Aeroengine variable topology fuel-oil self-matching thermal management system

Abstract

The application discloses an aero-engine variable topology fuel-oil self-matching heat management system, which comprises an oil tank, an oil pump, a fuel-oil heat exchanger, an air-oil heat exchanger, a bearing cavity, an oil supply system, a main combustion chamber and an oil return regulating valve, wherein the fuel-oil heat exchanger and the air-oil heat exchanger are connected with a first stepless adjustable valve, a second stepless adjustable valve, a third stepless adjustable valve, a fourth stepless adjustable valve, a fifth stepless adjustable valve, a sixth stepless adjustable valve, a seventh stepless adjustable valve and an eighth stepless adjustable valve to form a variable topology oil flow path, and the whole is connected in a "day" shape. According to the set control strategy and combined with working condition transformation, the application realizes efficient utilization of fuel heat sink and duct air heat sink to take away system heat load, meets the heat dissipation demand of the bearing cavity in the fuel-oil system, identifies different working conditions, automatically matches the corresponding optimal heat management system topology structure in combination with the control strategy, and realizes the variable topology structure of the fuel-oil system heat management system.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of aero-engine thermal management, in particular to a variable topology fuel-oil self-matching thermal management system of an aero-engine. BACKGROUND

[0002] With the continuous improvement of the overall performance of the aircraft, the flight speed of the aircraft is further improved to high Mach number. However, at high Mach number, the aircraft will face extremely serious aerodynamic heating, so that the aircraft surface and system components will bear higher thermal load. In addition, the improvement of performance leads to the increase of the number and power of on-board equipment, resulting in the continuous temperature rise of the internal environment of the aircraft. In this case, the fuel-oil thermal management system in the aero-engine thermal management system plays a key role in managing the heat load transfer between the internal systems of the aero-engine.

[0003] Fuel oil is the main cold source of the aero-engine oil system, and as the performance of the aero-engine improves, the heat dissipation thermal load borne by the fuel oil becomes larger and larger. Keeping the fuel oil temperature at a suitable high temperature can effectively improve the combustion efficiency of the combustion chamber, but excessively high fuel oil temperature will not only reduce the control accuracy and reliability of the engine fuel control system and shorten the working life of the fuel oil accessories, but also cause the heat return oil flow of the fuel oil system to be too large, resulting in excessively high oil tank temperature. And excessively high fuel oil temperature is prone to cause fuel coking.

[0004] With the improvement of the maneuvering performance of the aircraft, under the complete flight envelope, the engine needs to face more flight conditions, which also puts higher requirements on the fuel-oil thermal management system. The fixed and unchanged topology structure of the fuel-oil thermal management system cannot meet the heat exchange requirements under multiple different working conditions at the same time, or cannot realize efficient utilization of the heat sink under multiple working conditions at the same time.

[0005] Therefore, a variable topology fuel-oil self-matching thermal management system of an aero-engine is needed, which can self-match the optimal thermal management system topology structure when facing multiple complex working conditions, realize reasonable distribution of the heat sink in the system of the aircraft under the entire flight envelope, so as to realize efficient utilization of the heat sink and as low as possible heat return oil flow. SUMMARY

[0006] The purpose of the present application is to provide a variable topology fuel-oil self-matching thermal management system of an aero-engine, which can automatically match the optimal topology structure of the aero-engine fuel-oil system according to the change of working conditions, realize the function of optimizing the heat sink distribution under multiple working conditions, and meet the temperature limitation requirements of fuel oil and oil in the fuel-oil system.

[0007] The purpose of the present application is achieved by the following technical solutions:

