Tube Bank Research Articles

CFD investigation of a natural ventilation wind tower system with solid tube banks heat recovery for mild-cold climate

Passive strategies are increasingly being employed in buildings to minimize their total energy consumption and emissions. Wind tower, a natural ventilation device, can capture the wind flow from higher locations and supply the air into the buildings' indoor space without consuming electric power. However, its use in mild-cold climate is limited due to excessive heat loss and thermal discomfort, in particular, during winter. Limited research has been conducted to explore the pre-heating of the supply air in the wind tower to address the issue and potentially increase its adoption in mild-cold climates. Therefore, a novel wind tower ventilation system integrated with solid tube banks heat recovery (HR) is proposed in this research. By combining with a passive heat recovery device, a wind tower can be operated under winter conditions to improve indoor ventilation while reducing the building heating energy consumption. A three-dimensional computational fluid dynamics (CFD) model was developed to investigate the effects of the longitudinal pitch (SL) and the transverse pitch (ST) of the HR device on the ventilation and thermal performance. The overall performance of the wind tower was evaluated under different outdoor wind speeds. The results show that the heat recovery can be improved by reducing SL and ST, raising the supply fresh air temperature by up to 6.4 °C. Furthermore, the proposed wind tower can provide sufficient ventilation for a typical classroom occupied by 15 people when the external wind speed exceeds 3 m/s.

Numerical Simulation of Cross Flow in In-Line Square Tube Array to Estimate the Convective Heat Transfer Coefficient

Flow cross tube banks is an important application of different types of heat exchangers, such as compact heat exchangers, and baffled shell and tube heat exchangers. Baffles are used in shell and tube heat exchangers to allow the flow to become cross the tubes. This ensures the mix and increases the convective heat transfer coefficient. Design models of baffled shell and tube heat exchanger, which ensures the cross flow, relay on convective heat transfer coefficient. Which obtained from empirical correlation available in the literature. This work is a numerical approach to simulate flow cross a single tube and an in-line square tube array to estimate the convective heat transfer coefficient. This approach is an alternative to the experimental approach. In order to calculate the heat transfer coefficient and it’s relation to the Reynolds number for a single tube and for an in-line square tube array, a Computation Fluid Dynamic [CFD] software (ANSYS FLUENT), Which utilizes Reynolds Average Navier Stokes [RANS] method to solve the momentum equation in 3-D, was utilized to conduct the numerical simulations. Each model was simulated at 4 different entry velocities of (10, 15, 20, 25 m/s) for a Reynold’s number ranging between 6000-35000. The turbtulence model used was K-ω sst. The results obtained via the CFD simulations were validated with an empirical correlation for the two models. These results have deviations from the empirical results ranging between 5 to 22%. The numerical simulation and the empirical correlation results have identical trends for the case of a single tube and for the case of in-line square tube array. For further improvement in results validations, further studies should be made.

Thermal and flow characteristics of a channel formed by aligned round tube bank fins stamped with curve delta-winglet vortex generators

The primary purpose of present study is to reveal the roles played by CDVGs in vectoring the airflow and breeding longitudinal vortices for enhancing heat transfer. The thermal-hydraulic characteristics of a finned aligned round tube heat exchanger with stamped CDVGs were numerically studied by using the custom code with SIMPLE algorithm in a nonorthogonal curvilinear coordinate system. Four geometric parameters including vortex generator height, radial position, peripheral position and fin pitch, were programmed to ascertain parametric effects on fin side thermal performance, and the regression formulas of Nusselt number and resistance coefficient in the fin side passage with Reynolds numbers and four geometric parameters were secured. The findings suggest that the mean Nusselt number, resistance coefficient and secondary flow strength for the novel fin with CDVGs are all prevail over that in the reference plain fin under a certain Reynolds number. The body fitted CDVGs are ideally situated for finned round tube exchanger in aligned arrangement. The action of CDVGs can not only efficiently lessen the size of recirculation area behind tube but also produce strong longitudinal vortices, accordingly heighten evidently the fin side heat transfer ability in the recirculation zone. Using the studied optimum geometric parameters can make the thermal property factor up to 1.35 under the same pumping power constraint in comparison with the reference plain fin geometry when Reynolds number equals to 3000. This study can be a real application in compact heat exchanger with finned round tubes when heat transfer augmentation is under consideration.

