by Lin-jie Huang, Delphi Thermal and Interior, Lockport, NY, and Taeyoung Han, Delphi Research Labs, Shelby Township, MI

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Simulation of vehicle cabin climatic conditions is becoming increasingly important as a complement to wind tunnel and field testing to help achieve improved thermal comfort while reducing development time and cost. However, the tendency to use more glass in vehicle styling coupled with tightening fuel-economy constraints, the change to less-efficient, environmentally safe refrigerants, and reduced condenser air flow, particularly at idle, are significant challenges for achieving occupant thermal comfort. As a consequence, it has become necessary to develop tools that can predict the impact of various design choices on passenger thermal comfort early in the design process.

Schematic of virtual thermal comfort engineering process

Solar load on the glass and on the cabin interior of a full-size SUV

Air flow distribution in a full-size SUVcabin

To explore different climate control strategies as they relate to occupant thermal comfort in a quick and inexpensive manner, Delphi developed the Virtual Thermal Comfort Engineering (VTCE) process. The model has the ability to predict the local thermal comfort level of an occupant in a highly non-uniform thermal environment as a function of air temperature, surrounding surface temperatures, air velocity, humidity, direct solar flux, and the level of activity and clothing of each passenger. In its study, Delphi used test data to validate VTCE for a sport utility vehicle (SUV) cabin environment. VTCE was then used to perform sensitivity studies of various vehicle cabin environments on the occupant thermal comfort, including discharge temperature, breath level temperature and air velocity, solar intensity, and solar angles.

For the purpose of VTCE, the geometry of the passenger compartment was described by parameters that were carefully selected from early-stage vehicle architectural design. The key design parameters, such as A/C outlet location and size, windshield angle, and body vent locations were easily varied to accommodate potential design changes. Due to the readily available water-tight surface geometry from the Delphi compartment model, the mesh generation time was drastically reduced compared to the traditional CFD process. Once the compartment model was available, the benefits of the model for developing the HVAC system design were tremendous.

EHT index for 16 body segments for the baseline case near the end of the summer comfort ride.

Prediction of the comfort ratings during the summer comfort ride

Prediction of the comfort ratings during the winter comfort ride

During the simulations, it was necessary to specify the thermal environment around the passengers, such as air velocity, air temperature, and radiation load around body segments corresponding to a human physiology model. The human physiology model, which strives to simulate thermal comfort by modeling realistic blood flow, takes into account the layers of body tissue and clothing in addition to other descriptive characteristics of the body (e.g., height, weight, age, etc.). This data is used to calculate the equivalent homogenous temperature (EHT) [1, 4], which is plotted together with the comfort limits [2, 3] that have been established for different body parts. In general terms, the EHT attempts to account for both the physiological and psychological states of a person in order to assess thermal comfort in a non-uniform environment. Accurate prediction of EHT values for an occupant depends on the accuracy of predicting the cabin interior thermal environment, which includes the solar load, air velocities, breath level temperatures, and surrounding interior surface temperatures in the cabin.

In the VTCE process, FLUENT is used to predict the airflow, temperature, and humidity distribution around the vehicle occupants. These values are then used as boundary conditions for the human physiology model. The human physiology model computes the surface temperatures of the occupants, as described above, and passes these back to FLUENT so that the airflow can be recomputed. The exchange of data occurs at specified time intervals during the CFD analysis. For the case considered, the SUV solution converged very quickly since the thermal interaction between the occupants and the cabin thermal environment was not strong. Despite the weak thermal interaction, how-ever, the occupants in the cabin significantly affected the overall flow and air temperature distribution.

The EHT calculations from the VTCE process were validated against test data using a full-size SUV during both summer and winter driving conditions. Thermal comfort ratings were supplied by human subjects during soak and cool-down/warm-up vehicle comfort test rides. The overall predictions for occupant thermal comfort for the summer and the winter rides were within a quarter scale accuracy compared to the human subject data.

Beyond the validations, VTCE was subjected to a series of sensitivity studies, in which passenger compartment climate conditions such as solar loading and air velocity were varied to discern the effect on the EHT values. For the baseline case of a summer ride, a direct solar intensity of 1000 W/m2 and a diffused solar intensity of 50 W/m2 were specified with an incidence angle of noon time (90° altitude and 0° azimuth). Due to the vertical solar incidence angle, most of the occupant body surfaces were blocked from direct solar load by the roof of the vehicle. Only occupant hand and feet areas were impacted by direct solar load. For the case of the 45° altitude, -45° azimuth solar incidence angle, the right lower arm and the left upper arm areas had fairly high EHT values (33°C and 29.5°C respectively) due to direct solar heating.

The effects of the air velocity were simulated by assuming that the other thermal environment variables were the same as the baseline case. The air velocity was increased and decreased by 0.5 m/s to assess the effects of the air velocity on the thermal comfort. The increase of air velocity by 0.5 m/s influenced most of the body segments except the back and near the pelvis, which directly contacted the seat. The average EHT for the body segments increased by roughly 1°C when the air velocity decreased by 0.5 m/s and decreased by roughly 2°C when the air velocity was increased by 0.5 m/s. A large effect of air velocity was found on the head, arm, and hand body segments. Very little effect occurred for pelvis, back, and thigh because these body segments were in contact with the seat. As expected, the comfort ratings were improved as the air velocity magnitude increased around the occupant.

Overall, VTCE was found to be suitable for the evaluation of heat load and occupant thermal comfort in a vehicle cabin. This simulation tool allows for the rapid assessment of various parameters with respect to thermal comfort during the early stage of vehicle development without the need for time-consuming vehicle level tests.

References:

1. Currle, J. Numerical Study of the Flow in a Passenger Compartment and Evaluation of Thermal Comfort of the Occupants; SAE Paper 970529, March 1997.
2. Currle, J.; Maué, J. Numerical Study of the Influence of Air Vent Area and Air Mass Flux on the Thermal Comfort of Car Occupants; SAE Paper 2000-01-0980, March 2000.
3. Bohm M.; Browen, M.; Holmer, I.; Nilsson, H.; Noren, O. Evaluation of Vehicle Climate with a Thermal Manikin – The Relationship between Human Temperature Experience and Local Heat Loss; Swedish Institute of Agricultural Engineering. JTIReport 123, 1990.
4. Wyon, D.P.; Larsson, S.; Forsgren, B.; Lundgren, I. Standard Procedures for Assessing Vehicle Climate with a Thermal Manikin; SAE Paper 890049, 1989.

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