Heat transfer and friction in solar air heater duct with V-shaped rib roughness on absorber plate

In this work, results of an experimental investigation of the effect of geometrical parameters of V-shaped ribs on heat transfer and fluid flow characteristics of rectangular duct of solar air heater with absorber plate having V-shaped ribs on its underside have been reported. The range of parameters for this study has been decided on the basis of practical considerations of the system and operating conditions. The investigation has covered a Reynolds number (Re) range of 2500-18000, relative roughness height (e/D_h) of 0.02-0.034 and angle of attack of flow (α) of 30-90° for a fixed relative pitch of 10. Results have also been compared with those of smooth duct under similar flow conditions to determine the enhancement in heat transfer coefficient and friction factor. The correlations have been developed for heat transfer coefficient and friction factor for the roughened duct.     

Heat transfer and friction factor correlations for a duct having dimple-shape artificial roughness for solar air heaters

The heat transfer coefficient between the absorber plate and air can be considerably increased by using artificial roughness on the underside of the absorber plate of a solar air heater duct. Under the present work, an experimental study has been carried out to investigate the effect of roughness and operating parameters on heat transfer and friction factor in a roughened duct provided with dimple-shape roughness geometry. The investigation has covered the range of Reynolds number (Re) from 2000 to 12,000, relative roughness height (e/D) from 0.018 to 0.037 and relative pitch (p/e) from 8 to 12. Based on the experimental data, values of Nusselt number (Nu) and friction factor (f_r) have been determined for different values of roughness and operating parameters. In order to determine the enhancement in heat transfer and increment in friction factor values of Nusselt number and friction factor have been compared with those of smooth duct under similar flow conditions. Correlations for Nusselt number and friction factor have been developed for solar air heater duct provided such artificial roughness geometry.     

Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with multiple v-ribs

The use of artificial roughness on the underside of the absorber plate is an effective and economic way to improve the thermal performance of a solar air heater. Several experimental investigations, involving different types of roughness elements, have been carried out to improve the heat transfer from the absorber plate to air flowing in solar air heaters. This paper presents an experimental investigation carried out to study the effect of multiple v-rib roughness on heat transfer coefficient and friction factor in an artificially roughened solar air heater duct. The experiment encompassed Reynolds number (Re) from 2000 to 20000, relative roughness height (e/D) values of 0.019-0.043, relative roughness pitch (P/e) range of 6-12, angle of attack (α) range of 30-75° and relative roughness width (W/w) range of 1-10. Extensive experimentation has been conducted to collect data on heat transfer and fluid flow characteristics of a rectangular duct roughened with multiple v-ribs. Using these experimental data, correlations for Nusselt number and friction factor in terms of roughness geometry and flow parameters have been developed.     

Experimental investigation on heat-transfer enhancement due to a gap in an inclined continuous rib arrangement in a rectangular duct of solar air heater

Artificial roughness in the form of repeated ribs has been proposed as a convenient method for enhancement of 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 repeated square cross-section split-rib with a gap, on one broad wall arranged at an inclination with respect to the flow direction. The duct has a width to height ratio (W/H) of 5.84, relative roughness pitch (P/e) of 10, relative roughness height (e/D_h) of 0.0377, and angle of attack (α) of 60°. The gap width (g/e) and gap position (d/W) were varied in the range of 0.5–2 and 0.1667–0.667, respectively. The heat transfer and friction characteristics of this roughened duct have been compared with those of the smooth duct under similar flow condition. The effect of gap position and gap width has been investigated for the range of flow Reynolds numbers from 3000 to 18,000. The maximum enhancement in Nusselt number and friction factor is observed to be 2.59 and 2.87 times of that of the smooth duct, respectively. The thermo-hydraulic performance parameter is found to be the maximum for the relative gap width of 1.0 and the relative gap position of 0.25.     

Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate

As is well known, the heat transfer coefficient of a solar air heater duct can be increased by providing artificial roughness on the heated wall (i.e. the absorber plate). Experiments were performed to collect heat transfer and friction data for forced convection flow of air in solar air heater rectangular duct with one broad wall roughened by wedge shaped transverse integral ribs. The experiment encompassed the Reynolds number range from 3000 to 18000; relative roughness height 0.015 to 0.033; the relative roughness pitch 60.17φ^−1.0264<p/e<12.12; and rib wedge angle (φ) of 8, 10, 12 and 15°. The effect of parameters on the heat transfer coefficient and friction factor are compared with the result of smooth duct under similar flow conditions. Statistical correlations for the Nusselt number and friction factor have been developed in terms of geometrical parameters of the roughness elements and the flow Reynolds number.     

