Heat Transfer Augmentation Techniques Research Articles

Thermal performance augmentation of honeycomb metal matrix embedded phase change material in shell-tube latent heat storage unit

Due to the global warming challenge, there is a transition from the utilization of fossil fuels to harvesting new and renewable energy sources globally. Solar energy, being the infinite source with easy accessibility, leads the race among other allied sources. However intermittent nature of it demands an efficient thermal energy storage system. In the present experimental study, an innovative heat transfer augmentation technique i.e. aluminium honeycomb grill and fine mesh embedded in PCM is demonstrated. Both visualization of PCM melt fraction and capture of temperature contour complemented each other to identify the honeycomb version (HC-LHSU) as substantially better (43 % improvement in charge period) than the base model (BM-LHSU). On the other side, due to fine pores (porosity = 95 %), the fine meshmodel (FM-LHSU) yielded poorer results than BM-LHSU. Even the temperature gradient that exists during the charge or discharge period is gauged and recognized HC-LHSU as the best, followed by BM-LHSU and FM-LHSU. Since the onset of the phase transition zone reflects heat storage rate, the same is identified, and the charge period (sensible heat + latent heat) assessed to be 8000 s, 10000 s, and 12,000 s for HC-LHSU, BM-LHSU, and FM-LHSU, respectively. Further, an improvement in the average Nusselt number by 74 % and 67 % during the charge and discharge period respectively compared to the base model justified the honeycomb grill's role in augmenting the energy storage unit's performance.

Compound passive heat transfer augmentation techniques: A comprehensive review

AbstractThe function of energy is becoming increasingly vital in meeting the requirements of modern societies and sustaining rapid economic and industrial growth globally. Heat transfer equipment has been employed for heat recovery and conversion in various domestic and industrial applications. Therefore, due to the recent global energy crisis, boosting the thermal efficiency of energy systems has become an essential requirement, which would reduce both their size and rates of energy demand. There are active and passive methods for boosting heat transfer rates. As they have no moving components, passive methods are more affordable and dependable than active ones. Applying two or more passive techniques concurrently will result in a higher heat transmission rate than any approach working independently. The current article comprehensively reviews experimental and computational investigations of passive compound forced convection heat transfer augmentation techniques at laminar, transition, and turbulent flow regimes. This article focuses on compound techniques arrangement in the form of turbulators, typical twisted tapes (TTs), surface roughness, vortex generators, and so on. The pioneering research suggests that using a lower twist ratio, lower pitch, and smaller winglet angles in TTs can result in higher heat transfer rates, albeit with a slightly increased friction factor. The combination of alternate‐axes and wings in TTs leads to more effective heat transfer enhancement within the tube.

Experimental investigations on heat transfer enhancement in a double pipe heat exchanger using hybrid nanofluids

Abstract Heat exchanger (HE) is an instrument that facilitates the operation of HE between two fluids that are at various temperatures. Double-pipe HEs are used in many organizations because of their low installation, design, maintenance costs, flexibility, and their suitability for high pressure applications. Heat transfer (HT) augmentation techniques (passive, active or compound techniques) are used in heat exchangers to reduce the HT surface area, to increase HT capacity and to reduce pumping power. Passive augmentation techniques are much cheaper and do not involve any external power input. They aim to improve the effective surface area, the residence time of the HT fluid and its thermal conductivity by the usage of nanofluids. Nanofluids are used for cooling applications in organizations, transportation, nuclear reactors, electrical and electronic devices and for biomedical applications. Hybrid nanofluids have higher thermal conductivity, low PD and frictional losses and pumping power as compared to the mono nanofluids. In this present work, experiments are conducted in a double pipe HE using TiO2, and SiC-water nanofluids by varying the volume concentration and cold fluid mass flow rate ranging from 17.5 to 34.5 lpm by making constant hot fluid mass flow rate. Further, experiments are conducted using TiO2–SiC/water hybrid nanofluids. Influence of nano and hybrid nanofluids on the overall HTC and friction factor are experimentally investigated. From the experiments, TiO2–SiC/water hybrid nanofluid with nanoparticle ratio TiO2:SiC = 1:2 is found to be optimum as the heat transfer enhancement is more with less improvement in friction factor. The overall heat transfer, and friction factor enhancement is 22.92 %, and 11.20 % higher respectively when compared with base fluid for TiO2:SiC = 1:2.

