共查询到20条相似文献,搜索用时 15 毫秒
1.
超临界水在倾斜上升管内的非均匀传热特性数值模拟 总被引:1,自引:0,他引:1
对超临界压力水在管径为Φ32mm×3mm、长度为8000mm、倾角为20°的倾斜光管内的流动与传热特性进行数值模拟研究,重点考察超临界水在大比热区内的奇异物性变化对倾斜管内的传热特性的影响.通过与实验数据的对比,验证计算模型的正确性;计算压力P=26,34 MPa时,不同质量流速和热负荷下倾斜光管内壁温随超临界水焓值增大而变化的规律,以及倾斜光管内壁周向温度及热负荷分布的不均匀性;分析大比热区管内上下母线处内壁温度差随工质焓值变化的特性及机理,讨论大比热区水的物性变化对倾斜光管内传热不均匀性的影响;引入截面横向动能与截面相对横向动能两个变量对二次流的强度进行描述,重点考察超临界水在大比热区内的二次流的流动特点及其对倾斜管内传热特性影响的机理.并利用截面中垂线上的密度梯度定量分析二次流变化的规律,讨论超临界压力下大比热区内倾斜光管内壁温分布异常的机理. 相似文献
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This study reports an experimental investigation of evaporative heat transfer and pressure drop of R-134a flowing downward inside vertical corrugated tubes with different corrugation pitches. The double tube test section is 0.5 m long with refrigerant flowing in the inner tube and hot water flowing in the annulus. The inner tubes are comprised of one smooth tube and three corrugated tubes with different corrugation pitches of 6.35, 8.46, and 12.7 mm. The test runs are performed at evaporating temperatures of 10°C, 15°C, and 20°C; heat fluxes of 20, 25, and 30 kW/m2; and mass fluxes of 200, 300, and 400 kg/m2s. The experimental data obtained from the smooth tube are plotted with flow pattern map for vertical flow. Comparisons between smooth and corrugated tubes on the heat transfer and pressure drop are also discussed. It is observed that the heat transfer coefficient and frictional pressure drop obtained from the corrugated tubes are higher than those from the smooth tube. Furthermore, the heat transfer coefficient and frictional pressure drop increase as the corrugation pitch decreases. The maximum heat transfer enhancement factor and penalty factor are up to 1.22 and 4.0, respectively. 相似文献
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An experimental study was conducted to obtain a correlation for free convection heat transfer from isothermal elliptic tubes of minor to major axis ratios of ∈ = 0.53, 0.67, 0.8, and 1 confined between two adiabatic walls. Local and average Nusselt numbers were determined for several different tube axis ratios, Rayleigh numbers, and wall spacings by means of the Mach-Zehnder Interferometery (MZI) technique. For each tube axis ratio, the Rayleigh number varied from 1,000–2,750 and the wall spacing to tube minor axis ratio from 1.25–∞. For all the tube axis ratios, the periphery and length of the tubes were kept constant. Experimental data are presented with a correlation which gives the average Nusselt number as a function of the tube axis ratio, Rayleigh number, and wall spacing to tube minor axis ratio. As the tube axis ratio decreases the average heat transfer coefficient increases. Also, for a constant Rayleigh number and tube axis ratio there is an optimum wall spacing which maximizes the heat transferred from the tube. 相似文献
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Flames in narrow circular tubes 总被引:1,自引:0,他引:1
We examine an axi-symmetric deflagration located in a tube with thermally active walls. It is noted that the flame-in-tube configuration defines a classical edge-flame, a flame in a shear flow for which there is a water-shed solution for a critical value of the Damköhler number (D), ignition front solutions for larger values of D, and failure wave solutions for smaller values. We examine semi-infinite tubes with a heat flux imposed at the tube wall ends, to simulate experiments reported in the 30th Symposium. We identify parameters for which stable solutions are obtained at certain flow rates, but unstable solutions are generated at higher flow rates, followed by stable solutions at still higher flow rates. These solutions are consistent with the experimental record. Instability leads either to regular oscillations or to a violent process characterized by quenching and re-ignition. 相似文献
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Yashasvi Laxminarayan Peter Arendt Jensen Hao Wu Flemming Jappe Frandsen Bo Sander Peter Glarborg 《Proceedings of the Combustion Institute》2019,37(3):2689-2696
Fly ash deposition on boiler surfaces is a major operational problem encountered in biomass-fired boilers. Understanding deposit formation, and developing modelling tools, will allow improvements in boiler efficiency and availability. In this study, deposit formation of a model biomass ash species (K2Si4O9) on steel tubes, was investigated in a lab-scale Entrained Flow Reactor. K2Si4O9 was injected into the reactor, to form deposits on an air-cooled probe, simulating deposit formation on superheater tubes in boilers. The influence of flue gas temperature (589 – 968°C), probe surface temperature (300 – 550°C), flue gas velocity (0.7 – 3.5?m/s), fly ash flux (10,000 – 40,000?g/m2h), and probe residence time (up to 60?min) was investigated. The results revealed that increasing flue gas temperature and probe surface temperature increased the sticking probability of the fly ash particles, thereby increasing the rate of deposit formation. However, increasing flue gas velocity resulted in a decrease in the deposit formation rate, due to increased particle rebound. Furthermore, the deposit formation rate increased with probe residence time and fly ash flux. Inertial impaction was the primary mechanism of deposit formation, forming deposits only on the upstream side of the steel tube. A mechanistic model was developed for predicting deposit formation in the reactor. Deposit formation by thermophoresis and inertial impaction was incorporated into the model, and the sticking probability of the ash particles was estimated by accounting for energy dissipation due to particle deformation. The model reasonably predicted the influence of flue gas temperature and fly ash flux on the deposit formation rate. 相似文献
