Phase change transpiration cooling based on the porous structure is one of the most potential thermal protection technologies. Compared with an active transpiration cooling system, a self-adaptive transpiration cooling system does not need additional pressure devices, which reduces the instability factors and the system's weight. Utilizing the properties of liquid phase change that generates high-pressure vapor, which expands spontaneously, this paper proposes a new self-adaptive transpiration cooling scheme with liquid phase change. To evaluate its feasibility, reliability, and cooling capability, a mathematical model and corresponding numerical strategy are firstly established and validated by theoretical analysis, then are used in simulations of transient characteristics of fluid flow and fluid-structure coupled heat transfer in the self-adaptive transpiration cooling process with various operating conditions. The analysis indicates that the self-adaptive transpiration cooling system can successfully realize the self-generation and self-pumping of vapor flow within the structure and keep vapor flow stable and timely response to time-varying heat flow. Moreover, with the shortening of structure length, self -generation of vapor flow and cooling performance are significantly strengthened and are gradually approach-ing their optimal states, which verifies this new cooling scheme's extremely high cooling potential.

Phase change transpiration cooling based on porous structure is one of the most potential thermal protection technologies. Compared with active transpiration cooling system, self-adaptive transpiration cooling system does not need additional pressure devices, which reduces the instability factors as well as the system weight. Utilizing the properties of liquid phase change generating high pressure vapor and its spontaneous expansion, this paper proposes a new self-adaptive transpiration cooling scheme with liquid phase change. To exactly evaluate its feasibility, stability and cooling capability, a mathematical model and corresponding numerical strategy are firstly established and validated by theoretical analysis, then are used in simulations of transient characteristics of fluid flow and fluid-structure coupled heat transfer in the self-adaptive transpiration cooling process with various operating conditions. The analysis indicates that the self-adaptive transpiration cooling system can successfully realize the self-generation and self-pumping of vapor flow within structure, and keep vapor flow stably and timely respond to time-varying heat flow. What's more, with the shortening of structure length, self-generation of vapor flow and cooling performance are significantly strengthened, and are gradually approaching to their optimal states, which verifies the extreme high cooling potential of this new cooling scheme.

Artificial intelligence (AI)-based intelligent working parameter setting of cooling equipment is a promising strategy for energy savings in large data centers. For this purpose, the performance curves of each piece of equipment must be obtained under variable working conditions. In the present study, a porous media computational fluid dynamics (CFD) model was established to investigate the flow and thermal performance of a finned-tube heat exchanger in a precision air conditioner (PAC) of a large data center with 3000 server racks. The temperature and velocity at the air outlet of the PAC were measured at 15 measurement points, and the results agreed well with the simulation results with a difference less than 10%. Under various inlet temperatures and flow rates of both chilled water and air, the heat transfer rate and pressure drop were studied. From the viewpoint of economical operation of the PAC, a critical flow rate was revealed for water and air, respectively. In addition, empirical formulas of the heat transfer rate of the PAC were obtained, which can facilitate energy savings by determining the optimized temperature of the chilled water and operating parameters of the pump and fan.

以数据机房某一孔板上方的冷通道及其相邻机柜中的服务器作为研究对象,采用计算流体力学分析(FLUENT)方法,对冷通道和服务器建立3种不同的CFD模型:几何实体模型,出口面采用多孔阶跃的原始模型,将服务器整体视为多孔区域的优化模型.采用实验测量数据作为3种模型的边界条件,通过仿真计算获得它们的速度场及温度场并与实验测量数据比较,且分别沿着服务器的高度方向(Z轴)和深度方向(X轴)对3种模型中服务器内部的温度进行比较.结果表明:优化模型与实验测量数据的速度场和温度场最大误差分别为3.7％和0.6％,且优化模型与实体模型的温度场的最大误差为0.3％,同时优化模型的计算时间相较于实体模型可以减少58％.