Sustainable Energy Progress via Integration of Thermal Energy Storage and Other Performance Enhancement Strategies in FPCs: A Synergistic Review

Flat plate collectors (FPCs) are the leading solar thermal technology for low-medium range temperature applications. However, their expansion in developing countries is still lacking because of their poor thermal performance. Improving the thermal performance of flat plate collectors (FPCs) is a crucial concern addressed in this review This study comprehensively discussed the performance improvement methods of FPCs, such as design modification, reflectors, working fluid, and energy storage materials, by covering current issues and future recommendations. Design factors such as coating and glass cover thickness, thickness of absorber plate and material, air gap between the glass cover and absorber plate, and riser spacing, along with insulation materials, are examined for their impact on FPC performance. Absorber design changes with selective coatings for improving the heat transmission rate between the working fluid and absorber are critical for enhancing collectors’ thermal output. The nanofluids utilization improved FPC’s thermal performance in terms of energetic and exergetic outcomes in the 20–30% range. Moreover, adding a heat storage unit extends the operating hours and thermal output fluctuations of FPCs. Research suggests that employing turbulators and nanofluids as heat transfer fluids are particularly effective for enhancing heat transfer in FPCs. This comprehensive review serves as a critical tool for evaluating and comparing various heat transfer augmentation techniques, aiding in the selection of the most suitable option.

Thermo-hydraulic performance augmentation in residential heating applications using a novel multi-fluid heat exchanger with helical coil tube insertion

This study proposes a heat transfer augmentation technique using a brazed helix tube (BHT) fabricated from a helical tube with precision brazing between coil turns in a novel multi-fluid heat exchanger (NMFHE) for simultaneous heating of water and air using solar energy. The thermo-hydraulic performance of the present NMFHE for residential heating of water (CW) and air (CA) using hot water (HW) is tested experimentally. Nusselt number and friction factor for fluid flow inside the NMFHE are calculated as the thermo-hydraulic measure relating to variations in flow rate, inlet temperature, and flow configuration. Optimal flow parameters for overall optimized performances that is, maximum heat transfer and minimum pressure drop in NMFHE are determined using the Taguchi Grey relational approach. NMFHE performs efficiently in the Counterflow (cold water reverse) flow configuration with HW flow rate of 100 LPH, CW flow rate of 200 LPH, and HW inlet temperature of 70°C. The CW flow rate has the greatest impact on both the Nusselt number and friction factor, with a contribution of 82.37% and 93.42%, respectively. A confirmation test has been conducted to validate the findings, revealing a significant performance improvement of 32.19% when using the Grey relational grade model.

Comparative study of performance enhancement of latent thermal energy storage system with copper porous fin

Phase change material (PCM)-based thermal energy storage (TES) has attained significant attraction towards energy savings and ensures a continuous energy supply. However, the lower thermal conductivity of PCM impedes the efficient transfer in the storage systems, thereby causing longer charging time. Therefore, this paper introduces a novel heat transfer augmentation technique by incorporating copper porous fins in the shell-tube latent heat thermal energy storage. Further, to demonstrate the contribution of porous fins in heat transfer improvement, full-foam, solid-fin and without-fin TES systems are also evaluated and compared. A two-dimensional axisymmetric numerical model is developed and validated with in-house experimental results. It is found that conduction strongly affects the solidification process, exhibiting a greater response rate than the melting process of heat transfer enhancement techniques. Full-foam has higher melting and solidification rates, followed by solid-fin, porous-fin and without-fin. Compared to the without-fin configuration, the temperature response rates of the full-foam, solid-fin and porous-fin are improved by 169.9 %, 44.8 % and 41.88 %, respectively, during the melting process. Moreover, it is revealed that the porosity of foam has a marginally higher impact on the liquid fraction than the pore density. Further, the inclusion of porous-fin and full-foam leads to more uniform temperature distribution in PCM during melting and solidification compared to solid fin and without-fin. Therefore, the porous-fin and full-foam may be utilised for electronic cooling and battery thermal management, which requires uniform cooling at faster rates.

Heat transfer and pressure drop performance improvement using curved circular spines for flow through circular pipe

The heat transfer augmentation method of inserting curved circular spines increases the dwelling time of the fluid within the system by inducing swirling motion within the fluid. It is a promising passive heat transfer augmentation technique for improving average and local heat transfer coefficients in the flow across uniform & non-uniform cross-sectioned channels at a high Reynolds number. The spines of circular cross-sections can considerably enhance flow passages employed for various purposes. The heat transfer performance study of tubes mounted with curved circular spines is presented for turbulent fluid flow conditions for Reynolds numbers ranging from 10,000 to 50,000. The curved spines are placed in the inline arrangement at the inner wall of the circular tube, disturb the flow, and break the dominant thermal resistance close to the wall, improving the heat transfer to the flowing fluid. The heat transfer enhancement at the cost of a rise in the pressure drop is also studied. The overall performance factor, R3, at the same pumping power, is presented to account for cost-effectiveness. The numerical results show that the enhancement ratio, Nue/Nuo, and friction factor ratio, fe/fo, using curved spines are 1.5 to 3.6, and 2.0 to 20.0, respectively, compared with the smooth tube at similar flow conditions. The Nusselt number ratio, Nue/Nup, at the same pumping power of the equivalent smooth tube, is 0.8 to 1.6.

