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14 Section IV Relationship Between Emissions and Ambient Temperature Previous studies have observed that ambient temperature (expressed as grams of formaldehyde per kilogram of fuel) has a profound effect on the emissions of CO and HC when the measured for each of the CFM56-7B24 engines tested dur- engine is operating at near-idle state (Lyon et al. 1979). They ing the testing phase of this project is depicted as a function note that the change in emissions performance resulting from of ambient temperature in Figure IV-2. The formaldehyde the influence of ambient pressure on the combustor intake emission index measured at 7% thrust (N1 = 25%) and the pressure (P3 in Figure I-3) is not as significant as the effect of ground idle (no bleed air demand) during the JETS/APEX2 ambient temperature on the combustor inlet pressure (T3). test for the CFM56-7B24 engine have been included for Gas turbine engines are optimized for operation at cruise comparison (OAK g.i. and OAK 7%). The two dashed power, so when they are operating at idle power, far from lines added to Figure IV-2 suggest a simple linear tem- their optimal operation point, combustion efficiency can be perature dependence for the two idle states: ground idle non-ideal. Since the engine is operating at less than optimal (brown) and the N1 = 25% definition of idle (yellow). The efficiency, it is not surprising that it may be more sensitive data plotted in Figure IV-2 includes all test points, and to variations in ambient conditions. Physical conditions like deliberate variations in fuel flow are responsible for what temperature in the combustor strongly impact the combustion appears to be scatter in this depiction. Note that all of the process. Figure IV-1 depicts the output of a simple GasTurb cold weather (265K to 271K) data plotted in Figure IV-2 calculation to evaluate the potential magnitude that variations are also shown in Figure III-3 to vary systematically with in ambient temperature (T2) can impact the temperature at fuel flow. the combustor exit (T4). This simple model should not be Prior to the testing undertaken by this project, the test points taken to be quantitatively accurate; however, it suggests the typically probed during on-wing emissions test projects were effect at low fuel flow, indicative of near-idle operation, has an at ground idle (with no bleed air demand) and/or at the ICAO effect on combustor temperature. The combustion tempera- defined idle of N1 = 25% or 7% thrust. The previous section ture estimated using GasTurb (with combustor parameters described the significant effect of the fuel flow rate on emis- for a CFM56-7B24 engine) results in a roughly linear relation sions when operating the on-wing engine below N1 = 25%. ship between T2 and T4 (see Figure I-3 for locations of the The data plotted in Figure IV-2 suggest that the magnitude of numbered temperature stages). Although the relationship the influence of ambient temperature on the emission index between the ambient incoming air and the proxy combus- is similar to the effect of engine operation at rotational speeds tion temperature is essentially linear in this simulation, the net less than N1 = 25%. efficiency of combustion falls dramatically at lower T2 values. Less efficient combustion produces higher emission levels of HAP compounds and other hydrocarbons. IV.2Emissions Index Temperature Dependence The temperature dependence of the formaldehyde measure IV.1VOC Emissions and ments is very similar to the dependence observed for ethene, Ambient Temperature propene, acetaldehyde, and other VOC species. Qualitatively, The impact of ambient temperature on the combustion a similar temperature dependence was observed for the NASA efficiency is illustrated using the measurements made for DC-8 during APEX (2004). Because testing programs that formaldehyde. The absolute formaldehyde emission index use government-owned aircraft (such as APEX, JETS/APEX2,
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15 Figure IV-1. GasTurb simulation Figure IV-2. Formaldehyde emissions of combustor temperature for index versus ambient temperature for "idle" as a function of ambient the CFM56-7Bxx family of engines. The temperature. emission indices measured in this project are shaded by the N1 rotational fan speed to visually distinguish the N1 = 25% points APEX3, and the Alternative Aviation Fuel Experiment [AAFEX]) (yellow) from the tests at ground idle with have access to an aircraft for long stretches of time, those varying bleed air demand (darker reds). tests have been able to probe a range of ambient temperatures. The gray data points from the JETS/APEX2 at OAK are included for comparison The limitation, however, is that they only test the ground idle since the combustor type was the same and 7% idle conditions. In addition, the APEX and AAFEX as for the other data plotted here. 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 temperature is 288K (58.7°F). In the adopted normalization aircraft tests. scheme the emission index is normalized by a measured or To compare the temperature dependence from test results assumed emission index at 288K. for different CFM56 combustor types, as well as to examine the The normalized emission indices (at or near ground idle) temperature dependence using other available VOC measure for several measurement campaigns have been plotted as a ments, a normalization scheme is adopted. The ICAO reference function of ambient temperature in Figure IV-3. Overall, 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.
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16 the emission indices of the VOCs are approximately twice with the empirical results despite the potential misapplication as high between 0°C and -8°C as they are at 15°C. of BFFM2 to fuel flow below the 7% reference fuel flow. In order to query the functional dependence of the Figure IV-3 depicts the temperature dependence for the temperature correction implicit in BFFM2 for idle data, the ground idle results only. When this analysis is performed calculation was performed for temperatures ranging from on the ICAO 7% test data, the negative temperature depen- 258K to 308K. The resulting emissions index data was divided dence is still present, but not quite as steep. This result is not by the result at 288K in order to extract the embedded tem- unanticipated because at the increased fuel flow rate the perature dependence (DuBois and Paynter 2006). The resulting combustor temperature is increased and combustion efficiency curve is the light blue line in Figure IV-3 and agrees very well increases.