Heat transfer in a tunnel pasteuriser. Part III. Application of the Schultz-Olson theory

It was found that changes with time in the temperature of liquid heated in a glass bottle in a pasteuriser module can be represented by a straight line on semilogarithmic coordinates. Values of f (1/slope) and j (1/intercept) were calculated according to the method of Schultz-Olson. The influence on f of the volume-to-surface ratio of a bottle and of liquid concentration was determined. It was further established that the value of f does not depend on the initial temperature of the liquid or on the initial temperature difference. The value of the lag factor also does not depend on initial liquid temperature or on initial temperature difference. At the slowest heating point j is much larger than it is at other points in the bottle, and is greater than 1. The value of j does not depend on liquid concentration. Analysis of the relation of f and j to the ratio of heat transfer resistances showed that the lag factor, in principle, depends on the glass, whereas f depends on the properties of both the heated liquid and the glass.It was found that the overall heat transfer coefficient calculated from f differs from that obtained by the graphical integration method. The difference is a function of j.It is suggested that the heating of a liquid in a bottle may be treated as the heating of two media in series. The values of f and j depend on the relation between the accumulation of heat by the glass and the convective heating of liquid.     

Heat transfer in a tunnel pasteuriser. Part I. Factors affecting the rate of heating of liquid in a bottle

The purpose of this work was to investigate heat transfer conditions in tunnel pasteurisers used to pasteurise beer and fruit juices in glass containers. A module of a pasteuriser was built and the following process variables were investigated: bottle shape, hot water spray intensity, location of bottle in relation to the spray nozzle axis, and the spray nozzle pattern and the deflection angle of nozzles with respect to the vertical. It was found that all these variables affect the rate of heating of liquid in bottles. However, the shape of the bottle has little effect, while spray intensity and location of the bottle in relation to the nozzle axis are important factors in the heating process. The effect of the angle of the nozzle to the horizontal is rather ambiguous, since it improves heating at one location but worsens it at another.     

Heat transfer during contact cooking of minced meat patties

Experimental results on heat transfer during heating of four types of minced meat patties by contact with hot plates are given. Experimental equipment, method of measurement and experimental values of heat flux from heating plate to heated product surface during 4 min contact cooking are described. Simultaneous measurements of product and heating plate temperatures as a function of time were used to determine the contact heat transfer coefficient between heating plate and product surface. The results show that the heat flux varied during the heating process. The magnitude of the heat flux as a function of time varies with the type of material heated and the heating plate temperature. The heat transfer coefficients measured were in the range 200 to 1200 W m^?2 K^?1, depending on product type, contact plate temperature, contact pressure and stage in the heat treatment.     

Heat and mass transfer in combined convective and far-infrared drying of fruit leather

Combined convective and far-infrared drying is a challenging assignment due to complex relationship between heat and mass transfer. In this paper, heat and mass transfer of fruit leather drying with combination of hot air and far-infrared has been carried out. The heat and the mass transfer coefficients were analyzed by heat–mass analogy. It could be found that the ratio between heat and mass transfer coefficients for the combination technique could not be obtained from the heat–mass analogy classical model and a modification is needed. The modified correlations for predicting ratio of heat and mass transfer coefficients and the heat transfer coefficient in term of heat transfer Nusselt number are developed. The model could fit the experimental data quite well within ±10% deviation.     

Microwave Frying Compared with Conventional Frying via Numerical Simulation

Microwave heating can be combined with other means of heating to yield a unique heating profile. In the study, microwave frying, a combination of convective and microwave heating, was compared with conventional frying. Frying experiments were performed by inserting a single food sample (chicken breast meat) in the hot oil at 180?±?1°C for both frying methods. Center temperature of the sample and the oil temperature were recorded during both frying methods. Simulations were performed to predict heat transfer coefficients. Processing time was shorter with microwave frying. Simulations revealed a varying convective heat transfer coefficient, which was in the range of 160–490 W/m² K, during conventional frying. Higher convective heat transfer coefficient, 500 W/m² K, compared to conventional frying was observed during microwave frying with the simulations. This is suggested to be due to higher turbulence in microwave frying.     

