Abstract:
A simulated smoke generator method and apparatus is provided for generating a consistent smoke plume. By using a closed loop controller to maintain at least one property, affecting one or more characteristics of the oil, at a desired level, a consistent type of simulated smoke is generated.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to methods and apparatuses for generating simulated smoke, and in particular to methods and apparatuses for generating simulated smoke that may be used for testing smoke and fire detection equipment. 
   2. Background Description 
   Aircraft smoke detection testing, for example, used to test the performance of smoke detection systems for cargo compartments of aircraft, has been a highly uncertain and often costly component of the airplane certification process. Whenever a cargo compartment or a smoke detection system is designed or changed significantly, aircraft manufacturers are required to demonstrate acceptable smoke detector performance. This typically involves generating smoke in an affected compartment during a test flight, and showing that the smoke detection system produces an alarm within the specified period of time. 
   In connection with ongoing efforts to increase aircraft safety, the U.S. Federal Aviation Administration (“FAA”) has recently elevated test requirements by demanding swifter detection of smaller smoke quantities. The present allowable smoke rate that must be detected is near the limit of many of the most current smoke detection systems, and therefore small variations in the generation rate of smoke during testing, due to factors such as ambient temperature variations, can dramatically increase the likelihood of inconsistent test results. Thus, it has become a challenge to provide not only a quantity of smoke that meets test criteria for certification of smoke detection systems, but also a repeatable and consistent quantity of smoke for tests of aircraft smoke detection equipment. 
   Existing smoke generator systems produce thermal aerosols for testing aircraft cargo hold smoke detection systems. Examples of such smoke generator systems include, for example, the Aviator, manufactured by Corona Integrated Technologies, Inc. and the ZZ101, manufactured by Siemens SAS. Both of these smoke generators produce mineral oil thermal aerosols. However, recent lab tests have shown that the oil temperature in the reservoirs of these generators greatly affects smoke production. Tests of the Siemens ZZ101 showed that oil cold-soaked at 35° F. produced approximately 40% of the smoke produced by oil warm-soaked at 105° F. Oil viscosity likely caused this behavior, as it changes significantly in the range of temperatures tested (the oil freezes at 14° F.). Tests of the Aviator smoke generator system produced similar results. 
   This variability of output with temperature adds much risk to aircraft certification efforts, as a smoke detection system that passes ground detection tests on a warm day can fail a flight test with a cooler or unheated cargo compartment. Alternately, a generator whose output registers a given smoke density during lab calibration will release less simulated smoke in the following days if those days happen to be cooler. Such sequences of events may result in costlier test efforts. 
   Accordingly, there is a need for smoke generation systems and methods that precisely control smoke generation rates and other relevant parameters, such as, for example smoke particle size (droplet size) and heat plume energy. 
   The present invention is directed to overcoming one or more of the problems or disadvantages associated with the prior art. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, a method of generating simulated smoke for testing of fire detection systems is provided. The method includes: providing liquid oil; using closed loop control to maintain at least one property, affecting one or more characteristics of the oil, at a substantially constant desired level; and expelling the oil in droplet form to generate a consistent type of simulated smoke. The at least one property that may be maintained at a substantially constant desired level may be oil temperature, volumetric flow rate of air, and/or chimney air temperature. 
   According to another aspect of the invention, a simulated smoke generator includes a liquid oil tank, a closed loop controller to maintain at least one property, affecting one or more characteristics of liquid oil in the liquid oil tank, at a desired level, and a nozzle for dispersing the oil in droplet form to generate a consistent type of simulated smoke. The closed loop controller may be adapted to maintain liquid oil temperature at a desired level, control an effective air flow area of the chimney, and/or maintain chimney air temperature at a desired level. 
   The features, functions, and advantages can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a schematic diagram illustrating an exemplary embodiment of a smoke generator system according to the invention. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , a smoke generator system, generally indicated at  10 , includes an oil reservoir tank  12  containing oil  14  that may be placed under pressure, for example, by carbon dioxide gas  16  from a carbon dioxide (CO 2 ) tank  18 . The carbon dioxide tank  18  may be connected to the oil reservoir tank  12  via a supply line  20  and the oil in turn may be forced by the pressure of the carbon dioxide  16  to flow through an oil supply passage  22  that is in fluid communication with a heater block  24  via a solenoid on/off valve  26 . 
