Search Results (71)

The role of wall roughness in heat and mass transfer for fully developed viscous flows in square microchannels is investigated here. Since the roughness, which is the key geometrical feature to be investigated, introduces high velocity gradients at the wall, the effect of the viscous dissipation is considered. A fully developed flow in the forced convection regime is assumed. This assumption allows the two-dimensional treatment of the problem; thus, the velocity and temperature fields are simulated on the microchannel cross-section. The boundary roughness is modeled by randomly throwing points around the nominal square cross-section perimeter and by connecting those points to generate a simple polygon. This modification of the nominal square shape of the cross-section influences the velocity and temperature fields, which are computed by employing a finite element method solver. The heat and mass transfer is studied by calculating the Nusselt and the Poiseuille numbers as a function of roughness amplitude at the boundary. Each Nusselt and Poiseuille number is obtained by employing an averaging procedure over a sample of a thousand cases. Full article

The critical heat flux (CHF) of minichannel heat sinks is crucial, as it helps prevent thermal safety incidents and equipment failure. However, the underlying mechanisms of CHF in minichannels remain poorly understood, and existing CHF prediction models require further refinement. This study systematically investigates the characteristics and influencing factors of critical heat flux (CHF) in rectangular minichannels through combined experimental and theoretical approaches. Experiments were conducted using microchannels with hydraulic diameters ranging from 0.5 to 2.0 mm, with ethanol employed as the working fluid. Key parameters-including mass flux, channel geometry, system pressure, and inlet subcooling-were analyzed to assess their influence on CHF. Results indicate that CHF increases with mass flux; however, the increase rate diminishes under higher mass flux. Larger channel dimensions significantly enhance CHF by delaying liquid film dryout. System pressure further improves CHF by reducing bubble detachment frequency and promoting flow stability. Increased inlet subcooling enhances CHF by delaying the onset of nucleate boiling and improving convective heat transfer. Four classical CHF prediction models were evaluated, revealing significant overprediction-up to 148.69% mean absolute error (MAE)-particularly for channels with hydraulic diameters below 1.0 mm. An ANN deep learning model was developed, achieving a reduced MAE of 8.93%, with 93% of predictions falling within ±15% error. This study offers valuable insights and a robust predictive model for optimizing microchannel heat sink performance in high heat flux applications. Full article

The heat transfer enhancement of diamond-shaped ribs mounted in the periodic merging chambers of microchannel (MC) heat sinks is investigated using a numerical method for Reynolds number in the region of 300–700. Compared to triangular, rectangular, and cylindrical ribs, diamond-shaped ribs achieve 3.59%, 13.24%, and 6.34% higher enhancement effects, respectively, under the same mass flow rate. Further analysis of geometric parameters (length, width, and height) and rib positioning reveals that a rib height of h/H_ch = 0.8 provides optimal heat dissipation performance. For Re < 500, the optimal configuration is a rib length of l/L_merg = 0.55 and a width of b/W_ch = 0.8, while for 500 < Re < 700, it shifts to l/L_merg = 0.36 and b/W_ch = 1.6. For s/L_merg, the smaller it is, the shorter the main flow separation time, thereby improving heat transfer efficiency. Full article

Low-temperature Fischer–Tropsch (LTFT) synthesis converts syngas to diesel/wax at 200–250 °C. The LTFT reaction has recently received renewed interest, as it can be used for converting syngas from renewable sources (biomass and waste) to high-value fuels and chemicals. Conventional LTFT reactors, such as fixed-bed and slurry reactors, are not entirely suitable for bio-syngas conversion due to their smaller scale compared to fossil fuel-based syngas processes. This review explores advancements in intensifying LTFT reactors suitable for bio-syngas conversion, enabling smaller scale and dynamic operation. Various strategies for enhancing heat and mass transfer are discussed, including the use of microchannel reactors, structured reactors, and other designs where either one or both the heat and mass transfer are intensified. These technologies offer improved performance and economics for small LTFT units by allowing flexible operation, with increased syngas conversion and reduced risk of overheating. Additionally, this review presents our outlook and perspectives on strategies for future intensification. Full article

