FLOW PROFILES OF POWER LAW FLUIDS IN SCRAPED SURFACE HEAT EXCHANGER GEOMETRY USING MRI

Scraped surface heat exchangers (SSHE) are prevalent in the food industry to heat/cool viscous fluids and to provide enhanced mixing. This work focused on developing and verifying a theoretical model to characterize the flow patterns in two-dimensional angular flow in an SSHE geometry under isothermal conditions. Experimentally, the model was verified by a noninvasive magnetic resonance imaging (MRI) technique. Good agreement was achieved between the theoretical model and experimental data for a polyalkylene glycol, which behaved as a Newtonian fluid (n = 1), and a 1% CMC solution, a power law fluid with n = 0.77. However, the experimental flow profile tomato puree was characterized by apparent wall slip. The best fit, with respect to the power law model, was a flow behavior index of 0.11. This study provides the framework to quantify the effects of heat transfer on flow profiles in this geometry and the effectiveness of mixing.     

MODELING HEAT EFFICIENCY, FLOW AND SCALE-UP IN THE COROTATING DISC SCRAPED SURFACE HEAT EXCHANGER

A comparison of two different scale corotating disc scraped surface heat exchangers (CDHE) was performed experimentally. The findings were compared to predictions from a finite element model. We find that the model predicts well the flow pattern of the two CDHE's investigated. The heat transfer performance predicted by the model agrees well with experimental observations for the laboratory scale CDHE whereas the overall heat transfer in the scaled‐up version was not in equally good agreement. The lack of the model to predict the heat transfer performance in scale‐up leads us to identify the key dimensionless parameters relevant for scale‐up.     

EVAPORATION IN A HORIZONTAL THIN FILM SCRAPED SURFACE HEAT EXCHANGER

Literature surveys have revealed inadequate information on heat transfer characteristics during evaporation in straight-sided horizontal thin film scraped surface heat exchangers (SSHE). the evaporation of water, concentration of milk and dehydration of cream (30–40% total solids) at different rotor speeds, number of blades, flow rates and temperature differentials were studied in 108 tests with objectives to develop a predictive equation for overall heat transfer coefficient and to study its variation with regard to above parameters in the light of proposed mechanism governing fluid flow, film formation and heat transfer. Data were processed in HCL System-4 computer to fit in quadratic form by method of least squares. Experiments were conducted at higher temperatures compared to those encountered in milk evaporators. the information would be useful in designing SSHE for processing several Indian dairy products.     

MIXING IN SCRAPED SURFACE HEAT EXCHANGER GEOMETRY USING MRI

Mixing, as a unit operation, is a key component in the operation of scraped surface heat exchangers (SSHE) because of its importance in ensuring uniform heat treatment. In this study, two approaches were used to evaluate mixing effectiveness in a process geometry that simulates a closed type SSHE: the strain distribution function which was based on velocity profiles and statistical mixing indices based on concentration profiles. For the 1% carboxymethyl cellulose test solution under laminar flow conditions, the lower angular rotation speed and lower axial flow rate promoted mixing with respect to mixing intensity. The residence time, as characterized by the axial Reynolds Number, was the key factor for this statistic. The strain distribution function and mixing length scale were affected by the relative contribution of angular rotation to axial flow, which was expressed as the ratio of the angular Reynolds number to the axial Reynolds number. This work demonstrates the equivalence of evaluating mixedness by experimental concentration profiles and theoretical velocity profiles.     

EFFECTS OF FLOW INCIDENCE AND SECONDARY MASS FLOW RATE ON FLOW STRUCTURING CONTRIBUTIONS IN SCRAPED SURFACE HEAT EXCHANGERS

