Embed Size (px)
Citation preview
HEAT TRANSFER LAB
INDEX
S.No. NAME OF EXPERIMENT PAGE No.
1 Determination of Thermal conductivity of insulation powder
2 Determination of overall heat transfer coefficient of Composite Wall
3 Determination of over all heat transfer coefficient of Lagged Pipe
4 Determination of Thermal Conductivity of given Metal Rod
5 Determination of heat transfer coefficient of Pin-Fin (Natural and Forced Convection)
6 Determination of heat transfer coefficient of Natural Convection
7 Determination of heat transfer coefficient of Forced Convection.
8 Determination of Stefan Boltzman Constant
9 Determination of Emissivity of test plate
10 Determination of effectiveness and overall heat transfer coefficient using Parallel and Counter flow Heat Exchanger
11 Determination of heat transfer coefficient in drop and film wise condensation
12 Determination of Critical Heat flux
13 Study of heat pipe and its demonstration
THERMAL CONDUCTIVITY OF
INSULATING POWDER
THERMAL CONDUCTIVITY OF INSULATING POWDER
AIM To determine the thermal conductivity of insulating powder at average
temperature.
INTRODUCTION
Conduction of heat is flow of heat which occurs due to exchange of
energy from one molecule to another without appreciable motion of
molecules. In any heating process, heat is flowing outwards from heat
generation point. In order to reduce losses of heat, various types of
insulations are used in practice. Various powders e.g. asbestos powder,
plaster of paris etc. are also used for heat insulation. In order to
determine the appropriate thickness of insulation, knowledge of thermal
conductivity of insulating material is essential.
APPARATUS
The apparatus consists of a smaller (inner) sphere, inside, which is fitted
a mica electric heater. Smaller sphere is fitted at the center of outer
sphere. The insulating powder, whose thermal conductivity is to be
determined is filled in the gap between the two spheres. The heat
generated by heater flows through the powder to the outer sphere. The
outer sphere loses heat to atmosphere. The input to the heater is
controlled by a dimmerstat and is measured on voltmeter and ammeter.
Four thermocouples are provided on the outer surface of inner sphere
and six thermocouples are on the inner surface of outer sphere, which
are connected to a multi channel digital temperature indicator.
SPECIFICATIONS
1. Inner sphere- 100mm O.D., halved construction
2. Outer sphere- 200mm I.D., halved construction
EXPERIMENTAL PROCEDURE
1. Keep dimmerstat knob at ZERO position and switch ON the
equipment.
2. Slowly rotate the dimmerstat knob, so that voltage is applied
across the heater. Let the temperatures rise.
3. Wait until steady state is reached.
4. Note down all the temperatures and input of heater in terms of
volts and current.
OBSERVAATIONS Sl.
No.
Temperatures 0 C
Heat input
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 Volts Amps
THEORY
Consider the transfer of heat by heat conduction through the wall of a
hallow sphere formed of insulating powder (Ref.fig.)
Let, ri = raidus of inner sphere, m
r0 = radius of outer sphere, m
Ti = average inner sphere surface temp. OC
T0= average outer sphere surface temp. OC
Consider a thin spherical layer of thickness dr at radius r & temperature
difference of dT across the layer. Applying Fourier law of heat
conduction, heat transfer rate,
Q = -k. 4π . r2.[dT/dr]
Where k = thermal conductivity of insulating powder.
Therefore dTrdrx
k4Q
2 −=π
Integrating between ri to ro and Ti to To, we get
∫∫ −=0
i
0
i
T
T
r
r2 dT
rdr
k4Qπ
)TT(r1
r1x
k4Q
0i0i
−=−π
or )rr(
)TT(rkr4Qi0
oioi
−−
=π
From the measured values of Q, Ti and T0 thermal conductivity of
insulating powder can be determined as
)TT.(rr.4
)rr(Qkoio.i
i0
−−
=π
CALCULATIONS
1. Heater input = Q = V x I Watts
2. Average inner sphere surface temperature, 44321 TTTT
Ti+++
= 0C
6
.3
107650
TTTTT
etemperatursurfacesphereouterAverage+−−−+++
=
4. Inner sphere radius = 50 mm = 0.05 m
5. Outer sphere radius = 100 mm = 0.1 m.
Now )TT.(rr.4
)rr(Qkoio.i
i0
−−
=π
W / m K at CTT ooi
2+
PRECAUTIONS
1. Operate all the switches and controls gently.
2. If thermal conductivity of the powder other than supplied is to be
determined then gently dismantle the outer sphere and remove the
powder, taking care that heater connections and thermocouples
are not disturbed.
3. Earthling is essential for the unit.
RESULT
Thermal conductivity of insulating powder is ___________at temperature
of ________
Fig. 1 Apparatus of thermal conductivity of insulating powder
1. Shell 2. Voltmeter 3. Ammeter 4. Temperature indicator 5. Selector switch 6. Main switch 7. Heater control
Fig. 2 Location of thermocouples in spherical shell
COMPOSITE WALL APPARATUS
COMPOSITE WALL APPARATUS
AIM To determine the thermal resistance, thermal conductivity of composite
wall material and plot temperature gradient along composite wall
structure.
APPARATUS The apparatus consists of a plates of different materials sandwiched
between two aluminum plates. Three types of slabs are provided on both
sides of heater, which forms a composite structure. A small hand press
frame is provided to ensure the perfect contact between the slabs. A
dimmerstat is provided for varying the input to the heater and
measurement of input is carried out by a Voltmeter and Ammeter.
Thermocouples are embedded between interfaces of input slabs, to read
the temperatures at the surface.
SPECIFICATIONS
Slab size:
a. M.S. - ϕ 25 cm x 25 mm thick
b. Bakelite - ϕ 25 cm x 10 mm thick
c. Brass - ϕ 25 cm x 10 mm thick
EXPERIMENTAL PROCEDURE
1. Start the supply of heater. By varying the dimmerstat adjust the
input (range 30- 70 Watts) and start water supply.
2. Take readings of all the thermocouples at an interval of 10 minutes
until steady state is reached.
WALL THICKNESS CONDUCTIVITY
a. M.S 2.5 cm 46 W / m K
b. Bakelite 1.0 cm 0.223 W / m K
c. Brass 1.0 cm 110 W / m K
OBSERVATIONS Sl.No. Heat Supplied
(Watts)
Temperatures 0C
Voltmeter Ammeter T1 T2 T3 T4 T5 T6 T7 T8
CALCULATIONS
1) Mean readings,
a) 2
)( 21 TTTA+
= 0C
b) 2
)( 43 TTTB
+= 0C
c) 2
)( 65 TTTC
+= 0C
d) 2
)( 87 TTTD
+= 0C
2) Rate of heat supplied, Q = V x I Watts
For calculating the thermal conductivity of composite walls, it is assumed
that due to large diameter of the plates, heat flowing through central
portion is unidirectional i.e. axial flow. Thus for calculations central half
diameter area where unidirectional flow is assumed is considered.
Accordingly thermocouples are fixed at close to center of the plates.
Now, Heat flux, 2/ mWattAQq =
).(4
2 platesofdiahalfddxA ==π
Total thermal resistance of composite slab:
WKmq
TTR oDA
total /2−=
i) Thermal conductivity of composite slab, kmWTTbqK
DAcomposite //.
−=
b = Total thickness of composite slab = 0.045 m
ii) Plot thickness of slab material against temperature gradient.
PRECAUTIONS 1. Keep the dimmerstat zero before start
2. Increase voltage slowly
3. Keep all the assembly undisturbed
4. Do not increase voltage above 200 V
5. Operate selector switch of temperature indicator slowly.
GRAPH
RESULT Thermal resistance of composite wall =
Over all thermal conductivity of composite wall =
45 35 25
TD
TC
TB
TA
Bakelite Brass
Mild Steel
Tem
pera
ture
, 0 C
Thickness of slab
Fig. 1 Composite wall apparatus
1. Voltmeter 2. Ammeter 3. Temperature indicator 4 Main switch 5. Heater Control 6. Water connection
Fig. 2 Thermocouple setting in composite wall apparatus
A. Heater B & B1 M.S. Plate C & C1 Bakelite D & D1 Brass plate E & E1
Cooling plate
LAGGED PIPE
LAGGED PIPE
AIM
1. To determine heat flow rate through the lagged pipe and compare it
with the heater input for known value of thermal conductivity of
lagging material.
