On-Line Fuzzy-Logic-Based Temperature Control of a Concentric-Tube Heat Exchanger Facility

We have developed a fuzzy-rule-based controller to perform on-line temperature control of a concentric-tube heat exchanger facility. The rules for the controller were derived from dynamical values of the mass flow rates and fluid temperatures in the heat exchanger. The fuzzy controller was embedded in a closed-loop, single-input single-output system to control the outlet temperature of the cold fluid. The development of the controller was done in two phases, the difference between the phases being the information about the system that was used to build the corresponding control scheme. To validate this fuzzy controller, a series of experiments, corresponding to regulation and tracking of a setpoint, was carried out for maintaining specific values of the outlet temperature under different perturbations. Results from this investigation demonstrate that the fuzzy-rule-based controller is able to effect control of the system, and that the amount of information about the plant provided to it is important in terms of accuracy and efficiency.     

果蔬预冷冰浆式湿冷热湿交换器性能实验

针对果蔬预冷设备应用场合,提出并设计了一套以冰浆作为载冷介质的湿冷热湿交换器,并搭建单体性能测试台架,以出风温度和相对湿度为指标,通过改变填料类型(金属、纸质填料)、载冷介质种类(冰浆、冷水)和喷淋流量进行了性能实验研究。结果表明:实验工况下,金属填料的换热性能较纸质填料好;以冰浆作为载冷介质相比以冷水的情况,可以获得更低的出风温度,但出风相对湿度也有所降低;随着进风干球温度的降低,出风温度明显降低,而出风相对湿度变化并不明显;在一定范围内,提高载冷介质的喷淋流量,有利于湿冷热湿交换器出风温度的降低和出风相对湿度的升高;低浓度的冰浆可以在湿冷热湿交换器中稳定运行,且降温效果较冷水湿冷热湿交换器更加明显,虽然相对湿度略有下降但仍然可保持在90%左右,适用于果蔬预冷和保鲜。     

NUMERICAL SIMULATION OF THE THERMAL CONTROL OF HEAT EXCHANGERS

The temperature control of outlet air by changing the water flow rate in a single-pass waterto-air cross-flow heat exchanger is investigated. The conservation laws are applied to finite control volumes and an implicit formulation is used for transient numerical solutions. Conjugate forced convection heat transfer from the tube is solved to calculate the temperatures of the air and water coming out of the heat exchanger. In the simulations the outlet air temperature is controlled by changing the water flow rate entering the heat exchanger using a proportional-integral (PI) controller. The range of controllability of the heat exchanger was studied first. Then disturbances in the form of step changes in the inlet air temperature, the air flow rate, and the set point temperature were separately introduced. The effects of the limiting-condition constraints and different control parameters on controlling the outlet air temperature are presented. The results show that the control behavior can be simulated numerically and that this control methodology is effective within limits.     

New Characterization of Fluid Temperature Profiles in a Heat Exchanger

Although the determination of heat exchanger performance based on the log-mean-temperature-difference approach or the temperature effectiveness approach had been well established, the understanding of the fluid temperature variation along the heat exchanger was still essential in some situations in which the fluid properties were expected to vary substantially across the heat exchanger. In this study, two normalized temperature surpluses were employed to predict the temperature profiles of the hot and cold fluid streams. With specified capacitance rates for the hot and cold fluid streams and the overall heat transfer value of the heat exchanger, unified profiles for the normalized temperature surplus along the heat exchanger could be formulated irrespective of the actual fluid entering temperature. The differences between the mean normalized temperature surplus and that based on the arithmetic mean between the fluid inlet and outlet increased when the capacitance rates of the two fluid streams departed more. By comparing the simulated performances of a sample waste heat recuperator based on both mean-temperature approaches to evaluate the respective average fluid properties with those using a detailed numerical approach over a range of entering fluid temperatures, it was found that the mean normalized temperature surplus was better.     

