INVESTIGATION OF THERMO-HYDRAULIC PERFORMANCE OF SOLAR AIR HEATERS HAVING ARTIFICIALLY ROUGHENED ABSORBER PLATE

Abstract

Flat plate solar air heaters have been extensively employed to deliver heated air at low to moderate temperatures for space heating, crop drying and industrial applications. The thermal efficiency of a solar air heater is significantly low because of a low convective heat transfer coefficient between the absorber plate and air, leading to high absorber plate temperature and greater heat losses to the ambient. It has been found that the main thermal resistance to the convective heat transfer is due to the formation of boundary layer on the heat transferring surface. Efforts for enhancing heat transfer have been directed towards artificially destroying or disturbing this boundary layer. Artificial roughness in the form of wires or ribs of different shapes and in various arrangements have been used to create turbulence near the wall or to break the boundary layer. Thus, the artificial roughness can be employed for the enhancement of heat transfer coefficient between the absorber plate and air and thereby improving the thermal performance of solar air heaters. A critical review of the heat transfer and fluid flow characteristics in artificially roughened ducts has been carried out. Sufficient information is available in the literature about heat transfer and friction characteristics for single phase flow in the roughened circular tubes, annuli, and rectangular and square ducts (with all or two opposite roughened walls subjected to heat flux) pertaining to fully rough regime of flow. Investigations on heat transfer and friction characteristics have been reported pertinent to transitional flow of air in a rectangular duct with artificial roughness in the form of wires or expanded metal matrix on one broad wall which is subjected to uniform heat flux while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters. Further, the rib shape strongly affects the flow structure in the transitional flow regime. Use of artificial roughness to enhance heat transfer rate also results in considerably large increase in friction factor and hence in the pumping power. Chamfered integral ribs have been shown, for flow in circular annuli, to enhance the heat transfer rate considerably with minimum pressure drop penalty compared to other ribs. Thus, the application of such ribs to a solar air heater is worth exploring. It has been, therefore, planned to investigate the effect of geometrical parameters of integral chamfered repeated rib-roughness on the enhancement of thermal performance and also on the pressure drop characteristic of solar air heater. In view of above, the present investigation was planned with the following objectives: (i) Detailed experimental investigation of the effect of geometrical parameters of integral chamfered rib-roughness and of duct aspect ratio on heat transfer and friction behaviour with uniform heat flux to the roughened broad wall. (ii) Development of correlations for heat transfer coefficient and friction factor from data obtained from the experimental investigations. (iii) Experimental investigation of the performance of a solar air heater with such rib-roughened absorber plate and the comparison of these results with the performance predictions based on the correlations in ii order to validate the performance prediction procedure. (iv) Investigation of thermo-hydraulic performance of the roughened solar air heater and development of a design methodology for the selection of the preferred roughness parameters based on thermo-hydraulic performance. The present study involved experimental work in two parts. In the first part of experimentation, an-, indoor test facility has been designed and fabricated to generate heat transfer and friction data for flow in a rectangular section duct with one broad wall with artificially rib-roughened surface at different air flow rates for a range of roughness parameters and duct aspect ratio. The roughness parameters investigated are relative roughness height, relative roughness pitch, rib head chamfer angle, and flow angle of attack. Experimental data have also been collected for smooth ducts for ensuring the accuracy of the experimental data and also to compare the heat transfer and friction characteristics of roughened and smooth ducts. Electric strip heaters have been used for heating of one broad wall of the duct. Sixteen roughened plates having integral rectangular or chamfered rib-roughness have been tested for different duct depths resulting in 24 sets of test runs. Seven to eight flow rates corresponding to the flow Reynolds number of about 3000 to 20000 have been used for each test set and data were collected under steady state condition. The overall range of parameters for this experimental investigation are Reynolds number, Re 3000 to 20000 Channel aspect ratio, W/H 4.8 to 12.0 Test duct length 32 Dh to 66 Dh iii Relative roughness height, e/Dh 0.014 to 0.044 Relative roughness pitch, p/e 4.5 to 8.5 Rib head chamfer angle, -15°, and 0 to 18° Rib angle of attack, a 90° and 71° On the basis of this experimental investigation it is found that the presence of the ribs at one broad wall 9f the rectangular duct yields up to about 2-fold increase in the Stanton number