Repetitive control of tubular heat exchangers

Effective operation of heat exchangers involves optimum control of the fluid outlet temperature. Several ideas have been proposed in the literature to cope with intrinsic resonance dynamics aimed at widening the bandwidth of the closed-loop system to achieve fast, well-damped responses in controlling the outlet fluid temperature by means of the steam temperature. This is also the main purpose of this article, in which the use of a repetitive control scheme is proposed to take resonance dynamics into account when residence time is variable, as in this case the control variable is the fluid flow rate and not the steam temperature. The scheme is based on a model of the tubular heat exchanger dynamics in which the explicit terms of resonance are cancelled out by the controller. Simulation results are provided both for a typical tubular heat exchanger and for a special sort of heat exchanger, the distributed collector field of a solar power plant.     

On energy efficient flow provisioning in air-cooled data servers

This study considers how to efficiently manage the thermal state of data servers by provisioning their cooling airflow dynamically. It introduces a thermal network framework that is tailored to capture the temperature dynamics of the on-board thermal inertiae. On top of this modeling, it devises LeakageCooling, a novel flow provisioning controller that balances the temperature dependent leakage dissipation within the electronic chips and the power consumed by the local fans to produce the cooling airflow. The prediction capabilities of the proposed modeling and the overall control performance are assessed over several relevant experimental scenarios on an Open Compute Windmill V2 server.     

Grey box fault detection of heat exchangers

A grey-box model-based method for fault diagnosis is proposed in this paper. The method is based on a first principle model of the process unit, i.e. a heat exchanger, and on a grey-box model of the fault, i.e. the deterioration of the heat transfer surface by aging. During normal operating conditions the heat transfer coefficient is constant or slowly decreasing due to material settling on the heat transfer surface. In old heat exchangers big pieces of settled material can break off causing damage. When this happens, the heat transfer coefficients will rise sharply. In the proposed method a recursive least-squares estimator with forgetting factor is used to track the heat transfer coefficients. The settled material breakage fault is detected via detection of abrupt positive jump in the estimated heat transfer coefficients using a cumulative sum (CUSUM) test. The capability to detect faults in any industrial equipment is heavily dependent on the availability of suitable measurements. For heat exchangers the variables related to the in- and outflows of the equipment (flowrates and temperatures) are usually measured, but measurements along the equipment length are rarely available. Therefore, the possibilities of fault location in space are rather limited. However, simplified models can be used for fault detection in this case. Moreover, a fault detection method is proposed with the possibility of spatial fault location when measurements along the cold side are available. The proposed method is illustrated on simulated examples with different measurement situations.     

Modeling heat exchangers by quadratic Volterra polynomials

Different procedures based on special families of piecewise constant test inputs and intended for identification of the quadratic Volterra polynomials were compared as applied to the problem of automatic control of the nonlinear dynamic system. The results of computer-based experiments for the reference heat exchange model were presented.     

换热网络综合中管壳式换热器设计研究进展

管壳式换热器应用广泛尤其是对其设计的研究,回顾了近年来单台管壳式换热器的工艺设计方法,并指出这些方法难以兼顾换热网络的节能降耗。许多研究者,在换热网络最优综合的同时提出了多种面积裕量优化设计方法,虽然考虑了换热网络中各换热器换热面积的优化,但并未考虑换热器的选型与详细设计。近年来,也有研究者基于换热网络综合进行管壳式换热器的详细设计研究,同步优化换热网络和单台换热器的设计,但这些方法均是在给定的工况条件下进行设计,考虑到实际生产中工况条件经常发生变化,单台换热器的设计均采用超余设计难以保证总费用最小。最后,在此基础上,提出一种从灵敏度分析的角度出发以总费用最小为目标基于换热网络综合的管壳式换热器详细设计思路。     

Bounded positive control for double-pipe heat exchangers

In this work, an outlet temperature control scheme for double-pipe heat exchangers is proposed. Compared to previously proposed approaches, the algorithm developed here takes into account and actually exploits the analytical and stability properties inherent to the open-loop dynamics. As a result, outlet temperature regulation is achieved through a simple controller which does not need to feed back the whole state vector and does not depend on the exact value of the process parameters. Moreover, the proposed approach guarantees positivity and boundedness of the input flow rate without entailing a complex control algorithm. The analytical developments are corroborated through simulation and experimental results.     