Combined active-passive heat transfer control using slotted fins and oscillation: The cases of single cylinder and tube bank

In heat transfer augmenting methods such as radial fins, the heat transfer enhancement commonly leads to the drag force increment. In the present paper, slots are inserted over the fins to simultaneously reduce the drag coefficient. Turbulent convection heat transfer around a cylinder, as well as oscillating bundle of tubes including the slotted radial fins have been investigated. The governing equations are solved in two-dimension utilizing OpenFOAM software based on k-ω SST closure model. The cases with various slot location, slot width, the number of the slots, the fin height, and the oscillation frequencies are examined. In all cases, the Reynolds number is taken to be equal to 5000. Presence of three slots on the fins reduces the drag coefficient by 23% and augments the Nusselt number by 76%. In order to enhance the heat transfer from the bundle of tube, oscillation of tubes and utilization of the slotted fins are applied. The optimum situation occurs for a sample with the oscillating third column which shows 3% increment in heat transfer relative to that of the fixed case. This is while adding the slotted fins to the oscillating tube bank increases the heat transfer up to 2.5 times.

Experimental Investigation of Fiber-concentration Distributions of Pulp Liquid Flow through a Divergent Tube

An experimental study was performed on the fiber-concentration distributions of wood pulp liquid flowing through a divergent tube at a divergence angle of θ = 12°.The study was conducted to verify the tube’s effectiveness for application to the tube banks of papermaking machines. Experiments were conducted at bulk pulp-fiber concentrations of C<SUB>s</SUB> = 0.2-0.6 wt% and inflow velocities of U<SUB>a</SUB> = 0.1-2 m/s. The flow in the horizontal plane that contained the tube axis was visualized using the light-section method. In addition, the distributions of the time-averaged and fluctuating fiber concentrations (C<SUB>a</SUB> and C’, respectively) were measured. We then investigated the effect of the average flow rate on the variations in the directions of flow of the concentration distributions. Ata low flow rate (U<SUB>a</SUB><0.3m/s), the fluid of the annular water layer formed in the upstream tube divided the flow into two in the divergent tube, and the C<SUB>a</SUB> and C’ distributions exhibited shapes that had considerable differences in the cross-section. When the flow velocity was increased to U<SUB>a</SUB> ≥ 0.5 m/s, the shape of the C<SUB>a</SUB> distribution flattened, and C’ reduced gradually in the flow direction. The results related to the non-uniformity of fiber concentration demonstrated that the divergent tube is suitable for use in a tube-bank channel of the headbox of a papermaking machine.

A 2-D numerical analysis of heat and mass transfer in lithium bromide-water falling films and droplets

The absorber is the most crucial component of the vapor absorption system and has the largest impact on its performance. The horizontal tube bank geometry is by far the most commonly used absorber geometry, due to the high absorption efficiencies achievable without incurring commensurate pressure drops. The design of these falling film absorbers requires an understanding of the underlying heat and mass transfer mechanisms. A majority of the models proposed in the literature for absorption heat and mass transfer in horizontal tube banks make very simplistic assumptions about the flow profiles in the absorber. The effects on heat and mass transfer due to the mixing in the film caused by droplet impact are successfully modeled by the transient, 2-D spatial model developed in the present study. This 2-D model requires a significantly lower computational overhead compared to the transient, 3-D spatial model developed by the present authors in a previous study. The results from the baseline and parametric analyses are discussed. The key differences between the 2-D and 3-D models are also elucidated. The results obtained from this study will aid in the understanding and design of efficient vapor absorption systems.

