Design and performance evaluation of an ohmic heating unit for thermal processing of highly viscous liquids

The overall objective of this study is to design, build and test a prototype automated ohmic heating cell by using a fluid jet to serve as a basis for modification and scale-up for industrial use. The new technology consists of applying an alternative electrical current to a falling jet between two electrodes. The length of the jet is the primary critical process factor in ohmic heating using a fluid jet. A radar level sensor was validated at different operating conditions. Two air valves, that depressurize or pressurize the ohmic cell, ensure the control of the jet length for precision and reliability. The thermal performances and the technical feasibility of this innovated automated ohmic heater were approved. Tests with carboxymethylcellulose (CMC) solutions (2%, w/w) demonstrated that this technology can be efficient for thermal processing of highly viscous liquids.     

Influence of ohmic heating on rheological and electrical properties of reconstituted whey solutions

Reconstituted whey solutions (in the range of 8–24% w/v solute concentrations) were heated from 20 °C to prescribed temperatures (30, 40, 50, 60, 70 or 80 °C) ohmically by applying voltage gradients of 20, 30 or 40 V/cm, and conventionally at water bath. Electrical conductivity changes with increasing temperature were linear during ohmic heating. Whey solutions have non-Newtonian characteristics since Herschel–Bulkley model satisfactorily fitted the experimental shear stress–shear rate data. “n” values were in the range of 0.520–1.503. The whey solution having 24% concentration had a yield stress of 0.006–0.024 Pa at low temperatures. Although temperature and concentration were critical factors for the consistency and the electrical conductivity changes during heating (p < 0.01), the voltage gradient was not effective statistically. The high correlation (between ?0.910 and ?0.991) was obtained between changes of electrical conductivity and consistency coefficient values during ohmic heating. Since activation energies for ohmic heating (26.34–45.79 kJ/mol) depending on solute concentration were lower than conventional heating (26.70–50.04 kJ/mol), reconstituted whey solutions were less sensible to temperature changes during ohmic heating. It was recommended that ohmic heating could be applied as a faster alternative heating method in the whey processing.     

Effect of Blanching by Ohmic Heating on the Osmotic Dehydration Behavior of Apple Cubes

The effect of blanching by ohmic heating (OH) on the damage to apple tissues and subsequent osmotic dehydration kinetics was investigated. Apple cubes were heated ohmically to various blanching scales. Heating temperature and duration were, respectively, 60–95 ± 2°C and 0–6 min. After cooling, the treated samples were put into sucrose solutions (70 °B) for the osmotic dehydration (OD). The equilibrium state of osmotic dehydration was estimated using the Azuara model. Ohmic heating leads, even for short treatments, to significant changes in the cellular structure of apples and to the enhancement of mass transfer during osmotic dehydration.     

Numerical analysis of penetration lengths in submerged supercritical water jets

“Hydrothermal spallation drilling” is a possible alternative drilling technology that uses the properties of certain rock types to disintegrate into small fragments when heated up rapidly by a hot impinging fluid jet. Hot supercritical water jets are favored to provide the required heat for thermal rock fragmentation. However, the indispensable presence of a dense water-based drilling fluid during operation can cause considerable heat losses in the supercritical water jet before impingement on the rock surface. To predict these heat losses from the hot jet to the cold aqueous environment, a numerical model based on the commercial CFD tool ANSYS FLUENT^® was established. Penetration lengths of the supercritical jet plume at near-critical pressures were determined numerically and validated with experimental values for a wide range of conditions. Experiments and simulations showed an acceptable agreement and the experimental trends were satisfactorily predicted by the model.     

