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

Transcription

1 CTU Prague, Faculty of Civil Engineering Department of Materials Engineering and Chemistry Thermal properties of contemporary lightweight cavity bricks: a semi-scale experimental study Z. Pavlík, J. Fořt, M. Jerman, R. Černý

2 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 producer, cavities arrangement and filling Results and discussion thermal conductivity, thermal transmittance, valuation of cavity fillers effect Conclusions

3 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.

4 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. 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.

5 Determination of thermal conductivity λ (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 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.

6 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.

7 The measured brick block was provided with necessary temperature and heat flux sensors and thermally insulated in the tunnel between the climatic chambers.

8

9 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%, 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.

10 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 q = λ gradt, where q (W/m 2 ) 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.

11 THERMAL PROPERTIES OF CONTEMPORARY LIGHTWEIGHT CAVITY BRICKS: A SEMI-SCALE 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 masonry having a width of 500 mm. Bulk density (kg/m3) Matrix density (kg/m3) Total open porosity (-) 0.51

12

13 Results and discussion

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

15 Thermal conductivity of brick blocks (W/mK) Air cavities Polystyrene balls filled cavities Mineral wool filled cavities One can see improvement of bricks thermal resistance by application of 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.

16 Thermal transmittances of brick blocks (W/m 2 K) Air cavities Polystyrene balls filled cavities Mineral wool filled cavities According to the Czech standard ČSN , all the measured materials are up to this standard that specifies required value of thermal transmittance for external wall equal to 0.3 W/m 2 K and recommended value equal to 0.25 W/m 2 K.

17 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 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.

18 THERMAL PROPERTIES OF CONTEMPORARY LIGHTWEIGHT CAVITY BRICKS: A SEMI-SCALE 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.