Experimental study of augmented heat transfer and friction in solar air heater with different orientations of W-Rib roughness

Artificial roughness in the form of ribs is a convenient method for enhancing thermal performance of solar air heaters. This paper presents the experimental investigation of heat transfer and friction factor characteristics of a rectangular duct roughened with W-shaped ribs arranged at an inclination with respect to the flow direction on its underside on one broad wall. W ribs have been tested both pointing in downstream W-down and upstream W-up to the flow. The range of parameters for this study has been decided on the basis of practical considerations of the system and operating conditions. The duct has a width to height ratio (W/H) of 8.0, relative roughness pitch (p/e) of 10, relative roughness height (e/D_h) of 0.03375 and angle of attack of flow (α) of 30-75°. The air flow rate corresponds to Reynolds number between 2300-14,000. The heat transfer and friction factor results have been compared with those for smooth duct under similar flow and thermal boundary condition and thermo-hydraulic performance has been investigated. Thermo-hydraulic performance comparison for different angle of attack of flow shows that W-down arrangement with angle of attack of flow as 60° gives best thermo-hydraulic performance. In addition heat transfer and friction factor correlations have been developed.     

Thermal and friction characteristics of turbulent flow through rectangular and square ducts with transverse ribs and wire-coil inserts

The heat transfer and the pressure drop characteristics of turbulent flow of air through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and with wire-coil inserts have been studied experimentally. Circular duct has also been used. The transverse ribs in combination with wire-coil inserts have been found to perform better than either ribs or wire-coil inserts acting alone. The flow friction and thermal characteristics are governed by duct aspect ratio, coil helix angle and wire diameter of the coil, rib height and rib spacing, Reynolds number and Prandtl number. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. It has been found that on the basis of constant pumping power, up to 35% heat duty increase occurs for the combined ribs and wire-coil inserts case compared to the individual ribs and wire-coil inserts cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to 20% for the combined enhancement geometry than the individual enhancement geometries.     

Thermal and friction characteristics of laminar flow through rectangular and square ducts with transverse ribs and wire coil inserts

The heat transfer and the pressure drop characteristics of laminar flow of viscous oil (195 < Pr < 525) through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and with wire coil inserts have been studied experimentally. Circular duct has also been used. The transverse ribs in combination with wire coil inserts have been found to perform better than either ribs or wire coil inserts acting alone. The heat transfer and the pressure drop measurements have been taken in separate test sections. Heat transfer tests were carried out in electrically heated stainless steel ducts incorporating uniform wall heat flux boundary conditions. Pressure drop tests were carried out in acrylic ducts. The flow friction and thermal characteristics are governed by duct aspect ratio, coil helix angle and wire diameter of the coil, rib height and rib spacing, Reynolds number and Prandtl number. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. The performance of the geometry under investigation has been evaluated. It has been found that on the basis of constant pumping power, up to fifty per cent heat duty increase occurs for the combined ribs and wire coil inserts case compared to the individual ribs and wire coil inserts cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to forty per cent for the combined enhancement geometry than the individual enhancement geometries.     

Heat transfer performance comparison of steam and air in gas turbine cooling channels with different rib angles

Using steam as working fluid to replace compressed air is a promising cooling technology for internal cooling passages of blades and vanes. The local heat transfer characteristics and the thermal performance of steam flow in wide aspect ratio channels (W/H = 2) with different angled ribs on two opposite walls have been experimentally investigated in this paper. The averaged Nusselt number ratios and the friction factor ratios of steam and air in four ribbed channels were also measured under the same test conditions for comparison. The Reynolds number range is 6,000–70,000. The rib angles are 90°, 60°, 45°, and 30°, respectively. The rib height to hydraulic diameter ratio is 0.047. The pitch-to-rib height ratio is 10. The results show that the Nusselt number ratios of steam are 1.19–1.32 times greater than those of air over the range of Reynolds numbers studied. For wide aspect ratio channels using steam as the coolant, the 60° angled ribs has the best heat transfer performance and is recommended for cooling design.     

