Friday, 4 May 2018

homework and exercises - Heating transient of a composite rod?


A straight rod is made of two parts, [0,x1] (green in the figure) with thermal diffusivity κ1 and [x1,x2] (blue) with thermal diffusivity κ2. The rod is perfectly insulated. Zero y and z temperature gradients are assumed.


At x=0 temperature is maintained at constant T0. At x=x2 the rod is embedded into a perfect insulator (κ=0). At t=0 the rod has a uniform temperature T(x,0)=Ti.


Composite rod


Question: what is the temperature evolution of the rod?



1. The simple case where κ1=κ2=κ:


Let u(x,t)=T(x,t)T0.


Then Fourier's equation tells us:


ut=κuxx


Boundary conditions:


u(0,t)=0

ux(x2,t)=0


Initial condition:


u(x,0)=ui=TiT0


Using the Ansatz u(x,t)=X(x)Γ(t), separation constant k2 and the boundary conditions above, this solves easily to:


u(x,t)=+n=1Bnsin(nπx2x2)eκ(nπ2x2)2t



(for n=1,3,5,7,...)


The Bn coefficients can easily be obtained from the initial condition with the Fourier sine series:


Bn=4uinπ


Back-substituting we get:


T(x,t)=T0+4(TiT0)π+n=11nsin(nπx2x2)eκ(nπ2x2)2t


(for n=1,3,5,7,...)


A plot for the first three terms at t=0.1:


Homegeneous rod temperature


2. The case where κ1κ2:


We define two functions u1(x,t) for [0,x1] and u2(x,t) for [x1,x2]. We use the same Ansatz as under 1. We'll assume both functions have their own eigenvalues.



Boundary conditions:


u1(0,t)=0X1(0)=0

u2(x2)x=0X2(x2)=0


In addition (continuity):


u1(x1,t)=u2(x1,t)


With Fourier, the heat flux is the same at x=x1:


α1u1(x1)x=α2u2(x1)x


Where αi are the thermal conductivities.


a. for u1(x,t):


X1(x)=c1cosk1x+c2sink1x

X1(0)=0c1=0X1(x)=c2sink1x


b. for u2(x,t):



X2(x)=c3cosk2x+c4sink2x

X2(x2)=0
c3k2sink2x2+c4k2cosk2x2=0
Using the additional conditions (3) and (4):


c2sink1x1=c3cosk2x1+c4sink2x1

c2α1k1cosk1x1=c3α2k2sink2x1+c4α2k2cosk2x1


Problem:


(6), (7) and (8) form a system of three simultaneous equations but with five unknowns: c2, c3, c4, k1 and k2.


I'm tempted to set c3=0 as it would yield k2 from (6). I think this would yield also the remaining unknowns. But can I a priori assume c3=0? Or is there another approach possible?


I'm also left wondering whether perhaps k1=k2. The eigenvalues do not depend on κ, so perhaps the eigenvalues k are common to both functions. Due to (4), u1 and u2 would then still be distinct.




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