Temperature Measurement Of Stored Biomass Using Acoustic Method

As a type of renewable energy,biomass fuels are being widely adopted.Common biomass fuels are usually stored in a silo.During the storage of biomass fuels,there is a potential self-ignition risk.If not detected in time it will develop into a fire.Therefore,the internal temperature of stored biomass needs to be monitored.Existing contact temperature measurement techniques such as thermocouples are easy to be moved away,bent or even broken due to the pressure of surrounding biomass.This makes them unable to provide a reliable indication of the possible self-ignition location.Non-contact techniques such as infrared thermal imaging can only provide the surface temperature.Therefore,a new non-intrusive method needs to be invesitagted to measure the temperature of stored biomass.Acoustic technique has the advantage of non-intrusive measurement and can be used to acquire the temperature distribution image when combined with tomograp+hy technique.Existing research is mainly focused on the temperature measurement of air,flame and lake water.Compared with these media,the sound path is curved in stored biomass and there is a dispersion phenomenon in sound speed which makes it difficult to measure the sound speed.However,there is limited researches in this area.As low-frequency sound waves can propagate through the air gaps in stored biomass,the main work in this thesis focuses on the temperature measurement of stored biomass using low-frequency sound waves.The main work and innovation are as follows:(1)In order to consider the effect of air temperature,humidity,pressure and constituent concentrations on sound speed,a method of determining the sound speed through the calculation of air density is proposed.A theoretical analysis is conducted to investigate the effect of the humidity,pressure and constituent concentrations of air on temperature measurement results and corresponding solutions are provided.(2)In order to use low-frequency sound waves to measure the temperature of stored biomass,the feasibility of air temperature measurement using low-frequency sound waves needs to be validated.In the existing methods the distance between two acoustic sensors is used as the length of sound path and the systematic delay difference is negotiated which makes the accuacy of temperature measurement still need to be improved.Therefore,an air temperature measurement method using low-frequency sound waves is proposed.Time of flight(TOF)of sound waves under different air temperatures are acquired through reference temperature experiments and then substituted into the calibration equation.Then the equivalent length of sound path and systematic delay difference are estimated using the least squares method,which increases the accuracy of measured sound speed.Temperature measurements results indicate that the proposed method can be used to measure the air temperature accurately.(3)A temperature measurement method for stored biomass using low-frequency sound waves and cross-correlation technique is proposed.After analyzing the sound transmission theory in the porous medium,the model between sound speed in free space but under the same temperature with the biomass on sound path and the TOF of sound waves between two acoustic sensors is established.A characteristic factor is defined to calculate the sound speed in free space from measured TOF of sound waves.Initial value of the characteristic factor is calibrated experimentally at room temperature.This solves the problem of sound speed measurement caused by the tortuosity in sound path and frequency dispersion in sound speed.After analysing the influence of biomass temperature on characteristic factor,a cyclic updating process on characteristic factor is proposed.The characteristic factor needs to be updated based on the experimental relationship between characteristic factor and biomass temperature during temperature measurements.Temperature measurement results indicate that the proposed method is effective for the non-intrusive temperature measurement of stored biomass.(4)In order to select the suitable sound source signal,four common types of sound source including the chirp,white noise,maximum length sequence and exponentially decaying sinusoidal pulse are compared.Moreover,the type,frequency band and length of the chirp signal are also determined comparatively.Experimental results show that the TOF of sound waves in stored biomass can be measured by using a 0.1 s linear chirp signal of the frequency from 200 to 500 Hz as sound source combined with cross-correlation method.To ensure the signal-noise-ratio of sound signals received by acoustic sensors,the characteristic of sound absorption in stored biomass is investigated experimentally and the power requirement of sound source for different sensor distances are calculated.(5)Defined characteristic factor needs to be updated based on its experimental relationship with biomass temperature during temperature measurement.When biomass properties such as the type,particle size,density and bulk density changes,whether the relationship between the characterisitc factor and biomass temperature needs to re-calibrated through experiments needs investigation.Therefore the empirical equation proposed by Miki is used to model the relationship between the characteristic factor and biomass properties.Then it is used to analyse the influence of biomass properties on the relationship between the characteristic factor and biomass temperature.An acoustic temperature measurement system is constructed to measure the temperature of four common types of stored biomass,wood blocks,wood pellets,wood chips and wheat straws.The effect of bulk density of the four types of biomass on the relationship between the characteristic factor and biomass temperature is also investiagted experimentally.Experimental results show the acoustic system is effective for the temperature measurement of common types of stored biomass.For wheat straws,the experimental relationship between the characteristic factor and biomass temperature can be fitted with a linear equation.The gradient of the linear polynomial is the updating coefficient of characteristic factor on biomass temperature.The updating coefficient of characteristic factor has a linear relationship with the bulk density of wheat straws.This linear relationship can be used to derive the updating coefficient on characteristic factor for the acoustic temperature measurement,which can help reduce the necessity of the experimental work.