Relative Roughness Height Research Articles

Experimental determination of the effect of change in relative roughness pitch on the thermo-hydraulic performance of air heater working with solar energy

Solar Air Heater has recently developed the effective utilization of renewable, natural and non-conventional energy for environmental sustainability. It is the most appreciating natural resource available for sustainable development. The current work deals with the analysis of thermo-hydraulic properties of solar air heater with specific parameter variation i.e. Relative Roughness Pitch (RRP). As already diagnosed the effect of relative roughness height, width, etc. using MATLAB. Here the authors have specifically focused on the effect of RRP on the solar air heater’s performance. The system utilized some fixed parameters such as, 0.88 emittance of the glass, 0.9 emittance of plate and transmittance- absorptance as 0.8. Also the 0.004 m single glass cover thickness is considered with thermal conductivity of insulation as 0.037 W/m-K. For the analysis purpose, the range of RRP is kept between 6 and 12. Related to operating parameters, Ambient temperature is kept 300 K (fixed) and Wind velocity 1 m/s (assumed fixed) whereas Insulation is kept in the variable range of 500–1000 W/m2. Finally, the results are recorded and the interpretations are made based on the validated data and the outcome results.

Thermo-hydraulic performance analysis of a solar air heater (SAH) with quarter-circular ribs on the absorber plate: A comparative study

The thermo-hydraulic characteristics of a solar air heater with quarter-circular ribs on the absorber plate are studied numerically. A comparison of the three different arrangements of the quarter-circular ribs is performed. The ribs are aligned periodically in a transverse manner to the flow. The numerical simulation of the mass, momentum, and energy conservation equations are carried out for a two-dimensional computational domain using the finite volume multi-grid solver of ANSYS-Fluent-18. A constant and uniform heat flux of 1000 W/m2 is supplied to the aluminum collector plate. Many pertinent input parameters, viz. flow parameters like Reynolds number and roughness parameters like relative roughness pitch and relative roughness height are varied. Reynolds number is varied from 3800 to 18000, whereas the relative roughness pitch is varied from 7.14 to 17.86, and the value of relative roughness height is kept fixed, i.e., 0.042. The impact of relative roughness pitch and Reynolds number on the thermo-hydraulic performance of the ribbed solar air heater is analyzed here. The optimized rib geometry in the investigated range of parameters is obtained. Pumping power calculations are done. The contours of temperature, pressure, and velocity are illustrated pictorially to understand the flow physics with clarity. It is found that the solar air heater with quarter-circular ribs having a relative roughness pitch of 7.14 delivers a maximal thermal enhancement ratio (representing the overall energy performance) of 1.88 in the studied parametric range. The Nusselt number and friction factor correlations are proposed by performing a non-linear regression analysis.

Experimental investigation for thermal performance of inclined spherical ball roughened solar air duct

This work deals with determination of thermal efficiency of spherical ball roughened collector aligned inclined to flow direction. Roughened collector produces more heat transfer compared to smooth ones but heat transfer augmentation comes at the cost of increased frictional losses. Lesser thermal efficiency of solar air heater has necessitated researchers to focus on its improvement by providing artificial flow modification techniques in the flow field. Laminar sublayer beneath the absorber plate will get disturbed and this helps to increase turbulence level of air. Present work discusses the influence of inclined spherical ball shaped roughness element and improving its thermal performance. Design considerations, geometries used viz. relative roughness pitch 9–18, relative roughness height 0.024–0.040, height-diameter ratio 0.5–2, angle of attack 35°-75°, flow conditions and their influence on turbulence viz. Reynolds no 2500–18500, heat transfer rate, absorber temperature and thermal performance enhancement factor have been thoroughly discussed. Finally concluding remarks and future scope for additional enhancement in performance of solar air heater is proposed. Maximum thermal efficiency for roughened ducts was found as 81% compared to 38% for smooth duct at parameters viz. ‘relative roughness pitch = 12, ‘relative roughness height’ = 0.036, ‘height-diameter ratio’ = 1 and ‘angle of attack’ = 55֠ at mass flow rate = 0.0319 kg/s.

