16-02-2013, 12:42 PM
HEAT PIPES
[attachment=51178]
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.
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.