A method and system for measuring thermal conductivity of photothermal liquid

Abstract

The present invention discloses a method and system for measuring the thermal conductivity of a photothermal liquid, and relates to the field of optical measurement. The method for measuring the thermal conductivity of a photothermal liquid includes irradiating a phase diaphragm with a laser so that the image of the phase diaphragm is imaged on a sample to be measured; obtaining the light intensity change at the center position of the image of the phase diaphragm; and establishing a corroborative relationship between the light intensity change at the center position of the image of the phase diaphragm and the thermal conductivity of the sample to be measured. Compared with the existing steady-state thermal conductivity measurement method, the present invention has the advantages of not having to consider liquid convection, short measurement time, and small amount of liquid required. Compared with the existing transient thermal conductivity measurement method, the present invention has a simple device. Compared with the transient hot wire method, the present invention uses a focused laser beam to replace the hot wire, liquid light absorption to replace hot wire heating, and phase monitoring after the liquid is heated to replace temperature measurement, thereby improving the temperature detection accuracy, greatly simplifying the measurement device, and greatly reducing the amount of liquid to be measured.

Description

Technical Field

[0001] The invention relates to the technical field of optical measurement, and in particular to a system and method for measuring thermal conductivity of photothermal liquid. Background Art

[0002] The methods for measuring the thermal conductivity of liquids can be mainly divided into two categories: steady-state method and non-steady-state method according to the relationship between the temperature and time of the liquid to be measured.

[0003] The steady-state method refers to a method in which measurements are made only after the temperature distribution of the liquid to be measured has stabilized. Its principle is based on Fourier's law under one-dimensional steady-state heat transfer, and the thermal conductivity is calculated by directly measuring the heat flux and temperature gradient. It mainly includes three types: the plate method, the cylinder method, and the sphere method. This type of method is time-consuming because of its temperature field stabilization, often taking several hours or even longer, and the system is prone to natural convection. In order to ensure one-dimensional heat conduction, complex devices are required to control the boundary conditions, and the equipment processing requirements are high. At the same time, detecting the impact of natural convection requires comparing different experimental conditions, which is cumbersome.

[0004] The theoretical model of the steady-state method for measuring the thermal conductivity of liquids is based on the non-steady-state thermal conductivity differential equation, and the thermal conductivity of liquids is determined by the temperature distribution curve of the liquid to be measured over time. It mainly includes other methods such as the transient method, point heat source method, and laser flash method. Such methods are complex in design, high in cost, and require a large amount of liquid, and have high requirements for the experimental environment and operation. Summary of the invention

[0005] In view of the above problems existing in the prior art, the present invention is proposed.

[0006] Therefore, the problem to be solved by the present invention is that the existing method for measuring the thermal conductivity of liquid is complex, costly, and has high requirements on the experimental environment and operation.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions: a method for measuring the thermal conductivity of a photothermal liquid, comprising: irradiating a phase diaphragm with a laser to form an image of the phase diaphragm on a sample to be measured; obtaining the light intensity change at the center position of the image of the phase diaphragm; and establishing a corroborative relationship between the light intensity change at the center position of the image of the phase diaphragm and the thermal conductivity of the sample to be measured.

[0008] As a preferred solution of the method for measuring thermal conductivity of photothermal liquid of the present invention, the laser irradiating the phase aperture is a laser after collimation and beam expansion.

[0009] As a preferred solution of the method for measuring the thermal conductivity of the photothermal liquid described in the present invention, the laser passes through two convex lenses and an aperture diaphragm in sequence before irradiating the phase diaphragm, the two convex lenses are used to collimate and expand the laser, and the aperture diaphragm is used to adjust the size of the incident light spot.

[0010] As a preferred solution of the method for measuring the thermal conductivity of the photothermal liquid described in the present invention, the sample to be measured is placed at the confocal point of two convex lenses in a 4f optical system, and the phase stop is placed at the front focal plane of the 4f optical system.

[0011] As a preferred solution of the method for measuring thermal conductivity of photothermal liquid of the present invention, a photosensor is placed at the rear focal plane of the 4f optical system to detect the change of light intensity at the center position of the image of the phase stop.

[0012] As a preferred solution of the method for measuring thermal conductivity of photothermal liquid of the present invention, an attenuation plate and an aperture stop are placed in sequence between the rear focal plane of the 4f optical system and the photosensor.

