A method and system for measuring local convection heat transfer coefficient

Abstract

The present invention belongs to the technical field related to thermal property measurement, and discloses a method and system for measuring the local convective heat transfer coefficient. The method includes: using a transparent low-thermal-conductivity material with a metal film coated on the surface as a wall for convective heat transfer with a fluid; using a pump laser modulated by a square wave function to pass through the transparent low-thermal-conductivity material to heat the metal film, and simultaneously using a detection light to detect the metal film to obtain a detection signal, and processing the signal to obtain an experimental measurement temperature signal; establishing a bidirectional heat diffusion equation based on a double-layer structure, and using the convective heat transfer on the metal film surface as a boundary condition to solve and obtain a simulated temperature signal; with the goal of minimizing the deviation between the experimental measurement temperature signal and the simulated temperature signal, using the simulated temperature signal to fit the experimental measurement temperature signal to obtain the convective heat transfer coefficient to be measured. This application can accurately measure a temperature greater than 100W / (m 2 The local convection heat transfer coefficient in the range of 0.1K is measured with a spatial resolution of less than 0.1mm and a measurement error of less than 10%.

Description

Technical Field

[0001] The present invention belongs to the technical field related to thermal property measurement, and more specifically, relates to a method and system for measuring a local convection heat transfer coefficient. Background Art

[0002] Thermophysical property measurements are crucial in engineering, particularly when designing and optimizing heat exchangers, cooling systems, and thermal management devices. Accurately measuring the local convective heat transfer coefficient helps engineers adjust designs to ensure optimal thermal performance and energy efficiency. Furthermore, accurate local convective heat transfer coefficient data is crucial for validating computational models. A thorough understanding and precise measurement of this coefficient are essential for efficient and reliable heat transfer processes across a wide range of engineering applications.

[0003] Currently, the most commonly used method to determine the local convective heat transfer coefficient is the direct method. By applying a known amount of heat to the plate and measuring the local wall temperature (T s ) and the fluid inflow temperature (T ∞ ) is achieved. By assuming that the local heat flux density (q") on the plate is uniform, the local convection heat transfer coefficient can be directly calculated using Newton's law of cooling, that is, h = q" / (T s -T ∞ However, direct methods for measuring the local convective heat transfer coefficient can be affected by numerous factors, resulting in significant measurement errors. These factors include: errors in the local heat flux estimation due to heat loss, heat conduction along the wall due to uneven wall temperatures, disturbances in the original flow field caused by the insertion of temperature sensors, and temperature-dependent changes in the fluid's thermal properties due to high temperature gradients in the thermal boundary layer. These factors can also cause the measured local convective heat transfer coefficient to depend on the applied heat flux rather than the intrinsic value.

[0004] Another approach to determining the local convective heat transfer coefficient is to employ a mass transfer analogy, measuring the convective mass transfer coefficient to obtain the local convective heat transfer coefficient. However, a major challenge with this approach is the difficulty in ensuring consistent boundary conditions. For example, in mass transfer problems, the transfer of mass induces a normal velocity component near the wall, which contradicts the boundary condition of zero wall velocity in convective heat transfer. To more accurately measure the local convective heat transfer coefficient, it is necessary to explore more advanced measurement techniques to overcome the inherent limitations of traditional methods. Summary of the Invention

[0005] In view of the above defects or improvement needs of the prior art, the present invention provides a method and system for measuring the local convection heat transfer coefficient. The present invention can accurately measure the local convection heat transfer coefficient greater than 100W / (m 2 The local convection heat transfer coefficient in the range of 0.1K is measured with a spatial resolution of less than 0.1mm and a measurement error of less than 10%.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present invention, a method for measuring the local convective heat transfer coefficient is provided, including: S1: using a transparent low-thermal conductivity material with a metal film coated on the surface as a wall surface for convective heat transfer with the fluid, the fluid is in direct contact with the metal film, and then the fluid flows through the surface of the metal film to generate convective heat transfer; S2: using a pump laser modulated by a square wave function to pass through the transparent low-thermal conductivity material to heat the metal film, and at the same time using a detection light to detect the metal film to obtain a detection signal, and processing the detection signal to obtain an experimental measurement temperature signal; S3: establishing a bidirectional heat diffusion equation based on the double-layer structure of the wall, and using the convective heat transfer on the metal film surface as the boundary condition to solve and obtain a simulated temperature signal; S4: with the goal of minimizing the deviation between the experimental measurement temperature signal and the simulated temperature signal, using the simulated temperature signal to fit the experimental measurement temperature signal to obtain the convective heat transfer coefficient to be measured.