连续螺旋折流板管壳式换热器动态特性研究及预测

建立了可进行壳管式换热器动态特性试验研究系统,通过试验研究的方法对水-油为换热工质的连续螺旋折流板管壳式换热器动态特性进行了试验研究,进口流量扰动为等百分比流量特性,研究了4种流量扰动方式下水和油出口温度的动态响应。同时研究了在一定Re数下,不同的流体扰动量对换热器进出口温升的影响,得到了换热器进出口温升与流体扰动量之间的关联式。实验表明,液液换热系统温度的动态响应时间比较长,研究发现在正负的流量扰动下,换热器进出口温度变化呈现线性变化,进出口温升在正负流量扰动下其变化曲线具有对称特征。分别建立了有限差分数值预测模型及人工神经网络模型对换热器油侧的出口温度进行了动态预测,预测结果与试验值符合良好,人工神经网络的预测结果要好于数值模拟预测,其偏差绝对值在1.3%以内,表明人工神经网络在进行复杂的系统辨识时具有一定的参考及应用价值。     

Predicting the fluid temperature at the exit of the vertical ground heat exchangers

The energy analysis of ground source heat pump systems is based on the instantaneous fluid temperature at the ground heat exchanger outlet. This temperature defines the ground source heat pump coefficient of performance (COP) and hence the electricity consumption required in order to fulfill the energy demands of the building. The aim of this work is to present a model able to predict the fluid temperature at the ground heat exchanger outlet, taking into account the heat transfer phenomena in the soil and the temporal variation of the thermal load of the ground heat exchanger. The model developed was verified using experimental data, expanding over a three years period, of a vertical ground heat exchanger. It is proved that the model is able to satisfactorily predict the recorded temperature values throughout the verification period. The differences between measured and estimated outlet water temperatures impose a deviation between the estimated and the actually recorded electricity consumption of less than 4%.     

Dynamics of plate heat exchangers subject to flow variations

A predictive model has been presented to suggest the transient response of plate heat exchangers, subjected to a step flow variation. The work also brings out the effect of the port to channel maldistribution on the performance of plate heat exchangers under the condition of flow variation. The results indicate that flow maldistribution affects the performance of the plate heat exchangers in the transient regime. A wide range of the parametric study has been presented which brings out the effects of NTU and heat capacity rate ratio on the response of the plate heat exchanger, subjected flow perturbation.To verify the presented theoretical model, appropriate experiments have been carried out. Experiments include the responses of the outlet temperatures subjected to inlet temperature transient in the circuit followed by a sudden change in flow rate in one of the fluids. Simulated performance has been compared to the performance measured in the experiments. Comparisons indicate that theoretical model developed for flow transient is capable of predicting the transient performance of the plate heat exchangers satisfactorily, under the given conditions of changed flow rates.     

Improvement of thermal characteristics of latent heat thermal energy storage units using carbon-fiber brushes: experiments and modeling

Brushes made of carbon fibers with a high thermal conductivity are inserted on the shell side of a heat exchanger to enhance the conductive heat transfer rates in phase change materials. The experimental results show that the brushes essentially improve the heat exchange rate during the charge and discharge processes even when the volume fractions of the fibers are about one percent. A three-dimensional model describing the heat transfer in the heat exchanger is numerically solved. The model predicts well the experimental outlet fluid temperatures and the local temperatures in the composite.     

