Transient effectiveness characteristics of cross flow heat exchangers in data center cooling systems

Heat exchangers are important facilities used in data center cooling systems. The effectiveness of the heat exchangers strongly influences the thermal performance of the cooling systems. Rear door heat exchangers, in-row, overhead coolers and fully contained cabinets are some examples of executing liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is an important component of the Coolant Distribution Unit (CDU), which supplies chilled water to the heat exchangers mentioned above. Computer Room Air Conditioner (CRAC) units are also typically liquid to air cross flow heat exchangers. It is important to characterize the dynamic behavior of these heat exchangers for the thermal management of data centers, in order to improve the design of control strategies and energy efficiency. In this paper, the transient response of a two dimensional unmixed-unmixed cross flow heat exchanger is investigated. The transient effectiveness concept is used to build the mathematical models which will be used to solve and analyze the transient problems of cross flow heat exchangers. Numerical solutions for solving the mathematical model and capturing the transient effectiveness characteristics are developed. The solution accuracy is tested by comparison with several published analytical and analog solutions. An experimental study of the effectiveness of an IBM rear door heat exchanger is used to develop the heat exchanger models and to verify the effectiveness of the numerical solution at the same time. Several transient cases are investigated numerically for step, ramp and exponential variations in the inlet temperature of the minimum capacity rate fluid. The transient effectiveness characteristics are tested in parametric studies. The results obtained show that transient effectiveness is a useful method in analyzing and predicting the dynamic performance of heat exchangers.