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 coefficients to canned green peas during end-over-end sterilisation

The effect of temperature (110, 120 and 130 °C), rotation speed (5, 10 and 15 r.p.m.) and headspace (4, 8 and 12 mm) on heat transfer coefficients to canned green peas during end‐over‐end sterilisation was studied using response surface methodology. The models developed for fluid‐to‐particle heat transfer coefficient, h_fp, and overall heat transfer coefficient, U, were adequate, showing no significant lack of fit and satisfactory correlation coefficients. For the two responses, temperature, rotation speed and headspace have a significant effect. U, ranged between 477 and 905 W m^?2 °C^?1, while h_fp, fluctuated between 480 and 1950 W m^?2 °C^?1. The highest h_fp and U values are obtained at high temperatures, rotation speeds and headspaces. The verification of the prediction models was satisfactory. Dimensionless correlations were developed for h_fp and U, with equations showing a good agreement with the experimental data. Heat transfer to liquids and particles was modelled using the Reynolds number, the Prandtl number and adimensional headspace.     

Convective Heat Transfer at Particle-Liquid Interface in Continuous Tube Flow at Elevated Fluid Temperatures

Liquid-to-particle convective heat transfer coefficients are useful in developing aseptic food processing systems. They were determined for continuous flow through a holding tube at 115.5°C using liquid crystal and relative velocity methods with sodium carboxymethylcellulose solution to simulate non-Newtonian fluid characteristics. An on-line tube viscometer was used for in situ estimation of rheological characteristics. Minimum and maximum values of h_fp determined from the liquid crystal method ranged from 986 W/m²⁰K to 2270 W/m²⁰K, (Nusselt numbers from 26.4 to 54.6). Values from the relative velocity method ranged from 1143 to 2270 W/m²⁰K (Nusselt numbers from 33.2 to 63.1) when using the Ranz and Marshall relation, and from 598 to 1456 W/m²⁰C (Nusselt numbers from 13.6 to 24.1) with a flat-plate correlation. Heat transfer coefficients increased significantly with decreasing carrier medium viscosity and decreasing particle-to-tube diameter ratio and increased with flow rate.     

Determination of convective heat transfer coefficient and specific energy consumption of potato using an ingenious self tracking solar dryer

In the present work, an attempt has been made to experimentally determine the heat transfer properties of potato in terms of convective heat transfer coefficient, specific energy consumption and specific heating rate. Drying experiments with potato cylinders have been performed in an in-house fabricated laboratory scale natural convection indirect solar dryer with self tracking mechanism. The convective heat transfer coefficient of cylindrical potato samples was evaluated by considering the combined effects of heat capacities of food product as well as radiative heat transfer from drying chamber to the food product. This study revealed that the convective heat transfer coefficient for potato cylinders was varying from 11.73 to 16.23 W/m² °C with an experimental error of 7.86 %. The specific energy consumption was decreasing exponentially with drying time, and the average value was estimated to be 3,491 kJ/kg. It was also observed that the specific heating rate for potato cylinders decrease with dimensionless moisture content.     

A model for heat transfer in cryogenic food freezing

An experimental study of the heat transfer between liquid nitrogen sprays and a model food (a gelatine slab) was carried out under conditions similar to those in a cryogenic freezer. Measurements of heat flux and local mass flow rates of the spray were made at various liquid N₂ pressures and various temperature differences between the spray and the food surface.
At higher spray pressures, the heat transfer coefficient increases with the mass flux density of the liquid available at the food surface. The quantity of liquid nitrogen sprayed onto the solid surface, the mean droplet size, spray velocity, surface coverage and the mean temperature difference between the boiling nitrogen and the food surface are major factors influencing the rate of heat transfer during the freezing process. Although the heat transfer coefficients at the food surface are much less than those obtained for individual droplets, the model provides useful data.
The results are critically discussed in relation to cryogenic freezing of foods.     

