Transient Response of a Co-current Heat Exchanger to an Inlet Temperature Variation with Time Using an Analytical and Numerical Solution

In this article, we examine the transient response of a co-current double-pipe heat exchanger with respect to the inlet temperature change. For this investigation, a method whose main feature is analytical solution of the energy equation for one element of the heat exchanger is developed. The governing equations are linearized with respect to space and time in one element, and the temperature distribution for the element is obtained. Finally, the obtained temperature distribution for one element is utilized numerically for the whole length of the heat exchanger. Various boundary conditions (ramp, exponential, and sinusoidal) are applied, and the responses are investigated with respect to various parameters. A comparison of the results with data from previous studies shows that the method applied in this article is capable of predicting transient behavior accurately. In addition, because the energy equation is solved analytically, the present method has higher accuracy than those previously utilized, and the calculation time declines noticeably.     

Countercurrent Double-Pipe Heat Exchanger Subjected to Flow-Rate Step Change,Part II: Analytical and Experimental Transient Response

This article investigates the analytical time constants of temperatures transient response along a countercurrent heat exchanger when a mass flow-rate step change is applied on hot fluid flowing through the inner duct. The time constants of the hot and cold fluids are spatially linear and the fluid not submitted to step change shows two types of transient response. The first corresponds to a linear spatial decreasing of the time constant, while the second presents a uniform time constant along the heat exchanger. For each case, the analytical expressions of time constants are derived taking the conditions of the transient response on the boundaries of the heat exchanger. The condition which enables distinguishing the two cases is also proposed in this article. The comparison between theoretical and experimental results allows us to validate the analytical expressions, which depend on the initial and final steady states. The influence of the magnitude of flow-rate step change on the transient behavior is studied by maintaining the initial steady state. In the same way, the initial steady-state effect is investigated with the same magnitude of the flow-rate step change. The comparison of response time to positive and negative flow-rate step change is also presented for different values of cold-fluid flow rate.     

Second-Law and Experimental Analysis of a Cross-Flow Heat Exchanger

This article presents a cross-flow plate-type heat exchanger that has been studied and manufactured in laboratory conditions because of its effective use in waste heat recovery systems. This new heat exchanger was tested with an applicable experimental setup, considering temperatures, velocity of the air, and the pressure losses occurring in the system. These variables were measured and the efficiency of the system was determined. The irreversibility of the heat exchanger was taken into consideration, while the design of the heat exchanger was such that the minimum entropy generation number was analyzed with respect to the second law of thermodynamics in the cross-flow heat exchanger. The minimum entropy generation number depends on the parameters called the optimum flow path length and dimensionless mass velocity. Variations of the entropy generation number with these parameters are analyzed.     

Detection of Fouling in a Cross-Flow Heat Exchanger Using Wavelets

Detection of fouling in a heat exchanger experiencing perfect steady-state conditions is not very difficult. But the challenge is to detect fouling when all inputs (inlet temperature of the fluids and the mass flow rates) are simultaneously varying. In this paper it has been considered that the mass flow rates can vary in a ratio of 2, and that the inlet temperatures can vary by about ±20%. This first approach is dedicated to show the feasibility of using the wavelet transform. It has been considered that getting simulated data is the best way. In fact, it is then possible to introduce an arbitrary fouling factor. Thus, in the first part of the paper the model of the heat exchanger is presented. It is developed using Simulink. The validation is carried out on an electrical heater, for which it is possible to find an analytical solution for transient states. It is also shown that steady states are accurately computed over a large range of the number of transfer units and heat capacity rate ratios. Then a brief overview of the wavelet transform is given. Then basic examples show that the wavelet transform can help to find the trend of time series. It is then applied to the analysis of the “wavelet-transformed” effectiveness of the heat exchanger. This analysis is carried out on a sliding observation window (to be able to detect fouling on-line). It is shown that fouling is detected at a very early stage.     

