Search

The concentrated solar thermal systems, operating in the medium temperature range of 373–573 K, will be extremely useful for several industrial processes. However, the need for an in-depth understanding of the turbulent heat transfer in parabolic trough absorbers with pure and hybrid nano-oils, including the effect of buoyancy or gravity, is realized. This paper presents the Reynolds-averaged Navier–Stokes (RANS)-based turbulent heat transfer analyses in a 3D, long, straight for Reynolds number from 5000 to 20,000 and discrete heating conditions with different heat flux ratios such as 1, 5, 10, 20, 40, and 50 for pure oil and hybrid nano-oils having 1, 4, and 6% volume concentration of the nanoparticles. The major findings are, (a) gravity-induced anisotropy leads to high and low-speed fluid flows near the lower and upper walls, and temperature redistribution at a plane, which is beneficial, (b) the statistical axial-velocity deviates from the standard logarithmic law at a Reynolds number of 5000, and (c) the ratio of surface-area-averaged Nusselt number between the lower half and upper half of the tube is 4–12. Some important recommendations are (a) the effect of gravity must be included, (b) the local Richardson number may be used for improving the standard logarithmic law for the axial velocity, and (c) Nusselt number correlations are deduced for the upper half surface and lower half surfaces. The findings, albeit for limited parameters, will be useful for improving the heat transfer aspects in the parabolic trough absorber.

The conventional desert cooler is effective for dry seasons and the regenerative evaporative cooler (REC) is an effective device for humid seasons in composite climate zones. Hence, the dual-mode evaporative cooler (a two-in-one device) is an intelligent choice for air conditioning, which can operate in both direct and regenerative modes depending on the seasonal climatic condition. The exergy and economic analyses of this novel device for global climatic conditions are performed to check the suitability in different regions of the world. An experimental prototype of a dual-mode evaporative cooler is developed and tested to validate the simulation model. The effectiveness, coefficient of performance, exergy destruction, exergy efficiency, operating cost, and specific total cost (STC) are evaluated for both (direct and regenerative) modes of operation. The annual and month-wise performances of dual-mode evaporative cooler have been assessed for five cities of international climate zones. The operating cost of both modes is compared by considering electricity charges in different countries. The dual-mode device is compared with the single-mode device as well. The specific cost is similar for both modes in most of the ASHRAE climatic zones. The present study reveals that significant energy and cost savings are possible by using the dual-mode evaporative cooler. Practical application: This article considers the application of a dual-mode evaporative cooler (direct as well as regenerative mode) in different climate zones and, through investigating the exergy and economic performances, allows designers and operators to understand the potential benefits of employing various operating modes in particular climates.

Thermo-hydraulic behavior of the ribbed minichannel heat sink, having nine rectangular-shaped channels, is numerically investigated in the present study by using a multi-phase mixture model. Four different types of ribs (semi-circular, rectangular, triangular and trapezoidal ribs) and two hybrid nanofluids (Al2O3 + TiO2/DI water and Al2O3 + Cu/DI water) with a total nanoparticle volume concentration of 0.1% are considered. Heat transfer coefficient, Nusselt number, pressure drop, friction factor, performance evaluation criterion and thermal performance factor are evaluated for different Reynolds numbers (90–450) or flow rate (0.1–0.5 lpm). The maximum heat transfer coefficient (6690.12 W/m2 K) is observed for semi-circular ribs with Al2O3 + Cu hybrid nanofluid. The performance evaluation criterion is above 1 for all the cases except the plain channel with Al2O3 + TiO2 hybrid nanofluid at a lower flow rate. The combination of the semi-circular ribbed channel with Al2O3 + Cu hybrid nanofluid has a maximum performance evaluation criterion of 1.40 and that with Al2O3 + TiO2 hybrid nanofluid has a maximum thermal performance factor of 1.16. The thermal performance factor has an increasing trend with the flow rate for the semi-circular ribbed channel. The semi-circular ribbed channel shows better performance over other ribbed channels because the working fluid gets stuck and stagnant between two consecutive ribs in other ribbed channels, which leads to higher thermal resistance.

