# Interferometric Study of the Heat Transfer Phenomena Induced by Rapid Heating of Nickel Sheet
## 2 Experiments  
### Experimental Apparatus  
Figure 1a shows the schematic of an experimental chamber and circuit to obtain the pressure wave and the temperature in open space ([PERSON] et al., 2020). The experimental setup for an open space is composed of bus bars, connecting bolts, and a nickel sheet. For minimizing thermal loss, insulating materials are added under the nickel sheet. This is fixed on the bus bars to connect the heating circuit. The temperature of an experimental specimen (nickel sheet) with resistance can be increased suddenly according to the capacitor's characteristics, which allows the quick release of electric power. The designed R-C heating system of this study has charging and discharging circuits, an electric switch, and a trigger part near the capacitor (see Figure 1b). The charging circuit has a capacitor which has the ability to charge electric power, a power supplier to handle the direct current, a resistor to protect over current, and a power button. The discharging circuit contains a capacitor, a metal sheet which will be heated by the discharging electric power, and the power button for immediate opening of the circuit. Tables 1 and 2 show the details of the capacitor and the physical properties of the experimental specimen, respectively. Figure 1: The experimental chamber and circuit diagram ([PERSON] et al., 2020). (**a**) Modeled chamber, (**b**) Circuit diagram of resistance–capacitance (R-C) heating. The interference phenomenon of thermo-acoustic waves due to the variation of density is visualized by a Mach-Zehnder interferometer. The interferometer is equipped with a He-Ne laser with about 10-mW output, a beam expander, a beam splitter, a reflection mirror, and concave and convex lenses, as shown in Figure 2. This device measures the distribution of the density or temperature. The fundamental principle relies on the distinction between the refractive indices from the result of the interference between two laser beams separated by a half-silvered mirror, one passing to the test space and the other acting as a reference beam. The fringe shift between them is utilized to determine the density or temperature distributions. \begin{table}
\begin{tabular}{c c} \hline
**Capacitor** & **68,000 μF** \\ \hline Diameter (mm) & 76.0 \\ Length (mm) & 105.0 \\ Surging voltage (V) & 50 \\ Operating temperature (\({}^{\circ}\)C) & \(-\)40–85 \\ Max. equivalent series resistance (\(\Omega\)) & 0.012 \\ Max. ripple (RMS Amps) & 16.0 \\ Max. DC leakage current (A) & \(I=0.006\sqrt{C\cdot V}\) \\ Working voltage (V) & 50 \\ \hline \end{tabular}
\end{table} 
Table 1: Specification of capacitor applied. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Property** & **Nickel** \\ \hline \multirow{4}{*}{Physical} & Specific weight (kg/m\({}^{3}\)) & 8900 \\ \cline{2-3}  & Electrical resistance (μ\(\Omega\) cm) & 6.84 \\ \cline{2-3}  & Specific heat (J/kg K) & 445.9 \\ \cline{2-3}  & Thermal conductivity (W/m-K) & 83 \\ \cline{2-3}  & Thickness, \(\delta\) (mm) & 0.0125, 0.01, 0.0075 \\ \hline \multirow{4}{*}{Mechanical} & Yield strength (kg/mm\({}^{2}\)) & 15 \\ \cline{2-3}  & Tensile strength (kg/mm\({}^{2}\)) & 47 \\ \cline{1-1} \cline{2-3}  & Elongation length (\%) & 50 \\ \hline \end{tabular}
\end{table} 
Table 2: Physical and mechanical properties of the tested metal sheets. Figure 2: Schematic diagram and real image of Mach–Zehnder interferometer in this study. The operating sequence of the apparatus is as follows. First, the beam emitted from a He-Ne laser is diffused after passing through an objective lens and a space filter of 10-\(\upmu\)m depth. The beam is separated into two parallel beams by a beam splitter. The two beams are recombined after they pass through different paths, which have different densities. At this time, the interference phenomenon, which occurs due to the difference in wavelength, can show an interference fringe in the wall. This fringe is used to analyze the flow field.