Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 191

Table 5.2 Temperature after two minutes of microwave heating at the locations
where FBG sensors are mounted, as measured by the infrared thermal
camera and FBGs sensors and predicted using numerical simulation

Thermotracer
Readings at
t = 120 s (°C)

Analytically Predicted
Temperatures (°C)

Microwave
Heating
Power (W)

FBG Readings (°C)

Specimen S1 S2 S1 S2 S1 S2

C1 950 39.0 57.5 39.0 58.3 36.2 60.9

C2 1800 67.0 119.5 67.2 119.2 65.3 119.0

C3 950 38.5 59.0 37.6 58.2 36.2 60.9

C4 1800 66.5 120.0 65.7 120.4 65.3 119.0

C5 950 36.0 118.0 35.6 117.8 36.2 60.9

C6 1800 70.0 124.0 70.0 123.7 65.3 119.0

those obtained using the infrared thermal camera, and those predicted
through numerical simulation are compared in Table 5.2. As shown in
Figures 5.17 and 5.18, FBGs and numerical simulation indicated almost
similar temperature development trends and peak temperatures at the end
of the heating period. Similarly, the results of our case study show good
consistency and close agreement between the temperature measurements
during the cooling period obtained using FBGs and those predicted using
numerical simulation.
For both FBG sensors (FBG1 and FBG2), the experimental measurements
and numerical predictions showed an almost linear increase in the surface
temperature of concrete for the entire duration of the microwave heating
period. Again, the temperature measurement curves associated with FBG1
and FBG2 converged during the cooling period as heat was transferred
from hotter locations (measured by FBG2) to cooler locations (measured
by the FBG1).
It should also be noted that the small differences observed between
the FBG readings and the results of numerical simulations are likely to
be partially attributable to the differences between the actual magnitude
of the electromagnetic properties of concrete and those assumed in the
numerical simulation. As discussed in Chapter 1, each concrete specimen
has a unique set of electromagnetic properties determining the amount of
energy absorbed, reflected, and transferred when it is exposed to micro-
waves. Accurate measurement of the electromagnetic properties is usually
performed using a network analyser comprising a dielectric measurement
kit. However, in the illustrative example presented in this chapter, rather
than using the actual electromagnetic properties of each concrete specimen
tested, the average of the dielectric properties of typical saturated surface
dry concrete were used. This probably accounts for the minor discrepan-
cies observed.