[0007] Preferably, the modulation frequency of the square wave function is lower than 10 Hz, and the focused spot of the pump laser and the probe light is 1 / e 2 The radius is greater than 20μm.

[0008] Preferably, the thickness of the metal film is 70-100 nm; the extinction coefficient of the transparent low thermal conductivity material at the laser wavelength is 0, and its thermal conductivity is not greater than 1 W / (m·K).

[0009] Preferably, when the material of the metal film is aluminum, the wavelength of the pump laser is 400-900 nm, and the wavelength of the detection light is 770-810 nm; when the material of the metal film is gold, the wavelength of the pump laser is 400-600 nm, and the wavelength of the detection light is 488 or 532 nm.

[0010] Preferably, step S3 is specifically as follows: S31: solving the bidirectional heat diffusion equation based on the wall double-layer structure in the frequency domain, and obtaining the Green's function of the temperature response under the boundary conditions of convective heat transfer on the metal film surface; S32: applying a preset heat flux density at the middle interface of the double-layer structure, and multiplying the preset heat flux density by the Green's function to obtain the expression of the metal film temperature in the frequency domain; S33: performing Fourier and Hankel inverse transforms on the expression of the metal film temperature in the frequency domain, and further using a detection light with a radius of σ1 to perform weighted average detection on the metal film temperature to obtain a heat transfer model, and solving the heat transfer model to obtain a simulated temperature signal in the time domain.

[0011] Preferably, the heat transfer model is:

[0012]

[0013] Where ΔT(t) is the temperature signal of the metal film changing with time t, A0 is the pump light power absorbed by the heat absorption layer of the metal film, and A1 is the detection light power projected onto the surface of the metal film. is the thermal reflection coefficient of the metal film surface, R is the reflectivity of the metal film surface, is the temperature-response Green’s function, ρ and ω are frequency-domain variables, corresponding to the spatial variable r and the time variable t in the time domain, respectively; ω0 = 2πf0, f0 is the square-wave modulation frequency of the pump laser, and σ is the 1 / e-square of the pump laser and the probe light. 2 The root mean square value of the spot radius, is the imaginary unit, Re{z} is the real part of the complex number z, n = 1, 2, 3, ...;

[0014] Among them, Green's function for:

[0015]

[0016] in, k and C are the thermal conductivity and specific heat capacity of the material, respectively. Subscripts 1 and 2 correspond to the metal film and the transparent low thermal conductivity material, respectively. L is the thickness of the metal film, G is the interface thermal conductivity between the metal film and the transparent low thermal conductivity material, and h is the thickness of the metal film. c is the convective heat transfer coefficient, which is to be measured.

[0017] Preferably, step S3 further comprises performing a sensitivity analysis on the heat transfer model to evaluate the sensitivity of the simulated temperature signal to the convective heat transfer coefficient to be measured.

[0018] Preferably, the relative size of the absolute value of the sensitivity coefficient and the preset threshold is used to judge the sensitivity of the simulated temperature signal to the convective heat transfer coefficient to be measured. The sensitivity coefficient S α The expression is:

[0019]

[0020] Among them, A norm is the simulated temperature signal, and α is the convective heat transfer coefficient to be measured.

[0021] Preferably, step S2 processes the detection signal specifically as follows: using a digital-to-analog converter to collect the detection signal, and inputting it into a parallel computing graphics processing unit for processing to obtain an experimental measurement temperature signal with a high signal-to-noise ratio, and the sampling frequency of the digital-to-analog converter is greater than 1GS / s.

[0022] The second aspect of the present application provides a measurement system for a local convective heat transfer coefficient, including: a construction module: used to use a transparent low-thermal conductivity material coated with a metal film on the surface as a wall surface for convective heat transfer with a fluid, the fluid is in direct contact with the metal film, and then the fluid flows through the surface of the metal film to generate convective heat transfer; an experimental measurement temperature signal acquisition module: used to use a pump laser modulated by a square wave function to pass through the transparent low-thermal conductivity material to heat the metal film, and at the same time use a detection light to detect the metal film to obtain a detection signal, and process the detection signal to obtain an experimental measurement temperature signal; a simulation temperature signal acquisition module: used to establish a bidirectional heat diffusion equation based on the double-layer structure of the wall, and use the convective heat transfer on the metal film surface as a boundary condition to solve and obtain a simulation temperature signal; a convective heat transfer coefficient acquisition module: used to fit the experimental measurement temperature signal with the simulation temperature signal with the goal of minimizing the deviation between the experimental measurement temperature signal and the simulation temperature signal to obtain the convective heat transfer coefficient to be measured.

