Transverse Pitch Research Articles

Performance evaluation of evaporation from micropillar arrays with different pillar topologies

This paper reports the results of our studies on the performance of thin film evaporation, mostly used in the evaporator of an ultra-thin vapor chamber using micropillar arrays. The dry-out heat flux and evaporator surface temperature have been used as the performance metrics. A numerical model has been formulated to solve for the fluid flow and phase change heat transfer considering the capillary pumping action (wicking action) using a cell-by-cell forward approach and validated with results reported in the literature. The solutions of the model equation are then utilized to study the effects of geometry and arrangement of the micropillars within the vapor chamber and to gain further insights into the interplay of the various forces. It is seen that these variations strongly influence the pumping action as well as the shape of the liquid vapor interface thereby impacting the pressure field, flow rate and the dry-out heat flux. A conical shape of the micropillar (having the same volume as that of other shapes) has a significant impact on the dry-out heat flux e.g., reducing the top diameter from 10 to 8 μm results in an 18% reduction. For a rectangular arrangement of the micropillars, the performance is quite sensitive to the transverse and axial spacing. In the range of 30–40 μm of axial and transverse spacing, the dry out heat flux can be altered by as much as 20%. Higher transverse pitch with lower axial pitch is found to be a better combination for a given overall geometry and number of pillars.

Effect of new punched vortex generators in a rectangular channel on heat transfer using Taguchi method

ABSTRACT In this study, the effects of New Punched Rectangular Vortex Generators (NPRVGs) and Punched Rectangular Vortex Generators (PRVGs) inside a rectangular channel on heat transfer augmentation, Nusselt number, and friction factor were experimentally investigated. Temperature contours and average Nusselt numbers analyzed heat transfer characteristics resulting from the use of vortex generators. Furthermore, this study investigated the optimum values of the design parameters within the system of vortex generators by using the Taguchi method, and the orthogonal experimental plan was made for L16 (43x21). Reynolds numbers (5181, 10363, 20328, 25510), winglet arrangement, winglet attack angles (15°, 30°, 45°, 75°) and a distance of punched winglet from the channel bottom (b) to constant winglet height (e = 25 mm) (b/e = 0.04, 0.12, 0.2 and 0.36) were determined the working parameters in the experiments. Measurements are carried out for a rectangular channel with aspect ratio AR = 2, winglet transverse pitch (S) to longitudinal winglet height (e) ratio of S/e = 0.67 and a winglet height (e) to channel height (H = 25 mm) ratio of e/H = 1. Heat transfer rates of NPRVGs and PRVGs were compared to the obtained results for a smooth channel. The heat transfer performance is improved considerably by the NPRVGs. The maximum thermal performance factor increased from 0.99 to 1.89 owing to the combination of the vortex generator and the newly punched hole.

Computational modeling of thermal characteristics of hybrid nanofluid in micro-pin fin heat sink for electronic cooling

ABSTRACT Emerging technology in the electronic industry has led to miniaturization of electronic devices such as iPhones and laptops. Heat dissipation is required to maintain high performance and long-term reliability of these devices. Micro heat sinks and novel working fluids such as nanofluids are becoming popular. Until recent times the full potential of hybrid nanofluid in heat transfer has not yet been exploited. In order to address the lack of adequate modeling data in this field, the aim of this study is to systematically model and investigate computational heat transfer characteristics of Al2O3-Cu/water hybrid nanofluid in micro heat sink. Volume concentration in the range of 0.1%-0.5% was used for the working fluid. Hexagonal micro-pin fin heat sink with staggered arrangement was simulated in ANSYS Fluent. Reliable experimental result from literature was used to validate the accuracy of the results. The most obvious finding to emerge from this study is that the Nusselt number increases with increase in Reynolds number and this observation is consistently the same at different concentrations and pin spacing. Lower transverse pitch was observed to dominate the enhancement of Nusselt number. There was a 10% increase in heat transfer coefficient at 0.5% nanofluid concentration when compared to 0.1%. This phenomenon is highly influenced by the transition of the Re into turbulent situation which eventually enhances the heat transfer characteristics. Also, lower transverse pitch promotes swirling flow situation. At 0.5% hybrid nanofluid concentration, it was observed that the Nusselt number increased from 14.00 at transverse pitch of 3.81 mm to 17.00 at a transverse pitch of 1.81 mm and this corresponds to 16.05% enhancement of Nusselt number. The pressure drop penalty of the working fluid is a result of increase in viscous effect of the hybrid nanofluid, especially at high concentration.