Microchannel heat exchangers, with their large specific surface area, exhibit high heat/mass transfer efficiency and have a wide range of applications in chemical engineering and energy. To enhance microchannel flow boiling heat transfer, a top-connected microchannel heat exchanger with a Ni/Ag micro/nano composite surface was designed. Using anhydrous ethanol as the working fluid, comparative flow boiling heat transfer experiments were conducted on regular parallel microchannels (RMC), top-connected microchannels (TCMC), and TCMC with a Ni/Ag micro/nano composite surface (TCMC-Ni/Ag). Results show that the TCMC-Ni/Ag’s maximum local heat transfer coefficient reaches 179.84 kW/m²·K, which is 4.1 times that of RMC. Visualization reveals that its strongly hydrophilic micro/nano composite surface increases bubble nucleation density and nucleation frequency. Under medium-low heat flux, the vapor phase converges in the top-connected region while bubbles form on the microchannel surface; under high heat flux, its capillary liquid absorption triggers a thin-liquid-film convective evaporation mode, which is the key mechanism for improved heat transfer performance. Full article

In simulations of elastic flow using the lattice Boltzmann method (LBM), the steady-state behavior of the flow at low capillary numbers is typically poor and prone to the formation of bubbles with inhomogeneous lengths. This phenomenon undermines the precise control of heat transfer, mass transfer, and reactions within microchannels and microreactors. This paper establishes an LBM multiphase flow model enhanced by machine learning. The hyperparameters of the machine learning model are optimized using the particle swarm algorithm. In contrast, the non-dominated sorting genetic algorithm (NSGA-II) is incorporated to optimize bubble lengths and stability. This results in a coupled multiphase flow numerical simulation model that integrates LBM, machine learning, and the particle swarm algorithm. Using this model, we investigate the influence of elastic flow parameters on bubble length and stability in a T-shaped microchannel. The simulation results demonstrate that the proposed LBM multiphase flow model can effectively predict bubble elongation rates under complex conditions. Furthermore, multi-objective optimization determines the optimal gas–liquid two-phase inlet flow rate relationship, significantly mitigating elastic flow instability at low capillary numbers. This approach enhances the controllability of the elastic flow process and improves the efficiency of mass and heat transfer. Full article

Flow boiling heat transfer in micro-channels under hypergravity conditions is a crucial research area for developing efficient cooling systems in aerospace applications. This experimental study investigated the flow boiling heat transfer characteristics of deionized water in a tube with a 2 mm diameter under various gravitational conditions ranging from normal gravity (1 g) to hypergravity (up to 5.1 g) by employing a centrifugal rotating platform. The study systematically analyzes the effects of gravity level, vapor quality, mass flux, and heat flux on the flow boiling heat transfer coefficient (HTC) of deionized water. Experimental results reveal that hypergravity significantly influenced the HTC at a higher vapor quality with up to a 40% deviation and was less pronounced at lower values with an approximately 10% deviation. This deterioration is attributed to the complex interplay of centrifugal force, the Coriolis force, buoyancy, and the specific properties of water, leading to vapor–liquid stratification and hindering effective heat transfer. Meanwhile, critical heat flux was found to increase with increasing gravity acceleration at high vapor qualities. This enhancement is attributed to improved buoyancy effects, liquid replenishment, and altered flow patterns under hypergravity conditions. Furthermore, five existing correlations for predicting flow boiling HTCs were evaluated against the experimental data, and all the correlations showed overestimated results. The consistent over-prediction by these correlations highlights the need for modifications to better capture heat transfer mechanisms under hypergravity conditions. Full article

Microreactors have the advantages of high heat and mass transfer efficiency, strict control of reaction parameters, easy amplification, and good safety performance, and have been widely used in various fields such as chip manufacturing, fine chemicals, and biomanufacturing. However, narrow microchannels in microreactors often become filled with catalyst particles, leading to blockages. To address this challenge, this study proposes a multiphase flow heat transfer model based on the lattice Boltzmann method (LBM) to investigate the dynamic changes during the bubble collapse process and temperature distribution regularities. Based on the developed three-phase flow dynamics model, this study delves into the shock dynamic evolution process of bubble collapse and analyzes the temperature distribution regularities. Then, the flow patterns under different particle density conditions are explored. The study found that under the action of shock wave, the stable structure of the liquid film of the bubble is destroyed, and the bubble deforms and collapses. At the moment of bubble collapse, energy is rapidly transferred from the potential energy of the bubble to the kinetic energy of the flow field. Subsequently, the kinetic energy is converted into pressure waves. This results in the rapid generation of extremely high pressure in the flow field, creating high-velocity jets and intense turbulent vortices, which can enhance the mass transfer effects of the multiphase flows. At the moment of bubble collapse, a certain high temperature phenomenon will be formed at the collapse, and the high temperature phenomenon in this region is relatively chaotic and random. The pressure waves generated during bubble collapse have a significant impact on the motion trajectories of particles, while the influence on high-density particles is relatively small. The results offer a theoretical basis for understanding mass transfer mechanisms and particle flow patterns in three-phase flow. Moreover, these findings have significant practical implications for advancing technologies in industrial applications, including chip manufacturing and chemical process transport. Full article