Experimental investigation of exergetic efficiency of 3 side concave dimple roughened absorbers

The present work deals with evaluation of exergetic efficiency (ηII) of 3 side concave dimple roughened solar air heater duct system and the results were compared with 1 side roughened system experimentally and theoretically. Exergy analysis uses quality of useful energy i.e., what fraction of solar radiation is actually being converted into useful heat gain of air leaving beside possible losses. Experiments were performed under actual outdoor condition in open sun and relevant input data were recorded for exergy evaluation. Each day’s data was recorded under different sets of parameters viz., Reynolds number (Re) 2500–13500, relative roughness pitch (p/e) 8–15, relative roughness height (e/Dh) 0.018–0.045 and dimple’s depth to diameter ratio (e/d) 1–2. Variation in exergetic efficiency with Reynolds number and rise in temperature parameter (To-Ti)/I at varying p/e, e/Dh and e/d reveals that as Reynolds number changes, both 1 and 3 side roughness shows maximum exergetic efficiency at same Re but 1 side roughened duct shows maximum exergetic efficiency at (To-Ti/I) of 0.0172, 0.0125 and 0.017 for different p/e, e/Dh and e/d respectively. For 3 side roughened duct, maximum exergetic efficiency was seen at (To-Ti/I) of 0.023, 0.0212 and 0.022 for different p/e, e/Dh and e/d respectively.

Thermal enhancement study of a transverse inverted-T shaped ribbed solar air heater

In the present study, the heat transfers and flow fields analysis in solar air heater duct having of transverse inverted-T shaped ribs is performed. The significant outcomes are presented in terms of Nusselt number, friction factor, thermal enhancement factor, and visualization of different fluid properties for the solar air heater (SAH). The computational exercise is done by considering various non-dimensional parameters like Reynolds number (Re), relative roughness pitch (P/e), and relative roughness height (e/Dh) for the test to enhance the rate of heat transfer through the rectangular SAH duct. The relative roughness pitch and Reynolds number are varied from 7.14 to 17.86 and 3800 to 18,000 respectively, considering a fixed value of relative roughness height of 0.042. A two-dimensional computational domain is selected for the study where an absorber plate made up of aluminum is subjected to a constant and uniform heat flux of 1000 W/m2. Among the various turbulence models, the RNG k-ɛ model with enhanced wall treatment is accepted as the most appropriate model to simulate the governing eqs. A Thermal enhancement factor of 1.87 is obtained as a result of enhancement in heat transfer for the SAH with an inverted T-shaped rib roughened absorber plate. The correlations are obtained for the Nusselt number and friction factor as a function of Reynolds number and relative roughness pitch by carrying out a non-linear regression analysis.

Hydraulic Resistance of Pipes with Uniform Continuous Roughness in the Form of a Metric Thread of Varying Profile and an Inserted Twisted Tape

Results of experimental investigations into the hydraulic resistance of pipes with a metric thread of triangular, rectangular, and rounded profiles with an asperity height of 0.25 to 1.25 mm, which is cut inside, in air flow and with an inserted twisted tape have been given. The degree of swirling varied in the range from 2.5 to 7. A large data array has been obtained on the hydraulic resistance of pipes with a metric thread of varying profile and an inserted twisted tape. The influence of the Reynolds number Re, the degree of swirling, and the relative roughness height on the hydraulic resistance has been analyzed in the laminar regime of swirling flow with macrovortices and in theturbulent regime with macrovortices. Generalized dependences have been obtained for calculating the coefficient of hydraulic resistance of pipes with a triangular profile as a function of the degree of swirling and the relative roughness height at Re = 3000–80,000.

