Patent Publication Number: US-2010108661-A1

Title: Multi-layer heating assembly and method

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00019-02-C-3003 awarded by the United States Navy. 
    
    
     BACKGROUND 
     The present invention relates to a multi-layer heating assembly. More particularly, the present invention relates to a multi-layer heating assembly with failure immunity and variable watt density. 
     It is desirable to minimize or prevent the formation of ice on certain components of a gas turbine engine in order to avoid problems attributable to ice accumulation. There are many existing methods of removing or preventing the formation of ice on gas turbine engine components and airframe components. Among these methods is the incorporation (or embedding) of an electrothermal heating element into a gas turbine engine or airframe component that is susceptible to ice formation. The heating element may also be applied to a surface of the component. The heating element heats the susceptible areas of the component in order to prevent ice from forming. 
     The heating element may be a metallic heating element which typically converts electrical energy into heat energy. The metallic heating element is typically a part of a heater assembly that also includes at least one layer that electrically insulates the heating element. For example, the heater assembly may be formed of a metallic heating element embedded into a fiber-reinforced composite structure. 
     Typically, these types of heating elements have limitations. First, due to design space limitations, these heating elements generally do not offer failure immunity afforded by a redundant heating element. If a heating element fails or malfunctions, additional heating elements are not available to provide ice protection in that area. Second, the watt density of the heating element is determined at the time the heating element is constructed. Watt density is determined by the width and thickness of the heating element and the spacing of the heating element pattern. Once provided, the watt density of the heating element is fixed and cannot be changed. Increased watt density in a particular area of the gas turbine engine cannot be provided during flight where conditions might arise that require it. 
     SUMMARY 
     An exemplary embodiment of the present invention is an apparatus for ice protection. The apparatus includes a first heating element, a second heating element, and a dielectric support. The dielectric support has a first surface and a second surface opposite the first surface. The first heating element is located on the first surface and the second heating element is located on the second surface. 
     A further exemplary embodiment of the present invention is an apparatus having first, second, third, and fourth conductive layers. The apparatus also includes a plurality of dielectric supports spaced between the first and second conductive layers, the second and third conductive layers and the third and fourth conductive layers. At least one of the conductive layers is configured to provide ice protection. 
     Another exemplary embodiment of the present invention is a method of preventing ice accumulation on a component. The method includes mounting a multi-layer heating assembly to the component where the multi-layer heating assembly includes a first heating element, a second heating element, and a dielectric support. The method also includes providing electrical energy to at least one of the first and second heating elements to heat the component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of a multi-layer heating assembly having thin metal foil heating elements and a dielectric support. 
         FIG. 2  is a schematic end view of a multi-layer heating assembly having two heating elements and a dielectric support. 
         FIG. 3  is a schematic end view of a multi-layer heating assembly having multiple heating elements and multiple dielectric supports. 
         FIG. 4  is a schematic end view of a multi-layer heating assembly having multiple heating elements, dielectric supports and vias. 
         FIG. 5  is a schematic end view of a multi-layer heating assembly having multiple conductive layers, heating elements, dielectric supports and vias. 
         FIG. 6  is a schematic end view of a multi-layer heating assembly having multiple conductive layers, a sensor, a heating element, multiple dielectric supports and vias. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a multi-layer heating assembly configured to be embedded inside or mounted onto an engine or aircraft component. The multi-layer heating assembly includes at least one layer of a thin metal foil or thermal sprayed metal configured as a metallic heating element. The multi-layer heating assembly may be a composite structure formed from fabric layers or polymer films that surround the metal foil layers. The fabric layers or polymer films commonly include at least one non-conductive layer that electrically isolates the metal foil layers. 
     The heating assembly may be embedded inside or surface-mounted on any component that is susceptible to ice formation. For example, the component may be an aircraft component or a gas turbine engine component such as, but not limited to, a vane, an airfoil, a front bearing housing of the engine, a structural strut that supports the front bearing, a fan inlet shroud fairing or a duct. The component may be formed of materials such as, but not limited to, metal, polymer matrix composites (PMC) (which may be reinforced with polymeric, glass, carbon or ceramic fibers), metal matrix composites, metal, ceramic matrix composites (CMC), and carbon/carbon composites. 