The convective closed-loop solar-assisted heat pump dryer (batch type) has been designed, fabricated and experimentally investigated for both simple heat pump drying (HPD) and solar-assisted heat pump drying (SAHPD) modes. The banana chips have been dried up to moisture content of 11.5% for both modes. The next-generation environmental-friendly refrigerant R1234yf has been used in the heat pump cycle. Various thermodynamic, exergoeconomic and economic performance parameters have been evaluated and compared for HPD and SAHPD. Both exergy and energy efficiencies are found better in the case of SAHPD. The system coefficient of performance, moisture removal from the product, drying rate and specific moisture extraction rate are found better for the SAHPD than the HPD. The average drying rates of HPD and SAHPD are found 0.205 and 0.342 kg/kg min, respectively. The estimated payback period of the SAHPD instead of HPD is 3.9 years. The exergoeconomic factor is found lowest for the expansion device for both HPD and SAHPD with estimated values of 0.1335 and 0.2003, respectively. The components that need more improvement are evaporator and expansion device based on exergoeconomic factor. On the basis of the present study, it can be concluded that the SAHPD system performs better in all respects.

In the present study, a novel compact size convective batch-type solar-infrared assisted heat pump dryer is developed and experimentally investigated for closed air cycle drying of banana chips. Performances of four different operational modes of the system, simple heat pump drying, infrared-assisted heat pump drying, solar-assisted heat pump drying and solar-infrared-assisted heat pump drying are compared. Effects of drying time and product moisture content on various energetic and exergetic performance parameters are investigated. For all operational modes, the banana chips are dried to remove moisture from 83.8% to 11.5% at dryer inlet air velocity of 0.8 m/s. Both energy efficiency (58.5%) and exergy efficiency (24%) are found highest for solar-assisted heat pump dryer. The coefficient of performance is found highest for the simple heat pump dryer. The average moisture extraction rate is found highest (1.1618 kg/h) for solar-infrared-assisted heat pump dryer and lowest for the simple heat pump dryer. The specific moisture extraction rate is found highest for the solar-assisted heat pump drying and lowest in case of infrared-assisted heat pump drying. The present study shows the successful development of the combination of the different drying systems such as heat pump, solar and infrared in a single drying system for faster food chips drying.

In the present study, a batch-type solar-assisted heat pump dryer (SAHPD) has been developed and experimented in both simple heat pump dryer (HPD) and SAHPD modes for closed system drying of banana chips. The effect of drying time on various energy, exergy, economic and exergoeconomic performance parameters has been investigated. The banana chips have been dried to remove moisture from 83.5% to 11.5% at an air velocity of 1 m/s in the drying chamber. Both energy efficiency and exergy efficiency are found higher for SAHPD. The specific moisture extraction rate is higher for SAHPD as compared to simple HPD. On the basis of economic analysis, the payback period for the SAHPD over the simple HPD is found 46 months. The lowest exergoeconomic factor is for the expansion device in simple HPD and SAHPD with determined values as 0.1395 and 0.2053, respectively. Total exergy destruction cost for simple HPD and SAHPD is 0.1185 $/h and 0.1386 $/h, respectively. It is observed that the most important components that needed to improve based on exergoeconomic factor are expansion device and evaporator. The present study reveals that the SAHPD system is much better than the simple HPD system.

Impact of using V-cuts twisted tape insert and phase change material (PCM) dispersed mono or hybrid nanofluid on the hydrothermal characteristics of the double tube heat exchanger is experimentally...

Energetic and exergetic analyses of the plate heat exchanger are experimentally performed using Al2O3–TiO2 hybrid nanofluid as a coolant for sub-ambient temperature application to study the effect of nanoparticle volume ratio at various nanofluid flow rates ranging 2.0–4.0 lpm and inlet temperatures ranging 10–25 °C. The Al2O3–TiO2 hybrid nanofluids of 0.1% total volume concentration with different Al2O3–TiO2 ratios (5:0, 4:1, 3:2, 2:3, 1:4 and 0:5) are used as a coolant. Effects of nanoparticle mixture ratio, flow rate and inlet temperature on the heat transfer rate, heat transfer coefficient, pump work, performance index, and second law efficiency are investigated. Correlations are proposed to predict the Nusselt number for DI water as well as hybrid nanofluid. A maximum enhancement of around 16.91% and 4.5% are observed for convective heat transfer coefficient and heat transfer rate with Al2O3 (5:0) hybrid nanofluid along with 0.013% enhancement (insignificant) in the pump work for TiO2 (0:5) hybrid nanofluid. The maximum reduction in exergetic efficiency of 4.01% is observed for TiO2 (0:5) hybrid nanofluid. The study shows that the energetic and exergetic performances decrease continuously with the increase of the TiO2 ratio in the mixture, yielding no optimum nanoparticle mixture ratio.

