A method for determining the stable state of a flat plate liquid film sheared by air flow

Abstract

The present invention discloses a method for determining the stable state of a liquid film on a flat plate sheared by airflow, and belongs to the field of gas-liquid-solid multiphase flow and aerodynamic heat technology. The dimensionless relationship directly calculates dimensionless parameters based on parameters such as airflow density, velocity, viscosity, liquid film density, velocity, viscosity, and liquid nozzle diameter on the flat plate surface. The stable state of the liquid film is judged based on the size of the dimensionless parameter LFI value. The stable state of the liquid film is divided into four states: stable, relatively stable, transitional, and unstable. A criterion for judging the stable state of the liquid film is given. The present invention provides a dimensionless relationship and criterion for determining the stable state of a liquid film on a flat plate sheared by airflow, which can quickly estimate the stable state of the liquid film under the action of airflow shear, and provide a reference for the design of the liquid film state in the active sweat cooling of hypersonic aircraft, so as to keep the liquid film in a stable state as much as possible and avoid the waste of cooling medium caused by the instability of the liquid film due to airflow shear.

Description

Technical Field

[0001] The present invention relates to the field of gas-liquid-solid multiphase flow and aerodynamic heat technology, and in particular to a method for determining the stable state of a liquid film sheared by an airflow. Background Art

[0002] Due to the significant aerodynamic heating effects of high-speed aircraft, suitable thermal insulation layers are essential. Thermal protection has always been a key technology in the development of high-speed aircraft. Traditional hypersonic aircraft use passive ablative materials as thermal insulation layers. However, hypersonic aircraft are developing towards longer flight times, higher Mach speeds, longer ranges, and reusability. During these higher-speed flights, the aircraft will face a harsh service environment characterized by ultra-high temperatures, strong oxidation, and large temperature gradients. Furthermore, their structural surfaces are subjected to high pressure and strong airflow. Faced with these challenges, localized high-heat flux areas of the aircraft are facing a shortage of thermal insulation materials, making traditional thermal insulation designs unable to meet the increasingly stringent thermal protection requirements. Current research is exploring active transpiration cooling as a novel thermal protection method for hypersonic aircraft. This method uses phase change evaporation of cooling water to absorb the aerodynamic heating of the hypersonic aircraft. Cooling water is injected into the outer wall of the aircraft, where it forms a liquid film under the shearing action of the high-speed airflow. Under the shear of high-speed, high-temperature airflow, the liquid film evaporates and undergoes a phase transition, absorbing the heat from the hypersonic airflow and reducing the temperature of the vehicle wall, thereby protecting the hypersonic vehicle. However, the liquid film can become unstable and break into droplets under the shear of high-speed airflow, resulting in a waste of cooling fluid. Hypersonic vehicles cannot carry much cooling fluid, so studying the stability of liquid films sheared by airflow is essential. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for determining the stable state of a liquid film sheared on a flat plate by airflow to address the above-mentioned shortcomings, provide a judgment criterion for the stability of the liquid film, provide a dimensionless relationship for the stability of the liquid film, and be able to quickly and accurately determine the stable state of the liquid film, thereby providing a design basis for the design of active cooling thermal protection systems for hypersonic aircraft.

[0004] The present invention is achieved through the following solutions:

[0005] A method for determining the stable state of a flat plate liquid film sheared by air flow comprises the following steps:

[0006] Step 1: Analyze the key factors affecting the stability of the liquid film;

[0007] Step 2: determining the criterion for the stable state of the flat plate liquid film sheared by the air flow;

[0008] Step 3: Use dimensional analysis to obtain the dimensionless relationship between the liquid film stability and various factors;

[0009] Step 4: Determine the stable state of the liquid film through dimensionless relationships and criteria.

[0010] In step one, the process of liquid film formation caused by shearing of the aircraft flat-plate shape is numerically simulated. By changing the external airflow parameters of the aircraft, the properties of the coolant carried by the aircraft, and the coolant nozzle parameters of the aircraft, the factors affecting the stability of the liquid film formation are obtained.

[0011] In step 2, the liquid film morphologies under different working conditions are summarized to obtain the types of cooling liquid film morphologies.

