Microchannel Research Articles

In Vivo Femtosecond Laser Machined Transepithelial Nonlinear Optical Corneal Crosslinking Compared to Ultraviolet Corneal Crosslinking.

This study assessed the safety and efficacy of transepithelial crosslinking (CXL) using femtosecond (FS) laser-machined epithelial microchannels (MCs) followed by UVA CXL compared to FS laser (NLO CXL) in rabbits. The epithelium of 36 rabbits was machined to create 2- by 25-µm MCs at 400 MCs/mm2. Eyes were treated with 1% riboflavin (Rf) solution for 30 minutes, rinsed, and then crosslinked using UVA or NLO CXL. Rabbits were monitored by epithelial staining, optical coherence tomography (OCT) imaging, and esthesiometry. After sacrifice at 2, 4, or 8 weeks, corneas were examined for collagen autofluorescence and immunohistochemistry. NLO CXL showed no epithelial damage compared to UVA CXL, which produced on average 23.89 ± 5.6 mm2 epithelial defects that healed by day 3. UVA CXL also produced loss of corneal sensitivity averaging 0.83 ± 0.24 cm force to elicit a blink response that persisted for 28 days and remained significantly lower than control or NLO CXL. OCT imaging detected the presence of a demarcation line only following UVA CXL but not NLO CXL. Even with improved transepithelial Rf penetration, UVA CXL resulted in severe epithelial damage, loss of corneal sensitivity, and delayed wound healing persisting for a month. When MCs were paired with NLO CXL, however, these issues were mostly negated. This suggests that MC NLO CXL can achieve a faster visual recovery without postoperative pain or risk of infection. UVA CXL is a successful procedure, but there is a need for a transepithelial protocol. The combination of MCs and NLO CXL is able to keep the benefits of UVA CXL without causing epithelial damage.

Experimental study on flow optimization and thermal performance enhancement of an ultrathin silicon-based loop heat pipe

Excessive heat flux of integrated circuits (ICs) leads to the failure of electronic devices and the consumption of extra energy. Silicon-based loop heat pipe (sLHP) offers a simple, reliable and easily-integrate method for IC thermal management. In this study, the influence of two common wicks (micropillar arrays (MP) wick and microchannel (MC) wick) on fluid flow and performance of sLHP are systematically compared and studied through visual experiment. And two advanced wicks (in-line long rib (IR) wick and staggered long rib (SR) wick) are proposed to further optimize the sLHP. The results show that the fluctuant vapor-liquid interface in MP wick is beneficial to the supplement of working fluid from adjacent flow channel, but it also introduces the uncertainty of flow. The MC wick features the preset flow channel and stable fluid flow but limited heat transfer capacity. Notably, the IR wick and SR wick effectively enhance that capability of supplementing fluid by combining the advantages of the MP wick and MC wick showing better thermal performance. The effective thermal conductivity of SR-sLHP and IR-sLHP is better than that of MP-sLHP and MC-sLHP, with the maximum values of 860 W/(m·K) and 848 W/(m·K), respectively.

Multi-scale lumped modeling of micro-channels cooling structure for W7-X divertor unit target module

A novel concept for the cooling system of the W7-X divertor unit target module has been proposed, which involves tiles equipped with parallel arrays connected by hundreds of sub-millimeter rectangular micro-channels (MCs), obtained using Additive Manufacturing techniques. The structural material proposed for the heat sink substrate is galvanized copper, while the plasma facing material is tungsten. To reduce the high computational cost of thermal-hydraulic simulations of the tiles, a multi-scale lumped modeling approach has been built in previous studies. This involves replacing a group of hydraulic parallel MCs with a porous strip (PS), suitably calibrated to reproduce similar thermal-hydraulic behavior of the MCs. The aim of the current work is to verify that the PS-model is also suitable for the thermo-mechanical stress evaluation, comparing the results from arrays with MCs and PS. Since the heat sink and plasma-facing tiles are bonded, the interfacial delamination and shear stresses are analyzed, being crucial for structural integrity. The analyses, carried out in the elastic regime, show that the PS model returns conservative results when compared to the MCs model, with an overestimation of the delamination and shear stresses at the free edge. Results from both models reveal nearly identical interfacial stress predictions at the free surface edge. Moreover, it is found that both MCs and PS blocks, located near the bond interface, contribute to high-stress fluctuations that could lead to the delamination of the bond interface, suggesting that the distance of the microchannel from the interface should be increased. The PS model can reliably be used to design and verify the most convenient series/parallel connection of the tiles in the divertor unit target module, taking operational constraints into consideration.

