用颗粒热传递模型计算旋转管排式干燥机的传热系数

论述了应用基于移动加热板的颗粒热传递模型计算大型旋转管排式干燥机传热系数的方法。在料床完全混合的简化假设下,按物料在干燥机内的实际干燥过程,分35段计算各段的传热系数及整机总传热系数,所得结果与干燥机的实际运行情况基本相符。     

Fluid-to-particle Heat Transfer Coefficients for Continuous Flow of Suspensions in Coiled Tube and Straight Tube with Bends

Fluid-to-particle heat transfer coefficient (h_fp) for spherical particles in continuously flowing food suspensions (0-30% v/v green peas in 1.5% sodium carboxymethylcellulose solution) in the holding section of an aseptic processing system was determined. The effects of hold tube geometry, temperature, flow rate, and particle concentration were investigated. The h_fp values ranged from 538 to 1450 W/m²·°C in the coiled tube and from 204 to 1220 W/m²·°C in the straight tube. Significantly, higher h_fp value was observed for particles in the coiled tube as compared to that in the straight tube. The h_fp value increased with temperature, flow rate, and particle concentration. Dimensionless correlations relating the Nusselt number to Reynolds and Prandtl numbers were developed. Estimation of h_fp values based on the particle resistance temperature device (RTD) data and dimensionless correlations indicated a reduction in the degree of overprocessing of average particles when a coiled tube was used instead of a straight tube for the holding section.     

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.     

Impact of Overall and Particle Surface Heat Transfer Coefficients on Thermal Process Optimization in Rotary Retorts

ABSTRACT: This study attempts to examine the significance of recent research that has focused on efforts to estimate values for global and surface heat transfer coefficients under forced convection heating induced by end‐over‐end rotation in retorting of canned peas in brine. The study confirms the accuracy of regression analysis used to predict values for heat transfer coefficients as a function of rotating speed and headspace, and uses them to predict values over a range of process conditions, which make up the search domain for process optimization. These coefficients were used in a convective heat transfer model to establish a range of lethality‐equivalent retort temperature–time processes for various conditions of retort temperature, rotating speed, and headspace. Then, they were coupled with quality factor kinetics to predict the final volume average and surface quality retention resulting from each process and to find the optimal thermal process conditions for canned fresh green peas. Results showed that maximum quality retention (surface and volume average retention) was achieved with the shortest possible process time (made possible with highest retort temperature), and reached the similar level in all cases with small difference between surface and volume average quality retention. The highest heat transfer coefficients (associated with maximum rotating speed and headspace) showed a 10% reduction in process time over that required with minimum rotating speed and headspace. The study concludes with a discussion of the significance of these findings and degree to which they were expected.     

Heat Transfer Coefficients to Liquids with Food Particles in Axially Rotating Cans

Heat transfer rates were measured for steam-heated, rotating cans containing potato spheres in water. Can rotational speed (9.3–101 rpm), sphere size (22.2–35.0 mm), and potato volume fraction (0.107–0.506) were varied in 14 triplicated experiments. Overall heat transfer coefficients were correlated within ±25% with physical properties and operating variables by an equation derived by dimensional analysis. Film coefficients (h_p) for heat transfer from the water to potatoes in the can were determined by thermocouple measurement of potato surface temperatures; values were found to be finite and nearly invariant, averaging h_p= 160 ± 30 W/m²K. The lack of variation of h_p suggested that for the experimental conditions tested, there was little relative motion between liquid and particles.     

An Empirical Methodology for Evaluating the Fluid to Particle Heat Transfer Coefficient in Bi-axially Rotating Cans Using Liquid Temperature Data