[0023] In general, compared with the prior art, the above technical solution conceived by the present invention provides a method and system for measuring the local convective heat transfer coefficient, which has the following beneficial effects:

[0024] 1. By applying a small amount of heat flux disturbance locally on the wall surface through pump laser heating, the convective heat transfer coefficient can be measured. This will not cause a large temperature gradient in the flow field, thereby ensuring that the fluid thermal properties are constant, making the measurement result an eigenvalue and not dependent on the applied heat flux. The detection laser is used to measure the local temperature of the metal film based on the principle of thermal reflection. This method is non-contact and has no interference with the flow field, so the measurement result is an eigenvalue and is not easily affected by the probe like conventional methods. In addition, the position of laser detection is flexible and changeable, and local measurement can be performed at any position.

[0025] 2. In this application, the modulation frequency of the square wave function is not greater than 10 Hz, and the focused spot of the pump laser and the probe light is 1 / e 2 The radius is greater than 20 μm, the thickness of the metal film is 70-100 nm, the extinction coefficient of the transparent low thermal conductivity material at the laser wavelength is 0, and its thermal conductivity is not greater than 1 W / (m·K). These settings enable the technology of the present invention to accurately measure more than 100 W / (m 2 The local convection heat transfer coefficient in the range of 0.1K is measured with an error of less than 10% and a spatial resolution of less than 0.1mm.

[0026] 3. The heat transfer model of the present application effectively considers the three-dimensional heat conduction process in the wall, without the need to make a one-dimensional assumption on heat transfer as in conventional methods, so the measurement results are more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the steps of the method for measuring the local convection heat transfer coefficient in this application;

[0028] Figure 2 It is a device for realizing the method for measuring the local convection heat transfer coefficient in the embodiment of the present application;

[0029] Figure 3 The measurement signal and analysis of the local convective heat transfer coefficient in air impingement convective heat transfer implemented in this application, wherein (a) is the measured signal within one cycle and the best-fit simulation signal, (b) is the logarithmic representation of the cooling section of the temperature signal, and (c) is a schematic diagram of the sensitivity coefficient of the temperature signal to all parameters in the heat transfer model as a function of normalized time;

[0030] Figure 4 The present invention is based on the measurement results of the local convection heat transfer coefficient at different positions away from the impingement center in the air impingement convection heat transfer, and the comparison with the empirical correlation formula in the literature. DETAILED DESCRIPTION

[0031] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0032] The first aspect of the present application provides a method for measuring the local convective heat transfer coefficient, such as Figure 1 As shown, the method includes the following steps S1 to S4.

[0033] S1: A transparent low thermal conductivity material coated with a metal film is used as the wall surface for convective heat exchange with the fluid. The fluid is in direct contact with the metal film, and then the fluid flows through the surface of the metal film to generate convective heat exchange.

[0034] In this embodiment, the transparent low-thermal-conductivity material should meet the following requirements: 1) its absorptivity for the pump laser and probe light should be close to zero to ensure that the laser can penetrate the substrate material; 2) its surface roughness should be controlled below 20 nm to allow the probe laser to be specularly reflected at the interface between the transparent low-thermal-conductivity material and the metal film, facilitating thermal reflection measurement; and 3) its thermal conductivity should be no greater than 1 W / (m·K) to enhance the sensitivity of the measurement signal to the measured convective heat transfer coefficient, thereby improving measurement accuracy. Furthermore, the transparent low-thermal-conductivity material is preferably an optical-grade acrylic sheet (PMMA) or a glass slide.

[0035] The thickness of the metal film is preferably 70-100nm. The metal film serves as both a heat absorption layer and a temperature sensing layer. On the one hand, it absorbs the pump laser to achieve the preset heat flux density boundary condition; on the other hand, because the metal film has a high thermal reflectivity at the wavelength of the probe light, the temperature change of the metal film can be obtained by measuring the change in its reflectivity, thus achieving temperature sensing. Therefore, when selecting the metal film material, it is necessary to consider the wavelengths of the pump laser and the probe light, ensuring a high absorption rate at the wavelength of the pump laser and a high thermal reflectivity at the wavelength of the probe light. Specifically, when the metal film is made of aluminum, the wavelength of the pump laser is 400-900nm, and the wavelength of the probe light is 770-810nm; when the metal film is made of gold, the wavelength of the pump laser is 400-600nm, and the wavelength of the probe light is 488 or 532nm.

[0036] S2: using a pump laser modulated by a square wave function to pass through the transparent low thermal conductivity material to heat the metal film, and simultaneously using a detection light to detect the metal film to obtain a detection signal, and processing the detection signal to obtain an experimental measurement temperature signal.

