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Machine Design

Hot or cold interiors? Software assures they're just right

Recent automotive trends such as increased use of glass, tightening fuel constraints, and the move to environmentally safe refrigerants have made it more difficult to keep drivers and passengers comfortable.

CFD software shows a 3D view of thermal loads generated by the sun and airflow patterns inside the car created by air conditioning.

To simplify the task, designers at Delphi-Harrison Thermal Systems, Lockport, N.Y. (, developed a thermal-comfort model that accurately and quickly evaluates automotive HVAC systems.

Traditionally, HVAC systems were evaluated by testing prototypes in wind tunnels, a time-consuming and expensive process. (It typically costs nearly $500,000 to build a vehicle prototype and $400/hr to operate a wind tunnel.) And although automotive OEMs and suppliers have developed several cabin-climate prediction models over the years, Delphi's engineering model goes considerably further by integrating a 16-zone physiology model that calculates passenger skin temperatures, an indicator of comfort.

The model uses computational-fluid dynamics (CFD) to calculate local thermal comfort as a function of air temperature, surrounding surface temperatures, air velocity, humidity, direct solar heating, as well as the activity level and clothing of each individual. The model takes climactic conditions in which the vehicle is operating to develop a thermal model of the passenger compartment. Results serve as inputs to the model of the human thermal regulatory system, which predicts human comfort.

The model was validated using a test that simulates a passenger compartment. A pyranometer delivered solarlike radiation while a 2.5-liter engine and air-conditioning system cooled the compartment. Thermocouples measured air and surface temperatures, which closely matched those predicted by the model. This highlights an important advantage of numerical analysis: It can predict temperature and airflow at any point in the cabin even though data from physical experiments is limited by the number of sensors placed in the vehicle.

The model's physiology and comfort predictions were then validated using a thermal mannequin developed at the University of California, Berkeley. Air temperature and velocity around 16 body segments, interior surface temperatures, breath temperatures, air-conditioning outlet temperatures and humidity, solar load, and mannequin-body heat loss were all measured. Simulated temperatures matched measured temperatures to within a few degrees for each body segment. Measured and simulated thermal comfort index also showed a near-perfect match, according to Lin-Jie Huang, senior research scientist at Delphi.

In the future, thermal-comfort modeling is expected to play a major role in developing and evaluating sophisticated adaptive controls and algorithms that will make it possible to tune HVAC systems to more closely meet passenger-comfort requirements.

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