and 3-fold increase in the friction factor as compared to a smooth duct for the range of present investigation. The rib head chamfer angle has a significant effect on heat transfer coefficient and friction factor. Both the Stanton number and the friction factor have been found to be maximum at chamfer angle of 15°. The increase in the Stanton number and friction factor with the increase in the chamfer angle from 0° to 15° can be attributed to more frequent shedding of vortices. Typically, for a surface with ribs at a relative roughness pitch, of 4.5 and a relative roughness height of 0.032, the Stanton number and friction factor increased by 16 percent and 18 percent respectively for an increase in chamfer angle from 0° to 15°. Both the Stanton number and the friction factor are found to increase with an increase in the relative roughness height. The Stanton number is found to first increase sharply with an increase in the roughness Reynolds number e+ then either to decrease marginally or to attain nearly a constant value. The maxima of the Stanton number for cases with different flow and roughness parameters have been found to occur at roughness Reynolds number of about 20 to 25 which indicates the start of the fully rough regime for = (e/Dh) Re) and iv this particular type of roughness. For a rectangular duct with only one roughened and heated wall, a lower aspect ratio duct (i.e. duct with larger depth) is found to have higher heat transfer coefficient but lower friction factor as compared to higher aspect ratio ducts. The experimental results of friction factor and heat transfer coefficient are correlated in terms of roughness function R =IT +2.5 ln(2e/Dh)+3.75) and the heat transfer function g (= ((f/2St) - 1) + (a) Friction Factor Correlation (W/H)-o.4 03/02.695 R = 1.66 e-°•°°78cb exp {-0.762 (1n(p/e))2} for 5 s e+ < 20 and R = 1.325 e-0.00784) (W/H)-14 (p/e)2'695 exp {-0.762 (In (p/e))2} for 20 s e+ s 60 when W/H > 7.75 use W/H = 7.75 in both the above equations. (b) Heat Transfer Coefficient Correlation g = 103.77 e4"°60 (W/H)" (p/e)-2.56exp {0.7343 (In (p/e))2} (e+)-°'31 for 7 s e+ < 20 and g = 32.26 em.°°64' (W/H)" (p/e)-2.56 exp {0.7343 (In (p/e))2} (e-y.08 for 20 s e+ s 60 when W/11 > 10 use W/H = 10 in both the above equations. The standard deviations of the predicted and the experimental values of the roughness function and the heat transfer function have been found to be ±4.35 percent and ±6.3 percent respectively. Thus, the correlations predict the heat transfer and friction factor values quite satisfactorily in the range of parameters investigated. In the second part of the experimentation, outdoor tests have been conducted on three solar air heaters having 15° integral chamfered rib-roughness with different values of relative roughness heights (0.0192 to 0.044). The comparison of the experimental values of thermal efficiency with the values predicted based on the correlations showed that the experimental values and predicted values lie within ±7 percent with standard deviation of ±5.8%. Thus, the procedure of the performance prediction using the correlations can be used with confidence for the design of roughened solar air heaters. 10 to 40 percent enhancement in the thermal efficiency, as a result of using artificially roughned absorber plate has been observed as compared with a conventional smooth duct solar air heater. The enhancement in thermal efficiency of the solar air heaters as a result of artificial roughness on the absorber plates is found to be accompanied with a substantial increase in pumping power also. Effective efficiency, defined on the basis of the thermal gain reduced by the equivalent thermal energy for the pumping power, has been used for evaluation of thermo-hydraulic performance of roughened solar air heaters. vi The effective efficiency has been found to increase with a decrease in temperature rise parameter at/I (ratio of temperature rise to the solar insolation), attain a maximum value and then decrease. At lower mass flow rates (s about 0.05 kgs-1 per unit area of the plate) corresponding to higher values of the temperature rise parameter, the pumping power is not significant compared to the thermal energy collection rate and hence a solar air heater with roughness of high value of relative roughness height is found to perform better. While at the high flow rates (corresponding to low temperature rise parameter), the pumping power requirement is significant and a roughened solar air heater with small relative roughness height or a smooth duct air heater is seen to have better thermo-hydraulic performance. The results of thermal and thermo-hydraulic analysis have been presented in the form of design plots which will be useful to a designer. Summarizing, it can be stated that by providing artificial roughness on the absorber plate of a solar air heater, considerable enhancement in its performance can be achieved. The heat transfer coefficient and friction factor correlations developed in this work for integral chamfered rib-roughness can be used for the design of such solar air heaters. Thermo-hydraulic performance plots have been presented that can be utilized for the determination of optimum values of roughness parameters to be used for designing the solar air heaters with enhanced efficiency.