Tianyi Gao

Cross Flow Heat Exchanger Modeling of Transient Temperature Input Conditions

The effectiveness of heat exchangers strongly influences the thermal performance of data center cooling systems. Rear door heat exchangers, in-row and overhead coolers, and fully contained water cooled cabinets are some examples of liquid and hybrid cooling systems used in data centers. Modeling the dynamic behavior of heat exchangers is important for the design of control strategies to improve energy efficiency. In this paper, an existing 2-D model of the transient temperature response of an unmixed-unmixed cross flow heat exchanger, of the type that is widely used in data center cooling equipment, is solved numerically. A detailed analysis of the transient response to a step, ramp, or exponential change in the hot fluid inlet temperature is conducted. The heat capacity rate ratio (E), thermal resistance (R), capacitance ratio (V), and number of transfer units are varied over a wide range to determine their influence on the heat exchanger dynamic thermal performance under different transient input conditions. In addition, the thermal response to the magnitude and time period of the transient input functions are evaluated. The modeling results are used to analyze specific data center cooling scenarios, and provide a means for predicting the transient behavior of heat exchangers used in data center cooling equipment.

Dynamic Analysis of Cross Flow Heat Exchangers in Data Centers Using Transient Effectiveness Method

Heat exchangers are important facilities used in data center cooling systems. The effectiveness of heat exchangers strongly influences the thermal performance of 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, which supplies chilled water to the aforementioned heat exchangers. Computer room air handler 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, to improve the design of control strategies, and energy efficiency. The transient response of a 2-D unmixed-unmixed cross flow heat exchanger, with a finite wall capacitance, is investigated. The transient effectiveness concept is used to build the mathematical models that will be used to solve and analyze the transient problems involving cross flow heat exchangers. Several transient cases are investigated numerically for step, ramp, and exponential variations in the inlet temperature of the minimum capacity rate fluid, as well as step and ramp changes in the flow rate of hot and cold fluids. The transient effectiveness characteristics are tested in parametric studies. The results obtained show that transient effectiveness is a useful method of analyzing and predicting the dynamic performance of cross flow heat exchangers.

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.

Dynamic Analysis of Hybrid Cooling Data Centers Subjects to the Failure of CRAC Units

Thermal management of high power data centers poses challenges due to the high operational cost which is made worse due to the many inefficiencies that arise in them. Additional challenges arise due to the dynamic behaviors that occur during normal operation, and also during emergencies such as power outages or failure of some or all of the cooling equipment. Water and hybrid air plus water cooled data centers are an alternate cooling solution combining liquid cooling systems, such as rear door heat exchangers located within the racks themselves, in addition to the traditional raised floor cold aisle air cooling system. Such a solution may be used when some of the equipment in a data center is upgraded to higher end and higher power equipment which may not be manageable with the existing air cooling system. For a data center with a hybrid cooling system, the cold air supply and the cold water supply should increase in case of an emergency, such as a CRAC (Computer Room Air Conditioner) units’ failure. In this paper, a detailed computational study is conducted to investigate the dynamic response of the impact of a CRAC failure on both water side and air side in a representative hybrid cooling room. The room studied is an air cooled data center using the common cold aisle approach, with rear door heat exchangers installed on all of the racks. CRAC failure is investigated in a hybrid cooling room. The variation and fluctuation in an average rack inlet temperature, and inlet temperatures at different detail locations are presented in plots, showing the dynamic performance of a hybrid cooling data center subjected to the different CRAC failure scenarios. Different response time studies are also presented in this paper.Copyright

Steady State and Transient Experimentally Validated Analysis of Hybrid Data Centers

