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EXPERIMENT-2

zmir Institute of Technology

Chemical Engineering Department

2008-2009 Spring Semester

CHE 310

Chemical Engineering Laboratory I

Thermal Conductivity

THERMAL CONDUCTIVITY

1. OBJECTIVES

( To understand the use of the Fouriers law in determining heat rate through solids.

( To determine the thermal conductivityof a material, k.

( To determine the Overall Heat Transfer Coefficient for the flow of heat through a combination of different materials in use.

( To demostrate the effect of cross sectional area on the heat rate.

( To demostrate the effect of contact resistance on thermal conduction between adjacent materials.

( To measure the temperature distribution for unsteady state conduction of heat through the uniform plane wall and the wall of the thick cylinder.

2. THEORY AND PRINCIPLES

Conduction (heat transfer by diffusion) is the transport of energy from the more energetic to the less energetic particles of a substance due to a temperature gradient, and the physical mechanism is that of random atomic and molecular activity. For one-dimensional, steady-state heat conduction in a plane wall with no heat generation, temperature is a function of the x coordinate only and heat is transferred exclusively in this direction. Thus, the temperature distribution for the heat conduction through plane wall must be linear as shown in Figure 1.

Ts,1

Ts,2

qx

x x=L

Figure-1: Heat transfer through a plane wall

The heat transfer rate (qx) by conduction through a plane wall is directly proportional to the cross sectional area (A) and the temperature difference ((T), whereas it is inversely proportional to the wall thickness ((x).

In addition to single plane wall, heat transfer through composite wall is also important. Such walls may involve any number of series and parallel layers made of different materials. In the case of steady state one-dimensional heat conduction with no heat generation, temperature profile through each layer becomes linear as shown in Figure 2. Heat transfer through composite systems is usually described by an overall heat transfer coefficient. Simply, the overall heat transfer coefficient is related to the total thermal resistance.

Ts,1

Ts,4

A B C

xA xB xC

Figure-2: Heat transfer through composite systems.

Cylindrical and spherical systems often experience temperature gradients in the radial direction only and may therefore treated as one dimensional. A common example is the hollow cylinder, whose inner and outer surfaces are exposed to fluids at different temperatures, as shown in Figure 3.

Figure-3: Heat transfer through radial systems

The temperature distribution associated with radial conduction through a cylindrical wall is logarithmic, not linear, as it is for the plane wall under the same conditions.

3) EXPERIMENTAL

3.1 THERMAL CONDUCTIVITY CALCULATION IN LINEAR SYSTEMS

Experimental set-up for the linear conductive heat transfer system is shown in Figure 4.

Figure 4: Linear heat conduction unit.

A. Determine the effect of change of heat flow for steady state conduction of energy through a uniform plane

Procedure:

i) Smear the faces of the heated and cooled sections with thermal conducting paste and clamp them together without any intermediate section in place as illustrated in the following scheme.

ii) Ensure that the cooling water is flowing and then set the heater voltage V

iii) Monitor temperature T1, T2, T3, T6, T7 and T8 until steady-state is reached.

iv) When the temperatures are stabilized, record T1, T2, T3, T6, T7 and T8, V and I.

v) Reset the heater voltage and repeat the above procedure again recording the parameters T1, T2, T3, T6, T7 and T8, V and I when temperatures have stabilised.

vi) Reset the heater voltage and repeat the above procedure again recording the parameters T1, T2, T3, T6, T7 and T8, V and I when temperatures have stabilised.

B. Determine heat rate through solid materials for one dimensional, steady flow of heat

Procedure:

i) Smear the faces of the heated and cooled sections with thermal conducting paste and clamp them together with the Brass Intermediate Specimen in place as illustrated in the following scheme.

ii) Ensure that the cooling water is flowing and then set the heater voltage V

iii) Monitor temperature T1, T2, T3, T4, T5, T6, T7 and T8 until steady-state is reached.

iv) When the temperatures are stabilized, record T1, T2, T3, T4, T5, T6, T7 and T8, V and I.

v) Reset voltage and repeat the above procedure again recording the parameters T1, T2, T3, T4, T5, T6, T7 and T8, V and I when temperatures have stabilised.