Plate fin-and-tube heat exchanger computational fluid dynamics model

Insight into thermal–hydraulic correlations of plate fin-and-tube heat exchangers is of great interest in many industrial applications. Numerical simulations allow to efficiently and accurately obtain air-side heat transfer and pressure drop correlations for a broad variety of heat exchanger configurations, provided the numerical method is soundly validated against experimental measurements. In this contribution, we present a thoroughly validated computational fluid dynamics model applicable to solution of the conjugate heat-transfer problem in plate fin-and-tube heat exchangers. Favorable agreement with experimental work on four different geometries is demonstrated for high Reynolds numbers. Three out of four comparisons agree to within 20% with experiments. The computational model is applied to study the dependence of heat transfer and pressure drop in relation to the transverse tube array pitch. We show that minimizing the array angle results in enhanced fin efficiency.

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°.

Models for predicting the performance of fertilizer metering in seed cum fertilizer drill

ABSTRACT This study aimed to establish mathematical models that predict the performance of helical fertilizer metering according to the longitudinal and transverse inclinations, angular speed, and helical pitch. Laboratory tests were carried out with helical meterings with lateral and longitudinal discharge through overflow and with two helicoids, working at an angular speed of 16 and 46 rpm at the following longitudinal inclination angles: -20, -15, -10, -5, 0, +5, +10, +15, and +20° and on the transverse axis: -15, -10, -5, 0, +5, +10, and +15°. It was found that the variation of the transverse inclination has little effect on the dosage. When using fertilizer metering with a helicoid, the higher the longitudinal inclination, the greater the dosage in a linear manner, while the meterings with two helicoids provided less oscillation of the dosages according to the inclinations.

Design of two miniaturized and broadband polarization filters based on two types of square-lattice gold-filled photonic crystal fibers

Two ultra-broadband polarization filters based on Structure 1 and Structure 2 are reported and characterized in terms of attenuation, crosstalk, bandwidth, and polarization filtering properties by the finite element method. Structure 1 is formed by introducing partial elliptical air holes in the first layer of the square-lattice gold-filled photonic crystal fiber. Based on the design solution of Structure 1, Structure 2 is arranged by using circular air holes with the changed transverse pitch to replace elliptical ones. Numerical results show that high resonance intensities of 2265.07 and 1831.83 dB/cm for y polarization, together with the corresponding x polarization losses of 0.50 and 0.18 dB/cm at respective communication wavelengths of 1.55 and 1.31 μm can be achieved by varying the gold film thickness for Structure 1.Moreover, the design of Structure 2 with all circle holes can also realize a high resonance loss of 1942.54 dB/cm for y polarization with a low loss of 0.43 dB/cm for x polarization at 1.55 μm. Structure 1 and Structure 2 with the fixed fiber length of 300 μm can obtain the crosstalk values of −590.09 and −506.07 dB at 1.55 μm, together with the bandwidths of 1500 and 900 nm, respectively. Additionally, two types of designs represent good produce tolerances. These results show a high potential of the suggested fiber for application in polarization-dependent filtering and other polarizing devices.

Experimental study on the heat transfer and resistance characteristics of pin-fin tube

In order to obtain the influence of the structural parameters of the pin-fin tube on the heat transfer and resistance characteristics, ten different structural parameters of the pin-fin tube bundle were studied in this paper. The influence of the transverse pitch and the longitudinal pitch of the tube bundles, the transverse fin spacing and the height of the fin on the heat transfer and resistance characteristics of the pin-fin tube bundle were obtained, and the corresponding correlations were proposed. The results showed that the convection heat transfer coefficient and the flow resistance increased with the longitudinal pitch of the tube. The transverse fin spacing decreased, the heat transfer coefficient and the flow resistance increased. The change of the transverse pitch and the height of the pin-fin had little effect on the heat transfer and resistance performance of the tube bundles.

