Studies on Gas Flow through Smooth Microchannel Surface – Fabrication,Characterization, Analysis,and Tangential Momentum Accommodation Coefficient Comparison

Abstract

In the present work, experimental and numerical findings of tangential momentum accommodation coefficient (TMAC) on smooth microchannel surface and comparison with rough surface was performed. A modified silicon micromachining technique was used to achieve smooth surface of the microchannel. About two orders of magnitude decrease in surface roughness was obtained when compared to previous studies. Second order slip model was derived, and a special procedure was developed to simulate the gas flow in microchannel from slip-to-transition regimes. The microchannel system with 20 parallel channels were fabricated on a surface modified silicon surface and flow characteristics were studied by generating accurate and high resolution experimental data with comparison of simulation results. TMAC values were found up to an outlet Knudsen number of 0.851 with nitrogen and the second order slip coefficients were found. When compared to previous TMAC values for rough surfaces, low values were obtained in the present case. This decrease is an indication of the increase in slip velocity due to the reduction of surface roughness.     

On the eruptive boiling in silicon-based microchannels

This study investigates experimentally eruptive boiling in a silicon-based rectangular microchannel with a hydraulic diameter of 33.7 μm, a width of 99.8 μm and a depth-to-width ratio of 0.203. The microchannel is made of SOI wafer and prepared using bulk micro-machining and anodic bonding. The surface roughness for both the bottom and the side walls was measured using an atomic force microscope. The evolution of the eruptive boiling of water in the smooth microchannel was clearly observed using an ultra high-speed video camera (up to 50,000 frames/s) at mass fluxes of 417 and 625 kg/m² s and a heat flux from 14.9 to 372 kW/m². It is confirmed that eruptive boiling is a form of rapid bubble nucleation after which the bubble merges with a slug bubble downstream in a short distance or evolve to a slug bubble. The bubble frequency in all of the cases studied is provided. Eruptive boiling may be predicted classically with nano-sized cavities that are consistent with the measured surface roughness.     

Constructal Optimization of Microchannel Heat Sinks With Noncircular Cross Sections

This article reports the numerical geometric optimization of three-dimensional microchannel heat sinks with rectangular, elliptic, and isosceles triangular cross sections. The cross-sectional areas of the mentioned microchannels can change according to the degrees of freedom, that is, the aspect ratio and the solid volume fraction. Actually, the purpose of geometric optimization is to determine the optimal values of these parameters in such a way that the peak temperature of the wall is minimized. The effects of solid volume fraction and pressure drop upon the aspect ratio, hydraulic diameter, and peak temperature of the microchannels are investigated. Moreover, these microchannel heat sinks are compared with each other at their optimal conditions. Considering the constraints and geometric parameters for the optimization of the present study, it is revealed that microchannel heat sinks with rectangular and elliptic cross sections have similar performances, while microchannels with isosceles triangular cross sections show weaker performances. The optimal shapes of all three kinds of channels are achieved numerically and compared with the approximate results obtained from scale analysis, for which good agreements are observed.     

Unsteady jet impingement: Heat transfer on smooth and non-smooth surfaces

Often studies that determine the influence of unsteadiness on flat plate impinging jet heat transfer implicitly assume that the effect of unsteadiness found on smooth impingement surfaces also holds on surfaces with certain obstacles on them. In order to test this assumption a single roughness element was added to an otherwise smooth surface, and it was found that the steady heat transfer was almost the same as that for a totally smooth surface. The effect of unsteadiness, however, can be fundamentally different when roughness elements are added to a smooth surface. Slight changes in the surface geometry thus can have strong impact with respect to the effect of unsteadiness on heat transfer under impinging jets and cannot be neglected a priori.     

