ASHRAE NA-04-4-1-2004 Distribution of Cooling Airflow in a Raised-Floor Data Center《在活动地板数据中心的冷却气流的分配》.pdf

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1、NA-04-4-1 Distribution of Cooling Airflow in a Raised-Floor Data Center Subas V. Patankar, Pb.D. Kailash C. Karki, Ph.D. ABSTRACT For reliable operation of computer equipment in a data center; adequate cooling air must besupplied to the equipment. The distribution of cooling air through the perforat

2、ed tiles in a raised-floor data center is governed by the fluid mechanics of the underfloor space. The pressure variation in that space is shown to be the cause of nonuniform distribution of airflow. The various factors that influence the distribution are discussed. The efect offloor height and tile

3、 open area is illus- trated through the results for a simple configuration. The use of variable tile open area and other issues are also discussed. Calculated distributions of airflow rates are used to explain some of the observed behavior in data centers on the basis of fluid mechanics principles.

4、INTRODUCTION Raised-floor data centers are commonly used to house computer servers, telecommunications equipment, and data storage systems. The equipment dissipates a significant amount of heat and must be maintained at acceptable temper- atures for reliable operation. It is not sufficient that the

5、data center as a whole receives the required amount of cooling air. Each piece of equipment must be given the amount of cooling air that corresponds to its heat load. Therefore, special atten- tion must be paid to the distribution of cooling air in the data cent er . This paper uses the fluid mechan

6、ics of the underfloor space to calculate the distribution of the airflow rate through the perforated tiles. The effect of different factors such as the floor height and the open area of the perforated tiles is discussed. THE RAISED-FLOOR CONCEPT Raised-floor data centers use the underfloor plenum be

7、low a raised floor to supply cooling air to the computer equipment. As shown in Figure 1, the computer room air conditioner (CRAC) units push cold air into the plenum, from where it is introduced into the computer room via perforated floor tiles, tile cutouts, and other openings. The raised-floor de

8、sign offers considerable flexibility in placing the computer equipment above the raised floor. The underfloor plenum serves as the distribution chamber for the cooling air. Without the need for any ducting, cooling air can be delivered to any location simply by replacing a solid ti oor tile by a per

9、forated tile. A common arrangement for the perforated tiles and the computer equipment is the so-called “hot aisle-cold aisle” layout, which is shown in the plan view of Figure 1. Perforated tiles are placed in a region called the cold aisle. On each side of the cold aisle, computer racks are placed

10、 with their intake sides facing the cold aisle. A hot aisle is the region between the back ends of two rows ofracks. The cooling air delivered by the perforated tiles is drawn into the intake side of the racks. This air heats up inside the racks and is exhausted from the back of the racks into the h

11、ot aisle. From the hot aisle, the heated air returns to the CRAC units. REQUIREMENTS FOR AIRFLOW DISTRIBUTION A necessary condition for good thermal management is to supply the required airflow through the perforated tiie(s) located near the inlet of each computer server. The heat load can vary sign

12、ificantly across the computer room, and it changes with the addition or reconfiguration of hardware. For all computer servers to operate reliably, the data center design Suhas Patankar is the president and Kailash Karki is a principal engineer at Innovative Research, Inc., Plymouth, Mim. o2004 ASHAA

13、E. 629 CRAC Serverpack Perforated Tile AI I 150 4 Figure 1 A schematic of a raised-Joor data center: must ensure that the cooling air distributes properly, that is, the distribution of airflow rates through perforated tiles meets the cooling air needs of the equipment on the raised floor. When adequ

14、ate airflow is not supplied through the perfo- rated tiles, the internal fans in the server racks tend to draw air from the ceiling space. Since most of this air originates in the hot aisle, its temperature is high. Thus, the cooling of the upper parts of the server racks is seriously compromised. T

15、his behavior is schematically shown in Figure 2. Although the picture is a simplified representation of what really happens, it does capture the main physical phenomenon. Thus, the key to satisfactory cooling in a data center is to deliver the required amount of cooling airflow at the inlet of each

16、server. If the temperature rise of the air flowing through the server is to be limited to 20“F, the airflow requirement can be calculated from Required airflow in CFM = 154 x (the server heat load in kW). This formula is appropriate at sea level. For higher alti- tudes, the required airflow should b

17、e multiplied by the ratio (atmospheric pressure at sea level) / (atmospheric pressure at the local altitude). ROLE OF THE FLOW FIELD UNDER THE RAISED FLOOR Interestingly, the distribution of the cooling airflow through the perforated tiles is governed by the fluid mechanics of the space below the ra

