Abstract:
A total air temperature (TAT) probe for measuring TAT includes an inlet scoop which receives airflow from free stream airflow moving toward the inlet scoop from a first direction. A first portion of the airflow entering the inlet scoop exits the probe through a main exit channel. A second portion of the airflow enters a TAT sensor flow passage, which extends longitudinally along an axis. This axis is oriented to form an angle of less than 90 degrees with the first direction from which the free stream airflow moves toward the inlet scoop. A sensor assembly extends longitudinally in the sensor flow passage and measures a total air temperature of airflow through the sensor flow passage. By increasing the angle through which the internal air turns, better inertial extraction of ice and water particles is realized. As a result, sensor clogging from accreted ice is significantly reduced. A second improvement is achieved by repositioning the sensor element to be more in-line with the internal airflow direction. This helps lower DHE by minimizing heated boundary layer spillage onto the sensing element.

Description:
The present application is a continuation-in-part of U.S. patent application Ser. No. 09/960,594, filed Sep. 21, 2001, now U.S. Pat. No. 6,609,825 the content of which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to total air temperature (TAT) probes or sensors. More particularly, the present invention relates to improving anti-icing performance and reducing deicing heater error (DHE) in TAT probes. 
   Modern jet powered aircraft require very accurate measurement of outside air temperature (OAT) for inputs to the air data computer, engine thrust management computer, and other airborne systems. For these aircraft types, their associated flight conditions, and the use of total air temperature probes in general, air temperature is better defined by the following four temperatures: (1) Static air temperature (SAT) or (T S ), (2) Total air temperature (TAT) or (T t ), (3) recovery temperature (T r ), and (4) measured temperature (T m ). (1) Static air temperature (SAT) or (T S ) is the temperature of the undisturbed air through which the aircraft is about to fly. (2) Total air temperature (TAT) or (T t ) is the maximum air temperature that can be attained by 100% conversion of the kinetic energy of the flight. The measurement of TAT is derived from (3) the recovery temperature (T r ), which is the adiabatic value of local air temperature on each portion of the aircraft surface due to incomplete recovery of the kinetic energy. Temperature (T r ) is in turn obtained from (4) measured temperature (T m ), which is the actual temperature as measured, and which differs from recovery temperature because of heat transfer effects due to imposed environments. For measuring the TAT, TAT probes are well known in the art. 
   Conventional TAT probes, although often remarkably efficient as a TAT sensor, sometimes face the difficulty of working in icing conditions. During flight in icing conditions, water droplets, and/or ice crystals, are ingested into the TAT probe where, under moderate to severe conditions, they can accrete around the opening of the internal sensing element. An ice ridge can grow and eventually break free—clogging the sensor temporarily and causing an error in the TAT reading. To address this problem, conventional TAT probes have incorporated an elbow, or bend, to inertially separate these particles from the airflow before they reach the sensing element. These conventional TAT probe designs can be very effective at extracting particles having diameters of 5 microns or greater. However, the process of particle extraction becomes increasing less efficient in many conventional TAT probe designs when removing particles below this size. 
   Another phenomena which presents difficulties to some conventional TAT probe designs has to do with the problem of boundary layer separation, or “spillage”, at low mass flows. Flow separation creates two problems for the accurate measurement of TAT. The first has to do with turbulence and the creation of irrecoverable losses that reduce the measured value of TAT. The second is tied to the necessity of having to heat the probe in order to prevent ice formation during icing conditions. Anti-icing performance is facilitated by heater elements embedded in the housing walls. Unfortunately, external heating also heats the internal boundary layers of air which, if not properly controlled, provide an extraneous heat source in the measurement of TAT. This type of error, commonly referred to as DHE (Deicing Heater Error), is difficult to correct for. In conventional TAT probes, the inertial flow separation bend described above has vent, or bleed, holes distributed along its inner surface. The holes are vented to a pressure equal to roughly that of the static atmospheric pressure outside of the TAT probe. In this manner, a favorable pressure difference is created which removes a portion of the boundary layer through the bleed holes, and pins the remaining boundary layer against the elbow&#39;s inner wall. 
   In certain situations, the differential pressure across the bleed holes can drop to zero due to the higher flow velocity along the elbow&#39;s inner radius. This stagnation of flow through the bleed holes creates a loss in boundary layer control. The resulting perturbation, if large enough, can cause the boundary layer to separate from the inner surface and make contact with the sensing element. Because the housing walls are heated, so is the boundary layer. Hence, any contamination of the main airflow by the heated boundary layer will result in a corresponding error in the TAT measurement. In general, it is difficult to prevent the stagnation of some of the bleed holes. Thus, DHE is difficult to prevent or reduce. 
