I initially used @ t = 0, T2* = 55.1°C as per post #15, which gave T1* = 39.97°C. I thought thats pretty low as the outlet temperature of tank 1 when the water is switched back on shouldn't be that low. So I changed the initial conditions as follows:
@ t = 0; T1* = 94.9°C ; T2* = 81.3°C...
So T1 is T1,i (after cooling water is put back on) and T1 (∞) was T1 at inifinite time by setting differential to zero
T1* is T1,i+1
Subbed T1* into the differential term in place of T1 as it will vary with time and T (∞) is constant differential is now dT1*/dt
I'm most likely wrong here, setting the differentials to 0 in both eq.'s [19] and [20], I get for both eq.'s:
4486.2*T2 - 1500*T1 = 59724
as expected because eq.'s [20] is rearranged form of [19]