Thermal and hydraulic analysis of a brazed aluminum evaporator

This paper presents an analytical/computer model to predict the performance of a brazed aluminum evaporator operating under dehumidifying conditions. The evaporator uses small hydraulic diameter, flat multi-channel tubes and louver fins. The in-tube refrigerant flow was divided into three regions including the two-phase, liquid deficient and superheat regions. For each region, correlations were selected from the open literature to calculate the local heat transfer and pressure drop. The effects of refrigerant pressure drop along tube and pressure losses at the tube entrance and exit were accounted for in the heat transfer calculations. The air-side fins were assumed to operate at the fully wet condition and the sensible heat transfer coefficient of the wet fins was assumed to be equal to that of the dry fins. The overall heat transfer coefficient was calculated using the enthalpy driving potential method. The total heat transfer rate and refrigerant pressure drop depend on the ratio of the number of tubes in the first and second passes. Parametric studies were done to illustrate selection of the preferred number of tubes per pass. The average refrigerant side heat transfer coefficient is sensitive to the dry-out vapor quality. However, the total heat transfer rate is relatively insensitive to the dry-out vapor quality. As the air inlet humidity increases, the latent and total heat transfer rates increase, but the sensible heat transfer rate decreases. The program was used to design an R-404A evaporator, for which a prototype was built and tested. The program over-predicted the evaporator capacity by 8%. The over-prediction is believed due to flow mal-distribution in the branch tubes.     

某余热锅炉爆管原因分析和改造

水循环回路的合理布置和设计对余热锅炉的安全运行具有重要意义。循环回路结构复杂容易导致并联管内的流量分配较大的不均匀性。偏差管内的流量较小 ,循环倍率较小 ,容易发生水循环停滞。当余热锅炉的热负荷很低时 ,循环流速较小 ,自然循环发生停滞 ,容易导致余热锅炉发生爆管。水循环的脉动会使并联管内循环流速和管壁温度发生脉动 ,余热锅炉的安全性下降。消除并联管内的流量分配不均匀和水循环脉动 ,合理运行 ,能够提高自然循环的可靠性 ,对余热锅炉的安全运行是有利的     

600MW锅炉机组膜式水冷壁壁温的试验研究及理论分析

在６００ＭＷ锅炉机组水冷壁热力试验的基础上,为找到使壁温发生波动的根本原因,利用有限元分析的方法对低倍率锅炉膜式水冷壁管壁温度分布随传热工况的动态变化进行了分析。分析表明：导致水冷壁管壁温度波动最根本的原因是管内传热恶化;单面受热水冷壁在管内发生传热恶化时其向火这内外壁温差随时间的波动较小,而水冷壁周向温差则随向炎侧外壁的壁温波动而剧烈波动。     

余热锅炉水平管束的设计与计算

论述了余热锅炉在低碳经济中的重要性,介绍不同余热锅炉的容量计算及结构布置,对烟道式(立式)水平蒸发管束的自然循环水动力计算进行了分析,提出了简化的确定循环特性的方法,对一些计算参数作了推荐并附有计算案例.     

Condensation heat transfer on tubes in actual flue gas (Parametric study for condensation behavior)

In order to improve boiler efficiency, latent heat recovery from the flue gas is a very important concept. Condensation heat transfer on horizontal tubes was investigated experimentally by using flue gas from a natural gas boiler. The parametric studies concerning the effects of tube stages, gas velocities, SO₂ in the flue gas, and Teflon‐lined tubes were conducted at a wide range of tube surface temperature. The heat transfer was enhanced at the second stage due to the wake turbulence of the first stage. No significant decrease of the condensation in the second stage due to the condensate falling from the first stage could be observed. Even at the high gas velocity condition, the condensation pattern was similar to that observed in previous low gas velocity experiments. The effect of SO₂ in the flue gas on the condensation behavior could not be observed. The heat and mass transfer behaviors on stainless‐steel and Teflon‐lined surfaces were well predicted with the analogy correlation. The above results suggested that the heat and mass transfer were dominated by the convection and diffusion process on the gas side. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 153–166, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10079     