Dynamic Characteristics of a Free-Flow-Channel Heat Exchanger

The performance of sewage heat exchangers plays an important role in the sewage source heat pump systems when the sewage does not enter the heat pump unit directly, especially when untreated sewage is used as heat or cold source. In this article, a free-flow channel heat exchanger is proposed to be used in an untreated sewage source heat pump system. First the article designs the sizes of heat exchanger according to the real system requirements, and then it models and analyzes dynamic characteristics on the simulink platform. The transfer functions that are suitable for the heat exchanger are deduced. The analysis shows that the wall temperature is 9.1°C when the untreated sewage and water temperatures at the inlet are 15 and 7°C, respectively. The result is obtained when considering the temperature at the outlet is affected by the temperature at the inlet. The variable quantity of water temperature at the outlet is affected greatly by the variable quantity of water temperature at the inlet and the variable quantity of sewage temperature at the outlet by the variable quantity of sewage temperature at the inlet. Besides, the measured values of water temperatures at the inlet and outlet are also observed from the real system where the free-flow channel heat exchanger is adopted. The variable quantities of water temperature at the outlet are calculated according to the measured values, and are compared with the model results deduced by the transfer functions. The comparative result shows that the differences between the measured and model results of water temperature variable quantities at the outlet range from ?1 to 1°C, thus getting the water temperatures at the outlet according to the model results. The differences of the water temperature at the outlet between the measured values and the deduced results are from ?0.5 to 0.5°C. It illustrates the temperature at the outlet can be estimated by modeling the temperature variable quantity at the outlet.     

Performance prediction of plate-fin radiator for low temperature preheating system of proton exchange membrane fuel cells using CFD simulation

The objective of this paper is to analyze the heat transfer characteristics of plate-fin radiator for the cold air heating system of a PEMFC engine and to find the optimal parameter combination in order to reduce the power consumption. The effect of the coolant mass flow and temperature on the heat exchange performance of the radiator was investigated based on 3D porous medium model. The results, including the amount of heat transferred and temperature change and heat exchanger effectivity with the increasing of the air flow rate at different coolant flow rate were obtained using CFD method. Good agreement is found by comparing the simulation values with the test data and the deviation is less than 7% which indicate simulation model validation and research method feasibility used in this study. The simulation results indicate that bigger coolant flow rate and temperature result in higher outlet air temperature and the amount of heat transferred. The variation of the heat exchanger effectivity is predicted for different working conditions. Based on the Taguchi method, the influence of structural parameters of the corrugated fins on the heat transfer and pressure drop of the radiator is analyzed qualitatively. It is shown that fin length has the greatest impact on the comprehensive heat transfer performance of the radiator. This research provides a guide for optimizing the air preheating system and improving the amount of heat transferred.     

Design of A Single Tube Fuel Oil Preheater

This article presents design analysis of a fuel oil preheater. The preheater is a concentric tube heat exchanger where the fuel oil is on the tube-side and the heating medium flows in the annular space. Two situations were addressed in this study. In the first case, an analytical model was developed where the required heat exchanger length, diameter and the fluid velocity were determined for a given heat transfer duty and for an allowable pressure loss. A detailed study was conducted by individually varying parameters such as pressure loss, preheater discharge temperature, and mass flow rate of the fuel oil. In each instance their influence on the predicted design of the heat exchanger was investigated. In the second case, an optimization strategy was proposed for a certain heat transfer duty. The heat exchanger dimensions and the fluid flow rate were selected such that the annual operating cost of the heat exchanger was minimized. In addition, a detailed study was conducted to understand the total annual operating cost as a function of the fuel oil outlet temperature and the fuel oil mass flow rate.     

The benefit of dividing an indirect thermal storage into two compartments: Discharge experiments

Experiments are presented to demonstrate the benefits of dividing an indirect thermal storage into two compartments. The transient discharge experiments were conducted in an undivided and equally divided 126 l rectangular storage vessel, which has a height to depth aspect ratio of 9.3:1 and is inclined at 30° to the horizontal. A 240-tube copper heat exchanger with a total surface area of 2.38 m² was immersed in the storage fluid. For the divided storage, the heat exchanger flow path was in series through the two compartments. Water flow rate through the heat exchanger was varied from 0.05 to 0.15 kg/s to demonstrate the effect of varying the number of transfer units (NTU) from 2.2 to 7 on the relative performance of undivided and divided storage vessels. Reported measurements include transient storage temperature distribution, heat exchanger outlet temperature, delivered energy, and exergy of the divided and undivided storage. The divided storage provides higher energy delivery rates and higher heat exchanger outlet temperatures during most of the discharge. The magnitude of these benefits depends on NTU and the extent of discharge. For a flow rate of 0.05 kg/s, corresponding to a nominal NTU of 7, the divided storage delivers a maximum of 11% more energy than the undivided storage when 100 l of hot water or 55% of the stored energy has been delivered. For a flow rate of 0.15 kg/s, corresponding to a nominal NTU of 2.5, the divided storage delivers a maximum of 5% more energy at the same level of discharge. Data agree with first and second law analyses of a storage system comprised of two tanks in series.     

