As we saw before, if we look at transient heat transfer problems
in *dimensionless* form, we get a dimensionless temperature,
a dimensionless time, and the Biot number out. Not surprisingly, if
we have a problem whereby the Bi is **not** small, we also get a
dimensionless position (since spatial gradients do not
disappear).

One of the most *practical* reasons for making our equation
(and therefore our answers!) dimensionless is that our results are
then general. That is, since the same dimensionless equation arises
for many differing dimensional systems, we can use the SAME
dimensionless answer for all of them!

One particularly useful side-effect of this is that people have tabulated results of dimensionless solutions for transient problems.....we can use these for many problems as long as we know three of the four dimensionless parameters defining our system!

This approach works equally well for heat *or* mass
transfer problems!

Use Gurney-Lurie charts to solve heat and mass transfer problems

Let's look at an example...

A flat wall of brick which is 0.5 m thick and originally at 200K
has one side suddenly exposed to hot gas at 1200K. If the heat
transfer coefficient on the hot side is 7.38W/m^{2}K and
the other face is perfectly insulated determine

A. The time it takes to raise the center to 600K

B. The temperature of the insulated side of the wall at this
time.

The properties of the brick are as follows....k=1.125 W/mK;
c=919 J/kgK; $\rho$=2310 kg/m^{3}