Try making V a vector, and then using ./ and .^ because it's a vector:
clear
q=38.89 ; %W/m^3
r_o=0.04 ;
T_oo=5 ;
k=0.5 ;
c=10.1 ;
V = linspace(.1, 1.0, 500);
Temp = q/(6*k)*(r_o^2)+(q*r_o) ./ (3*(c*V.^0.425)) + T_oo
Temp(Temp==0)=[];
figure
plot(V, Temp, 'b-')
grid on;
xlabel('V *10^-1(m/s)')
xlim('auto')
ylim('auto')
ylabel('Temp (C)')
title('Temperature of center vs air velocity')