Continuous mixing and dispersing process flows produced by scraped surface heat exchangers (SSHE) in food technology influence the microstructure of multiphase food systems and hence desired quality aspects (e.g. specific texture properties and temperature resistance). Such process flows in general depict non‐Newtonian fluid behavior. To explain and optimize the structuring mechanism of food systems (due to mixing and dispersion) treated in such process apparatus the knowledge of the local flow behavior is necessary. In this paper a scraped surface apparatus with special narrow annular gaps including two wall scraper blades is chosen as model process (scraped surface heat exchanger (SSHE)). To get optimizing criteria in the SSHE for mixing and dispersion of shear‐thinning fluids which include structuring components, a numerical particle tracking method (NPT) was developed and used to investigate local flow behavior for various scraper blade geometries and rotational velocities. The flow fields considered are received from numerical flow simulations (finite volume method (FVM)), which have been validated with experimental velocity field measurements (digital‐particle image velocimetry method (D‐PIV)). Besides the flow field and pressure contours, values of elongational and shear rates (as components of the deformation rate tensor, causing flow structuring contributions) are compared along characteristic particle tracks in order to get a quantitative information on the mechanical history which is experienced by the structuring units. Related flow structuring contributions are defined in terms of elongational and shear energy dissipation, integrated over the particle residence times along the tracks. Hence the effects of the rotational velocity ω, the scraper blade angle β and the scraper blade gap r_s on the flow structuring contributions are discussed and suggestions of SSHE geometries for food processing are given.     

PARTICLE RESIDENCE TIME DISTRIBUTIONS IN A MODEL HORIZONTAL SCRAPED SURFACE HEAT EXCHANGER1

Residence times and their distribution characteristics of potato cubes with aqueous solutions of sodium carboxymethyl cellulose (CMC), simulating non-Newtonian fluid foods, in a horizontal scraped surface heat exchanger (SSHE) were investigated. Minimum and maximum normalized particle residence times (NPRTs) and standard deviations of mean values were not significantly affected by the particle concentration while mean NPRTs of up to 10% particle concentration were signficantly lower than those of 20–40% particle concentrations (P < 0.05). Mean NPRTs were significantly influenced by process parameters including concentration of carrier, viz., viscosity, mutator speed and particle size (P< 0.001) as well as 2-way interactions among flow rate, mutator speed and particle size (P < 0.05 or 0.001). Furthermore, most of the individual particle residence time distributions in a horizontal SSHE flow could be described by either normal or gamma distribution models.     

SCRAPED SURFACE HEAT EXCHANGERS

In this literature survey flow patterns, mixing effects, heat transfer and power required for rotation in scraped surface heat exchangers (SSHE) are thoroughly discussed, with the emphasis on assumptions and results, while the principal design of different SSHEs are only briefly discussed. The flow patterns control the desired radial mixing and the undesired axial mixing. the flow in a SSHE can be regarded as the sum of an axial flow and a rotational flow. the axial flow is laminar and the rotational flow is laminar or vortical. With laminar flow the radial mixing is poor, which causes poor heat transfer and allows the axial flow profile to control the residence time distribution. the precise onset of vortical flow in a SSHE is hard to predict. the vortical flow makes the radial mixing very efficient, giving good heat transfer and perhaps plug flow behavior. However, vortical flow also causes axial mixing which reduces the apparent heat transfer coefficient and increases the residence time distribution. The power required to rotate the shaft and blades is mainly determined by the design of the blades.     

TEMPERATURE VARIATIONS IN the OUTLET FROM SCRAPED SURFACE HEAT EXCHANGERS

The radial temperature differences and the stability of the temperatures in the product outlet from scraped surface heat exchangers (SSHE) have been investigated over wide ranges of operating conditions. When the flow was vortical, the temperature were stable and no radial differences occurred. When the flow was laminar, channelling effects occurred. the radial temperature differences of the outlet from SSHEs were sometimes very large. the flow was unstable; this was most obvious from the temperature variations close to the blades. In some cases the central parts of the product passed straight through the SSHE without being heated or cooled. Thus, efficient mixing of the product after the SSHE is of primary importance in many applications with high-viscosity products, e.g., aseptic processing, or the modelling of the performance of SSHEs. A static mixer after the SSHE eliminated the radial temperature differences, but in some cases unstable temperatures remained after the mixer due to unstable flow conditions in the SSHE. the degree of stability was influenced by the design of the SSHE and by the flow rate.     

