An Assessment of Dynamic and Steady State Solutions of Two-Phase Flows in a Radiator Heat Exchanger

Radiators (Crossflow) heat exchangers (HEXs) find extensive application in various cooling systems, including in Computer Room Air Conditioning (CRAC) units in data centers for high performance computers. Because of the large thermal loads, effective thermal management systems are needed to expand the capabilities of IT equipment. The radiator also finds extensive application in the automotive and aviation industries. To increase the cooling performance of radiators, two-phase flows with low-boiling-point refrigerants such as R134a are employed. Fluids in two phases characteristically have high heat transfer coefficients, which makes them highly desirable in heat transfer processes. In many applications, heat exchange under equilibrium phase change conditions suffices; that is, without the need for additional sensible heat transfer processes such as those associated with subcooling and superheating. In this case, the narrow temperature range over which a twophase flow normally operates leads to increased theoretical maximum thermodynamic efficiency of the process of heat exchange. Physics-based mathematical models derived from the conservation laws for mass, momentum, and energy form the basis of the study herein presented. Two-phase (air-R134a) calculations comprising of both steady state and dynamics are investigated and compared to assess the relevance of a steady state model. Other than the presence of the time-derivative terms, the equations solved for the two models are essentially the same. A satisfactory performance of the steady state models has been observed relative to the dynamic models.