8积分235热交换器设计(论文DWG图纸外文翻译文献综述开题报告)（说明书+图纸+三维）全套
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8积分235热交换器设计(论文DWG图纸外文翻译文献综述开题报告)（说明书+图纸+三维）全套,积分,235,热交换器,设计,论文,DWG,图纸,外文,翻译,文献,综述,开题,报告,说明书,三维,全套
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Heat In this chapter we examine the nature of heat, particularly the ways that it may be transferred. We also look at the 2nd law of thermodynamics, which is a fundamental law of nature with far-reaching implications. The nature of heat As mentioned in a previous chapter, heat can be thought of as the internal kinetic energy of the atoms and molecules that make up a substance. Being a form of energy, it is measured in the standard unit of Joule, but it is also commonly measured in the following units: calorie: this is the heat energy needed to raise 1 gm of water 1 oC. 1 calorie is equal to 4.186 Joules. Calorie: this is a common unit to measure the energy content of food, with 1 Calorie = 1000 calories. BTU: this is a British Thermal Unit, still used as a rating on some furnaces, and is the heat energy needed to raise 1 pound of water 1 oF. 1 BTU = 252 calories = 1,054 Joules. Temperature Heat refers to the total amount of heat energy in a substance - two liters of boiling water have more heat energy than one liter. Temperature, on the other hand, is a relative term, and refers to the average kinetic energy of the atoms and molecules in a substance. One fundamental property of temperature is that There are three main temperature scales used in the world - Celsius, Fahrenheit, and Kelvin. These are compared in the following table. Table 8.1: Temperature scale comparisonsoCKoFWater boils100373212Water freezes027332Absolute zero-2730-459One can see from this that the size of 1 oC and 1 K is the same, while both differ from the size of 1 oF; in fact, the difference between the Celsius scale and the Kelvin scale is simply a shift by 273 o. The Kelvin scale is more convenient to measure substances with very low temperatures. Moreover 0 Kelvin has a very deep physical significance: absolute zero, as the name suggests is the lowest temperature that can, even in principle, be achieved in Nature. It is the temperature associated with empty space that is completely devoid of all all motion and/or energy. In practice it is impossible to obtain, although one can get arbitrarily close. As we will see in the chapter on cosmology, not even the empty space between distance galaxies is at absolute zero: it contains energy and an associated temperature of about 2.7 Kelvin. Heat Capacity The heat capacity of a substance is a measure of how well the substance stores heat. Whenever we supply heat to a material, it will necessarily cause an increase in the materials temperature. The heat capacity is defined as the amount of heat required per unit increase in temperature, so that Thus, materials with large heat capacities, like water, hold heat well - their temperature wont rise much for a given amount of heat - whereas materials with small heat capacities, like copper, dont hold heat well - their temperature will rise significantly when heat is added. Heat Transfer There are three ways that heat may be transferred between substances at different temperatures - conduction, convection, and radiation. We consider each of these in turn. Conduction Convection Radiation Conduction The flow of heat by conduction occurs via collisions between atoms and molecules in the substance and the subsequent transfer of kinetic energy. Let us consider two substances at different temperatures separated by a barrier which is subsequently removed, as in the following figure. Figure 8.1: Heat transfer by conductionWhen the barrier is removed, the fast (hot) atoms collide with the slow (cold) ones. In such collisions the faster atoms lose some speed and the slower ones gain speed; thus, the fast ones transfer some of their kinetic energy to the slow ones. This transfer of kinetic energy from the hot to the cold side is called a flow of heat through conduction. Different materials transfer heat by conduction at different rates - this is measured by the materials thermal conductivity. Suppose we place a material in between two reservoirs at different temperatures, as in the following figure. Figure 8.2: Measurement of thermal conductivityLet us now measure the flow of heat through the material over time. Knowing the materials cross-sectional area and length, the thermal conductivity of the material is then defined as Thus, for a given temperature difference between the reservoirs, materials with a large thermal conductivity will transfer large amounts of heat over time - such materials, like copper, are good thermal conductors. Conversely, materials with low thermal conductivities will transfer small amounts of heat over time - these materials, like concrete, are poor thermal conductors. This is why if you throw a piece of copper and a piece of concrete into a campfire, the copper will heat up much more quickly than the concrete.Instead of being rated in terms of thermal conductivity, insulation is therefore usually rated in terms of its thermal resistance, which is defined as Materials which have a high thermal conductivity have, by definition, a low thermal resistance - they are poor heat insulators. On the other hand, materials with a low thermal conductivity have a high thermal resistance - they are good heat insulators. Good insulating materials therefore should have a high thermal resistance. In fact, the R value quoted for insulation is the thermal resistance (in British units). Convection Convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool region, as opposed to the microscopic transfer of heat between atoms involved with conduction. Suppose we consider heating up a local region of air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. For reasons discussed in the previous section, being less dense than the surrounding cooler air, the hot air will subsequently rise due to buoyant forces - this movement of hot air into a cooler region is then said to transfer heat by convection. Heating a pot of water on a stove is a good example of the transfer of heat by convection. When the stove is first turned on heat is transferred first by conduction between the element through the bottom of the pot to the