Fabrication of metallic heat exchangers using sacrificial polymermandrils

This paper demonstrates the use of poly(dimethylsiloxane) (PDMS), polyurethane (PU), epoxy, and poly(methyl methacrylate) (PMMA) as mandrils to fabricate metallic heat exchangers having 300-700 μm internal channels. The mandrils were prepared using two soft lithographic techniques-replica molding and microembossing. To fabricate the heat exchangers, the polymeric mandrils were coated with a thin layer of metal by thermal evaporation or sputtering; this layer acted as the cathode for electrodeposition of a shell of nickel or copper that was 100 μm thick. The polymers were removed by burning them out at 400°C in air, or by dissolving them with a tetrahydrofuran solution of tetrabutylammonium fluoride. Studies of heat dissipation showed that the nickel heat exchangers with features that range in size from 150-750 μm have thermal resistances ranging from 0.07 to 0.12°^-2 C W^-1 cm at flow rates of water of ~20 L h^-1 and pressures of 8.6-83×10³ N m^-2     

Model based control of compact heat exchangers independent of the heat transfer behavior

Compact heat exchangers have a wide range of applications where standard control strategies typically rely on the knowledge of the heat transfer model and thus on the overall heat transfer coefficient. In particular for compact plate heat exchangers, the overall heat transfer coefficient strongly varies with the manufacturer's plate design and has to be identified by means of extensive measurements. This paper presents an alternative approach for the control of compact heat exchangers which can be implemented without the knowledge of the heat transfer behavior and is robust against changes in the coolant supply system. For this, a model based control strategy is presented which relies on the total thermal energy stored in the fluids of the heat exchanger as control variable instead of the outlet temperature. Furthermore, two methods are developed in order to estimate the total thermal energy, one based on a Kalman Filter and the other one on quasi-static considerations. Finally, the proposed control and estimation strategies are validated by means of simulation and measurement results on an industrial plate heat exchanger.     

人字形波纹板式换热器性能数值模拟的研究

基于简化模型的计算结果难以准确描述换热器内完整的流体流动和换热特性.为此,本文建立与人字形波纹板片完全相同的,含分配区和传热区冷热双流道换热的计算模型,用计算流体力学软件Fluent 6.3,数值模拟4组不同名义速度下流体的流动和换热情况.分析流道内速度场和温度场发现,进口分配区对流体流动分布和换热都有显著影响,还将流体在流道内的流动情况详细描述.两侧流体的压降和进出口温差的计算值与实验值的误差小于6%,较准确地反映了换热器内整体的流动和换热特性,可直接用于研究板式换热器的性能,具有一定的工程实际意义.     

板翅式换热器平直翅片的数值模拟

利用CFD的方法建立了简单、准确计算板翅式换热器平直翅片在层流和湍流区域性能的数值模型。在湍流区域使用低雷诺数湍流模型来替代壁面函数的方法,f因子的平均绝对误差从22.7%降低到7.9%;并用湍流普朗特数模型代替FLUENT软件中默认的湍流普朗特数常量(0.85)使得j因子平均绝对误差从17.9%降低到6.9%;通过以上两个措施使得CFD方法能够更准确预测平直翅片性能。对18种具有不同几何结构参数的平直翅片进行数值模拟;计算结果表明:翅片间距、高度对性能有较大影响而翅片厚度影响较小。同时,回归了j和f因子关联式。     

微通道换热器的数值模拟和结构优化

为提高微通道换热器的换热效率,利用COMSOL耦合求解流动和传热方程,对微通道换热器换热特征进行数值模拟.通过分析微通道换热器的温度、微通道的入口与出口的压差以及微通道换热器的总热阻等参数,对其换热性能进行评估.优化微换热器的几何结构可以有效提高换热性能.数值模拟结果表明：当微通道的高宽比为0.8、微通道与间隔的宽度比为0.6、微通道数为71时热阻最小,换热性能最佳.     

Airside fin efficiencies for finned-tube heat exchangers with forced convection

The heat transfer and mass transfer fin efficiencies were analyzed numerically to show that popular models for heat transfer fin efficiency for circular fins are not always reasonable.The numerical results show that the effective heat transfer area of a circular fin increases several times faster than that of a straight fin for the same tube radius.Then,a simple but accurate heat transfer fin efficiency model was developed and verified by numerical results for a wide range of fin designs.This model predicts...     

Neural networks for cost estimation of shell and tube heat exchangers

The objective of this paper is to develop and test a model of cost estimating for the shell and tube heat exchangers in the early design phase via the application of artificial neural networks (ANN). An ANN model can help the designers to make decisions at the early phases of the design process. With an ANN model, it is possible to obtain a fairly accurate prediction, even when enough and adequate information is not available in the early stages of the design process. This model proved that neural networks are capable of reducing uncertainties related to the cost estimation of a shell and tube heat exchangers.     

