Heat Pipes - Energy And Economy热管-能源与经济.doc

Conference on New Energy Technologies for Environmental Development in the Arab World, Cairo, EgyptHeat Pipes:Theory and OperationDr. Eng. Mohammed M. El-KhayatNREA, P.O.Box 4544 Masakin Dobat El-Saff, El-Hay El-Sades,Nasr city, Cairo, Egypt.e-mail : mohamed_AbstractOne of the main objectives of this study is to obtain an understanding of heat pipes and their role in energy transmission, especially in the field of solar energy. Heat pipes are highly efficient heat transfer devices, which use the continuous evaporation/condensation of a suitable working fluid for two-phase heat transport in a closed system. Since the latent heat of vaporization is very large, heat pipes transport heat at small temperature difference, with high rates. Due to a variety of advantage features these devices have found a number of applications both in space and terrestrial technologies. The theory of operation and the characteristics of different types of heat pipes are described, and their performance limitations are discussed and presented in this study. Moreover, A description of using high-temperature non-concentrating collectors is illustrated.Key Words: Heat pipes, Composite wicks, Homogenous wicks, and Evacuated solar collectors.1- Introduction Historically, the first application of gravity heat pipes was in boilers and bakeries, the so-called Perkins tube was widely used in the 19th century. This is a bare, thick-walled carbon steel tube filled with a certain amount of water, about 1/3 of the total tube volume, and hermetically sealed. The lower tube end was heated by flue gases, the upper end extended into the boiler where it was used to generate steam. In 1938 a patent was granted in the USA which describes a tube incorporating capillary grooves to aid liquid distribution and hence vaporization in boilers. The first patent of a heat pipe employing a capillary wick for pumping liquid against gravity was applied by Gaugler in 1944 as a two-phase heat transport device for refrigerators. It was supposed to allow movement of the working fluid without pumps and without natural convection, by utilization of the capillary force generated by a capillary wick, Asselman (1973) and Groll (1992).In 1963 the capillary pumping principle was independently re-invented by Grover and his co-workers at the Los Alamos Scientific Laboratory in their search for a new means of efficient heat transport in space. Grover called the device “Heat Pipe” and characterized it in the following paragraph “With certain limitations on the manner of use, a heat pipe may be regarded as a synergistic engineering structure, which is equivalent to a material having a thermal conductivity greatly exceeding than of any known metal”.The first heat pipe had been built by Grover and his colleagues used water as a working fluid. Shortly after it, a sodium heat pipe was investigated, then lithium and silver as working fluids were followed. In addition, heat pipes may extend for several meters in length and have an equivalent diameter of up to several centimeters. They also come in miniature sizes for special medical applications, e.g. thermal treatment of cancer tumors, and industrial applications, and cooling of micro ships of high performance computer. The cross section and geometry of a heat pipe may take different shapes, i.e. circular, rectangular, . etc, and a single heat pipe may operate with a single or multiple heat sources depending on the application such as evacuated collectors, El-Genk (1995), and Gillet(1995). Nowadays, heat pipes applications span a wide range of temperatures from 270C below zero to 2000C, and for each temperature range there is a number of suitable working fluids. For example, for an application in which the temperature varies from 30C to 100C, water can be used as a working fluid. On the other hand, the wick and heat pipe walls are made of copper or stainless steel. 2-Theory Of Operation Fig. (1): Components of a heat pipe (After Groll and Rosler (1992)Heat pipe is a closed tube or chamber of different shapes whose inner surface is lined with a porous capillary wick as shown in Fig.(1). The wick is saturated with the liquid phase of a working fluid, and the remaining volume of the tube contains the vapor phase. Circulation of working fluid is an important heat pipe factor. Consequently, the maximum possible circulation is required to obtain the maximum heat transport capability of the heat pipe. Heat applied at the evaporator by an external source vaporizes the working fluid in that section. The resulting difference in pressure drives vapor from the evaporator to the condenser where it condenses releasing the latent heat of vaporization, to a heat sink in that section of the pipe. El- Khayat (1995) reported that there are several types of heat pipes, such as, heat pipe with flat cross section, in order to provide a wide area for both evaporator and condenser, and gravity assisted heat pipe as shown in Fig. (2.a) and Fig.