11/13/2015 1 ME 5337/7337 Notes-2005-003 Introduction to Computational Fluid Dynamics Lecture 3: System-level CFD analysis: Macroflow

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Introduction to Computational Fluid Dynamics

Lecture 3: System-level CFD analysis: Macroflow

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Remarks

All the slides presented in this lecture were provided by Kanchan Kelkar, Principal engineer, Innovative research

This material is copyrighted by Innovative Research, Inc.

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Outline

Overview of the Design Process and the Role of FNM Theory of FNM Demonstration of MacroFlow Validation of FNM Results Applications and Case studies

Copyright: Innovative Research

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Typical Challenges in the Design of Complex Flow Systems

Blower/Pump Sizing Flow Balancing Filter Degradation Bypass Effect Manifold Maldistribution Valve Selection Minimizing Pressure Loss Tube Sizing etc.


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Modeling Options for System Design

Hand Calculations (HC)

Tedious and very limited

Spreadsheets (SS)

System-specific, inflexible, and time intensive

Flow Network Modeling (FNM)

Simple, fast, and accurate

Computational Fluid Dynamics (CFD)

Time intensive for model definition, solution, and postprocessing

Suitable for component analysis, not suitable for system-level design


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Real System – Exhaust System of a Dump Truck Flow Network Modeling is the only feasible technique


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What is Flow Network Modeling?

Flow systems are like electrical circuits.

Just as a voltage drop drives a current, a pressure drop creates fluid flow.

The flow distribution through different flow paths depends upon their flow resistances.

A flow system can be represented as a network of flow resistances. This approach is called Flow Network Modeling (FNM).


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A Network for a Piping System


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FNM of Complex Flow Systems

A network model of the flow system is constructed by identifying flow paths through filters, screens, bends, tees, blowers/pumps, valves, orifices, etc.

The flow resistance relationships can be obtained from handbooks, vendor specs, in-house testing, or CFD analysis.

The flow rates, pressures, and temperatures throughout the system are calculated by solving mass, momentum, and energy equations.


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Advantages of the FNM Technique

FNM is simple, fast, and accurate

Simple because it is modular and object-oriented

Fast because it uses overall characteristics

Accurate because characteristics are empirically determined


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Limitations of FNM

The flow system must be described in terms of identifiable flow paths with definable resistance characteristics.

FNM provides gross (rather than detailed) predictions: FNM solution does not give local velocity vectors, flow separation, reattachment,

etc. Detailed temperature distributions, local heat fluxes, etc. are not calculated.

Accurate resistance correlations are needed for reliable prediction.


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Complementary Nature of FNM and CFD

FNM allows focused and efficient use of CFD Examine hundreds of design alternatives by FNM (Conceptual Design) and select a few

promising designs for CFD analysis (Detailed Design). Use FNM for an entire system and provide boundary conditions for the CFD analysis of

a subsystem.

Use of CFD at the component level for determining the flow resistances enhances the accuracy of the to the FNM technique.

Complementary use of System Analysis and CFD results in A comprehensive set of tools for complex flow systems. Shorter design cycle


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Time Required to Analyze a Typical System

Setup Time Run Time

FNM 1 Hour 10 Seconds

By using FNM, you save a substantial amount of design engineer’s time


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Thermal Design Process Conventional and Enhanced

Conventional

Test-Based

Enhanced

FNM-Based

HC Testing

FNM CFD Testing

FNM Testing


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Modified Bernoulli’s Equation

For constant density and no gravity head:

p1 + V12/2 =

p2 + V22/2 + Losses

The losses are due to viscous forces, flow separation, expansion/contraction, bends, etc.

Losses = K(V2/2) where K is the loss coefficient.

Thus, P = P1 - P2 = K(V2/2)

1 2


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Flow Resistance

P = P1 - P2 = K(V2/2)

Q = VA

P = K( Q2) / (2A2)Flow resistance:

P /Q = (K Q) / (2A2) (nonlinear) The values of K are available for screens, orifices, bends,

expansion/contraction, T-junctions, etc. For electronics cooling, the K values are needed for card arrays, heat

sinks, power supplies, etc.


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Flow Resistance

Flow resistance correlations for various flow

geometries are given in:

Idelchik, I.E., Handbook of Hydraulic Resistance, CRC Press, Florida, 1994. Blevins, R.D., Fluid Dynamics Handbook, Krieger Publishing Company,

1984.

Miller, D.S., Internal Flow Systems, Gulf Publishing Company, Texas, 1990.


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Flow Resistance of a DuctFriction factor:

f = p (D/L) / (V2/2)

Thus, K = f L/D

For duct flows, f is a well-established function of the Reynolds number Re and surface roughness.

Re = VD/, where = viscosity and

D = hydraulic diameter = (4)(area)/perimeter

D L

p1

p2


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Flow Through an Area Change

(The color shows total pressure)

For an abrupt expansion:K = (1 - A1/A2)2

For an abrupt contraction:K = 0.5 (1 - A2/A1)


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Flow Induced by Fans Whereas flow resistances cause a drop in pressure, a fan creates a rise

in pressure. The pressure rise p produced by a fan depends on the flow rate Q. This relationship is expressed by the fan performance curve.

p

Q

Qmax


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System Operating Point

What flow rate we get is determined by the intersection of the system resistance and fan performance curves.

