Metabolic Rate as total heat generation boundary condition

[hs.simanto]

Hi!
In online help of Autodesk Simulation CFD 2015, there is a tutorial named "Human Comfort Ventilation Study".The  Simulation CFD units of Metabolic rate are W/m², and the default value is 60 W/m². A single met is approximately the amount of energy produced per unit surface area of a person of average stature, seated at rest. The surface area of the average person is about 1.8 m² (19 ft²). The heat generation boundary condition is needed to compute an accurate temperature solution. Simulation CFD uses both the temperature distribution and the specified Metabolic rate factor to compute the thermal comfort.
 

So if i want to assign 60 W/m² metabolic rate as heat generation boundary condition,  i will have to multiply it by 1.8 m² (surface area of average human body) to convert it to total heat generation of 108W , am i right?

in the tutorial, total heat generation of 60W was assigned to the human model and the result of PMV was computed for 60W/m2 asdefault metabolic rate). 

if metabolic rate is used to assign total heat generation, isn't it obvious that it should be multiplied by the surface area of the occupant??

Please help me out!

Thanks in advance 🙂

[Royce_adsk · Accepted solution]

Normally, the way I see people use this informatio is that they will do the following:

1. Represent the human with a very simple block with the approximate size of the human.
2. Use the material 'Human' in the solids material database in CFD.
3. Apply the total heat generation to the entire volume of the human.  This is going to be based on the activity of that human (108W from you notes for instance)
4. Enable Thermal Comfort in the results quantity
5. If needed modify the variables that drive the thermal comfort calculations.  In this case you see the 60 W/m^2 that drives the 108W. If you were to have a different activity level this value and the above value would be similar.

Your last statement probably goes into what your real question is.  Why do I need to assign the 60 W/m^2 and the total heat generation?  Can't one drive the other? The answer to that is that your system could be very complex with variuos types of materials with heat generation.  The simulation doesn't know what is human and what might be a heater in the room.   By driving a 'comfort' off an assumed heat flux driven by a local temperature solution provides a more reasonable estimation of the human body comfort.

Best regards,

[ignassokolnikas]

Hello, I know this is an old topic, but I also had the same question in my head after going through thermal comfort study tutorial. I think the answer for this could be that when setting the heat generated by occupant in CFD, you need to take into account only sensible heat of the human body (not latent+ sensible). I got this information from my supervisor of bachelor thesis. And also sensible and latent heat ratio strongly depends on temperature.

For example if person emits 100 W of total heat in 22 C temperature, the share of sensible heat will be around 72W and 28 W for latent heat.

You can see the shares of sensible and latent heat in the link below.

http://www.engineeringtoolbox.com/metabolic-heat-persons-d_706.html

Also halton displacement ventilation program gives the value for active sedentary activity as 70W, when sensible+latent heat generation would be around 100.

[Eric.Dixon.SG]

Can anyone confirm that only the sensible load should be applied as the Total Heat Generation for humans?

The Autodesk Intro to Simulation CFD for AEC Applications website here contains the following statement: In all cases occupants generate heat and impede air flow, these are the characteristics that must be considered.  A reference about the amount of heat a person generates can be found in Building Science Fundamentals. The Building Science Fundamentals webpage goes on to define the thermal load for a sitting person as 100W. This conflicts with the Exhibit Hall tutorial previously mentioned that defines a sitting person Total Heat Generation as 60W. The original post refers to the Heat Generation in the Exhibit Hall tutorial being defined as 60W/m2. Is this the case and the online tutorial here has a mistake? Or is it true that only the sensible load should be applied as the above post and the post here about modeling humidity suggest? It would be great if Autodesk could either correct or clarify this in the help file.