Double Pass Solar Air Heater Research Articles

Double pass solar air heater with aerofoil and bio-inspired fins: A numerical analysis of thermohydraulic performance

In the present study, a numerical flow analysis is performed on a double pass solar air heater (DPSAH) with a bottom plate surface roughened using aerofoil fins to study its thermohydraulic performance. The experimentally validated DPSAH model was studied considering factors like different fin shapes, diurnal variation of solar radiation, absorber plate material, and the flow conditions at different relative height ratios (h/H) between 0.17 and 0.50 and fin length ratio (l/H) of 0.29. Using the RNG k-ε turbulence model, the relative Nusselt number (Nu/Nus), relative friction factor (f/fs), and thermohydraulic performance factor (THPF) were found for Reynolds number between 3000 and 21000 with an increment of 3000. The maximum enhancements of 4.23 and 2.51 times were achieved in the Nu/Nus and THPF values, respectively, for a Re value of 3000 in a 2-row setup with h/H of 0.50. The DPSAH with bio-inspired fins performed better than optimized aerofoil fins for majority of the tested range even with similar increase in effective heat transfer area. The values predicted using correlations developed in this study and numerically obtained values of the Nusselt number and friction factor matched closely with an extreme deviation of 5 % in f values.

Solar air heater energy and exergy enhancement using a v-corrugated wire mesh absorber: An experimental comparison

In this paper, the energy and exergy performance of a double-pass solar air heater (SAH) using a novel model of a v-corrugated wire mesh absorber (CM-SAH) is investigated experimentally. The v-corrugated wires are adjacent to each other in one layer along the airflow direction to reduce the hydraulic resistance. Simultaneously, this arrangement increases the surface absorptance rate of solar radiation and expands the heat exchange area. To evaluate the performance of CM-SAH, a standard SAH having a v-corrugated absorber plate (CP-SAH) and the same configuration dimensions is compared with CM-SAH. The comparison is achieved under the same operating conditions and using different air mass flow rates of (0.01, 0.02, and 0.03 kg/s). The results show that the CM-SAH outperformed the CP-SAH in terms of thermal efficiency, exergy efficiency, and output air temperature. The maximum thermal efficiencies reach 87.3 % and 65.1 %, while the maximum exergy efficiencies reach 4.4 % and 3.4 %, for CM-SAH and CP-SAH, respectively. In addition, the thermal efficiency of CP-SAH decreased after midday while that of CM-SAH continuously increased.

Prediction of thermal efficiency of double pass solar air heater having discrete multi V and staggered ribs

AbstractThis manuscript endeavors to elucidate a comprehensive analysis for the assessment of thermal efficiency in double pass solar air collectors augmented with fabricated roughness, instrumental in the generation of heated air for applications in heating and drying. The analytical procedure delineates comparisons between a smooth and a double‐sided, roughened absorber, characterized by discrete, multi V and staggered configurations of roughness. The analysis was performed for different roughness parameters including relative roughness width (W/w) from 4 to 8 and relative staggered rib size (r/e) from 1 to 4 and relative rib pitch (p′/p) from 0.2 to 0.8 while other parameters were kept constant. The research scrutinizes the impact of a constellation of parameters: the temperature rise coefficient (ΔT/I, varying between 0.002 and 0.02 K‐m²/W), Reynolds number (Re, spanning 2000–20,000), solar irradiance (I, ranging from 600 to 1000 W/m²), and the geometric variables of the artificial roughness, on both the thermal efficiency and efficiency enhancement factor of the collector. The study revealed that the roughened collector exhibits higher thermal efficiency compared to smooth collector for a similar condition. The extreme enhancement in thermal efficiency of the collector with roughness has been found to be 56.75% more than nonroughened collector for Re = 2000. A further observation was made that thermal efficiency has increased sharply for lower flow rate (Re < 10,000) whereas for higher flow rates (Re > 10,000), this increase becomes nearly asymptotic. In addition, the efficiency enhancement factor has also been found to increase with an increase in ΔT/I. A peak value of 2.321 has been found to be obtained for the efficiency enhancement factor for ΔT/I = 0.02 K‐m2/W and I = 1000 W/m2 as a result of the studies conducted.

