高速燃气喷射管道表面自然对流散热分析

将热阻比拟为等效电阻,采用有限体积元法对超音速燃气喷射管道传热及表面自然对流散热进行数值计算,得到喷射管壁温度分布和表面不同自然对流散热及参数变化对壁温的影响.由于管两端自然对流散热作用,管中部温度高于两端;随着表面自然对流换热增强管壁温度逐渐降低;选用不同材质时,随材质导热系数增大管内外壁温差线性增大.     

圆筒形设备保温层厚度优化计算

国家标准《设备及管道保温技术通则》(GB 4272)中规定,为减少保温结构散热损失的保温层厚度应按“经济厚度”的方法计算。而按《热力设备及管道保温》(87 R411-1)的经济厚度计算方法进行计算时,只考虑保温层本身热阻,未考虑保护层的热阻,有一定误差。而若设备器壁不采用金属     

保温管道经济评估的优化设计模型

本文介绍了保温管道经济评估优化设计方案和三个优化设计模型。包括保温管道的单双层结构和最佳保温材料的优化选取,最佳保温层厚度的设计计算。优化设计模型中的传热模型采用管道分段求解方式,管道散热损失的计算分成水平管和竖直管两种,并考虑了支架的散热损失。保温管道的经济模型分为单层结构和双层结构两种。     

聚氨酯预制直埋保温管道散热损失研究

针对聚氨酯预制直埋保温管道散热损失开展实验与数值模拟研究。通过实验测试了聚氨酯预制直埋保温管道的散热损失,同时对输送介质温度、聚氨酯导热系数、土壤温度及其导热系数进行了测试,对影响聚氨酯预制直埋保温管散热损失的相关因素进行分析。并根据实验数据开展数值模拟研究,分析了不同条件对聚氨酯预制直埋保温管道散热损失的影响。研究结果表明:聚氨酯预制直埋保温管道散热损失随输送介质温度的升高而增加,保温管道周围土壤温度与保温管道径向距离成反比,聚氨酯保温材料导热系数对保温管道的散热损失影响较大,土壤导热系数在1.082-1.561 W/m·K时,土壤导热系数与保温管道散热损失成正比,但对保温管道散热损失产生影响较小。     

方管保温优化设计研究

采取有限差分法和焓降法混合编程求解方管保温三维温度场,研究了方管保温复合传热机理,建立了优化设计模型,分析了最佳内、外层经济保温层厚度。研究表明:数值模拟与实测结果两者吻合良好;通过对双层保温的总费用分析,保温材料价格及性能、管内介质流速和热价是最佳内外经济保温层厚度的三大主要影响因素;在满足各约束条件下,内外保温层材料搭配应考虑适度控制高价位的保温层厚度,增厚低价位的保温层厚度,达到既可节能又减少投资和运行成本。此成果有助设计人员合理地选择保温材料并确定其最佳经济厚度,提高设计效率和设计质量,并已应用于某炉窑公司保温工程,得到了充分验证和取得良好的经济效益。     

回填土质对埋地热油管道传热影响的研究

针对不同导热系数的土样回填管沟,忽略管道轴向温降建立二维非稳态传热模型,环境温度采用周期性边界条件。利用有限容积法,对方程进行离散,数值模拟了管道投产第一年内正常运行非稳态传热过程,同时应用"焓-多孔度"技术,并考虑原油凝固潜热对温降的影响,数值模拟了管道停输非稳态温降过程。结果表明:采用导热系数相对较小的土质回填管沟,管壁及保温层外壁热流密度明显减小,而且回填区温度较高散热较慢,由计算的安全停输时间可知,回填土质导热系数越小安全停输时间越长,模拟结果符合实际。     

海水源热泵抛管式换热器管外传热特性研究

为给海水源热泵抛管式换热器的优化设计提供理论依据,通过计算流体力学(CFD)数值软件对该系统中的流动和传热过程进行数值模拟。结果表明,管内对流换热努谢尔特数Nui随抛管曲率比δ、无因次节距H_o的增大而减小;管外Nu_o随δ、H_o的增大而减小,而且δ和H_o对Nu_o的影响是复杂非线性关系。在一定模拟工况下,管外表面传热系数比管内小,热阻比管内大,换热器传热性能的决定性因素在管外,基于此,该文通过数值模拟拟合出适于南海岛礁海水源热泵抛管式换热器管外传热准则关联式。     