HEAT TRANSFER TO WATER AND SOME HIGHLY VISCOUS FOOD SYSTEMS IN A WATER-COOLED SCRAPED SURFACE HEAT EXCHANGER

Heat transfer in a water-cooled scraped surface heat exchanger has been investigated. The overall heat transfer coefficient in the heat exchanger is composed of three elements: heat transfer coefficient in the coolant jacket, resistance to heat flow in the separation wall and heat transfer coefficient inside the scraped cylinder. A method for assessing the heat transfer coefficient at the coolant side was developed. In contrast with studies published elsewhere, heat transfer was investigated with food systems which are non-newtonian and possess a complicated and unknown flowing behavior at higher shear rates. For water and three starch-based food products (starch content 12–18%) the heat transfer coefficients inside the scraped cylinder were measured for shaft speeds ranging from 1.67 to 10 revolutions/s. The experimental results were compared with heat transfer coefficients calculated with a model based on the penetration theory. For the starch-based products, in general, no consistent interactions between mass flow rates and internal heat transfer coefficients were observed. In the shaft speed range studied heat transfer coefficients at scraped surface varied from 3200 to 7800 W/m² K for water, from 500 to 3150 W/m² K for velouté sauce, from 670 to 1330 W/m² K for roux and from 780 to 1900 W/m² K for ragout.     

HEAT TRANSFER DURING EVAPORATION of MILK to HIGH SOLIDS IN THIN FILM SCRAPED SURFACE HEAT EXCHANGER

Thermal performance of thin film scraped surface heat exchanger was evaluated for concentration of milk to high solids with process variables such as mass flow rate, steam condensing, temperature, etc. Appropriate dimensionless groups were formulated and fitted in Cobb-Douglas model to obtain a correlation. This relationship which is in the form of a Nusselt equation will be useful in predicting the scraped film coefficient during milk concentration to high solids. the effect of process variables on scraped film coefficient were discussed.     

INACTIVATION of PROTEASES and LIPASE IN MILK IN THIN FILM SCRAPED SURFACE HEAT EXCHANGER

A cascade thin film scraped surface heat exchangers, having sterilizer, regenerator and cooler sections was designed and fabricated. It was employed for inactivation of thermostable proteases and lipases in milk. Buffalo milk was sterilized in the temperature range: 143–152°C for holding times of 0.75, 1.0 and 1.25 s. Samples collected aseptically were stored at 37°C to study the proteolytic and lypolytic activities. the study established that activity of enzymes in milk subjected to higher temperature was far less than that of milk processed at lower temperature for same holding time. the effect of longer holding times was similar.     

RESIDENCE TIME DISTRIBUTION IN HORIZONTAL THIN FILM SCRAPED SURFACE HEAT EXCHANGER (SSHE)

The residence time distribution (RTD) studies were conducted to predict the flow characteristics in thin film SSHE. the experiments were conducted at zero heat flux using water as working fluid. A pulse input in form of saturated sodium chloride solution was used as the disturbance at inlet and the response in form of electrical conductivity was measured at outlet. Thirty-two trials were conducted at different flow rates, number of blades and rotor speeds. the E-curves were plotted. It was inferred that the flow in thin film SSHE approximates to plug flow. Also, as the mass flow rate increased, RTD improved. the increasing number of blades resulted in appreciable improvement in RTD. the rotor speed had similar effect as that of number of blades on RTD. However, the number of blades had more profound effect as compared to rotor speed. If the rotor design is modified then RTD can be improved and simultaneously the power consumption can be kept at minimum.     

FLUID to PARTICLE CONVECTIVE HEAT TRANSFER COEFFICIENT IN A HORIZONTAL SCRAPED SURFACE HEAT EXCHANGER DETERMINED FROM RELATIVE VELOCITY MEASUREMENT1

Liquid-to-particle convective heat transfer coefficient (h_fp) between fluid and particle was investigated in continuous flow through a horizontal scraped surface heat exchanger. the relative velocities between fluid and particle were measured, and h_fp calculated from well known correlations. Heat transfer coefficients increased with increasing flow rate, rotational speed, and decreased with increasing carrier medium viscosity and particle size. the measured relative velocities during flow visualization studies ranged from 0.04 to 0.29 m/s with corresponding h_fp values of 597 W/m²°K and 1975 W/m²°K, respectively. Even with the most conservative correlation, Nusselt numbers ranged from 12.1 to 49.7; significantly greater than the value of 2.0 for a sphere in a stagnant fluid.     