[0012] In step three, based on the key factors affecting stability, dimensional analysis is used to establish a dimensionless relationship between the liquid film stability and various variables; through data analysis and processing, the relationship between the dimensionless relationship and the liquid film stability is obtained.

[0013] In step three, the key factors affecting stability are nozzle diameter d, coolant flow rate u2, coolant density ρ2, coolant viscosity μ2, incoming flow velocity u1, incoming flow density ρ1 and incoming flow viscosity μ1.

[0014] In step three, dimensional analysis is used to establish the dimensionless relationship between the liquid film stability and various variables:

[0015]

[0016] Where LFI is the degree of liquid film instability, k is a constant, ρ1 is the airflow density, u1 is the airflow velocity, μ1 is the airflow viscosity, ρ2 is the liquid density, u2 is the liquid velocity, μ2 is the liquid viscosity, and d is the diameter of the liquid nozzle on the flat surface; h 、 b 、 d and f is the exponential parameter of the relation.

[0017] In step three, through data analysis and processing, the relationship between the dimensionless relationship and the liquid film stability is obtained as follows:

[0018] By analyzing and processing the simulation data, it is found that when the value ranges of the exponents h, b, d, and f satisfy the following relationship, the dimensionless relationship above is monotonically related to the liquid film stability:

[0019] .

[0020] When b is 0.8, h, d, and f are 0.5, 0.9, and 0.05, respectively; the dimensionless relationship is:

[0021] .

[0022] In Step 2, under the action of air flow shear, the stable state of the flat liquid film is divided into four types according to the magnitude of the dimensionless parameter LFI value, namely stable, relatively stable, transitional, and unstable states.

[0023] In Step 4: Specifically:

[0024] According to the flow field state, coolant properties, and opening parameters to be determined, substitute them into the dimensionless relationship to obtain the dimensionless number LFI (liquid film instability); compare the dimensionless number LFI with the given criterion to know the stability degree after the liquid film is formed; specifically, judge the liquid film state according to the magnitude of the dimensionless parameter LFI value, where the liquid film with LFI ≤ 54k is in a stable state, the liquid film with 54k < LFI ≤ 81k is in a relatively stable state, the liquid film with 81k < LFI ≤ 94k is in a transitional state, and the liquid film with LFI > 94k is in an unstable state.

[0025] In this solution, for the liquid film in the "stable" state, the shape of the liquid film is finally stable, no cavity is formed at the front end of the downstream wall surface at the coolant outlet, no liquid droplets splash at the front end of the liquid film flow, no large number of small bubbles are formed inside the liquid film, and the liquid film cavity does not cause the liquid film to break with time evolution;

[0026] For the liquid film in the "relatively stable" state, the shape of the liquid film is finally stable, a cavity is formed at the front end of the downstream wall surface at the coolant outlet, liquid droplets splash at the front end of the liquid film flow, a large number of small bubbles are formed inside the liquid film, the liquid film cavity does not cause the downstream liquid film to break with time evolution, and the cavity volume changes little with time;

[0027] For the liquid film in the "transitional" state, the shape of the liquid film is finally unstable, a cavity is formed at the front end of the downstream wall surface at the coolant outlet, liquid droplets splash at the front end of the liquid film flow, a large number of small bubbles are formed inside the liquid film, the liquid film cavity does not cause the downstream liquid film to break with time evolution, and the cavity volume changes greatly with time;

[0028] For the liquid film in the "unstable" state, the shape of the liquid film is finally unstable, a cavity is formed at the front end of the downstream wall surface at the coolant outlet, liquid droplets splash at the front end of the liquid film flow, a large number of small bubbles are formed inside the liquid film, and the liquid film cavity causes the downstream liquid film to break with time evolution.

[0029] To sum up, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0030] 1. This proposal provides a dimensionless relationship and criterion for determining the stability of a flat-plate liquid film under airflow shear. This can quickly estimate the stability of the liquid film under airflow shear, providing a reference for the design of the liquid film state in active transpiration cooling for hypersonic vehicles. This helps maintain the liquid film in a stable state as much as possible and avoids waste of cooling fluid caused by liquid film instability due to airflow shear. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a flow chart of the present invention;

[0032] Figure 2 Schematic diagram of liquid film morphology;

[0033] Figure 3 Schematic diagram of the comparison of liquid film morphology for different examples at the same time. DETAILED DESCRIPTION

[0034] All features disclosed in this specification, or all steps in the disclosed methods or processes, except mutually exclusive features and / or steps, can be combined in any manner.