18、ised floor. It is not the large, visible, above-floor space that controls this flow distribution. It is the air movement in the tiny underfloor space that decides how much air will emerge from each perforated tile. To CRAC Unit E:._;+O CFM A 150 CFM, 55OF 150 CFM, 55OF Figure 2 Insuficient cooling a

19、ir-ow. At first sight, it may appear that, once the plenum is pres- surized (by the inflow from the CRAC units), each perforated tile will deliver the same amount of airflow (at least when the perforated tiles are identical in construction). Actually, there is a significant variation in the flow rat

20、es from different perfo- rated tiles. There are many factors that are responsible for the variation. A major factor is the fact that different perforated tiles are located at different distances from the CRAC unit. Further, the pattern of the airflow distribution is somewhat counterintuitive. One ma

21、y expect more flow near the CRAC unit and less away from it. In reality, there is very little flow near the CRAC and very large flow through the perforated tiles located far away. As a result, the computer equipment placed near the CRAC does not get much cooling air. PRESSURE VARIATIONS IN THE UNDER

22、FLOOR SPACE The flow rate through a perforated tile depends on the pressure drop across the tile, that is, the difference between the plenum pressure just below the tile and the room pressure above the raised floor. Pressure variations within the computer room are generally small compared to the pre

23、ssure drop across the perforated tiles. Thus, relative to the plenum, the pressure just above the perforated tiles can be assumed to be uniform. The flow rates through the perforated tiles, therefore, depend directly on the plenum pressure just below the tile. The nonunifonnity in the airflow distri

24、bution is caused by the hori- zontal pressure variations under the raised floor. THE BASIC CAUSE OF FLOW MALDISTRIBUTION The main reason for nonuniform distribution of airflow through the perforated tiles (and its counterintuitive nature) can be understood from the simple example shown in Figure 3.

25、Here, the flow in the vicinity of one CRAC unit is shown. The CRAC flow enters the plenum in a vertically downward direction, turns 90 degrees, and then proceeds horizontally. As the flow moves under the perforated tiles, some air exits from 630 ASHRAE Transactions: Symposia CRAC L _+ -., Velocity d

26、ecreases, pressure increases Figure 3 The basic cause offlow maldistribution. the plenum into the computer room. The horizontal air veloc- ity in the vicinity of the CRAC is very high since it has to carry the entire flow delivered by the CRAC. As air leaks out of the perforated tiles, successively

27、less and less flow moves in the horizontal direction. Thus, the horizontal velocity decreases from left to right. According to laws of fluid mechanics, a velocity decrease is accompanied by a pressure increase. (A simple form of this behavior is expressed by the well-known Bernoulli equation.) Thus,

28、 the static pressure in the plenum increases from left to right. Now, it is easy to see why the perforated tiles near the CRAC give smaller flow than those far away. Under some conditions, the nonuniformity of airflow distribution is so severe that the flow through the perforated tiles near the CRAC

29、 is not simply small but even negative. That is, the static pressure in the plenum in the region close to the CRAC is actually less than the room pressure. The high veloc- ity stream in the plenum entrains the room air through the perforated tiles located close to the CRAC. Obviously, the computer e

30、quipment placed in this area will not be properly cooled. FACTORS AFFECTING THE AIRFLOW DISTRIBUTION Whereas the main cause of flow nonuniformity is explained by the simple example in Figure 3, a number of factors influence the distribution of airflow through perforated tiles. They include the locat

31、ions of the CRACs and the corre- sponding spreading of the underfloor flow to various perfo- rated tile locations; collision or merging of the air streams coming from different CRACs; and the flow disturbance caused by underfloor blockages such as pipes and cable trays. Geometrical factors, such as

32、the height of the raised floor and the amount of open area of the perforated tiles, also affect the flow distribution. . A quantitative evaluation of all these factors in a real data center requires a detailed numerical calculation of the three- dimensional flow field in the underfloor space. Karki

33、et al. (2003) present a computational fluid dynamics (CFD) model ofthe underfloor space and compare the results with measure- 10 $5 -100 il“fil Tite Number Figure 4 Effect of raised-floor height on airflow rates. ments in a real-life data center. The CFD technique used is based on the methodology de