分离式热管倾斜蒸发段传热特性的试验研究

采用加热无缝钢管模拟倾斜布置的分离式热管蒸发段,分析了倾角、充液量、热流密度和工作压力对其传热特性的影响,并根据试验结果回归整理了相应的换热系数无量纲准则关系式,与试验数据吻合较好,计算误差小于15％。     

Thermal–hydraulic analysis of a steam boiler with rifled evaporating tubes

Rifled evaporating tubes are applied in the steam boilers with the aim to increase the steam–water mixture turbulization and to prevent the burnout of tubes walls. The rifled evaporating tubes and the working fluid forced circulation are applied in the steam boiler at the Thermal Power Plant “Kolubara B” that is being built by the Electric Power Utility of Serbia. In order to investigate operating characteristics of the steam boiler of such an advanced design, a simulation and analysis of complex coupled thermal processes on the furnace gas side and thermal–hydraulics inside the evaporating tubes were performed for the whole range of plant operating loads. In this paper a methodology for hydraulic calculations of both forced and natural circulation loops is presented. The increased thermal safety margin of rifled tubes is predicted and compared with the safety margin of smooth tubes for uniform and variable heat loads among walls of the boiler furnace. The rifled tube provides an acceptable temperature of its wall even under a high void fraction of the coolant and prevents the occurrence of critical heat transfer conditions. The influence of the rifled tubes on an increase of the hydraulic resistance in the circulation loop is analyzed. Also, a range of operating conditions under which there is no need for the circulation pumps operation is determined.     

一台DZL6.0-1.25蒸汽锅炉水冷壁管爆管事故的分析与改进

从对一台ＤＺＬ６．０－１.２５蒸汽锅炉近前炉门附近两侧炉膛水冷壁管多次发生局部爆管事故的调查出发,对该锅炉侧集箱排污系统存在的设计缺陷进行了流动分析、计算。计算表明,排污时前段集箱内流速过低是造成该段集箱区水渣大量沉积致使该区水冷壁管因水渣堵塞而引发局部爆管的根本原因。对侧集箱不同部位增开排污口方案进行的计算表明,在集箱前端增开排污口可有效地改善排污时该处集箱内水的流动性,从而消除泥渣沉积隐患。改造方案实施后,经多年的锅炉安全运行实践也证明了该方案的有效性。     

Heat Transfer Characteristics of Flood-type Evaporator Using R22 and R134a for Fishing Vessels

ABSTRACT

Most of fishing vessels pour plain ice into fish storages to maintain freshness of catches, which causes varying storage temperatures and changes in the salinity, and damages in catches. In contrast, seawater cooling systems that directly chill the seawater in the storage provide seawater with a constant temperature and salinity. Furthermore, the application of a flood-type evaporator enables the size of the system to be reduced due to the enhanced heat transfer performance of the evaporator. This study conducts experimental research on the boiling heat transfer characteristics of flood-type evaporators under various operating conditions. The test section of the experiment consists of the flood-type evaporator including tubes made of three different materials and having two different shapes, and R134a and R22 are used. As a result, low-fin tubes were found to present a higher heat transfer coefficient than that of the plain tube. Aluminum-brass low-fin tubes exhibited relatively 4.5% higher heat transfer coefficient than that of copper-nickel low-fin tubes when R22 was used, and relatively 5.3% higher value when the R134a was used. When R22 was used with an aluminum-brass low-fin tube, the heat transfer coefficient was relatively 6.9% higher than that when using R134a.     

The impact of bed temperature on heat transfer characteristic between fluidized bed and vertical rifled tubes

In the present work, the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes (38mm-O.D.) in a commercial circulating fluidized bed (CFB) boiler. Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246mm. The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler. A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach. The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.     

