Optimized electrode arrangement in solar air heater

Laminar forced convection inside the solar air heater with various wire electrode arrangements are numerically examined for heat transfer enhancement using electrohydrodynamic technique. The electric field is generated by the wire electrodes charged with DC high voltage ranging from 7.5 to 17.5 kV. Reynolds number corresponding to the flow considered is between 100 and 2000. The numerical modeling of computational fluid dynamics includes the interactions among electric field, flow field, and temperature field. It is found that the enhancement of heat transfer coefficient with the presence of electric field increases in relation with the supplied voltage but decreases when the Reynolds number and the distance between electrode and channel surface are augmented. The optimized electrode arrangement, which obtains the best heat transfer enhancement is investigated incorporating with the pressure drop consideration. The heat transfer enhancement is also depended on the number of electrodes per length and the channel dimension.     

Design and thermal performance evaluation of a novel solar air heater

In the present scenario, numerous applications perform on solar energy for cooking, heating and cooling, and power generation, globally. Solar air heaters are one of these applications purposely used for, drying, timber seasoning and space heating. In the present work, a solar air heater (SAH) has been designed to produce a good exhaust temperature for long hours especially in the case of poor ambient conditions or during off sunshine hours. A mixture of desert and granular carbon in the ratio of 4:6 has been used as thermal heat storage inside the SAH. Two halogen lights of 300 W are used to increase the exhaust temperature of the SAH by placing them in the inlet and outlet ducts. All the experiments have conducted on natural and forced convection for performance evaluation on two similar design solar air heaters (with and without heat storage). The comparisons are made with two similar design solar air heaters carrying desert and granular carbon, as an individual heat storing media, to find out an optimum design of a SAH with long term heating. The thermal efficiencies of the novel SAH range from 18.04% to 20.78% of natural convection and 52.21%–80.05% with forced convection.     

Enhanced heat transfer in inclined solar chimneys by electrohydrodynamic technique

Heat transfer enhancement of natural convection inside the inclined solar chimneys is investigated using electrohydrodynamic technique. The interactions between electric field, flow field, and temperature field are analyzed. The ranges of parameters considered are 10⁴Ra10⁷, 7.5 kVV₀17.5 kV, 30°θ120°, and 2aspect ratio14. Flow and heat transfer enhancements are significantly influenced at low Rayleigh number. The optimum inclined angle which obtains maximum volume flow rate and heat transfer is found to be at θ=60°. A maximum volume flow rate enhancement is expressed in relation with the number of electrodes. The relation between aspect ratio of chimney and number of electrodes that performs the optimum condition between efficiency and economy is analyzed incorporating with all concerning parameters.     

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.     

Investigation of heat transfer and friction characteristics of packed bed solar air heater using wire mesh as packing material

An experimental investigation has been carried out on a packed bed solar air heater using wire mesh as packing material. Data pertaining to heat transfer and friction characteristics were collected for air flow rates ranging from 0.0159 to 0.0347 kg/s-m² for eight sets of matrices with varying geometrical parameters. The thermal efficiency of a packed bed solar air heater was compared with that of a conventional solar air heater to determine the enhancement which was found to be strong function of system and operating parameters of the bed. It was found that an enhancement of the order of 76.9-89.5% can be obtained. Experimental data were utilised to develop correlations for Colburn J_h factor and friction factor as function of geometrical parameters of the bed and the flow Reynolds number. These correlations were found to predict the experimental results with reasonable accuracy. It has also been found that the present correlations show much better agreement as compared to the values predicted by earlier correlations for such systems.     

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.     

Analysis of a glass solar air heater

A matrix solar collector fabricated from broken glass pieces is being supposed to be a thermally efficient and economically cheap system for heating air. In this communication, we develop an analysis to study the performance of such a system. Typical cases considered are (i) top surface blackened, (ii) all glass pieces blackened and (iii) bottom surface blackened. Numerical calculations have been performed to study the effect of single and double glazing and insulation at the bottom, besides the effect of other physical parameters.     

Thermal behavior in solar air heater channel fitted with combined rib and delta-winglet

Effects of combined ribs and delta-winglet type vortex generators (DWs) on forced convection heat transfer and friction loss behaviors for turbulent airflow through a solar air heater channel are experimentally investigated in the present work. Measurements are carried out in the rectangular channel of aspect ratio, AR = 10 and height, H = 30 mm. The flow rate is presented in the form of Reynolds numbers based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The cross-section shape of the rib placed on the absorber plate to create a reverse-flow is an isosceles triangle with a single rib height, e/H = 0.2 and rib pitch, P_l/H = 1.33. Ten pairs of the DW with its height, b/H = 0.4; transverse pitch, P_t/H = 1 and three attack angles (α) of 60°, 45° and 30° are introduced and mounted on the lower plate entrance of the tested channel to generate longitudinal vortex flows. The experimental results show that the Nusselt number and friction factor values for combined rib and DW are found to be much higher than those for the rib/DW alone. The larger attack angle of the DW leads to higher heat transfer and friction loss than the lower one. In common with the rib, the DW pointing upstream (PU-DW) is found to give higher heat transfer rate and friction loss than the DW pointing downstream (PD-DW) at a similar operating condition. In comparison, the largest attack angle (α = 60°) of the PU-DW yields the highest increase in both the Nusselt number and friction factor while the lowest attack angle of the PD-DW provides the best thermal performance.     

