Centrifugal nozzle design method

Abstract

The invention belongs to the field of aero-engines and gas turbine combustion chambers, relates to an engine nozzle design technology, and provides a centrifugal nozzle design method which comprises the following steps: calculating a geometric constant initial value of a nozzle according to a given spray cone angle; according to the nozzle geometric constant initial value and the nozzle geometric constant increment, taking the relative radius of a hollow vortex as a constraint condition and taking a spray cone angle as a convergence condition, and obtaining a converged nozzle flow coefficient and a converged nozzle geometric constant; according to the flow coefficient of the nozzle after convergence, the geometric constant of the nozzle after convergence and the given flow number of the nozzle, the geometric dimension of the nozzle is obtained; and establishing a nozzle geometric model through the nozzle geometric dimension, and evaluating and correcting the nozzle geometric dimension by adopting a three-dimensional simulation method to obtain the final nozzle geometric dimension. According to the method, in the initial design stage of the nozzle, the main geometric parameters of the nozzle can be rapidly obtained, and the feasibility of the overall scheme of the combustion chamber can be evaluated.

Description

Technical Field

[0001] This invention belongs to the field of aero-engine and gas turbine combustion chambers, and relates to engine nozzle design technology, specifically to a centrifugal nozzle design method. Background Technology

[0002] Centrifugal nozzles are the core component of atomizing devices. They create a swirling motion against the wall through centrifugal force, forming a hollow cone-shaped spray after leaving the nozzle and rapidly atomizing into droplets. Centrifugal nozzles have the advantages of relatively simple design, high reliability, and good atomization quality over a wide range.

[0003] Centrifugal nozzles are one of the key components in the combustion chambers of aero-engines and gas turbine engines, and their design directly affects combustion efficiency and engine performance. Traditional centrifugal nozzle design methods rely on empirical formulas and trial-and-error approaches, which suffer from problems such as long design cycles, low accuracy, and low iteration efficiency.

[0004] Therefore, there is an urgent need for an efficient and precise centrifugal nozzle design method to meet the design requirements of modern aero engines and gas turbine engines. Summary of the Invention

[0005] To address the technical problems of long design cycles, low accuracy, and low iteration efficiency in designing centrifugal nozzles using empirical formulas and trial-and-error methods, this invention discloses a centrifugal nozzle design method, which includes the following steps: S1. Calculate the initial values ​​of the nozzle geometric constants based on the given spray cone angle; S2. Using the initial value and increment of the nozzle geometric constant, with the relative radius of the hollow vortex as the constraint and the spray cone angle as the convergence condition, obtain the converged nozzle flow coefficient and the converged nozzle geometric constant. S3. Obtain the nozzle geometry based on the converged nozzle flow coefficient, the converged nozzle geometric constant, and the given nozzle flow number; S4. Establish a nozzle geometric model based on the nozzle geometry, and use a three-dimensional simulation method to evaluate and correct the nozzle geometry to obtain the final nozzle geometry.

[0006] Further, in step S1, the initial values ​​of the nozzle geometric constants are calculated based on the given spray cone angle, including: S11. Calculate the initial value of the nozzle filling rate based on the given spray cone angle and the formula assuming constant nozzle axial velocity. S12. Calculate the initial value of the nozzle geometric constant using the nozzle geometric constant formula based on the initial value of the nozzle filling rate.

[0007] Further, in step S2, using the initial value and increment of the nozzle geometric constant, with the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition, the converged nozzle flow coefficient and the converged nozzle geometric constant are obtained, including: S21. Using the initial value of the nozzle geometric constant, according to the conditions... Design the nozzle geometric constant increment, where... α For a given spray cone angle, α 2 represents the incremental correction of the spray cone angle using geometric constants; S22. Calculate the sum of the previous iteration's input nozzle geometric constant and an increment of the nozzle geometric constant as the current nozzle geometric constant. Use the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition to perform iterative design to obtain the converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence condition. In the first iteration, the input nozzle geometric constant is the sum of the initial value of the nozzle geometric constant and an increment of the nozzle geometric constant.