[0037] A pump laser modulated by a square wave function is passed through a transparent low-thermal-conductivity material and focused at the interface between the material and the metal film, thereby periodically heating the measurement system. Simultaneously, a probe beam is also passed through the transparent low-thermal-conductivity material and focused at the heated location to detect the temperature response and obtain a detection signal.

[0038] Specifically, a signal generator generates a square wave function with a preset frequency and a 50% duty cycle. This square wave function is then transmitted to the pump laser to modulate the pump laser. The modulated pump laser becomes a heating laser, which is reflected by a dichroic mirror and enters the objective lens. It then passes through a transparent low-thermal-conductivity material and focuses on the interface between the transparent low-thermal-conductivity material and the metal film, periodically heating the metal film. Simultaneously, another laser beam with a different wavelength passes through the dichroic mirror, enters the same objective lens, and focuses on the interface between the transparent low-thermal-conductivity material and the metal film, detecting the temperature of the metal film. According to the principle of thermal reflection, the reflectivity of the metal film is linearly related to the temperature when the temperature change does not exceed 10K. Therefore, the detection light reflected from the metal film carries information about the temperature change of the metal film and is received by a photodetector. The photodetector converts the optical signal into an electrical signal, generating a detection signal.

[0039] The detection signal is collected by a digital-to-analog converter and input into a parallel computing graphics processing unit for processing to obtain an experimental measurement temperature signal with a high signal-to-noise ratio. The sampling frequency of the digital-to-analog converter is greater than 1GS / s.

[0040] S3: A bidirectional heat diffusion equation is established based on the double-layer structure of the wall, and the convective heat transfer on the metal film surface is used as the boundary condition to obtain the simulated temperature signal.

[0041] Step S3 specifically includes the following steps S31 to S33.

[0042] S31: Solve the bidirectional heat diffusion equation based on the double-layer wall structure in the frequency domain, and obtain the Green's function of the temperature response under the boundary conditions of convective heat transfer on the metal film surface.

[0043] Specifically, the expression of the Green's function of temperature response is:

[0044]

[0045] Where Q0 is the applied heat flux, Θ0 is the detected temperature response, k and C are the thermal conductivity and specific heat capacity of the material, respectively. Subscripts 1 and 2 represent the metal film and transparent substrate, respectively. L represents the thickness of the metal film. G is the interface thermal conductivity between the metal film and the substrate. h c is the convective heat transfer coefficient, which is to be measured.

[0046] S32: Apply a preset heat flux density at the middle interface of the double-layer structure to obtain an expression for the metal film temperature in the frequency domain.

[0047] The preset heat flux density is preferably:

[0048]

[0049] Where q0 is the preset heat flux applied at the interface, which is a function of the spatial coordinate r and time t; A0 is the average power of the pump laser absorbed by the metal film surface, and σ0 is 1 / e of the pump laser spot. 2 radius, ω0 = 2πf0, f0 is the square wave modulation frequency of the pump light, and n = 1, 2, 3… is an infinite series.

[0050] Based on the preset heat flux density expression, Fourier transform and Hankel transform are performed to obtain the heat flux density expression in the frequency domain:

[0051]

[0052] The expression of metal film temperature in the frequency domain is:

[0053] S33: Perform Fourier and Hankel inverse transforms on the expression of the metal film temperature in the frequency domain, further use detection light with a radius of σ1 to perform weighted average detection on the metal film temperature, obtain a heat transfer model, and solve the heat transfer model to obtain a simulated temperature signal in the time domain.

[0054] Specifically, the heat transfer model is:

[0055]

[0056] Where ΔT(t) is the temperature signal of the metal film changing with time t, A0 is the pump light power absorbed by the heat absorption layer of the metal film, and A1 is the detection light power projected onto the surface of the metal film. is the thermal reflection coefficient of the metal film surface, R is the reflectivity of the metal film surface, is the temperature-response Green’s function, ρ and ω are frequency-domain variables, corresponding to the spatial variable r and the time variable t in the time domain, respectively; ω0 = 2πf0, f0 is the square-wave modulation frequency of the pump laser, and σ is the 1 / e-square of the pump laser and the probe light. 2 The root mean square value of the spot radius, is the imaginary unit, Re{z} is the real part of the complex number z, n=1,2,3….

[0057] The heat transfer model of this application fully considers the conduction of heat in transparent low-thermal conductivity materials and the convective heat dissipation on the surface of the metal film. It can accurately measure the convective heat transfer coefficient on the surface of the metal film without making the assumption of one-dimensional heat transfer as in traditional convective heat transfer measurement technology.