Degradation kinetics of ascorbic acid during ohmic heating with stainless steel electrodes

Ascorbic acid degradation experiments were performed in buffer solution at pH 3.5 using a batch ohmic heater with uncoated stainless steel electrodes. The electrical conductivity of the buffer solution was adjusted using sodium chloride. The concentration of ascorbic acid was found using an HPLC technique. Kinetics of degradation can be described adequately by a first order model for both conventional and ohmic treatments, but unlike conventional heating, the temperature dependence of degradation for some ohmic treatments cannot be represented by the Arrhenius relation. During ohmic heating, power, temperature and NaCl content affect the degradation rate. A number of reactions, including electrode reactions, electrolysis of the solution, as well as reactions between electrode materials and electrolysis products may influence the reaction mechanism as well as kinetic parameters. At the highest power and salt content, citrate complexation and a significant loss of buffering capacity were noted, resulting in an increase in pH. The results underline the importance of inert electrode coatings, or the use of high frequency power for control of electrochemical reactions.     

Viscous Heating in a Wind Heater Device: CFD Simulation and Experiments

A wind heater device was developed based on three-dimensional computational fluid dynamics (CFD) simulation in order to generate heat using renewable energies, like wind. Frictional heat generation by viscous dissipation in a highly viscous liquid contained in the annular space between two rotating cylinders was investigated analytically and numerically. The results were also validated using experimental data, which showed very good agreement, the average error being low, with modeling data. The heat generated can be utilized for any general heating purpose. The effect of design, material, and viscous fluid properties on the efficiency of the heater was investigated. The simulation results showed that more heat energy is generated by increasing the oil viscosity and the rotational speed of the cylinders. Then, the best design, the one which generated more heat energy, was built. The results showed a reasonable compatibility with simulation outcomes.     

EFFECT OF LARGE TEMPERATURE DIFFERENCES ON LOCAL NUSSELT NUMBER UNDER TURBULENT SLOT IMPINGEMENT JET

In heating, cooling or drying applications involving large temperature differences between the jet and the target surface, it is necessary to incorporate the temperature-dependence of fluid properties on the flow and temperature fields. Despite their frequent occurrence in industrial practice, there is little research reported in the literature on this subject. It is also necessary to distinguish between heating and cooling applications since the thermo-physical properties of the fluid in the vicinity of the target surface vary in different directions for the two cases. The objective of this work is to present computational fluid dynamic model results for heat transfer under a semi-confined slot turbulent jet under thermal boundary conditions such that the temperature-dependence of the fluid properties affects the flow and thermal fields. A comparative analysis in the turbulent flow regimes is made of the standard k-ε and Reynolds stress turbulence models for constant target surface temperature. Nusselt number distributions with different definitions of Nusselt number were compared. The results show that, under large temperature differences between the jet and the target surface, the Nusselt number calculated at jet temperature shows the least spread. Results are consistent with the very limited experimental results available in the literature.     

柱形流化床传热特性的数值模拟

对Shedid等搭建的圆柱体流化床采用欧拉?欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。     

EFFECT OF LARGE TEMPERATURE DIFFERENCES ON LOCAL NUSSELT NUMBER UNDER TURBULENT SLOT IMPINGEMENT JET

ABSTRACT

In heating, cooling or drying applications involving large temperature differences between the jet and the target surface, it is necessary to incorporate the temperature-dependence of fluid properties on the flow and temperature fields. Despite their frequent occurrence in industrial practice, there is little research reported in the literature on this subject. It is also necessary to distinguish between heating and cooling applications since the thermo-physical properties of the fluid in the vicinity of the target surface vary in different directions for the two cases. The objective of this work is to present computational fluid dynamic model results for heat transfer under a semi-confined slot turbulent jet under thermal boundary conditions such that the temperature-dependence of the fluid properties affects the flow and thermal fields. A comparative analysis in the turbulent flow regimes is made of the standard k–ε and Reynolds stress turbulence models for constant target surface temperature. Nusselt number distributions with different definitions of Nusselt number were compared. The results show that, under large temperature differences between the jet and the target surface, the Nusselt number calculated at jet temperature shows the least spread. Results are consistent with the very limited experimental results available in the literature.     