[0008] Furthermore, in step S22, the sum of the previous iteration's input nozzle geometric constant and an increment of the nozzle geometric constant is calculated as the current nozzle geometric constant. Using the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition, iterative design is performed to obtain the converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence conditions, including: S221. Calculate the sum of the previous nozzle geometric constant input in the previous iteration and an increment of the nozzle geometric constant as the current nozzle geometric constant. Obtain the current nozzle filling rate through the current nozzle geometric constant. Calculate the current nozzle flow coefficient using the nozzle flow coefficient formula based on the current nozzle filling rate. S222, using the current nozzle flow coefficient and the current nozzle geometric constant, through the formula... The relative radius of the hollow vortex is obtained through iterative calculation using the relative radius of the hollow vortex as a constraint condition, where S is the relative radius of the hollow vortex. μ Where K is the nozzle flow coefficient and K is the nozzle geometric constant; S223. Calculate the current spray cone angle using the current relative radius of the hollow vortex, the current nozzle flow coefficient, and the current nozzle geometric constant. Use the spray cone angle as a convergence condition to judge the current spray cone angle. Based on the judgment result, evaluate and correct the current nozzle flow coefficient and the current nozzle geometric constant until a converged nozzle flow coefficient and a converged nozzle geometric constant that satisfy the convergence condition are obtained.

[0009] Preferably, in steps S222 and S223, the constraint condition is: ,and ,and The convergence condition is: ,in, The current spray cone angle is given by ζ, which is a constant and takes various values. .

[0010] Further, in step S3, the nozzle geometry is obtained based on the converged nozzle flow coefficient, the converged nozzle geometric constant, and the given nozzle flow number, including: S31. Obtain the nozzle diameter based on the given nozzle flow rate, the converged nozzle flow coefficient, and the fuel density, and give the tangential groove centerline radius based on the nozzle diameter; S32. Obtain the tangential groove diameter based on the converged nozzle geometric constant, the nozzle diameter, and the radius of the tangential groove centerline; obtain the nozzle length based on the nozzle diameter and the first coefficient of the spray cone angle. S33. Obtain the diameter of the swirl chamber based on the diameter of the tangential groove and the radius of the center line of the tangential groove, and obtain the length of the swirl chamber based on the diameter of the tangential groove and the second coefficient of the spray cone angle.

[0011] Further, in step S4, a nozzle geometric model is established based on the nozzle geometry, and the final nozzle geometry is obtained by evaluating and correcting the nozzle geometry using a three-dimensional simulation method, including: S41. Establish a nozzle geometric model based on the nozzle geometric dimensions, and use a three-dimensional simulation method to obtain the simulated values ​​of the spray cone angle and nozzle flow rate of the nozzle geometric model; S42. Evaluate and correct the nozzle geometry using the simulated spray cone angle, the simulated nozzle flow rate, the given spray cone angle, and the given nozzle flow rate.

[0012] Furthermore, in step S42, the nozzle geometry is evaluated and corrected using the simulated spray cone angle value, the simulated nozzle flow rate value, a given spray cone angle, and a given nozzle flow rate, including: S421. When the relative deviation between the simulated spray cone angle and the given spray cone angle is ≤ a first threshold, and the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate is ≤ a second threshold, the nozzle geometry is taken as the final high nozzle geometry. S43. When the relative deviation between the simulated spray cone angle and the given spray cone angle is greater than a first threshold, and / or the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate is greater than a second threshold, the nozzle geometry is corrected.

[0013] Compared with the prior art, the beneficial effects that at least one technical solution adopted in the embodiments of this specification can achieve include at least: The method of this invention is applicable to the design of centrifugal nozzles for combustion chambers of aero-engines and gas turbines. In the preliminary nozzle design phase, this method allows for the rapid acquisition of the nozzle's key geometric parameters, aiding in the evaluation of the feasibility of the overall combustion chamber design. Attached Figure Description

[0014] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a flowchart of the centrifugal nozzle design method of the present invention; Figure 2 This is the program execution flow of the centrifugal nozzle design method of the present invention. Detailed Implementation

[0016] The embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0017] The following specific examples illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. This application can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this application. It should be noted that, in the absence of conflict, the following embodiments and features of the embodiments can be combined with each other. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] This invention discloses a centrifugal nozzle design method, see [link to relevant documentation]. Figure 1 and Figure 2 As shown, the method includes the following steps: S1. Calculate the initial values ​​of the nozzle geometric constants based on the given spray cone angle; S2. Using the initial value and increment of the nozzle geometric constant, with the relative radius of the hollow vortex as the constraint and the spray cone angle as the convergence condition, obtain the converged nozzle flow coefficient and the converged nozzle geometric constant. S3. Obtain the nozzle geometry based on the converged nozzle flow coefficient, the converged nozzle geometric constant, and the given nozzle flow number; S4. Establish a nozzle geometric model based on the nozzle geometry, and use a three-dimensional simulation method to evaluate and correct the nozzle geometry to obtain the final nozzle geometry.

