Employing exergy-optimized pin fins in the design of an absorber in a solar air heater

Fins serve as heat transfer augmentation features in solar air heaters however they increase pressure drop in flow channels. Pin fins are relatively good heat transfer augmentation features with superior aerodynamic performance and as a result find application in some solar air heaters. The exergy optimization method is employed in sizing the pin fin. Results indicate that high efficiency of the optimized fin improves the heat absorption and dissipation potential of a solar air heater. With optimum fin efficiency and superior absorptive coating quality, useful energy losses can be minimized. Some important observations pertinent in design are made.     

Thermal performance enhancement of solar air heaters,by a fan-blown absorber plate with rectangular fins

The aim of this study is to provide a remedy for the low thermophysical properties of air, which is used as a fluid of transfer in solar collectors. A fully developed flow needs to be created by the use of staggered fin rows soldered under the absorber plate. The fluid flow undergoes contractions followed by expansions, which creates a fully developed turbulent flow, and increases the thermal heat transfer between the absorber plate and the air. The fins increase the heat transfer surface, from which an appreciable improvement of the thermal heat performance of solar air heaters has been found in comparison to those of solar air heaters with a plane absorber. In this work we have tested the influence of the dimension of the fins and the influence of the space between consecutive fin rows mounted in staggered rows.     

The effect of using transverse fins on a double pass flow solar air heater using wire mesh as an absorber

The thermal performance of a double pass solar air heater with 2, 4, and 6 fins attached was investigated experimentally. Wire mesh layers were used between the fins instead of an absorber plate. The effects of mass flow rate of air on the outlet temperature and thermal efficiency were studied. The indicated results show that the efficiency increases with increasing the mass flow rate for the range of the flow rate used in this work between 0.0121-0.042 kg/s. Moreover, the maximum efficiency was obtained by using 6 fins at the same mass flow rate. The maximum efficiency obtained for the 2, 4, 6 fins of SAH were 75.0%, 82.1% and 85.9% respectively for the mass flow rate of 0.042 kg/s. In addition, the maximum average temperature difference between the inlet and the outlet, ΔT, for the SAH with 6 fins was the highest for the same mass flow rates compared to 2 and 4 fins SAHs. The maximum average and instantaneous peak of ΔT obtained were 43.1 °C and 62.1 °C respectively for the 6 fins SAH when the mass flow rate was 0.0121 kg/s. Comparison of the results of a counter flow packed bed collector with those of a conventional collector shows a substantial enhancement in the thermal efficiency.     

Experimental performance of single and double pass solar air heater with fins and steel wire mesh as absorber

Thermal performance of a single and double pass solar air heater with fins attached and using a steel wire mesh as absorber plate was investigated experimentally. The effects of air mass flow rate range between 0.012 kg/s and 0.038 kg/s on the outlet temperature and thermal efficiency was studied. The bed heights were 7 cm and 3 cm for the lower and upper channels respectively.     

Energy analysis of a solar air collector with rows of fins

The necessity for improving the thermal performances of the solar air collector, for some needs, encouraged us to carry out this study. Initially, to improve the efficiency factor of these solar collectors, we create an increasingly turbulent flow between the absorber and the back wooden plate. For that, we use obstacles of various forms. In this study, we chose rectangular plate fins inserted perpendicular to the flow. The fluid flows out through the interstices between fins in the same row, this allows a good distribution of the fluid and reduces the dead zones. Secondly, and for the same configuration, we undertake a study on the evaluation of the transfer coefficient. The results are compared with those obtained with a solar air collector without fins, using two types of absorbers selective (in coppersun) or not selective (black-painted aluminium).     

Performance of plastic solar air heating collectors with a porous absorber

The thermal performance of solar air heaters consisting of a porous textile absorber between two PVC foils has been investigated. The efficiency of the heaters depends strongly on the characteristics of the textile forming the absorber and on the back insulation. For an incident solar radiation of 687 W/m² at the collector's surface, a temperature rise of 16-6°C in the air flowing through the solar collector at a rate of 800 m³/h, was achieved, thus yielding an efficiency of nearly 71 percent. Further it was found that the linear approximation for the Hottel-Bliss equation leads to erroneous estimations for the collector's parameters when the absorber is porous; for the same type of collector with a denser textile as absorber, however, such an approximation yields, as usual, correct numerical values for the characteristic parameters of the collector.     

Shape optimization for absorber plates of solar air collectors

Solar air heaters can be used for many applications at low and moderate temperatures. There are different factors affecting the solar air heater efficiency, e.g. collector length, collector depth, type of absorber plate, glass cover plate, wind speed, etc. The absorber shape factor is the most important parameter in the design for any type of solar air heater. Increasing the absorber shape area will increase the heat transfer to the flowing air, but on the other hand, will increase the pressure drop in the collector, this increases the required power consumption to pump the air flow crossing the collector. It was most important to find the optimizing angle of the triangular collector. The effect of the change of the absorber shape factor on the collector performance was studied. A theoretical model was constructed for the two types of collectors, taking into account the new parameter, called the absorber shape factor. The results can be used for all types of solar air heaters by changing the value of the absorber shape factor. The optimum angle of the triangular collector was deduced.     