   SUMMARY OF THE INVENTION 
   A total air temperature (TAT) probe includes an inlet scoop which receives airflow from free stream airflow moving toward the inlet scoop from a first direction. A first portion of the airflow entering the inlet scoop exits the probe through a main exit channel. A second portion of the airflow enters a TAT sensor flow passage, which extends longitudinally along an axis. This axis is oriented to form an angle of less than 90 degrees with the first direction from which the free stream airflow moves toward the inlet scoop. A sensor assembly extends longitudinally in the sensor flow passage and measures a TAT of the airflow through the sensor flow passage. By increasing the angle through which the internal air turns, better inertial extraction of ice and water particles is realized. As a result, sensor clogging from accreted ice is significantly reduced. 
   A second improvement is achieved by repositioning the sensor element to be more in-line with the internal airflow direction. This helps lower (Deicing Heater Error) DHE by minimizing heated boundary layer spillage onto the sensing element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic illustration, with portions shown in section, of a prior art total air temperature (TAT) probe. 
       FIG. 2  is a diagrammatic illustration of the prior art TAT probe shown in  FIG. 1 , which illustrates flow of air through the TAT probe. 
       FIGS. 3A and 3B  are diagrammatic illustrations of a prior art TAT probe and a TAT probe in accordance with the invention, respectively, which illustrate an angle θ between a direction of travel of free stream airflow into the TAT probe and an axis of the sensor flow passage. 
       FIGS. 4A and 4B  are diagrammatic illustrations of the prior art TAT probe and the TAT probe of the present invention, shown respectively in  FIGS. 3A and 3B , which illustrate flow of air through the TAT probes. 
       FIGS. 5A and 5B  are diagrammatic illustrations of a portion of each of the TAT probes shown in  FIGS. 3A and 4A  and in  FIGS. 3B and 4B , respectively, which illustrates a mechanism of particle capture in the TAT probes of the present invention. 
       FIGS. 6A and 6B  illustrate a prior art TAT probe and a TAT probe of the present invention, respectively, showing improvement in boundary layer spillage control. 
       FIG. 7  is a diagrammatic perspective view of a TAT probe of the present invention mounted to an aircraft engine. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a diagrammatic illustration of a conventional total air temperature (TAT) probe  100 , with portions shown in section. TAT probe or sensor  100  includes housing or walls  102  which form a primary inlet or inlet scoop  105  and a main exit channel  110  through which air from the free stream airflow (outside the probe) passes through. Also formed within housing  102  is a flow separation bend  115  which diverts a portion of the airflow between inlet scoop  105  and main exit channel  110  and redirects this portion into TAT sensor flow passage  120 . Positioned within sensor flow passage  120  is sensor assembly  125  that includes a sensing element  130  which senses the TAT, and a radiation shield  135  positioned annularly around sensing element  130 . 
   Flow separation bend  115  includes an inner elbow wall  140  which serves to redirect a portion of the airflow into sensor flow passage  120 . Contained within inner elbow wall  140  are bleed holes or ports  145 , which are maintained at a pressure differential occurring between the inner passage and an external air passage  140 A, to remove a portion of the airflow adjacent to the inner elbow wall  140  and to control the heated boundary layer of air to reduce deicing heater error (DHE). TAT sensor flow passage  120  includes a forward wall (relative to redirected airflow in the sensor passage)  150 , and an aft wall  155 . Forward wall  150  has an upper end at point  160  at which the arc of inner elbow wall  140  ends. 
     FIG. 2  is a diagrammatic illustration of prior art TAT probe  100  which demonstrates airflow patterns into, through and out of the probe. As can be seen in  FIG. 2 , free stream airflow enters inlet scoop  105  traveling in a direction, or along an axis, represented by arrows  165 . Once inside inlet scoop  105 , the airflow is redirected by flow separation bend  115  such that a portion of the airflow enters TAT sensor flow passage  120 . The portion of the airflow entering sensor flow passage  120  flows along sensing element  130 , between shield  135  and sensing element  130 , or between shield  135  and walls  150  and  155  before exiting an output port. The remaining air, which does not enter sensor flow passage  120 , leaves TAT probe  100  either through a bleed hole  145  or through main exit channel  110 . As discussed in the background section, during flight in icing conditions, water droplets and/or ice crystals are ingested into the area between sensing element  130  and radiation shield  135 , thereby occasionally temporarily clogging the sensor and causing an error in the TAT reading. 
     FIG. 2  also illustrates the location of TAT probe  100  relative to skin  175  of an aircraft. It also illustrates electronics housing  170  which contains electronics or electrical circuits of a type which are known to be used with conventional TAT probes for measuring the TAT. Although not specifically illustrated in other figures, TAT probes of the present invention can include an electronics housing and other common TAT probe features. 
