Crossflow heat exchangers are important facilities that are widely used in HAVC systems, such as data center cooling systems. The dynamic performance of the heat exchanger may have a significant effect on the performance of cooling systems. Characterizing and modeling the dynamic behaviors of cross flow heat exchangers in practical working conditions is important for both thermal management and control system design. Multiple variable input scenarios are common occurrences to the heat exchangers under practical operating conditions. In this paper, an established thermal dynamic model is solved numerically to predict the transient response of an unmixed-unmixed cross-flow heat exchanger, of the type that is widely used in data center cooling equipment. Detailed analyses of the transient response due to the time dependent fluid inlet temperature and flow rate are conducted. Cases for two fluid mass flow rate variation combinations, hot fluid inlet temperature and hot and cold fluid mass flow rate variation combinations are modeled and analyzed. Some of the most meaningful transient conditions such as step and linear ramp functions are used for the flow rate variation type for the hot and cold fluids, and step, ramp and exponential functions are applied to the fluid temperature at the entrance. Some of the modeling scenarios are based on control strategy design. The transient response results are also used to analyze some actual situations which are commonly seen for the cross flow heat exchangers used in data centers. The study and corresponding analysis provide fundamental insights into a cross flow heat exchanger which can be used to improve cooling unit performance and aid in the design of thermal control systems. (C) 2014 Elsevier Ltd. All rights reserved.