The ability to measure moisture is a challenging issue
Elsevier, Fire Safety Journal, shared under Creative Commons license
Shruti Ghanekar. Rajavasanth Rajasegar, Nicholas Traina, Constandinos Mitsingas, Richard M.Kesler, Gavin P.Hornb, Robin Zevotek, Stephen Kerber, and Tonghun Lee
Highlights
TDLAS based system for water-vapor measurement through varying smoke obscuration.
Pre- and post-suppression concentrations comparable except in metal structure.
Highest temperature and water-vapor concentration for pallet/straw/OSB as fuel load.
Non-conforming water-vapor trends in metal structure compared to other structures
Abstract
A robust tunable diode laser absorption spectroscopy (TDLAS) based system is developed and deployed to make real-time water-vapor concentration measurements in quasi-controlled live-fire experiments conducted in firefighter training props. This system targets the 1392.5 nm (7181.15 cm−1) water-vapor absorption line while employing a multi-tier detection sensitivity scheme that allows for measurements at multiple locations in fire environment through varying smoke obscuration levels. Temperature-corrected absorbance values are compared to HITRAN simulations to quantify water-vapor concentration. Upon validation in laboratory setting, the impact of firefighter hose stream application on water-vapor concentration is studied. Comparative effects of training structures (metal, concrete and drywall-lined) and fuel-loads (pallet/straw, lightweight furnishings and pallet/straw/oriented-strand-board (OSB)) on water-vapor concentration are characterized. Despite small increase in water-vapor concentration due to suppression, the post-suppression concentrations are found to be comparable or lower than the corresponding maximum pre-suppression concentrations in all scenarios except the metal structure. Irrespective of the structure, highest temperature and water-vapor concentrations are measured with pallet/straw/OSB fuel-load. Under identical fuel-loads, the drywall structure scenarios generate highest water-vapor concentration. Peak water-vapor concentrations are measured post-suppression in typical training structures (near-floor and crawling levels), but prior to suppression in the structure/fuel package combination that simulated a typical residential fire scenario.
Introduction
There exists a complex interplay between water and fire. Water is a primary byproduct in the combustion of hydrocarbon materials such as those that are common fuels in unwanted structure fires in the built environment. At the same time, water is the primary suppression agent used by firefighters to combat Class A fires. Fire service application of water is critical for successful and safe fire extinguishment. If not applied in appropriate flow rate, nozzle pattern and location, suppression streams may not reach the source of the fire and/or result in the generation of excess steam, potentially endangering occupants of the structure and the firefighter themselves. In recent years, there has been a conscious scientific effort geared towards understanding fires in terms of the underlying parameters: temperature, pressure, heat flux, concentrations of chemical species –oxygen, carbon dioxide, carbon monoxide, etc. [[1], [2], [3], [4]]. However, the relative magnitude of moisture introduced into the environment by steam production from application of water to a burning fuel compared to that generated by the fire itself is not well understood. In fact, the ability to measure moisture at elevated temperatures has been identified as a need for improved hazard assessments for occupants who are potentially trapped in the structure. In the SFPE Handbook [5], Purser suggests that: “… it is possible that the presence of water-vapor may be an important neglected hazard in fires.” and “Humid air, steam or smoke with a high thermal capacity of latent heat (due to vapor content or suspended liquid or solid particles) may be dangerous at temperatures of around 100 °C, causing burns throughout the respiratory tract.”
The ability to measure moisture concentration in such environments is a critical tool for research in firefighter safety as well as to fully understand the impact of tactical decisions on trapped occupants’ safety. While several instruments exist to characterize parameters such as temperature, heat flux and gas concentrations in a fire environment within a structure, the ability to measure moisture content in conditions applicable to describing fire environments, particularly after application of water to suppress the fire is a challenging issue. The amount of moisture content in an environment can be derived from humidity measured using instruments such as psychrometers, optical condensation hygrometers and dew cells which record dew point of the sample or from measurement of the change in electrical properties of certain hygroscopic materials that respond to changes in relative humidity. Most of these techniques are not suitable for moisture measurements at high temperatures. Although chilled mirror hygrometers which measure the dew point are used in high temperature commercial furnace environments and capacitive hygrometers have been used to make water-vapor measurements in prescribed grass fire, smoldering smoke and biomass combustion plumes [[6], [7], [8]], these instruments have a slow response time and are incapable of measuring water-vapor concentration as percentage of air. Moreover, sampling gases at high temperatures for transport for remote moisture measurements is challenging due to the need for bulky and cumbersome techniques to prevent condensation before it reaches the instrument. It is likewise impractical to sample and condense water-vapor to get a percentage of water-vapor in air at a sampling resolution of 1 Hz in a full-scale out-of-laboratory experiment. A more direct approach can be adopted to measure moisture content in situ using a spectroscopic technique such as tunable diode laser absorption spectroscopy (TDLAS), in which the amount of absorbed light at a particular wavelength is directly proportional to the moisture content in the environment. As the measurement is carried out by using a beam of laser across the medium, an apparatus to prevent condensation in the sampling lines is not required. Furthermore, this technique allows for accurate, continuous, in situ measurements without affecting the species composition at the measurement location.
TDLAS techniques are often used to measure species concentration in high-temperature and high-pressure reactive environments such as combustors and shock tubes [[9], [10], [11], [12], [13], [14], [15]] as well as in harsh environments such as furnaces, power plants and boilers where obscuration presents a major challenge to the successful measurement of the species concentration [[16], [17], [18], [19], [20], [21], [22], [23]]. Tunable diode laser absorption techniques have also been successfully used to make in situ molecular oxygen concentration measurements in fire environments, where varying levels of obscuration due to smoke and elevated temperatures present a serious challenge [24,25].
In addition to the scientific utility of this tool, such an instrument can be valuable for informing and training firefighters on the impact of hose stream application. A common concern in the US Fire Service is the impact of fire streams on occupant tenability, particularly with respect to steam generation and the risk for trapped occupant burns [26]. Firefighters are taught about hose stream application – and often steam generation – during live-fire training scenarios that are typically conducted in structures with concrete or metal walls using wood-based fuels such as pallets and/or engineered wood products such as oriented strand board (OSB). However, typical residential fires are suppressed in structures with drywall surface finishes and largely polymer-based fuels. If the feedback from the training fire environment does not appropriately simulate typical residential structure fires, an incorrect message may be reinforced to the firefighters.
In this work, development and laboratory assessment of a multi-tier TDLAS system is described first. This tool is then applied to a series of quasi-controlled experiments conducted at the Illinois Fire Service Institute (IFSI) using three types of fuel loads in three different training props. Changes in water-vapor concentration are characterized as the fire evolves and is suppressed by hose stream water application.