2. To determine the approximate thermal conductivity of lagging
material by assuming the heater input to be the heat flow rate
through lagged pipe.
3. To plot the temperature distribution across the lagging material.
APPARATUS
The apparatus consists of three concentric pipes mounted on suitable
stand. The hollow space of the innermost pipe consists of the heater.
Between first two cylinders the insulating material with which lagging is
to be done is filled compactly. Between second and third cylinders,
another material used for lagging is filled. The third cylinder is
concentric to other outer cylinder. Water flows between these two
cylinders. The thermocouples are attached to the surface of cylinders
appropriately to measure the temperatures. The input to the heater is
varied through a dimmerstat and measured on voltmeter and ammeter.
SPECIFICATIONS
Pipes –i) GI pipe inside Φ6 cm. (O.D)
ii) GI pipe middle Φ8.5 cm. (Mean dia.)
iii) GI pipe outer Φ10.7 cm. (I.D)
iv) Length of pipes 1 meter.
PROCEDURE
1) Start the supplies of heater and by varying dimmerstat adjust the
input for desired value (range 60 to 120 Watts) by using voltmeter and
ammeter, also start water supply.
2) Take readings of all the 6 thermocouples at an interval of 5 min until
the steady state is reached.
3) Note down steady readings in observation table.
OBSERVATIONS
1. Inner pipe O.D., D1 = 0.06 m
2. Middle pipe mean dia., D2 = 0.085 m
3. Outer pipe I.D., D3 = 0.107 m
Sl.No. Volt
meter Ammeter Thermocouple Readings
V I T1 T2 T3 T4 T5 T6
CALCULATIONS
Mean readings
C2
TTT
C2
TTT
C2
TTT
o65Outer
o43middle
o21inside
+=
+=
+=
ri = Inner pipe radius = 0.03 m
ro = outer pipe radius = 0.0535 m
rm = mean radius of middle pipe = 0.0425m
L = Length of pipe = 1m
k = thermal conductivity W / m K
Q= actual heat input = V x I Watts
Assumption: The pipe is so long as compared with diameter that heat
flows in radial direction only middle half length.
i) Now, first we find out the theoretical heat flow rate through the
composite cylinder
⎥⎦
⎤⎢⎣
⎡+
π
−=
m
0e
2i
me
1
outsideinside
rrLog
k1
rrLog
k1
L21
TTQ
k1 = 0.22 W/moK & k2 = 0.13 W/m0K, where Actual heat input, Qact = V.I.
ii) Now from known value of heat flow rate, value of combined thermal
conductivity of lagging material can be calculated.
)r/r(log)ToTi(Lk2Q
i0eact
−π=
mK/W)ToTi(L2
)r/r(logQk i0eact
−π=∴
The space between the pipes of φ6 cm and φ 8.5 cm contains commercial
asbestos powder and the space between pipes of φ8.5 cm and φ10.5 cm
contains saw dust.
.Km/W)TT(L2
)r/r(logQk
Km/W)TT(L2
)r/r(logQk
o
om
moeact2
o
mi
imeact1
−π=
−π=
iii) To plot the temperature distribution use formula
)r/r(Log)r/r(Log
S/VTT
TT
ioe
ie
io
i
−−
Where r is the selected radius for corresponding to temperature T,
between the two pipes of the same lagging material. Thus plot is made
for different values of ‘r’.
PRECAUTIONS
1) Keep dimmerstat to ZERO position before start.
2) Increase voltage gradually.
3) Keep the assembly undisturbed while testing.
4) While removing or changing the lagging materials do not disturb the
thermocouples.
5) Do not increase voltage above 150V
6) Operate selector switch of temperate indicator gently.
RESULTS
Theoretical Heat flow rate =
Thermal conductivity of lagged material =
Thermal conductivity of asbestos powder =
Thermal conductivity of saw dust =
Fig 1 Lagged pipe apparatus
1. Voltmenter 2. Ammeter 3. Temperature indicator 4. Selector switch 5. Main switch 6. Heater control 7. Assembly
Fig. 2 Thermocouple settings in lagged pipe
Fig.3 Graph of temperature gradient
THERMAL CONDUCTIVITY OF METAL ROD
THERMAL CONDUCTIVITY OF METAL ROD
AIM
To determine the thermal conductivity of copper bar at various sections
to study the variation of thermal conductivity with temperature.
INTRODUCTION
Thermal conductivity is the physical property of the material denoting the
ease with which 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 or substances of which it is a composed, the phase (i.e. gas,
liquid or solid) in which it exists, its crystalline structure if a solid, the
temperature and pressure to which it is subjected, and whether or not it
is a homogeneous material. For pure copper thermal conductivity is 380
W/ m. K at 200C.
Thermal energy can be conducted in solids by free electrons and by
lattice vibrations. Large number of free electrons moves about in the
lattice structure of the material, in good conductors. These electrons
carry thermal energy from higher temperature region to lower
temperature region, in a similar way they transport electric charge. In
fact, these electrons are frequently referred as electron gas. Energy may
also be transferred as vibrational energy in the lattice structure of the
material. In general, however, this mode of energy transfer is not as large
as electron transport and hence, good electrical conductors are always
good heat conductors, e.g. copper, silver etc. However, with increase in
temperature, lattice vibrations come in the way of transport by free
electrons and for most the metals thermal conductivity decreases with
increase in temperature.
APPARATUS
The apparatus consists of a copper bar, one end of which is heated by an
electric heater and the other end is cooled by a water-circulated heat
sink. The middle portion, i.e. Test section of the bar is covered by a shell
containing insulation. The bar temperature is measured at 8 different
section, while 2 thermocouples measure the temperature at the shell.
Two thermometers are provided to measure water inlet and outlet
temperatures. A dimmer is provided for the heater to control its input.
Constant water flow is circulated through the heat sink. A gate valve
provided controls the water flow.
SPECIFICATIONS 1. Metal bar – copper, 25mm O.D, approx. 430 mm long with
insulation shell along the test length and water cooled heat sink at
the outer end.
2. Test length of the bar – 240 mm
3. Measuring flask to measure water flow.
EXPERIMENTAL PROCEDURE 1. Start the electric supply.
2. Start heating the bar by adjusting the heater input to say 80 V or
100 V
3. Start cooling water supply through the heat sink and adjust it to
around 350- 400 cc per minute.
4. Bar temperature will start rising. Go on checking the temperatures
at time intervals of 5 minutes.
5. When all the temperatures remain steady, note down all the
observations and complete the observation table.
OBSERVATION TABLE
Sl. No.
TEST BAR TEMPERATURE OC Shell Temp.oC
Water Temp.oC
Water flow rate Lit/Sec.
T1 T2 T3 T4 T5 T6 T8 T9 T10 T11 T12
Using the temperatures of the bar at various points, plot the temperature
distribution along the length of the bar and determine the slopes of the
graph (i.e. temperature drop per unit length) dT/dx at the sections AA,
BB and CC as shown in figure.
(Note: As the value of temperature goes on decreasing along the length of
the bar, the value of the slope dT/dx is negative)
CALCULATIONS Heat is flowing through the bar from heater end to water heat sink.
When steady state is reached, heat passing through the section CC of the
bar is heat taken by water.
1) Heat passing through Section CC
Qcc = m. CP ΔT Watts.
Where,
m = mass flow rate of cooling water, kg/s.
CP = Specific heat of water= 4180 J / kgo C
Δ T = (Water outlet temperature) – (Water inlet temperature)
Now, Qcc = -kccccdx
dT⎥⎦⎤
⎢⎣⎡ .A
A = Cross sectional area of the bar = 2
4Dπ
Kcc = W / moC
2) Heat passing through section BB
Qbb = Qcc + Radial heat loss between CC & BB.
= Qcc +)/(log
)(.2
0
1061
ie rrTTLk −∏
Where k = Thermal conductivity of insulation = 0.35 W / m oC
L1 = Length of insulation cylinder = 0.060 m
ro = outer radius =0.105 m
ri = inner radius = 0.0125 m
Qbb = -kbb.bbdx
dT⎥⎦⎤
⎢⎣⎡ .A
Kbb = W/mo C
3) Heat passing through section AA
Qaa = Qbb + Radial heat loss between BB & AA.