     FIGS. 3A and 3B  are illustrations of prior art TAT probe  100  and TAT probe  300  of the present invention, respectively.  FIG. 3A  illustrates orientation of sensor flow passage  120  relative to axis or direction  165  of travel of the free stream airflow prior to entering primary inlet or inlet scoop  105 . As shown in  FIG. 3A , in prior art TAT probe  100 , sensor flow passage  120  is oriented generally along a longitudinal axis  180 . In the prior art, axis  180  of sensor flow passage  120  is oriented relative to axis or direction  165  of free stream airflow entering inlet  105  such that it forms an angle θ of approximately 90 degrees. 
     FIG. 3B  is a diagrammatic illustration of a TAT probe  300  in accordance with embodiments of the present invention. Although TAT probe  300  has structural differences relative to TAT probe  100  which provide enhanced anti-icing performance and reduced DHE, it has components which are similar to those of TAT probe  100 . TAT probe  300  includes housing  302  forming a flow passage between primary inlet or inlet scoop  305  and main exit channel  310 . Also formed in housing  302  is TAT sensor flow passage  320 , which receives a portion of the airflow entering the probe at inlet scoop  305 . Sensor flow passage  320  is formed between forward wall  350  and aft wall  355 . A sensor assembly  125  (having a sensing element  130  and a radiation shield  135  as discussed above) is provided and can be substantially the same as in TAT probe  100 . Flow separation bend  315  formed with inner elbow wall  340  acts to divert a portion of the airflow and to provide this portion of the airflow into sensor flow passage  320  for the TAT measurement. Inner elbow wall  340  also includes bleed holes or ports  345  to external air passage  340 A to maintain a differential pressure which removes a portion of the airflow and prevents spillage of the heated boundary layer. 
   In TAT probe  300 , sensor flow passage  320  is formed generally along a longitudinal axis  380 . As can be seen in  FIG. 3B , axis  380  forms an angle θ with axis  165  representing the direction of travel of the free stream airflow just prior to entering the probe primary inlet  305 . While this angle in prior art TAT probe  100  is approximately 90 degrees, in TAT probe  300  of the present invention, angle θ is substantially less than 90 degrees. For example, an angle of 45 degrees has been shown in simulations to reduce the total water mass impacting the sensor area as will be described in greater detail below. Stated another way, an elbow bend of approximately 135 degrees (180 degrees−θ) has been shown to be beneficial. While angles θ of between 35 degrees and 65 degrees are likely to produce significantly improved ice and water extraction and thus, enhanced performance in icing conditions, the present invention should be considered to include other ranges of angle θ. For example, angles θ of between 35 and 60 degrees or between 35 and 55 degrees will provide enhanced performance. Further, angles θ of less than approximately 80 degrees can also be used. 
   In comparing the conventional TAT probe  100  and TAT probe  300  of the present invention, it can be seen that the elbow angle (180 degrees−θ) is sharper in TAT probe  300 , creating a longer arc length of inner elbow wall  340 . The sensing element within sensor assembly  125  is also repositioned longitudinally along axis  380  to be more in-line with the airflow path provided by TAT sensor flow passage  320 . These two features help to provide significant performance improvements in TAT probe  300 .  FIGS. 4A  and  4 B diagrammatically illustrate the airflow in each of TAT probes  100  and  300 . 
     FIGS. 5A and 5B  are diagrammatic illustrations representing the inner elbow wall, the forward wall and the aft wall for each of TAT probes  100  and  300 , respectively. Curve  370  in  FIG. 5A  represents the trajectory of a 2.5 micron diameter particle into sensor flow passage  120  in TAT probe  100 . Curve  375  in  FIG. 5B  represents the trajectory of a 2.5 micron diameter particle into sensor flow passage  320  in TAT probe  300 . 
   As can be seen in  FIG. 5A , in the conventional TAT probe configuration, a 2.5 micron diameter particle will more frequently follow trajectory  370 , allowing it to easily exit the elbow region without impacting a wall. This increases the chance that the particle will impact, or accrete onto, the sensing element (for example by passing into the interior portion of radiation shield  135 ) during operation in atmospheric icing conditions. 
   In contrast,  FIG. 5B  illustrates a typical path of a 2.5 micron diameter particle in TAT probe  300  of the present invention during one example set of operational conditions. As can be seen in  FIG. 5B , the higher degree of elbow bend in TAT probe  300  causes the particle to impinge on the outer elbow wall or an upper portion of wall  355  of sensor flow passage  320 . Thus, the particle is less likely to enter the sensing area inside of radiation shield  135 , and can therefore be extracted from the flow with reduced likelihood for icing interference with the total air temperature measurement. The benefits offered by the sensor passage configuration and orientation in TAT probe  300  with regard to water droplet extraction have been demonstrated using analytical modeling techniques. Simulations show that the 135 degrees elbow bend (θ≈45 degrees) can reduce by up to 40% the total water mass impacting the sensor area. This reduction extends the time to accrete ice by almost a factor of two relative to conventional TAT probe designs during flight operation in moderate to severe liquid water icing conditions. 