[0008] An aero-engine variable topology fuel oil thermal management system, characterized in that it comprises a fuel oil tank, a fuel oil pump, a fuel oil heat exchanger, an air-oil heat exchanger, a bearing cavity, an oil supply system, a main combustion chamber, an oil return regulating valve, a first stepless regulating valve, a second stepless regulating valve, a third stepless regulating valve, a fourth stepless regulating valve, a fifth stepless regulating valve, a sixth stepless regulating valve, a seventh stepless regulating valve, and an eighth stepless regulating valve; the fuel oil tank is connected to the fuel oil pump, the fuel oil pump is connected to the air-oil heat exchanger through the first stepless regulating valve and to the fuel oil heat exchanger through the second stepless regulating valve; the bearing cavity is connected to the fuel oil heat exchanger through the seventh stepless regulating valve and to the air-oil heat exchanger through the eighth stepless regulating valve; the bearing cavity is connected to the fuel oil tank; the fuel oil heat exchanger is connected to the oil supply system, the main combustion chamber, and the oil return regulating valve; the third stepless regulating valve and the fifth stepless regulating valve are connected in series between the second stepless regulating valve and the eighth stepless regulating valve; the sixth stepless regulating valve is connected between the fifth stepless regulating valve and the seventh stepless regulating valve; and the fourth stepless regulating valve is connected between the first stepless regulating valve and the third stepless regulating valve.

[0009] The fuel oil heat exchanger, the air-oil heat exchanger, and the first stepless regulating valve, the second stepless regulating valve, the third stepless regulating valve, the fourth stepless regulating valve, the fifth stepless regulating valve, the sixth stepless regulating valve, the seventh stepless regulating valve, and the eighth stepless regulating valve form a fuel oil variable topology flow path, which is connected in a "day" shape as a whole.

[0010] In the fuel oil variable topology flow path, the first stepless regulating valve, the second stepless regulating valve, the third stepless regulating valve, and the fourth stepless regulating valve form a first valve group, the fifth stepless regulating valve, the sixth stepless regulating valve, the seventh stepless regulating valve, and the eighth stepless regulating valve form a second valve group, and the four three-ways are connected in sequence; the fifth stepless regulating valve and the sixth stepless regulating valve are connected to the third stepless regulating valve and the fourth stepless regulating valve through a pipe to realize mutual communication between the two valve groups; the fifth stepless regulating valve and the eighth stepless regulating valve are connected to one side of the air-oil heat exchanger and the fuel oil heat exchanger, respectively, and the sixth stepless regulating valve and the seventh stepless regulating valve are connected to the other side of the air-oil heat exchanger and the fuel oil heat exchanger, respectively; and the fifth stepless regulating valve and the sixth stepless regulating valve are connected to the third stepless regulating valve and the fourth stepless regulating valve through a pipe to realize mutual communication between the two valve groups.

[0011] When the second infinitely adjustable valve, the fourth infinitely adjustable valve, the sixth infinitely adjustable valve and the eighth infinitely adjustable valve in the variable topology flow path are closed, and the remaining infinitely adjustable valves are opened, the air-oil heat exchanger and the fuel-oil heat exchanger are in a series topology structure, and the air-oil heat exchanger is passed through first and then the fuel-oil heat exchanger; when the first infinitely adjustable valve, the third infinitely adjustable valve, the fifth infinitely adjustable valve and the seventh infinitely adjustable valve are closed, and the remaining infinitely adjustable valves are opened, the air-oil heat exchanger and the fuel-oil heat exchanger are in a series topology structure, and the fuel-oil heat exchanger is passed through first and then the air-oil heat exchanger; when the third infinitely adjustable valve, the fourth infinitely adjustable valve, the fifth infinitely adjustable valve and the sixth infinitely adjustable valve are closed, and the remaining infinitely adjustable valves are opened, the air-oil heat exchanger and the fuel-oil heat exchanger are in a parallel topology structure; when only the first infinitely adjustable valve and the eighth infinitely adjustable valve are opened, the air-oil heat exchanger is used alone; when only the second infinitely adjustable valve and the seventh infinitely adjustable valve are opened, the fuel-oil heat exchanger is used alone.

[0012] The fuel is divided into two paths after heat exchange with the fuel-oil heat exchanger and the oil, one path is returned to the oil through the oil return adjusting valve, and the other path enters the main combustion chamber for combustion.

[0013] The air-oil heat exchanger cools the oil by taking the duct air as a heat sink, and the fuel-oil heat exchanger cools the oil by taking the fuel as a heat sink.

[0014] The oil return adjusting valve adjusts the hot oil return flow according to the maximum fuel temperature, and when the maximum temperature exceeds the limit temperature, the oil return adjusting valve increases the hot oil return flow to stabilize the maximum temperature at a limited value.

