nirma university institute of technology · pdf filenirma university institute of technology...

NIRMA UNIVERSITY

INSTITUTE OF TECHNOLOGY

CHEMICAL ENGINEERING DEPARTMENT

LAB MANNUAL CH302- HEAT TRANSFER OPERATION

Chemical Engineering Department Institute of Technology, Nirma University

Heat Transfer Operation- Lab Manual 2

List of Experiments:

1. Thermal conductivity apparatus 2. Thermal conductivity of metal rod

3. Thermal conductivity of insulating powder

4. Heat Transfer in natural convection

5. Heat Transfer in forced convection

6. Extended surface equipment

7. Parallel flow heat exchanger

8. counter flow heat exchanger

9. Shell and Tube heat exchanger

10. Emissivity measurement apparatus

11. Drop wise and film wise condensation apparatus

12. Finned tube heat exchanger



Date: Roll no: Practical No:

THERMAL CONDUCTIVITY APPARATUS

Objective: After this experiment student will able to understand how to determine the Thermal Conductivity of given specimen. Apparatus: Two slab guarded hot plate thermal conductivity apparatus, Specimen, Insulation (Glass wool) Packets. Utility: Water, Electric Supply. Theory: Principle of the guarded hot plate method: A sketch of the apparatus is shown in Fig. (1). The essential parts the Hot plate, the cold plate, the heater assembly, thermocouples and the specimen, in position, are shown in the same figure. For measurement of the thermal conductivity K what is required is to have one dimensional heat flow through the flat specimen, an arrangement for maintaining its faces at the constant temperature and metering method to measure the heat flow through a known area. To eliminate the distortion caused by the edge losses in unidirectional heat flow from the central plate, it is surrounded by a guard ring heater separately. Temperatures are measured by calibrated thermocouples, attached to the plates or to the specimen at the hot and the cold faces. Two specimens are used to ensure that all the heat comes out to the specimen only. Knowing the heat input to the central plate heater, the temperature difference across the specimen, its thickness and the area, one can calculate the K by the following formula.

Where, K Thermal Conductivity of the sample, W /m °C q Heat flow rate in the specimen, W A Area of the specimen, m2 Th Hot plate temperature, °C Tc Cold plate temperature, °C L Thickness of the specimen, m If the specimen thickness are different and the respective hot and cold temperatures are different than,

Where suffix 1 stands for upper specimen and 2 stands for lower specimen.

)(*2*

ch TTALqK−

=

+=

c2h2

2

c1h1

1

T - TT - T2LL

AqK



Apparatus description: The heater plate is surrounded by a guard heater for stabilising the temperature of the primary heater and to prevent heat loss radially around the edges. The primary and guard heaters are made of Nichrome wire packed between upper and lower mica sheets. These heaters together with upper and lower copper plates and rings from the heater plate assembly. Two thermocouples (1 & 2) are used to measure the hot face temperature at the upper and lower central plate assembly copper plates. Two more thermocouples (3 & 4) are used to check balance in both the heater inputs (see figure 1). Specimens are held between the heater and cooling unit on each side of the apparatus. Thermocouples (5 & 6) measure the temperature of the upper cooling plate and lower cooling plate respectively. The cooling chamber is a composite assembly of spiral grooved Aluminium casting and aluminium cover with entry and exit adopters for water inlet and outlet. Procedure: The specimens are placed on either side of the heating plate assembly uniformly touching the cooling plates. The outer container is filled with loosely filled insulation such as glass wool supplied in small packets. The cooling circuit is started. Then calculated heat input is given to the central and guard heaters through separate single phase power supply lines with dimmerstat in each line and it is adjusted to maintain the desired temperature (For ensuring no radial heat transfer, generally outer heater input is 2.5 to 3.0 times more than the central heating input). The guard heater input is adjusted in such a way that there is no radial heat flow, which is checked from thermocouples readings are recorded accordingly. The input of the central heater (current & voltage, watts) and the thermocouple readings are recorded every 10 minutes till a reasonably steady state condition is reached. The readings are recorded in the observation table. The final steady state values are taken for calculations. Precautions: • Keep the dimmerstat to 0 voltage position at start. • Increase the voltage gradually of the two heaters during initial set-up experimentation. • Start the cooling circuit before switching on the heaters and adjust the flow rates so that

practically there is no temperature rise in the circulation fluid. • Keep the heater plate undisturbed and adjust the cooling plates after keeping the samples with

the help of nuts gently. • Keep the loose filled in insulation (glass wool) packets gently and remove them slowly so that

they do not disturb the thermocouples’ terminals and heater wires.



Observation table:

Sr. No.

Central Heater Guard Heater Cooling Plate

T1(0C) T2(0C) T3(0C) T4(0C) T5(0C) T6(0C)

The difference between the temperatures of central heater and guard heaters should not be more than 1 0C.

