These teachings relate generally to a system and method for measuring a 2- or 3-dimensional velocity vector in a detection system used in an emergency environment.
According to RAND Report on Emergency Responder Injuries and Fatalities, being caught or trapped, or being exposed to fire products or chemicals, is the second leading cause of injuries and fatalities within the fire service. All of these conditions can occur in a structural response when firefighters have limited information of the actual conditions inside the structure. These structures are not limited to buildings, but include other areas of interest such as tunnels and subway systems. The lack of real-time data and internal situational awareness contributes to the chaotic environment and the resulting loss of life. In addition to intentional acts of violence, this system of these teachings could have potential impacts to aid in the response to accidental fires in structures.
It is clear that current detection and preventions systems are limited in their ability to provide this critical information needed to enhance situational awareness of the responding firefighters and incident commanders who are responsible for making on-site decisions about the incident response. The majority of systems available today in the fire detection arena provide a single measure of information. This is typically in the form of a smoke or heat detection system relays only an indication that there is a problem. While these systems are a crucial component in addressing the life safety goals for the built environment, there are notorious for having faulty response rates. Most experts agree that the greatest shortcoming of fire detectors is a high rate of nuisance alarms that limit their credibility with the public.
Understanding the issues associated with existing detection systems, there is a trend toward the use of more elaborate and integrated sensing systems that would combine the fire detection and other safety and control systems together into one “intelligent” system. These types of systems are intended to increase the safety and response associated with fire related situations. Integrated building systems hold the potential for reducing false alarms, speeding building evacuation and assigning in fire fighting.
The ability of these advanced systems to provide such results are directly reliant upon the technology utilized and the design of the system. The purpose of detecting fires early is to provide an alarm when there is an environment which is deemed to be a threat to people or a building. High reliability detection is based on the supposition that it is possible to utilize a sufficient number of sensors to ascertain unequivocally that there is a growing threat either to people or to a building and provide an estimation of the seriousness of the threat. Therefore, reliability and design of the system is a critical component to ensure that system provides the correct data in enough time to allow both the people inside and the fire fighters responding to make informed decisions about their course of action. These types of systems have the ability to decrease loss of life and property.
Fire detection and suppression systems provide critical responses during a fire. In the early stages of a fire, it is critical to understand the flow behavior in order to conduct a proper fire protection assessment. The advancement in capability from the three dimensional velocity probe described herein has the potential of providing this increased level of flow behavior characterization needed for these advanced detection systems. This would allow for a more complex and accurate depiction of the actual situation within the structure allowing fire fighters and incident commanders to make more informed decisions regarding the structural integrity, smoke and toxic products faced.
One of the main requirements for such an emergency response system is reliable temperature and velocity prediction of fire induced flow fields. While thermocouples are relatively cheap and reliable, velocity measuring devices such as bidirectional probes and hot wire anemometers are almost 20 times the cost. Bidirectional probes cannot measure low velocity flows which originate from incipient or small fires, and hot wire anemometers normally cannot operate at a temperature range more than 50° C. Optical methods such as Laser Doppler anemometry and particle image velocimetry cannot be applied in large scale fire environments. The velocity measuring probe described herein allows accurate measurement of velocity at 1/10th the cost of current velocity measuring techniques opening up a new market for the fire sensing technology.
Previous studies have shown that the cross correlation velocity (CCV) measuring technique can be used to measure velocity. The CCV technique described herein is based in principle on the “frozen eddy” concept in turbulent flows put forward by Professor Geoffery Ingram Taylor in 1938. Taylor hypothesized that in a turbulent flow, there are eddy structures that retain their shape and characteristics over some time and space. A thermocouple pair can identify and trace these eddy structures to obtain the mean velocity of the flow. If the thermocouples are spaced d cm apart, the mean velocity of the flow, v, is simply equal to r/τ, where τ seconds is the offset (phase lag) between the two thermocouple signals. Professor Cox was the first to verify the “frozen eddy” hypothesis thereby developing the first one dimensional CCV probe in 1970. However, the high cost associated with expensive analogue correlators available in the 1970's caused the technique to gradually phase out. Hence, although conceptually sound, the idea was never implemented into a commercially available probe. The present teachings improve upon Cox's probe such that it can be mass produced and sold commercially, and then to develop a three-dimensional velocity measurement probe. The applications of a three dimensional probe are limitless and extend beyond fire applications to any environment that requires flow measurements in turbulent flows with a temperature gradient (e.g.: furnaces, coal fired power plants, aircraft engines, mining operations, ocean waves etc.). A probe design capable of measuring all the three components of a velocity vector simultaneously by correlating data from three or more thermocouples has never been designed.
The velocity probe to be described herein is inexpensive to construct (comprising of only seven thermocouples) and has the potential of yielding high accuracy with the recent advances in signal conditioning and data acquisition methods. The main advantage of the probe is its economy in construction and setting up cost. Ideally it could be placed anywhere in a fire room where thermocouples are usually installed, thereby allowing both temperature and velocity measurements simultaneously.