Prediction of thermal contact conductance based on the statistics of the roughness profile characteristics

The roughness profiles of some common machined surfaces were measured. Four different criteria for determining contact peaks are presented. The distributions of the roughness profile height and the contact peak height were calculated. Based on the statistics of roughness profile characteristics, the thermal contact conductances for surfaces with different roughnesses were predicted. The results showed that the contact peak’s criterion is crucial to the calculation of the distribution of the contact peak height. It has, however, limited influence on the prediction of thermal contact conductance. On the other hand, using several statistical roughness parameters or a single roughness profile is not adequate to describe the topography of the contact surface because of the contact surface’s anisotropy. The values of predicted thermal contact conductance for surfaces with different roughness profiles form a steady prediction strip.     

Numerical modeling of microchannel reactor with porous surface microstructure based on fractal geometry

A microchannel reactor with porous surface for hydrogen production can enhance fluid flow and heat transfer characteristics. To improve the fluid flow and heat transfer characteristics of a microreactor with a porous surface, a numerical model is proposed based on fractal geometry. The porous surface in the microreactor is fabricated using a layered powder sintering and dissolution method with NaCl particles, in which two sizes of NaCl particles (180–280 μm and 280–450 μm) are utilized. For the construction of the porous surface, these two types of fabricated surfaces are measured and the fractal dimensions are characterized as 1.905 and 1.849, respectively. Subsequently, a numerical model based on fractal geometry for a microchannel reactor with porous surface is developed to study the fluid flow and heat transfer characteristics. This is followed by the microchannel reactor fabrication and experimental testing. Both model calculation and experimental results demonstrate that a microreactor with a porous surface can enhance the heat transfer performances compared with that with a non-porous surface, and that a microchannel reactor fabricated with larger NaCl particles (280–450 μm) has better heat transfer characteristics compared with a microreactor with small NaCl particles (180–280 μm). Thus, the developed numerical model based on fractal geometry can be used to accurately predict the fluid flow and heat transfer characteristics of the microreactor for hydrogen production.     

Modeling of hydrodynamics in microchannel reactor for Fischer–Tropsch synthesis

Hydrodynamics of liquid and gaseous products in microchannel reactor for Fischer–Tropsch synthesis is considered. It is supposed, that liquid and gaseous products of the synthesis move downward in annular flow regime. A microchannel with irregular internal walls is investigated in cylindrical symmetry. In proposed numerical technique the peculiarities of coating the microchannel walls by cobalt-based catalytic particles are taken into account. System of equations of two-phase hydrodynamics is based on the generalized equations for mass flowrate and momentum of liquid film and gaseous phase. Stable numerical algorithm for calculation the thermodynamic equilibrium in gas–liquid mixtures of synthesis products is proposed. Calculation results illustrate thermophysical properties of liquid and gaseous products. In the hydrodynamics model variations along a microchannel mass fractions and thermophysical properties of liquid and gaseous products were taking into account. Principal hydrodynamical difference between a smooth microchannel and a microchannel with random roughness is explained. Hydrodynamical parameters and gradient of pressure are investigated as functions of pressure, temperature, averaged diameter of a microchannel and chain growth probability.     

Improvement of airfoil design using smooth curvature technique

The newly developed generalized function of airfoil profiles of wind turbine based on Trajkovski conformal transform theory can be used to fit the existing airfoil profiles and create the new ones by adjusting the coefficients of the generalized function. In this approach, the geometrical scale factor a， which was taken as a constant 0.25, has a significant impact on the curvature smooth continuity which will affect the aerodynamic performances of the airfoil. In this paper, the functional integral theory of wind turbine airfoils is studied. Furthermore, the advantage and the importance of curvature issue for airfoil surface are discussed in detail. It is found that, when different existing airfoils were analyzed using the generalized function, the geometrical scale factor a reaches an unexpected lower value. Based on curvature smooth continuity theory, a new method is presented to correct the geometrical scale factor a. As a result, the curvature smooth continuity of the fitting profile has been greatly improved, compared with that of the original profile. As an application of this new method, the DU93-W-210 airfoil is improved with the corrected geometrical scale factor a, and optimized using genetic algorithm (GA) method by controlling the coefficients of the shape function, leading to a new airfoil. Comparatively, the aerodynamic performances of the new airfoil such as maximum lift coefficient, maximum lift-drag ratio, roughness insensitivity and so forth are better than the DU93-W-210 airfoil performances. The achieved results show that this novel method is feasible to optimize airfoils of wind turbine.     