Noah-2100A and HFE-649, as two electronics fluorinated liquids (EFLs) with low saturation temperature, high safety, excellent insulation properties, and low environmental impact, are considered as replacements for the refrigerants with high Global Warming Potential (GWP), such as HFC-134a and HFC-245fa, in electronic cooling system. However, there is still a knowledge gap of boiling heat transfer for these two EFLs, especially in pin-fin microchannel. The effect of inlet temperatures, mass flow rates, and inlet vapor qualities on boiling heat transfer for two EFLs were studied experimentally in this paper. Overall, though the Noah-2100 has a higher pressure drop-in microchannel than HFE-649, Noah-2100A shows a higher overall thermal performance than HFE-649. Newly developed correlations of the Nusselt number (Nu) and pressure drop for two EFLs in a pin-fin microchannel heat sink were also presented. The proposed correlations can achieve a 10% and 11% mean average percentage error for Nu number and pressure drop. Full article

In thermal management applications using two-phase flow boiling, rectangular microchannels hold significant promise due to their ease of manufacturing and effective heat transfer characteristics. In this work, we combined experimental and theoretical analyses to propose a theoretical model based on thin liquid film evaporation for predicting heat transfer performance in rectangular cross-sectional microchannels. The heat transfer model is segmented into five zones based on two-phase flow patterns and transient liquid film thickness. These zones represent different flow boiling heat transfer mechanisms over time in microchannels: the liquid slug zone, elongated bubble zone, long-side wall dryout zone, corner liquid evaporation zone, and full dryout zone. The new model comprehensively explains experimental phenomena observed, including long-side wall dryout and thinning of the liquid film on the short-side wall. To validate our model, numerical solutions were computed to study the spatial and temporal variations in heat transfer coefficients. The results exhibited a consistent trend with experimental data regarding average heat transfer coefficients. We also analyzed factors influencing flow boiling characteristics, such as microchannel aspect ratio, hydraulic diameter, measurement location, fluid mass flux, and wall heat flux. Full article

The present review focuses on the recent studies carried out in passive micromixers for understanding the hydrodynamics and transport phenomena of miscible liquid–liquid (LL) systems in terms of pressure drop and mixing indices. First, the passive micromixers have been categorized based on the type of complexity in shape, size, and configuration. It is observed that the use of different aspect ratios of the microchannel width, presence of obstructions, flow and operating conditions, and fluid properties majorly affect the mixing characteristics and pressure drop in passive micromixers. A regime map for the micromixer selection based on optimization of mixing index (MI) and pressure drop has been identified based on the literature data for the Reynolds number (Re) range (1 ≤ Re ≤ 100). The map comprehensively summarizes the favorable, moderately favorable, or non-operable regimes of a micromixer. Further, regions for special applications of complex micromixer shapes and micromixers operating at low Re have been identified. Similarly, the operable limits for a micromixer based on pressure drop for Re range 0.1 < Re < 100,000 have been identified. A comparison of measured pressure drop with fundamentally derived analytical expressions show that Category 3 and 4 micromixers mostly have higher pressure drops, except for a few efficient ones. An MI regime map comprising diffusion, chaotic advection, and mixed advection-dominated zones has also been devised. An empirical correlation for pressure drop as a function of Reynolds number has been developed and a corresponding friction factor has been obtained. Predictions on heat and mass transfer based on analogies in micromixers have also been proposed. Full article

This paper investigates the heat transfer performance of flow boiling in microchannels under the dual effect of gravity and surface modification through both experimental studies and mechanistic analysis. Utilizing a test bench with microchannels featuring surfaces of varying wettability levels and adjustable flow directions, multiple experiments on R134-a flow boiling heat transfer under the effects of gravity and surface modification were conducted, resulting in 1220 sets of experimental data. The mass flux ranged from 735 kg/m²s to 1271 kg/m²s, and the heating heat flux density ranged from 9 × 10³ W/m² to 46 × 10³ W/m². The experimental results revealed the differences in the influence of different gravity and surface modification conditions on heat transfer performance. It was found that the heat transfer performance of super-hydrophilic surfaces in horizontal flow is optimal and more stable heat transfer performance is observed when gravity is aligned with the flow direction. And the impact of gravity and surface modification on heat transfer has been explained through mechanistic analysis. Therefore, two new dimensionless numbers, Fa and Co_new, were introduced to characterize the dual effects of gravity and surface modification on heat transfer. A new heat transfer model was developed based on these effects, and the prediction error of the heat transfer coefficient was reduced by 12–15% compared to existing models, significantly improving the prediction accuracy and expanding its application scope. The applicability and accuracy of the new model were also validated with other experimental data. Full article