Two-Dimensional Numerical Analysis of Membrane-Based Heat and Mass Cross-Flow Exchanger

ABSTRACT

A two-dimensional numerical simulation model for a membrane-based heat and mass exchanger was developed. The system model equations were used to determine the coupled heat and moisture transfer from the humid air to the high concentrated liquid desiccant solution (LiCl, lithium chloride) by means of a parallel stack hydrophobic permeable membrane. The two streams of air and liquid desiccant solution were arranged in cross-flow directions. The fourth-order Runge–Kutta method was employed to solve these system model equations in a steady-state condition. This model enables one to predict the latent effectiveness of a membrane-based parallel cross-flow exchanger for dehumidification purpose in response to air to liquid mass flow ratio and the mass transfer unit number.     

进出料换热器的制造

文章介绍了特殊结构大型换热器的球形封头内壁堆焊、下料、冷作、焊接、装配、热处理工艺及制造过程,并重点介绍了装配顺序。     

Experimental Study on Air Cooling Via a Multiport Mesochannel Cross-Flow Heat Exchanger

The air-side heat transfer and flow characteristics of cross-flow multiport slab mesochannel heat exchanger are investigated experimentally in this article. The multiport slab mesochannel heat exchanger consists of 15 finned aluminum slabs; each slab contains 68 flow channels of 1 mm circular diameter. The cold deionized water at a constant mass flow rate was forced to flow through the mesochannels, whereas the hot air at different velocities was allowed to pass through the finned passages of the heat exchanger core in cross-flow orientation. The heat transfer and fluid flow key parameters were examined in the region of the air-side Reynolds number in the range of 972–2758, with a constant water-side Reynolds number of 135. The effect of air-side Reynolds number on air-side Nusselt number was examined and a general correlation of Nusselt number with Reynolds number was obtained. The Nusselt number value was found to be higher in comparison with other research works for the corresponding Reynolds number range. The multiport mesochannel flat slab geometry has offered uniform temperature distribution into the core. This uniform temperature distribution leads to higher heat transfer over stand-alone inline flow tube bank.     

Construction of and Measurements with an Extremely Compact Cross-Flow Heat Exchanger

Abstract

An extremely compact cross-flow heat exchanger is described. The exchanger is constructed by furnace-brazing together a stack of hundreds of stainless-steel sheets. The resulting structure is leak-tight and very strong, but fluid channels as small as 51 μm (0.002 in) are not plugged by excess brazing material. Measurements of heat transfer between water and liquid propylene flowing through the heat exchanger are in excellent agreement with calculations based on exchanger geometry and fluid properties.     

Countercurrent Double-Pipe Heat Exchanger Subjected to Flow-Rate Step Change,Part I: New Steady-State Formulation

This article deals with a new steady-state formulation of temperatures along a double-pipe heat exchanger in counterflow configuration when the mass flow rate is submitted to step change. The steady-state method is based on estimation of the exponential factor of temperature profile. This method, compared to results obtained from correlations, gives an alternative approach to the well-known procedures for sizing and rating heat exchangers in industrial applications, such as the k -NTU and LMTD methods. It is based on the average convective heat transfer coefficients of the hot and cold sides, while the other methods are based on the overall heat transfer coefficient.     

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...     

Mathematical Model for Predicting the Heat Transfer Characteristics of a Helical-Coiled,Crimped, Spiral,Finned-Tube Heat Exchanger

This paper is a continuation of the authors’ previous work. Theoretical and experimental studies of the heat transfer characteristics of a helical-coiled, crimped, spiral, finned-tube heat exchanger in dry surface conditions are presented. The test section is a helical-coiled, finned-tube heat exchanger. The coil unit is composed of four concentric helical-coiled tubes of different diameters. All tubes are constructed by bending straight copper tube into seven layers of helical coil. Aluminum crimped spiral fins, with an outer diameter of 28.25 mm and a thickness of 0.5 mm, are connected around the tube. Hot water is used as a working fluid for the tube side, while ambient air is used for the shell side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. A mathematical model is developed and the simulation results show reasonable agreement with the experimental data.     

