Flotherm 教程 Cooling TechniquesFans and Heat Sinks.ppt

1、Lecture 6A,Cooling Techniques: Fans and Heat Sinks,2/42,Reducing Temperatures,Problem : We have a component that is too hot We can reduce the component temperature by increasing the heat transfer away it. We can: Add a fan Add a heat sink,3/42,How do we add a fan?,Drag and drop from Library Manager,

2、Create one using the fan smart part,Library Manager To access the FLOTHERM and user libraries, which contain frequently used designs and attributes, use the Library Manager. The Library Manager is not displayed by default, to save space. The quickest way to display the Library Manager is to click or

3、 choose Edit/Library. Alternatively, double-click F7. The Library Manager appears to the right of the display with the New Object Palette located horizontally above it. Saving Objects and Attributes to the Library Geometry and attributes can be dragged from the project data tree and dropped on a lib

4、rary node to be saved in the library. Loading Objects and Attributes from the Library Geometry can be dragged from the Library Manager and dropped on a project assembly node into which the geometry is copied. Attributes can be dragged from the Library Manager and dropped onto the project data tree a

5、t a geometry node that allows that type of attribute to be attached, attaching the attribute to the object. Dropping attributes onto multiple selections is not allowed.,5/42,Fan SmartPart,Modeling choices: Axial Fan Single Tube-Axial Fan Rectangular Fan Fan Tray Upstream Fan Outflow from Ducting Rad

6、ial Blower Fan Libraries: Papst, Comair Rotron, ETRI, NMB Micronel, Delta, Sanyo Denki www.SmartParts3D.com is an additional resource.,6/42,Fan Options,The fan specifications can be changed using the Construction dialog,7/42,Fan Options,Modelling Levels,Recommended for most applications,More “refine

7、d” modeling choices - 3D, 8 facets and 3D 12 facets are not always more accurate. A careful examination of the physics reveals that in most cases, the increase in accuracy is negligible. There are, of course, exceptions to this rule. In some cases, when a critical component is located very close to

8、the fan plane, a better representation of the circular contour may be necessary. In general, the user is advised to use the 3D, 4 facets option as the modeling level of choice when using FLOTHERM. You can locate the fan anywhere in the solution domain, as internal fans or on the face of the overall

9、domain. Fans can be located either: a) on the domain boundary, blowing air into or extracting air from the domain, or b) totally within the domain, stimulating fluid motion by imparting momentum to the fluid. More complex arrangements can be built up (e.g. fans in parallel and fans in series) by the

10、 introduction of several fans. Note that fans in series must be separated by at least one grid cell. Fans can be put inside the domain when they are connected on one side with a cutout. Further modifications can be made using Blocks with Holes, like locating in a hole in a cuboid, or combining a min

11、imum of four axial fans and a collapsed cuboid backplate to create a centrifugal fan.,9/42,Fan Flow Specification,Fixed Volume,Linear Fan,Non-linear Fan Fan Curve either in library or obtained from manufacturer,FLOTHERM allows for three types of fan flow specifications: Fixed Volume Flow - For an in

12、take fan on the domain boundary, the fluid is drawn in from the external environment. By default the fluid brought in this way is at the ambient temperature applied to the sides of the overall system domain using the Ambient properties. By default the program calculates the axial momentum (i.e. velo

13、city) of the incoming fluid from the velocity normal to the plane of the fan - it is possible to allow for the effect of an inclined flow by setting the Supply direction angle. For exhaust fans (i.e. outlets) the situation is quite different - the fluid leaving the enclosure leaves it with the condi

14、tions and properties that the fluid has on the enclosure side of the fan. Thus the temperature of the fluid leaving the enclosure through an exhaust fan is the temperature calculated by the program adjacent to the exit. The fluid leaves the enclosure with whatever angle the program computes for the

15、flow towards the fan. Linear Fan - The Linear Fan relates the volume flow linearly to the fan static pressure. The intercepts of the straight line with the axis are denoted P 0 and v 0 , which respectively signify the fan static pressure that causes zero flow through the fan, and the volume flow rat

16、e through the fan when the fan static pressure is zero (which pertains for a totally free standing fan, i.e. not enclosed in any way). If you use the linear fan curve option, you must decide how to best fit the linear characteristic to the actual characteristic. There are two ways of doing this. The

17、 first is simply to set P 0 and v 0 to those of the actual characteristic. The second method is to fit the line to cover the range over which you want the fan to operate. The linear fan representation gives a good fit to the fan characteristic in the normal operating range but a poor fit elsewhere.

