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SIMULATION OF THERMAL SYSTEMS Article 4 1 Article 4 SIMULATION OF THERMAL SYSTEMS Contents 1.1 Introduction to thermal system 1.2 System Description 1.3 Mathematical Modeling of the system 1.4 System Analysis Classical method 1.5 System Analysis in time domain w 1.6 System Analysis in frequency domain 1.7 System Analysis using MATLAB SIMULINK 1.1 Introduction: Thermal systems are those systems that involve the transfer of heat from one substance to another. In this article we will analyze the direct heat exchange process. In this process heat is transferred by the direct mixing of several mass flows of different temperature. In the tank itself additional heating also take place by electrical, steam or other heaters. 2 Article4 Simulation of Thermal System We will give the definitions of the thermal resistance and capacitance, and we will analyze the system in terms of thermal resistance and thermal capacitance. The thermal resistance and thermal capacitance are usually represented by distributed parameter model. Here, however to simplify the analysis we shall assume that the thermal system can be represented by a lumped- parameter model. We also assume that the mass flow rates are constant. Based on these two assumption the mathematical model of the system we obtain is described by a first order linear ordinary differential equations. Most thermal processes in control system conduction, convection, do not radiation. So here we consider only conduction and convection. 1.2 System Description: Consider the system shown in Figure 1.1. It is assumed that the tank is insulated to eliminate heat loss to surrounding air. It is also assumed that there is no heat storage in the insulation and that the liquid in the thanks is perfectly mixed so that it is at uniform temperature. Thus, a single temperature of is used to describe the temperature of the liquid in the tank and the out flowing liquid. Cold Liquid in Heate r Hot Liquid out Mixer Figure 1.1 Thermal System of Insulated heat exchanger The contents of the vessel shown above, Figure 1.1, are well mixed at all times, and the rate of volumetric flow in is balanced by the flow out so that the liquid volume remains constant. For conduction or convection heat transfer, q K (1.1) Where Q = heat flow rate, kcal/sec () = temperature difference, oC 3 Section 1.2 System Description K = coefficient, Kcal/sec oC The coefficient K is given by kA K (1.2) X Where K = the thermal conductivity, Kcal/m sec oC A= area normal to heat flow, m2 X= thickness of conductor, m. In order to develop the mathematical model of the thermal system we need to define the following parameters. Definition1.1 (Thermal Resistance): The thermal resistance R for heat transfer between two substance may be defined as d ( ) R (1.3) dq Where d() = change in temperature difference, oC dq = change in heat flow rate, kcal/sec From equation (1.1) and (1.3) it fellow that the thermal resistance for conduction heat transfer is given by d ( ) 1 R (1.4) dq K Definition 1.2 (Thermal Capacitance): The Thermal Capacitance C is defined by: C = mc (1.5) Where m is the mass of the substance considered, kg cp = c= Specific heat of the substance, kcal/kg oC 1.3 Mathematical Modeling of the System: To derive a model of dynamic temperature change at the thank outlet we need the following assumption.6 - inlet and outlet mass flow rates are constant, i.e., Gi = Go = G 4 Article4 Simulation of Thermal System - the thank is insulated - there is ideal mixing = 0. -the liquid has a constant specific heat cp = constant The dynamic equation that describe the thermal system may be written directly in terms of the deviations from the initial steady conditions. To do so let as define i =Steady-state temperature of inflowing liquid, 0C 0 = Steady-state temperature of outflowing liquid, 0C G= steady-state liquid flow rate, kg/sec M= mass of the liquid in tank, Kg c= Specific heat of liquid, kcal/kg 0C R= thermal resistance, 0C sec/kcal C=Mc thermal capacitance, kcal/0C