Experimental investigation of local heat transfer in a square duct with various-shaped ribs

In the present study, experimental studies are carried out to investigate the heat transfer and friction characteristics in a square duct roughened by various-shaped ribs on one wall. The ribs are oriented transversely to the main stream in a periodic arrangement. Liquid crystal thermography is employed to measure the local and average heat transfer coefficient on the ribbed surface. The rib height-to-duct hydraulic diameter ratio is fixed at 0.1; the rib pitch-to-height ratio varies from 8 to 15 and the test Reynolds number spans from 8,000 to 20,000. The results show that the trapezoidal-shaped ribs with decreasing height in the flow direction (case C) provide the highest heat transfer enhancement factor and are likely to be used to suppress the local hot spot which usually occurs in the region just behind the ribs.     

Heat transfer and friction factor of turbulent flow through a horizontal semi-circular duct

Forced convection heat transfer and pressure drop characteristics of air flow inside a horizontal semi-circular duct are investigated experimentally. The experiments are carried out on a semi-circular duct of 23 mm inner radius, 2 mm thickness, and 2,000 mm length within a range of Reynolds number (8,242 ≤ Re ≤ 57,794)., under uniform wall heat flux conditions. The friction factor is determined by measuring the axial static pressure at different selected axial stations along the semi-circular duct. The variations of surface and mean air temperatures, local heat transfer coefficient, local Nusselt number, and the friction factor with the axial dimensionless distance are presented. It is observed that, for a given value of Reynolds number, each of the local heat transfer coefficient and the friction factor has a relatively high value near the entrance of the semi-circular duct then it decreases with increasing the dimensionless axial distance until it approaches a nearly constant value at the fully developed region. Also, it is found that, with increasing the Reynolds number the average heat transfer coefficient is increased and the friction factor is decreased. Moreover, empirical correlations for the heat transfer coefficient and friction factor as a function of the Reynolds number are obtained.     

Detailed measurement of heat/mass transfer and pressure drop in a rotating two-pass duct with transverse ribs

The present study investigates the convective heat/mass transfer and pressure drop characteristics in a rotating two-pass duct with and without transverse ribs. The Reynolds number based on the hydraulic diameter is kept constant at 10,000 and the rotation number is varied from 0.0 to 0.2. When rib turbulators are installed, heat/mass transfer and friction loss are respectively augmented 2.5 times and 5.8 times higher than those of the smooth duct since the main flow is turbulated by reattaching and separating on the vicinity of the duct surfaces. Differences of heat/mass transfer and pressure coefficient between leading and trailing surfaces result from the rotation of duct, so that Sherwood number ratios and pressure coefficients are high on the trailing surface in the first-pass and on the leading surface in the second-pass. In the turning region, a pair of Dean vortices shown in the stationary case transform into one large asymmetric vortex cell, and subsequently heat/mass transfer and pressure drop characteristics also change. As the rotation number increases, the discrepancies of the heat/mass transfer and the pressure coefficient enlarge between the leading and trailing surfaces.     

Experimental investigation of the effect of variously-shaped ribs on local heat transfer on the outer wall of the turning portion of a U-channel inside solar air heater

In the present work, an experimental investigation of convective heat transfer and pressure drop was carried out for the turning portion of a U-channel where the outer wall was equipped with ribs. The shape of the ribs was varied. The investigation aims to give guidelines for improving the thermo-hydraulic performance of a solar air heater at the turning portion of a U-channel. Both the U-channel and the ribs were made in acrylic material to allow optical access for measuring the surface temperature by using a high-resolution technique based on narrow band thermochromic liquid crystals (TLC R35C5 W) and a CCD camera placed to face the turning portion of the U-channel. The uncertainties were estimated to 5 and 7 % for the Nusselt number and friction factor, respectively. The pressure drop was approximately the same for all the considered shapes of the ribs while the dimpled rib case gave the highest heat transfer coefficient while the grooved rib presented the highest performance index.     