[0058] S4: With the goal of minimizing the deviation between the experimentally measured temperature signal and the simulated temperature signal, the simulated temperature signal is used to fit the experimentally measured temperature signal to obtain the convective heat transfer coefficient to be measured.

[0059] The heat transfer model can be analyzed by numerical methods and programmed in assembly language to quickly and accurately obtain the simulated temperature signal. The simulated temperature signal is then normalized to obtain the normalized temperature signal A norm . norm The measured temperature signal is compared with the experimental temperature signal, and the best fitting is performed on the experimental temperature signal with the goal of minimizing the deviation to obtain the convective heat transfer coefficient to be measured.

[0060] In a further preferred embodiment, step S4 further includes performing a sensitivity analysis on the heat transfer model to evaluate the sensitivity of the simulated temperature signal to the convective heat transfer coefficient to be measured. Specifically, the sensitivity of the simulated temperature signal to the convective heat transfer coefficient to be measured is determined by comparing the absolute value of the sensitivity coefficient with the preset threshold. α The expression is:

[0061]

[0062] Among them, A normTo simulate the temperature signal, α is the convective heat transfer coefficient to be measured, and can also be other input parameters, thereby measuring the sensitivity of other input parameters.

[0063] Generally speaking, the sensitivity coefficient S α If the absolute value of is greater than 0.2, the signal is considered highly sensitive, and if it is less than 0.05, the signal is considered insensitive to the parameter α. In a further preferred embodiment, step S2 further includes optimizing the spot size and modulation frequency of the pump laser based on the sensitivity coefficient, so that the sensitivity of the measurement signal to the measured convective heat transfer coefficient is greater than 0.05, while reducing the sensitivity of the input parameters.

[0064] In a further preferred embodiment, the modulation frequency of the square wave function is not greater than 10 Hz, and the focused spot of the pump laser and the probe light is 1 / e 2 The radius is greater than 20μm. It is worth emphasizing that selecting a laser modulation frequency below 10Hz and a spot radius greater than 20μm is the key to successfully measuring the convective heat transfer coefficient. Only with this configuration can the measurement signal be sufficiently sensitive to the convective heat transfer coefficient to be measured. In contrast, other similar optical-based thermophysical property measurement techniques such as time-domain thermoreflectometry (TDTR) and frequency-domain thermoreflectometry (FDTR) cannot be used to measure the convective heat transfer coefficient because they cannot achieve such low-frequency measurements.

[0065] The above method of this application can be adopted as follows Figure 2 The device described in the embodiment of the present invention is implemented as follows: a first laser 1 receives a square wave signal with a preset frequency and a 50% duty cycle output by a signal generator 2, thereby emitting a laser light modulated by the square wave function (referred to as a pump laser or heating light). The pump laser light is reflected by a dichroic mirror 3 and enters a microscope objective 4, where it is focused on the surface of the metal film, periodically heating the metal film. A second laser 5 emits a linearly polarized continuous wave laser light of a different wavelength (referred to as a probe light). The laser light is adjusted in its linear polarization direction by a half-wave plate 6, then passes through a polarization beam splitter 7, then sequentially passes through a quarter-wave plate 8 and a dichroic mirror 3, enters the same microscope objective 4, passes through a transparent substrate, and is focused on the interface between a transparent low-thermal-conductivity material and the metal film, where the temperature response at the interface is detected. The response light reflected from the interface is received by a photodetector 9. The photodetector 9 averages the received light signals to obtain a thermal response signal with a high signal-to-noise ratio for one heating cycle. The signal is normalized, including normalization of the signal amplitude and time axis, to obtain a final experimental measurement temperature signal, and the signal is optimally fitted to obtain the convective heat transfer coefficient of the metal film surface to be measured.

[0066] The following is a specific example to illustrate the ability of the present method to measure the local convection heat transfer coefficient. The measurement results are as follows: Figure 3 and Figure 4 shown.

[0067] This example measures the local convective heat transfer coefficient of an air impingement jet. The wall material used is optical-grade acrylic (PMMA). A 100 nm thick aluminum film is deposited on the surface using magnetron sputtering technology, serving as a heat-absorbing and temperature-sensing layer. A step profiler measures the actual thickness of the aluminum film to be 93 nm. The thermal conductivity of the aluminum film, determined from its resistivity according to the Widmann-Franz law, is 45 W / (m·K), which can be measured using the van der Pauw method.