18、Non-linear fan The non-linear fan curve is used when simple modeling is not sufficient. It is possible to set up your own fan characteristic curve using the Fan Curve dialog to set the Volume Flow Rate and Static Flow at any number of points along the curve. Select Fan Curve. to display the fan curv

19、e dialog to set up the fan characteristics to be modeled.,11/42,Fan SmartPart: Non-linear Fans,Operating pressure shown on the fan curve Local minima ignored on fan curve .CSV import to create the fan curve points Or manually define the curve point by point.,Local Minimum,On-Line help excerpt,Why is

20、 the local minima ignored? Both numerically and physically, a fan cannot run at two different volume flow rates for one pressure point. FLOTHERM corrects the fan curve to ensure only one pressure versus volume flow rate per point. You should always check the fans operating point to make sure that th

21、e fan is not operating in the zone of the local minima. Where is the operating point for the fan? FLOTHERM determines the operating point on the fan characteristic line specified. The accuracy of the computed fan flow rate is determined by the accuracy with which the program calculates the pressure

22、head losses in the system. Therefore, for good results an accurate representation of the losses has to be present which will be achieved by a combination of judicious refinements of the grid and by correct settings of any loss factors for grilles, baffles, cabling etc. The computed operating point f

23、or each fan is given in the Tables application window, and also flags out-of-range operation.,13/42,Fan SmartPart: Swirl,Swirl Options Constant speed Flow dependent speed,What is the difference between the two swirl models in the fan construction menu? In the Constant Speed model, the swirl speed is

24、 imposed on the model (regardless of fan operating point). With the Flow Dependent Speed Model, the swirl speed is used in combination with the calculation of the swirl velocity (which does vary with fan operating point). The relationship used to determine the swirl component (Flow Dependent Speed m

25、odel) is based on empirical data gathered by Papst, should be valid for a wide range of tube-axial fans typical of electronics applications. With the Flow Dependent Speed Model, the swirl speed should be set to the rotational speed of the fan at zero impedance. (This value is usually provided on the

26、 fan data sheet from the manufacturer). Hint: If the design is sensitive to swirl, obtain the relevant swirl data from the fan manufacturer for the particular fan being considered.,15/42,Fan SmartPart: Model Options,Fan Failure Include the geometry of a failed fan Does not directly model the loss as

27、sociated with presence of fan blades Add fan blade losses by attaching a resistance attribute to the fan. Fan Power Include the heat dissipated by the fan,16/42,Fan SmartPart: Modeling Advice,Use 3D, 4 facets for most applications Include swirl when critical components are close to fans Include swir

28、l when fans are operating at high pressure Always check the fan operating point on the fan curve,17/42,Fan SmartPart: Tables Window,18/42,Fan Gridding,Use of Localized grid is important when using the more detailed modeling levels,19/42,Heat Sink Smart Part,Application: Create a Detailed Model of a

29、parallel or pin fin heat sink Create a Compact Model of a parallel or pin fin heat sink,20/42,Detailed Heat Sink Model,Set base dimensions Set fin type Choose modeling method,21/42,Detailed Heat Sink Model,The fin style can be uniform or full taper Number of grid cells between fins is controlled,Can

30、 I use the heat sink smart part to create a customized heat sink? A non-uniform heat sink model can be created by using the detail model option, and then decomposing the smart part and customizing the heat sink fins and base. How many grid cells do I need between the fins? In order to capture the pr

31、essure drop and heat transfer between the fins of a parallel fin heat sink, maintaining an adequate level of grid is extremely important. The pressure loss through a heat sink is made up of two components: skin friction loss, and exit/entry loss. The skin friction loss is induced by the fluid flowin

32、g over surfaces of the channels between the fins. The boundary layer that develops between the channels results in a frictional pressure drop that varies roughly in inverse proportion to the velocity through the fins gaps (as the flow there is typically laminar for parallel fin heat sinks used in el

33、ectronics cooling). The entry/exit loss is caused due to the flow streamlines contracting and expanding as the flow enters and leaves the heat sink, respectively. This loss is inertial in nature and varied roughly as the square of the velocity. To capture the skin friction losses a reasonable grid d

34、ensity must be maintained in the spanwise direction (see illustration above). 3 grid cells are recommended between a pair of fins to capture the thermal effects accurately. 4-5 cells are needed if an accurate pressure drop value is required,23/42,End Fins Can be the Same as the Internal Fins or Non-

35、Standard Can Always Decompose the Heat Sink and Customize FLO/MCAD can be used to create customized heat sinks as well,Detailed Heat Sink Model,24/42,Detailed Heat Sink Gridding,At least 3 grid cells between fins Use of Grid Inflation to capture contraction/expansion losses well Use of Localized Gri

36、d space to reduce solution time and increase stability,25/42,Compact Heat Sink Model,Volumetric resistance replaces fins Volumetric source adds heat Saves grid Good system level data obtained,Surface attribute,Volumetric Resistance,The compact model option in the heat sink smart part construction di

37、alog uses a standard friction factor and Nu correlations to determine the pressure drop and heat transfer in the fin volume based upon local flow conditions. These relations couple the heat to the height of the fin; creating a temperature gradient up the fin. This is the type of behavior that is see

38、n in a detailed model. The effective h for the heat sink should be based upon the base plate area, not the total fin area. How does the compact heat sink model calculate the flow resistance? This treatment replaces the fins by a volume flow resistance and planar flow resistances to account for the f

39、riction and contraction/ expansion losses as air passes through the fin or pin volume. In addition, the heat transfer is calculated in the fin/pin volume using a volume-based surface exchange attribute (see Surface Exchange attribute dialog). The settings within the volume are calculated using stand

