Dynamic Hot Box Test for
Lightweight Concrete Wall Made with EPS Beads Concrete Forms
DYNAMIC GUARDED HOT BOX THERMAL TEST OF THE RASTRA WALL
Measurements of wall systems are
typically carried out by apparatus such as the one described in ASTM C 236, Standard
Test Method for "Steady-State Thermal Transmission Properties of Building
Assemblies by Means of a Guarded Hot Box" [ASTM C-236].
A dynamic test typically consists of the
three basic stages:
-
steady-state stage (steady temperatures on both sides of the wall),
-
thermal ramp (rapid change of the temperature on the one side of the wall), and
- stabilizing stage (wall is kept under
the second set of steady boundary temperatures until steady state heat transfer occurs).
The precision of dynamic testing is close
to the precision of the steady-state test method which is reported to be
approximately 8% [ASTM, 1989].
The dynamic test results were used to calibrate the finite difference
computer model that served in the analytical part of this project.
At Oak Ridge National Laboratory -
Buildings Technology Center the Rastra wall was built and tested in the guarded
hot-box under steady state and dynamic conditions. Steady air temperatures and
air velocities were set to a minimum on both surfaces of the tested wall during
the test. Experimental data recorded during the hot box test is presented on
figure 2, figure 3, figure 4, and figure 5.
As presented in
figure 2 and figure 3, two steady-state periods were achieved during the hot box
test. Meter side temperature was kept relatively constant at about 79 deg F.
Climate side temperature was about 20 deg F during the first
steady-state period and later it was increased to about 40 deg F during the
second period. Dynamic change of temperatures is later used for dynamic thermal
performance analysis.
Figure 2, figure 3, figure 4, and figure 5 depict
the experimental data that was compiled during the Rastra wall hot box
testing. The temperature data that is
presented above enable calculation of the average temperature for the time
interval after steady-state had been achieved.
The
surface-to-surface thermal resistance (R-Value) is calculated by
where R = thermal
resistance of wall assembly, hr ft2 °F/Btu (m2
K/W);
A = area
of metering chamber, 64 ft2 (5.3 m2);
t1 = average
surface temperature of the wall assembly on the metering side,
°F (° C);
t2 = average
surface temperature of the wall assembly on the climate side,
°F (° C);
Qh = metering
heater energy input, Btu/hr (W); and
Qf = metering
fan energy input, Btu/hr (W).
The overall thermal
resistance (Ru-Value which includes surface film resistances ) is
calculated by
where Ru = overall
thermal resistance of wall assembly, hr ft2 °F/Btu
(m2 K/W);
A = area
of metering chamber, 64 ft2 (5.3 m2);
th = average
meter side air temperature, °F (° C);
tc = average
climate side air temperature, °F (° C);
Qh = metering
heater energy input, Btu/hr (W); and
Qf = metering
fan energy input, Btu/hr (W).
Figure 6, depicts
climate and meter side air film thermal resistances. The meter side air film
thermal resistance (Rms air) is calculated by
where Rms
air = meter side film thermal resistance of wall assembly, hr ft2
°F/Btu (m2 K/W);
A = area
of metering chamber, 64 ft2 (5.3 m2);
th = average
meter side air temperature, °F (° C);
t1 = average
surface temperature of the wall assembly on the metering side,
°F (° C);
Qh = metering
heater energy input, Btu/hr (W); and
Qf = metering
fan energy input, Btu/hr (W).
The climate side air
film thermal resistance (Rcms air)
is calculated by
where Rcs
air = climate side film thermal resistance of wall assembly, hr ft2
°F/Btu (m2 K/W);
A = area
of metering chamber, 64 ft2 (5.3 m2);
t2 = average
surface temperature of the wall assembly on the climate side,°F (°C);
tc = average
climate side air temperature, °F
(°C);
Qh = metering
heater energy input, Btu/hr (W); and
Qf = metering
fan energy input, Btu/hr (W).
Metering box wall
losses were not included in any of the energy balance calculations. In the worst case, the metering box wall
loss represents less than 0.2% of the energy input (Qh. + Qf).
The clear wall steady-state R-value,
which was achieved during the analysis of Rastra wall experimental results, is 7.68 hft2F/Btu.
The dynamic response
of the wall was analyzed for a 20oF thermal ramp ( it took 2 hours
to change the surface temperature on
the climate side of the wall from 20 to 40oF ). Temperatures on both sides of the wall were
stabilized and the experiment was continued until steady-state heat transfer
occurred. During the first stage of the test process, air temperatures on both sides of the wall were stabilized at 80
and 20oF. During the second
stage, the climate side air temperature was increased form 20 to 40oF. Air temperatures for the meter and climate
sides of the wall and measured heat flux on the meter chamber side of the wall
are presented in figure 2, figure 3, figure 4, and figure 5.
Validation of
developed computer model of Rastra wall was made by comparing computer heat
flow predictions to the hot box measured heat flow through an 8 ft by 8ft
Rastra clear test wall exposed to dynamic boundary conditions. As showed in
Figure 7, good agreement was found between test and computer modeling results.
The measured air
temperatures; 6-in. away from the surface of the metering side and 14-in. away
form the climate side, along with air velocities measured in the meter and
climate chambers were used as boundary conditions for dynamic modeling of the
Rastra wall. The computer program reproduced all recorded test boundary
conditions (temperatures and heat transfer coefficients) with one-hour time
intervals. The Rastra wall internal
geometry was numerically described to create the Heating 7.2 input file. The following thermal properties of
materials were used in calibration of dynamic model:
• thermal conductivity of Rastra light weight
concrete blocks 0.87 Btu-in./hft2F,
• thermal conductivity of mortar 9.09
Btu-in./hft2F.
Values of heat flux
on the surface of the wall generated by the program were compared with the
values measured during the dynamic test.
As depicted in Figure 7, computer program reproduced the test data very
well. The average discrepancy between
test generated and simulated heat fluxes was less than 5% (first 60 hours of
the simulation were neglected due to the different initial conditions). This comparison confirms the ability of
Heating 7.2 to reproduce the dynamic heat transfer process measured during the
dynamic hot box test of the actual Rastra wall.
