OD DVOSTRUKO ZASTAKLJENOG PROZORA DO ...kgh-kongres.rs/images/2015/3/3-nastase.pdf2015/03/03 · OD DVOSTRUKO ZASTAKLJENOG PROZORA DO DVOSTRUKE FASADE –INDIKATORI PRENOSA TOPLOTE

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 NĂSTASEAlexandru ȘERBANGeorge DRAGOMIR

Sorin BOLOCANAlin Ionuț BREZEANU

46. Međunarodni kongres i izložba o KGH, Beograd, 2–4. Decembar 201546th International HVAC&R Congress and Exhibition, Belgrade, 2–4 December 2015

CIVIL ENGINEERING FACULTY, BUILDING SERVICES DEPT., BRAȘOV, ROMANIA


The envelope (façade) is the part of the building which forms the primary

thermal barrier with its environment. It represents the most important factor in

determining the level of comfort, natural lighting and ventilation ability, and

finally how much energy is needed for heating and air –conditioning [1]. The

main purpose of this study is to identify and emphasize the difference between

single-skin façade and double-skin façade from heat transfer point of view. For

this purpose were taken into account total thermal resistances, transmittances

and heat fluxes.

I. INTRODUCTION


Designing a double glazed façade entails the detailed analysis of

several variables and then their implementation in a model which can be

subjected to a simulation which can take into account the atmospheric condition

for a one-year period in order to make decisions related to the optimal

configuration of a double glazed façade and also to the operation method and

the strategies for controlling it.

In double glazed façades the heat transfer involves several phenomena

which occur simultaneously and which overlap. A schematic model of the heat

transfer through a double glazed façade (b) and (c) is shown in Figure 1, in

comparison with heat transfer through a double pane window (a).


Heat transfer illustrated in Figure 1 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, without and with shading device inside cavity.


II. DOUBLE-GLAZED WINDOW HEAT TRANSFER

ALGORITHM

• double glazing with the height of 2 m;

• the heat transfer considered during the

night;

• the effect of the solar radiation can be

ignored;

• in stationary regime and in one direction;

• and the effect of the casement is ignored.


The expression of the global unitary flow for the proposed case has the

expression [3]:

recegegerbcbgigirici

oi

hhkhhkhh

TT

A

Q

11111

(1)


The global unitary flow is usually expressed by the global heat transfer

coefficient U:

(2))( oioi TTU

R

TT

A

Q


Comparing equations (1) and (2) it follows R=1/U that is [3]:

(3)

recegegerbcbgigirici hhkhhkhhR

11111


The value of the convective thermal transfer coefficient in the space

between the two glass sheets is then obtained with the formula:

b

cbk

LNuh

(4)

where kb is the thermal conductivity of the gas between the two glass sheets [3].

The thermal conductivity kb is evaluated at the average temperature from the

space between the two sheets namely Tbm=T2+T3.


III. FROM DOUBLE-GLAZED WINDOW TO DOUBLE-

SKIN FAÇADE

The exterior envelope is represented in the case of most double-skin facades by secure

glass; its thickness may vary between 8 and 12 mm. The main role of the exterior envelope is

to offer the entire construction adequate resistance against exterior climatic conditions, at the

same time contributing to good acoustic insulation for the entire building.

The cavity of the double façade which is located between the exterior cover and the

interior one is an intermediary air layer whose thickness can range between 25 cm and 2 m.

The role of this intermediary air layer is to contribute to the increase of the thermal insulation

degree of the entire façade, at the same time offering the possibility that, in certain conditions,

air can be preheated inside it for the building’s natural or mechanical ventilation.


Heat transfer in double-skin

façade and temperature

distribution during cold

season


Further on we suggest the extension of the method used for double

glazed window with the final purpose of determining the total heat

flow, which horizontally passes through the entire double glazed façade

system, consisting of the three components, the exterior envelope, the cavity

and the interior envelope.

The calculation is also made in an iterative way by determining a

convective coefficient and a radiation one for the cavity and afterwards we

obtain the conductive resistance through the secure glass as well as the

convective and radiation resistance at the exterior surface of the secure glass,

which is in contact with the exterior air.


In case of double-skin façade equation (2) remains the same but

in equation (3) and respectively in (1) we have two more terms and

equation (3) can be written as:

recegsgsrcavccavgegerbcbgigirici hhkhhkhhkhhR

1111111

(5)


IV. CONCLUSIONS

As can be seen the difficult part of this calculation is to determine convection

superficial heat transfer coefficients, for both inner envelope and for the double-skin cavity.

This coefficient varies depending on the type of envelope chosen for the interior of the

double façade and for the cavity it depends on ventilation mode, which can be natural,

mechanical or hybrid. As indicated in literature the energy performance of a double-skin

façade depends on various aspects as location, orientation, climatic conditions, type of inner

and outer envelope, thermophysical properties of materials included, type of double-skin

façade, type of ventilation inside cavity, control strategies. Study of steady state heat transfer

can provide first information on the impact of adopting a double glass facade to a certain

building but for this purpose it is important to create an universally classification for such

system.

THANK YOU VERY MUCH!!

Gabriel NĂSTASEAlexandru ȘERBANGeorge DRAGOMIR

Sorin BOLOCANAlin Ionuț BREZEANU


CIVIL ENGINEERING FACULTY, BUILDING SERVICES DEPT., BRAȘOV, ROMANIA

Documents

OD DVOSTRUKO ZASTAKLJENOG PROZORA DO ...kgh-kongres.rs/images/2015/3/3-nastase.pdf2015/03/03 · OD DVOSTRUKO ZASTAKLJENOG PROZORA DO DVOSTRUKE FASADE –INDIKATORI PRENOSA TOPLOTE