[0015] The application monitors the key node parameters of each flow path in real time, feeds back to the actuator, and the device executes the command according to the corresponding control strategy according to the identified working condition, finally meets the bearing cavity heat dissipation demand and the fuel maximum temperature limit demand, improves the fuel inlet temperature threshold and the combustion chamber fuel inlet temperature, reduces the hot oil return flow and the hot oil return temperature.

[0016] The oil passes through the fuel-oil heat exchanger, the air-oil heat exchanger and the eight infinitely adjustable valves to form a variable topology flow path, at this time the oil temperature is low, enters the bearing cavity to absorb heat, and finally returns to the oil tank to complete the whole cycle.

[0017] The fuel is divided into two paths after heat exchange with the fuel-oil heat exchanger and the oil, one path is returned to the oil through the oil return adjusting valve, and the other path enters the main combustion chamber for combustion.

[0018] The fuel oil system mainly comprises a main fuel oil circuit, a hot oil return circuit, an oil circulation circuit and a variable topology oil circuit.

[0019] The present application has the following advantages:

[0020] The present application avoids the fixed fuel oil system heat management strategy of traditional aeroengines, and innovatively designs a variable topology oil circuit. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The structure diagram of the variable topology fuel oil self-matching heat management system of the aeroengine.

[0022] Figure 2 The circuit state diagram of each topology structure of the variable topology heat management system, wherein a is a schematic diagram of the series topology structure of the air oil heat exchanger and the fuel oil heat exchanger, b is a schematic diagram of the series topology structure of the fuel oil heat exchanger and the air oil heat exchanger, c is a schematic diagram of the parallel topology structure of the air oil heat exchanger and the fuel oil heat exchanger, d is a schematic diagram of the air oil heat exchanger alone, and e is a schematic diagram of the fuel oil heat exchanger alone.

[0023] Markings in the figure: 1-lubricating oil tank, 2-lubricating oil pump, 3-first steplessly adjustable valve, 4-second steplessly adjustable valve, 5-third steplessly adjustable valve, 6-fourth steplessly adjustable valve, 7-air-lubricating oil heat exchanger, 8-fifth steplessly adjustable valve, 9-sixth steplessly adjustable valve, 10-seventh steplessly adjustable valve, 11-eighth steplessly adjustable valve, 12-bearing chamber, 13-oil supply system, 14-fuel-lubricating oil heat exchanger, 15-main combustion chamber, 16-return oil regulating valve. DETAILED DESCRIPTION

[0024] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0025] like Figure 1 As shown, an aircraft engine fuel and lubricating oil system can automatically match the topology of the thermal management system according to changes in operating conditions, thereby achieving the optimal topology under various operating conditions to optimize the system's utilization efficiency of the heat sink.

[0026] A fuel and lubricating oil thermal management system with a variable topology structure for an aircraft engine includes: a lubricating oil tank 1, a lubricating oil pump 2, a fuel and lubricating oil heat exchanger 14, an air-lubricating oil heat exchanger 7, a bearing cavity 12, an oil supply system 13, a main combustion chamber 15, an oil return regulating valve 16, a first steplessly adjustable valve 3, a second steplessly adjustable valve 4, a third steplessly adjustable valve 5, a fourth steplessly adjustable valve 6, a fifth steplessly adjustable valve 8, a sixth steplessly adjustable valve 9, a seventh steplessly adjustable valve 10, and an eighth steplessly adjustable valve 11; the lubricating oil tank 1 is connected to the lubricating oil pump 2, the lubricating oil pump 2 is connected to the air-lubricating oil heat exchanger 7 via the first steplessly adjustable valve 3, and is connected to the fuel and lubricating oil heat exchanger 7 via the second steplessly adjustable valve 4. The bearing cavity 12 is connected to the fuel and lubricating oil heat exchanger 14 through the seventh steplessly adjustable valve 10 and to the air and lubricating oil heat exchanger 7 through the eighth steplessly adjustable valve 11; the bearing cavity 12 is connected to the lubricating oil tank 1; the fuel and lubricating oil heat exchanger 14 is connected to the oil supply system 13, the main combustion chamber 15 and the return oil regulating valve 16; the third steplessly adjustable valve 5 and the fifth steplessly adjustable valve 8 are connected in series between the second steplessly adjustable valve 4 and the eighth steplessly adjustable valve 11; the sixth steplessly adjustable valve 9 is connected between the fifth steplessly adjustable valve 8 and the seventh steplessly adjustable valve 10; the fourth steplessly adjustable valve 6 is connected between the first steplessly adjustable valve 3 and the third steplessly adjustable valve 5.