Performance Optimization of Co-current Gas-to-Gas Micro Heat Exchanger Considering Radiation

Optimization of a parallel flow gas-to-gas tubular micro heat exchanger with hot core and cold annulus fluid is numerically analyzed, considering the beneficial role of surface radiation. Operating and geometric parameters are varied for fixed overall mass flow rate and temperature of cold core fluid, to study the effects on the following performance parameters: heat transferred to annulus fluid, logarithmic mean temperature difference, effectiveness, and volumetric heat transfer coefficient. The micro heat exchanger is optimized for high heat transfer to annulus fluid and volumetric heat transfer coefficient, for different operating and geometric conditions. Optimization for high volumetric heat transfer coefficient maximizes the micro heat exchanger effectiveness, heat transferred and improves logarithmic mean temperature difference.     

Heat exchanger fouling model and preventive maintenance scheduling tool

The crude preheat train (CPT) in a petroleum refinery consists of a set of large heat exchangers which recovers the waste heat from product streams to preheat the crude oil. In these exchangers the overall heat transfer coefficient reduces significantly during operation due to fouling. The rate of fouling is highly dependent on the properties of the crude blends being processed as well as the operating temperature and flow conditions. The objective of this paper is to develop a predictive model using statistical methods which can a priori predict the rate of the fouling and the decrease in heat transfer efficiency in a heat exchanger. A neural network based fouling model has been developed using historical plant operating data. Root mean square error (RMSE) of the predictions in tube- and shell-side outlet temperatures of 1.83% and 0.93%, respectively, with a correlation coefficient, R², of 0.98 and correct directional change (CDC) values of more than 92% show that the model is adequately accurate. A case study illustrates the methodology by which the predictive model can be used to develop a preventive maintenance scheduling tool.     

热负荷阶跃变化条件下热回流式换热系统缺陷机理及优化方法研究

针对国内某北方核电厂的设备冷却水系统热回流式换热器,分析了不同热负荷下热回流式换热系统的稳态特性及负荷阶跃变化下热回流式换热系统缺陷机理,提出了热回流式换热器系统优化方法。研究表明:在不同热负荷下热回流式换热器系统切换的关键是与不同热负荷对应的具有特定温度的伴流的形成;热负荷阶跃变化下状态转换瞬态过程中存在系统缺陷,其根本原因在于单纯采取调节热回流率的方法,具有较大的时间滞后性;调节换热器冷介质侧的流体流量,改变换热器传热系数,强化了对状态改变的快速响应;采取热回流叠加换热器旁流方法,可以解决原有系统状态转换瞬态过程中存在的缺陷。     

Full Model Simulation of Shellside Thermal Augmentation of Small Heat Exchanger with Two Tube Passes

A small heat exchanger with segmental baffles and two tube passes was built and experimented to investigate the shellside heat transfer enhancement in our previous research. In the present paper, the full numerical model is constructed for the experimented heat exchanger of two tube passes with the commercial software ANSYS Fluent, where the computation domain consists of not only the fluids on the shell and tube sides but also the solids of baffles and tubes. The conjugated heat transfer between the hot and cold fluids through tubes is modeled. To check the prediction precision of current full model, the computed fluid outlet temperature and heat transfer rate are compared with experimental counterparts. With current full model, the effects of baffle number on the performances of heat transfer and flow resistance are studied, and the discussions are conducted with the scalar contours and velocity vectors. The current research could be used for the industrial application of small segmental baffle heat exchangers with multiple tube passes.     