An empirical methodology was developed for evaluating the fluid to particle heat transfer coefficient (h _fp) and overall heat transfer coefficient (U) in bi-axially rotating cans. Conventional particle temperature measurement during thermal processing is generally difficult in cans undergoing agitation processing and is even more difficult in cans going through bi-axial free rotation as in continuous flow turbo cookers. Thin wire flexible thermocouples have helped in gathering temperature data of both particle and liquid in end-over-end batch processes. Wireless temperature loggers have been developed for liquid temperature measurements in continuous flow systems which can be used to estimate U. Evaluation of h _fp is still difficult in these systems due to difficulty in gathering particle temperatures. The proposed method involves developing correlations between h _fp and U using real time-temperature data gathered from test cans in fixed axial mode and then coupling them with experimentally evaluated U from fluid temperature gathered with wireless sensors to compute h _fp for bi-axially rotating cans. The methodology is based on the assumption that within a can, factors that influence U will also influence h _fp, and therefore h _fp and U are generally interrelated. A three factor, five level central composite rotatatable design and a response surface methodology was used to develop the correlation models for the U and h _fp in fixed axial mode with retort temperature (111.6–128.4 °C), glycerin concentration (80–100%), and rotational speed (4–24 rpm) as the main factors. The developed model was used to evaluate the U and h _fp in the free bi-axial mode, using a full factorial design (3 × 3 factorial). The method was successfully implemented and an analysis of variance study, as expected, indicated all three major factors to influence the U and h _fp values. Glycerin concentration and rotation speed were highly significant (<0.001), while temperature was marginally significant (p < 0.05) with respect to U while all factors were significant with h _fp.     

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.     

Determination of Heat Transfer Coefficients for Continuous Belt Freezers

Air-product heat-transfer coefficients for belt freezers were determined experimentally. Measurements were performed for foods (beef hamburgers) and a model substance (copper disk) using a prototype tunnel. Air flow parallel, as well as perpendicular, to the belt, was employed; in the latter case moving upwards or downwards through the belt. From the measured temperatures, heat-transfer coefficients were calculated using appropriate mathematical methods. Values obtained for both types of materials were compared, and the usefulness of the test substance was discussed. From these data, simple and precise regressions were obtained, useful to predict the coefficient for the above mentioned types of air flow and valid over a wide range of operating conditions in belt freezers.     

多通道烘缸通道截面变化的凝结换热研究综述

在造纸干燥部的革新中,多通道烘缸是一种创新的设计理念。本文介绍了多通道烘缸工作的基本原理和工作方式,综述了小通道在散热研究中的三个研究重点:换热系数、流型变化和压降。探讨了截面变化在通道凝结换热中的重要作用,指出了利用通道截面形状变化可以提高换热系数。从实验、理论和加工三个方面对不同截面形状通道内气液两相流的研究和应用前景进行了展望。     

Effect of Ultrasonic Vibration on Fluid-to-Particle Convective Heat Transfer Coefficients

Studies were conducted to determine the potential of ultrasonic energy as a heat transfer enhancing agent in sterilization applications. Metal transducer particles were immersed in warm liquids with and without sonic agitation, and temperature histories were recorded. Convective heat transfer coefficients were calculated from the solution to the lumped convective heat transfer equation. Results indicate that fluid-to-particle heat transfer was significantly enhanced by the use of sonic energy, and that within the range of variables investigated, the amount of enhancement was dependent more on natural convection and standing wave patterns than particle size and energy input per unit mass.     

优化拟合法计算黑液蒸发器传热系数

以四效混流蒸发器为背景，从系统的静态模型出发，提出了一种采用优化拟合的原理来计算蒸发器传热系数的算法。该算法可根据过程控制机采集的现场工作数据，实时地算出蒸发器的传热系数。该算法适合在集散型系统中监控机上运行，用于实时监控蒸发器的工作状况。     

Determination of Retortable Pouch Heat Transfer Coefficients by Optimization Method

A computer-based optimization technique was developed for determining the apparent heat transfer coefficient (h) for retortable pouches heated by circulated hot water under overriding air pressure. The technique incorporated actual process data, a finite difference model and an optimization criteria to converge on h values. A water flow channel was constructed to fit inside a vertical retort which allowed direct water exposure on the pouch surface at a quantifiable velocity. Because the optimization technique was not dependent on analytical equations, it could be applied to such areas as future heat transfer studies of new retortable pouch materials, designing new pouch-holding cassettes and improving water flow patterns in retorts.     

香菇脆片真空油炸过程中传热规律研究

真空油炸过程中同时发生热量和质量传递现象,这些现象对产品的品质和安全控制极为重要。该文研究真空度、油炸温度、预处理(漂烫、漂烫+浸渍+涂膜)对香菇脆片真空油炸过程中对流传热系数(h)的影响,结果表明:h值的变化趋势是先增大后减小;达到峰值的时间都在300 s～600 s;油炸温度越高,峰值越高;真空度越高,峰值越高;在真空油炸后期,油炸温度和真空度低的香菇脆片对流传热系数值高;油炸条件相同,漂烫处理香菇脆片的最大对流传热系数值高于漂烫+浸渍+涂膜处理香菇脆片。     

Measurement of Convective Heat Transfer Coefficients During Food Freezing Processes

Any prediction of freezing time for a food product depends on accurate heat transfer coefficients. The purpose of this investigation was to examine experimental procedures for accurate determination of convective transfer coefficients during freezing of food products. Measurements of heat transfer coefficients during cooling of any acrylic transducer and ground beef with similar size and shape were conducted at air temperatures below initial freezing point of the product. The results indicate that accurate measurements of the coefficients can be achieved through nonlinear regression analysis of temperature histories within an acrylic transducer.     

