John H. Lienhard IV, John H. Lienhard V. A Heat Transfer Textbook (776116), страница 73
Текст из файла (страница 73)
They are hard to see through, and one must keep the windshield wiper moving constantly to achieve any kind of visibility. A glasswindshield is normally quite clean and is free of any natural oxides, sothe water forms a contact angle on it and any film will be unstable. Thewater tends to pull into droplets, which intersect the surface at the contact angle. Visibility can be improved by mixing a surfactant chemicalinto the window-washing water to reduce surface tension. It can also be§9.9Dropwise condensationimproved by preparing the surface with a “wetting agent” to reduce thecontact angle.5Such behavior can also occur on a metallic condensing surface, butthere is an important difference: Such surfaces are generally wetting.Wetting can be temporarily suppressed, and dropwise condensation canbe encouraged, by treating an otherwise clean surface (or the vapor) withoil, kerosene, or a fatty acid.
But these contaminants wash away fairlyquickly. More permanent solutions have proven very elusive, with theresult the liquid condensed in heat exchangers almost always forms afilm.It is regrettable that this is the case, because what is called dropwise condensation is an extremely effective heat removal mechanism.Figure 9.21 shows how it works. Droplets grow from active nucleationsites on the surface, and in this sense there is a great similarity betweennucleate boiling and dropwise condensation.
The similarity persists asthe droplets grow, touch, and merge with one another until one is largeenough to be pulled away from its position by gravity. It then slides off,wiping away the smaller droplets in its path and leaving a dry swathe inits wake. New droplets immediately begin to grow at the nucleation sitesin the path.The repeated re-creation of the early droplet growth cycle creates avery efficient heat removal mechanism. It is typically ten times moreeffective than film condensation under the same temperature difference.Indeed, condensing heat transfer coefficients as high as 200,000 W/m2 Kcan be obtained with water at 1 atm. Were it possible to sustain dropwisecondensation, we would certainly design equipment in such a way as tomake use of it.Unfortunately, laboratory experiments on dropwise condensation arealmost always done on surfaces that have been prepared with oleic, stearic,or other fatty acids, or, more recently, with dioctadecyl disulphide.
Thesenonwetting agents, or promoters as they are called, are discussed in[9.60, 9.61]. While promoters are normally impractical for industrial use,since they either wash away or oxidize, experienced plant engineers havesometimes added rancid butter through the cup valves of commercialcondensers to get at least temporary dropwise condensation.Finally, we note that the obvious tactic of coating the surface with a5A way in which one can accomplish these ends is by wiping the wet window witha cigarette.
It is hard to tell which of the two effects the many nasty chemicals in thecigarette achieve.507a. The process of liquid removal during dropwise condensation.b. Typical photograph of dropwise condensation provided by Professor Borivoje B. Mikić. Notice the dry pathson the left and in the wake of the middle droplet.Figure 9.21 Dropwise condensation.508The heat pipe§9.10thin, nonwetting, polymer film (such as PTFE, or Teflon) adds just enoughconduction resistance to reduce the overall heat transfer coefficient to avalue similar to film condensation, fully defeating its purpose! (Sufficiently thin polymer layers have not been found to be durable.) Noblemetals, such as gold, platinum, and palladium, can also be used as nonwetting coating, and they have sufficiently high thermal conductivity toavoid the problem encountered with polymeric coatings. For gold, however, the minimum effective coating thickness is about 0.2 µm, or about1/8 Troy ounce per square meter [9.62].
Such coatings are far too expensive for the vast majority of technical applications.9.10The heat pipeA heat pipe is a device that combines the high efficiencies of boiling andcondensation. It is aptly named because it literally pipes heat from a hotregion to a cold one.The operation of a heat pipe is shown in Fig. 9.22. The pipe is a tubethat can be bent or turned in any way that is convenient. The inside ofthe tube is lined with a layer of wicking material. The wick is wetted withan appropriate liquid. One end of the tube is exposed to a heat sourcethat evaporates the liquid from the wick.
The vapor then flows from thehot end of the tube to the cold end, where it is condensed. Capillaryaction moves the condensed liquid axially along the wick, back to theevaporator where it is again vaporized.Placing a heat pipe between a hot region and a cold one is thus similar to connecting the regions with a material of extremely high thermalconductivity—potentially orders of magnitude higher than any solid material. Such devices are used not only for achieving high heat transferrates between a source and a sink but for a variety of less obvious purposes. They are used, for example, to level out temperatures in systems,since they function almost isothermally and offer very little thermal resistance.Design considerations in matching a heat pipe to a given applicationcenter on the following issues.• Selection of the right liquid.