Assessment of Perforated Double-Pass Solar Air Heater Using Parametric Interaction Analysis for Performance Enhancement in Solar Thermal Systems

تعتبر مشكلة الكفاءة القليلة لسخانات الهواء الشمسية عائقاً في تسويق الأنظمة الحرارية الشمسية. يتأثر أداء سخانات الهواء الشمسية بشكل مباشر بتغيير مستوى العوامل المؤثرة. يهدف هذا العمل الى تحديد العوامل المثلى عملياً من أجل تحسين أداء سخان هواء شمسي مثقب ذي الجريان المزدوج. تمت مناقشة تداخل العوامل المتعلقة بأداء سخان هواء شمسي مزدوج الجريان ذي صفيحة امتصاص مثقبة بثقوب دائرية ذات توزيع متداخل بناءً على طريقة تصميم التجارب. تمت دراسة عدد رينولدز بقيمة 10000 الى 30000 ونسبة التثقيب بقيمة 3 إلى 7 كعوامل تصميمية للنموذج بينما تم اعتماد فرق درجة الحرارة، وكمية الحرارة المكتسبة، والكفاءة الحرارية كاستجابات لنموذج الارتباط. تم الحصول على ثلاثة ارتباطات مقابلة لكل استجابة مع شكلين لكل منهما (الفعلي والمشفّر) من خلال تحليل طريقة تصميم التجارب هذه. بينت النتائج أن نسبة التداخل كانت 193٪ للكفاءة الحرارية و148٪ لكمية الحرارة المكتسبة، بينما لم تتعد 18٪ لفرق درجات الحرارة. عند المستوى الأعلى لعدد رينولدز، تم تسجيل زيادة بنسبة 12.36٪ في الكفاءة الحرارية بسبب التثقيب على عكس سخان الهواء الشمسي غير المثقوب. يغير عدد رينولدز تأثيره على الكفاءة الحرارية بشكل كبير عندما يتغير مستوى التثقيب.

Heat transfer and fluid flow analysis of PCM-based thermal energy storage concept for double pass solar air heater

Solar air heaters (SAHs) are flat plate collectors used for drying applications without causing any negative impact to the environment. However, SAH suffers from two drawbacks: first is the low convective heat transfer due to formation of boundary layer along the absorber plate, and second is the low thermal efficiency resulting from the lack of solar energy available after sunshine hours. The present study aims to examine the heat transfer and friction factor characteristics of double-pass SAH using two composite roughness geometries: perforated baffles and semicylindrical PCM tubes. The study is conducted at design parameters: blockage ratio (eb/Hd) of 0.4 to 0.8 and tube radius ratios (rt/Hd) of 0.2 to 0.5 under different Reynolds numbers (Re) from 3000 to 13,000. The maximum Nusselt number (Nu) and friction factor (f) are determined to be 200 at Re of 13,000 and 0.629 at Re of 3000, respectively, for the constant value of rt/Hd as 0.3 and eb/Hd as 0.8. In the range of parameters analyzed, maximum increase in Nu and f is determined to be 5.08 times and 65.80 times, respectively, over the smooth duct. Furthermore, correlations are derived for Nu and f with mean absolute error of 4.7% and 3.8%, respectively.

Effect of channel depth ratio and absorber plate configuration on performance of a solar air heater

An experimental evaluation of the effect of channel depth ratio and absorber plate configuration on the performance of a baffled counter flow double-pass solar air heater (BCDSAH) is main objective of current work. The BCDSAH was constructed so as to permit adjustments of channel depth ratio (CDR) to 2, 3 and 4. During experimentation, the lower channel depth was kept at 50 mm while the upper channel depth was adjusted to 100 mm, 150 mm and 200 mm, yielding various channel depth ratios. The assessments were conducted for three configurations of absorber plate: (i) flat absorber plate, (ii) baffled absorber plate and (iii) baffled absorber plate with phase change material (PCM). The performance of the BCDSAH was evaluated at an air mass flow rate of 0.07 kg/s for temperature rise, useful heat gain (UHG), energy efficiency, and exergy efficiency. For CDR = 2 and a baffled absorber plate with PCM, the obtained average values of temperature rise, UHG, energy efficiency and exergy efficiency were 15.3 °C, 1075.2 W, 76.2 % and 2.7 % respectively. For the baffled absorber plate with PCM, increases of 8.5 %, 9.4 %, 10.1 % and 10.9 % were obtained in temperature rise, UHG, energy efficiency and exergy efficiency respectively for CDR = 2 compared to CDR = 4. Finally, a value of CDR = 2 and a baffled absorber plate with PCM are suitable for applications like drying of agricultural products and space heating.

