Cross-flow Tube Research Articles

Laminar flow and conjugate heat and mass transfer in a hollow fiber membrane bundle used for seawater desalination

A hollow fiber membrane-based bundle or humidifier (HFMH) is used for air humidification in the process of membrane type seawater desalination, where seawater flows inside the fibers and the process air stream flows across the fiber bundle. The HFMH has a staggered arrangement of cylindrical fiber tubes with equal transverse and longitudinal pitches. The forced convection heat and mass transfer in the cross-flow tube bundle under naturally formed boundary conditions are investigated numerically on a periodic computational cell. A membrane-based air humidification-dehumidification type sea-water desalination (MHDD) system is constructed and used to validate the model. The mathematical model proposed here takes into account of the fiber-to-fiber interactions. The typical velocity vector profiles, temperature contours as well as the concentration contours for both the air stream and saline water stream are plotted to disclose the heat and mass transfer mechanisms. The effects of air Reynolds numbers (ranging from 50 to 300) as well as the module packing fractions (ranging from 0.126 to 0.503) on the fluid flow and heat mass transfer properties of the module are investigated. A set of correlations are also proposed for the prediction of the friction factor, mean Nusselt numbers and Sherwood numbers in air side. These fundamental data is of interest for the design and optimization of the HFMH, a promising membrane type seawater desalination system.

Selecting and Optimizing a Heat Exchanger for Automotive Vehicle Rankine Cycle Waste Heat Recovery Systems

Abstract This paper contributes for the selection and optimization of an adequate compact heat exchanger (HEX) for an automotive vehicle Rankine cycle Waste Heat Recovery Systems (RC-WHRS). Both tube type and plate type HEX were considered. Among the different HEX designs available, a robust cross-flow tube heat exchanger (CFT-HEX), with the working fluid circulating inside the tubes was selected. The selected HEX accomplishes with the specific pressure drops limits on both exhaust gas and working fluid side. The compactness of the CFT-HEX strongly depends on the optimization of the HEX pre-heater section, which mainly depends on the augmentation of the working fluid heat transfer coefficient. The present study data reveals that a transitional flow regime with a working fluid Reynolds number, Re > 3200, allows a significant increase of the Nusselt number (Nu > 20) as compared to laminar flow regime.

Experimental and CFD prediction of heat transfer and friction factor characteristics in cross flow tube bank with integral splitter plate

The experimental and the three-dimensional numerical simulation of cross flow tube bank in staggered arrangements with and without the splitter plate attachment is performed with (5500⩽Re⩽14,500) and splitter plate length to tube diameter (L/D) ratio as 1. The longitudinal and transverse tube pitch ratio SL/D and ST/D were maintained constant as 1.75 and 2.0 respectively. The provision of the splitter plate increases the Nusselt number for the fluid flow which results in the increase in the heat transfer along with the decrease in the pressure drop within the tube bank as compared to that of the bare cylinder. The overall thermal performance of the splitter plate attachment is much superior to that of the bare cylinder which indicates higher heat transfer with lower friction factor. An increase of maximum 60–82% in the overall thermal performance was observed by the use of the splitter plate over the bare cylinder at the Reynolds number of 5500.

Time-domain CFD computation and analysis of acoustic attenuation performance of water-filled silencers

The multi-dimensional time-domain computational fluid dynamics (CFD) approach is extended to calculate the acoustic attenuation performance of water-filled piping silencers. Transmission loss predictions from the time-domain CFD approach and the frequency-domain finite element method (FEM) agree well with each other for the dual expansion chamber silencer, straight-through and cross-flow perforated tube silencers without flow. Then, the time-domain CFD approach is used to investigate the effect of flow on the acoustic attenuation characteristics of perforated tube silencers. The numerical predictions demonstrate that the mean flow increases the transmission loss, especially at higher frequencies, and shifts the transmission loss curve to lower frequencies.

Effect of a cross-flow perforated tube on pressure pulsation and pressure loss in a reciprocating compressor piping system

This paper experimentally investigates the effects of a cross-flow perforated tube on the pressure pulsation attenuation and pressure loss in a reciprocating compressor piping network, with particular focus on the structure parameters and installation positions. The results demonstrate that significant pressure fluctuation attenuation and less pressure loss in the whole piping system can be achieved when a well-designed cross-flow perforated tube is installed downstream of the pulsating bottle. The pressure pulsation is reduced as the perforated rate decreases and as the perforated tube length increases, while the hole diameter has little effect upon the pulsation attenuation. In the aspect of reducing pressure loss, the perforated rate should be larger than 0.05 and the hole diameter should be larger than 8 mm. In addition, a pressure pulsation computation model based on the linear acoustic wave theory and transfer matrix method is developed to predict the pulsating pressure in compressor piping systems with an installed cross-flow perforated tube. With favorable agreement between the model prediction and the present experimental results (maximum deviation within 6.8%), the predicted pulsating pressure can be attenuated for the reciprocating compressor piping system with various compressor speeds when a cross-flow perforated tube is reasonably designed and installed.

