Numerical calculation method of lightning ignition source for composite fuel tank of aircraft

Abstract

The application discloses a numerical calculation method of a lightning ignition source of an aircraft composite fuel tank, comprising the following steps: step 1, an electro-thermal coupling simulation model is established to obtain a transient temperature field and an electric field intensity distribution; step 2, a thermal effect candidate region is extracted according to the temperature field, an electric field effect candidate region is extracted according to the electric field intensity, and the candidate regions are combined to form a candidate ignition source set; step 3, each candidate point in the set is classified according to a local temperature and a local electric field intensity, and potential ignition energies corresponding to different categories are calculated respectively; step 4, the potential ignition energies are compared with a safety threshold, and a flammable vapor region position judgment is combined to screen out an ignition source; and step 5, a spatial position, an energy value and a dominant mechanism of the ignition source are output. The evaluation method solves the problem that, in the existing evaluation method, thermal effects and electric effects are analyzed separately, the strong coupling relationship between the thermal effects and the electric effects cannot be considered, and the evaluation result is optimistic or a potential ignition source risk point is missed.

Description

Technical Field

[0001] This invention belongs to the field of fuel tank simulation analysis technology, specifically involving a numerical calculation method for lightning ignition sources in aircraft composite fuel tanks. Background Technology

[0002] With the widespread use of composite fuel tanks in aircraft, lightning protection has become an increasingly prominent issue. Composite materials have lower conductivity than metals, making them prone to creating localized strong electric fields or high temperatures due to poor contact at fasteners, lap joints, and structural gaps. These can become potential ignition sources for fuel vapors, making accurate assessment of fuel tank ignition sources essential for ensuring flight safety. Current technical challenges include: physical testing (such as the SAE ARP 5416A standard lightning strike test) is costly, time-consuming, and difficult to obtain internal field distribution data. Existing numerical calculation methods typically analyze thermal and electrical effects separately, failing to fully consider their strong coupling relationship, especially the field-induced electron emission mechanism under the combined effect of high temperature and strong electric field. Furthermore, existing methods provide a simplistic description of breakdown discharge phenomena, leading to overly optimistic assessments or missed risk points. Summary of the Invention

[0003] The purpose of this invention is to provide a numerical calculation method for lightning ignition sources in aircraft composite fuel tanks. This method solves the problem in existing evaluation methods that separate the analysis of thermal and electrical effects, fail to consider the strong coupling relationship between the two, and thus lead to optimistic evaluation results or omission of potential ignition source risks.

[0004] The technical solution adopted in this invention is a numerical calculation method for lightning ignition sources in aircraft composite fuel tanks, comprising the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source.

[0005] The invention is further characterized by: In step 1, the electrothermal coupling simulation model is a three-dimensional numerical calculation model established based on finite element software; the lightning current excitation source adopts the lightning current waveform specified in the SAE ARP 5416A standard.

[0006] The method for extracting the candidate region for thermal effects in step 2 is as follows: extract the temperature at all times. T > T min The spatial region, in which T min The temperature was set at 150°C, which is 210°C~250°C lower than the auto-ignition temperature of fuel to allow for a safety margin. The method for extracting the candidate region for electric field effect is as follows: extract the electric field intensity at all times. E >0.5× E bd0 The structural gap region, in which E bd0 The air breakdown field strength at standard atmospheric pressure is 3 × 10⁻⁶. 6 V / m, that is, the initial screening threshold is E >1.5×10 6 V / m; The candidate regions for thermal effects and electric field effects are combined to form a set of candidate ignition sources.

[0007] The specific criteria for classification in step 3 are as follows: When the temperature of the candidate point T c >150°C and electric field strength E c <1.5×10 6 When V / m, the candidate point is classified as purely thermally dominated. When the temperature of the candidate point T c <150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as a pure electric field dominated type; When the temperature of the candidate point T c >150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as thermoelectric coupling type.

[0008] In step 3, for purely thermally dominant candidate sites, the potential ignition energy is calculated using the following formula: ; in, The density of the material; Specific heat capacity of the material; The volume of the hot spot is taken as the volume of the simulated mesh cell or the volume enclosed by the temperature isosurface; This represents the rate of change of temperature over time. This represents the total duration of the lightning current.

