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December 2005
library  >  Case Studies  >  Innovative Research

Flow in a Fan-Cooled Cabinet Containing a Card Array


objective

 

with increasing power density and complexity of the electronic devices, appropriate cooling of the heat dissipating components is critical for reliable operation. typically, thermal designers use either hand calculations, spreadsheets, or computational fluid dynamics (cfd) analysis to predict flow and temperature distributions in the cooling systems to provide design guidance. hand calculations are prone to error and their use is very limited.

 

spreadsheets are system-specific and are therefore inflexible for use in a generalized manner for system-level design. cfd analysis provides valuable information about the flow and temperature distribution throughout the system.

 

however, such analysis for an entire system can be time-intensive in terms of model definition, computation, and visualization of results. further, such detail is not necessary for system-level thermal design and conceptual design during the early part of the design cycle. an alternate technique for flow and thermal analysis is flow network modeling (fnm). it allows quick and accurate analysis of flow distribution in electronics systems.

 

a commonly encountered situation in electronics cabinets is the cooling flow through a card array. it involves a manifold flow distribution encountered in a variety of applications such as industrial vme-based microcomputers, telecom, and avionics applications. it is known that the flow distribution in a manifold situation is nonuniform and knowledge of the flow distribution is necessary for good thermal design. manifold distribution problems are often modeled using cfd analysis.

 

the purpose of this case study is to illustrate the use of the flow network technique (fnm) in macroflow for quick and accurate prediction of the flow distribution in a fan-cooled electronic cabinet containing a card array.


physical system


the electronic cabinet analyzed in the present study is shown in fig. 1. the physical layout of the cards is representative of card rack systems. the physical system consists of a box with a centrally located card array and a fan tray located at the bottom left face for creating the throughflow of air. the flow through the screened inlet is in a direction perpendicular to the passages between the cards. after flowing through the card rack, the air leaves through the top of the cabinet. for simplicity, the cards are assumed to be equally spaced.

 

the relevant geometrical dimensions are indicated in fig. 1 in centimeters. the fan is assumed to be rotron-majorac-mr2b3 [1]. this fan provides a maximum head of 0.8 inches of water and a maximum flow rate of 240 cfm. the screened inlet and exit have 50% open area. flow distribution has been calculated for air with atmospheric conditions of 1 atm and 27°c. it is well known that a manifold design gives rise to a very nonuniform flow distribution in the card array.

 

therefore, design modifications are often sought to improve the flow distribution and two variations of the physical system are considered in this study. in the first design (design i), the passage has a constant cross-section while the second design (design ii) includes a taper to the bottom wall (angle of 18°) to create a passage that has a decreasing cross section. the focus of the application is to illustrate the calculation of the flow distribution. therefore, calculation of the temperature distribution is not considered in this study.


figure 1 - layout of the electronic cabinet.


network representation and flow impedence characteristics


the network representation of the physical system, constructed using macroflow, is shown in fig. 2. the flow network directly corresponds to the physical layout of the system. the following points are noteworthy:


figure 2 - flow network representation of the electronic cabinet.


each card passage is represented by a duct of appropriate rectangular cross-section and flow length. the tee junctions at the bottom of each of the card passage account for the flow inertia in predicting the bifurcation of the flow stream in the bottom passage. these flow streams meet in a plenum above the card array (shown by the generic node below the exit) and exit through the screen. the topology of the network distribution is the same for both designs.

 

the only difference for the second design corresponds to the specification of a progressively decreasing cross-section of the duct segments between the tee junctions. the losses in the various ducts (bottom duct segments and card passages) are calculated using the standard correlations available for ducts, screens, and tee junctions.

 

it is important to discuss the use of tee junctions in the network representation. the flow in the bottom passage splits into two streams under each passage between the card arrays - a fraction that flows into the card passage and the remaining stream that continues forward. this splitting of the flow is dependent on the downstream flow resistances and, more importantly, on the inertia of the flow streams. the loss-factor correlations for tee-junctions reflect this dependence on flow inertia through a nonlinear dependence of the loss factors on the flow split and also the configuration of the flow streams in the tee.

 

thus, tee junctions are used at the bottom of each card passage to represent the gradual bleeding of the main flow in the bottom passage into the individual card passages. the loss factors for various components in macroflow have been adopted from idelchik [2] and blevins [3].


figure 3. volumetric flow rate through card passages for design i.


figure 4. volumetric flow rate through card passages for design ii.


results


the predicted flow directions are shown by arrows on the network in figure 2. the direction of the flow in various passages is the same in two designs. figures 3 and 4 show volumetric flow rate in the card passages for the two designs. for design i, majority of the flow goes through the last three passages because of the inertia of the inlet flow.

 

note that, for this situation, the static pressure at the bottom of the card passage increases continuously away from main inlet. when the bottom wall is tapered, the decrease in the flow rate (increase in the pressure) caused by the bleeding of the main flow into the successive card passages is compensated by the decrease in the area of the bottom channel. this keeps the velocity (and hence static pressure) at the bottom of each card passage approximately the same.

 

since the flow in a card passage is governed by the change in static pressure across it, the distribution of the flow in the card passages is relatively uniform. the maldistribution of flow in a manifold design and the corresponding improvement in the flow distribution are consistent with the analysis of simple manifolds presented by blevins [3].

 

note that, the solution of the conservation equations in the fnm analysis allows characterization of the system impedence. thus, the overall flow rate in the system is determined by the balance between the system impedence and the fan characteristics. it is of interest to note that the fnm analysis of the cabinet for design i required only one hour for model setup, thirty seconds for calculation, and ten minutes for examination of the results.

 

further, investigation of the modification of the base required an additional 15 minutes. thus, fnm analysis is extremely efficient. it is also accurate because the correlations used in the analysis are empirically determined. this example illustrates the simplicity, speed, and utility of fnm technique for system-level thermal design.


conclusions

 

this case study paper illustrates the use of macroflow for the analysis of flow distribution in an air-cooled electronics cabinet containing a card rack. the flow configuration corresponds to a manifold distribution which is commonly encountered in practical electronics systems. two designs have been studied - a base design and its modification that includes the tapering of the bottom wall. macroflow predicts the highly nonuniform flow distribution typically observed in manifold situations and the effect of a tapering wall in obtaining improved performance.

 

macroflow enables a very efficient analysis of the flow distribution in the two designs for rapid and scientific investigation of the effect of a design modification on the flow distribution. the fnm approach offers significant benefits for system-level and conceptual design of the electronics systems. these include rapid evaluation of various system layouts for intelligent narrowing of design choices, investigation of "what if" scenarios, identification of performance limiting components or parts of the system and ideas for design improvements, and focused use of computational fluid dynamics analysis during thermal design.

 

these benefits suggest the adoption of an enhanced design cycle - fnm for system-level and conceptual thermal design, cfd for detailed design, and prototype testing for design refinement to arrive at a final design. the use of fnm significantly shortens the design cycle, allows the designer to investigate wider design options, and thereby optimizes the design process through improved productivity and product quality.


references

1. comair rotron, product catalog - fans, blowers, accessories, 2675 customhouse court, san ysidro, ca 92173.

2. idelchik i.e., handbook of hydraulic resistance, crc press, florida, 1994.

3. blevins r.d., fluid dynamics handbook, krieger publishing company, 1992.


 

 

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