Method for designing length of combustion chamber of ramjet

Abstract

The invention relates to a ramjet combustion chamber length design method, and belongs to the technical field of ramjets. Comprising the following steps: giving flight height and flight pressure of a designed working condition of the aircraft; according to the compression efficiency of the air inlet channel, the temperature rise ratio is designed, and combustion chamber inlet airflow parameters are obtained in combination with an isentropic relational expression; fuel is selected, the chemical reaction process is calculated through the Arrhenius law, and ignition delay time is obtained; an isobaric combustion hypothesis is adopted, and combustion chamber outlet airflow parameters are obtained through three conservation equations; obtaining the average speed of the combustion chamber according to mathematical averaging of inlet and outlet speeds of the combustion chamber; obtaining the combustion chamber length according to the combustion chamber average speed and the fuel ignition delay time. According to the method, chemical power in the combustion process is coupled with the airflow speed, rapid design of the length of the combustion chamber of the ramjet is achieved, combustion products can be obtained through calculation according to the working condition of the engine, and the combustion efficiency can be deduced according to the actual length of the engine.

Description

Technical Field

[0001] The invention relates to a method for designing the length of a ramjet combustion chamber, and belongs to the technical field of ramjet engines. Background Art

[0002] Ramjet engines are ideal power devices for supersonic flight, which use the forward motion of the aircraft to compress the air. Depending on whether the airflow entering the combustion chamber is supersonic or subsonic, it can also be subdivided into subsonic ramjet engines and scramjet engines. The main difference lies in the flight Mach number. The flight Mach number of subsonic ramjet engines is generally around 3, and the flight Mach number of scramjet engines is generally above 6. The difference in flight Mach number leads to differences in the flow field characteristics of the two types of ramjet engines, especially the combustion organization form of the combustion chamber. In subsonic ramjet engines, the airflow speed is subsonic, the residence time is long, and most of them can be fully burned, but for some inactive fuels, the combustion efficiency is still not high; in scramjet engines, the airflow speed is supersonic, the residence time is short, and most fuels are not easy to burn completely.

[0003] To address the above problems, flame stabilizers or swirlers are usually used to create local recirculation zones to extend the airflow residence time and achieve sufficient mixing and combustion of fuel / air. However, these structures will produce large internal resistance, affecting engine performance. If the airflow residence time is extended by increasing the length of the combustion chamber, how should the combustion chamber length be calculated? There is no relevant report in the existing public technology.

[0004] Therefore, it is urgent to design a ramjet engine combustion chamber length design method that can solve the above-mentioned technical problems. Summary of the invention

[0005] In view of the above problems, the present invention provides a method for designing the length of a ramjet combustion chamber. The method couples the chemical kinetics of the combustion process with the airflow velocity, thereby realizing a rapid design of the length of the ramjet combustion chamber. The combustion products can be calculated according to the engine operating conditions, and the combustion efficiency can also be inferred according to the actual length of the engine.

[0006] The present invention provides a method for designing the length of a ramjet combustion chamber, which is characterized in that it comprises the following steps:

[0007] Step 1, the flight altitude and flight pressure of the aircraft design condition are given;

[0008] Step 2, according to the intake duct compression efficiency, the design temperature rise ratio, and the isentropic relationship, the combustion chamber inlet airflow parameters are obtained;

[0009] Step 3, according to the type of fuel, select the typical intermediate products of the combustion process, use Arrhenius's law to calculate the chemical reaction process, and obtain the ignition delay time;

[0010] Step 4, using the isobaric combustion assumption, the combustion chamber outlet airflow parameters are obtained through the mass conservation, momentum conservation, and energy conservation equations;

[0011] Step 5, obtaining the average velocity of the combustion chamber airflow according to the mathematical average of the combustion chamber inlet and outlet velocities;

[0012] Step 6, obtaining the combustion chamber length according to the combustion chamber average velocity and the fuel ignition delay time.

[0013] Preferably, in step 1, the aircraft dynamic pressure is:

[0014]

[0015] Where q0 is the flight operating pressure, M0 is the flight Mach number, P0 is the atmospheric pressure at the flight altitude, and γ0 is the atmospheric specific heat ratio at the flight altitude.

[0016] Preferably, in step 2, the combustion chamber inlet airflow parameters are:

[0017]

[0018] ρ3=P3 / (R3T3)

[0019]

[0020] Where P3 is the static pressure of the combustion chamber inlet, T3 is the static temperature of the combustion chamber inlet, ψ is the design temperature rise ratio, η c is the intake compression efficiency, C p0 and C p3 represents the constant-pressure specific heat of the incoming air and the constant-pressure specific heat of the airflow at the combustion chamber inlet, U0 and U3 represent the incoming air velocity and the airflow velocity at the combustion chamber inlet, R3 is the gas constant at the combustion chamber inlet, ρ3 is the gas density at the combustion chamber inlet, and γ3 is the specific heat ratio of the airflow at the combustion chamber inlet.