= Qbb +)/(log
)(.2
0
932
ie rrTTLk −∏
Where,L2 = 0.090 m
Qaa = -kaa.aadx
dT⎥⎦⎤
⎢⎣⎡ .A
Kaa = W/moC
RESULTS
1) Temperature of the bar decreases from hot end to cool end, which
satisfies the
Fourier law heat conduction.
2) Thermal conductivity of bar at three different sections.
Thermal conductivity at section AA = kaa =
Thermal conductivity at section BB = kbb =
Thermal conductivity at section CC = kcc =
Fig.1 Apparatus of thermal conductivity of metal rod
1. Shell 2. Heater 3. Voltmeter 4. Ammeter 5. Temperature indicator 6. Main switch 7. Heater control
Fig.3 Thermocouple settings in metal rod
Distance between two thermocouple = 0.032 m
Fig.3 Graph for temperature gradient in metal rod
HEAT TRANSFER IN PIN FIN
HEAT TRANSFER IN PIN FIN
AIM
To study the temperature distribution, heat transfer coefficient and
efficiency of a pin fin in natural and forced convection heat transfer.
INTRODUCTION
Extended surfaces or fins are used to increase the heat transfer rates
from a surface to the surrounding fluid wherever it is not possible to
increase the value of the surface heat transfer coefficient or the
temperature difference between the surface and the fluid. Fins are
fabricated in variety of forms. Fins around the air cooled engines are a
common example. As the fins extend from primary heat transfer surface,
the temperature difference with the surrounding fluid diminishes towards
the tip of the fin.
APPARATUS
The apparatus consists of a simple pin fin which is fitted in a rectangular
duct. The duct is attached to suction end of a blower. One end of fin is
heated by an electrical heater. Thermocouples are mounted along the
length of fin and a thermocouple notes the duct fluid temperature. When
top cover over the fin is opened and heating started, performance of fin
with natural convection can be evaluated and with top cover closed and
blower started, fin can be tested in forced convection.
SPECIFICATIONS
1) Fins – 12 mm O. D., Effective length 102 mm with 5 nos of
thermocouple positions along the length, made of brass, mild steel and
aluminum - one each.
Fin is screwed in heater block which is heated by a band heater.
2) Duct- 150 x 100mm cross-section, 1000mm long connected to suction
side of blower
3) FHP centrifugal blower with orifice and flow control valve on discharge
side.
4) Orifice – dia 22mm, coefficient of discharge Cd = 0.64
5) Water manometer connected to orifice meter
THEORY
Let A= Cross sectional area of the fin, m2
P= Perimeter (circumference) of the fin, m
L= Length of the fin = 0.102 m
T1= Base temperature of fin
Tf= Duct fluid temperature (Channel No. 6 of temperature indicator)
θ = Temperature difference of fin and fluid temperature =T- Tf
h = Heat transfer coefficient, W / m2 oC
Kf = Thermal conductivity of fin material
= 110 W / m oC for brass
= 46 W / m oC for mild steel
= 232 W / m oC for aluminum
Heat is conducted along the length of fin and also lost to surroundings.
Applying first law of thermodynamics to a control volume along the
length of fin at a station which is at length ‘x’ from the base
0.
.2
2
=− θAk
PhdX
Td
f
(1)
).().( 21mxeCeC mx −+=∴ φ
(2)
AkPhmWhere
f ..
= (3)
With the boundary conditions of θ = θ1 at x = 0, θ1 = T1-Tf
Assuming tip is to be insulated, 0=dxdθ at x = L,
Results in obtaining equation (2) in the form
]..[
)]([
11 LmCoshXLmCosh
TTTT
f
f −=
−
−=
θθ (4)
This is the equation for temperature distribution along the length of the
fin. Temperatures T1 and Tf will be known for the given situation and the
value of ‘h’ depend upon mode of convection i.e. natural or forced.
EXPERIMENTAL PROCEDURE
A) NATURAL CONVECTION
Open the duct cover over the fin. Ensure proper earthing to the unit and
switch on the main supply. Adjust dimmerstat so that about 80 V are
supplied to the heater. The fin will start heating. When the
temperatures remain steady, note down the temperatures of the fin and
duct fluid temperature.
Sl.
NO. INPUT Fin temperatures oC
Duct fluid temperature oC
V I T1 T2 T3 T4 T5 T6 (Tf)
B) FORCED CONVECTION
Close the duct cover over the fin. Start the blower. Adjust the
dimmerstat so that about 100 – 110v are supplied to the heater. When
the temperatures become steady, note down all the temperatures and
manometer difference
CALCULATIONS
Nomenclature:
Tm = Average fin temperature = (T1 + T2 + T3 + T4 +T5) /5
fm TTT −=Δ
Tmf = Mean film temperature = (Tm+ Tf) / 2
ρa = Density of air, kg / m3
ρw = Density of water, kg / m3 = 1000 kg / m3
D = Diameter of pin fin = 12 x 10-3 m
d = Diameter of orifice = 22 x 10-3 m
Sl.
No.
INPUT Manometer
difference
Fin Temperature oC Duct fluid
temp. oC
V I H (m of water) T1 T2 T3 T4 T5 T6 (Tf)
Cd = coefficient of discharge of orifice = 0.64
μ = Dynamic viscosity of air, N-s/m2
Cp = Specific heat of air, kJ/kg.K
Kinematic viscosity, m2/s =ט
kair = Thermal conductivity of air, W/m K
β = volume expansion coefficient = 1 / (Tmf+273.15)
H = Manometer difference, m of water
V = velocity of air in duct, m/s
Q = volume flow rate of air, m3/s
Vtmf = velocity of air at mean film temperature
All properties are to be evaluated at mean film temperature.
NATURAL CONVECTION
The fin under consideration is horizontal cylinder losing heat by natural
convection. For horizontal cylinder, Nusselt number, from data book,
page number 122.
Nu= 1.02 (Gr.Pr)0.148 ----------for 10-2 < Gr.Pr < 102
Nu = 0.85 (Gr.Pr)0.188 --------- for 102 < Gr.Pr < 104.
Nu= 0.48 (Gr.Pr)0.25 ----------for 104 < Gr.Pr < 107
Nu = 0.125 (Gr.Pr)0.333 --------- for 107 < Gr.Pr < 1012.
Where Gr = Grashof number. 2
3 TD..gν
β Δ=
Pr = Prandtl number = airk
Cp μ. (take from data book.)
Determine Nusselt number.
Now, Nu = (hD)/kair
Therefore, h = Nu. Kair /D
From h determine m from equation (3)
Using h and m, determine temperature distribution in the fin from
equation (4)
The rate of heat transfer from the fin and efficiency can be calculated as,
)(... 1 fffin TTAkPhQ −= and mLmL ][tanh
=η
FORCED CONVECTION
For flow across Horizontal cylinder loosing heat by forced convection,
from data book, page number 100.
Nu = 0.911 (Re)0.385. Pr.0.333 ---------- for 4 < Re < 40
Nu = 0.683 (Re)0.466 . Pr.0.333 ---------- for 40 < Re < 4000
Nu= 0.193 (Re)0.618 . Pr.0.333 ----------for 4000 < Re < 40,000
Where, ν
DVR tmf
e
.=
)273()273(.
+
+=
f
mftmf T
TVV
Velocity of air is determined from air volume flow.
smXQareationalcrossDuctQV
smHgdCdQ aw
/)1.015.0/(sec/
/)/(..24
32
==
= ρρπ
From Nusselt Number, find out ‘h’ and from ‘h’, find out ‘m’
Now temperature distribution, heat transfer rate and effectiveness of the
fin can be calculated using equations 4, 5 and 6 respectively.
CONCLUSION
1. Comment on the observed temperature distribution and calculation
by theory, it is expected that observed temperatures should be
slightly less than their calculated values because of radiation and
non- insulated tip.
2. Plot the graphs of temperature distribution in both natural and
forced convection.