Temperature and Velocity Changes Across Tube Banks in One-directional and Bi-directional Flow Conditions

Temperature and Velocity Changes Across Tube Banks in One-directional and Bi-directional Flow Conditions

Heat transfer modelling of semi-suspension biomass fired industrial watertube boiler at full- and part-load using CFD

In the current work, a computational fluid dynamics model is presented of a semi-suspension fired bagasse boiler which pays special attention to resolving the heat transfer to the furnace walls, superheater tubes and evaporator tube bank. The boiler CFD model includes custom-developed sub-models programmed in the C language which captures the fuel particle grate interaction, cylindrical fuel particle drag effects, superheater steam side heat transfer and evaporator heat exchange using a porous media approach. The model is applied to simulate the case study boiler for 100% and 65% load cases. The CFD model results are verified using a steady-state process model which utilizes well-known empirical heat transfer correlations. The CFD model results are also validated using plant measurements. The results of the CFD model show that it can accurately predict the outlet steam temperatures of the superheaters with relative errors of between 0.89 and 1.6% compared to the site data. The model is also capable of accurately predicting the evaporator outlet gas temperatures with a relative error of approximately 1.4%. Once the model is verified and validated it is used to investigate heat flux and metal temperature distributions in the various heat exchangers. The results show that at part load conditions the combustion occurs much closer to the furnace rear wall due to deeper spreading of the fuel particles, which results in the front wall having a relatively low heat absorption rate compared to the side and rear walls.

Improving the thermal, productive, and environmental performance of a non-centrifugal cane sugar production module using a heat recovery system

Non-centrifugal cane sugar (NCS) is an unrefined sweetening product that has garnered growing interest in its commercialization for its nutritional attributes. However, NCS production modules present with thermal efficiency problems that may compromise the environmental and economic sustainability of the production process. Therefore, the objective of this study was to develop and implement a heat recovery system (HRS) within a traditional NCS production module to increase the production capacity and the overall thermal efficiency. A conceptual design of the system was carried out to recover residual heat from combustion gases. Later, a mathematical model was developed to predict the thermal performance of the HRS, and a techno-economic analysis was carried out as well. Finally, construction and experimental validation were carried out on an experimental NCS production module. The HRS consisted of a tube bank with staggered configuration and two coils located in the open exchangers (OEs) of the clarification and evaporation stages. The values predicted by the model, which represented average approximations of the experimental values, presented with a correlation coefficient of R 2 = 0.7378 and a mean relative error ≤8.13%. The implementation of the HRS achieved an increase in the capacity and overall thermal efficiency of the production module by 70% and 39%, respectively. Likewise, the emission rates of CO 2 and CO per unit of mass produced by NCS decreased by 28.23% and 29.14%, respectively. The economic analysis indicated that the internal rate of return on investment does not differ significantly between the traditional system and the HRS, which allowed the financial cost to be corrected in a period of time less than or similar to that required by the traditional system. • Mathematical model to predict the thermal performance. • Techno-economic analysis for two NCS production module Comparison. • Comparison of productivity, overall thermal efficiency and flue gases in two of NCS production module.

Assessment of Turbulence Models for Low Reynolds Number Flows and Their Computational Costs, Part 1: Staggered Tube Bank

Abstract This study investigates the accuracy of computational fluid dynamics (CFD) models to predict heat transfer in turbulent separated flows at low Reynolds numbers. This article will focus on flow in a staggered tube bank, while its companion articular will focus on a square prism (cylinder) in cross flow. Experimental data for both local heat transfer and velocity profiles are available for these cases and have been used extensively in the literature to evaluate various CFD methods. Six unsteady models were used and the results show that the unsteady shear stress transport (SST) model provided good overall accuracy relative to the mean Nusselt number for both cases. However, the SST model failed to accurately predict local variations. The partially averaged Navier–Stokes (PANS) variant of the SST model did show a marked improvement over the baseline SST model. The dynamic Smagorinsky large eddy simulation (LES) showed a much-improved fidelity to the local Nusselt number but unpredicted the actual values. The computational cost for the LES model was significant and it was found that the computationally expensive models with higher degrees of resolved turbulence did not necessarily return better results. Finally, the pressure drop results for the six models were scaled to predict the mean Nusselt number with the generalized Leveque method and were found to be very accurate. This method should prove useful to predict heat transfer performance with computationally less expensive cold flow results.

