Hi, I understand you are trying to solve these equations using vpasolve. In your code there are some undeclared variables as 'Re_c', 'qc', 'mwall', 'hl_c', 'lc', 'dc'. After declaring these variables the code is working. I have used random value for these variables. Here is the code for it.
Re_c=[100 200 300 433.464423 ];
qc=1;
mwall=1;
hl_c=1;
lc=1;
dc=1;
ds= 3;
rho_w= 3;
cp_w = 5;
hl_s = [2 , 2, 2, 2];
ls= 3;
mcv=9;
cp_al=7;
vs = [1, 2, 3, 4];
qs = pi*(ds/2)^2*vs;
%% Symbolic
tau_c_laminar = zeros(size(qs));
tau_s_laminar = zeros(size(qs));
ind = zeros(size(qs));
qcsolution_lam = zeros(size(qs));
qcsolutionlmin_lam = zeros(size(qs));
qcsolution_d_lam=[];
qclmin = qc*60000;
syms qcsyms qcsolution_lam
cont = 0;
range=[];
for i = 1:length(qs)
if Re_c(i) < 2300
fprintf("Re: %f \n",Re_c(i))
tau_c_laminar = (mwall*cp_al)/((rho_w*cp_w*qcsyms)*(1-exp((-hl_c*pi*lc*dc)/(rho_w*cp_w*qcsyms))));%s
fprintf("talsym: %s \n", char(tau_c_laminar))
tau_s_laminar(i) = (mcv*cp_al)/((rho_w*cp_w*qs(i))*(1-exp((-hl_s(i)*pi*ls*ds)/(rho_w*cp_w*qs(i)))));%s
fprintf("taunum: %f \n",tau_s_laminar(i))
qcsolution_lam(i) = vpasolve (tau_c_laminar == tau_s_laminar(i));
fprintf("vpasolve: %f \n",qcsolution_lam(i))
end
end
