Heat Transfer through Composite Walls

Aim: To determine total thermal resistance and thermal conductivity of composite walls. Introduction: Many engineering applications of practical utility involve heat transfer through a medium composed of two or more materials of different thermal conductivities arranged in series or parallel. Consider for example the walls of a refrigerator, hot cases, cold storage plants, hot water tanks etc., which always have some kind of insulating materials between the inner and the outer wall. A hot fluid flow inside the tube covered with a layer of thermal insulation is an example of composite system because in this case the thermal conductivities of tube metal insulation are different. The problem of heat transfer through the composite system can be solved by the application of thermal resistance concept. The procedure for solving one dimensional, steady state heat conduction problems for composite system comprising parallel plates, co-axial cylinders or concentric spheres are dealt here. Description: The apparatus consists of three slabs of different materials namely Mild Steel, Wood and Bakelite. The heater is provided to supply heat input across these composite walls. Total heater assembly comprises of a heater bound between two aluminium plates, on both side of these heater identical structures of composite walls are placed. Thermocouples are provided at proper positions in the composite walls to record desired inside temperature of composite wall. Multi-channel temperature indicator is used to measure this temperature. Small hand press provided to press the wall on each other and ensure that no air gap remaining between two plates. Heat input to heater is given through a dimmerstat variac and measured by Voltmeter and Ammeter. By varying heat input & combination of the composite structure, wide range of experiment can be performed. Specifications:

(1) Diameter of slabs (d) Wood : 0.3m Bakelite : 0.3m Mild Steel : 0.3m

(2) Thickness of slab (b) : 0.036m (3) Heater : 400W (4) Thickness of wood plate : 0.012m (5) Thickness of Bakelite plate : 0.012m (6) Thickness of Mild Steel plate : 0.012m (7) Temperature Indicator : 0 - 300ºC (8) Ammeter : 0 – 5A (9) Voltmeter : 0 – 300V

Precautions: (1) Keep the dimmerstat at zero position before switch on the power supply. (2) Increase the voltage gradually. (3) Operates selector switch of temperature indicator gently. (4) Do not exceed 120V so as to avoid the fluctuating result.

Procedure: (1) Arrange the plates in proper fashion (symmetrical) on both sides of the

heater plates. (2) See that plates are symmetrically arranged on both sides of the heater

plates. (3) Operate the hand press properly to ensure perfect contact between the

plates. (4) Close the box by cover sheet to achieve steady environmental

conditions. (5) Switch on the supply of heater. (6) Give known steady input to the heater with the help of dimmerstat. (7) Keep initially 100 V for 20 minutes almost and than reduce to 80 V till

steady state is reached so that steady state can be reached within less time.

(8) Check the input to the heater with selector switch, voltmeter & ammeter.

(9) Note down the temperature every 10 minutes till a steady condition is reached.

(10) Calculate the thermal resistance of the material based on the steady state condition readings.

Observation table: -

Voltmeter reading (V): Ammeter reading (I): Thermocouple Readings (ºC)

Time (min)

T1 T2 T3 T4 T5 T6 T7 T8

Calculations:

(1) Rate of heat supply Q = V x I W (For calculating the thermal conductivity of composite walls, it is assumed that due to large diameter of the plates, heat flowing through the central portion is unidirectional i.e. axial flow. Thus for calculating central half diameter area where unidirectional flow is considered. Accordingly thermocouples are fixed at closed to center of the plates.)

.q = 2

QW2

A m

A = 2π d4

m2

.q in any one direction = Q

2

1 2A

T +TT =2

3 4B

T +TT =2

5 6C

T +TT =2

7 8D

T +TT =2

(2) Total thermal resistance of composite slab:

A Dtotal Q

2

T -TR =

(3) Thermal conductivity of composite slab:

.

compositeA D

q xbK =T -T

b = total thickness of the slab in m (4) Thermal conductivity of individual materials

.

woodwood

A B

q xbK =T -T

.

bakelitebakelite

B C

q xbK =T -T

.

M.S.M.S.

