一种监测换热器污垢的新方法

在考虑污垢对换热器传热性能影响的基础上,提出换热器当量总污垢热阻和污垢函数的概念,并给出换热器当量总污垢热阻的监测方法,讨论了换热流型、传热有效度ε和冷热流体热容量率比R对换热器当量总污垢热阻的影响。     

Second-law performance of heat exchangers for waste heat recovery

Exergy change rate in an ideal gas flow or an incompressible flow can be divided into a thermal exergy change rate and a mechanical exergy loss rate. The mechanical exergy loss rates in the two flows were generalized using a pressure-drop factor. For heat exchangers using in waste heat recovery, the consumed mechanical exergy is usually more valuable than the recovered thermal exergy. A weighing factor was proposed to modify the pressure-drop factor. An exergy recovery index (η_II) was defined and it was expressed as a function of effectiveness (?), ratio of modified heat capacity rates (C^∗), hot stream-to-dead-state temperature ratio, cold stream-to-dead-state temperature ratio and modified overall pressure-drop factor. This η_II–? relation can be used to find the η_II value of a heat exchanger with any flow arrangement. The η_II−Ntu and η_II−Ntu_h relations of cross-flow heat exchanger with both fluids unmixed were established respectively. The former provides a minimum Ntu design principle and the latter provides a minimum Ntu_h design principle. A numerical example showed that, at a fixed heat capacity rate of the hot stream, the heat exchanger size yielded by the minimum Ntu_h principle is smaller than that yielded by the minimum Ntu principle.     

Cooling load density characteristics of an endoreversible variable‐temperature heat reservoir air refrigerator

The performance optimization of an endoreversible air refrigerator with variable‐temperature heat reservoirs is carried out by taking the cooling load density, i.e. the ratio of cooling load density to the maximum specific volume in the cycle, as the optimization objective in this paper. The analytical relations of cooling load, cooling load density and coefficient of performance are derived with the heat resistance losses in the hot‐ and cold‐side heat exchangers. The maximum cooling load density optimization is performed by searching the optimum pressure ratio of the compressor, the optimum distribution of heat conductance of the hot‐ and cold‐side heat exchangers for the fixed total heat exchanger inventory, and the heat capacity rate matching between the working fluid and the heat reservoirs. The influences of some design parameters, including the heat capacitance rate of the working fluid, the inlet temperature ratio of heat reservoirs and the total heat exchanger inventory on the maximum cooling load density, the optimum heat conductance distribution, the optimum pressure ratio and the heat capacity rate matching between the working fluid and the heat reservoirs are provided by numerical examples. The refrigeration plant design with optimization leads to a smaller size including the compressor, expander and the hot‐ and cold‐side heat exchangers. Copyright © 2002 John Wiley & Sons, Ltd.     

Effect of maximum and minimum heat capacity rate on entropy generation in a heat exchanger

This paper presents a theoretical analysis of a heat exchanger with a negligible fluid flow pressure drop to determine whether it is better to operate the heat exchanger with the minimum or maximum heat capacity rate of the hot fluid from entropy generation point of view. Entropy generation numbers are derived for both cases, and the results show that they are identical, when the heat exchanger is running at a heat capacity ratio of 0.5 with heat exchanger effectiveness equaling 1. An entropy generation number ratio is defined for the first time, which has a maximum value at ε = 1/(1+R) for any inlet temperature ratio. When R equals 0.1, 0.5 and 0.9, the entropy generation number ratio receives a maximum value at an effectiveness equaling 0.91, 0.67 and 0.526, respectively. When R=0.9, the entropy generation number ratio is the same for all inlet temperature ratios at ε=0.8. The results show that the entropy generation number ratio is far from 1 depending on the inlet temperature ratio of the cold and hot fluid. The results are valid for parallel‐flow and counterflow heat exchangers. Copyright © 2010 John Wiley & Sons, Ltd.     

考虑污垢时换热器热力学性能的评价

在分析污垢对换热器传热性能影响的基础上,在考虑污垢时采用Ｙｏｎｇ损率这一指标对换热器的热力学性能进行了评价,讨论了传热数和冷热流体热容量率比对其性能的影响,并把结果与不考虑污垢时的情况进行了比较,得到了一些有益的结论。     

Performance of Counterflow Microchannel Heat Exchangers Subjected to External Heat Transfer

This article analyzes the effect of external heat transfer on the thermal performance of counterflow microchannel heat exchangers. Equations for predicting the axial temperature and the effectiveness of both fluids as well as the heat transferred between the fluids, while operating under external heating or cooling conditions, are provided in this article. External heating may decrease and increase the effectiveness of the hot and cold fluids, respectively. External cooling may improve and degrade the effectiveness of the hot and cold fluids, respectively. For unbalanced flows, the thermal performance of the microchannel heat exchanger subjected to external heat transfer depends on the fluid with the lowest heat capacity. At a particular number of transfer units (NTU), the effectiveness of both the fluids increased with decrease in heat capacity ratio when the hot fluid had the lowest heat capacity. When the cold fluid had the lowest heat capacity, the effectiveness of both fluids increased with decrease in heat capacity ratio at low values of NTU but at high values of NTU the effectiveness increased with increase in heat capacity ratio. A term called the “performance factor” has been introduced in this article to assess the relative change in effectiveness due to external heat transfer.     