Data centers consume a considerable amount of energy which is estimated to be about 2% of the total electrical energy consumed in the U.S. in the year 2010, and this number continues to increase every year. Thermal management is becoming increasingly important in the effort to improve the energy efficiency and reliability of data centers. The goal is to keep the information technologies (IT) equipment temperature within the allowable range in high power density data centers while reducing the energy used for cooling. In this regard, liquid and hybrid air/water cooling systems are alternatives to traditional air cooling. In particular, these options offer advantages for localized cooling higher power racks which may not be manageable using the room level air cooling system without requiring significantly more energy. In this paper, a hybrid cooling system in data centers is investigated. In addition to traditional raised floor, cold aisle-hot aisle configuration, a liquid–air heat exchanger attached to the back of racks is considered. First of all, the paper presents a review of literature of the study of this heat exchanger strategy in the thermal management of a data center. The discussion focus on rear door heat exchanger (RDHx) performance, both the steady state and transient impact are analyzed. The studies show that under some circumstances, this hybrid approach could be a viable alternative to meet the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommended inlet air temperatures, while at the same time reducing the overall energy consumption in high density data centers. The hybrid design approach can also significantly improve the dynamic performance during rack power increases or computer room air conditioner (CRAC) unit failure. And then, additional parametric steady state and dynamic analyses, are presented in detail for the different scenarios.

Experimentally Verified Transient Models of Data Center Crossflow Heat Exchangers

Heat exchangers are key components that are commonly used in data center cooling systems. Rear door heat exchangers, in-row coolers, overhead coolers and fully contained cabinets are some examples of liquid and hybrid cooling systems used in data centers. A liquid to liquid heat exchanger is one of the main components of the Coolant Distribution Unit (CDU), which supplies chilled water to the heat exchangers mentioned above. Computer Room Air Conditioner (CRAC) units also consist of liquid to air cross flow heat exchangers. Optimizing the energy use and the reliability of IT equipment in data centers requires Computational Fluid Dynamics (CFD) tools that can accurately model data centers for both the steady state and dynamic operations. Typically, data centers operate in dynamic conditions due to workload allocations that change both spatially and temporally. Additional dynamic situations may also arise due to failures in the thermal management and electrical distribution systems. In the computational simulation, individual component models, such as transient heat exchanger models, are therefore needed. It is also important to develop simple, yet accurate, compact models for components, such as heat exchangers, to reduce the computational time without decreasing simulation accuracy.In this study, a method for modeling compact transient heat exchangers using CFD code is presented. The method describes an approach for installing thermal dynamic heat exchanger models in CFD codes. The transient effectiveness concept and model are used in the development of the methodology. Heat exchanger CFD compact models are developed and tested by comparing them with full thermal dynamic models, and also with experimental measurements. The transient responses of the CFD model are presented for step and ramp change in flow rates of the hot and cold fluids, as well as step, ramp, and exponential variation in the inlet temperature. Finally, some practical dynamic scenarios involving IBM buffer liquid to liquid heat exchanger, rear door heat exchanger, and CRAC unit, are parametrically modeled to test the developed methodology. It is shown that the compact heat exchanger model can be used to successfully predict dynamic scenarios in typical data centers.Copyright

A dynamic model of failure scenarios of the dry cooler in a liquid cooled chiller-less data center

Data centers take up a significant portion of the national energy consumption in the U.S., accounting for approximately 2% of the total electricity used in 2010. Around the world that percentage is growing at a rapid rate. Within data centers, cooling systems constitute a large portion of the energy consumed, accounting for roughly 25-35% of total expenditures. Many new data center cooling technologies have been developed to improve cooling energy efficiency. A data center cooling facility proposed by IBM was constructed to reduce cooling energy use to less than 5% of the total Information Technology (IT) energy use, which was accomplished through a combination of warm water cooling servers and liquid-side economization. The heat load of IT equipment is rejected out to the ambient air without using a refrigeration/chiller plant or a Computer Room Air Handlers unit (CRAH), which consume a large amount of energy. This work develops simulation tools for the dry cooler, which is an air-to-liquid cross flow heat exchanger used in this cooling technology. Steady state test data is incorporated into the models, and is then used to validate them. The dynamic prediction accuracy of the simulation tool is also compared with real time measurement results of the dry cooler. Results of the simulation and testing are then used to obtain a more complete understanding of the dry cooler performance data and analyze the performance of the dry cooler in the cooling infrastructure. Failure analyses of the dry cooler are also performed in this work. One failure category of the dry cooler is parametrically modeled. The dynamic effects of failures of the dry cooler are reported and analyzed. The effects of the thermal mass of liquid and ambient air as well as the effects of natural convection and thermal, hydraulic characteristics of the external loop on the dynamic behavior of the dry cooler in this failure scenario are presented.

