Nusselt Number Ratio Research Articles

Analysis of the effect of perforation in multi-v rib artificial roughened single pass solar air heater: - Part A

ABSTRACT A detailed study on Solar Air Heaters (SAHs) with single-pass roughening with plastic net perforated multi-V and continuous multi-V ribs was investigated and compared for its effectiveness. Improvements in the Nusselt number ratio (Nu/Nus) and the friction factor ratio (f/fs) studied with the Reynolds number (Re) ranged from 2000–18000 for a relative roughness height (e/Dh) of 0.043, a relative roughness pitch (P/e) of 10, an aspect ratio (W/H) of 12, and an angle of attack (α) of 60°, maintaining a similar flow scenario. For a syntactically roughened channel with a perforated multi-V rib, the significant enhancement was recorded in thermo-hydraulic performance with a slight decrease in pumping power. The study was extended to analyze the effect of perforation in a double pass parallel flow SAH.

Thermo-hydraulic performance and exergy analysis of a fin-and-tube heat exchanger with sinusoidal wavy winglet type vortex generators

The widely used fin-and-tube heat exchangers have a low air-side heat transfer capacity. Enhancing the air-side thermo-hydraulic performance is a key point of the relevant research. Vortex generators have been proven to be one of the most promising techniques to improve the air-side heat transfer capacity of the fin-and-tube heat exchangers. Herein, this paper proposed an original sinusoidal wavy winglet type vortex generator to achieve high heat transfer performance with moderate increased pressure drop penalty. A three-dimensional numerical simulation was conducted to explore the air-side thermo-hydraulic performance with Reynolds number ranging from 1027 to 2054. The longitudinal vortex is generated in the channel so as to enhance the intensify of convective heat transfer. Especially, the wave-shaped vortex generators can produce many local small-scale secondary flows. And it is confirmed that the frontal area of the vortex generator can be effectively extended by such wavy shape and a more intensified disturbance can be caused in the air-side. Furthermore, the effects of the attack angle (α), dimensionless height (Hp) and especially arrange position of the vortex generator were investigated. Additionally, the ranges of the Nusselt number ratio, friction factor ratio and surafece goodness factor in this study are 1.09–1.52, 1.09–2.31, and 1.06–1.24, respectively. Moreover, it is also very crucial to evalute the grade of energy and to explore the irreversible loss druing the heat tranfer process in view of the second law of thermodynamics. And the thermal dissipation can be reduced by up to 5.85% compared with the plain fins from the perspective of exergy analysis.

Aerothermal performances of two-pass wavy ribbed channels without and with internal effusion at three bypass angles

The detailed Nusselt number distributions, Fanning friction factors, and thermal performance indexes of four two-pass wavy ribbed channels are measured for highlighting the effect of the internal effusion on the aerothermal performance. The modified flow structures by the internal effusion at the bypass angles of 0° or +22.5° considerably alter the Nusselt number distribution from that without effusion. The local and spanwise-average heat transfer performances are respectively dominated by the effusion-rib induced flow phenomena and the local Reynolds number. The area-averaged Nusselt number (Fanning friction factor) ratio between the present two-pass channels with and without effusion is in the respective ranges of 91.39–187.68 (0.21–0.22), 100.43–196.08 (0.17–0.19), and 90.67–143.01 (0.22–0.22) for the bypass angles of 0°, +22.5°, and − 22.5° at the Reynolds numbers between 5000 and 15,000. The corresponding thermal performance indexes for present two-pass channels with the effusion at the bypass angles of 0°, +22.5°, and − 22.5° are 1.74–1.33, 2.07–1.47, and 1.71–1.01. The empirical correlations that evaluate the regional average Nusselt number and Fanning friction factor of the two-pass wavy ribbed channel with and without effusion are generated to assist the relevant applications.

