Thermal stresses in radiant tubes due to axial,circumferential and radial temperature distributions

This paper presents an analysis of the thermal stresses in radiant tubes. The analytical analysis is verified using a finite element model. It was found that axial temperature gradients are not a source of thermal stresses as long as the temperature distribution is linear. Spikes in the axial temperature gradient are a source of high thermal stresses. Symmetric circumferential gradients generate thermal stresses, which are low as compared to the stress rupture value of radiant tubes. Radial temperature gradients create bi-axial stresses and can be a major source of thermal stress in radiant tubes. A local hot spot generates stresses, which can lead to failure of the tube.     

内套管位置对预热式燃气辐射管NO_X浓度的影响

通过利用FLUENT软件,对辐射管内的燃烧过程进行了二维数值模拟。研究了内套管相对位置对烟气NOx浓度以及辐射管温度的影响。为自身预热式辐射管的开发与优化以及燃烧污染物控制提供了参考数据。     

P型蓄热式辐射管设计与数值模拟

开发了P型蓄热式辐射管。在开发设计过程中,应用CFD技术,对设计方案进行三维流动、传热与燃烧过程的数值模拟。根据模拟结果,改进和优化设计方案,大大减少了实验次数。实践表明,CFD技术是提高燃烧设备开发设计水平和增强技术创新能力的有效工具。     

开式直筒辐射管辐射换热性能的数值计算

针对一种设计用于长形炉膛的新型开式直筒辐射管,利用蒙特卡洛法对其辐射换热进行数值计算,从而对其工作性能作出预判,为开式直筒辐射管在工程领域的应用提供依据.结果表明：在长形筒状炉膛内使用开式直筒辐射管,炉膛壁面沿长度方向上的热流密度分布表现为先上升再下降、在尾部又上升的规律;在各负荷条件下,辐射管沿长度方向上的壁面温度分布规律同样为中间温度高,两侧温度低,其中前段温度平均低于后段温度.     

Residual stresses in girth butt welded pipes and treatments to modify these

A number of Type 304 stainless steel pipes are used in the primary cooling systems of nuclear plants. Intergranular stress corrosion cracks (IGSCC) were found at some welded joints in these piping systems due to very high tensile residual stress, sensitization of the material due to welding, and corrosive environment, all occurring simultaneously. Investigations have shown that at least one of the above factors must be eliminated to prevent IGSCC.

This report describes experimental results on the temperature variations during pipe welding by conventional techniques and by the heat sink welding (HSW) technique. The mechanism of residual stress generation due to welding is also discussed. The pipe used in these experiments was 4B Sch80 Type 304 stainless steel. It was found that the temperature distribution through the thickness of the pipes was almost uniform for the conventional welding technique, but had a very sharp gradient for HSW. In the pipe axial direction, the temperatures varied sharply for both welding techniques. This implies that the sensitization of metal due to HSW is lighter than that of conventional welding and that the residual stress on the inside surface of the heat sink welded pipe is compressive.

The induction heating stress improvement (IHSI) method has been investigated analytically and experimentally. In the IHSI method, a pipe is heated with an induction coil while cold water is pumped through it. This causes a temperature gradient throughout the pipe wall which generates high thermal stresses. This, in turn, generates compressive stresses on the inner surface of the pipe. This method is designed to eliminate tensile residual stresses near the weld heat affected zone on the inner surface.

Temperature analysis and subsequent thermoelastic-plastic analysis show that tensile weld residual stresses at a joint were changed into compressive stresses on the inner surface of a pipe. It was confirmed experimentally that these stresses suppressed fatigue crack propagation in the heat affected zone (HAZ) of a welded pipe. Therefore, the IHSI method is effective not only in preventing crack initiation but also in suppressing crack propagation.

As for the relaxation of residual stresses, no significant relaxation was measured when external loads were applied at as much as 80% of the yield strength in the experiments.     

