# Study on the Dynamic Generation of Subcooled Water Using a Compact Heat exchanger
## 2 Experimental Setup and Experimental Conditions  
### Selection of Compact Heat Exchanger  
When considering compact heat exchangers, the concept of high-performance heat exchangers, which refers to a heat transfer area of 500-700 per unit volume (m\({}^{2}\)/m\({}^{3}\)) and a hydraulic diameter of less than 5 mm, is relevant [17]. In this paper, the compact heat exchanger used for the dynamic generation of subcooled water had a heat transfer area of 2990 per unit volume (m\({}^{2}\)/m\({}^{3}\)) and a hydraulic diameter of 0.32 mm, being a heat exchanger with an excellent performance and high compactness [16]. The details are in Table 1. The compact heat exchanger was designed based on thermal resistance balance combined with a microstructural approach [16]. The heat exchanger contains multiple fluid channels, each separated by one layer of the same fluid channel, where the working fluid enters the center from its respective inlet, and the multi-layered channels are divided on this side and converge at the outlet for outflow. The compact heat exchanger has a counter-flow heat transfer structure, in which low-temperature non-freezing liquid (NFL, 40 wt% aqueous solution of ethylene glycol) and tap water flow into the heat exchanger on both sides of the channel through convective heat transfer to dynamically generate subcooled water. This process is shown in Figure 1. The flow of low-temperature non-freezing liquid is higher than the flow of tap water so as to enhance the turbulence strength and save energy. Water below the freezing point without freezing is called subcooled water, which exists in a metastable state and can reach \(-\)38 \({}^{\circ}\)C in homogeneous nucleation while still in the liquid phase [18]. The ice nucleation process forms ice and then undergoes ice growth. For water below zero degrees Celsius, the solid phase (ice) is thermodynamically more stable than the liquid phase, because the free energy of the solid phase per molecule is lower than that of the liquid phase. However, when a small ice embryo forms and grows in the liquid phase, the total free energy of the system increases as the ice embryo grows, owing to the increase in interfacial free energy. As a result, the water can be cooled below zero degrees Celsius and exist in a subcooled state without ice nuclei. Conversely, suppose that the ice embryo size exceeds the critical value, which depends on the degree of subcooling; the total free energy decreases as the ice embryo grows, and thus the ice nucleation process takes place. In liquid water, homogenous nucleation is extremely rare; in most cases, ice nucleation occurs on solid surfaces. To some extent, the presence of solid surfaces reduces the amount of interfacial free energy [19]. In this study, ice was formed by heterogeneous nucleation, considering the fact that certain factors, such as impurities in the water, small energy perturbations, and the cooling time, can play a catalytic role in ice nucleation. [PERSON] studied the influences of various factors on the dynamic generation of subcooled water [20]. After considering the relationship between the parameters of the subcooled \begin{table}
\begin{tabular}{c c c} \hline
**Item** & **Unit** & **Content** \\ \hline Dimensions & mm & 40 L x 40 W \(\times\) 12.6 H \\ Hydraulic diameter & mm & 0.32 \\ Heat exchange channel length & mm & 21.6 \\ Working fluid & - & High temperature: tap water; low temperature: NFL \\ Rated flow & L/min & 3.00 L/min, normal temperature \\ Material & - & SUS316L, mirror surface \\ Channel processing & - & Photetching \\ Structural processing & - & Atomic diffusion bonding \\ Performance testing & - & Leakage: \(<\)10\({}^{-10}\) Pa\(\cdot\)m\({}^{3}\)/s; pressure \textgreater{} 6.9 MPa \\ \hline \end{tabular}
\end{table} 
Table 1: Sizes, materials, and working fluid of compact heat exchanger. Figure 1: The compact heat exchanger and schematic of convection heat transfer.