Tapered wire coil insert is proposed as a novel enhancer in the double tube heat exchanger and experimental studies on Al2O3 + MgO hybrid nanofluid flowing under the turbulent condition are performed to investigate the hydrothermal characteristics. Effects of using tapered wire coil turbulator and hybrid nanofluid on the hydrothermal behaviors are examined for different coil configurations (Converging (C) type, Diverging (D) type and Conversing-Diverging (C-D) type) and hybrid nanofluid inlet temperatures and volume flow rates. Results show that D-type wire coil insert promotes better hydrothermal performance as compared to C-type and C-D type. Nusselt number and friction factor of hybrid nanofluid using D-type, C-D type and C-type wire coil inserts enhance up to 84%, 71% and 47%, and 68%, 57% and 46%, respectively than that of water in tube without insert. The entropy generation of hybrid nanofluid is lower than that of base fluid in all cases. The thermal performance factor for hybrid nanofluid is found more than one with all inserts. The thermal performance factor is observed a maximum of 1.69 for D-type coil. The study reveals that the hybrid nanofluid and tapered wire coil combination is promising option for improving the hydrothermal characteristics of double pipe heat exchanger.

Both mono and hybrid nanofluids, the engineered colloidal mixture made of the base fluid and nanoparticles, have shown many interesting properties and become a high potential nextgeneration heat transfer fluids in various heat exchangers for engineering applications. The present review focuses on improving the performance of tubular heat exchangers by using nanofluids. For this, the present review briefly summarizes the preparation, characterization and thermophysical properties (thermal conductivity, viscosity, specific heat capacity and density) of mono and hybrid nanofluids. Research works on heat transfer and pressure drop characteristics of nanofluids in the double tube and shell-tube heat exchangers with both straight and coiled tubes, and various engineering applications (power generation, refrigeration and air-conditioning, renewable energy, domestic cooling or heating, etc.) are well-grouped and thoroughly discussed. Physical mechanisms for the heat transfer enhancement using nanofluids are explored as well. Most of the studies reveal that there are significant enhancements in the heat transfer process and in the effectiveness of both straight and coiled tube heat exchangers with a slight increase in pressure drop using nanofluids. Hence, there is an excellent opportunity to use nanofluids in tubular heat exchangers; however, high cost (high payback period) and stability are the main challenges for practical implementation. Finally, some useful recommendations are also provided.

The turbulent flow of binary hybrid nano-oils is investigated and Nusselt number correlation is developed for futuristic concentrated solar thermal application. Available Nusselt number correlations for water-based hybrid nanofluids are nanoparticle-specific property correlations dependent and substantially over-predict the values for hybrid nano-oils. Therefore, a generalized Nusselt number correlation for turbulent flow of water and oil-based binary hybrid nanofluids is deduced using the separation approach. Dissimilar to the available correlations, the developed correlation needs only the thermophysical properties of base fluid and nanoparticles. It is valid for the Reynolds number range 10,000–30,000 and the Prandtl number range 0.5–2000. It is found that the proposed correlation predicts the published experimental values for different hybrid nanofluids mostly within ±10–20%. Computational fluid dynamics simulation is also performed for turbulent flow of different hybrid nano-oils to assess the developed correlation. The comparative assessment also confirms that the developed correlation predicts the numerical values for hybrid nano-oils within ±10–20%. The deduced Nusselt number correlation will be useful for a realistic heat transfer analysis with different water or oil-based hybrid nanofluids. The need for experiments with different hybrid nano-oils is realized for further improvement.