(2.b). The main components of the heat pipe will be described in the following paragraphs:- 2.1- Wick Structure El-Khayat (1995), mentioned that the purpose of a wick is to provide:-1- The necessary flow passages for the return of the condensed liquid,2- Surface pores at the liquid-vapor interface for the development of capillary pumping pressure, and3- A heat flow path between the inner wall of the container and the liquid-vapor interface.(a) Flat plate heat pipeOptional block Of wick material Heat input EvaporatorContainerLiquid flowWickVapor flow Heat output Condenser(b) Gravity assisted heat pipeFig.(2): Different types of heat pipesAdiabatic sectionLiquid flowEvaporatorHeat inputHeat outputCondenserVaporflowWickIn addition, mesh screen, fiberglass, sintered porous metal, and narrow grooves cut in the inner surface of the container wall have been used as wick material. Wick structure can be classified into two general classes, namely, homogenous wicks and composite wicks. The homogenous wicks are made of a single material and the composite wick consists of two or more materials. Several examples of homogenous and composite wicks are illustrated in Fig.(3).2.2- Working FluidFor operating a heat pipe it is essential that, its wick structure must remain saturated with the working fluid. Heat pipes have been developed with working fluids ranging from cryogenic liquids to liquid metals. Accordingly, heat pipes can be categorized into cryogenic, moderate-temperature, and liquid-metal types. As a result, heat pipes can be operated in a wide temperature range, from some degrees Kelvin, with liquid helium as a working fluid, up to about 2300K, with liquid silver as a working fluid. The most common working fluid is ammonia. The standard working fluid for space applications, and some refrigerants is water. Meanwhile, the standard working fluid for most terrestrial applications, some organic working fluids; and alkali-metals, especially sodium as the standard high temperature working fluid. (b)Types of composite wicks1- Wrapped screen 2- Sintered metal 3- Crescent 4- Annular(a) Homogenous wick structureFig.(3): Types of homogeneous and composite wicks 1- Slab 2- Composite3- Performance Limits Chi(1976) mentioned that, during the operation of heat pipes, there are some performance limits on these devices. These limits are; the limit on condenser pressure depression, viscous limit, heat transport ability, capillary limit, shocking of vapor flow, sonic limit, disruption of the liquid flow by nucleate boiling in the wick, boiling limit, and tearing of liquid off the liquid-vapor interface by vapor flowing at high velocity, entrainment limit. These limits will be described briefly in the following:- Viscous limit, for very low operating temperature, close to the melting point, and low vapor densities, the vapor flow is governed by viscous forces. The axial heat flow increases with decreasing pressure at the condenser end. If a condenser vapor pressure approaching zero, viscous limit is reached. When the total pressure drop in the heat pipe is equal to the maximum capillary pressure difference, then the maximum axial heat transport capability is reached. Another increase of the heat input will lead to an interruption of the condensate back flow to the evaporator, which will then dry out or burn out. This performance limit is called the capillary pumping limit or wick limit, Shibayama and Morooka (1980). Another condition occurs when the velocity in the adiabatic section becomes sonic. At this point the flow is shocked, and the maximum flow rate is attained. Clearly, the heat transported through the heat pipe can not be increased further beyond this point which is commonly called the sonic limit, El-Naquib(1990). For high radial heat fluxes to the evaporator nucleate boiling can occur. If the vapor bubbles would not expelled from the capillary structure a vapour layer will effectively insulate the heated evaporator which gets over-heated (burned out), this limit is called boiling limit, Chi(1976) and Shibayama and Morooka (1980). In addition, El-Khayat(1995) reported that, for high gas velocities, the interface between the gas and liquid becomes highly agitated and disturbance waves appear. These waves are torn from the surface giving rise to drop entrainment in the gas core. This can interrupt the liquid supply to the evaporator and the heated evaporator can dry out or burn out, this limit is called the inter action limit or entrainment limit. Finally, there are some additional factors that affect heat pipe performance such as the temperature characteristics of the heat pipe, the interface condition between the heat pipe and its external heat source and sink, and the effects of various heat pipe control techniques. 