Assessment of Turbulence Models for Low Reynolds Number Flows and Their Computational Costs, Part 2: Square Prism in Cross Flow

AbstractThe accuracy of six turbulent flow modeling techniques in an unsteady solution is evaluated against experimental data for a square prism in cross flow. The selected models, shear stress transport (SST), SST-SAS, Reynolds stress model (RSM), partially averaged Navier–Stokes (PANS)-SST, detached eddy simulation (DES), and large eddy simulations (LES), models are the same as those presented in Part 1 of this study, which focused on flow in a staggered tube bank. For this geometry, the SST model proved to be effective at capturing the averaged Nusselt values per side of the square with relatively low computational costs. The SST model, however, showed poorer fidelity to the local Nusselt number profile compared to the experimental data. The LES approach provided a more accurate representation of the local Nusselt number but the computational cost was significantly higher. The PANS modification to the SST model did provide a noticeable improvement in accuracy at a reasonable cost while the SAS modification did not see the same improvement. These conclusions are generally consistent with those found for the staggered tube bank in Part 1 of this study. This study can be used as a guide for the industrial user to select a turbulence model for a similar problem with a low Reynolds number and significant flow separation.

Numerical Investigation of The Effect of Finned Obstacle on Heat Transfer Characteristics in a Rectangular Channel

Nowadays, with the development of technology and science, heat transfer holds an important place in engineering applications. In industrial areas, heat increases give rise to overheating, causing system errors. Passive techniques are frequently used to prevent from such disruptions. In this work, fins which are passive techniques which provide heat transfer development with high efficiency and low cost were investigated. To improve heat transfer, finned structures should be well optimized. On the other hand, the designer can prevent mixing the incoming air with the heated air with a bad design, which may cause a negative effect rather than improve heat transfer. In this work, in contrast to previous works that is smooth tube and 0 degree rectangular finned tube, four fin structures were designed and flow and heat-transfer characteristics numerically analyzed. These are crescent finned tube, 20 degree symmetrical imperforated rectangular finned tube, 20 degree asymmetrical imperforated rectangular finned tube and 20 degree symmetrical perforated rectangular finned tube banks with six rows. In this investigation, the geometric parameters were not changed and their effects on flow and heat transfer properties in different Reynolds numbers on these models were examined. The results indicate that the symmetrical structure has better heat transferability and higher friction loss compared to the asymmetrical structure and the perforated fin is higher than imperforated fin but the overall performance is not always superior. Therefore, both symmetrical and perforated finned tube is designed and analyzed with the highest heat transfer potential, it is seen that in terms of heat transferability this model is better than other designs.

Fiber-Concentration Distributions of Papermaking Pulp Suspension Flowing in a Channel with a Sudden Expansion

In this study, we analyze the flow characteristics of pulp suspensions in a sudden expansion channel, which is used as a tube bank in papermaking machines. Flow visualization using a light section method and measurements of time averaged and fluctuating fiber concentrations, C<SUB>a</SUB> and C’, and pressure were performed. The experiments were carried out on pulp suspensions with a bulk fiber concentration (C<SUB>s</SUB>) of 0 2.0 wt% in a channel with an expansion ratio of β = 2 at varied average velocities (U<SUB>a</SUB>) of 0.04-6.6 m/s. We examined the effect of flow velocity on the changes in concentration and pressure distributions along the channel axis The distribution of the pulp fiber concentration behind the sudden expansion significantly changes depending on the flow state in the upstream channel before the expansion. In particular, for low and moderate velocities, the flow field is characterized in the main flow region consisting of flocculated plug fibers and a recirculation region with a low concentration near the expansion corner by a type of streamline originated from the thin water layer in the upstream channel. The distribution of C<SUB>a</SUB> shows a tongue shaped structure in the central part of the cross section and C’ is large near the annulus streamline As the velocity is further increased, the flocculated fibers gradually disperse and the distributions of C<SUB>a</SUB> and C’ become almost uniform. In addition, the uniformity of fiber concentration does not change much in the section downstream from a distance 10 times the step height from the sudden expansion plane.

