Cfd Simulations Comparing Hard Floor and Raised Floor Configurations in an Air Cooled Data Center

An increasing number of companies and organisations have started to outsource their data storage. Although the potential of future investments in data centers is prosperous, sustainability is an increasingly important factor. It is important to make sure that the server racks in data centers are sufficiently cooled whereas too much forced cooling leads to economical losses and a waste of energy. Computational Fluid Dynamics (CFD) is an excellent tool to analyze the flow field in data centers. This work aims to examine the performance of the cooling system in a data center using ANSYS CFX. A hard floor configuration is compared to a raised floor configuration. When a raised floor configuration is used, the cold air is supplied into an under-floor space and enters the room through perforated tiles in the floor, located in front of the server racks. The flow inside the main components and the under-floor space is not included in the simulations. Boundary conditions are applied to the sides where the flow goes out of or into the components. The cooling system is evaluated based on a combination of two different performance metrics. Results show that the performance of the cooling system is significantly improved when the hard floor configuration is replaced by a raised floor configuration. The flow field of the air differs in the two cases. It is considered to be improved when the raised floor configuration is used as a result of reduced hot air recirculation around the server racks. INTRODUCTION' The establishment of data centers is one of the most important growth factors in the IT business. One of the reasons for the growth is that an increasing number of companies and organisations have started to outsource their data storage. Although the potential of future investments in data centers is prosperous, sustainability is an increasingly important factor. In 2012, the total electricity used by data centers was about 1.4% of the total worldwide electricity consumption with an annual growth rate of 4.4% [1]. The main components in an air cooled data center are Computer Room Air Conditioner (CRAC) units and server racks. The server racks dissipate heat and need to be cooled in order to make sure that the electronics operate in the temperature range recommended by the manufacturer. Otherwise there is a risk of overheating, resulting in malfunction or shut down to prevent hardware damages. This interruption is costly for business and needs to be prevented. It is important to make sure that the data centers are sufficiently cooled whereas too much forced cooling leads to economical losses and a waste of energy. The CRAC units supply cold air into the data center. The cold air is supposed to enter through the front of the server racks and hot air will exit through the back. Depending on the distribution of CRAC units and server racks in the data center, there is a risk that the cold air does not necessarily reach all the server racks to the desired extent. Therefore, the cooling of data centers is crucially dependent on the flow field of the air. Computational Fluid Dynamics (CFD) is an excellent tool to provide detailed information about the temperature and flow field in a data center. CFD modeling can be used to analyze both existing configurations and proposed configurations before they are built [2]. It is important that the simulations are carried out with quality and trust. The cooling system in an air cooled data center might be based on a hard floor or raised floor configuration. When a raised floor configuration is used, the CRAC units supply cold air into an under-floor space and the cold air enters the room through perforated tiles in the floor [3]. The perforated tiles are removable and can be replaced by solid tiles which makes the configuration flexible. Strategies for the design in data centers are often based on hot and cold aisles where the server racks are placed into a serie of rows. Cold aisles are formed between the front sides of two rows of server racks and hot aisles are formed on the other sides. This design strategy has become the standard when raised floor configurations are used. Perforated tiles are placed in the cold aisles and solid tiles are placed in the hot aisles. The purpose of the design strategy is to prevent hot air exhausted by the back of a server rack to enter the front of another server rack. Hot air recirculation over and around the server racks should be avoided if possible [4]. A ceiling return strategy can be used in order to further prevent hot and cold air from mixing. The hot air is then guided back to the CRAC units through a void space with ceiling vents or ducts located above the hot aisles [5]. Another strategy that can be used for the same purpose is aisle containment where either the hot or cold aisles are isolated [6]. Recommended and maximum allowable air temperatures at the front of the server racks are specified by ASHRAE's thermal guidelines. The recommended temperature range is 18-27°C and the allowable temperature range is 15-32°C [7]. 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics