Largely due to cooling power consumption needed for their thermal management, data centers consume over 2% of the electricity produced in the United States. A fundamental understanding of data center energy management is both environmentally and economically important. This paper develops a model of data center energy utilization for thermal management based on thermo-fluid first principles. The model is applied to a single rack system that can be used as an experimental platform for its validation. Models are developed for conventional air-cooling and hybrid cooling, where microprocessors are liquid cooled and other components are air cooled. The air-cooling model considers a single chiller with a variable speed compressor and an intermediate air-to-water heat exchanger. The hybrid model considers cold plates rather than air-cooled heat sinks on the microprocessors and includes an additional chiller. Compiled results quantify the power consumption of the two cooling schemes over ranges of key variables, such as ambient temperature. For example, the air-cooling model shows a 10% reduction in energy consumption for thermal management when the chiller setpoint is raised by 5°C, and the water flow rate pumped through the air-to-liquid heat exchanger is increased to accommodate this change. Hybrid model results show that energy consumption decreases as much as 40% over the parametric ranges investigated relative to air-cooling. The models can be utilized for future system design and trade-off studies. These models do not include variations in air flow rates or power consumption by the server as part of the sensitivity study.