V (volts) I (amp) q(watt) L (m) 0.019 0.019 0.019 D (m) 0.18 0.18 0.18 A (m2)

K (Watt / m oC) Calculation: 1. Area of Heat transfer A = (Π / 4) * D2 2. Thermal Conductivity of specimen

Where, Th, av = (T1 + T2 ) / 2 Tc, av = (T5 + T6 ) / 2 L = Thickness of slab Result: Conclusion:

)av Tc, - av Th,(2*

ALqK =




THERMAL CONDUCTIVITY OF METAL ROD

OBJECTIVE: After this experiment student will able to understand how to determine the thermal conductivity of given metal rod. Apparatus: Thermal conductivity apparatus, metal rod. Utility: Water, Electric Supply. Theory: The thermal energy is conducted in solids by the following modes: (i) Lattice vibration (ii) Transport by free electrons In good electrical conductors a large number of free electrons move about in the lattice structure of the material. They carry thermal energy from a high temperature region to a low temperature region. Energy may also be transmitted as vibrational energy in the lattice structure of the material. In general, this latter mode of transfer is not as large as the electron transport. With increase in the temperature, however the increased lattice vibration come in the way of the transport by free electrons and for most of the pure metals the thermal conductivity decreases with increase in temperature. Apparatus Description: The experimental set up consists of metal bar, one end of which is heated by an electric heater while the other end of the bar projects inside the cooling water jacket. The middle portion of the bar is surrounded by cylindrical shell filled with asbestos magnesia insulating powder. The temperature of the bar is measured at different sections (figure1) from 1 to 7 while the radial temperature distribution is measured by separate thermocouples at two different sections in the insulating shell. The heater is provided with a dimmerstat for controlling the heat input. Water under constant heat condition is circulated through the jacket and its flow rate and temperature rise is noted. Procedure: Start the electric supply. Adjust the room temperature in the temperature indicator by means of rotating knob, for compensation of the temperature equal to the room temperature (normally this is readjusted). Give input to the heater by slowly rotating the dimmerstat and adjust it to voltage equal to 80 to 120 volts. Start the cooling water supply through the jacket and adjust it about 350 cc/ minute. Go on checking the temperatures at some specified time intervals say 5 minutes and continue this till a satisfactory steady state condition is reached. Note the mass flow rate of water in kg/ min. and temperature rise in it and also note the temperature readings from 1 to 11 using temperature indicators.



Observations: 1. Length of the metal bar (total) 450 mm 2. Size of the metal bar (diameter) 38 mm 3. Test length of the bar 230 mm 4. No. of thermocouples mounted on the bar 7 5. No. of thermocouple in the insulation shell 4 6. Heater coil (band Type) Nichrome 7. Cooling Jacket Diameter 90 mm 8. Radial distance of thermocouple in insulating shell ro = 55 mm=r2

ri = 35 mm =r1

9. Mass flow rate of water ‘m’ = ___________litre/ minute ___________kg/ minute 10. Inlet temperature of water Ti = ___________0C 11. Outlet temperature of water To= ___________0C

Observation Table:

Sr. No: V ( volt ) I ( amp ) W ( watt )

T/C No.

1 2 3 4 5 6 7 8 9 10 11

Temp (0C)

Plot the graph of the temperature distribution (at steady state) along the length of the metal rod using observed values 1 o 7 (as show in fig 2), for determining the slope at BB and CC sections, and also at AA section. Slope is dt / dx at various desired points on the plot of T vs. Distance. Nature of the graph will be probably as shown in figure 3. Heat transmitted in Radial direction by the cylinder can be calculated by following formula:

Calculations: 1. Heat transferred to cooling water = Heat passed from AA section

qw=qAA = m * Cp * (To - Ti) = -KAA * (dT / dx)AA * A

KAA = _____________ w/ m K

kLrr

TTQ oi

π2)/ln( 12

−=



2. Heat passed through = Heat passed in linear + Heat passed in redial

BB section direction from AA section direction

KBB = _____________w/ m K

3. Heat passed through = Heat passed in linear + Heat passed in redial

CC section direction from BB section direction

KCC = _____________w/ m K

Results: Conclusion:

AdxdTkrr

TTkLqq BBBB

ioWBB *)/(*

/ln)(2

1110 −=−

+=π

AdxdTkrrTTkL

qq CCCCio

BBCC *)/(*/ln

)(2 98 −=−

+=π




THERMAL CONDUCTIVITY OF INSULATING POWDER Objective: After this experiment student will able to understand how to determine the thermal conductivity of given insulating powder. Apparatus: Conductivity Instrument, Insulating Powder Utility: Electric Supply Apparatus Description: The apparatus consists of two thin walled concentric copper spheres. The inner sphere houses the heating coil. The insulating powder (Asbestos powder-lagging material) is packed between the two shells. The power supplied to the heating coil is varied by using a dimmerstat and is measured by a wattmeter or voltmeter and ammeter. Iron-Constantan thermocouple is used to measure the temperature. Thermocouple couple 1 to 4 are embedded on inner sphere and 5 to 10 are embedded on the outer shell (Figure 1). Position 1 to 10 is as shown on change over switch of temperature indicator. Under steady state condition the temperature 1 to 10 are noted and also particular power input reading is recorded. These readings in turn enable to find out the thermal conductivity of insulating powder packed between two shells. We assume the insulating powder as an isotropic material and the value of the thermal conductivity to be constant. The apparatus assume one-dimensional radial heat conduction across the powder layer and thermal conductivity can be determined as above under steady state condition. Observations: 1. Radius of the inner copper sphere (ri) = 50 mm 2. Radius of the outer copper sphere (ro) = 100 mm 3. Thermocouples 1 to 4 embedded on inner sphere to measure Ti 4. Thermocouples 5 to 10 embedded on outer sphere to measure To 5. Insulating powder Asbestos Magnesia packed between the two spheres Procedure: 1. Insert male socket of control panel and connect thermocouple band on temperature indicator

and test set-up in proper position and start the main switch of control panel. 2. Increase slowly the input to heater by the dimmerstat starting from 0 volts position. 3. Adjust input equal to 40 watts maximum by voltmeter and ammeter or watt meter. 4. See that this input remains constant throughout the experiment. 5. Wait till a satisfactory steady state condition is reached. This can be checked by reading

temperatures of thermocouples 1 to 10 & note changes in their readings with time. 6. Note down the readings in the observation table as given below.