高温空气燃烧技术蓄热式燃气辐射管燃烧器的研制开发和应用

介绍了应用高温空气燃烧技术研制开发W型和U型蓄热式燃气辐射管燃烧器的工作原理、技术特点和性能指标。研究指出．蓄热式燃气辐射管燃烧器是一种高效、节能、低污染的工业炉用加热装置。可广泛应用于机械、冶金、石化、电子、兵器、轻工、电器、交通、建材、纺织、汽车、航空航天和食品工业等加热炉设备上,具有广阔的应用前景。     

Radiant and convective heat transfer for flow of a transparent gas in a short tube with sinusoidal wall heat flux

The present analysis accounts for combined convective and radiant heat transfer to a fluid flowing in a short tube with prescribed wall heat flux. The heat flux distribution used was of sine shape with maximum at the middle of the tube. This solution is known to represent the axial power variation in a nuclear reactor core. The tube wall and gas bulk temperatures were obtained by successive substitutions for the wall and gas energy balance equations. The integrals were approximated by Sympson's rule and initial guesses for the iterative process were based upon limiting cases for pure radiation and pure convection. The results of the combined solution compared with the pure radiation approach show a decrease of 30 percent for the maximum wall temperature using black surface ($\text{ε=1}$). For this same situation, the increase in the gas temperature along the tube shows a reduction of 58 percent when compared to the pure convection solution.     

Simulation of gas species and temperature separation in the counter-flow Ranque–Hilsch vortex tube using the large eddy simulation technique

A computational fluid dynamic model is used to predict the species and temperature separation within a counter flow Ranque–Hilsch vortex tube. The large eddy simulation (LES) technique was employed for predicting the gas flow and temperature fields and the species mass fractions (nitrogen and helium) in the vortex tube. A vortex tube with a circumferential inlet stream of nitrogen–helium mixture and an axial (cold) outlet stream and a circumferential (hot) outlet stream was considered. The temporal evolutions of the axial, radial and azimuthal components of the velocity along with the temperature, pressure and mass density and species concentration fields within the vortex tube are simulated. Even though a large temperature separation was observed, only a very minimal gas separation occurred due to diffusion effects. Correlations between the fluctuating components of velocity, temperature and species mass fraction were calculated to understand the separation mechanism. The inner core flow was found to have large values of eddy heat flux and Reynold’s stresses. Simulations were carried out for varying amounts of cold outlet mass flow rates. Performance curves (temperature separation/gas separation versus cold outlet mass fraction) were obtained for a specific vortex tube with a given inlet mass flow rate.     

Stress and creep damage analysis for HTR heat exchangers at very high temperatures

The Helium-Cooled High Temperature Reactor (HTR) can supply process heat for methane reforming at temperatures of nearly 1000°C. As is the usual requirement for equipment used in a nuclear plant, the process heat exchangers must operate with a high reliability. This implies both that the exchanger tubes must be made from alloys which are very resistant to high temperatures and that the operational pressures are adjusted so that primary stresses in the tube walls are small. However, secondary stresses—an order of magnitude larger than primary stresses—are produced by the temperature profile through the wall.

With consideration of creep, the time-dependent stress-strain distributions in the tube walls were calculated using the Finite Element Programme ASKA. The creep damage was estimated using Robinson's rule. The results showed that operation was possible for 100 000 h.

Under accident conditions, load-controlled stresses in the tube walls may be an order of magnitude larger than under normal operation conditions. Here the stress distribution near the tube closure was investigated and it was found that lower stresses occurred in the transition and closure region than in the undisturbed tube area. In this way it could be shown that the closure is not seriously put at risk.     

Performance analysis of metallic concentric tube recuperator in parallel flow arrangement

A performance model for parallel flow arrangement in metallic concentric tube recuperator that can be used to utilize the waste heat in the temperature range of 1100–1800 K is presented. The arrangement consists of metallic concentric shells wherein flue gases pass through the inner shell and air to be preheated passes through annular gap in the same direction. The recuperator height is divided into small elements and an energy balance is performed on each element. Necessary information about axial shell-surface, gas and air temperature distribution, and the influence of operating conditions on recuperator performance is obtained. The recuperative effectiveness is found to be increased with increasing inlet gas temperature and with decreasing fuel flow rate. The present model provides a valuable tool for metallic concentric tube recuperator performance considerations in parallel flow arrangement.     