A solar-assisted heat pump (HP) dryer is fabricated for intermittent drying. The experiment is performed for different intermittency ratios for radish drying using future refrigerant, R1234yf. The effect of total drying time (on-period + off-period) on various energetic, exergetic, and economic performances is investigated. Radish chips were dried to extract moisture from 92.4% to 11.9%. Energy efficiency and drying efficiency (DE) are estimated higher for a lower intermittency ratio. The moisture extraction rate (MER) and the specific moisture extraction rate (SMER) are higher for intermittent drying as compared with continuous drying and increase with a decrease in intermittency ratio. The economic analysis concludes that the payback period is lower for a lower intermittency ratio. The payback period for intermittency ratio of 1, 0.66, 0.33, and 0.2 are estimated as 1.617 years, 1.459 years, 1.384 years, and 1.347 years, respectively. Present experimental thermoeconomic analysis reveals that intermittent drying is much better (maximum enhancement of specific moisture extraction rate is 60.6%, that of energy efficiency is 56.4% and maximum reduction of drying cost is 37.9% with studied conditions) than continuous drying.

The experimental studies on Al2O3 + TiO2 hybrid nanofluid flowing under the turbulent condition in a double-tube heat exchanger with various modified V-cuts twisted tape inserts are performed to study the hydrothermal characteristics. The hybrid nanofluid is prepared with a volume concentration of 0.1% by dispersing Al2O3 and TiO2 nanoparticles by equal volume ratio in distilled water. The effect of using twisted tape turbulator (with and without V-cuts) and hybrid nanofluid on the heat transfer and pressure drop characteristics are evaluated for different twist ratios, V-cut depth ratios, V-cut width ratios and hybrid nanofluid inlet temperatures. Results show that Nusselt number as well as friction factor increases with the decrease in twisting ratio, increase in depth ratio, decrease in width ratio and decrease in nanofluid inlet temperature. Maximum improvements in 132% for Nusselt number and 55% for friction factor are obtained as compared to that for the water in the tube without twisted tape. The values of thermal performance factor and entropy generation ratio are greater than unity for hybrid nanofluid for all modified twisted tape inserts.

The hydrothermal characteristics of minichannel heat sink are analyzed experimentally by using deionized (DI) water based different nanoparticles mixture dispersed hybrid nanofluids. Al2O3, MgO, SiC, AlN, MWCNT and Cu nanoparticles are considered for this study. Different nanoparticles combinations (oxide-oxide, oxide-carbide, oxide-nitride, oxide-carbon nanotube and oxide-metal) in 50/50 vol ratio with base fluid (DI water) have been taken as coolants for volume concentration of 0.01%. Effects of volume flow rate (0.1–0.5LPM), fluid inlet temperature (20–40 °C) and Reynolds number (50–500) are studied for heat flux of 50 W/cm2. Convective heat transfer coefficient and pressure drop are increased by about 42.24% and 22% for Al2O3 + MWCNT hybrid nanofluid. The maximum reduction of 21.36% in thermal resistance is obtained for Al2O3 + MWCNT hybrid nanofluid in comparison to DI water. Heat transfer effectiveness and figure of merit are above one for all the hybrid nanofluids which conclude that hybrid nanofluid is better option for electronics cooling over DI water. Al2O3 + MWCNT hybrid nanofluid is better in terms of heat transfer effectiveness; whereas, Al2O3 + AlN hybrid nanofluid (oxide-nitrite mixture) has maximum heat transfer coefficient to pressure drop ratio and coefficient of performance.

Hydrothermal and energy-economic performances of minichannel heat sink are experimentally compared by using water-based different nanoparticle-dispersed mono and hybrid nanofluids. Al2O3, AlN, CNT, Cu and capric acid as phase change material (PCM) are considered. Different nanoparticles combinations (oxide–PCM, oxide–nitride, oxide–carbon nanotube and oxide–metal) in 50/50 volume ratio with water (base fluid) are taken as working fluids. The effects of volume flow rate (0.1–0.5 LPM) or Reynolds number (50 to 500) and total particle volume concentration (0.01–0.1%) are investigated. Convective heat transfer coefficient and pressure drop increase by about 42.3% and 22%, respectively, for Al2O3 + CNT nanofluid. The maximum reduction of 26.6% in thermal resistance is obtained for Al2O3 + CNT nanofluid as compared to base fluid. Heat transfer effectiveness and figure of merit are above one for all the hybrid nanofluids, which conclude that hybrid nanofluid is a better option over base fluid for minichannel heat sink. Al2O3 + CNT hybrid nanofluid is better in terms of heat transfer effectiveness, but Al2O3 + AlN hybrid nanofluid yields higher heat transfer coefficient to pressure drop ratio and coefficient of performance. The lower nanoparticle volume concentration in nanofluid is preferable due to higher stability, lower clogging and lower cost per cooling capacity of heat sink.