Numerical Study About Nanofluids of Spherical and Tube-Shaped TiO2 Nanomaterials on the Thermal Performance and Entropy Generation of Different Cross-Section Microchannel Heat Sinks

We numerically evaluated the thermal performance of microchannel heat sinks, considering rectangular, hexagonal, and circular cross-sections. Moreover, as a passive heat transfer augmentation technique, dimples were added to improve the rectangular microchannel heat sinks. These simulations use nanofluids based on TiO2 nanoparticles or nanotubes dispersed in bidistilled water as working fluids. The mathematical model considered variable thermophysical properties of the nanofluids; for this purpose, polynomial fittings correlate the dependence of the thermophysical properties on the temperature. We considered a heat flux of q″ = 50 W/cm2 at the microchannel’s lower surface as a boundary condition along with laminar flow conditions. The numerical simulations allowed the Nusselt numbers and entropy generation calculation, which were the basis for the thermal performance calculation. Regarding the effect of TiO2 nanoparticles shape, spherical TiO2 nanoparticles based nanofluids using rectangular microchannels improve the Nusselt number. Moreover, the frictional entropy decreases with nanofluids based on TiO2 nanotubes, but the thermal entropy decreases with nanofluids based on TiO2 nanotubes. Incorporating dimples in the rectangular microchannel enhances the Nusselt numbers and lowers the entropy generation. Considering the Reynolds number range and from the perspective of Nusselt number and entropy generation, we concluded that the microchannels must be operated at a high Reynolds number to improve the microchannel heat sinks thermal performance.

Review on latent thermal energy storage using phase change material

One of the appealing technologies that contributes to raising the energy storage density is latent heat thermal energy storage. The heat of fusion is isothermally stored at a temperature representing the temperature at which a phase-change material transitions between phases. The current research provides a review of how phase transition materials are used in melting and solidification. Generally, the range of working temperature extends from -20 °C to 200 °C for solidification and melting applications. The first range (-20 to 5 °C) is employed for commercial and domestic refrigeration. The second range (5 to 40 °C) is utilized to lower the energy requirements for air-conditioning applications. The applications includes in third range (40 to 82 °C) are solar collector and heating of water. Applications of absorption cooling, waste electricity generations, and heat recovery are operated at high temperature range (82 to180 °C). There are various types of PCMs for all the above temperature ranges. The present review paper will discuss the application field, Geometry, PCM type, heat transfer augmentation technique and their effects on the performance. The conclusions are mentioned to give more insight about the PCM behavior in various applications.

Applications of metal hydride based thermal systems: A review

Conventionally, metal hydrides have been explored for their application in hydrogen storage systems. However, metal hydride based thermal systems have attracted huge attention for a wide variety of applications such as thermal storage, hydrogen compression, heating, cooling, etc due to the availability of hydride materials covering a wide range of operating temperatures and pressures. These can deliver high energy densities leading to compact systems for stationary as well as mobile applications. Moreover, the thermal properties of hydrides can be tailored by tuning the proportion of constituent metals to fit the required thermal application. As metal hydride based thermal systems are identified as potential candidates for global energy needs, it is essential to understand their modes of operations and the status of their development. This paper presents a comprehensive review of theoretical and experimental studies on thermal applications of metal hydrides which include refrigeration, heat transformers, heat pumps, and thermal energy storage. Also, hydride selection criteria for specific applications and heat transfer augmentation techniques adopted to improve the performance of hydride beds are discussed.

Numerical study on heat transfer augmentation techniques in concrete based thermal storage module for solar-thermal applications

• Thermal performance of sensible heat storage prototype is studied. • Heat transfer augmentation is investigated by varying the fin configuration. • The maximum reduction in charging time is 36.9%/. • Proposed optimized model increases the discharging rate by 75.6%. • Trapezoidal fin configuration is the best option with 50–75% cost reduction. In this study, the development and performance analysis of a concrete based thermal energy storage module with a capacity of 170 MJ operating in the temperature range of 523 K to 623 K is presented. To enhance the heat transfer rate in concrete based sensible heat thermal energy storage (SHTES) systems, the well-proven technique of fin incorporation is implemented. Longitudinal fins are attached on the outer side of the heat transfer fluid supply tube without compromising the thermal storage/discharge capacity of the storage module. Numerical simulations have been performed using COMSOL Multiphysics to study the thermal performance of the module by varying the design parameters such as the number of fins, fin dimension, and fin shape. The thermal performances of the models with fins, in terms of charging/discharging time and heat storage/retrieval rate, are compared with the model without fins for the same boundary condition. From the numerical study, it is observed that the maximum reduction in charging/discharging times of the storage module is 36.9%/48.1% and 36.9%/45.6% for rectangular and trapezoidal fin shaped models, respectively, in comparison with the model without fins. Whereas, the corresponding enhancement in heat storage/heat retrieval rate is 58.5%/92.7% and 58.5%/83.7% for rectangular and trapezoidal fin shaped models, respectively, in comparison with the model without fins. By adopting a trapezoidal shaped profile model, the fin material usage can be reduced by 50–75% without compromising on thermal performance. This numerical analysis would facilitate the design and development of efficient storage modules for solar thermal applications.