C D

q xbK =T -T

(5) Draw the temperature distribution along wall thickness ( dTdx

)

Conclusion:

Thermal Conductivity of Insulating Powder Aim: To determine the thermal conductivity of insulating powder. Introduction: Thermal conductivity is one of the important properties of the material and its knowledge is required for analyzing heat conduction problems. Physical meaning of thermal conductivity is how quickly heat passes through a given material. Thus the determination of this property is of considerable engineering significance. There are various methods of determination of thermal conductivity suitable for different material. The present apparatus is suitable for finding out thermal conductivity of the material in the powdered form. Apparatus: The apparatus consists of two thin walled concentric copper spheres. The inner sphere houses the heating coil. The insulating powder (Asbestos powder/ Plaster of paris-legging material) is packed between the inner and outer sphere. The power supply to the heating coil is adjusted by using a dimmerstat and is measured by a voltmeter and ammeter. Chromel-Alumel thermocouples are used to measure the temperature. Thermocouple No. 1 to 4 is embedded on inner sphere and No. 5 to 10 is embedded on outer sphere. Under steady state condition the temperature T1 to T10 are noted and also the voltmeter and ammeter readings are recorded. These readings in turn enable to find out the thermal conductivity of the insulating powder packed between the two spheres. The apparatus assumes one dimensional steady state radial heat conduction across the powder layer. Specifications:

(1) Radius of inner copper sphere (ri) = 50mm (2) Radius of outer copper sphere (ro) = 100mm (3) Voltmeter = 0 – 300V (4) Ammeter = 0 – 5A (5) Dimmerstat = 2A (6) Heater = mica type (7) Temperature indicator = 0 – 300ºC (8) Thermocouple No. 1 to 4 on inner sphere to measure Ti (Refer fig.) (9) Thermocouple No. 5 to 10 on outer sphere to measure To (Refer fig.) (10) Insulating powder (Asbestos in powdered form)

Procedure:

(1) Start the supply. (2) Increase slowly input to heater by dimmerstat and adjust equal to 60-100V (3) Take the temperature readings T1 to T10 after time interval of 10 minutes. (4) Take the readings till steady state is reached. (5) Note down the readings T1 to T10.

Precaution: (1) Keep the dimmerstat to zero position before starting the unit. (2) Operate selector switch of temperature indicator gently. (3) Never exceed the heater input to 100V.

Observation Table:

Voltmeter reading (V): Ammeter reading (I): Thermocouple Readings (ºC)

Time (min)

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10

Calculations: (1) Power input to heater Q:

Q = V x I Watt (2) Average surface temperature of inner sphere Ti

1 2 3 4i

(T +T +T +T )T = °C4

(3) Average surface temperature of outer sphere To

5 6 7 8 9 10o

(T +T +T +T +T +T )T = °C6

(4) Thermal conductivity of insulating powder (K)

i 0 i o

1 1 1 WK=Q( )r r 4π(T -T ) mK−

Heat Transfer From a Pin Fin Apparatus

Aim: To determine heat transfer co-efficient and efficiency of a pin fin. Introduction: Extended surfaces or fins are used to increase the heat transfer rate from a surface to a fluid whenever it is not possible to increase the value of the surface heat transfer co-efficient or the temperature difference between the surface and the fluid. The use of this is vary common and they are fabricated in variety of shapes circumferential fins around the cylinder of a motorcycle engines and fins attached to condenser tubes of a refrigerator are few familiar examples. It is obvious that a fin surface strikes out from the primary heat transfer surface. The temperature difference with surrounding fluid will steadily diminish as one moves out along the fin. The design of the fins therefore requires knowledge of the temperature distribution in the fin. The main object of this experimental set-up is to study the temperature distribution in a simple pin fin. Apparatus: A brass fin of circular cross section is fitted in a long rectangular duct. The other end of the duct is connected to the suction side of a blower and the airflows passed the fin perpendicular to its axis. One end of the pin projects outer sided the duct and is heated by a heater. Temperatures at five points along the length of the fin are measured by Chromel-Alumel thermocouples connected along the length of the fin. An orifice meter fitted on the delivery side of the blower measures the airflow rate.

