In this paper, we present numerical studies on multiphase flows related to food and pharmaceutical applications, focusing on the Discrete Element Method (DEM) and solid–fluid interaction simulations using the DEM. Specifically, we describe numerical models of the DEM coupled with the Volume-of-Fluid method and the DEM coupled with the Lattice Boltzmann Method, along with their application examples. In addition, we present the studies regarding the Reduced-Order Model based on Proper Orthogonal Decomposition.

Many food fluids exhibit non-Newtonian behavior, necessitating the measurement of flow curves, which describe the relationship between shear rate and shear stress, to understand their characteristic flow properties. Evaluating rheological properties from the relationship between pressure loss and flow rate in a pipe offers advantages over commercial rotational rheometers, including relatively low equipment costs, the potential to measure particle-dispersed systems by selecting appropriate piping, and the feasibility of in-line measurements. This research focuses on this approach to develop a table-scale, portable device using a small pressure-resistant container. Ketchup and mayonnaise were selected as model food fluids. By measuring the pressure within the container and the flow rate of the fluid through the pipe, we were able to successfully determine the characteristic parameters of the Herschel-Bulkley model. The results demonstrate the potential of this compact system for characterizing the viscous behavior of complex food fluids.

Vacuum freeze-drying is known as one of the most effective drying methods for maintaining quality among the many drying techniques. The main feature of freeze-drying is that it uses the sublimation of ice as the primary principle of dehydration. In this paper, the author will consider how freeze-drying process can be linked from a device perspective and a product quality perspective in terms of “fluid flow”. The drying rate is determined by the balance between the sublimation rate, water vapor transfer within the material, the water vapor velocity in the dryer, and the frosting velocity on the cold trap. When the water vapor velocity within the device reaches Mach 1, it enters a choked flow state. Therefore, the conductance of the device and the performance of the cold trap are critical factors determining the drying performance. During the freeze-drying process, when the product temperature remains above the glass transition temperature of the freeze-concentrated phase, the collapse of product occurs. The degree of collapse is one of the major factors affecting product quality (product shrinkage, rehydration ability, inactivation of active substances, etc.), but establishing a quantitative evaluation method for this remains a significant challenge. This study presents the results of an analysis using CFD simulation to investigate how fluid flow during the drying process determines the degree of collapse.

This special feature presents an application of electrical tomography (ET) for imaging air bubble dispersion in whipping cream during the agitation process. ET, a non-destructive and non-contact technique, reconstructs the conductivity or permittivity distribution by measuring impedance or capacitance, which is categorized into electrical impedance tomography (EIT) and electrical capacitance tomography (ECT). The ET system includes an impedance analyzer, capacitance meter, multiplexer, sensor, and control software, which together provide real-time insights into the internal structure of whipping cream. In this study, a fuzzy phase classification-implemented EIT is applied to image the air bubble dispersion. This EIT consists of non-linear conductivity σ reconstruction and fuzzy phase classification, which categorizes σ into probabilistic clusters uj (j=1: oil-in-water phase, j=2: air bubble). In the experiments, both σ and probabilistic air bubble cluster u₂ are reconstructed for whipping cream samples with three different fat contents to image air bubble dispersion. As a result, σ exhibited two distinct distribution patterns corresponding to changes in overrun (OR): a slight decrease due to initial air bubble formation and a sharp decline caused by significant changes in homogeneity and shrinkage. Through fuzzy phase classification, air bubble dispersion is imaged at the yield point of each whipping cream sample, during which the continuous liquid phase transitioned to the air bubble phase induced by agitation. The images also reveal that whipping cream with higher fat content reaches the maximum air bubble content at an earlier stage, which is consistent with OR. Local electrochemical impedance spectroscopy (EIS) is applied to validate the EIT results. For all whipping cream samples, the conductivity reduction rates obtained from both EIT and local EIS show a consistent trend, as both increased with agitation.

On-board CO₂ capture technology has attracted attention as a method for reducing GHG (greenhouse gas) from ships by scrubbing the ship’s exhaust gas with wash water to absorb CO₂. In this study, an acoustic levitation method was incorporated to visually observe and detail the CO₂ absorption process by droplets sprayed from a scrubber. First, an acoustic levitation device with enhanced structural strength and self-supporting capabilities was constructed, and droplets were levitated in an air atmosphere using the modified device. Subsequently, the ambient gas was replaced with CO₂, and the behavior of the levitated droplets was directly observed. Three types of aqueous solutions with varying pH values (water and two types of NaOH solutions) were used and a pH indicator (litmus reagent) was added beforehand at a concentration of 0.15wt%. As a result, it was calculated that pH of the droplet of NaOH aqueous solution with an initial pH of around 9 decreased by approximately 2.9 based on the hue change of the droplet. Under all CO₂ flow conditions, the pH was lower than that of the water droplets. On the other hand, the droplet in pH was slight, ranging from 0.1 to 0.3, for water droplets and the droplets of NaOH aqueous solution at pH over 13. The droplet shape started to flatten as CO₂ was supplied, eventually leading to a rapid decrease in the aspect ratio, and finally, a nearly spherical shape was obtained for all experimental conditions before the droplet fell. Finally, the CO₂ absorption amount and absorption rate were quantitatively evaluated from the pH changes of the levitated droplets measured through visualization.