First one has to solve the non-linear functional equation in eta. To avoid the Overflow error message, you define a function
In[77]:= Clear[leila]
leila[[Eta]_, R_] := Block[{x = (vLinf/Av)*(PL - Psat1)/(k*T) - N2*k*T/(N1*k*T - qc*Vv[R]*([Eta] *Psat1 - KH))}, Exp[x]]
this function is rather dull, because you can do
In[87]:= Table[{R, FindRoot[[Eta] == leila[[Eta], R], {[Eta], 0}]}, {R, 0, 10, 1}]
Out[87]= {{0, {[Eta] -> 1.0007}}, {1, {[Eta] -> 1.00072}}, {2, {[Eta] -> 1.00072}}, {3, {[Eta] -> 1.00072}},
{4, {[Eta] -> 1.00072}}, {5, {[Eta] -> 1.00072}}, {6, {[Eta] -> 1.00072}}, {7, {[Eta] -> 1.00072}},
{8, {[Eta] -> 1.00072}}, {9, {[Eta] -> 1.00072}}, {10, {[Eta] -> 1.00072}}}
with
In[95]:= leila[1.00072, 8]
Out[95]= 1.00072
as well as
In[96]:= Table[{R, FindRoot[[Eta] == leila[[Eta], R], {[Eta], 0}]}, {R, 0, 10^8, 10^7}]
Out[96]= {{0, {[Eta] -> 1.0007}}, {10000000, {[Eta] -> 1.00072}}, {20000000, {[Eta] -> 1.00072}},
{30000000, {[Eta] -> 1.00072}}, {40000000, {[Eta] -> 1.00072}}, {50000000, {[Eta] -> 1.00072}},
{60000000, {[Eta] -> 1.00072}}, {70000000, {[Eta] -> 1.00072}}, {80000000, {[Eta] -> 1.00072}},
{90000000, {[Eta] -> 1.00072}}, {100000000, {[Eta] -> 1.00072}}}
which means Eta is nearly a constant over R with the constants given. Anyway, in case you had a non-constant Eta you could fit its value table {R, Eta}
In[97]:= Table[{R, FindRoot[[Eta] == leila[[Eta], R], {[Eta], 0}][[-1, -1]]}, {R, 0,10^8, 10^7}]
Out[97]= {{0, 1.0007}, {10000000, 1.00072}, {20000000, 1.00072}, {30000000, 1.00072},
{40000000, 1.00072}, {50000000, 1.00072}, {60000000, 1.00072}, {70000000, 1.00072},
{80000000, 1.00072}, {90000000, 1.00072}, {100000000, 1.00072}}
e.g. with Interpolation and use it afterwards because you know it's a function of R now. A symbolic solution for Eta is unavailable as the simplest examples show
In[100]:= Reduce[[Eta] == Exp[b/(a - [Eta])], [Eta]]
During evaluation of In[100]:= Reduce::nsmet: This system cannot be solved with the methods available to Reduce. >>
Out[100]= Reduce[[Eta] == E^(b/(a - [Eta])), [Eta]]