A spray droplet temperature measurement system and measurement method based on fluorescence lifetime measurement

Abstract

The present invention discloses a spray droplet temperature measurement system based on fluorescence lifetime measurement and a measurement method thereof, wherein the temperature measurement system comprises a basic light source device, a frequency doubling device, an objective lens, an achromatic dichroic mirror, an optical fiber, a filter, a photomultiplier tube, a time-correlated single photon counting and a picosecond timer. The temperature measurement system of the present invention can be used to measure the transient temperature of various types of spray droplets. Compared with the prior art, the present invention has high measurement accuracy, is not affected by the intensity of the excitation source or the collection of fluorescence signals, and is particularly suitable for droplet temperature measurement in high spray density areas.

Description

Technical Field

[0001] The invention belongs to the application field of advanced laser measurement technology, and in particular relates to a spray droplet temperature measurement system based on fluorescence lifetime measurement and a measurement method thereof. Background Art

[0002] In the field of industrial spraying, the mutual transfer of energy and mass often plays an important role. In internal combustion engines, the measurement of spray thermodynamic characterization parameters is of great significance for studying potential emission reduction and overall optimization of combustion. Modeling droplet heating, evaporation and fuel-air mixing requires experimental acquisition of droplet dynamics and thermodynamics information, and the measurement of temperature is a key parameter that needs to be characterized in particular. In the field of spray drying, temperature and concentration changes on the droplet surface directly affect chemical reactions and phase changes, such as crystallization inside the droplet and changes in droplet shape and structure. In wet scrubbers, liquid sprays are used to capture gas phase and particulate pollutants. When fine droplets are atomized in the spray tower, the relatively large specific surface area provides a large liquid-gas interface, thereby maximizing the number of collisions between droplets and pollutant particles. The temperature of the droplets directly or indirectly affects the absorption efficiency because the chemical kinetics, equilibrium, absorption and diffusion coefficients involved are all thermally dependent. In the field of spray cooling, especially spray cooling in the steel processing industry and thermal management of advanced electronic components, the accurate measurement of droplet temperature directly affects the production efficiency and safety of related industries. In summary, accurate measurement of spray droplet temperature has practical engineering value for improving the efficiency of various industries involving spraying and basic research.

[0003] The present invention focuses on the application of advanced optical methods in droplet temperature measurement. Non-invasive and non-contact optical measurement methods can be applied to most complex and changeable practical environments. Among them, most of the measurement methods are based on the phenomenon of light-matter interaction, which is affected to some extent by the temperature or chemical composition of the measured medium. These technologies have a common feature, that is, they can all provide indirect measurement. In the history of droplet temperature measurement, the technical means of optical temperature measurement are mainly divided into two categories according to optical principles, namely, measurement means based on light scattering, measurement technology based on laser-induced fluorescence, and other diagnostic methods including thermochromic liquid crystal, infrared thermal imaging, and morphology-related resonance. Among the above temperature measurement methods, the intensity temperature measurement method based on laser-induced fluorescence has been considered as a flexible spray droplet temperature measurement method with high temperature sensitivity for some time in the past. However, intensity measurement always requires a reference, which must be carried out under experimental conditions close to the measurement conditions. Any interference (light source intensity, background light, light transmission through the detection optical device, or due to photobleaching and degradation of the dye) will cause some errors, thereby causing some errors in the measurement of temperature.

[0004] Compared with the above measurement method, the present invention obtains the fluorescence lifetime of organic fluorescent dye droplets such as Kiton red 620, Rhodamine 6G, Rhodamine B, etc. under femtosecond or picosecond high-frequency pulse laser irradiation by fluorescence lifetime measurement based on laser induced fluorescence, establishes a quantitative relationship between fluorescence lifetime and temperature, and thus obtains a direct measurement value of the droplet temperature. It is worth noting that the Chinese invention patent with authorization announcement number CN 106441628 A discloses a temperature measurement system based on YAG:Dy fluorescence lifetime measurement and its measurement method and application, which is specifically used in the measurement of gas temperature, in which the fluorescence excitation process and the laser induced fluorescence involved in the present invention are essentially different. Summary of the invention

[0005] Aiming at the measurement of industrial spray droplet temperature, the present invention proposes a spray droplet temperature measurement system and a measurement method based on fluorescence lifetime measurement, which can effectively overcome the interference of light source intensity, background light, light transmission through detection optical devices, or photobleaching and degradation of dyes under intensity measurement method, and realize efficient and rapid characterization of spray droplet temperature. The expected temperature measurement error can be controlled within 1°C.