Effect of chamfering on heat transfer and friction characteristics of solar air heater having absorber plate roughened with compound turbulators

Artificial roughness in the form of repeated transverse chamfered rib-groove roughness on one broad wall has been proposed as a convenient method for enhancement of thermal performance of solar air heater. An experimental investigation on heat and fluid flow characteristics of fully developed turbulent flow in a rectangular duct having repeated integral transverse chamfered rib-groove roughness on one broad wall has been carried out. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters. Six roughened plates have been tested placing a 60° V-groove at the centre line in between two consecutive chamfered ribs. The ribs' top have been chamfered having chamfer angles of 5°, 12°, 15°, 18°, 22° and 30°, while relative roughness pitch (P/e) and relative roughness height (e/D_h) of the ribs were kept constant having values of 10 and 0.03 respectively. The flow Reynolds number of the duct varied in the range of approximately 3000–21,000, most suitable for solar air heater. The effects of chamfer angle on Nusselt number and friction factor have been discussed and the results are compared with the square rib-grooved and smooth duct under similar flow conditions to investigate the enhancement in Nusselt number and friction factor. The conditions for the maximum enhancement of Nusselt number and friction factor have been determined. It has been found that the thermo-hydraulic performance of the solar air heater provided with such roughness is considerably enhanced.     

Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having arc-shaped wire as artificial roughness

An experimental study has been carried out for enhancement of heat transfer coefficient of a solar air heater having roughened air duct provided with artificial roughness in the form of arc-shape parallel wire as roughness element. Increment in friction factor by provided with such artificial roughness elements has also been studied. The effect of system parameters such as relative roughness height (e/d) and arc angle (α/90) have been studied on Nusselt number (Nu) and friction factor (f) with Reynolds number (Re) varied from 2000 to 17000. Considerable enhancement in heat transfer coefficient has been achieved with such roughness element. Using experimental data correlations for Nusselt number and friction factor have also been developed for such solar air heaters, which gives a good agreement between predicted values and experimental values of Nusselt number and friction factor.     

Correlations for solar air heater duct with V-shaped perforated baffles as roughness elements on absorber plate

An experimental investigation has been carried out to study the effect of heat transfer and friction characteristics of air passing through a rectangular duct which is roughened by V-down perforated baffles. The experiment encompassed Reynolds number (Re) from 3800 to 19,000, relative roughness height (e/H) values of 0.285–0.6, relative roughness pitch (P/e) range of 1–4 and open area ratio values from 12% to 44%. The effect of roughness parameters on Nusselt number (Nu) and friction factor (f) has been determined and increase in heat transfer and friction loss has been observed for ducts having a roughened test plate. Maximum Nusselt number is observed for the relative roughness pitch ranging from 1.5 to 3 for flow and geometrical parameters under consideration. The experimental data have been used to develop Nusselt number and friction factor correlations as a function of roughness and flow parameters.     

Heat transfer and friction characteristics of an equilateral triangular solar air heater duct using inclined continuous ribs as roughness element on the absorber plate

An experimental study has been carried out to determine the effect on the heat transfer and friction characteristics of an equilateral triangular solar air heater duct using inclined continuous ribs as roughness element on the absorber plate. The experimental study encompasses the range of Reynolds numbers from 5600 to 28,000, relative roughness height (e/D_h) 0.021–0.043, relative roughness pitch (p/e) 8–16 and angle of attack (α ) 30–60°. The duct has an aspect ratio (W/H) of 1.15. The effect of flow parameters and roughness parameters on heat transfer and friction factor is discussed. The thermohydraulic performance parameter has been determined for the given range of flow parameters and roughness parameters.     

Heat transfer and friction behaviors in rectangular channels with varying number of ribbed walls

An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with transverse ribs on one, two, three, and four walls is reported. Tests were performed for Reynolds numbers ranging from 10,000 to 80,000. The pitch-to-rib height ratio, P/e, was kept at 8 and rib-height-to-channel hydraulic diameter ratio, e/D_h was kept at 0.0625. The channel length-to-hydraulic diameter ratio, L/D_h, was 20. The heat transfer coefficient and friction factor results were enhanced with the increase in the number of ribbed walls. The friction roughness function, R(e⁺), was almost constant over the entire range of tests performed and was within comparable limits of the previously published data. The heat transfer roughness function, G(e⁺), increased with roughness Reynolds number and compared well with previous work in this area. Both correlations could be used to predict the friction factor and heat transfer coefficient in a rectangular channel with varying number of ribbed walls. The results of this investigation could be used in various applications of turbulent internal channel flows involving different number of rib roughened walls.     

