矩形冷却通道内超临界RP-3航空煤油对流传热数值研究

对超临界压力下RP-3航空煤油在内截面宽为4mm、高为4mm、固体壁面厚为1mm、加热段长度为500mm的水平矩形冷却通道内的对流传热特性进行了数值模拟研究。分析了通道内速度场的分布规律,讨论了热流密度、压力、进口温度对传热的影响。计算结果表明:当主流温度处于拟临界温度附近时,流体物性参数变化剧烈,导致传热系数降低,传热出现恶化。在超临界压力下,较低的热流密度、增大压力、降低进口流体温度或提高质量流速均有利于改善冷却通道内的传热性能。     

矩形通道干涸后膜态沸腾传热试验研究

在流动传热基础试验平台上进行了矩形通道干涸后膜态沸腾的传热试验,研究了各种热工水力参数对膜态沸腾传热的影响特性.结果表明:干涸后膜态沸腾是一个相对稳定的传热过程,其壁面温度不会出现明显的脉动;随着进口含汽率的增加,膜态沸腾热流密度减小,壁面温度升高,传热系数减小;随着质量流速的增大或系统压力的升高,膜态沸腾热流密度增大,壁面温度降低,传热系数增大.     

定位格架对超临界水冷堆堆芯换热的影响

在压力为25 MPa、质量流速800kg/(m~2·s)、热流密度600kW/(m2·K)的条件下,对超临界水冷堆堆芯(燃料棒直径D=8.0mm,节径比P/D=1.2)类三角形子通道内超临界水的换热特性进行研究。用结构化的六面体网格以及计算流体动力学软件ANSYS CFX对堆芯的换热情况进行了模拟。以无定位格架子通道为基准,对比阻流片型定位格架对通道内流体换热的作用。结果表明,在超临界压力下阻流片型定位格架能够明显增强换热,降低最高包壳温度。另外,不同焓值区定位格架对换热的影响存在差异。     

高热流密度回流式圆盘热沉构形设计中的应力-形变分析

采用圆心回流式微通道圆盘热沉三维模型,基于构形理论,考虑粘性耗散,研究在层流流动范围内质量流率、热流密度和微通道分支数对热沉最大热应力和最大热应变的影响。结果表明:随着质量流率的增大,热沉的最大热应力和最大形变均逐渐降低,但降低效果有所减弱;随着热流密度的增大,热沉的最大热应力和最大形变均近似呈线性增长;在相同质量流率和热流密度条件下,均是微通道分支数越多,最大热应力和最大形变越小,但微通道分支数由6增大到8对最大形变的影响相对较小。所得结果可为微通道圆盘热沉的实际热设计提供理论依据。     

工况参数对X型桁架阵列通道流动与 传热性能影响的数值研究

针对不同工况参数下桁架阵列通道冷却性能优劣的问题，采用数值方法模拟了 X型桁架阵列通道内 的流动与传热特性，分析了不同？诺数下人口湍流度和壁面热流密度对D型桁架阵列通道流动与传热性能 的影响规律。结果表明:人口湍流度从5%增大到15%时，通道摩擦系数增大了 1.69% ~3.23%，通道平均努 塞尔数提高了 2.47% ~2.57% ;壁面热流密度从3 000 W ? m-2增大到10 000 W ?m-2 时，通道摩擦系数增大 了 3.39% ~6.45%，通道平均努塞尔数降低了 11.14% ~17.44%。随着？诺数的增大，人口湍流度和壁面热 流密度对通道流动与传热性能的影响程度都有所减弱。增大人口湍流度可以较小幅度地提升通道的综合热 力性能，增大壁面热流密度则会降低通道的综合热力性能。     

颗粒状腐蚀产物在水跨临界区域近壁面传质特性的模拟

为研究(超)超临界机组汽水系统中腐蚀产物在跨临界区域内的沉积特性,以Fe3O4颗粒为典型腐蚀产物,采用RANS-DPM方法中SST k-ω湍流模型对腐蚀颗粒在直管内跨临界湍流中近壁面区域的传质特性进行了模拟,分析了颗粒传质随入口温度、热流密度和质量流量的变化规律.结果表明:最高近壁面温度的位置与超临界层的厚度有关;超临界层接近近壁面区域的边界时,近壁面颗粒质量浓度达到峰值;热流密度和质量流量的增加阻碍了颗粒向近壁面传质,而近壁面区域边界上的跨临界现象有利于颗粒向近壁面的传质.     

