Tomographic Techniques for Measuring Fluid Flow Properties

Experimental fluid velocity profiles can be readily obtained by using tomographic techniques. Combining measurements of a fluid velocity profile with a simultaneous pressure drop permits the evaluation of rheological properties. In order to control a process and to assure product quality, it is useful to monitor the rheological properties in‐line or on‐line. Two tomographic techniques, magnetic resonance imaging (MRI) and ultrasonic Doppler velocimetry (UDV), were used to obtain velocity profiles for a 65.7 °Brix corn syrup solution and a 4.3 °Brix tomato juice. The UDV technique provided velocity profiles that compared well with the MRI method. For the corn syrup, the shear viscosity of 1.37 Pa‐s (UDV) and 1.51 Pa‐s (MRI) agreed well with the offline measurement of 1.57 Pa‐s. The tomato juice was best characterized as a Bingham plastic fluid. The yield stress ranged from 4.44 Pa to 4.70 Pa, which matched well with the off‐line value of 4.50 Pa. The strengths and limitations of both techniques are presented.     

微管道阵列流体流动及其耦合传热的数值模拟

针对一种阵列式微管道换热器,利用FLUENT软件对其进行了数值模拟。将恒壁温、恒热流和流固耦合计算方法的模拟结果进行对比,结果表明:采用流固耦合传热边界条件更有助于揭示换热器局部温度场变化的实际情况,模拟结果与实际情况吻合较好,能够为微管道换热器结构优化设计提供参考依据。     

Fluid Flow and Heat Transfer in Air Jet Impingement in Food Processing

ABSTRACT: Air impingement technology is gaining popularity in food-processing operations such as baking, freezing, drying, and toasting. In this article, physical characteristics of impinging jets, such as turbulent mixing in the free jet region, stagnation, boundary-layer formation, recirculation, and their interactions with food products in terms of heat and mass transfer have been reviewed. The discussion includes experimental methods used for measurement of heat and mass transfer for single and multiple slot and circular jets. Procedures used for measurement of heat-transfer coefficient such as lumped sensor method, micro-calorimetric approach, and use of flux sensors are presented. Typical qualitative and quantitative flow-field studies using planar visualization and laser Doppler anemometry have been reviewed. Numerical modeling of air impingement systems is discussed with special consideration of problems arising in food-processing systems.     

Finite Element Modeling of Fluid Flow, Heat Transfer, and Melting of Biomaterials in a Single-screw Extruder

ABSTRACT: A mathematical model was developed to simulate non-isothermal plug flow of granular starch in feed zone, melting in transition zone, and non-Newtonian melt flow in metering zone and die channel during single-screw extrusion processing. The maximum deviations between the predicted and experimental pressure at the die entrance, product temperature at the die entrance, and minimum residence time were about 20%, 2.5%, and 8%, respectively, of the measured values when the barrel temperatures were 50°C in the feed zone and 120°C in the rest zones. The deviations were within 40%, 4%, and 15%, respectively, of the measured values when the barrel temperatures were 50°C in the feed zone and 140°C in the rest zones. Simulations were carried out to investigate the operating characteristics of an extruder, the melting degree, and profiles of pressure and bulk temperature during extrusion.     

空气冷却器开缝翅片传热与流动特性的数值模拟研究

文章采用CFD软件FLUENT对空压机中间冷却器用开缝翅片和平直翅片的流动和传热进行了三维数值模拟。通过建立管翅结构的局部模型,利用标准κ-ε湍流方程,研究了温度、速度等在翅片附近的分布,并对比了平直翅片和开缝翅片的模拟结果。结果显示:开缝翅片对空气流动有较大扰动作用,在开缝结构附近出现了大量涡流,强化了空气和冷却水之间的传热过程。在同样条件下,开缝翅片空气侧换热系数比平直翅片大8%,而开缝翅片的压力降比平直翅片也相应增大。     

Solubility and Viscous Properties of Casein and Caseinates

An integrated study was conducted on the effects of temperature, pH, salt type and concentration on the functionality of sodium caseinate, calcium caseinate and rennet casein. At 5.6 ≤ pH ≤ 6.2, all proteins exhibited greatest solubility when sodium phosphate was added. Viscosities under these conditions were one to two orders of magnitude greater than solutions at pH < 4.6. Protein solutions and viscosities with added sodium phosphate were not temperature dependent.     