Data center crossflow heat exchanger study under different transient temperature boundary conditions

Heat exchangers are primary components of HVAC systems. The effectiveness of the heat exchangers strongly influences the thermal performance of cooling systems. Rear door heat exchangers, in-row and overhead coolers, and fully contained cabinets, are some examples of liquid and hybrid cooling systems used in data centers. Characterizing the dynamic behavior of heat exchangers is important for thermal management of data centers, especially for the design of control strategies to improve energy efficiency. In this study, a model based on energy conservation of the transient temperature response of an unmixed-unmixed crossflow heat exchanger, of the type that is widely used in data center cooling equipment, is solved numerically. Detailed analysis of the transient temperature boundary conditions such as a step, ramp or exponential change to the inlet temperature of the hot fluid are conducted. Two governing parameters, heat capacity rate ratio (E) and number of transfer units (NTU), are varied in a wide range to determine their influence on the transient performance of heat exchangers under different transient input conditions. Also, the effects of the magnitude and time period of the transient input conditions are evaluated. Some specific data center cooling scenarios are analyzed based on the results from the heat exchanger dynamic performance study. The results obtained provide a means for predicting the transient behavior of heat exchangers used in data center cooling equipment.

Experimental characterization of the rear door fans and heat exchanger of a fully-enclosed, hybrid-cooled server cabinet

A water-cooled, fully-enclosed server cabinet is one type of hybrid cooling technology used in data centers to cool high power IT equipment. These types of cabinets employ a water/air heat exchanger. External chilled water is provided to the heat exchanger and circulating air inside the cabinet is supplied to the server intakes. One advantage of this type of hybrid system is that the heat removed by the water-side can be used for other purposes, which is a potential means for increasing the overall data center energy efficiency. A specific commercial hybrid server cabinet design has been characterized experimentally in this study. The cabinet utilizes a V-shaped, unmixed-unmixed liquid-to-air cross flow heat exchanger. Three robust rear door mounted fans produce the air circulation in the cabinet. They are capable of providing an air flow rate of up to 1.321m3/s (2800 CFM) depending on the air temperature inside the cabinet. In the experiments, the air velocity is measured at different positions at the inlet and outlet of the heat exchanger using an array of velocity sensors. The contribution of each fan to the airflow distribution at the inlet of the heat exchanger is characterized using the measured air velocity data. For the water-side, the flow rate and temperature of the chilled water supply is measured using a vortex flow meter. From the measured air and water flow rates and temperatures, the effectiveness of the heat exchanger is calculated. The experimental results are presented in terms of effectiveness curves for different air and water operating flow rates and the high performance V-shaped heat exchanger is characterized in detail using these results. The heat exchanger performance data provides a better understanding of the design and operation of this type of closed cabinet, which can be used to potentially improve the operating efficiency.

Transient Effectiveness Methods for the Dynamic Characterization of Heat Exchangers

This chapter introduces transient effectiveness methods for dynamic characterization of heat exchangers. The chapter provides a detailed description and review of the transient effectiveness methodology. In this chapter, all the transient effectiveness–related knowledge/works are summarized. The goal of this chapter is to provide a thorough understanding of the transient effectiveness for the reader and to provide guidance for utilizing this methodology in related heat exchanger transient characterization studies. Basically, there are three important applications for transient effectiveness methodology: (1) characterization of heat exchanger dynamic behaviors; (2) characterization of the transient response of closed-coupled cooling/heating systems with multiple heat exchanger units; and (3) development of compact transient heat exchanger models. This innovative modeling method can be used to assist in the development of physics-based predictive, capabilities, performance metrics, and design guidelines, which are important for the design and operation of highly reliable and energy efficient mechanical systems using heat exchangers.