Heat transfer augmentation for turbulent flow in circular tubes using inserts with multiple curved vortex generator elements

Measurements of local heat transfer coefficients on the circumference of a circular tube, fitted with two modified versions of an already existing axially inline curved Vortex Generator (VG) element insert are reported in this study. Measurements for average pressure drop are also presented and the air was used as the working fluid. The first modified insert is manufactured with a pair of two VG elements spaced symmetrically 180° apart but with the subsequent pairs along the axis staggered at an angle of 90°. The geometric features for the staggered insert were identical to the optimal one for the existing inline one. The staggered insert gives a more uniform circumferential and axial Nusselt number distribution. The ratio of the highest and lowest Nusselt number values for the staggered and inline case is equal to approximately 1.3 and 1.6 respectively. The average Nusselt number ratio at constant Reynolds number (10,000 < Re < 45,000) and constant pumping power however remained the same at about 2.6 and 1.5 respectively. The second modified insert was manufactured with three vortex generators placed symmetrically 120°apart at each axial location with all subsequent locations in line with one another. Two angles of attack 30°and 45° were used for the vortex generator elements and the pitch to projected length ratio of the VGs was varied between 1.6 and 12.2. The average heat transfer coefficient augmentation ratio using this insert at the equal Reynolds number and constant pumping power was obtained in the range of 1.5–5.5 and 1.0 - 1.8, respectively. The experimental data is presented in the form of empirical correlations for Nusselt number, and friction factor as a function of tube flow parameters, and non-dimensional geometrical parameters of VG inserts, which predict the data with an accuracy equal to ±20 %.

Thermal Performance of V-Shaped and X-Shaped Ribs in Trapezoidal Cooling Channels

Convective heat transfer enhancement using rib turbulators is effective for turbine blade internal cooling. Detailed heat transfer measurement of X-shaped ribs in a trapezoidal cooling channel was experimentally conducted using infrared thermography. The novel X-shaped ribs were designed by combining two V-shaped ribs, and more secondary flows generated by the X rib delivered higher heat transfer enhancement. The Reynolds numbers in this study were 10,000, 20,000, and 30,000. These ribs were installed on two opposite walls of a trapezoidal channel in a staggered arrangement. The rib pitch-to-height ratios were 10 and 20, and the rib height-to-hydraulic diameter ratio was 0.128. Results indicated that higher heat transfer distribution was observed in the vicinity of the shorter base in the trapezoidal channel. The full X-shaped ribs and the V-shaped ribs demonstrated the highest Nusselt number ratios among all the cases. Although full X-shaped ribs contributed to higher heat transfer improvement due to intensified secondary flows, they also caused significant pressure loss. Therefore, the cutback X-shaped ribs were proposed by removing a segment in the rib at either upstream or downstream region. Consequently, the upstream cutback X-shaped rib and the V-shaped rib produced the highest thermal performance in this trapezoidal channel.

Experimental Investigation of Micro Cooling Units on Impingement Jet Array Flow Pressure Loss and Heat Transfer Characteristics

With the development in additive manufacturing, the use of surface treatments for gas turbine design applications has greatly expanded. An experimental investigation of the pressure loss and heat transfer characteristics within impingement jet arrays with arrays of target surface micro cooling units is presented. The discharge coefficient and Nusselt number are measured and determined for an evaluation of the pressure loss of the flow system and heat transfer level, respectively. Considered are effects of impingement jet Reynolds number ranging from 1000 to 15,000 and micro cooling units (square pin fin) height (h) with associated values of 0.01, 0.02, 0.05, 0.2, and 0.4 D, where D is the impingement hole diameter. Presented are variations of Nusselt number, and Nusselt number ratio, discharge coefficient, discharge coefficient ratio, discharge coefficient correlation. Depending upon the micro cooling unit height, discharge coefficient ratios slightly decrease with height, and the ratio values generally remain unit value (1.0). When Rej = 1000 and 2500 for several cooling units height values, discharge coefficient ratios show the pressure loss decreases about 2–18% and 3–6%, respectively, when compared to the data of a baseline smooth target surface plate. The observed phenomenon is due to the effects of flow blockage of micro cooing units, local flow separation, and near-wall viscous sublayer reattachment. Results also show that heat transfer levels increase 20–300% for some of the tested toughened target surface plates when compared to smooth target surface plates. The heat transfer level enhancement is because of an increase in thermal transport and near-wall mixing, as well as the increased wetted area. In addition, micro cooling units elements break the viscous sublayer and cause greater turbulence intensity when compared to the smooth target surface. Overall, results demonstrate that the target surface micro cooling units do not result in a visible increment in pressure loss and reduce pressure loss of the flow system for some of the tested patterns. Moreover, results show the significant ability of micro cooling units to enhance the surface heat transfer capability of impingement cooling relative to smooth target surfaces.