Fouling effect on the resistance of different ship types

Considering that the ship hydrodynamic behaviours differ by the ship types and dimensions, the effect of biofouling on ship resistance can also vary with different ships. In this study, Unsteady Reynolds Averaged Navier-Stokes (URANS) based towed ship models were developed to simulate the roughness effect of biofouling on ship resistance. A container ship (KCS) and a tanker (KVLCC2), representing slender and a full hull forms, were modelled with various scale factors and speeds. The CFD simulations were conducted with several fouling conditions by embedding the roughness function of barnacles into the wall-function of the CFD model (i.e. modified wall-function approach). The fouling effects on the resistance components, form factors, wake fractions and the flow characteristics were investigated from the simulations. Significant differences were observed varying with the hull types, lengths (scales) and speeds of the ships and it was concluded that these differences are dominated by two parameters; relative roughness height and the roughness Reynolds number.

Experimental investigation on solar thermal air heater with a combination of roughness elements for heat transfer improvement

This work attempts to determine the influence of the integration of a combination of roughness elements on the heat dissipation and frictional losses occurring within the airflow channel pathways of a counterflow double-pass solar thermal air heater. The investigation considers three configurations, namely (1) circular rings with a relative width ratio of 4 (Case A), (2) circular rings with a relative width ratio of 6 (Case B), and (3) combination of circular rings and ribs (Case C). All three configurations were studied at different operating conditions of the Reynolds number (in the range of 4000 to 10,000) and relative roughness height (0.026 and 0.043). During the experiments, the operating parameters considered as constants for comparison were heat input (1000 W m−2), aspect ratio (20), and hydraulic diameter (0.048). The resultant observations of the three configurations were also compared with that of the smooth-plate absorber configuration. The observations revealed that with an increase in relative roughness height, there was only a marginal improvement in Nusselt number. The experimental findings reported that Case C resulted in maximum improvement in Nusselt number with a penalty of increase in friction factor among the different configurations. For Case C, the calculated maximum value of the Nusselt number ratio and friction factor ratio obtained corresponding to relative roughness height of 0.043 was 3.19 and 1.46, respectively. The maximum enhancement obtained in the thermohydraulic performance parameter for Case C configuration corresponding to the relative roughness height of 0.026 and 0.043 was 2.91 and 3.01, respectively.

Impact of multi-staggered rib parameters of the ‘W’ shaped roughness on the performance of a solar air heater channel

ABSTRACT 8The collection efficiency of a solar air heater can be enhanced by employing artificial roughness as one of the modus operandi to create turbulence which helps in breaking the laminar sub-layer. The present investigation takes into account the impact of multi-staggered rib parameters of the W-rib roughness on the performance of the solar air heater channel. The chosen geometry in the current investigation is novel in a way that no such combination of roughness in the form of ribs has ever been tried earlier. The dimensionless factors for investigation were varied to study the effect of staggered ribs in the form of relative staggered pitch (p/P) 0.45–0.75 and relative staggered rib length (w/e) 1.5–6.0 for Reynolds number in the workable range of solar air heater channel as 4000–14000. Other parameters/factors were kept constant by selecting the optimal values from available works as relative roughness pitch (P/e) 12, relative roughness height (e/Dh) 0.0423, angle of attack (α) 60°, relative gap width (g/e) 1, number of gaps (n) 2. The augmentation in Nusselt number ratio has emerged in the range of 2.3–4.1, optimum values of p/P and w/e correspond to 0.65 and 4.5, respectively. A comparison with geometries under the analogous flow and thermal conditions has also been carried out.

Efficient designs of double-pass curved solar air heaters

Recent research has shown that flat plate double-pass solar air heaters (DPSAH) exhibit higher thermal performance compared to conventional flat plate single-pass solar air heaters (SPSAH). However, scientific literature on design and performance evaluation of a curved DPSAH is scarce. In this paper, we systematically investigated various designs of DPSAH and reported its performance characteristics using a validated numerical model. Higher outlet air temperature by about 5 °C was observed when the DPSAH absorber plate is located at the mid of the insulating wall and transparent glass cover. Furthermore, putting asymmetric semi-circular roughened surfaces shows better performance than symmetric circular shapes as the reattachment of vortices with the absorber plate is more frequent in the former case. Two new correlations were developed for Nusselt number (Nu) and friction factor (f) as a function of Reynolds number (Re) and relative roughness height (dH). The values of Nu and f obtained from the developed correlations agree well with data from the model.