Schematic diagram of the set-up is shown in fig.2, while the detail of pin fin is as per fig.3 Specifications: (1) Diameter of the fin = 12.7mm (2) Length of the fin = 150mm (3) Diameter of orifice = 14mm (4) Internal Diameter of delivery pipe = 36mm (5) Co-efficient of discharge of orifice = 0.65 (6) Single phase motor for Centrifugal blower = 0.5 HP (7) Thermocouples on the fin = 5 nos. (8) Thermocouple for Ambient temperature = 1 no. (9) Fin material = Brass (10) Range of temperature indicator = 0 – 300 ºC (11) Range of Voltmeter = 0 – 300 V (12) Range of Ammeter = 0 – 5 Amp. (13) Heater (14) Dimmerstat for controlling heat supply

Procedure: (1) Switch on main switch. (2) Start heating the fin by switching on the heater. (3) Adjust dimmerstat voltage equal to 60 V to 90 V. (4) Start the blower. (5) Adjust the difference of level in manometer with the help of valve. (6) Take the reading of thermocouples in the time interval of 10 min. (7) Take the readings till steady state condition is reached.

Precautions:

(1) See that dimmerstat is at zero position before switching on heater. (2) Operate change over switch of temperature indicator gently. (3) Be sure the steady state is reached before taking the final readings.

Observation table: Input voltage = Input current = Manometer readings – h1 = h2 = h = h2 – h1= Ambient temperature (T6) = º C

Time in Min

Temperature T1 T2 T3 T4 T5 T6

Calculations:

(1) Average fin temperature (Tm)

1 2 3 4 5m

T +T +T +T +TT =5

º C

(2) Mean Fin temperature = m 6mf

T +TT =2

º C

(3) Air flow rate (Q)

md 1 2

a 3

2 21 2

ρC xa xa x 2ghρ

Q= m /sa - a

Where, Cd = 0.65 a1 = area of delivery pipe in m2 = 1.01x10-3 m2

a2 = area of orifice in m2 = 1.54 x 10-4 m2 g = 9.81 m/s2 h = difference in manometer reading in m ρw = density of water = 1000 Kg/m3 ρa = density of air = 1.207 Kg/m3 Note: Take the values of kair, υair from chart at Tmf

(4) Velocity of air at ambient temperature (T6)

QV= m/sA

A = area of duct = 0.0165 m2

(5) Velocity of air at Tmf mf

6

T +273V=VxT +273

m/s

(6) Reynolds No. (Re) mf

ea

V xdR =υ

(7) Nusselt No. (Nu) 0.466

u eN =0.615R when 40 < Re < 4000 0.618

u eN =0.174R when 4000 < Re < 40,000 0.805

u eN =0.024R when 40,000 < Re < 4, 00,000 (8) Heat transfer co-efficient (h)

u airN xKh=d

W/m2K

(9) Fin parameter (m) h.cm=k.A

m-1

Where, c = circumference of fin (π.d) = 0.0398m k = thermal conductivity of fin material (Brass) = 110.71 W/mº C A = Area of fin = 1.266x10-4 m2

(10) Efficiency (η) ml -ml

ml -ml

e -eη=tan h(ml)=e +e

= __________%

Heat Transfer In Natural Convection

Aim: To determine the heat transfer co-efficient for a vertical tube losing heat by natural convection.

Introduction: In contrast to forced convection, the natural convection phenomenon is

due to the temperature difference between the surface and the fluid and is not created by any external agency.

The present experimental set-up is designed and fabricated to study the natural convection phenomenon from a vertical tube in terms of average heat transfer co-efficient and its comparison with the value obtained using an appropriate co-relation.

Apparatus: The apparatus consists of a brass tube fitted in a rectangular duct in a

vertical fashion. The duct is open at the top & bottom & forms an enclosure & serves the purpose of undisturbed surroundings. One side of the duct is made up of Perspex for visualization. An electric heating element is kept in the vertical tube, which in turn heats the tube surface. The heat is lost from tube to surrounding air by natural convection. The temperature of the vertical tube is measured by seven thermocouples. The heat input to the heater is measured by an ammeter & Voltmeter, is varied by a dimmerstat. The vertical cylinder with the thermocouples position is shown in fig. (1).

Specifications: (1) Diameter of the tube = 38mm (2) Length of the tube = 530mm (3) No. of thermocouples = 7 are as shown as 1 to 7 & marked on temperature indicator switch (4) Thermocouple No.8 reads the temperature of air in the duct (5) Temperature indicator = 0 – 300º C (6) Ammeter = 0 – 5 A (7) Voltmeter = 0 – 300 V (8) Dimmerstat = 2 Amp. Procedure:

(1) Put on the supply & adjust the dimmerstat to obtain the required heat input. (2) Wait till the fairly steady state is reached, which is confirmed from temperature

readings (T1 – T7) (3) Note down surface temperatures at various points (T1 – T7). (4) Note down ambient temperature T8.

