Lateral motion and departure of vapor bubbles in nucleate pool boiling on thin wires in microgravity

A space experiment on bubble behavior and heat transfer in subcooled pool boiling phenomenon has been performed utilizing the temperature-controlled pool boiling (TCPB) device both in normal gravity in the laboratory and in microgravity aboard the 22(nd) Chinese recoverable satellite. The fluid is R113 at 0.1 MPa and subcooled by 26 degrees C nominally. A thin platinum wire of 60 mu m in diameter and 30mm in length is simultaneously used as heater and thermometer. Only the lateral motion and the departure of discrete vapor bubbles in nucleate pool boiling are reported and analyzed in the present paper. A scale analysis on the Marangoni convection surrounding a bubble in the process of subcooled nucleate pool boiling leads to formulas of the characteristic velocity of the lateral motion and its observability. The predictions consist with the experimental observations. Considering the Marangoni effect, a new qualitative model is proposed to reveal the mechanism underlying the bubble departure processes and a quantitative agreement can also be acquired.

Subcooled pool boiling on thin wire in microgravity

A new set of experimental data of subcooled pool boiling on a thin wire in rnicrogravity aboard the 22nd Chinese recoverable satellite is reported in the present paper. The temperature-control led heating method is used. The results of the experiments in normal gravity before and after the flight experiment are also presented, and compared with those in microgravity. The working fluid is degassed R113 at 0.1 MPa and subcooled by 26 degrees C nominally. A thin platinum wire of 60 mu m in diameter and 30 mm in length is simultaneously used as heater and thermometer. It is found that the heat transfer of nucleate pool boiling is slightly enhanced in microgravity comparing with those in normal gravity. It is also found that the correlation of Lienhard and Dhir can predict the CHF with good agreement, although the range of the dimensionless radius is extended by three or more decades above the originally set limit. Three critical bubble diameters are observed in microgravity, which divide the observed vapor bubbles into four regimes with different sizes. Considering the Marangoni effect, a qualitative model is proposed to reveal the mechanism underlying the bubble departure processes, and a quantitative agreement can also be acquired.

Dynamics of discrete bubble in nucleate pool boiling on thin wires in microgravity

A space experiment on bubble behavior and heat transfer in subcooled pool boiling phenomenon has been performed utilizing the temperature-controlled pool boiling (TCPB) device both in normal gravity in the laboratory and in microgravity aboard the 22(nd) Chinese recoverable satellite. The fluid is degassed R113 at 0.1 MPa and subcooled by 26 degrees C nominally. A thin platinum wire of 60 mu m in diameter and 30 mm in length is simultaneously used as heater and thermometer. Only the dynamics of the vapor bubbles, particularly the lateral motion and the departure of discrete vapor bubbles in nucleate pool boiling are reported and analyzed in the present paper. It's found that these distinct behaviors can be explained by the Marangoni convection in the liquid surrounding vapor bubbles. The origin of the Marangoni effect is also discussed.

Coupling Of Evaporation And Thermocapillary Convection In A Liquid Layer With Mass And Heat Exchanging Interface

We propose and analyse a new model of thermocapillary convection with evaporation in a cavity subjected to horizontal temperature gradient, rather than the previously studied model without evaporation. The pure liquid layer with a top free surface in contact with its own vapour is considered in microgravity condition. The computing programme developed for simulating this model integrates the two-dimensional, time-dependent Navier-Stokes equations and energy equation by a second-order accurate projection method. We focus on the coupling of evaporation and thermocapillary convection by investigating the influence of evaporation Biot number and Marangoni number on the interfacial mass and heat transfer. Three different regimes of the coupling mechanisms are found and explained from our numerical results.