Observation Table: Voltmeter Reading: V Ammeter Reading: amp. Heater Input: W Inner Sphere: Thermocouple No.

T1 T2 T3 T4 Mean Temp.(Ti) (T1 + T2 + T3 + T4)) /

4 Temp (C) Outer Sphere: Thermo-couple No.

T5 T6 T7 T8 T9 T10 Mean Temp. (To) (T5 + T6 + T7 + T8 + T9 + T10)

6

Temp (C) Calculations: Heat transfer through hollow sphere can calculate by following formula

Result: Conclusion:

rr*K* * 4

)()T - (T

0i

oi

πio rr

Q−

=

cmWrrQ

K oio /)T - (T r*r* * 4

)(*

oiioπ−

=⇒




PARALLEL FLOW AND COUNTER FLOW HEAT EXCHANGER

Objective: After this experiment student will able to understand how to (1) determine overall heat transfer coefficient for parallel and counter Flow pattern in double pipe heat exchanger (2) Temperature distribution in parallel flow and counter flow heat exchanger.

Apparatus: Double pipe heat exchanger assembly, Geyser, Thermometers. Utility: Water, Electric Supply. Apparatus Description: The apparatus consists of a tube in tube type (concentric) heat exchanger. The hot fluid (hot water) obtained from an electric geyser flows through inner tube while the cold fluid (cold water) flowing through the annulus. The direction of hot water flow can not be altered while the direction of cold water can be altered using valves so as to make the heat exchanger to function as parallel or counter flow heat exchanger s shown in Figure 1. The experiments are conducted by keeping the identical flow rates while running the unit as a parallel flow exchanger and counter flow exchanger. The temperatures are measured by mercury in glass thermometers and flow rates by graduated measuring flasks and stop clock. The readings are recorded when the steady state is reached. The outer tube is provided with adequate thermocol insulation to minimise the heat losses. Procedure: 1. Place thermometers in the positions and note down their readings when they are at room

temperature and no water is flowing at either side this is required to correct the temperature. 2. Start the flow on hot water side. 3. Start the flow through annulus and run the exchanger as parallel flow unit. Put on the geyser. 4. Adjust the flow rate on hot water side, between the rate of 1.5-4.0 litre/ min. 5. Adjust the flow on cold water side between range of 3.0-8.0 litres/ min. 6. Keeping the flowrate same, wait till the steady state conditions are reached. 7. Record the temperatures on hot water and cold water side and also the flow rates accurately. 8. Repeat the experiment with a counter flow under identical flow conditions. Observations:

1. Inner Tube Material: Copper I.D. (di) 10 mm O.D. (do ) 12 mm

2. Outer Tube Material: G. Iron I.D. (Di) 27.5 mm O.D. (Do ) 33.5 mm

3. Length of the heat exchanger (L) 2 m



4. Thermal conductivity of copper (K) 30 kcal/h m 0C

5. Thermometers: 0- 50 0C 2 Nos. (for cold water side)

0-100 0C 2 Nos. (for hot water side) Observation Table: PARALLEL FLOW RUN: Hot Water Side Cold Water Side

Sr. No: Flow Rate mh (kg/ h)

Thi

( 0C) Tho

( 0C) Flow Rate mc (kg/ h)

Tci

( 0C ) Tco

( 0C) 1 2 3

COUNTER FLOW RUN:

Hot Water Side Cold Water Side Sr. No: Flow Rate

mh (kg/ h) Thi

(0C) Tho (0C)

Flow Rate mc (kg/ h)

Tci

(0C) Tco (0C)

1 2 3

Calculations: Heat Transfer Rate, is calculated as, qh = Heat transfer rate from hot water = mh× Cph× (Thi - Tho) qc = Heat transfer rate to the cold water = mc× Cpc× (Tco - Tci) q = (qh + qc)/2 W (Assume Cph = Cpc= 1 kcal/kg °C) LMTD - Logarithmic Mean Temperature Difference. The temperature distribution in the two runs is plotted. LMTD can be calculated as

Overall Heat transfer coefficient can be calculated using, q= U*A*∆Tm U = q/ (A * ∆Tm)

( )oi

oim TT

TTTLMTD∆∆∆−∆

=∆=/ln



Calculated Ur i based on Ai = Π * di * L Uro based on Ao = Π * do * L Compare the values of ∆Tm and q in the parallel and counter flow runs. Note that if experiment is conducted very carefully then the superiority of counter flow arrangement in terms of higher value of ∆Tm and excess value of q for same flow rates conditions can be revealed. The value of overall heat transfer coefficient U is more or less same for both the runs. The overall heat transfer coefficient value can be predicted by using the force convention heat transfer correlations for flow through the tube and annulus and can be compared with the experimentally determined values. 1 / UO = 1 / ho + ( ro × ln (ro / rI )/ k ) + ( (ro / ri) × (1 / hi) ) Where, hi heat transfer coefficient for inner tube ho heat transfer coefficient for outer tube k thermal conductivity of material (for copper pipe) 30 kcal / hr m C hi is calculated by using the correlation NuDi = 0.023 (ReDi)0.8 (Pr)0.3

Evaluate properties of water at average bulk mean temperature (Thi + Tho) / 2 ho is calculated by using the correlation Nu (Di - do) = 0.023 (Re (Di - do))0.8 (Pr)0.4

Evaluate Di - do properties at average bulk mean temperatures (Tci + Tco) / 2 for cold water. Result: Sr. No.