An Experimental Study on the Performance of a Stainless Steel-Water Loop Heat Pipe under Natural Cooling Condition简

Aiming to improve the thermal characteristics of modern electronics, we experimentally study the performance of a stainless steel/water loop heat pipe （LHP） under natural cooling condition. The LHP heat transfer performance, including start-up performance, temperature oscillation and total thermal resistance at different heat loads and with different incline angles have been investigated systematically. Experimental results show that at an optimal heat load （i.e. 60 W） and with the LHP being inclined 60~ to the horizontal plane, the total thermal resistance is lowered to be -0.24 K/W, and the temperature of evaporator could be controlled steadily at around 90~C.     

Enhanced Boiling Heat Transfer of Water/Salt Mixtures in the Restricted Space of the Compact Tube Bundle

In desalinization devices and some heat exchangers making use of low-quality heat energy, both the wall temperature and the heat flux of the heated tubes are generally quite low, hence cannot cause boiling in flooded-type tube bundle evaporators with a large tube spacing. But when the tube spacing is quite small, incipient boiling can occur in the restricted space and results in higher heat transfer than that in a falling-film evaporator or during pool boiling at the same heat flux. This study experimentally investigates the effects of the tube spacing, the positions of tubes, and the salt-water concentration on bundle boiling heat transfer of salt water in the restricted space of the compact tube bundle evaporator under atmospheric pressure. The experimental results provide a restricted space boiling database for salt water in the compact tube bundle. Of particular importance is information concerning the influences of the tube spacing of the tube bundle and the concentration of salt water in desalination evaporators.     

电加热倾斜管温度场分布计算

考虑自然对流对倾斜上升管内流体传热的影响,将外壁温度与外壁热负荷作为边界条件,同时将内热源作为并联网格电阻放热来进行处理,为电加热倾斜管温度场分布建立二维数学模型。根据空间节点推进的控制容积差分法求解流体换热和管壁导热耦合决定的电加热倾斜管二维温度场导热反问题,编制计算程序,针对我国第一台超临界锅炉螺旋管圈水冷壁的管型进行了管壁温度场的计算。在亚临界及超临界压力工况下,计算结果都可以很好地反映倾斜管壁温的分布规律,同时计算收敛性良好。     

Heat transfer performance of a double-loop separate-type heat pipe: Measurement results

An experimental study is presented for the heat transfer performance of a rectangular double-loop natural circulation system, in which the condensers are made of double tubes with water-steam as the working fluid. Detailed temperature measurements of the core fluid and the wall are made, from which overall heat transfer coefficients for the evaporator, condensers, and entire system are obtained. Parametric studies of the liquid charge level, fluid properties, and heating or cooling conditions on the heat transfer performance are presented and correlation equations are given. The results show that the overall heat transfer coefficients for the evaporator, condensers, and entire loop are all increasing with decreasing liquid charge level. Overhead phenomena at low liquid charge level and thermal oscillation at some situations are also observed and discussed.     

一种用于铬盐生产中余热回收新型锅炉的设计开发

根据铬盐生产的工艺流程以及其烟气特性,设计开发出一种用于铬盐生产余热回收的新型锅炉。该型锅炉的主要特点是:受热面U型结构布置,自然循环、蒸发器三级并联布置,蒸发器采用加肋片结构,省煤器采用H型翅片管布置,蒸发器和省煤器管束呈交叉布置等。实践表明,该型锅炉对窑炉烟气温度、粉尘具有很好的适应性。     

Flow boiling characteristics of refrigerant mixture R22/R114 in the annulus of enhanced surface tubing

The paper presents an experimental study of flow boiling heat transfer characteristics of refrigerant mixture R22/R114 in the annuli of a horizontal enhanced surface tubing evaporator. The test section had an inner tube bore diameter of 17.3 mm, an envelope diameter of 28.6 mm and an outer smooth tube of 32.3 mm internal diameter. The ranges of heat flux and mass velocity covered in the tests were 5–25 kW/m² and 180–290 kg/m²/s, respectively, at a pressure of 570 kPa. The enhanced surface tubing data shows a significant enhancement of the heat transfer compared with an equivalent smooth tube depending on the mixture components and their concentrations. Correlations are proposed to predict such heat transfer characteristics as the average heat transfer coefficients as well as pressure drops of R22/R114 nonazeotropic refrigerant mixture flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture composition.     

