The heat pipe that rotates about its longitudinal or radial axes is called a rotating heat pipe. The rotating heat pipe is a very effective device for transmitting the heat at high rates within small temperature difference with a simple construction and an easy control. It has been employed in the cooling systems of the thermal turbine components, automobile brakes, the rotating metal cutting tools, and the electric machines. Therefore, alternative operating conditions for the rotating heat pipe, such as varying the rotational speed, varying the taper angle, and varying the input heat load, are presented and well investigated.
The main objective of this study is to compare the performance between two different taper angle heat pipes of 0° and 4°, by varying the remaining parameters on them both. The rotational speeds experimented were within the range from 300 to 700 [rev/min]. Different theoretical and experimental models of the rotating heat pipes are presented and comparisons between them are discussed. And finally this study gives a semi-comprehensive picture for the rotating heat pipes performance.
One limitation of the basic thermosyphon is that in order for the condensate to be returned to the evaporator region by gravitational force, the latter must be situated at the lowest point.
The basic heat pipe differs from the thermosyphon in that a wick, constructed for example from a few layers of fine gauze, is fixed to the inside surface and capillary forces return the condensate to the evaporator. (See Fig. 1.1b.) In the heat pipe the evaporator position is not restricted and it may be used in any orientation. If, of course, the heat pipe evaporator happens to be in the lowest position, gravitational forces will assist the capillary forces. The term ‘heat pipe’ is also used to describe high thermal conductance devices in which the condensate return is achieved by other means, for example centripetal force, osmosis or electrohydrodynamics.
Heat pipes and Capillary-driven two-phase systems offer significant advantages over traditional single-phase systems. With the typically increased thermal capacity associated with the phase change of a working fluid, considerably smaller mass flow rates are required to transport equivalent amounts than in single-phase liquid or gas systems for a given temperature range. Moreover, heat transfer coefficients of two phase systems are much greater than in single-phase flows and result in enhanced heat transfer. Lower mass flow rates and enhanced thermal characteristics provide the benefits of smaller system size (and weight) while providing increased performance.
The thermal capacity of a single-phase system depends on the temperature change of the working fluid; thus, a large temperature gradient or a high mass flow rate is required to transfer a large amount of heat. However, a two-phase system can provide essentially isothermal operation regardless of variations in the heat load. Additionally, single phase systems require the use of mechanical pumps and fans to circulate the working fluid, while capillary-driven two-phase systems have no external power requirements, which make such systems more reliable and free of vibration.
Theoretically, heat pipe operation is possible at any temperature between the triple state and the critical point of the working fluid utilized, albeit at significantly reduced transport capabilities near the two extremes due to the fluid property characteristics of surface tension and viscosity. Several typical heat pipe working fluids are given in Table 1.2, along with the corresponding triple point, critical point, and most widely utilized temperature range for each individual fluid. Classification of heat pipes may
be in terms of geometry , intended applications, or the type of working fluid utilized.
Each heat pipe application has a temperature range in which the heat pipe is intended to operate. Therefore, the working fluid must be chosen to take into account this operating temperature (along with the pressure condition), but also its chemical compatibility with the container and wick materials (see Table 1.3)
Depending on operating temperature, four different types of heat pipes are usually described with regard to commonly used working fluids:
1. Cryogenic heat pipes designed to operate from 1 to 200 K, with working fluids such as helium, argon, neon, nitrogen, and oxygen. These typically have relatively low heat transfer capabilities, due to very low values of the latent heat of vaporization, hfg, and low surface tensions of the working fluids. In addition, startup of the heat pipe involves transitioning from a supercritical state to an operating liquid–vapor condition.
2. Room (low)-temperature heat pipes with operating temperatures ranging between 200 and 550 K. Working fluids typically used in this range include methanol, ethanol, ammonia, acetone, and water.
3. Medium-temperature heat pipes with operating temperatures ranging from 550 to 700 K. Mercury and sulfur are typical fluids in this range, along with some organic fluids (e.g., naphthalene and biphenyl).
4. High (liquid-metal)-temperature heat pipes operating above 700 K. Very high heat fluxes can be obtained using liquid metals due to the characteristics of the fluid: namely, very large surface tensions and high latent heats of vaporization. Examples of liquid metals commonly used include potassium, sodium, and silver. In the case of liquid metal heat pipes, startup typically involves starting from an initially frozen working fluid.
1.2 Heat pipe thermodynamics:
Zuo and Faghri in 1998 presented a network thermodynamics analysis of the heat pipe that provided a unique into the heat pipe operation physics behind. This physics behind was considered a thermal network of various components. In addition, they described the transient heat pipe behavior with a first order linear ordinary differential equation and they described the working fluid undergoing a thermodynamic cycle.
One of the strongest opposing arguments was that the working fluid inside the heat pipe is seldom in an equilibrium state and consequently the working fluid cannot be specified on a T-s diagram. But Zuo and Faghri used the internal combustion engine (ICE) as an example to clarify their thermodynamics cycle approach. As background knowledge in the typical internal combustion cylinder, the fuel is mixed with air to undergo a four–process cycle of intake, compression, power and exhaust. Inside the cylinder is a mixture of air, water vapor hydrocarbons and other substances resulting from the combustion. Each cycle lasts approx. 0.01 sec for 5000 rev/min in two stroke engine. complete equilibrium is an impossible thing to reach in such a rapid cycle containing so many chemical components. However, the internal combustion cycle has been successfully analyzed on a T-s diagram in numerous textbooks.
Depending upon this example Zuo and Faghri provided a simplified engineering model for the heat pipe by representing a heat pipe working fluid circulation on T-s diagram as shown in Figure 1.3. Furthermore this model is able to use the thermodynamic cycle approach to analyze the heat pipe characteristics such as the condensation and evaporation process as well as the internal flow analysis. As the thermodynamic cycle is the heat pipe occurs at a much slower pace than in the internal combustion cycle, so the thermodynamic analysis in the heat pipe is more accurate than it is in the case of internal combustion engine ( ICE ).
1.3 Modes of heat transfer:
The thermal energy is transported through the heat pipe in two forms, transferring form and carrying form:
- The transferring form is the conventional heat transfer mechanisms through both the evaporator and the condenser sections that mainly depend upon the heat pipe geometric dimensions, the thermal properties of the heat pipe materials and the heat transfer coefficient of the heat source and the heat sink.
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