# Experimental Study of a Novel Prototype of Dual-Cycle Two-Phase Cooling System for a Data Cabinet
## 3 Results and Discussion
### The Effects of the Flow Rate of Each Loop on the Cooling System
#### 3.2.3 The Effects of PBTCS Flow Rate  
The temperature of the equipment room is maintained at 27 \({}^{\circ}\)C. The effect of flow on temperature and pressure drop is studied by adjusting the pump speed of PBTCS. As depicted in Figure 8, the experimental results indicate that an increase in flow rate leads to a steady rise in the pressure drop across the PFHE \(\Delta\)P\({}_{301-302}\), the vapor line pressure  
Figure 7: Effects of external loop flow rate on temperature and pressure difference: (**a**) temperature of main measuring point; (**b**) internal and external loop pressure difference.  
drop \(\Delta\)P\({}_{302-304}\), and the PHE condensation pressure drop \(\Delta\)P\({}_{304-305}\), although the rate of increase diminishes with higher flow rates. Each increment in flow rate results in a rapid increase in the saturation temperature at the PFHE inlet, followed by a gradual stabilization, while the temperature at the PFHE outlet decreases noticeably with each increase in flow rate due to the increased resistance in various parts of the system. In fact, the temperature difference caused by the resistance of the PFHE constitutes the majority, exceeding 50% of the total fluid temperature difference. Assuming no overheating within the PFHE, augmenting the flow rate elevates the evaporation temperature while lowering the condensation temperature. The peak temperature difference within the fluid loop, designated as \(\Delta\)T\({}_{301-305}\), rises from 2.3 \({}^{\circ}\)C at a flow rate of 1.5 L/min to 6.5 \({}^{\circ}\)C at 3.1 L/min. The pump, responsible for powering the entire loop, exerts a minimal impact on the convective heat transfer coefficient. As the system nears the \"ideal cycle\", further increasing the flow rate may adversely affect the system [16].  
The \"ideal cycle\" typically assumes the absence of superheating, subcooling, or pressure drop [16]. However, in practical applications, these conditions are often unattainable. Experiments conducted in this study on various pump-driven loops reveal that compared to two-phase natural circulation, the primary function of the pump is not to enhance heat transfer but to provide mechanical power for flow distribution. As shown in Figure 9, the control strategy of the pump significantly impacts the performance of the cooling system. Before reaching the control point, insufficient evaporator flow leads to extensive superheating, with the heat transfer rate gradually increasing as the flow rate rises. Once the pump's rotational speed reaches the control point depicted in the figure, further increases in speed result in excessive flow, thereby augmenting the pressure drop and heat transfer temperature difference, which in turn diminishes the system's heat transfer rate. For chip cooling, influenced by chip power fluctuations and flow distribution, the pump's rotational speed typically exceeds the control point to stabilize chip temperature. Provided that chip temperature remains unaffected, the cooling system's performance can be enhanced by reducing the pump's rotational speed. For backplane cooling, which targets air cooling, it is recommended to maximize the dryness at the backplane outlet, even permitting slight superheating at the exit, to bring the system closer to the ideal cycle.  
Figure 8: Effects of PBTCS flow rate on temperature and pressure difference: (**a**) temperature of main measuring point; (**b**) pressure difference.