Effects of blockage and fluid inertia on drag and heat transfer of a solid sphere translating in FENE-P viscoelastic fluids in a tube

An extensive numerical investigation of the flow and heat transfer phenomena of a solid sphere translating in a cylindrical tube filled with FENE-P viscoelastic fluids is reported herein. The governing equations, namely, mass, momentum, energy, and viscoelastic constitutive equations, have been solved over the following ranges of conditions: Reynolds number, 1≤Re≤100, Weissenberg number, 0≤Wi≤10, polymer extensibility parameter, 10≤L2≤500 and blockage ratio, 0≤BR≤0.7 for a fixed value of the polymer viscosity ratio β=0.5 and Prandtl number Pr = 10. Limited simulations with the FENE-CR viscoelastic fluid model have also been carried out to make a comparison between the two viscoelastic models. At low Reynolds numbers, the velocity overshoot and/or negative wake downstream the sphere has been observed under appropriate conditions. This tendency of their appearing decreases with the increasing Reynolds number, and decreasing blockage ratio, polymer extensibility parameter and Weissenberg number. The size of the recirculation region (wake length) increases with the Weissenberg number at low values of the polymer extensibility parameter, whereas a reverse trend is seen at high values of L2. The drag coefficient decreases with the Reynolds and Weissenberg numbers, whereas it increases with the blockage ratio. On the other hand, the average Nusselt number always increases with the Reynolds number irrespective of the values of Wi, L2 and BR. However, the corresponding effect of the blockage ratio and polymer extensibility parameter is seen to be more complex, modulated by the values of Re and Wi. For instance, at high Reynolds numbers, the average Nusselt number always increases with the blockage ratio; however, at low values of it, there is a critical value of the blockage ratio present up to which the average Nusselt number increases, and beyond that, it decreases. Furthermore, the average Nusselt number always initially increases up to a certain value of the Weissenberg number, and after that, it remains almost constant or decreases depending upon the values of L2, BR and Re. Finally, simple correlations for the average Nusselt number and drag ratio are presented, which not only capture the functional dependence of the governing parameters, but also can be used for the interpolation of the present results for the intermediate values of the governing parameters in a new application.

Comparison of Co- and counter-current modes of operation for wavy minichannel heat sinks (WMHSs)

This study aims to make a comprehensive comparison between the applications of co- and counter-current modes in the wavy minichannel heat sinks (WMHSs). In this regard, the effects of wave-amplitude (wa) and wave-length (wl) are studied at different levels. Water is used as coolant over the Reynolds number (Re) range of 100–900. Nusselt number ratio, friction factor ratio, performance index, max. base temperature difference, and pumping power are reported as quantitative results. An experimental study is also carried out to establish required data for the validation. The outputs illustrate that for both the current modes, the dependence of thermal and hydraulic characteristics on Re increases with increasing wa and decreasing wl, while the counter-current mode is more impressed by the increase of Re compared to the co-current mode. In summary, the Nusselt number ratio ranges from 1.22 to 2.96 for the co-current mode and from 1.23 to 3.17 for the counter-current mode, and the friction factor ratio ranges from 1.09 to 2.73 for the co-current mode and from 1.10 to 2.92 for the counter-current mode, and the range of performance index is from 1.18 to 2.12 for the co-current mode and from 1.19 to 2.22 for the counter-current mode. Eventually, it is found that at the same pumping powers, replacing the co-current mode with the counter-current mode can significantly boost the temperature uniformity in WMHS. The max. base temperature of about 0.84 K is recorded for WMHS with wl = 5 mm at Re = 900. It is worthy to note that the superiority of the counter-current mode increases with increasing wa and decreasing wl.