Heat transfer coefficient and thermal losses of solar collector and Nusselt number correlation for rectangular solar air heater duct with longitudinal fins hold under the absorber plate

An experimental investigation has been carried out for a series of system and operating parameters in order to analyze the effect of mass flow rate on heat transfer and Nusselt number characteristics in solar air heater. Experiments are performed at different air mass flow rates; varying from 0.012 to 0.016 kg/s, about hot summer days of Mai 2012. Hourly values of global solar radiation and some meteorological data (temperature, wind speed, relative humidities, etc.) for measuring days are obtained from the Biskra city of Algeria. The experiments encompassed the flow Reynolds number in the range 965.48–1301.4. Longitudinal fins were used inferior the absorber plate for an increase the heat exchange and render the flow fluid in the channel uniform. The effects of mass flow rate of air on the outlet temperature, Nusselt Number, Reynolds Number, Prandtl Number, the heat transfer in the thickness and length of the solar air collector were studied. For this effect was have created a new correlation correspondent of solar air collector with using fins it was written Nu = κ₀Re^1.36Pr^?0.68exp(0.342m)h ^[?0.018Pr].     

Analysis of exergetic performance for a solar air heater with metal foam fins

In this experimental study, an exergetic analysis is presented for solar air heaters (SAHs) that have absorber plates equipped with different fin arrangements. The following two types of fins were used: solid fins (conventional) and metal foam fins. Longitudinal, staggered, and corrugated fin arrangements in SAHs were investigated under the weather conditions experienced in Baghdad, Iraq in February to April 2018. The exergy efficiency and exergy loss of the SAHs were evaluated for five air mass flow rates ranging from 0.011 to 0.059 kg/s. Based on the exergy analysis, SAHs with metal foam fins are more efficient than those with solid fins. In addition, corrugated metal foam fins introduce more turbulent flow than the other fin configurations. It was found that the exergy loss and the exergy efficiency were directly proportional to the values of solar irradiance and air mass flow rates. At solar noon, the maximum values for exergy change were 127 and 89 W/m² for air mass flow rates of 0.011 and 0.059 kg/s, respectively.     

Thermal performance of an air solar collector with an absorber plate made of recyclable aluminum cans

The present paper describes the development and testing of an efficient single-glass air solar collector with an absorber plate made of recyclable aluminum cans (RAC). This collector was designed as a proposal to use recycle recyclable materials to build absorber plates of air solar collectors at an acceptable cost. The absorber plate of the collector consisted of eight circular cross section air flow channels of 128 recyclable aluminium cans. Each channel was built with 16 recyclable cans blackened with common opaque black paint of 0.903 absorptance and 0.097 reflectance. The design parameters to determine the size of the collector were obtained by implementing a simulation model for double flow air solar collectors. Also, to determine the appropriate configuration for a uniform air flow distribution inside the eight RAC air channels, a hydrodynamic numerical study was carried out. The RAC air solar collector designed and built was tested outdoors following the ASHRAE 93-86 standard to determine the time constant, the thermal efficiency and the incidence angle modifier. Comparison between the predicted theoretical temperatures and the measured ones were in good agreement. Comparison between the thermal efficiency of the RAC air solar collector with the ones reported in the literature is presented.     

Collector efficiency of upward-type double-pass solar air heaters with fins attached

The collector efficiency of upward-type double-pass flat plate solar air heaters with fins attached and external recycle is investigated theoretically. The double-pass device was constructed by inserting the absorbing plate into the air conduit to divide it into two channels (the upper and lower channels). The double-pass device introduced here was designed for creating a solar collector with heat transfer area double as well as the extended area of fins between the absorbing plate and heated air. Moreover, the advantage of external recycle application to solar air heaters is the enhancement of forced heat convection strength, resulting in considerable device heat transfer performance improvement. This advantage may compensate for the remixing at the inlet which decreases the heat transfer transfer-driving force decrement (temperature difference).     

Optimization of short micro pin fins in minichannels

Finned minichannels are modeled in order to optimize microstructure geometry and maximize heat transfer dissipation through convection from a heated surface. Six pin fin shapes – circle, square, triangle, ellipse, diamond and hexagon – are used in a staggered array and attached to the bottom heated surface of a rectangular minichannel and analyzed. Also, using square pin fins, different channel clearance over fins are investigated to optimize the fin height of the fins with respect to that of the channel. Fin width and spacing are investigated using a ratio of fin width area to the channel width. Fin material is then varied to investigate the heat dissipation effects. Triangular fins with larger fin height, smaller fin width, and spacing double the fin width maximizes the number of fins in each row and yields better performance. Correlations describing the Nusselt number and the Darcy friction factor are obtained and compared to previous ones from recent studies. These correlations only apply to short fins in the laminar regime. Completely understanding the effects of micro pin fins in a minichannel is essential to maximizing the performance in small scale cooling apparatuses to keep up with future electronic advancements.     