34、scribed in Patankar (1980). In this paper, instead of presenting results for complex layouts, simplified scenarios are used to provide a qualitative understanding of the flow parameters and to establish some general trends. This is described in the following sections. EFFECT OF THE RAISED-FLOOR HEIG

35、HT As seen from Figure 3, the variation of static pressure in the plenum results from the variation of the horizontal veloc- ity. It, therefore, follows that, if the magnitude of the maxi- mum velocity is small, the corresponding pressure variations will be mild. The magnitude of the velocity depend

36、s on the height of the raised floor (which decides the area available for the flow supplied by the CRAC). For the same CRAC flow rate, the magnitude of the velocities in a 24-inch (0.61 m) raised floor will be only half as much as that in a 12-inch (0.3 m) raised floor. The result is that a larger h

37、eight will lead to a more uniform pressure distribution. The pressure distribution in the plenum governs the distribution of airflow through the perforated tiles. Thus, larger floor heights lead to a more uniform airflow distribution. This effect is shown in Figure 4 for a particular case. The CFM s

38、upplied by each tile is plotted for a row of 15 perforated tiles in front of a CRAC. Each curve corresponds to a different height of the raised floor. Whereas the flow distribution is quite nonuniform for small heights such as 6 or 12 inches (O. 15 or 0.3 m), it becomes progressively more uniform as

39、 larger heights are used. For the small heights, the nonuniform distri- bution includes regions of negative flow, where air is drawn from the computer room into the plenum. An additional observation is relevant here. What is truly significant is the actual flow area available for the air. If a 12- i

40、nch (0.61 m) raised floor space is filled with cables and other obstructions, it may behave like an 8-inch or 6-inch (0.2 m or ASHRAE Transactions: Symposia 63 1 -100 “:11111111 io 15 Tile Number Figure 5 Effect of tile open area on airflow rates. O. 15 m) floor leading to more severe nonuniform dis

41、tribution of airflow. EFFECT OF THE OPEN AREA OF THE PERFORATED TILES The flow resistance of the perforated tiles depends on their open area. For a given airflow rate, a tile with only 10% open area will require a much greater pressure drop than a tile with 25% open area. As described above, the non

42、uniformity in the airflow distribution is caused by the horizontal pressure variations in the plenum. These variations should be judged in comparison with the pressure drop across the perforated tiles. A pressure variation that is significant for a 25% open tile may not be very significant for a 10%

43、 open tile (in comparison with the much larger pressure drop across it.) It, therefore, follows that restrictive tiles (such as 10% open) give a nearly uniform airflow distribution, while open tiles (25%) create nonunifor- mities. The open tiles readily respond to the pressure varia- tions in the pl

44、enum. The restrictive tiles (due to the large pressure drop across them) are insensitive to the pressure vari- ations. This behavior is shown in Figure 5 for a typical case. Again, the CFM supplied by each tile is plotted for a row of 15 perforated tiles in front of a CRAC. Each curve corresponds to

45、 a particular value of the fractional open area of the perforated tiles. The flow distribution is significantly nonuniform for more open tiles such as 50% or 70%. They lead to large regions of negative flow near the CRAC. The flow uniformity improves as more restrictive tiles are used. Some addition

46、al comments are relevant here. First, there is a common misconception that having more open tiles increases the airflow rate. Obviously, for the same staticpres- sure in theplenum, more open tiles will produce more airflow than more restrictive tiles. However, the static pressure in the 80 CI I Vari

47、able Open Area - Uniform Open Area t“ E 40 U 0 20 o! i= o 12 3 4 5 6 7 8 Q101112131415 Tile Number O 40 111111111 II 12 3 4 5 6 7 8 9 1011 12131415 Tile Number Figure 6 Flow rates for a variable tile open area arrangement. plenum is not a given constant; it is a result of the flow resis- tance of th

48、e tiles. The airflow rate is controlled by the amount of flow the CRAC blower is able to supply. For the blower, the controlling resistance is internal to the CRAC unit. For most situations, the additional flow resistance offered by the perfo- rated tiles is rather insignificant. By lowering this re

49、sistance, it is not possible to increase the flow. In general, the open area of the perforated tiles will influence the flow uniformity (as seen in Figure 5), not the amount of flow. Second, the flow resistance of more restrictive tiles (such as 10% open) is comparable to the flow resistance of other flow openings such as cable cutouts and cracks. When restric- tive tiles are used, the air will have an increased tendency to leak through these other openings. This may be undesirable in some circumstances. Third, when very restrictive tiles (for example, 5% open) are used, their flo