激光与微波协同加热碳纤维的温度与热应力分布规律

利用仿真软件建立微波加热-激光加热-热辐射-流动传热-固体力学多物理场模型，研究了激光加热直径1mm碳纤维丝束的温度场分布与热应力大小，以及不同激光功率对于温度场与热应力的影响。同时，首次提出激光加热与微波加热结合的方式调节温度场分布与热应力大小的方法，仿真结果显示激光加热与微波加热结合的方式可以改善碳纤维丝束温度场的分布，有效降低丝束加热过程中的热应力。     

防爆集束式井口加热器设计

为了解决井口加热原油实验仪器的贫乏,加热不均匀及加热效率低等问题,根据牛顿冷却公式和傅里叶定律,并考虑了原油温度和压力的影响,对加热器加热效率进行了研究。在此基础上设计了一种油井井口加热原油的实验装置。该装置在加热管上安装旋流片增大原油流动距离,在进口处安装涡流装置用以改变原油流态,增大传热系数,并起到充分混合油水的作用。该实验装置能够研究流体流态对加热效率的影响,为井口加热原油工艺设计提供实验依据,为高效的井口加热器设计提供实验基础。     

Influence of heat transfer and curing on the quality of pultruded composites. II: Modeling and simulation

In this study, a computer simulation code is developed to predict the dynamics of heat transfer in the pultrusion process. The die block and heater arrangement are included in the heat transfer analysis so that the simulation can provide the temperature profile at the interface between the die and the composite. The measured interface temperature profiles are then used to validate the simulation code. Energy management, i.e. heater power control along the heating die, is also considered in the simulation code. The code is capable of carrying out transient thermal analysis for both start-up and steady-state operation of the pultrusion process. From the experimental observations on the part quality in terms of blister formation, a processing window was obtained by showing the relation of the die length and the line speed to the part quality. The processing window is then generated numerically using the computer code based on the definition of a critical die length proposed in this work, and the result shows good agreement with the experimental data.     

电热水器内部结构优化及储能特性研究

将相变储能技术应用于电热水器,并通过添加石墨纳米颗粒改善相变材料的导热特性,对其储能过程进行调节,可以起到"移峰填谷"的作用.建立了四种不同结构的电热水器三维模型,模拟了电热水器内部速度场与温度场分布特性.考察了进出口水管结构、电加热管布置方式、保温层结构等因素对热水器内部流场及传热特性的影响,研究了不同储能层厚度对电...     

利用铂电阻测量油水两相流含油率

利用铂电阻基于流体传热方程测量油水两相流的含油率。研究了油水两相流含油率与电热器上下游铂电阻的温差和油水总流量之间的关系,提出用温差、流量校准系数对铂电阻的测量温差和油水总流量进行校正,对校正后得到的含油率测量模型进行了实验验证。实验结果表明含油率在0-50%范围内,平均测量误差为3.89%,提高了含油率的测量精度。     

基于膨胀机-热泵（st-hp）的大型吸收式热电联产机组集中供热方法

提出基于螺杆膨胀机-热泵（st-hp）的热电联产供热方法来挖掘汽水系统的余热余压利用潜力,实现了更好的节能效果和更高的供热功率,基于热力学第二定律和理论对主要新增元件在变工况下进行了?分析和分析。通过让中压缸排汽“余压发电、余热采暖”的手段提高集中供热能力;利用算法设计热力站处吸收式换热装置以提升一次热供回网水温差,并予以模型验证;以某600MW机组为算例,利用Ebsilon软件分析变工况下电厂侧新增主要供热元件效率和效率。结果表明：st-hp系统不仅能在不增加市政热网运输管径和机组出力的同时使供热量提高50%,而且满足部分厂用电需求;吸收式热泵和尖峰加热器的节能研究重点是提高热力循环的完善程度以减少?损;螺杆膨胀机和尖峰加热器应着重于提高传热过程中工质的速度场和温度场协同程度。运行工况对机组热经济性影响很大,热负荷不同时,发电煤耗率极差极小值和极大值分别为-5g/(kW·h)和22.97g/(kW·h)。     