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Accurate, repeatable heat transfer and pressure-drop measurements have been made for condensation of CFC-113 with downflow inside enhanced microfin tubes and tubes containing twisted-wire inserts. In the latter case measurements have also been made for CFC-113/air mixtures. The heat transfer rate was calculated from the coolant flow rate and temperature rise, the latter measured using a 10-junction thermopile with careful attention paid to adequate coolant mixing and isothermal immersion of the thermopile leads. The surface temperature was found from thermocouples embedded in the tube wall. One plain tube, nine microfin tubes (with different fin heights, helix angles, and number of fins), and four twisted-wire inserts (with different wire densities) were tested. Enhancement ratios (i.e., vapor-side heat transfer coefficient for the enhanced tube divided by that for a smooth tube at the same vapor-side temperature difference and vapor inlet velocity) between 1.6 and 5.6 for the microfin tubes and between 1.2 and 1.6 for the twisted-wire inserts were found, with values depending on vapor-side temperature difference, vapor inlet velocity, and air inlet mole fraction in the case of CFC-113/air mixtures. The microfin tubes showed moderate pressure-drop penalties of around 50% compared to the plain tube, while the twisted-wire inserts showed increasing pressure-drop penalty with increasing wire density. 相似文献
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In this paper, transient and steady natural convection heat transfer
in an elliptical annulus has been investigated. The annulus occupies
the space between two horizontal concentric tubes of elliptic
cross-section. The resulting velocity and thermal fields are
predicted at different annulus orientations assuming isothermal
surfaces. The full governing equations of mass, momentum and energy
are solved numerically using the Fourier Spectral method. The heat
convection process between the two tubes depends on Rayleigh number,
Prandtl number, angle of inclination of tube axes and the geometry
and dimensions of both tubes. The Prandtl number and inner tube axis
ratio are fixed at 0.7 and 0.5, respectively. The problem is solved
for the two Rayleigh numbers of $10^4$ and $10^5$ considering a
ratio between the two major axes up to 3 while the angle of
orientation of the minor axes varies from $0^\circ$ to $90^\circ$. The
results for local and average Nusselt numbers are obtained and
discussed together with the details of both flow and thermal fields.
For isothermal heating conditions, the study has shown an optimum
value for major axes ratio that minimizes the rate of heat transfer
between the two tubes. Another important aspect of this paper is to
prove the successful use of the Fourier Spectral Method in solving
confined flow and heat convection problems. 相似文献
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The effect of thermal environment on boiling heat transfer performance in a section of a horizontal tube bundle was investigated using R-113 as the working fluid. The in-line tube bundle has five columns and 27 rows with a pitch-to-diameter ratio of 1.3. Heal transfer coefficients obtained from the instrumented tube in the tube bundle with only one tube heated while the other tubes remained unhealed and with all the lubes in the bundle heated are reported for a range of heat flux, pressure, mass flow rate, and quality. The results showed that heat transfer coefficient of a tube in a heated bundle is slightly higher than that in an unhealed bundle, with the variation of heat transfer coefficient decreasing as heat flux, mass flow rate, or pressure increased. It was also found that higher quality would tend to improve the heat transfer. However, the effect of quality disappeared as heat flux, mass flow rate, and pressure increased. Based on the experimental data, the mechanism of the heat transfer augment due to thermal environment was analyzed. It was proposed that fluid agitation and thin liquid film formation are two main factors for a heated bundle to have better transfer performance than an unhealed bundle, 相似文献
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Sung Hoon Kim Ik-Bu Sohn Sungho Jeong 《Applied Physics A: Materials Science & Processing》2011,103(4):1053-1057
The self-organized formation of ripples in the direction parallel to the groove during the femtosecond laser machining of
microgrooves on aluminum nitride ceramic at laser fluences much higher than the single-pulse ablation threshold is reported.