Parallel Flow run Counter Flow run Pract. U (W/m2 0C) Theo. U (W/m2 0C) Pract. U (W/m2 0C) Theo. U (W/m2 0C)

1 2 3 4

Conclusion:



Date: Roll No: Practical No:

SHELL AND TUBE EXCHANGER

Objective: After this experiment student will able to understand how to determine Overall Heat Transfer Coefficient for the shell and tube heat exchanger. Apparatus: 1-2 pass Shell and Tube heat exchanger, Thermometer Utility: Water, Electric Supply. Apparatus Description: Shell Material M.S.

Inner Diameter. Di 208 mm Thickness 6 mm Length 500 mm 25% cut baffle plates at 100 mm apart – 4 numbers Tubes Material Copper

Inner Diameter. Di 13 mm Outer Diameter. D0 16 mm

Length L 500 mm No. of Tubes N 32 Fluid Water Hot water from the geysers enters the heat exchanger at the bottom into the header. The header has a partition at the centre. The hot water flows through the bottom bank of tubes and comes to the other end, changes direction and passes through the top bank of tubes and leaves the chamber at top. The cooled water enters at the bottom of shell and after passing through the five compartments made by four baffle plates leaves the shell at top. Procedure: Adjust the flow rate such that there is applicable temperature difference between the outlet and inlet temps. of two fluid. Put on the geyser and set up pre determined hot water temp. After steady state read four thermometer readings and measure the hot and cold flow rates. Observation: 1. Heat transfer area of tubes and baffle plates = 1m2 2. Correction factor for heat exchanger (F) = ____ 3. Inlet temperature of hot water (thi) = ____ °C 4. Cold water flow rate for 1 Lit. = ____ Kg/s 5. Inlet temperature for cold water = ____°C



6. Outlet temperature for cold water(tco) = ____°C 7. Hot water flow rate for 1 Lit. = ____ Kg/s 8. Outlet temperature for hot water (tho) = ____°C Observation Table: Hot Water Side Cold Water Side

Sr. No:

Flow Rate mh (kg/ hr)

Thi

( 0C) Tho (0C)

Flow Rate mc (kg/ hr)

tci

( 0C) tco

( 0C) 1 2 3

Sample Calculations: For Shell and Tube heat exchanger, (LMTD) 1-2 pass shell & tube = F *(LMTD) Counter Flow

where, LMTD = Log mean temperature difference F correction factor (From LMTD correction factor chart) *(LMTD) Counter Flow = [ ( Thi – tco ) – ( Tho – t ci ) ] / ln [ ( Thi – tco ) / ( Tho – t ci ) ] = _______ 0 C Now q is given by:

q = mc Cp (tco - tci ) for cold water.

where Cp =Specific heat of water =4.1868 kJ/kg-k

Now for Overall Heat Transfer coefficient ‘U’

q = U*A*F (LMTD) Counter Flow.

U = q / A* F *(LMTD)Counter flow

= ______ KW/m2 °C Result: Conclusion:




DROP WISE AND FILMWISE CONDENSATION APPARATUS Objective: After this experiment student will able to understand how to study the dropwise and filmwise condensation phenomena. Apparatus: Condensers, main unit, heating elements, temperature indicator, rota meter, pressure gauge Apparatus Description: Condensers: One chromium plated for dropwise condensation and one natural finish for filmwise condensation otherwise identical in construction. Dimensions: 15 cm dia.*12 cm length Surface area: cm2

Fabricated from copper with reverse flow in concentric tubes Fitted with thermocouple for surface temp. Measurements Main unit: M.S. fabricated construction comprising test section and steam generation section. Test section provided with glass converse on front and rear sides of test section for visualisation of the process. Heating elements: 1.5KW water heater operated through 30 amps D.P.switch Procedure: 1. Fill up the water in the main unit through bottom valve upto the sight glass fitted on the

unit. 2. After filling the water close the valve. Start the water flow rate through one of the

condensers which is to be tested and note water flow rate in the rotameter. Ensure that during measurement water is flowing only through the condenser under test and second valve is closed.

3. Connect supply socket to the mains and switch on the heater. 4. Slowly steam generation will start in the bottom portion of the unit and as the steam rises

to test section and get condensed on the tube and fells back in the trough. 5. Depending upon the type of condenser under test dropwise or filmwise condensation can

be visualised. 6. If water flow rate is low than steam pressure in chamber will rise and pressure gauge will

read the pressure. 7. If the water flow rate is matched than condensation will occur at more or less at

atmospheric pressure. 8. Process of dropwise and filmwise condensation can be easily viewed through the front

glass window of main unit. 9. Observation like temperatures, water flow rates, pressures are noted down in the

observation table at the end of the each set.



Observation Table: Filmwise Condensation: Sr. No

Steam Pressure

Water Rate

Temperature

Plated Conden.

Plain Conden.

