Shroud heat transfer measurements from a rotating cavity with an axial throughflow of air

The paper discusses measurements of heat transfer obtained from the inside surface of the peripheral shroud. The experiments were carried out on a rotating cavity, comprising two 0.985-m-dia disks, separated by an axial gap of 0.065 m and bounded at the circumference by a carbon fiber shroud. Tests were conducted with a heated shroud and either unheated or heated disks. When heated, the disks had the same temperature level and surface temperature distribution. Two different temperature distributions were tested; the surface temperature either increased, or decreased with radius. The effects of disk, shroud, and air temperature levels were also studied. Tests were carried out for the range of axial throughflow rates and speeds: 0.0025 ≤ m ≤ 0.2 kg/s and 12.5 ≤ Ω ≤ 125 rad/s, respectively. Measurements were also made of the temperature of the air inside the cavity. The shroud Nusselt numbers are found to depend on a Grashof number, which is defined using the centripetal acceleration. Providing the correct reference temperature is used, the measured Nusselt numbers also show similarity to those predicted by an established correlation for a horizontal plate in air. The heat transfer from the shroud is only weakly affected by the disk surface temperature distribution and temperature level. The heat transfer from the shroud appears to be affected by the Rossby number. A significant enhancement to the rotationally induced free convection occurs in the regions 2 ≤ Ro ≤ 4 and Ro ≥ 20. The first of these corresponds to a region where vortex breakdown has been observed. In the second region, the Rossby number may be sufficiently large for the central throughflow to affect the shroud heat transfer directly. Heating the shroud does not appear to affect the heat transfer from the disks significantly.

Lift and the leading edge vortex

Leading edge vortices are considered to be important in generating the high lift coefficients observed in insect flight and may therefore be relevant to micro-air vehicles. A potential flow model of an impulsively started flat plate, featuring a leading edge vortex (LEV) and a trailing edge vortex (TEV) is fitted to experimental data in order to provide insight into the mechanisms that influence the convection of the LEV and to study how the LEV contributes to lift. The potential flow model fits the experimental data best with no bound circulation, which is in accordance with Kelvin's circulation theorem. The lift-to-drag ratio is well approximated by the function 'cot α' for α > 15°, which supports the tentative conclusion that shortly after an impulsive start, at post-stall angles of attack, lift is caused non-circulatory forces and by the action of the LEV as opposed to bound circulation. Copyright © 2012 by C. W. Pitt Ford.

Scaling laws for planetary dynamos

We propose new scaling laws for the properties of planetary dynamos. In particular, the Rossby number, the magnetic Reynolds number, the ratio of magnetic to kinetic energy, the Ohmic dissipation timescale and the characteristic aspect ratio of the columnar convection cells are all predicted to be power-law functions of two observable quantities: the magnetic dipole moment and the planetary rotation rate. The resulting scaling laws constitute a somewhat modified version of the scalings proposed by Christensen and Aubert. The main difference is that, in view of the small value of the Rossby number in planetary cores, we insist that the non-linear inertial term, uu, is negligible. This changes the exponents in the power-laws which relate the various properties of the fluid dynamo to the planetary dipole moment and rotation rate. Our scaling laws are consistent with the available numerical evidence. © The Authors 2013 Published by Oxford University Press on behalf of The Royal Astronomical Society.

Integrated ZnO surface acoustic wave microfluidic components for biosensor

This paper describes the novel nanocrystalline film ZnO surface acoustic wave devices, which demonstrate their great potential for the portable disease diagnostic system with integrated functions of microfluidic transport, mixing and biosensing. The devices can be easily integrated with electronic control circuitry and fabricated with low temperature process on Si, glass or even polymer substrates. The liquid convection and internal streaming patterns was easily induced by acoustic wave at signal voltages. With further increase in applied voltage to above 20V, the liquid droplet was pushed forward. Immunoreaction-based bio-detection PSA/ACT, all based on SAW devices on thin film piezoelectric ZnO on Si substrate was demonstrated. © 2009 CBMS.

Numerical analysis of LEC growth of GaAs with an axial magnetic field

A set of numerical analyses for momentum and heat transfer For a 3 in. (0.075 m) diameter Liquid Encapsulant Czochralski (LEC) growth of single-crystal GaAs with or without all axial magnetic field was carried Out using the finite-element method. The analyses assume a pseudosteady axisymmetric state with laminar floats. Convective and conductive heat transfers. radiative heat transfer between diffuse surfaces and the Navier-Stokes equations for both melt and encapsulant and electric current stream function equations Cor melt and crystal Lire considered together and solved simultaneously. The effect of the thickness of encapsulant. the imposed magnetic field strength as well as the rotation rate of crystal and crucible on the flow and heat transfer were investigated. (C) 2002 Published by Elsevier Science Ltd.

