Lecture Objectives Discuss: Project 1 Diffuser modeling Ventilation effectiveness

Meshing (Project 1) Pat a) Numerical diffusion The purpose of this project part is to analyze how mesh size and orientation affects the accuracy of result. outlet inlet T1 T2 T1=30C T2=20C outlet inlet

Concentration equation Conservation of mass of considered gas (chemical species): mgas=const mgas=C∙mair= C∙ρ∙dxdydz=const mgas,in mgas,out dy incompressible flow dz C=const dx Diffusion coefficient C – concentration of: H2O , VOC, CO, CO2 , and other gasses What about particles?

Inlets Diffuser Types Valve diffuser swirl diffusers ceiling diffuser wall or ceiling floor

Diffuser Types Grill (side wall) diffusers Linear diffusers Vertical Horizontal one side

Displacement ventilation diffusers

Diffuser modeling Complex geometry - Δ~10-4m We can spend all our momentum sources Momentum method Complex geometry - Δ~10-4m We can spend all our computing power for one small detail

Diffuser Modeling Fine mesh or box method for diffuser modeling

IAQ parameters Number of ACH quantitative indicator ACH - for total air - for fresh air Ventilation effectiveness qualitative indicator takes into account air distribution in the space Exposure takes into account air distribution and source position and intensity

IAQ parameters Age-of-air air-change effectiveness (EV) Specific Contaminant Concentration contaminant removal effectiveness e

Single value IAQ indicators Ev and ε Contaminant removal effectiveness (e) concentration at exhaust average contaminant concentration Contamination level 2. Air-change efficiency (Ev) shortest time for replacing the air average of local values of age of air Air freshness

Air-change efficiency (Ev) Depends only on airflow pattern in a room We need to calculate age of air (t) Average time of exchange What is the age of air at the exhaust? Type of flow Perfect mixing Piston (unidirectional) flow Flow with stagnation and short-circuiting flow

Air exchange efficiency for characteristic room ventilation flow types Flow pattern Air-change efficiency Comparison with average time of exchange Unidirectional flow 1 - 2 tn < texc < 2tn Perfect mixing 1 texc = tn Short Circuiting 0 - 1 texc > tn

Contaminant removal effectiveness (e) Depends on: position of a contaminant source Airflow in the room Questions 1) Is the concentration of pollutant in the room with stratified flow larger or smaller that the concentration with perfect mixing? 2) How to find the concentration at exhaust of the room?

Differences and similarities of Ev and e Depending on the source position: - similar or - completely different air quality Ev = 0.41 e = 0.19 e = 2.20

Thermal comfort Temperature and relative humidity

Thermal comfort Velocity Can create draft Draft is related to air temperature, air velocity, and turbulence intensity.

Thermal comfort Mean radiant temperature potential problems Asymmetry Warm ceiling (----) Cool wall (---) Cool ceiling (--) Warm wall (-)

Prediction of thermal comfort Predicted Mean Vote (PMV) + 3 hot + 2 warm + 1 slightly warm PMV = 0 neutral -1 slightly cool -2 cool -3 cold PMV = [0.303 exp ( -0.036 M ) + 0.028 ] L L - Thermal load on the body L = Internal heat production – heat loss to the actual environment L = M - W - [( Csk + Rsk + Esk ) + ( Cres + Eres )] Predicted Percentage Dissatisfied (PPD) PPD = 100 - 95 exp [ - (0.03353 PMV4 + 0.2179 PMV2)] Empirical correlations Ole Fanger Further Details: ANSI/ASHRAE standard 55, ISO standard 7730