Improved Thermal Management of Lithium‐Ion Battery Module through Microtexturing of Serpentine Cooling Channels

Elevated battery temperature and its heterogeneity are responsible for many battery‐related problems such as short lifespan and thermal runaway. For longer life and safe functionality of lithium‐ion battery (LIB), both the average battery temperature and temperature heterogeneity must be controlled. The present work analyzes a modified cooling channel design that can control both the average battery temperature and its heterogeneity. The design adopts microtexturing approach for the heat transfer augmentation of the serpentine cooling channels. First, the transient thermal analysis of different geometry of microtextures, viz., microchannels (MCs) and microdimples (MDs), is carried out on an aluminum slab. The rectangular MCs and square MDs are found to be the most efficient among the other considered microtexture geometries. Further, the modified design is employed for a 19‐cell battery pack to analyze the thermal performance, pressure drop, and coolant pump power requirements. A decrease in maximum temperature difference (MTD) by 6.03 % and 3.72% is observed for 380 MCs (rectangular) and 960 MDs (squared), respectively. While MDs improve the temperature homogeneity within the LIBs, the coolant pump power requirement and pressure drop (by ≈15.42%) are higher for it compared to the MC‐based design for a given MTD.

Accelerating melting time in a triplex tube heat exchanger thermal energy storage unit with micro-channels

This article studies optimal conditions to maximize heat transfer efficiency within a triplex tube heat exchanger (TTHX) filled with phase-change material. In this context, micro-channels (MCs) incorporated into TTHX for the first time for acceleration melting time. A two-dimensional numerical model has been developed, and pure conduction and natural convection are both simulated. The Reynolds numbers range from 1 to 100. The types of temporal velocity profiles into micro-channels were uniform, step, elliptic, sinusoidal, and power. The effect of gravity states, namely no gravity, with gravity, and (MCHE), has been studied with different numbers of microchannels (4, 6, 8, 10, 12, and 14). The results showed that there was no variation in melting duration observed when increasing the Re number from 50 to 100. Thus, the maximum reduction in melting period occurred at Re = 50. Moreover, sinusoidal velocity profiles were more effective and could help the process finish sooner. In addition, micro channel tube exchanger (MCTE) fitted or equipped with 10 micro-channels has an optimum condition considering pressure drop and complete melting time. Using the sinusoidal profile reduced the total fusion time by 325 s compared to uniform flow (8.47 % reduction). This indicates that the sinusoidal profile is an effective way to reduce the melting period.

Numerical study on flow and heat transfer in a novel symmetric sinusoidal wavy microchannel heat sink with rectangular rib prisms

To enhance thermal and overall performances, a novel microchannel heat sink (SSW-RP) was devised by integrating symmetric sinusoidal wavy sidewalls with rectangular rib prisms within the channels. A numerical comparative study was conducted to explore heat transfer mechanism in channels by investigating the flow and heat transfer characteristics of SSW-RP in comparison with those of smooth straight microchannel (MC), smooth straight microchannel with rectangular rib prisms (MC-RP), and symmetric sinusoidal wavy microchannel (SSW). The results show that the addition of rib prisms makes the fluid flow into wave peaks, reducing and even eliminating vortices from wave peaks of the symmetric sinusoidal wavy microchannel, thus significantly improving heat transfer performance. The new design SSW-RP had the highest relative Nusselt number, which reaches 4.16 at Re = 800. Further, the hydrothermal performance sensitivity of SSW-RP to the relative width (α) and length (β) of rib prisms is studied, and the best overall performance factor of SSW-RP finally can be achieved up to 1.88 at α = 0.35, β = 0.4 and Re = 800.

Optimal Design of Microchannel Heat Sink with Staggered Fan-Shaped Cavities and Ribs

In the present study, a three-dimensional numerical simulation is conducted to study the characteristics of fluid flow and heat transfer in a newly designed microchannel heat sink with fan-shaped cavities and ribs (MC-FCFR). The Reynolds number ranges from 147 to 736. A straight rectangular microchannel (MC-RC) is used as the baseline. The geometric parameters include the relative spacing θ, relative width α, and relative height β of the fan-shaped cavities and ribs. The overall performance index (OPI) of the MC-FCFR is evaluated in terms of the friction factor and Nusselt number. The OPI for the MC-FCFR with θ=3.56, α=1.63, and β=0.5 reaches 1.70.