Numerical and experimental investigation of different rib roughness in a solar air heater

A numerical study is aimed at comparing dissimilar types of repeated-rib roughness: arc-shape with gap and V-shape with gap on the absorber plate of the solar air heater having similar shape and size working under identical boundary conditions. Relative gap width (g/e) of 1, relative roughness height of 0.04, the relative roughness pitch of 10, p/P = 0.65, d/W = 0.69, No. of gaps = 1, attack angle α = 600 of the rib, heat flux 1000 W/m2, etc. are kept constant and variation in range of Reynolds number is 3000–15000. Numerical simulations are conducted with the help of ANSYS CFD tool, and results have been validated with the experimental studies. The V-shape ribs have maximum frictional loss penalty of 4.05 and Nusselt number enhancement of 3.4 at Re 3000. The arc-shape ribs have maximum frictional loss penalty of 3.94 and Nusselt number enhancement of 2.4 for Re 15000. The maximum THPP for the V-shape ribs and arc shape ribs are 2.21 and 1.5 respectively at Re 3000.

Acoustic excitation of Tollmien–Schlichting waves due to localised surface roughness

Experiments on the receptivity of two-dimensional boundary layers to acoustic disturbances from two-dimensional roughness strips were performed in a low-turbulence wind tunnel on a at plate model. The freestream was subjected to a plane acoustic wave so that a Stokes Layer (SL) was created on the plate, thus generating a Tollmien-Schlichting (T-S) wave through the receptivity process. An improved technique to measure the T-S component is described based on a retracting two-dimensional roughness, which allowed for phase-locked measurements at the acoustic wave frequency to be made. This improved technique enables both protuberances and cavities to be explored in the range 30�m < jhj < 750�m (equivalent to 0:025 < jhj=��B < 0:630 in relative roughness height to the local unperturbed Blasius boundary layer displacement thickness). These depths are designed to cover both the predicted linear and non-linear response of the T-S excitation. Experimentally, cavities had not previously been explored. Results show that a linear regime is identifiable for both positive and negative roughness heights up to � 150 �m (jhj=��B � 0:126). The departure from the linear behaviour is, however, dependent on the geometry of the surface imperfection. For cavities of signi�cant depth, the non-linear behaviour is found to be milder than in the case of protuberances - this is attributed to the flow physics in the near field of the surface features. Nonetheless, results for positive heights agree well with previous theoretical work which predicted a linear disturbance response for small-height perturbations.

Analytical predictions of exergoeconomic performance of a solar air heater with surface roughness of metal waste

The current study is the authors’ next work from the perspectives of the second law and economics of an air collector having artificial roughness of metal waste. In a previous study, heat and fluid flow characteristics and thermo-hydraulic performance were experimentally investigated. In the present paper, thermal efficiency, effective efficiency, exergetic efficiency and economic-based optimization are analytically appraised. The influences of the relative roughness pitch, relative roughness height, collector area and Reynolds number on the above parameters are examined. The optimal roughness parameters to achieve the best exergetic performance are formulated as design plots for the design and operation of a solar air heater. The weighted sum method is used to optimize the objectives of exergetic performance and total annual cost. The results reveal that too high Reynolds numbers result in low effective performance and unexpected exergetic performance. In that case, a lower roughness should be used, i.e. a smaller relative roughness height and larger relative roughness pitch. This is because the pressure loss penalty and irreversibility are quite large at high Reynolds numbers. An absorber plate area of 1.3 m2 was found to achieve the maximum exergetic performance and minimum total annual cost.

