ASHRAE ST-16-001-2016 Analyzing the Performance of a Kitchen Exhaust Air Duct with Regards to Recent Standards-A CFD Thermal-Stress Simulation.pdf

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1、 2016 ASHRAE 3ABSTRACTRecent standards have defined a lower minimum airvelocity in kitchen exhaust air ducts without compromisingperformance. Steel ducts are usually specified to a standardsteel gage thickness capable of handling the extreme loadconditions of high temperatures, possible corrosive co

2、ndi-tions, and negative pressure levels within exhaust duct. Thispaper simulates a typical kitchen exhaust air duct performingunder extreme temperature conditions and low air duct veloc-ities. Lower exhaust air velocities should correspond to lowerin-duct negative pressure values and, therefore, pos

3、sibly areduction in steel duct wall thickness.A computational fluid dynamics (CFD)/thermal stressanalysis was carried out under the most extreme load condi-tions specified under recently issued standards. This analysisdemonstrates that a lower-gage steel duct thickness is morethan sufficient than wh

4、at is specified in recent standards, and,therefore, a lower-gage steel thickness can be used.INTRODUCTIONKitchen exhaust air ducts are widely used and are built tospecific standards; ASHRAE HandbookHVAC Applications(2015) is widely used for this purpose. This Handbook referstoareducedexhaustairveloc

5、itythatwasregardedassufficientto expel obnoxious particles/fluids from cooking facilities,practically reducing exhaust fan capacity and saving energy.Now that these reductions in exhaust air duct velocity arepart of a standard, computational fluid dynamics (CFD)/ther-mal stress analysis can be used

6、to carry out a comprehensiveanalysisandexplorehowsuchanairductbehavesandwhetherthere is a possibility to reduce steel duct thickness by usingstandardindustrialsoftwaresimulationmethods.Naturally,toensure the integrity of a reduced air duct steel wall thickness,steel mechanical characteristics under

7、elevated temperaturesspecified under ASHRAE HandbookHVAC Applications(2015) will have to be accounted for. Therefore, it is possibleto say that a criterion can be set to accept a reduced steel ductthickness operating under elevated temperatures. Such a deci-sion will have to be justified technically

8、, such as by the use ofCFD/thermal stress analysis.KITCHEN STEEL EXHAUST AIR DUCTSAccording to ASHRAE HandbookHVAC Applications(2015), Type 1 kitchen hoods are designed to cater to a rangeof appliances categorized as light, medium, heavy duty, andextra heavy duty, with operating temperatures ranging

9、 from200Cto370C(392Fto698F).Assumedairducttempera-ture in this paper will be based on the most extreme tempera-ture condition, 370C (698F).William Gerstler (2002) notes the following: “Commercialkitchen design professionals were concerned that the minimumexhaust air duct velocity of 7.62 m/s (1500 f

10、pm),” which was anNFPArequirement,“wastoorestrictive.”Thecompletedresearchwork addresses “the relationship between grease deposits andexhaust velocityResearch resulted in several published docu-ments.” NFPA 96, Standard for Ventilation Control and FireProtection of Commercial Cooking Operations, the

11、n in March2002,reducedexhaustvelocityto2.54m/s(500fpm).Thisreduc-tioninexhaustvelocitycanbefurtherenhancedbyprovidingductinsulation, as explained by William Gerstler (2002): “Loweringvelocityreducesgreasedepositioninvirtuallyallcaseswhenductwork with insulation of R-10 or greater is used.”John Clark

12、e (2012) notes: “Type 1 duct constructionconsists of at least 16-gage black steel or 18-gage stainlessAnalyzing the Performance of aKitchen Exhaust Air Ductwith Regards to Recent StandardsA CFD/Thermal-Stress SimulationAli M. Hasan, CEngMember ASHRAEAli M. Hasan is a senior mechanical engineer at KE