Brines (ethylene glycol, calcium chloride, propylene glycol and potassium acetate)-based hybrid (combinations of alumina, copper oxide, silica and titania with copper nanoparticles) nanofluids have been used as a secondary refrigerant to improve the heat transfer characteristics of the plate evaporator for milk chilling. Effect of nanoparticle combination, shape and size on heat transfer area, pump work, the ratio of heat transfer coefficient to pressure drop, coefficient of performance, performance index, thermal performance factor and exergetic efficiency has been examined theoretically. Copper oxide–copper hybrid nanofluid gives superior performance, while silica–copper hybrid nanofluid performs well in terms of exergetic efficiency. The maximum decrease in effective heat transfer area (5.9%) is found for propylene glycol brine-based copper oxide hybrid nanofluid. Percentage change in heat transfer area and performance index reduces with an increase in the particle size and is maximum for alumina–copper hybrid nanofluid. However, thermal performance factor increases with particle size. Brick-shaped particles show maximum changes in heat transfer area and performance index, while platelet-shaped particles show worse performance. The study reveals that the nanoparticle shape has a strong influence on the plate heat exchanger performance due to a significant deviation in surface area-to-volume ratio.

The simulation of batch type open cycle heat pump dryer has been performed to study the system performances and food quality using promising low GWP refrigerants (R290, R600a, R32, R152a and R1234yf) as a substitute of R134a for carrot and banana drying. The experimentation with heat pump dryer test facility using R134a has been performed for model validation. Effects of drying time and material moisture content on the coefficient of performance, moisture extraction rate, specific moisture extraction rate, specific energy consumption, drying efficiency, exergy destruction, exergy efficiency and product water activity have been investigated. Results show that moisture extraction rate is maximum for R32, specific moisture extraction rate is maximum for R152a, specific energy consumption is minimum for R152a, drying efficiency is maximum for R152a (average value is 29.74%) and exergy destruction is minimum for R152a for the same air inlet velocity of 2 m/s, respectively. The present study reveals that R152a may be a potential option for heat pump dryer by considering performances, environmental safety (low GWP) and personal safety (low flammability).

To enhance the energetic and exergetic performances of the counter-flow plate heat exchanger with corrugations, several experiments were conducted using different Al2O3 hybrid nanofluids as a coolant. The combinations studied were Al2O3–SiC, Al2O3–AlN, Al2O3–MgO, Al2O3–CuO and Al2O3–MWCNT in 4:1 nanoparticle volume ratio and 100% Al2O3 with 0.1 vol% concentration suspended in DI water. Parameters which were varied are the coolant flow rate ranging from 2.0 to 4.0 lpm and coolant inlet temperature from 10 to 25 °C. Effect of coolant flow rates and inlet temperatures on different parameters like heat transfer coefficient ratio, heat transfer coefficient to pressure drop ratio, pump work, performance index and irreversibility has been investigated. A maximum enhancement of around 31.2% has been observed in the heat transfer coefficient for Al2O3–MWCNT (4:1) hybrid nanofluid with the negligible enhancement of 0.08% in the pump work and 12.46% enhancement in the performance index. The maximum enhancement in the irreversibility is around 1.6% for Al2O3–CuO (4:1) hybrid nanofluid. Among the studied nanoparticle combinations, Al2O3–MWCNT (4:1) gives better performance. This study suggests that Al2O3–MWCNT hybrid nanofluid can be a good alternative to enhance thermal performance.