Parametric performance optimization of three sides roughened solar air heater

ABSTRACT Solar clean energy can be harnessed by several methods using technologies like solar heating, photovoltaic cell, solar architecture, photosynthesis. Solar energy is converted either by active technologies or by passive technologies by the method they capture and distribute solar energy. In view of this, the paper represents the optimization result on Nusselt number and thermal efficiency for 3 sides roughened solar air heater using the hybrid approach of response surface method (RSM) and multi objective particle swarm optimization (MOPSO) algorithm. The values of input parameters are taken according to the experimental values. The value of the output parameters, Nusselt Number and thermal efficiency are found to be in the range of 65–80 and 63–75%, respectively, for the optimum value of the input parameters are Reynolds number, 12000–13000, the relative roughness pitch, 10, the relative roughness height 0.03–0.04 and the rate of mass flow is 0.04 kg/s using MOPSO approach, and the optimum solution is identified by application of maximum deviation theory (MDT) approach. A confirmative test has been conducted to validate the optimum results obtained with an error of 3.39 percentages.

Experimental study of thermal performance and pressure drop on a solar air heater with different orientations of arc-shape rib roughness

An exhaustive experimental investigation was carried out for the thermal performance of artificially roughened solar air heaters (SAHs) under the actual outdoor conditions. Circular shape wire ribs have been arranged to form an arc-shape geometry on top of the absorber plates, and it has been tested for two configurations of SAHs named as arc-shape apex-downstream flow and arc-shape apex-upstream flow SAH. Roughness parameters have been taken as relative roughness pitch (P/e) in the range of 8–15 (three values), angle of attack (α) 45°–75° (three values) for fixed relative roughness height (e/D) of 0.0454, and duct width-to-duct height ratio (W/H) = 11. The range of mass flow rate (m) and Reynolds number (Re) covered during experimentation was 0.0100–0.0471 kg s−1 and 2983–13,955, respectively. Based on the experimental results, it was found that arc-shape wire with apex-upstream flow SAH is having a higher value of thermal efficiency ( $$\eta_{\text{th}}$$ ), heat removal factor (FR), collector efficiency factor (F′) and duct pressure drop (ΔPD) as compared to arc-shape wire with apex-downstream flow SAH and smooth absorber plate SAH. In the range of the operating parameters investigated, the maximum $$\eta_{\text{th}}$$ , FR, F′ and ΔPD have been found for e/D = 0.0454, α = 60° and P/e = 8 for m = 0.0471 kg s−1. At these conditions, apex-upstream and apex-downstream configurations enhance the thermal performance by 71.6% and 55.8% when compared with the conventional plane solar air heater.

Experimental investigation of solar air heater duct with discrete V-rib integrated with staggered elements

ABSTRACT This paper presents an experimental investigation of heat transfer and friction factor in a rectangular duct with discrete V-rib integrated with staggered elements. The Reynolds number (Re) of 3000–12,000, ratio of staggered element size to rib height (r/e) of 2–5.5, relative roughness pitch (p/e) of 12, relative roughness height (e/Dh) of 0.045, flow angle of attack (α) of 60°, relative gap width (g/e) of 1, relative staggered element position (p’/p) of 0.65 and number of gaps (Ng) of 6. The maximum augmentation in Stanton number is 1.90 times over the smooth duct and maximum thermal efficiency is obtained 86% corresponding to ratio of staggered element size to rib height (r/e) of 3.5.

Thermal performance and heat transfer analysis of arc shaped roughened solar air heater – An experimental study

In the present study, thermal performance and heat transfer of arc shaped roughened solar air heater (SAH) with apex up and apex down air flow has been examined through experiments. To achieve this aim, experiments were conducted with mass flow rate from 0.007 to 0.022 kg/s, and variations of mass flow rate on the thermal efficiency and outlet temperature were examined. The arc shape roughness with relative roughness pitch, relative roughness height, rib roughness and arc angle were 10, 0.0395, 2.5 and 60° respectively have been taken. It has been observed that the thermal efficiency increases with increase in mass flow rate of air, for the same mass flow rate the average enhancement of apex upstream flow is 6.9% higher than the apex downstream flow. The maximum efficiency obtained for apex up and apex down air flow arc shaped roughened SAH are 73.2% and 69.4% respectively, at mass flow rate 0.022 kg/s. Also, it has been observed that the heat transfer of apex up arrangement performs 33.2% better as compared to apex down. The above result reveals that apex up air flow wire rib roughness absorber plate is more efficient than the apex down roughness solar air heater.