13、O International Consulting Engineers, Doha, Qatar.ST-16-001Published in ASHRAE Transactions, Volume 122, Part 2 4 ASHRAE Transactionssteel. This duct must slope 6 mm per 0.3 m (0.25 in. per ft)toward the hood or an approved grease reservoir.”This paper will simulate (at the first instant) a typicalk

14、itchenexhaustairductconstructedoutofa16-gagesteelductsheet material but without insulation. The aim is to investigateair duct mechanical characteristics near 2.54 m/s (500 fpm)exhaust air velocity and an extreme inlet air temperature of370C (698F) (see Discussions section). Investigation willthen co

15、ntinue to simulate an 18- and 24-gage steel duct (notstainlesssteel)andvalidateperformanceasanalternativethin-ner material but with lower cost than stainless steel.CFD MODELThe model was prepared as shown in Figure 1. A CFDanalysis was carried out using Fluentsoftware (ANSYS2016) with the following

16、assumptions and settings:a. Steady-state analysis with CFD/thermal stress analysis:Once the CFD analysis was setup and run using the cus-tom system, select from the software toolbox Fluid Flow(Fluent) Static Structural tool, thermal stress analysis.This action will link the CFD software tool with th

17、e ther-mal/stress analysis software tool.b. Number of cells used is 98,103. It is important to checkthe quality of mesh, therefore select the following: MeshMetrics Element Quality; Minimum Jacobian element0.27, Maximum Jacobian element 0.99, Average Jaco-bian element 0.82, Standard Deviation 9.86 e

18、-2. A Jaco-bian element close to 1 indicates a good element quality,while a Jacobian element close to 0 indicates a poor ele-ment quality. Element sizing, use advanced Size func-tion; On: Proximity and Curvature. Number of cellsacross gap, 3.c. Settings not mentioned in this paper can be left at sof

19、t-ware default settings.d. Air as an ideal gas was assumed.e. Used the k-omega shear stress transport (k- SST) tur-bulence model. See section “Discussions” on perfor-mance of this model.f. Inlets shown in Figure 1 were set at 0 gage pressure, tem-perature at 370C or 643 K (698F). Outlet was assumedt

20、o be exhaust fan with a capacity of 30 Pa (0.0003 bar).g. In the Boundary Conditions settings, select Thermal tab.Enter the following information to define duct walls ther-mal settings:Select from main menu: Thermal Conditions Mixed,and then enter the following data:Heat transfer coefficient 7.9 W/(

21、m2K)(1.40 Btu/hft2F)Free stream temperature 26.8C (80.24F)External emissivity 0.9External Radiation Temperature 26.8C (80.24F)Wall thickness, varies depending on selected sheetmetal thickness.h. Duct wall 16-gage galvanized sheet steel, assumed to be1.61e-3 m (0.0635 in.). This gage thickness was th

22、enchanged for the various simulations. See Tables 1 and 2for simulated gage thickness.i. A check on maximum air velocity can be checked in thepostprocessor. In this example, maximum velocity is3.339 m/s (657.11 fpm), as shown in Figure 1.Figure 1 CFD stream lines for a 370C (698F) air temperature at

23、 inlets. Duct shown to scale, with uniform sectional areaof 200 300 mm (8 12 in.) Inlets 200 mm (8 in.) diameter 150 mm (6 in.) Inlets are connected to kitchen hoods,not shown.Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 5j. Link the two Geometries items shown in the Flui

24、d Flowand Static Structural menus. Link the Solution item in theFluid Flow menu with the Setup item in the Static Struc-tural menu.k. At the structural model settings, select steel sheet fromthe Engineering Data tool, and then select and open theModel item. Fix supports as shown Figure 1, and thenim

25、port solution data, which is pressure in duct acting onall duct walls. Carry out the same for thermal solution.Import from Fluid Flow thermal settings.l. When meshing the steel duct walls, select the mesh tooland allow for a medium mesh setting: Number of ele-ments used is 1608. To check the quality