Thermal performance analysis of a double-pass solar air heater with lava rock as porous and sensible heat storage material

This study investigates the utilization of lava rock as a sensitive heat storage material in a double-pass solar air heater. Three configurations were examined: (i) Double-pass solar air heater without the lava rock, (ii) Double-pass solar air heater with a 50 % lava rock packed bed, and (iii) Double-pass solar air heater with a 100 % lava rock packed bed. The experiments were performed indoors under the solar simulator with solar irradiances ranging from 590 to 1000 W/m2 with varying mass flow rates of 0.02, 0.06, and 0.12 kg/s. Results indicate a significant improvement in thermal efficiency with the addition of lava rock. At 1000 W/m2 solar irradiance, the configurations (i), (ii), and (iii) exhibited thermal efficiencies ranging from 18.2 % to 65.02 %, 25.27 % to 72.17 %, and 26.28 % to 76.41 %, respectively. Lava rock's integration into the double-pass solar air heater significantly lowered the temperature of the absorber plate as compared to the conventional double-pass solar air heater, showcasing the thermal storage properties of the lava rock. Under solar irradiance of 1000 W/m2 and a mass flow rate of 0.06 kg/s, the temperature of the absorber plate dropped from 91.3 °C in configuration (i) to 80.0 °C in configuration (ii), a decrease of 12.56 %, and to 71.4 °C in configuration (iii), a decrease of 21.74 %, respectively. The configuration (iii), utilizing a 100 % lava rock packed bed, exhibited the most enhanced thermal performance, representing the potential of lava rock as a sensible heating storage material in double-pass solar air heaters for improved heat retention and utilization under varying solar conditions. Moreover, the use of a double-pass solar air heater with lava rock results in a consistent and even distribution of temperature throughout the heater.

Investigation on energy, economic, and environmental aspects of double-pass solar air heater.

Numerous research studies have found that a double-pass solar air heater (DPSAH) performs better than a single-pass solar air heater (SAH). This suggested study aims to evaluate the performance of a DPSAH setup in Southern Tamil Nadu, India. Several artificial roughness features have been incorporated into the solar black-coated absorber plate for this examination. Broken ribs with semi-circular and semi-polygonal shapes are used and explicitly tested on the absorber plate. Next, the efficiency of these rib designs is contrasted with a typical flat plate DPSAH. These studies also employ three different mass flow rates (0.01kg/s, 0.02kg/s, and 0.03kg/s), enabling a thorough assessment of the DPSAH system's performance at each rate. These studies' findings demonstrate that adding artificial roughness to the solar collector plate has a beneficial effect on the turbulence of fluid flows. As a result, this innovation increases the double-pass solar air heater's (DPSAH) heat transfer rate. It is noteworthy that compared to the DPSAH with flat plates, both rib designs perform better. It is important to remember that the semi-polygonal ribs function better than the semi-circular ones. The average efficiency values for the semi-polygonal rib structure are 17.1%, 18.7%, and 19.1% greater than those seen for flat plates. These efficiency values are additionally 4.4%, 7.4%, and 8.7% higher than those attained with the semi-circular rib topologies at flow rates of 0.01kg/s, 0.02kg/s, and 0.03kg/s, respectively. The study goes into considerable detail on how particular rib patterns can be advantageous economically and environmentally.