[0035] Any feature disclosed in this specification (including any appended claims and abstract), unless otherwise stated, may be replaced by other equivalent or similar features. In other words, unless otherwise stated, each feature is only an example of a series of equivalent or similar features.

[0036] In the description of the present invention, it should be understood that the terms "up", "down", "left", "right", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a predetermined direction, be constructed and operated in a predetermined direction, and therefore cannot be understood as a limitation on the present invention.

[0037] Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be understood to indicate or imply relative importance or to implicitly indicate the quantity of the technical features being referred to. Thus, a feature defined as "first," "second," etc. may explicitly or implicitly include one or more of such features.

[0038] Example 1

[0039] The present invention provides a technical solution:

[0040] like Figure 1 As shown, a method for determining the stable state of an airflow shearing flat plate liquid film comprises the following steps:

[0041] Step 1: Analyze the key factors affecting the stability of the liquid film:

[0042] The formation process of the airflow shear liquid film on the flat-plate shape of the aircraft was numerically simulated. By changing the external airflow parameters of the aircraft, the properties of the coolant carried by the aircraft, and the parameters of the aircraft coolant nozzle (the specific parameter settings for different examples are shown in Tables 1 and 2), the key factors affecting the stability of the liquid film formation were obtained.

[0043] Table 1 Parameter settings for different examples

[0044]

[0045] Table 2 Parameter settings for different examples

[0046]

[0047] Step 2: Criteria for determining the stable state of the flat plate liquid film sheared by air flow:

[0048] Summarizing the liquid film morphology under different working conditions, it can be found that there are four types of cooling liquid film morphology (see Figure 2 ), Figure 2 Form 1 is a stable state, Form 2 is a relatively stable state, Form 3 is a transitional state, and Form 4 is an unstable state. The detailed differences between the different liquid film forms and the degree of stability of the liquid film are shown in Table 3 below.

[0049] Table 3 Description of liquid film morphology

[0050]

[0051] Step 3: Use dimensional analysis to obtain the dimensionless relationship between the liquid film stability and various factors;

[0052] According to the key factors affecting stability (nozzle diameter d, coolant flow rate u2, coolant density ρ2, coolant viscosity μ2, incoming flow velocity u1, incoming flow density ρ1, incoming flow viscosity μ1), the dimensionless relationship between liquid film stability and various variables is established using dimensional analysis:

[0053]

[0054] By analyzing and processing the data of all simulation examples, it can be concluded that when the value range of the exponents h, b, d, and f satisfies the following relationship, the dimensionless relationship above is monotonically related to the liquid film stability.

[0055]

[0056] When b is 0.8, h, d, and f can be 0.5, 0.9, and 0.05, respectively. The dimensionless relationship is:

[0057]

[0058] All the simulated liquid film cooling cases are sorted out to obtain the dimensionless number LFI and the corresponding liquid film morphology as shown in Table 4 and Figure 3 :

[0059] Table 4 Dimensionless number LFI and corresponding liquid film morphology

[0060]

[0061] Step 4: Determine the liquid film stability state through dimensionless relationship and criteria:

[0062] The dimensionless equation provided by this invention is substituted into the desired flow field state, coolant properties, and aperture parameters to obtain the dimensionless number LFI. Comparing the dimensionless number LFI with the provided criteria directly reveals the stability of the liquid film after formation.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for determining the stable state of a flat-plate liquid film sheared by air flow, characterized in that: The following steps are involved: Step 1: Analyze the key factors affecting the stability of the liquid film; Step 2: determining the criterion for the stable state of the flat plate liquid film sheared by the air flow; Step three, using dimensional analysis to obtain the dimensionless relationship between the liquid film stability and various factors; In step three, using dimensional analysis to establish the dimensionless relationship between the liquid film stability and various variables is specifically: Where LFI is the degree of liquid film instability, k is a constant, ρ1 is the airflow density, u1 is the airflow velocity, μ1 is the airflow viscosity, ρ2 is the liquid density, u2 is the liquid velocity, μ2 is the liquid viscosity, and d is the diameter of the liquid nozzle on the flat surface; h 、 b 、 d and f is the exponential parameter of the relation; Step 4: Determine the stable state of the liquid film through dimensionless relationships and criteria.

