Comfort Parameters

See Ref. 21, Ref. 10, and Ref. 11. These criteria are used when designing occupied spaces and their HVAC systems and are intended to determine whether environmental conditions are acceptable in terms of general thermal comfort and air quality or represent discomfort. The calculation of the comfort criteria assumes that the analyzed fluid is Air.

Mean Radiant Temperature (MRT) is the uniform surface temperature of an imaginary black enclosure in which an occupant would exchange the same amount of radiant heat as in the actual non-uniform space.

The mean radiant temperature (Tr) is defined as follows:

where Idif is the intensity of the diffuse (thermal) radiation (W/m2/rad), Isun is the intensity of the solar radiation (W/m2), σ is the Stefan-Boltzmann constant.

To calculate the Mean Radiant Temperature, it is assumed that the emissivity of all the surfaces within the computational domain equals to unity.

Operative Temperature is the uniform temperature of an imaginary black enclosure, in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual non-uniform environment.

The operative temperature (Tc) is defined as follows (Ref. 10):

where Tr is the mean radiant temperature (°C), T is the fluid temperature (°C), V is the fluid velocity (m/s).

Predicted Mean Vote (PMV) is an index that predicts the mean value of the votes of a large group of persons on the 7-point thermal sensation scale, based on the heat balance of the human body. Thermal balance is obtained when the internal heat production in the body is equal to the loss of heat to the environment. In a moderate environment, the human thermoregulatory system will automatically attempt to modify skin temperature and sweat secretion to maintain heat balance (Ref. 11).

The PMV is defined as follows:

where:

where:

M is the metabolic rate (W/m2 of the body area). It is the rate of transformation of chemical energy into heat and mechanical work by metabolic activities within an organism (by default it is set to 70 W/m2);

W is the external work (W/m2 of the body area). It accounts for the effective mechanical power (by default it is set to 0 W/m2);

Icl is the clothing thermal resistance (m2K/W). It is the resistance to sensible heat transfer provided by a clothing ensemble. The definition of clothing insulation relates to heat transfer from the whole body and, thus, also includes the uncovered parts of the body, such as head and hands. The typical values of thermal resistance for a certain clothing ensemble can be found in Ref. 11 (by default it is set to 0.11 m2K/W);

fcl is the ratio of clothed surface area to nude surface area;

Ta is the air temperature (°C);

Tr is the mean radiant temperature (°C);

V is the relative air velocity (m/s);

pa is the water vapor partial pressure (Pa) calculated in accordance with the saturation curve, the air temperature Ta and the Relative Humidity (see “Water vapor condensation and relative humidity”);

hc is the convective heat transfer coefficient (W/m2/K);

Tcl is the clothing surface temperature (°C).

Predicted Percent Dissatisfied (PPD) is an index that provides information on thermal discomfort or thermal dissatisfaction by predicting the percentage of people likely to feel too warm or too cool in a given environment (Ref. 11). It can be obtained from the PMV using the following equation:

Draught Rate is the percentage of people predicted to be bothered by draught. The draught rate (DR) is defined as follows:

where:

Ta,l is the local air temperature (°C), 20°C to 26°C;

va,l is the local mean air velocity (m/s), < 0.5 m/s;

Tu is the local turbulence intensity (%), 10% to 60%. If the Laminar Only flow type is selected, Tu equals 40%.

This model applies to people at light, mainly sedentary activity with a thermal sensation for the whole body close to neutral and for prediction of draught at the neck. At the level of arms and feet, the model could overestimate the predicted draught rate. The sensation of draught is lower at activities higher than sedentary (> 1.2 met) and for people feeling warmer than neutral.

Draft Temperature is the difference in temperature between any point in the occupied zone and the control condition. “Draft” is defined as any localized feeling of coolness or warmth of any portion of the body due to both air movement and air temperature, with humidity and radiation considered constant (Ref. 21).

The draft temperature (Td) is defined as follows:

where:

T is the local fluid temperature (°C);

Tm is the average fluid temperature within the control space (°C);

V is the local fluid velocity (m/s).

Air Diffusion Performance Index (ADPI) is the percentage of the space in which the air speed is less than 0.35 m/s and the Draft Temperature falls between -1.7 ºC and 1.1 ºC (Ref. 21).

If the Draft Temperature or ADPI are calculated in Volume Parameters, then the “control space” will correspond to the specified volume region. In all other cases the “control space” corresponds to the whole computational domain.

Contaminant Removal Effectiveness (CRE) is an index that provides information on the effectiveness of a ventilation system in removing contaminated air from the whole space. For a perfect mixing system CRE = 1. Values above 1 are good, values below 1 are poor.

This parameter is only available if more than one fluid is present in the control space. Its value is defined as follows:

where:

Ce is the bulk average mass fraction of the contaminant calculated over all faces through which the fluids outflow from the computational domain;

<C> is the bulk average mass fraction of the contaminant calculated over the whole computational domain.

Local Air Quality Index (LAQI) is an index that provides information on the effectiveness of a ventilation system in removing contaminated air from a point. For a perfect mixing system LAQI = 1. For other systems, the higher the value at a point, the better the capability of the ventilation system in removing contaminated air from that point. This parameter is only available if more than one fluid is present in the control space. Its value is defined as follows:

where:

Ce is the bulk average mass fraction of the contaminant calculated over all faces through which the fluids outflow from the computational domain;

C is the mass fraction of the contaminant at a point.

The Flow Angle shows the flow deviation from the design flow direction. Consider one of the axis of selected coordinate system as the design flow direction, the results can then be viewed as the deviation from the design. Typically, flow angles of less than 15° might be considered as good. The flow angle components (αi) are defined as follows:

where Vx, Vy, Vz are the X, Y, and Z components of the fluid velocity and V is the absolute value of the fluid velocity vector.