class 13 - mathematical modeling of thermal system
TRANSCRIPT
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System Modeling Coursework
P.R. VENKATESWARANFaculty, Instrumentation and Control Engineering,
Manipal Institute of Technology, ManipalKarnataka 576 104 INDIAPh: 0820 2925154, 2925152
Fax: 0820 2571071Email: [email protected], [email protected]
Web address: http://www.esnips.com/web/SystemModelingClassNotes
Class 13: Modeling of Thermal systems
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July – December 2008 prv/System Modeling Coursework/MIT-Manipal 2
WARNING!
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I claim no originality in all these notes. These are the compilation from various sources for the purpose of delivering lectures. I humbly acknowledge the wonderful help provided by the original sources in this compilation.
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For best results, it is always suggested you read the source material.
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Of late, this has becoming supplement to what is taught in the class. So BEWARE! You are on two tracks!
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Contents
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Description of a Thermal system•
Model of the Thermal system
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Introduction
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Thermal systems are those that involve the transfer of heat from one substance to another.
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Thermal system may be analyzed in terms of thermal resistance and thermal capacitance although they may not be represented as lumped parameters.
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But by making some assumptions, they can be represented as distributed parameters, which make the analysis simple.
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A Thermal System
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Assumptions for the system
• Fluid in the tank is perfectly mixed so that it is at uniform temperature
• The tank is insulated to eliminate heat loss to the surrounding air.
• There is no heat storage in the insulation.
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Definitions for variables of the system
• θi
= Steady state temperature of inflowing liquid,•
θ
= Steady state temperature of out-flowing liquid,
•
H = Steady state heat input rate from heater.
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What happens when you move from steady state?
• Let ∆H be a small change in the heat input rate from its steady state value. This change in H will result in the following changes.
– Change in heat output rate by an amount ∆H1
.–
Change in heat storage rate of liquid in the tank by an amount ∆H2
.–
Change in temperature of out-flowing liquid by an amount ∆θ.
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Thermal Resistance
•
Change in outflow heat rate is given by∆H1
= Q Cs ∆θ•
WhereQ = Steady state liquid flow rateCs = Specific heat of liquid
• ∆H1
= ∆θ/R–
If R = 1/QCs which is defined as the Thermal Resistance
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Thermal Capacitance
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Change in heat storage rate is given by∆H2
= MCs d∆θ/dt•
Where–
M = mass of the liquid in the tank
–
∆dθ/dt
= rate of rise of temperature in the tank•
∆H2
= C d∆θ/dt–
Where C = MCs which is defined as Thermal Capacitance.
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Transfer function for the system
∆H= ∆H1 + ∆H2 •
The mathematical model of a thermal system shown in figure is
•
Applying Laplace transform
H= + C dR dtθ θΔ Δ
Δ
( )H(s)= + Cs ( )
1 = + Cs ( )
1 Cs = ( )
s sR
sR
R sR
θ θ
θ
θ
ΔΔ Δ
⎡ ⎤ Δ⎢ ⎥⎣ ⎦+⎡ ⎤ Δ⎢ ⎥⎣ ⎦
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Transfer function of the system
( ) = H(s) 1 Cs
s RR
θΔ ⎡ ⎤⎢ ⎥Δ +⎣ ⎦
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Summary
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Thermal systems are simple systems like level and do not yield to complexities like pneumatic or hydraulic systems.
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Hence, it is easy and possible to associate analogous situations and derive the transfer function.
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References
1.
Advanced Control Systems Engineering, Ronald Burns
2.
Modern Control Engineering, Ogata3.
Control Systems, Nagoor
Kani
4.
A course in Electrical, Electronic Measurements and Instrumentation, A.K. Sawhney
…amongst others
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And, before we break…
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Love has the patience to endure the fault we cannot cure.
Thanks for listening…