Experimental studies of the efficiency of a single-row bundle of bimetallic finned tubes with different finning heights in free convective heat exchange with air

Considerer the influence of a change in the height of the height of the finning of bimetallic finned tubes of a single-row bundle on the thermal, overall and metal-capacity characteristics in the mode of free air convection at different angles of inclination (γ = 0, 15, 30, 45, 60 and 90°) to the horizontal plane. The investigated tubes have the following characteristics: the material of the supporting tube was carbon steel (outer diameter d out = 25 mm, wall thickness δ = 2 mm), the material of the rolling finned shell was aluminum alloy AD1M. Geometric parameters of the fins: d×d 0 ×h×s×Δ×φ× l = 56,0×26,8×14,6×2,5× 0,5×19,3×300 mm (type I). The studies were carried out by the method of complete thermal modeling on a specially designed experimental stand. Then the fins were ground by grinding with the formation of new types of tubes (II–VI), which were assembled into a single-row bundle with a constant relative transverse pitch σ 1 = S 1 / d = 1,14 = const. The article describes the criterion dependences for heat transfer of a finned single-row bundle of tubes with different finning heights at different angles of inclination to the horizontal plane, as well as a graphical dependenc e of the correction factor for the angle of inclination of bundle (γ = 0–90°) were obtained. It has been established that an increase in the angle of inclination and the height of the finning of tubes of a single -row bundle is generally accompanied by a decrease in heat transfer. In terms of overall and metal-capacity characteristics, the most effective is a single-row bundle at various angles of inclination to the horizontal plane with a tube finning height 2,0 mm.

Computational simulation of air-side heat transfer and pressure drop performance in staggered mannered twisted oval tube bundle operating in crossflow

With a high demand for reduced energy consumption, twisted oval tube (TWT) heat exchangers are increasingly convenient for industrial applications. After a comprehensive literature survey, it was found that the literature so far discussed the counterflow efficiency of such heat exchangers, unfortunately, there is minimal emphasis on the thermohydraulic behaviour of the staggered TWT bundle operating in air-water cross-flows. And in particular, there are almost no computational studies to explore the thermal-hydraulic behaviour of staggered TWT bundles operating in crossflow. Further investigations with design optimisation are therefore required for efficient and cost-effective operation. In this study, a three-dimensional (3D) computational fluid dynamics (CFD) model is developed to analyse the air-side heat transfer and pressure drop performances in TWT bundle with staggered layout operating in air-water crossflow (600 < Re < 17,000). The Realizable k-ϵ model is used to evaluate the turbulence effect of the 3D fluid flow. The simulation results is firstly validated against the measured results and then the effects of geometrical parameters on the overall performance were analysed, such as ratio of outer major to outer minor axis (A/B = 1.491, 2.063 and 3.034), twisted pitch length (S = 0.2, 0.3 and 0.4 m), transverse tube pitch, (S1 = 0.033, 0.038 and 0.043 m), longitudinal tube pitch (S2 = 0.050, 0.065 and 0.080 m), diagonal tube pitch (S2/= 0.0527, 0.0671 and 0.0827 m), longitudinal tube rows (Z = 4, 6 and 8), and the number of TWTs (tn = 6, 9 and 12). The air-side overall heat transfer coefficient, h, the Nusselt number, Nu, and pressure drop, ΔP, were observed to be positively correlated to the Reynolds number, Re. As the Re increased, both Nu and ΔP increased with the increase of A/B, S2, and S2/, Z, respectively. However, both Nu and ΔP decreased with the geometric increase in S and S1, respectively. Finally, a generalised Nu correlation (independent of tn) is developed and validated against a recent measurement, offering a useful reference for the design of such heat exchangers.

