Research On Measurement Method Of Time Constant Of High Speed Thermocouple Based On Laser Excitation

Transient temperature is an important parameter for thermodynamic analysis in combustion explosion,engine running state and friction characteristics of parts.At present,the response time of high-speed thermocouples has reached microsecond level,which can meet the needs of most transient temperature measurement scenarios.Although the high-speed thermocouple temperature measurement technology has been developed rapidly,the corresponding measurement and evaluation standards for the dynamic response performance of thermocouples are relatively backward.The industrial thermocouple dynamic response test method is not suitable for the measurement of response time in milliseconds or less.There are some problems with some developed measurement methods,which may affect the judgement of the actual temperature measurement results.The test method based on laser heating has the advantage of fast temperature excitation.However,the corresponding heat transfer mode does not conform to the theoretical model of time constant.It is not conducive to the evaluation and analysis of the dynamic performance of thermocouples.For this reason,a high-speed thermocouple dynamic performance test scheme based on the negative step response process of pulsed laser is presented.Theoretical analysis and experimental study on thermocouple dynamic temperature response mechanism,photothermal response model and its influencing factors are carried out.The main work and results of this thesis are as follows:(1)According to the requirement of high-speed thermocouple dynamic performance test.A high-speed thermocouple dynamic performance test scheme based on negative step response of pulsed laser is presented.The time constant measurem method based on the negative step response of pulsed laser can effectively solve the problems of the conventional methods,including slow temperature excitation,inconsistent heat transfer mode,incomplete response curve and so on.(2)Simulation of thermocouple dynamic temperature response model.Based on the calculation models of laser absorption rate and heat flow density of metal materials.The effects of laser modulation,model size,convection conditions and laser energy on the time constant measurements are analyzed.The results show that the heating effect of pulse modulated laser is better than that of step modulated laser.Time constant increases with model size.As the convection heat transfer coefficient increases,it decreases gradually.Laser power does not affect the time constants of each size model.(3)Software and Hardware Integration Design of High Speed Thermocouple Time Constant Test System.A test system consisting of a laser light source,a high-speed thermocouple and its cold-end compensation,a data acquisition circuit and an upper computer is set up.Based on the response signal amplitude and response speed of the Ktype thermocouple.Select six-bit semi-precision voltmeter to collect response signal and transfer it to the upper computer through USB.Thermoelectric potential temperature conversion,signal processing and time constant calculation are carried out in the upper computer.(4)Experimental test of time constant of high-speed thermocouple and analysis of influencing factors.The time constants of 0.05 mm K-type thermocouples under the same conditions are 73.78 ms and 41.34 ms,respectively.The experimental measurements of different laser parameters and different samples are carried out by using the pulsed laser excitation method.The experimental results and the influencing factors are analyzed,which are in accordance with the theoretical analysis and simulation.The theoretical and experimental studies show that the time constant measurement method proposed in this paper is based on the negative step response of the pulsed laser.It conforms to the theory of time constant measurement.The maximum temperature measurement speed of the high-speed thermocouple can be more reflected.The scheme has reference significance for the relevant research on dynamic performance test of highspeed response thermocouple.