Physical Property and Rotational Speed Effects on Heat Transfer in Axially Rotating Liquid/Particulate Canned Foods

Particle surface temperatures were measured by liquid crystals that allowed unrestricted motion of the monitored particles. Liquid-particle film heat transfer coefficients (h_p) were considerably higher than published values which were determined with particles attached to thermocouple wires. In general, as a result of the particle motion, h_p increased with increasing rotational speed and increasing fluid viscosity. Furthermore, h_p values were higher for Teflon spheres than for aluminum spheres. Results for the overall (heating medium/container wall/internal liquid) heat transfer coefficients were consistent with published observations.     

VISCOSITY and HEAT TRANSFER COEFFICIENTS FOR CANOLA, CORN, PALM, and SOYBEAN OIL

To understand the influence of frying oil's physical properties on heat transfer, heat transfer coefficient and oil viscosity were measured for combinations of oil type, temperature, and condition. the lumped capacity method for heat transfer in a high thermally conductive metal gave convective heat transfer coefficients. A capillary viscometer in a convective air heater provided viscosity data at frying temperatures. Frying time and oil temperature significantly affected viscosity. Oil viscosities were not statistically different between fresh and 12 h frying oil or 12 and 24 h frying oil, while between the remaining frying times the oil viscosities were statistically different. Corn oil viscosity showed the greatest increase over 36 h and the highest correlation between viscosity and heat transfer coefficient (−0.959).     

矩形肋槽烘缸内壁面的传热系数

将传热学的理论和实验成果应用于矩形肋槽烘缸内壁面的传热工作中,分析了饱和水蒸气在烘缸内壁的肋顶面及肋侧面的冷凝传热和冷凝水在肋槽底部的对流传热并推导出这3种情况下传热系数的具体计算公式,以期为烘缸向纸张的传热计算提供依据。初步研究表明,宽和短的肋有利于增强肋槽烘缸的传热,内壁面的热阻最小、壁壳的热阻次之、外壁面的热阻最大。     

Convection and radiation combined surface heat transfer coefficient in baking ovens

The combined surface heat transfer coefficient is a determining parameter of convective baking process time and efficiency, as well as the resulting food product quality. By this study, the combined surface heat transfer coefficient term was determined at the convective oven temperature range of 70–220 °C, with fan (turbo) and without fan (static oven) applications. The methods of “Lumped Capacity” and “Time–Temperature Matching” were used. Both methods utilize the time–temperature data at a fixed position of a definite material, during unsteady state heating up period inside the convective oven. The increase in oven temperature and the fan application in the oven derived higher calculated values of surface heat transfer coefficients. Good agreement was observed between both methods and the literature values. The given methods are applicable to other oven types and heating modes.     

HEAT TRANSFER IN A VERTICAL, LIQUID-FULL SCRAPED-SURFACE HEAT EXCHANGER. APPLICATION OF THE PENETRATION THEORY AND WILSON PLOTS MODELS

Heat transfer characteristics of a scraped-surface heat exchanger (Contherm Model 6 × 2) were evaluated at ultra high temperatures using water and soybean water extracts as model systems. The resistance equation was used to calculate internal (scraped-side) heat transfer coefficients (h_i) from the overall heat transfer coefficient, the wall coefficient and the external (steam-side) coefficient (calculated from the Nusselt Theory using an iteration procedure). The Penetration Theory of Harriot (1958) predicted h_i values quite well at low axial mass flow rates, where laminar flow conditions prevail. However, turbulent axial flow resulted in experimental higreater than predicted by the theory. A correction factor based on Prandtl number suggested by Trommelen et al. (1971) did not improve the prediction. The Wilson Plots method was useful for explicitly accounting for axial and rotational velocity effects, both of which significantly affected heat transfer, especially at high values of either variable.     

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.     

