This study proposes a kind of jet impingement microchannel heat sink with pinfins and then investigates the effects of jets arragement, pinfins placement angle, and pinfins shape on the flow characteristics and heat transfer of various heat sinks. It is found that the arrangement of jets has a minor influence on the flow pressure drop but has a significant influence on the heat transfer. Considering the heat transfer coefficient and pressure drop, the 5 x 6 array jet heat sink performs the best. The shape and placement angle of pinfins greatly affect the flow characteristics and heat transfer. Elliptical fins have a more advantageous influence on the overall performance of the heat sink compared with rectangular fins. The angle of the elliptical fins corresponding to maximum thermal performance factor value decreases with Reynolds number, the trend not seen in rectangular fins. The maximum thermal performance factor of heat sink, with rectangular fins at the angle of 15 degrees is increase by 6.73 %, and it is increased by 7.86 % for the case with elliptical fins at the angle of 30 degrees. The numerical simulations of the present design are justified by the analysis from the perspectives of entropy generation and Field Synergy Principle.

The overall safety of a Supercritical Water-cooled Reactor (SCWR) highly lies in the heat transfer performance of cooling system. This paper investigates the upward flow and heat transfer of supercritical water in a 2 x 2 rod bundle channel with a new Y-shaped longitudinal ribs through numerical simulation. When the Y -type longitudinal ribs are presented, the cooling channel is divided into several sub-channels, preventing fluid from flowing to the center of the channel due to pressure differences, making the fuel rod outer wall temperature more uniform and alleviating the heat transfer deterioration. At the same time, the introduction of Y-shaped ribs increases the mass flow rate of fluid in the cooling channel by 30% to 65%, and due to the existence of heat conduction effect, the temperature of Y-shaped ribs reaches 70% - 85% of the fuel rod wall temperature, which increases the heat transfer area. The increase in mass flow rate and heat transfer area improves the heat transfer efficiency between the coolant and the fuel rods. The Y-shaped ribs can increase the heat transfer coefficient by 10% - 56%, and the pressure loss is only increased by 10% - 40%, which is better than traditional positioning lattice frames.

Falling film flow and heat transfer are extensively encountered in various industries of renewable and sustainable energy due to the outstanding heat transfer performance. The predictions in falling film hydrodynamics and heat transfer are crucial for the optimal design of falling film heat exchangers, and there have been considerable advancements made in this area over the past few decades. Considering the gaps in current literature predictions, this paper aims to present a comprehensive review on the correlations of flow pattern transition, film thickness, heat transfer, and liquid film rupture within falling film flowing over horizontal tubes and tube bundles. Through comparisons and summarizations, it can be found that the documented correlations appear to be significantly out of alignment with one another, which greatly restricts the applicability of these predictions. The correlations of falling film thickness that are obtained using regression approach based on dimensional analysis may take into account more factors than the ones modified from Nusselt theory. The predictions of flow pattern transition of falling liquid film with horizontal column are often described as the functions of the film Reynolds number, derived from the Kapitza number or the Galileo number. The functions of Nusselt number, film Reynolds number, Prandtl number, and Archimedes number are typically used to represent sensible and evaporative heat transfer correlations, whereas boiling heat transfer correlations also take into account the influence of heat flux, such as Boiling number. The full process of actual heat transfer is taken into account from both the standpoints of sensible convection and nucleate boiling using the combination functions of various heat transfer forms. Within the ranges of their own parameters, the well-established heat transfer models often offer good accuracy. While the predictions of phase change heat transfer often behave worse, the predictions of sensible heat transfer are more general and accurate. Generally speaking, the minimum film flow rate or the maximum heat flux were the functions that correlated with the inceptions of liquid film rupture.

In this paper, single-phase heat transfer characteristics of water inside internal enhanced tubes were investigated. Refrigerants are boiling or condensing outside the tube. The experimental tubes have internal helical rib heights of 0.25-0.36 mm, helix angles of 40-60 degrees, rib base thicknesses of 0.40-0.79 mm, rib tip thicknesses of 0.078-0.283 mm, and Ns (starts number per circle) of 40-50. It shows that the heat transfer enhanced ratios usually range from 2.3 to 3.64. The friction factors relative to the smooth tube are about 1.8 to 3.3 times higher. Analyzing the effect of rib geometry on flow and heat transfer, it was found that the higher the height of the internal rib, the better the enhancement of convective heat transfer in the tube. Also, the greater the thickness of the rib tip and base, the more detrimental to the friction factor in the tube. There was no noticeable influence on the heat transfer performance as the helix angle increased from 45 degrees to 50 degrees. For the increase of Ns, it appears that 45 ribs per circle is the best value in the present study when considering the increase in pressure loss. The thermal-hydraulic performance of 11 tubes was also evaluated. It shows that Tube-1 had the best performance in the condensing tubes and Tube-7 had the best performance in the boiling tubes.