ELEKTRİKTE PİK TALEBİ TÖRPÜLEME İÇİN KULLANILACAK FDM-HAVA SOĞUK ENERJİ DEPOLAMA CİHAZI TASARIMI VE OPTİMİZASYONU

Parametric analysis and dimensional optimization study for the melting process of the phase changing material placed in cold energy storage devices have been conducted. Phase changing material, PCM, is used to store the cold energy. A cross-flow tubular PCM-air heat exchanger is used. An electricity peak shaving for a summer day in Antalya for 5 hours (10:00-15:00) is aimed. The cooling load of a room (33 m2) in Antalya is taken as a reference value. A minimum cooling load of 2850 W is supplied throughout 5 hours to maintain the building's comfort temperature. Contrary to the studies handling air velocity constant, it is varied during melting in the present study to satisfy the minimum cooling load. The melting process is analyzed using a numerical model. A computational algorithm is implemented in MATLAB environment. The numerical model is validated for the same conditions and the dimensions with an analytical model presented in the literature. A transient analysis has been employed, and the problem is discretized for the time and space domain. A dimensional optimization algorithm is employed. A parameter “performance ratio/ cooling index”, the ratio of total cooling to fan energy consumption, is defined. Spacing between the tubes giving the maximum performance ratio is sought for aligned and staggered tube bank arrangements using this algorithm. The best performance ratio is obtained at 0.2 m/s initial velocities for both arrangements. In addition, the effect of sizing/arrangement of the tubes, PCM mass, and thermal conductivity of the PCM on the melting characteristics is investigated. As a result of increasing the thermal conductivity from 0.2 W/(m·K) to 0.6 W/(m·K), the performance ratios are raised 4.58 and 3.52 times for aligned and staggered orders, respectively. It is calculated that 10 kg PCM can be saved thanks to enhancing the thermal conductivity of the PCM.

Critical review of the definitions of the Bejan number - first law of thermodynamics

This paper is an extension of our recently published paper (Zimparov et al., 2020 [1]), proving that the unique form of the Bejan number (Be) is derived from the classic fluid mechanics and that the other forms, derived from the first law of thermodynamics (related to the convection heat or mass transfer), are combination of unique Bejan number with Prandtl (Pr) or Schmidt (Sc) numbers respectively. In this study we continue to maintain this idea through presenting many new examples of problems in the field of periodic or time dependent pure laminar duct flows and laminar flows in channels filled with porous medium, with critical review. The evaluation of the friction factor for channel flow or external cross flow over tube bank have also been critically discussed. The discriminated dimensional analysis has been again applied as a tool to reveal the role and physical meaning of the unique Bejan number (Be) as criterion of similarity or dimensionless variable.

Performance enhancement for tube bank staggered configuration heat exchanger – CFD Study

The effect of attaching a splitter plate to the circular tube on the performance of a heat exchanger with a circular tube bank is investigated. A CFD model is developed to simulate the cross-flow behavior inside the heat exchanger. The study of the thermal performance of the tube bank heat exchanger with staggered configuration with and without attached splitter plates to the tubes is conducted. The numerical results are compared with previous experimental and numerical data. Results show that attaching the splitter plates to the tubes can reduce the pressure drop and increase the Nusselt number leading to better overall thermal performance for the heat exchangers and indicate that the splitter plates should be as thin as possible to keep the optimum overall performance. As the splitter thickness is increased to about 20 % of the pipe diameter the heat transfer is slightly increased by about 7 % but the pressure drop is dramatically increased by about 20%. As splitter length is increased up to L/D ratio of 1.5, the heat transfer is doubled, the pressure drop as well is increased by about 20%.