water. However, eventually the water starts bubbling - these bubbles are actually local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated. These convection currents are illustrated in the following figure. Figure 8.3: Convection currents in boiling waterConsider now two regions separated by a barrier, one at a higher pressure relative to the other, and subsequently remove the barrier, as in the following figure. These convection currents are illustrated in the following figure. Figure 8.4: Flow of material through a pressure differenceWhen the barrier is removed, material in the high pressure (high density) area will flow to the low pressure (low density) area. If the low pressure region was originally created by heating of the material, one sees that movement of material in this way is an example of heat flow by convection. Radiation The third and last form of heat transfer we shall consider is that of radiation, which in this context means light (visible or not). This is the means by which heat is transferred, for example, from the sun to the earth through mostly empty space - such a transfer cannot occur via convection nor conduction, which require the movement of material from one place to another or the collisions of molecules within the material. Often the energy of heat can go into making light, such as that coming from a hot campfire. This light, being a wave, carries energy, as we saw in the last chapter, and so can move from one place to another without requiring an intervening medium. When this light reaches you, part of the energy of the wave gets converted back into heat, which is why you feel warm sitting beside a campfire. Some of the light can be in the form of visible light that we can see, but a great deal of the light emitted is infrared light, whose longer wavelength is detectable only with special infrared detectors. The hotter the object is, the less infrared light is emitted, and the more visible light. For example, human beings, at a temperature of about 37 o Celsius, emit almost exclusively infrared light, which is why we dont see each other glowing in the dark. On other hand, the hot filament of a light bulb emits considerably more visible light. 2nd Law of Thermodynamics Heat, being a form of energy, is subject to the principle of energy conservation discussed in the last chapter. In the context of heat energy, this principle is called the 1st law of thermodynamics: By closed, we mean a system that is completely cut-off, or insulated from its surroundings, so that no material or energy enters or leaves. Also as discussed in the last chapter, heat, being a form of energy, can be transformed into work and other forms of energy, and vice versa. However, this transformation of heat energy is subject to a very important restriction, called the 2nd law of thermodynamics. This law is in fact necessary to resolve the following apparent paradox: The answer lies in the statement of the 2nd law of thermodynamics, which can be given in three equivalent forms: When phrased in a precise mathematical language, one can show that any one of the forms of the 2nd law imply the other two. We shall discuss each of these three forms in turn. Heat flows from hot to cold The first statement of the 2nd law of thermodynamics - heat flows spontaneously from a hot to a cold body - tells us that an ice cube must melt on a hot day, rather than becoming colder. An explanation for this form of the 2nd law can be obtained from Newtons laws and our microscopic description of the nature of temperature. We have already seen that the flow of heat through conduction occurs when fast (hot) atoms collide with slow (cool) atoms, transferring some of their kinetic energy in the process. One might wonder why the fast atoms dont collide with the cool ones and subsequently speed up, thereby gaining kinetic energy as the cool ones lose kinetic energy - this would involve the spontaneous transfer of heat from a cool object to a hot one, in violation of the 2nd law. The answer lies in energy and momentum conservation in a collision - one can show, using these two principles, that in a collision between two objects which conserves energy (called an elastic collision the faster object slows down and the slower object speeds up. It is important to emphasize that this statement of the 2nd law applies to the spontaneous flow of heat from hot to cold. It is possible, of course, to make a cool object in a warm place cooler - this is what a refrigerator does - but this involves the input of some external energy. As such, the flow of heat is not spontaneous in this case. The generic way that this works is pictured below. Figure 8.6: A generic heat pumpA useful analogy in this regard is to think of heat flowing from hot to cold objects as running down hill, which is what objects naturally do in Newtonian mechanics. It is possible to make objects go up hill, but only by doing external work on them. This movement of heat from a cool to a warm reservoir through some external work is the basis of the following three devices. In a refrigerator, the cool reservoir is the inside of the refrigerator, and the warm reservoir is the room itself. From this, one can see that leaving a refrigerator door open will not cool off the room that it is in. In an air conditioner, the cool reservoir is the inside of a house, and the warm reservoir is the outside. This is used to cool a house in the summer. In a heat pump, the cool reservoir is the outside of a house, and the warm reservoir is the inside. This can be used to warm a house in the winter. The heat pump is thus just the reverse of an air conditioner, and indeed some heat pumps have a switch which allows them to function as an air conditioner in the summer. A practical refrigerator cycle uses a special liquid which, when the pressure is reduced, evaporates to become a gas. Such a cycle is illustrated below. Figure 8.7: A refrigerator cycleIn this cycle, liquid enters the refrigerator in a region of low pressure, where it evaporates to become a gas, absorbing heat in the process. This gas then passes through a pump into a region of high pressure, where it condenses to become a liquid, thereby releasing heat. This cycle thus requires a special liquid which evaporates and condenses within the given operating pressure