Seminar Heat Pipe
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Transcript of Seminar Heat Pipe
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CONTENTS_
Introduction
Basic components of a heat pipe
Container
Working fluid
Construction
Operating principleMechanism
Experimental Procedure
Experiment
Study of various parameters
Limitations
Conclusion
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HEAT PIPES
INTRODUCTIONA heat pipe is a device that efficiently transports thermal energy from
its one point to the other. It utilizes the latent heat of the vaporized
working fluid instead of the sensible heat. As a result, the effective
thermal conductivity may be several orders of magnitudes higher than
that of the good solid conductors. A
heat pipe consists of a sealed container, a wick structure, a small
amount of working fluid that is just sufficient to saturate the wick and
it is in equilibrium with its own vapor. The operating pressure inside
the heat pipe is the vapor pressure of its working fluid. The length of
the heat pipe can be divided into three parts viz. evaporator section,
adiabatic section and condenser section. In a standard heat pipe, the
inside of the container is lined with a wicking material. Space for the
vapor travel is provided inside the container.
Basic components of a heat pipe
The basic components of a heat pipe are
1.The container2.The working fluid
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3.The wick or capillary structure
Container
The function of the container is to isolate the working fluid from the
outside environment. It has to be there for leak proof, maintain the
pressure differential across the walls, and enable transfer of thermal
energy to take place from and into the working fluid.
The prime requirements are:
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1.Compatibility (Both with working fluid and Externalenvironment)
2.Porosity3.Wettability4.Ease of fabrication including welding, machinability and
ductility
5.Thermal conductivity6.Strength to weight ratio
Working fluid
The first consideration in the identification of the working fluid is the
operating vapor temperature range. Within the approximate
temperature band, several possible working fluids may exist and a
variety of characteristics must be examined in order to determine the
most acceptable of these fluids for the application considered.
The prime requirements are:
7.Compatibility with wick and wall materials8.Good thermal stability9.Wettability of wick and wall materials10. High latent heat11. High thermal conductivity12. Low liquid and vapor viscosities
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13. High surface tension
Wick
The wick structure in a heat pipe facilitates liquid return from the
evaporator from the condenser. The main purposes of wick are to
generate the capillary pressure, and to distribute the liquid around the
evaporator section of heat pipe. The commonly used wick structure is
a wrapped screen wick.
ConstructionA typical heat pipe consists of a sealed hollow tube, which is made
from a thermoconductive metal such as copper or aluminium. The
pipe contains a relatively small quantity of "working fluid" (such aswater, ethanol or mercury) with the remainder of the pipe being filled
with vapor phase of the working fluid.
On the internal side of the tube's side-walls a wick structure exerts a
capillary force on the liquid phase of the working fluid. This is
typically a sintered metal powder (sintering is a method for making
objects from powder, by heating the material until its particles adhere
to each other) or a series of grooves etched in the tube's inner surface.
The basic idea of the wick is to soak up the coolant.
Axial Groove Fine Fiber Screen Mesh Sintering
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Heat pipes contain no moving parts and require no maintenance and
are completely noiseless. In theory, it is possible that gasses may
diffuse through the pipe's walls over time, thus reducing this
effeciveness.
The vast majority of heat pipes uses either ammonia or water as
working fluid. Extreme applications may call for different materials,
such as liquid helium (for low temperature applications) or mercury
(for extreme high temperature applications).
The advantage of heat pipes is their great efficiency in transferring
heat. They are actually a better heat conductor than an mass of solid
copper.
Wicking
MaterialConductivity
Overcome
Gravity
Thermal
ResistanceStability
Conductivity
Lost
Axial Groove Good Poor Low Good Average
Screen Mesh Average Average Average Average Low
Fine Fiber Poor Good High Poor Average
Sintering r) Average Excellent High Average High
Operating principle
Figure shows the working principle of a heat pipe. Thermal input at
the evaporator region vaporizes the working fluid and this vapor
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travels to the condenser section through the inner core of heat pipe. At
the condenser region, the vapor of the working fluid condenses and
the latent heat is rejected via condensation. The condensate returns to
the evaporator by means of capillary action in the wick.
