Thermal properties of contemporary lightweight cavity

Thermal properties of contemporary lightweight cavi ty bricks: a semi-scale experimental study

Z. Pavlík, J. Fo řt, M. Jerman, R. Černý

■■■ CTU Prague, Faculty of Civil Engineering

Department of Materials Engineering and Chemistry

THERMAL PROPERTIES OF CONTEMPORARY LIGHTWEIGHT CAVI TY BRICKS: A SEMI-SCALE EXPERIMENTAL STUDYcontact: [email protected]

Outline:

• Introduction – motivation of the work – design of new advanced types of

cavity bricks

• Semi-scale experiment and its evaluation

• Studied cavity bricks – basic properties of the brick body, materials • Studied cavity bricks – basic properties of the brick body, materials

producer, cavities arrangement and filling

• Results and discussion – thermal conductivity, thermal transmittance,

valuation of cavity fillers effect

• Conclusions

Introduction – work motivation

• Thermal comfort of buildings occupants and indoor air quality are important

factors to be considered during building design and operation.

• With reference to the sustainable development principles, especially the

passive and low energy buildings should be constructed - the key features


of passive and low energy buildings design: building location and

orientation on the site, window design, building layout, shading and

ventilation, insulation and thermal mass (heat storage capacity) of building

envelopes.

• In order to meet requirements on thermophysical properties of materials for

envelopes of passive and low energy buildings, new advanced materials

are being developed.

Introduction – work motivation II

• In Central European countries, there is paid specific attention to the design

and manufacturing of single-layer building envelope systems based on

lightweight cavity bricks, whereas the cavities are filled either by air or

thermal insulation materials as polystyrene balls, mineral wool, crushed

polyurethane foam, etc.


polyurethane foam, etc.

• Filling the cavities by thermal insulation materials reduces the heat transfer

by radiation and contributes to the higher thermal resistance of brick block

compared to materials with air cavities only.

• In this contribution we refer about determination of thermophysical

properties of new types of cavity bricks produced in Czech Republic.

Determination of thermal conductivity λλλλ (W/mK)

• Main material parameter describing the heat transport in materials.

• From the engineering point of view this parameter includes heat

conduction, convection, and radiation.

• For determination of thermal conductivity, steady state methods can be

used - as for the practical experimental setups, the guarded hot plate


used - as for the practical experimental setups, the guarded hot plate

arrangement is the most frequently applied method.

• Application of the transient methods of thermal conductivity measurement

- hot wire method, heat flow impulse method, laser flash, etc.

• Most of the laboratory methods allow determination of thermal conductivity

on small samples or for a plate sample configuration with small specimen

thickness. This makes possible measurement of homogeneous materials

with no large-scale discontinuities only – new steady state technique is

designed and applied for entire brick blocks testing.

SEMI-SCALE EXPERIMENT

Within the semi-scale experiment, the climatic chamber system originally

developed for investigation of hygrothermal performance of building materials

and structures was used.


• The measured brick block was provided with necessary temperature and

heat flux sensors and thermally insulated in the tunnel between the climatic

chambers.



• In the climatic chambers, specific different temperatures were set,

nominally temperature difference 15/30°C and 30% re lative humidity were

used. In this way, the 1-D heat transport through the studied brick block

was simulated until the steady state heat flux was obtained.

• Sensors form Ahlborn Germany - the capacitive relative humidity sensors

are applicable in the 5-98% relative humidity range with accuracy ± 2%,


are applicable in the 5-98% relative humidity range with accuracy ± 2%,

the thermometers have an accuracy ± 0.1°C in the temp erature range from

0°C to 70°C.

• The heat flux through the studied brick was monitored by the heat flux plate

sensors Ahlborn FQA020C of a cylindrical shape having diameter 33 mm,

which were fixed on both front sides of the brick block. The accuracy of

these sensors was ± 5% of the measured value.

Evaluation of Semi-Scale Experiment

• Calculation of thermal conductivity λ (W/m/K) was done using the

measured data of heat fluxes according to the equation


,gradTq ⋅−= λ

where q (W/m2) is the heat flux and T (K) temperature.

• The heat flux data were averaged over a time interval of several days and

then used for calculations.

Studied cavity bricks

• Three types of brick blocks with internal cavities produced by company

Heluz Brick Industry, Czech Republic, were analyzed.

• Two types of bricks had the cavities filled by mineral wool or polystyrene

balls, whereas the third brick had air cavities only, without additional

thermal insulation filling. The bricks were designed for application in


thermal insulation filling. The bricks were designed for application in

thermal insulation masonry having

a width of 500 mm.

Bulk density(kg/m 3)

Matrix density(kg/m 3)

Total open porosity

(-)

1 389 2 830 0.51


Results and discussion


Typical heat flux measured at air-cavities

brick surface.


Typical heat flux measured at polystyrene-

balls filled cavities brick surface.

Typical heat flux measured at mineral-wool filled cavities brick surface.

Thermal conductivity of brick blocks (W/mK)

• One can see improvement of bricks thermal resistance by application of


Air cavities

Polystyrene balls filled cavities

Mineral wool filled cavities

0.123 0.091 0.075

thermal insulation materials as cavity fillers.

• From the quantitative point of view, the best thermal insulation performance

exhibited brick block with mineral-wool filled cavities. Its thermal

conductivity was about 40% lower compared to brick block with air cavities.

• Also the application of polystyrene filler led to the significant improvement

of thermal insulation properties.

Thermal transmittances of brick blocks (W/m2K)


Air cavities

Polystyrene balls filled cavities

Mineral wool filled cavities

0.246 0.182 0.150

• According to the Czech standard ČSN 730540-3, all the measured

materials are up to this standard that specifies required value of thermal

transmittance for external wall equal to 0.3 W/m2K and recommended

value equal to 0.25 W/m2K.

Conclusions

• A steady-state experiment in semi-scale conditions was applied for the

determination of the effective thermal conductivity of a brick blocks with

internal cavities.

• The result of the experiment presents valuable information for the building

practice, where the measured thermal conductivities can find use at the


practice, where the measured thermal conductivities can find use at the

thermal design of brick based structures.

• The obtained data pointed to the effectiveness of tested cavity fillers for

improvement of thermal resistance of single-layer brick building envelopes.

• In future work, the real climatic data for the Prague test reference year will

be applied on the exterior surface of the brick block in order to simulate real

climatic loading and hygrothermal performance of the brick blocks.


THANK YOU FOR YOUR ATTENTION!

This research has been supported by the Ministry of Industry and Trade of the Czech Republic, under project No FR-TI2/007.

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Thermal properties of contemporary lightweight cavity