If the resulting Q is not acceptable, you reduce the system resistance, choose a different fan, or use multiple fans in series or parallel.

p

Q

fan curve

system resistance

operatingpoint


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Overall Thermal Resistance

For heat transfer through a composite solid from the hot fluid at Ta1 to the cold fluid at Ta2

Overall Resistance =

1/h1A + (L/kA)A + (L/kA)B + (L/kA)C + 1/h2A

Heat flow: q = (Ta1 - Ta2)/ overall resistance

Ta1

Ta2

h1 h2

A B C

q


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ME 5337/7337Notes-2005-003Heat Transfer in FNM

Solve the flow equations to get the flow rates through individual components.

For a known heat load in a component: Calculate the exit fluid temperature as:

Tout = Tin + (q / mcp ) Use a suitable correlation to get the heat transfer coefficient h for the

calculated flow rate. Then calculate Twall from:

(Twall -Tout) / (Twall -Tin) = exp (- hA /mcp )

•

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ME 5337/7337Notes-2005-003Heat Transfer in FNM (continued)

For heat loss from the fluid within a component to the ambient fluid: Calculate the overall heat transfer coefficient U as: U = [(1/hinside) + (L/k)wall

+ (1/houtside)]-1

Obtain the exit fluid temperature from:

Tout - Tamb = (Tin - Tamb) exp (- UA / mcp ) Get the heat loss as:

q = mcp (Tout - Tin)

•

•


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FNM for System Design

FNM empowers the design engineer to visualize the flow behavior of the entire system to predict system-wide effects of a design change to identify performance-limiting components to quickly evaluate different physical layouts to explore many available design options to perform “what-if” studies to avoid costly design changes later in the cycle to meet tight design schedules


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MacroFlowTM

A Flow Network Modeling Tool for the Design of Flow Systems


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Capabilities of MacroFlow

An integrated and easy-to-use GUI for network construction, solution control, and display of results

A comprehensive component library for modeling complex flow systems

Customizable and expandable structure Robust and efficient direct solution technique Steady/unsteady, Incompressible/compressible flow and heat transfer Input and output in mixed units Comprehensive post-processing in terms of plots, tables, animation


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Ease of use Speed of execution Flexibility and user control Expandable and customizable structure Tool for enhanced productivity

MF

Demonstration of MacroFlow


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Flow through an “S” Manifold


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Flow through an “S” Manifold

0.06

0.08

0.1

0.12

1 2 3 4 5 6 7 8 9 10

Branch Number

Vol

. Flo

w R

ate

(CF

M)

Exp.

FNM

Comparison with Experimental DataCopyright: Innovative Research

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Flow through a “U” Manifold


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Flow through a “U” Manifold

0.08

0.09

0.1

0.11

0.12

1 2 3 4 5 6 7 8 9 10

Branch Number

Vol

. Flo

w R

ate

(CF

M)

Exp.

FNM

Comparison with Experimental DataCopyright: Innovative Research

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Exhaust System of a Dump Truck – Physical System


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Design Goals: Minimize the back pressure while recovering waste heat

Technical Approach: Large size and widely different length scales made CFD analysis of the entire

system impractical Use of MacroFlow at the system level and CFD for localized analysis of complex

regions

Exhaust System – The Design Problem


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Exhaust System – Flow Network Model


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Exhaust System – Design Cycle

MacroFlow-based analysis indicated regions of system with large pressure losses.

CFD analysis was used to optimize the local geometry (e.g. gradual expansion instead of a sudden expansion) and to determine the flow characteristics for use in MacroFlow.

The final design met the stipulated backpressure requirements. MacroFlow enabled scientific analysis during the design process and

resulted in an order-of-magnitude reduction in the associated costs.


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Liquid Cooling System for Automatic Test Equipment

Manifold System Liquid Cooling Module


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Design Goals: Achieve a uniform distribution of flow to each LCM

Design Iterations: Selected the tube sizes for the main and the side branches Determined the orifice sizes in the individual branches for flow balancing Sized the pump for the cooling system

ATE System – The Design Problem


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Liquid Cooling System – Characteristics of One LCM

Flow Characteristics A Single LCM


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Liquid Cooling - Flow Network Model for the Complete System


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ME 5337/7337Notes-2005-003Liquid Cooling System – Distribution of

Flow rates in One Branch of the Manifold


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Engineering Applications of MacroFlow

Electronics Cooling Telecommunications Computer Avionics Peripheral Equipment

Semiconductor Processing Gas and Liquid Distribution Systems

Automobile Intake and Exhaust Manifolds Engine Cooling Systems


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11/13/2015 1 ME 5337/7337 Notes-2005-003 Introduction to Computational Fluid Dynamics Lecture 3: System-level CFD analysis: Macroflow