   Gaseous CO 2  pressurizes the reservoir and forces oil into the oil supply passage  22 , where a small orifice (not shown) drilled into the side of the oil supply passage  22  allows CO 2  to enter the oil supply passage  22  and mix with the oil. The resulting CO 2 -oil mixture travels through the on/off solenoid valve  26  to the heater block  24 , where the oil is vaporized and forced through a nozzle  28  into a chimney  30 . The CO 2 -oil mixture exits the nozzle  28 , cools and condenses upon discharge, and forms a thermal aerosol of microscopic (e.g., micron-sized) oil droplets. This thermal aerosol is carried upward and out of the chimney  30  by a heat plume maintained by a heater  32 , that may be positioned within the chimney  30 , and that heats air within the chimney  30 . 
   The temperature of the oil  14  in the oil reservoir tank  12  may be regulated by an oil tank heater  34  that may be regulated by a controller, such as, for example, a digital proportional integral derivative (PID) controller  36 , that may be operatively connected to the oil tank heater  34  and to an oil temperature sensor or thermocouple  38  for providing closed-loop control of the temperature of the oil  14  in the oil reservoir tank  12 . 
   The temperature of the air in the chimney  30 , and thus the size of the oil droplets dispersed by the nozzle  28 , may also be controlled by the PID controller  36 , that may be operatively connected to the heater  32  and to a chimney temperature sensor or thermocouple  40 . The PID controller  36  may also be operatively connected to the heater block  24 . 
   The oil droplet size is a function of a number of factors. Higher air temperature in the chimney  30  and/or the heater block  24  tends to produce a smaller droplet size in the thermal aerosol exiting the chimney  30 , and makes the thermal aerosol more buoyant as it exits the chimney  30 . A certain level of buoyancy may be desirable, since it makes the thermal aerosol behave in a manner similar to smoke from an actual fire, by rising upward. A higher flow rate of air through the chimney  30  prevents oil droplets from colliding with one another and coalescing, thereby preventing the formation of a fog of larger oil droplets (such a fog is likely to sink, rather than rise, and therefore not behave similar to smoke that typically rises). Accordingly, by flowing more air and/or hotter air through the chimney  30 , a low droplet size may be maintained. Higher gas pressure applied to the liquid oil in the oil reservoir tank  12  tends to produce a larger droplet size in the thermal aerosol exiting the chimney  30 . 
   The volumetric flow rate of air through the chimney  30  is a function of a number of variables, including air temperature in the chimney  30  and the effective flow area of the chimney  30 . The average diameter of the oil droplets exiting the chimney  30  is a function of mass flow of oil exiting the nozzle  28 , the temperature of the oil exiting the nozzle  28 , the pressure of the oil exiting the nozzle  28 , and the volumetric flow rate of air through the chimney  30 . The buoyancy of the plume exiting the chimney  30  is a function of a number of variables, including the mass and temperature of the oil introduced into the chimney  30 , as well as the mass and temperature of the air flowing through the chimney  30 . The smoke density of the plume exiting the chimney  30  is a function of a number of variables, including the mass flow of oil exiting the nozzle  28  and the volumetric flow rate of air through the chimney  30 . The mass flow of oil exiting the nozzle  28  is a function of a number of variables, including the oil temperature, oil pressure, the geometry of the nozzle  28 , and the flow resistance of the fluid path (e.g., the flow resistance through the oil supply valve  22 , solenoid valve  26 , etc.). 
   Droplet size of the thermal aerosol may be affected by varying the volumetric flow rate of air through the chimney  30 , for example, by varying the effective air flow area through the chimney  30 . Providing a larger effective air flow area through the chimney  30  tends to spread the oil droplets apart from one another and prevents the oil droplets from coalescing. The effective air flow area through the chimney  30  may be regulated, for example, using movable louvers  46  that may be operatively connected to the controller  36 . Of course, other methods and/or structures, such as one or more fans (not shown) may be used to vary the volumetric flow rate of air through the chimney  30 . 
   A purge valve  42  may be connected to the conduit  22 , downstream of the solenoid on/off valve  26 , in order to purge excess oil from the system at startup using a secondary source of pressurized carbon dioxide  44 . 
   Initial testing of a smoke generating system with an oil reservoir temperature control device according to the invention has shown that through this addition, unprecedented precision may be achieved in controlling smoke output. Together with the benefits of control over chimney air temperature, the smoke generator improvements in accordance with the invention reduce a significant portion of the risk in testing aircraft smoke detection systems. Cost savings from such improvements can be realized not only in reduced lab, ground, and flight test costs, but also in reduced risk of rushed redesigns that result from failed tests due to inconsistent smoke generation. 
   Other aspects and features of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims. 
   Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutes are possible, without departing from the scope and spirit of the invention as disclosed herein and in the accompanying claims. For example, although the invention has been described primarily for use with smoke generator systems that produce thermal aerosols, the invention may of course be used with other smoke generator systems, such as, for example, wood and/or paper based smoke generators, e.g., by controlling air temperature and volume of a smoke plume to get consistent smoke characteristics, according to the invention.