Page 246 - Fluid mechanics, heat transfer, and mass transfer

DIMENSIONLESS NUMBERS, TEMPERATURE MEASUREMENT, AND CONDUCTION HEAT TRANSFER
226
surface tension forces are greater than gravity
forces for the remainder of the fin.
. “Biot number (B i ) expresses the relative importance of
the thermal resistance of a body to that of the convection
resistance at its surface”. True/False?
& True.
. “If B i > 40, the surface temperature of a solid may be
assumed to be equal to the temperature of the
surroundings.” True/False?
& True.
Illustration for triple point.
FIGURE 8.1

8.2 TEMPERATURE MEASUREMENT
& Figure 8.1 gives the phase diagram illustrating the
. What are reference temperatures? Give some reference
triple point.
temperatures.
. Name important types of temperature measuring
& Temperatures established by physical phenomena
devices.
that are easily observed and consistent in nature.
& Thermocouples.
& The International Temperature Scale (ITS) is based
& Resistance temperature detectors (RTDs).
on such phenomena. It establishes 17 fixed points and
corresponding temperatures. A sampling is given in & Thermistor and integrated circuit temperature sen-
Table 8.1. sors (ICTS).
. Give a typical pressure–temperature phase diagram. & Other temperature-sensing devices: Liquid-in-glass
& If matter is heated to a high enough temperature, it thermometers, bimetallic strip thermometers, pres-
becomes gaseous. If matter is subjected to a high sure thermometers, pyrometers, and infrared
enough pressure, it becomes a solid. At combinations thermometers.
of pressure and temperature in between these limits, . What are the effects associated with temperature mea-
matter can exist as a liquid. surement by thermocouples?
& The boundaries that separate these states of matter & Seebeck effect: It states that thevoltage produced in a
are called the melting (or freezing) curve, the vapor- thermocouple is proportional to the temperature
ization (or condensation) curve, and the sublimation between the two junctions.
curve.The intersection of all three curves is called the & Peltier effect: It states that if a current flows through
triple point. All three states of matter can coexist at a thermocouple, one junction is heated (puts out
that pressure and temperature. Triple point is used as energy) and the other junction is cooled (absorbs
a fixed temperature point in the International Tem- energy).
perature Scale. & Thompson effect: It states that when a current flows
in a conductor along which there is a temperature
difference, heat is produced or absorbed, depending
TABLE 8.1 Reference Temperatures
upon the direction of the current and the variation of

Element Type Temperature ( C)
temperature.
Hydrogen (H 2 ) Triple point 259.3467 & In practice, the Seebeck voltage is the sum of the
Neon (Ne) Triple point 248.5939 electromotive forces generated by the Peltier and
Oxygen (O 2 ) Triple point 218.7916 Thompson effects.
Argon (Ar) Triple point 189.3442 . What is a thermocouple? How does it work?
Mercury (Hg) Triple point 38.8344
Water (H 2 O) Triple point þ 0.01 (273.16K) & Thermocouples consist essentially of two strips or
Gallium (Ga) Melting point 29.7646 wires made of different metals and joined at one end.
Indium (In) Freezing point 156.5985 Changes in the temperature at that junction induce a
Tin (Sn) Freezing point 231.928 change in electromotive force (emf) between the
Zinc (Zn) Freezing point 419.527 other ends. As temperature goes up, this output emf
Aluminium (Al) Freezing point 660.323
of the thermocouple rises although not necessarily
Silver (Ag) Freezing point 961.78
linearly. Temperature ranges for thermocouples are
Gild (Au) Freezing point 1064.18

from 270 to 2300 C.