The Influence of Core Capacitance on the Dynamic Performance of a Single-Phase Natural Circulation Loop With End Heat Exchangers

The effect of wall-core capacitance of heat exchangers on the dynamic behavior of a natural circulation loop (NCL) with end heat exchangers is studied under various excitations such as step, ramp, exponential, and sinusoidal. The transient one-dimensional conservation equations are derived for loop fluid, hot and cold fluid streams, and wall core of both heat exchangers. The solution of a set of transient partial differential equations and one integro-differential equation for loop fluid circulation rate is achieved through a finite-element technique. Imposing the excitations to the inlet temperature of hot fluid, the effects of wall-core capacitance on the responses of outlet temperatures of both hot and cold fluid streams and flow rate of loop fluid are studied. Wall-core capacitance diminishes the initial transients and delays the inception of hot and cold fluids outlet temperature profiles as well as loop fluid flow profile. Further, it has the ability to bring even unstable system behavior with reverse flows into a stable system with steady loop flow rate through quickly decaying oscillations. System responses are also greatly influenced by boundary conditions such as hot and cold fluids flow rates and their inlet temperature excitations such as step, ramp, and exponential. As flow stability is an important subject for single-phase NCLs, a stability map is constructed and compared with zero wall-core capacitance. Inclusion of wall-core capacitance in the present study reveals the important fact that the stable state operating zone widens with the wall-core capacitance.     

Issues on the Testing of Small Cryogenic Recuperators and Experimental Studies on Perforated Plate Heat Exchangers

This paper deals with various issues on testing heat exchangers at cryogenic temperatures, the test setup and experimental studies on perforated plate heat exchangers. A number of fluid flow circuits are used in practice for testing a heat exchanger at cryogenic temperatures. These circuits are reviewed and a simplified fluid circuit is proposed with justifications. Effects of the fluid inlet/outlet tubes on the performance evaluation of the heat exchanger were investigated numerically. It is found that convection heat transfer between fluid and the tube as well as conduction heat transfer through the tube walls can cause significant errors in the evaluation. These errors are considered while designing the inlet/outlet tubes for the test setup. Details of the test setup including mounting of temperature sensors, instrumentation and data acquisition are discussed. Some selected data from our experimental studies on perforated plate heat exchangers are presented. Transient study on the heat exchanger by using the setup is also demonstrated. For pressure drop studies, a separate test setup is used.     

板式相变贮能换热器传热模型和热性能分析

建立了板式相变贮能换热器的无量纲传热模型。它对流体入口流量、入口温度随时间变化情况和需考虑入口效应及添加肋片的情况均适用。模型解和文献准稳态解吻合。作为算例,藉此模型从各时刻的流体温度、相变界面随空间的分布情况和相变蓄热比、相变传热效率、传热系数、完全相变截面位置随时间的变化情况六个角度分析了一板式相变贮能换热器的相变传热性能。该模型可为板式相变贮能换热器的结构优化设计和热性能分析提供帮助。     

Numerical Analysis of a Small Size Baffled Shell-and-Tube Heat Exchanger Using Different Nano-Fluids

In this study, numerical simulation was used to investigate the effect of adding different nano-particles into the fluid on the performance of a baffled shell-and-tube heat exchanger. A three-dimensional modeling approach was followed to analyze the effect of different nano-fluids, at various volume fractions, as applied in a baffled shell-and-tube heat exchanger. Once finished with validating the grid independency and results, we proceeded to obtain heat transfer rate, pressure drop, outlet shell temperature and exchanger effectiveness for different volume fractions and particle size of different nano-fluids. The studied nano-particles in the present work included Al₂O₃, CuO, Fe₂O₃, Cu, Fe, SiO₂, and Au, with water and ethylene glycol employed as base fluids. With constant mass flow rate for all cases, the results indicated that, the addition of nano-particles had reduced the heat transfer coefficient, pressure drop and the rate of heat transfer through the shell, even though it had increased outlet shell temperature. In other words, considering a constant heat transfer rate, the presence of nano-fluids in a baffled shell-and-tube heat exchanger is likely to be associated with increased outlet shell temperature. Another consequence presents that using ethylene glycol as base fluid leads to higher effectiveness compared with water as a base fluid in exchanger.