Precautions:

(1) Before starting the unit, keep dimmerstat to zero volt position & increase it slowly.

(2) Operate selector switch of temperature indicator gently. (3) Never exceed 100V.

Observation Table: Input voltage (V) = Input current (I) = Thermocouple

Location Time (min.)

10 20 30 40 50 60 70 80 1 2 3 4 5 6 7

8(Ta) Calculation:

(1) Average heat transfer co-efficient (h)

as s a

qh =A (T -T )

Where, h = heat transfer co-efficient q = V x I Watts As = Surface area of cylinder in m2 = 2.π.r.l r = radius of pipe in m l = length of pipe Ts = Average of surface temperature T1 – T7 Ta = Ambient temperature

Heat Transfer In Forced Convection

Aim: To determine heat transfer co-efficient for internal pipe in forced convection. Introduction: If a heater is exposed to ambient room having air without any external movement there is a movement as a result of density gradient neat plate. This is natural convection as opposed to the forced convection, which is found in case of a fan blowing air over the plate. The quantity “h” is called convection heat transfer convection. Apparatus: The apparatus consists of blower unit fitted with the test pipe. The test section is surrounded by nichrome band heater. Six thermocouples are placed in the air system at the entrance & exit, of the test section & thermocouples are embedded on the test section to measure the air temperature. Test pipe is connected to the delivery side of the blower along with the orifice to measure flow of air through the pipe. Input to the heater is given through a dimmerstat & measured by voltmeter & ammeter. A temperature indicator is provided to measure temperatures of pipe wall in the test section. Airflow is measured with the help of orifice meter and the water manometer fitted on the panel. Specification: (1) Delivery pipe dia. (Di) = 0.036 m (2) Length of test section (L) = 0.45 m (3) Blower ( 1 – Ø with motor ) = 0.5 HP (4) Orifice Diameter = 0.014 m (5) Voltmeter = 0 – 300 V (6) Ammeter = 0 – 5 A (7) Thermocouples = Chromel – Alumel (8) Temperature Indicator = 0 – 300 ºC (9) Heater = Nichrome wire heater Procedure:

(1) Start the supply. (2) Start the blower and adjust the flow by means of valves to some desired

difference in manometer level (preferably open control valve fully.) (3) Start the heating of the test section with the help of dimmerstat and adjust

desired heater input with the help of ammeter and voltmeter (100 V). (4) Take the readings of all 8 thermocouples at an interval of 10 min. until the

steady state is reached. (5) Also note down the heater input (in terms of voltage and current)

Thermocouple positions: T1- T6 = Temperature of pipe wall in the test section T7 = Air inlet temperature T8 = Air outlet temperature

Precautions: (1) Keep the dimmerstat to zero position before switch on the power supply. (2) Increase the voltage gradually. (3) Do not stop the blower in between the testing period. (4) Operate selector switch of temperature indicator gently. (5) Do not exceed the heater input to 100V.

Observation Table: Input voltage (V) = Input current (I) = Manometer reading h1 = mm h2 = mm H = h1 – h2 = mm

Temperatures Time (min.) 10 20 30 40 50 60 70 80

T1 T2 T3 T4 T5 T6 T7 T8

Calculation:

(1) Bulk mean temperature of air ( )i o

bm

T +TT =

2

Properties of air at bmT

aρ = Kg/m3

aCp = KJ/Kg-K (2) Discharge of air

wd 1 2 a

aa 2 2

1 2

ρC ×a ×a × 2g×hρ

Q =a -a

Where a1 = Area of delivery pipe = 1.01 x 10-3 m2 a2 = Area of orifice = 1.54 x 10-4 m2 Cd = Co-efficient of discharge = 0.65 ha = Manometer reading in m = h1-h2 g = Gravitational acceleration = 9.81 m/s2 aρ = Density of air = 1.1238 Kg/ m3 aCp = Density of air at

(3) Mean mass flow rate of air (ma) ma = Qa x aρ Kg/s

(4) Heat of air (q) ( )a o iq = m×Cp × T -T KW

(5) Heat transfer co-efficient (h)

