Thermal analysis on metal-foam filled heat exchangers. Part I: Metal-foam filled pipes

This paper presents an analytical study of the forced convection heat transfer characteristics in high porosity open-cell metal-foam filled pipes. The Brinkman-extended Darcy momentum model and two-equation heat transfer model for porous media were employed. Based on the analytical solutions, the velocity and temperature distributions in metal-foam filled pipes were obtained. The effects of the microstructure of metal foams on overall heat transfer were examined. The results show that the pore size and porosity of metal-foams play important roles on overall heat transfer performance. The use of metal-foam can dramatically enhance the heat transfer but at the expense of higher pressure drop.     

Algebraic solution of capillary tube flows. Part II: Capillary tube suction line heat exchangers

Capillary tube suction line heat exchangers have been modeled using both numerical and analytical approaches. The former requires a reasonable understanding of the governing heat and fluid flow equations, thermodynamic relations, numerical methods, and computer programming, and therefore are not suitable for most refrigeration and air-conditioning practitioners. Alternatively, empirical algebraic formulations for diabatic capillary tube flows have been proposed in the literature, in spite of their lack of generality and accuracy. This paper introduces a physically consistent, unconditionally convergent, easy-to-implement semi-empirical algebraic model for capillary tube suction line heat exchangers, with the same level of accuracy as found with more sophisticated first-principles models. The methodology treats the refrigerant flow and the heat transfer as independent phenomena, thus allowing the derivation of explicit algebraic expressions for the refrigerant mass flow rate and the heat exchanger effectiveness. The thermal and hydraulic models are then conflated through the so-called Buckingham-π theorem using in-house experimental data collected for diabatic capillary tube flows of refrigerants HFC-134a and HC-600a. Comparisons between the model predictions and the experimental data revealed that more than 90% and nearly 100% of all data can be predicted within ±10% and ±15% error bands, respectively.     

Theoretical analysis of triple concentric-tube heat exchangers Part 1: Mathematical modelling

A theoretical study consisting of two parts is conducted on triple concentric tube heat exchangers. This paper presents the first part of the study and deals with mathematical modelling. The model includes the derivation and possible solutions of the governing differential equations for both counter-flow and parallel-flow arrangements. In the second part of the study, which is under its way for publication, the results of several case studies will be presented.     

Theoretical analysis of triple concentric-tube heat exchangers Part 2: Case studies

A two-part theoretical study on counter-flow triple concentric-tube heat exchangers is conducted. In the present part of the study, several case studies are conducted based on the solutions obtained in the first part. The case studies include both performance calculations and design calculations through which it is demonstrated that the relative sizes of the three tubes with respect to each other are the most important parameters that influence the performance (or size) of the exchanger.     

Thermal design of multi-stream heat exchangers

The thermal design of multi-stream heat exchangers of the plate and fin type is presented. Although originally used in low temperature processes, their application is extrapolated to above temperature processes and it is shown that, conceptually, multi-stream exchangers could replace whole heat recovery networks. The approach is based on the use of temperature vs. enthalpy diagrams or composite curves, which show how a multi-stream exchanger can be subdivided into block sections that correspond to enthalpy intervals and indicate the entry and exit points of the streams. A design methodology for plate and fin exchangers in countercurrent arrangement, characterized by the maximization of allowable pressure as a design objective is extended to the design of multi-fluid exchangers. The methodology uses a thermo-hydraulic model which relates pressure drop, heat transfer coefficient and exchanger volume. The potential applicability of the methodology is demonstrated on a case study.     

Thermal evaluation of nanofluids in heat exchangers

Nanofluids, suspensions of nanoparticles (less than 100 nm) in a basefluid, have shown enhanced heat transfer characteristics. In this study, thermal performances of nanofluids in industrial type heat exchangers are investigated. Three mass particle concentrations of 2%, 4%, and 6% of silicon dioxide–water (SiO₂–water) nanofluids are formulated by dispersing 20 nm diameter nanoparticles in distilled water. Experiments are conducted to compare the overall heat transfer coefficient and pressure drop of water vs. nanofluids in laboratory-scale plate and shell-and-tube heat exchangers. Experimental results show both augmentation and deterioration of heat transfer coefficient for nanofluids depending on the flow rate and nanofluid concentration through the heat exchangers. This trend could be explained by the counter effect of the changes in thermo-physical properties of fluids together with the fouling on the contact surfaces in the heat exchangers. The measured pressure drop while using nanofluids show an increase when compared to that of basefluid which could limit the use of nanofluids in industrial applications.     

Thermal evaluation of coaxial deep borehole heat exchangers

This paper presents a performance study of deep borehole heat exchangers. The coaxial borehole heat exchanger (BHE) has been selected because for the present conditions it has a better performance than the conventional U-tube BHE. A numerical model has been developed to study the coaxial BHE. The model predictions are compared to detailed distributed temperature measurements obtained during a thermal response test. The model is found to accurately predict the behavior of a coaxial BHE. The influence of the flow direction of the mass flow is studied for BHEs in the range 200 m–500 m. A parametric performance study is then carried out for the coaxial case with different borehole depths, flow rates and collector properties. The results clearly show a significant increase in the system performance with depth. In addition, it is shown that with increasing borehole depth, the heat load that can be sustained by the BHE is significantly increased. An overall performance chart for coaxial BHEs for the depths of 300–1000 m is presented. The chart can be used as a guide when sizing deep BHE installations.     