Thermal performance of a small oil-in-glass tube thermal energy storage system during charging

A very small oil-in-glass tube thermal energy storage (TES) system is designed to allow for rapid heat transfer experiments. An electrical hot plate in thermal contact with a steel spiral coil (SSC) is used to charge the TES system under different hot plate temperatures and under different average charging flow rates. Thermal performance during charging is presented in terms of the axial temperature distribution, the axial degree of thermal stratification, the total energy stored and the total exergy stored. The energy and exergy delivery rates of the energy delivery device (EDD) are also evaluated in relation to the thermal performance of the storage system. Results of charging the storage system under different hot plate temperatures indicate that there is an optimal charging temperature for optimal thermal performance. The results also indicate that exceeding this optimal temperature leads to a degradation of the thermal performance due to increased heat losses. Charging at the same temperature conditions under different flow rate regimes suggests that there is an optimal charging flow rate. This optimal flow rate is a compromise between achieving a greater heat transfer rate in the EDD and achieving a greater degree of thermal stratification in the TES system.     

Thermal stress analysis of eccentric tube receiver using concentrated solar radiation

In the parabolic trough concentrator with tube receiver system, the heat transfer fluid flowing through the tube receiver can induce high thermal stress and deflection. In this study, the eccentric tube receiver is introduced with the aim to reduce the thermal stresses of tube receiver. The ray-thermal-structural sequential coupled numerical analyses are adopted to obtain the concentrated heat flux distributions, temperature distributions and thermal stress fields of both the eccentric and concentric tube receivers. During the sequential coupled numerical analyses, the concentrated heat flux distribution on the bottom half periphery of tube receiver is obtained by Monte-Carlo ray tracing method, and the fitting function method is introduced for the calculated heat flux distribution transformation from the Monte-Carlo ray tracing model to the CFD analysis model. The temperature distributions and thermal stress fields are obtained by the CFD and FEA analyses, respectively. The effects of eccentricity and oriented angle variation on the thermal stresses of eccentric tube receiver are also investigated. It is recommended to adopt the eccentric tube receiver with optimum eccentricity and 90° oriented angle as tube receiver for the parabolic trough concentrator system to reduce the thermal stresses.     

高温熔盐吸热器的传热研究和系统设计

本文对非均匀辐射热流密度太阳能熔盐吸热器传热过程进行了模拟研究,得到了熔盐吸热器内部的温度、传热性能等特征参数。结果表明在轴向和径向上熔盐流体温度和管壁的温度都非常不均匀,同时其综合传热性能要高于按照Sieder-Tate公式的计算值。并对10 MW塔式太阳能热发电的熔盐吸热器进行了设计和分析。     

Development of a high efficiency radiation converter using a spiral heat exchanger

Heating by radiation is widely used for materials processing. Electrical radiant heaters are the most commonly used heaters. Electricity is expensive and the combustion of fossils fuels for electricity production emits CO₂. In order to convert the energy from the fuel to radiation energy directly and efficiently, our group has developed a compact, high efficiency, radiation converter using a spiral heat exchanger to recover the energy from high-temperature exhaust gas. The spiral heat exchanger has a weld-free construction to prevent cyclic thermal stress, and is constructed from inexpensive ferrite steel plates. The combustion chamber, equipped with a swirler to mix the gas fuel and air, can achieve stable combustion. The distribution of the surface temperature on the radiant tube was measured by a radiation thermometer, called a thermo viewer, and then the radiant energy emitted from the radiant tube was estimated. The efficiency of the spiral heat exchanger was measured from the temperature of the inlet air and exhaust gas. The heat exchanger achieved a high effectiveness, and heat loss from the exhaust gas was minimized. Consequently, a highly efficient radiation converter was produced to convert the fuel energy to radiation energy.     

Thermal stresses in an operating micro-tubular solid oxide fuel cell

A multi-physics model is developed to investigate the thermal stresses in a micro-tubular SOFC, based on a previously developed thermal-fluids model predicting cell operation. Mechanical properties of the anode and cathode are determined theoretically through composite structure approximation. Residual stresses arisen during the fabrication of the cell due to the mismatch in thermal expansion coefficients are calculated by accounting for each fabrication process separately. The interactions between the cell, the sealant and the alumina tube are accounted for a better representation of the actual fuel cell test setup. The effect of sealant and alumina tube on the stress distribution in the cell is investigated and it is found out that near the fuel cell-sealant interface stress distribution changes significantly. The effect of spatial temperature gradient on the stress distribution is also analyzed and found to have a minimal impact for a typical fuel cell operation at mid-range current densities. The effects of oxygen vacancies caused by the reduction of the GDC electrolyte on the overall stress distribution are also shown. Oxygen vacancies of the electrolyte result in relaxation of the stresses due to the alleviation of mismatch in Young's modulus between different layers of the cell.     