( )s bm

qh =A× T -T

W/m2-K

Where A = Area of test section. = π.Di.L m2 Ts = (T1 + T2 +…+ T6) / 6 ºC

Stefan-Boltzman Constant

Aim: To determine Stefan-Boltzman constant. Introduction: The most commonly used law of thermal radiation is the Stefan Boltzman law which states that thermal radiation heat flux or (emissive power) of a black surface is proportional to the forth power of absolute temperature of the surface and is given by,

4b

q = e =σ TA

⋅ W/m2

The constant of proportionality is called the Stefan Boltzman constant and has the value of 5.67 x 10-8 W/m2 K4. The Stefan Boltzman law can be derived by integrating the Plank’s law over the entire spectrum of wavelengths from 0 to ∞ . Through historically it is worth noting that the Stefan Boltzman Law was independently developed before Plank’s Law. The object of this experimental set-up is to verify the value of this constant fairly close by an arrangement. Apparatus: The apparatus as illustrated in fig. 1 is centered on a flanged copper hemisphere B fixed on flat non-conducting plate A. The outer surface of B is enclosed in a metal water jacket to heat B to some suitable constant temperature. The hemispherical shape of B is chosen solely on grounds that it simplify the task of drawing the water between B and C. Four Chromel –Alumel thermocouples are attached to various point on surface of B to measure its mean temperature to be read by a temperature indicator. The disc D that is mounted in an insulating bakelite sleeve S is fitted in a hole drilled in the centre of base plate A. the base of S is conveniently supported from underside of A. A Chromel-Alumel thermocouple is used to measure temperature of D (T5). A thermocouple is mounted on the disc to study the rise of its temperature. When the disc is inserted at the temperatureT (T1, T2,… the temperature of enclosure) the response of temperature change of disc with time is used to calculate the Stefan-Boltzman constant. Specification: (1) Hemisphere diameter = 0.2m (2) Base Bakelite plate diameter = 0.3m (3) Test disc diameter = 0.02m (4) Thickness of test disc = 0.002m (5) Thermocouples on hemisphere = 4 Nos.

(6) Water tank with immersion heater (7) Thermocouple on test disc = 1 No. (8) Temperature indicator = 1 No.

Procedure:

(1) First boil the water in the water tank with the help of immersion heater upto boiling temperature (T6 = 92-95 ºC)

(2) The disc D is removed before pouring the hot water in the jacket. (3) The hot water is poured in the water jacket.

(4) The hemispherical enclosure B will come to some uniform temperature T in short time after filling the hot water in the jacket.

(5) The enclosure will soon come to thermal equilibrium conditions. (6) Now insert the disc D at a time when its temperature is say Ta ( to be sensed by

a separate thermocouple). The radiation energy falling on D from the enclosure is given by

E = σ .AD.T4 ----------(1) Where AD = area of the disc D T = Avg. temperature of the enclosure recorded by

the thermocouples (T1 to T4) The absorptivity if the disc D is assumed to be unity (black disc). The radiant

energy disc D is emitting into the enclosure will be E1 = σ .AD.T1

4 ---------(2) The net heat input to disc ‘D’ is given by (1) – (2) E – E1 = σ .AD.(T4 – T1

4) If the disc ‘D’ has mass “m” and specific heat ‘S’ then after a short time Disc ‘D’ is inserted in ‘A’.

m.S.(dT/ds) = σ .AD.(T4 – T14)

OR 441

dTm.S.( )dtσ =

A(T -T )

In this equation (dT/dt) t = 0 denotes the rate of rise of temperature of the disc ‘D’ at the instant when its temperature is T and will vary with t. It is clearly best measured at time t=0 before heat conducted from ‘A’ to ‘D’ begins to have any significant effect. This is obtained from plot of temperature rise of ‘D’ with respect to time and obtaining its slope at t=0 when temperature = T. This will be the required value of (dT/dt) at t=0. The thermocouple mounted on the disc is to be used for this purpose. Note that the disc ‘D’ with its insulating sleeve is placed quickly in position and start the timer and record the temperature at fixed time intervals. The whole process is completed in about 120 seconds. Longer ‘D’ is left in position, the greater is probability of errors due to the heat conduction from ‘A’ to ‘D’.

Precautions:

(1) Operate the selector switch of the temperature indicator gently. (2) Stir the water with stirrer before taking the readings. (Inside the water tank).