Three-Dimensional Thermo-Hydraulic Analysis of Solar Air Heater With Equilateral Prism-Shaped Rib Roughness

Abstract Thermal transport and flow friction characteristics due to roughness on the absorber plate of solar air heater are evaluated by applying three-dimensional finite volume based code. Renormalization group (RNG) k–ɛ model is employed to capture the turbulent nature of the flow. The effect of equilateral prism-shaped rib roughness geometrical parameters in terms of relative roughness height (e/D) and relative roughness pitch (p/e) on heat transfer and flow friction is analyzed. Further, the effect of flow parameter, Re in the range of 4000–18,000 is also explored. Results are elucidated in terms of average Nusselt number, friction factor, turbulent kinetic energy, and eddy dissipation. Results are compared with a smooth absorber plate solar air heater. Thermo-hydraulic performance of the roughened solar air heater is analyzed. Noteworthy augmentation in heat transport is obtained. The thermal enhancement factor is calculated for optimal performance and found to vary from 1.7 to 3.5. However, friction factor and pressure loss for roughened plate is significantly higher than its smooth counterpart. The pressure drop across the test section increases with the rise in roughness height due to flow obstruction. A minimum value of the friction factor enhancement ratio worth 2.13 is obtained. Enhancement in thermal transport and pressure losses are combined by introducing a thermo-hydraulic performance factor (THHP). For the range of parameters investigated, the optimum value of the thermo-hydraulic performance factor is found to be 3.41. Correlations for average Nusselt number and friction factor are offered at the end.

Innovatively Arranged Curved-Ribbed Solar-Assisted Air Heater: Performance and Correlation Development for Heat and Flow Characteristics

Abstract Solar radiation is a rich and clean source of energy. It can be collected and converted to thermal energy with the help of flat plate collectors called the solar-assisted air heater. Because of the low coefficient of heat transfer of air, the solar-assisted air heater has low thermal performance which can be improved by creating local turbulence using surface roughness on the heat transferring plate. The present investigation has been conducted to perceive the influence of the curved-ribbed element with gap on flow and heat transfer. The roughness element is defined by using five non-dimensionlized parameters, i.e., relative roughness width (W/w), relative roughness pitch (P/e), relative gap width (g/e), relative roughness height (e/D), and relative gap distance (d/x). The radius of the curvature of the curved rib-element is kept constant and the experimental measurements were done under quasi-steady state. The thermohydraulic performance parameter improved by 3.61 times the smooth flat plate solar air heater (SAH), in curved-ribbed SAH for W/w = 3, P/e = 8, g/e = 1, e/D = 0.045, and d/x = 0.65 at Reynolds number of 23,000. The generalized relation for heat transfer and flow characteristics is also being developed and the predicted Nusselt number and friction factor with the accuracy of ± 7.5% and ± 6.7%, respectively.

Thermo-hydraulic performance analysis of a solar air heater roughened with reverse NACA profile ribs

The solar air heater is subjected to a low convective heat transfer coefficient on account of the formation of the laminar sub-layer between the absorber plate and flowing air. To increase the heat transfer coefficient, the laminar sub-layer can be interrupted with the use of artificial roughness (turbulator) on the absorber plate. In the present analysis, the effects of novel type of roughness element in the form of NACA 0040 profile ribs in the reverse position are studied on the performance of the solar air heater. The effects of relative roughness pitch (P/e) and relative roughness height (e/D) were studied for the Reynolds number (Re) ranging from 6000 to 18,000. Among the various roughness parameters, NACA 0040 profile ribs provided in the reverse direction with P/e as 5 and e/D as 0.065 was found to be the optimum. The thermo-hydraulic performance parameter for this case was found to be 2.53 at a Reynolds number of 6000. The experimental results validated the numerical results, and the maximum deviation was found to be 4.84%. An empirical correlation was developed for prediction of Nu and f in terms of Re and e/D and found to conform within ±3% of the corresponding experimental values.