[0013] As a preferred solution of the method for measuring the thermal conductivity of the photothermal liquid described in the present invention, the phase aperture includes, from the inside to the outside, a central light-transmitting area, an outer ring light-transmitting area and a peripheral light-blocking area, and the laser will produce a phase difference when passing through the central light-transmitting area.

[0014] As a preferred solution of the method for measuring thermal conductivity of photothermal liquid of the present invention, the central light-transmitting area of ​​the phase diaphragm generates phase difference by coating.

[0015] Another object of the present invention is to provide a system for measuring the thermal conductivity of photothermal liquids, which is suitable for the above-mentioned method for measuring the thermal conductivity of photothermal liquids. The system includes a light source expansion and collimation unit, which includes a first convex lens, a second convex lens and a first aperture diaphragm arranged in sequence along the optical path; an imaging unit, which includes a phase diaphragm, a third convex lens, a sample dish and a fourth convex lens arranged in sequence on the subsequent optical path of the first aperture diaphragm; and a signal receiving unit, which includes an attenuation plate, a second aperture diaphragm and a photodiode arranged in sequence on the subsequent optical path of the fourth convex lens.

[0016] As a preferred solution of the system for measuring thermal conductivity of photothermal liquid of the present invention, the third convex lens and the fourth convex lens together constitute a 4f imaging system, and the sample dish is located at the confocal plane of the 4f imaging system.

[0017] The beneficial effects of the present invention are as follows: compared with the existing steady-state thermal conductivity measurement method, this patent has the advantages of no need to consider liquid convection, short measurement time and small amount of liquid required, etc. Compared with the existing transient thermal conductivity measurement method, this patent has a simple device. Compared with the transient hot wire method, the present invention uses a focused laser beam to replace the hot wire, liquid light absorption to replace hot wire heating, and phase monitoring after the liquid is heated to replace temperature measurement, which improves the temperature detection accuracy, greatly simplifies the measurement device, and greatly reduces the amount of liquid to be measured. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0019] Figure 1 Schematic diagram of the measurement system structure of the thermal conductivity of photothermal liquid.

[0020] Figure 2 Schematic diagram of the phase aperture structure.

[0021] Figure 3 Schematic diagram of the light beam irradiating the sample dish.

[0022] In the figure: 100, light source beam expansion and collimation unit; 101, first convex lens; 102, second convex lens; 103, first aperture diaphragm; 200, imaging unit; 201, phase diaphragm; 202, third convex lens; 203, sample dish; 204, fourth convex lens; 300, signal receiving unit; 301, attenuation plate; 302, second aperture diaphragm; 303, photosensor; 400, laser. DETAILED DESCRIPTION

[0023] 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 in conjunction with the accompanying drawings.

[0024] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0025] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it an embodiment that is mutually exclusive with other embodiments, either individually or selectively.

[0026] Example 1

[0027] Reference Figure 1~Figure 3 , which is the first embodiment of the present invention, and provides a method for measuring the thermal conductivity of a photothermal liquid. The method for measuring the thermal conductivity of a photothermal liquid includes:

[0028] S1: irradiating a phase diaphragm with laser light so that the image of the phase diaphragm is formed on the sample to be measured.

[0029] Specifically, in order to better describe the measurement method of this embodiment, it is now described based on a set of measurements of the thermal conductivity of the photothermal liquid, such as Figure 1 As shown, the system includes a laser 400, a first convex lens 101, a second convex lens 102, a first aperture stop 103, a phase stop 201, a third convex lens 202, a sample dish 203, a fourth convex lens 204, an attenuation plate 301, a second aperture stop 302 and a photosensor 303 arranged in sequence along the optical path.

[0030] The third convex lens 202 and the fourth convex lens 204 together form a 4f imaging system, and the sample dish 203 is placed at the confocal plane of the 4f imaging system. The phase stop 201 is placed at the front focal plane of the 4f imaging system.

[0031] After the laser is collimated and expanded by the first convex lens 101 and the second convex lens 102, the size of the incident light spot is adjusted by the first aperture diaphragm 103. The light spot is irradiated on the phase diaphragm 201, so that the phase diaphragm 201 is imaged on the sample dish 203, in which the liquid to be measured is placed.

[0032] S2: Acquire the light intensity change at the center position of the image of the phase stop.

[0033] Specifically, an attenuation plate 301, a second aperture stop 302 and a photosensor 303 are sequentially placed on the optical path following the fourth convex lens 204. The attenuation plate 301 is used to reduce the light intensity, and the second aperture stop 302 is used to adjust the size of the light spot. The photosensor 303 can be used to measure the light intensity change at the center position of the image of the phase stop.