This paper attempts to develop an intelligent plate fin-and-tube heat exchanger (PFTHE) design system, which is entirely self-programming, to achieve quickly design. The proposed design system consists of four modules: (1) formulation, (2) optimization, (3) post-processing, and (4) decision-making. The proposed design system is implemented and validated with the application of shape optimization of ellipse tubes of plate-fin heat exchanger. In the formulation module, the physical problem to be studied is mathematized and the main design variables will be determined. In the optimization module, a famous algorithm, non-dominated sorting genetic algorithm of type II (NSGA-II), is embedded in an in-house Multi-concept Heat Transfer (MHT) code to achieve call CFD simulation during optimization process. To reduce computation time, Open multi-processing (OpenMP) is employed. The optimal solutions (Pareto solutions) obtained by the optimization module will be stored in the database and also taken as the input for the post-processing module and the decision-making module. In the post-processing module, Artificial neural network (ANN) is utilized to establish the correlation between design variables and heat transfer performance indicators assisting engineers to quickly design. As a short cut for heat exchanger design, both forward and backward designs have been implemented. Finally, in the decision-making module, technique for order preference by similarity to an ideal solution (TOPSIS) is applied to determine the best compromise solution from Pareto solutions according to the actual requirements provided by users. Results show that the proposed design system could determine a best compromise solution by reducing the pressure drop (80%) of the tube bundle without sacrificing too much heat transfer performance (5%) and also save much time for designers. For the forward design, the ANNs taken six decision variables as inputs are modelled to forecast two objectives has reached acceptable precisions. This research provides a promising tool for PFTHE optimization to improve heat transfer and comprehensive performance, also for quickly design based on historical simulation or experimental results.

The optimized structures for the liquid-cooled microchannel heat sinks with different pin-fin arrays were designed via using the topology optimization method for the better performance, including minimization of the flow energy dissipation and the average temperature of the bottom surface, aiming to facilitate the more efficient design of microchannel heat sink for electronic chips' cooling. The heat transfer and flowing performance was simulated with steady-state incompressible Navier-Stokes equation and the energy equation. Based on the rational approximation of material properties (RAMP) method, the mathematical optimization model for heat sink was established according to the multi-objective function. The Galerkin finite element method (GFEM) was employed to calculate the flow and heat transfer system. Furthermore, the globally convergent method of moving asymptotes (GCMMA) was adopted to deal with the mathematical optimization problem as the optimization method. The entropy generation analysis and comprehensive heat transfer factor were employed to evaluate the performance of the conjugate heat transfer problem of 3D microchannel heat sinks with various pin-fin structures. The calculation results reveal that the multi-objective optimal structure has good heat transfer performance, and the temperature distribution of the optimized structure is more uniform. As the same pumping power, the comprehensive heat transfer efficiency of the optimized inline and staggered pin-fin arrays increases by 31.20% and 32.18% compared to the smooth rectangular channel. Compared with the cylindrical pin-fin configuration with the same heat transfer area, the entropy generation rates of the optimized inline and staggered pin-fin configurations decrease by 4.67% and 4.25%, respectively. The results can provide theoretical basis and application support for the electronic chip cooling technology.

Inundation effect on condensation heat transfer is ubiquitous and a crucial factor in designing a large-scale condenser. In the present study, the effect of condensation liquid inundation on heat transfer is experimentally studied for R134a on a plain tube and three kinds of enhanced tubes. A specially designed test method with a liquid distributor for studying the effect of inundation liquid has been adopted, by which the inundation liquid flow rate can be adjusted easily within a wide range with high accuracy of inundation liquid flow rate. It is found that the inundation liquid may enhance the heat transfer at low heat flux of 20 kW/m2 when Rer is less than 600 and 800 for the 3D finned tube and C9 + tube (a kind of 3D finned tube with smaller fins on each fin to break the liquid film), respectively, and for the 2D low fin tube within the entire test range of Rer, with the maximum enhancement factor being 1.21. At high heat flux of 80 kW/m2 the inundation liquid always deteriorate heat transfer for the three enhanced tubes tested . At middle heat flux of 40 kW/m2, the inundation usually has negative effect but for C9 + tube heat transfer is enhanced when Rer is less than 1100 . It is also found that the advantage of C9 + tube in enhancing condensation heat transfer over the finned tubes is not so appreciable under the inundation case as it is for non-inundation case. Some flow visualization pictures are provided to deepen the understanding of physical process.(c) 2023 Elsevier Ltd. All rights reserved.