Observations: (1) Diameter of the test disc d = 0.020m (2) Mass of the test disc m = 0.007Kg (3) Specific heat of test disc (Brass) C = 877 J/KgK (4) Area of test disc Ad = 3.14 x 10-4 m2

Observation Table: (1) For hemisphere: -

(2) For Test disc: -

Calculations:

(1) Stefan-Boltzman constant,

441

dTm.S.( )dtσ =

A(T - T ) Where, σ = Stefan-Boltzman constant

m = mass of the trest disc in Kg S = Specific heat of test disc (brass) Ad = Area of the test disc (π/4.d2) T = mean temperature of hemisphere i.e.

1 2 3 4T +T +T +TT =4

ºC

Ta = Avg. Temperature of test disc (T5) ºC dT = TFinal - Tinitial TFinal = Steady state reading of test disc Tinitial = Temp. of disc from which it starts increasing dt = Total time required for achieving the steady state from the time when

temp of the disc start increasing

Sr. No. Water Temp.(T6)

Temp. of the hemisphere T1 T2 T3 T4

Thermocouple location

Time in Sec. 10 20 30 40 50 60 70 80 90 100 110 120 130

T5

Emissivity Measurement Apparatus

Aim: To determine emmisivity of a given plate. Introduction: All substances at all temperature emit thermal radiation. Thermal radiation is an electromagnetic wave and does not require any material medium for propagation. All bodies can emit radiation and have also capacity to absorb all or part of the radiation coming from the surrounding towards it. An idealized black surface is one, which absorbs all the incident radiations with reflectivity and transmitivity equally zero. The radiant energy per unit area from the surface of the body is called as the emissive power & is usually denoted by ‘e’. The emissivity of the surface is the ratio of the emissive power of the surface to the emissive power of the black surface at the same temperature. It is denoted by ‘E’. Thus, E = e / eb For a black body, absorptivity = 1 & by the knowledge of Kirchoff’s Law emissivity of the black body becomes unity. Emissivity being a property of the surface depends on the nature of the surface & temperature. It is obvious from the Stefan-Bolztman Law that the prediction of emissive power of a surface requires knowledge about the value of its emissivity & therefore much experimental research in radiation has been on measuring the values of emissivity as a function of surface temperature.

The present experimental set-up is designed to measure the property of emissivity of the test plate surface at various temperatures. Table-1 gives approximate values of emissivity for some common materials for ready reference. Temperature Emissivity Metal polished copper, steel, Stainless steel, Nickel

20ºC 0.15 Increase with temperature

Aluminium (oxidized) 95-540 ºC 0.2 – 0.33 Non metal bricks, Wood, marble, Water 29 – 100ºC 0.8 – 1.00 Apparatus: The experimental set-up consists of two circular aluminum plates of identical in size & is provided with heating coils sandwiched. The plates are mounted on brackets are kept in an enclosure so as to provide undisturbed natural convection surrounding. The heater input to the heater is varied by separate dimmerstat and is measured by using an ammeter and voltmeter, with the help of double plate toggle switches. The temperature of the plates is measured by temperature indicator. Another thermocouple is kept in enclosure. Plate-1 is blacked whereas plate-2 is the test plate whose emissivity is to be determined. The heater inputs to two plates are dissipated from the plates by conduction, convection and radiation. The experimental set-up is designed in such a way that under

steady state conditions the heat dissipation by conduction & convection is same for both the plates, when the surface temperatures are same & the difference in the heater input readings are because of the difference in radiation characteristics due to their different emisivities. Theory: Under steady state condition, Let, Wb = Heater input to black plate = V1I1 WT = Heater input to test plate = V2I2

A = Area of plates = 2π d4

m2

Ts = Temperature of black plate Td = Ambient temperature εb = Emissivity of black plate to be assumed equal to unity ε = Emissivity of non-black test plate

σ = Stefan-Boltzman constant = 5.67 x 10-8 2 4

Wm K

By using Stefan-Boltzman Law: (W1 – W2) = (εb – ε). σ.A (Ts

4 – Td4)