Impingement cooling in a rotating curved square annular duct with crossflow effect from Rib-roughened surfaces

An experimental investigation was carried out to determine the effects of jets in crossflow on impingement heat transfer from rib-roughened curved duct with rotational speeds of 120 and 240 rpm. The jet impinged on curved surface in crossflow having repeated square ribs (3 2 3mm). The curvature of the duct was fixed and has a value of 300 mm. The rib height (e) was fixed at 1.5 mm and pitch-to-height ratio (p/e) kept at 2. The study covered jet Reynolds number in the range 6500 to 26000, and duct stream crossflow Reynolds number from 3250 to 13000 based on jet nozzle diameter, respectively, which gave M = 0.12 to 2. Results were presented for rotational, crossflow, roughened surfaces and curvature effects on local Nusselt numbers. Significant heat transfer enhancement was found for the present physical geometry and within the ranges of operating parameters considered in the study.     

Convective heat transfer in a square channel with angled ribs on two opposite walls

Detailed quantitative maps of the heat transfer distribution in a square channel with angled rib turbulators are measured by means of infrared (IR) thermography associated with the heated-thin-foil technique. Air flows in the channel where square ribs are mounted on two opposite walls at an angle of either 30° or 45° with respect to the duct axis. Two rib pitches, two different rib arrangements and two heating conditions are investigated. Results are presented in terms of local and averaged Nusselt numbers, which are normalised with the classical Dittus and Boelter correlation, for three different Reynolds numbers.     

Experimental study of slot jet impingement heat transfer on a wedge-shaped surface

An experimental investigation was conducted to study the convective heat transfer rate from a wedge-shaped surface to a rectangular subsonic air jet impinging onto the apex of the wedge. The jet Reynolds number, nozzle-to-surface distance and the wedge angle were considered as the main parameters. Jet Reynolds number was ranged from 5,000 to 20,000 and two dimensionless nozzle-to-surface distances h/w?=?4 and 10 were examined. The apex angle of the wedge ranged from 30° to 180° where the latter case corresponds with that of a flat surface. Velocity profile and turbulence intensity were provided for free jet flow using hot wire anemometer. Local and average Nusselt numbers on the impinged surface are presented for all the configurations. Based on the results presented, the local Nusselt number at the stagnation region increases as the wedge angle is decreased but, it then decreases over the remaining area of the impinged surface. Average Nusselt number over the whole surface is maximum when the wedge angle is 180° (i.e. plane surface) for any jet and nozzle-to-surface configuration.     

Heat transfer distribution in rectangular ducts with V-shaped ribs

 Heat transfer distributions are presented for a rectangular duct with two opposite wide walls arranged with V-shaped ribs pointing upstream or downstream relative to the main flow direction. The rectangular duct has an aspect ratio of 1/8. The parallel V-shaped circular ribs are arranged staggered on the two wide walls. The rib height-to-hydraulic diameter ratio is 0.06, with an attack angle of 60°. The pitch-to-height ratio equals 10. The tested Reynolds numbers range from 1000 to 6000. The test surface is sprayed with black paint and then liquid crystal, and a steady state method is adopted to obtain the temperature distribution between adjacent ribs. The secondary flow caused by the angled ribs creates different spanwise variation of the heat transfer coefficient on the rib-roughened wall for different V-rib orientations. Interaction between heat transfer and secondary flow is analyzed. In the streamwise direction, the temperature distribution shows a sawtooth behavior between a pair of adjacent ribs. Local Nusselt numbers are presented between a pair of adjacent ribs, and based on these the average Nusselt numbers are calculated to investigate the augmentation of heat transfer by the presence of the V-shaped ribs. Received on 15 May 2000     

Groove geometry effects on turbulent heat transfer and fluid flow

The present work represents a two-dimensional numerical prediction of forced turbulent flow heat transfer through a grooved tube. Four geometric groove shapes (circular, rectangular, trapezoidal and triangular) were selected to perform the study, as well as two aspect ratios of groove-depth to tube diameter (e/D = 0.1 and 0.2). The study focuses on the influence of the geometrical shapes of grooves and groove-depth on heat transfer and fluid flow characteristics for Reynolds number ranging from 10,000 to 20,000. The characteristics of Nusselt number, friction factor and entropy generation are studied numerically by the aid of the computational fluid dynamics (CFD) commercial code of FLUENT. It is observed that the best performance occurs with the lower depth-groove ratio, whereas it is found that the grooved tube provides a considerable increase in heat transfer at about 64.4 % over the smooth tube and a maximum gain of 1.52 on thermal performance factor is obtained for the triangular groove with (e/D = 0.1).     