[0006] The present invention can be implemented through the following technical solutions:

[0007] A spray droplet temperature measurement system based on fluorescence lifetime measurement comprises a basic light source device, a frequency doubling device, an objective lens, an achromatic dichroic mirror, an optical fiber, a filter, a photomultiplier tube, a time-correlated single photon counting and a picosecond timer. The laser generated by the basic light source device is converted into an excitation wavelength suitable for fluorescence of a solution to be measured by the frequency doubling device, and then the laser is focused on a spray area to be measured by the objective lens. The laser-induced fluorescence signal is collected by the achromatic dichroic mirror, and then the fluorescence is introduced into the photomultiplier tube by the optical fiber. The optimal collection of the fluorescence signal is achieved by the filter. Finally, the fluorescence signal attenuation curve is obtained by the time-correlated single photon counting and the picosecond timer. A quantitative relationship between the fluorescence lifetime and the temperature is established by a calibration method, and finally the direct measurement value of the spray temperature can be obtained by the calibration relationship.

[0008] Furthermore, the basic light source device is a femtosecond titanium sapphire laser, a picosecond dye laser or a coaxial flash lamp. The laser wavelength generated by the basic light source device should be in the range of 200nm to 1200nm, and the pulse width should be not less than 500ps.

[0009] Furthermore, the frequency doubling device contains a frequency doubling crystal or a frequency tripling crystal.

[0010] Furthermore, the focal length of the objective lens is greater than the spray radius at the height.

[0011] Furthermore, the focal length of the achromatic dichroic mirror is greater than the spray radius at the height, and is placed at a 90° angle to the laser plane.

[0012] Furthermore, the filter is a bandpass filter, and its central wavelength selection interval is 450-650nm.

[0013] Furthermore, the timing resolution of the photomultiplier tube is lower than 300 ps, ​​and the detection wavelength range is between 200 nm and 1200 nm.

[0014] Furthermore, the time-correlated single photon counting and picosecond timer have a minimum time resolution of less than 100 ps, ​​making the time width less than 100 ns.

[0015] The present invention also proposes a measurement method implemented by the above-mentioned spray droplet temperature measurement system based on fluorescence lifetime measurement. The method for obtaining the spray temperature through the fluorescence signal attenuation curve specifically includes the following steps:

[0016] (1) The theoretical value of fluorescence intensity decay should satisfy I(t) = I 0 exp(-t / τ), where I 0 is the initial strength;

[0017] (2) The basic principle of the fluorescence lifetime calculation method is consistent with:

[0018]

[0019] That is, the fluorescence lifetime can be determined by obtaining the slope value of the fluorescence signal decay curve;

[0020] (3) Fluorescence signal decay curve: photon events are counted by photomultiplier tube, time-correlated single photon counting and picosecond timer;

[0021] (4) The fluorescence signal decay curve obtained by experimental measurement needs to be fitted to obtain the slope;

[0022] (5) The specific steps of the calibration experiment are as follows: First, a solution containing a specific concentration of Kiton red 640, Rhodamine 6G or Rhodamine B is heated to a fixed temperature by a constant temperature heating device, and the above-mentioned fluorescence lifetime is obtained for dye solutions at different fixed temperatures. Finally, a calibration curve between fluorescence lifetime and temperature is established. The data show that the fluorescence lifetime is strongly linearly correlated with the temperature, and the calibration curve of the spray measurement can be fitted.

[0023] Compared with other existing droplet temperature measurement technologies, the present invention has the following advantages:

[0024] 1) Fluorescence lifetime is an intrinsic molecular property and is not affected by the intensity of the excitation source or the collection of the fluorescence signal. When used in dense spray conditions, its measurement method is more robust and reliable;

[0025] 2) Time-correlated single photon counting is particularly suitable for temperature measurements because it is the most sensitive method for determining fluorescence lifetime. Under all conditions of the tested spray, a very high signal-to-noise ratio can be achieved, which provides an accuracy and robustness to the measurement that may be unmatched by other temperature measurement techniques in sprays. In most cases, the absolute error is usually no more than ±1°C, and even reduced to about ±0.5°C if any uncertainties affecting the calibration are set aside. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the structure of the temperature measurement system of the present invention;

[0027] Figure 2 Schematic diagram of the fluorescence signal attenuation curve and the method for obtaining the slope thereof of the present invention;

[0028] Figure 3 This is the temperature-fluorescence lifetime calibration curve of the present invention.

