ASHRAE HVAC APPLICATIONS SI CH 12-2015 AIRCRAFT.pdf

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1、12.1CHAPTER 12AIRCRAFTDesign Conditions. 12.1Typical Flight 12.11Air Quality. 12.13Regulations 12.14NVIRONMENTAL control system (ECS) is a generic termEused in the aircraft industry for the systems and equipment asso-ciated with ventilation, heating, cooling, humidity/contaminationcontrol, and press

2、urization in the occupied compartments, cargocompartments, and electronic equipment bays. The term ECS oftenencompasses other functions such as windshield defog, airfoil anti-ice, oxygen systems, and other pneumatic demands. The regulatoryor design requirements of these related functions are not cov

3、ered inthis chapter.1. DESIGN CONDITIONSDesign conditions for aircraft applications differ in several waysfrom other HVAC applications. Commercial transport aircraft oftenoperate in a physical environment that is not survivable by the unpro-tected. In flight, the ambient air may be extremely cold an

4、d dry, andcan contain high levels of ozone. On the ground, the ambient air maybe hot, humid, and contain many pollutants such as particulatematter, aerosols, and hydrocarbons. These conditions changequickly from ground operations to flight. A hot-day, high-humidityground condition usually dictates t

5、he thermal capacity of the air-conditioning equipment, and flight conditions determine the supplyair compressors capacity. Maximum heating requirements can bedetermined by either col d-day ground or flight operations.In addition to essential safety requirements, the ECS should pro-vide a comfortable

6、 cabin environment for the passengers and crew.This presents a unique challenge because of the high-density seatingof the passengers. Furthermore, aircraft systems must be low inmass, accessible for quick inspection and servicing, highly reliable,able to withstand aircraft vibratory and maneuver loa

7、ds, and able tocompensate for various possible system failures.Ambient Temperature, Humidity, and PressureFigure 1 shows typical design ambient temperature profiles forhot, standard, and cold days. The ambient temperatures used for thedesign of a particular aircraft may be higher or lower than those

8、shown in Figure 1, depending on the regions in which the aircraft isto be operated. The design ambient moisture content at various alti-tudes that is recommended for commercial aircraft is shown inFigure 2. However, operation at moisture levels exceeding 30 g/kg ofdry air is possible in some regions

9、. The variation in ambient pressurewith altitude is shown in Figure 3. Refer to the psychrometric chartfor higher altitudes for cabin humidity calculations. Figure 4 showsa psychrometric chart for 2440 m altitude.Heating/Air Conditioning Load DeterminationThe cooling and heating loads for a particul

10、ar aircraft model aredetermined by a heat transfer study of the several elements that com-prise the air-conditioning load. Heat transfer involves the followingfactors:Convection between the boundary layer and the outer aircraft skinRadiation between the outer aircraft skin and the external environ-m

11、entThe preparation of this chapter is assigned to TC 9.3, Transportation AirConditioning.Fig. 1 Ambient Temperature ProfilesFig. 2 Design Humidity Ratio12.2 2015 ASHRAE HandbookHVAC Applications (SI)Solar radiation through windows, on the fuselage and reflectedfrom the ground.Conduction through cabi

12、n walls and the aircraft structureConvection between the interior cabin surface and the cabin airConvection and radiation between the cabin and occupantsConvection and radiation from internal sources of heat (e.g., elec-trical equipment)Latent heat from vapor cycle systemsAmbient Air Temperature in

13、FlightDuring flight, very cold ambient air adjacent to the outer surfaceof the aircraft increases in temperature through ram effects, and maybe calculated from the following equations:TAW= T+ r(TT T)TT= TorTAW= Tr = Pr1/3wherePr = Prandtl number for air (e.g., Pr = 0.73 at 240 KT= ambient static tem

14、perature, KTT= ambient total temperature, Kk = ratio of specific heat; for air, k = 1.4M = airplane Mach numberFig. 3 Cabin Pressure Versus AltitudeFig. 4 Psychrometric Chart for Cabin Altitude of 2440 m1k 12- M2+1 rk 12- M2+Aircraft 12.3r = recovery factor for turbulent boundary layer (i.e., fracti

15、on of total temperature recovered in boundary layer as air molecules rest on the surface)TAW= recovery temperature (or adiabatic wall temperature), KExample 1. The International Civil Aviation Organization (ICAO) coldday at 9144 to 12 192 m altitude has a static temperature of 65C(208 K) and a Prand

16、tl number of 0.739. If an airplane is traveling at0.8 Mach, what would the external temperature be at the airplanes skin?Solution: Iteration is usually required. First guess for r 0.9:Pr = 0.728 at 0.9(240 208) + 208 = 236.8 Kr = Pr = (0.728)1/3= 0.8996Air Speed and Mach NumberThe airplane airspeed