以数据中心机房某一孔板及其相邻机柜为研究对象,采用ANSYS有限元分析方法,对孔板和机柜建立3种不同的有限元模型:几何实体模型、缩小送风孔板风口面积的基础模型、将孔板视为多孔介质面并添加源项的优化模型.根据实测数据确定3种模型的边界条件,通过有限元仿真计算获得它们的压力、速度云图,将基础模型和优化模型分别与几何实体模型进行对比.结果表明,优化模型与几何实体模型的速度场和压力场的大小误差在5％以内,同时优化模型的计算时间相较于几何实体模型可以减少60％.

In this work, we experimentally investigate the impact of microdrops on diverse soft substrates at low Weber numbers. We show that the dynamic behaviors of the impinging microdrops are very different from that of macroscopic droplets reported in the literature. Whereas various impact phenomena have been identified for millimeter-sized water droplets in previous studies, we only observed regular deposition for droplets of a few tens of micrometers in the experiment. In contrast to the significant slowdown of the receding millimeter-sized droplets, no effects of substrate stiffness on the impact process have been identified at microscale. These different dynamics are explained by the different dynamic response of soft viscoelastic materials during droplet impact over different timescales. Finally, the analysis of the post-impact droplet oscillation with the theory for damped harmonic oscillators reveals that the damping coefficient is also size-dependent while the spring constant is not. A simple scaling argument was proposed to understand this finding.

设计并采用MEMS工艺制造了一种特征尺寸10μm并能产生单一稳定微米气泡的Pt薄膜加热器.在不同加热脉冲宽度条件下(100～1000 ms),用CCD摄像头和MATLAB程序定量研究了在乙醇、除气水和纳米流体中微气泡的动力学过程.研究发现:不同流体中,随着脉冲宽度的增加气泡直径增大,所需的起始功率减小.乙醇中气泡成核所需的功率最小,气泡直径最小;而纳米流体中所需功率最大,气泡直径也最大.通过表面张力与成核的理论模型对此进行了初步的理论分析.

A simple structure, high-force and high isolation metal contact RF MEMS switch was fabricated based on micro electroplating technology, and the mechanical and RF performance of the switch were measured. After release and annealing, the switch’s cantilever beam showed a tip-up of less than 1 µm over the beam length of 485 µm, indicating a good stress control and thermal stability of the electroplated gold. The pull-in voltage and the switching characteristics at different temperatures, ranging from 25 to 90 °C, were investigated. A fast, stable and temperature independent switching process was observed as the actuation voltage was 10% larger than the pull-in voltage. Based on the classical cantilever beam model, both the electrostatic actuation force and the return force of the switch were larger than 1000 μN, and the extracted effective spring constant of the free-standing beam was 718 N/m. Cold RF tests were carried out at various temperatures by a network analyzer, and we found that temperature showed very less influence on the insertion and isolation of the switch in the present case. At 25 °C, the insertion loss was −0.13 dB at 10 GHz, and was lower than −0.3 dB over the 0.05−20 GHz frequency range. The isolation was −36 dB at 10 GHz, and was higher than −27 dB over the entire scanning frequency range. The high isolation performance is consistent with a lumped-element equivalent circuit analysis of the switch.

In this paper, we present two novel “quasi-Bayer” micro-polarizer (MP) patterns for the polarization imaging based on the division-of-focal-plane polarimeters (DoFP). Compared with the traditional equally-weighted MP pattern with four different micro-polarizers, the “quasi-Bayer” pattern requires less photo-lithography-based selective etching steps, leading to a significant reduction of the MP array's fabrication complexity. In addition, for the mainstream bilinear interpolation algorithm, the proposed “quasi-Bayer” pattern with three micro-polarizers exhibits the lowest mean square error (MSE) of 0.43%. Moreover, the “quasi-Bayer” patterns take advantages not only at the fixed illumination level, but also for different illumination levels. Reported experimental results validate the effectiveness of the “quasi-Bayer” patterns by varying the input light intensity from 13lx to 213lx.

