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INTRODUCTION
A
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
container
2. The
working fluid
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:
1. Compatibility
(Both with working fluid and External environment)
2. Porosity
3. Wettability
4. Ease
of fabrication including welding, machinability and ductility
5. Thermal
conductivity
6. 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 materials
8. Good
thermal stability
9. Wettability
of wick and wall materials
10. High
latent heat
11. High
thermal conductivity
12. Low
liquid and vapor viscosities
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.
Operating principle
Figure
shows the working principle of a heat pipe. Thermal input at the evaporator
region vaporizes the working fluid and this vapor 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 circulates the fluid
against the liquid and vapor pressure losses and body forces such as
gravity.
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 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
cosФ, where cosФ 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 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 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
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.
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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