重力热管两相传热特性影响参数的数值研究

重力热管以其极佳的传热能力广泛应用于工程领域,而对于单根热管传热性能的提升则能够更好地提高换热设备的效率。本文通过数值模拟方法,分析了重力热管参数变化对其传热特性的影响。结果表明:VOF(Volume of Fluid)模型能够捕捉重力热管内部的蒸发冷凝现象;热管热阻随着充液率或加热功率的增大而减小;在一定范围内,热管热阻随热管内径增大而减小,随冷凝段长度增大而先增大后减小,但冷凝段长度过小会导致气液循环效果变差,因此,在实际设计中应考虑热管传热极限及安全长度比,保证热管的安全运行。

Computed tomography-guided chemical renal sympathetic nerve modulation in the treatment of resistant hypertension: A case report

BACKGROUND Resistant hypertension(RH)has always been a difficult problem in clinical diagnosis and treatment.At present,there is no recognized safe and effective minimally invasive treatment.CASE SUMMARY An 80-year-old woman was admitted to hospital due to trigeminal neuralgia(TN).The patient had a history of RH for more than 10 years and her blood pressure(BP)was not well-controlled.Before the treatment for TN,we decided to perform chemical renal sympathetic denervation with ethanol in the Pain Department of our hospital.One year after the operation,she stopped taking antihypertensive drugs,and her BP was satisfactorily controlled within 4 years after surgery.CONCLUSION Computed tomography-guided chemical renal sympathetic modulation may be a feasible method for the treatment of RH.

Evaluation of polarization field in InGaN/GaN multiple quantum well structures by using electroluminescence spectra shift

In order to investigate the inherent polarization intensity in InGaN/GaN multiple quantum well(MQW) structures,the electroluminescence(EL) spectra of three samples with different GaN barrier thicknesses of 21.3 nm, 11.4 nm, and 6.5 nm are experimentally studied. All of the EL spectra present a similar blue-shift under the low-level current injection,and then turns to a red-shift tendency when the current increases to a specific value, which is defined as the turning point.The value of this turning point differs from one another for the three InGaN/GaN MQW samples. Sample A, which has the GaN barrier thickness of 21.3 nm, shows the highest current injection level at the turning point as well as the largest value of blue-shift. It indicates that sample A has the maximum intensity of the polarization field. The red-shift of the EL spectra results from the vertical electron leakage in InGaN/GaN MQWs and the corresponding self-heating effect under the high-level current injection. As a result, it is an effective approach to evaluate the polarization field in the InGaN/GaN MQW structures by using the injection current level at the turning point and the blue-shift of the EL spectra profiles.

Blast wave characteristics of multi-layer composite charge:Theoretical analysis,numerical simulation,and experimental validation

This article investigates the characteristics of shock wave overpressure generated by multi-layer composite charge under different detonation modes.Combining dimensional analysis and the explosion mechanism of the charge,a peak overpressure prediction model for the composite charge under singlepoint detonation and simultaneous detonation was established.The effects of the charge structure and initiation method on the overpressure field characteristics were investigated in AUTODYN simulation.The accuracy of the prediction model and the reliability of the numerical simulation method were subsequently verified in a series of static explosion experiments.The results reveal that the mass of the inner charge was the key factor determining the peak overpressure of the composite charge under single-point detonation.The peak overpressure in the radial direction improved apparently with an increase in the aspect ratio of the charge.The overpressure curves in the axial direction exhibited a multi-peak phenomenon,and the secondary peak overpressure even exceeded the primary peak at distances of 30D and 40D(where D is the charge diameter).The difference in peak overpressure among azimuth angles of 0-90°gradually decreased with an increase in the propagation distance of the shock wave.The coupled effect of the detonation energy of the inner and outer charge under simultaneous detonation improved the overpressure in both radial and axial directions.The difference in peak overpressure obtained from model prediction and experimental measurements was less than 16.4%.

