. The separation and reattachment locations were documented and compared with numerical and experimental data. The length of the primary recirculation bubble in current simulations are consistent with the literature while the comparison of the secondary recirculation bubble with experimental data had limited agreement. This secondary recirculation bubble disappeared for the cases at transitional Reynolds number. Instead, the strong primary recirculation bubble induced a corner eddy at the rear of the fence. ‘Packets’ of cross stream velocity fluctuations were observed as the initially 2D, steady, laminar shear layer breakdowns to a 3D turbulent flow. The space–time diagram also showed ripple-like fluctuations of the shear layer further downstream of the fence with a single distinct peak observed in the streamwise and cross flow velocity fluctuations spectra. This peak in the spectra can be predicted by linear stability analysis of a mixing layer (Yasuo et al., 1986). In the near wake, scale-like features were observed while patches of fluctuations of a longer timescale were noted in the recirculation region.

Non-isothermal flow past a heated circular cylinder in subcritical regime: a numerical investigation based on large-eddy simulation

Compressible flow past a heated circular cylinder at subcritical Reynolds number of 3900 is numerically investigated by using the large-eddy simulation method. Rigorous validations of the numerical model are carefully performed under isothermal conditions on the basis of available experimental data. The calculated mean flow and Reynolds stresses show good agreement with the published experimental data. The effects of temperature difference between the cylinder surface and the freestream on the flow statistics and thermal characteristics are further studied in detail by setting two kinds of wall temperature boundary conditions. It is manifested that increasing the wall temperature leads to the augmentation of skin friction drag, suppression of turbulent intensity, enhancement of flow mixing and extension of recirculation zone. In addition, it is found that the variations of thermo-physical properties pose a slight effect on the wall heat flux before the boundary layer separates from the cylinder surface. It is worth noting that the recirculation bubble length can be used as a distance scaling parameter to weaken the temperature dependence of the flow and thermal statistics. These results provide a more detailed insight into the statistical difference in the wake region of cylinder when the temperature effect is taken into account.

Taylor bubble formation and flowing in a straight millimetric channel with a cross-junction inlet geometry. Part I: Bubble dynamics

• Taylor bubble formation dynamics were studied at a cross-junction. • Filling and squeezing stage frequencies were not controlled by the same parameters. • Gas finger lengths were measured to identify different bubble pinch-off patterns. • A simple relationship linking relative bubble lengths with j G 0 / j L was established. The investigations of bubble formation dynamics at a cross-junction in a straight milli-channel were reported. The bubble formation process could be divided into the filling and squeezing stages, and their frequencies were compared at various conditions. It was found that the filling and squeezing frequencies were controlled mainly by the gas and liquid superficial velocities, respectively. The bubble formation frequencies could be related to the gas-liquid superficial velocity ratios, the liquid superficial velocities, and the bubble length. The bubble formation process was then analyzed considering the length of the gas finger right after the bubble pinch-off. Two patterns were identified depending on whether the pinch-off occurred inside of the cross-junction or not. The transition between these two patterns was described by a critical liquid Capillary number. Furthermore, the squeezing and dripping patterns were distinguished as the gas finger could fully block the channel or not. The transitions between the bubble formation patterns were determined by the gas Weber number and liquid Capillary number. Finally, the bubble length, the liquid slug length, and the bubble length normalized by the unit cell length could all be predicted based on the gas-liquid superficial velocity ratios η 0 .

Gas-liquid hydrodynamics with different liquid viscosities in a split-and-recombine microchannel

Split-and-recombine (SAR) technology has shown great potential in the passive intensification of mass transfer. However, the gas-liquid flow is rarely encountered in the SAR microdevices. In this work, the flow regime, void fraction, and pressure drop of gas-liquid flow were experimentally studied in a common SAR microchannel designed with a curved divergence-convergence channel and rectangular obstacles. Parameter studies were conducted by varying the flow rates of nitrogen/glycerol aqueous solutions and glycerol contents. Four flow regimes, namely, total breakup, partial breakup, abnormal breakup, and annular flow are observed with a high-speed camera. The flow map and correlations to predict the regime transitions are presented based on the liquid-phase Capillary number (0.0022-1.016) and gas/liquid rate (0.5-5). Furthermore, the modified frictional factor is correlated with Capillary and Reynolds numbers based on the flow regimes. Overall, the SAR microchannel is a promising candidate as the high-efficient gas-liquid contactor in terms of mass transfer rate, product selectivity, and energy efficiency for highly viscous liquids. A reasonable match of bubble length and the number of SAR units for the total breakup of bubbles is required. This study can contribute to the design, optimization, and regulation of the SAR microchannel for gas-liquid flows.

A hybrid smoothed particle hydrodynamics coupled to a fictitious domain method for particulate flows and its application in a three-dimensional printing process

A hybrid smoothed particle hydrodynamics (SPH) coupled to a direct-forcing fictitious domain (FD) scheme is proposed for the simulation of particulate flows. The new method is effective in calculating the particle-fluid interaction forces and uses a kernel interpolation function to realize the momentum exchange between fluids and particles. A higher-order interpolation scheme is applied to preserve the consistency and accuracy of the kernel function. We first test our method by simulating the benchmark of flow past a cylinder at Reynolds number ranging from 20 to 200 and obtain the convergent results, including accurate lift and drag coefficients, Strouhal number, length of the recirculation bubble, and separation angle. Subsequently, we verify our new method for the typical particulate flows, including the motion of a neutrally-buoyant cylinder in Couette flow, the motion of a neutrally-buoyant cylinder in a planar Poiseuille flow, the sedimentation of a circular cylinder in a channel, and the motion of a neutrally-buoyant slender particle in Couette flow. A good agreement with the existing experimental data, with other numerical results, and with available theoretical solutions is obtained indicating the accuracy and the robustness of the new method. Subsequently, our SPH-FD is applied to simulate a slender fiber in a three-dimensional (3D) printing process, showing a good potential and advantages in simulating particulate flows of some industrial processes.