These parallel ripples are notably different from the commonly observed polarization-perpendicular ripples and are produced
in grooves having an appropriate width and depth, irrespective of laser polarization. From subsequent experiments with narrow
and wide groove widths, it could be considered that the groove walls play an important role in the formation of these parallel
ripples, possibly by confining the laser-induced plasma. 相似文献
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Raspet R Slaton WV Hickey CJ Hiller RA 《The Journal of the Acoustical Society of America》2002,112(4):1414-1422
The theory of acoustic propagation in an inert gas-condensing vapor mixture contained in a cylindrical pore with wet walls and an imposed temperature gradient is developed. It is shown that the vapor diffusion effects in the mixture are analogous to the heat diffusion effects in the thermoacoustics of inert gases, and that these effects occur in parallel with the heat diffusion effects in the wet system. The vapor diffusion effects can be expressed in terms of the thermoviscous function F(lambda) used in the theory of sound propagation of constant cross-section tubes. As such, these results can be extended to any shape parallel-walled tube. The propagation equations predict that the temperature gradient required for onset of sound amplification in a wet-walled prime mover is much lower than the corresponding temperature gradient for an inert gas prime mover. The results of a measurement of the onset temperature of a simple demonstration prime mover in air with a dry stack and with a stack wetted with water provide a qualitative verification of the theory. 相似文献
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螺旋折流片强化壳侧传热的四管模型数值模拟 总被引:2,自引:0,他引:2
针对螺旋折流片管壳式换热器的正方形布管方式,建立了相间套螺旋折流片的四管数学物理模型,利用FLUENT软件对该模型的流动与传热情况进行了数值模拟;并与光滑通道中及单管螺旋折流片模型的流动和传热结果进行了对比.结果显示旋向相反的相邻螺旋折流片所诱导的两股旋流通过相互作用可提高通道内流体流速,并有效地形成对相邻传热管外的斜向冲刷,这对于减薄边界层,促进近壁流体与主流区流体的动量和质量交换进而强化传热有明显的作用,算例显示其传热系数可比相同尺寸的光管通道中的情形提高约44%~57%. 相似文献
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Adiabatic expansion of plasma behind the reflected shock front in an electromagnetic shock tube has been verified. Electron densities and temperatures have been measured spectroscopically. Assuming complete local thermal equilibrium, the specific heat ratios of hydrogen plasmas have been determined out of expansion rate per a heavy particle directly from the adiabatic expansion law. Noticeably lower values of the ratio are determined at temperatures where ionization and recombination processes are abundant. The ratios determined in this work are in excellent agreement with theoretical values determined in an earlier work. 相似文献
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对氟利昂 R123 在水平单管外的凝结换热性能进行了试验研究,试验管为光管和五根强化管.目的是获得不凝气体对 R123 蒸气凝结时最佳肋密度的影响.试验结果表明:光管管外 Nusselt 理论值与实验数据偏差在±5%以内.对于含 8%不凝气体的 R123 在低肋管外的凝结换热,在肋密度为 1475 翅/米时可以获得最佳的换热性能.含不凝气体的 R123凝结换热系数显著下降,其管外换热系数约为纯蒸气的 20%~25%.随着肋密度的减小,不凝气体对凝结换热的影响逐渐减弱,但其最佳肋间隙仍保持不变,均为 0.32 mm. 相似文献
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A new w-type spirally fluted tube with enhanced heat transfer characteristics was developed and manufactured. The heat transfer and pressure drop characteristics of tubes with various geometric parameters were measured over a range of Reynolds numbers from 8,000 to 30,000 for water flowing horizontally through the tube with steam condensing on the outside surface. Experimental correlations for the pressure drop and the heat transfer coefficient were developed from a large amount of data using multivariable linear regression analysis. Factors influencing the heat transfer and flow were analyzed. The experiments showed that heat transfer coefficient can be increased by 3–8% with the pressure drop reduced by 5–10% compared with an ordinary spirally fluted tube. 相似文献
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Heat transfer with vapor condensation inside a longitudinally finned tube is numerically studied. The proposed model considers vapor condensation on two initial flow areas, namely, annular and rivulet. The model allows prediction of pressure difference along the tube length, vapor velocity profiles in the central channel and an interfin groove, and also a velocity profile in the condensate rivulet at the bottom of the interfin channel, local heat transfer coefficients at different fin points, and average heat transfer coefficients over tube section and length. The calculations showed that in the case of vapor condensation in longitudinally finned tubes of a small diameter it is of fundamental importance to divide the flow tube section into a central channel and interfin channels. The governing vapor velocities in these channels may differ by more than an order of magnitude. The reduced vapor velocity, used in engineering calculations, does not reflect the character of dynamic vapor impact on a condensate film on the most part of the heat transfer surface. For tubes with relatively large fins the proposed model describes vapor condensation almost completely,meanwhile, the mass vapor quality by the time of filling of the grooves reaches 0.01–0.05. The highest heat transfer intensification was obtained for “sharp fins” with a high value of the fin head curvature. Comparison of results of calculation by the model with results of the known experiments on water vapor condensation yields a good qualitative and quantitative agreement for low vapor velocities at the channel inlet (under 30 m/s). The wall thermal conductivity coefficient value affects significantly the condensation efficiency. 相似文献