Hydrothermal performance through multiple shapes of microchannels (MCHS) using nanofluids: an exhaustive review

Hydrothermal performance through multiple shapes of microchannels (MCHS) using nanofluids: an exhaustive review

Impact of the diameter of airflow passing holes between curved microchannels on the cooling process of a vertical LED using nanofluid: DPM two-phase simulations

In this paper, a circular heatsink (HSK) with a new geometry cooled by nanofluids (NFs) flow and free airflow is numerically examined to reduce the temperature of an LED. The HSK has a number of curved microchannels (MCH) in which alumina/water NFs flows. A number of holes with different diameters are installed on the heatsink. NFs flow is simulated using the two-phase discrete phase model (DPM) and equations are solved utilizing the finite element method (FEM). The results demonstrate that an enhancement in the curvature of the MCH can reduce the average HSK temperature (THS) by 2.34 degrees, and an increment in the radius of the MCH intensifies the maximum THS (T-max) by about 3 degrees. The use of models M1 and M3 leads to the highest and lowest T-max, respectively. The use of a MCH with greater curvature can improve the thermal coefficient by 6.4%. The T-max uniformity in the HSK corresponds to model M3 with a MCH radius (MCR) of 12 cm, and the minimum temperature (T-min) uniformity is related to model M1 with a MCR of 4 cm. Increasing the radius of MCHs from 4 to 12 cm reduces the pressure drop (Δp) by 44.8%.

Heat-Flux Effect on Fluid Flow and Convective Heat Transfer through a Rotating Curved Micro-Channel

The present paper investigates heat-flux effect and the dissemination of energy in a rotating bent square micro-channel (MC) subject to a temperature gradient between the vertical sidewalls. The flow structure prevailing the problem is solved by applying a highly accurate spectral-based numerical scheme. The flow controlling parameters are the Dean number (0<Dn≤5000) and the Taylor number (-500 ≤ Tr ≤ 2000) for curvature 0.01 and the Grashof number, Gr=1000. After applying the arc-length path continuation technique to obtain steady solution (SS) curves, two branches of SS consisting of 2- to 8-vortex solutions are prevailed for the non-rotating case while a single branch with a symmetric 2-vortex solution is for positive rotation of the channel. Unsteady flow (UF) properties are simulated by the time-average of the solutions, and the transitional behavior is well predicted by contemplating the power spectrum and phase spaces of the solutions. Results manifest that the UF experiences a consequence ‘steady-state àmulti-periodic àsteady-state’ for no rotation of the channel as Dn is increased. For the rotating case, on the other hand, the flow advances in the scenario ‘steady-state àmulti-periodic àsteady-state’ for negative rotation and only a steady-state solution for rotation in the positive direction. Streamlines and isotherms of SS and UF for various values of the flow-controlling parameters are obtained. Centrifugal force impacts the fluid mixer, which then assists to turn the flow into chaos and prompts to intensify the convective heat transfer (CHT).
 J. of Sci. and Tech. Res. 4(1): 129-144, 2022

Computational study of hydrothermal characteristics in straight microchannels with base triangular and wavy profile designs for electronic cooling

In the current study, computational analysis of the hydrothermal behavior of fluid flow in straight microchannels (MCs) with base triangular and wavy profile designs was conducted and the results were compared with that of straight MCs. The computational study was conducted in the 100–300 Reynolds number (Re) range. According to computational analysis, the thickness of the boundary layer grows as the fluid moves through straight MC, resulting in lower heat dissipation. Continual thinning and thickening of the boundary layer is seen to form in straight MC with triangular base and wavy profile designs. The fluid passing through the MC material can transfer heat more effectively as a result. Additionally, in straight MC with base wavy profiles designs, it was observed that there were more Dean vortices, reduction in stagnation zones, and lowering of channel base temperature. From the current study, it was noticed that straight MC with base wavy profiles designs showed higher heat transfer (0.5% to 21.14%), lower pressure drop (18.65% to 14.25%) compared to straight MC at Re range from 100 to 300, and higher overall performance factor compared to other designs.

