共查询到20条相似文献,搜索用时 15 毫秒
1.
Abstract The geometric shape of a passage's cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, the diameter ratio of the annular space plays an important role. The purpose of this study was to determine to what extent research has been done on convective heat transfer in smooth concentric annuli and, if possible, to improve on or contribute to existing theories. It was found that although various correlations exist, they are not in good agreement. For this study, experiments were conducted with a wide range of annular diameter ratios. The Wilson plot method was used to develop a convective heat transfer correlation for annular diameter ratios of 1.7 to 3.2. For Reynolds numbers (based on the hydraulic diameter), in the range of 4000 to 30000, the deduced correlation predicted Nusselt numbers accurately within 3% of experimental values. 相似文献
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本文基于一侧含有三个均匀分布的凸出热源竖壁、另一侧为绝热竖壁的二维垂直换热通道,研究了物性变化相比于常物性和Boussinesq假设对通道内最大温度和平均Nu数的影响。通过网格无关性验证,证实了计算程序的有效性,并修正了无量纲最大温度的公式为幂函数形式。计算结果验证了,在Gr〉7.5×10。时,变物性对非对称通道换热的必要性。 相似文献
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Minsoo Kim 《传热工程》2019,40(12):973-984
ABSTRACTThe present study investigated the evaporation heat transfer coefficients of R-446A, as a low global warming potential alternative refrigerant to R-410A. The evaporation heat transfer coefficients were obtained by measuring the wall temperature of a straight stainless tube and refrigerant pressure. The heat transfer coefficients were measured for the quality range from 0.05 to 0.95, the mass flux from 100 to 400 kg/m2s, heat flux from 10 to 30 kW/m2, and saturation temperature from 5 to 10°C. The evaporation heat transfer coefficient of R-410A was verified by comparing the measured evaporation heat transfer coefficient with the value predicted by the existing correlation. The evaporation heat transfer coefficient of R-446A was measured using a proven experimental apparatus. When the heat flux was 10 kW/m2, the evaporation heat transfer coefficient of R-446A was always higher than that of R-410A. But, when the heat flux was 30 kW/m2, the evaporation heat transfer coefficient of R-446A was measured to be lower than that of R-410A near the dry-out point. The effect of the tube diameter on the R-446A evaporation heat transfer coefficient was negligible. The effect of saturation pressure on the evaporation heat transfer coefficient was prominent in the low quality region where the nucleate boiling was dominant. 相似文献
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Gian Luca Morini 《传热工程》2019,40(9-10):695-710
ABSTRACTThe determination of local convective heat transfer coefficients in microfluidics is a very hard task. Due to the small dimensions of channels and walls, the use of conventional measurement techniques is only partially suitable in microfluidics. For this reason, a strong effort has been made during the last decades in order to propose innovative techniques which use internal (to microdevices) sensors of reduced dimensions and/or external conventional sensors. In this paper a review of the main experimental techniques proposed for the determination of the local near-wall fluid temperature, the local wall temperature, and the local fluid bulk temperature will be given by putting in evidence for each technique's positive and negative aspects as well as their actual limitations with the aim to stimulate and address the research on this topic in the near future. The problems and the limitations existing nowadays for the accurate measurements of the local thermal properties of a convective microflow demonstrate that for the analysis of microconvection experimental data have to be always integrated by a numerical modeling of the observed system. 相似文献
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Rashid A. Ahmad 《传热工程》2013,34(6):40-61
Computational fluid dynamics (CFD) analysis was used to compute effective nozzle discharge coefficients for subscale sharp-edged converging/diverging nozzles, with a variety of convergence half-angles, motor operating conditions, and two propellants with different ballistics. Convergence half-angles ranged from 10° to 90°. Analysis was conducted at total temperatures from 2,946 K (5303°R) to 3,346 K (6023°R) and total pressures ranging from 2.72 MPa (395 psia) to 20.68 MPa (3,000 psia). Area ratios (A e /A*) ranged from 7.43 to 9.39. Ratio of specific heats ( n ) ranged from 1.13 to 1.18. The maximum throat and exit Reynolds' numbers based on axial diameter ranged from 6.73 2 105 to 3.61 2 106 and 3.26 2 105 to 1.99 2 106, respectively. Present results of nozzle discharge coefficients are reported and correlated as a function of nozzle convergence half-angle ( / c ), area ratios (A e /A*), and pressure ratio (P o /P X ) for a constant divergence half-angle ( / d ) of 15°. Computed discharge coefficients ranged from 0.88 to 0.97. They are compared with theory and experimental data available in the literature. Available turbulence models with respect to grid refinements and heat transfer are discussed. Heat transfer is calculated from a modified Reynolds' analogy for laminar flow over a flat plate, the Dittus - Boelter correlation for fully developed turbulent pipe flow, and the Bartz correlation for nozzle flows, and the results are compared with available experimental data. 相似文献
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设计搭建了喷雾冷却实验台,以去离子水为冷却工质,研究了喷雾高度和流量对光滑表面和方肋表面传热系数的影响。喷雾高度从29 mm降低到10 mm,喷雾流量的变化范围为20~32 L/h。实验结果表明:喷雾高度从29 mm降低到14 mm,表面传热系数增加72%,而从14 mm降低到10 mm,表面传热系数仅增加2.7%;喷雾流量从20 L/h增加到32 L/h,表面温度降低2.4 ℃,表面传热系数增加10.6%;在相同的实验工况下,方肋表面的传热系数始终大于光滑表面。 相似文献
7.
