## Not getting the right root when using solve

on 26 Apr 2013

### Walter Roberson (view profile)

The answer I'm getting is - 775.94006610198792379207269683591 + 890.01055416454391218342305188902*i My answer should be around 515 any suggestions

here's how I was solving, all variables are given except T2

```%Set up Variable T2 = final Temperature for Isentropic process
syms T2
%s2 = s(T) of equation 9
%Utilize Matlab solve for Variable T2
Tfinal=solve(s2-s0 == a.*log(T2 ./T0)+b.*(T2 -T0)+.5*c.*(T2.^2-T0^2)+d.*(T2.^3-T0^3)/3+.25*e.*(T2.^4-T0^4),T2)
```

Tyler

### Tyler (view profile)

on 27 Apr 2013

a,b,c,d,e are constants based on experimental data. T0 is the reference property temperature (it is 300K for both gases)

Tyler

### Tyler (view profile)

on 27 Apr 2013

Note I get the right answer when using Air

Zhang lu

### Zhang lu (view profile)

on 27 Apr 2013

function [ ] = Isentropic( TA, P1, P2 )

%This Mfile finds the final temperature of an isentropic process for Air

% Intial Temp and Initial / Final pressures are given

prompt='What Gas Are We Dealing With?(Type 1 for Air or 2 for Oxygen)';

result=input(prompt);

if result == 1

T0=300; %K

s0=1.70203;%kJ/kg-K

%For Ideal Air %Gas Constant

R=.2869; %kJ/kg K

alpha=3.653;

beta=-1.337*10^-3;

gamma=3.294*10^-6;

delta=-1.913*10^-9;

elpison=0.2763*10^-12;

elseif result == 2 %Oxygen

```    %Set Reference State @ 300K
T0= 300; %K
s0=205.213;%kJ/kmol-K
R=8.314; %kJ/kg-mol-K
alpha=3.626;
beta=-1.878*10^-3;
gamma=7.055*10^-6;
delta=-6.764*10^-9;
elpison=2.156*10^-12;
else error('Please Try Again and Choose Air(1) or Oxygen(2)')```

end

%Equation 3

a=alpha*R;

b=beta*R;

c=gamma*R;

d=delta*R;

e=elpison*R;

%Find Specific entropy 1 = s1

s1 = a.*log(TA ./T0)+b.*(TA -T0)+.5*c.*(TA.^2-T0^2)+d.*(TA.^3-T0^3)/3+.25*e.*(TA.^4-T0^4)+s0;

%Find Specific entropy 2 = s2

s2=s1+R.*log(P2./P1);

%Set up Variable T2 = final Temperature for Isentropic process

syms T2

%s2 = s(T) of equation 9

%Utilize Matlab solve for Variable T2

Tfinal=solve(a*log(T2 /T0)+b*(T2 -T0)+.5*c*(T2^2-T0^2)+d*(T2^3-T0^3)/3+.25*e*(T2^4-T0^4)-s2+s0,'T2')

end

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### Walter Roberson (view profile)

on 27 Apr 2013

Try

```syms T2 real
```

Tyler

### Tyler (view profile)

on 27 Apr 2013

Thanks you're a life saver...I really appreciate it, The program works now...that was easier then I thought

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