Effect of microchannel wall dimensions and temperature on ethylene glycol fluid's thermal performance in two-dimensional microchannels using molecular dynamics simulation

Increasing the transfer(HT) coefficient used in thermal industries is very important. Various methods are used to improve the efficiency of thermal heat HT so that maximum HT takes place in a smaller space. Ethylene glycol (EG) is generally used as an agent for convective HT. EG obtains energy from a hot source and discharges it to the required location. At present, the most consumption of EG is to produce engine cooling fluid. In the upcoming research, the TB of EG fluid in two-dimensional microchannels(MCs) has been investigated using molecular dynamics (MD) simulations, and the effect of variables such as MC dimensions and MC wall temperature(Temp) on the TB of the simulated fluid has been investigated. The results revealed that by increasing the Temp difference of the MC wall from 10 to 50 K, the maximum temperature (Max-Temp) and velocity (Max-Vel) of the target sample increased to 640.94 K and 0.024 Å/ps. It can be concluded that the increase in the cross-sectional area and the wall Temp difference leads to an increase in the HT rate in the MC.

Pericoronary Adipose Tissue Radiomics from Coronary Computed Tomography Angiography Identifies Vulnerable Plaques

Pericoronary adipose tissue (PCAT) features on Computed Tomography (CT) have been shown to reflect local inflammation and increased cardiovascular risk. Our goal was to determine whether PCAT radiomics extracted from coronary CT angiography (CCTA) images are associated with intravascular optical coherence tomography (IVOCT)-identified vulnerable-plaque characteristics (e.g., microchannels (MC) and thin-cap fibroatheroma (TCFA)). The CCTA and IVOCT images of 30 lesions from 25 patients were registered. The vessels with vulnerable plaques were identified from the registered IVOCT images. The PCAT-radiomics features were extracted from the CCTA images for the lesion region of interest (PCAT-LOI) and the entire vessel (PCAT-Vessel). We extracted 1356 radiomic features, including intensity (first-order), shape, and texture features. The features were reduced using standard approaches (e.g., high feature correlation). Using stratified three-fold cross-validation with 1000 repeats, we determined the ability of PCAT-radiomics features from CCTA to predict IVOCT vulnerable-plaque characteristics. In the identification of TCFA lesions, the PCAT-LOI and PCAT-Vessel radiomics models performed comparably (Area Under the Curve (AUC) ± standard deviation 0.78 ± 0.13, 0.77 ± 0.14). For the identification of MC lesions, the PCAT-Vessel radiomics model (0.89 ± 0.09) was moderately better associated than the PCAT-LOI model (0.83 ± 0.12). In addition, both the PCAT-LOI and the PCAT-Vessel radiomics model identified coronary vessels thought to be highly vulnerable to a similar standard (i.e., both TCFA and MC; 0.88 ± 0.10, 0.91 ± 0.09). The most favorable radiomic features tended to be those describing the texture and size of the PCAT. The application of PCAT radiomics can identify coronary vessels with TCFA or MC, consistent with IVOCT. Furthermore, the use of CCTA radiomics may improve risk stratification by noninvasively detecting vulnerable-plaque characteristics that are only visible with IVOCT.

A multi-scale hybrid approach to the modelling and design of a novel micro-channel cooling structure for the W7X divertor

The second operating phase of the W7X stellarator, with an expanded set of plasma-facing components, includes the test of divertor tiles with a continuous heat load reaching 10 MW/m2. The divertor tiles are cooled by subcooled water. Here a novel cooling concept, based on a network of parallel arrays of micro-channels (MC) with sub-millimetre dimensions, is investigated on a 0.1 m × 0.1 m tile, realizable by Additive Manufacturing. Detailed CFD simulations of the mock-up are performed to check the cooling uniformity using a multi-scale approach, aiming at limiting the dimension of the computational grid without a major loss of accuracy. First, the detailed hydraulic and thermal characterization on a sub-domain with of a small group of MC is performed. Then, the block of MC is substituted with an equivalent porous strip (PS), calibrating the hydraulic and thermal characteristics of the porous medium. The model is verified on an array of MCs or PSs connected to the same manifolds, showing the capability to reproduce the pressure drop and temperature increase with maximum errors of 1.05% and ∽20% in nominal conditions, respectively. The numerical model of the entire tile equipped with PSs is then reliably adopted to evaluate the thermal-hydraulic performance of the cooling device.