Influence of carrier gas H2 flow rate on quality of p-type GaN epilayer grown and annealed at lower temperatures

In this work, we study the influence of carrier gas H2flow rate on the quality of p-type GaN grown and annealed at lower temperatures. It is found that the concentration of H atoms in Mg-doped GaN epilayer can effectively decrease with appropriately reducing the carrier gas H2flow rate, and a high-quality p-type GaN layer could be obtained at a comparatively low annealing temperature by reducing the carrier gas H2flow rate. Meanwhile, it is found that the intensity and wavelength of DAP peak are changed as the annealing temperature varies, which shows that the thermal annealing has a remarkable effect not only on the activation of acceptors but also on the compensation donors.

Analysis of localization effect in blue-violet light emitting InGaN/GaN multiple quantum wells with different well widths

Four blue-violet light emitting InGaN/GaN multiple quantum well（MQW） structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.

A MOVPE method for improving InGaN growth quality by pre-introducing TMIn

We propose a metal organic vapor phase epitaxy(MOVPE) method of pre-introducing TMIn during the growth of uGa N to improve the subsequent growth of In Ga N and discuss the impact of this method in detail. Monitoring the MOVPE by the interference curve generated by the laser incident on the film surface, we found that this method avoided the problem of the excessive In Ga N growth rate. Further x-ray diffraction(XRD), photoluminescence(PL), and atomic force microscope(AFM) tests showed that the quality of In Ga N is improved. It is inferred that by introducing TMIn in advance, the indium atom can replace the gallium atom in the reactor walls, delivery pipes, and other corners. Hence the auto-incorporation of gallium can be reduced when In Ga N is grown, so as to improve the material quality.

Numerical investigation of the interactions between a laser-generated bubble and a particle near a solid wall

The interactions between the bubbles and the particles near structures are important issues for the applications of the cavitation in the fluid machinery.To study the hidden microscopic mechanisms,a numerical method for simulating the laser-generated bubble between the solid wall and a particle is developed in this paper with considerations of the viscosities and the compressibility of the gas and the liquid phases,as well as the surface tension between them.The gas-liquid interface is tracked by the coupling level set and the volume of fluid(CLSVOF)method.The numerical results clearly reveal that the particle can influence the cavitation bubble behaviors.The potential damage of the nearby structures is numerically quantified in terms of the wall pressure,which helps better understand the synergetic effects of the particle on the cavitation.The effects of three dimensionless parameters on the wall pressure are also investigated,especially,on the peak pressure,namely,γ1(defined as L_(1)/R_(max),where L_(1)is the distance from the center of the initial bubble to the solid wall and R_(max)is the maximum bubble radius),γ_(2)(defined as L_(2)/R_(max),where L_(2)is the distance from the lower surface of the spherical particle to the initial bubble center)andθ(defined as Rp/R_(max),where Rp is the spherical particle radius).Further numerical results show that these parameters play a dominant role in determining the peak pressure.When y_(1)<1.00,the peak pressure on the solid wall during the bubble collapse is mainly resulted from the liquid jet.Whenγ1>1.00,the peak pressure is caused by the shock wave.With the increase ofθor decrease ofγ_(2),the peak pressure increases.Whenγ_(2)>2.00,the effect of the particle on the bubble behavior can be neglected.

Influence of administrative intervention on the availability,utilization, and competency for the use of defibrillators in primary hospitals

To the Editor:Sudden cardiac death is a major public health issue across the world.[1] Timely cardiopulmonary resuscitation (CPR) and defibrillation helps improve prognosis and avert the neurological sequelae of ventricular fibrillation.[2] However,there is low availability and utilization of defibrillators in China.[3] Hence,we investigated the availability and utilization of defibrillators during two consecutive years (before and after an administrative intervention) at all primary (public and private) hospitals in Suzhou city (Jiangsu province,China) to assess the effect of administrative intervention.