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Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions (2012)

Chapter: Section IV - Relationship Between Emissions and Ambient Temperature

« Previous: Section III - Key Project Findings
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Suggested Citation:"Section IV - Relationship Between Emissions and Ambient Temperature." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Suggested Citation:"Section IV - Relationship Between Emissions and Ambient Temperature." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Suggested Citation:"Section IV - Relationship Between Emissions and Ambient Temperature." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Page 16

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14 Previous studies have observed that ambient temperature has a profound effect on the emissions of CO and HC when the engine is operating at near-idle state (Lyon et al. 1979). They note that the change in emissions performance resulting from the influence of ambient pressure on the combustor intake pressure (P3 in Figure I-3) is not as significant as the effect of ambient temperature on the combustor inlet pressure (T3). Gas turbine engines are optimized for operation at cruise power, so when they are operating at idle power, far from their optimal operation point, combustion efficiency can be non-ideal. Since the engine is operating at less than optimal efficiency, it is not surprising that it may be more sensitive to variations in ambient conditions. Physical conditions like temperature in the combustor strongly impact the combustion process. Figure IV-1 depicts the output of a simple GasTurb calculation to evaluate the potential magnitude that variations in ambient temperature (T2) can impact the temperature at the combustor exit (T4). This simple model should not be taken to be quantitatively accurate; however, it suggests the effect at low fuel flow, indicative of near-idle operation, has an effect on combustor temperature. The combustion tempera- ture estimated using GasTurb (with combustor parameters for a CFM56-7B24 engine) results in a roughly linear relation- ship between T2 and T4 (see Figure I-3 for locations of the numbered temperature stages). Although the relationship between the ambient incoming air and the proxy combus- tion temperature is essentially linear in this simulation, the net efficiency of combustion falls dramatically at lower T2 values. Less efficient combustion produces higher emission levels of HAP compounds and other hydrocarbons. IV.1 VOC Emissions and Ambient Temperature The impact of ambient temperature on the combustion efficiency is illustrated using the measurements made for formaldehyde. The absolute formaldehyde emission index (expressed as grams of formaldehyde per kilogram of fuel) measured for each of the CFM56-7B24 engines tested dur- ing the testing phase of this project is depicted as a function of ambient temperature in Figure IV-2. The formaldehyde emission index measured at 7% thrust (N1 = 25%) and the ground idle (no bleed air demand) during the JETS/APEX2 test for the CFM56-7B24 engine have been included for comparison (OAK g.i. and OAK 7%). The two dashed lines added to Figure IV-2 suggest a simple linear tem- perature dependence for the two idle states: ground idle (brown) and the N1 = 25% definition of idle (yellow). The data plotted in Figure IV-2 includes all test points, and deliberate variations in fuel flow are responsible for what appears to be scatter in this depiction. Note that all of the cold weather (265K to 271K) data plotted in Figure IV-2 are also shown in Figure III-3 to vary systematically with fuel flow. Prior to the testing undertaken by this project, the test points typically probed during on-wing emissions test projects were at ground idle (with no bleed air demand) and/or at the ICAO defined idle of N1 = 25% or 7% thrust. The previous section described the significant effect of the fuel flow rate on emis- sions when operating the on-wing engine below N1 = 25%. The data plotted in Figure IV-2 suggest that the magnitude of the influence of ambient temperature on the emission index is similar to the effect of engine operation at rotational speeds less than N1 = 25%. IV.2 Emissions Index Temperature Dependence The temperature dependence of the formaldehyde measure- ments is very similar to the dependence observed for ethene, propene, acetaldehyde, and other VOC species. Qualitatively, a similar temperature dependence was observed for the NASA DC-8 during APEX (2004). Because testing programs that use government-owned aircraft (such as APEX, JETS/APEX2, S e c t i o n i V Relationship Between Emissions and Ambient Temperature

15 APEX3, and the Alternative Aviation Fuel Experiment [AAFEX]) have access to an aircraft for long stretches of time, those tests have been able to probe a range of ambient temperatures. The limitation, however, is that they only test the ground idle and 7% idle conditions. In addition, the APEX and AAFEX testing on the NASA DC-8 do not have a measured record of the fuel flow that parallels the data collected by the digital flight data recorder (DFDR) during subsequent commercial aircraft tests. To compare the temperature dependence from test results for different CFM56 combustor types, as well as to examine the temperature dependence using other available VOC measure- ments, a normalization scheme is adopted. The ICAO reference temperature is 288K (58.7°F). In the adopted normalization scheme the emission index is normalized by a measured or assumed emission index at 288K. The normalized emission indices (at or near ground idle) for several measurement campaigns have been plotted as a function of ambient temperature in Figure IV-3. Overall, Figure IV-1. GasTurb simulation of combustor temperature for “idle” as a function of ambient temperature. Figure IV-2. Formaldehyde emissions index versus ambient temperature for the CFM56-7Bxx family of engines. The emission indices measured in this project are shaded by the N1 rotational fan speed to visually distinguish the N1 = 25% points (yellow) from the tests at ground idle with varying bleed air demand (darker reds). The gray data points from the JETS/APEX2 at OAK are included for comparison since the combustor type was the same as for the other data plotted here. Figure IV-3. Normalized emission index versus ambient temperature. The measured emission indices for formaldehyde and ethene have been normalized by the value at 288K for several datasets. The light blue line is a representation of the BFFM2, and the dark black line is a quadratic fit to the data.

16 the emission indices of the VOCs are approximately twice as high between 0°C and -8°C as they are at 15°C. In order to query the functional dependence of the temperature correction implicit in BFFM2 for idle data, the calculation was performed for temperatures ranging from 258K to 308K. The resulting emissions index data was divided by the result at 288K in order to extract the embedded tem- perature dependence (DuBois and Paynter 2006). The resulting curve is the light blue line in Figure IV-3 and agrees very well with the empirical results despite the potential misapplication of BFFM2 to fuel flow below the 7% reference fuel flow. Figure IV-3 depicts the temperature dependence for the ground idle results only. When this analysis is performed on the ICAO 7% test data, the negative temperature depen- dence is still present, but not quite as steep. This result is not unanticipated because at the increased fuel flow rate the combustor temperature is increased and combustion efficiency increases.

Next: Section V - Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature »
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TRB’s Airport Cooperative Research Program (ACRP) Report 63: Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions is designed to help improve the assessment of hazardous air pollutants (HAP) emissions at airports based on specific aircraft operating parameters and changes in ambient conditions.

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