Thermal decomposition behavior of tobacco stem Part II: Kinetic analysis

As a continuation of the previous study on the thermal degradation behavior of tobacco stem, this work is focused on the kinetics of pyrolytic decomposition. Thermogravimetric analysis of tobacco stem samples was conducted under nitrogen atmosphere at different heating rates of 5, 10, 15, and 20°C/min at a temperature range of 25–1,000°C. The kinetic parameters, such as activation energy, pre-exponential factor, and reaction order, were determined by applying the Coats–Redfern method for the main pyrolysis occurred in the second zone by means of the decomposition of hemicellulose, cellulose, and lignin at a temperature range 180–540°C. In addition, the activation energy was calculated using various degradation models, including Kissinger, Friedman (FR), Flynn–Wall–Ozawa (FWO), and Kissinger–Akahira–Sunose (KAS). The average activation energy of tobacco stem was calculated to be 150.40, 230.76, 216.97, and 218.56 kJ/mol by the Kissinger, FR, FWO, and KAS models, respectively.     

Thermal resistance and capacity models for borehole heat exchangers

The detailed design and energy analysis of ground source heat pump systems requires the ability to predict the short‐term behavior of borehole heat exchangers (BHE). The application of fully discretized models leads to extensive computation times and a substantial effort in terms of pre‐processing work. On the contrary, analytical models offer simple, parameter input‐based modeling and short computation times, but they usually disregard the transient effects of heat and mass transport in the borehole and hence are not suitable for the prediction of the short‐time behavior. In order to combine the advantages of both types of models, the authors developed two‐dimensional thermal resistance and capacity models for different types of BHE. These models take the capacity of the grouting material with one capacity per tube into account and, therefore, the range of validity is extended to shorter times. The correct consideration of all thermal resistances between the fluid in the pipes, the grout capacities and the borehole wall is important because of the significant influence on the validity of the models. With the developed models, the modeling work and the computation time can be significantly reduced compared with fully discretized computations while precise results are still achieved. The validation of the suggested models against fully discretized FEM models shows a very good agreement. Copyright © 2010 John Wiley & Sons, Ltd.     

Tube bundle replacement for segmental and helical shell and tube heat exchangers: Experimental test and economic analysis

In this investigation, the second test analysis with more comprehensive evaluation with a focus on fouling mitigation, increased running-time and economic analysis are shown and then, the thermal design procedure for tube bundle replacement of critical heat exchanger of Butene-1 unit in Petrochemical Company as a case study are described. Finally, experimental data for the average heat transfer coefficient and pressure drop of shell-side in segmental and helix bundles are measured and calculated for the mass flow rate of 14.24 kg/s and then these data are compared with the data from code and EXPRESS. Moreover, additional comparison between code and EXPRESS results are provided to ensure the accuracy of calculation program in various mass flow rates. Based on the same shell in the case studies, the results showed that in addition to improved heat transfer performance of the helix bundle over segmental bundle, helix bundle achieved two to three times longer operational run times. From economic point of view, the results for replacement of segmental bundle with a helix bundle showed that initial and installation costs of helix bundle to segmental bundle could be increased, but maintenance and operating costs can be decreased in the helix bundle, 60% and 20%, respectively. Comparison between code and EXPRESS results with experimental data for the mass flow rate of 14.24 kg/s showed that the deviation in heat transfer coefficient and pressure drop are quite reliable for segmental and helix bundles.     

Thermal performance evaluation of heat exchangers fitted with twisted-ring turbulators

Heat transfer, friction factor and thermal performance characteristics in a tube equipped with twisted-rings (TRs) are experimentally investigated. The experiments were conducted using TRs with three different width ratios (W/D = 0.05, 0.1 and 0.15) and three pitch ratios of (p/D = 1, 1.5 and 2) for Reynolds numbers ranging from 6000 to 20,000 using air as a test fluid. The typical circular rings (CRs) were also tested for an assessment. The experimental results reveal that most TRs yield lower Nusselt numbers and friction factor than CRs, except at the largest width ratio (W/D = 0.15) and the smallest pitch ratio (p/D = 1.0). In addition, Nusselt number and friction factor increase as width ratio increases and pitch ratio decreases. However, a maximum thermal performance factor is associated by TRs with the smallest width ratio and pitch ratio. The empirical correlations of the heat transfer (Nu) and friction factor (f) are also included in this paper.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.     