26、 of mesh selectMesh Metrics Element Quality; this gives MinimumJacobian element 0.24, Maximum Jacobian element 0.99,Average Jacobian element 0.70, Standard Deviation 0.17.A Jacobian element close to 1 indicates a good elementquality, while a Jacobian element close to 0 indicates apoor element qualit

27、y.m. Finally, select outputs which are Equivalent Stress (vonMises) and Total deformation.RESULTSGraphical and numerical results were generated using thesoftware postprocessor. Results for minimum and maximumvon Mises stress values are shown in Table 1, extrapolatedfromFigures2to4,whileTable2showsma

28、ximumtotaldefor-mation figures, extrapolated from Figures 5 to 7.Refer to Figure 2 for an example of how maximum andminimum stress readings were extrapolated and entered inTable 1. The minimum value shows that this stress contoursubstantially covers the duct surface area.Figures 2 to 4 are thermal s

29、tress analysis results:Figure 2 shows a kitchen exhaust duct made of sheetmetal 0.7 mm (0.028 in.) thick, 24-gage galvanized steelsheet metal. The top image represents an inlet tempera-ture of 200C (392F), while the bottom image rep-resents an inlet at 370C (698F).Figure 3 shows a kitchen exhaust du

30、ct made of sheetmetal 1.31 mm (0.0524 in.) thick, 24-gage galvanizedsteel sheet metal. The top image represents an inlet tem-perature of 200C (392F), while the bottom image rep-resents an inlet at 370C (698F).Figure 4 shows a kitchen exhaust duct made of sheetmetal 1.61 mm (0.0644 in.) thick, 24-gag

31、e galvanizedsteel sheet metal. The top image represents an inlet tem-perature of 200C (392F), while the bottom image rep-resents an inlet at 370C (698F).Figures5to7aretotal(exaggerated)deformationimages:Figure 5 shows a kitchen exhaust duct made of sheetmetal 0.7 mm (0.028 in.) thick, 24-gage galvan

32、ized steelsheet metal. The top image represents an inlet tempera-ture of 200C (392F), while the bottom image rep-resents an inlet at 370C (698F).Figure 6 shows kitchen exhaust duct made of sheetmetal 1.31 mm (0.0524 in.) thick, 24-gage galvanizedTable 1. Maximum and Minimum Equivalent(von Mises) Str

33、ess Obtainedper a Simulated Duct Category:Light Duty 20C (392F)and Extra Heavy Duty 370C (698F)Light DutyDuct CategoryEquivalent Stress,Min MPa (kPsi)MaxMPa (kPsi)16-gage 28.462 256.0918-gage 28.47 256.1724-gage 28.492 256.37Heavy DutyDuct CategoryEquivalent Stress,Min MPa (kPsi)MaxMPa (kPsi)16-gage

34、 28.495 256.3918-gage 28.51 256.5224-gage 28.55 256.92Results obtained at the postprocessor stage. See Figures 2 to 4 for locations.Table 2. Total Deformations Obtained per aSimulated Duct Category:Light Duty 200C (392F)and Extra Heavy Duty 370C (698F)Light DutyDuct Category Total Deformation, mm (i

35、n.)16-gage 0.18741 (0.00749)18-gage 0.18822 (0.00753)24-gage 0.18972 (0.00759)Heavy DutyDuct Category Total Deformation, mm (in.)16-gage 0.36544 (0.01462)18-gage 0.36654 (0.01466)24-gage 0.36857 (0.01474)Results obtained at the pos tprocessor stage. See Figures 2 to 4 for locations. Maximumreadings

36、recorded.Published in ASHRAE Transactions, Volume 122, Part 2 6 ASHRAE Transactionssteel sheet metal. The top image represents an inlet tem-perature of 200C (392F), while the bottom image rep-resents an inlet at 370C (698F).Figure 7 shows kitchen exhaust duct made of sheetmetal 1.61 mm (0.0644 in.)