Analysis of radiative heat transfer impact in cross-flow tube and fin heat exchangers

AbstractA cross-flow, tube and fin heat exchanger of the water – air type is the subject of the analysis. The analysis had experimental and computational form and was aimed for evaluation of radiative heat transfer impact on the heat exchanger performance. The main element of the test facility was an enlarged recurrent segment of the heat exchanger under consideration. The main results of measurements are heat transfer rates, as well as temperature distributions on the surface of the first fin obtained by using the infrared camera. The experimental results have been next compared to computational ones coming from a numerical model of the test station. The model has been elaborated using computational fluid dynamics software. The computations have been accomplished for two cases: without radiative heat transfer and taking this phenomenon into account. Evaluation of the radiative heat transfer impact in considered system has been done by comparing all the received results.

Triple-finned tubes – Increasing efficiency, decreasing CO2 pollution of a steam boiler

This paper presents the results of techno-economical analysis of a proposal of the utilization of TFT (triple-finned tubes) in a boiler's tube banks. The following three tube bank arrangements were compared: (A) the base – staggered plain tube bank; (B) the combination of triple-finned and plain tubes – two rows of triple-finned tubes every 10 rows; (C) the alternating tube bank, where half of the plain tubes were replaced by triple-finned tubes in an in-line arrangement.The results of CFD calculations of heat load q let to obtain relative values of the Nu number on the tube bank in (B) and (C) arrangements and the base arrangement (A) were carried out in a 2D model using the CFD (computational fluid dynamics) code Fluent.Heat transfer values of an ECO (economizer) with triple-finned tubes were obtained for the calculation of outlet flue gas temperature and outlet loss. Calculations of the Nu number of the tube were performed for all arrangements. Next, boiler efficiency and fuel consumption were calculated by 0D modelling. Received results were used to make economical analysis. In the case of modernization in the (C) arrangement, efficiency would be 90.67%, which is an increase of 1.1%. This is related to the reduction of flue gas temperature to 138 °C from 158.8 °C.Economically, the most advantageous solutions, are the variants of modernization (B) and (C) for the newly formed boiler. In both cases, SPBT (simple payback time) capital payback period is very short - close to half of a year.It is worth noting that the proposed modernizations of the arrangement (B) and (C), will respectively reduce CO2 emissions by 550 t/a and 826 t/a.The results show that triple-finned tubes can be used to increase the performance of cross-flow tube banks.

Improving the Thermal Performance of Parallel and Cross-Flow Concentric Tube Heat Exchangers Using Fly-Ash Nanofluid

This study experimentally demonstrated the effect of using a nanofluid obtained from fly ash, which is comprised of various types of metal oxides in varying ratios, on improving the performance of a parallel flow (PFCTHE) or a cross-flow (CFCTHE) concentric tube heat exchanger. Fly-ash nanofluid/water and water/water hot/cold working fluids were used for monitoring the differences in the performance of the heat exchangers. The Triton X-110 dispersant was used in the study to produce 2% (wt) concentration of the fly-ash nanofluid via direct synthesis. The heat exchanger is of the double-pipe type with hot water flowing through the central tube while cooling water flows through the annular space. A double-pipe heat exchanger with co-current or countercurrent flow was utilized along with all ancillary equipment and instrumentation for the determination of overall heat transfer coefficients and heat transfer coefficients during turbulent flow. An improvement of 31.2% and 6.9% was determined in the study for the efficiency of the PFCTHE and CFCTHE, respectively, when the fly-ash nanofluid was used as the working fluid.

Effect of blockage ratio on pressure drag and heat transfer of a cam-shaped tube

An experimental investigation has been conducted to clarify the effects of blockage ratio on forced convective heat transfer and pressure drag of an isothermal cam-shaped tube in cross-flow of air. The blockage ratio varies between 1.5 ≤ H/Deq ≤ 7 and Reynolds number based on equivalent diameter varies in range of 7.5 × 103 to 17.5 × 103. Results show that by increasing blockage ratio from 1.5 to 7, drag coefficient of the cam-shaped tube decreases about 55 % and increasing Reynolds number from 7.5 × 103 to 17.5 × 103 results in 40–48 % increment in Nusselt number. For better comparison, ratio of pressure drag over heat transfer of cam-shaped tube is compared with circular tube. In all ranges of Reynolds number and blockage ratios, thermal–hydraulic of cam-shaped tube is about 40–171 % greater than circular tube.

