Nusselt number and friction factor correlations for solar air heater duct having artificially roughened absorber plate

An experimental investigation has been carried out for a range of system and operating parameters in order to analyse effect of artificial roughness on heat transfer and friction in solar air heater duct having protrusions as roughness geometry. An increase in heat transfer and friction loss has been observed for duct having roughened absorber plate. Experimental data have been used to develop Nusselt number and friction factor correlations as function of system and operating parameters for predicting performance of the system having investigated type of roughness geometry.     

A review on methodology of artificial roughness used in duct of solar air heaters

In order to enhance rate of heat transfer to flowing air in the duct of a solar air heater, artificially roughened surface of absorber plate is considered to be an effective technique. Investigators reported various roughness geometries in literature for studying heat transfer and friction characteristics of an artificially roughened duct of solar air heaters. In the present paper an attempt has been made to categorize and review the reported roughness geometries used for creating artificial roughness. Heat transfer coefficient and friction factor correlations developed by various investigators for roughened ducts of solar air heaters have also been reported in the present paper.     

Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having multi v-shaped with gap rib as artificial roughness

This paper presents the results of an experimental investigation of heat transfer and friction in the flow of air in rectangular ducts having multi v-shaped rib with gap roughness on one broad wall. The investigation encompassed Reynolds number (Re) from 2000 to 20,000, relative gap distance (G_d/L_v) values of 0.24–0.80, relative gap width (g/e) values of 0.5–1.5, relative roughness height (e/D) values of 0.022–0.043, relative roughness pitch (P/e) values of 6–12, relative roughness width ratio (W/w) values of 1–10, angle of attack (α) range of 30°–75°. The optimum values of geometrical parameters of roughness have been obtained and discussed. For Nusselt number (Nu), the maximum enhancement of the order of 6.74 times of the corresponding value of the smooth duct has been obtained, however the friction factor (f) has also been seen to increase by 6.37 times of that of the smooth duct. The rib parameters corresponding to maximum increase in Nu and f were G_d/L_v = 0.69, g/e = 1.0, e/D = 0.043, P/e = 8, W/w = 6 and α = 60°. Based on the experimental data, correlations for Nu and f have been developed as function of roughness parameters of multi v-shaped with gap rib and flow Reynolds number.     

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.     

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.     

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.     

Effective efficiency of solar air heaters having different types of roughness elements on the absorber plate

The use of an artificial roughness on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the duct of a solar air heater. This paper presents a comparison of effective efficiency of solar air heaters having different types of geometry of roughness elements on the absorber plate. The effective efficiency has been computed by using the correlations for heat transfer and friction factor developed by various investigators within the investigated range of operating and system parameters.     

A review on roughness geometry used in solar air heaters

The use of an artificial roughness on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the duct of a solar air heater. Number of geometries of roughness elements has been investigated on the heat transfer and friction characteristics of solar air heater ducts. In this paper an attempt has been made to review on element geometries used as artificial roughness in solar air heaters in order to improve the heat transfer capability of solar air heater ducts. The correlations developed for heat transfer and friction factor in roughened ducts of solar air heaters by various investigators have been reviewed and presented.     

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

Artificial roughness in form of ribs is convenient method for enhancement of heat transfer coefficient in solar air heater. This paper presents experimental investigation of heat transfer and friction factor characteristics of rectangular duct roughened with W-shaped ribs on its underside on one broad wall arranged at an inclination with respect to flow direction. Range of parameters for this study has been decided on basis of practical considerations of system and operating conditions. Duct has width to height ratio (W/H) of 8.0, relative roughness pitch (p/e) of 10, relative roughness height (e/D_h) 0.018-0.03375 and angle of attack of flow (α) 30-75°. Air flow rate corresponds to Reynolds number between 2300-14,000. Heat transfer and friction factor results have been compared with those for smooth duct under similar flow and thermal boundary condition to determine thermo-hydraulic performance. Correlations have been developed for heat transfer coefficient and friction factor for roughened duct.     

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.     

Performance evaluation of solar air heaters having v-down discrete rib roughness on the absorber plate

This paper presents results of a study of the performance of solar air heaters with 60 ° v-down discrete rectangular cross-section repeated rib roughness on the air flow side of the absorber plate. A detailed investigation has been carried out using a mathematical model to study the effects of various ambient, operating and design parameters on the thermal efficiency and effective efficiency (based on the net gain after taking account of the pumping power) of such air heaters. The study shows that, at air mass flow rates less than about 0.04 kg s⁻¹ per m² of the absorber plate, roughened duct solar air heaters provide significant performance advantage over the smooth duct air heater. The thermal and effective efficiencies differ only marginally at low flow rates. With the increase in the flow rate, the difference between the thermal and effective efficiencies increases because of the increase in the pumping power. At the mass flow rate of about 0.045 kg s⁻¹ m⁻², the effective efficiencies of the roughened and smooth duct solar air heaters are practically the same. The results of the study are presented in the form of design plots.     