Theoretical and experimental investigations on the effect of double pass solar air heater with staggered-diamond shaped fins arrangement

This study presents the research study, both theoretical and experimental, on the effect of double pass solar air heater (DPSAH) using staggered-diamond shaped fins effects. Diamond shaped fins are considered to have the potential to improve the convective heat transfer and increase energy efficiency as well. Forty number of diamond shaped fins have been used in this experiment which have been fixed to the plate absorber where there are four mass flow rates (0.0104 kg/s, 0.0157 kg/s, 0.0209 kg/s, 0.0261 kg/s) used and three variations of solar radiations (600 W/m2, 800 W/m2 and 1000 W/m2). The effect of mass flow rates, solar radiation and outlet temperature has greatly affected the overall performance of DPSAH with staggered-diamond shaped fins. For the computational domain fluid, the configuration of mass flow rates and solar radiation is analyzed using COMSOL Multiphysics 5.6.1, hence the actual experiment has been carried out based on the simulation results. The highest thermal efficiency obtained for experiment and simulation were 59.34% and 62.01% for mass flow rates of 0.0261 kg/s using solar irradiance of 1000 W/m2.

Thermal and dynamic improvement of a double pass solar air heater for drying applications

The low efficiency of solar thermal systems is a drawback that hinders the effective utilization of renewable energy. In this context, a feasibility study was conducted on a double-pass solar air heater designed for solar drying applications. Given that low efficiency often results from inefficient thermal conversion, an optimization of the V-shape absorber was performed by varying the corrugation angle. The CFD method, validated by experimental results, was employed in the current study. The meteorological station located at Mohamed First University in Morocco provided real and unstable weather conditions that were inserted into the software. The results obtained were analyzed and discussed regarding the air domain temperature, collector outlet temperature, collector efficiency, and average drying tray temperature together with its distribution. The findings show that the average air temperature and outlet temperature reach a maximum of 345.53 k and 365.5 k, respectively. For absorber corrugation angles of Φ=100°, a maximum collector efficiency of 86% was achieved. In addition, different mass flow rates (MFRs) ranging from 0.003 to 0.068 kg/s were evaluated in this study. From the results, it was noticed that the optimal MFR varies depending on the products to be dried, with values of 0.028, 0.035, 0.049, and 0.068 kg/s for sweet potato, fig, cauliflower, and orange, respectively.

Red pepper drying with a double pass solar air heater integrated with aluminium cans

In this paper, an experimental evaluation of a newly developed flat plate double pass solar air heater combined with aluminum cans for drying red pepper was presented. The proposed solar dryer system was designed, modeled, and evaluated. Solar air heater trials were carried out using the absorber’s top and bottom plate and aluminum cans for red pepper drying at Bahir Dar, Ethiopia. To test the solar dryer, 100 pieces of red paper were obtained from the Bahir Dar region of Ethiopia for the purpose of experimentation. Microsoft Excel was used to perform statistical analysis of eleven mathematical models. The results show that the mixed-mode solar greenhouse dryer takes less time to dry red pepper than the open solar dryer. In the midday, the solar insolation reached 973 W/m2 and the minimum solar insolation was 220 W/m2 and air is expelled at a rate of 0.0383 kg/s. According to the experimental results, the dryers chamber temperature ranged from 30.9 to 54 °C, while the ambient temperature was between 22.6 and 28.2 °C. The mixed-mode double pass achieves up to 46% and 28% efficiency when used with aluminum can dryers and conventional open sun dryers, respectively. A drying rate of 0.0003395 kg/s was achieved for the open sun dryer system and 0.0000365 kg/s for the mixed mode solar dryer. Using mixed-mode and open-sun solar dryers, the logarithmic model was found to be most effective in explaining the red pepper behavior. Furthermore, a comparison was made between the experimental and predicted moisture ratios through the calculation of the coefficient of determination (R2), the reduced chi-square (X2), and the root mean square error (RMSE). The results show that the logarithmic model achieved the highest value of the correlation coefficient (R2), which was determined to be 0.9978 and 0.9989, while the logarithmic model achieved the lowest value of Chi-square (X2).