       FIG. 1  is a schematic top view of a multi-layer heating assembly  10  having a thin metal foil heating element  12  and a dielectric support  14 . Because it is a top view,  FIG. 1  shows only one heating element  12  (on top of dielectric support  14 ) on the multi-layer heating assembly  10 . Additional heating elements may be located underneath dielectric support  14 . Heating element  12  is a thin metal foil and is designed as a metallic heating element, which converts electrical energy into thermal heat, as is known in the art. Heating element  12  may be titanium as described in U.S. patent application Ser. No. 11/591,327, the contents of which are incorporated herein by reference in their entirety. Other suitable materials for thin metal foil heating elements  12  include resistive heating elements such as stainless steel, copper and wire cloth heating elements. Multi-layer heating assembly  10  is embedded into a component or mounted on its surface. 
     As illustrated in  FIG. 1 , heating element  12  has a generally serpentine circuitous pattern. In this embodiment, heating element  12  is one continuous segment. In alternative embodiments, heating element  12  may have more complex geometry and may include multiple segments that are interconnected. The generally serpentine circuitous pattern of heating element  12  defines the heating path trace. Heating element  12  includes termini  12   a,  turns (or “racetrack” portions)  12   b,  and legs  12   c,  which connect turns  12   b  to termini  12   a  or to other turns  12   b . Heating element  12  is electrically connected to an electrical power source using any suitable conductor, such as a wire or a flexible circuit at a terminus  12   a  (not shown). The electrical energy may be intermittently or continuously supplied to heating element  12 , depending upon whether a deicing or anti-icing function is desired. Electrical energy follows the heating path trace of heating element  12  from one terminus  12   a  to the other along the generally serpentine circuitous pattern. As electrical energy travels through heating element  12 , its resistance causes a portion of that electrical energy to be emitted as thermal energy from the heating path trace. Open areas  16  indicate spacing between adjacent legs  12   c  of the heating path trace and are regions (cold regions) that are not directly heated by heating element  12 . 
     In the embodiment of  FIG. 1 , heating element  12  has a thickness of approximately 0.001 inches (0.0254 mm). A suitable range for the thickness of heating element  12  is approximately 0.0005 inches to 0.005 inches (0.0127 to 0.127 mm), while an exemplary range is approximately 0.001 inches to 0.003 inches (0.0254 to 0.0762 mm). A thin foil may be ideal due to limited space inside the component. Other factors which may limit the thickness of the foil include weight restrictions of the component and an overall efficiency of the foil as a heater. 
     The width of the paths of heating element  12  will vary depending on the amount of watt density needed for a particular multi-layer heating assembly  10 . In the embodiment of  FIG. 1 , heating element  12  has a width of about 0.1 inches (2.54 mm). The widths of open areas  16  in between the paths of heating element  12  will also vary depending on the amount of watt density needed and the voltage applied across heating element  12 . Open areas  16  have a minimum width of about 0.05 inches (1.27 mm) or greater. 
     Each metal foil heating element  12  is generally attached to one or more dielectric supports  14 . Heating elements  12  may be attached to a dielectric support  14  by film adhesives. Film adhesives may be fiberglass scrim supported bismaleimide (BMI) film adhesives. Other materials that may be used in film adhesives include, but are not limited to, polyimide, polyester, phenolic, cyanate ester, epoxy, fluoropolymer, silicone, elastomers and phthalonitrile. 
     Dielectric supports  14  are made up of electrically non-conductive materials, such as polyimide film. Dielectric supports  14  may also be electrically non-conductive fabric polymer matrix composites (PMC) or other non-conductive polymer films. In the embodiment of  FIG. 1 , dielectric support  14  generally has a thickness of approximately 0.002 inches (0.0508 mm). A suitable range for the thickness of dielectric supports  14  is approximately 0.001 inches to 0.003 inches (0.0254 to 0.0762 mm), while an exemplary range is approximately 0.001 inches to 0.002 inches (0.0254 to 0.0508 mm). 
     Watt density is a result of both a trace watt density, which is the density along the trace of the resistive heating element circuit pattern, and a substrate watt density, which is the amount of coverage of the resistive circuit pattern across the dielectric support. In areas where the spacing between heating element trace paths are closer, the watt density is higher. Conversely, in areas where the heating element trace paths are farther apart, the watt density is lower. In areas where the heating element trace is thinner, the watt density is higher. Conversely, in areas where the heating element trace is thicker or wider, the watt density is lower. Heating elements in the prior art have used these principles to vary the watt density of heating assemblies. However, these heating elements are unable to provide a variable watt density once they have been fixed to the substrate. Once applied, the watt densities are not modifiable. 