hi =the heat input rate that supplied by the heater, kcal/sec H =Steady-state heat input rate, kcal/0C??? From the equation for the conservation of heat (i.e., inflow- outflow = accumulation) it follows that dE Gc i hi Gc o (1.6) dt Where E Mc 0 When we arrange the last expression, we get M d0 h i i o (1.7) G dt Gc If the temperature of the inflowing liquid is kept constant and that the heat input rate to the system (heat supplied by the heater) is suddenly changed from H to H +hi, where hi represents a small change in the heat input rate. The heat outflow rate will then change gradually from H to H +ho. The temperature of the outflowing liquid will also be changed from 0 to 0 +. For this case ho, C and R obtained, respectively, as 5 Section 1.3 Mathematical Modeling h0 Gc C Mc (1.8) 1 R h0 Gc If we plug the relations given by equation (1.8) in equation (1.6) we obtain d C hi h0 i (1.9) dt If we put i 0 , equation (1.7) will be d RC Rhi (2.0) dt If the temperature of the inflowing liquid is suddenly changed to from i to i + i while the heat input rate H and the liquid flow rate G are kept constant, then the heat outflow rate will be changed from H to H +h0, and the temperature of the outflowing liquid will be changed from 0 to 0 +. The differential equation for this case is d C hi h0 Gc i (2.1) dt If we put hi 0 , equation (1.9) will be d RC i (2.2) dt 1.4 System Analysis Classical Method: In articles 1 and 2 we already steady two different techniques to solve a differential equation of type (1.8). In fact in the article 1(RC circuit analysis) we apply the general formula which is given by 1 t 1 hi (t ) e RC [ 0 e RC d ] 0 c and in the article 2 (Self regulated liquid tank analysis) we use forced and transient response techniques. Since the equations (2.0) and (2.2) are a variable separable differential equation, we can apply a variable separable techniques to analyze this system. From equation (2.0) it follows that 6 Article 2 Simulation of Dynamic System d 1 ( Rhi ) (2.3) dt RC Separating the variables given as d dt (2.4) Rhi RC We integrate equation (2.4) d t dt Rhi RC 0 0 (2.5) Equation (2.5) integrate as 1 ln( Rhi ) t k1 (2.6) RC where k1 is a constant that depend on the initial condition of the system. After a few algebraic steps equation (2.6) simplified to t (t ) [ Rhi k 2 e RC ] (2.7) where k 2 e k1 . Putting in the boundary condition the initial temperature (0)=0 at t = 0 gives k2 =Rhi, t (t ) Rhi (1 e ) (2.8) where RC= , and which is called the time constant of the system. Equation (2.8) relates the temperature and change in the heat input rate hi From equation (2.8) we note that as t increased without bound, the temperature approaches a limiting value, Rhi. Equation 2.2 is also variable separable differential equation, by applying the same procedure we obtain t (t ) i (1 e ) (2.9) Equation (2.9) relates the temperature and change in the input temperature i 7 Section 1.5 System Simulation in the time domain 1.5 System simulation in the time domain In this section we write short Matalb codes to analyze the response of the thermal system in time domain. In particular we investigate the step and the exponentially decay response of the system by varying its time constant =RC. %Script1.1 This program compute the output temperature of % hot water from the tank with different values of the %time constant tau. In this program inflowing liquid %temperature is kept constant while the heat supplied by % heater is suddenly changed from zero to one % t=0:0.01:10; %vector of time input c=1 %specific heat of the water m=[1 2 3];% the mass of the water in the tank C=c.*m; % the thermal capacitance of the water R=[0.5 0.3 0.2];%different values of thermal resistance tau=R.*C;% the time constant of the system j=1;% counter hi=1-exp(-100*t); % the input heat supplied by the heater. for j=1:3; Hi=R(j).