Thermal conversion of Shengli residue and its constituents

Shengli residue was separated into several fractions by using alumina column chromatography and supercritical fluid separation. The various fractions, and the residue itself, were decomposed in a quartz boat heated by a tubular heater. The products of thermal conversion (volatiles, benzene-solubles and benzene-insolubles) were recovered, and the variation of these product yields with time and temperature was investigated. The data show that most of the benzene-insolubles came from the thermal conversion of the resins, and only an insignificant amount was obtained from the aromatics. In the present study, the relation between product yields of thermal conversion of the residue and its fractions was investigated, and the thermal conversion of the resin fractions of different molecular weight was also discussed.     

Wirtschaftliches Aufheizen von Flüssigkeiten mit Hilfe von Dampfstrahl-Anwärmern

Economic Heating of Liquids Using Steam Jet Heaters In the processing of fats and oils many processing steps involve heating. Several examples given include heating of oilseed cookers, rendering of fat-bearing materials in cookers, heating of driers, heating of flow-type and stirred tanks, and direct heating of liquids. Heat transfer is accomplished, either indirectly from a medium at a higher temperature via a heat exchange surface or directly to the product to be heated. The heating capacity must be adjusted to ensure the desired temperature for the respective course of reaction. During this process, the flow of heat to be transferred can vary strongly. Therefore, the flow of steam added to a system must be adjusted with the aid of a vapour pressure regulator. A new type of steam jet heater features such a high regulating capability.     

Model parameterisation of nonlinear devices using impedance spectroscopy

Spatially resolved simulation models are valuable tools to predict the behaviour (electrical, thermal, ageing) of energy storage systems. However, parameterisation of the electrical impedance-based model is often difficult because local measurements are hardly possible. For homogeneous conditions, e.g. homogeneous current, state of charge and temperature distribution, the measured overall impedance of a cell or electrode can be divided equally over the volume elements of the spatially resolved model. Homogeneous conditions are though very rare, so all larger devices show at least a current distribution because of non-zero ohmic resistances in current collectors, active masses and electrolyte.In this paper, a method is presented and validated for calculating the local impedance of nonlinear inhomogeneous devices from the overall impedance of the device. The method consists of iteration between impedance parameter identification and calculation of current distribution.     

PHOTOVOLTAIC DRYER WITH DUAL PACKED BEDS FOR DRYING MEDICAL HERB

Abstract

This work presents design and optimization of a cylindrical photovoltaic dryer with dual packed beds thermal energy storage for drying medical herb. The dryer is provided with electrical heater where the electrical energy is generated by using photovoltaic system. The electrical heater is designed and sized to realize continuouse drying (day and night) to minimize the drying time. Two packed beds are used to fix the drying temperature in dryer during day and night. The main packed bed thermal energy storage is charged during the sun-hours directly, to realize continued drying after sunset. An efficient PV dryer is devised to work under forced air created by air blower and heated by the electrical coils.     

空气源热泵热水系统即刻加热模式和循环加热模式的对比

建立了空气源热泵在即刻加热（即热）模式和循环加热（循环）模式下制取热水的对比试验台,并对两种模式制取热水的运行性能作了对比研究。在环境温度为(19±0.5)℃条件下,分别将176 kg热水从初温20℃加热至55℃,即热模式平均COP比循环模式高出24%,同时冷凝加热功率也提高了约20%。结果表明,即热模式不仅具有更高的COP,还具有更高的冷凝加热功率,节约了电能消耗,缩短了加热时间。同时即热模式下冷凝压力、压缩比、压缩机最高出口温度等重要参数都要优于循环模式。显示出了空气源热泵热水系统在即热模式下具有更优越的热泵性能。