[0009] In step 3, the potential ignition energy for candidate sites dominated by the pure electric field is calculated as follows: First, regarding the electric field strength E c >1.5×10 6 Electrical breakdown detection was performed on the gap of V / m, and the gap distance was extracted. D The electric field strength on both sides of the gap is used to establish an electric breakdown model based on the electrostatic and electric breakdown detection physical fields in the finite element software to detect whether the gap has undergone electric breakdown. If no breakdown occurs, the candidate point is considered safe; if breakdown occurs, the breakdown field strength is recorded. And calculate the spark energy using the following formula: ; in, For gap capacitance, This is the breakdown voltage.

[0010] In step 3, the potential ignition energy for thermoelectric coupling candidate sites is calculated as follows: The emission current density was calculated using a temperature-dependent modified Fowler-Nordheim thermo-field emission equation. ; ; in, This represents the thermal field emission current density. This is the first Fowler-Nordheim constant, usually taken as... ; This is the second Fowler-Nordheim constant, usually taken as... ; Let be the normal electric field intensity of the emitting surface. If the candidate point is a tip type, then... Take the normal electric field intensity of the tip surface. If the candidate point belongs to the gap type, then Take the larger value of the normal electric field intensity on both sides of the gap; The field enhancement factor is dimensionless, with a value of 20-50 for metal fastener edges and 50-150 for carbon fiber edges. It is a temperature-dependent work function, which decreases as the temperature increases; It is a thermal auxiliary factor, dimensionless, used to correct the effect of temperature on tunneling probability, reflecting the degree to which thermally excited electrons participate in tunneling; Temperature-dependent work function φ ( T The formula for calculating ) is: ; in, This represents the electron work function at the material surface at a temperature of 0 K. ω The temperature coefficient is on the order of 10. -5 eV / K 2 ; The energy is obtained by integrating the emitted current density over the duration of the lightning current: ; in, Given the emission area, the electric field strength at the candidate point is taken as... The area of ​​the region; This is the gap voltage.

[0011] The safety threshold in step 4 is 200 μJ; the specific method for determination is as follows: When the potential ignition energy of a candidate point is greater than or equal to 200 μJ and the candidate point is located in the combustible vapor region, the candidate point is determined to be an ignition source. When the potential ignition energy of a candidate point is less than 200 μJ, the candidate point is determined to be safe.

[0012] The dominant mechanism in step 5 is one of three types: pure thermal effect, pure electric field effect, and thermoelectric coupled field emission; the ignition source information output in step 5 is sorted from largest to smallest energy value.

[0013] It also includes optimization design steps: based on the spatial location, energy value and dominant mechanism of the output ignition source, guide the optimization design of the fuel tank structure, including increasing the conductive path, eliminating the tip structure and filling the gap.

[0014] The beneficial effects of this invention are: The numerical calculation method for lightning ignition sources in aircraft composite fuel tanks provided by this invention employs a dual-channel (temperature field and electric field) initial screening strategy, performing detailed energy calculations only on candidate regions, significantly reducing the computational load. A temperature-dependent field-induced electron emission model is introduced to realistically reflect the nonlinear coupling effect of high temperatures reducing work function and enhancing emission, more closely approximating the actual physical process of lightning. The ignition source risk point list output by the evaluation process can directly guide protection design, save testing costs, shorten the airworthiness certification cycle, and solve the problems of traditional methods failing to detect pure electric field breakdown and underestimating coupling effects. Attached Figure Description

[0015] Figure 1 This is a flowchart illustrating the numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to the present invention; Figure 2 This is a flowchart of the thermoelectric coupling energy calculation sub-process in Embodiment 5 of the present invention; Figure 3 This is a simplified model diagram of the fuel tank in Embodiment 6 of the present invention; Figure 4 This is the waveform of the applied current in Embodiment 6 of the present invention; Figure 5 This is a schematic diagram of the breakdown detection of the gap between the second fastener and the wall of the composite oil tank hole in Embodiment 6 of the present invention; Figure 6 This is a schematic diagram of the breakdown detection of the gap between the fuel pump and the composite fuel tank hole wall in Embodiment 6 of the present invention. Detailed Implementation

