Air Curtains Combined with Smoke Exhaust for Smoke Control in Case of Fire: Full-Size Experiments

Abstract

This paper analyses the possibility of using air curtains to prevent smoke flow from fire compartments. Full size experiments have been carried out and several relevant conditions to assess smoke-tightness have been tested. The smoke temperature during the tests was ranging from 182°C to 351°C, the angle measured between the curtain axis and the vertical plane was ranging from 18° and 26°, the nozzle thickness was ranging from 0.017 m to 0.045 m and the velocity at the nozzle was ranging from 8.3 m/s to 19.9 m/s. During the tests, the air curtain’s nozzle was positioned horizontally at the top of a permanent opening (door). With this configuration, we obtained an approximately vertical downward jet through the used opening. This paper includes the final results of the tests and develops an analytical tool for predicting the performance of air curtains. It was concluded that it is possible to achieve smoke-tightness, provided that the adequate plane jet parameters and the compartment’s smoke exhaust are correctly adjusted. According to this analysis, the smoke-tightness limit corresponds to equation $B = \Delta {\text{P}}_{\text{a}} /\Delta {\text{P}}_{\text{s}} = - 0.30 \,u_{a} /u_{a\_min} + 1.25$ (with $1.30 \le u_{a} /u_{a\_min} \le 1.67$).

Appendix: Uncertainty Assessment

1.1 Exhaust Volume Flow Rate from the Compartment

The exhaust volume flow rate was assessed by comparing the frequency of the fan variable speed drive with the average flow velocity in the exhaust outlet. The components of uncertainty are:

• Anemometer Calibration

• Anemometer measurement

• Average of the measurements in the exhaust outlet

• Calculation of volume flow rate

• Frequency display

• Linear regression

• Flow correction due to temperature

The standard uncertainty was determined following the document JCGM 100:2008 [18].

It was used a fan anemometer. The standard uncertainty due to the calibration of the anemometer is U (v_cal-cRL) = 0.285 m/s. The standard uncertainty of the reading of the anemometer was evaluated as U (v_amem) = 0.005 m/s; as it is very small it was neglected.

The average ${\bar{\text{v}}}$ of n velocity measurements ${\bar{\text{v}}}_{i}$ in the exhaust outlet was assessed and its combined standard uncertainty $U\left( {{\bar{\text{v}}}} \right)$ was determined according to the following formula, where U $\left( {{\text{X}}_{\text{j}} } \right)$ is the standard uncertainty of the variable ${\text{x}}_{\text{j}}$:

$$ {\text{U}}\left( {{\bar{\text{v}}}} \right) = \left[ {\sum \left( {\frac{{\partial {\bar{\text{v}}}}}{{\partial {\text{x}}_{\text{j}} }}} \right)^{2} {\text{U}}^{2} \left( {{\text{X}}_{\text{j}} } \right)} \right]^{0.5} $$

(15)

The calculated value is $U\left( {{\bar{\text{v}}}} \right) = 0.127$  m/s.

The standard uncertainty due to the calculation of the volume flow rate is $U\left( {\acute{V}} \right) = 0.334$  m/s.

The standard uncertainty due to the frequency display was estimated according to method B of the document JCGM 100:2008 [18] and is $U\left( F \right) = 8,33 \times 10^{ - 6}$  Hz, and was neglected due to its very low value.

The standard uncertainty associated with linear regression between the frequency of the fan controller and the volume flow rate is less than or equal to 1.4% of the measurement in the relevant range (flows greater than 1.58 m³/s). The value of the standard uncertainty of the volume flow rate, including the linear regression component, is $U\left( {\acute{V}} \right) = 0.339$  m³/s.

The calibration of the volume flow rate measurement system was carried out with isothermal flow. During tests there were significant heat losses in the duct between the compartment outlet and the fan. Due to these heat losses the volume flow rate at the compartment outlet $\dot{V}_{exaust}$ is higher than the volume flow rate at the fan inlet $\dot{V}$, but the correlation between frequency of the fan controller and the volume flow rate is valid for the fan only. The temperature of the smoke was measured at the compartment outlet $T_{ex}$ and at the exhaust fan inlet $T_{fan}$ and the volume flow rate was corrected according to the following expression:

$$ {\dot{\text{V}}}_{\text{exaust}} = {\dot{\text{V}}}\frac{{{\text{T}}_{\text{ex}} }}{{{\text{T}}_{\text{fan}} }} $$

(16)

In the less favourable case $T_{ex} = 613$  K, $T_{fan} = 529$  K and $\dot{V} = 3.47$  m³/s. The standard uncertainty of the thermocouples was assessed as 0.90 K.

The combined standard uncertainty was calculated considering both temperatures and $\dot{V}$ as uncertainty sources and the final value of the standard uncertainty of the corrected volume flow rate is $U\left( {\dot{V}_{exaust} } \right) = 0.393$  m³/s.

