Graciela Alvarez

The rheology of starch dispersions at high temperatures and high shear rates: a review

Engineering design of continuous processes for liquid and semi-liquid foods involves a complex heat transfer and fluid flow coupling, which strongly depends on the rheological behaviour of fluid foods. Most of these processes are performed at temperatures above 95°C. Lack of rheological data is one major problem encountered during numerical simulation and equipment design of liquid food processing at high temperatures. Insufficient rheological studies in the literature have been conducted with food products under these conditions. This paper presents a literature review of experimental devices and methods used to obtain rheological data for starch dispersions at temperatures below and above 95°C. Results and gelatinization models from the literature are analysed and discussed.

Two-dimensional simulation of turbulent flow and transfer through stacked spheres

Abstract We propose a one-equation model for two-dimensional turbulent flow through porous media. The momentum equation is derived from the space averaging of Navier–Stokes equations, leading to the so-called Darcy–Forchheimer equations. In the turbulent kinetic energy transport equation, the production term is assumed to be proportional to the cube of velocity. The dissipation term is not estimated with a transport equation, it is explicitly given by a law involving turbulent kinetic energy and velocity. The model requires only four experimentally determined parameters. The local Nusselt number was correlated to local Reynolds number, and to local turbulence intensity. Good agreement between the simulated and the experimental local Nusselt number is obtained.

Analysis of heat transfer during mist chilling of a packed bed of spheres simulating foodstuffs

Abstract Heterogeneity of heat transfer was studied in a packed bed of 8 mm spheres cooled by a two-phase flow composed of air and fine ( 8 μm diameter) water droplets. Local heat fluxes in the packed bed were measured in both single- and two-phase flow and analyzed as a function of water mass flow rate, the surface temperature of the spheres and air velocity. Heat transfer was enhanced in the two-phase flow: the maximum value rose to 2.8 under experimental conditions. The study shows that heat transfer depends on several factors: axial position, temperature difference, water mass flow rate and air velocity. A marked heterogeneity of heat transfer was demonstrated and was correlated with the local mass flow rate of the collected water.

Analyse du transfert thermique entre un cylindre et un écoulement d'air faiblement chargé en gouttelettes d'eau

Abstract Chilling of fruit and vegetables by forced air convection is slow and causes a weight loss of several percent. ‘Mist-chilling’, which uses a two-phase flow composed of air and suspended water droplets as a cooling medium, can reduce these drawbacks. In this process, the water supply must be controlled to avoid water excess at the surface of the product. It is known that the water supply must be high at the beginning of the treatment, when the products are still warm, and must be reduced during treatment to avoid water accumulation, which can cause spoilage of vegetable products. We measured heat-transfer between heated cylinders and two-phase flow. We studied the influence of several factors: diameter of the cylinders, temperature of surface and captured water mass flow rate. When the captured water mass flow rate is low, all the captured water evaporates and the analytical prediction given by the thermal balance agrees well with experimental data. As the captured water mass flow rate increases, the wet area on the cylinder surface increases. Water excess appears when the area of evaporation reaches almost 50% of the total surface of the cylinder.

Steady-state and stability analysis of a population balance based nonlinear ice cream crystallization model

The process of crystallization can be modelled by a population balance equation coupled with an energy balance equation. Such models are highly complex to study due to the infinite dimensional and nonlinear characteristics, especially when all the phenomena of nucleation, growth and breakage are considered. In the present paper, we have performed the stability analysis on a reduced order model obtained by the method of moments, which remains still highly complex. The considered model has been developed by the Cemagref and validated on experimental data. After computation, we get a scalar equation whose solutions correspond to the equilibrium points of the system. This equation is finally solved numerically for a concrete physical configuration of the crystallizer. We show that in most instances, there is only one steady state. The possibility of multiple steady-states is discussed.

Control of a Nonlinear Ice Cream Crystallization Process

In the ice cream industry, the type of final desired product (large cartons (sqrounds) or ice creams on a stick) determine the viscosity at which the ice cream has to be produced. One of the objectives of the ice cream crystallization processes is therefore to produce an ice cream of specified viscosity. In this paper, a nonlinear control strategy is proposed for the control of the viscosity of the ice cream in a continuous crystallizer. It has been designed on the basis of a reduced order model obtained by application of the method of moments, on a population balance equation describing the evolution of the crystal size distribution. The control strategy is based on a linearizing control law coupled with a Smith predictor to account for the measurement delay. It has been validated on a pilot plant located at IRSTEA (Antony, France).