Three climatic zones (composite, hot-dry, hot-humid) cover most of the regions in India. Direct evaporative cooler is suitable for dry and indirect regenerative evaporative cooler is suitable for humid months. To avoid the use of two different coolers for the same purpose during the composite climate (significant seasonal variations of temperature and humidity), a novel two-in-one evaporative cooler is proposed and numerically investigated in this study. Proposed cooler takes advantage of both types of evaporative cooler and be operated in dual mode: acts as direct evaporative cooler for hot-dry and acts as regenerative indirect evaporative cooler for hot-wet seasons. The working of both modes (direct and regenerative) is explained with the help of a diagram. Effects of channels height and water flow rate on cooling capacity, wet bulb effectiveness, and dew point effectiveness are discussed. The month-wise performance of the proposed cooler is evaluated for five Indian cities (Bhopal, Lucknow and Varanasi of composite climate; Ahmedabad of hot-dry climate; Kolkata of hot-humid climate). Suitability of the modes in hot months (April to September) and cities are evaluated with the help of a psychometric chart. Study reveals that the proposed cooler can be effectively and economically used for composite climate.

A batch-type open cycle heat pump dryer has been numerically simulated to study the energy and exergy performances using various low-GWP refrigerants (R290, R600, R600a, R32, R152a, R1234yf, R1234z...

Heat transfer and pressure drop characteristics of minichannel heat sink are analysed experimentally and numerically (two-phase mixture model) by using hybrid nanofluids. Minichannel heat sink consists of 9 parallel rectangular shaped minichannels having depth of 3 mm and width of 1 mm. The Al2O3–TiO2 hybrid nanofluid with nanoparticles volume concentration of 0.1% with different nanoparticle mixture ratios (10:0 to 0:10) has been used as coolant to analyze the effect of volume flow rate (0.1 to 0.5 LPM), Reynolds number (90

The polymer–particle matrix structure has attracted great interest for daytime radiative cooling due to its various advantages; however, cost-effective fabrication process development and proper material selection are crucial tasks. Herein, we illustrate the selection of fabrication process and materials based on development, characterization, and experimental performance. The initial selection is based on solar radiation backscattering and isotropic thermal emission properties of polymers and particles. Among potential polymers (PMMA, PVDF, and PU), PMMA is experimentally found more suitable for radiative cooling because of negligible absorption in the solar radiation spectrum. Among the selected particles (CaCO3, BaSO4, ZnO, and SiO2), the experimental performance of BaSO4 in the polymer matrix is found to be excellent. Furthermore, the optimum particle volumetric concentration is found at 70%, with an optimum thickness of 500 μm for the substrate independent radiative cooler. Spray-coating is found to be a better option for fabrication as compared to drop casting. The performance evaluation was carried out at Varanasi, India for over two months to investigate the environmental parameters' effect on performance. For an optimized structure, the observed maximum temperature drop from the ambient is 6 °C (17 °C lower than bare roof surface) during noontime and 9 °C during the evening. At ambient temperature, the recorded cooling power is 35 W/m2 at noon (solar peak) and 78 W/m2 in the evening for the experimentally optimized structure.

Steady-state and transient behaviors of single-phase natural circulation loop (SPNCL) are investigated using four thermal oils (Therminol VP1, Paratherm CR, Dowtherm A, and Dowtherm Q) and water-based ternary hybrid (various combinations of different nature and shaped nanoparticles: Al2O3, Cu, carbon nanotube (CNT) and graphene) nanofluids as loop fluid. The influences of nanoparticle volume concentration and loop height-to-width ratio on the mass flow rate and total entropy generation rate of SPNCL are investigated. Results disclose that ternary hybrid nanofluids enhance flow initiation, reduce fluctuation and are expected to attain a steady-state faster than water. Steady-state mass flow rate increases/decreases for ternary hybrid nanofluid depending on the shape of the nanoparticle, and the total entropy generation rate decreases as compared to water. Thermal oil shows a higher mass flow rate and total entropy generation rate as compared to water. Al2O3–Cu–CNT–water and Paratherm CR show the best result among all ternary hybrid nanofluids and thermal oils, respectively. The nanoparticle shape decides the optimum nanoparticle volume fraction. Increasing the height-to-width ratio decreases the total entropy generation and upsurges the mass flow rate at specified input power. The optimum height-to-width ratio depends on the loop fluid.