Influence of Fluid Rheological Properties and Particle Location on Ultrasound-assisted Heat Transfer between Liquid and Particles

Convective heat transfer between fluids and a particle was investigated as a function of fluid rheological properties and position in the ultrasonic field, expressed as dimensionless parameters. The convective heat transfer coefficient was determined for an irregularly shaped particle immersed in sodium carboxymethylcellulose solutions of various concentrations. The extent of sonic enhancement was strongly dependent on fluid rheological properties. At low viscosity, the distance of the particle to the nearest corner of the tank was found to be significantly correlated with heat transfer coefficient; this effect decreased as viscosity increased. Dimensionless correlations were developed.     

A METHOD FOR DETERMINING THE CONVECTIVE HEAT TRANSFER COEFFICIENT DURING IMMERSION FRYING

The convective heat transfer coefficient is a useful parameter in characterizing heat flow across a fluid/solid interface when the fluid flow field is complex and solution of the coupled transport equations impractical. While convective heat transfer coefficient values for many unit operations have been tabulated, the boiling phase of immersion frying has not been quantified. The objective of this study was to develop a laboratory method for the measurement of the convective heat transfer coefficient during immersion frying. The method that was developed was applied to the immersion frying of potato cylinders at an oil temperature of 180C. The convective heat transfer coefficient was initially 300 W/m²K, it increased sharply to 1100 W/m²K, and gradually decreased to 300 W/m²K over the duration of the process. Use of this new method will allow the study of the effects of oil temperature, oil quality, product shape/size, and product quality on heat transfer coefficients.     

Heat Transfer Study on Flame Pasteurization of Liquids in Aluminum Cans

Heat transfer studies were performed in 209 × 413 aluminum cans using 1% bentonite suspension. The cans were heated in a pilot flame sterilizer. The temperature profile was practically uniform radially and axially after 4.5 min of heating. The slowest heating was experienced in the bottom (heel) of these cans. The temperature difference between the slowest heating point and the hot region over the flame decreased as the can was heated: from 15.6°C after 1 min to 6.7°C after 4.5 min of heating. The internal heat transfer coefficient at 40 rpm was measured and showed an increase with heating time.     

Prediction of convective heat transfer coefficient during deep-fat frying of pantoa using neurocomputing approaches

Deep-fat frying (DFF) is the major processing step in preparation of pantoa, a popular Indian dairy sweetmeat. In this study, the dough for pantoa was rolled into balls of 15 g, and fried in sunflower oil at 125, 135 and 145 °C for 8 min. Convective heat transfer coefficient, which defines the heat transfer characteristics of the product during DFF, was determined using one-dimensional transient heat conduction equation as 92.71–332.92 W·m^− 2·K^− 1. Neurocomputing techniques such as connectionist models and adaptive neurofuzzy inference system (ANFIS) were compared vis-à-vis multiple linear regression (MLR) models for prediction of heat transfer coefficient. A back-propagation algorithm with Bayesian regularization optimization technique was employed to develop connectionist models while the ANFIS model was based on Sugeno-type fuzzy inference system. Both connectionist and ANFIS models exhibited superior prediction abilities than the classical MLR model. Amongst the three approaches, the hybrid ANFIS model with triangular membership function and frying time and temperature as input factors gave the best fit of convective heat transfer coefficient with R² as high as 0.9984 (99.84% accuracy) and %RMS value of 0.1649.Industrial relevanceConvective heat transfer coefficient defines the heat transfer characteristics of a product during frying. Accurate prediction of heat transfer coefficient is important for design of process equipment and saving energy during commercial production. Developing models to predict heat transfer and the coefficients have been a challenge. Neurocomputing is one of the emerging intelligent technologies with analogies to biological neural systems. Therefore, it has the capability to predict complex relationships in food systems. Neurocomputing approaches such as connectionist and ANFIS models are now widely used in the food industry to predict various engineering properties of food, optimization of various transport processes, unit operations and formulating new products and product characteristics. No attempt has been made to predict the heat transfer coefficient during frying of pantoa. In this study, the convective heat transfer coefficient of pantoa was predicted using connectionist models and ANFIS techniques. These neurocomputing techniques are expected to alleviate the difficulties in conventional heat transfer modeling.