[0034] S3: Establishing a correlation between the light intensity change at the center position of the image of the phase stop and the thermal conductivity of the sample being measured.

[0035] Specifically, it should be noted that the wavelength of the laser should match the absorption characteristics of the sample to ensure that the sample can absorb light and convert it into heat. After the collimated laser beam passes through the phase stop 201, it is focused on the 4f system spectrum plane through the third convex lens 202 in the 4f system.

[0036] The sample placed on the spectrum surface of the 4f system generates heat by absorbing the incident light. Since the light intensity distribution on the spectrum surface is a first-order Bessel function, the light intensity at the center of the spot is much greater than the light intensity at the boundary. After the sample is heated on the spectrum surface, the temperature at the center of the spot is high, resulting in a low-density area. The light spot on the outer ring of the spot absorbs less heat and has a lower temperature than the center. The heated sample generates a density gradient in the local area. By controlling the incident light power, the temperature difference at the center of the sample is small, and an approximate steady state is reached within seconds, which can avoid the influence of liquid convection on the thermal conductivity measurement.

[0037] When the laser beam irradiates the sample initially, the sample is heated within the range of the beam. At this time, the temperature difference between the center of the focused beam and the outer ring of the beam is small, and the heat conducted outward is small. As the heat accumulates, the center temperature gradually rises, and the heat conducted outward from the center increases. When the heat conducted per unit time is close to the heat absorbed, the system approximately reaches thermal equilibrium. At this time, the temperature difference between the center and the outer ring reaches the maximum, and the refractive index of the heated liquid in the center is also the smallest. The phase difference generated by the laser passing through the center and the outer ring area also reaches the maximum, such as Figure 3 Therefore, the temperature can be measured by measuring the phase difference between the center of the focused beam and the outer ring area. When the center phase difference is calculated, the temperature difference can be calculated through the phase as long as the refractive index temperature coefficient (thermo-optic coefficient) of the sample under test is known. The thermal conductivity of the sample under test can be calculated through the thermal conductivity equation based on the relationship between the phase (temperature) and time.

[0038] However, the phase difference is not easy to measure directly. Furthermore, the relationship between the phase change of the output light and the temperature rise of the liquid can be obtained from the Gaussian beam properties of the laser and geometric optics:

[0039] ,

[0040] Among them is Laser wavelength, is the liquid thermo-optic coefficient, L is the width of the sample dish 203. The phase difference of the sample illumination center can be obtained by the above formula. With temperature difference When the incident wavelength, sample dish size and incident focusing lens focal length are determined, the coefficient is a constant.

[0041] The phase difference of the center of the light beam after the sample absorbs heat on the 4f confocal plane can be measured by the phase contrast principle. The introduction of the phase stop 201 can ultimately convert the phase signal into the intensity change of the central light spot on the image plane. The principle is as follows:

[0042] like Figure 2 The black part of the phase stop 201 placed on the incident surface of the 4f system is the light-blocking area, the white part is the light-transmitting area, and the shadow part has a certain additional phase difference compared to the white area through the coating method, L p and R a are the radii of the phase region and the aperture, respectively. After focusing at 4f, the shadow part has a larger focus spot on the focal plane due to its small incident area, while the outer ring white part has a larger area and a smaller focus spot on the focal plane. When the focused light beam passes through the sample material, the sample's absorption coefficient of light is a constant. Therefore, the amount of heat absorbed by the sample at different positions is proportional to the light intensity at this position. The greater the amount of heat absorbed, the greater the phase difference, and the greater the phase modulation of the incident light. Therefore, the phase generated by the central heat mainly modulates the incident light of the outer ring white part, while the shadow part has little effect on the light.

[0043] On the receiving plane, the zero-frequency light phase-modulated by the sample reaches each area of ​​the receiving surface, and the incident light in the shadow part is basically unaffected. After passing through the sample, it is focused to the center of the receiving surface and interferes with the zero-frequency modulated light. According to the phase contrast principle, since the shadow part has phase modulation compared to other incident lights, the phase of the zero-frequency light after interference is converted into the light intensity change in the central area. In this way, by monitoring the light intensity change of the light spot in the center of the shadow part on the image plane, the phase change in the center of the heating area can be judged, that is, the temperature monitoring is converted into the detection of the central light intensity change.