Experimental exergy and entransy analyses on designed and fabricated crossflow heat exchanger

A crossflow heat exchanger (CFHEx) is designed and fabricated in a workshop. For designing this heat exchanger (HEx), the number of passes, frontal areas, HEx volumes, heat transfer areas, free-flow areas, ratios of minimum free-flow area to frontal area, densities, mass flow rates of flowing fluids, maximum/minimum heat capacities, heat capacity ratio, outlet temperatures of hot/cold fluids, average temperatures, mass velocities, Reynolds numbers, and convective heat transfer coefficients are evaluated by considering Colburn/friction factors. After fabrication of the HEx, effectiveness, exergy destruction, entransy dissipation, entransy dissipation-based thermal resistance, entransy dissipation number, and entransy effectiveness for hot/cold fluids sides are found at different flow rates and inlet temperatures of fluids. By experimental results, optimum operating conditions are found, which gives maximum effectiveness and entransy effectiveness but minimum rates of exergy destruction, entransy dissipation, entransy dissipation-based thermal resistance, and entransy dissipation number for the fabricated CFHEx. This study is concluded as follows: minimum exergy destruction and entransy dissipation rates (ie, 3.061 kJ/s·K and 1125.44 kJ·K/s, respectively) are found during experiment 2. Maximum entransy effectiveness of hot/cold fluids (ie, 0.689/0.21) is achieved in experiment 1. Moderate values of entransy dissipation number (ie, 4.689), entransy dissipation-based thermal resistance (ie, 0.04 s·K/J), exergy destruction (ie, 3.845 kJ/s·K), and entransy dissipation (ie, 1374.04 kJ·K/s) rates are found during experiment 1. Maximum effectiveness (ie, 0.4) for the fabricated HEx is also obtained through experiment 1. After comparative analyses, it is found that experiment 1 provides optimum results, which shows the best performance of the fabricated HEx.     

Oil fouling in double‐pipe heat exchanger under liquid‐liquid dispersion and the influence of copper oxide nanofluid

Dispersions of oil in water are encountered in a variety of industrial processes leading to a reduction in the performance of the heat exchangers when thermally treating such two phase fluids. This reduction is mainly due to changes in the thermal and hydrodynamical behavior of the two phase fluid. In the present work, an experimental investigation was performed to study the effects of light oil fouling on the heat transfer coefficient in a double‐pipe heat exchanger under turbulent flow conditions. The effects of different operating conditions on the fouling rate were investigated including: hot fluid Reynolds number (the dispersion), cold fluid Reynolds number, and time. The oil fouling rate was analyzed by determining the growth of fouling resistance with time and through pressure drop measurements. The influence of copper oxide (CuO) nanofluid on the fouling rate in the dispersion was also determined. It was found that the presence of dispersed oil causes a reduction in heat transfer coefficient by percentages depending on the Reynolds number of both cold and hot fluids and the concentration of oil. In addition, the time history of fouling resistance exhibited different trends with the flow rates of both fluids and its trend was influenced appreciably by the presence of CuO nanofluid.     

Optimization for a heat exchanger couple based on the minimum thermal resistance principle

Following the brief introduction to the concept of a physical quantity, entransy, the equivalent thermal resistance of a heat exchanger couple is defined based on the entransy dissipation. The minimum thermal resistance principle is applied to obtain the optimal heat capacity rate of the medium fluid and the optimal allocation of heat exchangers thermal conductance, which correspond to the maximum heat transfer rate in the heat exchanger couple. In addition, analytical expression for the optimal heat capacity rate of the medium fluid is derived, whose reciprocal equals the sum of the reciprocal of the individual heat capacity rate of the hot and cold fluids, just like the case of two electrical capacitors in series. Numerical results in the variation of the thermal resistance and the heat transfer rate with the medium fluid heat capacity rate or the thermal conductance allocation agree with the theoretical analyses. Finally, for comparison, the entropy generation rate is also calculated to obtain its relation with the thermal performance of the heat exchanger couple. The results show that there is no one-to-one correspondence of the minimum entropy generation rate and the maximum heat transfer rate. This indicates that the minimum entropy generation principle cannot be used for optimizing the heat exchanger couple.     

