空气源热泵机组结霜问题的实验分析

基于空气源热泵在低温寒冷地区运行中遇到的结霜问题,对不同风速工况下,结霜过程中设备性能的变化进行分析,以换热量、换热系数为指标对不同翅型换热器的换热特性进行研究.实验结果显示:换热器结霜过程中,换热过程主要分为初始增加段、换热平稳段、缓慢衰减段、后期平稳等四段,结晶体在增加空气湍流度强化换热的同时,也增加了换热热阻使换...     

半干旱草原下垫面动量和感热总体输送系数参数化关系研究

本文利用锡林郭勒草原2008年春季近地层涡旋相关系统和铁塔的风、 温平均梯度观测资料, 分析了总体输送系数随梯度Richardson数的变化特征, 建立了动量总体输送系数随大气稳定度、近地层风速以及感热总体输送系数随大气稳定度和近地层气温的关系. 中性条件下, 半干旱草原植被下垫面动量总体输送系数与近地层大气动力状态之间存在明显的相互作用, 总体输送系数与近地面层风速之间满足二次曲线拟合关系; 风速较小时, 大气动力特征对地表粗糙度长度的改变不是很明显, 动量总体输送系数随气流增强而增大; 而当风速较大时, 强风速会使植被高度发生改变, 动量总体输送系数随气流增强而减小. 另外, 感热总体输送系数与近地层气温之间也存在二次曲线关系. 动量总体输送系数与近地层风速之间的关系、感热总体输送系数与近地层气温之间关系的建立为总体输送系数参数化提供了重要途径, 同时该方案避免了对动力学粗糙度长度和热力学粗糙度长度的求解. 关键词： 总体输送系数 参数化 湍流通量 相似性函数     

Flow boiling heat transfer of refrigerant R21 in microchannel heat sink

Boiling heat transfer in a refrigerant R 21 flow in a microchannel heat sink is studied. A stainless steel heat sink with a length of 120 mm contains ten microchannels with a size of 640×2050 μm at cross-section with a wall roughness of 10 μm. The local heat-transfer coefficient distribution along the heat sink length is obtained. The ranges of parameters are: mass flow from 68 to 172 kg/m²s, heat fluxes from 16 to 152 kW/m², and vapor quality from 0 to 1. The maximum values of the heat transfer coefficient are observed at the inlet of microchannels. The heat transfer coefficients decrease substantially along the length of channels under high heat flux conditions and, on the contrary, change insignificantly under low heat flux condition. A comparison with the well-known models of flow boiling heat transfer is performed and the range of applicability is defined.     

Heat Transfer Experiments in a Rotary Drum for a Variety of Granular Materials

The influence of material properties on the contact heat transfer coefficient between the covered wall surface and the solid bed was investigated. The contact heat transfer coefficients were calculated from the measured radial and circumferential temperature profiles. Experiments were carried out with six different materials, including steel spheres, animal powder, cement clinker, quartz sand, glass beads, and expanded clay. The rotational speeds were varied from 1 to 6 rpm to evaluate the influence of rotational speed on the contact heat transfer coefficient. The measured contact heat transfer coefficients were compared with four models from the literature.     

机载C波段全极化SAR海面风矢量反演理论研究及实验验证

机载全极化SAR海面风矢量反演研究对于近海岸复杂气象条件下风矢量探测具有重要意义. 本文从极化散射理论出发,通过分析全极化SAR探测数据与海面风矢量的关系, 设计了全极化SAR海面风矢量反演方案.依据机载SAR高机动性和全极化两个探测特点, 针对VV极化探测数据,提出了基于最大似然估计的海面风矢量反演方法,并设计了飞行实验方案; 针对VH极化探测数据,提出了通过带约束最优拟合的VH极化海面散射模型反演风速, 再利用CMOD5地球物理模型函数计算风向的海面风矢量反演方法. 利用机载全极化SAR探测的台风'海葵'边缘数据,开展了海面风矢量反演实验研究. 研究结果表明,两种风矢量反演方法均可不借助辅助信息,反演复杂气象条件下的海面风矢量. 前者反演风向、风速的均方根误差分别为18.0°, 1.8 m/s, 后者反演风向、风速的均方根误差分别为9.3°, 1.2 m/s,后者的反演精度优于前者. 这是因为VH极化归一化雷达截面与风向和雷达入射角无关,仅与风速密切相关, 更适合复杂气象条件下的海面风矢量反演. 关键词： 机载全极化SAR 海面风矢量 理论研究 实验验证     

