Veerendra Mulay

Effect of the location and the properties of thermostatic expansion valve sensor bulb on the stability of a refrigeration system

The combination of increased power dissipation and increased packaging density has led to substantial increases in chip and module heat flux in high-end computers. The challenge has been to limit the rise in chip temperature. In the past, virtually all commercial computers were designed to operate at temperatures above the ambient. However, researchers have identified the advantages of operating electronics at low temperatures. The primary purpose of low-temperature cooling using a vapor compression system are faster switching times of semiconductor devices, increased circuit speed due to lower electrical resistance of interconnecting materials, and a reduction in thermally induced failures of devices and components. The current research focuses on IBMs mainframe, which uses a conventional refrigeration system to maintain chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperatures. Although performance has been the key driver in the use of this technology, the second major reason for designing a system with low-temperature cooling is the improvement achieved in reliability to counteract detrimental effects, which rise as technology is pushed to the extremes. A mathematical model is developed to determine the time constant for an expansion valve sensor bulb. This time constant varies with variation in the thermophysical properties of the sensor element; that is, bulb size and bulb liquid. An experimental bench is built to study the effect of variation of evaporator outlet superheat on system performance. The heat load is varied from no load to full load (I KW) to find out the system response at various loads. Experimental investigation is also done to see how the changes in thermophysical properties of the liquid in the sensor bulb of the expansion valve affect the overall system performance. Different types of thermostatic expansion valves are tested to investigate that bulb size, bulb constant, and bulb location have significant effects on the behavior of the system

Effect of the thermostatic expansion valve characteristics on the stability of a refrigeration system-Part I

The combination of increased power dissipation and increased packaging density has led to substantial increases in chip and module heat flux in high-end computers. The challenge has been to limit the rise in chip temperature. In the past virtually all-commercial computers were designed to operate at temperatures above the ambient. However researchers have identified the advantages of operating electronics at low temperatures. The current research focuses on IBMs S/390 mainframe, which uses a conventional refrigeration system to maintain chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperatures. Attention will be to investigate the characteristics of the thermostatic expansion valve, specifically, the effect of variation of evaporator outlet superheat on the flow through the TXV at varying evaporator temperature, and the effect of sudden changes in evaporator heat load and condenser pressure variation on the temperature oscillations at the evaporator. The paper also discusses the effect of changes in the thermostatic bulb location and bulb time constant on the hunting phenomena at the evaporator.

Computational study of hybrid cooling solution for thermal management of data centers

The power trend for server systems continues to grow thereby making thermal management of data centers a very challenging task. Although various configurations exist, the raised floor plenum with Computer Room Air Conditioners (CRACs) providing cold air is a popular operating strategy. In prior work, numerous data center layouts employing raised floor plenum and the impact of design parameters such as plenum depth, ceiling height, cold isle location, tile openings and others on thermal performance of data center were presented. The air cooling of data center however, may not address the situation where more energy is expended in cooling infrastructure than the thermal load of data center. Revised power trend projections by ASHRAE TC 9.9 predict heat loads as high as 5000W per square feet of compute servers’ equipment footprint by year 2010. These trend charts also indicate that heat load per product footprint has doubled for storage servers during 2000–2004. For the same period, heat load per product footprint for compute servers has tripled. Amongst the systems that are currently available and being shipped, many racks exceed 20kW. Such high heat loads have raised concerns over air cooling limits of data centers similar to that of microprocessors. A hybrid cooling strategy that incorporates liquid cooling along with air cooling can be very efficient in such situations. The impact of such an operating strategy on thermal management of data center is discussed in this paper. A representative data center is modeled using commercially available CFD code. The change in rack temperature gradients, recirculation cells and CRAC demand due to use of hybrid cooling is presented in a detailed parametric study. It is shown that the hybrid cooling strategy improves the cooling of data center which may enable full population of rack and better management of system infrastructure.Copyright

Thermal design considerations of air-cooled high powered telecommunication cabinets

Telecommunication cabinets house numerous electronic components which dissipate heat to varying degrees. The thermal management of these components is of utmost importance in the design of these cabinets. CFD allows designers to try out and compare various cabinet configurations and enable an optimal design thus reducing unnecessary construction of cabinet prototypes and elaborate experimental tests, thereby resulting in cost savings and reduction in the lead time. This paper deals with design and thermal analysis of a CommScope outdoor integrated telecommunication enclosure, named RBA84-3036. This cabinet is air-cooled by a series of DC axial door mounted fans. Various design and cooling configurations will have to be considered and verified to meet the tight thermal requirements within the enclosure.