Inverse problem and variation method to optimize cascade heat exchange network in central heating system

Urban heating in northern China accounts for 40％ of total building energy usage.In central heating systems,heat is often transfened from heat source to users by the heat network where several heat exchangers arc installed at heat source,substations and terminals respectively.For given overall heating capacity and heat source temperarure,increasing the terminal fluid temperature is an effective way to improve the thermal performance of such cascade heat exchange network for energy saving.In this paper,the mathematical optimization model of the cascade heat exchange network with three-stage heat exchangers in series is established.Aim at maximizing the cold fluid temperature for given hot fluid temperature and overall heating capacity,the optimal heat exchange area distribution and the medium fluids' flow rates are determined through inverse problem and variation method.The preliminary results show that the heat exchange areas should be distributed equally for each heat exchanger.It also indicates that in order to improve the thernmal performance of the whole system,more heat exchange areas should be allocated to the heat exchanger where flow rate difference between two fluids is relatively small.This work is important for guiding the optimization design of practical cascade heating systems.     

Numerical study on coupled fluid flow and heat transfer process in parabolic trough solar collector tube

A unified two-dimensional numerical model was developed for the coupled heat transfer process in parabolic solar collector tube, which includes nature convection, forced convection, heat conduction and fluid-solid conjugate problem. The effects of Rayleigh number (Ra), tube diameter ratio and thermal conductivity of the tube wall on the heat transfer and fluid flow performance were numerically analyzed. The distributions of flow field, temperature field, local Nu and local temperature gradient were examined. The results show that when Ra is larger than 10⁵, the effects of nature convection must be taken into account. With the increase of tube diameter ratio, the Nusselt number in inner tube (Nu₁) increases and the Nusselt number in annuli space (Nu₂) decreases. With the increase of tube wall thermal conductivity, Nu₁ decreases and Nu₂ increases. When thermal conductivity is larger than 200 W/(m K), it would have little effects on Nu and average temperatures. Due to the effect of the nature convection, along the circumferential direction (from top to down), the temperature in the cross-section decreases and the temperature gradient on inner tube surface increases at first. Then, the temperature and temperature gradients would present a converse variation at θ near π. The local Nu on inner tube outer surface increases along circumferential direction until it reaches a maximum value then it decreases again.     

Study on oil fouling in a double pipe heat exchanger with mitigation by a surfactant

Fouling of oils on heat exchanger surfaces and pipelines is a common problem in a variety of industrial applications. This is because the oil deposits on the heat transfer surface causes an increase in pressure drop and a decrease in heat exchanger efficiency. In the current work, oil fouling in double pipe heat exchanger was investigated and mitigated using a surface‐active agent for the flow of a dispersion fluid containing different dispersed oil fractions in water. The effect of the dispersed oil fraction (5%vol and 10%vol) and temperature (35°C‐55°C) on the oil fouling rate was studied and discussed under turbulent flow conditions for both hot and cold fluids. Different amounts of alkylbenzene sulfonate as a surfactant were added to reduce the fouling rate under turbulent flow. It was found that the fouling thermal resistance (R_f) increases when the fluid temperature decreases. The higher the dispersed oil fraction, the higher the R_f for all temperatures due to higher oil deposition. Addition of 0.2%vol to 0.5%vol of alkylbenzene sulfonate caused an appreciable reduction in R_f depending on oil fraction and Reynolds number. The mitigation percent was higher for a lower Reynolds number, reaching up to 96%.     

换热器系统的热力学性能评价

引入可用能损率这一指标对串联组合的换热器系统的热力学性能进行了 分析和评价,得到了换热器系统可用能损率的一般计算式,讨论了换热器系统的总流动趋势、冷热流体热容量流率比、传热单元数及单台换热器的流型、传热有效度和数目等对换热器系统可用能损率的影响。     

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...     

Second Law Efficiency Analysis of Heat Exchangers

Analytical analysis of unbalanced heat exchangers is carried out to study the second law thermodynamic performance parameter through second law efficiency by varying length‐to‐diameter ratio for counter flow and parallel flow configurations. In a single closed form expression, three important irreversibilities occurring in the heat exchangers—namely, due to heat transfer, pressure drop, and imbalance between the mass flow streams—are considered, which is not possible in first law thermodynamic analysis. The study is carried out by giving special influence to geometric characteristics like tube length‐to‐diameter dimensions; working conditions like changing heat capacity ratio, changing the value of maximum heat capacity rate on the hot stream and cold stream separately and fluid flow type, i.e., laminar and turbulent flows for a fully developed condition. Further, second law efficiency analysis is carried out for condenser and evaporator heat exchangers by varying the effectiveness and number of heat transfer units for different values of inlet temperature to reference the temperature ratio by considering heat transfer irreversibility. Optimum heat exchanger geometrical dimensions, namely length‐to‐diameter ratio can be obtained from the second law analysis corresponding to lower total entropy generation and higher second law efficiency. Second law analysis incorporates all the heat exchanger irreversibilities. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21109     