粗糙度对压气机叶栅性能影响研究

受工作环境的影响,空气中的杂质进入到压气机内部,这些杂质冲击压气机部件表面造成壁面粗糙度增加,严重时会造成叶片损伤,严重影响压气机的安全稳定运行。为了分析粗糙度造成压气机性能衰退的气动原因,首先需要研究粗糙度对叶栅性能的影响。选择某双弧形压气机叶栅作为研究对象,首次使用一种多控制点叶型型线变化的方法模拟二维粗糙叶片表面,从而实现粗糙叶片的物理模型体现。数值模拟计算过程中避开了传统的、经验式的壁面函数方程,提高了数值模拟对于粗糙表面的计算精度。研究了不同粗糙度以及不同粗糙位置条件下叶栅各方面的损失变化。研究结果表明：当叶栅表面完全粗糙时,尾迹损失值较光滑叶栅升高33%;叶背前缘20%位置的粗糙度对叶栅性能影响最大。     

Numerical simulation of wall roughness on gaseous flow and heat transfer in a microchannel

A flow and heat transfer numerical simulation was performed for a 2D compressible gas flow through a microchannel in the slip regime to investigate the effects of wall roughness. The wall roughness is simulated by rectangular microelements. This effect is examined for gas flows under inlet Mach number ranging from 0.0055 to 0.202. The numerical results demonstrate that the roughness elements have a significant impact on the flow characteristics. For rarefied gases, it is found that roughness effect leads to an increase in the Poiseuille number with increasing roughness height and decreasing element spacing. The surface roughness has a more significant effect on the flow with a lower inlet Kn. Compressible gas flow is also sensitive to the height of the wall roughness elements. In addition, an increase of the relative roughness height leads to a pronounced decrease in the local heat flux for both rarefied and compressible flow. The average Nusselt numbers have a much more significant reduction for a rarefied flow than a compressible flow. The influence of wall roughness on the average heat transfer rate is smaller than that on the Poiseuille number.     

Numerical simulation of stacked microchannel heat sink with mixing-enhanced passive structure

The paper is focused on the investigation of numerical simulation of stacked two-layer microchannel heat sink with enhanced mixing passive microstructure. In contrast to the smooth microchannel studies in the literature, the microchannel with embedded passive microstructure is chosen. The computational fluid dynamics (CFD) will be used to simulate the flow and heat transfer in a stacked two-layer microchannels with multiple MEMS easy-processing passive microstructures. To simulate the conjugated heat transfer among the heatsink and fluid, the three-dimensional conjugated model is used to solve this problem. The important parameters (e.g. the ratio of embedded structure height to microchannel height and fluid property) are investigated. The ratio of embedded structure height to microchannel height is changed from 0.13 to 0.26. The microchannel Reynolds number is fixed at 14.8. The stacked microchannel with passive structures has better performance than the smooth microchannels.     

Flow and Heat Transfer Characteristics of Liquid Nitrogen in Mini-/Microchannels

Flow and heat transfer of liquid nitrogen in mini-/microchannels have many particular characteristics and are very important for many cooling applications. In this study, the investigation of flow and heat transfer characteristics of liquid nitrogen in mini-/microchannels is presented by summarizing the experimental studies carried out in the author's group. In addition, some recent results about flow and heat transfer of liquid nitrogen in microchannel heat sink are also presented. It is found that small viscosity of liquid nitrogen enables the single-phase liquid flow in mini-/microchannels to be turbulent state, which proves that the classical theory for pressure drop is still valid if the surface roughness of the passage is properly taken into consideration. Experiments of flow boiling of liquid nitrogen are conducted under both adiabatic and diabatic conditions. It is shown that confinement number can be applicable in classifying the heat transfer characteristics of liquid nitrogen in macro- and microchannels. Flow visualization in microchannels at low temperatures poses big challenges on experimental aspects, which have been subtly overcome and clear images have been obtained. The flow patterns and flow regimes of two-phase flow of liquid nitrogen exhibit different features from the room-temperature fluids. Furthermore, three-dimensional (3D) flow visualization by only one high-speed camera is conducted to obtain more detailed information of flow patterns. Finally, the experiments of flow boiling of liquid nitrogen in microchannel heat sink are also presented and discussed.     