Thermal performance of aluminium‐foam CPU heat exchangers

This study investigates the performance of existing central processing unit (CPU) heat exchangers and compares it with aluminium‐foam heat exchangers in natural convection using an industrial set‐up. Kapton flexible heaters are used to replicate the heat produced by a computer's CPU. A number of thermocouples are connected between the heater and the heat sink being used to measure the component's temperature. The thermocouples are also connected to a data‐acquisition card to collect the data using LabVIEW program. The values obtained for traditional heat exchangers are compared to published data to validate experiments and set‐up. The validated set‐up was then utilized to test the aluminium‐foam heat exchangers and compare its performance to that of common heat sinks. It is found that thermal resistance is reduced more than 70% by employing aluminium‐foam CPU heat exchangers. The results demonstrate that this material provides an advantage on thermal dissipation under natural convection over most available technologies, as it considerably increases the surface‐area‐to‐volume ratio. Furthermore, the aluminium‐foam heat exchangers reduce the overall weight. Copyright © 2005 John wiley & Sons, Ltd.     

Plate heat exchangers: Recent advances

This study presents the advances in plate heat exchangers both in theory and application. It dresses the direction of various technical research and developments in the field of energy handling and conservation. The selected areas of heat transfer performance and pressure drop characteristics, general models and calculations change of phase; boiling and condensation, fouling and corrosion, and welded type plate heat exchangers and finally other related areas are highlighted.     

Performance evaluation criteria for heat exchangers based on second law analysis

This paper presents second-law based performance evaluation criteria to evaluate the performance of heat exchangers. First, the need for the systematic design of heat exchangers using a second law-based procedure is recalled and discussed. Then, a classification of second-law based performance criteria is presented:

• 1.criteria that use entropy as evaluation parameter, and
• 2.criteria that use exergy as evaluation parameter.

Both classes are collectively presented and reviewed, and their respective characteristics and constraints are given. It is shown how some of these criteria are related to each other. Emphasis is also placed on the importance of second law-based thermoeconomic analysis of heat exchangers, and these methods are discussed briefly.     

流动参数对双级螺旋套管换热器传热性能的数值分析

高效热泵系统性能研究一直是热泵空调领域普遍关注的热点问题,针对设计开发的双级套管串联式热泵系统。采用3D有限容积法和可实现的k-ε模型,数值分析入口流体温度、流动速度对换热系数以及内外管努塞尔数的影响规律。结果表明,降低入口水温或者增加入口制冷剂温度能够提高整体传热性能,Nui随着水和制冷剂流率的增加有所增加,而Nuo随着水流率的增加而增加,但随着制冷剂流率则增加而减小,Nui 和Nuo都随着水温的减小或制冷剂温度的增大而增大。     

Performance analysis and optimization of heat exchangers: a new thermoeconomic approach

A thermoeconomic performance optimization has been carried out for a single pass counter-flow heat exchanger model. In the considered model, the irreversibilities due to heat transfer between the hot and cold stream are taken into account and other irreversibilities such as pressure drops and flow imbalance are ignored. The objective function is defined as the actual heat transfer rate per unit total cost considering lost exergy and investment costs. The optimal performance and design parameters which maximize the objective function have been investigated. The effects of the technical and economical parameters on the general and optimal thermoeconomical performances have been also discussed.     

Comparative analysis of corrugation effect on thermohydraulic performance of double-pipe heat exchangers

Heat transfer augmentation in heat exchangers has been a key research topic in recent times. Over the years, many methods have been proposed for heat transfer enhancement, such as providing fins, changing the cross-sectional area of tubes, vortex generator, twisted tape inserts, and so forth. In addition to the above-mentioned techniques, corrugation of tubes was also proposed by a few authors who demonstrated that this method could effectively increase the heat transfer rate. To address the same in this study, the different corrugation profiles have been created with the help of CATIA software for the study. The simulations were performed using ANSYS R19.2. The results so obtained were used to calculate the various thermal and hydraulic perfoallrmance parameters of the heat exchanger with the help of macros created in MS Excel. The result shows that the use of corrugation on the inner tube of the heat exchanger increased the heat transfer coefficient, fanning friction factor, and rate of cooling by 5%–21%, 90%–355%, and 25.67%–157.40%, respectively, in case of the plain double-pipe heat exchanger for the mass flow rate variation of 5–25 kg/min. It is also observed that the smooth tube has more thermohydraulic performance as 1.2152.     

Heat transfer analysis of boreholes in vertical ground heat exchangers

A ground heat exchanger (GHE) is devised for extraction or injection of thermal energy from/into the ground. Bearing strong impact on GHE performance, the borehole thermal resistance is defined by the thermal properties of the construction materials and the arrangement of flow channels of the GHEs. Taking the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs into account, a new quasi-three-dimensional model for vertical GHEs is established in this paper, which provides a better understanding of the heat transfer processes in the GHEs. Analytical solutions of the fluid temperature profiles along the borehole depth have been obtained. On this basis analytical expressions of the borehole resistance have been derived for different configurations of single and double U-tube boreholes. Then, different borehole configurations and flow circuit arrangements are assessed in regard to their borehole resistance. Calculations show that the double U-tubes boreholes are superior to those of the single U-tube with reduction in borehole resistance of 30-90%. And double U-tubes in parallel demonstrate better performance than those in series.