Effect of the Thermostatic Expansion Valve Characteristics on the Stability of a Refrigeration System

The combination of increased power dissipation and increased packaging density has led to substantial increases in chip and module heat flux in high-end computers. The challenge has been to limit the rise in chip temperature. In the past virtually all-commercial computers were designed to operate at temperatures above the ambient. However researchers have identified the advantages of operating electronics at low temperatures. The primary purpose of low temperature cooling using vapor compression system are faster switching times of semiconductor devices, increased circuit speed due to lower electrical resistance of interconnecting materials, and a reduction in thermally induced failures of devices and components. Achievable performance improvements range from 1 to 3% for every 10°C lower transistor temperature, depending on the doping characteristics of the chip. The current research focuses on IBM’s mainframe, which uses a conventional refrigeration system to maintain chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperatures. Although performance has been the key driver in the use of this technology, the second major reason for designing a system with low temperature cooling is the improvement achieved in reliability to counteract detrimental effects, which rise as technology is pushed to the extremes. A mathematical model is developed to determine the time constant for expansion valve sensor blub. This time constant varies with variation in thermo-physical properties of sensor element that is bulb size and blub liquid. An experimental bench is built to study the effect of variation of evaporator outlet superheat on system performance. The heat load is varied from no load to full load (1KW) to find out the system response at various loads. Experimental investigation is also done to see how the changes in thermo-physical properties of the liquid in sensor bulb of expansion valve affect the overall system performance. Different types of thermostatic expansion valves are tested to investigate that bulb size; bulb constant and bulb location have significant effect on the behavior of the system. Thermal resistance between the bulb and evaporator return line can considerably affect the system stability and by increasing the thermal resistance, the stability can be further increased.Copyright

Improving cooling efficiency of servers by replacing smaller chassis enclosed fans with larger rack-mount fans

As a common practice in the data center industry, chassis fans are used to direct air flow independent from neighboring servers. In general, smaller fans are less efficient compared to geometrically similar larger fans. In this study, a novel approach is proposed whereby chassis enclosed fans are replaced with a smaller number of larger fans installed behind a stacked array of servers which share airflow. As a baseline study, a CPU dominated 1.5U Open Compute server with four 60mm fans installed within its chassis is characterized experimentally for its flow impedance, fan speed dependent flow rate, effect on die temperature and power consumption at various compute utilization levels. Larger fans with a square frame size of 80mm and 120mm are selected and individually characterized for their air moving capacity and power consumption. Primary emphasis is placed on the 80mm fan case, with discussion of the 120mm fans included. CFD is used to simulate a system of stacked servers serviced by larger fans to obtain its flow characteristics and operating points. The fan power consumption of the larger fans is determined experimentally at these operating points replicated in an air flow bench. Comparing with the base line experiments, replacing smaller fans with larger units results in a significant decrease in fan power consumption without conceding flow rate and static pressure requirements.

Thermal management of telecommunication cabinets using thermoelectric coolers

Telecommunication cabinets are standalone outdoor enclosures, which houses electronic components and switching devices. These electronics are powered by DC power and have backup batteries to support them in the event of power supply failure. The heat loads on these cabinets are dependent not only on the heat dissipated by the internal components but also by solar heating. Therefore the ambient temperatures around the cabinet, based on the location and time of day can vary anywhere from −40°C to +50°C. The life of a battery is dependent on the nature of the load applied, recharging conditions and most importantly ambient temperatures. Batteries supporting the cabinet electronics are either housed within the cabinet or in compartments attached to the cabinet. For long standing battery life, the temperature inside these battery compartments should be kept below 25°C [1]. Active cooling using air-conditioners are often used to achieve this, but air-conditioners are difficult to backup and are high in maintenance. A more convenient way to cool the battery compartments are to use Thermo-electric Coolers (TEC) as they are less bulky and quite. This paper discusses the validation of results of numerical modeling of a telecommunication cabinet, which uses TEC to cool its battery compartment, with experimental data for the corresponding real world model.Copyright

LIQUID COOLING FOR THERMAL MANAGEMENT OF DATA CENTERS

The power trend for Server systems continues to grow thereby making thermal management of Data centers a very challenging task. Although various configurations exist, the raised floor plenum with Computer Room Air Conditioners (CRACs) providing cold air is a popular operating strategy. The air cooling of data center however, may not address the situation where more energy is expended in cooling infrastructure than the thermal load of data center. Revised power trend projections by ASHRAE TC 9.9 predict heat load as high as 5000W per square feet of compute servers’ equipment footprint by year 2010. These trend charts also indicate that heat load per product footprint has doubled for storage servers during 2000–2004. For the same period, heat load per product footprint for compute servers has tripled. Amongst the systems that are currently available and being shipped, many racks exceed 20kW. Such high heat loads have raised concerns over limits of air cooling of data centers similar to air cooling of microprocessors. Thermal management of such dense data center clusters using liquid cooling is presented.Copyright

Parametric study of hybrid cooling solution for thermal management of data centers

The power trend for Server systems continues to grow thereby making thermal management of Data centers a very challenging task. Although various configurations exist, the raised floor plenum with Computer Room Air Conditioners (CRACs) providing cold air is a popular operating strategy. The air cooling of data center however, may not address the situation where more energy is expended in cooling infrastructure than the thermal load of data center. Revised power trend projections by ASHRAE TC 9.9 predict heat load as high as 5000W per square feet of compute servers’ equipment footprint by year 2010. These trend charts also indicate that heat load per product footprint has doubled for storage servers during 2000–2004. For the same period, heat load per product footprint for compute servers has tripled. Amongst the systems that are currently available and being shipped, many racks exceed 20kW. Such high heat loads have raised concerns over limits of air cooling of data centers similar to air cooling of microprocessors. A hybrid cooling strategy that incorporates liquid cooling along with air cooling can be very efficient in these situations. A parametric study of such solution is presented in this paper. A representative data center with 40 racks is modeled using commercially available CFD code. The variation in rack inlet temperature due to tile openings, underfloor plenum depths is reported.Copyright