Measuring and testing – Gas analysis – By thermal property
Reexamination Certificate
2002-03-22
2003-08-26
Raevis, Robert Roy (Department: 2856)
Measuring and testing
Gas analysis
By thermal property
C250S288000
Reexamination Certificate
active
06609412
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to a wet gas probe. More particularly, this invention relates to a sensor probe capable of accurately measuring the temperature and liquid volumetric fraction of a hot gas laden with liquid droplets.
2. Description of Related Art
For many liquids, there is a temperature well above the boiling point called the Leidenfrost point or Leidenfrost transition. As an example, water has a Leidenfrost point of 300-350° C. at atmospheric conditions. Consider a simple experiment where a droplet of water is placed on a surface kept at a temperature above the boiling point of water. If the temperature of the surface is below the Leidenfrost point, then the droplet starts to spread out and vaporizes rather quickly.
However, if the temperature of the surface is at or above the Leidenfrost point, the bottom layer of the droplet vaporizes almost immediately on contact, creating a cushion of vapor that repels the rest of the droplet from the surface. Furthermore, the evaporation of the bottom layer of the droplet from the surface produces a cooling effect, which detrimentally affects the heating of the surface. The remaining portion of the droplet does not make contact with the surface, and thus no heat can be transferred directly from the surface to the droplet. At such high temperatures, one might expect that the vapor layer would quickly transfer enough heat to the rest of the droplet to vaporize the droplet. Water vapor, however, is a very poor conductor of heat at these temperatures. Hence, the vapor layer actually acts as an insulator.
Currently it is quite difficult to accurately measure gas temperature in a liquid droplet and gas mixture. Most common gas measurement devices, such as thermocouples and resistance temperature detectors (RTD), work on the principal that the electrical resistance of most materials varies with temperature. Hence, knowledge of the functional relationship between temperature and change in resistance of a given material(s) and measurement of this change allows for inference of the temperature of the gas stream. Generally, these devices are comprised of different metals, which have high thermal conductivity. In a liquid-droplet gas mixture, the droplets will tend to impact and coat the surface of these devices. The liquid on the surface of the probe draws heat from the measurement device, in a process known as evaporative cooling. This process results in measurement of temperatures much below the true gas temperature. This phenomenon prevents these common temperature measurement devices from accurately measuring gas temperature in this environment. As described above, the invention eliminates this problem by preventing the droplet from impacting the surface of the probe by keeping the measurement surface above the Leidenfrost point.
The above-identified phenomena give rise to an unsolved problem in the prior art of measuring the temperature and other characteristics of gases laden with liquid droplets. The need to solve this problem arises in a wide variety of contexts, including, for example, measurement of gas temperature in propulsion and power generation systems, which often use water or other liquid introduced into the system to, for example, control emissions (e.g., pollution control) or augment power. In such applications, determination and control of gas temperature may be important to performance. Many other systems, devices, and situations arise, such as a gas turbine, combustion engine, tank, pipe, duct, manifold, chamber, or the like, as well as external flows, including, for example, droplets in an open air environment, in which the presence of liquid droplets in a gas can produce difficulties in measuring gas temperature and other properties of the gas.
As an example, one simple conventional temperature and liquid sensitive system for which this problem can arise is a fire detection system, an example of application of which is as follows. As is well known, fire detection systems are installed in residential and commercial buildings to protect property and occupants from fire. An important characteristic of the fire detection system is the capability to detect a fire in the early stages, when the fire is still small. Early detection and activation by fire suppression devices are important to allow more time for the evacuation of the occupants as well as to increase the chance of successfully suppressing the fire before extensive damage is caused to the buildings. Therefore, the early detection of a fire is very important.
Ceiling mounted devices that do not interfere with normal room arrangements are generally preferred for fire protection purposes. Automatic sprinklers are devices that distribute water onto a fire in sufficient quantity either to extinguish the fire in its entirety or to prevent the spread of the fire in case the fire is too far from the water discharged by the sprinklers. Typically, the water is fed to the sprinklers through a system of piping, suspended from the ceiling, with the sprinklers placed at regular intervals along the pipes. The orifice of the sprinkler head is normally closed by a disk or cap held in place by a temperature sensitive releasing element. The temperature sensitive releasing element of the sprinkler will be referred to hereinafter as a sprinkler link.
Automatic sprinklers have several temperature ratings that are based on standardized tests in which a sprinkler is immersed in a liquid and the temperature of the liquid raised very slowly until the sprinkler activates. The temperature rating of most automatic sprinklers is stamped on the sprinkler. The time delay between the onset of the fire and the activation of the sprinkler depends upon several parameters, such as the placement of the sprinkler with respect to the fire, the dimensions of the enclosed space, the energy generated by the combustion and the sensitivity of the sprinkler.
Buoyancy pushes the hot products generated by a fire toward the ceiling while mixing with room air to form a hot-gas plume. Impingement of the hot-gas plume on the ceiling results in a gas flow near the ceiling, even at a considerable distance from the core of the fire. This flow is responsible for directing hot gases to the thermally actuated fire detection devices.
The rate of heat released by the fire and the room dimensions are the main parameters of considerable importance in any discussion of fire-induced convection near the room ceiling. Also, the size and the composition of the sprinkler link influences the sensitivity of the sprinkler. Other conditions being equal, the sensitivity of a sprinkler is inversely proportional to the time required for the sprinkler link to melt. Therefore, sprinklers are rated according to their Response Time Index, hereinafter referred to as RTI, which characterizes the speed of the sprinkler's response to a fire.
The RTI is the product of the thermal time constant of the sprinkler link and the square root of the flow velocity of the hot gas. This parameter is reasonably constant for any given sprinkler and is considered sufficient for predicting the sprinkler response for known gas temperatures and velocities near the sprinkler. However, recent full-scale tests on warehouse fires uncovered a behavior of sprinklers that does not correspond to the predictions of the RTI model.
The RTI model considers the sprinkler link as a cylinder in cross-flow. It is assumed that the heat transfer between the hot gases flowing under the ceiling and the sprinkler is convective and radiative, thus, among other factors, the RTI model neglects the presence of water droplets in the airflow.
The first sprinkler to activate in case of fire is referred to as a primary sprinkler and the surrounding sprinklers are identified as secondary sprinklers. Tests show that the primary sprinkler activates as predicted, but the secondary sprinklers respond after a much longer delay than suggested by the RTI model. In some cases, the sprinklers immediately surrounding the primary sprin
Dimarzo Marino
Ruffino Paolo
Arent Fox Kintner & Plotkin & Kahn, PLLC
Politzer Jay L.
Raevis Robert Roy
University of Maryland
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