Steam Water in

plated

Water out

plated

Water in

plain

Water out

plain Dropwise Condensation: Sr. No

Steam Pressure

Water Rate

Temperature

Plated Plain Steam Water in

plated

Water out

plated

Water in

plain

Water out

plain Calculations: Normally steam will not be pressurised, but the pressure gauge reads some pressure than properties of steam should be taken at that pressure or otherwise atmospheric pressure will be taken. First calculate the heat transfer coefficient inside the condenser under test. For this properties of water are taken at bulk mean temperature of water i.e. (Twi + Two ) / 2. Following properties are required: Density of water ρ1 kg / m3 Kinematics Viscosity ν1 m2/ sec Thermal Conductivity K1 kcal / hr m C Prandtl Number Pr Reynolds Number ReD = 4 * mw / D1

where D1 Inner Diameter of Condenser = 2.5 cm If this value of ReD > 2100 then flow is turbulent, below this value flow is laminar. Normally flow will be turbulent in the tube. Nusselt Number NuD = 0.023 (ReD)0.4 (Pr)0.4 hi = NuD * K / D1 kcal / hr m2 C Inside heat transfer coefficient



Calculate heat transfer coefficient on outer surface of the condenser ho. For this properties of water are taken at bulk mean temperature of condensate (Ts + Tw ) / 2. Where, Ts Temperature of steam, Tw Temperature of condenser wall Density of water ρ2 kg / m3 Kinemtic Viscosity ν2 m2/ sec Thermal Conductivity K2 kcal / hr m C Heat of evaporation ho kcal / kg ho = 0.725 * 2 * g * K2

3 / ( Ts - Tw) * Do , Do Outside diameter of condenser = 2.75 cm From these values overall heat transfer coefficient (U) can be calculated, 1/ U = 1 / hi + (Di / Do) (1 / ho) kcal / hr m2 C same procdure can be repeated for next condenser. Except for some exceptional cases overall heat transfer coefficient for dropwise condensation will be higher than that of filmwise condensation. Results may vary from theory to some degree due to unavoidable heat losses from front and rear glass windows and through walls. Precautions: Do not start heater supply unless water is filled in the test unit. Operate gently the selector switch of temperature indicator to read various temperatures. Result: Conclusion:




HEAT TRANSFER IN NATURAL CONVECTION

Objective: After this experiment student will able to understand how to determine the surface heat transfer coefficient for a vertical tube losing heat by natural convection. Apparatus: Vertical brass tube with housing. Utility: Electric Supply. Theory: When a hot body is kept in a still atmosphere, heat is transferred to the surrounding fluid by natural convection. The fluid layer in contact with the hot body gets heated, rises up due to the decrease in its density and the cold fluid rushes in to take place. The process continues and the heat transfer takes place due to the relative motion of hot and cold fluid particles. The heat transfer coefficient is given by: h = q / [ AS * ( Ts - Ta ) ] Where, h = Average surface heat transfer coefficient (kcal / hr.m2.0C), AS = Area of heat transfer surface , m2

Ts = Average surface temperature , 0C Ta = Ambient temperature of the duct . 0C The surface heat transfer coefficient of a system transferring heat by natural convection depends on the shape, dimensions and orientation of the fluid and the temperature difference between heat transferring surface and the fluid. The dependence of h on all the above mentioned parameters is generally expressed in terms of non-dimensional groups as follows:

Where, hL / k = Nusselt number, g L3 β ∆T / ν2 = Grasshof number, Cp µ / k = Prandtl number . A and n are constants depending on the shape and orientation of the heat transferring surface. L = Characteristic dimension of the surface, m. k = Thermal conductivity of the fluid , w/(m 0C) µ = Dynamic viscosity of fluid, kg/m s . ν = Kinematics Viscosity, m2/s CP = Specific heat of fluid, J/kg 0C β = Coefficient of volumetric expansion of fluid, K-1(0C-1) g = Acceleration due to gravity , m/s2

∆T = Ts - Ta, 0C

n**

2

3

∆=

kC

vTngLA

khL p µβ



For gases , β = _____1_____ where Tf = Ts + Ta_ Tf + 273 2 For a vertical cylinder losing heat by natural convection, the constants A and n have been determined and the following empirical correlation obtained , McAdams equation: h L = 0.59 ( Gr * Pr ) 0.25 for 104 < Gr * Pr < 109

k h L = 0.13 ( Gr * Pr )1/3 for 109 < Gr * Pr < 1012

k where L = Length of the cylinder. All the properties of fluid are determined at the mean film temperature Tf .

Apparatus Description: The apparatus consist of a brass tube fitted in a rectangular duct in a vertical fashion. The duct is open at the top and bottom and forms an enclosure and serves the purpose of undisturbed surroundings. One side of the duct is made up of perplex for visualisation. An electric heating element is kept in the vertical tube which in turn heats the tube surface. The heat is lost from the tube to the surroundings to the air by natural convection. The temperature of vertical tube is measured by 7 thermocouple. The heat input to the tube is measured by an ammeter and voltmeter and is varied by dimmerstat. The vertical cylinder with the thermocouples, position are shown in Figure 1; while the possible flow patterns and also expected variation of local heat transfer coefficient shown in figure 2. The tube surface is polished to minimise the radiation losses. 1. Diameter of the tube (D) 38 mm 2. Length of tube (L) 500 mm 3. Duct size 20 cm * 20 cm * 0.75 m length 4. No. of thermocouples 7 and are shown as 1 to 7 Thermocouple 8 reads the ambient temperature and is kept in the duct. Procedure: 1. Switch on the supply and adjust the dimmerstat to obtain the required heat input. 2. Wait till the steady state is reached which is confirmed from temperature readings T1 to T7. 3. Measure surface temperatures at the various points at T1 to T7. 4. Note the ambient temperature T8. 5. Repeat the experiment at different heat inputs. Precautions: 1. Adjust the temperature indicator to ambient level by using compensation screw before

starting the experiment. 2. Keep dimmerstat to zero volt position and increase it slowly. 3. Use the proper range of ammeter and voltmeter.



4. Operate the change over switch of temperature indicator gently from one position to other, i.e. from 1 to 8 positions.

