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MacroFlow-based
Flow Network Modeling (FNM) technique is now widely used for system-level thermal design. Prospective as well as existing users of MacroFlow have questions about the FNM technique,
capabilities of MacroFlow, and its application to specific problems. In what follows, responses to commonly asked questions are provided.
How does MacroFlow apply to electronics cooling?
How is MacroFlow used in the design process?
How is MacroFlow different from CFD tools?
Can MacroFlow be used in conjunction with CFD tools?
How is MacroFlow different from spreadsheets?
Does MacroFlow have a library of components for building flow networks?
How long does a typical model construction and solution take?
How does one represent an air-cooled system as a flow network?
What is the accuracy of the results obtained from MacroFlow?
Can MacroFlow be used for both air- and liquid-cooled systems?
Does MacroFlow handle different fluids in the physical systems?
Can users input their own correlations for flow and thermal resistances into MacroFlow?
Is it possible to create separate databases for fan or pump characteristics?
Why is volumetric flow rate not constant in different components along a single
flow path in air-cooled systems?
Is MacroFlow applicable for passively cooled systems?
How many components can be present in a flow network?
Can MacroFlow predict component temperatures?
What are the common applications of MacroFlow?
Is MacroFlow a One-Dimensional tool? How can real three-dimensional systems be modeled using MacroFlow?
Does MacroFlow account for flow inertia?
How long does it take for a new user to learn MacroFlow? Is any training available for MacroFlow?
Is MacroFlow useful for board-level analysis?
What kind of post-processing options are available in MacroFlow?
What are the licensing options for MacroFlow?
What technical support does Innovative Research provide to users of MacroFlow?
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How does MacroFlow apply to electronics cooling?
MacroFlow is ideally suited for system-level thermal design. In MacroFlow, the
flow system is represented as a network of components (card passages, filters, screens, fans/blowers, heat sinks, power supplies, bends, orifices, tee-junctions
etc.) and flow paths (ducts and tubes). The behavior of each component is represented using an overall correlation describing the variation of pressure drop and
heat transfer coefficient with flow rate. The resulting solution of the systemwide flow and temperature distributions using the technique of Flow Network Modeling (FNM) is very rapid (in a few minutes) and accurate.
How is MacroFlow used in the design process?
MacroFlow-based analysis allows examination of all possible design options and scientific selection of the feasible options for later investigation. Further, sizing of
fans, heat sinks, and screens can performed very quickly and reliably. Finally, analyses of "what if" scenarios such as fan failure and rise in the ambient
temperature can also be carried out. MacroFlow is very easy to use and occasional users can come back to it without any need for "relearning". Thus, MacroFlow is a
powerful tool for shortening the design cycle and improving the productivity of thermal design process. Benefits from the use of MacroFlow for thermal design of
electronic equipment are evident from the testimonials provided by our customers.
How is MacroFlow different from CFD tools?
CFD analysis involves creating a mesh over the entire electronic system for the
solution of the Navier-Stokes and energy transport equations for a detailed prediction of the velocity, pressure, and temperature fields. MacroFlow, on the other
hand, uses the technique of Flow Network Modeling (FNM) based on the overall
behavior of different flow components. Whereas a CFD solution for an electronic system may involve solving for velocity, pressure, and temperature at 500,000 or
more grid points, the MacroFlow solution is usually limited to calculating 50 to 100 quantities. Therefore, model construction, solution, and examination of results takes only a few minutes while the corresponding CFD analysis will take many hours.
Given the speed and accuracy of MacroFlow, it is ideally suited for system-level thermal design at the Conceptual Design stage. It allows examination of all the
possible design options accurately and quickly so that risk in the design process is significantly reduced.
Can MacroFlow be used in conjunction with CFD tools?
CFD analysis can then be used in a complementary manner with MacroFlow in the following manner:
- MacroFlow allows CFD analysis to be focused on the few feasible designs and
to the critical areas of the design such as flow and temperature distribution in a card array.
- Boundary conditions for CFD analysis of the subsystem can be determined from the MacroFlow-based system-level analysis.
- CFD analysis can be used to determine overall flow and heat transfer characteristics of non-standard components (such as card arrays with a large
number of different components mounted on the boards) for use in the system-level analysis with MacroFlow.