基于变导热系数的蒸汽长距离输送模拟

为了提高长距离输送蒸汽参数的计算精度,建立了蒸汽热力和水力耦合计算方程,并在能量方程中考虑了蒸汽流动过程中的相态变化。对于具有多层保温结构管道的节点热阻,提出了一个导热系数随温度变化的热阻计算方法,在介质流动方向上将长距离管道保温热阻进行分段计算。通过实测数据对模型进行验证,结果表明,与将平均温度作为保温材料定性温度相比,新计算模型模拟出的蒸汽温降和压降值更小且计算精度更高,其误差分别不超过1%和4%。     

壁面厚度和材料对微通道内H_2/空气燃烧特性影响的数值模拟

通过数值计算对二维平板微燃烧器内H_2/空气的预混燃烧特性进行研究,讨论了壁面厚度和材料对火焰位置、外壁面温度、燃烧效率、热循环比和散热损失比的影响.结果表明:平板的热导率越高、平板厚度越大,火焰的稳定性就越好,吹熄极限和偏斜极限就越大;平板厚度越大、热导率越高,火焰位置越靠近燃烧器的上游,且平板外壁面温度分布更加均匀,燃烧效率和热循环比更高;燃烧器的散热损失比受到火焰温度的影响,并随着火焰温度的升高而降低,但是当进气速度达到偏斜极限时,燃烧效率和火焰温度开始下降,从而导致燃烧的散热损失比开始急剧下降.     

周向非均匀热流下太阳能吸热管局部传热特性

利用数值模拟方法研究周向非均匀热流下太阳能吸热管局部传热特性,分析吸热管壁厚、熔盐入口温度、熔盐流速对局部传热性能的影响规律,结果表明:吸热管周向加热量相同,管壁越厚其外壁温度越高,管内壁热流分布越均匀。壁厚对低热流侧周向及轴向局部Nu影响较大,且低热流侧局部Nu均随壁厚的减小而增大。同一流速,熔盐入口温度越高,周向局部Nu越大。不同流速吸热管周向局部Nu均随周向热流的减小而增大,流速越大,周向局部Nu越大。不同壁厚吸热管内低热流侧Nu明显大于高热流侧Nu,平均Nu略大于高热流侧Nu,且平均Nu基本相等。     

中深层地热地埋管实际运行影响因素研究

为研究中深层地热地埋管运行的影响因素,分析西咸新区中深层地热地埋管供暖系统的长期运行结果,并结合关中地区地质数据,建立深度为2510 m的中深层地埋管换热器全尺寸模型,采用数值模拟法研究实际岩层分布下地埋管的运行、结构和材料因素对其取热能力的影响。结果表明,西咸新区某项目1号地埋管和2号地埋管两个地埋管,其平均取热功率均在310 kW以上,具有优良的取热能力。地埋管进水温度随季节变化明显,并引起用户侧负荷及热泵回水温度的波动。在结构方面,随内管径由63 mm增至125 mm,平均出口水温和换热功率分别降低1.9%和4.8%,但内管径过小将影响内管运行的安全性,综合安全和换热两方面因素,最佳内管径应选取ϕ110 × 10mm规格;随外管径由168.3 mm增至244.5 mm,平均出口水温和换热功率分别增加3.5%和9%,综合成本和换热两方面因素,最佳外管径应选取ϕ 177.8 × 19 mm规格;在运行方面,地埋管出口水温随着流量的增加而减小,换热功率随着流量增加而增加;出口水温随着进水温度的升高而上升,换热功率也随之减小。在材料方面,减小内管导热系数和增加固井材料导热系数均能增加地埋管出口水温和换热功率,考虑换热功率变化和成本因素,在工程中导热系数为0.42 W/（m∙K）的内管和导热系数为3 W/（m∙K）左右的固井材料。     