超临界和超超临界汽轮机汽缸传热系数的研究 总被引:6,自引:2,他引:6
提出了汽轮机汽缸传热系数的计算方法。介绍了超,临界和超超临界压力汽轮机汽缸光滑内表面和安装镶片式汽封表面的对流换热表面传热系数的计算公式,安装整体车制式汽封的汽缸内表面、安装静叶的汽缸内表面和安装隔板的汽缸内表面的传热过程总传热系数的计算方法。采用圆筒壁与肋片传热等简化模型来计算汽封块、静叶和隔板的传热过程总传热系数。给出了某型号超,临界600MW汽轮机高压内缸内表面传热系数的计算结果。该方法考虑了不同运行工况下汽缸不同部位的传热过程,在超临界和超超临界压力汽轮机汽缸的温度场与热应力场的有限元法数值计算和寿命评定中,为确定传热边界条件提供了依据。 相似文献
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ABSTRACTThe present work uses a perturbation procedure to deduce the small perturbation differential equations for velocity, temperature, and the diffusion equation for nanoparticle volume concentration. Thermophysical variables are obtained from conventional means (e.g., mixture and field theory estimates) for nanofluids consisting of alumina nanoparticles dispersed in water (alumina–water nanofluid) and gold nanoparticles dispersed in water (gold–water nanofluid), and, in the case of gold–water nanofluid, molecular dynamics results are used to estimate such properties, including the transport coefficients. The very thin diffusion layer, at large Schmidt numbers, is found to have a great impact on the velocity and temperature profiles, owing to the transport property dependency and has a profound influence on surface conduction heat transfer rate enhancement and skin friction suppression for the case of nanofluid concentration withdrawal at the wall. In this case, the diffusional heat transfer rate is negligible, again, owing to the large Schmidt numbers encountered. Possible experiments directed at this interesting phenomenon are discussed. 相似文献
10.
Volker Gnielinski 《传热工程》2013,34(6):431-436
On the basis of a large number of experimental data from the literature, correlations were developed for the heat transfer coefficient for turbulent flow in concentric annular ducts. A proven correlation for heat transfer in circular tubes was extended by factors that take into consideration the effect of the diameter ratio of the annulus and the different boundary conditions for heating or cooling. 相似文献
11.
Results of an investigation into the sensitivity of natural convection heat transfer correlations with respect to relative humidity are presented. Given the relatively small values of natural convection heat transfer coefficients, small changes in the thermophysical properties can have a significant impact on the values predicted by theoretical/empirical correlations. In this study, the thermophysical properties are assumed to be those of a dry air and water vapor mixture. The mole fractions are determined as a function of relative humidity. Several widely used natural convection heat transfer correlations have been examined to determine the impact of varying the relative humidity on the predicted Nusselt number. The results show a general trend of an increasing Nusselt number with relative humidity. The results presented in this paper provide an engineering tool for obtaining accurate values of natural convection heat transfer coefficients for a moist air environment using only the thermophysical properties of dry air. 相似文献
12.
Walls' cooling of aeronautic propeller combustion chamber is performed with the injection, through the combustion chamber wall, of a part of the air coming from compressors placed upstream. Measurements of the wall thermal fields are made by infrared thermography along the injection wall. This injection wall is pierced by 9 rows of 8 holes (α=90°) in staggered configuration(p/D=s/D=6). We propose a model using two heat transfer coefficients to represent the convective exchanges. The results are non-dimensioned and presented in comparison with the case without holes. The use of this model allows us to define 4 zones. Those 4 zones exist for the 5 blowing rates. 相似文献
13.
Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on their convective heat transfer capabilities. Linear and nonlinear regression models exist for determining the heat transfer coefficients; however, these are complex and time-consuming, and require much experimental data in order to obtain accurate solutions. A large data set of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593, and 0.712 with respective hydraulic diameters of 17.01 mm, 13.84 mm, 10.88 mm, and 7.71 mm was gathered. Mean Nusselt numbers were determined using the modified Wilson plot method, a nonlinear regression scheme, and the logarithmic mean temperature difference method. These three methods presented disagreements with existing correlations based on local wall temperatures. The local Nusselt numbers were determined using the logarithmic mean temperature difference method. Local wall temperature measurements were made using a novel method that minimized obstructions within the annulus. Friction factors were calculated directly from measured pressure drops across the annuli. Both heated and cooled horizontal annuli in fully turbulent flow with Reynolds numbers based on the hydraulic diameter varying from 10,000 to 45,000 with water as the working medium were investigated. 相似文献
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Experimental measurements were carried out to compare the air-side heat transfer coefficients of three oval tubes (axis ratio 2, 3, and 4) with those of an equivalent circular tube (o. d. 18 mm). The tubes were tested as single tubes in a cross-flow of air. The range of the investigated Reynolds numbers Re D was between 1000 and 11000. The effects of the area blockage and the free stream turbulence were taken into consideration in the evaluation of the thermal performance. The measurement results indicate that Nusselt numbers Nu D for the tested circular and oval tubes are close at the lower range of the tested Reynolds numbers (Re D < 4000) corresponding to an air velocity < 4 ms ?1 in this work, which is the air velocity for most air conditioning applications. For Re D > 4000, the Nu D for the circular tube are higher than those for the oval tubes, and the Nu D for the oval tubes decrease with the increase of the axis ratio. 相似文献
16.
The two-phase convective heat transfer coefficients for nitrogen inside the flow path of plate-fin type heat exchangers operating at cryogenic temperatures are calculated using CFX Release 13.0. Using a homogeneous two-phase model, the governing equations are solved to find pressure, velocity, and enthalpy distributions for three types of fin geometries: plain, wavy, and serrated. The results are further processed to evaluate the wall shear stress and heat flux, which in turn yield the friction coefficients and convective heat transfer coefficients. The coefficients are presented as functions of system pressure, flow rate, and local quality. The results can be used for the design of plate-fin type exchangers with the same fin configurations and operating conditions as the calculation. 相似文献
17.
根据螺旋槽管的结构特点及传热特性,建立了三种不同槽口形状的螺旋槽管与光滑管换热器的三维模型。以水为工质,运用 Fluent流体分析软件,采用k-ε湍流模型,研究了三种不同槽口形状的螺旋槽管与光滑管换热器在换热过程中的速度场和温度场,得到了不同槽口形状和光滑管的壁面Nusselt数。结果表明。在相同壳程和雷诺数的情况下,螺旋槽管比光滑管的换热能力提高了6.7%-37.6%,其中三角彤槽和矩形槽螺旋槽管的换热能力提高最大,从而强化了传热。为谊产品的理论进一步研究和实验研究奠定了基础,为谊产品的设计和推广应用提供了依据。 相似文献
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采用共轭传热计算方法,对某柴油发动机废气涡轮增压器径流涡轮流场进行了数值模拟,对比了叶轮在绝热和传热条件下温度分布的差别,研究了传热边界对叶轮温度分布的影响。结果表明,绝热条件下,叶片两侧的温度存在显著的温差,并且从叶轮进口到出口也有明显的温降;而传热条件下,相同叶高下的叶片表面温度分布近乎于一条直线,压力面和吸力面的温度几乎相同;在热平衡条件下,叶轮实体内的温度范围很小,不超过10 K,温度梯度较小表明由温差引起的热应力很小。 相似文献
20.
A simplified method, successfully tested previously for flow in circular pipes, is used in this work to estimate the friction factor and Nusselt number in fully-developed laminar flow between parallel plates of non-Newtonian fluids. Both constant wall temperature and constant wall heat flux cases are considered. The methodology was tested using several constitutive equations, including generalized Newtonian fluids, such as the Herschel–Bulkley, Bingham, Casson, and Carreau–Yasuda models, and also the simplified Phan-Thien–Tanner viscoelastic model. The error of the approximate methodology was found to be small, below 3.4%, except for the fluids with yield stress for which the maximum error increased to 8.4% for the cases analyzed, which cover a wide range of shear viscosity curves. 相似文献