PRECAUTIONS
1. Operate all the switches and controls gently
2. Do not obstruct the suction of the duct or discharge pipe
3. Open the duct cover over the fin for natural convection experiment
4. Fill up water in the manometer and close duct cover for forced
convection experiment
5. Proper earthing to the unit is necessary
6. While replacing the fins, be careful for fixing the thermocouples.
Incorrectly fixed thermocouples may show erratic readings
Fig.1: Thermocouple position on fin
GRAPH
Fig 2: Variation of fin temperature along the length of fin with natural
convection and forced convection.
RESULTS
Natural convection:
Heat transfer coefficient =
Efficiency of pin fin =
Forced convection:
Heat transfer coefficient =
10 23 23 23 23
T1 T2 T3 T4 T5
Natural Convection
Forced Convection
Fin
Tem
pera
ture
, 0 C
Thermo couple distance, x
Efficiency of pin fin =
Fig. 3 Pin fin apparatus
1. Manometer 2. Ammeter 3. Voltmeter 4. Temperature indicator 5. Selector switch 6. Blower switch 7. Heater control 8. Main switch 9. Suction duct 10. Orifice meter
HEAT TRANSFER IN PIN FIN
HEAT TRANSFER IN NATURAL CONVECTION
AIM
To determine the experimental and theoretical heat transfer coefficient
for vertical tube losing heat by natural convection.
INTRODUCTION
In contrast to the forced convection, 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 cylinder in terms of the variation of local heat transfer
coefficient along the length and also the average heat transfer coefficient
and its comparison with the value obtained by using and appropriate
correlation.
APPARATUS
The apparatus consists of a brass tube fitted in a rectangular vertical
duct. The duct is open at the top and bottom and forms an enclosure
and serves the purpose of undisturbed surrounding. 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 the tube to the 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
and a voltmeter and is varied by a dimmerstat. The vertical cylinder with
the thermocouple positions is shown in figure. The tube surface is
polished to minimize the radiation losses.
SPECIFICATIONS 1. Diameter of the tube (d)= 38mm
2. Length of tube (L) = 500mm
3. Duct size 200mm x 200mm x 800mm Length
THEORY When a hot body is kept in 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 is continuous and the
heat transfer takes place due to the relative motion of hot and cold fluid
particles.
The heat transfer coefficient is given by:
)TT(AQQh
ass
R
−−
= (1)
Where h = average surface heat transfer coefficient (W/m2 oC)
Q = Heat transfer rate V. I (watts)
As = Area of the heat transferring surface =∏ .d L (m2)
Ts = Average surface temperature 7
)TTTTTTT( 7654321 ++++++= 0 C
Ta = Ambient temperature in the duct = T8 0C
QR = Heat loss by radiation = )(.. 44as TTA −∈σ
Where
.σ = Stefan Boltzmann constant = 5.667x 10-8 W/m2 K4
∈ = Emissivity of pipe material = 0.06
Ts & Ta = Surface and ambient temperatures in o K respectively.
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: n
KCxTLgA
khxL
⎥⎦
⎤⎢⎣
⎡ Δ= μρ
υβ
2
3 .. (2)
Where k
hxL is called the Nusselt number.
2
3..v
TLg Δβ = is called the Grashof Number and
kC μρ .
= is the Prandtl Number.
A and n are constants depending on the shape and orientation of the
heat transferring surface.
Where L = A characteristic dimension of the surface.
K= Thermal conductivity of fluid
Kinematic viscosity of fluid =ט
μ = Dynamic viscosity of fluid
Cp = Specific heat of fluid
β = Coefficient of volumetric expansion for the fluid
g = Acceleration due to gravity.
TΔ = [Ts – Ta]
For gases ⎥⎥⎦
⎤
⎢⎢⎣
⎡
+=
)273(1
fTβ K-1, Tf = (Ts + Ta)/2
For a vertical cylinder losing heat by natural convection, the constants A
and n of equation (2) have been determined and the following empirical
correlation’s obtained from data book, page number 120.
khL = 0.59 (Gr.Pr.)0.25 for 104 < Gr.Pr. < 109 (3)
khL = 0.10 (Gr.Pr.)1/3 for 109 < Gr.Pr. < 1013 (4)
L = Length of the cylinder.
All the properties of the fluid are determined at the mean film
temperature (Tf)
PROCEDURE 1. Put ON the supply and adjust the dimmerstat to obtain the
required heat input (say 40 W, 60 W, 70 W etc).
2. Wait till the steady state is reached, which is confirmed from
temperature readings ( T1- T7)
3. Measure surface temperature at the various points i.e. T1 to T7
4. Note the ambient temperature i.e. T8
OBSERVATIONS
1) O.D. Cylinder = 38 mm
2) Length of cylinder = 500 mm
3) Input to heater = V. I Watts
Sl.No. Volt Amp Temperature, 0C
T1 T2 T3 T4 T5 T6 T7 T8
CALCULATIONS
1) Calculate the value of average surface heat transfer coefficient
neglecting end losses using equation (1)
2) Calculate and plot (Fig.4) the variation of local heat transfer
coefficient along the length of the tube using:
T = T1 to T7 and h = Q/[As(T-Ta)]
3) Compare the experimentally obtained value with the predictions of
the correlation equation (3) or (4).
PRECAUTIONS 1. Proper earthing is necessary for the equipment.
2. Keep dimmerstat to ZERO volt position before putting on main
switch and increase it slowly.
3. Keep at least 200 mm space behind the equipment.
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 input above 80 Watts.
RESULTS AND DISCUSSIONS The heat transfer coefficient is having a maximum value at the beginning
as expected because of the just staring of the boundary layer and it
decreases as expected in the upward direction due to thickening of layer
and which is laminar one. This trend is maintained up to half of the
lengths (approx) and beyond that there is little variation in the value of
local heat transfer coefficient because of the transition and turbulent
boundary layers. The last point shows some what increase in the value
of heat transfer coefficient which is attributed to end loss causing a
temperature drop. The comparison of average heat transfer coefficient is
also made with predicted values are some what less than experimental
values due to the heat loss by radiation.
RESULTS
Experimental heat transfer coefficient =
Theoretical heat transfer coefficient =
Fig. 1 Natural convection apparatus
Fig. 2 Thermocouple locations
Fig.3 Variation of local heat transfer coefficient
HEAT TRANSFER IN FORCED CONVECTION
HEAT TRANSFER IN FORCED CONVECTION
AIM
To determine the experimental and theoretical heat transfer coefficient in
forced convection heat transfer for internal flow.
APPARATUS
The apparatus consists of a circular pipe, through which cold fluid, i.e.
air is being forced. Pipe is heated by a band heater outside the pipe.
Temperature of pipe is measured with thermocouples attached to pipe
surface. Heater input is measured by a voltmeter and ammeter. Thus,
heat transfer rate and heat transfer coefficient can be calculated.
SPECIFICATIONS
• Test pipe – 33mm I.D. 500 mm long
• Band heater for pipe.
• Blower to force the air through test pipe
• Orifice meter with water manometer.
EXPERIMENTAL PROCEDURE
1. Put ON main supply.
2. Adjust the heater input with the help of dimmerstat.
3. Start the blower and adjust the air flow with valve
4. Wait till steady state is reached and note down the reading in
the observation table
Sl.
No
Vol
t
Amp. Temperatures O C Manomete
r
Difference
V I T1 T2 T3 T4 T5 T6 T7
hw
CALCULATIONS 1. Air inlet temp T1 = 0 C
2. Air outlet temp. T7 = 0 C
3. Density of air, 1273
273293.1T
xa +=ρ kg/m3
4. Diameter of orifice = 22 mm
Manometer difference = Water head = hw meters
Air head, ha = hw (ρw / ρa)
Where ρw = density of water = 1000 Kg/m3
Air volume flow rate, i.e., discharge
ad ghaCQ 20= m3 /sec.
Where Cd = 0.64
ao = cross section area of orifice
5. Mass flow rate of air, ma = Q x ρa kg/sec.
Velocity of air, ρa
QV = m/sec.
