Software Cradle SC/Tetra V7 CFD Fluid/Thermal Software Uses a Fully Integrated Thermoregulation Model to Predict Human Body Skin Temperatures and Improve HVAC System Designs Yuya Ando Software Cradle Properly designing a Heating, Ventilation, and Air Conditioning (HVAC) system to consistently provide a comfortable environment for human occupants is an extremely challenging fluid/thermal problem. Today, high customer satisfaction means ensuring proper temperatures and air flows are produced at all points in the operating space and not just for specific design conditions. Computational modeling is a key enabler for evaluating more design concepts over a broad range of operating conditions.

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Despite advancements in Computational Fluid Dynamics (CFD) as a design tool, a critical piece of the HVAC design puzzle has still been missing. This missing link is the skin temperature of the human subjects themselves. Failure to consider the temperature of the human subjects in the HVAC environment oversimplifies the analysis in two critical ways. First, it neglects thermal coupling between the body(s) and the surroundings. Second, it does not consider how the person actually feels.

The skin temperature is the temperature the human body feels and determines whether the person feels comfortable in the environment. The skin has been likened to being an infinite number of temperature sensors that are critical for alerting the body of changing environmental conditions so it can implement controls to maintain a constant internal core temperature. When the surrounding temperature increases and the skin senses this, the skin temperature sensors send signals to the brain and the body starts to perspire. Evaporation lowers the skin temperature which lowers the blood temperature and keeps the body from overheating. 

From a simulation perspective, the primary challenge is developing an accurate model of the human body.   The human body is an extremely complicated fluid/thermal system. It is physically complex (skin, tissues, blood flow) and it also comes in many different shapes and sizes. Heretofore, commercial CFD codes have not included a heat transfer model for the human body despite it being the critical piece of the HVAC design puzzle. Recently, third party heat transfer software products have started to include human body models. However, these software products require separately coupling the air flow around the body with the human body internal heat transfer calculations in order to pass temperature and convection coefficient information between the CFD code and the heat transfer software. The ideal situation is to embed the human body heat transfer model within the CFD code. This fully couples the body temperature calculations with the calculations for the surrounding flow field.

Professor Shin-ichi Tanabe at the Waseda University in Tokyo, Japan developed an accurate thermoregulation model of the human body. This model is called JOS (JOint System) Thermoregulation Model. To meet the needs of the HVAC simulation market, Software Cradle Co, Ltd , developer of SC/Tetra CFD software, worked with Professor Tanabe to integrate JOS into its recently released SC/Tetra Version 7 CFD software product. The integration of JOS within SC/Tetra is comprised of three parts.

1. 1. JOS computes the skin temperature of the human body. JOS is a physical model based on the heat balance equations for divided body segments. JOS inputs include body size, sex, age, basal metabolic rate, and fat rate. JOS considers thermal conductance between tissues, the detailed vascular system, and the thermoregulatory system consisting of perspiration, vasomotion, shivering heat production, and arterio-venous anatomies (AVA). 
   
2. 2. CFD is used to compute the temperature and air velocities in the fluid environment. The fluid domain is modeled in a traditional manor. Grid meshing is only required in the fluid domain and not within the human body(s).
   
3. 3. Boundary conditions couple JOS to the fluid domain. Energy is passed between the human body thermoregulation model and the fluid CFD environment through the body skin. Thermal resistance due to clothing and water vapor concentration from perspiration diffusing into the air at the skin surface form the boundary conditions for the CFD calculations. The air temperature and the water vapor at the skin surface are the boundary conditions for the thermoregulation model.

The JOS Model

JOS models the human body by dividing it into seventeen body segments. Individual body segments consist of a core layer and a skin layer. In the center of the core layer are both an artery blood pool and a vein blood pool used for modeling the vascular system. In addition, a superficial vein blood pool is modeled in the skin layer of limb segments. 

JOS Body Segmentation Model

The blood pools for each segment constitute the vascular system around the heart. Pathways flow from the heart to the head, chest, back, hands and feet. Arterio-venous anatomies (AVA) accounts for changes in blood flow due to changes in the ambient environment. AVA is a vessel between the artery blood pool and the superficial vein blood pool to model the change in blood flow. For example, in a hot environment AVA is opened which promotes additional blood flow and increased heat release from the skin surface. 

Heat exchange occurs within each body segment and includes heat production (except at the extremities). Heat loss at the skin surface, by convection and radiation (sensible heat loss), and evaporation (latent heat loss) are the boundary conditions for the segment heat transfer model. Sensible heat loss accounts for the thermal resistance caused by different types of clothing. Heat is conducted through the tissues and considers transfer between the core layer and blood vessels, core layer and skin, and countercurrent heat exchange between the arteries and veins. Conduction heat transfer between segments is negligible compared to heat transfer from the blood flow. Heat production within each segment by basal metabolism occurs in the core and skin layers. 

Physiological factors of the human body must be considered within the thermoregulation model calculations. These include thermal conductance between tissues, thermal capacity of tissues, basal metabolic rate, and the basal blood flow rate. These factors are largely a function of body size, sex, age, and percent body fat. Body size is determined from the CFD calculation mesh. While no CFD calculation mesh is required within the body, the boundary cells around the body define the surface area and consequent size of the body.

Using JOS within SC/Tetra

SC/Tetra provides a user friendly interface to input JOS specific data. Inputs consist of sex, age, body fat rate, and metabolic rate. Body surface areas must be registered as either being in contact with air or with a solid surface. Additional commands can be used to define computational requirements such as outputs, time steps, number of CFD calculations per loop, and convergence criteria. Outputs can be viewed numerically or graphically using SC/Tetra’s post processor. 

Sample Results

Graphical output from SC/Tetra and JOS shows how a particular automotive HVAC system performs. The velocity vector plot shows a typical CFD output for the air velocities and temperatures. The value of JOS is demonstrated by displaying the skin temperatures for each person are displayed. The skin temperatures are also calculated where the body contacts the seat. This information can be translated into more accurate comfort indices which, in turn, result in better designs.

                 Body Skin Temperature Distribution at Contact Interface