and temperature regions; these liquids, such as Freon, usually require special care in their handling and disposal. Heat cannot be completely converted The 2nd form of the 2nd law - heat cannot be completely converted into other forms of energy - places some practical restrictions on the efficiency of, for example, internal combustion and steam powered engines. Before discussing this in more detail, however, let us see why this statement is surprising. As with the example of the ice cube melting on a hot day, nothing from energy conservation would prevent work from being completely converted into other forms of energy, and indeed such total conversion can happen for energies other than heat. For example, a ball released from rest which falls to the ground has all gravitational potential energy at the top and all kinetic energy at the bottom - the potential energy at the top thus gets completely converted to kinetic energy at the bottom. Or if the ball is connected to a rope and pulley of some sort, its gravitational potential energy can be converted into useful work (churning butter for example). Although in practice some of the potential energy will be lost to friction, there is no reason in principle that all the potential energy cannot be converted into work. If we go back to our analogy between heat flowing from hot to cold and an object running down hill, we can understand that in principle, instead of going into kinetic energy to raise the temperature of the cooler substance heat can be harnessed to do useful work. In this context the second law makes the very surprising statement that some of the heat energy must always be lost, so that the conversion from heat to work is never 100% efficient. Note, however, that this form of the 2nd law places no restriction on converting other forms of energy into heat - it is the conversion of heat into other forms of energy that turns out never to be 100% efficient, even in principle. An machine which converts heat into other forms of energy is called a heat engine; the generic operation, in accordance with the 2nd law, is pictured below. Figure 8.8: A generic heat engineThe important aspect here is that some waste heat is always expelled into the cooler reservoir; no heat engine could operate without such expulsion. This is why, for example, one notices in the winter near a steam powered electrical generating plant that nearby ice on a river is melted - this comes from the waste heat of the plant being expelled into the river. One can show that there is an ideal maximum efficiency present for the conversion of heat into external work: this is where the temperatures are expressed in the Kelvin scale. It is important to note that this is an ideal efficiency - real engines also lose some efficiency due to friction, etc., but this is above this theoretical limit. Thus, a heat engine would operate with 100% efficiency in converting heat into useful work only if the cool reservoir was at 0 K ( -273 o), which is not possible. For example, a steam powered electrical generating plant which operates between 500 K and 300 K (room temperature) has a maximum efficiency of 40%. Similar considerations hold for an internal combustion engine, the basic operation of which is illustrated below. Figure 8.9: A simplified internal combustion engineIn this segment of the cycle, the fuel mixture explodes, either from a spark plug for a gas engine or from the high pressure for a diesel engine. This drives the piston downwards, which subsequently turns the crankshaft and eventually the wheels - this is the part which converts the energy of heat into useful work. The piston then rises, expelling the exhaust gases which carry away the waste heat. The cycle then goes on to draw in more fuel mixture to repeat the cycle. The major point here is that the exhaust gases carry with them excess heat which could not be converted into useful work. Isolated systems become disordered The final form of the 2nd law - systems become more disorganized over time - is at the same time perhaps the most abstract and profound statement of the three. A trivial example of Natures tendency towards disorder can be achieved by the following experiment. Take a jar full of pennies that have carefully been arranged so that all the heads are facing up. Then tip the jar so that all the pennies fall to the ground - theoretically, it is possible that all the pennies will land with heads facing up. Our experience though is that most of the time, the some pennies will have heads up and some will have tails up. This example illustrates clearly, and quite accurately what is meant by order, and why nature prefers disorder. Having 50 pennies in a jar all with heads up is clearly an ordered state. Having 50 pennies on the ground with some, unspecified number heads, and the rest tails is clearly disordered. The reason the pennies never land all heads up is simply that it is too unlikely. There are far more disordered states possible than the one, and only one ordered state. This example also shows that ordered states are by no means impossible to achieve in Nature, they just require work: someone must put a lot of effort into arranging the pennies so that they are all facing the same way. However, in the absence of such external intervention, a disordered state is overwhelmingly more probably, which is in effect the 3rd version of the 2nd Law. Heat transfer by conduction works is also an illustration of this statement of the 2nd Law. When a hot and a cold body are initially in contact, the system is somewhat ordered, in that we know most of the molecules in the hot side are moving faster than those in the cool side. However, this degree of order is lost after the system has attained a uniform temperature. The concept of entropy is introduced to characterize the order of a system. Qualitatively speaking, Here, state is a very general term, and could include position, speed, etc. What the 2nd law states is then that This is equivalent to saying that the number of states available to a system increases in general, by which the system thus becomes more disordered. In the coin example, the initial state (all heads up) was unique, whereas the likely finally state (roughly half heads, half tails) c
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