[0021] Preferably, in step 3, the typical intermediate product of the combustion process is selected, usually OH for hydrogen fuel and CH or OH for hydrocarbon fuel, and the time required for the mass proportion to reach a maximum value is the ignition delay time τ.

[0022] Preferably, in step 4, the combustion chamber outlet airflow parameters are:

[0023] P4=P3

[0024] Mass equation:

[0025] Momentum equation:

[0026] Energy equation:

[0027] Where P4 is the static pressure of the combustion chamber outlet, T4 is the static temperature of the combustion chamber outlet, ρ4 is the gas density at the combustion chamber outlet, U4 represents the velocity of the combustion chamber outlet, h k represents the formation enthalpy of component k, is the mass flow rate of fuel, represents the generation rate of component k, A3 and A4 represent the inlet and outlet cross-sectional areas of the combustion chamber, in a subsonic ramjet engine, A4 / A3 is 1.0, and in a scramjet engine, it is 1.6;

[0028] Preferably, in step 5, the average velocity of the combustion chamber airflow is for:

[0029]

[0030] Preferably, in step 6, the combustion chamber length L is:

[0031]

[0032] The present invention provides a method for designing the combustion chamber length of a ramjet engine. By giving a given aircraft design operating condition, the combustion chamber inlet and outlet airflow parameters are determined in sequence, the average airflow velocity of the combustion chamber is obtained, and the ignition delay time obtained in the chemical reaction solution process is combined to finally obtain the combustion chamber design length. In the process of calculating the chemical reaction, different fuels have different chemical compositions, resulting in large differences in the fuel / air elementary reactions, and the obtained combustion chamber design lengths are also different. The present invention provides an effective method for the early combustion chamber design of ramjet engines. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The engine configuration of the embodiment of the present invention;

[0034] Figure 2 It is a schematic flow chart of a method for designing the length of a ramjet combustion chamber according to an embodiment of the present invention;

[0035] Figure 3 This is a diagram showing the influence of flight pressure and flight altitude on combustion chamber length and ignition delay time obtained by using the ramjet engine combustion chamber length design method in this embodiment;

[0036] Figure 4 This is a diagram showing the influence of the equivalence ratio on the combustion chamber length and the ignition delay time obtained by using the ramjet engine combustion chamber length design method in this embodiment;

[0037] Figure 5 This is the curve of the mass proportion of the intermediate product OH in the combustion chamber changing with time. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] This embodiment discloses a method for designing the length of a ramjet combustion chamber, by which the combustion chamber length required for complete combustion of fuel can be obtained. Figure 1 , is the engine configuration given in this embodiment, and multiple cross sections are divided in the figure, among which, cross section 0 is the incoming flow cross section, cross section 1 is the isolation section inlet cross section, cross section 3 is the combustion chamber inlet cross section, cross section 4 is the combustion chamber outlet cross section, cross section 9 is the tail nozzle restricted expansion section outlet cross section, and cross section 10 is the tail nozzle free expansion section outlet cross section. In this embodiment, the inlet compression efficiency of the ramjet engine is 0.9, and the temperature rise ratio is 6.5.

[0040] refer to Figure 2 In this embodiment, the method for determining the performance of a ramjet engine specifically includes the following steps:

[0041] Step 1: Given the design flight altitude and flight Mach number, calculate the flight pressure, specifically:

[0042]

[0043] Where q0 is the flight operating pressure, M0 is the flight Mach number, P0 is the atmospheric pressure at the flight altitude, and γ0 is the atmospheric specific heat ratio at the flight altitude.

[0044] Step 2: According to the inlet compression efficiency and temperature rise ratio, combined with the isentropic relationship, the combustion chamber inlet airflow parameters are obtained, which are specifically:

[0045]

[0046] ρ3=P3 / (R3T3)

[0047]

[0048] Where P3 is the static pressure of the combustion chamber inlet, T3 is the static temperature of the combustion chamber inlet, ψ is the design temperature rise ratio, η c is the intake compression efficiency, C p0 and C p3represents the constant-pressure specific heat of the incoming air and the constant-pressure specific heat of the airflow at the combustion chamber inlet, U0 and U3 represent the incoming air velocity and the airflow velocity at the combustion chamber inlet, R3 is the gas constant at the combustion chamber inlet, ρ3 is the gas density at the combustion chamber inlet, and γ3 is the specific heat ratio of the airflow at the combustion chamber inlet.

[0049] Step 3, hydrogen is selected as the fuel, OH is selected as the typical intermediate product, the Arrhenius law is used to calculate the chemical reaction process, the static pressure and static temperature of the combustion chamber inlet airflow are set as the initial pressure and temperature of the chemical reaction, and the elementary reaction parameters are shown in Table 1.