*hi;% the input temperature. j=j+1; subplot(1,2,1)%select the left corner of the screen to plot plot(t,Hi); % plot the graph of xlabel('time[sec]'), ylabel(' Amplitude of the input temperature[C]') title('The curve of input temperature'), grid if ishold~=1, hold on end end hold off i=1; for i=1:3 temp_out=Hi(i)*(1-exp(-t/tau(i)));%evaluate the output tempe i=i+1; 8 Article 2 Simulation of Thermal System % Script 1.1 continue subplot(1,2,2) plot(t,(temp_out))% plot the graphs of the output %temperature xlabel('time[sec]'), ylabel('Amplitiude output temperature'), title('the curve of the output temperature'), grid if ishold ~=1 hold on end end hold off Figure 1.2. The temperature of the cold water. Figure1.3. The temperature of the hot water 9 Section 1.5 System Simulation in time domain %Script1.2 This program compute the output temperatures of % the hot water from the tank with different values of the time constant %tau. In this program inflowing liquid temperature is kept constant %while the heat supplied by heater is gradually changed. % t=1:0.01:10; %vector of time input c=1 %specific heat of the water m=[5 10 15];% the mass of the water in the tank C=c.*m; % the thermal capacitance of the water R=[0.2 0.5 0.8];%three different values of thermal resistance % tau=R.*C;% the time constant of the system j=1;% counter hi=1-exp(-0.5*t); % the input heat supplied by the heater. for j=1:3; Hi=R(j)*hi;% the input temperature. j=j+1; subplot(2,2,1) % select the left side of the screen to plot plot(t,Hi); % plot the input temperature xlabel('time[sec]'), ylabel(' Amplitude of the input temperature[C]') title('The curve of input temperature'), grid if ishold~=1, hold on end end hold off i=1; for i=1:3 temp_out= Hi(i)*(1-exp(-t/tau(i)));% evaluate the output temp i=i+1, subplot(2,2,2) % select the left side of the screen to plot plot(t,temp_out)% plot the graphs of the output tempe. xlabel('time[sec]'), ylabel('Amplitude output temperature'), title('the curve of the output temperature'), grid if ishold ~=1 hold on, end end hold off 10 Article 2 Simulation of Thermal System The curve of input tempereture The curve of the output tempereture 0.8 0.35 R=0.2 tau=1 0.7 0.3 Amplitiud of the output temperature[C] Amplitiud of the input tempereture[C] 0.6 R=0.5 0.25 tau=5 0.5 0.2 tau=12 0.4 0.15 0.3 R=0.8 0.1 0.2 0.1 0.05 0 0 Figure 1.3c. The temperature of the cold water. Figure1.3d. The temperature of the hot water 0 5 10 0 5 10 time[sec] time[sec] FFf Figure 1.4 The input temperature Figure 1.5 The output of the hot water %Script 1.3 The program compute the response of the system %with different values of time constant tau=RC. Assume that %the heat input rate and the liquid flow rate are kept constant while the temperature of the inflowing liquid is: % (1) suddenly change from 0 to 1{step input}) % (2) linearly changed with time {ramp input} t=0:0.01:10; %vector of time input R=[0.2 0.5 1];%three different values of thermal resistance teta2_in=20.*t; % generate a ramp input teta1_in=1-(exp(-1000.*t));% generate a step input c=1 %spesific heat of the water 11 Section 1.5 System Simulation in time domain % m=[5 10 15];% the mass of the water in the tank C=c.*m % the thermal capacitance of the water tau=R.*C;% the time constant of the system i=1% countre for i=1:3% teta1_out= teta1_in.*(1-exp(-t/tau(i)));% %the response of the output temp due to the step input teta2_out= teta2_in.*(1-exp(-t/tau(i))); % the response output tempe due to the ramp input i=i+1 subplot(2,2,1)% pick the upper left corner of the screen plot(t,teta1_in)% plot inflowing liquid temp graph ylabel('Amplitude of input temp'), title('the graph of input temp'), subplot(2,2,2)%pick the upper right corner of the screen plot(t, teta1_out)% plot outflowing liquid temp graph. ylabel('Amplitude of output temp'), title('the responses of output temp'), if ishold~=1 hold on end grid subplot(2,2,3)% pick the lower left corner of the screen plot(t, teta2_in)%plot the graph of the inflowing liquid xlabel('time[ sec]'), ylabel('Amplitude of input temp'), title('the graph of the input temp'), grid subplot(2,2,4)%pick the lower right corner of the graph plot(t, teta2_out)%plot the graph of the inflowing liquid xlabel('time[ sec]'), ylabel('Amplitude of output