JOS (JOint System) Thermoregulation Model
An
invaluable feature for HVAC engineers: SC/Tetra V7 CFD
Software
Integrated Human Body Thermoregulation Model that Predicts Human Body
Skin Temperatures.
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. 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.
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. Not calculating the temperature of the human
subjects in the HVAC environment is akin to only calculating the air
temperature and heat transfer coefficient around a large mass being
heated in an oven and assuming the mass will be properly
baked. The analogous problem becomes even more complicated
when multiple masses (people) are involved.
From
a simulation perspective, the primary modeling challenge is accurately
representing 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, coupling the air flow around the body with the human body
internal heat transfer physics requires passing 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 so the body temperature and the
surrounding air flow calculations are fully coupled.
 Professor
Shin-ichi Tanabe at the Waseda University in Tokyo, Japan has 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.
- JOS
computes the temperature of the human body.
JOS computes the temperature of the human body. JOS is a physical
model based on the heat balance equations for divided body
segments. JOS considers body size, sex, and age. It
includes thermal conductance between tissues and models the detailed
vascular system consisting of perspiration, vasomotion, shivering heat
production, and arterio-venous anatomies (AVA).
-
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). 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.
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 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, 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 RESULTSGraphical
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.
 
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