Due to the rapid development in micro manufacturing and 3D printing technology, the complex shape of microchannel heat sink (MCHS) can be realized, instead of just the traditional rectangular channels. It is recognized that twisted square channels which are similar to a kind of internal thread channel have favorable effects on heat transfer because of its enhancement of turbulence intensity. In this work, a geometry of twisted tube is applied to the microchannel and the thermal and hydrodynamic performance of twisted square microchannels with jet impingement are investigated. The arrangement of the jet channels is the research focus and one-jet and multi-jet microchannel heat sinks are compared by discussing the temperature profile, pressure drop and heat transfer characteristics. By applying the twisted geometry to the microchannel, the Nusselt number can be increased by 16.48% with little increase in pressure drop. However, with the increase in torsion angle of the twisted microchannel (from 90° to 720°), the thermal performance changes little for the limited effect on the temperature uniformity of the cross section. Integration of jet impingement further improves the thermal-hydrodynamic performance of MCHS. Specifically, the thermal resistance of MCHS was reduced by 41% at most. Finally, by comparing the comprehensive performance of the cases with varied jet arrangement, it is concluded that jets distributed at the cross section with the torsion angle of 45°, 135°, and 225° can induce better performance. The present investigation combines the twisted geometry and jet impingement into microchannel, quantitatively characterizes the performance of MCHS and gives the guidance for the jet arrangement design.

Particulate transport from surfaces governs a variety of phenomena including fungal spore dispersal, bioaerosol transmission, and self-cleaning. Here, we report a previously unidentified mechanism governing passive particulate removal from superhydrophobic surfaces, where a particle coalescing with a water droplet (∼10 to ∼100 μm) spontaneously launches. Compared to previously discovered coalescence-induced binary droplet jumping, the reported mechanism represents a more general capillary-inertial dominated transport mode coupled with particle/droplet properties and is typically mediated by rotation in addition to translation. Through wetting and momentum analyses, we show that transport physics depends on particle/droplet density, size, and wettability. The observed mechanism presents a simple and passive pathway to achieve self-cleaning on both artificial as well as biological materials as confirmed here with experiments conducted on butterfly wings, cicada wings, and clover leaves. Our findings provide insights into particle-droplet interaction and spontaneous particulate transport, which may facilitate the development of functional surfaces for medical, optical, thermal, and energy applications.

A water droplet impacting onto a supercooled surface is typically considered to freeze and adhere to the substrate. This ice accretion poses safety and economic threats to transportation infrastructure, power generation/transmission systems, and telecommunication facilities. Here we report the observation of ultra-low ice-substrate adhesion (0-50 kPa) and remarkable self-deicing during droplet-impact freezing on copper surfaces having medium to high supercooling (30 degrees C-80 degrees C). Mechano-thermo-hydraulic coupling during droplet-impact freezing governs the ice-substrate adhesion by gap. ping the droplet-substrate contact, enabling self-peeling facilitated by thermal-mechanical stress relaxation. We observe a strong adhesion region in the center of the frozen droplet, which determines the adhesion strength, and develop a regime map to delineate the dependence of adhesion/peeling on droplet inertia, substrate supercooling, and surface wettability. Our work demonstrates key mechanisms governing ice-substrate adhesion during impact icing and presents an approach to passive self-deicing.

In this study, thin liquid film boiling heat transfer characteristics of R134a on a plain and two finned tubular surfaces were experimentally investigated. The dependence of heat transfer coefficient (HTC) on the composite effects of microstructure and hydrophobic coating was characterized. It is found that HTC increases monotonously with increasing heat flux until reaching a threshold heat flux (THF), beyond which the HTC starts to descend. The THF is highest for the plain surface and lowest for the boiling-enhanced surface due to the effect of reentrant cavities. The boiling-enhanced surface shows larger HTC under low heat fluxes (less than 60 kW m(-2)) compared with the condensation-enhanced counterpart, whereas the latter one is superior when the heat flux exceeds 60 kW m(-2). Hydrophobic coating can substantially intensify the liquid film boiling heat transfer on boiling-enhanced finned surface with reentrant cavities. The intensification of heat transfer is more prominent in cases of high heat flux.