[0029] Explanation of the symbols in the figure: 1- basic light source equipment; 2- frequency doubling equipment; 3- objective lens; 4- atomizing nozzle; 5- achromatic dichroic mirror; 6- optical fiber; 7- filter; 8- photomultiplier tube; 9- time-correlated single photon counting and picosecond timer DETAILED DESCRIPTION

[0030] The following describes the implementation of the present invention through specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0031] like Figure 1 As shown, a spray droplet temperature measurement system based on fluorescence lifetime measurement includes a basic light source device 1, a frequency doubling device 2, an objective lens 3, an achromatic dichroic mirror 5, an optical fiber 6, a filter 7, a photomultiplier tube 8, and a time-correlated single photon counting and picosecond timer 9. The laser generated by the basic light source device 1 is converted into an excitation wavelength suitable for the fluorescence of the solution to be measured by the frequency doubling device 2, and then the laser is focused on the spray area to be measured by the objective lens 3, and the laser-induced fluorescence signal is collected by the achromatic dichroic mirror 5, and then the fluorescence is introduced into the photomultiplier tube 8 through the optical fiber, and the optimal collection of the fluorescence signal is achieved through the filter 7. Finally, the fluorescence signal attenuation curve is obtained by time-correlated single photon counting and picosecond timer 9, and a quantitative relationship between the fluorescence lifetime and the temperature is established by using a calibration method, and finally the direct measurement value of the spray temperature can be obtained by the calibration relationship.

[0032] The frequency doubling device 2 contains a frequency doubling crystal or a frequency tripling crystal; the focal length of the objective lens 3 is greater than the spray radius at the height.

[0033] The focal length of the achromatic dichroic mirror 5 is larger than the spray radius at the height, and is placed at 90° to the laser plane.

[0034] The filter 7 is a bandpass filter, and its central wavelength (CWL) should be selected in the range of 450 to 650 nm.

[0035] The timing resolution of the photomultiplier tube 8 is lower than 300 ps, ​​and the detection wavelength range is between 200 nm and 1200 nm.

[0036] Among them, the minimum time resolution of the time-correlated single photon counting and picosecond timer 9 should be less than 100ps, and the dead time width should be less than 100ns.

[0037] The method for obtaining the spray temperature through the fluorescence signal attenuation curve of the present invention specifically comprises the following steps:

[0038] (1) The theoretical value of fluorescence intensity decay should satisfy I(t) = I 0 exp(-t / τ), where I 0 is the initial strength;

[0039] (2) The basic principle of fluorescence lifetime calculation method should comply with:

[0040]

[0041] That is, the fluorescence lifetime can be determined by obtaining the slope value of the fluorescence decay curve;

[0042] (3) Fluorescence decay curves can be used to count photon events using photomultiplier tubes, time-correlated single photon counting, and picosecond timers;

[0043] (4) The fluorescence decay curve obtained by experimental measurement needs to be fitted to obtain the slope;

[0044] (5) The specific steps of the calibration experiment are as follows: First, a solution containing a specific concentration such as Kiton red 640, Rhodamine 6G, Rhodamine B, etc. is heated to a fixed temperature by a constant temperature heating device, and the above fluorescence lifetime is obtained for dye solutions at different fixed temperatures. Finally, a calibration curve between fluorescence lifetime and temperature is established. The data shows that the fluorescence lifetime is strongly linearly correlated with the temperature, and the calibration curve of the spray measurement can be fitted.

[0045] The specific examples are described as follows:

[0046] The preparation method of the aqueous solution containing dye is as follows (taking Kiton Red 620 as an example): the molar content of the aqueous solution containing dye is 10 -4 mol / L、10 -5 mol / L、10 -6 mol / L dye solution, the boiling point, evaporation rate and other thermal properties of the dye solution with the above ratio are almost the same as those of deionized water.

