DEVELOPMENT OF AN INSTRUMENT FOR MEASUREMENT OF THERMAL CONDUCTIVITY AND THERMAL DIFFUSIVITY OF TROPICAL FRUIT JUICE

Abstract

This research work is on the development of an instrument for measuring the thermal conductivity and thermal diffusivity of tropical fruit juice using a transient thermal probe technique based on line heat principle. The system consists of two thermocouple sensors, constant current source, heater, thermocouple amplifier, microcontroller, microSD card shield and liquid crystal display. A fixed distance of 6.50 mm was maintained between the two probes. When heat is applied, the temperature rise at the heater probe was measured at time interval of 4s for 240s. The measuring element conforms as closely as possible to an infinite line source of heat in an infinite fluid. Under these conditions, thermal conductivity and thermal diffusivity was simultaneously measured, with thermal conductivity determined from the slope of a plot of the temperature rise of the heating element against the logarithm of time while thermal diffusivity was determined from the time it took the sample to attain a peak temperature and the time duration over a fixed diffusivity distance. A constant current source was designed to apply a power input of 16.33𝑊/𝑚 to the probe throughout the experiment. The thermal probe was interfaced with a digital display and data logger by using an application program written in C++. Calibration of the instrument was done by determining the thermal properties of distilled water. Error due to convection was avoided by adding 1.5% agar to the water. The instrument has been used to investigate the thermophysical properties of some tropical fruit juice samples namely; banana, orange and water melon. Thermal conductivity values of 0.593, 0.598, 0.586 𝑊/𝑚𝑜𝐶 and thermal diffusivity values of 1.053 ×10−7, 1.086 ×10−7, and 0.959 ×10−7𝑚/𝑠2 were obtained for banana, orange and water melon samples respectively. Measured values were stored in a microSD card. The instrument performed very well as it measured the thermal conductivity and thermal diffusivity of the tropical fruit juice samples with statistical analysis (ANOVA) showing no significant difference (𝑝>0.05) between the literature standards and estimated averages of each sample investigated with the developed instrument.