Log In   |   Sign up

New User Registration

Article / Abstract Submission
Register here
Register
Press Release Submission
Register here
Register
coolingZONE Supplier
Register here
Register

Existing User


            Forgot your password
December 2005
library  >  Case Studies  >  Innovative Research

Flow Distribution in an AS-400 Entry System


objective

 

during the conceptual design stage of a computer system package, distribution of airflow in competing system layouts needs to be assessed in a rapid and reliable manner. use of computational fluid dynamics at this stage is too detailed and time consuming while back of the envelope calculations are unreliable. flow network modeling provides a fast and accurate method for determining the airflow performance of competing designs.

 

at the as-400 division of ibm corporation, flow network modeling has been the method of choice since the early 90s [1]. more recently, ibm has adopted macroflow for performing network modeling [1]. the purpose of this study is to demonstrate the use of macroflow for quick and accurate prediction of airflow and bulk temperature distribution in an as-400 entry system.


physical system

 

the present study illustrates the application of macroflow for calculating the flow distribution in an as-400 entry system [1]. the physical layout of the system is shown in fig. 1. the components in the system are enclosed in a box. the airflow through the box is created by fans situated in the plenum region in the central part of the system. two disk unit cages, each consisting of five disk drives are located in the front-left portion of the system.

 

the cd-rom and tape drive are situated in the front-right region. the flow enters the system through the front cover. a part of the flow goes through the disk drives, after passing through an electromagnetic compatibility (emc) screen. the remaining part goes through the screen located above the cd-rom and tape drive units. these streams then meet in the central plenum.

 

the fans redistribute the flow into streams that cool the pci card array, the processor card, and the power supply. note that all these components have screens attached downstream at the exit for emc and flow control. a well-designed cooling system allows control over the flow distribution so that effective cooling of heat dissipating components is ensured.

figure 1 - layout for an as-400 entry system.

 


network representation

 

the network representation of the as-400 is shown in fig. 2. the network is constructed by tracing the path the air follows through the system and representing it within the macroflow framework by using the appropriate components and links from the available library.

 

the important features of the model are as follows:

  • the front cover of the system is a slotted plate and is represented by a screened inlet/exhaust.

  • the resistance in the flow path through the screen above the cd-rom/tape drive and the passage after it is represented by the cdtap-screen followed by the cdtap-loss resistance. the resistance correlation for the secondary loss resistance is available from experimental measurements. the screen has a cross-section of 100mm x 50mm with a 60% open area.

  • each of the disk unit cages is represented as an array of five parallel flow resistances (a multiplier of five) corresponding to the five disk drives within it. the perforated plate preceding each of the disk unit cages has 5 mm round holes and a 60% open area.

  • the central-plenum designates the central open region in which the incoming flow streams meet.

  • the three fans, pci-fan, pro-fan, and ps-fan redistribute the incoming flow so that appropriate cooling is provided for the heat dissipating units. in particular, the pci-fan and the pro-fan force the flow into the secondary plenum (pci-pro-plenum) upstream of the pci and the processor cards. the fans for the pci and the processor are identical while the fan for the power supply is more powerful. the fan characteristics are represented in macroflow by specifying the fan curves as piecewise-linear functions.

  • the pci card array is represented by three flow resistances parallel to each other (a multiplier of three) followed by the emc/flow control screen.

  • the processor is enclosed in a box and its resistance is represented by the processor resistance. the screen emc-pro follows the processor card.

  • the flow path through the power supply and the corresponding downstream screen is represented by the ps resistance and the emc-ps screen component.