Geometrical investigation of bluff bodies array subjected to forced convective flows for different aspect ratios of frontal body

The present computational study comprises the geometrical investigation using the Constructal Design of a triangular array of bluff bodies subjected to incompressible, transient, and forced convective flows in a two-dimensional domain. It is considered a Reynolds and Prandtl numbers of ReD = 100 and Pr = 0.71. The body areas and the maximum occupation area of the array are the problem constraints. The problem has three degrees of freedom (DOF): ST/D, SL/D (ratios between transverse and longitudinal pitch over characteristic dimension D, respectively), and H1/L1 (height and length ratio of the upstream body of the arrangement). The objectives are to minimize the drag coefficient (CD¯) and maximization of heat transfer rate per unit length (q¯′) of the arrangement. Conservation equations of mass, momentum, and energy are solved with the Finite Volume Method (FVM). Results indicated a significant gain in the fluid dynamic and thermal performances of 68.85% and 100.34%, respectively when the best and worst shapes are compared. Moreover, variations of the ratio H1/L1 strongly affected the behavior of CD¯ and q¯′ as a function of ST/D and SL/D and optimal designs. Thermal streams with complex vortex structures distributed in tree-shaped patterns led to the highest heat transfer rate magnitudes.

Numerical investigation of flow and heat transfer in axially-finned in-line tube banks

ABSTRACT Numerical analysis of heat and fluid flow over tube bank heat exchangers with axial fins is performed in this work. Finite volume method is used to solve the governing equations numerically. Simulations are carried out in the range of 400–1500 Reynolds number. To ensure the numerical model reliability, unfinned in-line tube bank is simulated and compared with results from the literature. It is shown that the results are in accordance with the experimental studies in the literature. Heat transfer and pressure loss computations are carried out for four different axial and transversal pitch ratios; namely, and , and , and , and . Air and water are used as working fluids for determined mass flow rates where Prandtl numbers are 0.7 and 7, respectively. Moreover, velocity streamlines and temperature contours along the flow field are given. Effects of axial () and transversal () pitch ratios and fin lengths on the average Nusselt number and friction factor are also presented in graphical forms. It is found that the maximum average Nusselt number is observed at Re = 1500 for the pitch ratios of and where fin length equals to 2D mm. Besides, adding axial-fins to in-line tube bank results in a decrement in friction factors.

Influence of deltoid tabs on flow dynamics and heat transfer in the separation region behind a back-facing step

The results of an experimental study of the influence of deltoid tabs on flow dynamics and heat transfer in the separation region behind a back-facing step at Re = 4000 are presented. The studied tabs are triangular in shape with a height of 6 mm. The transverse pitch of their arrangement varies. The flow pattern behind a ledge with mounted tabs, whose structure differs significantly from the flow pattern behind a classical step, is obtained. The presence of tabs brings the maximum heat transfer region closer to the step, while with a decreasing pitch between them the vortex generators become more effective.

An experimental comparison of delta winglet and novel type vortex generators for heat transfer enhancement in a rectangular channel and flow visualization with stereoscopic PIV

Vortex generators (VGs) make a very important contribution to enhancement of heat transfer in many applications. In this study, the comparison of a novel type of vortex generator (NTVG) producing longitudinal vortex (LV) with delta winglet pair (DWP) in terms of improving heat transfer was investigated experimentally. VGs were placed on the bottom surface of the rectangular channel. In order to make a correct comparison, projected areas of each vortex generator were kept equal. The position of the DWP, which ensured that the thermal enhancement factor (TEF) was maximal, was determined from the literature and placed (CFU, common flow up) accordingly on the bottom surface of the channel. As a result of the experiments, the highest TEF for DWP was obtained at about 1.15 and the results were found to be compatible with the literature. The highest TEF value for the NTVG was obtained of about 1.34. Consequently, enhancement in the heat transfer by NTVG was obtained higher than that of DWP. The transverse pitch ratio, Pt (=P/W) where the highest thermal enhancement was provided, was determined for the NTVG. When the transverse pitch ratio Pt was 0.4, the highest TEF value was obtained. LVs generated by the NTVG were visualized by using the PIV (Particle Image Velocimetry) system. The single NTVG has capacity to be able to produce two longitudinal vortices. It was seen that CFD (common flow down) and CFU, flow arrangements were generated at the back of VG and between VGs, respectively. According to the experimental results obtained, we can suggest that using NTVG instead of DWP is more effective in improving heat transfer.