Fiber Bragg grating is inscribed on microfiber with femtosecond laser pulses irradiation. The microfiber is fabricated by stretching a section of single mode fiber over a flame. Periodic grooves are carved on the microfiber by the laser as have been observed experimentally. The microfiber Bragg grating is demonstrated for temperature and strain sensing, and the strain sensitivity is improved with decreased diameters of the microfibers.

Significant successes have recently been reported in the study of the generation of spectrally pure state in group-velocity-matched (GVM) nonlinear crystals. However, the GVM condition can only be realized in limited kinds of crystals and at limited wavelengths. Here, we investigate pure state generation in the isomorphs of PPKTP crystal: i.e., periodically poled RTP, KTA, RTA and CTA crystals. By numerical simulation, we find that these crystals from the KTP family can generate pure photons with high spectral purity (over 0.8), wide tunability (more than 400 nm), reasonable nonlinearity at a variety of wavelengths (from 1300 nm to 2100 nm). It is also discovered that the PPCTA crystal may achieve purity of 0.97 at 1506 nm. This study may provide more and better choices for quantum state engineering at telecom wavelengths.

In this work, we experimentally investigate the impact of water droplets onto soft viscoelastic surfaces with a wide range of impact velocities. Several impact phenomena, which depend on the dynamic interaction between the droplets and viscoelastic surfaces, have been identified and analyzed. At low We, complete rebound is observed when the impact velocity is between a lower and an upper threshold, beyond which droplets are deposited on the surface after impact. At intermediate We, entrapment of an air bubble inside the impinging droplets is found on soft surfaces, while a bubble entrapment on the surface is observed on rigid surfaces. At high We, partial rebound is only identified on the most rigid surface at We≳92. Rebounding droplets behave similarly to elastic drops rebounding on superhydrophobic surfaces and the impact process is independent of surface viscoelasticity. Further, surface viscoelasticity does not influence drop spreading after impact-as the surfaces behave like rigid surfaces-but it does affect drop recoiling. Also, the postimpact drop oscillation on soft viscoelastic surfaces is influenced by dynamic wettability of these surfaces. Comparing sessile drop oscillation with a damped harmonic oscillator allows us to conclude that surface viscoelasticity affects the damping coefficient and liquid surface tension sets the spring constant of the system.

设计了一种新型薄膜热电阻温度传感器.传感器感温结构由基片(Si)/绝缘层(SiO2)/感温部(Pt)组成,Pt薄膜片以悬空的微桥连接方式搭接在SiO2片上,SiO2片也以同样的方式搭接在Si片上,以此构成两级微桥机构.较之传统温度传感器,该感温部件采用悬空布置结构可使测温过程中的热损失大为减少,并能保证温度传感器热响应的线性度和可靠性.通过ANSYS有限元软件仿真Pt薄膜片在不同厚度SiO2片下的温度分布情况.当SiO2片厚度为2μm,该传感器热响应时间常数达到最小的10 ms,与SiO2片厚度为5μm和10μm相比其时间常数减小了50％以上.研究结果表明:在温度测量过程中,SiO2片厚度对感温的Pt薄膜片热损失影响很大,在设计中应尽可能减小SiO2片厚度.

The level shifters are crucial primitives in the multi-supply voltage circuits and systems. In this paper, an energy-efficient level shifter is proposed to achieve the conversion from the subthreshold voltage to the above threshold voltage. It is a hybrid structure consisting of the Wilson current mirror and the cross-coupled level shifter. By addressing the voltage drop issue of the level shifter based on the Wilson current mirror, the leakage power is significantly reduced, with the advantage of wide input voltage range for the Wilson current mirror level shifter well-preserved. In addition, the multi-threshold CMOS (MTCMOS) technology is employed to provide more flexibility for our ultra-low power design. The reported simulation results using 65 nm CMOS process validate our proposed implementation and an ultra-low power consumption of 19.44 fJ per conversion from 0.2 V to 1.2 V at 1 MHz is achieved without the need of any intermediate power supply.

