Capillary Wick Designs and Structures in Heat Pipes
From Thermal-FluidsPedia
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The wick structure within the heat pipe is present to return condensate to the evaporator section. While small pores are needed at the liquid-vapor interface to develop high capillary pressures, large pores are preferred within the wick so that the movement of the liquid is not restricted too greatly. For this reason, many different types of wick structures have been developed in order to optimize the performance of the capillary heat pipe. The types of wick structures can be divided into two categories: homogeneous and composite wicks. Homogeneous wicks (Table 1) [1][2] have the benefit of being relatively simple to design, manufacture and install. Composite wicks (Table 2) [2], however, can significantly increase the capillary limit of the heat pipe, but have the drawback of high manufacturing costs. When selecting a wick structure for a particular application, one must keep in mind the benefits and drawbacks of each type of wick.
There are three properties of wicks that are important in heat pipe design:
Minimum capillary radius: This parameter should be small if a large capillary pressure difference is required, such as in terrestrial operation for a long heat pipe with the evaporator above the condenser, or in cases where a high heat transport capability is needed.
Permeability: Permeability is a measure of the wick resistance to axial liquid flow. This parameter should be large in order to have a small liquid pressure drop, and therefore, higher heat transport capability.
Effective thermal conductivity: A large value for this parameter gives a small temperature drop across the wick, which is a favorable condition in heat pipe design.
A high thermal conductivity and permeability, and a low minimum capillary radius are somewhat contradictory properties in most wick designs. For example, a homogeneous wick may have a small minimum capillary radius and a large effective thermal conductivity, but have a small permeability. Therefore, the designer must always make trade-offs between these competing factors to obtain an optimal wick design.
Homogeneous Wicks
Homogeneous wicks (Table 1) [2] are constructed with one type of material or machining technique. The screen wick is seemingly the simplest and most common type of wick structure. It consists of a metal or cloth fabric which is wrapped around a mandrel and inserted into the heat pipe. After placement, the mandrel is removed, leaving the wick held by the tension of the wrapped screen, in the case of a metal fabric. For a cloth fabric, a spring may be inserted into the heat pipe to hold the wick against the inside of the pipe wall. The capillary pressure generated by a screen wick is determined by the size of the rectangular pores between the individual threads. The permeability is determined by the number of wraps and the looseness of the wraps, which create annular gaps through which the condensate can flow.
Table 1. Typical homogeneous wick designs.
|
Wick type |
Capillary Pumping |
Thermal Conductivity |
Permeability |
Comments |
|
|
High |
High |
Low-average |
Single or multiple wraps of wire screen mesh. |
|
|
High |
Average |
Low-average |
Packed spherical particles, felt metal fibers or powder. |
|
|
Low |
High |
Average-High |
Rectangular, circular, triangular, or trapezoidal grooves. |
|
|
Low |
Low |
High |
Wire screen mesh spaced from wall. |
|
|
Low |
High |
High |
Wire screen mesh formed into artery and wall lining |
|
|
High |
High |
Average-high |
Homogeneous material with built-in arteries |
Sintered metal wicks are manufactured by packing tiny metal particles between the inner heat pipe wall and a mandrel in powder form. This assembly is then heated until the metal spheres are sintered to each other and to the inner wall of the heat pipe. Special materials are used for the mandrel so that it can be removed, leaving an open vapor space. This type of wick is obviously more difficult to manufacture compared to the simple screen wick. However, the capillary pressure developed by the sintered metal wick (and therefore capillary limit) is more easily predicted, as the annular gaps present in a screen wick present uncertainties in the permeability. Also, since a metal powder is sintered, the effective thermal conductivity is much higher than a comparable screen wick due to the poor thermal contact between the screen wraps.
Axial groove wicks are formed by the extrusion or broaching of grooves into the inner radius of the pipe. Several different types of grooves have been used and proposed, which have rectangular, triangular, trapezoidal, or nearly circular cross-sections. Trapezoidal grooves are currently the most common type. The performance of axial groove wicks is excellent, provided that the application does not call for a significant adverse elevation against gravity. Since the size of the grooves are large compared to those of a screen or sintered metal wick, the capillary pumping pressure is quite small. However, the permeability and the effective thermal conductivity are very high. Drawbacks to this type of wick are the difficulty in machining the grooves for long heat pipes, which might prove to be excessively expensive, and the liquid-vapor interaction at the opening of the groove at high vapor velocities. Other attractive features of axial groove wicks include the field-tested performance, reliability, and simplicity of design.