Numerical assessments of flow pattern and heat transfer profile for the round tube equipped with different configurations of the dual-inclined baffle

Numerical studies of the heat exchanger tube (HXT) equipped with the dual-inclined baffle (DIB) on fluid streams and heat transfer profiles are reported. The DIB configurations can be separated into three types: 1. The DIB is inserted in the middle of the HXT called “type I″, 2. The DIB is placed on the HXT wall called “type II” and 3. The combination of the type I and II DIB called “type III”. The three DIB types are designed with the main purpose to create three different flow profiles. The influences of DIB height with a single pitch distance (P/D = 1) and DIB attack angle of 30° for heat transfer characteristics and fluid streams are discussed in a laminar flow region at Re = 100–2000 (considered at the entry condition). The numerical problem of the HXT inserted with the DIB is solved with a commercial code (the finite volume method). The computation domain is validated to confirm the reliability and accuracy of the simulated results. The flow configurations: streamlines in transverse planes (y-z planes), and heat transfer behaviors: temperature contours in transverse planes (y-z planes) and local Nusselt number contours (Nux), in the HXT equipped with the DIB are proposed in the simulated-result section. The relations of the average Nusselt number ratio (Nu/Nu0), friction factor ratio (f/f0) and thermal enhancement factor (TEF) with the Reynolds numbers in the HXT inserted with the DIB are presented. As the simulated results, the equipment of the DIB in the HXT brings upper heat transfer rate and thermal performance than those of the smooth tube because of the creations of the vortex streams and impinging streams. The different flow profiles are detected when varying the DIB types that impact for the variations of the heat transfer profile. For the studied range, the increased heat transfer rate in the HXT installed with the DIB is observed to be around 1.03–17.46 times above the plain tube depended on the DIB type, DIB blockage and Reynolds number. Additionally, the maximal TEF of 3.70 is found for the type II DIB at b/D = 0.25 and Re = 2000.

Study on flow and heat performance of thermal-hydrolyzed sludge in a double-pipe heat exchanger with a series of inner corrugated tubes

In this paper, a double-pipe heat exchanger with a series of corrugated tubes as inner tubes is studied for the adequate cooling of thermal-hydrolyzed sludge. A numerical model, built on the Spalart-Allmaras model, is validated to have the ability for characterizing the flow and heat transfer performance of sludge. The influencing factors, including the flow parameters of the sludge and cooling water, and the geometric configurations of the corrugated tube are parametrically investigated. The ratio of Nusselt number (Nu/Nu0), the coefficient of flow resistance (f/f0) of the sludge, the synthesis factor of performance (η) and the ratio of the heat transfer increase over the pump power increase (ζ) are used as the performance assessment indexes. The results show that increasing Resludge can negatively affect η and η tends to be stable when Rewater is higher than 1.2 × 104. Decreasing S/D and increasing H/D result in a higher ratio of flow resistance f/f0 and a higher ratio of Nu/Nu0. The corrugation height (H) shows a larger effect than the corrugation length (S). In practical applications, the optimal flow rates of the sludge for the studied corrugated tubes are recommended to be within the range of 1.2–1.5 m/s.

Numerical and Experimental Investigations of Micro Thermal Performance in a Tube with Delta Winglet Pairs.

In this research, a novel vortex generator (VG) is presented. The experimental and numerical investigations were carried out to study the micro thermal-hydraulic performance in a heated tube. The numerical results showed that the fluid in the core flow region and the near-wall region was fully mixed because of the longitudinal vortices created by the vortex generators. In addition, the experimental results showed that the heat transfer coefficient () decreased with the increasing pitch ratio () value, while the friction coefficient exhibited the opposite trend. With the increasing ration angle () numbers, the values decreased while the numbers increased. In addition, the maximum and minimum values of the fraction ratio were 1.66 and 4.27, while these values of the Nusselt number ratio were 1.24 and 1.83. The maximum thermal enhancement factor (TEF) was 1.21 when = 0.5, = 0° and = 9090. The heat transfer enhancement mechanism of the vortex generator is explained from the microscopic point of view.

Heat Transfer and Fluid Flow Analysisfor Turbulent Flow in Circular Pipe with Vortex Generator