5. Never exceed 80 watts. Observation Table: Input to heater

Sr. No: V ( volt ) I ( amp ) W ( watt ) Sr. No.

T1

( 0C ) T2

( 0C ) T3

( 0C ) T4

( 0C ) T5

( 0C ) T6

( 0C ) T7

( 0C ) T8 = Ta

( 0C ) Calculations: Experimental Calculations: The average surface heat transfer coefficient, neglecting end losses using the equation, h = q / (As [Ts - Ta]) q = V I cosϕ =______ Watt As = Π D L = m2 Ts = ( T1 + T2 + T3 + T4 + T5 + T6 + T7 ) / 7 = _________ 0C Theoretical Calculations : Compare the experimentally obtained value with the prediction of the correlation equation, McAdams Equation: Laminar Flow over vertical plates and cylinder: h* L / K = 0.59 (Gr * Pr) 0.25 for 104 < Gr * Pr < 108

Turbulent Flow over vertical plates and cylinder: h * L / K = 0.13 (Gr * Pr) 0.33 for 109 < Gr * Pr < 1012

Calculate and plot the variation of local heat transfer coefficient h along the length of the tube L.



Result: Sr. No h ( Experimental )

( W/m2.0C ) h ( Theoretical )

( W/m2.0C ) 1 2 3

Conclusion:




HEAT TRANSFER IN FORCED CONVECTION

Objective: After this experiment student will able to understand how to determine average surface heat transfer coefficient for pipe losing heat by forced convection. Apparatus: Forced convection apparatus equipped with blower, manometer. Utility: Electric Supply. Apparatus Description: The apparatus consists of a blower unit fitted with the test pipe. The test section is surrounded by Nichrome bend heater. 4 Thermocouples are embedded on the test section and 2 thermocouples are placed in the air stream at the entrance and the exit of the test section to measure air temperatures. Test pipe is connected to the delivery side of the blower along with the orifice to measure the flow of air through the pipe. Input to the heater is given through dimmerstat and is measured by meter. It is also noted that only a part of the total heat supplied is utilised in heating the air. A temperature indicator with cold junction compensation is provided to measure the temperature in pipe wall in test section. Air flow is measured with the help of an orifice meter and the water manometer fitted on the board. Schematic diagram of the setup is shown in the figure 1 and details of the test pipe are given in figure 2. Procedure: 1. Start and adjust the flow by means of a valve to some desired difference in the manometer

levels. 2. Start the heating of the test section with the help of dimmerstat and adjust the desired heat

input. 3. Take the readings of all thermocouples at an interval of 10 minutes until the steady state is

reached. 4. Wait for steady state and take readings of all six thermocouples at steady state. 5. Note down the heater input with the help of ammeter and voltmeter. Precautions: 1. Adjust the temperature indicator to ambient level by using compensation screw before

starting the experiment. 2. Keep dimmerstat to zero volt position and increase it slowly. 3. Use the proper range of ammeter and voltmeter. 4. Do not stop the blower between the testing period. 5. Do not disturb the thermocouples while testing. 6. Operate the change over switch of temperature indicator gently from one position to other,

i.e. from 1 to 6 positions. 7. Never exceed 200 watts.



Data: 1. Outer Diameter of the tube (D) 34 mm 2. Inner Diameter of the tube (D) 28 mm 3. Length of the test section (L) 500 mm 4. Blower motor 1 H.P. 5. Orifice Diameter 14 mm connected to water manometer 6. Coefficient of discharge 0.64 Observations: Sr. No.

T1A T6A T6A - T1A = ∆T

Ta AV = (T1A + T6A)/2

T2S T3S T4S T5S TS,,AV =(T2S+ T3S+ T4S+ T5S) /4

1. 2. 3. 4. 5.

Calculations: Pressure differences, velocities, and mass flow rate of air, Va = Velocity of air =

Where, Cd Coefficient of discharge = 0.64 g 9.81 m / sec2 ∆h Differential pressure expressed in meters of air = (H) * (ρw,T / ρair,T) H Manometer reading, m ρw,T Density of water at temperature at temperature T, kg/m3

ρair,T Density of ait at temperature at temperature T, kg/m3

QA Volume of air flow, m3/ sec

Sr. No.

H ρw,T ρair,T ∆h Va QA = A* Va (m3/ sec)

m = QA * ρair,T

(kg / sec) 1. 2. 3. 4. 5.

Rate of air heating and heat transfer Coefficient hT at temperature T, Air heating rate = qA = m * Cp * ∆T , kcal / sec

sec / m **2 hgCV d ∆=



Where Cp = Sp. heat of air at temperature (T1A + T6A ) / 2 Heat transfer coefficient of wall = h = qA / (Awall [Ts,AV - Ta, AV])

Sr. No.

m (kg / sec)

Cp

(kcal / kg C) ∆T (C)

qA

kcal / sec H(practical)

(w/m2 0C) 1. 2. 3. 4. 5.

Theoretical Calculation: Reynolds Number (Re) Re = Va * Di / νair, TAV , (νair, TAV Kinematic Viscosity of air at TAV)

Nusselt Number Nu = (ha, T * Di) / KTAV Where ha, T =Average heat transfer coefficient of film over the length of the pipe KTAV = Thermal conductivity of air at TAV = ( T1A + T6A) / 2

Sr. No.

Di νair, TAV Re ha, T KTAV

Nu (Re)0.8 (Pr)0.4 Nu,tur

1. 2. 3.