应用于日光温室墙体的相变材料热物性优化研究

建立日光温室计算传热模型,以室内空气温度和墙体内表面温度为指标,通过实验方法验证了所建立的传热模型准确性,最后分析相变材料相变温度、相变焓、导热系数、密度等热物性对室内最低温度和相变蓄热率的影响规律,确定被动式相变蓄热墙体和主-被动式相变蓄热墙体的最佳相变材料热物性,阐明了实际应用时相变材料选择原则。研究结果表明,所建立的日光温室传热模型具有较高准确性,可用于日光温室墙体相变材料热物性优化;主-被动式相变蓄热墙体最佳相变材料的相变温度为27 ℃,相变焓为200 kJ/kg,导热系数为0.35 W/（m·K）,密度为440 kg/m³,被动式相变蓄热墙体最佳相变材料的相变温度为26 ℃,相变焓为200 kJ/kg,导热系数为0.35 W/（m·K）,密度为792 kg/m³;最佳相变材料热物性应用时,2种墙体室内最低温度均可达到15.0 ℃,但是被动式相变蓄热墙体的相变蓄热率较主-被动式相变蓄热墙体减小29.5%。本研究可为相变材料在日光温室的高效利用提供参考。     

Thermal Analysis of a Pipe Insulation with a Phase Change Material: Material Selection and Sizing

This research investigates the thermal characteristics of a thermal insulation for a pipe with a phase change material (PCM) for an unsteady operating condition. A layer of the PCM located at the inner surface of the insulation is aimed to minimize the heat loss from the pipe by absorbing and storing the heat loss in the form of latent and sensible heats. A convection boundary condition is applied at the inner and outer surfaces of the insulation, and one-dimensional finite element method is utilized to solve the problem. The effectiveness of the insulation with the PCM is evaluated by comparing the heat loss to insulation without a PCM. The effect of the PCM type, the PCM layer thickness, and temperature cycle of the inner surface is studied. The results indicate that heat loss is reduced significantly when the PCM layer is used for a significant amount of time, and the heat loss is reduced more when the quantity of the PCM is increased. The temperature cycle has an insignificant influence on the thermal performance of the insulation with the PCM.     

金属直通型集热管非选择性吸收涂层段热性能研究

考虑金属连接裸管和膨胀节的影响,建立了太阳能槽式集热器能量转换数学模型,分析其吸热、散热和光热转换效率并与常规计算方式进行了对比。计算表明金属连接裸管段的热损失较大,其对集热器性能的影响较膨胀节更为显著。随着太阳直射辐射强度的降低和金属集热管温度的升高,太阳能槽式集热器光热转换效率逐渐降低。对比等同计算方式,本文所采用的差异化计算所得的吸热量较低,集热器热效率与文献中的实测值相比误差在3%左右,更加贴近实际情况。     

Vertical temperature profile in ground heat exchanger during in-situ test

Ground source heat pump systems often use closed-loop heat exchangers, which depend on the soil thermal conductivity and borehole thermal resistance as parameters. An in-situ thermal response test provides a method to estimate these two parameters. In analyzing the in-situ test, one commonly uses the mean of the measured inlet and outlet temperatures of the circulating fluid as a representative fluid temperature along the entire ground loop. This assumption is convenient but not rigorous. In this paper an analytical model of the actual vertical temperature profile in the ground loop has been developed for the late-time period of the in-situ test. With this model one can estimate the soil thermal conductivity and borehole thermal resistance without the mean temperature approximation. In addition, the model identifies dimensionless groups that guide a sensitivity study of the errors introduced by the mean temperature approximation. The error in the total thermal resistance is less than 5% if the volume flow rate is maintained above a threshold, which increases linearly with the total length of the borehole. The analysis also indicates a p-linear averaging method gives smaller errors than the mean temperature approximation.     

附加阳光间型被动式太阳房传热量简化计算方法研究

附加阳光间型被动房传热量受室外空气温度和太阳辐射双波动扰动影响,导致其热负荷具有显著波动特征,传统稳态计算方法难以适用,而动态方法过于复杂,不适用于工程设计使用。基于此,该文采用周期反应系数法研究结构参数和被动房传热量之间的定量关系,并结合结构参数,提出附加阳光间型被动房热负荷简化计算新方法。结果表明阳光间总传热量受公用墙体构造变化影响较大,内门传热量与公用墙体热阻呈正相关,墙体传热量与墙体热阻呈负相关;同类围护结构下,不同参数组合下公用墙导热量与阳光间空气温度变化规律趋势一致,进而提出附加阳光间导热变化系数和温度变化系数,以该参数为基础建立了关键结构参数与传热量之间的多元回归模型,该简化计算方法与实测值对比结果表明吻合度较大,简化计算方法精度较高,可供实际工程设计使用。     