[0016] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0017] The present invention provides a numerical calculation method for the lightning ignition source of aircraft composite fuel tanks, such as... Figure 1 As shown, it includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; In step 1, the electrothermal coupling simulation model is a three-dimensional numerical calculation model established based on finite element software; the lightning current excitation source adopts the lightning current waveform specified in the SAE ARP 5416A standard. Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; The method for extracting the candidate region for thermal effects in step 2 is as follows: extract the temperature at all times. T > T min The spatial region, in which T min The temperature was set at 150°C, which is 210°C~250°C lower than the auto-ignition temperature of fuel to allow for a safety margin. The method for extracting the candidate region for electric field effect is as follows: extract the electric field intensity at all times. E >0.5× E bd0 The structural gap region, in which E bd0 The air breakdown field strength at standard atmospheric pressure is 3 × 10⁻⁶. 6 V / m, that is, the initial screening threshold is E >1.5×10 6 V / m; The union of the candidate regions for thermal effects and the candidate regions for electric field effects is used to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. The specific criteria for classification in step 3 are as follows: When the temperature of the candidate point T c >150°C and electric field strengthE c <1.5×10 6 When V / m, the candidate point is classified as purely thermally dominated. In step 3, for purely thermally dominant candidate sites, the potential ignition energy is calculated using the following formula: ; in, The density of the material; Specific heat capacity of the material; The volume of the hot spot is taken as the volume of the simulated mesh cell or the volume enclosed by the temperature isosurface; This represents the rate of change of temperature over time. This represents the total duration of the lightning current. When the temperature of the candidate point T c <150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as a pure electric field dominated type; In step 3, the potential ignition energy for candidate sites dominated by the pure electric field is calculated as follows: First, regarding the electric field strength E c >1.5×10 6 Electrical breakdown detection was performed on the gap of V / m, and the gap distance was extracted. D The electric field strength on both sides of the gap is used to establish an electric breakdown model based on the electrostatic and electric breakdown detection physical fields in the finite element software to detect whether the gap has undergone electric breakdown. If no breakdown occurs, the candidate point is considered safe; if breakdown occurs, the breakdown field strength is recorded. And calculate the spark energy using the following formula: ; in, For gap capacitance, This is the breakdown voltage; When the temperature of the candidate point T c >150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as thermoelectric coupling type; In step 3, the potential ignition energy for thermoelectric coupling candidate sites is calculated as follows: The emission current density was calculated using a temperature-dependent modified Fowler-Nordheim thermo-field emission equation. ; ; in, This represents the thermal field emission current density. This is the first Fowler-Nordheim constant, usually taken as... ; This is the second Fowler-Nordheim constant, usually taken as... ; Let be the normal electric field intensity of the emitting surface. If the candidate point is a tip type, then... Take the normal electric field intensity of the tip surface. If the candidate point belongs to the gap type, then Take the larger value of the normal electric field intensity on both sides of the gap; The field enhancement factor is dimensionless, with a value of 20-50 for metal fastener edges and 50-150 for carbon fiber edges. It is a temperature-dependent work function, which decreases as the temperature increases; It is a thermal auxiliary factor, dimensionless, used to correct the effect of temperature on tunneling probability, reflecting the degree to which thermally excited electrons participate in tunneling; Temperature-dependent work function φ ( T The formula for calculating ) is: ; in, This represents the electron work function at the material surface at a temperature of 0 K. ω The temperature coefficient is on the order of 10. -5 eV / K 2 ; The energy is obtained by integrating the emitted current density over the duration of the lightning current: ; in, Given the emission area, the electric field strength at the candidate point is taken as... The area of ​​the region; This is the gap voltage; Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; The safety threshold in step 4 is 200 μJ; the specific method for determination is as follows: When the potential ignition energy of a candidate point is greater than or equal to 200 μJ and the candidate point is located in the combustible vapor region, the candidate point is determined to be an ignition source. When the potential ignition energy of a candidate point is less than 200 μJ, the candidate point is deemed safe. Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source; The dominant mechanism in step 5 is one of three types: pure thermal effect, pure electric field effect, and thermoelectric coupled field emission; the ignition source information output in step 5 is sorted from largest to smallest energy value. It also includes optimization design steps: based on the spatial location, energy value and dominant mechanism of the output ignition source, guide the optimization design of the fuel tank structure, including increasing the conductive path, eliminating the tip structure and filling the gap.

[0018] Example 1 The numerical calculation method for lightning ignition sources in aircraft composite fuel tanks proposed in this embodiment is as follows: Figure 1 As shown, it includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source.

[0019] Example 2 The numerical calculation method for lightning ignition sources in aircraft composite fuel tanks proposed in this embodiment is as follows: Figure 1 As shown, it includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; In step 1, the electrothermal coupling simulation model is a three-dimensional numerical calculation model established based on finite element software; the lightning current excitation source adopts the lightning current waveform specified in the SAE ARP 5416A standard. Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; The method for extracting the candidate region for thermal effects in step 2 is as follows: extract the temperature at all times. T > T min The spatial region, in which T min The temperature was set at 150°C, which is 210°C~250°C lower than the auto-ignition temperature of fuel to allow for a safety margin. The method for extracting the candidate region for electric field effect is as follows: extract the electric field intensity at all times. E >0.5× Ebd0 The structural gap region, in which E bd0 The air breakdown field strength at standard atmospheric pressure is 3 × 10⁻⁶. 6 V / m, that is, the initial screening threshold is E >1.5×10 6 V / m; The union of the candidate regions for thermal effects and the candidate regions for electric field effects is used to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source.

