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In this article, thermal performance of a solar air heater (SAH) having broken arc-shaped ribs has been investigated experimentally, and then, performance parameters have been optimized. Study reveals that the use of artificial roughness enhances both heat transfer rate and pumping power requirement, and hence, the overall performance of solar air heater increases. Thus, it is imperative to optimize the performance-defining criteria viz. Nusselt number enhancement ratio, friction factor enhancement ratio, and thermohydraulic performance parameter, to propose an optimal set of flow and roughness parameters. The hybrid entropy-VIKOR technique has been employed to outline the optimal set of parameters using performance-defining criterions, to obtain maximum profit with the minimum incurred cost. The optimal set of parameters attained using entropy-VIKOR approach among all the alternatives is as follows: relative gap width = 4, relative roughness pitch = 10, and arc angle = 45 deg at Reynolds number = 9000. [ABSTRACT FROM AUTHOR]

This article presents the heat transfer augmentation in a solar air heater (SAH) using arc-shaped ribs with multiple gaps. Multiple gaps in arc-shaped ribs are used to enhance the heat transfer by interrupting the growth of the boundary layer. Thermal performance analysis of a SAH has been carried out experimentally for the roughness parameters: number of gaps (Ng) ranging from 2 to 5 and flow parameters; Reynolds number (Re) ranging from 3000 to 18000 at a constant heat flux of 1000 W/m2. The data obtained from the experimental investigation are validated and compared with the smooth duct under similar conditions. For roughened SAH duct, the heat transfer and pressure drop are found to be 274% and 169% higher than that for smooth SAH duct at Ng = 3. Thermo-hydraulic performance is determined for considered parameters and maximum value is observed to be 2.75. [ABSTRACT FROM AUTHOR]

Thermal performance of ground-air heat exchanger (GAHE) depends on the rate of heat transfer between air and soil, which is governed by thermal properties of soil surrounding the GAHE pipe. Soil thermal properties around GAHE pipe can be improved either by increasing its moisture contents or by using some thermally enhanced backfilling materials. In the present study experimentally investigates the thermal performance of ground-air heat exchanger system using a sand-bentonite mixture (dry as well as wet) and compare their performance with the ground air heat exchanger system having native soil (dry as well as wet) as backfilling material. The study acknowledges the highest cooling capacity (125 W) for GAHE with wet sand-bentonite as backfilling material, and after 6 h of continuous operation, it is 38.4%, 18.4% and 11.1% higher than that obtained with dry native soil, dry sand-bentonite and wet native soil, respectively. The study also revealed that thermal performance deterioration factor (TPDF) increases with the duration of the operation. At airflow velocity of 5 m/s, after 6 h of continuous operation highest TPDF is noticed for ground-air heat exchanger with dry soil (0.22); whereas, lowest TPDF is observed for ground-air heat exchanger system with wet sand-bentonite (0.07). Schematic of laboratory experimental setup of GAHE system. Image 1 • Development of a laboratory scale experimental set up for GAHE system. • To evaluate the effect of sand-bentonite backfilling material (dry as well as wet) on performance of GAHE system. • Maintaining a certain level of moisture into the soil in the close vicinity of GAHE pipe. • Better thermal performance of GAHE due to BFM resulting reduction in required pipe length. [ABSTRACT FROM AUTHOR]

Solar air heater (SAH) is a popular and economical device which collects the solar energy and is employed for space heating, drying of agricultural products, food items, and leather, seasoning of timber, etc. Attachment of artificial roughness on the absorber surface is an appropriate method to augment the heat transfer from the heated surface to the air flowing through SAH duct. Use of artificial roughness has been an area of great interest for researchers as far as the cooling of turbine blades, and combustion chambers are concerned. This technique is effective in enhancing the heat transfer in a micro-channel heating/cooling system. Present paper holistically furnishes concise information about various kinds of V-shaped roughness geometries used in SAHs for improving its performance covering experimental, analytical, numerical and computational fluid dynamics (CFD) approaches. In this paper, 124 research articles have been referred, which provide a detailed comprehensive and comparative study revealing the effect of various geometrical parameters and different V-shaped roughness patterns on the performance of SAH. This article also brings in the information about the correlation developed by researchers for heat transfer and friction factor. This comprehensive review will be quite useful for technical persons and researchers working in the area of SAH. [ABSTRACT FROM AUTHOR]

A comparative experimental study of two box-type solar cookers with different cooking environments was conducted. The first one with natural convection heat transfer-based and the second one with forced convection heat transfer (fan for flow augmentation)-based environment. The experimental results show that the forced convection-based environment remarkably improves the thermal performance of box-type solar cooker. Forced convection-based cooker is able to cook food at much faster rates compared with natural convection-based cooker. This reduces the cooking time considerably. Results of thermal performance tests show (i) stagnation absorber plate temperatures of 133.9°C and 119.3°C and (ii) boiling times of 52 min (3120 s) and 75 min (4500 s) for heating 1 kg of water, for the cooker with and without fan, respectively. The solar cooker performance has been rated using the first figure of merit (F1) on the no-load test and the second figure of merit (F2) on the sensible heat tests. The cooking time was considerably reduced by 30.6%. The tests were carried out on the research unit in Government Engineering College Ajmer, Rajasthan located at 26°27′ N latitude and 74°42′ E longitude in India. [ABSTRACT FROM PUBLISHER]

Large-scale adaptation of solar air heating in industries and agro-processing will lead to clean energy processing as well as reducing the production cost for these industries. The solar air heater uses the principle of the greenhouse effect to heat air through the collected heat in the absorber. Among the various techniques employed by the researchers to augment heat transfer, the addition of artificial roughness elements/fins/corrugations on the heated surface is the promising one for heat transfer augmentation. In the present work, the effect of broken inclined ribs with rectangular cross-section on heat transfer and friction characteristics of the equilateral triangular passage duct has been analyzed using computational fluid dynamics. The effect of roughness parameters, viz., relative gap position and relative gap width, is being investigated for the Reynolds number (Re) ranging from 4000 to 18,000. The values of relative gap position (d/W) and relative gap width (g/e) are varied from 0.16 to 0.67 and 0.5 to 2, respectively, while a constant heat flux is supplied on the absorber side, other surfaces being insulated. The Nusselt number increased up to 2.16 times by using broken ribs than that of the smooth duct at d/W = 0.25 and g/e = 1. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents results of a theoretical study carried out to investigate the effect of flow maldistribution, caused by the manufacturing imperfections and tolerances, on the thermal efficiency of a solar air heater array with subcollectors in parallel. The air mass flow rate, ambient temperature, solar insolation and wind heat transfer coefficient have been systematically varied to study the effect under a wide range of these parameters. The collector length, duct height, and plate emissivity were also changed to study their effect. It has been found that the maximum reduction in thermal efficiency due to flow maldistribution is less than about 3% for an array with a commercial grade finish of duct surfaces and ±10% manufacturing tolerance for the duct height. [Copyright &y& Elsevier]