Experimental and numerical study of turbulent flow and heat transfer inside hexagonal duct

Flow and heat transfer characteristics in transition and turbulent regions are studied experimentally and numerically in a horizontal smooth regular hexagonal duct under constant wall temperature boundary condition covering a range of Reynolds number from 2.3 × 10³ to 52 × 10³. Two types of k-omega (standard and shear stress transport (SST)) and three types of k-ε (standard, renormalization (RNG), and realizable) turbulence model are employed for transition and turbulent regions, respectively. Both average and fully developed Darcy friction factor and Nusselt number are presented as a function of Reynolds number. It is seen that k-omega SST and k-ε realizable turbulence models gave the best agreement with the experimental data in transition and turbulent regions, respectively. All the experimental results are correlated within an accuracy of ±13 % and ±7 % for Nusselt number and Darcy friction factor, respectively. Results obtained in this study are compared with circular duct results using hydraulic diameter.     

Chaotic advection induced heat transfer enhancement in a chevron-type plate heat exchanger

The present work examines the role of chaotic mixing as a means of heat transfer enhancement in plate heat exchangers. In order to demonstrate the chaotic behavior, sensitivity to initial conditions and horseshoe maps are visualized. The Nusselt number and the friction factor were computed in the range of reynolds number, 1 < Re < 10. The Nusselt number increases considerably in chaotic models whereas the friction factor increases only marginally.     

Forced convection and friction in triangular duct with uniformly spaced square ribs on inner surfaces

Experimental investigation had been conducted to study the steady-state forced convection heat transfer and pressure drop characteristics of the hydrodynamic fully-developed turbulent flow in the air-cooled horizontal equilateral triangular ducts, which were fabricated with the same length and hydraulic diameter. Inner surfaces of the ducts were fixed with square ribs with different side lengths of 6.35, 9.525 and 12.7 mm, respectively, and the uniform separation between the centre lines of two adjacent ribs was kept constant at 57.15 mm. Both the triangular ducts and the ribs were fabricated with duralumin. The experiments were performed with the hydraulic diameter based Reynolds number ranged from 3100 to 11300. The entire inner wall of the duct was heated uniformly, while the outer surface was thermally insulated. It was found that the Darcy friction factor of the duct was increasing rather linearly with the rib size, and forced convection could be enhanced by an internally ribbed surface. However, the heat transfer enhancement was not proportional to the rib size but a maximum forced convection heat transfer augmentation was obtained at the smallest rib of 6.35 mm. Non-dimensional expressions for the determination of the steady-state heat transfer coefficient and Darcy friction factor of the equilateral triangular ducts, which were internally fabricated with uniformly spaced square ribs of different sizes, were also developed. Received on 25 May 1999     

Heat transfer and pressure drop measurements in rib-roughened rectangular ducts

In the present study, the thermal and hydraulic performance of three rib-roughened rectangular ducts is investigated. The aspect ratio of the ducts was 1 to 8, and the ribs were arranged staggered on the two wide walls. Three rib configurations were tested: parallel ribs and V-shaped ribs pointing upstream or downstream of the main flow direction. For all cases, the rib height-to-hydraulic diameter ratio was 0.06, with an attack angle of 60° and a pitch-to-height ratio of 10. The Reynolds number range was from 1000 to 6000. Liquid crystal thermography was employed in the heat transfer experiment to demonstrate detailed temperature distribution between a pair of ribs on the ribbed surfaces. The secondary flows caused by the inclined ribs create a significant spanwise variation of the heat transfer coefficients on the rib-roughened wall with high heat transfer coefficient at one end of the rib and low value at the other. In the streamwise direction between two consecutive ribs, the temperature distribution shows a sawtooth fashion because of flow reattachment. Based on the local heat transfer coefficients, the average Nusselt numbers were estimated as weighted mean values. Isothermal pressure drop data were taken and presented as Fanning friction factors. The ducts are compared to each other by considering both heat transfer and friction factor performance.     