Research on the Thermal Hydraulic Performance and Entropy Generation Characteristics of Finned Tube Heat Exchanger with Streamline Tube

Thermal hydraulic performance of the fin-and-tube heat exchanger is presented in this paper. The purpose of this investigation was to investigate the heat transfer mechanism and flow characteristics in the finned tube heat exchanger with streamline tube. The streamline tube in this paper had the streamline cross section which was composed of a semicircle and a half diamond. Three-dimensional numerical simulation was presented and validated by the experiment and the other numerical simulation from public articles. The present simulation had good agreement with the experimental results. The difference of the j factor and f factor between the experimental results and present simulation results by k-ε-enhance model was less than 7.6%. The geometrical parameters were considered as every single variable to investigate the thermal hydraulic performance. The results showed that smaller transversal and larger tube pitch provided greater compactness and better thermal performance. Moreover, a larger angle was not only beneficial to enhance the thermal performance, but also helpful to improve the overall performance. Secondly, the effects of angle on the heat transfer performance and fluid flow characteristics were investigated as the perimeter kept constant. It was shown that the overall performance of the streamline tube was better than the circular tube. Lastly, the entropy generation including frictional entropy generation and the thermal entropy generation were analyzed. It can be concluded that by using the streamline tube, the wake region can be obviously reduced, and thermal performance can be improved.

Experimental Behavior of Concrete Columns Confined by Transverse Reinforcement with Different Details

Introduction:Most of the existing reinforced concrete buildings often have columns with poor transverse reinforcement details. Models for computing the confined concrete strength were developed using experimental tests performed on specimens with transverse reinforcement typical of seismic design. The paper presents the results of an experimental program performed to investigate the effect of type, amount and pitch of transverse reinforcement on the behavior of confined concrete.Aim:The paper is also aimed at evaluating whether the current code models are suitable for estimating the confined strength of concrete in existing buildings.Methods:A total of 45 reinforced concrete columns with four volume ratios of transverse reinforcement were tested under axial loads. Type and pitch of transverse reinforcement typical of existing r/c buildings not designed according to seismic standards were considered. Therefore, columns reinforced by spiral and hoops with 135° or 90° hooks at the end are investigated for comparing their behavior. The confinement of spirals and hoops to core concrete is discussed as the amount of transverse and longitudinal reinforcement varies. Small increases in strength due to the concrete confinement were measured for hoop pitch of 150 mm (ranging between 2% and 7%), but also for hoops with 90° hook and pitch of 75 mm. Greater increments were obtained by spirals and hoops with 135° hook in the case of 75 mm pitch and when rhomboidal hoops or cross-ties were arranged in addition to the perimeter hoops. A comparison with some similar experimental results is also performed, achieving quite similar results. The mean experimental stress-strain curves are also analyzed.Results:The results show how the increase in concrete strength due to the confinement is more dependent on the transverse reinforcement pitch than the type and detail of transverse reinforcement or even less diameter of longitudinal bars. Finally, the experimental strength of confined concrete is then compared with the values provided by Eurocode 8 and the new Italian Building Code, showing that the higher the volumetric percentage of transverse reinforcement, the greater the overestimation of code models.Conclusion:An overestimation of codes up to 30% is assessed, systematically lower in the case of spirals, and higher in the case of hoops with 90° hooks at the end. The results highlight the need to develop specific equations to determine the strength increase due to the concrete confinement in the case of existing buildings with poor transverse reinforcement.