Preparation of monodisperse water-in-oil emulsions using microchannel homogenization

The droplet size and uniformity of water-in-oil (W/O) emulsions are important properties governing their stability in diverse applications. Monodisperse emulsions are preferred over polydisperse emulsions because their droplet size and distribution are easier to control. Here, we prepared (relatively) monodisperse W/O emulsions using microchannel homogenization (MCH) with an asymmetric straight-through microchannel (MC) array chip. Numerous asymmetric straight-through MCs, each comprising a microslot and circular microhole (diameter: 10 µm), were microfabricated on a silicon-on-insulator chip. We investigated the effects of emulsifier concentration, continuous-phase viscosity, dispersed-phase volume fraction, and the operating pressure of coarse W/O emulsions on the preparation of W/O emulsions, droplet size, and size distribution. The continuous phase was composed of soybean or medium-chain triglyceride oil solutions with tetraglycerin monolaurate condensed ricinoleic acid ester as the emulsifier. The dispersed phase was a Milli-Q water solution containing NaCl and/or polyethylene glycol (m.w. 20,000). The results demonstrate the suitability of MCH for the preparation of monodisperse W/O emulsions with an average droplet diameter of 12–15 µm and 8% lowest coefficient of variation using hydrophobically surface-treated asymmetric straight-through MCs with appropriately selected emulsion compositions and operating conditions.

Investigation of atomic behavior and pool boiling heat transfer of water/Fe nanofluid under different external heat fluxes and forces: A molecular dynamics approach

The boiling process is an efficient and effective heat transfer mode. Generally, different parameters such as temperature, pressure, external forces, etc., amend the pool boiling heat transfer (PBHT) rate. The present article uses molecular dynamics (MD) simulation to study the efficacy of different external forces(EFs) and heat fluxes (HFs) on the atomic and PBHT of water/Fe nanofluid (NF) flow. This study is performed in a microchannel (MC) with Fe-walls. The atomic behavior of the simulated structure is examined using the change in maximum temperature (T), maximum velocity(V), and maximum density(D), and the PBHT is studied by the phase change time (PCT) and HF. Results show that the maximum values of T, V, and D increase with increasing the EF and HF. Numerically, with increasing EF from 0.001 to 0.005 eV/Å, the maximum D, maximum V, and maximum T increase from 0.033 atom/Å 3 , 0.038 Å/fs, and 789 K to 0.034 atom/Å 3 , 0.039 Å/fs, and 900 K, respectively. Also, the result appears that the HF increases by increasing the applied EF, and the PCT reduces from 0.33 to 0.32 ns. So, the PBHT in the NF improves with increasing EF. On the other hand, the increase in external HF led to a reduction in the PCT (from 0.33 to 0.21 ns).

Using molecular dynamics simulation to investigate the number of wall layers and pyramidal surface roughness on atomic behavior and boiling characteristics of water/Fe nanofluid flow

In today’s heating and cooling industry, all investigators assume environmentally friendly and economic procedures because of environmental and energy crisis issues. Boiling heat transfer (BHT) is one of the impressive heat transfer (HT) procedures known in various engineering applications. The thermal conductivity of nanoparticles (NPs) is several times that of the base fluid. One way to amend a fluid’s thermophysical attributes is to utilize nanofluids (NFs) as boiling fluid. In this research, Fe NPs have been used in the water-based fluid. Fe NPs to the base fluid for reasons like enhancement of the HT surface, enhancement of the heat capacity of the fluid, enhancement of the effective thermal conductivity, and monotony of the temperature gradient in the fluid cause a substantial enhancement in HT coefficient. So far, the effect of phase change time and the effects of pyramidal surface height on the Fe/water NF have been rarely done numerically using molecular dynamics simulation (MDS). The main purpose of the current modeling is to improve the atomic behavior and pool boiling heat transfer (PBHT) of water/Fe NF. This paper investigates the effect of atomic layers and atomic pyramidal surface roughness (SR) with different heights on atomic behavior and PBHT by the MDS. The outcomes display that increasing the number of layers increases the maximum density value from 0.029 to 0.033 atom/Å3 and reduces the maximum temperature from 789 K to 653 K. The increase in the number of atomic layers in the microchannel (MC) wall corresponds to the decrease in heat flux (HF) in the MC. With addition of pyramidal SR, the HF in the MC increases.