As previously mentioned there is liquid vapor equilibrium inside the
heat pipe. When thermal energy is supplied to the evaporator, this
equilibrium breaks down as the working fluid evaporates. The
generated vapor is at a higher pressure than the section through the
vapor space provided. Vapor condenses giving away its latent heat of
vaporization to the heat sink. The capillary pressure created in the
menisci of the wick, pumps the condensed fluid back to the
evaporator section. The cycle repeats and the thermal energy is
continuously transported from the evaporator to condenser in the form
of latent heat of vaporization. When the thermal energy is applied to
the evaporator, the liquid recedes into the pores of the wick and thus
the menisci at the liquid-vapor interface are highly curved. This
phenomenon is shown in figure. At the condenser end, the menisci at
the liquid-vapor interface are nearly flat during the condensation due
to the difference in the curvature of menisci driving force that
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circulates the fluid against the liquid and vapor pressure losses and
body forces such as gravity.
MechanismHeat pipes use evaporation and condensation to move heat quickly
from one place to another. A typical heat pipe is a sealed tube
containing a liquid and a wick. The wick extends from one end of the
tube to the other and is made of a material that attracts the liquid--the
liquid "wets" the wick. The liquid is called the "working fluid" and is
chosen so that it tends to be a liquid the temperature of the colder end
of the pipe and tends to be a gas at the temperature of the hotter end
of the pipe. Air is removed from the pipe so the only gas it contains isthe gaseous form of the working fluid.
The pipe functions by evaporating the liquid working fluid into gas at
its hotter end and allowing that gaseous working fluid to condense
back into a liquid at its colder end. Since it takes thermal energy to
convert a liquid to a gas, heat is absorbed at the hotter end. And
because a gas gives up thermal energy when it converts from a gas to
a liquid, heat is released at the colder end.
After a brief start-up period, the heat pipe functions smoothly as a
rapid conveyor of heat. The working fluid cycles around the pipe,
evaporating from the wick at the hot end of the pipe, traveling as a gas
to the cold end of the pipe, condensing on the wick, and then traveling
as a liquid to the hot end of the pipe.
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The vapor pressure over the hot liquid working fluid at the hot end of
the pipe is higher than the vapour pressure over fluid at the cooler end
of the pipe (where it condenses), and this pressure difference drives a
rapid mass transfer to the condensing end where the excess vapour
releases its latent heat, warming the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the
vapour impede the gas flow, and reduce the effectiveness of the heat
pipe, where vapor pressures are low.
The condensed working fluid then flows back to the hot end of the
pipe. In the case of vertically-oriented heat pipes the fluid may be
moved by the force of gravity. In the case of heat pipes containing
wicks, the fluid is returned by capillary action. Most heatpipes used
in heatsinks today have wicks, and are effective in vertical orhorizontal orientation.
In summary: inside a heat pipe, "hot" vapor flows in one direction,
condenses to the liquid phase, which flows back in the other direction
to evaporate again and close the cycle.
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Experimental Procedure
The heat pipe construction is as follows. A copper tube of suitable
length is cleaned thoroughly with suitable cleaning agents. Screen
mesh acts as a wick is wound around a coil in layers and inserted into
the copper tube intact. It is then closed by end caps at both ends.
Thermocouples are equally spaced at various positions of the heat
pipe. The mica sheet is wound over the evaporator region of the heat
pipe since mica is a good electrical insulator and a thermal conductor.
A heating coil is wound over the mica sheet in a uniformly spaced
manner. The two end of the heating coil are connected to the electric
power input. A few centimeter thick cover of glass wool is provided
over the entire region of the heat pipe over the glass wool covering,
the heat pipe is covered with thick PUF insulation which is normally
provided n automobiles.
The heat pipe is evacuated to a pressure of -1.36atm for about 2hours
using a vacuum pump. The heat pipe is tested for holding the vacuum
for about twelve hours. After vacuum test,R-12 working fluid is filled
in the heat pipe for specified pressure which can be indicated by the
pressure gauge.
The coolant water supply is provided to the heat pipe and can be
controlled by a valve. The thermocouples on the heat pipe are
connected to the temperature scanner. A voltmeter is connected in
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parallel to the dimmerstat. Dimmerstat is supplied with ac current.