Smart optimization and investigation of a PCMs-filled helical finned-tubes double-pass solar air heater: An experimental data-driven deep learning approach

Solar Air Heaters (SAHs), widely utilized for space heating and drying in residential and commercial settings, exhibit performance dependent on several operational parameters, including solar radiation, ambient air temperature, exposure time, and temperature absorbance. Addressing these variables, a Deep Neural Network (DNN) was developed to predict the heat output of a solar air heater system. The research included a double-pass SAH equipped with circular and semi-circular fin-based aluminum tubes, filled with RT44HC and RT18HC as energy storage materials. Parametric data from three different heater configurations were collected to train and test the model, which yielded accurate heat output predictions (R2 = 0.995, RMSE = 0.018). The developed DNN model was then combined with a Genetic Algorithm (GA) to determine the optimal parameter combinations that achieved an efficiency of 76.328 %, surpassing the average efficiency of 64.51 % obtained without optimization. Furthermore, an Explainable AI (XAI) approach was utilized to analyze the influence of each input parameter on the system's heat output. The results demonstrated that temperature rise was the most influential factor impacting heat output, followed by absorber plate temperature, heat input, exposure time, ambient air temperature, and solar radiation. The dependence analysis further explains the relationship between input variables and how the interaction between the input parameters can affect the output, providing valuable insights for optimizing solar air heater systems.

Effect of the partial blockage in the exit of the mixing channel on thermo - Hydraulic performance of the multi – Pass jet plate solar air heater

The present experimental work deals with the effect of partial blockage in the exit of the mixing channel on thermo-hydraulic performance of multi-pass jet plate solar air heater. This investigation is performed at IIT(ISM) Dhanbad campus (India) to obtain the results in terms of outlet temperature, collector efficiency, Temperature rise parameter (TRP), thermo - hydraulic performance parameter (THPP) and Nusselt number for double–pass cross and non–cross flow inline holes Jet Plate Solar Air Heater (JPSAH). Results for different ranges of operating and geometrical parameters such as, mass flow rate = 0.10–0.14 kg/s, Reynolds number, Re = 4741–10632, daily average solar flux incident on glass cover, Itavg. = 611–1113 W/m2 and blockage ratio (H/Z2) = 0, 0.33, 0.66 are presented. This work also compares the present results with simple double – pass solar air heater. It is found that the maximum percentage increments in THPP and Nu are 37 % and 79 % respectively at Re = 10247 and 9500 for BR = 0 compared to non-cross flow and simple double pass solar air heater. Based on the experimental data, correlations for the Nusselt number, friction factor and THPP have been developed.

Thermo-hydraulic performance investigation of double pass solar air heater integrated with PCM-based thermal energy storage

The solar air heater (SAH) is one of the ways to utilize solar energy for the purpose of drying, greenhouse heating and air conditioning. However, the SAHs are unable to deliver reliable thermo-hydraulic performance as a result of the intermittent nature of solar energy and the low convective heat transfer coefficient. Thus, in the present study, a three-dimensional numerical investigation is performed to study the thermo-hydraulic performance of the double pass solar air heater (DPSAH) having semi-cylindrical PCM tubes and perforated baffles as roughness geometry. The parameters selected for this study include baffle height ratio (eb/H) from 0.4 and 0.8, tube radius ratio (rt/H) from 0.2 and 0.5, and Reynolds numbers (Re) from 3000 and 13,000. Experimental validation has been conducted to verify the numerical results, and it has been found to be well-validated with mean absolute errors of 6.5 % and 1.2 % for the Nusselt number (Nur) and friction factor (fr), respectively. The maximum Nusselt number (Nur) is found to be 200.08 at Re of 13,000, while the maximum friction factor (fr) is measured to be 0.6290 at Re of 3000, which corresponds to eb/H of 0.8 and rt/H of 0.3. The maximum heat transfer enhancement (Nur/Nus) in comparison to the smooth duct is found to be 5.08 times with a maximum friction factor increment (fr/fs) of 65.80 times, which corresponds to eb/H of 0.8, Re of 13,000 and rt/H of 0.3. The maximum thermo-hydraulic performance parameter (THPP) is observed to be 1.83 at Re of 13,000, rt/H of 0.2 and eb/H of 0.4. This study contributes to the design and optimisation of SAHs by incorporating thermal energy storage and roughness geometries.