工质回流式微通道圆盘热沉的热有效性构形研究

建立了圆心回流式微通道圆盘热沉的三维模型,基于构形理论,考虑粘性耗散,研究了在层流流动范围内质量流率、热流密度和微通道分支数对圆心回流式微通道圆盘热沉压降、最高温度和热有效性的影响。数值计算结果表明:随着质量流率的增大,热沉压降逐渐增大,且增大幅度逐渐加大,最高温度和热有效性逐渐减小,但减小趋势逐渐趋缓;随着热流密度的增大,热沉压降基本保持不变,最高温度和热有效性近似呈线性增长;不同质量流率和不同热流密度条件下,均是微通道分支数越多,热沉压降和最高温度越小,热有效性越高,热沉整体传热性能越好。所得结果可为工质回流式微通道圆盘热沉的实际热设计提供理论支撑。     

内螺纹管中超临界水的传热数值模拟

通过数值模拟研究了超临界水在半周加热内螺纹管中的流动传热过程。采用SST k-ω湍流模型求解流固耦合换热,在压力25 MPa、质量流速600 kg/(m2•s)、热流密度分别为280 kW/m2和470 kW/m2条件下研究了螺纹高度、螺距和螺纹形状等结构参数对超临界水传热的影响,比较了全周加热和半周加热条件下螺纹结构参数对传热的影响差异,揭示了螺纹结构参数变化引起的传热强化机理。结果表明：与全周加热相比,半周加热条件下螺纹结构参数增强了对加热侧换热性能的影响,削弱了对整体平均换热性能的影响,冷侧壁面温度主要受周向导热的影响,仅与热流密度有关,不同螺纹结构参数下冷侧温度分布几乎没有变化;当浮升力准则数Bo>10-5时,优化螺纹结构对改善超临界水换热性能的效果更突出,增大螺纹高度、减小螺距能够强化换热,矩形内螺纹管的换热性能优于梯形内螺纹管;旋流是内螺纹管中超临界水传热强化的主要因素,结构参数主要通过强化边界层30相似文献   

超临界压力下航空煤油在竖直管内的传热研究

对竖直上升管内超临界压力下航空煤油的传热特性进行了实验研究。分析了不同质量流量、热流密度、压力和进口温度对超临界压力下航空煤油传热特性的影响。实验结果表明,提高质量流量或进口温度均使煤油传热效果变好。而热流密度对流体传热的影响主要在于改变了流体和壁面温度,热流密度越大,传热系数越高。压力对煤油传热影响不大,一般情况下,提高压力会恶化传热。超临界状态下,煤油物性变化很大,因此对煤油的传输和热力学性质的准确计算是研究超临界压力下传热现象的关键。利用拓展的对比态法来计算煤油的密度和传输特性,如黏度、热导率等。给出了煤油在超临界压力下的传热关联式,其计算值和实验值吻合良好。     

楔形仿螺旋肋片内冷通道的流动与换热

利用有限体积法对三维不可压缩的N-S方程进行离散,对上下表面带有错排间断性楔形肋片且对置的仿螺旋内冷通道进行了数值模拟。网格划分采用非结构化混合网格,湍流模型为kε-两方程模型,在近壁面处采用标准壁面函数法进行处理,速度和压力的耦合采用S IM PLE算法。计算获得了楔形仿螺旋肋片内冷通道在楔形肋片与主流方向夹角分别为0°、15°、30°时的三维流场分布。结果表明楔形仿螺旋肋片内冷通道的流场结构比较复杂,通道内流体流动达到了预期的仿螺旋流动效果。通道的平均努谢尔数随楔形肋片与主流夹角的增大而呈增大趋势,通道换热强度得到了明显的提高,但同时流动阻力也显著增加。     

Entropy generation in transpiration cooling of concentric spherical shells

The present paper deals with transpiration cooling of two concentric spherical shells. The analysis includes the calculation for the radial distribution of temperature and volumetric entropy generation, and the total rate of entropy generation in the thermal system. Standard air is considered as the cooling fluid. Results showed that the entropy generation increases with increasing temperature difference between the sphere surfaces. Variation of either mass flow rate or radius ratio affects volumetric entropy distribution and the total rate of entropy generation of the processes. The increase of mass flow rate or radius ratio increases the total rate of entropy generation. The performance of the system is analyzed by calculating irreversibility to heat transfer ratio at both inner and outer sphere surfaces. It was found that irreversibility to heat transfer ratio at the inner sphere surface increases with increasing mass flow rate, or decreasing radius ratio. The opposite is true for the outer sphere surface.     

Heat Transfer and Entropy Generation in Concentric/Eccentric Double-Pipe Helical Heat Exchangers

Abstract

Double-pipe helical heat exchangers are integral to contemporary mechanical refrigeration equipment. Modification of flow geometry has been widely adopted to enhance heat transfer performance of a heat exchanger. The objective of this study is to numerically investigate heat transfer and entropy generation in a double pipe helical heat exchanger with various cross-sections. A computational model for laminar convective heat transfer was developed and validated against the results from previously published literature. To capture entropy generation, the entropy balance equation for open system is adopted. Effect of inner pipe Dean number, inner pipe and annulus inlet mass flow rate ratio, eccentricity, and flow configuration (co-flow and counter-flow) were examined and discussed in light of computational results. To ensure fair comparison, the considered geometries have same inner pipe cross-section area, same annulus cross-section area, and same outer surface area of inner pipe. The results suggest that square cross-section offers best performance in term of heat transfer, pressure drop and entropy generation. In addition, concentric configuration is more appropriate for low flow rate application while eccentric outer configuration is more suitable for high flow rate application.     