The performance of heat transfer enhancement (HTE) using modified inserts (MIs) as a vortex generator in pipe flow and fluid flow analysis using computational fluid dynamics (CFD) are evaluated in this article. The MIs are fastened to the central rod, and the circular sections of the MIs touched the circular wall of the test pipe. Heat transfer and fluid flow analyses are carried out for the various pitch to diameter ratios (P/D) and angles of the MIs. P/D ratios of 3, 4 and 6 and MIs angles of 15°, 30°, 45°, 60° and 90° are considered for experimental analysis. CFD analysis is carried out for P/D ratios of 3, 4 and 6 and MIs angles of 30°, 45° and 90°. Nusselt number (Nu/Nus) and friction factor (f/fs) ratios are evaluated using the same Reynolds number between 8000 and 17,000 in the experimental study. The MIs encourage the wall and core fluid to be combined thus helps in HTE. It is found that, as the P/D ratio increases, the Nu/Nus and f/fs decrease. If the distance between the MIs increases, the mixing of fluid weakens. With decreasing the P/D ratio, Nu/Nus increases. Increased fluid mixing leads to a higher coefficient of heat transfer and higher values of pressure drop. A P/D ratio of 4 and MIs angle of 45° results in greater heat interaction than others. Finally, recommendations for the best P/D ratio and angles of MIs are made for improved HTE on fluid flow through a circular pipe.Article HighlightsModified inserts (MIs) are used inside the test pipe to check the heat transfer enhancement at various angles. Also, compared the performance with and without MIs.Fluid flow analysis is checked by CFD (Fluent) in Ansys software.Fluid flow patterns for various MIs angles and P/D ratios are compared.

The effect of linearly increasing/decreasing pitch ratio twisted tape with various progression rate and nanofluid towards the system performance

The overall enhancement ratio decreases with increasing Reynolds number for most of the existing modification of twisted tape. Hence, in this study, a new innovative twisted tape with linearly increasing/decreasing pitch ratio at various progression rate of 0.2, 0.25 and 0.3 (LIPR/LDPR) combined with three different nanofluid (Al2O3/water, SiC/water and CuO/water) are designed and analyzed. The analysis is conducted at Reynolds number ranges from 4000 to 16,000 and volume fraction ranges from 1% to 3% by using numerical and experimental approaches. ANSYS FLUENT software is selected for simulation analysis by adopting RNG K-ε turbulent model using enhanced wall treatment method. Five different geometries of twisted tape including standard twisted tape with constant PR of 2 (TT PR 2), linearly-increasing-progression-rate of 0.2 twisted tape (TT LIPR 0,2), linearly-increasing-progression-rate of 0.25 twisted tape (TT LIPR 0.25), linearly-increasing-progression-rate of 0.3 twisted tape (TT LIPR 0.3) and linearly-decreasing-progression-rate of 0.25 twisted tape (TT LDPR 0.25) are used for analysis. Among all geometries of twisted tapes, TT LIPR 0.25 creates the highest turbulent kinetic energy value at the cross-sectional area. TT LIPR is the best choice to be utilized under turbulent flow regime where Reynolds number >10,000 as it creates higher overall enhancement ratio compared to TT PR 2 and TT DPR. The simultaneous use of 1% SiC/water nanofluid and TT LIPR 0.25 at 10,000 Reynolds number has increased overall enhancement ratio up to 2.985. The empirical correlation is generated for future prediction of Nusselt number, friction factor and overall enhancement ratio with wide range of nanofluid type, fluid flow range and Reynolds number of twisted tapes.

Influence of Narrow Rectangular Channel ( AR = 1 : 4 ) on Heat Transfer and Friction for V- and W-Shaped Ribs in Turbine Blade Applications

Effective cooling of blades with a nominal pressure drop is essential for performance augmentation and thermal management of gas turbines. Hence, present work is aimed at determining the heat transfer enhancement and friction for W- and V-shaped ribs inside a rectangular cooling channel having hydraulic diameter ( D h ) of 0.048 m and aspect ratio ( AR ) 1 : 4. Ribs are fixed facing downstream with angle of attack ( α ) 45° on opposite walls. Pitch ( P ) between two successive ribs is 25 mm for both cases. Continuous V- and W-shaped ribs with height to channel hydraulic diameter ratio ( e / D h ) 0.052 and 0.0416 and pitch to height ratio ( P / e ) 10 and 12.5, respectively, have been examined for Reynolds number ( Re ) range 20000-80000. Heat transfer augmentation achieved at Re 80000 is 1.94 and 1.8 times higher than Re 20000 for V- and W-shaped ribs, respectively. Streamwise and spanwise variations in local Nusselt number ratio are highest for V-shaped ribs, which are estimated to be 31% and 12%. For W-shaped ribs, variations are 17.5% and 3.5%. Nusselt number ( Nu ) is highest along span length 0.5 w for V-shaped ribs due to dominance of apex induced secondary flow. For W-shaped ribs, Nusselt number along the span lengths is found to be nearly same view uniformity in secondary flow. Maximum enhancement ( Nu / N u o ) estimated for both the rib shapes is 3.9 at Re 20000. Due to increased rib height, friction losses for V-shaped ribs are higher than W-shaped ribs. Maximum friction loss increment is estimated to be 85% for V-shaped ribs and 42% for W-shaped ribs between Re 20000 and 40000. For both rib shapes, impact of ribs is found to be greatest at Re 40000. Thermohydraulic performance ( THP ) for W-shaped ribs is superior to V-shaped ribs. Best THP achieved for W- and V-shaped ribs are 3.7 and 3.4 at Re 20000.