[0027] The fuel and lubricating oil heat exchanger 14, the air and lubricating oil heat exchanger 7 and the first steplessly adjustable valve 3, the second steplessly adjustable valve 4, the third steplessly adjustable valve 5, the fourth steplessly adjustable valve 6, the fifth steplessly adjustable valve 8, the sixth steplessly adjustable valve 9, the seventh steplessly adjustable valve 10 and the eighth steplessly adjustable valve 11 constitute a variable topology flow path for the lubricating oil, which is connected in a "sun" shape as a whole.

[0028] In the variable topology flow path of the lubricating oil, the first infinitely variable valve 3, the second infinitely variable valve 4, the third infinitely variable valve 5 and the fourth infinitely variable valve 6 form a group, and the fifth infinitely variable valve 8, the sixth infinitely variable valve 9, the seventh infinitely variable valve 10 and the eighth infinitely variable valve 11 form another group, which are connected in sequence by four three-ways; the three-way between the fifth infinitely variable valve 8 and the sixth infinitely variable valve 9 and the third infinitely variable valve 5 and the fourth infinitely variable valve 6 is connected by a pipeline to realize the intercommunication between the two valve groups; the three-ways between the fifth infinitely variable valve 8 and the eighth infinitely variable valve 11 and the sixth infinitely variable valve 9 and the seventh infinitely variable valve 10 are respectively connected to one side of the air lubricating oil heat exchanger 7 and the fuel lubricating oil heat exchanger 14, and the three-ways between the first infinitely variable valve 3 and the fourth infinitely variable valve 6 and the second infinitely variable valve 4 and the third infinitely variable valve 5 are respectively connected to the other side of the air lubricating oil heat exchanger 7 and the fuel lubricating oil heat exchanger 14.

[0029] An aero-engine variable topology fuel lubricating oil self-matching heat management system mainly consists of a main fuel oil circuit, a hot return oil circuit, a lubricating oil circulation circuit and a lubricating oil variable topology flow path.

[0030] In the lubricating oil circulation circuit, the lubricating oil is discharged from the lubricating oil tank 1, passes through the lubricating oil pump 2, and enters the lubricating oil variable topology flow path composed of the air lubricating oil heat exchanger 7, the fuel lubricating oil heat exchanger 14 and eight infinitely variable valves. In the variable topology lubricating oil flow path, the topology structure will be identified according to the pre-set variable topology scheme and the topology structure will be changed. When the second infinitely variable valve 4, the fourth infinitely variable valve 6, the sixth infinitely variable valve 9 and the eighth infinitely variable valve 11 in the variable topology flow path are closed and the remaining infinitely variable valves are opened, the air lubricating oil heat exchanger 7 and the fuel lubricating oil heat exchanger 14 are in series topology structure and pass through the air lubricating oil heat exchanger 7 first and then pass through the fuel lubricating oil heat exchanger 14, as shown in FIG. a; when the first infinitely variable valve 3, the third infinitely variable valve 5, the fifth infinitely variable valve 8 and the seventh infinitely variable valve 10 are closed and the remaining infinitely variable valves are opened, the air lubricating oil heat exchanger 7 and the fuel lubricating oil heat exchanger 14 are in series topology structure and pass through the fuel lubricating oil heat exchanger 14 first and then pass through the air lubricating oil heat exchanger 7, as shown in FIG. b; when the third infinitely variable valve 5, the fourth infinitely variable valve 6, the fifth infinitely variable valve 8 and the sixth infinitely variable valve 9 are closed and the remaining infinitely variable valves are opened, the air lubricating oil heat exchanger 7 and the fuel lubricating oil heat exchanger 14 are in parallel topology structure, as shown in FIG. c; when only the first infinitely variable valve 3 and the eighth infinitely variable valve 11 are opened, the air lubricating oil heat exchanger 7 is used alone, as shown in FIG. d; and when only the second infinitely variable valve 4 and the seventh infinitely variable valve 10 are opened, the fuel lubricating oil heat exchanger 14 is used alone, as shown in FIG. e. Figure 2 a Figure 2 b Figure 2 c Figure 2 d Figure 2e; by the above-mentioned mode, the topology structure is switched according to the working condition change under the whole flight envelope, so that the heat management system is always in the optimal topology structure, so as to realize the efficient utilization of the heat sink;

[0031] Finally, the lubricating oil is cooled by the front end variable topology flow path, the lubricating oil temperature reaches the lowest, enters the bearing cavity 8 to absorb heat, and finally returns to the lubricating oil tank 1 to complete a whole cycle.