Thermal performance of in-line diamond-shaped pin fins in a rectangular duct

This work experimentally studied the pressure drop and heat transfer of an in-line diamond-shaped pin-fin array in a rectangular duct by using the transient single-blow technique. The variable parameters are the relative longitudinal pitch (X_L = 1.060, 1.414, 1.979) and the relative transverse pitch (X_T = 1.060, 1.414, 1.979). The empirical formula for the heat transfer is suggested. Besides, the optimal inter-fin pitches, X_T = 1.414 and X_L = 1.060, are provided based on the largest heat dissipation under the same pumping power.     

Thermohydraulic performance of solar air heaters with roughened absorber plates

Artificial roughness has been found to enhance the heat transfer from the absorber plate to the air in a solar air heater duct. However, this improvement is invariably accompanied by increased pumping power. In this work, the effect of roughness and operating parameters on the thermal as well as the hydraulic performance of roughened solar air heaters is discussed and the thermohydraulic performance of roughened solar air heaters is compared with that of conventional smooth solar air heaters. 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.     

Thermal analysis and design of a volumetric solar absorber depending on the porosity

The thermal evaluation of different absorber configurations for a volumetric solar receiver designed for a solar furnace has been carried out by means of commercial Computational Fluid Dynamics (CFD) software in a 2D numerical model. Simulation results for proposed configurations depending on the porosity are discussed and compared to find the optimum configuration for which flow instabilities and thermal stresses are minimized and higher efficiencies are reached. The results obtained from the comparison of air velocity and thermal profiles at the absorber outlet propose a gradual-porosity configuration as an alternative to a previous design of a porous silicon-carbide honeycomb structure in order to heat an air stream up to temperatures suited for several high-temperature industrial processes.     

Thermal performance analysis and optimum design parameters of heat exchanger having perforated pin fins

This paper reports the heat transfer enhancement and corresponding pressure drop over a flat surface equipped with circular cross section perforated pin fins in a rectangular channel. The channel had a cross section area of 100–250 mm². The experiments covered the following ranges: Reynolds number 13500–42,000, clearance ratio (C/H) 0, 0.33 and 1 and interfin spacing ratio (S_y/D) 1.208, 1.524, 1.944 and 3.417. Correlation equations were developed for the heat transfer, friction factor and enhancement efficiency. The experimental results showed that the use of circular cross section pin fins may lead to heat transfer enhancement. Enhancement efficiencies varied between 1.4 and 2.6 depending on clearance ratio and interfin spacing ratio. Using a Taguchi experimental design method, optimum design parameters and their levels were investigated. Nusselt number and friction factor were considered as performance parameters. An L₉(3³) orthogonal array was selected as an experimental plan. First of all, each goal was optimized separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be Reynolds number of 42,000, fin height of 50 mm and streamwise distance between fins of 51 mm.     

Heat transfer enhancement of concentrated solar absorber using hollow cylindrical fins filled with phase change material

A concentrated solar absorber with finned phase change materials was experimentally studied using a Scheffler type parabolic dish concentrator. The absorber's inner surface was fixed with hollow cylindrical containers filled with phase change material (PCM) for heat transfer augmentation. The absorber's selected PCM was acetanilide (Melting point of 116 °C)—the cylindrical capsules protruding into the fluid side to create turbulence and mixing and acting as fins. The absorber surface temperature was observed to be about 130–150 °C during the outdoor tests while passing fluid through the absorber. The fluid flow rate varied from 60 to 100 kg/h during the outdoor experiments. The peak energy and exergy efficiency of parabolic dish collector (PDC) at the fluid flow rate of 80 kg/h with PCM integrated solar absorber was found to be about 67.88% and 6.96%, respectively. The integration of cylindrical PCM containers resulted in more heat transfer augmentation in the solar absorbers. The optimized solar absorber could be suitable for various applications like steam generation, biomass gasification, space heating, and hydrogen generation.     

Heat transfer enhancement from micro pin fins subjected to an impinging jet

An experimental investigation was carried out to study the single-phase stagnation point jet impingement heat transfer on smooth and micro pin fin structures using water and R134a. The experiments were carried out for a single jet (d_j = 2.0 mm) impinging on a 2 × 2 mm micro-heater over a wide range of Reynolds numbers. Both an unfinned and a micro structured impingement surfaces were investigated. The micro structures consisted of an array of 64 circular micro pin fins fabricated using MEMS microfabrication. The micro pin fins had diameters of 125 μm, heights of 230 μm, and pitches of 250 μm with an area enhancement of A_total/A_base = 2.44. The jet stand-off ratio and area ratio (A_j/A_base) were 0.86 and 0.785, respectively. Nusselt numbers were found to increase with increasing Reynolds numbers. Correlations from the literature for impingement zone Nusselt numbers were found to underpredict the experimental results. Significant enhancement of the heat transfer coefficients were observed as a result of the presence of the micro pin fins on the impingement surface. Enhancement factors as high as 3.03 or about 200% increase in the heat transfer coefficients were demonstrated. Enhancements are attributed to flow mixing, interruption of the boundary layers, and augmentation of turbulent transport.