[0020] Example 3 The numerical calculation method for lightning ignition sources in aircraft composite fuel tanks proposed in this embodiment is as follows: Figure 1 As shown, it includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; In step 1, the electrothermal coupling simulation model is a three-dimensional numerical calculation model established based on finite element software; the lightning current excitation source adopts the lightning current waveform specified in the SAE ARP 5416A standard. Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; The method for extracting the candidate region for thermal effects in step 2 is as follows: extract the temperature at all times. T > T min The spatial region, in which T min The temperature was set at 150°C, which is 210°C~250°C lower than the auto-ignition temperature of fuel to allow for a safety margin. The method for extracting the candidate region for electric field effect is as follows: extract the electric field intensity at all times. E >0.5× E bd0 The structural gap region, in which E bd0 The air breakdown field strength at standard atmospheric pressure is 3 × 10⁻⁶. 6 V / m, that is, the initial screening threshold is E >1.5×10 6 V / m; The union of the candidate regions for thermal effects and the candidate regions for electric field effects is used to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. The specific criteria for classification in step 3 are as follows: When the temperature of the candidate point T c >150°C and electric field strength E c <1.5×10 6 When V / m, the candidate point is classified as purely thermally dominated. In step 3, for purely thermally dominant candidate sites, the potential ignition energy is calculated using the following formula: ; in, The density of the material; Specific heat capacity of the material; The volume of the hot spot is taken as the volume of the simulated mesh cell or the volume enclosed by the temperature isosurface; This represents the rate of change of temperature over time. This represents the total duration of the lightning current. When the temperature of the candidate point T c <150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as a pure electric field dominated type; In step 3, the potential ignition energy for candidate sites dominated by the pure electric field is calculated as follows: First, regarding the electric field strength E c >1.5×10 6 Electrical breakdown detection was performed on the gap of V / m, and the gap distance was extracted. D The electric field strength on both sides of the gap is used to establish an electric breakdown model based on the electrostatic and electric breakdown detection physical fields in the finite element software to detect whether the gap has undergone electric breakdown. If no breakdown occurs, the candidate point is considered safe; if breakdown occurs, the breakdown field strength is recorded. And calculate the spark energy using the following formula: ; in, For gap capacitance, This is the breakdown voltage; When the temperature of the candidate point T c >150°C and electric field strength E c >1.5×10 6When V / m, the candidate point is classified as thermoelectric coupling type; In step 3, the potential ignition energy for thermoelectric coupling candidate sites is calculated as follows: The emission current density was calculated using a temperature-dependent modified Fowler-Nordheim thermo-field emission equation. ; ; in, This represents the thermal field emission current density. This is the first Fowler-Nordheim constant, usually taken as... ; This is the second Fowler-Nordheim constant, usually taken as... ; Let be the normal electric field intensity of the emitting surface. If the candidate point is a tip type, then... Take the normal electric field intensity of the tip surface. If the candidate point belongs to the gap type, then Take the larger value of the normal electric field intensity on both sides of the gap; The field enhancement factor is dimensionless, with a value of 20-50 for metal fastener edges and 50-150 for carbon fiber edges. It is a temperature-dependent work function, which decreases as the temperature increases; It is a thermal auxiliary factor, dimensionless, used to correct the effect of temperature on tunneling probability, reflecting the degree to which thermally excited electrons participate in tunneling; Temperature-dependent work function φ ( T The formula for calculating ) is: ; in, This represents the electron work function at the material surface at a temperature of 0 K. ω The temperature coefficient is on the order of 10. -5 eV / K 2 ; The energy is obtained by integrating the emitted current density over the duration of the lightning current: ; in, Given the emission area, the electric field strength at the candidate point is taken as... The area of ​​the region; This is the gap voltage; Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source.