[0047] The basic light source device 1 generates a femtosecond laser beam with a repetition rate of 80MHz, a wavelength of 980nm, a pulse width of 140fs, and an energy of 1W, which is converted into a 490nm laser beam by the frequency doubling device 2 (doubled). The light beam is focused on a completely atomized area 10mm directly below the spray nozzle 4 by using a microscope objective 3 with a focal length of f=150mm. At this time, the droplets of the dye aqueous solution containing Kiton Red 620 are stimulated to generate fluorescence, and the fluorescence signal is collected by the achromatic dichroic mirror 5 and transmitted to the photomultiplier tube 8 almost losslessly through the optical fiber 6. Before entering the photomultiplier tube 8, it needs to be initially filtered by the filter 7 (CWL=640nm) so that the fluorescence signal in the temperature-sensitive band is photoelectrically converted. Finally, the fluorescence intensity decay curve with a time resolution of 32ps is measured by high-resolution collection of time-correlated single photon counting and picosecond timer 9. Its decay curve is fitted and analyzed by MATLAB mathematical tools (such as Figure 2 As shown in the figure), determine the exact slope of the height match, and then obtain the following Figure 3 The temperature-fluorescence lifetime calibration curve is shown.

[0048] By referring to the calibration curve of temperature-fluorescence lifetime, the droplet temperature can be directly determined by obtaining the fluorescence decay curve. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A spray droplet temperature measurement system based on fluorescence lifetime measurement, characterized in that: The invention comprises a basic light source device, a frequency doubling device, an objective lens, an achromatic dichroic mirror, an optical fiber, a filter, a photomultiplier tube, a time-correlated single photon counting and a picosecond timer. The laser generated by the basic light source device is converted into an excitation wavelength suitable for fluorescence of a solution to be tested by the frequency doubling device, and then the laser is focused on a spray area to be tested by the objective lens. The laser-induced fluorescence signal is collected by the achromatic dichroic mirror, and then the fluorescence is introduced into the photomultiplier tube by the optical fiber. The optimal collection of the fluorescence signal is achieved by the filter. Finally, the fluorescence signal attenuation curve is obtained by the time-correlated single photon counting and the picosecond timer. A quantitative relationship between the fluorescence lifetime and the temperature is established by a calibration method, and finally a direct measurement value of the spray temperature can be obtained by the calibration relationship.

2. A spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The basic light source equipment is a femtosecond titanium sapphire laser, a picosecond dye laser or a coaxial flash lamp. The laser wavelength generated by the basic light source equipment is in the range of 200nm to 1200nm, and the pulse width is not less than 500ps.

3. A spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The frequency doubling device contains a frequency doubling crystal or a frequency tripling crystal.

4. The spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The focal length of the objective lens is greater than the spray radius at the height.

5. The spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The focal length of the achromatic dichroic mirror is larger than the spray radius at the height, and is placed at a 90° angle to the laser plane.

6. The spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The filter is a bandpass filter, and its central wavelength selection interval is 450-650nm.

7. The spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The timing resolution of the photomultiplier tube is lower than 300 ps, ​​and the detection wavelength range is between 200 nm and 1200 nm.

8. The spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The time-correlated single photon counting and picosecond timer have a minimum time resolution of less than 100 ps, ​​making the time width less than 100 ns.

9. The spray droplet temperature measurement system based on fluorescence lifetime measurement according to claim 1, characterized in that: The method for obtaining the spray temperature through the fluorescence signal attenuation curve specifically comprises the following steps: (1) The theoretical value of fluorescence intensity decay should satisfy I(t) = I0exp(-t / τ), where I0 is the initial intensity; (2) The basic principle of the fluorescence lifetime calculation method is consistent with: That is, the fluorescence lifetime can be determined by obtaining the slope value of the fluorescence signal decay curve; (3) Fluorescence signal decay curve: photon events are counted by photomultiplier tube, time-correlated single photon counting and picosecond timer; (4) The fluorescence signal decay curve obtained by experimental measurement needs to be fitted to obtain the slope; (5) The specific steps of the calibration experiment are as follows: First, a solution containing a specific concentration of Kiton red 640, Rhodamine 6G or Rhodamine B is heated to a fixed temperature by a constant temperature heating device, and the above-mentioned fluorescence lifetime is obtained for dye solutions at different fixed temperatures. Finally, a calibration curve between fluorescence lifetime and temperature is established. The data show that the fluorescence lifetime is strongly linearly correlated with the temperature, and the calibration curve of the spray measurement can be fitted.

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