McAdams Equation: Turbulent flow inside tube: Nu, TUR = 0.023 * (Re)0.8 * (Pr) 0.4 Result: Conclusion




EXTENDED SURFACE EQUIPMENT Objective: After this experiment student will able to understand how to study the temperature profile along with length in case of an extended surface. Apparatus: Hot Plate with attached rod, dimmerstat, thermocouples. Utility: Electric supply. Theory: When one of the two fluid streams has a much lower heat transfer coefficient than the other, extended surface heat exchangers are used. In this exchangers , the outside area of the tube is extended by fins, pegs, disks and other appendages and the outside area in contact with the fluid thereby made much larger than the inside area . Ui = ______1_______ 1 + _ Ai_

hi Ao ho

This equation shows that if ho is small and hi is large , the value of Ui will be small , but if A0 is made much larger than Ai , then Ui increases . The temperature profile of extended surface is given by the following equation: T - Ta = ( T0 - Ta) cos h [ m ( L : x ) ]- cos h [ m L ] _________ m = √(h P / KA) Where, T0 = Temperature of the base plate.= Ts Ta = Temperature of the ambient air = T∞ T = Temperature at distance x from base plate ( theoretical ) h = film coefficient of air K = thermal conductivity of extended surface L = Height of the extended surface P = Perimeter of the extended surface A = Area of cross section of the extended surface Apparatus Description: The apparatus consists of a hot plate with a M.S rod attached to the centre of the plate. A groove is made at the centre of the plate and the M.S rod is joined to it. The temperature of the hot plate can be varied by the dimmerstat. The temperature at various points of the rod is measured by thermocouples. One thermocouple measures the ambient temperature of air.



Apparatus Specifications: Diameter of the rod , D = 25 mm. Total length of the rod, L = 60 cm. Distance between the thermocouple = 10 cm. Procedure: 1. Switch on the power supply and adjust the dimmerstat to obtain the required heat input. 2. Keep the hot plate temperature at 250 0C maximum. 3. Wait until the steady state is reached, which is confirmed from the consistent temperature

readings T1 to T8 . 4. Measure the surface temperature at various points of the extended surfaces, base of the

hot plate and ambient temperature of air by rotating the selector switch. 5. Repeat the experiment at different heat inputs. Precautions: 1. Keep dimmerstat to zero volt position before switching on the supply and then increase it

slowly. 2. Use proper range of ammeter and voltmeter. 3. Operate the selector switch of the temperature gently from 1 to 8. 4. Take the readings only when steady state is reached. Data: Thermal conductivity of M.S rod , K = 40 W / m.0K Heat transfer coefficient of air , h = 0.4 W/m2.

0K

Observations: Distance from base x

(cm) 0 10 20 30 40 50 60 Ambient

air Temperature of rod

( 0C) T7 = T0

( base plate )

T1 T2 T3 T4 T5 T6 T8 = Ta

Obs.1 ( 150V) Obs.2 ( 200V ) Obs.3 (250V )

Calculations: Perimeter of the rod , P = 3.14 * D = m. Area of cross-section of rod , A = (3.14 * D2 ) / 4 = m 2.



_________ m = √(h P / KA) = m-1 T - Ta = ( T0 - Ta) cos h [ m ( L - x ) ] cos h [ m L ] Where T0 = Temperature of the base plate. Ta = Temperature of the ambient air T = Temperature at distance x from base plate ( theoretical )

Distance from base

plate x cms.

Temperature ( experimental ) 0C

Temperature ( Theoretical ) 0C

% Error (TExp – TTheo)/(TExp)

Obs.1 Obs.2 Obs.3 Obs.1 Obs.2 Obs.3 Obs.1 Obs.2 Obs.3 10 20 30 40 50 60

Graph: (1) Temperature vs Distance (experimental) (2) Temperature vs Distance (theoretical) Result: Conclusion:




FINNED TUBE HEAT EXCHANGER Objective: After this experiment student will able to understand how to study and compare temperature distribution, heat transfer rate, overall heat transfer coefficient of the finned tube heat exchanger. Apparatus: Stop clock, measuring flask, thermometers. Utility: Water, electricity supply. Theory: Finned tube heat exchangers are used when there is a huge difference between the values of the heat transfer coefficients of the fluids involved e.g. heat transfer between air and water. So to increase the total transmission of heat, extended surfaces like fins are added. The fins are of many types like longitudinal, transverse, pegs or studs, spines etc. Apparatus description: The apparatus consists of a concentric tube heat exchanger. The hot fluid i.e. Hot water is obtained from an electric geyser and it flows through the inner finned tube, in both directions. The direction can be changed by operating different valves. The cold air is admitted through one end. Temperatures of the fluids can be measured using stop and measuring flask. The outer tube is provided with adequate asbestos rope insulation to minimise the heat loss to the surrounding. Apparatus Specification: Length of the heat exchanger = 1m Inner copper finned tube I.D = 0.016 m, O.D = 0.02 m with longitudinal fin 0.022m height and 1m. long. Total number of fins = 8 Outer tube, I.D = 0.064 m, O.D = 0.075 m Fin thickness = 0.00166 m Annulus flow area = 1.288 m2 Perimeter of the fin = 0.4016 m Procedure: Keep the thermometers in position. • Start the hot water flow. Start the flow of cold air. • Put on the electric geyser. • Adjust the flow rate on hot water side (approx.: 500 ml/min.) .Keep the flow rate same till

the steady state condition is reached. • Note the temperatures and measure the flow rate. • Repeat the experiment for different flow rates.