[0021] Example 4 The numerical calculation method for lightning ignition sources in aircraft composite fuel tanks proposed in this embodiment is as follows: Figure 1 As shown, it includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; In step 1, the electrothermal coupling simulation model is a three-dimensional numerical calculation model established based on finite element software; the lightning current excitation source adopts the lightning current waveform specified in the SAE ARP 5416A standard. Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; The method for extracting the candidate region for thermal effects in step 2 is as follows: extract the temperature at all times. T > T min The spatial region, in which T min The temperature was set at 150°C, which is 210°C~250°C lower than the auto-ignition temperature of fuel to allow for a safety margin. The method for extracting the candidate region for electric field effect is as follows: extract the electric field intensity at all times. E >0.5× E bd0 The structural gap region, in which E bd0 The air breakdown field strength at standard atmospheric pressure is 3 × 10⁻⁶. 6 V / m, that is, the initial screening threshold is E >1.5×10 6 V / m; The union of the candidate regions for thermal effects and the candidate regions for electric field effects is used to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. The specific criteria for classification in step 3 are as follows: When the temperature of the candidate point T c >150°C and electric field strength E c <1.5×10 6 When V / m, the candidate point is classified as purely thermally dominated. In step 3, for purely thermally dominant candidate sites, the potential ignition energy is calculated using the following formula: ; in, The density of the material; Specific heat capacity of the material; The volume of the hot spot is taken as the volume of the simulated mesh cell or the volume enclosed by the temperature isosurface; This represents the rate of change of temperature over time. This represents the total duration of the lightning current. When the temperature of the candidate point T c <150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as a pure electric field dominated type; In step 3, the potential ignition energy for candidate sites dominated by the pure electric field is calculated as follows: First, regarding the electric field strength E c >1.5×10 6 Electrical breakdown detection was performed on the gap of V / m, and the gap distance was extracted. D The electric field strength on both sides of the gap is used to establish an electric breakdown model based on the electrostatic and electric breakdown detection physical fields in the finite element software to detect whether the gap has undergone electric breakdown. If no breakdown occurs, the candidate point is considered safe; if breakdown occurs, the breakdown field strength is recorded. And calculate the spark energy using the following formula: ; in, For gap capacitance, This is the breakdown voltage; When the temperature of the candidate point T c >150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as thermoelectric coupling type; In step 3, the potential ignition energy for thermoelectric coupling candidate sites is calculated as follows: The emission current density was calculated using a temperature-dependent modified Fowler-Nordheim thermo-field emission equation. ; ; in, This represents the thermal field emission current density. This is the first Fowler-Nordheim constant, usually taken as... ; This is the second Fowler-Nordheim constant, usually taken as... ; Let be the normal electric field intensity of the emitting surface. If the candidate point is a tip type, then... Take the normal electric field intensity of the tip surface. If the candidate point belongs to the gap type, then Take the larger value of the normal electric field intensity on both sides of the gap; The field enhancement factor is dimensionless, with a value of 20-50 for metal fastener edges and 50-150 for carbon fiber edges. It is a temperature-dependent work function, which decreases as the temperature increases; It is a thermal auxiliary factor, dimensionless, used to correct the effect of temperature on tunneling probability, reflecting the degree to which thermally excited electrons participate in tunneling; Temperature-dependent work function φ ( T The formula for calculating ) is: ; in, This represents the electron work function at the material surface at a temperature of 0 K. ω The temperature coefficient is on the order of 10. -5 eV / K 2 ; The energy is obtained by integrating the emitted current density over the duration of the lightning current: ; in, Given the emission area, the electric field strength at the candidate point is taken as... The area of ​​the region; This is the gap voltage; Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; The safety threshold in step 4 is 200 μJ; the specific method for determination is as follows: When the potential ignition energy of a candidate point is greater than or equal to 200 μJ and the candidate point is located in the combustible vapor region, the candidate point is determined to be an ignition source. When the potential ignition energy of a candidate point is less than 200 μJ, the candidate point is deemed safe. Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source.