Passive radiative cooling is an emerging field and needs further development of material. Hence, the computational approach needs to establish for effective metamaterial design before fabrication. The finite difference time domain (FDTD) method is a promising numerical strategy to study electromagnetic interaction with the material. Here, we simulate using the FDTD method and report the behavior of various nanoparticles (SiO2, TiO2, Si3N4) and void dispersed polymers for the solar and thermal infrared spectrums. We propose the algorithm to simulate the surface emissive properties of various material nanostructures in both solar and thermal infrared spectrums, followed by cooling performance estimation. It is indeed found out that staggered and randomly distributed nanoparticle reflects efficiently in the solar radiation spectrum, become highly reflective for thin slab and emits efficiently in the atmospheric window (8–13 µm) over the parallel arrangement with slight variation. Higher slab thickness and concentration yield better reflectivity in the solar spectrum. SiO2-nanopores in a polymer, Si3N4 and TiO2 with/without voids in polymer efficiently achieve above 97% reflection in the solar spectrum and exhibits substrate independent radiative cooling properties. SiO2 and polymer combination alone is unable to reflect as desired in the solar spectrum and need a highly reflective substrate like silver.

An experimental investigation is conducted with graphene–Al2O3 hybrid nanofluid as a coolant in corrugated surface plate heat exchanger to study energetic performance. The combinations studied are Al2O3–graphene in 4:1 nanoparticle ratio and 100% Al2O3 nanoparticle with 0.01 v % concentration in DI water. Effect of coolant flow rates (2.0–4.0 lpm) and coolant inlet temperatures (10–25 °C) on various factors like coolant outlet temperature, heat transfer rate, convective heat transfer coefficient, pressure drop, and heat transfer coefficient to pressure drop ratio has been investigated. Maximum enhancements of around 2.5%, 5.34%, 25.36%, and 23.8% are observed in the coolant outlet temperature, heat transfer rate, heat transfer coefficient, and heat transfer coefficient to pressure drop ratio, respectively, for Al2O3–graphene hybrid nanofluid.

Nanocomposite is a nanotechnology-based multiphase, high-performance composite material having combination of properties, which has been emerged as one of the promising research and development activities. Recently, nanocomposites have shown significant application opportunities for several sectors of biotechnology, engineering, and medical sciences, and its dispersed fluids (composite nanofluids) have gained huge interest and demand in wide area of engineering applications, particularly for heat transfer intensification. The main aim of the present review is to summarize the recent advances in nanocomposite-dispersed nanofluids. The synthesized methods, characterization, and applications of nanocomposites as well as the preparation, stability analysis, thermophysical, optical and electrical properties, heat transfer and pressure drop characteristics and applications of nanocomposite-dispersed nanofluids are well-grouped and discussed. Present review reveals that the nanocomposite with proper combination of nanomaterials yields superior performance characteristics compared to the individual alone for application in nanofluids, although many related aspects are still unexplored. Hence, the challenges and opportunities for future research are also identified, which will be useful for the newcomers and manufacturers in this field.

Theoretical study on the energetic and exergetic performances of a counter-flow corrugated plate heat exchanger using hybrid nanofluids for the milk chilling application has been done in th...

In view of increasing tendency of power density of electronic systems, cooling performance improvement of micro-channel heat sink is an emerging issue. In the present article, supercritical CO2 is proposed as a heat transfer fluid in micro-channel heat sink for power electronics cooling. Energetic and exergetic performance analyses of micro-channel heat sink using supercritical CO2 have been done and compared with conventional coolant, water. To take care of sharp change in properties in near critical region, the discretization technique has been used for simulation. Effects of both operating and geometric parameters (heat flux, flow rate, fluid inlet temperature, channel width ratio, and channel numbers) on thermal resistance, heat source (chip) temperature, pressure drop, pumping power and entropy generation are presented. Study shows that the thermal resistance, heat source temperature and pumping power are highly dependent on CO2 inlet pressure and temperature. Supercritical CO2 yields better performance than water for certain range of fluid inlet temperature. For the studied ranges, maximum reduction of thermal resistance by using CO2 is evaluated as 30%. Present study reveals that there is an opportunity to use supercritical CO2 as coolant for power electronic cooling at lower ambient temperature.