[0068] Compressed air was ejected at a flow rate of Q = 5 L / min from a micronozzle with a diameter of D = 2.0 mm, flowing perpendicularly toward the metal film surface at a distance of H = 15 mm from the nozzle. The intensity of convective heat transfer varies at different locations on the metal film surface. By adjusting the relative position of the laser focus and the center of the impinging jet using a translation stage, the local convective heat transfer coefficient at different locations on the metal film surface can be measured. A 29 μm spot radius and a 1 Hz modulation frequency were used for the measurements.

[0069] Figure 3 The measurement signal and analysis are shown as an example at the center of the impinging jet area. Figure 3 Figure (a) shows the signal for a complete heating cycle on a linear scale, with the horizontal axis representing normalized time and the vertical axis representing the normalized temperature rise. The symbol represents the measured signal, and the curve represents the simulated signal generated by the best-fit heat transfer model. The cooling signal of this temperature response is particularly sensitive to the convective heat transfer coefficient on the metal film surface. Figure 3 (b) in Figure 1 shows the local signal from 0.5 to 0.54 in the normalized time period in the logarithmic coordinate format to examine the quality of the heat transfer model fitting the measured signal in more detail. The sensitivity coefficients of this signal to all parameters in the heat transfer system are shown in Figure 3 (c) According to the sensitivity curve, the signal is mainly affected by the spot radius σ and the thermal conductivity k of the PMMA substrate. PMMA and specific heat capacity C P,,A , and the convective heat transfer coefficient h on the metal film surface c Sensitive. Among them, h c The absolute value of the sensitivity coefficient is as high as 0.4, indicating that h can be accurately determined by fitting this set of signals. cAmong other sensitive parameters, the exact value of the laser spot radius σ can be measured by the knife-edge method, or calibrated by measuring a fused quartz standard sample without convective heat transfer using the same optical method and apparatus, with an estimated error of 2%. The thermal conductivity and specific heat capacity of the PMMA substrate material can be determined by measuring the PMMA substrate material without convective heat transfer using the same optical method and apparatus alone (i.e., all other conditions are the same, only the air flow rate is set to zero), with an estimated error of 7%. After determining these parameters, the local convective heat transfer coefficient at this position under this operating condition can be determined as h by optimally fitting this set of signals. c =700±50W / (m 2 ·K), with an estimated error of 7%.

[0070] Figure 4 The local convective heat transfer coefficients at different locations on the metal film surface relative to the center point are measured by adjusting the relative position of the laser focus point and the center point of the impact jet using a translation stage, and are compared with empirical correlations known in the literature.

[0071] like Figure 4 As shown in the convective heat transfer problem of impinging jets, the regions at different distances from the jet center can be divided into the acceleration stagnation region, the transition region, and the deceleration wall jet region. The literature can only find the average Nusselt number of the deceleration wall jet region. The empirical correlation formula is:

[0072]

[0073] in, is the average Nusselt number, is the average convective heat transfer coefficient from the center of the jet to the disk area with a radius of R, k f is the thermal conductivity of the fluid, r is the radius coordinate with the jet center as the zero point, D is the nozzle diameter, H is the distance from the nozzle to the wall, Re = 4Q / πDν is the Reynolds number, Q is the volume flow rate of the fluid, ν is the kinematic viscosity of the fluid, and Pr is the Prandtl number, which is also a thermophysical property of the fluid. The scope of application of this empirical correlation is:

[0074] 2,000≤Re≤400,000

[0075] 2≤H / D≤12

[0076] 2.5≤r / D≤7.5

[0077] According to the average Nusselt number Relationship with the local Nusselt number Nu The expression of the local Nusselt number Nu can be derived as

[0078]

[0079] The thermal conductivity of dry air at room temperature is obtained by looking up the database as k f =0.026W / (m·K), kinematic viscosity v=15×10 -6 m 2 / s, Prandtl number Pr = 0.703.

[0080] Based on the air flow rate, nozzle diameter, and kinematic viscosity of the air, the Reynolds number in this embodiment can be calculated to be Re = 3520. Substituting the Reynolds number, Prandtl number, and nozzle distance to aperture ratio H / D into the expression for the local Nusselt number, the local Nu number under this operating condition can be calculated, as follows: Figure 4 The solid curve shows that its effective range is 2.5 ≤ r / D ≤ 7.5. The predictions from this empirical correlation agree perfectly with the measured results of this example within the deceleration wall jet region (r / D ≥ 4), mutually verifying their accuracy. However, there is a lack of effective empirical correlations in the literature for the acceleration stagnation and transition regions.