Complementary use of MacroFlow and CFD tools significantly reduces the overall
time required for analysis and improves the productivity of the thermal design process. Such use for a practical problem is illustrated in the papers by Steinbrecher
et al. (1999) and Kowalski and Radmehr (2000). Indeed, many of the MacroFlow users utilize the Enhanced Design Cycle to shorten the thermal design process and improve the productivity.
How is MacroFlow different from spreadsheets?
Spreadsheets are often used by thermal engineers for system-level thermal design. A spreadsheet is written for a specific configuration of the components in the system
and a network-specific procedure for the calculation of the flow rates and the pressure drops is used. Spreadheets are therefore inflexible; a separate spreadsheet
needs to be written for each system configuration. For a complex system that involves multiple branches, it may not be possible to construct an efficient
network-specific solution procedure within the spreadsheet. Further, component correlations need to be programmed within a spreadsheet. Finally, a spreadsheet
can be conveniently used by its author but is very inconvenient to transfer to other members of the team.
In contrast, MacroFlow incorporates a comprehensive component library, a generalized solution procedure, and a powerful postprocessing capability. Thus,
network representations for many different designs can be analyzed in a very rapid manner. With the ease of use and the generality of MacroFlow, its use for thermal
design of electronics cooling systems is orders of magnitude more productive than spreadsheets. Indeed, many users of MacroFlow have recognized its power and
abandoned any use of spreadsheets for such analysis.
Does MacroFlow have a library of components for building flow networks?
MacroFlow includes an extensive library of components for constructing flow networks of complex electronic systems. It consists of standard flow components
(such as bend, orifice, tee-, wye- and cross-junction, area change, screen, inlet with and without screen, and plenum), flow paths (duct, tube, and zero-resistance link), and electronics cooling components (heat sink, power supply, filter, and heat
exchanger). The flow and heat transfer characteristics of these components are determined internally in MacroFlow based on the geometrical details specified in
component dialogs. The correlations are taken from the following handbooks:
- I.E. Idelchik, Handbook of Hydraulic Resistance, CRC Press, 1994.
- R.D. Blevins, Applied Fluid Dynamics Handbook, Krieger Publishing Company, 1992.
- D.S. Miller, Internal Flow Systems, Gulf Publishing Company.
With a comprehensive component set and accurate correlations, MacroFlow enables
reliable prediction of the thermal performance of a wide variety of electronics cooling systems.
How long does a typical model construction and solution take?
MacroFlow has an intuitive Graphical User Interface (GUI) for easy construction of flow networks and postprocessing of results. Network representation of a flow
system is carried out by selecting the components in the component palette, placing them in the work area, and connecting them in the desired manner using a
point-and-click interface. The network is characterized by specifying the component properties in the corresponding component dialogs. The solution of the equations is
almost instantaneous. Comprehensive postprocessing capabilities are provided for viewing results in the form of bar charts, tables, on-screen display, and animation.
Thus, network model for a practical electronics systems (e.g. a telecommunications cabinet) requires one hour to construct, less than a minute to
solve, and five minutes to examine the results. Further, the modifications to the system design can be incorporated very quickly in the network model and hundreds
of design options can be analyzed within a day.
How does one represent an air-cooled system as a flow network?
Network representation of an air-cooled electronics cooling system is constructed
by identifying the various paths that the air streams follow through the system and representing these paths using the components in the MacroFlow component library.
Typically, there is a one-to-one correspondence between the physical layout of the system and its flow network representation. The network of a physical system can
be constructed at different levels of detail and complexity. For example, consider the problem of flow distribution from a clearance below a card rack into the individual
card passages. A simple representation will assume that the pressure variation in the open region is small so that it can be treated as a plenum. A more involved
representation will use the Tee-junction component to account for the maldistribution in the distant card passages that results due to the inertia of the main flow stream.
What is the accuracy of the results obtained from MacroFlow?
The Flow Network Modeling (FNM) approach of MacroFlow utilizes empirical
correlations for characterizing the overall flow and thermal behavior of the individual components. For each component in the MacroFlow library, the most accurate
empirical correlation available from the various handbooks has been chosen. Component characteristics can also be provided in user-defined form based on
vendor data or in-house testing. MacroFlow analyzes the interaction among the components to predict the flow and thermal behavior of the entire system. Since the
characteristics of the individual components are accurate (within 10%), the performance of the entire system is also typically predicted within 10 to 15% of the observed behavior. This is illustrated in the case studies involving comparison of
measurements or CFD analyses and MacroFlow-based analyses (Minichiello, 2000; Kowalski and Radmehr, 2000).