17、is related to the airplane Mach number bythe local speed of sound:u= Mwherek = ratio of specific heats; 1.4 for airR = gas constant; 287 m2/(s2K)M = airplane Mach numberu= airplane airspeed, m/sAmbient Pressure in FlightThe static pressure over most of the fuselage (the structurearound the cabin) is

18、 essentially equal to the ambient pressure at theappropriate altitude.Ps= Pinf+ CpwherePs= pressure surrounding the fuselage, N/m2CP= pressure coefficient, dimensionless; approximately zero for passenger section of fuselage= free-stream or ambient air density, kg/m3)External Heat Transfer Coefficien

19、t in FlightThe fact that the fuselage is essentially at free-stream static pres-sure implies that a flat-plate analogy can be used to determine theexternal heat transfer coefficient at any point on the fuselage:Rex= q = hA(T TAW)whereh = external heat transfer coefficient, W/(m2K)Rex= local Reynolds

20、 number, dimensionlessx = distance along the fuselage from nose to point of interest, mcp= constant-pressure specific heat; for air, J/(kgK)w= ambient air (weight) density at film temperature T*, kg/m3 = absolute viscosity of air at T*; 3.673 109(T*)3/2408.2/(T* + 120) kg/(ms)(mPa s)A = outside surf

21、ace area, m2T = outer skin temperature, Kq = convective heat loss from outer skin, Wuinf= airplane airspeed, m/sExternal Heat Transfer Coefficient on GroundThe dominant means of convective heat transfer depends onwind speed, fuselage temperature, and other factors. The (free con-vection) heat transf

22、er coefficient for a large, horizontal cylinder instill air is entirely buoyancy-driven and is represented as follows:Gr = for 109 GrPr 1012:hfree= whereg = gravitational acceleration, 9.8 m/s2k = thermal conductivity of air, W/(mK) = kinematic viscosity, m2/sd = fuselage diameter, mhfree= free-conv

23、ection heat transfer coefficient, W/(m2K) = expansion coefficient of air = 1/Tf, where Tf = (Tskin+ Tinf)/2, KT = Tskin TinfTskin= skin temperature, KTinf= ambient temperature, KGr = Grashof numberPr = Prandtl numberA relatively light breeze introduces a significant amount of heatloss from the same

24、horizontal cylinder. The forced-convection heattransfer coefficient for a cylinder may be extrapolated from the fol-lowing:Re = for 4 104 Re 4 105hforced= where V is wind speed in m/s, and is evaluated at Tf = (Tskin+ Tinf)/2.Example 2. One approximation of the fuselage is a cylinder in cross-flow.T

25、he fuselage is 3.7 m in diameter and 37 m long, in a 4.3 m/s crosswindand a film temperature of 319 K. The surface temperature varies withthe paint color and the degree of solar heating. For instance, a typicalwhite paint could be 17 K higher than the ambient air temperature, sothe heat transfer fro

26、m the fuselage would beFree convection:Gr = = 8.43 1010for 109 GrPr 1012.TTT1k 12- M2+208 11.4 12- 0 . 82+235 K= =TAWTr+ TTT208 0.8996 235 208+232.3 K 41C=kRT12-u2h wcpuinf0.185 log10Rex2.584Pr23=note: 107Rex1094000, soC = 0.193 and n = 0.618, which leads toThese two correction factors have been com

27、bined see Equation(37) in Chapter 9 of the 2013 ASHRAE HandbookFundamentalsto produce a simpler relationship that applies to the full velocityrange (0 1 means that concentrations in the breathing zone are lowerthan in a perfectly mixed system; VE 1 means they are higher.There is a distinction betwee

28、n VE for bleed air and VE for totalventilation. For bleed air, the inlet concentration cinis the concen-tration of gases in the supply air to the entire system (i.e., bleed airconcentration). The local concentration will be larger than the inletconcentration only if the contaminant is generated with

29、in the cabin.For total ventilation, VE uses the cinat the nozzle (i.e., supply mix-ture concentration) and includes contaminants from the recircula-tion system. The practical use of this VE applies to particulate levelsin the cabin, because the recirculated air is equivalent to bleed air inthis rega

30、rd.Contaminant concentrations in the cabin can be converted toflows delivered to the breathing zone Qlocalusing the following rela-tionship:Qlocal= Fig. 7 Cabin Air Velocities from CFD, m/s(Lin et al. 2005)cmixedcinclocalcin-QlocalQcabin-Table 2 FAA-Specified Bleed Airflow per PersonCabin Pressure,

31、kPa Altitude, mRequired Flow per Person at 24C, L/s101.325 0 3.4999.505 150 3.5497.719 305 3.6395.954 460 3.6894.210 610 3.7892.500 760 3.8290.811 915 3.9289.149 1070 3.9687.508 1220 4.0685.895 1370 4.1184.309 1525 4.2082.744 1680 4.2981.200 1830 4.3479.683 1980 4.4478.187 2135 4.5376.711 2290 4.637

