Effect of inundation for condensation of steam on smooth and enhanced condenser tubes

The paper presents new measurements on the effect of inundation during condensation of steam in tubes banks. Most of the data relate to wire-wrapped enhanced tubes but measurements are also reported for low-finned and smooth tubes. The technique of artificial inundation has been used where liquid is supplied above a single horizontal test condenser tube to simulate condensate draining from higher tubes. Inundation rates have been used to simulate a column of up to almost 30 tubes. The surface temperature of the condenser tube was measured at four locations around the tube using buried thermocouples. The heat transfer and hence condensation rate was determined from the mass flow rate and temperature rise from coolant. The temperature and flow rate of the simulated inundation was carefully controlled. All tests were carried out at atmospheric pressure with constant vapour downflow approach velocity and constant coolant flow rate. For the given coolant and vapour flow rates and temperatures (same for all tests), and in the absence of inundation, the vapour-side heat-transfer coefficient for the finned tube was around four times that of the smooth tube while the heat-transfer coefficient for the wire-wrapped tubes was independent of winding pitch and around 30% higher than for the smooth tube. For inundation conditions the smooth tube data are in line with the widely used Kern equation relating the heat-transfer coefficient to the depth of a tube in the bank. The heat-transfer coefficient for the finned tube was virtually unaffected by inundation up to the maximum used which was equivalent to a depth of about 20 finned tubes in a bank. At this depth level the heat-transfer coefficient for the finned tube was around six times that of the smooth tube. For the wire-wrapped tubes the deterioration in performance with increasing inundation was least for the smallest winding pitch used for which the heat-transfer coefficient fell by around 9% at an equivalent depth in a bank of 25 tubes. At this depth level the heat-transfer coefficient for the wire-wrapped tube was almost twice that of the smooth tube.     

新型钢制散热器翅片管的传热分析及优化

通过试验测得了翅片管的温度分布 ,得出这种新型散热器翅片管的自然对流换热规律。通过对试验数据的分析整理得到了肋壁平均换热系数的经验公式。分析了翅高比、翅片夹角对平均换热系数、平均辐射换热系数的影响。以金属热强度最大为目标函数 ,采用解多变量最优化问题的坐标轮换法得出了该形式翅片管的最佳结构尺寸。     

Experimental thermal behavior of a power plant reheater

The process conditions of power plant components subjected to high pressures and temperatures are essential to determine their remaining life, availability and efficiency. It is, therefore, expedient to pay special attention to critical components, such as superheater and reheater heat exchangers, headers, and main and reheated steam lines. In this paper, on-line and off-line variables of a power plant reheater that has presented problems of thickness losses and repetitive tube fissures are studied. The fissures are associated with the effect of a thermal–mechanical mechanism. Off-line measurements were taken of the following variables: pressure, temperature, velocity and composition of the gases. On-line instrumentation was completed by the installation of specific thermocouples to ascertain the temperatures in the tubes outlet. Various angles for the fuel inlet of the burners and variations in the number and location of the working burners were also assayed. As a consequence of this analysis, it can be deduced that there are important differences in the outlet temperature of the reheater tubes that decrease for lower powers. Finally, it is pointed that a non-uniform distribution of the steam flow in the reheater might be the cause of the problem.     

椭圆翅片管空冷器流动传热特性的研究

用稳态的恒壁温法对3个椭圆翅片管空冷器和1个圆翅片管空冷器的传热和阻力特性进行了研究,得到空冷器空气侧的传热与阻力性能,在相同的迎风面流速下,椭圆翅片管比圆翅片管空气侧换热系数约大3－7倍;在相同的换热系数下,椭圆翅片管比圆翅片管的压降低。     

Technological and economical analysis of water recovery in steam injected gas turbines

Steam injected gas turbines are an interesting technology for co-generation applications. In these gas turbines the heat of the exhaust gases is used to produce steam. This steam is injected in the combustion chamber, resulting in a high specific power and a high thermal efficiency. A major disadvantage of steam injected gas turbines is the large water consumption. Placing a condenser in the cycle makes it possible to recover all the injected steam. An analysis is made of different types of condensers. Condensers based on finned tubes and direct-contact-condensers are considered. The dimensions of the condensers are determined for existing steam injected gas turbines. Furthermore, the investment costs and the exploitation costs for each type are compared.     

