John H. Lienhard IV, John H. Lienhard V. A Heat Transfer Textbook, страница 3

To emphasize this point we suggestthat the reader make an experiment.Experiment 1.1Generate as much power as you can, in some way that permits you tomeasure your own work output. You might lift a weight, or run your ownweight up a stairwell, against a stopwatch. Express the result in watts (W).Perhaps you might collect the results in your class. They should generallybe less than 1 kW or even 1 horsepower (746 W). How much less mightbe surprising.Thus, when we do so small a thing as turning on a 150 W light bulb,we are manipulating a quantity of energy substantially greater than ahuman being could produce in sustained effort.

The power consumedby an oven, toaster, or hot water heater is an order of magnitude beyondour capacity. The power consumed by an automobile can easily be threeorders of magnitude greater. If all the people in the United States workedcontinuously like galley slaves, they could barely equal the output of evena single city power plant.Our voracious appetite for energy has steadily driven the intensityof actual heat transfer processes upward until they are far greater thanthose normally involved with life forms on earth. Until the middle of thethirteenth century, the energy we use was drawn indirectly from the sun1Some anthropologists think that the term Homo technologicus (technological man)serves to define human beings, as apart from animals, better than the older term Homosapiens (man, the wise).

We may not be as much wiser than the animals as we think weare, but only we do serious sustained tool making.§1.1Heat transferusing comparatively gentle processes — animal power, wind and waterpower, and the combustion of wood. Then population growth and deforestation drove the English to using coal. By the end of the seventeenthcentury, England had almost completely converted to coal in place ofwood. At the turn of the eighteenth century, the first commercial steamengines were developed, and that set the stage for enormously increasedconsumption of coal.

Europe and America followed England in thesedevelopments.The development of fossil energy sources has been a bit like JulesVerne’s description in Around the World in Eighty Days in which, to wina race, a crew burns the inside of a ship to power the steam engine.

Thecombustion of nonrenewable fossil energy sources (and, more recently,the fission of uranium) has led to remarkably intense energy releases inpower-generating equipment. The energy transferred as heat in a nuclearreactor is on the order of one million watts per square meter.A complex system of heat and work transfer processes is invariablyneeded to bring these concentrations of energy back down to human proportions. We must understand and control the processes that divide anddiffuse intense heat flows down to the level on which we can interact withthem. To see how this works, consider a specific situation. Suppose welive in a town where coal is processed into fuel-gas and coke. Such powersupplies used to be common, and they may return if natural gas suppliesever dwindle. Let us list a few of the process heat transfer problems thatmust be solved before we can drink a glass of iced tea.• A variety of high-intensity heat transfer processes are involved withcombustion and chemical reaction in the gasifier unit itself.• The gas goes through various cleanup and pipe-delivery processesto get to our stoves.

The heat transfer processes involved in thesestages are generally less intense.• The gas is burned in the stove. Heat is transferred from the flame tothe bottom of the teakettle. While this process is small, it is intensebecause boiling is a very efficient way to remove heat.• The coke is burned in a steam power plant. The heat transfer ratesfrom the combustion chamber to the boiler, and from the wall ofthe boiler to the water inside, are very intense.5Introduction6§1.2• The steam passes through a turbine where it is involved with manyheat transfer processes, including some condensation in the laststages. The spent steam is then condensed in any of a variety ofheat transfer devices.• Cooling must be provided in each stage of the electrical supply system: the winding and bearings of the generator, the transformers,the switches, the power lines, and the wiring in our houses.• The ice cubes for our tea are made in an electrical refrigerator.

Itinvolves three major heat exchange processes and several lesserones. The major ones are the condensation of refrigerant at roomtemperature to reject heat, the absorption of heat from within therefrigerator by evaporating the refrigerant, and the balancing heatleakage from the room to the inside.• Let’s drink our iced tea quickly because heat transfer from the roomto the water and from the water to the ice will first dilute, and thenwarm, our tea if we linger.A society based on power technology teems with heat transfer problems.

Our aim is to learn the principles of heat transfer so we can solvethese problems and design the equipment needed to transfer thermalenergy from one substance to another. In a broad sense, all these problems resolve themselves into collecting and focusing large quantities ofenergy for the use of people, and then distributing and interfacing thisenergy with people in such a way that they can use it on their own punylevel.We begin our study by recollecting how heat transfer was treated inthe study of thermodynamics and by seeing why thermodynamics is notadequate to the task of solving heat transfer problems.1.2Relation of heat transfer to thermodynamicsThe First Law with work equal to zeroThe subject of thermodynamics, as taught in engineering programs, makesconstant reference to the heat transfer between systems.

The First Lawof Thermodynamics for a closed system takes the following form on aRelation of heat transfer to thermodynamics§1.2Figure 1.1 The First Law of Thermodynamics for a closed system.rate basis:Qpositive towardthe system=Wk+positive awayfrom the systemdUdt(1.1)positive whenthe system’senergy increaseswhere Q is the heat transfer rate and Wk is the work transfer rate. Theymay be expressed in joules per second (J/s) or watts (W). The derivativedU/dt is the rate of change of internal thermal energy, U, with time, t.This interaction is sketched schematically in Fig. 1.1a.The analysis of heat transfer processes can generally be done without reference to any work processes, although heat transfer might subsequently be combined with work in the analysis of real systems. If p dVwork is the only work occuring, then eqn.

(1.1) isQ=pdUdV+dtdt(1.2a)This equation has two well-known special cases:Constant volume process:Constant pressure process:dU= mcvdtdHQ== mcpdtQ=dTdtdTdt(1.2b)(1.2c)where H ≡ U + pV is the enthalpy, and cv and cp are the specific heatcapacities at constant volume and constant pressure, respectively.When the substance undergoing the process is incompressible (so thatV is constant for any pressure variation), the two specific heats are equal:7Introduction8§1.2cv = cp ≡ c.

The proper form of eqn. (1.2a) is thenQ=dTdU= mcdtdt(1.3)Since solids and liquids can frequently be approximated as being incompressible, we shall often make use of eqn. (1.3).If the heat transfer were reversible, then eqn. (1.2a) would become2dSdV dUT=p+dt dt dtQrev(1.4)Wk revThat might seem to suggest that Q can be evaluated independently for inclusion in either eqn.

(1.1) or (1.3). However, it cannot be evaluated usingT dS, because real heat transfer processes are all irreversible and S is notdefined as a function of T in an irreversible process. The reader will recallthat engineering thermodynamics might better be named thermostatics,because it only describes the equilibrium states on either side of irreversible processes.Since the rate of heat transfer cannot be predicted using T dS, howcan it be determined? If U (t) were known, then (when Wk = 0) eqn. (1.3)would give Q, but U (t) is seldom known a priori.The answer is that a new set of physical principles must be introducedto predict Q. The principles are transport laws, which are not a part ofthe subject of thermodynamics. They include Fourier’s law, Newton’s lawof cooling, and the Stefan-Boltzmann law.

We introduce these laws laterin the chapter. The important thing to remember is that a descriptionof heat transfer requires that additional principles be combined with theFirst Law of Thermodynamics.Reversible heat transfer as the temperature gradient vanishesConsider a wall connecting two thermal reservoirs as shown in Fig. 1.2.As long as T1 > T2 , heat will flow spontaneously and irreversibly from 1to 2. In accordance with our understanding of the Second Law of Thermodynamics, we expect the entropy of the universe to increase as a consequence of this process.