[0032] The fuel in the fuel system is divided into two ways after heat exchange with the lubricating oil through the fuel lubricating oil heat exchanger 14 and the lubricating oil, one way returns to the oil through the oil return regulating valve 16, and the other way enters the main combustion chamber 15 for combustion.

[0033] In the hot oil return oil path, the oil return regulating valve 16 adjusts the hot oil return flow according to the highest fuel temperature in the fuel system, when the highest temperature in the fuel system exceeds the limit temperature, the oil return regulating valve will increase the hot oil return flow, and the highest temperature will be stabilized at the limited value.

[0034] The lubricating oil variable topology flow path is innovatively designed, under different working conditions, the heat management system topology structure is changed by changing the state of the regulating valve group, so as to improve the utilization efficiency of the overall architecture to the heat sink, and then improve the fuel temperature and reduce the oil return flow. While avoiding the fuel temperature being too high in the fuel system, the hot oil return flow and temperature are controlled to be minimum, and the service life of each accessory in the aero-engine fuel lubricating oil system is prolonged. The variable topology design can provide a basis for the heat management and environmental control system of the aircraft system.

[0035] The above-mentioned is only the preferred embodiment of the present application, it should be pointed out that, for ordinary skilled in the art, without departing from the principles of the present application, several improvements can be made, these improvements should also be considered as the protection scope of the present application.

Claims

1. A variable topology fuel and lubricating oil self-matching thermal management system for an aircraft engine, characterized in that: Including: A lubricating oil tank (1), a lubricating oil pump (2), a fuel-oil / lubricating oil heat exchanger (14), an air-lubricating oil heat exchanger (7), a bearing chamber (12), a fuel supply system (13), a main combustion chamber (15), a return oil regulating valve (16), a first continuously adjustable valve (3), a second continuously adjustable valve (4), a third continuously adjustable valve (5), a fourth continuously adjustable valve (6), a fifth continuously adjustable valve (8), a sixth continuously adjustable valve (9), a seventh continuously adjustable valve (10) and an eighth continuously adjustable valve (11); The lubricating oil tank (1) is connected to the lubricating oil pump (2), and the lubricating oil pump (2) is connected to the air-lubricating oil heat exchanger (7) through the first continuously adjustable valve (3) and connected to the fuel-oil / lubricating oil heat exchanger (14) through the second continuously adjustable valve (4); The bearing chamber (12) is connected to the fuel-oil / lubricating oil heat exchanger (14) through the seventh continuously adjustable valve (10) and connected to the air-lubricating oil heat exchanger (7) through the eighth continuously adjustable valve (11); The bearing chamber (12) is connected to the lubricating oil tank (1); The fuel-oil / lubricating oil heat exchanger (14) is connected to the fuel supply system (13), the main combustion chamber (15) and the return oil regulating valve (16); The third continuously adjustable valve (5) and the fifth continuously adjustable valve (8) are connected in series between the second continuously adjustable valve (4) and the eighth continuously adjustable valve (11); The sixth continuously adjustable valve (9) is connected between the fifth continuously adjustable valve (8) and the seventh continuously adjustable valve (10); The fourth continuously adjustable valve (6) is connected between the first continuously adjustable valve (3) and the third continuously adjustable valve (5); The fuel-oil / lubricating oil heat exchanger (14), the air-lubricating oil heat exchanger (7) and the first continuously adjustable valve (3), the second continuously adjustable valve (4), the third continuously adjustable valve (5), the fourth continuously adjustable valve (6), the fifth continuously adjustable valve (8), the sixth continuously adjustable valve (9), the seventh continuously adjustable valve (10) and the eighth continuously adjustable valve (11) form a lubricating oil variable topology flow path, which is integrally connected in a "day" shape; In the lubricating oil variable topology flow path, the four regulating valves, namely the first steplessly adjustable valve (3), the second steplessly adjustable valve (4), the third steplessly adjustable valve (5), and the fourth steplessly adjustable valve (6), form a group, and the four regulating valves, namely the fifth steplessly adjustable valve (8), the sixth steplessly adjustable valve (9), the seventh steplessly adjustable valve (10), and the eighth steplessly adjustable valve (11), form another group, which are connected in sequence by four three-way valves; the fifth steplessly adjustable valve (8) and the sixth steplessly adjustable valve (9) are connected to the third steplessly adjustable valve (5) and the fourth steplessly adjustable valve (6). The three-way valves are connected by a pipeline to realize the mutual communication between the two valve groups; the three-way valves between the fifth steplessly adjustable valve (8) and the eighth steplessly adjustable valve (11) and the sixth steplessly adjustable valve (9) and the seventh steplessly adjustable valve (10) are respectively connected to one side of the air-lubricating oil heat exchanger (7) and the fuel-lubricating oil heat exchanger (14); the three-way valves between the first steplessly adjustable valve (3) and the fourth steplessly adjustable valve (6) and the second steplessly adjustable valve (4) and the third steplessly adjustable valve (5) are respectively connected to the other side of the air-lubricating oil heat exchanger (7) and the fuel-lubricating oil heat exchanger (14).