Can MacroFlow be used for both air- and liquid-cooled systems?
Since the basic approach is general, it is applicable for liquid as well as air cooled
systems. Practical applications of MacroFlow for the design of air- and liquid-cooled systems are discussed in detail in the section on Case Studies.
Does MacroFlow handle different fluids in the physical systems?
MacroFlow contains a library of fluid properties for modeling a variety of flow
systems. Using this database, properties (density, specific heat, viscosity, etc.) of the selected fluid are calculated as functions of pressure and temperature
throughout the system. Further, the user can specify property variations using various functional forms (Piecewise Linear, Polynomial, Ideal Gas Law for density
etc.) for fluids that are not available in the library. Finally, the user-defined fluid properties can be stored in the property database for ongoing use.
Can users input their own correlations for flow and thermal resistances into MacroFlow?
MacroFlow allows user-defined correlations for describing flow and thermal
characteristics of components. The data can be specified in a variety of functional forms that include Piecewise Linear, Polynomial, and Power Law variations. Such data
are obtained from in-house testing, CFD analysis, or vendor data for the component of interest. Card Arrays and Power Supplies are examples of components for which
in-house testing or CFD analysis is commonly undertaken. Fans, Heat Sinks, Filters are examples of components for which vendor data is often available. The user-defined characteristics can be stored in a database. This is very useful since
the same components are used in different systems and availability of the characteristics in a database is very convenient for rapid analysis of a variety of systems.
Is it possible to create separate databases for fan or pump characteristics?
MacroFlow already contains a library of fans from Rotron in the standard database
of fan characteristics. Further, the User-Defined option allows the user to enter the fan and pump curve, if it is not included in the standard library. The newly defined
fan curve can also be added to the database for repetitive use in subsequent modeling.
Why is volumetric flow rate not constant in different components along a single flow path in air-cooled systems?
In the network analysis approach, the properties of the fluid (typically air) in any component or flow path are dependent on the local pressure and temperature and
therefore vary throughout the network. Further, along a single flow path, the mass flow rate remains constant. Since the density changes due to changes in the
pressure and temperature of the air, the volumetric flow rates are different upstream and downstream of a component.
Is MacroFlow applicable for passively cooled systems?
MacroFlow can be used for a natural convection cooled system as long as the flow system can be accurately described as a flow network. For example, in vented
cabinet containing a heat dissipating card rack, the flow network consists of the flow paths through the screens the inlet and exit of the cabinet and through the card
passages. The flow is driven by the chimney effect. However, the cooling of a sealed hot box due to natural convection on the outer surfaces cannot be adequately
modeled using MacroFlow. This is because the air flowing over the various surfaces of the heated box cannot be described adequately within a flow network framework.
In this situation, a detailed CFD-based analysis is required for accurate prediction of the cooling effect.
How many components can be present in a flow network?
There is no restriction on the number of components that can be included in a flow
network representation of a cooling system. For most practical systems, flow network representations contain less than a hundred components for which the analysis is very rapid (less than a minute). However, the time required for the
solution of equations in MacroFlow increases with an increase in the number of components. For example, for a liquid-cooled system with 900 components,
MacroFlow required a solution time of 2 hours on a Pentium 300 PC.
Typically, a big flow system contains the same set of components repeated many
times throughout the system. In such a case, it is possible to use a two-level approach. First a separate network is constructed for the repeating module and this
network model is run for different flow rates to determine the overall flow and heat transfer characteristics. Then, in the network model for the complete system, the
characteristics determined from the submodel are used to represent each occurrence of the repeating module. This keeps the model of the complete system very compact without any loss of accuracy. The resulting approach is very efficient and has been
illustrated in the technical papers on the analysis of liquid cooling system for automatic test equipment (Verma, 2000) and for a power supply system (Ahmed, 2000).
Can MacroFlow predict component temperatures?
MacroFlow uses overall characteristics for determining the flow and temperature
distribution within a cooling system. Thus, it calculates average quantities (flow rates and temperatures) and cannot predict the local temperature variation over a
surface. Thus, for a card passage, it can determine the heat transfer coefficient from the average velocity within the card passage and hence calculate the average
surface temperature of the card for a given heat dissipation. However, it cannot predict the local temperature of the heat-dissipating component since it would
require a detailed calculation of the temperature field in the air and conduction in the board. Click here for more information the benefits and limitations of MacroFlow.
