Browsing by Author "Melo, Luis F."
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- A comparison between heat transfer correlations obtained from experimental data and numerical simulation of flow of stirred yoghurt during cooling in plate heat exchangersPublication . Afonso, Isabel M.; Fernandes, Carla S.; Maia, João M.; Melo, Luis F.Thermal processing is widely used in the food industry mainly to improve quality and safety of food products. The investigation of heat transfer problems of non-Newtonian fluids during heating and cooling in heat exchangers is of major interest since the main factor limiting heat transfer is the viscous behaviour of these fluids. Therefore, the knowledge of the interface heat transfer coefficients is important in the design of food processes and processing equipment. In the present work, simulations of stirred yoghurt cooling in a plate heat exchanger were performed using computational fluid dynamics (CFD) calculations and the obtained results were compared with experimental data. Simulations were carried out using the commercial finite element method package POLYFLOW, being the geometrical domain the representation of a single 3D channel of the plate heat exchanger with a 30º corrugation angle. The correlation obtained numerically was compared to the one obtained from previous experimental work and they were found to be very similar to the experimental one. The constitutive model, under the assumptions used, was found to be a very good approximation for predicting the convective coefficients of stirred yoghurt during cooling in a plate heat exchanger.
- Influence of corrugation angle in the stirred yoghurt processing in plate heat exchangersPublication . Fernandes, Carla S.; Afonso, Isabel M.; Melo, Luis F.; Maia, João M.Research on heat transfer of non-Newtonian fluids during thermal processing is useful to optimise the heat exchanger design, as well as to define quality levels of the final product. In the present study, a nonisothermal and non-Newtonian flow in plate heat exchangers was simulated numerically, in order to evaluate the influence of corrugation angle on the thermal and hydrodynamics characteristics of yoghurt during cooling in a plate heat exchanger (PHE). Cooling treatment of stirred yoghurt is usually carried out in PHEs since these equipments are suitable for liquid-liquid heat transfer duties that require uniform and rapid cooling or heating. In this operation, two mechanisms of heat transfer occur: conduction, in the plates, and convection inside the channels. The set of equations that describe mathematically the problem were the Navier-Stokes equations, for incompressible and stationary flow, and Fourier’s law for the conduction problems. Additionally, a constitutive model that describes the rheological properties of yoghurt under the cooling conditions has to be established in order to define totally the problem. The used model was proposed by Afonso et al. (2003) and takes into account the influence of shear rate and temperature
- Influence of corrugation angle in the stirred yoghurt processing in plate heat exchangersPublication . Fernandes, Carla S.; Afonso, Isabel M.; Melo, Luis F.; Maia, João M.Research on heat transfer of non-Newtonian fluids during thermal processing is very useful for optimise heat exchanger design, as well as to define quality levels of the final product. In the present study non-isothermal, non-Newtonian flow in plate heat exchangers were simulated numerically, in order to evaluate the influence of corrugation angles on the thermal and hydrodynamics characteristics of yoghurt cooling in a plate heat exchanger. Simulations were carried out using the commercial finite element method package POLYFLOW, being the geometrical domain the representation of a single 3D channel of the plate heat exchanger. Two corrugation angles were considered, 30º and 60º, being the obtained numerical results for the first one compared with experimental data. A laminar flow was observed for the two cases and the decrease of the angle conducted to a decrease of pressure drop and maximum shear rate.
- Simulation of stirred yoghurt processing in plate heat exchangersPublication . Fernandes, Carla S.; Dias, Ricardo P.; Nóbrega, João M.; Afonso, Isabel M.; Melo, Luis F.; Maia, João M.In the present work, simulations of stirred yoghurt processing in a plate heat exchanger were performed using computational fluid dynamics (CFD) calculations and the results compared with experimental data, showing a very good agreement. A Herschel–Bulkley model for the viscosity and an Arrhenius-type term for the temperature dependence were used to model the thermo-rheological behaviour of yoghurt. The heat exchanger used in this study operates in a parallel arrangement, thus simplifying the problem to the construction of a single complete 3D channel. After analysis of the velocity field and fanning friction factors, laminar flow was observed for all the operating conditions used and relations are proposed for the present heat exchanger between fanning factors and Reynolds number and between mean shear rate and mean velocity of yoghurt.