An integrated thermoelectric heating-cooling system for air sterilization— a simulation study

Semiconductor thermoelectric devices are broadly used for various heat-pump applications, where they function as either a heater or a cooler. In this article, an integrated system combining thermoelectric heating and cooling is introduced, which not only improves the overall energy efficiency but has a great potential for applications in air sterilization and purification. In the proposed system, the air is firstly heated up for air sterilization by fins thermally grounded to the hot side of multiple thermoelectric devices. The air is then cooled down by the cold side of the same thermoelectric devices. The heat exchange and flow conditions of the system are analyzed, and it is found that the Nusselt number ratio after adding the fins is increased by two times compared with the smooth tube. The air temperature in the system can reach up to more than 65°C using multiple thermoelectric devices. Such a temperature is able to inactivate many viruses. The coefficient of performance of the overall system can reach 5.4. This work demonstrates new applications of thermoelectric devices for energy-efficient air sterilization applications.

Thermal analysis of flow in a porous flat tube in the presence of a nanofluid: Numerical approach

Heat transfer enhancement is an important subject in heat exchangers research. An understanding of different shapes and designs of tubes and pipes means of improving heat extraction will have a direct effect on a new design for a heat exchanger. The present paper involved various investigations of thermal efficiency in flat tubes and regular pipes. The results revealed that a flat tube with an aspect ratio of 0.15 (Width/Height) exhibited enhanced heat extraction when compared to a circular pipe for the same cross section. The insertion of a porous material in the flat tube further improved the heat extraction, as seen by examining the thermal efficiency, known to be the ratio of the local Nusselt number to the fanning friction coefficient. Furthermore, the presence of a nanofluid in the flat tube flow led to an improvement in heat extraction depending on the concentration of the nanoparticle. A 5% enhancement was noticeable for a Reynolds number of 1000 with 1%vol Al2O3/water nanofluid for a flat tube. The enhancement increased to 12% in the presence of 2%vol Al2O3/water nanofluid. This enhancement was independent of the Reynolds number.

Computational Study on the Effect of the Staggered Ribs on Heat Transfer Phenomena Between the Horizontal Plates

In terms of multifarious technical applications, various kinds of passive methods are preferred to the active techniques when it comes to increase the amount of convection heat transfer via less energy consumption. As a vortex generator within the scope of the passive method, rib is usually employed to induce the heat transfer enhancement. In this study, the rectangular cross-sectional ribs have been placed to increase the amount of the heat transfer for the staggered arrangement between the horizontal parallel plates. Numerical simulations have been conducted by using k-ω SST turbulence model at Re = 10000. The rib effect has been comparatively investigated in case of thermal and hydraulic performance presented via the numerical results. Time-averaged results for temperature, pressure, streamwise velocity component and streamline patterns have been presented in terms of contour graphics. Furthermore, heat transfer enhancement by using the ribs has been given depending on the increment ratio of Nusselt numbers. Including friction losses due to the ribs mounted on the plates, the values of thermal performance factor for all ducts have been calculated. According to these results for heat transfer augmentation at Re = 10000, h' = 0.1 with S' = 0.5 having η = 1.049 and h' = 0.1 with S' = 0.75 having η = 1.019 have been recommended rather than the smooth duct.