Where, ρa = Cross Sectional area of pipe = m2
6. Heat gained by air, Qair = ma x Cpa x (T7-T1)
Where Cpa = Specific heat of air = 1KJ/Kg.K. or 103 J/Kg K
7. Average inside surface temperature, Ts 5
)( 65432 TTTTT ++++= 0C
8. Bulk mean temperature of air, Tm 2
)( 71 TT += 0C
9. Average heat transfer coefficient
Actual Heat Loss Due to Forced Convection= Qair– QR
Heat Loss due to radiation, QR = σ 0.4 x A x (Ts4- Ta4)
Ta = atmospheric temperature = T1
hexpt )( ms
Rair
TTAQQ
−−
= W/m2 K
Where A = Inside surface are of the pipe =π x di x L
10. Reynolds number, ReD υ
VxD=
υ = Kinematic Viscosity at Tm
D = 0.033 m
If ReD < 2300, flow is laminar.
For laminar flow, NuD = airkDh.
= 4.36, from data book, page number 109.
If Reynolds number exceeds 2300, flow is turbulent.
For turbulent flow, NuD = 0.023 (ReD)0.8 (Pr.)n from data book, page
number 110.
Where n = 0.4 when fluid is being heated.
n = 0.3 When fluid is being cooled.
Determine htheo from NuD.
Note: The calculated values and actual values may differ appreciably
because of heat losses. The heat loss through natural convection,
conduction and heat loss through insulation over the heater is not
considered, but they are present. Also, the heat flux is not uniform
practically, as assumed in theory, which gives difference between actual
and theoretical value.
PRECAUTIONS
1. While putting ON the supply, keep dimmerstat at zero position and
blower switch OFF
2. Operate all the switches and controls gently.
3. Do not obstruct the flow of air while experiment is going on.
RESULT
Experimental heat transfer coefficient, hexpt =
Theoretical heat transfer coefficient = htheo =
Fig.1 Forced convection apparatus
1. Manometer 2. Voltmeter 3. Ammeter 4. Temperature indicator 5. Selector switch 6. Blower switch 7. Heater control 8. Main switch 9. Blower 10. Orifice meter 11. Test section 12. Thermocouple setting
STEFAN BOLTZMANN APPARATUS
STEFAN BOLTZMANN APPARATUS
AIM
To determine the Stefan- Boltzmann’s constant in the radiation heat
transfer.
INTRODUCTION
All the substances emit thermal radiation. When heat radiation is
incident over a body, part of radiation is absorbed, transmitted through
and reflected by the body. A surface which absorbs all thermal radiation
incidents over it is called black surface. For black surface, transmissivity
and reflectivity are zero and absorptivity is unity. Stefan Boltzmann Law
states that emissivity of a surface is proportional to fourth power of
absolute surface temperature i.e.
E α T4
or E = σ ε T4
Where E = emissive power of surface, W / m2
T = absolute temperature
σ = Stefan Boltzmann constant
ε = emissivity of the surface
Value of Stefan Boltzmann constant is taken as
σ = 5.667 x 10-8 W / m2 K4
For black surface ε = 1, hence above equation reduces to
E = σ. T4
APPARATUS
The Apparatus consists of a water-heated jacket of hemispherical
shape. A copper test disc is fitted at the center of jacket. The hot
water is obtained from a hot water tank, fitted to the panel, in which
water is heated by an electric immersion heater. The hot water is
taken around the hemisphere, so that hemisphere temperature rises.
The test disc is then inserted at the center. Thermocouples are fitted
inside hemisphere to average out hemisphere temperature. Another
thermocouple fitted at the center of test disc measures the
temperature of test disc. A timer with a small buzzer is provided to
note down the disc temperatures at the time intervals of 5 seconds.
EXPERIMENTAL PROCEDURE
1. See that water inlet cock of water jacket is closed and fill up
sufficient water in the heater tank.
2. Put ‘ON’ the heater.
3. Blacken the test disc with the help of lamp black and let it cool.
4. Put the thermometer and check water temperature.
5. Boil the water and switch ‘OFF’ the heater
6. See that drain cock of water jacket is closed and open water inlet
cock.
7. See that there is sufficient water above the top of hemisphere
(A piezometer tube is fitted to indicate water level)
8. Note down the hemisphere temperatures (up to channel 1 to 4)
9. Note down the test disc temperature (i.e.. channel 5)
10. Start the timer. Buzzer will start ringing. At the start of timer
cycle, insert test disc into the hole at the bottom of hemisphere.
11. Note down the temperatures of disc, every five times of the buzzer
rings. Take at least 8-10 readings
OBSERVATIONS
Hemisphere Temperature (oC)
T1 =
T2 =
T3 =
T4 =
Time Interval
(Sec)
Test disc Temperature
(oC)
0
25
50
75
100
125
150
175
CALCULATIONS
1) Area of test disc, A = (π/4)d2 = m2 (d = 20 mm)
2) mass of test disc, m = 7.6 gr = 7.6 x 10-3 kg.
3) Plot a graph of temperature rise of test disc with time as base and
find out its slope at origin. i.e. 0=
⎥⎦⎤
⎢⎣⎡
attdtdT K / sec
Hemisphere temperature, TH = KTTTT
15.2734
4321 ++++
4) Initial Test disc temperature
TD = T5 + 273.15 k
As area of hemisphere is very large as compared to that disc, we can put
Q = σ є.A (TH4 – TD4)
Where Q = heat gained by disc/sec.
Q = m. cP. (dT/dt)t=0
σ = Stefan Boltzmann Constant
m = Mass of test disk = 7.6 x 10-3 kg.
є = Emissivity of test disc = 1
A = Area of Test disc
cP= Specific heat of copper = 381 J/Kg 0 C
σ = ).(
)/.(.44
0
DH
tP
TTAdtdTcm
−= W/ m2 K4
Theoretical value of σ is 5.667x 10-8 W/m2 K4.
In the experiment this value may deviate due to reasons like convection,
temperature drop of hemisphere, heat losses etc.
PRECAUTIONS
1) Never put ‘ON” the heater before putting water in the tank.
2) Put ‘OFF’ the heater before draining the water from heater tank.
3) Drain the water after completion of experiment.
4) Operate all the switches and controls gently
RESULT
Stefan – Boltzmann’s constant, σ = ________W / m2 K4
Fig.1 Stefan Boltzmann apparatus
1. Water tank 2. Main switch 3. Temperature indicator 4. Temperature selector switch 5. Buzzer switch 6. Heater switch 7. Shell
Fig.2 Thermocouple setting
Fig.3 Variation of temperature of disc with time
EMISSIVITY MEASUREMENT
EMISSIVITY MEASUREMENT
AIM
To determine the emissivity of the test plate.
INTRODUCTION
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 radiations are electromagnetic waves and
they do not require any 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) and 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, ρ+α+τ = 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. Thus,
ε = E / Eb
APPARATUS The apparatus uses comparator method for determining the emissivity of
test plate. It consists of two aluminum 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 comparator (because black
surface has ε = 1). Another plate is having natural surface finish. Input
to heaters can be controlled by separator dimmer stats. 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 same temperature, so that conduction and
convection losses form both the plates are equal and difference in input
is due to different emissivities. 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.
EXPERIMENTAL PROCEDURE
1. Blacken one of the plates with the help of lamp black (Normally
this is blackened at the works, but if blackening 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
earthed) and switch ‘ON’ the mains supply.
4. Switch ON the mains switch on the panel.
5. Keep the meter selector switch (toggle switch) at the black plate
side 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 on other side.
8. Adjust test plate voltage slightly less than that of black plate (say
100- 110 volts)
9. Check the temperatures (after, say 10 min) and adjust the
dimmers so that temperatures 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.
OBSERVATIONS
Plate Input Surface temperature,
0C V I
Test plate T1 =
Black plate T2 =
Enclose temperature, T3 = 0C
CALCULATIONS
1. Enclose temperature:
TE =T3 = 0C = (T3 + 273.15) 0K
2. Plate surface temp.
T = T1 =T2 = 0 C
TS = (T+273.15) 0K
3. Heat input to black plate, Wb = V x I Watts
4. Heat input to test plate, WT = V x I Watts
5. Surface area of test plates, A = 2 x (π/4) D2 + (π D t) =
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 + WCdT + WRT (ii)
As both plates are of same physical dimensions, same material and
at same temperatures,
WCvb = WCvT and WCdb = WCdT
Subtracting equation (ii) from (i) we get,
Wb - WT = WRb - WRT
= [σ A (Ts4 – TE4)] - [σ A εT (Ts4 – TE4)]
= σ A (Ts4 – TE4) (εb – ε)
As emissivity of black plate is 1,
Wb - WT = σ A (Ts4 – TE4) (1 – ε)
Where, ε = Emissivity of test plate
σ = Stefan Boltzman constant = 5.667 x 10-8 W/m2K4
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 backside of unit and
the wall.