temp'), title('the graph of the output temp') if ishold~=1 hold on end grid end 12 Article 2 Simulation of Thermal System the graph of input temp the responses of output temp 1 1 Amplitude of output temp Amplitude of input temp 0.8 0.8 0.6 0.6 tau deacreses 0.4 0.4 0.2 0.2 0 0 0 5 10 0 5 10 the graph of the input temp the graph of the output temp 200 200 Amplitude of output temp Amplitude of input temp 150 150 100 100 tau decreases 50 50 0 0 0 5 10 0 5 10 time[ sec] time[ sec] Figure 1.5 While the tope two graphs from left to right represent the input and the out put temperatures of the thermal system when the input is a unit step. The bottom two graphs from left to right represent input and the out put temperature of the plant when the input is a unit ramp respectively. 1.6 System Analysis the Frequency Domain: In this section we analyze the response of the output temperature based on the transfer function of the system. In order to obtain the transfer function of the system we need to apply the Laplace transform, i. e, we have to change the system from time domain to frequency domain. 13 Section 1.6 System Simulation in time frequency domain The differential equation of the system that relate the temperature of the outflowing liquid to the heat supplied by heater while the inflowing liquid temperature is kept constant is given by equation (2.8), for commodity we rewrite it here d RC Rhi (2.0) dt The Lapalce transform of the equation (2.0) gives: d L ( RC Rhi ) dt d RCL ( ) RCL ( ) RL (hi ) dt RC[ s s RC s R( H i ( s) (3.0) where (s) = L [(t)] and Hi(s) = L [hi(t)] and we assume that the output initial temperature at t= 0 is (0)=0. From equation (3.0) it follows that the transfer function relating the temperature of the outflowing liquid and the heat supplied by heater hi is given by ( s ) R (3.1) H i ( s ) RCs 1 Now we exam the transfer function that relate outflowing liquid temperature with the inflowing liquid temperature. The differential equation of the system that relate the temperature of the outflowing liquid to the inflowing liquid while the heat supplied by heater is kept constant is given by equation (2.1), for commodity we rewrite it here. d RC i (2.1bis) dt The Lapalce transform of the equation (2.1) gives: d L [ RC i ] dt 14 Article 2 Simulation of Thermal System d RCL L L i dt RC s (0) s) i s) (3.2) where (s) = L [(t)] and i(s) = L [i(t)] and we assume that the output initial temperature at t=0 is (0)=0. From equation (3.2) it flows that the transfer function relating temperature of the outflowing liquid to the temperature of the inflowing liquid is given i is given by ( s ) 1 (3.3) i ( s ) RCs 1 From equations (3.2) and (3.3) we obtain the following block diagram of the system. i(s) + Hi(s) R + 1 _ RCs (s) Figure 1.6 The block diagram of the Thermal system 15 Section 1.6 System simulation in the frequency domain % Script1.4 The program compute the step response of the %system when the inputs are the inflowing liquid and heat %supplied %by heater. The program evaluate the response of %the system with different values of time constant tau=RC m=[1 2 5 10 15]; % the mass of the water in the tank c=1; % specific heat of the water C=c.*m; %the thermal capacity of the water R=[2 2 2 2 2]; % the thermal resistance tau= R.*C; % the time constant of the system j=1; % counter % t1=0:10; input_1=step(1,1);% generate the graph of the infolwing temp for j=1:5 num_1= R(j); %numerator of the transfer function when the % input is the heat supplied by heater num_2=1; % numerator of the transfer function when the % when the inflowing liquid temp den=[tau(j) 1];%denominator of the transfer function input_2=input_1*R(j); %generate the graph of the input %temp due to the heat supplied by heater subplot(2,2,1)% pick the upper left side of the screen %to plot plot(t1,input_1); % plot the inflowing temperature output_1(:,j)= step(num_1,den,t);% the response of the out- %flowing liquid temp subplot(2,2,2)% pick the upper left side of the screen %to plot plot(t,out_1) % plot the outflowing temperature