Experimental and numerical investigation of turbulent flow heat transfer in a serpentine channel with multiple short ribbed passes and turning vanes

Detailed experimental and numerical investigations are conducted on the heat transfer, pressure loss and flow structure characteristics of a serpentine channel with multiple short ribbed passes and turning vanes, and the channel Reynolds numbers range from 10,000 to 60,000. The results are compared with the counterparts of a smooth three-pass serpentine channel and another one without turning vanes in the bend regions. The globally averaged heat transfer and the pressure loss characteristics of the three-pass serpentine channels were obtained by steady-state heat transfer experiments. The local heat transfer distributions of each ribbed pass were obtained by using transient liquid crystal thermography technique. Furthermore, the SST k-ω turbulence model was adopted for the steady-state three-dimensional numerical simulations to examine the detailed flow characteristics in the three-pass channels. The experimental results show that within the studied Reynolds number range the ribbed channel with the turning vanes shows averaged Nusselt number ratios of 1.56–1.72 in the first pass, 2.05–2.18 in the second pass and 2.15–2.23 in the third pass respectively, which are correspondingly about 10% lower than those of the counterpart without the turning vanes. The total pressure loss of the serpentine channel with the turning vanes is reduced by 13.4%–48.4% compared to the case without the turning vanes. The experiments by the transient liquid crystal thermography indicate that the turning vanes in the bend regions appreciably improve the heat transfer uniformity in the second and third passes. The numerical simulations show similar heat transfer characteristics as the experiments for the ribbed serpentine channels, and further show that the turning vanes improve the flow structure in the bend regions, and reduce the flow separation in the second and third passes, which contributes a more uniform heat transfer in the three-pass serpentine channel.

Effects of Attack Angle and Relative Thickness of Novel Wing-Shaped Turbulators on Turbulent Hydrothermal Performance in a Two-Pass Square Channel

Abstract This work aims to combine the effects of the near-wall and core flow disturbance by proposing novel wing-shaped turbulators. The new turbulators are fabricated with the fused deposition modeling (3D printing) technology. To explore their effects on detailed flow fields, local temperature distributions, and pressure drops in a two-pass square channel, particle image velocimetry (PIV), infrared thermography (IR camera), and pressure transducer measurements are performed. The turbulator pitch, clearance, and truncation gap ratio based on the channel hydraulic diameter of 45.5 mm are, respectively, fixed at 0.7, 0.25, and 0.06. Varied parameters include turbulator attack angle (α = 10 deg, 15 deg, 20 deg, and 30 deg), maximum thickness to chord line ratio (t/C = 0.08, 0.13, 0.16, 0.20, and 0.23), and bulk Reynolds number (Re = 5000–20,000). From the experimental results and flow parameters analyzed, the dimensionless spanwise-averaged mean transverse velocity and cross-sectionally averaged vorticity magnitude are identified to be the most relevant ones to spanwise-averaged local Nusselt number ratio in the first and second pass. Among all examined cases and previous data with Fanning friction factor ratio (f¯/fo) less than 50, the case with α = 20 deg and t/C = 0.20 attains the highest thermal performance factor (TPF) and overall Nusselt number ratio Nu¯/Nuo up to 1.68 and 5.36, respectively. Furthermore, empirical correlations of Nu¯/Nuoandf¯/fo versus α, t/C, and Re are proposed.

A comprehensive review of parabolic trough solar collectors equipped with turbulators and numerical evaluation of hydrothermal performance of a novel model

Absorber tube in parabolic trough concentrating systems is located at the location of the focal line and a fluid passes through it, the temperature of which increases due to receiving heat from sunlight. The more heat the fluid can absorb from the concentrated solar energy, the better the performance of the system. Since turbulators can be used in the absorber tube for this purpose, a lot of experimental and numerical research have been done on the performance of parabolic trough collectors equipped with various turbulators. In this study, the most important findings of these studies are presented and then, Nusselt number ratio, mean friction factor ratio, and thermal–hydraulic performance evaluation criterion are used to compare the hydrothermal performance of the systems. After reviewing previous researches and identifying advantages and disadvantages of each turbulator, a novel parabolic trough collector equipped with a turbulator is introduced and its hydrothermal performance is evaluated through simulation. The outcomes revealed that the ribbed absorber tube and the concentric rod insert are the most optimum turbulators. In addition, the parabolic trough collector equipped with a combination of corrugated channel and obstacle has a much better hydrothermal performance than the collectors using each of these turbulators separately.