[0044] On the rear focal plane of the 4f system, the image of the phase diaphragm is obtained. The light intensity of the central part of the phase diaphragm is modulated by the phase change of the sample center, resulting in grayscale changes. The phase contrast signal of the rear focal plane of the 4f system is defined as the difference between the average intensity inside and outside the phase diaphragm 201 image, that is, the grayscale ratio of the central shadow part to the outer ring white part image:

[0045] ,

[0046] in, I 0 is the average light intensity of the image of the phase stop 201 on the receiving surface, and They are respectively the light intensity averages of the central shadow part and the outer ring part of the phase stop 201 image, which can be obtained by measuring the grayscale averages of different areas of the received plane phase stop 201 image or by collecting the light intensity of different areas.

[0047] The phase change relationship between the back focal plane phase contrast signal of the 4f system and the modulation of the sample dish 203 is calculated as follows:

[0048] ,

[0049] in, is the ratio of P0 to the radius of the phase stop 201, is the phase shift of the phase object in the phase stop 201, is the coefficient difference of the two top-hat light Taylor expansions of the incident electric field. This coefficient is related to the size of the phase stop 201 and the focal length of the lens. For a fixed phase stop 201 and the imaging system, it is a constant and can be obtained by calibration of a standard sample. The temperature difference of the liquid after heating can be calculated by measuring changes.

[0050] Referring to the data processing method in the hot wire method, a focused laser beam is used to replace the heating wire in the hot wire method, and the heating power is replaced by the absorption energy. Combined with the heat conduction differential equation, the thermal conductivity can be calculated from the laser power, the sample absorption coefficient and the sample thickness:

[0051] ,

[0052] in q is the heat absorption power per unit length of the liquid under fixed laser power conditions , The change in is proportional to the phase change, and thus proportional to the change in central light intensity, so the temperature changes with time The intensity change of the image center with phase stop 201 The proportional coefficient can be obtained by calibrating the thermal conductivity sample. When measuring the thermal conductivity of liquid, the expression can be used:

[0053] ,

[0054] Calculate, where K is the calibration factor. and The relationship can be obtained by collecting the central light intensity change of the phase stop 201 by the photosensitive sensor 303.

[0055] From the above, the thermal conductivity of the liquid sample can be measured by measuring the change of the light intensity at the center of the phase aperture 201.

[0056] Example 2

[0057] This embodiment is the second embodiment of the present invention, and this embodiment is based on the previous embodiment. This embodiment provides an application example of measuring the thermal conductivity of copper sulfate solution.

[0058] Specifically, a laser with an absorption peak wavelength for the copper sulfate solution is selected. In this example, a 6328 helium-neon laser is selected as the heating light source.

[0059] S1: The laser beam is expanded by the method described in Example 1. After expansion, the laser beam can be regarded as an approximately parallel spot with a spot diameter greater than 4 mm.

[0060] S2: Place the phase object on the front focal plane of the 4f system described in Example 1, use a CCD camera to receive the phase object image on the back focal plane of the 4f system, and adjust the position of the CCD camera until a clear imaging position of the phase object is found.

[0061] S3: Place the standard sample in the confocal plane of the 4f system, and the CCD camera can see that the central light spot becomes darker.

[0062] S4: Adjust the size of the second aperture diaphragm 302 so that only the central black spot of the emitted light spot passes through, replace the CCD camera with a photodiode, and connect the photodiode to an oscilloscope.

[0063] S5: Block the incident laser light source, and after the thermal lens is fully restored, emit the laser and the oscilloscope records the laser heating process.

[0064] S6: The recorded curve is the relationship between the thermal lens phase and time. Since the central phase change is proportional to the temperature change, the curve can be used to calculate , using the relationship between light intensity change, incident light intensity and sample thermal conductivity to calculate the coefficient K between the system light intensity change and temperature change.

[0065] S7: Replace the standard sample with the unknown thermal conductivity copper sulfate solution, repeat steps 3 to 6, and use the calibration coefficient K to obtain the light intensity change curve. And the incident light intensity to calculate the thermal conductivity λ of copper sulfate solution.

[0066] Example 3

[0067] This embodiment is the third embodiment of the present invention, which is based on the first two embodiments. This embodiment provides a system for measuring the thermal conductivity of a photothermal liquid. The system includes a light source beam expansion and collimation unit 100, an imaging unit 200, and a signal receiving unit 300.

[0068] Specifically, the light source beam expansion and collimation unit 100 includes a laser 400, a first convex lens 101, a second convex lens 102, and a first aperture stop 103, which are arranged in sequence from left to right along the optical path. The first convex lens 101 and the second convex lens 102 are used to expand and shape the laser beam. The first aperture stop 103 is used to adjust the size of the incident light spot.