Deterioration in heat transfer of endothermal hydrocarbon fuel

Numerical studies under supercritical pressure are carried out to study the heat transfer characteristics in a single-root coolant channel of the active regenerative cooling system of the scramjet engine, using actual physical properties of pentane. The relationships between wall temperature and inlet temperature, mass flow rate, wall heat flux, inlet pressure, as well as center stream temperature are obtained. The results suggest that the heat transfer deterioration occurs when the fuel temperature approaches the pseudo-critical temperature, and the wall temperature increases rapidly and heat transfer coefficient decreases sharply. The decrease of wall heat flux, as well as the increase of mass flow rate and inlet pressure makes the starting point of the heat transfer deterioration and the peak point of the wall temperature move backward. The wall temperature increment induced by heat transfer deterioration decreases, which could reduce the severity of the heat transfer deterioration. The relational expression of the heat transfer deterioration critical heat flux derives from the relationship of the mass flow rate and the inlet pressure.     

基于有限元方法的溴化锂降膜吸收传热传质的数值研究

吸收器是吸收式制冷系统的重要部件。溴化锂溶液的降膜吸收是吸收器中最常见的传质传热形式之一。通过对溴化锂溶液在降膜吸收过程中传质和传热特性的分析,使用基于有限元法的COMSOL Multiphysics软件,建立了溴化锂溶液和水蒸汽降膜吸收的物理数学模型,计算了液膜内部温度和质量分数的分布、界面处传质通量、界面处传热通量、传质传热速率和液相传质传热系数。根据计算结果分析了喷淋密度对平均传质通量、平均传热通量和平均传质传热系数的影响。结果表明:平均传质系数和平均传热系数均随着喷淋密度的增大而增大;平均传质通量和平均传热通量均随喷淋密度的增大而先增后减。     

Improved Scaling Analysis for Heat Transfer in a Circular Tube With Various Supercritical Fluids Using Computational Fluid Dynamics Simulations

The operating conditions of supercritical water cooler reactor (SCWR) are well above the critical point of water, so it is not possible to investigate its heat transfer aspects through laboratory experiments without industry-scale support. The most feasible alternative can be to scale-down the operating parameters by fluid-to-fluid scaling with a suitably chosen scaling fluid. However, it is impossible to incorporate all phenomenological factors of an intricate system like the SCWR through simple analytical scaling. This study demonstrates the limitation of fluid-to-fluid scaling in such situations and suggests the incorporation of computational fluid dynamics simulation as a subsequent step for better scaling. A scaling methodology from the published literature is adopted. Carbon dioxide and R134a have been considered as scaling fluids to identify the parameter ranges suitable for lab-scale simulation of the SCWR. A circular tube of 8 mm diameter and 1500 mm length is taken for simulation. A grid dependency test is done and the standard κ ? ? turbulence model is selected. The developed computational model showed amicable agreement with existing experimental data. Analytically scaled-down parameters failed to simulate the axial and radial temperature profiles of the prototype. Increase in wall heat flux and reduction in mass flow rate are suggested as two possible options for achieving better profile matching. The modified values of scaled parameters with respect to a particular prototypical condition are reported. Profiles with CO₂ as model fluid show better agreement with water as compared to R134a and hence this is recommended for use in lab experiments.     

Numerical Investigation of Conjugate Heat Transfer to Supercritical CO2 in a Vertical Tube-in-Tube Heat Exchanger

Conjugate heat transfer to supercritical CO₂ in a vertical tube-in-tube heat exchanger was numerically investigated. The results demonstrate that most models considered are able to reproduce the heat transfer processes qualitatively, and the Abe, Kondoh, and Nagano model shows optimal agreement with the experimental data. The influences of hot fluid mass flux and temperature of the shell side, supercritical fluid mass flux of the tube side, flow direction, and pipe diameter on conjugate heat transfer were investigated based on velocity and turbulence fields. It is concluded that hot fluid mass flux and temperature of the shell side significantly affect heat transfer of the tube side. Mixed convection is the main heat transfer mechanism for the supercritical CO₂ conjugate heat transfer process when the inner diameter of the tube is greater than 1 mm. In addition, density variation is highly significant for heat transfer of supercritical CO₂ while high viscosity hinders the distortion of the flow field and reduces deterioration in heat transfer.     

标准型定位格架对超临界水在堆芯传热和流动影响的数值研究

采用非结构化多面体网格,利用商业软件STARCCM+6.04对标准型定位格架对超临界水在反应堆堆芯子通道内流动及传热特性的影响进行了数值研究。研究结果表明,由于流动阻塞效应的影响,燃料棒覆层表面温度周向分布不均,窄缝区最高、中心区最低。标准定位格架能够强化定位格架内部传热,但会造成其下游局部传热弱化,周向温度分布差异增大,不利于降低覆层温度。标准型定位格架下游的传热弱化现象或许是超临界反应堆紧凑型堆芯的特有现象,须在堆芯设计中加以重视。