2. A method for determining the stable state of a flat plate liquid film sheared by air flow according to claim 1, characterized in that: In step one, the process of liquid film formation caused by shearing of the aircraft flat-plate shape is numerically simulated. By changing the external airflow parameters of the aircraft, the properties of the coolant carried by the aircraft, and the coolant nozzle parameters of the aircraft, the factors affecting the stability of the liquid film formation are obtained.

3. The method for determining the stable state of a flat plate liquid film sheared by air flow according to claim 1, wherein: In step 2, the liquid film morphologies under different working conditions are summarized to obtain the types of cooling liquid film morphologies.

4. A method for determining the stable state of a flat plate liquid film sheared by air flow according to claim 1, characterized in that: In step three, based on the key factors affecting stability, dimensional analysis is used to establish a dimensionless relationship between the liquid film stability and various variables; through data analysis and processing, the relationship between the dimensionless relationship and the liquid film stability is obtained.

5. A method for determining the stable state of a flat plate liquid film sheared by air flow according to claim 4, characterized in that: In step three, the key factors affecting stability are nozzle diameter d, coolant flow rate u2, coolant density ρ2, coolant viscosity μ2, incoming flow velocity u1, incoming flow density ρ1 and incoming flow viscosity μ1.

6. A method for determining the stable state of a flat plate liquid film sheared by air flow according to claim 5, characterized in that: In step three, through data analysis and processing, the relationship between the dimensionless relationship and the liquid film stability is obtained as follows: By analyzing and processing the simulation data, it is found that when the value ranges of the exponents h, b, d, and f satisfy the following relationship, the dimensionless relationship above is monotonically related to the liquid film stability: 。 7. A method for determining the stable state of a flat plate liquid film sheared by air flow according to claim 6, characterized in that: When b is 0.8, h, d, and f are 0.5, 0.9, and 0.05, respectively; the dimensionless relationship is: 。 8. A method for determining the stable state of a flat-plate liquid film sheared by air flow according to any one of claims 1 to 7, characterized in that: In step 2, under the action of airflow shear, the stable states of the flat liquid film are divided into four types according to the value of the dimensionless parameter LFI: stable, relatively stable, transitional, and unstable. The liquid film in the "stable" state is characterized by a stable shape, no cavity forming in front of the downstream wall of the coolant outlet, no droplets splashing in front of the liquid film flow, no large number of small bubbles forming inside the liquid film, and no cavities in the liquid film causing the liquid film to break over time. In the "relatively stable" state, the liquid film shape is ultimately stable, a cavity is formed at the front end of the downstream wall of the coolant outlet, droplets splash at the front end of the liquid film flow, a large number of small bubbles are formed inside the liquid film, and the liquid film cavity does not cause the liquid film downstream to break as time evolves; In the "transition" state, the liquid film shape is ultimately unstable, a cavity is formed in the liquid film at the front end of the downstream wall of the coolant outlet, droplets splash at the front end of the liquid film flow, a large number of small bubbles are formed inside the liquid film, and the liquid film cavity does not cause the downstream liquid film to break as time evolves; The liquid film in the "unstable" state is ultimately unstable in shape, with a cavity forming in front of the downstream wall of the coolant outlet, droplets splashing at the front of the liquid film flow, a large number of small bubbles forming inside the liquid film, and the liquid film cavity causing the liquid film to break downstream as time evolves.

9. A method for determining the stable state of a flat plate liquid film sheared by air flow according to claim 8, characterized in that: In step 4: Specifically: According to the flow field state, coolant properties, and opening parameters to be determined as needed, substitute them into the dimensionless relationship to obtain the dimensionless number LFI; compare the dimensionless number LFI with the given criterion to know the stability degree after the liquid film is formed; specifically, judge the liquid film state according to the value of the dimensionless parameter LFI, where the liquid film is in a stable state when LFI ≤ 54k, in a relatively stable state when 54k < LFI ≤ 81k, in a transitional state when 81k < LFI ≤ 94k, and in an unstable state when LFI > 94k.

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