What are the common applications of MacroFlow?
MacroFlow has been used for thermal design of a variety of electronic systems. These include computers (workstations and servers), telecommunications
equipment, automatic test equipment, peripherals (overhead and screen projectors), and power supplies. It has been used for liquid- and air-cooled systems.
Further, for air-cooled systems, it has been used for closed and open systems. The Case Studies section includes detailed discussions of various applications of MacroFlow in the design of practical systems.
Is MacroFlow a One-Dimensional tool? How can real three-dimensional systems be modeled using MacroFlow?
MacroFlow should not be characterized as a one-, two- or three-dimensional tool.
It uses the flow Network Modeling (FNM) approach in which individual components are represented using overall characteristics. The components can be connected to
form a one-, two-, or three-dimensional network. Most flow systems modeled are three-dimensional and MacroFlow calculates the three-dimensional motion of the fluid within it.
Does MacroFlow account for flow inertia?
The flow inertia affects the behavior of the flow system in two ways. For a steady flow, inertia of the flow imparts directionality to it. Thus, a stream of air flowing
perpendicular to the card passages will cause flow maldistribution within the card passages. This is because the flow inertia prevents the incoming stream from turning
easily. This effect can be properly modeled using the Tee-junction component.
In an unsteady flow, the inertia of the fluid results in additional changes to accelerate/decelerate the flow. This effect is accounted in MacroFlow by including
the unsteady term in the momentum equations during transient calculations.
How long does it take for a new user to learn MacroFlow? Is any training available for MacroFlow?
MacroFlow has an intuitive and easy to use Graphical User Interface (GUI) to
facilitate easy construction of flow network and comprehensive postprocessing of results. The networks are constructed using drag-and-drop point-and-click method.
Network construction of a flow system is very straightforward since there is a one-to-one correspondence bewteeen the flow system and the corresponding
network layout. Thus, MacroFlow is extremely easy to learn and use. Further, users can come back to using MacroFlow after a long period without the need for any relearning.
Typically, training is provided to a new user over the phone that normally lasts for one hour. Such training along with use of MacroFlow for one or two practical
problems is sufficient for using MacroFlow in an effective manner. We also provide on-site training and seminars for additional fees.
Is MacroFlow useful for board-level analysis?
Components in the MacroFlow library such as area change, heat sink, and ducts
can be used to construct a network representation of the flow in a card passage. Such analysis can predict the average velocity through the card passage from which
suitable heat transfer coefficient can be determined. However, MacroFlow cannot calculate the conduction heat transfer within the board. Such analysis is best
performed using board-level tools.
What kind of post-processing options are available in MacroFlow?
Postprocessing capability in MacroFlow is very comprehensive and allows viewing
the results in the form of bar charts, tables, on-screen display, and animation. Bar charts allow a visual comparison of the relative behavior of different components
(e.g flow rates or pressure drops in different components). Tabular output provides numerical values of all the calculated quantities in any set of components. On-screen
display allows printing the desired quantities for the chosen components on the network. This feature is very useful during parametric studies in which a particular
behavior is sought in certain parts of the system. Finally, the movement of the fluid in through the network can be animated using moving dots. The speed of the dots
correspond to the velocity while the color of dots can represent the pressure or the temperature, Thus, the flow animation provides an overview of the flow and thermal
behavior of the entire system.
What are the licensing options for MacroFlow?
MacroFlow is available for use on PCs with Windows 95/NT/98/2000 operating systems in the form of annual licenses. MaroFlow can be configured in a node-locked
(usable on a specific PC) or a floating (installed over a network of PCs) license. Please contact us for further information on the licensing options and the licensing
fees.
What technical support does Innovative Research provide to users of MacroFlow?
Under the annual license of MacroFlow, the user is entitled to unlimited technical
support. The technical support is aimed at facilitating the use of MacroFlow for effective solution of the thermal problem at hand. During the technical support, we
help users to construct and refine network representations of the problems of interest, identify causes of any problems such as unrealistic behavior, and finally provide suggestions regarding the solution of the engineering problem at hand.
Innovative Research is proud of the quality of the technical support that we provide.
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