We shall now define a number of terms concerned with the emission characteristics of surfaces. It is useful to introduce first the concept of an ideal surface called a black surface or a black body. A black surface is one, which absorbs all the radiation falling on it regardless of its wavelength or direction. For a given temperature & wavelength, it emits the maximum amount of energy. It thus serves as a standard on which to base the emission characteristics of real surfaces. The radiant flux emitted from the surface of a body is called the total hemispherical emissive power & will be denoted by symbol ‘e’ (W/m2). The total hemispherical emissive power of a black surface will be denoted by ‘eb’. The adjectives total and hemispherical indicate that the quantity being defines is a summation of radiation emitted over all wavelength & over all directions. Specifications: (1) Test Plate 170mm (2) Black Plate 170mm

(3) Heater – Nichrome strip wound on mica sheet (4) Dimmerstat (5) Voltmeter 0 – 300V (6) Ammeter 0 – 5 A (7) Enclosure With one side of perpex sheet (8) Thermocouple Chromel – Alumel – 3 Nos. (9) Temperature Indicator 0 – 300ºC

Procedure: (1) Start the supply. (2) When both the toggle switches are in downward direction black plate

dimmerstat operate whereas when both the toggle switches are in upward direction test plate dimmerstat operate.

(3) Gradually increase the input to the heater to black plate & adjusts the heater input to test plate slightly less than the black plate.

(4) Check the temperature of the two plates with small time intervals & adjust the input to test plate only by the dimmerstat so the two plates will be maintained at the same temperature.

(5) This will require some trial & error and has to wait sufficiently to obtain the steady state condition.

(6) After attaining the steady state condition, record the temperatures & voltmeter & ammeter both the plates.

Precautions:

(1) Use stabilized A.C. single-phase supply. (2) Always keep the dimmerstat at zero position before start. (3) Gradually increase the heater input. (4) See that the black plate is having layer uniformly. (5) Operate the selector switch of temperature indicator gently.

Note: There is a possibility of getting absurd result if the supply voltage fluctuating. Observation table: 1. Heater input to black plate- 2. Heater input to test plate - V = V = I = I =

Temperatures Steady state readings Black plate (T1) Test Plate (T2) Enclosure (T3) Calculations:

(1) Emissivity of test plate: (Wb – WT) = (εb – ε). σ.A (Ts

4 – Td4)

Where, Wb = Heater input to black plate (watt) WT= Heater input to test plate (Watt)

A = Area of plates = (2. 2π d4

) + (π .d.t)

d = 170mm t = 6mm Ts= temperature of plates after steady state (T1 = T2) (ºK) TE= Enclosure temperature (ºK) ε = Emissivity of test plate which is to be determine σ = Stefan-Boltzman constant = 5.67 x 10-8 W/m2. K4 Assuming εb =1 (for black plate the emissivity of test plate can be calculated at various surface temperatures of the plate. With increase in temperature, the test surface becomes somewhat dull and therefore its emissivity increases with increase of surface temperature.

Shell & Tube Type Heat Exchanger

Aim: To determine LMTD, heat transfer rate & overall heat transfer co-efficient of shell & tube type heat exchanger.

Introduction:

Heat exchangers are devices in which heat is transferred from one fluid to another. The necessity for doing this arises in a wide variety of applications, including space heating and cooling, thermal power production and chemical processing systems. Theory:

A shell & tube type heat exchanger is direct type of heat exchanger. It consists of bundle of round tubes packed together inside a cylindrical shell with the tube axis parallel to that of the shell. One fluid flows inside the tubes while the other flows on the outside of the tubes. Shell & tube heat exchangers are normally used for transferring heat between liquids, either with or without a change of phase. The major components of the exchanger are the tubes, the shell, the front and rear end heads and the tube header sheets. Baffles are also provided on the shell side to prevent stagnation of the shell side fluid and promote better heat transfer.

The shell & tube heat exchanger is one of the most widely used heat exchangers and can be designed for almost any capacity.

Specifications:

(1) Shell : I.D. = 230mm

Thickness = 6mm Material = M.S.

(2) Tubes : O.D. = 16mm

I.D. = 13mm Material = Copper No. of tubes = 32 Length = 550mm Surface area of tubes = π.di.l.n where, π = 3.14 di= 13 x 10-3m l = 0.55m n = 32

(3) Flow rate can be adjusted by valve on hot and cold side. The temperature at hot inlet and outlet and those at cold inlet and outlet are measured by thermocouple & digital temperature indicator.

