Benoît Haut

A comprehensive analysis of the evaporation of a liquid spherical drop.

In this paper, a new comprehensive analysis of a suspended drop of a pure liquid evaporating into air is presented. Based on mass and energy conservation equations, a quasi-steady model is developed including diffusive and convective transports, and considering the non-isothermia of the gas phase. The main original feature of this simple analytical model lies in the consideration of the local dependence of the physico-chemical properties of the gas on the gas temperature, which has a significant influence on the evaporation process at high temperatures. The influence of the atmospheric conditions on the interfacial evaporation flux, molar fraction and temperature is investigated. Simplified versions of the model are developed to highlight the key mechanisms governing the evaporation process. For the conditions considered in this work, the convective transport appears to be opposed to the evaporation process leading to a decrease of the evaporation flux. However, this effect is relatively limited, the Péclet numbers happening to be small. In addition, the gas isothermia assumption never appears to be valid here, even at room temperature, due to the large temperature gradient that develops in the gas phase. These two conclusions are explained by the fact that heat transfer from the gas to the liquid appears to be the step limiting the evaporation process. Regardless of the complexity of the developed model, yet excluding extremely small droplets, the square of the drop radius decreases linearly over time (R(2) law). The assumptions of the model are rigorously discussed and general criteria are established, independently of the liquid-gas couple considered.

Structured model of VERO cells metabolism in a fixed-bed bioreactor

In this work, a structured mathematical model of the VERvet Origin (VERO) cell metabolism in a fixed-bed bioreactor is developed. Mass balance equations for the extra- and intracellular species are written considering the main pathways of the animal cell metabolism. From the model equations, an undetermined set of equations relating the consumption or production velocities of the extracellular species and the velocities of the metabolic reactions can be developed. The monitoring of a reference cell culture enables to transform this undetermined set of equations in a determined one. The resolution of this determined set of equations enables to analyze and characterize the cell metabolism. From the model and the cell metabolism characterization, a monitoring tool is developed to simulate the time evolution of the species concentrations when the time evolution of the cell concentration is known. This simulation tool is used to simulate five experimental cell cultures. The comparison of the experimentally determined and computed time evolutions of the species concentrations enables to validate the developed monitoring tool. The validated monitoring tool can be used to optimize the operating conditions of the culture, as the medium feeding rate, the range of the species concentrations.

Ventilated tunnel solar dryers for small-scale farmers communities: Theoretical and practical aspects

ABSTRACT The design of so-called “ventilated tunnel solar dryers is presented in this work under theoretical and practical aspects. These dryers consist in a tunnel with a black floor and a plastic top, ventilated with fans supplied by a photovoltaic module. Along this tunnel, the air first passes through a heating part, which is product free and where the air is heated due to greenhouse effect. It then enters the second part of the dryer, the drying part, where the products to dry are spread. A rational and general design procedure based on the solving of energy and mass balance equations is developed and followed. This procedure allows designing the ventilation system and calculating the lengths of the heating and the drying parts of a dryer according to a scope statement and to design conditions insuring an efficient and homogeneous drying. Two dryers were designed, built and operated, in the frame of two development aid programs, in Cambodia and in Uganda. The fieldwork highlighted the importance of practical building and operating aspects, reported in the present paper, and of including people of the local communities in these programs. Results of field drying experiments are shown and demonstrate the validity of the design procedure. In particular, the validity of the energy balance equations at the basis of the procedure is demonstrated.

Implementation of a rational drying process for fish conservation

Fishing is a traditional activity that is widespread in West Africa. One of the greatest problems for fishermen and a cause of lack of food accessibility is the difficulty in conserving fish. Drying is a widely used technique in sub-Saharan Africa for preservation of fish. However, drying is a complex process, making the construction and calibration of efficient drying devices challenging. This paper presents the construction and calibration of five mobile fish dryers in Mali and, for one of them, development of a method for its use. The performances achieved far exceeded those of traditional solar dryers as drying was faster and the fish were not contaminated by being exposed to flies. Furthermore, construction and user manuals were written for the local fishermen which were well understood as the fishermen were able to disassemble and reassemble the dryers when they were required to be moved.

Computational Fluid Dynamics Characterization of a Bioreactor Mixing Device for the Animal Cell Culture

During a cell culture in a bioreactor, the cells are exposed to the shear stresses mainly generated in the culture medium by the mixing device. Beyond a critical shear stress, this exposition induces cell damages. Therefore, the limitation of the shear stress is an important criterion for the design of bioreactors. An accurate modeling of the flow and the induced shear stresses in the medium is a tool to achieve an effective design of a bioreactor. In this work, a new design of a mixing device is considered. The aims of this work are to develop a methodology to study the flow and the induced shear stresses in the device, to study and to model the relation between the flow, the induced shear stresses and the cell viability, to use the developed model as an optimization tool, and to study the design of the bioreactor mixing device and its scale-up. In a first step, the flow and the induced shear stresses in the device are simulated by Computational Fluid Dynamics. In a second step, the model of the influence of the flow and the induced shear stresses on the cell viability is established by a comparison between the computed flow and the induced shear stresses and experimental measurements of cellular viabilities for different impeller rotation speeds. Finally, the influence of another design of the mixing device on the cell viability is studied.

Segregated Model of Adherent Cell Culture in a Fixed-Bed Bioreactor

In this work, a mathematical model of a fixed-bed bioreactor for the animal cell culture is developed to study the optimization and the scale-up of this bioreactor. Several cell populations are considered: the cells in suspension in the medium at the beginning of the process and the adhering cells to the fixed-bed. The model includes a capture rate kinetic of the cells in suspension by the fixed-bed and a spatial distribution of the nutrient and by-product concentrations in the fixed-bed. Therefore, the model reports the potential gradients of the cell concentrations in the fixed-bed. Some model parameters are experimentally identified and the model is validated using experimental data obtained with two pilot bioreactors. The model is used as a simulation tool to study the influence of the bioreactor design or the velocity field of the culture medium on the cell concentration gradients in the fixed-bed bioreactor and to optimize the operating conditions, the design, and the scale-up of this bioreactor.