A new 600V Partial Silicon-on-Insulator (PSOI) lateral double-diffused metal-oxide-semiconductor (LDMOS) field-effect transistor with step-doped drift region (SDD) is introduced to improve breakdown voltage (BV) and reduce on-resistance (Ron). The step-doped profile induces an electric field peak in the surface of the device, which can improve the surface field distribution and the doping accommodation in the drift region. The adjusted drift region can allow higher doping concentration under the drain end which results in higher breakdown voltage, and accommodate more impurity atoms as a whole which provides more electrons to support higher current and thus reduce on-resistance. The proposed LDMOS transistor with SDD in partial PSOI (SDD-PSOI) is analyzed by 2-D numerical simulations, compared with conventional SOI (CSOI) and conventional PSOI (CPSOI) LDMOS transistors. The results indicate that the proposed structure can significantly improve BV up to 607V and reduce on-resistance by 12.6% in comparison to CPSOI.

Electroporation (EP) is a process of applying a pulsed intense electric field on the cell membrane to temporarily induce nanoscale electropores on the plasma membrane of biological cells. A nonlinear size-dependent equivalent circuit model of a single-cell electroporation system is proposed to investigate dynamic electromechanical behavior of cells on microfluidic chips during EP. This model consists of size-dependent electromechanical components of a cell, electrical components of poration media, and a microfluidic chip. A single-cell microfluidic EP chip with 3D microelectrode arrays along a microchannel is designed and fabricated to experimentally analyze the permeabilization of a cell. Predicted electrical current responses of the model are in good agreement (average error of 6%) with that of single-cell EP. The proposed model can successfully predict the time responses of transmembrane voltage, pore diameter, and pore density at four different stages of permeabilization. These stages are categorized based on electromechanical changes of the lipid membrane. The current-voltage characteristic curve of the cell membrane during EP is also investigated at different EP stages in detail. The model can precisely predict the electric breakdown of different cell lines at a specific critical cell membrane voltage of the target cell lines.

A nonlinear electromechanical coupled model is proposed to study the pore expansion of single-cell electroporation (EP) due to intense applied electric field. By introducing the strain energy accumulated in the membrane during EP, the proposed model can predict the critical transmembrane potential, the activation energy, and the equilibrium pore size. To verirfy the model, a 3D single-cell micro EP chip was developed by using micro electroplating technology, and the EP experiments were conducted on chicken red blood cells (RBC). An EP “phase diagram” for chicken RBCs was obtained, which delineates the boundary of the areas for cell lysis, electroporation and absence of electroporation. The critical transmembrane potential of EP predicted from our model shows a good agreement with the experimental results.

Under an electric field, the electric tractions acting on a cell membrane containing a pore-nucleus are investigated by using a nonlinear electromechanical coupling model, in which the cell membrane is treated as a hyperelastic material. Iterations between the electric field and the structure field are performed to reveal the electrical forces exerting on the pore region and the subsequent pore expansion process. An explicit exponential decay of the membrane's edge energy as a function of pore radius is defined for a hydrophilic pore and the transition energy as a hydrophobic pore converts to a hydrophilic pore during the initial stage of pore formation is investigated. It is found that the edge energy for the creation of an electropore edge plays an important role at the atomistic scale and it determines the hydrophobic-hydrophilic transition energy barrier. Various free energy evolution paths are exhibited, depending on the applied electric field, which provides further insight towards the electroporation (EP) phenomenon. In comparison with previous EP models, the proposed model has the ability to predict the metastable point on the free energy curve that is relevant to the lipid ion channel. In addition, the proposed model can also predict the critical transmembrane potential for the activation of an effective electroporation that is in a good agreement with previously published experimental data.

In this work, we systematically investigate the nonlinear phenomenon of an electrostatic comb driven micro scanning mirror. The system governing equation is illustrated by Mathieu equation. A numerical model is built in MatLab based on the governing equation. The scanning mirror response in both time and frequency domain is simulated. Results are verified by the investigation vehicle, a fabricated MEMS scanning mirror. The experimental and simulation results have a great agreement on showing the similar parametric response and stiffness softening. Unstable region is also observed and well validated by the model.