[0019] Further, in step S1, the initial values ​​of the nozzle geometric constants are calculated based on the given spray cone angle, including: S11. Based on the given spray cone angle, calculate the initial value of the nozzle filling rate using the constant nozzle axial velocity assumption formula. In practice, the constant nozzle axial velocity assumption formula is as follows: The calculation, among which, α For a given spray cone angle, ε This refers to the nozzle fill rate; S12. Based on the initial value of the nozzle filling rate, use the nozzle geometric constant formula... Calculate the initial values ​​of the nozzle geometric constants, where K is the nozzle geometric constant.

[0020] Further, in step S2, using the initial value and increment of the nozzle geometric constant, with the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition, the converged nozzle flow coefficient and the converged nozzle geometric constant are obtained, including: S21. Using the initial value of the nozzle geometric constant, according to the conditions... Design the nozzle geometric constant increment, where... α For a given spray cone angle, α 2 is the geometric constant increment for correcting the spray cone angle; S22. Calculate the sum of the previous iteration's input nozzle geometric constant and an increment of the nozzle geometric constant as the current nozzle geometric constant. Use the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition to perform iterative design to obtain the converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence condition. In the first iteration, the input nozzle geometric constant is the sum of the initial value of the nozzle geometric constant and an increment of the nozzle geometric constant.

[0021] Furthermore, in step S22, the sum of the previous iteration's input nozzle geometric constant and an increment of the nozzle geometric constant is calculated as the current nozzle geometric constant. Using the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition, iterative design is performed to obtain the converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence conditions, including: S221. Calculate the sum of the previous nozzle geometric constant input and an increment of the nozzle geometric constant as the current nozzle geometric constant. Obtain the current nozzle filling rate using the current nozzle geometric constant. Use the nozzle flow coefficient formula based on the current nozzle filling rate. Calculate the current nozzle flow coefficient.

[0022] S222, using the current nozzle flow coefficient and the current nozzle geometric constant, through the formula... The relative radius of the hollow vortex is obtained through iterative calculation using the relative radius of the hollow vortex as a constraint condition, where S is the relative radius of the hollow vortex. μ Let K be the nozzle flow coefficient, and K be the nozzle geometric constant. In this invention, iterative design can be performed using the bisection method or Newton's method, and the constraints are as follows: ,and ,and During the iteration process, if the relative radius of the hollow vortex in the current iteration does not meet the constraint conditions, then an increment of the nozzle geometric constant needs to be added to the current nozzle geometric constant as the input for the next iteration to recalculate the relative radius of the hollow vortex.

[0023] S223. Calculate the current spray cone angle using the current relative radius of the hollow vortex, the current nozzle flow coefficient, and the current nozzle geometric constant. Use the spray cone angle as a convergence condition to judge the current spray cone angle. Based on the judgment result, evaluate and correct the current nozzle flow coefficient and the current nozzle geometric constant until a converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence condition are obtained. In this invention, the convergence condition is... ,in, The current spray cone angle is given by ζ, which is a constant and takes various values. During convergence iteration, if the current spray cone angle does not meet the convergence condition, it is necessary to return to step S211 to recalculate the relative radius of the hollow vortex, the nozzle flow coefficient, and the current nozzle geometric constant.

[0024] Further, in step S3, the nozzle geometry is obtained based on the converged nozzle flow coefficient, the converged nozzle geometric constant, and the given nozzle flow number, including: S31. Obtain the nozzle diameter based on the given nozzle flow rate, the converged nozzle flow coefficient, and the fuel density, and give the tangential groove centerline radius based on the nozzle diameter.

[0025] Nozzle diameter D The formula for calculating 0 is: Calculate the nozzle diameter D 0, FN is the nozzle flow rate, μ The nozzle flow coefficient is... Fuel density. Radius of the tangential groove centerline. R The value range is (1~2.5). D 0.

[0026] S32. Obtain the tangential groove diameter based on the converged nozzle geometric constant, the nozzle diameter, and the radius of the tangential groove centerline; obtain the nozzle length based on the nozzle diameter and the first coefficient of the spray cone angle.

[0027] Tangential groove diameter D p Through formula Calculation, where i constant ， The value ranges from 2 to 4. Nozzle length. In the formula The first coefficient for the spray cone angle ranges from 0.5 to 1.0.

[0028] S33. Obtain the diameter of the swirl chamber based on the diameter of the tangential groove and the radius of the center line of the tangential groove, and obtain the length of the swirl chamber based on the diameter of the tangential groove and the second coefficient of the spray cone angle.

[0029] cyclone chamber diameter D s Through formula D s= 2R + D p Calculate the length of the cyclone chamber. L s Through formula Calculate, where It is the second coefficient of the spray cone angle, with a value range of 0.75~1.5.

