MEASUREMENT AND PREDICTION OF HEAT TRANSFER AND MASS FLOW OF A

VENTILATED FAÇADE

Tarantino Sergio (Stanford University)Belleri Annamaria (EURAC)

Arlati Ezio (Politecnico di Milano)Lollini Roberto (EURAC)

Opaque Ventilated Façade (OVF)

Cladding system made up of panels anchored to the building by means of

a metal structure.

Air flows in the intermediate cavity driven by the buoyancy effect.

An insulation layer is usually applied to the existing wall.

Reduce energy consumption of building by providing high thermal insulation, shading from solar radiation, protection against humidity penetration, an

opportunity for PV technologies and solar thermal systems integration.

source: Façade White papers by Leads Façade (2013)

Opaque Ventilated Façade (OVF)

Airflow network model

Provides fast useful information about bulk flows

Can be coupled with the thermal network

Our research:

- Modelling approach based on coupling thermal and airflow network in simulation

- Experimental validation on a test building

Difficult to predict influence on indoor environment

due to the unsteady convection flows in the air cavity

Case Study

Location

Koppen Climate Classification Map

credits: Murray C. Peel et al. – University of Melbourne

Case Study

Building

Concrete prefabricated panels Opaque ventilated façade

Case Study

Building

Typical Section

South Façade

Case Study

Envelope

Three cladding panel types are installed at different façade heights:• Stone panel (from ground to 1m height)• Brick panel (from 1m to 3.8m height)• Fibre cement panel (from 3.8m to 6m height)

Typical wall section source: technical sheets provided by manufacturers

Sensors nodes in each section of the façade at different heights: • at the base (0.4m from floor level)• around the higher-middle (2.6m)• at the top (5.5m) of every vertical section

Measured data• Wall inside surface temperature (T1)• Wall outside surface temperature (T2)• Surface temperature of the insulation layer (T3)• Surface temperature on the back side of the cladding panel (T4)• Air Temperature (Tair) at the base (0.4m) and the top (5.5m)

• Air Velocity (Vair) at the base (0.4m) and the top (5.5m)

Monitoring

Summer week: from August 28th to September 3rd 2015 Winter week: from December 3rd to december 10th 2015

All measurements are taken at 10min intervals

ModelingTwo thermal zones: • indoor environment• ventilated air cavity

Vertical sequence of three air nodes

Building external surface is splitted into three sub-surfaces

Model predicts temperatures at three building heightsSketch-up model view with

airflow network scheme

Air temperature of indoor thermal zone is set equal to the measured one

Modeling

The modelling approach combines:

• the airflow network model (Trnflow)

• the features of a wall model with exterior air gap (Type 56, Type 1230)

Two iterative steps:

1. estimate air mass flow rate induced by natural draft

2. estimate surface temperature on the inner side of the air cavity

Trnsys simulation software

Modeling

The flow through the façade air cavity is assumed fully developedand the pressure does not vary over a cross section

Discharge coefficient is derived by the overall resistance coefficient

Wind pressure coefficients used in the airflow network model derived from the AIVC database

Step 1 - Assumptions

[Etheridge D, 2012]

[Liddament M., 1986]

Open joints between the cladding panels are not modelled[Marinosci et al., 2011]

Modeling

Building thermal zone that represents the indoor environment (Type 56) coupled with a module (Type 1230) able to model an exterior wall where the outside surface is an high mass surface and has a ventilated air gap behind it

The model represents the ventilated wall as an opaque solar collector

Step 2 - Assumptions

[Duffie J.A. and Beckam W.A, 2013]

Type 1230 is coupled to Type 56 through the surface

temperature of the wall within the air cavity (T3)

ResultsMeasured vs simulation predicted temperature in the cavity

summer week winter week

at the façade base

at the façade top

summer week

winter week

Air cavity temperatures during the summer week vary within a much broader range(13 – 35°C) compared to the winter week (5 – 13°C) because of the solar radiation effect

Surface temperature of the inner side of cladding panels

Results

summer week

winter week

Air velocity within the air cavity

Results

summer week

winter week

Air temperatures within the ventilated cavity

ConclusionsThe performance of OVFs is strictly dependent on:• climate variables• their interaction with the building envelope (wind pressure distribution)• the urban surroundings (wind obstructions, heat island effect, ground reflectance)

Predicted airflow rate is affected by uncertainty in input parameters:• opening model used to calculate inlet and outlet flow resistances• discharge coefficients and dynamic loss coefficients• wind pressure coefficients and wind speed profile

Measurements required for detailed analysis:• inlet air temperature at façade base• wind pressure at façade base and top

The airflow network model does not estimate in detail the air velocity patterns:“reverse” flow was noted within the air cavity

Conclusions

Predicted air cavity temperatures are generally in good agreement with the measured ones

except during peak solar radiation timewhen the model over estimates the inner surface temperatures of the cladding panels

The modelling approach can be suitable to model OVFs using building energy simulation for energy savings assessment

OVFs design optimization requiresa better definition of airflow network input data

Thank you!

Questions?