Simple and Clear Approach to Industrial Boiler Circulation Analysis

Circulation in the industrial boiler is the continuous supply of water to the boiler heated tubes in order to sustain steady steam output without overheating tubes. Adequate circulation occurs when there is sufficient flow of water into tubes for adequate cooling. In the natural circulation industrial boiler system the rate of flow in the circulation system is governed by flow resistances and differences in density between downcomer tube circuits and heated riser circuits. Control of these resistances allows adequate flow of water to parallel circuits. Adequate water flow through the boiler's heat-absorbing circuits is necessary to cool the tubes. To insure proper boiler circulation, a simple, clear, and comprehensive approach is described in this article covering all aspects in boiler circulation analysis. The analysis details all boiler circulation circuits, explaining all forces imposed on them, including thermosyphon as well as friction forces.     

Solved and Thermal-Hydraulic Problems Systems: Learning from Accidents Unsolved in Boiler

This article begins with a brief review of the similarity law of conventional fossil fuel-fired boilers. The concept is based on the fact that the heat release due to combustion in the furnace is restricted by the furnace volume but the heat absorption is restricted by the heat transfer surface area. This means that a small-capacity boiler has relatively high specific furnace heat release rate, about 10 MW/m 3 , and on the contrary, a large-capacity boiler has a lower value. The surface heat flux limit is dominated mainly by the critical heat flux (CHF) inside the water wall tubes of the boiler furnace. This heat flux limit is almost the same order, independent of the capacity of the boiler. For the safety of water walls, it is essential to retain a good amount of water circulation, i.e., the circulation ratio and velocity of the water must be high enough above the respective critical values. This principle is common knowledge in boiler designer, but the actual situation is not the case. Newly designed boilers often suffer from similar accidents, especially burnout due to water circulation problems. This article demonstrates recent accidents encountered in practical boilers, and raises rather classical but important problems related to two-phase flow and heat transfer.     

大容量燃气热水锅炉对流管束的振动分析和预防措施

针对一台58 MW燃气热水锅炉部分对流管束产生的开裂现象进行了深入的原因分析.对断口部分进行了宏观检查、金相组织分析和扫描电镜分析,对烟气冲刷对流管束时气柱的固有(驻波)频率、卡门涡流频率和水管固有振动频率分别进行了计算,并对该锅炉的对流管束进行了振动测试,得出了对流管束产生管子的共振会引发部分对流管脆性断裂的结论.并据此提出了一系列具体的预防和控制措施及建议,通过改变管子的固有频率解决此类问题的发生,为锅炉的安全稳定运行提供保障.     

Effect of tube diameter on boiling heat transfer of R-134a in horizontal small-diameter tubes

The boiling heat transfer of refrigerant R-134a flow in horizontal small-diameter tubes with inner diameter of 0.51, 1.12, and 3.1 mm was experimentally investigated. Local heat transfer coefficient and pressure drop were measured for a heat flux ranging from 5 to 39 kW/m², mass flux from 150 to 450 kg/m² s, evaporating temperature from 278.15 to 288.15 K, and inlet vapor quality from 0 to 0.2. Flow patterns were observed by using a high-speed video camera through a sight glass at the entrance of an evaporator. Results showed that with decreasing tube diameter, the local heat transfer coefficient starts decreasing at lower vapor quality. Although the effect of mass flux on the local heat transfer coefficient decreased with decreasing tube diameter, the effect of heat flux was strong in all three tubes. The measured pressure drop for the 3.1-mm-ID tube agreed well with that predicted by the Lockhart–Martinelli correlation, but when the inner tube diameter was 0.51 mm, the measured pressure drop agreed well with that predicted by the homogenous pressure drop model. With decreasing tube diameter, the flow inside a tube approached homogeneous flow. The contribution of forced convective evaporation to the boiling heat transfer decreases with decreasing the inner tube diameter.