板式换热器性能的数值模拟

建立了人字形板式换热器冷热双流道的流体流动与传热计算模型,利用计算流体力学软件对5组不同速度工况下换热器内流体的流动和传热进行了数值模拟,分析了换热器流道内的速度场、温度场和压力场.结果表明:数值模拟得到的板式换热器进、出口温差和压降与试验测量值的误差均小于6%;换热器内流体的流动和传热存在明显的不均匀性,导致其进、出口的另一侧出现明显的传热"死区";换热器的总传热系数和流道阻力均随着流体流速的增大而增大.     

Unsteady conjugated heat transfer in laminar pipe flows with step change in ambient temperature

This study aims to numerically investigate the effects of wall heat capacity on the unsteady forced convection heat transfer in the flow in a long circular pipe resulting from a step change in the ambient temperature. Two non-dimensional parameters, Nu_o and a^*, are identified in the study, and their effects on the non-dimensional duct wall heat flux and the time evolution of temperature field are discussed in great detail. Comparisons are made with zero thermal capacity wall solution. It is found that the wall heat capacity has decisive impact on the unsteady heat transfer in the flow.     

Computational and statistical analysis on the U-tube heat exchanger with different passes configuration: Taguchi method

In the present study, the passive technique of heat transfer in which single pass and double passes are included in a simple U-tube heat exchanger is analyzed. The computational fluid dynamics (CFD)-based parametric analysis is carried out to optimize the parameters affecting the temperature drop and heat transfer achieved from the U-tube heat exchanger. ANSYS Fluentv20 is used for the CFD analysis, and the RNG k-ɛ model and energy equation were considered to define the turbulence and heat transfer phenomena. The Taguchi method is used to formulate the experimental work and analyze the working parameters of the U-tube heat exchanger, such as hot and cold mass flow rate and hoRenew Energyt inlet temperature and cold inlet temperature. For the U-tube heat exchanger, four operating parameters are considered at four different levels in the Taguchi method. The best combination of parameters for achieving a maximum temperature drop is A₄B₁C₂D₃, and it is A₃B₄C₁D₂ in case of heat transfer. A U-tube single-pass heat exchanger is more effective as compared with other U-tube heat exchangers (zero- and double-pass). Experimental results are provided to validate the suitability of the purpose of the approach.     

Experimental investigation of Al2O3–MgO hot hybrid nanofluid in a plate heat exchanger

Exergy–energy analysis of the plate heat exchanger is experimentally performed with different Al₂O₃–MgO hybrid nanofluid (HyNf) as a hot fluid. There were six combinations of fluids, namely, deionized (DI) water, ethylene glycol–DI water brine (1:9 volume ratio), propylene glycol–DI water brine (1:9 volume ratio), base fluids and their respective Al₂O₃–MgO (4:1 particle volume ratio) HyNfs of 0.1% total volume concentration. The effects of different flow rates and hot inlet temperatures on the heat transfer rate, heat transfer coefficient, pump work, irreversibility, and performance index (PI) are investigated. It is witnessed that the heat transfer rate, heat transfer coefficient, pump work, and irreversibility enhances with the flow rate and nanoparticle suspension. While the PI declines with a rise in the flow rate, the heat transfer rate, heat transfer coefficient, PI, and irreversibility rise up maximum for MgO–alumina (1:4) DI water HyNf upto 11.8%, 31.7%, 11.1%, and 4.05%, respectively. The pump work enhances upto 1.6% for MgO–alumina (1:4)/EG–DI water (1:9) HyNf.     

Numerical simulation and experimental research on heat transfer and flow resistance characteristics of asymmetric plate heat exchangers

The asymmetric plate heat exchanger (APHE) has the possibility of achieving balanced pressure drops on both hot and cold sides for situations with unbalanced flow, which may in turn enhance the heat transfer. In this paper, the single-phase water flow and heat transfer of an APHE consisted of two types of plates are numerically (400≤Re≤12000) and experimentally (400≤Re≤ 3400) investigated. The numerical model is verified by the experimental results. Simulations are conducted to study the effects of N, an asymmetric index proposed to describe the geometry of APHEs. The correlations of the Nusselt number and friction factor in the APHEs are determined by taking N and working fluids into account. It is found that an optimal N exists where the pressure drops are balanced and the heat transfer area reaches the minimum. The comparison between heat transfer and flow characteristics of the APHEs and the conventional plate heat exchanger (CPHE) is made under various flow rate ratios of the hot side and the cold side and different allowable pressure drops. The situations under which APHE may perform better are identified based on a comprehensive index Nu/f^1/3.