Experimental Study of Heat Transfer and Pressure Drop of Two-Pass Tube Banks.(Dept.M)

Tube bandles are widely used in heat exchangers for energy carriers and cooling product process, . as a cooler for compressor in gas turbine unit and refrigeration plants. Heat transfer and pressure drop of isolated two-pass tube banks bandles with baffles at different locations are experimentally studied. In the present study, five-heated test tubes are located at inlet of the first p ass. The test. tubes are heated at different rates-(Q). The measurements are taken for the five test tubes. The length of the baffles and its location are taken into account. Also the effect of the tube banks aspect ratio is studied. From the measurements, the local heat transfer coefficient and pressure drop are calculated. All the measurements are conducted at low Reynolds number range, (Re = 1000 to 3500) .

Numerical Investigation of Heat and Fluid Flow Characteristics through Steam Coil Air Preheater Installed in 100 MWe Utillity Boiler

This paper study the flow characteristic through a set of tube bank used as air preheater in a 100 MWe utility boiler. The study using numerical simulation commercial Computational Fluid Dynamic (CFD) software. A 3D model uses to analyze the flow characteristics with 3 load variations. The model using a steady-state flow assumption. The Steam Coil Air Preheater modelled as a porous medium (PM) to represent its effect on the airflow through it. Porous Medium model can avoid a time consuming mess generation and simulation. This paper using an empirical equation to define inertial resistance coefficient (C2). The model using Heat Exchanger Model, Energy Model, and Realizable k -Epsilon for turbulence viscous model. 100%, 75%, and 50% load variation simulated with the model. The simulation result in both quantitative and qualitative data. The velocity and pressure distribution affected by the load variation, geometrical shape change, and tube bank set. Some secondary flow observed due to geometrical change along with the flow. The heat transfer characteristics observed with temperature distribution show that it affected by the flow characteristic.

Multi-objective optimization of heat transfer through the various types of tube banks arrangements

In the present study, numerical method is used to investigate the exergy and energy characteristics of the air flow through tube banks which their arrangements are common inline or staggered. Furthermore, new arrangement called “twisted tube bank” is introduced and perused. Realizable K-ε model is selected for the investigation of the turbulence impact. The influence of dimensionless transverse and longitudinal pitches, Reynolds number, twist angle and dimensionless duct diameter on three objective parameters, COP, CEP and second law efficiency are analyzed. Equations are suggested for prediction of these three objectives parameters. Furthermore, a multi-objective method will be introduced and used to obtain the optimum cases of the geometrical and operational variables. Results indicate, if the Reynolds number is doubled, the CEP increases by 39.5% while, the second law efficiency and the COP reduce by 75.7% and 10.2%, respectively. Furthermore, it was realized, the application of the twisted tube bank is superior when the CEP and the second law efficiency are the objective parameters simultaneously while, for other multi-objects the inline arrangement is more appropriate. The optimum cases will be achieved at twist angle equal to 43° and 84°.

Heat Transfer Across Tube Banks With a Passive Control Vortex Generator in Steady One-Directional and Oscillatory Flows

Fluid can flow in one-directional (normal flow) or oscillatory conditions. Fluid flow in some energy system involved oscillatory flow condition. The use of vortex generator has been proven to improve heat transfer in the case of one-directional flow but the impact of vortex generator in oscillatory flow condition is yet unknown. This study focusses on the heat transfer performance across a heated tube banks using a Computational Fluid Dynamics (CFD) model. Two flow conditions were modelled: steady one-directional and oscillatory flow conditions. Two-dimensional CFD models of steady flow and oscillatory flow were solved using the SST k-? turbulence model for two different cases of heated tube banks with and without the vortex generators. The heat transfer performance for both flow conditions were analysed by considering a heat transfer parameter known as Colburn-j factor. Results showed that the use of a vortex generator increased the heat transfer enhancement, regardless of the flow conditions. However, it is also noted that the heat transfer behaviour in a steady flow and an oscillatory flow is not the same, especially with the appearance of secondary flows in the system. The difference is discussed with respect to dimensionless quantity of Colburn j-factor, the non-dimensionless quantity, and the amplitude of temperature field. The result indicates that the heat equation in the steady flow condition is not very suitable to be directly used in oscillatory flow conditions. Appropriate heat equation needs to be properly addressed for situations that involve oscillatory flow motion.