Effect of apex angle variation on thermal and hydraulic performance of roughened triangular duct

The experiments were performed to analyze the influence of apex angle on thermal and hydraulic characteristics of triangular duct for Reynolds number range from 2000 to 16,000. Four different triangular ducts of apex angle 30°, 60°, 90° and 110° were fabricated for the experimentation. The one side of the duct is roughened with dimple shaped roughness element. The both relative shortway length (s/e) and relative longway length (l/e) of dimple shaped roughened element was kept constant (i.e., 10) but relative roughness height (e/D) is varied from 0.016 to 0.038. Result shows that apex angle plays an important role in heat transfer. The experimental results are presented in a form of correlation for Nusselt number (Nu) and friction factor (f).     

Investigation of thermal performance of solar air heater having roughness elements as a combination of inclined and transverse ribs on the absorber plate

An experimental investigation has been carried out to study the heat transfer and friction characteristics by using a combination of inclined as well as transverse ribs on the absorber plate of a solar air heater. The experimental investigation encompassed the Reynolds number (Re) ranges from 2000 to 14 000, relative roughness pitch (p/e) 3–8 and relative roughness height (e/D_h) 0.030. The effect of these parameters on the heat transfer coefficient and friction factor has been discussed in the present paper and correlations for Nusselt number and friction factor has been developed within the reasonable limits. A procedure to compute the thermal efficiency based on heat transfer processes in the system is also given and the effect of these parameters on thermal efficiency has been discussed.     

Experimental investigations on heat transfer and frictional characteristics of a turbulator roughened solar air heater duct

An experimental investigation has been carried out to study the heat transfer coefficient and friction factor by using artificial roughness in the form of specially prepared inverted U-shaped turbulators on the absorber surface of an air heater duct. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters.The experiments encompassed the Reynolds number range from 3800 to 18000; ratio of turbulator height to duct hydraulic mean diameter is varied from, e/D_h = 0.0186 to 0.03986 (D_h = 37.63 mm and e = 0.7 to 1.5 mm) and turbulator pitch to height ratio is varied from, p/e = 6.67 to 57.14 (p = 10 to 40 mm). The angle of attack of flow on turbulators, α = 90° kept constant during the whole experimentation. The heat transfer and friction factor data obtained is compared with the data obtained from smooth duct under similar geometrical and flow conditions. As compared to the smooth duct, the turbulator roughened duct enhances the heat transfer and friction factor by 2.82 and 3.72 times, respectively. The correlations have been developed for area averaged Nusselt number and friction factor for turbulator roughened duct.     

Heat transfer and friction factor correlation for artificially roughened duct with metal grit ribs

This paper presents the results of an experimental investigation of heat transfer to the airflow in the rectangular duct of an aspect ratio 10:1. The top wall surface is made rough with metal ribs of circular cross section in staggered manner to form defined grid. The roughened wall is uniformly heated and the other walls are insulated. This geometry of duct closely corresponds to that used in solar air heaters. The effect of grit geometry [i.e., relative roughness height of grid (e/D_h), relative roughness pitch of grit (p/e), relative length of grit (l/s)] on the heat transfer coefficient and friction factor is investigated. The range of variation of system parameters and operating parameters is investigated within the limits, as e/D_h: 0.035 to 0.044, p/e: 12.5–36 and l/s: 1.72–1, against variation of Reynolds number: 4000–17,000. It is observed that the plate of roughness parameters l/s = 1.72, e/D_h = 0.044, p/e = 17.5 shows optimum performance. Correlations for Nusselt number and friction factor in terms of above parameters are developed which reasonably correlate the experimental data.     

Experimental investigation on heat transfer and fluid flow characteristics of air flow in a rectangular duct with Multi v-shaped rib with gap roughness on the heated plate

In this work, results of an experimental investigation of the effect of geometrical parameters of Multi v-shaped ribs with gap on heat transfer and fluid flow characteristics of rectangular duct with heated plate having rib roughness on its underside have been reported. The range of parameters for this study has been decided on the basis of practical considerations of the system and operating conditions of solar air heaters. The experimental investigation encompassed the Reynolds number (Re) range from 2000 to 20,000, relative width ratio (W/w) of 6, relative gap distance (Gd/Lv) of 0.24–0.80, relative gap width (g/e) of 0.5–1.5, relative roughness height (e/D) of 0.043,relative roughness pitch (P/e) of 10, angle of attack (α) of 60°. The maximum enhancement in Nusselt number and friction factor is observed to be 6.32–6.12 times of that of the smooth duct, respectively. The thermo-hydraulic performance parameter is found to be the best for the relative gap distance of 0.69 and the relative gap width of 1.0.     

