2.1 Principle
The pulse echo measurement system is made up of an ultrasonic generator and a sensor (hereinafter, referred to as a probe) which is placed on the surface of an object (Shen Yi, 2011). A piezoelectric crystal oscillator is placed in front of the probe to transmit ultrasonic pulses to the object. When the ultrasonic wave encounters an interface composed of media with different acoustic impedances, it will be reflected. The ultrasonic pulse reflected from the interface is received by the sensor, and the signal is collected by a signal acquisition system after signal amplification and detection. Then, the position of the solid–liquid interface and its time dependence are determined according to the amplitude of the reflected wave and its position on the time base axis, as illustrated in Fig. 1.
The echo flight distance is calculated by time difference, as shown in formular 1, where s is the distance travelled by ultrasonic wave in solid, and c is the velocity of ultrasound. The distance travelled by ultrasonic pulse is obtained by multiplying the measured time \({\tau }_{m}\) from sending to receiving of ultrasonic wave by the ultrasound velocity in the metal material c (Li Yan-qin, 2014). By dividing this distance by 2, the distance from probe interface to solid–liquid interface can be obtained. The propagation velocity of ultrasonic waves in different metal materials is listed in Table 1.
$$\text{s}=\text{c}\bullet {{\tau }}_{m}/2$$ 1
Table 1
Velocity of ultrasound in metals
No.
|
Material
|
Sound Velocity (m/s)
|
1
|
Aluminium
|
6300
|
2
|
Zinc
|
4170
|
3
|
Silver
|
3600
|
4
|
Gold
|
3240
|
5
|
Steel
|
5900
|
6
|
Copper
|
4700
|
7
|
Stainless steel
|
5790
|
8
|
Tin
|
3320
|
2.2 Data Acquisition System
The schematic diagram of the data acquisition system is shown in Fig. 2. After the ultrasonic signal is sent out, the echo is collected, and the central processor unit controls the signal acquisition chip to transmit the collected data to a host computer through the data interface. In addition to measuring the ultrasonic signals, the signals of five thermocouples distributed along the length of the sample are also measured simultaneously. These five thermocouples are used to obtain the axial temperature distribution of the sample surface during the experiment.
The data acquisition system consists of four parts: signal filtering and amplification, data acquisition, processor, and communication interface. The processor is the core device of the acquisition system. The analogue signal is firstly processed by the signal filtering and amplification circuit, and then analog-to-digital conversion is performed by an multichannel A/D converter. The analog-to-digital conversion results are processed by the processor, and the echo signals are denoised using the wavelet algorithm. Then, the processed results are transmitted to the data processing system through the communication interface.
2.3 Solid–liquid Interface Measurement System
The experimental facility for the solid–liquid interface measurement system comprises a heating furnace, a sample cartridge assembly, a cooling system, a sample moving system and the ultrasonic detection system, as shown in Fig. 3.
The heating furnace comprises a furnace chamber, a heater, a thermal insulation layer, and a thermocouple (T0). The furnace chamber is made of ceramic, and its diameter is 36mm. The length of the furnace is 190mm. The heater is made of iron-chromium-aluminium wire wound inside a ceramic framework. The thermocouple is mounted on the wall of the ceramic chamber. The outside of the heater is covered with thermal insulator, and the whole furnace is wrapped and fixed by an aluminium shell. The thermocouple in the furnace is used to control the temperature. The temperature control system controls the heating power according to the temperature inside the furnace to maintain the temperature at 1160°C, which are higher than the melting temperature of aluminium.
The sample cartridge assembly comprises the quartz ampoule, the ceramic crucible, and the sample. The outer most layer is a quartz ampoule. The outer diameter of the quartz ampoule is 32mm, and its inner diameter is 28mm. A ceramic crucible is in the quartz ampoule. The outer diameter of the ceramic crucible is 24mm, and its inner diameter is 18mm. Five grooves are evenly distributed along the ceramic crucible, where the thermocouples (defined as T1, T2, T3, T4, T5) are embedded. The thermocouples are evenly distributed along the length, and they are used to measure the surface temperature distribution along the length of the sample. The sample is made of aluminium with a purity of 99.99%. The sample is cylinder, the length of the sample is 240mm, and the cross-sectional diameter is 8mm.
This facility is also equipped with a sample moving mechanical structure. The holder of sample cartridge assembly is driven by a motor. It can move the sample in the furnace. It is used to study the velocity of solid-liquid interface movement during the sample moving.