The microchannel type effects on water-Fe3O4 nanofluid atomic behavior: Molecular dynamics approach

BackgroundToday, thermal management in electronic and industrial equipment at nano and micro scales is an important issue and has a significant impact on the price and reliability of the system. For this reason, the use of microchannels (MCs) was considered by researchers. MC type is an important parameter for heat and mass transition applications. MC wall type change can optimize their performance for various industrial aims. MethodsMolecular Dynamics simulation (MDS) is utilized in the current computational examination to describe water/Fe3O4 nanofluid (NF) behavior inside MCs with Cu and Pt wall types. Our simulations consist of two main steps as equilibrium and atomic evolution steps. In the first step, the temperature and total energy (TE) of the MC-NF system are computed for the equilibrium phase detection process. Furthermore, density/velocity/temperature profile, potential energy (PE) and aggregation time are reported to atomic behavior description of simulated structures. MD outputs indicate that by changing the MC type from Pt to Cu, the atomic stability of the total system increases. Significant findingsUsing Cu and Pt atoms for the MC walls, the PEs of simulated structures were equal to −611,228 eV and −742,726 eV, respectively. Also, the nanoparticles (NPs) aggregation phenomenon was affected by MC wall type changes, and this phenomenon was occurred in 1.40 ns/1.45 ns time using Pt/Cu MC.

Enhancing the Formation and Stability of Oil-In-Water Emulsions Prepared by Microchannels Using Mixed Protein Emulsifiers.

Although natural emulsifiers often have many drawbacks when used alone, their emulsifying ability and stability can usually be improved unexpectedly when used in combination. In this study, monodisperse emulsions stabilized by combining two natural protein emulsifiers, i.e., whey protein isolate (WPI) and sodium caseinate (SC), in different proportions were prepared using microchannel (MC) emulsification. The influences of temperature, pH, ionic strength, and storage time on the microstructure and stability of the emulsions were examined. Analysis of the microstructure and droplet size distribution revealed that the WPI-, SC-, and mixed protein-stabilized emulsions exhibited uniform droplet distribution. The droplet size and ξ-potential of the MC emulsions stabilized by mixed protein emulsifiers were higher than those of the emulsions stabilized by WPI or SC separately. The emulsions stabilized by the two types of proteins and mixed emulsifiers had better stability under high salt concentrations than the synthetic emulsifier Tween 20. WPI-SC-stabilized emulsions were more resistant to high temperatures (70–90°C) and exhibited excellent stabilization than those stabilized by WPI and SC, which was attributed to the more sufficient coverage provided by the two types of protein emulsifier layers and better protein adsorption at the oil-water interface. These results indicate that WPI-SC is a potential stabilizer for MC emulsion requirements. This study provides a basis for the formulation of monodisperse and stable natural emulsion systems.

Effect of using a heatsink with nanofluid flow and phase change material on thermal management of plate lithium-ion battery

The thermal management of a lithium-ion battery employing a heatsink is evaluated in this study.The heatsink employs nanofluids (NFs) and phase change material (PCM) and has a revolutionary shape. The NFs flows through a helical microchannel (MCH) into the heatsink. A number of pin-fins with a spiral arrangement are used in the heatsink. The MCHs are filled with nano-PCM containing graphene nanoparticles. This study is performed in a time interval of 0 to 500 s at the Reynolds number (Re) range of 100 to 500. NFs outlet temperature (T-O), maximum (T-M) and average temperature (T-A) of PCM, heat transfer coefficient (HTC) and amount of molten PCM at different times, and different Re are estimated. COMSOL Multiphysics software is employed to do the simulations. The results demonstrate that the amount of molten PCM is enhanced except for Re = 500 until all the PCM inside the heatsink is melted. At Re ≥ 500, the PCM melting process does not occur within the heatsink and the PCM remains solid. The output temperature of NFs and PCM temperature are intensified with time. An increment in the Re reduces the amount of NFs temperature at the outlet as well as the T-A and T-M of PCM inside the heatsink. The amount of HTC between PCM and NFs depends on Re and increases over time.