Optimum Design of Cross-Flow In-Line Tube Banks at Constant Wall Temperature

For many applications, it is important to transfer a given amount of heat with minimum possible volume of heat exchanger for a given pressure loss and efficiency. In this study, such optimum design of cross-flow in-line tube banks at constant wall temperature is carried out. For this purpose, two different methods, namely, the Gnielinski + Gaddis and Zukauskas methods, for the calculation of heat transfer coefficient and pressure loss in tube banks are used. By prescribing the tube diameter, heat exchanger efficiency, and pressure loss of heat exchanger, the pitch length and tube numbers in the flow direction are determined for the condition of the maximum heat flow per heat exchanger volume. For a given value of total heat to be transferred, the length, the height, and the width of heat exchanger can be determined. Equations for this optimum design are derived using appropriate dimensionless numbers. The methodology of the optimum design is explained for a practical example.

Heat Transfer and Pressure Loss In Combined Tube Banks With Triple-Finned Tubes

This paper presents experimental and analytical results of an investigation of heat transfer and pressure loss in a new type of convection heat exchanger tube bank with triple-finned tubes (TFTs). An initial evaluation of the optimal geometry of TFT banks using the Lusas code is presented. The heat transfer between the external environment and the medium inside the TFTs was modeled. Fins were arranged on a tube in the following three configurations: three fins with the same geometry with 45° between two symmetric fins and a vertical line of symmetry, denoted by T90, three fins with the same geometry with 30° between two symmetric fins and a vertical line of symmetry, denoted by T60, and one fin and angle steel plate placed parallel to the line of symmetry of the triple-finned tube, denoted by TAS90. The naphthalene sublimation technique is used as a heat/mass transfer analogy to evaluate the Sherwood (Nusselt) number ratios in the tube banks. The heat transfer performance of staggered S1T90 tube banks and the basic plain tube bank SP was compared. The second possibility for using TFTs is in a tube bank with all tubes finned was studied using the naphthalene analogy technique. The results are then compared with those computed using the computational fluid dynamics code. A comparison of the numerical and experimental heat transfer results shows that the trends in the heat transfer increase are very similar. The results show that triple-finned tubes can be used to increase the performance of cross-flow tube banks.

The Effect of Position of Heated Rod in Tube Banks on the Heat Transfer Coefficient

Heat transfer in flow across a bank of tubes is of a particular importance in the design of heat exchangers. Heat exchangers are used in numerous services and industrial applications. Experimental studies were performed, carried out in cross-flow tube banks. They contain copper heated rod and 17 Aluminum rods arranged in the form of staggered and at arrange of Reynolds number 2700 to 21530. The experimental results indicated that the location of the heated rod in the work section affects the value of heat transfer coefficient thus this value is increased when the heated rod location changes both the y and x-directions. The experimental data revealed good agreement with Zukauskas correlation in four columns.

An unsteady model for fluidelastic instability in an array of flexible tubes in two-phase cross-flow

The streamtube model for fluidelastic instability in single-phase flow was extended to two-phase flow by two kinds of two-phase flow model. One was the traditional HEM, and the other was the slip ratio model. The model based on slip ratio can represent the effect of velocity difference between gas and liquid phases, and was verified with the experimental data from the literature. Comparing to the conventional HEM model, the slip ratio model and the introduction of interfacial velocity could collapse the stability data from various studies, which is beneficial for predicting the fluidelastic instability of tube bundles from existing stability maps and methods.

On the use of the periodicity condition in cross-flow tube

This paper presents the results of measurements and numerical predictions of turbulent cross-flow through an in-line 7×7 bundle configuration with a constant transverse and longitudinal pitch-to-diameter ratio of 1.44. The experiments are conducted to measure the pressure around tubes, using DPS differential pressure scanner with air flow, in square channel at a Reynolds number of 35000 based on the gap velocity and the tube diameter. The commercial ANSYS FLUENT is used to solve the unsteady Reynolds–Averaged Navier–Stokes (RANS) equations. The primary aim of the present study is to search for a turbulent model that could serve as an engineering design tool at a relatively low computational cost. The performances of the Spalart-Allmaras, the RNG k-ε, the Shear Stress Transport k-ω and the second moment closure RSM models are evaluated by comparing their simulation results against experimental data. The second objective is to verify the validity of the periodicity assumption taken account in the most previous numerical works by considering the filled bundle geometry. The CFD results show that the Spalart-Allmaras model on the fine mesh are comparable to the experiments while the periodicity statement did not produce consistently the flow behavior in the 7×7 tube bundle configuration.

