Abstract:
A method and apparatus for reducing the emission of pollutants resulting from static test firings of rocket and jet engines. The apparatus comprises a test stand for capturing the exhaust gas from an engine undergoing a static test. The test stand includes a duct system for guiding the exhaust gas from a duct inlet to a duct outlet through stages that reduce the exhaust gas pollutants. Liquid oxygen (“LOX”) is introduced into the duct system to interact with the engine exhaust gas stream. By introducing LOX, the pollutant CO is converted to non-pollutant carbon dioxide (“CO 2 ”) as the exhaust stream moves through the duct system.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the static testing of rocket and jet engines and more particularly to a method and apparatus for treating engine exhaust gas to reduce pollutants discharged to the atmosphere. 
       BACKGROUND OF THE INVENTION 
       [0002]    Rocket and jet engines use the combustion of propellant chemicals to propel vehicles and missiles into or through Earth&#39;s atmosphere. To assure the proper functioning of such engines, extensive tests and static firings of engines are performed at ground based test facilities. During such static test firings, hydrocarbon fueled engines typically emit a high mass fraction of the pollutant carbon monoxide (“CO”) into the atmosphere where further reactions can form pollutant oxides of nitrogen (“NOx”). Also, particulate matter may be discharged as a result of incomplete combustion and the levels of such pollutants emitted into the atmosphere may be unacceptable. Additionally, depending upon the size of the engine, the testing may produce unacceptably high noise levels. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention is directed to a method and apparatus for treating engine exhaust to reduce the emission of chemical pollutants resulting from static test firings of rocket and jet engines. 
         [0004]    In accordance with the present invention, a test stand is provided for capturing the exhaust gas from an engine undergoing a static test. The test stand is characterized by a duct system which guides the exhaust gas from a duct inlet to a duct outlet through stages that reduce the exhaust gas pollutants. 
         [0005]    In accordance with one significant aspect of the invention, an oxidizer, e.g., liquid oxygen (“LOX”), is introduced into the duct system to interact with the engine exhaust gas stream. By properly introducing the oxidizer, the pollutant CO is converted to a non-pollutant carbon dioxide (“CO 2 ”) as the exhaust stream moves through the duct system. In accordance with a further significant aspect, a coolant, e.g., water, is also introduced into the exhaust stream to reduce the exhaust gas temperature to avoid the formation of pollutant oxides of nitrogen NOx. 
         [0006]    In accordance with a further feature of a preferred embodiment, the amount of free air entrained by the exhaust gas entering the duct system is minimized in order to further reduce the formation of NOx. 
         [0007]    In a preferred embodiment of the invention, the engine exhaust gas entering the duct inlet will first flow through a diffuser duct section designed to reduce the velocity of the exhaust gas stream and then through a CO converter duct section. With the exhaust gas stream above a threshold temperature, an oxidizer, preferably LOX, is injected into the stream within the CO converter duct section where it combines with the CO in the exhaust gas to form CO 2 . Coolant is also injected into the exhaust stream in the CO converter duct section to reduce the temperature of the stream below the threshold temperature that promotes the formation of NOx in free (i.e. atmospheric) air, and to assist in converting CO into CO 2 . 
     
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0008]      FIG. 1  is a side view of an exemplary test stand comprising a duct system including a diffuser section, a CO converter section, and an exhaust stack; 
           [0009]      FIG. 2  is a schematic representation of the diffuser and CO converter sections of  FIG. 1  in accordance with the present invention; 
           [0010]      FIG. 3  is an isometric view of a portion of the duct system of  FIG. 1 ; 
           [0011]      FIG. 4  is an enlarged cross section view showing the junction of preferred spool sections in the duct system of  FIG. 1 ; 
           [0012]      FIG. 5  is a cross section view of the preferred injection ring shown in  FIG. 4  indicating the radial and cordial injection pattern; and 
           [0013]      FIG. 6  is an enlarged cross section view showing a preferred interface between a nozzle of the engine under test and the inlet of the duct system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a side view of a representative test stand  100  in accordance with the invention showing an exemplary rocket or jet engine  102  under test, restrained by an engine thrust mount  104 . The engine&#39;s exhaust nozzle  106  is mounted to discharge engine exhaust into the inlet  108  of a duct system  110  formed primarily of flanged pipe sections, or spools, coupled in series. The duct system  110  can be viewed as comprising the following major functional units; a diffuser section  112 , a CO converter section  114 , and an exhaust stack section  116  leading to an outlet  118  to the atmosphere. 