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.     

PBO纤维纸基复合材料的热老化及高温力学性能研究

本研究采用湿法成形技术制备了聚对苯撑苯并二噁唑（PBO）纸,将其浸渍聚酰亚胺（PI）树脂后,得到PBO纤维纸基复合材料（PBO/PI),随后对PBO/PI进行300℃的老化,并在300℃下测试了其拉伸性能。将PBO/PI与模拟蜂窝格壁的间位芳纶浸渍纸（PMIA/PI）进行对比,分析了老化和高温对PBO/PI和PMIA/PI力学性能的影响。结果表明,在300℃的高温老化下,由于材料微裂纹的产生及扩展,二者拉伸强度均呈下降趋势,但老化前后PBO/PI的强度均比PMIA/PI更强。动态力学性能显示,老化前后PBO/PI的储能模量大于PMIA/PI的储能模量,说明PBO/PI的刚性比PMIA/PI大,在高温下仍不易发生变形。在300℃的高温拉伸测试下,PBO/PI的拉伸强度和保持率均比PMIA/PI要高。PBO/PI在常温及300℃高温下的力学性能均优于PMIA/PI,PBO纤维制备的复合材料可用于需要高的抗变形和热稳定性的承重结构和蜂窝部件中。     

山楂果胶的黏度特性及其与化学构造的关联性

山楂果实中含有大量的果胶,其果胶溶液的黏度高于同等条件下市售柠檬果胶的黏度.山楂果胶与柠檬果胶黏度不同的主要是由山楂果胶与柠檬果胶中中性糖组分(Hf1、Lf1)的化学组成与化学构造的不同以及山楂果胶中存在而柠檬果胶中不存在的酸性糖组分(Lf2)所引起的.对山楂果胶中中性糖组分Hf1的化学构造特征进行分析的结果表明,Hf1是一种含有Glc(1→3)Xyl与Xyl(1→6)Glc这两种键合样式的Arabinoxyloglucan.     

Influence of Heat Treatment of Milk on the Flow Properties of Yoghurt

Milk was processed by vat heating (VAT, 85°C for 10 to 40 min), high temperature-short time heating (HTST, 98°C for 0.5 to 1.87 min) and ultra-high-temperature heating (UHT, 140°C for 2 to 8 sec) and made into yoghurt. Apparent viscosity (shear rate 57.75^5-1) ranged from 550–568 mPa.sec for VAT treatments, 260–462 mPa.sec for HTST treatments and 170–333 mPa.s for UHT treatments. Flow behavior indices for all treatments were between 0.55 to 0.65 while consistency index values ranged from 0.3 to 1. Milk processed by HTST systems appeared to be viable alternatives to vat processing. UHT processed milk, however, yielded yoghurts of low viscosity.     

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.     

Temperature and Moisture Dependent Dielectric Properties of Chinese Steamed Bread Using Mixture Equations Related to Microwave Heating

In-package pasteurization treatments based on radio frequency or microwave energy can greatly reduce the danger of microbial spoilage for Chinese steamed bread. The mixture equations methods were used to obtain dielectric properties at the frequency from 1 to 2450 MHz for the temperature range of 25–100ºC and the moisture content range of 40.1–48.5%. The effects of frequency, moisture content, and temperature on Chinese steamed bread dielectric properties were analyzed. The Bottcher Equation was adapted for calculating dielectric constant, the Landau and Lifshitz, Looyenga Equation for dielectric loss factor. Both dielectric constants and dielectric loss factors increased with increasing moisture content and temperature. Radio frequency energies had several times deeper in Chinese steamed bread than microwave energies.     

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.     