[0050] Table 1: Elementary reaction parameters

[0051]

[0052]

[0053] Step 4, using the isobaric combustion assumption, obtain the combustion chamber outlet airflow parameters through the mass, momentum, and energy conservation equations, specifically:

[0054] P4=P3

[0055]

[0056] Where P4 is the static pressure of the combustion chamber outlet, T4 is the static temperature of the combustion chamber outlet, ρ4 is the gas density at the combustion chamber outlet, U4 represents the velocity of the combustion chamber outlet, h k represents the formation enthalpy of component k, is the mass flow rate of fuel, represents the generation rate of component k. A3 and A4 represent the inlet and outlet cross-sectional areas of the combustion chamber, and A4 / A3 is 1.6.

[0057] Step 5, obtaining the average velocity of the combustion chamber airflow, specifically:

[0058]

[0059] Step 6, get the design length of the combustion chamber:

[0060]

[0061] refer to Figure 3 , the dotted line represents the ignition delay time, and the solid line represents the combustion chamber size. Taking H = 30km as an example, as the dynamic pressure increases, the ignition delay time remains unchanged. However, the combustion chamber length is gradually increasing. Taking q = 50kPa as an example, the higher the flight altitude, the longer the ignition delay time and the larger the combustion chamber length.

[0062] refer to Figure 4, the ignition delay time and the combustion chamber length gradually increase with the increase of the equivalence ratio, and then decrease. When the equivalence ratio is equal to 0.9, the ignition delay reaches the maximum value of 0.0287ms, and the combustion chamber length also reaches the maximum value of 54.68mm.

[0063] refer to Figure 5 The combustion chamber temperature increases sharply with time and finally stabilizes at around 2500K. The OH mass ratio first increases sharply, reaches a maximum value, then gradually decreases and finally stabilizes. The ignition delay time calculated is 2.4x10 - 2 ms. The oxygen mass ratio dropped rapidly from the initial 0.23 to 0.11, and then remained unchanged.

[0064] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification and drawings under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for designing the length of a ramjet combustion chamber, characterized in that: The steps include: Step 1, the flight altitude and flight pressure of the aircraft design condition are given; Step 2, according to the intake duct compression efficiency, the design temperature rise ratio, and the isentropic relationship, the combustion chamber inlet airflow parameters are obtained; Step 3, according to the type of fuel, select typical intermediate products of the combustion process, use Arrhenius law to calculate the chemical reaction process, and obtain the ignition delay time; Step 4, using the isobaric combustion assumption, the combustion chamber outlet airflow parameters are obtained through the mass conservation, momentum conservation, and energy conservation equations; Step 5, obtaining the average velocity of the combustion chamber airflow according to the mathematical average of the combustion chamber inlet and outlet velocities; Step 6, obtaining the combustion chamber length according to the combustion chamber average velocity and the fuel ignition delay time.

2. The method for designing the length of a ramjet combustion chamber according to claim 1, characterized in that: In step 1, the aircraft dynamic pressure is: Where q0 is the flight operating pressure, M0 is the flight Mach number, P0 is the atmospheric pressure at the flight altitude, and γ0 is the atmospheric specific heat ratio at the flight altitude.

3. The method for designing the length of a ramjet combustion chamber according to claim 2, characterized in that: In step 2, the combustion chamber inlet airflow parameters are: ρ3=P3 / (R3T3) Where P3 is the static pressure of the combustion chamber inlet, T3 is the static temperature of the combustion chamber inlet, ψ is the design temperature rise ratio, η c is the intake compression efficiency, C p0 and C p3 represents the constant-pressure specific heat of the incoming air and the constant-pressure specific heat of the airflow at the combustion chamber inlet, U0 and U3 represent the incoming air velocity and the airflow velocity at the combustion chamber inlet, R3 is the gas constant at the combustion chamber inlet, ρ3 is the gas density at the combustion chamber inlet, and γ3 is the specific heat ratio of the airflow at the combustion chamber inlet.

4. A method for designing the length of a ramjet combustion chamber according to claim 1, characterized in that: In step 3, the typical intermediate product of the combustion process is selected. Usually, OH is selected for hydrogen fuel, and CH or OH is selected for hydrocarbon fuel. The time required for the mass proportion to reach a maximum value is the ignition delay time τ.

5. A method for designing the length of a ramjet combustion chamber according to claim 3, characterized in that: In step 4, the combustion chamber outlet airflow parameters are: P4=P3 Mass equation: Momentum equation: Energy equation: Where P4 is the static pressure of the combustion chamber outlet, T4 is the static temperature of the combustion chamber outlet, ρ4 is the gas density at the combustion chamber outlet, U4 represents the velocity of the combustion chamber outlet, h k represents the formation enthalpy of component k, is the mass flow rate of fuel, represents the generation rate of component k, A3 and A4 represent the inlet and outlet cross-sectional areas of the combustion chamber. In a ramjet engine, A4 / A3 is 1.0, and in a scramjet engine, it is 1.

6.

6. A method for designing the length of a ramjet combustion chamber according to claim 5, characterized in that: In step 5, the average velocity of the combustion chamber airflow is for:

7. A method for designing the length of a ramjet combustion chamber according to claim 6, characterized in that: In step 6, the combustion chamber length L is:

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