[0081] The measurement results of this example reveal the complexity of heat transfer in an impinging jet convective heat transfer system. First, in the acceleration stagnation region, the local Nu number and local convective heat transfer intensity reach a peak. This is due to the extremely high local heat transfer efficiency caused by the high-speed impact and strong turbulence of the jet in this region. As the radial position r / D of the jet increases, the local Nu number first decreases. This is because the diffusion slows down due to the increased contact area between the jet and the surface, resulting in a decrease in turbulence and jet kinetic energy. Then, a second peak appears in the transition region. This may be due to the interaction between the jet and the surrounding fluid, such as the formation of vortices or the reattachment of the fluid to the surface, which causes a sudden increase in turbulence intensity. As the jet further diffuses outward, its momentum continues to weaken, eventually tending to a standard wall jet, at which point the heat transfer capacity continues to decrease with increasing r / D. A deep understanding of these thermal flow dynamics is crucial for optimizing impingement cooling systems because it allows engineers to predict the locations where cooling is most effective and design nozzle arrangements and operating conditions to maximize heat transfer at the desired locations.

[0082] A second aspect of the present application provides a local heat transfer coefficient measurement system, which includes a construction module, an experimental measurement temperature signal acquisition module, a simulation temperature signal acquisition module, and a convection heat transfer coefficient acquisition module. Specifically:

[0083] Building module: It is used to use a transparent low thermal conductivity material coated with a metal film as a wall surface for convective heat exchange with the fluid. The fluid is in direct contact with the metal film, and then the fluid flows through the metal film surface to generate convective heat exchange;

[0084] An experimental measurement temperature signal acquisition module is used to heat the metal film by passing a pump laser modulated by a square wave function through the transparent low thermal conductivity material, and simultaneously detect the metal film using a detection light to obtain a detection signal, and process the detection signal to obtain an experimental measurement temperature signal;

[0085] Simulated temperature signal acquisition module: used to establish a bidirectional heat diffusion equation based on the double-layer structure of the wall, and use the convection heat transfer on the metal film surface as the boundary condition to solve and obtain the simulated temperature signal;

[0086] Convective heat transfer coefficient acquisition module: used to fit the experimental measurement temperature signal with the simulated temperature signal with the goal of minimizing the deviation between the experimental measurement temperature signal and the simulated temperature signal to obtain the convective heat transfer coefficient to be measured.

[0087] It will be easily understood by those skilled in the art that the above description is merely 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 measuring local convection heat transfer coefficient, characterized in that: include: S1: A transparent low thermal conductivity material coated with a metal film is used as the wall surface for convective heat transfer with the fluid. The fluid is in direct contact with the metal film, and then the fluid flows over the surface of the metal film to generate convective heat transfer; S2: using a pump laser modulated by a square wave function to pass through the transparent low thermal conductivity material to heat the metal film, and simultaneously using a detection light to detect the metal film to obtain a detection signal, and processing the detection signal to obtain an experimental measurement temperature signal; S3: A bidirectional heat diffusion equation is established based on the double-layer structure of the wall, and the convection heat transfer on the metal film surface is used as the boundary condition to obtain the simulated temperature signal; S4: with the goal of minimizing the deviation between the experimentally measured temperature signal and the simulated temperature signal, fitting the experimentally measured temperature signal with the simulated temperature signal to obtain the convective heat transfer coefficient to be measured; Step S3 is specifically as follows: S31: Solve the bidirectional heat diffusion equation based on the double-layer wall structure in the frequency domain, and obtain the Green's function of the temperature response under the boundary conditions of convective heat transfer on the metal film surface; S32: applying a preset heat flux density at the middle interface of the double-layer structure, and multiplying the preset heat flux density by the Green's function to obtain an expression of the metal film temperature in the frequency domain; S33: Perform Fourier and Hankel inverse transforms on the expression of metal film temperature in the frequency domain, and further use the radius The detection light performs weighted average detection on the temperature of the metal film to obtain a heat transfer model, and the heat transfer model is solved to obtain a simulated temperature signal in the time domain; The heat transfer model is: in, The metal film changes with time t Changing temperature signal, is the pump light power absorbed by the metal film heat absorption layer, is the detection light power projected onto the metal film surface, is the thermal reflection coefficient of the metal film surface, R is the reflectivity of the metal film surface, is the temperature-responsive Green’s function, and are variables in the frequency domain, corresponding to spatial variables in the time domain and time variables ; , is the square wave modulation frequency of the pump laser, The pump laser and the probe light The root mean square value of the spot radius, is the imaginary unit, To take the real part of the complex number z, =1, 2, 3…; Among them, Green's function for: in, , and are the thermal conductivity and specific heat capacity of the material, respectively. Subscripts 1 and 2 correspond to metal film and transparent low thermal conductivity material, respectively. L is the metal film thickness, G is the interface thermal conductivity between the metal film and the transparent low thermal conductivity material, is the convective heat transfer coefficient, which is to be measured.