This study aims to provide condensation heat transfer coefficients of R600a (isobutane) refrigerant under mass fluxes between 50 and 98 kg/m²·s at saturation temperatures of 35 °C, 40 °C and 45 °C. Additionally, experiments are conducted with varying inlet vapour quality to understand its effect on the condensation heat transfer measurement. An aluminium multiport microchannel with a hydraulic diameter ($D_h$) of 0.399 mm is used, where a plexiglass cover is mounted on the top of the microchannels to observe the flow conditions. A 1D heat transfer through the aluminium block is assumed, and heat flux through the refrigerant to the coolant is measured to obtain condensation heat transfer coefficients of R600a. The results showed that decreasing saturation temperature and increasing vapour quality increase the condensation heat transfer coefficient. Increasing refrigerant mass flux increases the heat transfer coefficient up to a specific mass flux. It is observed that the effect of inlet vapour quality becomes significant as introduced quality decreases due to increasing fluctuation. Full article

Biodiesel stands at the forefront as a replacement for fossil diesel in compression ignition engines, particularly in the transportation sector where diesel engines are the primary movers. However, biodiesel production is hampered by poor heat and mass transfer during the transesterification reaction, leading to long production times and high costs due to inefficient energy utilisation. This study targets heat and mass transfer issues during the production of biodiesel via a synergic approach that combines microwave-assisted heating and microfluidics via a polyethylene terephthalate glycol (PETG) microchannel reactor. The transesterification reaction of palm oil and methanol was investigated using a full factorial design of experiments (DOE) method. Biodiesel yield was quantified via gas chromatographic analysis, and the results were optimised using statistical analysis. Optical analysis of slug quantification within the microchannel revealed that small slugs, smaller than 1 mm, accelerated the transesterification reaction. The composite-optimised experimental results, aimed at minimising energy costs and environmental impacts while maximising fatty acid methyl ester (FAME) yield, indicate a reaction temperature of 50 °C, a catalyst loading of 1.0 wt.%, and a 3:1 methanol to oil molar ratio. Regression analysis revealed that the reaction temperature was statistically insignificant when utilising the PETG microchannel reactor. This key finding positively impacts biodiesel production as it relates to significantly reduced energy intensity, costs, and emissions. Overall, this research work paves a pathway toward an energy-efficient and sub-minute rapid transesterification reaction, highlighting the effectiveness of microwave heat delivery and effects of microfluidics via the PETG microchannel reactor in overcoming heat and mass transfer barriers in biodiesel production. Full article

Interconnected microchannels (IMCs) in flow boiling have the advantages of optimized heat transfer performance, energy savings and high efficiency, compact size, and strong customizability. They provide new solutions for thermal management and heat transfer at the microscale and have broad application prospects. To further investigate the effect of microchannels with different numbers of transverse sections on the flow boiling heat transfer, we performed numerical simulations on a rectangular microchannel (RMC) and IMCs with 3, 5, and 7 transverse microchannels at high and low mass flux. It was found that fluid experiences similar bubble and slug flow in different numbers of IMCs and the RMC at low mass flux. At a heat flux of q = 90 W/cm², the downstream regions of the IMCs produce vapor films that span the channels, obstructing the cross-section and weakening the flow exchange between the channels, which lead the heat transfer performance factor of IMC-3, reaching 148.43%, 110.04%, and 116.92% of the RMC, IMC-5, and IMC-7. Under high-quality flux, as the heat flux increases, the heat transfer coefficient increases and the pressure drop decreases due to the existence of lateral microchannels introduced in the interconnected microchannels. Whether at high or low mass flux, structural reasons pertaining to the RMC can easily lead to the accumulation of bubbles and the occurrence of slugs, and the flow boiling instability increases with the increase of heat flux, which leads to a pressure drop and heat transfer performance generally lower than that of IMCs under the same conditions. At q = 120 W/cm², IMC-7 showed the best heat transfer enhancement. Its heat transfer performance factor was 129.37%, 120.594% and 107.98% of the RMC, IMC-3, and IMC-5, respectively. This article provides theoretical support for the design of interconnected microchannels in thermal management. Full article