Principles of naval engineering - Historic Naval Ships Association

Chapter 8 -INTRODUCTION TO THERMODYNAMICSare poor absorbers. In general, good reflectorsare poor radiators and poor absorbers.In considering thermal radiation, the conceptof black body radiation is frequently a usefulconstruct. A black body is conceived of as anideal or theoretical body which, being perfectlyblack, is a perfect radiator, a perfect absorber,and a perfect nonreflector of radiant energy.The thermal radiation emitted by such a perfectblack body is proportional to T^— that is, to theabsolute temperature raised to the fourth power.Because of the fourth power relationship, doublingthe absolute temperature increases theradiation 16 times, tripling the absolute temperatureincreases the radiation 81 times, andso forth. The thermal radiation emitted by realbodies is also proportional to the fourth power ofthe absolute temperature, although the totalradiation emitted by a real body depends alsoupon the surface of the body. Consideration ofthe relationship between the thermal radiationof a body and the fourth power of the absolutetemperature of that body explains why the problemof thermal insulation against radiationlosses increases so enormously as the temperatureincreases.CONVECTION.— Although convection is oftenloosely classified as a mode of heat transfer, itis more accurately regarded as the mechanicaltransportation of a mass of fluid (liquid or gas)from one place to another. In the process of thistransportation, all the thermal energy storedwithin the fluid remains in stored form unless itis transferred by radiation or by conduction.Since convection does not involve thermal energyin transition, we cannot in the most fundamentalsense regard it as a modeof heat transfer.Convection is the transportation or the move -ment of some portions within amass of fluid. Asthis movement occurs, the moving portions ofthe fluid transport their contained thermalenergy to other parts of the fluid. The effect ofconvection is thus to mix the various portions ofthe fluid. The part that was at the bottom ofthe container may move to the top or the partthat was at one side may move to the other side.As this mixing takes place, heat transfer occursby conduction and radiation from one part of thefluid to another and between the fluid and itssurroundings. In other words, convection transportsportions of the fluid from one place to another,mixes the fluid, and thus provides anopportunity for heat transfer to occur. Butconvection does not, in and of itself, "transfer"thermal energy.Convection serves a vital purpose in bringingthe different parts of a fluid into closecontact with each other so that heat transfer canoccur. Without convection, there would be littleheat transfer from, to, or within fluids, sincemost fluids are very poor at transferring heatexcept when they are in motion.Two kinds of convection may be distinguished.Natural convection occurs when there are differencesin the density of different parts of thefluid. The differences in density are usuallycaused by unequal temperatures within the massof fluid. As the air over a hot radiator is heated,for example, it becomes less dense and thereforebegins to rise. Cooler, heavier air is drawnin to replace the heated air that has moved upward,and convection currents are thus set up.Another example of natural convection, and onethat may be quite readily observed, may befound in a pan of water that is being heated ona stove. As the water near the bottom of the panis heated first, it becomes less dense and movesupward. This displaces the cooler, heavier waterand forces it downward; as the cooler water isheated in turn, it rises and displaces the waternear the top. By the time the water has almostreached the boiling point, a considerable amountof motion can be observed in the water.Forced convection occurs when some mechanicaldevice such as a pump or a fan producesmovement of a fluid. Many examples of forcedconvection may be observed in the shipboardengineering plant: feed pumps transporting waterto the boilers, fuel oil pumps moving fuel oilthrough heaters and meters, lubricating oilpumps forcing lubricating oil through coolers,and forced draft blowers pushing air throughboiler double casings, to name but a few.The mathematical treatment of convectionis extremely complex, largely because theamount of heat gained or lost through the convectionprocess depends upon so many differentfactors. Empirically determined convection coefficientswhich take account of these manyfactors are available for most kinds of engineeringequipment.Sensible Heat and Latent HeatThe terms sensible heat and latent heat areoften used to indicate the effect that the transferof heat has upon a substance. The flow of167