Bubble formation in liquids is frequently observed in nature and applied in various industrial processes. These include pool and flow boiling for thermal management systems, where bubbles may form asymmetrically at narrow slits and in convective flows. While previous studies have focused on symmetric bubble formation at circular orifices, the dynamics of asymmetric bubble formation remains poorly understood. Here, we experimentally investigate bubble formation at rectangular orifices and examine the effects of the orifice size and aspect ratio and the gas flow rate on the bubble size. The asymmetric bubble shape evolution at the rectangular orifice is analyzed, and we find that the size of the bubble neck is controlled either by the orifice size or by the capillary length. Based on these findings, we develop a static force balance model to predict the bubble size in the quasi-static regime, where the roles of Bond number and aspect ratio are identified. The bubble size evolution in the dynamic regime is further understood by introducing a Weber number that evaluates the effect of the virtual mass force induced by gas flow. Our study provides physical understanding of the dynamics of asymmetric bubble formation and guidance to predict the bubble size at asymmetric orifices.

Jet electrolyte micromachining (JEMM) exploits water-jet-assisted electrochemistry to achieve metal processing with spatial localization, precision, and flexibility. Currently, JEMM enables both micromilling and deposition, with the manufacturing precision and efficiency limited by the preparation and installation of the microscale tool electrodes (typically > 100 μm). Here, we develop a facile and low-cost platform for integrated in situ micro-subtractive and additive JEMM. Our technology is capable of machining micrometric grooves and pillars with controllable length scales (>20 μm) and topologies (patterns or spatial geometries) on metallic substrates. The integrated platform pumps electrolyte toward a workpiece through a nozzle to perform multiple tasks on the same setup, including micronozzle tool preparation, subtractive manufacturing, and additive manufacturing. We achieve this by controlling electrode polarity and electrolyte. We demonstrate our platform for microfabrication of grooves having a variety of widths ranging from 20 to 100 μm when working in the subtractive JEMM mode. In the additive JEMM mode, we demonstrate the fabrication of complex three-dimensional high-aspect-ratio micropillars having customized geometries beyond what is currently available with conventional methods. The proposed technology enables precise, controllable, efficient, and scalable additive and subtractive micromanufacturing for a plethora of applications.

The peripheral heat transfer correlation of the subcooled falling liquid film outside the horizontal tube was established using the present numerical data under different liquid flow rates, heat fluxes, tube sizes, distributor heights, and liquid temperatures. The results indicated that the present correlation can predicate 93% of 2654 data within ±25% in θ = 2°–15°, 92% of 2254 data within ±15% in θ = 15°–165°, and 80% of 1442 data within ±30% in θ = 165°–178°. The peripheral heat transfer correlation can predict 74% of 903 data and 80% of 700 data, respectively, within 0°–180° and 15°–165° with the deviations of ±30%. The sensitivity analysis indicated that the parameter angle and the distributor height have the most and least significant impact on the peripheral heat transfer correlation, respectively.

实验研究了不同热流密度、不同液膜流量下,饱和温度对R134a在水平光管外降膜蒸发传热系数的影响.结果 表明:当液膜流量较小时,降膜蒸发传热系数随着液膜流量的增大迅速增大,当液膜流量较大时,降膜蒸发传热系数随着液膜流量的增大维持在一个稳定值;随着热流密度的增大,降膜蒸发传热系数表现出先增大后减小的趋势,并且随着饱和温度的升高,这一趋势增强;当热流密度较小时,饱和温度的升高有利于降膜蒸发传热,当热流密度较大时,较高的饱和温度不利于维持液膜的完整性,从而不利于降膜蒸发传热.

Falling film evaporation (FFE) involves complicated physical phenomena and mechanisms such as wavy liquid film, bubbly flow, capillary-driven evaporation and nucleate boiling. FFE heat transfer characteristics in four doubly-enhanced tube bundles were investigated experimentally with R134a. For single tube, heat transfer coefficient (HTC) first increases then decreases with increase in heat flux, the turning points occurs around 20 kW/m(2). Tubes with different positions in tube bundle own similar HTCs when tested individually. In tube bundle, with decreasing film Reynolds number (Ref), HTC firstly keeps a quasi -plateau stage (increasing or keeping constant for upper tubes, decreasing for lower tubes), then after a certain threshold film Reynolds number, HTC decreases sharply with Ref.. At lower heat fluxes (10 kW/m(2) and 20 kW/m(2)), tubes with different positions exhibit similar HTCs and threshold Rec. At higher heat fluxes (30 kW/m(2) and 40 kW/m(2)), bottom tubes own much smaller HTCs and larger threshold Ref than upper ones due to partial dryout occurrence. The tube bundle with top plate exhibits higher HTCs and lower threshold Rer than those of the open-ended tube bundle indicating that counter-current vapor flow can deteriorate the heat transfer of FFE. Effect of heat flux on the bundle -averaged HTC increases with tube pitch.