Heat exchange in short microtubes and micro heat exchangers with low hydraulic losses

 Miniaturization of heat exchangers opens up the way to a considerable increase of their volumetric characteristics. We examine the changes in heat transfer due to decreasing tube size and relative length. To get a higher overall thermal power, we have considered a “checkmate” design that allows to combine a number of micro cross-flow heat exchange modules. The requirement for low overall pumping power limits the pressure drop and also the minimal diameter of heat exchange tubes. An estimated volumetric heat transfer coefficient of a single symmetric air–air module of 50 mm³ volume (including supply channels) and 6 mm height is 4.3 MW/m³ K (at 260 Pa pressure drop, temperature difference of 5 K, pumping power comprising 4.3% of the thermal power, and micro tube size of 128×1200 μm). Micro machine-tools and assembly devices will allow fabrication of low-cost modules of this and smaller sizes and the ability to combine them into compact heat exchangers with high thermal power. Received: 27 September 1997/Accepted: 20 November 1997     

Thermo-flow characteristics and air flow field leading of the air-cooled condenser cell in a power plant

Special A-frame geometry of the air-cooled condenser cell and the complicated flow field at the exit of the axial flow fan bring on the air mal-distribution on the surface of the finned tube bundles and the deteriorated thermo-flow performances of a condenser cell.It is of benefit to the design and operation optimization of the direct dry cooling system in a power plant to investigate the thermo-flow characteristics of the condenser cell and propose the flow leading measures of cooling air.On the basis of t...     

基于数字滤波算法的风冷热泵机组实验台测控系统设计

为了提高测控系统的测量精度,消除系统中多种干扰噪声的影响,根据风冷热泵机组自身性能及要求,对现有数字滤波算法进行改进,提出一种多级数字滤波算法,有效地消除了测控系统在稳定工作状态下的各种噪声干扰,算法运算效率高、实时性强,滤波效果良好.此外,系统采用通道与测量点双向选择,改善了传统测控系统中测量点与通道固化的缺陷,提高了系统的灵活性和可靠性.实验结果表明,该测控系统功能强大,可操作性强,性能稳定可靠,容错性强,人性化程度高,适合在其他测控系统中推广应用.     

The equivalent solid plate concept in the finite element analysis of tube bundle heat exchangers

The equivalent solid plate concept as used previously in analytical approaches to the mechanical design of tube bundle heat exchangers is derived within the context of finite element analysis. The concept is then applied to the finite element analysis of a typical chemical reactor, and the results are compared with some extensive measurements made during proof testing.     

Particle and thermohydraulic maldistribution of nanofluids in parallel microchannel systems

A deep understanding of fluidic maldistribution in microscale multichannel devices is necessary to achieve optimized flow and heat transfer characteristics. A detailed computational study has been performed using an Eulerian–Lagrangian twin-phase model to determine the concentration and thermohydraulic maldistributions of nanofluids in parallel microchannel systems. The study reveals that nanofluids cannot be treated as homogeneous single-phase fluids in such complex flow situations, and effective property models drastically fail to predict the performance parameters. To comprehend the distribution of the particulate phase, a novel concentration maldistribution factor has been proposed. It has been observed that the distribution of particles does not entirely follow the fluid flow pattern, leading to thermal performance that deviates from those predicted by homogeneous models. Particle maldistribution has been conclusively shown to be due to various migration and diffusive phenomena such as Stokesian drag, Brownian motion and thermophoretic drift. The implications of particle distribution on the cooling performance have been illustrated, and smart fluid effects (reduced magnitude of maximum temperature in critical zones) have been observed for nanofluids. A comprehensive mathematical model to predict the enhanced cooling performance in such flow geometries has been proposed. The article clearly highlights the effectiveness of discrete phase approach in modeling nanofluid thermohydraulics and sheds insight on the specialized behavior of nanofluids in complex flow domains.     

Investigation of parallel heat flow path in electro-thermal microsystems

Due to the miniaturization seen in the last decades, several macro models that neglect some parameters have to be revised in order to evaluate the behavior of different types of microsystems precisely. In this paper we present the analysis of one such parameter which affects the thermal functionality, namely, the conduction of natural gases. In macro scale this parameter is usually neglected, because the conductivity of gases is several magnitudes smaller than that of the base materials. With the advance of microfabrication really good thermal isolation can be achieved and so the values of the conductivity of the air and of the base material can be evenly compared and as a consequence the former models have to be revised. This effect can be easily modeled on microstructures whose thermal resistance can be compared to the thermal resistance of natural gases. In order to observe its behavior appropriately, different types of cantilevers with embedded thermopiles and heating resistors were used. The modeling and experimental results show that in microsystems that are sensitive to temperature change, the parallel heat flow created by the surrounding gases may have significant impact on the operation.