Define thermal shock. Suppose a rod of 90% pure alumina (Al_2O_3) with a diamete
ID: 483053 • Letter: D
Question
Define thermal shock. Suppose a rod of 90% pure alumina (Al_2O_3) with a diameter of 1 cm and a length of 10 cm is held in a furnace until the temperature of the rod is uniformly 600 degree C. The rod is then thrust into a bath of ice water such that, initially, its surface temperature changes to 0 degree C while the center remains at 600 degree C. Over time the entire rod will cool to 0 degree C. Estimate what length change the external surface would like to undergo when the rod is first immersed in ice bath. What constraint prevents the surface from changing its dimension?Explanation / Answer
Answer (A) Thermal shock is the failure of structure in material / metal / object. It occurs due to the temperature gradient in material / metal / object. Basically it is understood in terms of stress and strain equivalently. If the crack is not stopped then it can cause to the material / metal / object failure also.
Answer (B)
(i) When an object is heated or cooled its length changes by an amount proportional to the original length and the change in temperature.
The linear thermal expansion of an object can be expressed as dL = L (t1 – t0)
dL = change in object length
L = initial length of object
= linear expansion coefficient
t0 = initial temperature
t1 = final temperature
so the answer is dL= 10x0.000023x(600-0)=0.14 and the new length will be= 10+0.14=10.14cm
(ii) Generally the metallurgy is the main resistance to the thermal shock or expansion but if we use some cooling or heating media against the object as we do for heaters and coolers in industry like heat exchangers this expansion can be prevented. say for example we are boiling the water in oven/cooker or any other utensil if the object does not have any water the metal expands or contracts depending upon the metallurgy like vise the heat or cooling exchange makes this to restrict or resist the changing dimensions.
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