[0069] The imaging unit 200 includes a phase stop 201, a third convex lens 202, a sample dish 203, and a fourth convex lens 204, which are sequentially arranged on the optical path following the first aperture stop 103. The third convex lens 202 and the fourth convex lens 204 together form a 4f optical imaging system. The phase stop 201 is placed on the front focal plane of the 4f optical imaging system, and the sample dish 203 is placed at the confocal plane of the third convex lens 202 and the fourth convex lens 204.

[0070] The signal receiving unit 300 includes an attenuation plate 301, a second aperture diaphragm 302, and a photosensor 303, which are sequentially arranged on the optical path following the fourth convex lens 204. The attenuation plate 301 is used to reduce the light intensity of the laser beam, the second aperture diaphragm 302 is used to adjust the size of the exiting light spot, and the photosensor 303 is connected to the oscilloscope to detect the change of light intensity.

[0071] This system is applicable to the measurement method described in Example 1.

[0072] In summary, the beneficial effects of the present invention are:

[0073] 1. Compared with the existing steady-state thermal conductivity measurement method, this patent has the advantages of no need to consider liquid convection, short measurement time and small amount of liquid required.

[0074] 2. Compared with the existing transient thermal conductivity measurement method, this patented device is simple and easy to operate.

[0075] 3. Compared with the transient hot wire method, the present invention uses a focused laser beam to replace the hot wire, liquid absorption of light to replace hot wire heating, and phase monitoring of the liquid after heating to replace temperature measurement, which improves the temperature detection accuracy, greatly simplifies the measuring device, and greatly reduces the amount of liquid to be measured.

[0076] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A method for measuring thermal conductivity of a photothermal liquid, characterized in that: include, The phase diaphragm is irradiated by laser so that the image of the phase diaphragm is formed on the sample to be measured; Acquire the light intensity change at the center position of the image of the phase stop; Establishing a corroborative relationship between the light intensity change at the center position of the image of the phase stop and the thermal conductivity of the sample being measured; The sample to be tested is placed at the confocal position of two convex lenses in a 4f optical system, and the phase stop is placed at the front focal plane of the 4f optical system; The phase aperture includes, from the inside to the outside, a central light-transmitting area, an outer ring light-transmitting area, and a peripheral light-blocking area. When the laser passes through the central light-transmitting area, a phase difference will be generated.

2. The method for measuring thermal conductivity of photothermal liquid according to claim 1, characterized in that: The laser beam irradiating the phase stop is a laser beam that has been collimated and expanded.

3. The method for measuring the thermal conductivity of a photothermal liquid according to claim 2, characterized in that: Before irradiating the phase diaphragm, the laser passes through two convex lenses and an aperture diaphragm in sequence. The two convex lenses are used to collimate and expand the laser, and the aperture diaphragm is used to adjust the size of the incident light spot.

4. The method for measuring thermal conductivity of photothermal liquid according to claim 3, characterized in that: A photosensitive sensor is placed at the rear focal plane of the 4f optical system to detect changes in light intensity at the center position of the image of the phase stop.

5. The method for measuring thermal conductivity of photothermal liquid according to claim 4, characterized in that: An attenuation plate and an aperture stop are placed in sequence between the rear focal plane of the 4f optical system and the photosensor.

6. The method for measuring thermal conductivity of photothermal liquid according to claim 5, characterized in that: The central light-transmitting area of ​​the phase stop generates a phase difference by coating.

7. A system for measuring thermal conductivity of photothermal liquid, characterized in that: Applicable to the measurement method according to any one of claims 1 to 6, the system comprising: A light source beam expansion and collimation unit (100), comprising a first convex lens (101), a second convex lens (102), and a first aperture stop (103) which are sequentially arranged along an optical path; An imaging unit (200) comprising a phase diaphragm (201), a third convex lens (202), a sample dish (203), and a fourth convex lens (204) which are sequentially arranged on a subsequent optical path of the first aperture diaphragm (103); The signal receiving unit (300) comprises an attenuation plate (301), a second aperture stop (302) and a photosensor (303) which are sequentially arranged on the subsequent optical path of the fourth convex lens (204).

8. The system for measuring thermal conductivity of photothermal liquid according to claim 7, characterized in that: The third convex lens (202) and the fourth convex lens (204) together form a 4f imaging system, and the sample dish (203) is located at the confocal plane of the 4f imaging system.

Images