Because the ultrasonic probe can only withstand the temperature of 100°C, a cooling water circulation system is installed at the measurement end of the sample to ensure that the temperature is not higher than 100°C. The heat sink at the measurement end is made of an annular copper pipe with water running inside. The cooling system is provided by a water pump on the ground.
2.4 The temperature profile
There are five S-type thermocouples at the surface of the ceramic crucible in the sample cartridge assembly used for monitoring the temperature of the aluminium sample, as shown in Fig. 4.
The distances between the measuring end and T1 is 127mm. The intervals between the thermocouples T1, T2, T3, T4 and T5 are all 15mm. The extent between the end of the sample and T5 is 53mm.
During the solid-liquid interface monitoring experiment, the temperature control process of the furnace is shown in Fig. 5.
The temperatures of the thermocouples in the sample cartridge assembly are shown in Fig. 6. “Measure” is the temperature profile at the surface of the ceramic crucible.
According to the melting temperature of the sample and the temperature at the end of the sample outside the furnace, combined with the thermal conductivity, density, specific heat, emissivity, and transmission coefficient of components of the furnace (shown in Table 2.), the temperature distribution on the surface of the aluminium sample is obtained by simulation.
Table 2
Thermal parameters of materials
Name
(Material)
|
Thermal
conductivity
(W/(m·K))
|
Density
(g/cm3)
|
Specific heat
( kJ/(kg·K))
|
Emissivity
|
Transmission
coefficient
|
Furnace
Chamber
(Ceramic)
|
0.105
|
7.75
|
0.5
|
0.85
|
/
|
Sample
(aluminium)
|
230–240
|
2.7
|
0.88
|
0.2
|
/
|
crucible
(ceramic)
|
25–35
|
3.72
|
0.85
|
0.85
|
/
|
ampoule
(quartz)
|
1.4
|
2.5
|
750
|
0.85
|
0.78–0.82
|
The curve labelled simulation without cooling system is the temperature distribution of the aluminium sample without a cooling system from the simulation. The temperature line labelled simulation cooling system run is simulation with a cooling system. The temperature gradient of 50℃/cm is attained.
As can be seen in Fig. 6, the temperature of the outer surface of the ampoule is more than 300 degrees higher than that of the sample. This is because the inner diameter of the ceramic ampoule is larger (18mm) while the diameter of the sample is smaller (8mm). The heat transfer conducted by radiation and convection. The temperature profile is close to linear distribution.
2.5 Cooling system
Cooling system is an important part of this facility. It enables the ultrasonic sensors to measure the solid-liquid interface of materials at higher temperatures.
For ultrasonic measuring, the end temperature should be below 80℃ for reliable solid-liquid interface measuring. According to the sample temperature distribution, it is necessary to design a cooling system to meet the experimental requirements.
During experiment on ground, the cooling system is shown in Fig. 7. It consists of a cooling water pipe, a water pump, a water circulation controller, a computer and a cold water tank. The cold cooling pipe is wound around the sample end to cool it. Computer set cooling temperature, and issued command to control the water pump. The pump drives the cold water in the water tank to the cooling water pipe for circulation, in order to achieve the purpose of cooling.
According to the temperature measured on the surface of the ceramic crucible, we calculated the temperature profile on the surface of the sample, as shown in Fig. 6.
The heating power of the furnace is 250W, and the temperature at the measuring end of the aluminium sample is 293℃ without cooling. In order to ensure that the ultrasonic sensor can work in a reliable condition, we set the temperature of the measuring end of the aluminium sample at 50℃. The inlet temperature of water in the cooling pipe is 25℃ and the outlet temperature is 40℃. The flow rate of water in the cooling system is 0.14m/s, the cross-section area of water pipe is 4mm2, the length of cooling pipe is 0.321m, the density of water \({\rho }_{\text{water}}\)is 1000Kg/m3, the heat capacity of water\({c}_{\text{water}}\) is 4.2×103 J/Kg℃.
Thus, the quantity of heat absorbed by the cooling system in steady state can be obtained:
$$Q={\rho }_{\text{water}}*V*{c}_{\text{water}}*\varDelta T=80.89J$$
Heat dissipation power is 35.32W.
This experiment performs on the ground. During the process of temperature rising and stabilizing, a cooling water circulation pump was operated to ensure that the temperature of the end of the sample was lower than the temperature that the sensors can withstand. During the experiment, the maximum temperature of the end of the sample is 52℃ actually, the solid-liquid interface was measured by the ultrasonic sensors successfully.