40、ard Nu number and fanning friction factor correlations, appropriate for fully-developed duct flow, and utilizes the local flow data in each grid cell along with the defined geometry of the heat sink. The Nusselt Number is calculated from: Nu = max (7.54, 0.024 Re 0.786 Pr 0.45 ) The fanning friction

41、 factor, f is calculated from: f = max(24/Re, 0.0791/Re 0.25 ) This is represented in FLOTHERM using the existing advanced resistance formula f = k1/Re + k2/Re a For situations where an engineering approximation of heat sink performance is sufficient for assessing the overall cooling strategy in a s

42、ystem, this modeling option offers an efficient alternative to modeling each fin or pin explicitly.,27/42,Manual Compact Heat Sink Model,Volumetric resistance replaces fins Volumetric Surface Exchange attribute controls heat transfer (.CSV import),28/42,Results Animations,Particle Animations Numeric

43、al Smoke Test Plot Animations Animating a Visualization Plane Animating an Iso-Surface,29/42,Particle Animations,Position with Snap Target, and Manipulator tools Snap Target to position particle source approximately Translate, Rotate, and Scale to refine the position precisely Control the appearance

44、 of the particles Number of Particles, Life-time, time-step control where the particles are displayed Smear, width, energy, particle type and others control the appearance of the particles,30/42,Particle Animations,The Global Source Properties Sets up the properties for the display of any source. Th

45、e options available are: Smear: Controls the amount of time each individual particle is displayed. The larger the smear value the longer the particle will appear. To change the smear value, use the thumbwheel or enter a value in the text box and press . As each particle is displayed for a fixed time

46、 faster moving particles will appear longer than slower moving particles. This enhances the impression of speed. Large values for smear, particularly associated with small Time Step values, may cause particles to overlap producing a continuous ribbon-type effect. Width: Sets the radius of the partic

47、les. Distribution: Sets the random distribution of particles as a ratio value. A Distribution value of 0 will release all particles at the same time. A Distribution value of 1 will set the release time of particles as 100% randomly distributed. Energy: Sets the amount of energy of each particle. To

48、change the energy, use the thumbwheels or enter a value in the text box and press . Life Time: Sets the life expectancy of a particle. To change the life time, use the thumbwheels or enter a value in the text box and press . Time Step: Sets the timespan between each frame of the animation. To change

49、 the timespan use the thumbwheels or enter a value in the text box and press . Int Time Step: Sets the integral time step. To change the Int Time Step, use the thumbwheels or enter a value in the text box and press . Reset All:Resets the values under Global Particle Properties back to their default

50、settings. Individual Source Properties Sets up the appearance of the source selected in the list. The list contains all the source plots known to this project. The settings made in the Individual Source Properties panel apply to the source selected in the list. Sources are selected by clicking the l

51、ist. The selected source becomes highlighted, and its name appears in the text box below the list. Source names may be changed by overtyping in the text box, and pressing Enter. The properties set and the operations performed on the selected source are as follows: Create: Creates a new source which

52、will appear in the sources list as the selected source. Delete: Deletes the selected source. Streamlines: Sets the number of particles to emit from the source. To change the number of particles use the thumbwheel or enter a value in the text box and press . The particles are distributed evenly over

53、the particular line(s) or area of release. On: Activates the display of the particles emitting from the source within the Animation Display Area.,The Global Source Properties (cont.) Visible: Displays the source in the Animation Display Area. Reverse: Reverses the direction of the particle flow. Thi

54、s is useful for locating a particle source. Snap to source target: Once you have moved the Snap Target into the required position click on the Snap button to move the source to meet the target. (See Snapping Source.) The Snap Target is activated via Activate Snap Target from the Selection menu of th

55、e Main Window or by clicking the middle mouse button over the model while the pick cursor is displayed. Particles: Draws the flow stream with individual shapes selected from the Appearance pop-up menu. The particles can be represented as ellipses, arrows or triangles. The figure below shows a partic

56、le stream of ellipses. Ribbon: Draws the flow stream in solid bands. Line: Draws the flow stream as solid thin lines. Manipulator Type: Displays the Manipulator which, when the mouse is in Pick Mode ( ), sets how the source can be manipulated within the Animation Display Area. Please note that only

57、one manipulator will be displayed at a time and its shape is determined by its function chosen from four mutually exclusive options as follows: none, no manipulator is selected (source cannot be moved) translate, displays the manipulator which may move the source along an axis direction. The manipul

58、ator is a set of arrows pointing in the directions that the source can move. To move the manipulator, click on the arrow pointing in the desired direction and then drag the mouse. rotate, displays the manipulator which may rotate the source. The manipulator is a ball made up of circular elements. To

59、 rotate the source, select and drag the circle that is angled in the direction you want the source to rotate. scale, displays the manipulator which may rescale the source. The manipulator has four corner handles. To resize the source, select and drag one of the corner handles to change the size. A box outline will appear to indicate the change in scale while the drag is being done. Source Type: Sets the shape of the source. You can chose from either elliptic, rectangular or line source shapes with particles released from the sides or evenly ac