Chapter 8 -INTRODUCTION TO THERMODYNAMICS 



are poor absorbers. In general, good reflectors 

 are poor radiators and poor absorbers. 



In considering thermal radiation, the concept 

 of black body radiation is frequently a useful 

 construct. A black body is conceived of as an 

 ideal or theoretical body which, being perfectly 

 black, is a perfect radiator, a perfect absorber, 

 and a perfect nonreflector of radiant energy. 

 The thermal radiation emitted by such a perfect 

 black body is proportional to T^— that is, to the 

 absolute temperature raised to the fourth power. 

 Because of the fourth power relationship, dou- 

 bling the absolute temperature increases the 

 radiation 16 times, tripling the absolute tem- 

 perature increases the radiation 81 times, and 

 so forth. The thermal radiation emitted by real 

 bodies is also proportional to the fourth power of 

 the absolute temperature, although the total 

 radiation emitted by a real body depends also 

 upon the surface of the body. Consideration of 

 the relationship between the thermal radiation 

 of a body and the fourth power of the absolute 

 temperature of that body explains why the prob- 

 lem of thermal insulation against radiation 

 losses increases so enormously as the tempera- 

 ture increases. 



CONVECTION.— Although convection is often 

 loosely classified as a mode of heat transfer, it 

 is more accurately regarded as the mechanical 

 transportation of a mass of fluid (liquid or gas) 

 from one place to another. In the process of this 

 transportation, all the thermal energy stored 

 within the fluid remains in stored form unless it 

 is transferred by radiation or by conduction. 

 Since convection does not involve thermal energy 

 in transition, we cannot in the most fundamental 

 sense regard it as a modeof heat transfer. 



Convection is the transportation or the move - 

 ment of some portions within amass of fluid. As 

 this movement occurs, the moving portions of 

 the fluid transport their contained thermal 

 energy to other parts of the fluid. The effect of 

 convection is thus to mix the various portions of 

 the fluid. The part that was at the bottom of 

 the container may move to the top or the part 

 that was at one side may move to the other side. 

 As this mixing takes place, heat transfer occurs 

 by conduction and radiation from one part of the 

 fluid to another and between the fluid and its 

 surroundings. In other words, convection trans- 

 ports portions of the fluid from one place to an- 

 other, mixes the fluid, and thus provides an 

 opportunity for heat transfer to occur. But 



convection does not, in and of itself, "transfer" 

 thermal energy. 



Convection serves a vital purpose in bring- 

 ing the different parts of a fluid into close 

 contact with each other so that heat transfer can 

 occur. Without convection, there would be little 

 heat transfer from, to, or within fluids, since 

 most fluids are very poor at transferring heat 

 except when they are in motion. 



Two kinds of convection may be distinguished. 

 Natural convection occurs when there are differ- 

 ences in the density of different parts of the 

 fluid. The differences in density are usually 

 caused by unequal temperatures within the mass 

 of fluid. As the air over a hot radiator is heated, 

 for example, it becomes less dense and there- 

 fore begins to rise. Cooler, heavier air is drawn 

 in to replace the heated air that has moved up- 

 ward, and convection currents are thus set up. 

 Another example of natural convection, and one 

 that may be quite readily observed, may be 

 found in a pan of water that is being heated on 

 a stove. As the water near the bottom of the pan 

 is heated first, it becomes less dense and moves 

 upward. This displaces the cooler, heavier water 

 and forces it downward; as the cooler water is 

 heated in turn, it rises and displaces the water 

 near the top. By the time the water has almost 

 reached the boiling point, a considerable amount 

 of motion can be observed in the water. 



Forced convection occurs when some me- 

 chanical device such as a pump or a fan produces 

 movement of a fluid. Many examples of forced 

 convection may be observed in the shipboard 

 engineering plant: feed pumps transporting water 

 to the boilers, fuel oil pumps moving fuel oil 

 through heaters and meters, lubricating oil 

 pumps forcing lubricating oil through coolers, 

 and forced draft blowers pushing air through 

 boiler double casings, to name but a few. 



The mathematical treatment of convection 

 is extremely complex, largely because the 

 amount of heat gained or lost through the con- 

 vection process depends upon so many different 

 factors. Empirically determined convection co- 

 efficients which take account of these many 

 factors are available for most kinds of engineer- 

 ing equipment. 



Sensible Heat and Latent Heat 



The terms sensible heat and latent heat are 

 often used to indicate the effect that the trans- 

 fer of heat has upon a substance. The flow of 



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