Flow Properties of Low-Pulp Concentrated Orange Juice: Effect of Temperature and Concentration

Three 65° Brix low-pulp concentrated orange juice (COJ) samples, between −19 and 30°C were shear-thinning (pseudoplastic) fluids with negligible magnitudes of yield stress. The simple power law model fit welt the shear rate-shear stress data. For one sample, the Powell-Eyring model also described the data well. The Arrhenius model described the effect of temperature on the apparent viscosity and the consistency index of the power law model. The activation energy of flow (E_a) was 10.7 ± 0.2 kcal/g mole. The models of Harper and El Sahrigi and Christiansen and Craig were suitable for describing the combined effect of temperature and shear rate. The magnitude of E_a decreased with decrease in concentration. Apparent viscosity and K increased exponeitially with concentration.     

Apparent Activation Energy of Viscous Flow in Pectin Jellies

A change of state in a cooling jelly sol is preceded by a narrow temperature interval of viscous flow that may be measured, in order to compute the activation energy of such flow. The contribution of decreasing temperatures to the viscosity-increase was 10⁶ times greater than that of time. The apparent activation energy of viscous flow in citrus and apple jelly was found to be of the same order of magnitude, 10⁴ cal/mole. Of the pectins studied, those that were commercially designated as slow-setting tended to have a higher activation energy than those that were rapid-setting. The flow pattern of some jellies, notably from pure citrus and rapid-set pectins, was characterized by a different activation energy at high and low temperatures.     

LED路灯新型热管散热器散热研究

LED(light emitting diode)结温过高不仅会使光波波长漂移,还会使发光效率和寿命下降,为此设计了一种热管翅片散热结构,并运用ANSYS Icepak热分析软件对该结构进行了参数化分析设计,得到了LED最高结温小于70℃时的最优结构参数。同时建立了翅顶为恒温的数学方程,计算验证翅片温度分布,结果显示翅片温度分布相近,散热器整体温度分布均匀,整体温差小于5℃。表明设计的热管翅片散热结构满足大功率LED路灯的使用要求。     

Simultaneous Estimation of the Thermophysical Properties of Selected Liquid Dairy Products by a Transient Heat Flow Probe Method

The thermophysical properties of reconstituted whole milk, skim milk, and whey were measured at various temperatures and concentrations. A new method for the simultaneous determination of thermophysical properties using a modified version of current probe theory method was proposed. Two new correction coefficients were introduced, and using the values of these coefficients and an approximate solution of the heat conduction equation, the thermal conductivity and thermal diffusivity of each sample were determined. The specific heat of each sample was estimated from the definition of thermal diffusivity. These properties were expressed as a function of concentration and temperature.     

Numerical and Experimental Validation of Natural Fiber Orientation in Viscous Fluid of Injection Cavity

In an injection moulded product, the orientation of short fiber in polymer composite influences the strength of the product. A method was developed to predict orientation of natural fiber in thermoset composite. The purpose of this investigation is to predict the orientation of natural fiber in a viscous fluid and to study fiber flow in mould cavity. An experimental set up was developed on injecting viscous fluid with short natural inside the transparent mould cavity and visualize the orientation of short natural fiber and flow front during filling period of cavity. The proposed model for natural fiber orientation was derived by coupling the tangential orientation of natural fiber in flow front and constant curling factor in angular velocity of fluid element. The orientation angle was predicted through proposed model at specified position and was validated with experimental method through digitized image analysis technique.     

Temperature Histories in a UHT Indirect Heat Exchanger

An indirect UHT processing system was modeled by a computer program to predict the temperature profile of a mass element of fluid as it progressed through heating, hold and cooling units. The computer program consisted of successive calculations of the temperature at the exit of numerous increments in each unit based on the overall heat transfer coefficient determined for the total length of the unit. The validity of the computer program was determined by correlation of computer results with experimental determinations of sucrose inversion. The percentage of total inversion of sucrose occurring in the heater was 36%, indicating the error associated with the assumption of chemical reactions based exclusively on hold tube temperature, residence time and laminar flow pattern.