2. The aircraft engine variable topology fuel and lubricant self-matching thermal management system according to claim 1, characterized in that: When the second steplessly adjustable valve (4), the fourth steplessly adjustable valve (6), the sixth steplessly adjustable valve (9), and the eighth steplessly adjustable valve (11) in the variable topology flow path are closed, and the remaining steplessly adjustable valves are opened, the air-lubricating oil heat exchanger (7) and the fuel-lubricating oil heat exchanger (14) are in a series topology structure and first pass through the air-lubricating oil heat exchanger (7) and then pass through the fuel-lubricating oil heat exchanger (14); when the first steplessly adjustable valve (3), the third steplessly adjustable valve (5), the fifth steplessly adjustable valve (8), and the seventh steplessly adjustable valve (10) are closed, and the remaining steplessly adjustable valves are opened, the air-lubricating oil heat exchanger (7) and the fuel-lubricating oil heat exchanger (14) are in a series topology structure. The air-lubricating oil heat exchanger (7) is a topological structure and first passes through the fuel-lubricating oil heat exchanger (14) and then passes through the air-lubricating oil heat exchanger (7); when the third steplessly adjustable valve (5), the fourth steplessly adjustable valve (6), the fifth steplessly adjustable valve (8), and the sixth steplessly adjustable valve (9) are closed and the remaining steplessly adjustable valves are opened, the air-lubricating oil heat exchanger (7) and the fuel-lubricating oil heat exchanger (14) are in a parallel topological structure; when only the first steplessly adjustable valve (3) and the eighth steplessly adjustable valve (11) are opened, the air-lubricating oil heat exchanger (7) is used alone; when only the second steplessly adjustable valve (4) and the seventh steplessly adjustable valve (10) are opened, the fuel-lubricating oil heat exchanger (14) is used alone.

3. The aircraft engine variable topology fuel and lubricant self-matching thermal management system according to claim 1, characterized in that: After the fuel passes through the fuel-lubricating oil heat exchanger (14) and exchanges heat with the lubricating oil, the fuel is divided into two paths. One path is returned through the oil return regulating valve (16), and the other path enters the main combustion chamber (15) for combustion.

4. The aircraft engine variable topology fuel and lubricant self-matching thermal management system according to claim 1, characterized in that: The air-lubricating oil heat exchanger (7) uses the bypass air as a heat sink to cool the lubricating oil, and the fuel-lubricating oil heat exchanger (14) uses the fuel oil as a heat sink to cool the lubricating oil.

5. The aircraft engine variable topology fuel and lubricant self-matching thermal management system according to claim 1, characterized in that: The oil return regulating valve (16) adjusts the hot oil return flow rate according to the maximum fuel temperature. When the maximum temperature exceeds the limit temperature, the oil return regulating valve (16) will increase the hot oil return flow rate to stabilize the maximum temperature at the limit value.

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