Numerical simulation of tissue freezing by liquid nitrogen based cryoprobe

A model is presented to simulate the cooling processes during tumor cryosurgery with different kinds of flows through the cryoprobe. The heat flux between the cryoprobe wall and the tumor, the heat transfer coefficient under different inflow conditions are obtained numerically. The impact of the inlet mass flow rate, gas volume fraction on these parameters is investigated. It is found that the heat transfer coefficient decreased significantly when inflow changed from two-phase annular flow to droplet flow, and to gas flow. The inlet gas volume fraction and flow velocity only significantly affect the freezing ability of the probe when the inflow is gas or in droplet phase. Simulation of the tumor temperature profiles under different flow conditions show that the heat transfer coefficient is a crucial parameter in temperature prediction during cryosurgery. Results indicate that when the cryoprobe wall is assumed at a constant temperature conventionally, the cooling effect could be overestimated. It would be more reasonable to use the constant wall heat transfer coefficient to simulate the cooling progress under a specific flow.     

竖直圆管内超临界压力煤油传热特性实验研究

在压力2.5~4 MPa, 质量流量0.7~1.7 g/s, 热流密度0.06~1 MW/m2的实验条件下, 对煤油在内径1 mm, 长度300 mm竖直上升圆管内的流动与传热特性开展了实验研究, 并分析了传热系数随局部油温的变化及不同实验参数对传热的影响.结果表明, 超临界压力下煤油传热主要由自身物性和流动状态决定.超临界压力煤油传热过程大致可以分为3个区域:正常传热区传热强化区和传热恶化区.传热强化主要是湍流掺混增强和近壁面流体在拟临界温度附近物性剧烈变化的综合作用; 传热恶化则是因为壁温及近壁面流体温度远高于拟临界温度, 在近壁面发生了类似于亚临界状态下的“拟膜态沸腾”.      

水平梯度表面能材料表面上的滴状凝结换热系数

本文在均质表面的单个球缺形液滴换热模型和液滴通用尺度分布规律的基础上,结合梯度表面能材料表面的液滴分布和凝结换热特性,得到了一维水平梯度表面能材料表面上的滴状凝结换热计算式。在此基础上,研究了壁面过冷度、接触角梯度、工质物性等参数对梯度表面能材料表面滴状凝结换热性能的影响。结果表明:随着过冷度的增加和凝结工质汽化潜热的增大和表面张力的减小和接触角梯度的增大,平均表面凝结换热系数会增大。     

HL-2M装置第一壁传热结构设计及传热性能测试

在 HL-2M 第一壁传热结构设计中,利用导热管的轴向快速导热特性及较短的传热路径,将面对等离 子体的第一壁表面热量快速传至真空室内壁上。第一壁背板和真空室内壁上分别焊接导热铜块作为冷热连接端, 导热管嵌入其内,导热管与铜块之间增垫导热金属箔并用压板固定压紧,以增强接触界面传热。根据此传热结构 设计,设计加工了相应的传热性能测试试验件。通过对试验件进行传热性能测试及实验条件外推可知,试验件冷 热端面间的最大传热功率为 4kW,端面间最大对流换热系数为 6kW·m^‒2。     

新型沟槽道微热管散热性能实验研究

微热管以其效率高、响应快且无能耗,在高功率集成微电子散热方面应用广泛。针对电子器件的小型化、高能耗发展趋势,本文提出一种新型沟槽道微热管结构,对该沟槽道微热管进行稳态和瞬态热性能实验研究,研究了风速、角度、加热功率等因素对该新型热管的热性能影响规律。结果表明,该微热管在整个散热器传热上起主导作用,性能比达到0.88,冷凝端温差为0.8℃,具有良好的均温性,该微热管加热功率为140 W,空气流速1.5 m/s时,换热系数可达2 359 W/(m^2·℃),热阻为0.27℃/W;高功率状态下可保持良好的热扩散性能,有效避免微热管的热应力集中,有望高效解决集成电子器件的散热问题。     