Thermal Performance of Double Pass Solar Air Collector: Numerical and Experimental Investigation

Abstract In this research study, the development of a theoretical model to estimate the efficiency of a double pass solar air heater (DPSAH) was the main objective. The mathematical model was compared to experimental results and found to be in agreement, both qualitatively and quantitatively. The model used energy balance equations to determine the outlet temperature and could solve the temperature equations of each component of the heater simultaneously. A matrix inversion method was used for this purpose. The study found that increasing the Nusselt number led to an increase in energy efficiency, but also caused a drop in outlet temperature at any solar radiation. The mass flow rate (MFR), with larger flow rates resulting in better efficiencies, had a significant impact on the double pass air heater's efficiency. Thus, increasing of DPSAH’s length increases its thermal performances and reach its maximum values for L=9m. These findings could be beneficial in selecting the most efficient design parameters for DPSAH.

Investigation on solar drying system with double pass solar air heater coupled with paraffin wax based latent heat storage: Experimental and numerical study

Since solar energy uses the sun's free energy and is environmentally friendly, it is generally accepted for various applications like drying products. However, the drawback of this system is the inconsistent and unpredictable nature of solar radiation. The best solution for the above said problem is using thermal energy storage. In this study forced convection indirect solar drying system that has a double pass solar air heater (DPSAH), shell and tube type latent heat storage (LHS) with paraffin wax as phase change material (PCM), blower and drying chamber is developed and tested. Numerical study is conducted on the DPSAH to determine the effect of mass flow rate on its performance and appropriate flow rate is selected for the drying experiment. It was found that outlet temperatures of the DPSAH is higher at lower mass flow rates and lower at higher mass flow rates and mass flow rate of 0.01 kg/s gives the required outlet temperature. Whereas numerical simulation is done on the LHS to predict the charging and discharging behavior of the PCM and found that the storage can reach a fully charged state within 133 min, which can be achieved around the peak hours during the experiment. The performance of the components of the dryer and the overall performance of the dryer is investigated with experiment. From the experimental result, a maximum temperature of 69 °C is obtained at the outlet of the DPSAH and the energy and exergy efficiencies are 25 % and 1.5 %, respectively. The LHS reduces the fluctuations in the outlet temperature of the SAH and extends the drying process for extra 3 h a day. The drying experiment is conducted by drying 5 kg fish and took 21 h to reduce the moisture content of the fish from an initial value of 75 %–12.5 %. The efficiency of the drying chamber was 35 % and the overall efficiency of the dryer was 9.4 %.

Transient analysis and techno-economic assessment of thermal energy storage integrated with solar air heater for energy management in drying

The integration of thermal energy storage (TES) system with solar air heater holds an immense potential for optimising the energy management in drying application. In this pioneer research, a transient thermodynamic model of conical shaped rock bed TES has been developed, and the economic analysis of hybrid V-groove double pass solar air heater (SAH)-TES has been studied, offering a unique perspective on the feasibility and economic viability of hybrid SAH-TES for drying application. By considering the time dependent dynamics of energy demand, the model predicts energy stored and released by the system during charging and discharging with a considerable error of 9.9%, as validated with a pilot scale experimental setup. The numerical and experimental results showed that the stored and recovered energy is highly dependent on the inlet temperature and flow rate of air. Notably, the optimised conical shaped TES has the capacity of storing 15.6 kWh of energy during six-hour charging period enabling the recovery of 33% of the stored energy to power the dryer during the discharge period. Moreover, the pressure drop across the TES observed to be 1306.37 Pa/m at 0.050 kg/s, which quantify the power required for fan to overcome such pressure drop to be 4.22 kW Furthermore, the economic assessment showcased remarkable reduction in energy payback period, dropping from 0.7 years to 0.5 years using hybrid SAH-TES dryer compared to SAH dryer. This demonstrates the superior economic value of the hybrid SAH-TES dryer paving the way for the scalability and widespread adoption of low-carbon drying industry.