Numerical study on the effects of design parameters on the heat transfer performance of coaxial deep borehole heat exchanger

Deep borehole heat exchanger (DBHE) is attracting attention intensively owing to much more geothermal extraction, higher efficiency for heat pumps, and lesser land demand compared with shallow borehole heat exchanger. DBHE is usually dipped into several thousand meters in the subsurface, having a complicated heat transfer with surrounding rock–soil. However, the heat transfer characteristics below surface under different conditions are rarely studied. In this study, a numerical model considering the comprehensive effects of geothermal gradients and heat loss from inner pipe was proposed. The model was validated with experimental data and Beier analytical solution. Based on the model, the effects of primary design parameters on the heat transfer performance below surface along the pipe were investigated. The results indicate that temperature at pipe bottom increases with inlet flow rate decreasing, while the heat load cannot be extracted fully to the surface because of the heat loss of inner pipe. When the inlet flow rates decrease from 41.39 to 4.52 m³/h, the heat loss ratio increases from 25.5% to 63.7%. It is an effective way of insulating inner pipe to reduce heat loss under low inlet flow rates. Increasing the velocity in inner pipe by lessening the inner pipe diameter can also decline the heat loss well. While by this way, the increasing pumping power resulting from the higher velocity in inner pipe has to be considered. This study is significant to effective optimization of DBHE and energy conservation of buildings.     

Wall Resistance in Graphite-Block Heat Exchangers

Abstract

Accurate data have been obtained on the thermal conductivity of six grades of manufactured graphite. In particular, the effect of orientation, manufacturing process, and impregnant content on the thermal conductivity was investigated. Using the thermal conductivity data, a three-dimensional finite-element model has been set up to determine the wall resistance of a particular graphite block. The effect of the anisotropy of the extruded graphite on the wall resistance was investigated. It was found that the anisotropy can be neglected if the wall resistance is calculated by assuming that the blocks have an isotropic thermal conductivity equal to the thermal conductivity perpendicular to the direction of extrusion. Also, the phenol formaldehyde film on the surface of the heat exchanger channels was determined to have a significant thermal resistance. It was calculated that, for typical operating film coefficients with water on both sides, the overall heat transfer coefficient can be expected to increase by more than 50% if the resin layer is removed.     

稠油地面蒸汽管线热力参数影响及保温厚度优化

提高蒸汽注汽品质和优化保温层厚度是改善地面蒸汽管线热力输运、实现稠油高效开采的关键。建立了油田地面蒸汽管线热力参数计算模型和保温层厚度经济性分析模型,基于分段微元方法求解沿程蒸汽干度、热损等特性参数,分析了锅炉出口蒸汽参数和注汽流量的影响规律,并结合经济厚度法的计算原理优化了保温层厚度。结果表明：提高锅炉出口蒸汽温度和压力,沿程蒸汽干度降低速度变快,沿程热损增加变快,与锅炉出口蒸汽温度313 oC,压力10.2 MPa相比,其热损最大增加11.15%;增加注汽流量,蒸汽干度提升且随管线沿程降幅缩小,等梯度注汽流量差下,高注汽流量时蒸汽干度降幅较小,其降幅为3.58%;经济厚度为0.33m时,其热损费用同比原有厚度可降低68.22%。     

添加物对石蜡相变螺旋盘管蓄热器蓄热和放热性能的影响

对以石蜡为相变材料的螺旋盘管蓄热器的蓄热和放热性能进行了实验研究,探讨了在石蜡中添加铜粉、硅粉和不锈钢丝带对石蜡螺旋盘管蓄热器蓄热和放热性能的影响。结果表明：在蓄热过程中,随着加热时间的增加,蓄热器内的温度分布不均匀性逐渐增大;纯石蜡蓄热器内温度分布不均匀性最为严重;插入不锈钢丝带的蓄热器内温度分布最均匀。在放热过程中,纯石蜡蓄热器的出口水温下降最快;而石蜡加不锈钢丝带的蓄热器出口水温最高。