A new three-dimensional CFD model for efficiency optimisation of fluid-to-air multi-fin heat exchanger

New simulation and experimental results have been obtained and are presented for a multi-tube fin heat exchanger unit, from which semi-analytical correlations for the Fanning friction and Colburn factors were developed. The multi-tube and fin heat exchanger represents the main component of the Fan Coil Unit, an essential component of HVAC systems used for domestic and commercial heating and cooling. Improving the efficiency of the heat exchanger typically comes at the expense of higher pressure drops or costlier materials and production costs. Here, an experimental setup was designed and constructed to evaluate the thermal performance of such a heat exchanger. Geometrical modifications were explored for thermal performance enhancement. Furthermore, full three-dimensional CFD case studies of the heat exchanger were investigated to examine the effect of the geometrical features on the air side of the heat exchanger to study the effect of fin spacing, transverse and longitudinal pitches. The CFD model developed was first globally validated against experimental results. The model results were used to predict the Fanning and Colburn factors and the local fin efficiency based on the carefully selected geometric parameters. The data obtained was utilised to develop two new semi-analytical models for the Fanning and Colburn friction factors which were well within ±10% error bands and showed strong correlation coefficients of more than 98 and 97% respectively.

Optimization of Heat Transfer and Pressure Drop in Axially Finned Staggered Tube Banks

Numerical optimization of heat and fluid flow over staggered tube banks are investigated for both unfinned and axially finned tube banks. Ansys Fluent software is utilized for numerical computations. To find maximum total heat transfer and minimum volume of tube bank for the given mass flow rate, allowable pressure drop and tube bank effectiveness; A Multi-objective Genetic Algorithm II is used. Optimum designs are given on tabular forms and variations of total heat transfer with total volume are presented. Moreover, effects of transversal and longitudinal pitches on objective functions are graphically demonstrated. In order to confirm reliability of the numerical studies, unfinned staggered tube bank configuration is compared with experimental studies in literature. As a result of optimization studies, axial fin usage results in 42.7% decrement in volume of staggered tube bank with respect to unfinned case, while heat transfer values are equal with maximum obtainable heat transfer rate for the given optimization problem.

Optimization design with an advanced genetic algorithm for a compact air-air heat exchanger applied in aero engine

This work presents an optimal design method for a compact air-air serpentine tube heat exchanger (STHE), which is applied as the core component of the bearing sealing cooling system in the aero engine. A new optimization tool, coupling with the Logarithmic Mean Temperature Difference (LMTD) method and an advanced genetic algorithm (AGA) using Gray code, is proposed to optimize the STHE with unknown outlet parameters under variable working conditions. Under the given performance and structural constraints, six optimization design variables, including tube outer diameter, the transverse tube pitch, the longitudinal tube pitch, the transverse tube rows, the height and the number of elbows, are selected to obtain the maximum heat transfer capability KA per total weight. The optimization results show that, by using AGA, the target parameter distribution is narrowed from 88.31~93.86 kW/(K•kg) to 91.90~93.92 kW/(K•kg), and the proportion of the optimal solutions higher than 93 kW/(K•kg) increases from 22.62% to 94.32%. The optimization also improves the heat transfer capability KA per total weight to 93.92 kW/(K•kg), comparing with the original value of 70.08 kW/(K•kg). As the key limiting factors, the hot side maximum outlet temperature and pressure drop in the optimized STHE do not appear at the same operation mode and both of them are quite close to the given limits. It well demonstrates the validity and applicability of the proposed optimization design method using under multiple restrictions and conditions.

Optimization of air-water microchannel heat exchanger using response surface methodology

Multivariate optimization of the air-water heat exchanger with microchannel coil has been presented in this paper. The effects of four geometry parameters, i.e. fin pitch, microchannel flat tube transversal row pitch, number of small channels and partition wall thickness, on heat transfer rate per volume and mass, pressure drops on air and water side, heat transfer efficiency and rate of compactness have been examined using the Response Surface Methodology. Data set, composed of 27 different geometry models, has been prescribed using Box-Behnken design method. Experimentally validated, 3D mathematical model and numerical procedure have been used for numerical simulations. Fluid flow and heat transfer simulations have been performed using CFD solver FLUENT. Numerical results have been employed to calculate the objective function responses. Obtained contour plots of the objective functions have been shown for various setups. According to objective function results, nonlinear regression models (polynomial functions) have been derived and the Genetic Algorithm has been used to obtain optimal setup. The comparison of results has been given, followed by discussion and conclusions.