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.     

Analysis of heat transfer augmentation and flow characteristics due to rib roughness over absorber plate of a solar air heater

A computational analysis of heat transfer augmentation and flow characteristics due to artificial roughness in the form of ribs on a broad, heated wall of a rectangular duct for turbulent flow (Reynolds number range 3000–20,000, which is relevant in solar air heater) has been carried out. Shear stress transport k−ω turbulence model is selected by comparing the predictions of different turbulence models with experimental results available in the literature. A detailed analysis of heat transfer variation within inter rib region is done by using the selected turbulence model. The analysis shows that peak in local heat transfer coefficient occurs at the point of reattachment of the separated flow as observed experimentally. The results predict a significant enhancement of heat transfer in comparison to that for a smooth surface. There is a good matching between the predictions by SST k−ω and experimental results. In this work, nine different shapes of rib are examined using SST k−ω model and compared on the basis of heat transfer enhancement, friction characteristics and performance index considering heat transfer enhancement with the same pumping power.     

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.     

Experimental investigation of optimum thermohydraulic performance of solar air heaters with metal rib grits roughness

Artificial roughness has been found to enhance the heat transfer from the collector plate to the air in a solar air heater. However, it would result in increase in frictional losses and hence, power required by fan or blower. This paper presents the results of an experimental investigation of thermohydraulic performance of roughened solar air heaters with metal rib grits. The range of variation of system and operating parameters is investigated within the limits of, e/D_h: 0.035-0.044, p/e: 15-17.5 and l/s as 1.72, against variation of Reynolds number, Re: 3600-17000. The study shows substantial enhancement in thermal efficiency (10-35%), over solar air heater with smooth collector plate. The thermal efficiency enhancement is also accompanied by a considerable increase in the pumping power requirement due to the increase in the friction factor (80-250%). The optimum design and operating conditions have been determined on the basis of thermohydraulic considerations. It has been found that, the systems operating in a specified range of Reynolds number show better thermohydraulic performance depending upon the insolation. A relationship between the system and operating parameters that combine to yield optimum performance has been developed.     

Single and double pass solar air heaters with wire mesh as packing bed

The thermal performances of single and double pass solar air heaters with steel wire mesh layers are used instead of a flat absorber plate are investigated experimentally. The effects of mass flow rate of air on the outlet temperature and thermal efficiency were studied. The results indicate that the efficiency increases with increasing the mass flow rate for the range of the flow rate used in this work between 0.012 and 0.038 kg/s. For the same flow rate, the efficiency of the double pass is found to be higher than the single pass by 34–45%. Moreover, the maximum efficiencies obtained for the single and the double pass air collectors are 45.93 and 83.65% respectively for the mass flow rate of 0.038 kg/s. Comparison of the results of a packed bed collector with those of a conventional collector shows a substantial enhancement in the thermal efficiency.     

Studies on heat transfer and friction factor characteristics of thermosyphon solar water heating system with helical twisted tapes

Experimental investigation of heat transfer, friction factor and thermal performance of thermosyphon solar water heater system fitted with helical twisted tape of various twist ratios has been performed and presented. The helical twisted tape induces swirl flow inside the riser tubes, which increases the heat transfer and pressure drop. The empirical correlations developed for Nusselt number and friction factor with various twist ratios (Y = 3, 4, 5, 6) are fitted with the experimental data with a discrepancy of less than ±4.54% and ±6.13% respectively. The results are compared with a plain tube collector at the same operating conditions. Conclusions made from the results show that heat transfer enhancement in twisted tape collector is higher than the plain tube collector with minimum twist ratio and gradually decreases with increase in twist ratio. The overall thermal performance of twisted tape collector is found to increase with increase in solar intensity.     

Experimental studies on heat transfer and friction factor characteristics of forced circulation solar water heater system fitted with helical twisted tapes

Experimental investigation of heat transfer, friction factor and thermal performance of twisted tape solar water heater with various twist ratios has been conducted and the results are compared with plain tube collector for the same operating conditions with Reynolds number varied from 3000 to 23,000. Experimental data from plain tube collector is validated with the fundamental equations and found that the discrepancy is less than ±5.35% and ±8.80% for Nusselt number and friction factor, respectively. Correlations have been developed for Nusselt number and friction factor with various twist ratios (Y = 3, 4, 5, 6) and are compared with the experimental values. Results conclude that, heat transfer and pressure drop are higher in twisted tape collector compared to the plain one. Among the various twist ratios, the minimum twist ratio 3 is found to enhance the heat transfer and pressure drop due to swirl generation. As the twist ratio increases, the swirl generation decreases and minimizes the heat transfer and friction factor.     

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.     

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.