The falling film evaporation and the effect of cross vapor stream in falling film evaporator with triangular tube bundle using R123 was experimentally investigated. The variations of local and bundle average heat transfer coefficients were described. The tube bundle consisted of 4 x 3 (column x row) of triangular horizontal copper tubes. The tests without vapor effect were conducted with nominal heat fluxes of 20 to 60 kWm(-2), saturation temperatures of 6 to 16 degrees C and film flow rates of 0.016 to 0.18 kgm(-1) s(-1). Cross vapor stream effect experiments were operated at three heat fluxes of 20, 30 and 40 kWm(-2) and two film flow rates of 0.035 and 0.07 kgm(-1) s(-1), and the vapor velocity at the narrowest interstice in the tube bundle varies from 0 to 5.0 ms(-1). It is indicated that the heat transfer is seriously influenced by the bundle effect while less affected by changing of saturation temperature. With the increase in vapor velocity, the heat transfer performance is generally weakened; the cross vapor stream has a strong influence on falling film evaporation of R123. (C) 2020 Elsevier Ltd and IIR. All rights reserved.

In this study, the falling film evaporation heat transfers of R290 on an array composed of five enhancement horizontal tubes (groove tubes) are studied. The tests are performed at constant saturation temperatures 5.5 degrees C with change of heat flux from 10 to 40 kW/m(2). The film Reynolds number ranges from 200 to 2200 and the film flow rate of refrigerant is between 100 and 660 kg/h. The results show that the film flow rate and heat flux have significant effects on R290 falling film evaporation heat transfer coefficients (HTCs) of the tubes in the tube array. With decreasing the film flow rate on the five tubes the tube HTCs display two stages, a plateau stage and a sharp drop stage. The heat transfer coefficients firstly keep more or less constant at the plateau stage and then decreasing rapidly. At the same nominal film Reynolds number the tube averaged heat transfer coefficients of tube No. 1 to tube No. 5 decrease in order of the increasing tube number from top to bottom of the array. The falling film evaporation HTC of R290 on single enhanced tube is about 4.5 times of single smooth tube HTC, and the HTCs of the enhanced tube array is higher than single smooth tube by more than 2.5 times. In addition, the R290 HTCs of the tube array are higher than those of R134a for the same tube array in the plateau region by about 25%. It is found that at high heat flux of 30-40 kW/m(2) , the heat transfer coefficient variation with film Reynolds number of the lower enhanced tubes in the tube array exhibits severe undulating characteristics. (C) 2020 Elsevier Ltd. All rights reserved.

As the softest part in a proton exchange membrane fuel cell (PEMFC), the gas diffusion layer (GDL) could have a large deformation under assembly pressure imposed by bipolar plate, which would have an impact on the cell performance. So, there is an urgent need to clearly reveal the mechanical behavior of GDL under certain pressure. In this paper, the mechanical behavior of paper-type GDL of PEMFC is studied, considering the complex contact environment in the fibrous layered structure. The microstructure of GDL is reconstructed stochastically, then the stress-strain relationship of GDL is explored from the perspective of solid mechanics by using the finite element method. Based on microstructure morphology, it is found that contact pairs and pore space of microstructure are two key factors determining the nonlinearity of the compressive curve. The equivalent Young's modulus increases with the decrease of porosity and carbon fiber diameter but it is not very sensitive to the carbon paper thickness. The results indicate that with the increase in acting pressure, the average porosity of the carbon paper decreases, and the nonuniformity of porosity along the through-plane direction increases. Furthermore, a reasonable explanation for the increase of concentration loss and the decrease of ohmic loss is given from the microstructure findings of the present study.

The laminar liquid film falling on a horizontal smooth tube is studied numerically. The instantaneous hydrodynamic characteristics of falling film flow, the importance of surface tension in calculation and the effects of film flow rate, tube diameter, liquid distributor height and inlet liquid temperature on the flow field and film thickness are elucidated. The results indicate that: (1) The surface tension is important in the calculations of falling film flow on a horizontal tube; (2) The film falling on a circular tube has obvious instantaneous behaviors; (3) The film thickness increases with increase of film flow rate, while decreases with increase of the tube diameter, liquid distributor height and liquid temperature, respectively; (4) The film distribution along the peripheral angle is unsymmetrical, and the minimum thickness appears in 110-150 of peripheral angle depending on the working conditions. Furthermore, new correlations of falling film thickness on a horizontal tube based on the present data are established, which fit 97% of 84 data in theta = 2-15 degrees within 20%, 90% of 632 data in theta = 15-165 degrees within 20%, and 73% of 112 data in theta = 165-178 within +/- 30%. (C) 2017 Elsevier Ltd. All rights reserved.