4. Operate all the switches and knobs gently.
Note: Emissivity of oxidized aluminum plate i.e. test plate is normally
with in the range of 0.3 to 0.7.
RESULT
Emisvity of the test plate surface = ________at temperature of _____
Fig.1 Emissivity measurement apparatus
1. Voltmeter 2. Ammeter 3. Temperature indicator 4. Meter selector switch 5. Heater control 6. Heater control 7. Black plate 8 Test plate
CONCENTRIC TUBE HEAT EXCHANGER
CONCENTRIC TUBE HEAT EXCHANGER
AIM
To determine the heat transfer rate, LMTD, over all heat transfer
coefficient and effectiveness of heat exchangers in parallel flow and
counter flow concentric tube heat exchanger.
INTRODUCTION
Heat exchangers are the devices in which the heat is transferred from one
fluid to another. Exchange of heat is required at many industrial
operations as well as chemical process Common examples of heat
exchangers are radiator of a car, condenser of a refrigeration unit or
cooling coil of an air conditioner.
Heat exchanger are of basically three types – i) Transfer type- in which
both fluids pass through the exchanger and heat gets transferred
through the separating walls between the fluids, ii) Storage type – in this,
firstly the hot fluid passes through a medium having high heat capacity
and then cold fluid is passed through the medium to collect the heat.
Thus hot and cold fluids are alternately passed through the medium. iii)
Direct contact type – in this type, the fluids are not separated but they
mix with each other and heat passes directly from one fluid to the other.
Transfer type heat exchangers are the type most widely used. In transfer
type heat exchangers, three types of flow arrangements are used, viz.
parallel, counter or cross flow. In parallel flow, the fluids flow in the same
direction while in counter flow, they flow in the opposite direction. In
cross flow, they flow at right angles to each other.
APPARATUS The apparatus consists of two concentric tubes in which fluids pass. The
hot fluid is hot water, which is obtained from an electric geyser. Hot
water flows through the inner tube in one direction. Cold fluid is cold
water, which flows through annulus. Control valves are provided so that
direction of cold water can be kept parallel or opposite to that of hot
water. Thus, the heat exchanger can be operated either as parallel or
counter flow heat exchanger. The temperatures are measured with
thermometers. Thus, the heat transfer rate, heat transfer coefficient,
LMTD and effectiveness of heat exchanger can be calculated for both
parallel and counter flow.
SPECIFICATIONS
1. Heat exchanger- a) Inner tube - ϕ 12.7 mm O.D., ϕ 11.7 mm I.D.
copper tube
b) Outer tube ϕ 25 mm G.I. Pipe.
c) Length of heat exchanger – 1 m.
2. Valves for flow and direction control – 5nos
3. Thermometers to measure temperatures – 10 to 1100 C – 4nos
4. Measuring flask and stop clock for flow measurement.
EXPERIMENTAL PROCEDURE
1. Start the water supply. Adjust the water supply on hot and cold
sides. Firstly, keep the valves V2 and V3 closed and V1 and V4
opened so that arrangement is parallel flow.
2. Put few drops of oil in thermometer pockets. Put the thermometer
in the thermometer pockets.
3. Switch ‘ON’ the geyser. Temperature of water will start rising.
After temperatures become steady, note down the readings and fill
up the observation table.
4. Repeat the experiment by changing the flow.
5. Now open the valvesV2 and V3 and then close the valves V1 and V4.
The arrangement is in now counter flow.
6. Wait until the steady state is reached and note down the readings.
OBSERVATION
TYPE OF
FLOW
HOT WATER COLD WATER
Temperatures Time for 1
Lit. Water Temperatures Time for 1
Lit. Water
Inlet 0C
Outlet 0C
xh sec Inlet
0C
Outlet 0C
xc sec
Parallel
Flow
Counter
Flow
CALCULATIONS
1. Hot water inlet temperature, thi = 0C
Hot water outlet temperature, tho = 0C
2. Hot water flow rate, mh
Let time required for 1lit of water be xh sec
Mass of 1lit water = 1kg
There fore, mh = 1/ xh kg/s
3. Heat given by hot water (inside heat transfer rate)
Qh = mh cp (thi – tho) Watts
where cp =specific heat of water = 4200 J/kgK
4. Similarly, for cold water
Heat collected by cold water (out side heat transfer rate)
Qc = mc cp (tco – tci) Watts
5. Logarithmic mean temperature difference (LMTD)
LMTD = ΔTm = (Ti – To) / ln (Ti / To)
Where for parallel flow, for counter flow
Ti = thi - tci Ti = thi - tco
To = tho - tco To = tho - tci
6. Overall heat transfer coefficient, U
a) Inside overall heat transfer coefficient, Ui
Inside diameter of tube = 0.011 m
Inside surface area of the tube, Ai = Π x 0.011 x L
Now Qh = Ui ΔTm Ai
Therefore Ui = Qh / (ΔTm Ai) W/m2 0C
b) Outside overall heat transfer coefficient, Uo
Outside diameter of tube = 0.012 m
Outside surface area of the tube, Ao = Π x 0.012 x L
Similarly Qc = Uo ΔTm Ao
Therefore Uo = Qc / (ΔTm Ao) W/m2 0C
7. Effectiveness of heat exchanger
Є = Rate of heat transfer in heat exchanger / Max. possible heat
transfer rate
)(][)(
min cihip
hohiph
ttmcttcm−
−=ε
Where [mcp]min is smaller of two capacity rates of mh or mc
PRECAUTIONS
1. Never switch on the geyser unless there is water supply through it.
2. If the red indicator on geyser goes off during operation, increase
the water supply, because it indicates that water temperature
exceeds the set limit.
3. Ensure steady water flow rate and temperatures before noting
down the readings, as fluctuating water supply can give erratic
results.
TYPE
OF
FLOW
HEAT TRANSFER
RATE LMTD Heat Transfer
Coefficient
Effective
- ness
Inside
(Watts)
Outside
(Watts) 0C
Ui
W/m2k
UO
W/m2k ε
Parallel
Flow
Counter
Flow
RESULTS
Fig.1 Concentric tube heat exchanger (plain tube type)
1. Tci
2. Tho
3. V4
4. V3
5. Thi
6. V1
7. Tco
8. V2
Parallel flow Cross flow
V1 & V4 open V2 & V3 open
V2 & V3 close V1 & V4 close
HEAT TRANSFER IN DROP AND
FILM WISE CONDENSATION
HEAT TRANSFER IN DROP AND FILM WISE CONDENSATION
AIM
To determine the experimental and theoretical heat transfer coefficient
for drop wise and film wise condensation.
INTRODUCTION
Condensation of vapor is needed in many of the processes, like steam
condensers, refrigeration etc. When vapor comes in contact with surface
having temperature lower than saturation temperature, condensation
occurs. When the condensate formed wets the surface, a film is formed
over surface and the condensation is film wise condensation. When
condensate does not wet the surface, drops are formed over the surface
and condensation is drop wise condensation
APPARATUS
The apparatus consists of two condensers, which are fitted inside a
glass cylinder, which is clamped between two flanges. Steam from
steam generator enters the cylinder through a separator. Water is
circulated through the condensers. One of the condensers is with
natural surface finish to promote film wise condensation and the
other is chrome plated to create drop wise condensation. Water flow
is measured by a Rota meter. A digital temperature indicator
measures various temperatures. Steam pressure is measured by a
pressure gauge. Thus heat transfer coefficients in drop wise and film
wise condensation cab be calculated.
SPECIFICATION
1. Condensers: Made of copper, 19 mm O.D., 150 mm long, one with
natural surface and one with chrome-plated surface.
2. Pressure gauge to measure steam pressure
3. Necessary valves for water and steam control.
EXPERIMENTAL PROCEDURE
• Fill up the water in the steam generator and close the water-filling
valve.
