OD DVOSTRUKO ZASTAKLJENOG PROZORA DO DVOSTRUKE FASADE INDIKATORI PRENOSA TOPLOTE STACIONARNOG STANJA

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1 OD DVOSTRUKO ZASTAKLJENOG PROZORA DO DVOSTRUKE FASADE INDIKATORI PRENOSA TOPLOTE STACIONARNOG STANJA FROM DOUBLE-GLAZED WINDOW TO DOUBLE-SKIN FACADE STEADY STATE HEAT TRANSFER INDICATORS Gabriel NĂSTASE Alexandru ȘERBAN George DRAGOMIR Sorin BOLOCAN Alin Ionuț BREZEANU CIVIL ENGINEERING FACULTY, BUILDING SERVICES DEPT., BRAȘOV, ROMANIA

2 I. INTRODUCTION Te envelope (façade) is te part of te building wic forms te primary termal barrier wit its environment. It represents te most important factor in determining te level of comfort, natural ligting and ventilation ability, and finally ow muc energy is needed for eating and air conditioning []. Te main purpose of tis study is to identify and empasize te difference between single-skin façade and double-skin façade from eat transfer point of view. For tis purpose were taken into account total termal resistances, transmittances and eat fluxes.

3 Designing a double glazed façade entails te detailed analysis of several variables and ten teir implementation in a model wic can be subjected to a simulation wic can take into account te atmosperic condition for a one-year period in order to make decisions related to te optimal configuration of a double glazed façade and also to te operation metod and te strategies for controlling it. In double glazed façades te eat transfer involves several penomena wic occur simultaneously and wic overlap. A scematic model of te eat transfer troug a double glazed façade (b) and (c) is sown in Figure, in comparison wit eat transfer troug a double pane window (a).

4 Heat transfer illustrated in Figure include direct and diffuse solar radiation, conduction, convection, and long-wave radiation. (a) (b) (c) Heat transfer in double pane window and in double skin façade, witout and wit sading device inside cavity.

5 II. DOUBLE-GLAZED WINDOW HEAT TRANSFER ALGORITHM double glazing wit te eigt of 2 m; te eat transfer considered during te nigt; te effect of te solar radiation can be ignored; in stationary regime and in one direction; and te effect of te casement is ignored.

6 Te expression of te global unitary flow for te proposed case as te expression [3]: Q A ci ri k gi gi T cb i T o rb k ge ge ce re ()

7 Te global unitary flow is usually expressed by te global eat transfer coefficient U: Q T i T o U ( T T ) i o (2) A R

8 Comparing equations () and (2) it follows R=/U tat is [3]: R ci ri k gi gi cb rb k ge ge ce re (3)

9 Te value of te convective termal transfer coefficient in te space between te two glass seets is ten obtained wit te formula: Nu (4) cb kb were k b is te termal conductivity of te gas between te two glass seets [3]. Te termal conductivity kb is evaluated at te average temperature from te space between te two seets namely T bm =T 2 +T 3. L

10 III. FROM DOUBLE-GLAZED WINDOW TO DOUBLE- SKIN FAÇADE Te exterior envelope is represented in te case of most double-skin facades by secure glass; its tickness may vary between 8 and 2 mm. Te main role of te exterior envelope is to offer te entire construction adequate resistance against exterior climatic conditions, at te same time contributing to good acoustic insulation for te entire building. Te cavity of te double façade wic is located between te exterior cover and te interior one is an intermediary air layer wose tickness can range between 25 cm and 2 m. Te role of tis intermediary air layer is to contribute to te increase of te termal insulation degree of te entire façade, at te same time offering te possibility tat, in certain conditions, air can be preeated inside it for te building s natural or mecanical ventilation.

11 Heat transfer in double-skin façade and temperature distribution during cold season

12 Furter on we suggest te extension of te metod used for double glazed window wit te final purpose of determining te total eat flow, wic orizontally passes troug te entire double glazed façade system, consisting of te tree components, te exterior envelope, te cavity and te interior envelope. Te calculation is also made in an iterative way by determining a convective coefficient and a radiation one for te cavity and afterwards we obtain te conductive resistance troug te secure glass as well as te convective and radiation resistance at te exterior surface of te secure glass, wic is in contact wit te exterior air.

13 In case of double-skin façade equation (2) remains te same but in equation (3) and respectively in () we ave two more terms and equation (3) can be written as: R ci ri k gi gi cb rb k ge ge ccav rcav k gs gs ce re (5)

14 IV. CONCLUSIONS As can be seen te difficult part of tis calculation is to determine convection superficial eat transfer coefficients, for bot inner envelope and for te double-skin cavity. Tis coefficient varies depending on te type of envelope cosen for te interior of te double façade and for te cavity it depends on ventilation mode, wic can be natural, mecanical or ybrid. As indicated in literature te energy performance of a double-skin façade depends on various aspects as location, orientation, climatic conditions, type of inner and outer envelope, termopysical properties of materials included, type of double-skin façade, type of ventilation inside cavity, control strategies. Study of steady state eat transfer can provide first information on te impact of adopting a double glass facade to a certain building but for tis purpose it is important to create an universally classification for suc system.

15 THANK YOU VERY MUCH!! Gabriel NĂSTASE Alexandru ȘERBAN George DRAGOMIR Sorin BOLOCAN Alin Ionuț BREZEANU CIVIL ENGINEERING FACULTY, BUILDING SERVICES DEPT., BRAȘOV, ROMANIA