- Simulations of stirred yoghurt processing in plate heat exchangers. Part I: Rheological behaviourPublication . Fernandes, Carla S.; Afonso, Isabel M.; Melo, Luis F.; Maia, João M.Thermal processing and manufacturing in the food industry involves heating and cooling of highly viscous fluids. In general, these fluids exhibit complex flow patterns and are dependent on temperature, shear rate, duration of shear and elastic properties. Since the main factor limiting heat transfer is the viscous behaviour of the fluids, models that can capture this are of major interest to optimize heat exchanger design and to proper control of the manufacturing processes. A typical non-Newtonian food fluid is yoghurt, the rheological properties of which are influenced by several factors related with the physical nature of yoghurt and the processing conditions. Rheologically, stirred yoghurt shows a typical Herschel-Bulkley-type behaviour, with a yield stress at low shear rates and a power-law behaviour at higher stresses. Also, its viscosity varies from being highly temperature dependent to being almost temperature independent, depending whether it is being processed at temperatures above or below 22 ºC, respectively. The aim of the first part of this work is to simulate the non-isothermal flow of stirred yoghurt in a plate heat-exchanger. In order to do so, three problems were solved simultaneously: two of heat conduction in the plates and one on non-isothermal flow in the channel. The simulation was carried out using POLYFLOW, the geometrical domain being constituted by three three-dimensional components: superior and inferior plates and the flow channel. The corrugation of the plates was assumed to have a sinusoidal variation along the heat-exchanger.
- Simulations of stirred yoghurt processing in plate heat exchangers. Part II: Thermal behaviourPublication . Afonso, Isabel M.; Fernandes, Carla S.; Maia, João M.; Melo, Luis F.Thermal processing and manufacturing in the food industry involves heating and cooling of highly viscous fluids. In general, these fluids exhibit complex flow patterns and are dependent on temperature, shear rate, duration of shear and elastic properties. Since the main factor limiting heat transfer is the viscous behaviour of the fluids, models that can capture this are of major interest to optimize heat exchanger design and to proper control of the manufacturing processes. A typical non-Newtonian food fluid is yoghurt, the rheological properties of which are influenced by several factors related with the physical nature of yoghurt and the processing conditions. Rheologically, stirred yoghurt shows a typical Herschel-Bulkley-type behaviour, with a yield stress at low shear rates and a power-law behaviour at higher stresses. Also, its viscosity varies from being highly temperature dependent to being almost temperature independent, depending whether it is being processed at temperatures above or below 22 ºC, respectively. The aim of the second part of this work is to compare the correlations concerning stirred yoghurt convective heat transfer, obtained from numerical approaches, to the ones obtained from previous experimental work carried out in a plate heat exchanger during the cooling stage. These correlations included both rheological and thermal aspects, taking into account the complex flow behaviour of stirred yoghurt.
- Thermal behaviour of stirred yoghurt during cooling in plate heat exchangersPublication . Fernandes, Carla S.; Dias, Ricardo P.; Nóbrega, João M.; Afonso, Isabel M.; Melo, Luis F.; Maia, João M.In this study, CFD calculations were made in order to analyse the thermal behaviour of yoghurt in a plate heat exchanger, the yoghurt viscosity being described by a Herschel-Bulkley model. The influence of Reynolds number on local Nusselt numbers was analysed as well as the influence of entry effects on average Nusselt number. Numerical runs considering and discarding the influence of temperature on the viscosity were performed and the impact on average Nusselt numbers was analysed. Transversal variations of viscosity were studied and thermal correlations including the ratio between bulk and wall viscosities are proposed. The obtained thermal correlations were compared with the experimental ones (described in the literature) and a very good agreement was found, mainly when considering the effect of temperature on viscosity. Simulations with a non-Newtonian fluid having lower Prandtl numbers than yoghurt were performed in order to analyse the influence of this variation on the Reynolds number exponent of the thermal correlation.