Read time: 10 minutes          

Target audience: Thermal Researchers/ EV Automobile Engineers/ Cabin Comfort/Thermal-Fluid Industry/ Aero Industry

Written by: Dr. Tabish Wahidi

1. Background

With the rapid transition toward electrified and energy-efficient mobility, HVAC system design has evolved into a critical engineering challenge. Unlike conventional vehicles, electric vehicles must balance thermal comfort and energy optimisation, as HVAC loads directly affect battery performance and driving range. Conventional HVAC design workflows often assume a fixed occupant posture, typically a standard driving position. However, this assumption introduces uncertainty, as human physiology and positioning directly influence convective heat transfer, airflow distribution, and localized thermal gradients. Recent advancements in simulation tools enable high-fidelity modeling of human thermal response coupled with CFD, making it possible to quantify the impact of posture variability on cabin comfort metrics.

2. Introduction

Thermal comfort inside a vehicle cabin is governed by a multi-physics interaction involving:

• Convective heat transfer (airflow from HVAC vents)
• Radiative heat exchange (solar load and interior surfaces)
• Conductive heat transfer (contact with seat, steering wheel, and other interfaces)
• Human thermoregulation (metabolism, blood perfusion, sweating)

The simulation framework integrates:

Human Thermal Model (HTM)

• Metabolic heat generation
• Blood perfusion and thermoregulation
• Sweating and shivering response
• Segmented human body representation

Comfort Metrics

• PMV (Predicted Mean Vote)
• PPD (Predicted Percentage of Dissatisfied)
• MRT (Mean Radiant Temperature)
• Berkeley Thermal Sensation Model (local and global)

The objective is to evaluate how driver hand position on the steering wheel alters thermal boundary conditions and impacts both global and local comfort indices.

3. Methodology

The objective is to evaluate how driver hand position on the steering wheel alters thermal boundary conditions and impacts both global and local comfort indices. A detailed human model representing a 50th percentile male with a sedentary activity level (1.0 met) and light clothing was used to capture physiological heat transfer and thermoregulation. Thermal interactions between the human body and cabin surfaces were modeled to account for conductive effects. Three driving postures, namely: (i) standard grip, (ii) one-hand grip, and (iii) low grip, were simulated under identical conditions to isolate the impact of human positioning. Airflow was modeled using RapidFlow™, coupled with TAITherm™ for heat transfer, enabling a fully integrated thermal-fluid analysis.

The evaluation focused on both global and localized thermal comfort, including sensation levels, convective heat transfer, airflow distribution, and surface temperatures, to quantify the effect of posture on cabin thermal performance.

4. Case Study: Posture-Induced Variations in Thermal Comfort

• Transient Comfort Response

Simulation results indicate that posture significantly influences transient thermal response:

• The standard grip configuration achieves near-neutral comfort approximately 3 minutes faster than the low grip case
• This improvement is attributed to more effective airflow impingement on critical body regions, enhancing convective cooling

• Flow Field Distortion

CFD results show that human limbs act as flow obstructions within the cabin airflow domain:

• The left arm introduces a strong streamline deflection
• Airflow distribution shifts depending on posture, altering which body regions receive cooling
• This results in non-uniform thermal exposure across the body

• Local Thermal Sensation Analysis

• Hands and forearms show high sensitivity to airflow variations
• Low grip posture results in:
• Higher skin temperature persistence
• Slower transition from warm to neutral sensation
• Standard grip provides:
• More uniform airflow exposure
• Faster stabilization of thermal comfort
  • Surface Temperature Distribution
• Spatial temperature gradients across the human body are strongly posture-dependent
• Contact regions (e.g., steering interface) influence localized heat accumulation and dissipation

• Implications for Comfort Modeling

Even under identical HVAC boundary conditions:

• Global comfort metrics alone are insufficient
• Local discomfort persists due to geometry-driven airflow distribution effects

This highlights the importance of segment-wise thermal comfort evaluation.

5. Conclusion:

This study demonstrates that human posture is a critical parameter in cabin thermal simulations and should not be treated as a fixed or secondary input. Below are the major conclusions:

• Incorporate multiple posture configurations in HVAC simulation workflows
• Human geometry alters airflow topology, impacting convective heat transfer
• Posture variations introduce non-uniform thermal boundary conditions
• Local comfort metrics are highly sensitive to airflow exposure
• By leveraging advanced simulation tools such as TAITherm™ and RapidFlow™, engineering teams can:
• Reduce dependency on physical prototyping
• Enable predictive, human-centric HVAC design
• Optimize thermal comfort and energy efficiency trade-offs, particularly for electric vehicles

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