   Conventional TAT probe  100  and TAT probe  300  of the present invention can also be compared to illustrate the differences between the two designs with regard to boundary layer spillage. The differences in boundary layer spillage performance are depicted in  FIGS. 6A and 6B . As discussed above, the boundary layer of air which separates from the heated elbow surface and enters the sensing element will result in an increased DHE. As shown in  FIG. 6A , a portion  400  of the airflow entering the probe also exits through the main exit channel. Another portion  405  of the air entering the TAT probe is redirected by the flow separation bend (i.e., including inner elbow wall  140 ) into the TAT sensor flow passage where it flows across the sensing element  130 . As illustrated in  FIG. 6A , this portion  405  of the airflow flows in the inside of radiation shield  135 . With inner elbow wall  140  being a heated surface, a boundary layer  410  of air along the inner elbow wall will also be heated. 
   As illustrated in  FIG. 6A , the configuration of TAT probe  100  is such that a number of bleed holes  145  become stagnant (as indicated by cross-hatching). This is due to the fact that, under certain situations, the differential pressure across the bleed holes can drop to zero due to the higher flow velocity along the elbow&#39;s inner radius. This stagnation of flow through the bleed holes creates a loss in boundary layer control. The resulting perturbation, if large enough, can cause the boundary layer  410  to separate from the inner surface and make contact with the sensing element as shown in FIG.  6 A. Because the housing walls are heated, so is the boundary layer, and hence any contamination of the main airflow by the heated boundary layer will result in a corresponding error in the TAT measurement. In general, it is difficult to prevent the stagnation of some of the bleed holes. However, as will be discussed with reference to  FIG. 6B , the sensing element in the present invention can be favorably repositioned in the sensor flow passage  320  to help reduce the impact of boundary layer spillage. 
   Referring to  FIG. 6B , TAT probe  300  of the present invention is shown with sensor flow passage  320  angled relative to the direction of incoming airflow (just prior to entering the inlet scoop) as was discussed above. As was the case with TAT probe  100  shown in  FIG. 6A , a portion  500  of the airflow entering the probe exits through main exit channel  110 , while another portion  505  is redirected by the flow separation bend such that it enters the area of sensor  130  within sensor flow passage  320 . Also like TAT sensor  100 , the heated surface of the inner elbow wall results in a boundary layer  510  having a temperature which can adversely affect the total air temperature measurement. Bleed ports  345  are again used to control boundary layer  510 . The TAT probe configuration of the present invention helps to reduce the number of stagnant bleed ports to reduce boundary layer spillage, and to position sensor assembly  125  (i.e., sensing element  130  and radiation shield  135 ) such that the effect of boundary layer spillage is lowered. 
   As can be seen in  FIG. 6A , in TAT probe  100  inner elbow wall  140  ends at point  160  at which it meets forward wall  150 . The tangent to inner elbow wall  140  at point  160  forms an angle φ, relative to forward wall  150 , of approximately 90 degrees. This sharp angle enables flow re-circulation and can result in boundary layer spillage possibly entering the sensor area and causing DHE. 
   Referring now to  FIG. 6B , in the present invention, point  360  is where the inner elbow wall  340  meets forward wall  350  of sensor flow passage  320 . At point  360 , the arc or curved surface of the inner elbow wall has a tangent which forms a much smaller angle φ with wall  350 . Reduction of angle φ reduces flow re-circulation and boundary layer spillage. Because the sensing element axis in the TAT probe  300  configuration is aligned with the flow, any flow separation that does occur will be less likely to enter the sensor when compared to the standard configuration shown in FIG.  6 A. Furthermore, the configuration of the present invention shown in  FIG. 6B  eliminates a region of separation and re-circulation located at the base of the elbow (i.e., at point  360 ), thus encouraging a more uniform flow over the sensing element. 
     FIG. 7  is a diagrammatic perspective view of a portion of an aircraft  700  on which TAT probe  300  of the present invention is mounted. As shown in  FIG. 7 , aircraft  700  includes a fuselage  705  and an aircraft engine  710 . While TAT probe  300  of the present invention can be positioned or mounted on other surfaces of aircraft  700 , in this particular embodiment, instead of being mounted to the skin of fuselage  705 , TAT probe  300  is mounted on surface  715  of engine  710 . Surface  715  forms part of the inlet portion of engine  710 , upstream of fan blades  720 . However, other aircraft engine surfaces can also be used. Of course, the present invention is not limited to TAT probes mounted to surfaces of aircraft engines, but instead applies more generally to TAT probes mounted to any aircraft surfaces for purposes of measuring TAT. Still more generally, the present invention applies to TAT probes having features as described above with reference to  FIGS. 1-6B , regardless of the surface they are to be mounted to. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.