At the fundamental level, thermal comfort is the computation of a heat transfer energy balance. The heat transfer contributions of radiation, convection, and conduction are balanced against the heat generated by the occupant. Other factors such as relative humidity and occupant clothing play a role as well.
A simple expression of thermal comfort in terms of the heat entering or leaving the occupant is this:
- If heat leaving > heat entering, the occupant is cold.
- If heat entering > heat leaving, the occupant is hot.
The result of a thermal comfort calculation is an assessment of whether the occupant is too cold (negative thermal comfort) or too hot (positive thermal comfort) The available results quantities are the Mean Radiation Temperature, the Predicted Mean Vote, and the Predicted Percentage of Dissatisfaction.
Mean Radiant Temperature (MRT)
Mean radiant temperature (MRT) is the temperature that is achieved when the occupant absorptivity and emissivity are that of a radiative black body. It is the area-weighted mean temperature of all the objects surrounding the body.
Plot MRT as a global scalar, on results planes, and with iso surfaces to visualize the temperature distribution throughout the occupant space.
Predicted Mean Vote (PMV)
PMV is an index that predicts the mean value of the votes of a large group of persons on the seven point thermal sensation scale:
|
Value |
Sensation |
|
-3 |
cold |
|
-2 |
cool |
|
-1 |
slightly cool |
|
0 |
neutral |
|
+1 |
slightly warm |
|
+2 |
warm |
|
+3 |
hot |
Use PMV to assess the impact of the room design on the occupants. The best way to display PMV is on the surfaces of the model. To plot PMV, right click off the model, select Global result, and select it from the menu.
In the image, velocity is plotted on the results plane, and PMV is plotted on the surfaces:
The PMV values indicate that the human occupant is might not be too comfortable--the left arm and leg are really cold!
Predicted Percentage of Dissatisfied (PPD)
PPD is an index that establishes a quantitative prediction of the percentage of people that are dissatisfied with the thermal conditions, as determined from PMV. The PPD is highest at the ends of the PMV range:
This image shows the PPD of the same scenario:
The PPD is highest on the left arm and leg, confirming what the PMV indicated about this occupant.