Composite wall - Heat and Mass Transfer Experiment PDF

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Summary

Composite WallExperimental Setup:(1) Hand Wheel (2) Screw (3) Cabinet (4) Fabricated Frame(5) Mild Steel Plate ( 6) Bakelite Plate ( 7) Brass Plate (9) HeaterIntroduction:Thermal conductivity is the physical property of the material denoting the case with a particular substance can accomplish the tr...

Description

Composite Wall

Composite Wall Experimental Setup:

(1) Hand Wheel (2) Screw (3) Cabinet

(4) Fabricated Frame

(5) Mild Steel Plate (6) Bakelite Plate (7) Brass Plate (9) Heater Introduction: Thermal conductivity is the physical property of the material denoting the case with a particular substance can accomplish the transmission of thermal energy by molecular motion. Thermal conductivity of a material is found to depend on the chemical composition of the substance of which it is composed, the phase ( i.e solid, liquid or gas ) in which it exists , its crystalline structure of a solid, the temperature and pressure to which it is subjected, and whether or not it is a homogeneous material. Description: At many places composite structure are used mainly to reduce the heat loss. Such examples are cold storage walls, refrigerator walls etc. this composite structure comprises of layer of different types of insulating materials. The thickness of theses can be varied according to the requirements. This apparatus is designed and fabricated mainly to study the above mentioned characteristics of the composite structures. Here three types of slabs are provided namely, mild steel Bakelite 1

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Composite Wall and brass. The thicknesses of the three slabs are different. Heat input across these composite walls is given by a Nichrome heater. Total heater assembly comprises of a heater bound between two aluminium plates. On the both sides of this heater identical structures of composite walls are placed. Thermocouples are provided at proper position in the composite walls. Multichannel temperature indicator is used to measure these temperatures. Small hand press is provided to press the walls on each other and to ensure that no air gap is remaining between two plates. Heat input to the heater is given through a dimmerstat and measured by a voltmeter and ammeter. An enclosure is provided around the walls to ensure the steady atmospheric condition with one side of Perspex for visualization. All instruments and the apparatus are mounted on separate panels. By varying the heater input and combination of the composite structures wide range of experiments can be performed. Specification 1) 2) 3) 4) 5) 6) 7) 8) 9)

Mild steel 250mm×25mm Bakelite 250mm×10mm Brass 250mm×10mm No of thermocouples 8 Nos. Heater Coil-Nichrome heater Temperature Indicator- 0-300oc Dimmerstat 2 Ampere open type Voltmeter 0-300 V Ammeter 0-2 A

Theory: No. microscopic motion is involved in heat conduction in a solids, the two fundamental laws, namely the law of conservation of mass and Newton’s second law are trivially satisfied. The only fundamental law which will need to be satisfied is the first law of thermodynamics as applied to a system. Heat Transfer through composite wall Q=

kxA (TA − TD) b

Where Q=Heat flow rate in W A=Area of slab in m2 2

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Composite Wall b=Thickness of slab in m(Total b1+b2+b3) TA= Temperature of the composite slab in oc(First Location Average) TD= Temperature of the composite slab in oc(Last Location Average) K=Thermal conductivity of composite slab in w/m-k Sample Calculation Observations 1) Mild steel 2) Bakelite 3) Brass

250mm×25mm 250mm×10mm 250mm×10mm

K(MS) = 54 w/mK K(Bak) = 0.17 w/mK K(Brass) = 110 w/mK

Observations Table T1oC 41.1

T2oC 40.8

T3oC 39.1

T4oC 38

T5oC 31.2

T6oC 30.8

T7oC 30.5

T8oC 31

V in Volt 100

I in Ampere 0.5

Calculation Area of the plate A =

πd2 π(0.25)2 = 0.049 m2 = 4 4

Heater Input 1) Q = V×I/2 = 100×0.5/2 = 25 W T1+T2 41.1+40.8 = 2) TA = = 40.95 oC 2 2 TB = TC =

TD =

T3+T4 2 T5+T6 2

T7+T8 2

= = =

39.1+38 2

= 38.5 oC

31.2+30.8 = 31 oC 2 30.5+31 2

= 30.75 oC

3) Practical Thermal Resistance 40.95+30.75 71.7 TA +TD = = Rth(Pra) = 25 Q 25 4) Theoretical thermal resistance 3

= 2.868 k/w HMT Lab Manual

Composite Wall Rth(Theo) =

1 L1

Rth(Theo) =

1

L2

L3

[ + K2 + K3] A K1

[ 0.049

0.025 54

0.01

0.01

+ 0.17 + 110 ]

=1.21 k/w 5) Thermal Conductivity of Composite wall

K=

K=

25×L A(TA−TD) 25×0.045

0.049(40.95−30.75)

K = 2.45 w/m k

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Composite Wall

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