增压柴油机EGR阶跃工况瞬态响应特性仿真分析

建立了带EGR回路的增压柴油机瞬态仿真平台并验证了准确性.为了缩短EGR阶跃工况的响应时间,进行了EGR阀开度阶跃时间、开阀特性、目标EGR率等影响因素的不同控制策略的对比.结果表明,缩短EGR阀开度阶跃时间,采用“上抛型”开阀控制策略,可改善EGR流量等参数的瞬态响应特性,但会造成转矩波动增加.为减少转矩波动,提出了...     

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.     

进风角度对椭圆管翅式换热器流动性能的影响

基于多孔介质模型,对两排椭圆管翅式换热器实验元件在不同进风角度(30°、45°、60°和90°)下的流动性能进行了数值模拟,并与实验结果进行对比,研究了不同进风角度下换热器迎风面气流速度的分布.结果表明:在进风角度测试范围内,随着进风角度的减小,换热器流动阻力损失增大;不同进风角度时总压差Δp计算结果与实验结果的差别均小于7%;进风角度为90°时两相邻椭圆管间通道速度分布最均匀.     

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.     

Experimental Study on the Air-Side Performance of a Multipass Parallel and Counter Cross-Flow L-Footed Spiral Fin-and-Tube Heat Exchanger

This study conducted experiments on the air-side performance of novel L-footed spiral fin-and-tube heat exchangers that were faced with airflow at high Reynolds numbers (3500–13,000). The examined heat exchangers have a multipass parallel-and-counter cross-flow type of water flow arrangement. This flow arrangement is a combination of the parallel cross-flow and the counter cross-flow. This type of water flow arrangement may be the best fit for the reverse-flow system, because it can provide constant heat-exchange effectiveness for every flow reversal direction at the same airflow rate. Ambient air was used as a working fluid on the air side and hot water for the tube side. This way the effect of the number of tube rows on the heat transfer and friction characteristics of L-footed spiral fin-and-tube heat exchangers was clearly observed. The effect of the fin's outside diameter on the pressure drop was also studied. The results show that the number of tube rows has no significant effect on the air-side heat transfer or on friction characteristics at high Reynolds numbers. However, the fin's outside diameter shows a significant effect on the pressure drop. The pressure drop increases as the fin's outside diameter increases for the same number of tube rows.     

土壤源热泵U型埋管换热器传热的非稳态数值模拟

本文基于能量平衡方程,建立了土壤源热泵U型埋管换热器周围土壤的非稳态传热模型,用所建立的传热模型对土壤源热泵冬季取热过程进行了动态模拟.研究了不同物性土壤温度及U型埋管出口流体温度的变化规律,分析了土壤物性对换热器换热性能的影响,采用间歇运行方式提高了换热器换热能力.通过建立数学模型得出的结果,可供设计参考.     

Experimental Investigation of Heat Transfer and Resistance Characteristics of a Finned Oval-Tube Heat Exchanger With Different Air Inlet Angles

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger.     

多股流板翅式换热器温度交叉的数值分析

以平行多股流板翅式换热器为研究对象,给出了考虑翅片旁通作用的多股流板翅式换热器流体和翅片的能量方程。在改变多股流板翅式换热器各通道的流体参数、流动方式及换热器的结构参数等情况下,对能量方程进行数值求解,获得了各通道的流体温度分布情况及相邻通道的流体温度差,并分析了流体参数、流动方式和结构参数的变化对相邻通道流体温度交叉的影响。     

Fast Simulation of a Vertical U-Tube Ground Heat Exchanger by Using a One-Dimensional Transient Numerical Model

An explicit one-dimensional transient numerical model has been built for a single-borehole ground heat exchanger, and two computing algorithms are given. The outlet temperature of the U-tube and the soil temperature can be predicted by using arbitrary time-varying load or inlet temperatures as inputs. This numerical model has been compared with analytical models and validated by using test data of three boreholes. Conclusions are that this numerical model is considerably accurate and efficient; the computational time of a one-year-period simulation is about 52 s; recommended is the discrete scheme with 60-s step time and spatial increment of 0.033–0.067 m.