(4) Water flow measurement - with measuring flask and stop watch (For hot & cold water)

(5) For hot water, gyser of capacity 3 KW is provided. (6) Electric supply : 230V, 15A with neutral & earthing connection

Procedure: (1) Start the flow of water through hot and cold side. (2) Adjust the flow as per requirement. (3) Put on the gyser. (4) Wait till the hot water temperature is rising & then take the reading. (5) Measure the flow rate of hot & cold water.

Observation table:

Sr. No.

Cold water (Shell side) Hot water (Tube side) Flow rate

Mcw Inlet temp.

Tci = T3 Outlet Temp.

Tco = T4 Flow rate

Mhw Inlet temp.

Thi = T1 Outlet Temp.

Tho = T2 Calculations:

(1) Hot water side: Qh = mh. Cph. [Thi – Tho]

(2) Cold water side: Qc = mc. Cpc. [Tco – Tci]

(3) i e

i

e

ΔT -ΔTLMTD = ΔTlnΔT

Where, i hi ci

e ho co

ΔT =T -TΔT =T -T

(3) For overall heat transfer co-efficient U: q = U. A. LMTD

U = qA.LMTD

(then h cQ +Qq=2

)

Heat Transfer From a Pin Fin Apparatus

Aim: To determine heat transfer co-efficient and efficiency of a pin fin for natural convection. Introduction: Extended surfaces or fins are used to increase the heat transfer rate from a surface to a fluid whenever it is not possible to increase the value of the surface heat transfer co-efficient or the temperature difference between the surface and the fluid. The use of this is very common and they are fabricated in variety of shapes circumferential fins around the cylinder of a motorcycle engines and fins attached to condenser tubes of a refrigerator are few familiar examples. It is obvious that a fin surface strikes out from the primary heat transfer surface. The temperature difference with surrounding fluid will steadily diminish as one moves out along the fin. The design of the fins therefore requires knowledge of the temperature distribution in the fin. The main object of this experimental set-up is to study the temperature distribution in a simple pin fin. Apparatus: A brass fin of circular cross section is fitted in a long rectangular duct. The other end of the duct is connected to the suction side of a blower and the airflows passed the fin perpendicular to its axis. One end of the pin projects outer sided the duct and is heated by a heater. Temperatures at five points along the length of the fin are measured by Chromel-Alumel thermocouples connected along the length of the fin. Specifications: (1) Diameter of the fin = 14.5mm (2) Length of the fin = 150mm (3) Thermocouples on the fin = 5 nos. (4) Thermocouple for Ambient temperature = 1 no. (5) Fin material = Brass (6) Range of temperature indicator = 0 – 300 ºC (7) Range of Voltmeter = 0 – 300 V (8) Range of Ammeter = 0 – 5 Amp. (9) Heater (10) Dimmerstat for controlling heat supply Procedure:

(8) Switch on main switch. (9) Start heating the fin by switching on the heater. (10) Adjust dimmerstat voltage equal to 60 V to 90 V. (11) Take the reading of thermocouples in the time interval of 10 min. (12) Take the readings till steady state condition is reached.

Precautions:

(4) See that dimmerstat is at zero position before switching on heater. (5) Operate change over switch of temperature indicator gently. (6) Be sure the steady state is reached before taking the final readings.

Observation table: Input voltage = Input current = Ambient temperature (T6) = º C

Time in Min

Temperature T1 T2 T3 T4 T5 T6

`` Calculations:

(11) Average fin temperature (Tm):

1 2 3 4 5m

T +T +T +T +TT =5

º C

(12) Mean Fin temperature = m 6mf

T +TT =2

º C = ___________.

Note: Take the values of kair, from chart at Tmf

(13) 2

3)..(υβ LtgGr ∆

=

kCp µ.Pr =

Nu = 0.53 (Gr. Pr )1/4 for (104 < Gr.Pr <109 ) Nu = 0.13 (Gr. Pr )1/3 for (109 <Gr. Pr <1012 )

(14) Heat transfer co-efficient (h):

u airN xKh=

dW/m2K

(15) Fin parameter (m) :

AkPhm..

= m-1

Where, P = circumference of fin (π.d) = ________m k = thermal conductivity of fin material (Brass) = 110.71 W/mº C A = Area of fin [(π/4)d2] = ________ m2

(16) Efficiency (η):

mLmL

fin)tanh(

=η

= __________%