Biomedical Engineering Reference
In-Depth Information
0.4
0.3
0.2
0.1
0
1
2
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4
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10
FIGURE 4.15
Value of
Q
.
a
p
Ê
Ë
ˆ
¯
1
W
=
f H AlQ
m
2
¥
10
W
(4.11)
2
-
7
0
a
2
where
f
is the frequency (in hertz), m the permeability,
a
the cylinder radius,
k
the conductivity,
H
the magnetic field at the surface of cylinder, and
p
the skin
depth of the current near the heating body surface. The value of
Q
is obtained
from Figure 4.15. Since the size of the heating material chosen in the analyti-
cal model is small, centimeter-gram-second (cgs) units are used in this analy-
sis. The characteristics of the maximum power loss appear clearly in Figure
4.15: The electric power absorption goes through a maximum value when
is equal to 2.5, that is, when
a
/
p
nearly equals 1.75. Expression (4.11)
is related to the skin depth
p
of the current, which becomes small when the
frequency becomes high. Therefore, when higher frequency is used, only the
surface vicinity can be heated.
2
ap
4.2.5
Actual Inductive Heating Applicator
To easily understand the difference between dielectric heating and inductive
heating, let us first introduce experimental heating characteristics [10, 11]
before describing actual inductive heating applicators. Figure 4.16 shows a
thermographic view of an experimental result of RF capacitive heating of a
rectangular agar phantom which is sandwiched between two layers of pig
fat perpendicularly to the RF electric field at 3 MHz. A conductive sphere is
buried in the center of the phantom. White areas indicate high temperature.
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