对于动力学反应隐式有限差分热方程解的matlab [英] Matlab solution for implicit finite difference heat equation with kinetic reactions
问题描述
我试图用隐式有限差分方法木气缸内的热传导模型。我使用圆柱形和球形一般的热方程式为：
I am trying to model heat conduction within a wood cylinder using implicit finite difference methods. The general heat equation that I'm using for cylindrical and spherical shapes is:
其中p是形状因数，p值= 1汽缸和p = 2球体。边界条件包括在表面的对流。有关模式的详细信息，请参见下面的Matlab的code的意见。
Where p is the shape factor, p = 1 for cylinder and p = 2 for sphere. Boundary conditions include convection at the surface. For more details about the model, please see the comments in the Matlab code below.
主m文件是：
%--- main parameters
rhow = 650; % density of wood, kg/m^3
d = 0.02; % wood particle diameter, m
Ti = 300; % initial particle temp, K
Tinf = 673; % ambient temp, K
h = 60; % heat transfer coefficient, W/m^2*K
% A = pre-exponential factor, 1/s and E = activation energy, kJ/mol
A1 = 1.3e8; E1 = 140; % wood -> gas
A2 = 2e8; E2 = 133; % wood -> tar
A3 = 1.08e7; E3 = 121; % wood -> char
R = 0.008314; % universal gas constant, kJ/mol*K
%--- initial calculations
b = 1; % shape factor, b = 1 cylinder, b = 2 sphere
r = d/2; % particle radius, m
nt = 1000; % number of time steps
tmax = 840; % max time, s
dt = tmax/nt; % time step spacing, delta t
t = 0:dt:tmax; % time vector, s
m = 20; % number of radius nodes
steps = m-1; % number of radius steps
dr = r/steps; % radius step spacing, delta r
%--- build initial vectors for temperature and thermal properties
i = 1:m;
T(i,1) = Ti; % column vector of temperatures
TT(1,i) = Ti; % row vector to store temperatures
pw(1,i) = rhow; % initial density at each node is wood density, rhow
pg(1,i) = 0; % initial density of gas
pt(1,i) = 0; % inital density of tar
pc(1,i) = 0; % initial density of char
%--- solve system of equations [A][T]=[C] where T = A\C
for i = 2:nt+1
% kinetics at n
[rww, rwg, rwt, rwc] = funcY(A1,E1,A2,E2,A3,E3,R,T',pw(i-1,:));
pw(i,:) = pw(i-1,:) + rww.*dt; % update wood density
pg(i,:) = pg(i-1,:) + rwg.*dt; % update gas density
pt(i,:) = pt(i-1,:) + rwt.*dt; % update tar density
pc(i,:) = pc(i-1,:) + rwc.*dt; % update char density
Yw = pw(i,:)./(pw(i,:) + pc(i,:)); % wood fraction
Yc = pc(i,:)./(pw(i,:) + pc(i,:)); % char fraction
% thermal properties at n
cpw = 1112.0 + 4.85.*(T'-273.15); % wood heat capacity, J/(kg*K)
kw = 0.13 + (3e-4).*(T'-273.15); % wood thermal conductivity, W/(m*K)
cpc = 1003.2 + 2.09.*(T'-273.15); % char heat capacity, J/(kg*K)
kc = 0.08 - (1e-4).*(T'-273.15); % char thermal conductivity, W/(m*K)
cpbar = Yw.*cpw + Yc.*cpc; % effective heat capacity
kbar = Yw.*kw + Yc.*kc; % effective thermal conductivity
pbar = pw(i,:) + pc(i,:); % effective density
% temperature at n+1
Tn = funcACbar(pbar,cpbar,kbar,h,Tinf,b,m,dr,dt,T);
% kinetics at n+1
[rww, rwg, rwt, rwc] = funcY(A1,E1,A2,E2,A3,E3,R,Tn',pw(i-1,:));
pw(i,:) = pw(i-1,:) + rww.*dt;
pg(i,:) = pg(i-1,:) + rwg.*dt;
pt(i,:) = pt(i-1,:) + rwt.*dt;
pc(i,:) = pc(i-1,:) + rwc.*dt;
Yw = pw(i,:)./(pw(i,:) + pc(i,:));
Yc = pc(i,:)./(pw(i,:) + pc(i,:));
% thermal properties at n+1
cpw = 1112.0 + 4.85.*(Tn'-273.15);
kw = 0.13 + (3e-4).*(Tn'-273.15);
cpc = 1003.2 + 2.09.*(Tn'-273.15);
kc = 0.08 - (1e-4).*(Tn'-273.15);
cpbar = Yw.*cpw + Yc.*cpc;
kbar = Yw.*kw + Yc.*cpc;
pbar = pw(i,:) + pc(i,:);
% revise temperature at n+1
Tn = funcACbar(pbar,cpbar,kbar,h,Tinf,b,m,dr,dt,T);
% store temperature at n+1
T = Tn;
TT(i,:) = T';
end
%--- plot data
figure(1)
plot(t./60,TT(:,1),'-b',t./60,TT(:,m),'-r')
hold on
plot([0 tmax/60],[Tinf Tinf],':k')
hold off
xlabel('Time (min)'); ylabel('Temperature (K)');
sh = num2str(h); snt = num2str(nt); sm = num2str(m);