Numerical simulation of impact initiation of a matchhead composition for different in-process contact materials

In the processing of matchhead composition accidental initiation due to the interaction of different materials, even at low velocity, leads to fire and explosions. Experimental investigation of these transient events is expensive and time-consuming, and nowadays the use of numerical approaches is on the increase. In this study, the influence of different types of contact materials and their surface roughness, on initiation of a matchhead composition by impact load was analysed using LS-DYNA®. The results demonstrated that the contact materials and their surface roughness imparted variations in initiation time histories, critical load and peak pressure of the matchhead composition. However, the initiation response of the matchhead composition was quicker in case of steel–steel anvil combinations as compared to those of aluminium–aluminium combinations. For the first time, the numerical investigation identified a new value of peak over pressure called critical pressure which initiated the matchhead composition at the minimum impact load. The results when compared with the experimental results to validate the numerical approach matched well. It is suggested that the computational approach can be used as an alternative tool to predict impact initiation parameters of an energetic compound.     

A Reynolds analogy for real component surface roughness

A new Reynolds analogy equation is presented, which is based on flow and thermal behavior in a rectangular cross-section channel with real component surface roughness. This roughness is similar to that which exists on some turbine surfaces under extreme operating conditions, or on surfaces of other industrial devices, with deposit accumulation such as heat exchangers. Skin friction coefficient, Nusselt number, Stanton number, and performance factor experimental results are given over a range of Reynolds numbers for one polished smooth surface, and for two other surfaces with different levels of irregularly shaped and irregularly distributed, three-dimensional surface roughness. The Reynolds analogy deduced from these data, with such roughness, is important because it is different from Reynolds analogy equations for surfaces with uniformly shaped elements arranged in a regular, periodic pattern.     

Splashing of molten tin droplets on a rough steel surface

We photographed the impact of molten metal droplets on a flat plate. From these images we measured droplet dimensions during spreading and counted the number of fingers around a splashing drop. Experiments were done using stainless steel substrates with average roughness of 0.06, 0.07, 0.56, and 3.45 μm respectively. The temperature of the substrate was kept at either 25 or 240 °C. Droplet diameter (2.2 mm) and impact velocity (4 m/s) were kept constant, giving a Reynolds number (Re) of 31 135 and Weber number (We) of 463.Raising substrate roughness from 0.06 to 0.56 μm enhanced the tendency of droplet to splash, whereas increasing roughness even further to 3.45 μm suppressed splashing. This behaviour was attributed to changes in droplet solidification rate with surface roughness. A simple model of droplet spreading was used to estimate thermal contact resistance between the droplet and surface. Increasing surface roughness was found to raise thermal contact resistance and reduce heat transfer from the droplet to the substrate, delaying the onset of solidification and reducing splashing. The number of fingers formed around a droplet splashing on a smooth surface could be predicted reasonably well by a model based on Rayleigh-Taylor instability theory. Increasing surface roughness reduced the number of fingers while enlarging their size.     