Experimental study of heat transfer of dielectric liquid perfluorohexane at flowboiling in a microchannel heat exchanger

Presented are results of an experimental study of local heat transfer characteristics in boiling of the dielectric liquid perfluorohexane under forced convection in a horizontal microchannel heat exchanger. The experiments with a copper microchannel heat exchanger comprising 21 channels with sections of 335 × 930 μm were conducted with a mass velocity of 250 to 1000 kg/m²s and a heat flux through the outer wall of the heat exchanger of 3 to 60 W/cm². The dependence of the local heat transfer coefficient on the heat flux density on the inner wall of the microchannels was established, as well as the critical heat flux. The experimental data are compared with calculations based on known models of heat transfer.     

Transient Heat Transfer for Forced Convection Flow of Helium Gas Over a Horizontal Plate

Transient heat transfer coefficients for helium gas flowing over a horizontal plate (ribbon) were measured under wide experimental conditions. The platinum plate with a thickness of 0.1 mm was used as the test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q₀ exp(t/τ). The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 290 to 353 K, and the periods of heat generation rate, τ, ranged from 50 ms to 17 s. The surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period τ longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlation for transient heat transfer was also obtained based on the experimental data.     

浅海起伏海面下气泡层对声传播的影响

分析了起伏海面下风浪引起的气泡层对海面反射损失和对声传播的影响.一方面,气泡层会改变原来水中的声速剖面;另一方面,气泡层会对声波产生散射和吸收作用.考虑以上两方面的因素,分析了不同风速下气泡层对海面反射损失和声传播损失的影响,仿真发现,在风速大于10 m/s时,对于2 k Hz以上频率时气泡层对小掠射角下海面反射损失的影响不可忽视.在给定的水声环境中,当声源深度和接收深度都为7 m时,风速为16 m/s的风浪下生成的气泡层,在10 km处对3 k Hz的声传播损失的影响达到8.1 d B.当声源深度和接收深度都为18 m时,风速为16 m/s的风浪下生成的气泡层,在10 km处对3 k Hz的声传播损失的影响达到4 d B.     

Performance optimization of solar chimney power plant using electric/corona wind

The effect of the corona wind on the natural convection at absorber of a solar chimney power plant pilot was investigated experimentally. The aim of the study is to improve the efficiency of SCPP through enhanced the heat transfer coefficient of absorber with corona wind. The results show that corona wind enhanced the absorber convective heat transfer coefficient leading to increment in air the velocity and the output power of the SCPP. The amount of heat transfer of pilot increased more than 14.5% when applying voltage of 15 KV and the speed in chimney experienced about 72% amelioration.     

辐射平衡壁温下有限催化模型数值模拟

文章发展了高超声速飞行器辐射平衡壁温下有限催化的数值方法,将数值模拟结果与基于返回舱外形的风洞实验数据进行了对比,并进一步针对典型高超声速飞行器钝双锥研究了辐射平衡壁温下有限催化对气动热环境的影响规律.针对返回舱外形的数值实验表明,完全催化与完全非催化边界条件下壁面热流密度均与风洞实验结果偏差较大,而采用合适的有限催化模型获得的壁面热流密度与风洞实验结果符合良好.针对典型高超声速飞行器钝双锥的研究表明,在辐射平衡温度边界条件下驻点附近氧原子的催化复合系数约为0.17,氮原子的催化复合系数约为0.026,大面积区则分别降为0.005 3和0.01.在驻点热流密度方面,完全催化的壁面热流峰值比有限催化高约21%,而完全非催化的壁面热流峰值比有限催化低约29%.      