2. The method for measuring the local convection heat transfer coefficient according to claim 1, characterized in that: The modulation frequency of the square wave function is lower than 10 Hz, and the focused spots of the pump laser and the probe light are Radius greater than 20 .

3. The method for measuring the local convection heat transfer coefficient according to claim 1, characterized in that: The thickness of the metal film is 70-100 nm; the extinction coefficient of the transparent low thermal conductivity material at the laser wavelength is 0, and its thermal conductivity is not greater than 1 .

4. The method for measuring the local convection heat transfer coefficient according to claim 1, characterized in that: When the material of the metal film is aluminum, the wavelength of the pump laser is 400-900 nm, and the wavelength of the probe light is 770-810 nm; when the material of the metal film is gold, the wavelength of the pump laser is 400-600 nm, and the wavelength of the probe light is 488 or 532 nm.

5. The method for measuring the local convection heat transfer coefficient according to claim 1, characterized in that: Step S3 further includes performing a sensitivity analysis on the heat transfer model to evaluate the sensitivity of the simulated temperature signal to the convective heat transfer coefficient to be measured.

6. The method for measuring the local convection heat transfer coefficient according to claim 5, characterized in that: The relative size of the absolute value of the sensitivity coefficient and the preset threshold is used to judge the sensitivity of the simulated temperature signal to the convective heat transfer coefficient to be measured. The expression is: in, To simulate the temperature signal, is the convective heat transfer coefficient to be measured.

7. The method for measuring the local convection heat transfer coefficient according to claim 1, characterized in that: Step S2 processes the detection signal specifically as follows: The detection signal is collected by a digital-to-analog converter and input into a parallel computing graphics processing unit for processing to obtain an experimental measurement temperature signal with a high signal-to-noise ratio. The sampling frequency of the digital-to-analog converter is greater than 1 GS / s.

8. A system for measuring local convection heat transfer coefficient, characterized in that: include: Building module: It is used to use a transparent low thermal conductivity material coated with a metal film as a wall surface for convective heat exchange with the fluid. The fluid is in direct contact with the metal film, and then the fluid flows through the metal film surface to generate convective heat exchange; An experimental measurement temperature signal acquisition module is used to heat the metal film by passing a pump laser modulated by a square wave function through the transparent low thermal conductivity material, and simultaneously detect the metal film using a detection light to obtain a detection signal, and process the detection signal to obtain an experimental measurement temperature signal; Simulated temperature signal acquisition module: used to establish a bidirectional heat diffusion equation based on the double-layer structure of the wall, and use the convection heat transfer on the metal film surface as the boundary condition to solve and obtain the simulated temperature signal. Specifically: Solve the bidirectional heat diffusion equation based on the double-layer wall structure in the frequency domain, and obtain the Green's function of the temperature response under the boundary conditions of convective heat transfer on the metal film surface; Applying a preset heat flux density at the middle interface of the double-layer structure, and multiplying the preset heat flux density by the Green's function to obtain an expression of the metal film temperature in the frequency domain; The expression of the metal film temperature in the frequency domain is subjected to Fourier and Hankel inverse transforms, and further the radius is used to The detection light performs weighted average detection on the temperature of the metal film to obtain a heat transfer model, and the heat transfer model is solved to obtain a simulated temperature signal in the time domain; The heat transfer model is: in, The metal film changes with time t Changing temperature signal, is the pump light power absorbed by the metal film heat absorption layer, is the detection light power projected onto the metal film surface, is the thermal reflection coefficient of the metal film surface, R is the reflectivity of the metal film surface, is the temperature-responsive Green’s function, and are variables in the frequency domain, corresponding to spatial variables in the time domain and time variables ; , is the square wave modulation frequency of the pump laser, The pump laser and the probe light The root mean square value of the spot radius, is the imaginary unit, To take the real part of the complex number z, =1, 2, 3…; Among them, Green's function for: in, , and are the thermal conductivity and specific heat capacity of the material, respectively. Subscripts 1 and 2 correspond to metal film and transparent low thermal conductivity material, respectively. L is the metal film thickness, G is the interface thermal conductivity between the metal film and the transparent low thermal conductivity material, is the convective heat transfer coefficient, which is to be measured; Convective heat transfer coefficient acquisition module: used to fit the experimental measurement temperature signal with the simulated temperature signal with the goal of minimizing the deviation between the experimental measurement temperature signal and the simulated temperature signal to obtain the convective heat transfer coefficient to be measured.

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