Stresses from temperature transients

Catalytic combustion for gas turbines has received much attention in recent years in view of its unique capability of simultaneous control of NOX) CO, and unbumed hydrocarbon emissions.1 One of the major challenges to be faced in the development of industrial devices is associated with the severe requirements on catalytic materials posed by extreme operating conditions of gas turbine combustors. The catalytic combustor has to ignite the mixture of fuel (typically natural gas) and air at low temperature, preferably at the compressor outlet temperature (about 350 °C), guarantee complete combustion in few milliseconds, and withstand strong thermal stresses arising from long-term operation at temperatures above 1000°C and rapid temperature transients. 

Effect of temperature on the creep of NT154 (a) Strain as a function of exposure time (b) Minimum creep rate as a function of temperature. Transient creep effects are evident from the time required for the data to approach a common Arrhenius curve. Approximately 1500 hr of creep was required to approach steady state creep. Stress 100 MPa. 

Plow and pressure and temperature transients occurring in the ex reactor primary coolant system have been analysed from the viewpoint of stress and fatigue and effect on process Many conditions have been Studied ranging from the normal scram tranaienta to significant pipe rtgitures and lose of the heat removal eyacem ... 

In many instances such as transient and upset conditions the temperature distribution through the wall of a vessel cannot be represented by a mathematical expression. In this case a graphical solution can be obtained from the thermal stress. From Eq. 5.38... 

The measurement of normal stress differences in transient deformations is extremely sensitive to small variations in gap spacing, which can arise from instrument compliance or minute temperature variations. Venerus and Kahvand [43] have shown how to evaluate the effect of instrument compliance by measuring the response using several sets of cone-plate fixtures. If a Force Rebalance Transducer is used for a transient normal stress measurement to compensate continuously for compliance in order to keep the gap constant, the response time of the transducer may affect the data. Also, the thermal expansion that results from the power dissipated in the transducer can affect the gap spacing and is of particular concern when normal stresses are being measured [104]. 

Thermal Stresses. When the wak of a cylindrical pressure vessel is subjected to a temperature gradient, every part expands in accordance with the thermal coefficient of linear expansion of the steel. Those parts of the cylinder at a lower temperature resist the expansion of those parts at a higher temperature, so setting up thermal stresses. To estimate the transient thermal stresses which arise during start-up or shutdown of continuous processes or as a result of process intermptions, it is necessary to know the temperature across the wak thickness as a function of radius and time. Techniques for evaluating transient thermal stresses are available (59) but here only steady-state thermal stresses are considered. The steady-state thermal stresses in the radial, tangential, and axial directions at a point sufficiently far away from the ends of the cylinder for there to be no end effects are as fokows ... 

In reactor construction, the tubesheet is the most expensive part and the possibility of tubes getting loose from the tubesheet is the most dangerous problem. Therefore, large reactors have some limitations on the rate at which they can be heated (for example, 25°C/hour), to avoid developing temperature differences in the body which lead to transient thermal stresses. These stresses can cause tubes to bend and break loose from the tube sheets. 

Another distinctive feature of strong tunnelling recombination could be seen after a step-like (sudden) increase (decrease) of temperature (or diffusion coefficient - see equation (4.2.20)) when the steady-state profile has already been reached. Such mobility stimulation leads to the prolonged transient stage from one steady-state y(r,T ) to another y(r,T2), corresponding to the diffusion coefficients D(T ) and >(72) respectively. This process is shown schematicaly in Fig. 4.2 by a broken curve. It should be stressed that if tunnelling recombination is not involved, there is no transient stage at all since the relevant steady state profile y(r) — 1 - R/r, equation (4.1.62), doesn t depend on >( ). 

Any oxygen bomb calorimeter consists of four essential parts (I) a bomb or vessel in which the sample is burned (2) a bucket or container which holds the bomb as well as a precisely measured quantity of water to absorb the heat released from the bomb and a stirring device to aid in achieving rapid thermal equilibrium (3) a jacket for protecting the bucket from transient thermal stresses and (4) a thermometer for measuring temperature changes within the bucket The cross section of a simple calorimeter is shown in Fig. I. 

---