In this study, three novel biomass gasification based tetra-generation systems for syngas, heating, cooling and power generations have been proposed and theoretically assessed for selected biomass materials. The systems, instead of using the chemical energy of the gasification products, use the thermal energy to operate both Rankine and Refrigeration cycles in three configurations, namely, steam Rankine cycle with organic Rankine Cycle and ejector refrigeration cycle (Cycle-1), binary Rankine cycle with ejector refrigeration cycle (Cycle-2) and steam Rankine cycle with combined power ejector refrigeration cycle (Cycle-3). Syngas (a mixture of hydrogen and carbon monoxide gas), which has the potential for various domestic applications, has been obtained as the gasification product. The effects of different operating parameters such as water to biomass ratio, total biomass-water mass flow rate, mass flow rate of refrigerant, generator pressure, gasification temperature and types of biomass material have been studied on the syngas and hydrogen yields, as well as coefficient of performance and overall performance index of the system. The study shows that Cycle-2 is more effective in terms of coefficient of performance and refrigeration effect whereas Cycle-1 is best for higher performance index. Hence, either Cycle-1 or Cycle-2 can be preferred depending on the requirement.

The energy, exergy and economic assessments of a shell and tube condenser of 580 MW nuclear power plant have been carried out using different water-based hybrid nanofluids such as Al2O3 + MWCNT, Al2O3 + Ag, Al2O3 + Cu and Al2O3 + TiO2 as coolant. The effect of nanoparticle concentration on reductions of coolant requirement, pumping power and operating cost has been investigated. The results reveal that Al2O3 + MWCNT shows the highest reduction of coolant mass flow rate of 3.6%. The effectiveness of shell and tube condenser enhances with the addition of nanoparticles. The reduction of the pressure drop is maximum for Al2O3 + MWCNT hybrid nanofluid (5.8%) and minimum for Al2O3 + Ag (3.7%) at 1% volume concentration. Therefore, Al2O3 + MWCNT requires 11.2% less pumping power followed by Al2O3 + TiO2 (10.9%), Al2O3 + Cu (10.2%) and Al2O3 + Ag (9.6%) as compared to the base fluid. Al2O3 + Ag shows maximum (29.97%) and Al2O3 + MWCNT shows minimum (29.81%) second law efficiency at 1% volume concentration. Al2O3 + MWCNT hybrid nanofluid yields maximum saving of operating cost (11.1%) and Al2O3 + Ag shows minimum (9.62%) at 1% volume concentration. However, Al2O3 + Ag shows payback period about 247 years and Al2O3 + TiO2 about 9.8 years. Hence, the payback period needs to be reduced before practical implementation, which may be possible by reducing the nanoparticle cost and increasing nanofluid stability.

The increase in energy demand for water cooling in nearby future encourages the use of metamaterial-based passive radiative water cooler, which has application potential in a highly-populated country like India. However, an energy-exergy-economic feasibility study with the best tube design is essential before implementation due to climatic diversity, but not yet been studied. Hence, this paper deals with the analysis of the passive radiative water cooler and the study of its performance with respect to the variable water mass flow rate and water tube geometry. The research presents the monthly variation of cooling performance, cooler efficiency and COP concerning Indian cities of different climatic conditions. The study also carries out exergy, economic and ecological analyses and proposes a new exergy model. Results reveal that the cooling performance is low for humid climate as compared to arid climate. For a total cooling panel area of 10.75 m2 and water flow rate of 0.03 kg/s, a maximum water temperature drop of 3.63 °C with an associated cooling capacity of 42.46 W/m2 is obtained at Jaisalmer in October (arid climate). The least performance is obtained at Chennai in May (humid climate). Results also reveal that the semicircular channel increases temperature drop and cooling capacity by 15.8% and 14.8%, respectively, over the circular channel. Exergy analysis shows very high entropy generation and hence very low exergy efficiency for radiative cooling. The use of the radiative cooler over the electric cooler produces a maximum annual operational cost saving of 19.25% and carbon dioxide mitigation of 1.987 kg.