The intended operating temperature ofthe heat pipe can be met only with a fluid whose saturation temperatures cover the design temperature range. Depending on thetemperature range needed, the liquid can be a cryogen, an organic509510Heat transfer in boiling and other phase-change configurations§9.10Figure 9.22 A typical heat pipe configuration.liquid, water, a liquid metal, or, in principle, almost any fluid. However, the following characteristics will serve to limit the vapor massflow per watt, provide good capillary action in the wick, and controlthe temperature rise between the wall and the wick:i) High latent heatii) High surface tensioniii) Low liquid viscositiesiv) High thermal conductivityTwo liquids that meet these four criteria admirably are water andmercury, although toxicity and wetting problems discourage theuse of the latter. Ammonia is useful at temperatures that are abit too low for water.
At high temperatures, sodium and lithiumhave good characteristics, while nitrogen is good for cryogenic temperatures. Fluids can be compared using the merit number, M =hfg σ /νf (see Problem 9.36).• Selection of the tube material. The tube material must be compatiblewith the working fluid. Gas generation and corrosion are particularconsiderations.
Copper tubes are widely used with water, methanol,and acetone, but they cannot be used with ammonia. Stainless steel§9.10The heat pipetubes can be used with ammonia and many liquid metals, but arenot suitable for long term service with water. In some aerospaceapplications, aluminum is used for its low weight; however, it iscompatible with working fluids other than ammonia.• Selection and installation of the wick.
Like the tube material, thewick material must be compatible with the working fluid. In addition, the working fluid must be able to wet the wick. Wicks canbe fabricated from a metallic mesh, from a layer of sintered beads,or simply by scoring grooves along the inside surface of the tube.Many ingenious schemes have been created for bonding the wick tothe inside of the pipe and keeping it at optimum porosity.• Operating limits of the heat pipe. The heat transfer through a heatpipe is restricted byi) Viscous drag in the wick at low temperatureii) The sonic, or choking, speed of the vaporiii) Drag of the vapor on the counterflowing liquid in the wickiv) Ability of capillary forces in the wick to pump the liquid throughthe pressure rise between evaporator and condenserv) The boiling burnout heat flux in the evaporator section.These items much each be dealt with in detail during the design ofa new heat pipe [9.63].• Control of the pipe performance.
Often a given heat pipe will becalled upon to function over a range of conditions—under varyingevaporator heat loads, for example. One way to vary its performance is through the introduction of a non-condensible gas in thepipe. This gas will collect at the condenser, limiting the area ofthe condenser that vapor can reach. By varying the amount of gas,the thermal resistance of the heat pipe can be controlled.
In theabsence of active control of the gas, an increase in the heat loadat the evaporator will raise the pressure in the pipe, compressingthe noncondensible gas and lowering the thermal resistance of thepipe. The result is that the temperature at the evaporator remainsessentially constant even as the heat load rises as falls.Heat pipes have proven useful in cooling high power-density electronic devices. The evaporator is located on a small electronic component511512Chapter 9: Heat transfer in boiling and other phase-change configurationsFigure 9.23 A heat sink for cooling a microprocessor. Courtesy of Dr. A. B.
Patel, Aavid Thermalloy LLC.to be cooled, perhaps a microprocessor, and the condenser is finned andcooled by a forced air flow (in a desktop or mainframe computer) or isunfinned and cooled by conduction into the exterior casing or structuralframe (in a laptop computer). These applications rely on having a heatpipe with much larger condenser area than evaporator area. Thus, theheat fluxes on the condenser are kept relatively low. This facilitates suchuncomplicated means for the ultimate heat disposal as using a small fanto blow air over the condenser.One heat-pipe-based electronics heat sink is shown in Fig. 9.23.
Thecopper block at center is attached to a microprocessor, and the evaporator sections of two heat pipes are embedded in the block. The condensersections of the pipes have copper fins pressed along their length. A pairof spring clips holds the unit in place. These particular heat pipes havecopper tubes with water as the working fluid.The reader interested in designing or selecting a heat pipe will find abroad discussion of such devices in the book by Dunn and Reay [9.63].Problems513Problems9.1A large square tank with insulated sides has a copper base1.27 cm thick.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