The temperature scanner is connected to an electric power inlet
through a voltage stabilizer.
The ambient pressure and ambient temperature are noted. The heat
pipe evaporator region heating coil is connected to the electric power
inlet. Coolant water is supplied to the condenser coolant chamber.
The dimmerstat initially is at no-load condition. The load on the
dimmerstat is varied very slowly till the required power is obtained.
Power can be calculated using the equation P=VI cos, where cos
is the power factor, (0.8 for A.C supply).
Heat pipe test rig
The copper tube heat pipe of 25.4 mm inner diameter and thickness
employs a five layered 100x100 brass screen mesh wick. The length
of the evaporator, adiabatic and condenser sections are 100, 50 and
150 mm respectively. The temperature of the heat pipe are measured
using a copper-constantan T-type thermocouples arranged at ten
positions equally spaced along a line on the periphery of the heat pipe.Additionally, two thermocouples are provided to measure the
temperature of coolant inlet and outlet temperatures. The evaporator
region is heated by an electric heating coil wound over a mica sheet.
The condenser region is cooled using coolant flowing through
condenser coolant chamber. Electric power input is varied by using
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dimmerstat. The thermocouples are connected to the 16-channel
temperature scanner.
Experiment
The experimental heat pipe is initially at room temperature. The
coolant water enters the condenser cooling chamber at this
temperature and coolant is allowed to flow at a particular flow rate.
The initial pressure reading is to be noted from the pressure gauge
connected at the evaporator end. The ambient thermocouple
temperatures are noted using thermocouples. Initially, the dimmerstat
is to be kept at no-load condition. The load on the dimmerstat is
slowly varied till it reaches the required value. The electric power is
supplied to the electric heating coil which is wound over the
evaporator section. The temperature at each position on the heat pipe
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can be measured by using the thermocouples connected at equal
intervals. The initial temperature readings are taken in steps of 2
minutes and in later stages the time interval increases to 5 minutes.
After 30-35 minutes the system will reach the steady state conditions.
Study of various parameters
Effect of power
The first experiment is to find out how the temperature profile varies
with respect to the variation provided to the electric power supply
given the electric heating coil in the evaporator region. The
temperature profile with electric power supply of 25 W is plotted for a
time period of nearly one hour till it reaches steady state condition.
Then the electric power varied to 50 Wand 80 W to plot
corresponding temperature profiles. The variation of temperature
profile is then compared.
Effect of pressure
The pressure inside the heat pipe plays an important role in the
temperature profile plotting. The pressure is varied by controlling the
quantity of the working fluid supplied to heat pipe. The plotting of
temperature profile is done on different pressure values. The
combined effect of pressure inside the heat pipe and the power
supplied to electric coil at the evaporator can also be obtained by
varying both parameters
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Effect of coolant supply
The coolant supplied for circulation over the condenser coolant
chamber is water. The coolant flow rate is the ratio of coolant volume
circulating in unit time. The variation in temperature profile is
analyzed at various coolant flow rates.
Effect of working fluid
The effects of various working fluids like R-12,R-22 and R-134a are
analyzed in the experiment. These are commonly used refrigerants.
Limitations
When heated above a certain temperature, all of the working fluid in the heat
pipe will vaporize and the condensation process will cease to occur; in such
conditions, the heat pipe's thermal conductivity is reduced to the heat
conduction properties of its solid metal casing alone. As most heat pipes are
constructed of copper, an overheated heatpipe will generally continue to
conduct heat at only around 1/80th of their original conductivity.
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Conclusion and scope for future work
Our work involves the study of variation on the temperature profile
due to the varied power supply to the evaporator, variation in the
coolant flow rate, variation in the working fluid, variation in the
average pressure inside the heat pipe, variation due to different wick
structures and finally the variation due to geometrical configuration of
the heat pipe. The results of our proposed work provide an insight into
the effect of parameter variation on the temperature profile of a heat
pipe and experiment results will be compared with analyzed results.
Thus we could suggest better ways to improve the performance of a
heat pipe. Further studies on the velocity profile and pressure profile
of a heat pipe can be made and it would be further used for modeling
a heat pipe using any software package such as CFD.
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REFERENCE
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