Basic equations for suspension flows with interphase mass-transfer

In the case of suspension flows, the rate of interphase momentum transfer M(k) and that of interphase energy transfer E(k), which were expressed as a sum of infinite discontinuities by Ishii, have been reduced to the sum of several terms which have concise physical significance. M(k) is composed of the following terms: (i) the momentum carried by the interphase mass transfer; (ii) the interphase drag force due to the relative motion between phases; (iii) the interphase force produced by the concentration gradient of the dispersed phase in a pressure field. And E(k) is composed of the following four terms, that is, the energy carried by the interphase mass transfer, the work produced by the interphase forces of the second and third parts above, and the heat transfer between phases. It is concluded from the results that (i) the term, (-alpha-k-nabla-p), which is related to the pressure gradient in the momentum equation, can be derived from the basic conservation laws without introducing the "shared-pressure presumption"; (ii) the mean velocity of the action point of the interphase drag is the mean velocity of the interface displacement, upsilonBAR-i. It is approximately equal to the mean velocity of the dispersed phase, upsilonBAR-d. Hence the work terms produced by the drag forces are f(dc) . upsilonBAR-d, and f(cd) . upsilonBAR-d, respectively, with upsilonBAR-i not being replaced by the mean velocity of the continuous phase, upsilonBAR-c; (iii) by analogy, the terms of the momentum transfer due to phase change are upsilonBAR-d-GAMMA-c, and upsilonBAR-d-GAMMA-d, respectively; (iv) since the transformation between explicit heat and latent heat occurs in the process of phase change, the algebraic sum of the heat transfer between phases is not equal to zero. Q(ic) and Q(id) are composed of the explicit heat and latent heat, so that the sum Q(ic) + Q(id)) is equal to zero.

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。

种子汽泡传热原理与实验研究

两相流动不稳定性是在各种水力学直径通道内所遇到的复杂现象，这些不稳定性对热能系统可能造成有害影响。本文以丙酮为工质，在受热微通道入口处设置微汽泡发生器，采用脉冲电压激励产生可控微汽泡，并以微汽泡为种子尝试对热微通道内沸腾不稳定性及传热的控制。实验研究表明低频种子汽泡热控能够减轻各参数的脉动幅度；高频种子汽泡热控能彻底抑制沸腾不稳定性；种子汽泡技术的优点是能够在各发泡频率下抑制沸腾不稳定性，强化换热。本研究结果为解决微通道内沸腾不稳定性提供了一种崭新有效的手段，大大延长了待冷却芯片的使用寿命。

Numerical simulations of interrupted and conventional microchannel heat sinks

We provide three-dimensional numerical simulations of conjugate heat transfer in conventional and the newly proposed interrupted microchannel heat sinks. The new microchannel heat sink consists of a set of separated zones adjoining shortened parallel microchannels and transverse microchambers. Multi-channel effect, physical property variations, and axial thermal conduction are considered. It is found that flow rate variations in different channels can be neglected, while heat received by different channels accounts for 2% deviations from the averaged value when the heat flux at the back surface of the silicon chip reaches 100 W/cm(2). The computed hydraulic and thermal boundary layers are redeveloping in each separated zone due to shortened flow length for the interrupted microchannel heat sink. The periodic thermal developing flow is responsible for the significant heat transfer enhancement. Two effects influence pressure drops across the newly proposed microchannel heat sink. The first one is the pressure recovery effect in the microchamber, while the second one is the head loss when liquid leaves the microchamber and enters the next zone. The first effect compensates or suppresses the second one, leading to similar or decreased pressure drop than that for the conventional microchannel heat sink, with the fluid Prandtl number larger than unity.

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. 

Combined Effects of Hot Curing Conditions and Reaction Heat on Rubber Vulcanization Efficiency and Vulcanizate Uniformity

A mathematical model of the chemical kinetics of silicone rubber Vulcanization is developed, with the thermal effects being computed using the increment method, and the hot Vulcanization process estimated with the finite element method. The results show that the reaction heat of rubber vulcanization is important for energy saving, and that a proper curing medium temperature is important when considering both vulcanization efficiency and vulcanizate uniformity. The results also indicate that increases in the forced convective heat transfer coefficient have no significant effect above a certain level. The validity of the numerical model is indirectly proven by comparison with existing data.

Effect of ambient pressure and radiation reabsorption of atmosphere on the flame spreading over thermally thin combustibles in microgravity

For the flame spread over thermally thin combustibles in an atmosphere, if the atmosphere cannot emit and absorb the thermal radiation (e.g. for atmosphere Of O-2-N-2), the conductive heat transfer from the flame to the fuel surface dominates the flame spread at lower ambient atmosphere. As the ambient pressure increases, the flame spread rate increases, and the radiant heat transfer from the flame to the fuel surface gradually becomes the dominant driving force for the flame spread. In contrast, if the atmosphere is able to emit and absorb the thermal radiation (e.g. for atmosphere Of O-2-CO2), at lower pressure, the heat transfer from flame to the fuel surface is enhanced by the radiation reabsorption of the atmosphere at the leading edge of the flame, and both conduction and thermal radiation play important roles in the mechanism of flame spread. With the increase in ambient pressure, the oxygen diffuses more quickly from ambient atmosphere into the flame, the chemical reaction in the flame is enhanced, and the flame spread rate increases. When the ambient pressure is greater than a critical value, the thermal radiation from the flame to the solid surface is hampered by the radiation reabsorption of ambient atmosphere with the further increase in ambient pressure. As a result, with the increase in ambient pressure, the flame spread rate decreases and the heat conduction gradually dominates the flame spread over the fuel surface.