Observation: Sr. No Type

of flow Hot Water Air LMTD

0C Mass Flow (kg/s)

Mh

Inlet temp

0C Thi

Outlet temp 0C

Tho

Mass Flow (kg/s)

mc

Inlet temp 0C

Tci

Outlet temp

0C Tco

Calculation: Heat transfer from hot water : Qh = mh * Ch * ( Thi - Tho ) Heat transfer from cold air : Qc = mc * Cc* ( Tco – Tci ) Where, Ch = specific heat of hot water , kJ/kg Cc = specific heat of cold air , kJ/kg mh = mass flow rate hot water , kg/hr mc = mass flow rate cold air , kg/hr LMTD = 0C Area of heat transfer (based on outer diameter) A = ( 8 × 0.022 × 1 ) + ( ∏ × 0.02 × 1) + 0.025 Overall heat transfer coefficient U = Q / ( A * LMTD ) = W/m2.0C Effectiveness = mc * Cc* ( Tco – Tci _) = MH * CH * ( THI - THO ) Result:

Sr. No Heat Flow ( Watts ) Heat transfer coefficient

( W/ m2.0C )

Effectiveness




EMISSIVITY MEASUREMENT APPARATUS

Objective: After this experiment student will able to understand how to determine emissivity of test plate. Apparatus: Black plate, Test plate (aluminium) Theory: All the bodies emit and absorb the thermal radiation to and from surroundings. The rate of thermal radiation depends upon the temperature of body. Thermal radiation is electromagnetic wave and does not require any material medium for propagation. When thermal radiation strikes a body, part of it is reflected, part of it is absorbed and part of it is transmitted through body. The fraction of incident energy, reflected by the surface is called reflectivity (ρ). The fraction of incident energy, absorbed by the surface is called absorptivity (α). The fraction of incident energy transmitted through body is called transmissivity ( τ ). The surface which absorbs all the incident radiation is called a black surface. FOR A BLACK SURFACE (α=1)

The radiant flux, emitted from the surface is called emissive power (e). The emissivity of a surface is ratio of emissive power of a surface to that of black surface at the same temperature. Construction of apparatus: The apparatus uses comparator method for determining the emissivity of test plate. It consists of two aluminium plates, of equal physical dimensions. Mica heaters are provided inside the plates. The plates are mounted in an enclosure to provide undisturbed surroundings. One of the plates is blackened outside for use as a compurgator because black surface has ε=1. Another plate is having natural surface finish. Input to heaters can be controlled by separate dimmerstats. Heater input is measured on common ammeter and voltmeter. One thermocouple is fitted on surface of each plate to measure the surface temperature with digital temperature indicator. By adjusting input to the heaters, both the plates are brought to the same temperature, so that conduction and convection losses from both the plates are equal and difference in input is due to different emissivity. Holes are provided at backside bottom and at the top of enclosure for natural circulation of air over the plates. The plate enclosure is provided with Perspex acrylic sheet at the front. Thus, ε =e/eb



Procedure: 1. Blacken one of the plates with the help of lamp black (normally this is blackened at the

works, but if blacking is wiped out, then blackening is necessary) 2. Keep both the dimmer Knobs at ZERO position. 3. Insert the supply pin-top in the socket (which is properly earthen) and switch ON the main

supply. 4. Switch ON the main switch on the panel. 5. Switch the meter selector switch (toggle switch) in downward position. 6. Adjust dimmer of black plate, so that around 110-120 volts are supplied to black plate. 7. Now, switch the meter selector switch in upward position. 8. Adjust test plate voltage slightly less than that of black plate (says 100- 110 volts) 9. Check the temperature (after, say 10 minutes) and adjust the dimmers so that temperature

of both the plates are equal and steady. Normally, very minor adjustments are required for this.

10. Note down the readings after the plate temperatures reach steady state. Precautions: 1. Black plate should be perfectly blackened 2. Never put your hand or papers over the holes provided at the top of enclosure. 3. Keep at least 200 mm distance between the back side of unit and wall Operate all the switches sand Observations: Enclosure temp. - T3 =__________ C

Calculation: 1. Enclosure temp.

TD = TE= ----------- C =( T3 + 273.15 ) K

2. Flate surface temp. T1 = T2 = C Ts=T3 = (T1 + 273.15) K

3. Heat input to black plate WB = V * I watts

4. Heat input to test plate WT = V * I watts

Test plate Black plate Input v*I

Surface temperature

T1 T2



5. Surface area of plates

A = 2* ∏/4{D2 + (∏.D.t)} =0.0447 m2

Where, D= dia. of plates=0.16 m And t= thickness of plates = 0.009 m 6. For black plate

WB = WCVB+WCDB+WRB -------------------(I) Where, WCVB = Convection losses WCDB= Conduction losses WRB = Radiation losses

Similarly, for test plate, WT=WCVT+WCDB+WRT ----------------(ii) As both plates are of same physical dimensions, same material and at same temperature. WCVB = WCVT and WCDB = WCDT Subtracting equation (ii) from (i), we get WB _- WT = WRB - WRT = [σ A εB(TS

4 - TD4)]- [σ A εT(TS

4 - TD4)]

= σ A (TS4 - TD

4)( εB-εT) As emissivity of black plate is 1. WB-WT =σ A (TS

4 - TD4) (1-εT)

Where, εT =Emissivity of test plate σ = Stefan Boltzmann constant

= 5.667*10-8 W/m2 K4

Result: The emissivity of test plate was found to be ___________at the temperature of __________ K Conclusion:

nirma university institute of technology · pdf filenirma university institute of technology...

Documents