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Experimental and numerical study of energy loss through double-glazed windows

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Abstract

A double-glazed window separating a small scale room and the cold environment outside it are modeled by such a window joining a hot box to a cold one. The effects of different parameters on the heat transfer from the hot box (room) to the cold box (outside) are investigated experimentally. These parameters include: the size of the air gap between the double-glazed window, installation of a curtain in the air gap, the curtain to window distance behind the double-glazed window and tilting the double-glazed window from its vertical position. Although the results of these investigations are qualitatively predictable, a comprehensive experimental study to quantify the share of each individual item on the heat loss through the window is a reasonable research. The results indicate that increasing the air gap from 12 to 21 mm, results in 5.1% less energy loss through the window. By installing the curtain in the middle of a 21 mm air gap, the energy loss reduces by 19.9% compared to the same air gap but without a curtain. Also, by installing the curtain at a distance of 0.4 to 4 cm behind the window on the hot side, a 22% to 33.9% reduction in energy loss is observed. A tilt angle of 10° from vertical position can decrease the energy loss by 6.5%. Numerical studies with the Ansys Fluent 16 software provide good agreement with the experimental data. These numerical studies are conducted for one case in each set of experiments and help to study parameters that changing them in experimental work is costly and time consumable.

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Abbreviations

\( {\overline{Q}}_{heater} \) :

The average heat generation rate of hot box heater(W)

Δt :

Summation of all time periods in which the hot box heater is on(s)

ΔT :

A long time period in which the average heat generation rate of hot box heater is measured(s)

t :

Time(s)

P :

Pressure(pa)

V :

Velocity(\( \frac{m}{s} \), \( \frac{cm}{s} \))

g :

Gravitational constant(\( \frac{m}{s^2} \))

e :

Internal energy per unit mass(\( \frac{j}{kg} \))

T :

Temperature(K)

u j :

The component of velocity in direction j(\( \frac{m}{s} \))

K :

Conductivity (\( \frac{W}{mK} \))

K glass :

Glass conductivity (\( \frac{W}{mK} \))

A glass :

Surface of hot or cold side glass(m2)

\( {\dot{q}}_{generation} \) :

Heat generation rate per unit volume(\( \frac{W}{m^3} \))

T 0 :

reference temperature in Boussinesq equation(K)

q i :

The net rate at which radiation leaves surface i(W)

A i :

The area of surface i(m2)

J i :

Radiosity of surface i(\( \frac{W}{m^2} \))

G i :

Irradiance of surface i(\( \frac{W}{m^2} \))

E bi :

The rate at which black surface i emits(\( \frac{W}{m^2} \))

q ij :

The net rate at which radiative heat transfer occurs from surface i to j(W)

F ij :

View factor(\( \frac{q_{ij}}{A_i{j}_i} \))

\( {\overline{q^{"}}}_{heater} \) :

The average heat flux of hot box heater(\( \frac{W}{m^2} \))

T middle :

Temperature of a point inside the hot box or cold box which is located at middle (K)

T down _ left :

Temperature of a point inside the hot box which is located at down_left (K)

T down _ right :

Temperature of a point inside the hot box which is located at down_right (K)

T top :

Temperature of a point inside the hot box or cold box which is located at top (K)

T down :

Temperature of a point inside the cold box which is located at top (K)

T right :

Temperature of a point inside the cold box which is located at right (K)

\( {\overline{T}}_{glazing\ surface\ next\ to\ hot\ box} \) :

The mean temperature of glazing surface next to hot box(0K)

\( {\overline{T}}_{hot\ side\ glazing} \) :

The mean temperature of total glazing surface next to hot and guard box (0C)

\( {\overline{T}}_{\mathrm{cold}\ side\ glazing} \) :

The mean temperature of total glazing surface next to cold box (0C)

T 1 :

The mean surface temperature of hot glass on the side adjacent to the air gap (0C)

T 2 :

The mean surface temperature of cold glass on the side adjacent to the air gap (0C)

L :

The distance between the two pieces of glass (mm)

\( {\overline{Q}}_{air\ gap} \) :

The average heat transfer rate through double glazed window (W)

\( {\overline{Q}}_{air\ gap\_ radiative} \) :

The average radiative heat transfer rate through double glazed window (W)

\( {\overline{Q}}_{air\ gap\_ convective} \) :

The average convective heat transfer rate through double glazed window (W)

Ra air gap :

Flow Rayleigh number in the air gap

Nu air gap :

Flow Nusselt number in the air gap

\( {\overline{T}}_{curtain} \) :

The average temperature of curtain surface(K)

\( {\overline{T}}_{\mathrm{hot}\ \mathrm{box}} \) :

The volume average temperature of hot box (0C)

d :

The distance between curtain and glazing surface next to hot box(m)

h :

Convective heat transfer coefficient (\( \frac{W}{m^2K} \))

A glazing surface next to hot box :

The area of glazing surface next to hot box(m2)

μ :

Dynamic viscosity(pa. s)

ρ :

Density(\( \frac{kg}{m^3} \))

ρ 0 :

reference density in boussinesq equation(\( \frac{kg}{m^3} \))

β :

Thermal expansion coefficient (K−1)

β air :

Thermal expansion coefficient of air (K−1)

ν air :

Kinematic viscosity of air (\( \frac{m^2}{s} \))

α air :

Thermal diffusivity (\( \frac{m^2}{s} \))

θ :

Tilt angle of double glazed window from vertical position(°)

τ ij :

Stress on an element surface(pa)

ε i :

Emissivity of surface i

ε curtain :

Emissivity of curtain surface

ε glazing :

Emissivity of glazing surface

σ :

Stefan–Boltzmann constant(\( \frac{W}{m^2{K}^4} \))

δ glass :

Thickness of glass(m)

Nu :

Nusselt number

Ra :

Rayleigh number

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Authors and Affiliations

  1. Mechanical Engineering Department, Amirkabir University of Technology, 424 Hafez Avenue, Tehran, Iran

    Mohammadhassan Bitaab, Reza Hosseini Abardeh & Saeid Movahhed

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  1. Mohammadhassan Bitaab
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  2. Reza Hosseini Abardeh
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  3. Saeid Movahhed
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Correspondence to Reza Hosseini Abardeh.

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Bitaab, M., Hosseini Abardeh, R. & Movahhed, S. Experimental and numerical study of energy loss through double-glazed windows. Heat Mass Transfer 56, 727–747 (2020). https://doi.org/10.1007/s00231-019-02729-4

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