[0022] Example 5 The numerical calculation method for lightning ignition sources in aircraft composite fuel tanks proposed in this embodiment is as follows: Figure 1 As shown, it includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; In step 1, the electrothermal coupling simulation model is a three-dimensional numerical calculation model established based on finite element software; the lightning current excitation source adopts the lightning current waveform specified in the SAE ARP 5416A standard. Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; The method for extracting the candidate region for thermal effects in step 2 is as follows: extract the temperature at all times. T > T min The spatial region, in which T min The temperature was set at 150°C, which is 210°C~250°C lower than the auto-ignition temperature of fuel to allow for a safety margin. The method for extracting the candidate region for electric field effect is as follows: extract the electric field intensity at all times. E >0.5× E bd0 The structural gap region, in which E bd0 The air breakdown field strength at standard atmospheric pressure is 3 × 10⁻⁶. 6 V / m, that is, the initial screening threshold is E >1.5×10 6 V / m; The union of the candidate regions for thermal effects and the candidate regions for electric field effects is used to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. The specific criteria for classification in step 3 are as follows: When the temperature of the candidate point T c >150°C and electric field strength E c <1.5×10 6 When V / m, the candidate point is classified as purely thermally dominated. In step 3, for purely thermally dominant candidate sites, the potential ignition energy is calculated using the following formula: ; in, The density of the material; Specific heat capacity of the material; The volume of the hot spot is taken as the volume of the simulated mesh cell or the volume enclosed by the temperature isosurface; This represents the rate of change of temperature over time. This represents the total duration of the lightning current. When the temperature of the candidate point T c <150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as a pure electric field dominated type; In step 3, the potential ignition energy for candidate sites dominated by the pure electric field is calculated as follows: First, regarding the electric field strength E c >1.5×10 6 Electrical breakdown detection was performed on the gap of V / m, and the gap distance was extracted. D The electric field strength on both sides of the gap is used to establish an electric breakdown model based on the electrostatic and electric breakdown detection physical fields in the finite element software to detect whether the gap has undergone electric breakdown. If no breakdown occurs, the candidate point is considered safe; if breakdown occurs, the breakdown field strength is recorded. And calculate the spark energy using the following formula: ; in, For gap capacitance, This is the breakdown voltage; When the temperature of the candidate point T c >150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as thermoelectric coupling type; In step 3, the potential ignition energy for thermoelectric coupling candidate sites is calculated as follows: like Figure 2 As shown, the emission current density is calculated using the temperature-dependent modified Fowler-Nordheim thermo-field emission equation. ; ; in, This represents the thermal field emission current density. This is the first Fowler-Nordheim constant, usually taken as... ; This is the second Fowler-Nordheim constant, usually taken as... ; Let be the normal electric field intensity of the emitting surface. If the candidate point is a tip type, then... Take the normal electric field intensity of the tip surface. If the candidate point belongs to the gap type, then Take the larger value of the normal electric field intensity on both sides of the gap; The field enhancement factor is dimensionless, with a value of 20-50 for metal fastener edges and 50-150 for carbon fiber edges. It is a temperature-dependent work function, which decreases as the temperature increases; It is a thermal auxiliary factor, dimensionless, used to correct the effect of temperature on tunneling probability, reflecting the degree to which thermally excited electrons participate in tunneling; Temperature-dependent work function φ ( T The formula for calculating ) is: ; in, This represents the electron work function at the material surface at a temperature of 0 K. ω The temperature coefficient is on the order of 10. -5 eV / K 2 ; The energy is obtained by integrating the emitted current density over the duration of the lightning current: ; in, Given the emission area, the electric field strength at the candidate point is taken as... The area of ​​the region; This is the gap voltage; Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; The safety threshold in step 4 is 200 μJ; the specific method for determination is as follows: When the potential ignition energy of a candidate point is greater than or equal to 200 μJ and the candidate point is located in the combustible vapor region, the candidate point is determined to be an ignition source. When the potential ignition energy of a candidate point is less than 200 μJ, the candidate point is deemed safe. Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source; The dominant mechanism in step 5 is one of three types: pure thermal effect, pure electric field effect, and thermoelectric coupled field emission; the ignition source information output in step 5 is sorted from largest to smallest energy value. It also includes optimization design steps: based on the spatial location, energy value and dominant mechanism of the output ignition source, guide the optimization design of the fuel tank structure, including increasing the conductive path, eliminating the tip structure and filling the gap.