32、5.263 2440 4.72qgenclocalcin-12.8 2015 ASHRAE HandbookHVAC Applications (SI)Substituteqgen= Qsupplied (cmixed cin)whereqgen=CO2generation rate, 0.005 L/s at standard conditionsclocal=local CO2concentration by volumecin=inlet CO2concentrationQsupplied= flow to cabin or zone, L/sQlocal= flow delivered

33、 to breathing zone, L/sSome consideration can be given to distribution effectiveness(DE), where flows to higher-occupant-density sections of the cabin(such as coach) are used to set minimum flows to the cabin, andlower-density sections (such as first class) may subsequently beoverventilated:DE = whe

34、reQzone/nzone= flow per person in zoneQcabin/ncabin= average flow per person for entire cabinDistribution effectiveness accounts for a system that provides auniform flow per length of cabin yet has varying seating densitiesalong the length. For bleed air distribution, this effectiveness is tem-pered

35、 somewhat by occupant diversity D (see ASHRAE Standard62.1), because underventilated zones feed into the same recircula-tion flow. For total flow (bleed + recirculated) and for systems with-out recirculation, however, occupant diversity does not apply.System ventilation efficiency (SVE) is a measure

36、 of how wellmixed the recirculated air is with the bleed air before it enters thecabin. The SVE can be determined from the concentration varia-tions in the ducts leaving the mix manifold (see Figure 11), forinstance. The SVE is similar to VE in formulation:SVE = wherecall zones= average concentratio

37、n of all supply ductsczone= concentration in individual supply ductcamb= ambient reference concentration = Cfr(bleed air concentration)Dilution Ventilation and TLVContaminants that are present in the supply air and are also gen-erated within the cabin require increasing dilution flows to avoidreachi

38、ng Threshold Limit Values (TLVs) (ACGIH). For example,suppose carbon monoxide is present in the atmosphere at 210 ppband that each person generates 0.168 mL CO per minute, or 2.8 106L/s (Owens and Rossano 1969). The amount of bleed airrequired to stay below the EPA guideline of 9000 ppb will dependo

39、n the ambient CO levels, the human generation rate, and the COcontribution of the ventilation system:Qreq= whereCTLV= allowable concentrationCsystem= concentration rise from systemCsupply= concentration in supply (air entering cabin)Cfr= concentration in bleed airqgen= CO generated per personExample

40、 3. If the ventilation system does not contribute carbon monoxideto the supply air, then the required ventilation rate to stay below thethreshold isCTLV= 9000 ppb = 0.000009qgen= 2.8 106L/sCsystem= 0Cfr = 210 ppb = 2.1 107If, however, the ventilation system produces a 1000 ppb rise in car-bon monoxi

41、de, then the required ventilation isCTLV= 9000 ppb = 0.000009qgen= 2.8 106L/sCsystem= 1000 ppb = 0.000001Cfr= 210 ppb = 2.1 107It is important to note that, under certain circumstances, qgenandCsystemmay change sign as contaminant sources become contaminantsinks. This simplified approach shown here

42、is more conservative, andcould overpredict contaminant levels in real situations.Air ExchangeHigh occupant density ventilation systems have higher airexchange rates than most buildings (i.e., offices). The typical air-plane may have an air exchange rate of 10 to 20 air changes per hour(ach), whereas

43、 an office might have 1 ach. The air is not replaced ina mixed system at every air exchange. Actually, the ratio Q/V (airexchange rate) is more like the inverse of decay time constant . Anairplane cabin can be approximated as a partially mixed volume (avolume with ventilation effectiveness) as long

44、as the contaminantsources are uniformly distributed throughout the volume. For awell-mixed volume, contaminant in equals contaminant out pluscontaminant accumulated in the volume, orQcin= Qcout+ VAccounting for ventilation effectiveness, the concentration leav-ing the volume coutis related to the co

45、ncentration within the volumec and the concentration entering the volume cinby the ventilationeffectiveness VE:VE = = cin+ VE(c cin)Substituting,Qcin= Qwhich leads toQlocalQsuppliedcmixedcinclocalcin-=QzonenzoneQcabinncabin-call zonescambczonecamb-qgenCTLVCsystem Cfr-qgenCTLVCsupply-=QreqqgenCTLVCsy

46、stem Cfr-2.8 1060.000009 0 2.1 107-=0.32 L/(sperson)=QreqqgenCTLVCsystem Cfr-2.8 1060.000009 0.000001 2.1 107-=0.36 L/(sperson)=dcdt-cmixedcambclocalcamb-coutcambclocalcamb- cout=cmVE ccin+Vdcdt-+ccincincoeQ VEV- t=Aircraft 12.9Although air exchange rates are occasionally used as require-ments on th

47、e ventilation system, in the case of cabin ventilation,there is no basis for setting one. Air exchange rate can be a surro-gate (only for similarly sized volumes) for temperature uniformity,air quality, or smoke clearance. The flow-per-person specificationis preferred, because it can be related to the predominant pollutantsource more directly. Air exchange rates therefore indirec