Dimensionless analysis of the flow of spherical particles in two-phase flow in straight tubes

In the aseptic processing of particulate fluid foods, the residence time distribution of particles and the fluid-to-particle heat transfer are intimately related to the linear and rotational velocities of the particles. The development of models correlating these velocities with fluid and particle characteristics and with operating conditions is therefore very useful, as it provides a fast method to estimate fluid-to-particle heat transfer coefficients without measuring the actual particle velocities. The models can further be used to estimate the average residence time of the particles. Data for modelling particle linear and rotational velocities were obtained by videotaping the flow, along the wall, of individual spherical particles in transparent 2-m long straight tubes. Linear and rotational velocities were correlated with generalized Reynolds, Archimedes and Froude numbers, to the particle-to-fluid density simplex and to the particle/tube diameter ratio. The models obtained had correlation coefficients of 0.89 and 0.86 for the particle linear and rotational velocities, respectively

Modelling respiration rate of fresh fruits and vegetables for modified atmosphere packages: a review

Respiration rate and gas exchange through the package material are the processes involved in creating a modified atmosphere inside a package that will extend shelf life of fresh fruits and vegetables. Thus, modelling respiration rate of the selected produce is crucial to the design of a successful modified atmosphere packaging (MAP) system. In this paper, general aspects of the respiration process are presented. The major methods for measuring respiration rates, along with their advantages and limitations are discussed. Factors affecting the respiration rate and respiratory quotient are outlined, stressing the importance of temperature, O2 and CO2 concentrations, and storage time. Respiration rate models in the literature are also reviewed. 2002 Elsevier Science Ltd. All rights reserved

Dimensionless analysis of fluid-to-particle heat transfer coefficients

Average fluid-to-particle heat transfer coefficients were experimentally determined for spherical aluminium particles heated in car☐ymethylcellulose solutions. Two situations were considered: a still panicle immersed in a moving fluid, and a particle rotating in an otherwise stagnant fluid. Fluid flow rate, rotating particle velocity, particle diameter and fluid rheological properties were varied, covering a large range of the generalized Reynolds (0 to 801) and Prandtl (69 to 5358) numbers. Average heat transfer coefficients ranged between 56 and 2612 W/m2K. The results were compared with values predicted by published dimensionless correlations, showing that correlations based on a Fro¨szling-type equation were more adequate. It was found that the contribution due to natural convection should be considered for proper correlation of the results at low Reynolds numbers. The results also show the importance of the fluid velocity profile

Modelling O2 and CO2 exchange for development of perforation mediated modifed atmosphere packaging

Perforation-mediated modi®ed atmosphere packaging relies on the use of macro perforations or tubes to control the O2 and CO2 exchange and create the desired atmosphere inside an otherwise gas-tight package. In this work, the O2 and CO2 exchange through a single tube was studied. Di erent temperatures (5±20°C) and tube dimensions (length from 9 to 17 mm and diameter from 6 to 30 mm) were tested. O2 and CO2 mass transfer coe cients were determined according to a lumped mass transfer capacitance model that yielded a good description of the gas transfer. Temperature in the range tested did not show a signi®cant e ect on the mass transfer coe cients. A multiplicative non-linear equation was found to yield a good prediction of the dependence of the O2 mass transfer coe cient on tube diameter and length. The ratio between the CO2 and O2 mass transfer coe cients, an important parameter in the design of MAP, was 0.81 and none of the factors tested in¯uenced its value

Modelling respiration rate of shredded Galega kale for development of modified atmosphere packaging

The design of modified atmosphere packaging (MAP) for fresh-cut produce requires an adequate model for prediction of respiration rate as a function of both temperature and gas composition. In this work, the O2 consumption and CO2 production rates of shredded Galega kale were studied. The storage temperatures used were 1, 5, 10, 15 and 20 C. The atmospheres tested were all combinations of 1, 5 and 10% v/v O2 plus 0, 10 and 20% v/v CO2 with the balance being N2, as well as ambient air. Temperature was the variable with the greatest influence on respiration rate and the effect of gas composition increased with temperature. The dependence of respiration rate on gas composition was well described by a Michaelis–Menten type equation with uncompetitive CO2 inhibition. The respiratory quotient (RQ) was found to be constant for the range of temperatures and gas compositions tested and was equal to 0:93 0:01. The constants of the Michaelis–Menten equation increased exponentially with temperature. The change over time of respiration rate of leaves exposed to air at 20 C was also analysed. It was observed that respiration rate decreased with time and that the ratio between the respiration rate of shredded and intact leaves was approximately constant in the period tested and equal to 2.8. 2002 Elsevier Science Ltd. All rights reserved