Study on the thermal performance of double pass solar air heater with PCM-based thermal energy storage system

The solar air heater (SAH) is an efficient, eco-friendly, and cost-effective way for solar drying, space heating, crop yielding, and HVAC applications. However, SAH has two drawbacks: low heat transfer coefficient and the intermittent nature of solar energy. Different roughness elements were employed to enhance the heat transfer coefficient, while different PCM arrangements were used to increase thermal efficiency and operational hours. Accordingly, the composite geometry of semicircular PCM tubes and perforated blocks are investigated at non-dimensional parameters such as tube amplitude ratios (a/H) from 0.2 to 0.5 and relative blockage height ratios (e/H) from 0.4 to 0.8. In this study, the three-dimensional transient numerical investigation is carried out along with experimental validation. The effect of a/H and e/H on the thermal efficiency, heat storage rate and pumping power are analyzed at a mass flow rate (ṁ) from 0.011 kg/s to 0.047 kg/s. For the range of parameters studied, the maximum thermal efficiency is determined to be 97 %, corresponding to mass flow rate of 0.047 kg/s, a/H value of 0.2 and e/H value of 0.4, while the maximum heat storage rate is determined to be 637 W at a/H value of 0.5, mass flow rate of 0.011 kg/s and e/H value of 0.4. The improvement in thermal performance is considered at the expense of pumping power. Moreover, the maximum pumping power is determined to be 38.9 W at e/H value of 0.8, mass flow rate of 0.047 kg/s and a/H value of 0.3.

Experimental testing of mixed-mode double pass solar air heater integrated with aluminium cans for red pepper drying

A solar air heater that operates in the mixed-mode double pass and an absorber plate has aluminum cans at the top and bottom of the absorber for red pepper drying experiments was conducted in Bahir Dar, Ethiopia climatic conditions. A total dying period of red pepper in a mixed-mode solar greenhouse dryer takes less time than an open solar drying system. From the experiment, the average daily dryer chamber temperature ranges between 39.93 0C and 47.02 0C and the average daily ambient temperature ranges between 24.890C and 27.530C. Solar insolation reached 973 W/m2 when the sun was at mid-day and minimum solar insolation was 220 W/m2 and 0.0383 kg per second of air are expelled. A newly designed mixed-mode double pass, integrated with an Aluminum can dryer and the traditional open sun dryer, achieves an efficiency of 46% and 28%, respectively. In the open sun dryer system and the mixed-mode solar dryer, the drying rates are 0.0003395 kg/s and 0.0000365 kg/s respectively. On eleven mathematical models, Microsoft Excel was used to perform the statistical analysis. Logarithmic models were found to be the best explanation of red pepper behavior in both mixed-mode and open-sun solar dryer systems. These models were tested by comparing the maximum value of the coefficient of determination R2 and the minimum value of reduced chi-square X2 and root mean square error RMSE between experimental and predicted moisture ratios.

Development of Nusselt number and friction factor correlations for double-pass solar air heater duct

A solar air heater exhibits a low heat transfer rate due to the formation of a laminar thermal boundary layer across the absorber plate. Furthermore, the artificial roughness contributes to the intensification of turbulence along the absorber plate, which interrupts and breaks the thermal boundary layer. Keeping in this view, a three-dimensional simulation is performed to investigate the heat transfer and frictional characteristics of a double-pass solar air heater with semicircular tubes and perforated baffles as artificial roughness. This numerical investigation is performed for the parameters considered in the study: tube amplitude ratios (a/H) from 0.2 to 0.5, baffle height ratios (e/H) from 0.4 to 0.8, open area ratios (β) from 13 % to 30 % and Reynolds number (Re) from 3000 to 19000. The experimental testing rig is designed to validate numerical results corresponding to e/H of 0.4, an open area ratio of 30 %, a/H of 0.3, and Reynolds number varying from 3000 to 15000. Numerical results are found to be in good agreement with the experimental results, with a maximum absolute percentage error of 1.82 % and 5.28 % for the Nusselt number (Nur) and friction factor (fr), respectively. The maximum Nusselt number is observed as 274.29 at Reynolds number of 19,000 and e/H of 0.8; however, the maximum friction factor is observed as 0.6290 at Reynolds number of 3000 and e/H of 0.8 at a constant value of open area ratio of 30 % and a/H of 0.3. Within the range parameters studied, the maximum THPP value is reported to be 2.51 at Reynolds number of 3000, β of 30 %, e/H of 0.4, and a/H of 0.3. Further, correlations for the Nusselt number and friction factor in the function of a/H, e/H, open area ratio, and Reynolds number have been derived based on simulation results.