       FIG. 2  is a schematic end view of a multi-layer heating assembly  10  having two heating elements and a dielectric support. Unlike the heating elements of the prior art described above, this embodiment is able to provide failure immunity and variable watt density. As illustrated in  FIG. 2 , dielectric support  14  includes a first surface  16  and a second surface  18 . Second surface  18  is on the opposite side of dielectric support  14  as first surface  16 . In this illustration, the first surface  16  is a top surface, while the second surface  18  is a bottom surface. 
     First heating element  12  is located on first surface  16 . First heating element  12  is a thin metal foil heating element as described above in reference to  FIG. 1 .  FIG. 2  illustrates a side view of the heating path trace. First heating element  12  includes termini  12   a,  turns  12   b  and legs  12   c  (not shown) as described above in reference to  FIG. 1 . Open areas  20  indicate spacing between legs of the heating path trace. Second heating element  22  is located on second surface  18 . Second heating element  22  is also a thin metal foil heating element as described above. Second heating element  22  includes termini  22   a,  turns  22   b  and legs  22   c  (not shown). Open areas  24  indicate spacing between legs of the heating path trace. 
     The heating path traces of first heating element  12  and second heating element  22  may be arranged in an overlapping (mirror image) or offset configuration. The embodiment illustrated in  FIG. 2  shows that the heating path trace of first heating element  12  and the heating path trace of second heating element  22  are stacked and offset with respect to each other. Thus, the heating path trace of first heating element  12  is generally located so that legs  12   c  are positioned above open areas  24  of second heating element  22  Likewise, the heating path trace of second heating element  22  is generally located so that legs  22   c  are positioned below open areas  20  of first heating element  12 . This configuration allows thermal energy to be delivered with little or no overlap so that heat is distributed evenly and the number of cold zones (areas that do not receive heat) are eliminated or reduced. 
     This arrangement provides heating assembly  10  with the potential for failure immunity. In the event that first heating element  12  fails or malfunctions, second heating element  22  may still provide heat and function to protect the component it is embedded in from ice accumulation. If second heating element  22  fails or malfunctions, first heating element  12  may still provide heat. Thus, this arrangement provides for heating element redundancy (failure immunity) should one of the heating elements become inoperable. 
     This arrangement also provides multi-layer heating assembly  10  with the potential for increased watt density. Activation of first heating element  12  provides multi-layer heating assembly  10  with a first watt density. Further activation of the second heating element  22  provides additional wattage to the same general area of the substrate (dielectric support  14 ). When both heating elements  12 ,  22  are activated, additional watt density is provided to multi-layer heating assembly  10 . Thus, first heating element  12  may be activated to provide a first watt density. If additional watt density is needed, to de-ice a component in certain conditions, for example, second heating element  22  may also be activated to provide the additional watt density. 
     First heating element  12  and second heating element  22  may be connected in various ways depending on the desired function of the multi-layer heating assembly  10 . In one embodiment, first heating element  12  and second heating element  22  may be parallel circuits (each element is on a different circuit but both are controlled by a single controller). A parallel arrangement allows for watt density higher than that provided by a single heating element. In another embodiment, first heating element  12  and second heating element  22  may be connected in series. As in the parallel arrangement, the series circuits are controlled by a single controller. An electrically conductive through hole (plated through hole or via) extending through dielectric support  14  may provide the series connection. A series arrangement also allows for watt density higher than that provided by a single heating element. 
     In other embodiments, first heating element  12  and second heating element  22  are controlled independently. In an embodiment with redundant heating elements, first heating element  12  heats the component while second heating element  22  remains inactive. When first heating element  12  fails, second heating element  22  is activated and heats the component. In another embodiment, first heating element  12  and second heating element  22  are controlled by two independent controllers. Since the two heating elements are controlled independently, the watt density is controllable between a minimum value of the lowest watt density heating element and a maximum value of the combined watt density of both heating elements. 