C. Determine overall heat transfer coefficient for the flow of heat through a combination of different materials in use and determine the thermal conductivity k of a metal specimen

Procedure:

i) Smear the faces of the heated and cooled sections with thermal conducting paste and clamp them together with the Stainless steel and Aluminium Intermediate Specimens in place as illustrated in the following scheme.

ii) Ensure that the cooling water is flowing and then set the heater voltage V for stainless steel specimen and for aluminium specimen.

iii) Monitor temperature T1, T2, T3, T6, T7 and T8 until steady-state is reached.

iv) When the temperatures are stabilized, record T1, T2, T3, T6, T7 and T8, V and I.

v) Reseet the voltage and repeat the above procedure again recording the parameters T1, T2, T3, T4, T5, T6, T7 and T8, V and I when temperatures have stabilised.

D. Determine the effect of cross sectional area on the heat rate

Procedure:

i) Smear the faces of the heated and cooled sections with thermal conducting paste and clamp them together with the reduced diameter brass intermediate specimen in place as illustrated in the following scheme.

ii) Ensure that the cooling water is flowing and then set the heater voltage V

iii) Monitor temperature T1, T2, T3, T6, T7 and T8 until steady-state is reached.

iv) When the temperatures are stabilized, record T1, T2, T3, T6, T7 and T8, V and I.

v) Reset the voltage and repeat the above procedure again recording the parameters T1, T2, T3, T4, T5, T6, T7 and T8, V and I when temperatures have stabilised.

E. Determine the effect of contact resistance on thermal conduction between adjacent materials

Procedure:

i) Ensure that the faces of the heated and the cooled sections are cleaned of thermal conducting paste and that the brass intermediate section is also similarly cleaned.

ii) Lightly coat the mating faces between the cooled section and the brass intermediate specimen with thermal paste and assemble them together.

iii) Do not coat the mating faces of the heated section and the brass intermediate specimen with thermal paste and assemble.

iv) Finally, do not clamp the assembly together as normal but leave the clamps open as illustrated in the following scheme.

v) Ensure that the cooling water is flowing and then set the heater voltage V to approximately 12 volts

vi) Monitor temperature T1, T2, T3, T4, T5, T6, T7 and T8 until steady-state is reached.

vii) When the temperatures are stabilized, record T1, T2, T3, T4, T5, T6, T7 and T8, V and I.

viii) Reset the voltage and repeat the above procedure again recording the parameters T1, T2, T3, T4, T5, T6, T7 and T8, V and I when temperatures have stabilised.

ix) Clamp the sections together on the unit. Monitor temperatures T1, T2, T3, T4, T5, T6, T7 and T8 until they become stable and then repeat the above readings.

F. Determine the thermal conductivity,k of an insulation material

Procedure:

i) Ensure that the faces of the heated and cooled sections are cleaned of thermal conducting paste.

ii) Select the thin cork disc provided, measure and record the thickness (xint of the disc as accurately as possible ( A vernier gauge or micrometer is suitable). Place this between the heated and cooled sections then clamp the assembly together as illustrated in the following scheme.

ii) Ensure that the cooling water is flowing and then set the heater voltage V

iii) Monitor temperature T1, T2, T3, T6, T7 and T8 until steady-state is reached.

iv) When the temperatures are stabilized, record T1, T2, T3, T6, T7 and T8, V and I.

v) Reset the voltage and repeat the above procedure again recording the parameters T1, T2, T3, T4, T5, T6, T7 and T8, V and I when temperatures have stabilised.

G. Observe unsteady state conduction of heat

Procedure:

i) Ensure that the faces of the heated and cooled sections are cleaned of thermal conducting paste.

ii) Select the thin cork disc provided and place this between the heated and cooled sections then clamp the assembly together as illustrated in the following scheme.

ii) Ensure that the cooling water is flowing.

iii) Then disconnect the heater dc supply and then set the heater voltage V

iv) Start a stopwatch to record regular time intervals and then reconnect the dc supply to the heater with the voltage still set