不同风速下侧向散射激光雷达的回波信号分析

对不同风速条件下,CCD侧向散射激光雷达的回波信号进行了分析.根据Mie散射原理及侧向散射激光雷达工作原理,确定了大气气溶胶浓度与激光雷达侧向散射光强的相关性;进一步考虑气溶胶浓度与风速的关联,利用实验装置获取了激光雷达侧向散射光强与风速的关系,分析了不同风速条件下侧向散射激光雷达的回波信号.在轴流风机以及自然风两种条件下进行对比实验发现,当风速范围在1~4.5 m/s时,侧向散射光强随风速的增加而增加;当风速范围在4.5~6.0 m/s时,光强随风速的增加较少.对实验结果归一化处理,得到风速范围在1~4.5 m/s时,风速每增加1 m/s,侧向散射光强在轴流风机及自然风的实验条件下分别增加了3.7%和3.9%;风速范围在4.5~6.0 m/s时,风速每增加1 m/s,侧向散射光强分别减少了3.1%和3.8%,在自然风况的各个风向上都基本符合这一变化趋势.     

HEAT TRANSFER TO DISPERSED SWIRL FLOW OF HIGH-PRESSURE WATER WITH LOW WALL SUPERHEAT

Heat transfer was studied to two-phase flow in the post-CHF dispersed flow regime under swirl conditions. Data were used for post-CHF water flow at high pressure, 15.9 MPa, inside a vertical tube with twisted-tape inserts. Data used were obtained under test conditions typical of heat exchanger operation with low wall superheats of 5-40°C. The mass flux range of the data was 910-1878 kg/m2 s, and three tape-twist ratios were used: 2.55, 5.03, and 7.52. The low wall superheat and the relatively high mass fluxes resulted in higher than predicted drop-wall heat transfer. The only existing heat transfer correlation for this flow situation was modified to include this heat exchanger parameter range.     

螺旋管内超临界氮的对流换热研究

在内径为2mm曲率为0.057的螺旋管内进行了超临界氮的对流换热实验,研究探讨了螺旋管入口温度、壁面热流密度对沿程壁面温度分布以及平均换热系数的影响,与前人关于螺旋管内常规流体流动换热的平均Nu的经验关系式进行了比较。并基于FLUENT软件进行了数值计算,并与实验结果进行了比较。分析表明,数值计算对壁面温度的预测有一定的适用性。     

熔渣颗粒碰壁的相变沉积热过程模拟

离心粒化方法在高温熔渣余热回收方面具有结构紧凑、能耗低、得到的渣粒粒径小等优势。但受到粒化仓空间限制,会出现高温熔融渣粒碰撞、黏附在壁面的现象,影响装置运行的稳定性。本文针对该问题建立了熔融渣粒碰撞壁面的三维对称模型,结合VOF(流体体积)方法和凝固/融化模型来模拟熔渣动态形变和凝固换热过程。模拟得到,两个同粒径熔融渣粒在壁面上相继沉积会出现碰撞、铺展、回缩、飞溅、稳定的动态行为。进一步,讨论了熔渣初始温度和冷却风速对其形变和凝固换热的影响。结果表明,熔渣初始温度越小,铺展因数越小,凝固所需时间延长。而冷却风速(1~3 m/s)对熔渣碰壁过程中的铺展形变和换热的影响都很微小。     

EFFECTS OF SURFACE ROUGHNESS ON FORCED CONVECTION AND FRICTION IN TRIANGULAR DUCTS

Experimental investigations had been conducted to study the forced convective heat transfer and pressure drop characteristics of the hydrodynamic fully developed turbulent flow in horizontal equilateral triangular ducts fabricated with Ike same length and hydraulic diameter but different surface roughness of 1.2, 3.0, and 11.5 μm. The experiments were performed with hydraulic diameter-based Reynolds number ranging from 7,000 to 20,000. The entire inner wall of the duct was heated uniformly, while the outer surface was thermally insulated. It was found that the variation of Stanton number (St) with friction factor (f) can be expressed by a relationship of St = C * f, where the constant (C) increases from 0.41 to 0.50 when the surface roughness is increased from 1,2 to 11.5 μm. It was also concluded that the duct with a higher surface roughness will have better heat transfer performance. Nondimensional expressions for the determination of the heat transfer coefficient and friction factor of the equilateral triangular ducts with different surface roughness were also developed.