Heat transfer and friction factor correlations of solar air heater ducts artificially roughened with discrete V-down ribs

The heat and fluid flow characteristics of rectangular duct having its one broad wall heated and roughened with periodic ‘discrete V-down rib’ are experimentally investigated. Reynolds number (Re) has been varied from 3000-15000 with relative gap width (g/e) and relative gap position (d/w) range of 0.5-2.0 and 0.20-0.80 respectively. The respective variation in relative roughness pitch (P/e), angle of attack (α) and relative roughness height (e/D_h) have been 4-12, 30°-75° and 0.015-0.043. The effect of roughness parameters on Nusselt number (Nu) and friction factor (f) has been determined and the results obtained were compared with those of smooth duct. The maximum increase in Nu and f over that of smooth duct was 3.04 and 3.11 folds respectively. The rib parameters corresponding to maximum increase in Nu and f were d/w = 0.65, g/e = 1.0, P/e = 8.0, α = 60° and e/D_h = 0.043. Correlations for the Nu and f in terms of Re and rib parameters have been developed.     

Second law optimization of a solar air heater having chamfered rib–groove roughness on absorber plate

Artificially roughened solar air heaters perform better than the plane ones under the same operating conditions. However, artificial roughness leads to even more fluid pressure thereby increasing the pumping power. The entropy generation in the duct of solar air heater having repeated transverse chamfered rib–groove roughness on one broad wall is studied numerically. Roughness parameters, viz., relative roughness pitch P/e, relative roughness height e/D_h relative groove position g/P, chamfer angle φ and flow Reynolds number Re have a combined effect on the heat transfer as well as fluid friction. The entropy generation is minimized and reasonably optimized designs of roughness are found.     

Correlations for solar air heater duct with dimpled shape roughness elements on absorber plate

An experimental investigation has been carried out for a range of system and operating parameters in order to analyse the effect of artificial roughness on heat transfer and friction characteristics in solar air heater duct which is having dimple shaped elements arranged in angular fashion (arc) as roughness elements on absorber plate. Duct has an aspect ratio (W/H) of 11, relative roughness pitch (p/e) range of 10–20, relative roughness height (e/D_h) range of 0.021–0.036, arc angle (α) range of 45–75° and Reynolds number (Re) ranges from 3600 to 18,000. A considerable increase in heat transfer and friction loss has been observed. The experimental data have been used to develop Nusselt number and friction factor correlations as a function of roughness parameters and operating parameters.     

Heat transfer and friction characteristics of rectangular solar air heater duct using rib-grooved artificial roughness

Experimental investigation on the heat transfer and friction characteristics of rib-grooved artificial roughness on one broad heated wall of a large aspect ratio duct shows that Nusselt number can be further enhanced beyond that of ribbed duct while keeping the friction factor enhancement low. The experimental investigation encompassed the Reynolds number range from 3000 to 21,000; relative roughness height 0.0181–0.0363; relative roughness pitch 4.5–10.0, and groove position to pitch ratio 0.3–0.7. The effect of important parameters on the heat transfer coefficient and friction factor has been discussed and the results are compared with the results of ribbed and smooth duct under similar flow conditions. The present investigation clearly demonstrates that the heat transfer coefficient for rib-grooved arrangement is higher than that for the transverse ribs, whereas the friction factor is slightly higher for rib-grooved arrangement as compared to that of rectangular transverse ribs of similar rib height and rib spacing. The conditions for best performance have been determined. Correlations for Nusselt number and friction factor have been developed that predict the values within reasonable limits.     

Investigation for the optimal thermohydraulic performance of artificially roughened solar air heaters

Artificially roughened solar air heaters perform better than the plane ones under the same operating conditions. However, artificial roughness leads to even more fluid pressure thereby increasing the pumping power. Roughness and flow parameters viz. relative roughness pitch p/e, relative roughness height e/D and flow Reynolds number Re have a combined effect on the heat transfer as well as fluid pressure (friction factor). Investigation for the optimal thermohydraulic performance (i.e. maximum heat transfer for minimum friction loss) of artificially roughened solar air heaters has been carried out. An optimisation parameter known as roughness Reynolds number which combines the roughness and flow effect and is expressed as has been considered. Thermohydraulic performance has been defined by the equation η_thermo=(St_r/St_s)³/(f_r/f_s). It has been found that e⁺_opt24 gives the optimal thermohydraulic performance in such collectors and therefore the optimal thermohydraulic performance curves [3], for designing such collectors for practical applications are suitable. The value of optimal thermohydraulic performance has been found to be about 71% corresponding to e⁺_opt=24.