         [0015]      FIG. 2  schematically represents the duct system  110  of the test stand  100  of  FIG. 1  and shows the exemplary engine  102  mounted with its nozzle  106  adjacent to duct inlet  108 . Typically, propellant  120  fed into the engine  102  is ignited and burns in the combustion chamber (not shown) to produce thrust. The process of igniting and burning the propellant  120  in the engine  102  releases energy in the form of heat and pressure. The products of combustion then expand and exit with a high velocity from the nozzle  106  to produce thrust. But for the engine being restrained by the thrust mount  104  (see  FIG. 1 ), the thrust produced would propel the engine  102  in a direction opposite to the exhaust discharged from the nozzle  106 . During static test in the test stand  100 , the exhaust stream discharge from the nozzle  106  passes through the duct system inlet  108  and enters the diffuser section  112 . 
         [0016]    The primary function of the diffuser section  112  is to reduce the velocity of the entering exhaust stream  122  to below the speed of sound, Mach 1, at the diffuser section exit. Once the flow has become subsonic, the static pressure will increase sufficiently to drive the exhaust gas through the downstream converter and stack sections to the atmosphere. The temperature of the exhaust gas exiting the nozzle  106  is extremely high (typically in excess of 4000° F.) and the heat transfer from the gas to the wall of the diffuser section  112  is extremely high. To survive the extreme thermal load, it is preferable that the diffuser section  112  include a actively cooled water jackets  124 . 
         [0017]    The exhaust stream  122  exiting the diffuser section  112  enters the CO converter section  114 . A primary function of the CO converter section  114  is to reduce the CO content of the exhaust stream  122  by converting CO to CO 2 . This is accomplished in accordance with the invention by introducing coolant  126 , e.g., deionized water, into the exhaust stream near the upstream end of the converter section  114  to cool the exhaust stream to a temperature in excess of ˜3000° F. Above this temperature, the introduction of oxidizer at  128  combines with CO in the exhaust stream to form CO 2 . The preferred oxidizer is liquid oxygen (LOX) because it constitutes a pure and dense form of oxygen. However, alternative oxidizers, e.g., hydrogen peroxide (H 2 O 2 ), can be used. Regardless, the process is exothermic. Accordingly, the CO converter section  114  introduces further coolant downstream at  130  to reduce the exhaust stream temperature to below a threshold temperature (˜2,780° F.) at which NOx is formed in free air. In the preferred embodiment described herein, the coolant  126  will be assumed to be deionized water and the oxidizer will be assumed to be LOX. 
         [0018]    The converter section  114  is typically constructed of multiple flanged pipe, or spool, sections preferably including actively cooled water jackets  124 . The introduction of water and LOX into the exhaust stream flowing through converter section  114  is preferably implemented via injection rings to be discussed in greater detail in conjunction with  FIG. 5 . 
         [0019]    In an exemplary embodiment, the converter section  114  will typically reduce the exhaust stream temperature to ˜2000° F. at the exit of the converter section, i.e., at the entrance to the stack section  116 . 
         [0020]    The primary purpose of the stack section  116  is to further cool and maintain the exhaust stream below the threshold temperature required to form NOx. The stack section  116  is preferably vertically oriented with a cross section dimension considerably larger than the cross section dimension of converter section  114  for the purpose of slowing the exhaust stream velocity to below Mach 0.25. At this low velocity, the exhaust stack will be a phase separator for excess liquid water, any unburned fuels and any soot or particulate. These will be collected at the base of the vertical exhaust stack. All chemical reactions within the exhaust gases will be quenched by water sprays contained near the upper portion of the exhaust stack assembly. The design exit temperature of the exhaust gases into the atmosphere is ˜900° F. 
         [0021]    The length and diameter of the diffuser section  112  are selected in relation to the thrust rating of the engine  102  under test; and should be sized to reduce the velocity of the exhaust stream  118  to below Mach 1 at the diffuser section exit. To achieve this result, the ratio of diffuser section length to diameter is typically greater than twelve. 
         [0022]    The length and diameter of the CO converter section  114  are selected with regard to the diffuser section dimensions. Typically, the CO converter section  114  will have a diameter of about twice that of the diffuser section and a length at least six times the diameter of the converter section  114 . 