4- Applications of Heat PipesAs mentioned before, heat pipes have a wide range of the industrial applications such as boilers, evaporators, bakeries, and special medical applications. In this section a brief description for using heat pipes in designing high-temperature non-concentrating collectors will be presented.4.1- High-Temperature Non-concentrating Collectors Loss of heat from the absorber plate by conduction and convection can be eliminated by removing nearly all the air in the space between the absorber plate and the glazing. Effective suppression of conduction requires a pressure of about 10-4 mmHg, or a hard vacuum. Unless glass supports are provided in the typical flat-plate collector, a flat-glass surface would collapse with this pressure difference, and the maintenance of a vacuum seal would also be difficult. For these reasons, evacuated collectors have generally involved tubular designs, which have inherently higher strength to withstand external pressure. Some of the designs involve flat absorbers, others employ cylindrical absorbing surfaces, but all are of the non-concentrating type Dickmson and Cheremisinoff (1980). In one U.S. type, sketched in Fig. (4.A), the absorber is a thin, blackened flat metal sheet supported across the diameter of a single evacuated glass tube, with pipe connections for liquid circulation sealed into one end of the tube. Liquid is circulated through the pipe, which is in close thermal contact with the absorber plate. A selective black absorbing surface suppresses thermal radiation, so all forms of heat loss are small at ordinary space-heating temperatures, and operation at temperatures approaching 2000C is possible at satisfactory efficiency. In another type, the conventional vacuum bottle principle is used, there being a double glass wall with an evacuated space. Fluid is circulated through the hollow interior of the inside tube, Fig. (4.B), so the energy absorbed by a black coating deposited on the outer surface of the inner glass tube is transferred to the fluid. The open ends of individual tubes are inserted into insulated manifolds which provide the proper fluid flow pattern through tube multiples. Fig. (4): Types of evacuated tube collectorsA third U.S. type is also a double-walled, internally blackened glass tube with a vacuum between the two glass surfaces, Fig. (4.C). The inner glass tube contains a thin cylindrical copper sleeve to which a copper pipe in the form of a long narrow U is attached. Liquid is circulated through the copper pipe in series flow with adjacent tubes. An experimental evacuated tube collector from Germany comprises a corrugated aluminum heat-transfer surface, with internal liquid passages, tightly fitting a closely packed array of evacuated single glass tubes which are internally coated with a selective black absorber, as shown in Fig. (4.D). A flat glass cover is used primarily for weather protection. A Japanese design involves straight-through flow of liquid in a small copper pipe bonded to a flat metal absorber plate inside a single evacuated glass tube, Fig. (4.E). Expansion differences are handled by use of metal bellows bonded to the ends of the glass tube and to the metal pipe. The principle involved is similar in all the types. A selective surface and high vacuum result in suppression of both radiative and convective losses. With a glass-to- metal permanent seal or an all-glass seal, loss of vacuum is only a rare occurrence. Even if vacuum is lost, in a few of many tubes, performance of the entire collector decreases only slightly. Since the black coating is inside the evacuated space in all designs, it is fully protected from oxidization, moisture, or any other form of attack. Although the glass tubes do not involve costly materials and their designs are simple, manifolding can be expensive. Distribution of liquid flow in series and parallel channels requires extensive plumbing assemblies, with accompanying requirements for insulation and weather protection. Freezing of water inside the tubes is unlikely because of the excellent insulating properties of a vacuum, but the danger of freezing in the manifolds makes the use of nonfreezing fluids advisable in severe climates. Compared with most flat-plate designs, evacuated tubular collectors normally have a smaller fraction of total occupied area actually intercepting solar radiation. Spacing between tubes, areas required for manifolds, and piping access all require space, which limits coverage by solar absorbing surface. The operation of evacuated tubular collectors is accompanied by such a low heat-loss rate that high-temperature (approaching 200C) fluid can be obtained at efficiencies comparable to those realized with non-evacuated types operating at only 100C. They are uniquely suited, therefore, for applications requiring temperatures considerably above those involved in space -heating and water-heating uses. 5- Conclusion Heat pipes are very efficient heat transport elements. They can be described as light weight devices with high thermal con