Analysis of skim milk powder deposition on stainless steel tubes in cross-flow

Particulate fouling on the gas side of heat recovery equipment is a common industrial problem. The aim of this study is to characterize the deposition of skim milk powder on a single bare tube in cross-flow. A custom built rig is applied to simulate exhaust air conditions that is experienced in an exhaust exchanger. For a constant airflow rate, increasing particle stickiness resulted in greater deposition coverage around the front of the round and elliptical tubes peaking in the middle, whereas the turned square tube tended to be either clear or covered. Results show that the skim milk powder particle impact angle on the tube, as opposed to the wall shear stress, is an important determinant for deposition. During each test and at the conclusion of each test, which was when fouling reached an asymptote, the amount of deposition coverage and build-up on the tube varied depending on the tube shape, particle stickiness, and air velocity.

Analyses of Flow Field and Heat Exchange about Limestone Grain in the Preheater

According to the structural features of the vertical preheater, the 2-dimention finite element models are set up for the section of the preheater with different arrangement and diameters for the limestone grains. The situation of gas-solid two-phase flow between the limestone grains and the hot air in the preheater is simulated by the discrete particle model. Based on the heat transfer theory, the convection heat transfer coefficient between the limestone grains and the hot air is deduced by means of the experimental correlation of the air cross-flow tube bundles. On the simulation of the 2-dimention finite element model of the vertical preheater based on the FLOTRAN module of ANSYS, the temperature field, pressure field and velocity field of the limestone grains are obtained. The results show that there is a little change of convection heat transfer coefficient by varying the arrangement of limestone grains. The convection heat transfer coefficient is improved by employing the limestone grains with little diameter. The larger the gap between the grains is, the larger the convection heat transfer coefficient is.

Mathematical Modeling of Cross-Flow Tube Heat Exchangers With a Complex Flow Arrangement

The general principles of mathematical modeling of heat transfer in cross-flow tube heat exchangers with complex flow arrangements that allow the simulation of multipass heat exchangers with many tube rows are presented. The finite-volume method is used to solve the system of differential equations for temperature of the both fluids and the tube wall with appropriate boundary conditions. A numerical model of a multipass steam superheater with 12 passes is presented. The convection and radiation heat transfer on the flue gas side are accounted for. In addition, the deposit layer is assumed to cover the outer surface of the tubes. Comparing the computed and measured steam temperature increase over the entire superheater allows for determining the thermal resistance of the deposits layer on the outer surface of the superheater. The developed modeling technique can especially be used for modeling tube heat exchangers when detailed information on the tube wall temperature distribution is needed.

A parameter study of tube bundle heat exchangers for fouling rate reduction

The formation of particulate deposits on flue gas heat exchanger surfaces will reduce heat transfer efficiency, increase the instability of equipment operation and introduce a major uncertainty into the heat exchanger design. In this paper, a numerical model was developed to predict the flue-ash particle deposition rate by considering particles transport, sticking and rebound behaviors based on the software FLUENT, extended by user-defined functions (UDFs). The numerical model was applied to cross-flow tube bundle heat exchangers with a 6-row tube arrangement. The effects of six parameters (particle diameter, flow velocity, spanwise tube pitch, longitudinal tube pitch, tube geometry shape, and arrangement) on fouling rate, as well as on the heat transfer and hydrodynamics performance, were examined. It was found that particle deposits accumulated primarily in the flow stagnation region, recirculation region, the vortex separation and reattachment regions. Increasing particle diameter moved the deposition zones towards the windward side of tubes. Using both oval tubes and staggered arrangements can reduce the fouling rate. With the increase in longitudinal tube pitch, both the particulate deposit rate and the heat transfer performance increased. To account for fouling and heat transfer performance, a tube spacing value of 2 was recommended.

Systematic analysis of the heat exchanger arrangement problem using multi-objective genetic optimization

A two-dimensional cross-flow tube bank heat exchanger arrangement problem with internal laminar flow is considered in this work. The objective is to optimize the arrangement of tubes and find the most favorable geometries, in order to simultaneously maximize the rate of heat exchange while obtaining a minimum pressure loss. A systematic study was performed involving a large number of simulations. The global optimization method NSGA-II was retained. A fully automatized in-house optimization environment was used to solve the problem, including mesh generation and CFD (computational fluid dynamics) simulations. The optimization was performed in parallel on a Linux cluster with a very good speed-up.The main purpose of this article is to illustrate and analyze a heat exchanger arrangement problem in its most general form and to provide a fundamental understanding of the structure of the Pareto front and optimal geometries. The considered conditions are particularly suited for low-power applications, as found in a growing number of practical systems in an effort toward increasing energy efficiency. For such a detailed analysis with more than 140 000 CFD-based evaluations, a design-of-experiment study involving a response surface would not be sufficient. Instead, all evaluations rely on a direct solution using a CFD solver.

Multimode Fluid Elastic Instability of Heat Exchanger Tubes

Multimode fluid elastic instability analysis is made of heat exchanger tubes in cross flow. The stability analysis predicts that the flow velocity for onset of tube instability in nonuniform flow is lowered by participation of multiple tube modes with similar natural frequencies.