Small corporate aircraft often sit on the tarmac for hours prior to boarding VIP guests, corporate executives and the occasional public official. The focus of this sample model is to use thermal design to improve the aircraft’s human comfort while reducing the power load on its climate control system. Two cases were created: a baseline aircraft case and a second case with improved thermal design and reduced air flow from the climate control system.
A generic aircraft geometry was meshed and prepared for analysis in a natural environment. Seats, cockpit, human thermal manikins, and air zones were configured. The environment was setup in RadTherm as Yuma, Arizona in July, with the aircraft sitting idle from 2-4:00 PM prior to 'boarding.’ This provided a substantial solar thermal load; therefore the aircraft’s temperature was predicted without human models up until the final 4:00 PM boarding call. A pilot, seats, and yokes were included in the cockpit. Two passengers and seats were included in the cabin.
Common Settings to Baseline and Improved Cases
Both cases employed common geometry, external environments, and overall model structures. The variations were created by changing insulation, window glass, and paint and by reducing the AC airflow rate. In both cases there was an air node assigned to the cockpit and four air nodes partitioned by cross-sectional quadrants in the passenger seating area. This allowed for lateral and vertical temperature variations with mixing flows between zones. An AC source air reference temperature was assigned and an advection link set to the air zones.
Thermal Effect of Paint on Corporate Jet
The baseline corporate jet was assigned a black paint with solar absorptivity of 0.95 and thermal emissivity of 0.95. The improved case used white paint with solar absorptivity of 0.27 and thermal emissivity of 0.92. Because RadTherm can easily define these properties within the user interface or by command line utilities, an optimization code could be employed to determine the optimal paint across multiple environments.
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Baseline |
Improved |
|
Black Solar Absorptivity: Paint 0.95 |
White Solar Absorptivity: Paint 0.27 |
|
Thermal Emissivity: 0.95 |
Thermal Emissivity: 0.92 |
Insulation Comparison for Corporate Aircraft
The baseline corporate jet model was assigned a 5mm aluminum external skin with 25mm fiberglass insulation and a 6mm plastic interior liner. Because these values are parametrically defined with a planar mesh in RadTherm, it required only a moment to change the design for the improved case to 50mm fiberglass insulation. No return to CAD was needed, nor was it necessary to recompute view factors. RadTherm automatically updated the thermal network and thermal mass of the layers.
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Baseline |
Improved |
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Aluminum (5 mm) |
Aluminum (5 mm) |
|
Fiberglass (25 mm) |
AC-550 (50 mm) |
|
Plastic (6 mm) |
Plastic (6 mm) |
Window Design for Aircraft Thermal Performance
The baseline case had standard glass windows with 75% transmission of incident visible (solar) radiation; the improved case used tinted windows with transmission reduced to 34%. This increased the temperature of the windows while keeping the energy closer to the "sink" of the external environment, rather than loading objects deeper into the system like seats, floors and passengers. Because our software stores the solar load to every element in the model, it is possible to examine in detail the solar contribution on the aircraft and in the cabin.
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Conventional |
76% Transmissive |
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Reflective |
34% Transmissive |
Aircraft Climate Control System Design
The airflow rate for the baseline case was 20,000 liter/min of conditioned air at 11ºC. Because our goal was to reduce the required airflow while improving thermal comfort, we cut this by 75% to 4,000 liter/min. Although this is a drastic reduction, we will demonstrate below that the other design changes to the improved case resulted in a net increase in passenger human comfort.
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Baseline |
20,000 L/min |
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Optimized |
4,000 L/min |
Aircraft Climate Control System
The climate control system was engaged five minutes after boarding. The airflow rate was held constant but the AC source air temperature was ramped from 15 C to 7C over 10 minutes. Each of the four air zones in the cabin received 25% of the total AC air flow.
Aircraft Thermal Comfort
The passenger thermal comfort was analyzed using our human thermal analysis module and the included Berkeley Comfort Model. This module computes the physiologically accurate human thermal response to the aircraft’s interior environment and determines both human thermal sensation and thermal comfort.
Results
The improved thermal designs of windows, paint and insulation had a substantial effect on the aircraft skin temperature, interior temperature, and human comfort. The change in paint reduced the overall solar load while maintaining the thermal band radiant cooling emissions outwards toward the sky.
The increased insulation reduced conductive load into the cabin, and tinted glass further cut the total load into the aircraft. The result was that the overall system was not as "saturated" with energy prior to engaging the climate control system.
The results for the human comfort analysis demonstrate that even with a 75% reduction in cooling air flow, the improved aircraft design parameters improved thermal comfort. This type of analysis can be executed early in the design phase of an aircraft and refined along with each engineering stage.
The thermal performance of an aircraft and its climate control system can therefore be easily studied over transient conditions using RadTherm. The global location of the airport, orientation of the aircraft, weather conditions, and design parameters are easily modified and the simulation repeated. Optimization codes can also be employed to vary multiple design parameters and achieve an optimal thermal design.
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