subplot(2,2,3) plot(t1,input_2); pick the lower left side of the screen %to plot out_2(:,j)=step(num_2,den,t); %the response of the out- %flowing liquid temp subplot(2,2,4) ); pick the lower right side of the screen %to plot plot(t, out_2) % plot the outflowing temperature j=j+1; end 16 Article4 Simulation of Thermal System 17 Section 1.6 System Simulation in the frequency domain Script1.5 The program compute the response of the system when the % inflowing liquid and heat supplied by heater are change %linearly (i.e., ramp input). The program evaluate the response % of the system with different values of time constant tau=RC m=[1 2 5 10 15];c=1;C=c.*m; R=[2 2 2 2 2]; tau= R.*C; j=1; t1=0:10; input_1=step(1, [1 0]); for j=1:5 num_1= R(j) ; num_2=1; den=[tau(j) 1 0]; H_1=tf(num_1, den); H_2=tf(num_2, den); input_2=input_1*R(j);%the input temperature subplot(2,2,1) plot(100.*input_1); grid ylabel('Amplitude') title( 'the input temp') out_1(:,j)= step(num_1,den,t); subplot(2,2,2) plot(t,out_1) grid ylabel('Amplitude') title( 'the output temp due to the input temp') subplot(2,2,3) plot(100.*input_2); grid xlabel('time[sec]'); ylabel('Amplitude') title( 'the input temp') out_2(:,j)=step(num_2,den,t); subplot(2,2,4) plot(t, out_2) grid xlabel('time[sec]'); ylabel('Amplitude') title( 'the output temp due to the heater') j=j+1; end 18 Chapter 3 Simulation of Thermal System Script1.6 This MATLAB cods creates a mesh plot showing the effect of increasing the time constant R=2; n=[5.1:0.1:20.1]; y=zeros(200,1); for i=1:140 num=1/n(i); num_1=R.*num; den=[1 num]; t=[0:.1:19.9]'; y(:,i)=step(num,den,t); y1(:,i)=step(num_1,den,t); i=i+1; end subplot(2,1,1); mesh(fliplr(y),[-120 30]) subplot(2,1,2); mesh(fliplr(y1),[-120 30]) ylabel('time'), xlabel('timeconstantau') ,zlabel('Amplitiud'),title('Mesh Plots Showing Step Response for 14 time constante') 19 Section 1.6 System Simulation in the frequency domain 1.7 System Analysis using SIMULINK: Now we apply the Matlab Simulink to analyze the step, ramp responses of the thermal system. In this section we use two different methods to analyze the response of the system. The first method is based on the transfer function of the system, and in the second one is based feedback block of the system. In these paragraph we analyze the step responses of the system based on the transfer function of the system. The block diagram of the transfer function of the system with different values of time constant is shown below. Since the system has two inputs we divide the block diagram in to two sub block diagrams. The sub block diagram with light blue color relate the heat supplied by heater with the output temperature of the hot water while the sub-block diagram with orange color relate the input temperature of the cold water with the output temperature of the hot water. 20 Article 2 Simulation of Thermal System Fig 1.7a. The block diagram of the thermal system with different values of time constant The corresponding step responses of the system the system is shown in the figure 1.7b. From top to bottom the first three step responses are due to input heat supplied by the heater while the last three responses are due to the input temperature of the cold water. 21 Section 1.7 System Analysis using Simulink Figure 1.7b The step responses corresponding to the block diagram shown above 1.7b Feedback Simulink: In this section we exam the step responses of the system by building the feedback block diagram that simulate of the differential equation of our system. For example the feedback block diagram that simulate the differential equation : RC Rhi is shown below hi 1/C + d/dt - 1/RC Fig 1.7b Block diagram for simulating fist order differential equation 22 Article 2 Simulation of Thermal System The feedback block diagram that simulate the differential equation of the system with different values of time constant and the corresponding step response is shown figure 1.7c and 1.7d respectively. 23 Section 1.7 Simulation with Simulink 24

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