Visualization of dust explosion under microgravity conditions

The objective of this work was to apply visualization methods to the experimental study of cornstarch dust-air mixture combustion in a closed vessel volume under microgravity conditions. A dispersion system with a small scale of turbulence was used in the experiments. A gas igniter initiated combustion of the dust-air mixture in the central or top part of the vessel. Flame propagation through the quiescent mixture was recorded by a high-speed video camera. Experiments showed a very irregular flame front and irregular distribution of the regions with local reactions of re-burning behind the flame front. at a later stage of combustion. Heat transfer from the hot combustion products to the walls is shown to have an important role in the combustion development. The maximum pressure and maximum rate of pressure rise were higher for flame propagation from the vessel center than for flame developed from the top pan of the vessel. The reason for smaller increase of the rate of pressure rise, for the flame developed from the top of the vessel. in comparison with that developed from the vessel center, was much faster increase of the contact surface of the combustion gases with the vessel walls. It was found that in dust flames only small part of hear was released at the flame front, the remaining part being released far behind it.

Modeling Study on the Characteristics of Laminar and Turbulent Argon Plasma Jets Impinging Normally upon a Flat Plate in Ambient Air

Modeling study is performed to compare the flow and heat transfer characteristics of laminar and turbulent argon thermal-plasma jets impinging normally upon a flat plate in ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of argon in the argon-air mixture for the laminar and the turbulent cases, respectively. Modeling results presented include the flow, temperature and argon concentration fields, the air mass flow-rates entrained into the impinging plasma jets, and the distributions of the heat flux density on the plate surface. It is found that the formation of a radial wall jet on the plate surface appreciably enhances the mass flow rate of the ambient air entrained into the laminar or turbulent plasma impinging-jet. When the plate standoff distance is comparatively small, there exists a significant difference between the laminar and turbulent plasma impinging-jets in their flow fields due to the occurrence of a large closed recirculation vortex in the turbulent plasma impinging-jet, and no appreciable difference is found between the two types of jets in their maximum values and distributions of the heat flux density at the plate surface. At larger plate standoff distances, the effect of the plate on the jet flow fields only appears in the region near the plate, and the axial decaying-rates of the plasma temperature, axial velocity and argon mass fraction along the axis of the laminar plasma impinging-jet become appreciably less than their turbulent counterparts.

Thermocapillary flow in an annular liquid layer painted on a moving fiber

In the present paper, a theoretical model is studied on the flow in the liquid annular film, which is ejected from a vessel with relatively higher temperature and painted on the moving solid fiber. A temperature gradient, driving a thermocapillary flow, is formed on the free surface because of the heat transfer from the liquid with relatively higher temperature to the environmental gas with relatively lower temperature. The thermocapillary flow may change the radii profile of the liquid film. This process analyzed is based on the approximations of lubrication theory and perturbation theory, and the equation of the liquid layer radii and the process of thermal hydrodynamics in the liquid layer are solved for a temperature distribution on the solid fiber.

Research on Dynamic Micro-Deformation under Laser Point Source

The dynamic micro-deformation of the specimen under laser point source is measured using a laser beam reflex amplifier system and numerically simulated by Msc.Marc software. Compared with experimental result and calculated result, the final deformation direction of the specimen depends on the result of the thermal strain and the phase transformation strain cooperation, away from the laser beam or towards the laser beam, the final deformation angle depends on temperature gradient in the thickness direction and the geometry constraint of the specimen. The conclusion lays the foundation for further research on the mechanism of laser bending. At the same time, it is proposed that the model of calculation based on classical Fourier heat transfer theory cannot be enough to simulate the dynamic micro-deformation of the specimen under laser point source, the model of calculation should be modified in the future.