**3. Microwave heating system for rock**

The microwave heating equipment includes a power supply, a magnetron, an isolator, a coupler, an impedance tuner, a rectangular waveguide, a microwave applicator, and a shielded cavity (**Figure 2**). The power supply is used to convert alternating-current into direct-current to thus create conditions for the operation

**Figure 2.** *Schematic view of the microwave heating system.*

### *Experimental Investigation on the Effect of Microwave Heating on Rock Cracking… DOI: http://dx.doi.org/10.5772/intechopen.95436*

of the magnetron. The magnetron converts direct-current electrical energy into microwave energy, thus providing continuous microwave power. The isolator is used for the unidirectional circular transmission of microwave energy. The function of the water cycle is to absorb the reflected microwave energy, thus protecting the magnetron from damage. The microwave applicator is used to emit microwave energy to the surface of rocks, where it is used to heat and crack rocks. The impedance tuner is used for impedance matching. Compared with the microwave source with a frequency of 915 MHz, the microwave source with a frequency of 2450 MHz has higher heating efficiency and smaller volume, which is conducive to the combination with the mechanical rock-breaking device. During testing, a metal net is used as shielding to avoid microwave interference with signal transmission to/from the other apparatuses.

#### **Figure 3.**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

By combining Eqs. (3) and (5), the rate of heating may be given by

t

An increase in material temperature causes the volume of the material to

<sup>T</sup> <sup>P</sup>

where *Q* is the energy absorbed by the material; C is the specific heat capacity; *m* is the mass; ∆*T* is the temperature increase after absorbing energy; ∆*t* is the time

<sup>2</sup> T 2 "E f <sup>0</sup>

where, *V*0 is the volume at some reference temperature, and *α* is the coefficient

After being irradiated by microwaves, rocks absorb electromagnetic energy that is transformed into thermal energy, causing the temperature rising of rocks. After microwave irradiation, the temperature of the rocks is not uniform, resulting in uneven thermal expansion in the rocks. Different minerals within rocks have different dielectric properties, leading to different rocks have different microwave absorption capacities. Different minerals also have different thermal expansion coefficients, so the thermal expansion property is different after heating. Therefore, due to the different microwave sensitivity and thermal expansibility of different types of rocks, different types of rocks show different heating characteristics and

The microwave heating equipment includes a power supply, a magnetron, an isolator, a coupler, an impedance tuner, a rectangular waveguide, a microwave applicator, and a shielded cavity (**Figure 2**). The power supply is used to convert alternating-current into direct-current to thus create conditions for the operation

ρ*C*

ρ

t

*<sup>C</sup>* <sup>∆</sup> <sup>=</sup> <sup>∆</sup> (5)

<sup>∆</sup> πε ε <sup>=</sup> <sup>∆</sup> (6)

V(T) (1 T) = +α∆ *V*<sup>0</sup> (7)

**150**

**Figure 2.**

difference.

increase

fracturing effects.

of thermal expansion of the medium [43].

**3. Microwave heating system for rock**

*Schematic view of the microwave heating system.*

*Temperature distribution on the surface of samples measured by infrared camera (ambient temperature at 13.5°C) [44]. Tmax = 92.0°C, Tave = 68.7°C Tmax = 90.8°C, Tave = 63.7°C Tmax = 91.9°C, Tave = 67.3°C (a) 5 kW, 10 s. Tmax = 158.2°C, Tave = 109.6°C Tmax = 169.3°C, Tave = 119.0°C Tmax = 166.8°C, Tave = 116.9°C (b) 5 kW, 20 s. Tmax = 240.8°C, Tave = 169.9°C Tmax = 225.9°C, Tave = 157.0°C Tmax = 218.0°C, Tave = 163.3°C. (c) 5 kW, 30 s.*