1.2 Nozzle Velocity

The velocity at the nozzle was measured at several points of the nozzle by a hot wire anemometer (model Airflow TSI 8455) and its average was compared with the velocity measured by a fixed anemometer (of the same model). The procedure was repeated for diferent levels of nozzle velocity and a linear regression was obtained. The velocity measurements during tests were made by the fixed anemometer and corrected by the linear regression results.

The components of uncertainty are:

• Anemometer Calibration

• Anemometer measurement

• Linear regression

The standard uncertainty of the anemometer, including the calibration and the measurement, was assessed as U (v) = 0.04 m/s. The standard uncertainty of the linear regression was determined following the document JCGM 100:2008 [18] and it is less of 2.5% of the reference velocity reading.

The combined standard uncertainty was calculated considering these uncertainty sources and the final value of the standard uncertainty of the nozzle velocity is $U\left( {\text{v}} \right) = 0.462$  m/s.

1.3 Exhaust Convected Part of the Heat Release Rate

For the calculation of the exhaust convected part of the heat release rate the Eq. (8) is followed. The components of the uncertainty are:

• Exhaust volume flow rate from the compartment

• Temperature

• Density

• Specific heat at constant pressure

It was assessed in Sect. 5.1 that the final value of the standard uncertainty of the corrected volume flow rate is $U\left( {\dot{V}_{exaust} } \right) = 0.39$  m³/s.

The standard uncertainty of the thermocouples was assessed as 0.90 K.

The density $\rho$ was calculated according to the formula, being T the temperature:

$$ \uprho = \frac{353.55}{\text{T}}\,{\text{kg}}/{\text{m}}^{3} $$

(17)

The standard uncertainty $U\left( \rho \right)$ was determined according to the following formula, where U(T) is the standard uncertainty of the temperature:

$$ {\text{U}}\left(\uprho \right) = \left( {\frac{{\partial\uprho}}{{\partial {\text{T}}}}} \right){\text{U}}\left( {\text{T}} \right) $$

(B4)

The calculated value is ${\text{U}}\left( \rho \right) = 1.36 \times 10^{ - 3}$  kg/m³.

The specific heat at constant pressure was calculated according to the formula, being T the temperature:

$$ {\text{C}}_{\text{p}} = 0.1835 {\text{T}} + 948.42\,{\text{J}}/\left( {{\text{kg}}\;{\text{K}}} \right) $$

(18)

The standard uncertainty $U\left( {C_{p} } \right)$ was determined according to the following formula, where U $\left( {\text{T}} \right)$ is the standard uncertainty of the temperature:

$$ {\text{U}}\left( {{\text{C}}_{\text{p}} } \right) = \left( {\frac{{\partial {\text{C}}_{\text{p}} }}{{\partial {\text{T}}}}} \right){\text{U}}\left( {\text{T}} \right) $$

(19)

The calculated value is ${\text{U}}\left( {C_{p} } \right) = 1.65$  J/(kg K)

The combined standard uncertainty of the exhaust convected part of the heat release rate $U\left( {{\dot{\text{Q}}}_{\text{c}} } \right)$ was determined according to the following formula, taking into account the Eq. (8), where U $\left( {{\text{x}}_{\text{j}} } \right)$ is the standard uncertainty of the variable ${\text{x}}_{\text{j}}$:

$$ {\text{U}}\left( {{\dot{\text{Q}}}_{\text{c}} } \right) = \left[ {\sum \left( {\frac{{\partial {\dot{\text{Q}}}_{\text{c}} }}{{\partial {\text{x}}_{\text{j}} }}} \right)^{2} {\text{U}}^{2} \left( {{\text{x}}_{\text{j}} } \right)} \right]^{0.5} $$

(20)

The standard uncertainty in the estimation of the exhaust convected part of the heat release rate was less than 17%, except for test 8, in which it was 23%.

1.4 Heat Release Rate

The fire source heat release rate was assessed through the measurement of the gasoline consumption. The gasoline consumption was evaluated by weight variation (by a load cell model HBM U2B/1kN) and this method was calibrated by oxygen depletion. The components of uncertainty are:

• Weight measurement

• Linear regression of the gasoline mass loss during calibration

• Oxygen depletion analysis

• Linear regression of the heat release rate measured by oxygen depletion

The uncertainty due to the weight measurement is very small when compared to the other components of the uncertainty; therefore, it was neglected.

In the pool fire the mass loss of gasoline was assumed to be linear with the time during the calibration; therefore, there is a source of the uncertainty in the linearization of the mass loss. For the assessment of the standard uncertainty of the linear regression of the gasoline mass loss during calibration was followed the document JCGM 100:2008 [18] and its value is 0.00832 kg/s.

The standard uncertainty of the oxygen depletion analysis was previously assessed by the laboratory and the value does not exceed 9% of the measurement.

During the calibration the heat release rate readings made by oxygen depletion, the variance was calculated and the standard uncertainty (due to the assumption that heat release rate was constant) was assessed, being its value 4946 W.

The combined standard uncertainty is $U\left( {\dot{Q}} \right) = \sqrt {0.0081 \dot{Q}^{2} + 24.5 \times 10^{6} } W$.