Experimental and numerical investigations on dissolution and recrystallization processes of GaSb/InSb/GaSb under microgravity and terrestrial conditions

The effects of gravity and crystal orientation on the dissolution of GaSb into InSb melt and the recrystallization of InGaSb were investigated under microgravity condition using a Chinese recoverable satellite and under normal gravity condition on earth. To investigate the effect of gravity on the solid/liquid interface and compositional profiles. a numerical simulation was carried out. The InSb crystal melted at 525 degrees C and then a part of GaSb dissolved into the InSb melt during heating to 706 degrees C and this process led to the formation of InGaSb solution. InGaSb solidified during the cooling process. The experimental and calculation results clearly show that the shape of the solid/liquid interface and compositional profiles in the solution were significantly affected by gravity. Under microgravity, as the Ga compositional profiles were uniform in the radial direction. the interfaces were almost parallel. On the contrary, for normal gravity condition, as large amounts of Ga moved up in the upper region due to buoyancy, the dissolved zone broadened towards gravitational direction. Also. during the cooling process, needle crystals of InGaSb started appearing and the value of x of InxGa1-xSb crystals increased with the decrease of temperature. The GaSb with the (111)B plane dissolved into the InSb melt much more than that of the (111)A plane. (C) 2000 Elsevier Science B.V. All rights reserved.

热学微流控系统中瞬态相变传热研究

微尺度相变传热广泛存在于微反应器、微型燃料电池、微蒸发器、微冷凝器、微热管、微汽泡执行器等微流控器件中，研究微流控系统中的相变问题对于微流控器件的设计和运行具有重要的科学意义。本文针对三类典型的微尺度相变问题，即微尺度流动沸腾、微尺度流动凝结以及微加热器上的汽泡动力学进行了深入细致的研究，实验研究中所采用的实验件均为标准MEMS微加工工艺制作，克服了常规机械加工所造成的表面粗糙度的影响。 考虑到微流控系统中大量应用交叉型、弯曲型等复杂结构的微通道，在微尺度流动沸腾研究中，设计了一种具有交错微通道结构的微流控芯片，并以丙酮为工质，对该芯片内的流动沸腾进行了研究。发现了周期为毫秒量级微时间尺度的流型结构，整个周期包括单相液体充液、两相分层流以及部分蒸干的液膜流三个阶段；在单个微通道区域，由于蒸发动量力的作用，液膜沿流动方向呈非均匀分布，蒸干首先发生在上游；由于液相弗劳德数较小，导致微通道中依然存在分层流流型。由于毕渥数较小，芯片背面温度几乎与芯片内壁面温度保持同步变化。虽然红外热像仪的响应频率较低，但仍然可以鉴别出由于流型周期性转换导致的壁面温度脉动。 在微尺度流动凝结换热研究中，为便于获取凝结过程的动态流动特性，设计了一种低高宽比的单微通道，并以水为工质，对该微通道中的流动凝结换热进行了研究。实验中采取了空气自然对流冷却和 水强制对流冷却两种冷却强度。研究发现，该微通道中的凝结换热呈周期性，其周期在毫秒量级。在通道上游入口处，存在一个呈准静止状态的长汽弹，汽弹前端周期性脱离汽泡。增加冷却强度会使汽泡的脱离频率增大，脱离直径减小；长汽弹前端周期性脱离汽泡是由于汽液界面具有较大的韦伯数。汽泡在该微通道内的运动过程中直径基本不变是由于汽泡在通道内的滞留时间远小于汽泡完全冷凝所需的总时间。 为澄清并联通道的多通道效应对微尺度凝结换热的影响，作者设计了由三个矩形通道组成的并联微通道冷凝器。研究发现，通道中的流型结构与单通道凝结过程类似，均为上游呈准静止状态的长汽弹和下游周期性的汽泡脱离。在中间通道和侧通道中，总共发现了三种不同的汽泡脱离模式，即单汽丝断裂模式、双汽丝同步断裂模式以及双汽丝非同步断裂模式。多通道效应主要表现在由于硅基固体导热的影响，三个通道中具有不同的温度分布，中间通道的温度关于其中心线成对称分布，而两侧通道中的高温区域均靠向中间通道。虽然硅具有良好的导热性，整个硅基上的温差很小，但在微尺度下，小温差依然可以导致较大的温度梯度，造成中间通道的双汽丝关于其中心线成对称分布，并且总是发生同步断裂；侧通道中的双汽丝偏向中间通道，并且在靠近中间通道的一侧汽丝总是首先发生断裂。由于温度梯度引起的Maragnoni对流效应，侧通道中的汽泡脱离后便靠向高温侧。 在微汽泡动力学研究中，设计了一种尺寸为 的Pt薄膜微加热器，研究了脉冲控制参数对微加热器上汽泡动力学特性的影响。研究发现在该微加热器上发生汽泡核化时，核化温度均达到液体的过热极限，因此为均质核化过程。在不同的脉冲控制参数下，存在三类不同的汽泡动力学特性，即（1）汽泡爆炸性生长和冷凝以及汽泡二次生长；（2）汽泡爆炸性生长继而分裂、吸引并聚合；（3）汽泡振荡生长而后持续生长并最终达到稳定状态。在第（1）类中，汽泡二次生长是由于脉冲加热过程中在玻璃基片上储存了热量；在第（2）类中，汽泡冷凝过程中的Marangoni效应导致分裂后的汽泡互相吸引并最终聚合。在第（3）类中，汽泡尺寸最终达到稳定是由于汽泡内蒸汽的发生量与汽液界面上蒸汽的凝结量相等。 本文的研究将为微反应器、微型燃料电池、微换热器、微汽泡执行器等相变微流控系统的设计和运行提供科学指导。