• Start water supply through the condensers.
• Close the steam control valve, switch on the supply and start the
heater.
• After some time, steam will be generated. Close water flow through
one of the condensers.
• Open steam control valve and allow steam to enter the cylinder and
pressure gauge will show some reading.
• Open drain valve and ensure that air in the cylinder is expelled
out.
• Close the drain valve and observe the condensers.
• Depending upon the condenser in operation, drop wise or film wise
condensation will be observed.
• Wait for some time for steady state, and note down all the readings.
• Repeat the procedure for the other condenser.
OBSERVATIONS
Drop wise Condensation Steam Pressure, kg/cm2
Water flow rate, LPH
Steam temperature, T1, 0C
Drop wise condensation surface temperature, T2, 0C
Water inlet temperature , T4, 0C
Water outlet temperature , T5, 0C
Film wise Condensation Steam Pressure, kg/cm2
Water flow rate, LPH
Steam temperature, T1, 0C
Film wise condensation surface temperature, T3, 0C
Water inlet temperature , T4, 0C
Water outlet temperature , T6, 0C
CALCULATIONS
(Film wise condensation)
Water flow mw= LPH = kg/sec
Water inlet temperature T4= oC
Water outlet temperature = oC
(T5 for drop-wise condensation and T6 for film-wise condensation)
Heat transfer rate at the condenser wall,
Q = mw.cP. (T6-T4) Watts
Where cp = Specific heat of water = 4.2 x 103 J / Kg K
Surface area of the condenser, A =πdL m3
Experimental heat transfer coefficient, CmWTTA
Qh o
ws
2/)( −
= (for both film
wise and drop wise condensation)
Where Ts = Temperature of steam (T1)
TW = Condenser wall temperature (T2 or T3)
Theoretically, for film wise condensation 25.032
.).(...
943.0⎥⎥⎦
⎤
⎢⎢⎣
⎡
−=
LTTkgh
hws
fg
μρ
Where
hfg = Latent heat of steam at TS J/kg (take from temperature tables
in steam tables)
ρ = Density of water, Kg / m3
g = Gravitational acceleration, m / sec2
k = Thermal conductivity of water W / mo C
μ = Viscosity of water, N.s/m2
and, L = Length of condenser = 0.15 m
Above values at mean temperature, CTT
T oWsm 2
)( += (from data book)
(For drop wise condensation, determine experimental heat transfer
coefficient only)
In film wise condensation, film of water acts as barrier to heat transfer
where as, in case of drop formation, there is no barrier to heat transfer,
Hence heat transfer coefficient in drop wise condensation is much greater
than film wise condensation, and is preferred for condensation. But
practically, it is difficult to prolong the drop wise condensation and after
a period of condensation the surface becomes wetted by the liquid.
Hence slowly film wise condensation starts.
PRECAUTIONS
1. Operate all the switches and controls gently
2. Never allow steam to enter the cylinder unless the water is flowing
through condenser.
3. Always ensure that the equipment is earthed properly before
switching on the supply.
RESULTS
Film wise condensation
Experimental average heat transfer coefficient =
Theoretical average heat transfer coefficient =
Drop wise condensation
Experimental average heat transfer coefficient =
Fig. 1 Condensation in drop and film forms
1. Steam generator
2. Water level
3. Rota meter
4. Steam pressure
5. Condensers
6. Temperature indicator
7. Selector switch
8. Heater control
9. Main switch
CRITICAL HEAT FLUX APPARATUS
CRITICAL HEAT FLUX APPARATUS
AIM
To determine the experimental and theoretical value of critical heat flux
in pool boiling of water.
INTRODUCTION
When heat is added to a liquid from a submerged solid surface which is
at a temperature higher than the saturation temperature of the liquid, it
is usual for a part of the liquid to change phase. This change of phase is
called boiling. Boiling is of various types, the type depending upon the
temperature difference between the surface and the liquid. The different
types are indicated in figure, in which a typical experimental boiling
curve obtained in a saturated pool of liquid is drawn. The heat flux
supplied to the surface is plotted against (Tw – Ts) the difference between
the temperature of the surface and the saturation temperature of the
liquid. It is seen that the boiling curve can be divided into three regions.
I) Natural convection region
II) Nucleate boiling region and
III) Film boiling region
Heat flux, W/m2
106
107
C
B
A
Stable film
II b
Bubbles rise
Bubbles condense
Nucleate boiling
Unstable film
Excess temperature (Tw - Ts)
Radiation coming in to
play
Film boiling I II III
II aIII a III b
1 10 10 100010
The region of natural convection occurs at low temperature differences (of
the order of 100C or less). Heat transfer from the heated surface to the
liquid in its vicinity causes the liquid to be superheated. This
superheated liquid rises to the free liquid surface by natural convection,
where vapour is produced by evaporation. As the temperature difference
(Tw – Ts) is increased, nucleate boiling starts. In this region, it is observed
that bubbles start to form at certain locations on the heated surface
region (II) consists of two parts. In the first part (II-a) the bubbles formed
are very few in number. They condense in the liquid and do not reach the
free surface. In the second part (II-b) the rate of bubble formation as well
as the number of locations where they are formed increase. Some of the
bubbles now rise all the way to the free surface.
With increasing temperature difference, a stage is finally reached when
the rate of formation of bubbles is so high, that they start to coalesce and
blanket the surface with a vapour film. This is the beginning of region (III)
viz, film boiling. In the first part of this region (III-a) the vapour film is
unstable, so that film boiling may be occurring on a portion of the heated
surface area, while nucleate boiling may be occurring on the remaining
area. In the second part (III-b) a stable film covers the entire surface. At
the end of region (II) the boiling curve reaches a peak (point A). Beyond
this, in region (III-A) in spite of increasing temperature difference, the
heat flow increases with the formation of a vapour film. The heat flux
passes through a minimum (point B) at the end of region (III-a). It starts
to increase again with (Tw – Ts) only when stable film boiling begins and
radiation becomes increasingly important.
It is of interest to note how the temperature of the heating surface
changes as the heat flux is steadily increased from zero. Up to the point
A, natural convection boiling and then nucleate boiling occur and the
temperature of the heating surface is obtained by reading off the value of
(Tw – Ts) from the boiling curve and adding to it the value of Ts. If the heat
flux is increased even a little beyond the value of A, the temperature of
the surface will shoot up to the value corresponding to the point C. It is
obvious from figure that the surface temperature corresponding to point
C is high. For most surfaces it is high enough to cause the material to
melt. Thus in most practical situation, it is undesirable to exceed the
value of heat flux corresponding to point A. This value is therefore of
considerable engineering significance and is called the critical or peak
heat flux. The discussions so far has been concerned with the various
type of boiling which occurring saturated pool boiling. If the liquid is
below the saturation temperature we say that sub-cooled pool boiling is
taking place. Also in many practical situations, e.g. steam generators,
one is interested in boiling in a liquid flowing through tubes. This is
called forced convection boiling may also be saturated or sub cooled and
of the nucleate or film type. Thus in order to completely specify billing
occurring in any process, one must state that (i) whether it is forced
convection boiling or pool boiling, (ii) whether the liquid is saturated or
sub cooled and (iii) whether is in the natural convection nucleate of film
region.
APPARATUS
The apparatus consists of a cylindrical glass housing the test heater and
heater coil for heating of the water. This heater coil is direct connected to
the mains (Heater R1) and the test wire is also connected to mains via.
variac. An ammeter is connected in series while a voltmeter across it to
read the current and voltage respectively. The glass container is kept on
a stand. There is provision of observing the test heater wire with the help
of a lamp light from back and the heater wire can be view a lens.
SPECEIFICATIONS
1) Glass container – Diameter – 250 mm
Height – 100 cm
2) Heater for initial heating, Nichrome Heater (R–1) – 1 kW
3) Test Heater (R-2), Nichrome wire size - Φ mm
(To be calculated according to wire used say 36 SWG to 40 SWG.)