地板辐射与下送风复合式供冷系统运行特性实验研究

测量地板辐射与下送风复合式供冷系统运行过程中的室内空气温湿度、围护结构表面温度等室内环境参数,分析室内温湿度、热舒适性、系统换热量的变化规律,并对室内空气环境进行影响因素分析。实验结果表明:室内空气绝对湿度较室内空气温度达到稳定需要的时间更短;MRT(mean radian temperature)、OT(operation temperature)和PMV-PPD值在系统开启后第1.0 h减小速率最大,1.5 h后逐渐趋于稳定,此时,PMV约为0.49,PPD约为10%,在热舒适范围内;地板净辐射换热量、对流换热量和总换热量在系统开启后的1.5 h内递增,然后趋于稳定,此时,地板辐射换热量约为37 W/m~2,占总换热量的47%;室内空气温度和作用温度均随室外综合温度、室内发热量、供回水平均温度和送风温度的增加而增加,当室外综合温度较低或较高,或室内发热量较低,或供回水平均温度较低时,室内空气温度和作用温度变化梯度较小,室内空气温度和作用温度随送风温度增加而增加的速率近似呈线性。     

镀锌板生产线退火炉辐射管燃烧实验研究

建造一座模拟退火炉作为煤气燃烧试验装置，通过模拟不同热值的发生炉煤气在炉内辐射管中燃烧，探索辐射管换热性能和烟气热损失大小的变化规律，为将燃烧半水煤气更换成燃烧发生炉煤气提供技术依据。模拟在线辐射管的运行状态，找出改变助燃空气量和排烟负压对辐射管壁温和炉温的影响，寻求一种最佳的运行方案，从而使生产线达到节能降耗目的。     

引射式中温燃气辐射管热工性能的试验研究

对吸风式直通型辐射管进行了试验研究,该燃气辐射管是由低压引射式大气燃烧器和石英玻璃辐射管组成。介绍了引射式燃烧器的研制过程及其特点,通过试验对燃气辐射管性能进行了研究,分析了燃气负荷、空气系数,在辐射管内加入插入物这几个因素对辐射管的温度分布及热效率的影响规律,为燃气辐射管的工业化应用提供理论依据。     

Numerical analysis of temperature non-uniformity and cooling capacity for capillary ceiling radiant cooling panel

Capillary ceiling radiant cooling panel is a high temperature cooling system, which could pose low energy consumption to meet thermal comfort requirements. A computational fluid dynamics (CFD) simulation study on heat transfer of chilled water flow in the capillary of ceiling radiant cooling panel was performed to attain surface temperature distributions and cooling capacities. Six influencing factors included chilled water inlet parameters, conditions of gypsum plaster and capillary mats structural parameters were considered to obtain the complicated relationships between capillary radiant panel conditions and heat transfer performance. The index of temperature non-uniformity coefficient was proposed to evaluate temperature profiles of ceiling panel surface. The results of the simulation were compared with the values depicted in ASHRAE Handbook and good agreement had been achieved. The average difference between simulation results and the values reported by ASHRAE handbook was within the region of 15%. The research results showed that temperature non-uniformity coefficient was negatively correlated with temperature of chilled inlet water (linear correlation), water velocity (correlation coefficient R = −0.85), and pipe diameter (correlation coefficient R = −0.93), but positively and linearly correlated with tube spacing. Cooling capacity was found to have negative linear correlation with temperature of chilled inlet water, covering thickness and tube spacing.     

辐射管对加热炉内部燃烧影响的数值模拟与试验研究

以大庆油田火筒式加热炉为研究对象,利用试验测试和数值模拟方法,分析加热炉内增加辐射管对火筒壁面温度场的影响。试验中采用的是大气式燃烧器,燃烧功率为400 k W。研究结果表明:数值模拟结果与试验测试基本一致;安装辐射管后,火筒壁面整体温度有所降低,但分布更加均匀;火筒上部、中部和下部非均匀温度系数ε分别降低36%、26%和32%;热烟气向上流动导致火筒壁面下部的非均匀温度系数比中、上部大。