Solutions Manual 1-1 for Heat and Mass Transfer: Fundamentals & Applications 5th Edition Yunus A. Cengel & Afshin J. Ghajar McGraw-Hill,

1-1


Solutions Manual
for
Heat and Mass Transfer: Fundamentals & Applications
5th Edition
Yunus A. Cengel & Afshin J. Ghajar
McGraw-Hill, 2015




Chapter 1
INTRODUCTION AND BASIC CONCEPTS




PROPRIETARY AND CONFIDENTIAL


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Thermodynamics and Heat Transfer


1-1C Thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to
another. Heat transfer, on the other hand, deals with the rate of heat transfer as well as the temperature distribution within the
system at a specified time.




1-2C (a) The driving force for heat transfer is the temperature difference. (b) The driving force for electric current flow is the
electric potential difference (voltage). (a) The driving force for fluid flow is the pressure difference.




1-3C The caloric theory is based on the assumption that heat is a fluid-like substance called the "caloric" which is a massless,
colorless, odorless substance. It was abandoned in the middle of the nineteenth century after it was shown that there is no
such thing as the caloric.




1-4C The rating problems deal with the determination of the heat transfer rate for an existing system at a specified
temperature difference. The sizing problems deal with the determination of the size of a system in order to transfer heat at a
specified rate for a specified temperature difference.




1-5C The experimental approach (testing and taking measurements) has the advantage of dealing with the actual physical
system, and getting a physical value within the limits of experimental error. However, this approach is expensive, time
consuming, and often impractical. The analytical approach (analysis or calculations) has the advantage that it is fast and
inexpensive, but the results obtained are subject to the accuracy of the assumptions and idealizations made in the analysis.




1-6C The description of most scientific problems involves equations that relate the changes in some key variables to each
other, and the smaller the increment chosen in the changing variables, the more accurate the description. In the limiting case
of infinitesimal changes in variables, we obtain differential equations, which provide precise mathematical formulations for
the physical principles and laws by representing the rates of changes as derivatives.
As we shall see in later chapters, the differential equations of fluid mechanics are known, but very difficult to solve
except for very simple geometries. Computers are extremely helpful in this area.




1-7C Modeling makes it possible to predict the course of an event before it actually occurs, or to study various aspects of an
event mathematically without actually running expensive and time-consuming experiments. When preparing a mathematical
model, all the variables that affect the phenomena are identified, reasonable assumptions and approximations are made, and
the interdependence of these variables are studied. The relevant physical laws and principles are invoked, and the problem is
formulated mathematically. Finally, the problem is solved using an appropriate approach, and the results are interpreted.




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preparation. If you are a student using this Manual, you are using it without permission.

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1-8C The right choice between a crude and complex model is usually the simplest model which yields adequate results.
Preparing very accurate but complex models is not necessarily a better choice since such models are not much use to
an analyst if they are very difficult and time consuming to solve. At the minimum, the model should reflect the essential
features of the physical problem it represents.




1-9C Warmer. Because energy is added to the room air in the form of electrical work.




1-10C Warmer. If we take the room that contains the refrigerator as our system, we will see that electrical work is supplied to
this room to run the refrigerator, which is eventually dissipated to the room as waste heat.




1-11C For the constant pressure case. This is because the heat transfer to an ideal gas is mcpT at constant pressure and
mcvT at constant volume, and cp is always greater than cv.




1-12C Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life.




1-13C The rate of heat transfer per unit surface area is called heat flux q . It is related to the rate of heat transfer by
Q   qdA .
A




1-14C Energy can be transferred by heat, work, and mass. An energy transfer is heat transfer when its driving force is
temperature difference.




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1-15 The filament of a 150 W incandescent lamp is 5 cm long and has a diameter of 0.5 mm. The heat flux on the surface of
the filament, the heat flux on the surface of the glass bulb, and the annual electricity cost of the bulb are to be determined.
Assumptions Heat transfer from the surface of the filament and the bulb of the lamp is uniform.
Analysis (a) The heat transfer surface area and the heat flux on the surface of the filament are

As  DL   (0.05 cm)(5 cm)  0.785 cm 2
Q
Q 150 W Lamp
q s    191 W/cm2  1.91 10 6 W/m2
As 0.785 cm 2 150 W

(b) The heat flux on the surface of glass bulb is

As  D 2   (8 cm) 2  201.1 cm 2

Q 150 W
q s    0.75 W/cm2  7500 W/m2
As 201.1 cm 2

(c) The amount and cost of electrical energy consumed during a one-year period is

Electricit y Consumption  Q t  (0.15 kW)(365  8 h/yr)  438 kWh/yr
Annual Cost = (438 kWh/yr)($0.08 / kWh)  $35.04/yr




1-16E A logic chip in a computer dissipates 3 W of power. The amount heat dissipated in 8 h and the heat flux on the surface
of the chip are to be determined.
Assumptions Heat transfer from the surface is uniform.
Analysis (a) The amount of heat the chip dissipates during an 8-hour period is Logic chip
Q  3 W
Q  Q t  (3 W)(8 h)  24 Wh  0.024 kWh

(b) The heat flux on the surface of the chip is
Q 3W
q    37.5 W/in2
A 0.08 in 2




PROPRIETARY MATERIAL. © 2015 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.