title(['Cylinder Model, d = 20mm, h = ',sh,', nt = ',snt,', m = ',sm])
legend('Tcenter','Tsurface',['T\infty = ',num2str(Tinf),'K'],'location','southeast')
figure(2)
plot(t./60,pw(:,1),'--',t./60,pw(:,m),'-','color',[0 0.7 0])
hold on
plot(t./60,pg(:,1),'--b',t./60,pg(:,m),'b')
hold on
plot(t./60,pt(:,1),'--k',t./60,pt(:,m),'k')
hold on
plot(t./60,pc(:,1),'--r',t./60,pc(:,m),'r')
hold off
xlabel('Time (min)'); ylabel('Density (kg/m^3)');
函数m文件，funcACbar，创建方程解决的制度是：
The function m-file, funcACbar, that creates the system of equations to solve is:
% Finite difference equations for cylinder and sphere
% for 1D transient heat conduction with convection at surface
% general equation is:
% 1/alpha*dT/dt = d^2T/dr^2 + p/r*dT/dr for r ~= 0
% 1/alpha*dT/dt = (1 + p)*d^2T/dr^2 for r = 0
% where p is shape factor, p = 1 for cylinder, p = 2 for sphere
function T = funcACbar(pbar,cpbar,kbar,h,Tinf,b,m,dr,dt,T)
alpha = kbar./(pbar.*cpbar); % effective thermal diffusivity
Fo = alpha.*dt./(dr^2); % effective Fourier number
Bi = h.*dr./kbar; % effective Biot number
% [A] is coefficient matrix at time level n+1
% {C} is column vector at time level n
A(1,1) = 1 + 2*(1+b)*Fo(1);
A(1,2) = -2*(1+b)*Fo(2);
C(1,1) = T(1);
for k = 2:m-1
A(k,k-1) = -Fo(k-1)*(1 - b/(2*(k-1))); % Tm-1
A(k,k) = 1 + 2*Fo(k); % Tm
A(k,k+1) = -Fo(k+1)*(1 + b/(2*(k-1))); % Tm+1
C(k,1) = T(k);
end
A(m,m-1) = -2*Fo(m-1);
A(m,m) = 1 + 2*Fo(m)*(1 + Bi(m) + (b/(2*m))*Bi(m));
C(m,1) = T(m) + 2*Fo(m)*Bi(m)*(1 + b/(2*m))*Tinf;
% solve system of equations [A]{T} = {C} where temperature T = [A]\{C}
T = A\C;
end
最后，随着动力学反应，funcY交易的功能，方法是：
And finally the function that deals with the kinetic reactions, funcY, is:
% Kinetic equations for reactions of wood, first-order, Arrhenious type equations
% K = A*exp(-E/RT) where A = pre-exponential factor, 1/s
% and E = activation energy, kJ/mol
function [rww, rwg, rwt, rwc] = funcY(A1,E1,A2,E2,A3,E3,R,T,pww)
K1 = A1.*exp(-E1./(R.*T)); % wood -> gas (1/s)
K2 = A2.*exp(-E2./(R.*T)); % wood -> tar (1/s)
K3 = A3.*exp(-E3./(R.*T)); % wood -> char (1/s)
rww = -(K1+K2+K3).*pww; % rate of wood consumption (rho/s)
rwg = K1.*pww; % rate of gas production from wood (rho/s)
rwt = K2.*pww; % rate of tar production from wood (rho/s)
rwc = K3.*pww; % rate of char production from wood (rho/s)
end
运行上面code给出在木材气缸中心和表面的温度分布：
Running the above code gives a temperature profile at the center and surface of the wood cylinder:
你可以从这个图看到，由于某些原因，中心和表面温度快速收敛在2分钟大关这是不正确的。
As you can see from this plot, for some reason the center and surface temperatures rapidly converge at the 2 min mark which isn't correct.
这是如何解决这一问题或创建来解决问题更有效的方法有什么建议？
Any suggestions on how to fix this or create a more efficient way to solve the problem?
推荐答案
它看起来像你正在使用的扩散PDE的离散化落后的欧拉隐式方法。更精确的方法是曲柄尼科尔森方法。这两种方法都无条件稳定。
It looks like you are using a backward Euler implicit method of discretization of a diffusion PDE. A more accurate approach is the Crank-Nicolson method. Both methods are unconditionally stable.
引进了相关的T-扩散系数需要特殊治疗，最好的可能是线性的形式，如简要说明的此处。这将是确定的稳定标准有用，以确保时间和距离步长是合适的下面的介绍依赖T-系数。
The introduction of a T-dependent diffusion coefficient requires special treatment, best probably in the form of linearization, as explained briefly here. It would be useful to identify stability criteria to ensure that the time and distance step lengths are appropriate following introduction of T-dependent coefficients.
注意，MATLAB提供 PDE工具箱这可能是对你有用，虽然我有未选中了如何在细节中使用它。
Note that matlab offers a PDE toolbox which might be useful to you, although I have not checked how you might use it in detail.
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