Transient experiments on an air-to-refrigerant pumped crossflow evaporator under air temperature perturbations for data center applications

Abstract

Crossflow heat exchangers are used widely in different applications, such as air conditioning systems, data centers, and the automobile industry. In data centers, crossflow heat exchangers are present in applications known as close-coupled hybrid cooling systems. These technologies are installed in the proximity of data center racks. As rack power density has increased over the years, these systems have transitioned from using chilled water to two-phase refrigerant cooling to dissipate the higher level of generated heat. Existing literature provides abundant data on crossflow heat exchangers in single-phase flow. However, data from experiments in which one of the working fluids is a refrigerant undergoing phase change are either unavailable or lack detail. The current work presents detailed data from both single- and two-phase flow experiments in an air-to-refrigerant crossflow heat exchanger with transient changes in the air temperature. A novel technique to generate air temperature perturbations is described, which resembles scenarios in which computing demand changes over time. Experiments were performed utilizing refrigerant (R134a) as a working fluid to analyze systems with and without subcooling control. The crossflow heat exchanger exhibits different behavior under subcooled control during the boiling process in a pumped refrigerant system. In situations where the inlet pressure is not controlled, the level of subcooling can fluctuate by up to 3 °C. To evaluate the heat exchanger’s response, both the inlet and outlet temperatures, as well as the pressure, are measured. Actively controlling the subcooling demonstrates pressure variations in the dynamic behavior of the heat exchanger, in addition to influencing the sensible heat.