Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 222

mode stirrers tends to be minimal [11]. However, mode stirrers can consid-
erably improve heating uniformity when small and low-loss material loads
are heated. The location of the mode stirrer also affects its effectiveness.
Placing the mode stirrer close to the microwave feed point can improve
efficiency [12].
The third general approach to improve heating uniformity in a multi-
mode cavity is by using higher frequencies. In a multimode cavity with
fixed dimensions, heating uniformity increases generally with an increase
in the microwave frequency. This is mainly because a larger number of
wavelengths can be fitted in a multimode cavity with a particular fixed
dimension, so a larger number of operating modes can exist at higher fre-
quencies than at lower frequencies. In addition, at higher frequencies, the
distance between the nodes and antinodes of standing waves decreases
with an increase in microwave frequency, leading to improved conven-
tional heat transfer between adjacent nodes and nulls. Aside from con-
ventional 2.45-GHz magnetrons, those of higher frequencies, including
5.8-GHz magnetrons are readily available and are gaining popularity [13].
Considerably higher frequencies, including 24 GHz generated by gyrotrons,
have also been used in applications such as ceramic sintering.
The fourth method used to improve heating uniformity in multimode
cavities is called frequency sweeping . The working principle of this method
is based on the fact that a larger number of modes may be excited in wider
bandwidths. The presence of a larger number of modes in turn leads to
a better heating uniformity. However, it is well known that achieving a
wide bandwidth around a fixed frequency at high powers is challenging.
One innovative alternative method for creating broad bandwidths is by
high-speed frequency sweeping, which may create a relatively uniform
time-averaged power density in the multimode microwave cavity. A vari-
ety of microwave electron devices, including helix TWTs, can be used to
achieve the desired frequency sweeping to cover a broad bandwidth, such
as 2-8 GHz. However, the variable-frequency microwave generators are
highly expensive and are therefore not commonly used in normal heat-
ing applications. An alternative method to improve field uniformity is to
use two or more magnetrons with different frequencies instead of a single
variable magnetron. This method can be used to excite a large number of
modes at considerably less cost than variable-frequency magnetrons.
6.4.2 Open- ended applicators
The multimode cavities discussed in the previous section are suitable appli-
cators for applications such as microwave separation of mortar from RCA
and microwave-assisted accelerated curing of concrete, for which the load
shape and configuration allow it to be surrounded by the applicator and be
uniformly heated from all the faces of the load as desired. However, there