METHODS to DISTINGUISH BETWEEN LAMINAR and VORTICAL FLOW IN SCRAPED SURFACE HEAT EXCHANGERS

The transition between laminar and vortical flow has been investigated in scraped surface heat exchangers (SSHEs), using new methods based on viscosity and temperature measurements. The transition occurred close to the critical Reynolds number for annular flow of Newtonian fluids. Thus, the methods used to predict the viscosity can be successfully applied to SSHEs despite the very complex flow behaviour of the products, the complex geometry of the SSHEs, and the heat transfer in the SSHEs. In industrial applications, the transition can be detected using the new method based on temperature measurements at the outlet of the SSHEs together with some calculations. This is a very important result, since with this “sensor” the flow pattern can be controlled towards the optimal flow pattern in SSHEs, which in turn is a fundamental requirement when the taste of aseptic products containing particulates are to be improved by the introduction of continuous sterilization.     

MODELLING of VORTICAL HEAT TRANSFER IN SCRAPED SURFACE HEAT EXCHANGERS

Heat transfer experiments have been performed in pilot plant scraped surface heat exchangers, using starch pastes and water as model products. When the flow was vortical, the backmixing effects were considered using the plug flow and axial dispersion model; the true surface heat transfer coefficients and axial dispersion coefficients were determined from temperature measurements. These coefficients were modelled and the following variables were considered: rotational speed, viscosity, flow rate, number of blades, radius ratio and heat transfer direction. Using these models, the overall heat transfer coefficients and the axial temperature profiles can be predicted quite precisely for SSHEs over a wide range of operating conditions. the first blade improved the heat transfer coefficients while additional blades did not; and heat transfer was higher when the radius ratio was 0.5 than when it was 0.75. the differences between previously proposed models are explained. The most favorable flow pattern in SSHEs occurred when the flow was vortical, but close to the transition point to laminar flow.     

MODELLING of LAMINAR HEAT TRANSFER IN SCRAPED SURFACE HEAT EXCHANGERS

Heat transfer experiments with starch pastes have been performed in pilot plant scraped surface heat exchangers (SSHE). Effects of the following variables were considered in the modelling of the laminar heat transfer: rotational speed; viscosity, flow rate, number of blades, radius ratio and heat transfer direction. The poor radial mixing restricted the heat transfer coefficients. When the radius ratio increased, the heat transfer rate increased. This was probably due to increasing mixing effects from the blades. the flow rate and the heat transfer directions did not affect the heat transfer coefficients. The scatter in the heat transfer coefficients was considerable, due to unstable flow conditions in the SSHEs. However, the scatter was reduced, compared with previous investigations, by the use of a static mixer after the SSHEs, since the radial temperature differences were eliminated. Better design of rotor and blades may improve the performance of SSHEs operating with laminar flow.     

MODELLING of the MEDIA-SIDE HEAT TRANSFER IN SCRAPED SURFACE HEAT EXCHANGERS

The heat transfer on the media-side and in the tube wall of scraped surface heat exchangers was investigated when the product was heated with steam or cooled with water. The heat transfer coefficients found experimentally were 15% higher than predicted with the Nusselt theory for steam condensation, and 30% lower than predicted with a traditionally recommended model for cooling with water. New, much better models for the media-side heat transfer were developed. the choice of model for the media-side affects a subsequent modelling of the heat transfer on the product-side. When the product-flow was laminar in the scraped surface heat exchangers, the heat transfer was controlled mainly by the resistance on the product-side; while the resistance on the media-side was very small. Vortical flow decreased the resistance on the product-side considerably and made the choice of material in the tube wall important from a heat transfer point of view.     

HYDRODYNAMICS and HEAT TRANSFER IN LIQUID FULL SCRAPED SURFACE HEAT EXCHANGERS — A REVIEW

Theoretical and empirical models pertaining to the hydrodynamics and heat transfer in liquid full scraped surface heat exchangers have been reviewed up to date. In hydrodynamics, various aspects viz. fluid flow, residence time distribution, power requirement are covered. the heat transfer characteristics have been reviewed from stand points of heating, cooling and ultra high temperature applications. the limitations of various models are explained. the logical conclusions and the areas needing further investigations have been delineated.