[0030] Further, in step S4, a nozzle geometric model is established based on the nozzle geometry, and the final nozzle geometry is obtained by evaluating and correcting the nozzle geometry using a three-dimensional simulation method, including: S41. Establish a nozzle geometric model based on the nozzle's geometric dimensions, and obtain the simulated spray cone angle value of the nozzle geometric model using a three-dimensional simulation method. α s Simulated nozzle flow rate FN s .

[0031] In practice, the nozzle geometric model can be meshed, and a three-dimensional simulation analysis of the atomization process can be performed. The simulation process requires the use of a two-phase model and a high-precision turbulence model. The simulation results are then analyzed to extract the simulated spray cone angle value. α s Simulated nozzle flow rate FN s .

[0032] S42. Evaluate and correct the nozzle geometry using the simulated spray cone angle, the simulated nozzle flow rate, a given spray cone angle, and a given nozzle flow rate. This process includes the following steps: S421, when the simulated spray cone angle value is different from the given spray cone angle α When the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate FN is less than or equal to the first threshold, and the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate FN is less than or equal to the second threshold, the nozzle geometry is taken as the final high nozzle geometry. S43, when the simulated spray cone angle value is different from the given spray cone angle α When the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate FN is greater than a first threshold, and / or the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate FN is greater than a second threshold, the nozzle geometry is corrected.

[0033] Specifically, when if and If the nozzle geometry meets the design requirements; if or If the nozzle geometry does not meet the design requirements, adjustments are needed. One adjustment method is to modify the radius of the tangential groove centerline. R Length of cyclone chamber L s and nozzle length L o Perform matching design.

[0034] The present invention will illustrate the above method in detail through the following examples: The design input is: given spray cone angle. =80°, given nozzle flow rate FN=4 .

[0035] Step 1: Calculate the initial values ​​of the nozzle geometric constants K 0.

[0036] according to Calculate the initial fill rate of the nozzle =0.62.

[0037] according to Calculate the initial values ​​of the nozzle geometric constants K 0 = 1.09.

[0038] Step 2: Calculate geometric constants K Nozzle filling rate ε Flow coefficient μ Relative radius of air nucleus S .

[0039] Pick δK =10 -4 ,by As a convergence variable, the nozzle fill rate after convergence is calculated through two-level iterative calculation. The nozzle flow coefficient after convergence and the relative radius of the converged air nucleus .

[0040] Step 3: Calculate the main geometric dimensions of the nozzle.

[0041] Based on a given nozzle flow rate Calculate the nozzle diameter D 0 = 0.66 mm.

[0042] Pick i =2, R = 2D 0 = 1.32 mm, according to geometric constants Calculate the diameter of the tangential groove D p =0.71mm.

[0043] cyclone chamber diameter D s = 2R + D p =3.35mm, cyclone chamber length L s = D p =0.71mm, nozzle length L 0= D 0 = 0.66 mm.

[0044] Step 4: 3D simulation analysis of the nozzle Based on the main geometric dimensions of the nozzle obtained in step 3, a geometric model of the nozzle was established and a simulation analysis of the nozzle atomization process was carried out. The spray cone angle and flow number of the nozzle were found to be 78° and 4.15, respectively.

[0045] Step 5: Verify the design results Comparing the calculation results of step 4 with the design input, the relative deviations of the nozzle's spray cone angle and flow number are 2.5% and 3.75%, respectively, indicating that the nozzle design meets the requirements.

[0046] The method of this invention is applicable to the design of centrifugal nozzles for combustion chambers of aero-engines and gas turbines. In the preliminary nozzle design phase, this method allows for the rapid acquisition of the nozzle's key geometric parameters, aiding in the evaluation of the feasibility of the overall combustion chamber design.

[0047] Obviously, those skilled in the art should understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Various modifications and variations of the embodiments of the present invention are possible for those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A centrifugal nozzle design method, characterized in that, include: Calculate the initial values ​​of the nozzle geometric constants based on the given spray cone angle; Using the initial value and increment of the nozzle geometric constant, with the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition, the converged nozzle flow coefficient and the converged nozzle geometric constant are obtained. The nozzle geometry is obtained based on the converged nozzle flow coefficient, the converged nozzle geometric constant, and the given nozzle flow number; A nozzle geometric model is established based on the nozzle geometry, and the final nozzle geometry is obtained by evaluating and correcting the nozzle geometry using a three-dimensional simulation method.

2. The centrifugal nozzle design method according to claim 1, characterized in that, Calculate the initial values ​​of the nozzle geometric constants based on the given spray cone angle, including: Based on the given spray cone angle, the initial value of the nozzle filling rate is calculated using the formula assuming constant nozzle axial velocity. Based on the initial value of the nozzle filling rate, the initial value of the nozzle geometric constant is calculated using the nozzle geometric constant formula.