连续流动式聚合酶链式反应生物芯片／微装置中脱氧核糖核酸样品的驱动控制技术进展

介绍国内外连续流动式聚合酶链式反应生物芯片/微装置中脱氧核糖核酸样品的驱动控制技术进展,主要包括恒流泵(注射泵驱动和蠕动泵驱动)、旋转泵驱动、磁流体动力驱动以及自然对流驱动等。并对这几种驱动方式的优缺点作简要的讨论(引用文献43篇)。

Effect of channel surface wettability and temperature gradients on the boiling flow pattern in a single microchannel

The objective of this paper is to investigate the effects of channel surface wettability and temperature gradients on the boiling flow pattern in a single microchannel. The test section consists of a bottom silicon substrate bonded with a top glass cover. Three consecutive parts of an inlet fluid plenum, a central microchannel and an outlet fluid plenum were etched in the silicon substrate. The central microchannel had a width of 800 mu m and a depth of 30 mu m. Acetone liquid was used as the working fluid. High outlet vapor qualities were dealt with here. The flow pattern consists of a fluid triangle (shrinkage of the liquid films) and a connected long liquid rivulet, which is generated in the central microchannel in the timescale of milliseconds. The peculiar flow pattern is formed due to the following reasons: (1) the liquid rivulet tends to have a large contact area with the top hydrophilic channel surface of the glass cover, but a smaller contact area with the bottom silicon hydrophobic surface. (2) The temperature gradient in the chip width direction at the top channel surface of the glass cover not only causes the shrinkage of the liquid films in the central microchannel upstream, but also attracts the liquid rivulet populated near the microchannel centerline. (3) The zigzag pattern is formed due to the competition between the evaporation momentum forces at the vapor-liquid interfaces and the force due to the Marangoni effect. The former causes the rivulet to deviate from the channel centerline and the latter draws the rivulet toward the channel centerline. (4) The temperature gradient along the flow direction in the central microchannel downstream causes the breakup of the rivulet to form isolated droplets there. (5) Liquid stripes inside the upstream fluid triangle were caused by the small capillary number of the liquid film, at which the large surface tension force relative to the viscous force tends to populate the liquid film locally on the top glass cover surface.

Experimental investigation on TBAB clathrate hydrate slurry flows in a horizontal tube: Forced convective heat transfer behaviors

Tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is one kind of secondary refrigerants, which is promising to be applied into air-conditioning or latent-heat transportation systems as a thermal storage or cold carrying medium for energy saving. It is a solid-liquid two phase mixture which is easy to produce and has high latent heat and good fluidity. In this paper, the heat transfer characteristics of TBAB slurry were investigated in a horizontal stainless steel tube under different solid mass fractions and flow velocities with constant heat flux. One velocity region of weakened heat transfer was found. Moreover, TBAB CHS was treated as a kind of Bingham fluids, and the influences of the solid particles, flow velocity and types of flow on the forced convective heat transfer coefficients of TBAB CHS were investigated. At last, criterial correlations of Nusselt number for laminar and turbulent flows in the form of power function were summarized, and the error with experimental results was within 20%.

Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (similar to 10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (similar to 100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000-2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.