       FIG. 3  is a schematic view of a multi-layer heating assembly  30  having multiple heating elements and multiple dielectric supports. Multi-layer heating assembly  30  includes heating elements  32 ,  34 ,  36  and  38  and dielectric supports  40 ,  42 ,  44  and  46 . The heating elements and dielectric supports have the same properties as those described above. In this embodiment, dielectric support  40  is located between heating elements  32  and  34 . Dielectric support  42  is located between heating elements  34  and  36 . Dielectric support  44  is located between heating elements  36  and  38 . Dielectric support  46  is located underneath heating element  38 . Multi-layer heating assembly  30  may optionally include an electrically non-conductive cover layer  48  above heating element  32  to isolate the assembly from other structural components. Cover layer  48  may be made up of electrically non-conductive materials, such as polyimide. Cover layer  48  may also be electrically non-conductive fabric polymer matrix composites (PMC) or other non-conductive polymer films. 
     In the embodiment illustrated in  FIG. 3 , each heating element is stacked and offset with respect to adjacent heating elements. Thus, heating elements  32  and  34  are stacked and offset with respect to each other, heating elements  34  and  36  are stacked and offset with respect to each other and heating elements  36  and  38  are stacked and offset with respect to each other. As a result of the stacked and offset configuration, heating elements  32  and  36  overlap and heating elements  34  and  38  also overlap. The stacked and offset configuration allows the heating elements to evenly distribute heat throughout multi-layer heating assembly  30 . Heating elements  32 ,  34 ,  36  and  38  may be connected in series or in parallel or may be controlled independently to provide failure immunity or variable watt density. 
       FIG. 4  is a schematic view of a multi-layer heating assembly  50  having multiple heating elements, dielectric supports and vias. Multi-layer heating assembly  50  includes heating elements  52 ,  54 ,  56  and  58  and dielectric supports  60 ,  62 ,  64  and  66 . The heating elements and dielectric supports have the same properties as those described above. In this embodiment, dielectric support  60  is located between heating elements  52  and  54 . Dielectric support  62  is located between heating elements  54  and  56 . Dielectric support  64  is located between heating elements  56  and  58 . Dielectric support  66  is located underneath heating element  58 . Multi-layer heating assembly  50  may optionally include an electrically non-conductive cover layer  68  above heating element  52  to isolate the assembly from other structural components. 
     In the embodiment illustrated in  FIG. 4 , heating elements  52  and  54  overlap and heating elements  56  and  58  overlap. Heating elements  54  and  56  are stacked and offset with respect to each other. Thus, heating elements  52  and  54  form a first group, heating elements  56  and  58  form a second group, and the two groups are stacked and offset with respect to each other. The heating elements in each group are connected by vias  70 . Heating elements  52  and  54  are connected by vias  70  and heating elements  56  and  58  are also connected by vias  70 .  FIG. 4  shows two via connections within each group, but more or fewer via connections may be present depending on whether the two heating elements are connected in series or parallel and the number of heating element segments present in a heating element layer. 
       FIG. 5  is a schematic view of a multi-layer heating assembly  80  having multiple conductive layers, heating elements, dielectric supports and vias. Multi-layer heating assembly  80  includes conductor layers  82  and  84 , heating elements  86  and  88 , and dielectric supports  90 ,  92 ,  94  and  96 . The heating elements and dielectric supports have the same properties as those described above. Conductor layers  82 ,  84  are configured to conduct electrical energy with minimal resistance. Conductor layers  82 ,  84  do not produce large amounts of thermal energy as electrical energy flows through them, but rather serves as conductive paths for the delivery of electrical energy to other structures within or near the component. Conductor layers  82 ,  84  are typically copper, but other conductive materials with low resistance may be suitable. 
     In this embodiment, dielectric support  90  is located between conductor layer  82  and heating element  86 . Dielectric support  92  is located between heating elements  86  and  88 . Dielectric support  94  is located between heating element  88  and conductor layer  84 . Dielectric support  96  is located underneath conductor layer  84 . Multi-layer heating assembly  80  may optionally include an electrically non-conductive cover layer  98  above conductor layer  82  to isolate the assembly from other structural components. 
     In the embodiment illustrated in  FIG. 5 , conductor layer  82  and heating element  86  overlap and heating element  88  and conductor layer  84  overlap. Heating elements  86  and  88  are stacked and offset with respect to each other. Thus, conductor layer  82  and heating element  86  form a first group, heating element  88  and conductor layer  84  form a second group, and the two groups are stacked and offset with respect to each other. The conductor layer and heating element in each group are connected by vias  100 . Conductor layer  82  and heating element  86  are connected by vias  100  and heating element  88  and conductor layer  84  are also connected by vias  100 .  FIG. 5  shows two via connections within each group, but more or fewer via connections may be present depending on whether the conductor layers and heating elements are connected in series or parallel and the number of conductor or heating element segments present in a conductor or heating element layer. 