The open annulus wick is simply a single-wrap screen wick, held away from the inner pipe wall by stand-offs. This provides an unimpeded return flow path between the screen and the pipe, which greatly increases the permeability over the simple screen wick, while maintaining the high capillary pressure. However, the effective thermal conductivity of this wick is very low for most working fluids due to the low liquid thermal conductivity. Difficulties in priming during startup, and depriming during near-dryout operation are common in this type of wick. The idea of an open condensate return path is also present in the artery wick, but the problem of a low effective thermal conductivity, as is present in the open annulus wick, is significantly reduced.
The artery wick combines the two necessities of having a small pore radius for capillary pressure generation and having a large pore radius for high permeability. In the artery wick structure, the interior of the heat pipe is either covered by a screen wick or a sintered metal powder in the usual manner, but a hollow passage(s) running the length of the pipe is fashioned and is in communication with the rest of the wick structure. Condensate is collected within this passage(s) or artery and is pumped back to the evaporator section by the capillary forces generated at the liquid-vapor interface. Since the inner diameter of the artery is much larger than the effective pore radius of the wick, the liquid is able to easily traverse the length of the heat pipe with a minimum pressure drop. As with any design, there are difficulties associated with the artery wick. First, problems associated with the manufacturing of arteries must be addressed. The startup of artery wick heat pipes is also critical due to the inevitable presence of vapor bubbles within the artery, which effectively block liquid return. Methods of collapsing these bubbles are complicated and not always effective.
Composite Wicks
Composite wick structures (Table 2) [2] employ the benefits of having small pores for generating high capillary pumping pressures and having large pores for increasing the permeability of the liquid return path. Again, the simplest type of composite wick is the screen wick, except that two screens with different pore sizes are used. Several wraps of a screen with a large pore size is used against the inner pipe wall for the liquid return path, and a single wrap of a screen with a much smaller pore size is placed adjacent to the vapor space to develop high capillary pressures. Similarly, axial grooves covered by a single wrap of a small-pore screen can solve many of the problems associated with the homogeneous axial groove wick. Since the screen effectively separates the liquid and vapor flows, the entrainment of the liquid into the vapor flow by the interfacial shear is nearly eliminated. Also, this composite wick can be used in adverse gravity fields because the screen generates the needed capillary pressures.
Table 2: Typical composite wick designs.
|
Wick type |
Capillary Pumping |
Thermal Conductivity |
Permeability |
Comments |
|
|
High |
Low-average |
Average |
Two or more layers of homogeneous material. The material next to wall has largest pore size. |
|
|
High |
High |
Average-high |
Axial grooves covered with one screen. |
|
|
High |
High |
High |
Slab of homogeneous or non-homogeneous material with circumferential grooves. |
|
|
High |
High |
Average-high |
Spiral wire screen mesh and circumferential grooves. |
|
|
Average |
High |
High |
Slab of non-homogeneous material with circumferential grooves. |
|
|
High |
High |
Average-high |
Concentric perforated externally grooved inner tube wall. Inner-annuli screen mesh wick. |
A slab wick in conjunction with circumferential grooves is often used for applications where vapor velocities are generally low. The slab wick, which is usually a felt or several layers of screen, is made in the shape of a bar with a rectangular cross section. The longest side of the cross section is the same as the inner diameter of the heat pipe. Circumferential grooves distribute the condensate around the entire circumference. Obviously, this type of wick is not appropriate for very long heat pipes due to the difficulty in machining circumferential grooves on the inside of the pipe. The tunnel or spiral artery also uses a felt or several layers of screen, but is fashioned in the shape of a tube, which is smaller than the inside diameter of the pipe. The artery is then located on the pipe axis by slab wick stand-offs, which provide condensate communication between the hollow artery and the circumferential grooves on the inside diameter of the pipe. In the monogroove heat pipe design, the vapor and liquid condensate flow in different channels separated by a narrow groove. As with other arterial type wick designs, circumferential grooves in the vapor channel help distribute the condensate. The double-walled artery consists of an inner tube with external grooves, which is perforated at the evaporator and condenser sections to allow vapor to escape into the interior. This inner tube is concentrically placed in an outer tube with a screen mesh wick at the inner wall. The double-walled artery wick design produces high capillary pressures, and since condensate returns to the evaporator by both a screen wick and arteries on the exterior of the inner pipe, total blockages of the liquid return are unlikely.