37、thick, 24-gage galvanizedsteel sheet metal. The top image represents an inlet tem-perature of 200C (392F), while the bottom image rep-resents an inlet at 370C (698F).DISCUSSIONSCFD Discussions on Turbulence Model UsedAccording to Ghiaasiaan (2011), the shear stress trans-port (SST) model uses the k-

38、 model at near wall conditions,taking an advantage of its effectiveness at the inner boundarylayer, and then switches to k- at the free-stream conditions,where k- performs better than the k- model.As expected in this simulation, where adverse pressuregradients do exist, particularly near duct walls

39、and aroundcorners,themodelperformedwellandconfirmswhatisstatedabove.Discussions on Heat TransferHeat transfer selections made in the CFD Model section,item g, were comprehensive, considering convective, conduc-tive,andthermalradiation.Hence,themixedfunctionsettingswas selected.Figure 2 Thermal stres

40、s analysis based on CFD simula-tion. Top image based on 200C (392F) inlet airtemperature, lower image based on a 370C(698F) inlet temperature. Using 24-gage galva-nized sheet metal, 0.7 mm (0.028 in.) sheet thick-ness.Figure 3 Thermal stress analysis based on CFD simula-tion. Top image based on 200C

41、 (392F) inlet airtemperature, lower image based on a 370C(698F) inlet temperature. Using 24-gage galva-nized sheet metal, 1.31 mm (0.0524 in.) sheetthickness.Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 7Faye McQuiston et al. (2000) describes thermal radiation as“the tran

42、sfer of thermal energy by electromagnetic waves, anentirelydifferentphenomenonfromconductionandconvection.”In fact, thermal radiation can occur in a perfect vacuum and isactually impeded by an interrupting medium. Therefore, thedifferentmethodsinheattransferwereaddressedseparatelyorasmixed settings

43、as shown in the software settings.Thefollowingparagraphsaremechanicaldiscussionsandobservations made on simulation/results:1. Because the steel duct in this analysis is subjected to hightemperatures, the first discussion will focus on creep fail-ure. According to S.S. Manson and G.R. Halford (2009),

44、“Creep is a time dependent deformation that occurs athigh temperature relative to the melting point of metallicmaterials. The creep regime for metals is commonlyregarded to begin at a temperature of approximately halfthe absolute temperature (degrees Kelvin or Rankine) ofthe metal melting point.”The

45、refore, if the melting point of carbon steel is1698 K (2597F), the creep range will begin at 849 K(1068F). This is well above the 643 K (698F) figurementioned in item F in the FD Model section. Thermalstress analysis results will exclude this concern of creepfailure.Figure 4 Total deformation analys

46、is based on CFD simu-lation. Top image based on 200C (392F) inletair temperature, lower image based on a 370C(698F) inlet temperature. Using 24-gage galva-nized sheet metal, 0.7 mm (0.028 in.) sheet thick-ness.Figure 5 Total deformation analysis based on CFD simu-lation. Top image based on 200C (392

47、F) inletair temperature, lower image based on a 370C(698F)inlettemperature.Using24-gagegalva-nized sheet metal, 1.31 mm (0.0524 in.) sheetthickness.Published in ASHRAE Transactions, Volume 122, Part 2 8 ASHRAE Transactions2. Fatigue limit as explained by N.E. Frost et al. (1974)notes that the “fatig

48、ue limit/tensile strength ratio laybetween 0.4 and 0.5.” For practical purposes and littleerror, steels having tensile strengths up to about1250 MN/m2(181.3 kpsi) that the fatigue limit/ultimatetensile strength, ratio can be assumed to be 0.5. In otherwords,Fatiguelimitforsteel=0.5460=230MN/m2(33,358.7 psi)Note, wrought steels have tensile strengths rangingfrom310to2000 MN/m2(45to290kpsi).The460MN/m2(67 kps