         [0023]    Whereas  FIG. 2  schematically depicts a duct system  110  in accordance with the present invention,  FIGS. 3-6  illustrate a preferred structural embodiment of such a duct system. More particularly,  FIG. 3  shows multiple identical pipe sections, i.e. spools,  132 A,  132 B,  132 C,  132 D, and  132 E. Each spool has a flange  134  on each end. The flanges are bolted together to complete the spools in series to form the converter section  114  to define a continuous interior passageway for guiding the exhaust stream. Although  FIG. 3  depicts a representative converter section  114 , it should be understood that the diffuser section  112  would have a very similar appearance but, as previously mentioned would differ significantly dimensionally. 
         [0024]    It should be noted in  FIG. 3  that each spool section  132  includes a bellows-like section  136  to accommodate the differential longitudinal expansion/contraction of the spool section  132  outer wall relative to the inner wall (as will be discussed in conjunction with  FIG. 4 ) attributable to thermal effects. It should also be noted in  FIG. 3  that fluid injection rings  138  are mounted between adjacent flanges. As will be discussed in greater detail hereafter, the rings  138  function to inject fluid (e.g., LOX, water) into the exhaust stream. 
         [0025]      FIG. 4  depicts a typical junction between flanges  134 A and  134 B of adjacent spools  132 A and  132 B. Initially note that each spool section  132  includes a double walled cooling manifold  140 , with cooling water  142  flowing in  144  between the inner wall  146  and the outer wall  148 , at the downstream flange end of each spool  132 , through the cooling manifold  140 , and out  150  at the upstream flange end of each spool  132 . The outer wall  148  of each spool section  132  includes the bellows-like section  136  to allow for the differing rate of thermal expansion of the duct outer wall  148  compared to the thermal expansion of the duct inner wall  146 . Note also in  FIG. 4 , the fluid injection ring  138  mounted between the flanges  134  of adjacent spools  132 A, 132 B. 
         [0026]      FIG. 5  is a cross section view of an exemplary fluid injection ring  138  for use in the CO converter section  114  (see  FIGS. 3 and 4 ). Either water or LOX can be injected through injection jets  152  into the exhaust stream  118 . It will be recalled that water is injected in order to cool the exhaust stream  118 . LOX is injected to combine with CO in the exhaust stream to produce CO 2 . In the exemplary injection ring  138  shown in  FIG. 5 , fluid from a source  154  is supplied to an outer circumferential manifold  156 . Radially oriented passages  158  couple manifold  156  to an inner circumferential manifold  160  which supplies fluid to the injection jets  152 . In the preferred embodiment illustrated, the jets are organized into four quadrants with the jets of each quadrant oriented parallel to each other and perpendicular to the jets of each neighboring quadrant. In this manner, the jets will discharge into the exhaust stream both radially and cordially to optimize the cooling and conversion of CO to CO 2 . 
         [0027]    Attention is now directed to  FIG. 6  which illustrates a preferred interface adapter  162  for coupling the engine nozzle  106  to the diffuser section duct inlet  108 . The purpose of the adapter  162  is to minimize the amount of free air entrained by the exhaust gas entering the duct inlet  108 , yet not physically constrain the engine under test so as to influence thrust measurements. By minimizing the amount of free air (typically 78% nitrogen [N 2 ]) entrained, the amount of pollutant NOx in the gas exhausted by the duct system  110  will be reduced. 
         [0028]    The adapter  162  is comprised of a closure plate  164  configured to seal around the duct inlet  108 . The plate  164  defines a large central opening  166 , large enough to pass the engine nozzle  106  as shown in  FIG. 6 . The plate  164  carries a resilient seal, or flap,  168  extending around the opening  166  and configured to narrow the clearance gap between the nozzle exterior surface and the opening  166 . For example, in exemplary embodiments of the invention, the flap  168  reduces the gap to approximately 0.1 inch to restrict the amount of free air (and N 2 ) which can be drawn into the diffuser section  112 . 
         [0029]    From the foregoing, it should now be appreciated that a test stand has been described for capturing and treating exhaust gas from an engine undergoing a static test firing to minimize pollutants discharged to the atmosphere. The test stand embodiment described is characterized by a duct system extending between a duct system inlet which captures exhaust gas from the engine and a duct system outlet which discharges to the atmosphere and which system introduces a liquid coolant and an oxidizer into the exhaust stream for converting CO to CO 2  and for minimizing the formation of NOx. 
         [0030]    Although a preferred embodiment has been described in detail herein, it is recognized that many variations and modifications will readily occur to persons skilled in the art which are consistent with the teachings of this application and within the intended scope of the appended claims.