[0023] Example 6 The numerical calculation method for lightning ignition sources in aircraft composite fuel tanks proposed in this embodiment is as follows: Figure 1 As shown, it includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; In step 1, the electrothermal coupling simulation model is a three-dimensional numerical calculation model established using finite element software. The three-dimensional model is a simplified model of the fuel tank, including composite material skin, metal fasteners, and a fuel pump, as shown below. Figure 3As shown, the input materials for the fuel tank composite skin, metal fasteners, and metal fuel pump include: electrical conductivity, heat capacity, thermal conductivity, and density. The lightning current excitation source adopts the current component A waveform (peak value 200 kA, action integral) specified in the SAE ARP 5416A standard. ),like Figure 4 As shown; The temperature field and electric field intensity distribution of the oil tank were obtained by using the "Joule heating" and "current" physical field coupling solution in COMSOL Multiphysics with a time step of 0.5μs and a total duration of 100μs. Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; Channel A: Extraction T In an area >150°C, the head of the first fastener (the point where the lightning current flows in, with a maximum temperature of 1320°C) was found. The interface between the first fastener and the composite fuel tank hole wall (maximum temperature 720°C) ) and current output point (maximum temperature 380) There are hot topics; Channel B: Extraction E >1.5×10 6 The gap of V / m was found to be 0.05mm at the interface between the first fastener and the composite oil tank hole wall, with an electric field of 9.1×10. 7 V / m), the interface between the second fastener and the composite oil tank hole wall (0.05 mm, electric field 4.2×10), 6 V / m) and the interface between the fuel pump and the composite fuel tank bore wall (0.2mm, electric field 2.8×10). 6 There are high points in the electric field at (V / m); After merging the candidate sets, there are a total of 5 candidate points; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. In step 3, for purely thermally dominant candidate sites, the potential ignition energy is calculated using the following formula: The heat released by the head of the first fastener was calculated: The heat released at the point where the current flows out: ; in, The density of the material; Specific heat capacity of the material; The volume of the hot spot is taken as the volume of the simulated mesh cell or the volume enclosed by the temperature isosurface; This represents the rate of change of temperature over time. This represents the total duration of the lightning current. In step 3, the potential ignition energy for candidate sites dominated by the pure electric field is calculated as follows: Electrical breakdown detection: Using the "Electrostatics" and "Electrical Breakdown Detection" physics fields of COMSOL Multiphysics, electrical breakdown models were established for the gaps between the second fastener and the composite fuel tank, and between the fuel pump and the composite fuel tank, respectively, and electrical breakdown detection was performed. Simulation results proved that electrical breakdown occurred in both gaps, as shown below. Figure 5 , 6 As shown; Calculate spark energy: based on The energy released by the electric spark between the second fastener and the wall of the composite fuel tank hole was calculated to be... The energy released by the electric spark between the fuel pump and the composite fuel tank bore wall is ; In step 3, the potential ignition energy for thermoelectric coupling candidate sites is calculated as follows: Based on the thermal-field emission theory, using The energy released by the current generated between the first fastener and the wall of the composite fuel tank hole was calculated to be... ; Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; The energy calculated for each candidate ignition source is compared with the aviation safety threshold. Comparison: a. First fastener head (pure heat-driven type): However, since it is located outside the fuel tank and is not within the flammable vapor area, it is deemed safe. b. Interface between the first fastener and the composite fuel tank hole wall (thermoelectric coupling type): And since it is located in a flammable vapor area, it can be identified as an ignition source; c. Current outflow point (pure heat-dominated type): However, the ablation point was located outside the fuel tank, and there was almost no temperature rise inside. It was not in the area of ​​flammable vapors and was therefore deemed safe. d. Interface between the second fastener and the composite fuel tank hole wall (pure electric dominant type): And since it is located in a flammable vapor area, it can be identified as an ignition source; e. Fuel pump and composite fuel tank bore wall overlap interface (pure electric dominant type): It was determined to be safe; Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source; The composite fuel tank has two ignition sources in a lightning environment: the first fastener overlaps with the fuel tank hole wall and the second fastener overlaps with the fuel tank hole wall. According to energy classification, the ignition source of the first fastener overlaps with the fuel tank hole wall is more dangerous than the latter. Based on the above output, the following suggestions are made: a) A conductive cap can be added to the head of the fastener to disperse the current; b) Add an insulating seal to the end of the fastener to prevent sparks from being ejected. c) The installation of metal parts and composite skin adopts wet installation with interference fit to eliminate gaps.

Claims

1. A numerical calculation method for lightning ignition sources in aircraft composite fuel tanks, characterized in that, Includes the following steps: Step 1: Establish an electrothermal coupling simulation model, apply a lightning current excitation source, and solve for the transient temperature field and electric field intensity distribution; Step 2: Extract candidate regions for thermal effects based on the temperature field, extract candidate regions for electric field effects based on the electric field strength, and merge them to form a set of candidate ignition sources; Step 3: For each candidate point in the set, classify them according to local temperature and local electric field strength, and calculate the corresponding potential ignition energy for each category. Step 4: Compare each potential ignition energy with the safety threshold, and make a judgment based on the location of the combustible vapor area to screen out the ignition source; Step 5: Output the spatial location, energy value, and dominant mechanism of the ignition source.

2. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 1, characterized in that, The electrothermal coupling simulation model mentioned in step 1 is a three-dimensional numerical calculation model established based on finite element software; the lightning current excitation source adopts the lightning current waveform specified in the SAE ARP 5416A standard.

3. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 1, characterized in that, The method for extracting the candidate region of thermal effect in step 2 is as follows: extract the temperature at all times. T > T min The spatial region, in which T min The temperature is set at 150°C, which is 210°C~250°C lower than the auto-ignition temperature of fuel to allow for a safety margin; the method for extracting the candidate region of the electric field effect is as follows: extract the electric field intensity at all times. E >0.5× E bd0 The structural gap region, in which E bd0 The air breakdown field strength at standard atmospheric pressure is 3 × 10⁻⁶. 6 V / m, that is, the initial screening threshold is E >1.5×10 6 V / m; The candidate regions for thermal effects and electric field effects are combined to form a set of candidate ignition sources.

4. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 1, characterized in that, The specific criteria for classification in step 3 are as follows: When the temperature of the candidate point T c >150°C and electric field strength E c <1.5×10 6 When V / m, the candidate point is classified as purely thermally dominated. When the temperature of the candidate point T c <150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as a pure electric field dominated type; When the temperature of the candidate point T c >150°C and electric field strength E c >1.5×10 6 When V / m, the candidate point is classified as thermoelectric coupling type.

5. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 4, characterized in that, In step 3, for the pure heat-dominated candidate points, the potential ignition energy is calculated using the following formula: ； in, The density of the material; Specific heat capacity of the material; The volume of the hot spot is taken as the volume of the simulated mesh cell or the volume enclosed by the temperature isosurface; This represents the rate of change of temperature over time. This represents the total duration of the lightning current.

6. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 4, characterized in that, In step 3, the potential ignition energy for the pure electric field-dominated candidate sites is calculated as follows: First, regarding the electric field strength E c >1.5×10 6 Electrical breakdown detection was performed on the gap of V / m, and the gap distance was extracted. D The electric field strength on both sides of the gap is used to establish an electric breakdown model based on the electrostatic and electric breakdown detection physical fields in the finite element software to detect whether the gap has undergone electric breakdown. If no breakdown occurs, the candidate point is considered safe; if breakdown occurs, the breakdown field strength is recorded. And calculate the spark energy using the following formula: ； in, For gap capacitance, This is the breakdown voltage.

7. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 4, characterized in that, In step 3, the potential ignition energy for the thermoelectric coupling candidate point is calculated as follows: The emission current density was calculated using a temperature-dependent modified Fowler-Nordheim thermo-field emission equation. ; ； in, This represents the thermal field emission current density. This is the first Fowler-Nordheim constant, usually taken as... ; This is the second Fowler-Nordheim constant, usually taken as... ; Let be the normal electric field intensity of the emitting surface. If the candidate point is a tip type, then... Take the normal electric field intensity of the tip surface. If the candidate point belongs to the gap type, then Take the larger value of the normal electric field intensity on both sides of the gap; The field enhancement factor is dimensionless, with a value of 20-50 for metal fastener edges and 50-150 for carbon fiber edges. It is a temperature-dependent work function, which decreases as the temperature increases; It is a thermal auxiliary factor, dimensionless, used to correct the effect of temperature on tunneling probability, reflecting the degree to which thermally excited electrons participate in tunneling; The temperature-dependent work function φ ( T The formula for calculating ) is: ； in, This represents the electron work function at the material surface at a temperature of 0 K. ω The temperature coefficient is on the order of 10. -5 eV / K 2 ; The energy is obtained by integrating the emitted current density over the duration of the lightning current: ； in, Given the emission area, the electric field strength at the candidate point is taken as... The area of ​​the region; This is the gap voltage.

8. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 1, characterized in that, The safety threshold mentioned in step 4 is 200 μJ; the specific method for determination is as follows: When the potential ignition energy of a candidate point is greater than or equal to 200 μJ and the candidate point is located in the combustible vapor region, the candidate point is determined to be an ignition source. When the potential ignition energy of a candidate point is less than 200 μJ, the candidate point is determined to be safe.

9. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 1, characterized in that, The dominant mechanism described in step 5 is one of three types: pure thermal effect, pure electric field effect, and thermoelectric coupled field emission; the ignition source information output in step 5 is sorted from largest to smallest energy value.

10. The numerical calculation method for the lightning ignition source of the aircraft composite fuel tank according to claim 1, characterized in that, It also includes optimization design steps: based on the spatial location, energy value and dominant mechanism of the output ignition source, the fuel tank structure is optimized, and the optimization design includes increasing the conductive path, eliminating the tip structure and filling the gap.

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