Effects of processing parameters on the convective heat transfer rate during ultrasound assisted low temperature immersion treatment of a stationary sphere Original Research Article

The effect of ultrasound irradiation, as a novel method, on the enhancement of convective heat transfer between a stationary copper sphere and cooling medium was experimentally studied at different Re and Pr numbers. The high thermal conductivity of copper allowed the application of lumped system analysis which led to more accurate results on convective heat transfer. The ultrasonic cooling system included a refrigerated circulator, a flow meter, an ultrasound generator (25 kHz) and an ultrasonic bath. The studied parameters were sphere diameter (0.01 or 0.02 m), flow rate (1.67 × 10−5, 2.5 × 10−5 m3 s−1), fluid temperature (0, −5, −10, −15 and −20 °C) and ultrasound intensity (0, 190, 890 and 2800 W m−2), leading to Re range of 0.98–3.4 and Pr range of 68.3–188.9. The Nu number varied from 6.8 to 19 for non-irradiated samples. When ultrasound was irradiated (890 W m−2) the range of Nu number increased to 11–31. Enhancement factors from 30% to 119% were observed for the irradiated samples. The largest values of the enhancement factor were observed for low values of Re and Pr which demonstrated a better efficiency of ultrasound irradiation at higher viscosities and lower flow rates as a result of its mixing and cavitation effects. The results obtained in this study confirmed that ultrasound irradiation is able to enhance the convective heat transfer rate between the cooling medium and the submerged object. Irradiation effect was independent of sphere diameter and a linear relationship was detected between the ultrasound intensity and the Nu number. A correlation was suggested to predict the values of Nu at the presence or absence of ultrasound for different Re and Pr values with good agreement with the experimental data (R2 = 0.90). The obtained data for copper can be generalized for food products with similar geometries and can be used for process design, as the convective heat transfer is mostly controlled by the cooling medium.