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17 Section V Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature The essential trend observed when an engine is operating Jet A (proxy fuel for JP-8) data have been considered here. at near-idle conditions is an increase in VOC emission indices The JETS/APEX2 testing, also conducted at zero bleed air with decreasing temperature. This negative temperature demand and N1 = 25% fuel flows, was performed on the dependence was demonstrated for several VOCs. Scatter is CFM56-7B combustor, but only modest ambient temperature present in the observations and the apparent engine-to-engine variations were encountered during the test. The testing variability will be discussed later. An empirical model of protocol included tests at the ground idle (zero bleed) and the temperature dependence of near-idle emissions can be N1 = 25% fuel flow points, but also probed intermediate developed and locked to the ICAO emissions performance fuel flows. These latter tests were conducted at both cold databank values. This empirical model can also account for weather (MDW 2009/2010) and warm weather (DAL 2009) the effect of ambient temperature and engine operation at venues. rotation speeds less than the 7% thrust condition. Figure V-1 indicates that, to the extent that the normal- ization of datasets can allow comparisons between different CFM56 combustors, empirical data from all quadrants of V.1 Proposed Empirical Model the fuel flow and ambient temperature space are available. In the empirical model proposed here, the effect of ambient The essential functional forms are a linear representation temperature and sub-7% fuel flow are treated as independent of the dependence of the emission index on fuel flow and factors that influence a base reference emission index defined a semilinear (weakly quadratic) dependence of the emission at 288K and 7% thrust. This is how the ICAO databank index on ambient temperature. A smooth interpolation has idle reference conditions are incorporated into this model been applied between the temperature dependence observed approach. Near-idle emissions scaling suggests that it is for the ground idle fuel flow data and the N1 = 25% data. plausible to rely on the FAA and EPA's Speciate database An abbreviated encapsulation of the model developed (EPA 2008) VOC emissions profile to convert the tabulated for CFM56-7B22 engines is tabulated in Table V-1. At the UHC emission index to specific VOC (or specific HAP) species reference fuel flow rate of 0.105 kg s-1 (7% thrust fuel flow) emission indices for a reference emission index. and temperature (288K) the emission index is by definition 1. The datasets and the measurement conditions they span are If an application of this model were directed to estimate the used to develop the empirical emissions index model shown effect of ambient temperature and non-7% fuel flow on an in Figure V-1. The APEX1 datasets, which first showed a emission rate, the emission index could be multiplied by the strong dependence of emissions on ambient temperature at the factor in Table V-1. For example, the model predicts that ground idle engine condition, were conducted at temperatures emission rate for any VOC or HAP compound at 278 K and at or greater than the ICAO reference temperature (288K). The a fuel flow of 0.095 kg s-1 would be estimated by the following two idle conditions defined in that test matrix were ground equation: idle (with no bleed air demand) and idle defined by N1 = 25%. Similarly, the AAFEX tests were conducted at the same engine Emission Rate ( g s -1 ) = 2.1 × 0.095 ( kg s -1 ) conditions; however, the ambient temperatures encountered × Reference Emission Index ( g kg -1 ) during that testing were essentially at or lower than the ICAO reference temperatures. Note that while the study goals of The data tabulated in Table V-1 are graphically depicted AAFEX were to study alternative fuel emissions, only the in Figure V-2.
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18 Figure V-1. Datasets available to develop fuel flow/ambient temperature model for predicting near-idle emissions. The red datasets were collected for the CFM56-2C combustor on the NASA DC-8. The blue datasets for the CFM56-7B24 combustors were collected as part of this project. The emission index (grams of HAP per kilogram of The multiplicative factor is applied to the emissions index, fuel) multiplicative factor for the CFM56-7B22 depicted in with the total emission rate as the product of EI and fuel Figure V-2 and tabulated in Table V-1 should not be inter- flow rate. Thus, at lower fuel flow rates, the emission rate preted as a direct multiplicative factor on the total emission is linearly reduced with the reduced fuel burn. This effect is rate (grams of HAP per second) when a fuel flow rate besides accounted for in Figure V-3, which depicts the direct factor the reference fuel flow rate is considered. for the temperature dependent emission rate that accounts for Table V-1. Emissions index correction factor for near-idle CFM56-7B22 operation. ° ° ° ° ° °
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19 Figure V-2. Relative emission index as a function of ambient temperature and fuel flow. The gray-scale shading and labeled contours are the multiplicative factor for the reference emission index. Figure V-3. Near-idle emission rates (not emission index) as a function of ambient temperature and fuel flow. The contours and shading in the figure are derived from the data in Figure V-2. The effect of reduced fuel flow rate has been applied.