Heat transfer investigation of an array of jets impinging on a target plate with detached ribs

This combined experimental and numerical study focuses on impingement jet cooling in combination with detached rib turbulators on a flat target for turbomachinery applications. The investigated impingement array consists of an impingement plate with 9 × 9 jet holes with diameter D and a target plate with detached ribs installed beneath the jet hole. The effects of different separation distances (H/D=3-5), jet Reynolds numbers (15,000-35,000) and rib clearances (0.3D and 0.08D) are investigated. The heat transfer is investigated experimentally by the transient liquid crystal (TLC) method. A computational fluid dynamics (CFD) model is carried out within the software package ANSYS CFX. This model uses a steady-state three-dimensional Reynolds-Averaged Navier-Stokes (RANS) approach with the Shear Stress Transport (SST) turbulence model. Numerical simulations allow detailed insight into the fluid mechanics of the complex flow field and complement experimental measurements. Detached ribs in the impingement channel have a strong influence on the flow field and can increase the global Nusselt number by up to 4% if the ribs have adiabatic boundary conditions. The usage of the detached rib reduces the relative discharge coefficient by up to 11% compared to a smooth target.     

The Effects of duct height on heat transfer enhancement of a co-rotating type rectangular finned surface in duct

Experiments were performed to investigate the effect of duct height on heat transfer enhancement of a surface affixed with arrays (7 × 7) of short rectangular plate fins of a co-rotating type pattern in the duct. An infrared imaging system is used to measure detailed distributions of the heat transfer at the endwall along with the fin base. An infrared camera of TVS 8000 with 160 × 120 point In–Sb sensor was used to measure the temperature distributions in order to calculate the local heat transfer coefficients of the representative fin regions. Pressure drop and heat transfer experiments were performed for a co-rotating fin pattern varying the duct height from 20?50 mm. The friction factor calculated from the pressure drop shows that comparatively larger friction occurs for the smaller duct cases and the friction factor slowly decreases with increasing Reynolds number. The effect of duct height on the area-averaged heat transfer results show that heat transfer initially increases with duct height and then finally decreases with increasing the duct height. Detailed heat transfer analysis and iso-heat transfer coefficient contour gives a clear picture of heat transfer characteristics of the overall surface. The relative performance graph indicates that a 25 mm duct is the optimum duct height for the highest thermal performance. In addition, a significant thermal enhancement, 2.8?3.8 times the smooth surface, can be achieved at lower Reynolds number with a co-rotating fin pattern in the duct.     

Effects of the surface roughness on heat transfer of perpendicularly impinging co-axial jet

Current work presents the comparison of the cooling characteristics of roughened and smooth heated surfaces subjected to co-axial impinging jet. The work fluid is air and the data runs are performed for jet Reynolds numbers for 10,000, 20,000 and 40,000, and non-dimensional surface to jet exit spacing, H/D, from 1 to 10. The co-axial jet configuration is based on a fully developed pipe flow encountering a double-pipe arrangement and splitting between the two pipes. The inner to outer diameter ratio is 0.5. A straight pipe without inner section is used as the circular jet. The impingement of circular jets to the roughened and smooth surfaces is also performed for comparison. Average Nusslet numbers were obtained to show the heat transfer enhancement from the surface. A good agreement between the literature and present paper was obtained. As a result, average Nusselt number with co-axial jet impingement to the roughened surface increased by up to 27% comparing to the circular jet impingement. In addition, the average Nusselt number increased with roughened surface by up to 6% over the whole surface area, comparing to the smooth surface.