Experiments on solid state hydrogen storage device with a finned tube heat exchanger

The absorption and desorption performances of a solid state (metal hydride) hydrogen storage device with a finned tube heat exchanger are experimentally investigated. The heat exchanger design consists of two “U” shaped cooling tubes and perforated annular copper fins. Copper flakes are also inserted in between the fins to increase the overall effective thermal conductivity of the metal hydride bed. Experiments are performed on the storage device containing 1 kg of hydriding alloy LaNi₅, at various hydrogen supply pressures. Water is used as the heat transfer fluid. The performance of the storage device is investigated for different operating parameters such as hydrogen supply pressure, cooling fluid temperature and heating fluid temperature. The shortest charging time found is 490 s for the absorption capacity of 1.2 wt% at a supply pressure of 15 bar and cooling fluid temperature and velocity of 288 K and 1 m/s respectively. The effect of copper flakes on absorption performance is also investigated and compared with a similar storage device without copper flakes.     

Performance investigation of plain and finned tube evaporatively cooled heat exchangers

The performance of two evaporatively cooled heat exchangers is investigated under similar operating conditions of air flow rates and inlet hot water temperatures. The heat exchangers are plain and plate-finned circular tube types which occupy the same volume. Spray water, which is circulated in a closed circuit, is injected onto the exposed surfaces of the tubes and fins. The contact between air and spray water results in evaporative heat transfer. The tubes are copper, 10 mm o.d. The finned configuration is constructed by introducing 0.5 mm thick copper plates between the tubes, with a total area ratio of four. A substantial increase in heat transfer takes place for the plate-finned tubes. The increase is 92–140% for air velocities from 1.66 to 3.57 m s⁻¹. A model is used to calculate the thermal performance of the plain and finned tubes assuming a constant spray water temperature in the heat exchanger. The wet-finned surfaces show low fin efficiency compared with dry surfaces. An energy index defined as the ratio of volumetric thermal conductance to air pressure drop per unit length is found to be close for the two heat exchangers. This reveals higher thermal utilisation of the occupied volume by the finned tubes with the same energy index.     

不同材质花瓣形扁通道内翅换热管的实验研究

通过实验的方法研究了一种花瓣形扁通道内翅换热管的对流换热和阻力特性，拟合了所测Re范围内对流换热和阻力实验关联式，并运用相同质量流量、相同泵功率和相同阻力降这3种准则比较了采用不同材质时翅片管与普通光管之间的传热效果。结果表明，翅片材质对换热强化效果有较大影响，无论采用哪种材质，花瓣形扁通道内翅管均有较强的换热效果，特别是在低盅条件下，强化效果更加明显。     

A technical note on application of internally finned tubes in solar parabolic trough absorber pipes

The heterogeneous incoming heat flux in solar parabolic trough absorber tubes generates huge temperature difference in each pipe section. Helical internal fins can reduce this effect, homogenising the temperature profile and reducing thermal stress with the drawback of increasing pressure drop. Another effect is the decreasing of the outer surface temperature and thermal losses, improving the thermal efficiency of the collector. The application of internal finned tubes for the design of parabolic trough collectors is analysed with computational fluid dynamics tools. Our numerical approach has been qualified with the computational estimation of reported experimental data regarding phenomena involved in finned tube applications and solar irradiation of parabolic trough collector. The application of finned tubes to the design of parabolic trough collectors must take into account issues as the pressure losses, thermal losses and thermo-mechanical stress, and thermal fatigue. Our analysis shows an improvement potential in parabolic trough solar plants efficiency by the application of internal finned tubes.     

Heat Transfer Characteristics of Laminar Flow in Internally Finned Tubes under Various Boundary Conditions

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