3. The centrifugal nozzle design method according to claim 1, characterized in that, Using the initial value and increment of the nozzle geometric constant, with the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition, the converged nozzle flow coefficient and converged nozzle geometric constant are obtained, including: Based on the initial value of the nozzle geometric constant, and according to the conditions Design the nozzle geometric constant increment, where... α For a given spray cone angle, α 2 is the geometric constant increment for correcting the spray cone angle; The sum of the previous iteration's input nozzle geometric constant and an increment of the nozzle geometric constant is calculated as the current nozzle geometric constant. The relative radius of the hollow vortex is used as a constraint condition, and the spray cone angle is used as a convergence condition. Iterative design is performed to obtain the converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence condition. In the first iteration, the input nozzle geometric constant is the sum of the initial value of the nozzle geometric constant and an increment of the nozzle geometric constant.

4. The centrifugal nozzle design method according to claim 3, characterized in that, The sum of the previous iteration's input nozzle geometric constant and an increment of the nozzle geometric constant is calculated as the current nozzle geometric constant. Using the relative radius of the hollow vortex as a constraint and the spray cone angle as a convergence condition, iterative design is performed to obtain the converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence conditions, including: The sum of the previous nozzle geometric constant input in the previous iteration and an increment of the nozzle geometric constant is used as the current nozzle geometric constant. The current nozzle filling rate is obtained through the current nozzle geometric constant. The current nozzle flow coefficient is calculated using the nozzle flow coefficient formula based on the current nozzle filling rate. Using the current nozzle flow coefficient and the current nozzle geometric constant, through the formula The relative radius of the hollow vortex is obtained through iterative calculation using the relative radius of the hollow vortex as a constraint condition, where S is the relative radius of the hollow vortex. μ Let K be the nozzle flow coefficient, and K be the nozzle geometric constant. The current spray cone angle is calculated using the current relative radius of the hollow vortex, the current nozzle flow coefficient, and the current nozzle geometric constant. The current spray cone angle is used as a convergence condition to judge the current spray cone angle. Based on the judgment result, the current nozzle flow coefficient and the current nozzle geometric constant are evaluated and corrected until the converged nozzle flow coefficient and converged nozzle geometric constant that satisfy the convergence condition are obtained.

5. The centrifugal nozzle design method according to claim 4, characterized in that, The constraint condition is: ,and ,and The convergence condition is: ,in, The current spray cone angle is given by ζ, which is a constant and takes various values. .

6. The centrifugal nozzle design method according to claim 1, characterized in that, Based on the converged nozzle flow coefficient, the converged nozzle geometric constant, and the given nozzle flow number, the nozzle geometry is obtained, including: Based on the given nozzle flow rate, the converged nozzle flow coefficient, and the fuel density, the nozzle diameter is obtained, and the radius of the tangential groove centerline is given based on the nozzle diameter; The tangential groove diameter is obtained based on the converged nozzle geometric constant, the nozzle diameter, and the radius of the tangential groove centerline; the nozzle length is obtained based on the nozzle diameter and the first coefficient of the spray cone angle. The diameter of the vortex chamber is obtained based on the diameter of the tangential groove and the radius of the centerline of the tangential groove, and the length of the vortex chamber is obtained based on the diameter of the tangential groove and the second coefficient of the spray cone angle.

7. The centrifugal nozzle design method according to claim 1, characterized in that, A nozzle geometric model is established based on the nozzle geometry, and the final nozzle geometry is obtained by evaluating and correcting the nozzle geometry using a three-dimensional simulation method, including: A nozzle geometric model is established based on the nozzle geometry, and the simulated values ​​of the spray cone angle and nozzle flow number of the nozzle geometric model are obtained by three-dimensional simulation method. The nozzle geometry is evaluated and corrected using the simulated spray cone angle, the simulated nozzle flow rate, a given spray cone angle, and a given nozzle flow rate.

8. The centrifugal nozzle design method according to claim 7, characterized in that, The nozzle geometry is evaluated and corrected using the simulated spray cone angle, the simulated nozzle flow rate, a given spray cone angle, and a given nozzle flow rate, including: When the relative deviation between the simulated spray cone angle and the given spray cone angle is less than or equal to a first threshold, and the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate is less than or equal to a second threshold, the nozzle geometry is taken as the final height of the nozzle geometry. When the relative deviation between the simulated spray cone angle and the given spray cone angle is greater than a first threshold, and / or the relative deviation between the simulated nozzle flow rate and the given nozzle flow rate is greater than a second threshold, the nozzle geometry is corrected.

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