29-03-2011, 09:29 AM
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HEAT PIPES
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.
HEAT PIPES
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.