Steady flow past elliptic cylinders with blockage effects

In the present work, steady two-dimensional flow past elliptic cylinders with different aspect ratios (Ar) have been investigated for Reynolds number (Re) up to 200. Main characteristics such as the bubble length, bubble width, separation Reynolds number, separation angle, drag coefficient, coefficients of the front and rear stagnation pressure, and the maximum vorticity on the cylinder surface have been obtained. Least squares fit equations for some of these quantities have also been presented. In the special case of a circular cylinder, results computed with the largest domain show that the bubble length, bubble width, maximum vorticity on the cylinder surface, coefficient of pressure drag, coefficient of total drag, and flow separation angle are linear functions of Re, Re0.5, Re0.35, Re−0.4, Re−0.5, and Re−0.1, respectively. As Ar changes, the trends in the growth, with respect to Ren (with the corresponding index), for the bubble length, bubble width, and the maximum surface vorticity deviate from their linear nature. The effect of blockage on the steady flow characteristics has been studied by changing the location of the side boundaries. In certain cases, the flow properties are found to vary in a non-monotonic fashion with the change in the blockage. An explanation based on the dual role of the side boundaries in promoting and suppressing the growth of the corresponding flow property is offered.

Stability and uniformity of gas–liquid two-phase flow in shear-thinning fluids in parallelized microchannels

The stability of gas–liquid two-phase flow and uniformity of bubble size in shear-thinning fluids within symmetrical parallelized microchannels are investigated experimentally. Air and sodium carboxymethyl cellulose (CMC) aqueous solutions with 0.3% SDS are used as the dispersed and continuous phases, respectively. The process of gas pressure buildup and rapid release is observed under conditions of exceeding the critical gas and liquid flow rates. Various slug bubble shapes are identified in different concentrations of CMC solutions, gas and liquid flow rates. The results show that the increase of liquid flow rates and the decrease of CMC concentration are beneficial for the stability of bubble formation and uniformity of bubble size in the parallelized microchannels. Finally, semi-empirical formulas are proposed, based on the rheological property of CMC aqueous solutions, to estimate the bubble length.

Low Reynolds Number Effects on the Separation and Wake of a Compressor Blade

Abstract This paper investigates the surface boundary layer and wake development of a compressor blade at a range of low Reynolds number from 45,000 to 120,000. Experiments in a miniature linear compressor cascade facility have been performed with detailed surface pressure measurements and flow visualization to track variations in the separation bubble size. These have been combined with high-resolution pneumatic pressure and hot-wire probe traverses in the downstream wake. High-fidelity direct numerical simulations have been completed on the same compressor blade section across the same range of operating conditions. The results show that large laminar separation bubbles exist on both blade surfaces. As Reynolds number increases, these separation bubbles shorten in length and reduce in thickness. Correspondingly, the downstream wake narrows, although the peak wake loss coefficient remains approximately constant. As the Reynolds number is increased from 45,000 to 120,000, the bubble length on the suction side reduced from 48% to 28% chord and on the pressure side reduced from 35% to 20% chord, while the loss coefficient reduced from 9% to 5%. The flow features are examined further within the high-fidelity computations, which reveal the dependence of the wake turbulence on the laminar separation bubbles. The separation bubbles are found to generate turbulent kinetic energy, which convects downstream to form the outer part of wake. As Re increases, a shorter bubble produces less turbulence in the outer part of the boundary layer leading to a narrower wake. However, the trailing edge separation is largely independent of Reynolds number, leading to the constant peak loss coefficient observed. The overall loss is shown to vary linearly with the total turbulence production, and this depends on the size of the separation bubbles. Overall, this research provides new insight into the connection between the blade surface flow field and the wake characteristics at low Reynolds number. The findings suggest that changes that minimize the extent of the blade separation bubbles could provide significant improvements to both the steady and unsteady properties of the wake.

Effect of solvent on CO2 absorption performance in the microchannel

The absorption performances of CO2 into 2–amino–2–methyl–1–propanol (AMP) solutions mixed with solvents water, ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG) were investigated and compared in a T–junction microchannel. The absorption rate under different liquid flow rates (10–40 ml·h−1), gas/liquid flow rate ratios (0.5–10) and AMP concentrations (0.2–0.8 mol·L–1) was determined by the online photographic method. The dynamics (bubble and slug lengths, film thickness, recirculation frequency in slug, film–slug exchange and pressure drop) in the slug–flow region were analyzed and discussed. The results show that solvents affect the absorption rate with the order of AMP–water < AMP–EG < AMP–DEG < AMP–TEG, which is opposite to their solubility. The increased pressure drop and shorter bubble surrounded with thicker film are the dominant factors affecting the absorption rate, which increase the concentration driving of CO2 and film–slug mass exchange at the higher solvent viscosity respectively. Under current conditions, water is a good solvent candidate by joint consideration of absorption rate and energy consumption. This study provides guidance for solvent selection and manipulation of CO2 absorption in the microreactor.