       FIG. 6  is a schematic view of a multi-layer heating assembly  110  having multiple conductive layers and dielectric supports, a sensor, a heating element and vias. Multi-layer heating assembly  110  includes conductor layers  112  and  114 , sensor  116 , heating element  118 , and dielectric supports  120 ,  122 ,  124  and  126 . The conductor layers, heating elements and dielectric supports have the same properties as those described above. Sensor  116  may be a temperature sensor. Sensor  116  may be a thermocouple, thermistor or resistance temperature detector (RTD). In the embodiment illustrated in  FIG. 6 , sensor  116  is a resistance temperature detector (RTD), as is known in the art. An RTD sensor  116  is generally platinum. Sensor  116  uses the resistance of an electric current passed through it to determine temperature. Sensor  116  may activate heating element  118  if the resistance indicates that the component temperature is too low. 
     In this embodiment, dielectric support  120  is located between conductor layer  112  and sensor  116 . Dielectric support  122  is located between sensor  116  and heating element  118 . Dielectric support  124  is located between heating element  118  and conductor layer  114 . Dielectric support  126  is located underneath conductor layer  114 . Multi-layer heating assembly  110  may optionally include an electrically non-conductive cover layer  128  above conductor layer  112  to isolate the assembly from other structural components. 
     In the embodiment illustrated in  FIG. 6 , conductor layer  112  and sensor  116  overlap and heating element  118  and conductor layer  114  overlap. Sensor  116  and heating element  118  are stacked and offset with respect to each other. Thus, conductor layer  112  and sensor  116  form a first group, heating element  118  and conductor layer  114  form a second group, and the two groups are stacked and offset with respect to each other. The conductor layer and sensor in the first group and the heating element and conductor layer in the second group are connected by vias  130 . Conductor layer  112  and sensor  116  are connected by vias  130  and heating element  118  and conductor layer  114  are also connected by vias  130 .  FIG. 6  shows two via connections within each group, but more or fewer via connections may be present depending on whether the conductor layers and sensors or heating elements are connected in series or parallel and the number of conductor, sensor or heating element segments present in a conductor, sensor or heating element layer. 
     The multi-layer heating assemblies  10 ,  30 ,  50 ,  80 , and  110  illustrated in  FIGS. 2 through 6  have a thickness depending on the number of layers in the assembly and the thicknesses of the layers. Gas turbine components are often composite components having several plies of fabric materials. Generally, the heating assemblies occupy a single ply or part of a single ply of a component&#39;s structure. Heating assemblies with four conductive layers and five non-conductive layers, such as those illustrated in  FIGS. 3 through 6 , may occupy a single ply. If additional conductive and non-conductive layers are added to the multi-layer heating assembly, the multi-layer heating assembly may occupy two or more plies of the component. Heating assemblies may or may not span the entire length or width of a component ply. In an embodiment where the multi-layer heating assembly is located on only a portion of the component ply, a layer of fabric material may be positioned near or around the multi-layer heating assembly to “fill in” the component ply layer and/or provide structure necessary for the strength and integrity of the component. 
     Multi-layer heating assemblies  10 ,  30 ,  50 ,  80 , and  110  provide for a method of preventing ice accumulation on a component. Multi-layer heating assemblies  10 ,  30 ,  50 ,  80 , and  110  are embedded in a component or mounted to a component surface. Electrical energy is provided to one or more heating elements of multi-layer heating assemblies  10 ,  30 ,  50 ,  80 , and  110  to heat the component. The method provides for failure immunity. When one heating element fails, electrical energy is delivered to a second heating element to heat the component. The method also provides for variable watt density. When a higher watt density is needed, electrical energy is provided to additional heating elements. When a lower watt density is needed electrical energy is provided to fewer than all heating elements. 
     In summary, the present invention relates to a multi-layer heating assembly and a method of preventing ice accumulation on a component having a multi-layer heating assembly. Multi-layer heating assemblies may be used for failure immunity and to provide variable watt density. 
     Although the present invention has been described with reference to exemplary 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.