Numerical Analysis of Constructal Water-Cooled Microchannel Heat Sinks with Multiple Bifurcations in the Entrance Region

The Constructal Theory is applied to obtain better thermal performance from a type of microchannel heat sink. Based on a smooth, straight, rectangular microchannel heat sink (Case 1), three different configurations of constructal multiple bifurcation are designed for the entrance region of each microchannel. These types are one bifurcation (Case 2), two bifurcations with the second placed in the front part (Case 3), and two bifurcations with the second bifurcation placed in the front part (Case 4). The corresponding laminar flow and heat transfer fields are investigated numerically by means of computational fluid dynamics. The effects of the bifurcation number and length ratio on pressure drop and overall thermal resistance are observed. The overall thermal resistance for the four microchannel heat sinks is compared when subjected to pumping power. It is found that designing one or two bifurcations (Cases 2, 3, 4) in the entrance region can improve thermal performance effectively. It is also recommended to place the second bifurcation in the back part (Case 4) of the microchannel heat sinks to obtain good overall thermal performance by proper design of the bifurcation position and number of channels.     

Molecular dynamics simulation of electroosmotic flow in rough nanochannels

A three-dimensional molecular dynamics model of electroosmotic flow in rough nanochannels is developed and numerically analyzed to investigate the role of surface roughness on microscale electroosmotic flow. The water and ion concentration distributions in the fluid, velocity profiles in rough nanochannels are examined and compared with the corresponding smooth nanochannel. In addition, the role of roughness height on electroosmotic velocity and zeta potential is presented. The results indicate that the electroosmotic behavior in nanochannels is sensitive to the surface roughness. The plug-like velocity in nanochannel is reduced by the presence of surface roughness, which owes to the variation in electrical double layer and additional viscous dissipation for flow past rough surface. There is a layering distribution of water molecules and Cl⁻ ions in the near wall region, and some ions and molecules are confined at the concave region due to fluid/solid interaction. In addition, increases in roughness height lead to a smaller electroosmotic velocity in bulk solution and also a smaller zeta potential.     

Numerical investigation of the influence of roughness on the laminar incompressible fluid flow through annular microchannels

The main objective of this work is to investigate, by means of numerical simulation, the effects of the surface roughness on the laminar fluid flow through annular microchannels, and to propose a method to take into account the surface roughness effects in the calculation or simulation of the fluid flow through these microchannels. This method is based on the classical viscous flow equations, and consists in building an equivalent smooth channel with the same flow resistance as the “rough” one.     

Thermal characteristics of tree-shaped microchannel nets with/without loops

Several tree-shaped microchannel networks with/without loops are numerically examined and compared for application in cooling of electronic components. The physical model of microchannel electronic cooling system is set up with tree-shaped networks. The tree-shaped microchannel nets are embedded in a disk-shaped heat sink, which is attached to a chip to remove the heat dissipated by a chip. The effects of total branching level and loops on the thermal and flow performances of heat sink system are investigated numerically. Results show that tree-shaped nets with loops provide a great advantage when the structure experiences accidental damage to one or more channel segments since the loop assures continuity of coolant flow. Under blockage of some branches, the channel networks only experience an increase of pressure drop while maintaining the capability to remove the heat generated by the chip.     

Comparison of Roughness Parameters for Various Microchannel Surfaces in Single-Phase Flow Applications

Recently, a set of new roughness parameters was proposed by Kandlikar et al. and Taylor et al. for reporting surface roughness as related to fluid flow. The average roughness R _a parameter is often used in microfluidic applications, but this parameter alone is insufficient for describing surface roughness; a specimen with deep grooves and sharp obstructions can share the same average roughness value as a relatively smooth surface with low uniform surface roughness. Because the average roughness parameter is broad, it is difficult to assess the surface topography features that result from different machining processes or etches. A profilometer and a digital microscope are used to examine the surface roughness profiles of various materials submitted to different machining techniques. The materials studied will be similar to those used for microchannels including aluminum, stainless steel, copper, and silicon. Depending on the material, these samples are submitted to several machining processes, including milling, grinding, fly cutting, and microfabrication techniques. These machining processes and microfabrication techniques are of practical interest in microfluidics applications. After studying the surface roughness patterns exhibited in these samples, the roughness parameters employed in some of the recent roughness models are evaluated. This study is expected to provide more understanding of assorted surface roughness.