  • the flow streams from the pci, the processor, and the power supply discharge into the atmosphere.

flow impedance characteristics


systems such as the as-400 are built from individual components or subsystems provided by vendors or that have been used previously in similar machines. therefore, their flow characteristics (pressure drop - flow rate relationship) are already known. in the present as-400 system, the variation of the pressure drop with the flow rate is available for all subsystems from empirical correlations. these characteristics are listed in table 1.


table 1- flow characteristics of individual components component

component

loss coefficient b in

front cover 6.5e-7
secondary loss for cd-rom and the tape drive 1.5e-6
disk drive 1.5e-4
pci cards 3.0e-6
processor 1.1e-6
power supply 2.0e-5

 

 

 

figure 2 - flow network representation of the as-400 entry system.


results


the direction of flow through the system is shown by the arrows on the network as shown in fig. 2. comparison of the volumetric flow rate through various parts of the system is shown in the bar chart in fig. 3. note that the disk units are represented as a set of five disks parallel to each other. the flow rate plotted in fig. 3 for the disk unit 1 represents the flow through one disk within the unit.

 

similarly, the flow rate reported for the pci array corresponds to the flow rate through a single passage in the card array. note that it is this flow rate, rather than the total flow through the entire assembly, that provides the necessary cooling. the flow is lowest in each pci card array passage while it is highest for the power supply. the system is well balanced in that it provides the air flow in the pci, processor, and power supply components according to their heat flux requirements. figure 4 shows the bulk temperature of the air streams exiting the heat dissipating components.

 

it is of interest to examine the operation of the fans and the pressure losses in various parts of the system. figure 5 shows the head or the pressure rise across the three fans. the fan for the power supply is more powerful than the pci and processor fans. therefore, it creates the extra head necessary to overcome the higher resistance of the power supply and still force more flow through it. figures 6 and 7 show the pressure losses in the front and back portions of the system respectively.

 

note that the pressure losses through the various screens and the front cover constitute a significant portion of the overall pressure losses. among the heat dissipating components, the power supply exhibits the highest pressure loss. the predicted flow distribution among the pci, processor, and power supply components was within 15% of the experimentally measured values.


figure 3. comparison of the volumetric flow rate through
various parts of the system.



figure 4. bulk temperature of the air streams.

 


figure 5. the head provided by the individual fans.



figure 6. pressure losses in the front portion of the system.



figure 7. pressure losses in the back portion of the system.


concluding remarks


in the present study, the easy-to-use graphical environment and the generalized network modeling capability of macroflow enabled rapid construction of the network model and fast and accurate prediction of the airflow distribution in an actual as-400 entry system. there is a real need, in the early stages of a design cycle, for determining the airflow performance of plausible system layouts, the present study demonstrates that macroflow fulfills this need.

 

macroflow is useful for comparing airflow distributions in alternate system layouts, for sizing fans, and for incorporation of flow balancing elements (screens, orifices etc.). this allows intelligent narrowing of design choices and avoids the costly changes that may otherwise be necessary during later phases of the design cycle.

 

macroflow can also be used to perform contingency studies, such as determining the effect of fan failure, rise in ambient temperature, and presence of flow obstructions at the inlet and exit, on system performance. thus, macroflow constitutes an easy to use, fast, and accurate tool for solving design problems in practical electronic cooling systems.


references

1. kang s. s., ibm corporation, rochester, minnesota; 'modeling of the air flow distribution in computer system packages using macroflow," personal communication, december 1997.

 


 

 

Choose category and click GO to search for thermal solutions

 
 

Subscribe to Qpedia

a subscription to qpedia monthly thermal magazine from the media partner advanced thermal solutions, inc. (ats)  will give you the most comprehensive and up-to-date source of information about the thermal management of electronics

subscribe

Submit Article

if you have a technical article, and would like it to be published on coolingzone
please send your article in word format to [email protected] or upload it here

Subscribe to coolingZONE

Submit Press Release

if you have a press release and would like it to be published on coolingzone please upload your pr  here

Member Login

Supplier's Directory

Search coolingZONE's Supplier Directory
GO
become a coolingzone supplier

list your company in the coolingzone supplier directory

suppliers log in

Media Partner, Qpedia

qpedia_158_120






Heat Transfer Calculators