To mitigate power transmission losses between power plants and consumers, Medium-Voltage Direct Current (MVDC) systems, which transmit power as DC and convert it back to AC at the destination, are increasingly being developed. However, these MVDC conversion systems, which are rich in semiconductor devices, generate significantly more heat than conventional transformers. Consequently, the thermal management capabilities of traditional air-cooling methods are proving insufficient, necessitating the adoption of liquid cooling systems. This study aims to optimize the energy efficiency of MVDC liquid cooling systems. Finally, we demonstrated a optimized system that minimizes the pump's power consumption by dynamically adjusting the coolant flow rate to each thermal module based on the automatic control valve system.

Active electronically scanned array (AESA) multi-function radars (MFRs) comprise numerous transmit/receive modules (TRMs) whose maximum temperature and temperature uniformity must be tightly controlled. This study proposes a new liquid-cooling-plate flow-channel design for an X-band AESA MFR: a two-layer straight channel incorporating multiple fins irregularly spaced along the flow channel. The proposed design (Type-4) is compared with three baseline channel designs. At the same coolant flow rate, Type-4 reduces the TRM maximum temperature by 28.2 K and the maximum inter-module temperature difference by 19.7 K relative to Type-1. However, the pressure drop increases by 726% because of the added internal surfaces and fins which are flow obstructions. A comprehensive thermo-hydraulic comparison, including pumping power criteria, is conducted over multiple flow-rate conditions. Overall performance was highest for Type-4, followed by Type-2, Type-3, and Type-1. When designs achieve similar maximum temperature and temperature difference with various coolant flowrate condition, Type-2 requires 83.6% less pumping power than Type-1, and Type-4 requires 33.8% less pumping power than Type-2.

The residual stress analysis and fatigue test of a steel plate with a hole were performed in order to investigate the effects of the cold expansion on the fatigue life of the plate. The cold expansion method is a metal forming process by expanding a hole in order to induce compressive tangential residual stresses near the hole. In this research, a S45C steel plate of 3.2 mm thickness with a hole of 4.318 mm diameter was cold expanded by using a mandrel, resulting in a degree of expansion 1.47%. A significant amount of compressive tangential residual stress of 502 MPa at the hole surface was calculated using the finite element analysis and the compressive residual stress was more widely spreaded in the entry and exit planes than the mid plane in the plate thickness. The compressive tangential residual stresses showed a very beneficial influence on the fatigue life of the plate by increasing 1.2 to 2 times longer lives compared to the plate without cold expansion, depending on the applied load levels. Fatigue crack initiated and propagated at the hole surface of the mandrel entrance region, where relatively lower magnitude of compressive residual stress was found than the other region.