3) Length of test Heater (R-2) = 100 mm
4) Thermometer – 0 to 100o C
EXPERIMENTS This experimental set up is designed to study the pool boiling
phenomenon up to critical heat flux point. The pool boiling over the
heater wire can be visualized in the different regions up to the critical
heat flux point at which the wire melts. The heat flux from the wire is
slowly increased by gradually increasing the applied voltage across the
test wire the change over from natural convection to nucleate boiling can
be seen. The formation of bubbles and their growth in size and number
can be visualized followed by the vigorous bubbles formation and their
immediate carrying over to surface and ending this in the breaking of
wire indicating the occurrence of critical heat flux point.
PROCEDURE 1) Take sufficient amount of distilled water in the container.
2) See that both the heaters are completely submerged.
3) Connect the heater coil R-1 (1KW Nichrome coil) and test heater
wire across the studs and make the necessary electrical
connections.
4) Switch on the heater R-1(Let variac be at O position.)
5) Keep it ON till you get the required bulk temperature of water in
the container say 50O C, 60O C, 70O C temperature.
6) Switch off the heater R-1.
7) Very gradually increase the voltage across test heater by slowly
changing the variac position and stop a while at each position to
observe the boiling phenomenon on wire.
8) Go on increasing the voltage till wire brakes and carefully note the
voltage and current at this point.
PRECAUTIONS 1) Keep the variac to zero voltage position before starting the
experiments.
2) Take sufficient amount of distilled water in the container so that
both the heaters are completely immersed.
3) Connect the test heater wire across the stud.
4) Do not touch the water or terminal points when the main switch
ON.
5) Operate the variac gently in steps and sufficient time in between.
6) After the attainment of critical heat flux decrease slowly the voltage
and bring it to zero position.
OBSERVATIONS 1) Diameter of test heater wire, d = m.
2) Length of the test heater, L = 0.1 m
3) Surface area A = π.d.L m2 =
Note: - The ammeter and voltmeter readings are to be note down when
wire melts.
CALCULATIONS The critical heat flux at various bulk temperatures water can be
calculated by the following procedure.
1. Heat input Q = V. l Watts
2. Critical Heat flux 2texp m/W
AI.V
AQq ==
3. Zubfer has given following equation for calculating peak heat flux in
saturated pool boiling. 4/1
2
(.18.0 ⎥⎦
⎤⎢⎣
⎡ −==
v
vLLVvfgtheor
ghAQq
ρρρσρ
Where Q/A = Heat Flux, W/m2
hfg = Latent Heat of vaporization J/kg ----------- (from steam table)
LVσ = Liquid vapour surface tension N/m ----------- (from Chart)
Lρ = Density of Liquid
Bulk
Temperature of
water OC
Ammeter Reading
(I Amps)
Voltmeter Reading
(V volts)
νρ = Density of vapour = 1/vg kg/m3 (from steam table)
hfg, LVσ , Lρ and νρ are evaluated at the water temperature.
The experimental value of critical heat flux at the sat temperature is
comparable to that obtained by Zuber’s correlation.
RESULT
1. Experimental critical heat flux =
2. Theoretical critical heat flux =
Fig.1 Critical heat flux apparatus
1. Voltmeter
2. Ammeter
3. Heater switch
4. Lamp switch
5. Main switch
6. Heater control
7. Class container
8. Heater fitting
HEAT PIPE DEMONSTRATOR
HEAT PIPE DEMONSTRATOR
AIM
To study the variation of heat sink temperature and longitudinal
temperature distribution for heat pipe, stainless steel and copper pipe
with comparison.
INTRODUCTION
Heat pipe is an interesting device, which is used to transfer heat from
one location to another. It works with the help of evaporation and
condensation of liquid, which is filled inside heat pipe as a working
medium.
Heat pipe basically consist of a stainless steel pipe, sealed at both the
ends. It is evacuated and filled partially with distilled water. Stainless
steel mesh is provided at inside periphery of the pipe. When heat is
applied at the lower end of the heat pipe, water inside it evaporates and
vapor passes to upper end of the pipe. The heat is taken by the medium
surrounding upper portion of heat pipe. The vapor condenses giving its
latent heat of evaporation to the surrounding medium. The condensed
vapor returns to bottom through the mesh packing, thus because of
circulation of vapor, heat pipe operates at near to isothermal operation
and conducts much heat than conventional conductors.
APPARATUS
The apparatus consists of three pipes, viz., a heat pipe, copper pipe and a
stainless steel pipe. All the pipes have same physical dimensions.
Copper and Stainless steel pipes serve the purpose of comparison of heat
pipe performance with copper pipe as good conductor of heat and with
stainless steel pipe as same material of construction. All pipes are
mounted vertically with a band heater at lower end and a water filled
heat sink at upper end. When heaters start heating the pipes, begin to
transfer the heat to heat sinks. Rapid rise of temperature of water in the
heat pipe heat sink demonstrates high [apparent] thermal conductivity of
heat pipe. Nearly isothermal operation of heat pipe is clearly visualized
from longitudinal temperature distribution of pipes.
SPECIFICATIONS
1. Heat pipe- Stainless steel pipe, φ 25 mm OD, 400 mm long at both
ends, evacuated & filled partially with distilled water – one no.
2. Copper and stainless steel pipes of same size as that of heat pipe –
one each
3. Equal capacity heaters at bottom end of each pipe.
4. Water filled heat sinks at upper end of each pipe.
5. Thermometers to note down water temperatures in heat sinks –
3nos.
6. Thermocouples for longitudinal temperatures and three
thermometers to sink temperatures.
EXPERIMENTAL PROCEDURE
Fill up sufficient water in heat sinks. Insure proper earthling to the
unit; put the thermometers in the grommets provided at the top of
heat sinks. Keep dimmer stat zero position and start electric supply
to unit. Slowly increase the dimmer so that power is supplied to
heaters. As same dimmer stat supplies power to all heaters and all
heaters are of same capacity, power input to all the heaters remains
same. This makes the comparison simpler. Go on noting down the
temperatures of water in heat sinks every 5 min (stir the water before
noting down the temperature. After around 30 minutes, note down the
longitudinal temperature of the pipes, from the temperatures
indicator. Replace the water when pipes become cool lower than 45oC
otherwise removing water at high temperature of pipe may burn the
seals at the bottom of heat sinks. If an experiment is conducted for
more time, it is merely to raise the water temperature and ultimately
evaporation of water. Hence it is not recommended to conduct the
experiment for more times than 30 minutes.
OBSERVATIONS
I) Heat sink water temperatures
Time, minutes S.S Pipe
Heat sink
Copper Pipe
Heat sink
Heat Pipe
Heat sink
0 min
5 min
10 min
15 min
20 min
25 min
30 min
II) Longitudinal temperature distribution
S.S. Pipe Copper Pipe
(Switch at right
hand position)
Heat Pipe
T6 T1 T1
T7 T2 T2
T8 T3 T3
T9 T4 T4
T10 T5 T5
PRECAUTIONS
1. Proper earthing is necessary.
2. Stir the water before noting the water temperature in heat sink.
3. Do not remove water from heat sink till the pipes become cool.
4. Operate only one meter selector switch at a time in upward
position. Other two switches must be in downward position.
GRAPHS
1. Plot the graph of heat sink water temperature rise up to 30
minutes
2. Plot longitudinal temperature distribution for pipes.
Fig.1: Thermocouple settings along the pipe length.
Fig.2: Variation of water temperature in three sinks with increase in time.
Fig.3: Variation of surface temperature along the length of the pipe after
30 min.
Long
itudi
nal
Tem
pera
ture
, 0 C
Height of pipe, mm
0 10 120 200 290 440 450
Hea
t sin
k w
ater
te
mpe
ratu
re, 0 C
Time, t, min
0 5 10 15 20 25 30
450
10 150 90 80 110 10
T1 T2 T3 T4 T5
RESULT
Graphs are plotted and the performance of heat pipe is compared with
stainless steel pipe and copper pipe. Heat pipe conducts much heat than
conventional conductors. In longitudinal temperature distribution graph,
the heat pipe curve is almost straight line.
Fig.4 Heat pipe demonstrator
1. Heater 2. Water container 3. Thermometer 4. Heat pipe 5. S.S. pipe 6. Copper pipe 7. Voltmeter 8. Ammeter 9. Temperature indicator 10 selector switch 11. Temperature selector switch 12 Pipe toggle switches (only one switch is to be put in upward direction at time) 13. Main switch 14. Heater control