Multi-channel effect of condensation flow in a micro triple-channel condenser

Multi-channel effect is important to understand transport phenomenon in phase change systems with parallel channels. In this paper, visualization studies were performed to study the multi-channel effect in a silicon triple-channel condenser with an aspect ratio of 0.04. Saturated water vapor was pumped into the microcondenser, which was horizontally positioned. The condenser was cooled by the air natural convention heat transfer in the air environment. Flow patterns are either the annular flow at high inlet vapor pressures, or a quasi-stable elongated bubble at the microchannel upstream followed by a detaching or detached miniature bubble at smaller inlet vapor pressures. The downstream miniature bubble was detached from the elongated bubble tip induced by the maximum Weber number there. It is observed that either a single vapor thread or dual vapor threads are at the front of the elongated bubble. A miniature bubble is fully formed by breaking up the vapor thread or threads. The transient vapor thread formation and breakup process is exactly symmetry against the centerline of the center channel. In side channels, the Marangoni effect induced by the small temperature variation over the channel width direction causes the vapor thread formation and breakup process deviating from the side channel centerline and approaching the center channel. The Marangoni effect further forces the detached bubble to rotate and approach the center channel, because the center channel always has higher temperatures, indicating the multi-channel effect. 

Pool boiling heat transfer of ultra-light copper foam with open cells

High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0 mm by 12.0 mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0 mm, and porosity of 0.88 and 0.95. The surface superheats are from -20 to 190 K, and the heat fluxes reach 140 W/cm(2). The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid. Boiling curves of copper foams show three distinct regions. Region I and II are those of natural convection heat transfer, and nucleate boiling heat transfer for all the foam covers. Region III is that of either a resistance to vapor release for the 30 and 60 ppi foam covers, or a capillary-assist liquid flow towards foam cells for the 90 ppi foam covers. The value of ppi has an important effect on the thermal performance. Boiling curves are crossed between the high and low ppi foam covers. Low ppi foams have better thermal performance at low surface superheats, but high ppi foams have better one at moderate or large surface superheats and extend the operation range of surface superheats. The effects of other factors such as pool liquid temperature, foam cover thickness on the thermal performance are also discussed.

A three-dimensional analytical solution of the microheater temperature before bubble nucleation

A three-dimensional analytical solution of the microheater temperature based on heat diffusion equation is developed and compared with experimental results. Dimensionless parameters are introduced to analyze the temperature rise time and the distribution under steady state. To study the microheater temperatures before bubble nucleation, a set of working fluids and microheaters are considered. It is shown that the dimensionless time xi(-)(0) required for the temperature rise from room to 95% of the steady state temperature is about 75, not dependent on working fluids and microheaters. Heat transfer to the surrounding liquid is mainly caused by conduction, not by convection and radiation mechanisms. The microheater length affects the surface temperature uniformity, while its width influences the steady temperatures significantly, yielding the transition from heterogeneous to homogeneous nucleation mechanism from square microheaters to narrow line microheaters. 

Periodic bubble emission and appearance of an ordered bubble sequence (train) during condensation in a single microchannel

Condensation of steam in a single microchannel, silicon test section was investigated visually at low flow rates. The microchannel was rectangular in cross-section with a depth of 30 pm, a width of 800 mu m and a length of 5.0 mm, covered with a Pyrex glass to allow for visualization of the bubble formation process. By varying the cooling rate during condensation of the saturated water vapor, it was possible to control the shape, size and frequency of the bubbles formed. At low cooling rates using only natural air convection from the ambient environment, the flow pattern in the microchannel consisted of a nearly stable elongated bubble attached upstream (near the inlet) that pinched off into a train of elliptical bubbles downstream of the elongated bubble. It was observed that these elliptical bubbles were emitted periodically from the tip of the elongated bubble at a high frequency, with smaller size than the channel width. The shape of the emitted bubbles underwent modifications shortly after their generation until finally becoming a stable vertical ellipse, maintaining its shape and size as it flowed downstream at a constant speed. These periodically emitted elliptical bubbles thus formed an ordered bubble sequence (train). At higher cooling rates using chilled water in a copper heat sink attached to the test section, the bubble formation frequency increased significantly while the bubble size decreased, all the while forming a perfect bubble train flowing downstream of the microchannel. The emitted bubbles in this case immediately formed into a circular shape without any further modification after their separation from the elongated bubble upstream. The present study suggests that a method for controlling the size and generation frequency of microbubbles could be so developed, which may be of interest for microfluidic applications. The breakup of the elongated bubble is caused by the large Weber number at the tip of the elongated bubble induced by the maximum vapor velocity at the centerline of the microchannel inside the elongated bubble and the smaller surface tension force of water at the tip of the elongated bubble.