Velocity Distributions of Food Particle Suspensions in Holding Tube Flow: Experimental and Modeling Studies on Average Particle Velocities

Average particle velocities of model food particles were investigated by videotaping particles suspended in sodium carboxymethylcellulose solutions during passage through a transparent holding tube similar in dimension to that of commercial aseptic processing systems. Results indicated the mean normalized velocity of particles was significantly affected by particle Froude number and a dimensionless viscosity. Within the range of this study, particle concentration effects were significant only with respect to standard deviations of distributions. Generalized Reynolds number poorly correlated with mean normalized particle velocity. A mathematical model for particle action was in qualitative agreement with experimental results.     

逆流式三套管换热器的传热计算

三重套管换热器适用于食品、化学等行业冷却（加热）工作介质,并兼作输送管路.本文根据传热基本方程推导纯逆流传热套管的总传热系数,沿传热管长度的流体轴向温度分布,模拟二重套管的对数平均温差和有效总传热系数以便于工程设计计算.     

Heat Transfer to Non-Newtonian Fluid Foods under Laminar Flow Conditions in Horizontal Tubes

This paper presents a study of rates of heat transfer to non-Newtonian fluid foods: mango and papaya nectars, passing through horizontal tubes of circular cross section under laminar flow conditions. Determination of the flow and thermal properties of the fluids under study was performed to adequately explain heat transfer experiences. A laminar mixed convection equation was used for describing experimental heat transfer coefficients: Nu = 1.75 [Gz + 0.0083 (Gr_w . Pr_W)^0.75]^1/3 . δ^1/3 . (K_b/K_w)^0.14 The experimental data covered Graetz numbers between 500 and 7000, and were correlated with a mean deviation of 8.4%. The results should be helpful for interpreting heat transfer to pseudoplastic materials in the design of double tube heat exchangers.     

换热管内置直齿扭带的阻力与传热特性研究

为了研究直齿扭带的阻力与传热特性,将12根不同结构参数的扭带分别置入换热管内进行了冷态与热态实验。实验结果表明,直齿扭带的宽度、扭距、齿间距是影响换热系数和阻力系数的主要因素,并由此建立了实验状况下包含各影响因子的阻力系数关联式和换热系数关联式。直齿扭带比光滑扭带的阻力系数增加了约8.3%~35.4%,而努塞尔系数提高13.1%~74.1%。     

Heat and Mass Transfer Coefficients in Drying of Starch Based Food Systems

Experiments were conducted to measure the temperature and moisture content of starch and starch-fructose samples during drying. A two-dimensional finite element model was developed. An exhaustive parameter estimation technique was applied in conjunction with the model and drying data to estimate the moisture filtration coefficient k_p and the surface heat and mass transfer coefficients, h_q and h_m. Minimization of the deviation between experimental and simulation data resulted in a k_p value of 0.51*10^?7 kg.m/N.h and 0.52*10^?7 kg.m/N.h for the starch and starch-fructose samples. Corresponding values of h_q were 15.8 W/m²K and 21.2 W/m²K and that of h_m were 1.12*10^?6 m/s and 0.94*10^?6 m/s respectively.     

Heat Transfer Coefficients on Cakes Baked in a Tunnel Type Industrial Oven

Using an h-monitor, surface heat flux and effective surface heat transfer coefficients were evaluated during baking of two cakes in a tunnel-type multi-zone industrial oven. An average 75–80% of total heat flux was counted as radiation heat. Air-mass temperature outside the boundary layer was determined from the experimental temperature profiles over the h-monitor top plate. In the range of baking temperatures (186–22 5°C), relative air velocities (0.02-0.437 m/s) and absolute humidities (0.0267–0.0428 kg H₂O/kg dry air) heat transfer coefficients were 20 to 48.0 W/m²K. A simple regression model was developed based on experimental data.