Analysis of fluid flow and heat transfer in a rib grit roughened surface solar air heater using CFD

This paper presents the study of fluid flow and heat transfer in a solar air heater by using Computational Fluid Dynamics (CFD) which reduces time and cost. Lower side of collector plate is made rough with metal ribs of circular, square and triangular cross-section, having 60° inclinations to the air flow. The grit rib elements are fixed on the surface in staggered manner to form defined grid. The system and operating parameters studied are: e/D_h = 0.044, p/e = 17.5 and l/s = 1.72, for the Reynolds number range 3600-17,000. To validate CFD results, experimental investigations were carried out in the laboratory. It is found that experimental and CFD analysis results give the good agreement. The optimization of rib geometry and its angle of attack is also done. The square cross-section ribs with 58° angle of attack give maximum heat transfer. The percentage enhancement in the heat transfer for square plate over smooth surface is 30%.     

An experimental study on a finned type and non-porous type solar air heater with a solar simulator

The results of experiments performed on two non-porous solar absorber solar air heaters with and without fins have been reported in this communication. The experiments were performed under laboratory conditions. Air heaters with fins are seen to be more efficient in comparison to the air heater without fins for air flow rates ≤0.0388 kg/sec per m². The experimental results have been used to validate a simple theoretical model.     

Transient analysis of a solar air heater of the second kind

This paper presents a theoretical analysis along with the experimental validation study of a solar air heater of the second kind. The heater consists of a flat passage between two metallic plates through which the heat transfer fluid air is made to pass by some auxiliary means. Study of the periodic response of different parameters of this solar air heater is attempted. The heat balance equations governing the behaviour of the system are solved explicitly. The results obtained from the analytical expressions for the transient variation of outlet air temperature compare well with experimental data. Predictions are also made regarding effects of different performance parameters of the air heater with variations of air mass flow rate and plate emissivity with the hope of optimizing the collector configuration.     

Numerical modeling of the electrohydrodynamic effect to natural convection in vertical channels

The electrohydrodynamic effect to natural convection inside the vertical channels is numerically investigated by computational fluid dynamics technique. The range of parameters considered are 10⁴ = Ra = 10⁷, 7.5 = V₀ = 17.5 kV, and 2 = aspect ratio = 10. Flow and temperature distributions are affected with supplied voltage at the wire electrodes, and the heat transfer enhancement is significantly influenced at low Rayleigh number. The augmented volume flow rate of fluid is indicated in relation with the number of electrodes. Moreover, heat transfer enhancement also depended on the electrode arrangement while the number of electrodes is initially fixed. The relation between channel aspect ratio and number of electrodes that performs the maximum heat transfer is expressed incorporating with the optimum concerning parameters.     

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.     

Investigation on double-glazed solar air heater connected in series with rock bed solar collector-cum-storage system

This paper presents an experimental investigation along with a theoretical model of a double-glazed flat plate solar air heater connected in series with an integrated rock bed collector-cumstorage unit. Predictions are made regarding the effects of heat transfer coefficient and number of glazings on the performance of the air heater. The model fits quite well with the experimental observations.     

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.     

Augmentation of heat transfer coefficient by using 90° broken transverse ribs on absorber plate of solar air heater

An experimental investigation has been carried out to study the heat transfer coefficient by using 90° broken transverse ribs on absorber plate of a solar air heater; the roughened wall being heated while the remaining three walls are insulated. The roughened wall has roughness with pitch (P), ranging from 10–30 mm, height of the rib of 1.5 mm and duct aspect ratio of 8. The air flow rate corresponds to Reynolds number between 3000–12,000. The heat transfer results have been compared with those for smooth ducts under similar flow and thermal boundary condition to determine the thermal efficiency of solar air heater.     

The effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer

We present a theoretical study of a solar photovoltaic-thermal (hybrid) system consisting of a flat-plate solar air heater mounted with solar cells and a plane booster. A conventional flat-plate collector is converted into a hybrid system by mounting solar cells directly on the absorber plate. A hybrid system is self-sufficient in the sense that the electrical energy required by the pump is supplied by the panel. Such systems are well suited to applications such as solar drying. The combined system is analysed for the case when the radiative and absorptive properties of the cell surface and the absorber plate are nearly the same. The solar cell efficiency is a linearly-decreasing function of the absorber plate temperature. The performance of the system has been evaluated for various combinations of boosters. The minimum area of the solar cells required to run the pump at a given flow rate has been calculated as a function of time, with and without boosters. The minimum cell area required decreases with the use of boosters. High cost cells may be replaced by low cost reflectors. The solar air heaters presently available on the market are not suitable for direct conversion to hybrid systems.     

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.     

Experimental investigation of thermal performance of solar air heater having different obstacles on absorber plates

This paper presents the results of an experimental investigation of the performance for a new flat plate solar air heater (SAH) with several obstacles (Type I, Type II, and Type III) and without obstacles (Type IV). The efficiencies, the heat gain factors and heat loss coefficients are determined for the collectors and comparisons were made among them. The experimental data along with the correlations obtained by linear regression are presented. The optimal value of efficiency was determined for the solar air heater with Type II absorbent plate in flow channel duct for all operating conditions and the collector supplied with obstacles appears significantly better than that without obstacles.