Experimental evaluation of hydrothermal performance of solar air heater with discrete roughened plate

• The novel discrete double arc roughness has been used in a SAH and performed numerical and outdoor experimental studies. • Optimum results were found at a p/e of 8.33 and α of 60° for Nu, h and f & THP. • Optimum results were found at a p/e of 11.67 and α of 60° for η th . • Nu and f enhanced by 2.87 and 2.47 times respectively in comparison with smooth plate. • Maximum value of THP was found to be 2.82 at a Re of 13,935 and p/e of 8.33. • Correlations has been developed for Nusselt number and friction factor and deviation were found with experimental values within ± 18%. The solar air heater has a low convective heat transfer coefficient because the absorber plate and moving air form a laminar sub-layer. Adding artificial roughness to the absorber surface is one of the most efficient heat transfer and thermal efficiency augmentation techniques utilised in SAH duct construction. The absorber plate is given new double arc discrete shaped roughness geometry. The Nu, f, and η th characteristics of a rectangular duct were evaluated employing a new double arc discrete roughness element on one side of the SAH wall and insulation on the other three sides.The outdoor investigation has been carried out with roughness parameters are relative roughness pitch (p/e) of 6.67, 8.33,10,11.67, relative roughness height (e/D h ) of 0.027, angle of arc or attack angle (α) of 60°, Reynolds number (Re) in the range of 3000–14000, and as aspect ratio (W/ H) of 8 were selected. Solar radiation intensity ranged from 860 W/m 2 to 895 W/m 2 on a bright sunny day was used. Between 11:00 A.M. and 2:00P.M., from morning to afternoon, the outside test was also done. Optimum results were found at a p/e of 8.33 and α of 60° for Nu, h and f & THP. Optimum results were found at a p/e of 11.67 and α of 60° for η th . The maximum enhancement of Nu & f by 2.87 & 2.47 of the roughened plate respectively was found when compared to the smooth absorber plate under similar flow conditions. Maximum value of Nu and f were found 121.66 and 0.0342 respectively. Maximum value of h and η th were noticed 75.23 W/m 2 k and 87.95% respectively. The maximum enhancement of thermo-hydraulic performance (THP) was noticed under-investigated parameters of 2.82 at p/e of 8.33, and Re of 13935. Heat transfer and friction factor correlations have been developed deviation were found with experimental values within ± 18%.The mathematical model, which is utilised for SAH duct design and performance prediction under various operating conditions, has been validated using experimental data.

Experimental and numerical study of the thermal-frictional behavior of a horizontal heated tube equipped with a vibrating oscillator turbulator

This study investigates a novel heat transfer enhancement method named electromagnetic vibration (EMV) method. For the first time a stretched oscillator with geometries of string and strip was placed coaxially inside a heated copper tube and induced to vibrate at the natural frequencies. Longitudinal vibration of the stretched oscillators creates harsh turbulence, radial flow and vortexes inside the heated tube. Tests were conducted for various vibrational frequencies ranging 0 to 1 KHz in different mass flow rates. The experimental results indicated that oscillator with the geometry of strip has the better thermal performance; also the heat transfer coefficient can be increased with increasing vibration frequency from 2.18 to 2.50 folds. For comprehensive analyze at high frequency vibrations, a numerical model was conducted for the vibrating strip geometry to evaluate this new method capability in heat transfer performance enhancement. The numerical study shows that at 1 KHz frequency, heat transfer coefficient and thermal enhancement factor (TEF) can reach up to 32.2 and 14.2 fold compare to the plain tube, respectively. This magnificent increment shows the high potential of the EMV method as a new heat transfer augmentation technique.

Shape optimization of hotspot targeted micro pin fins for heterogeneous integration applications

In the field of high-performance computing (HPC), growing power demands makes Heterogeneous Integration (HI) the future of next-generation computing systems to sustain Moore's law. HI refers to the assembly of different separately-manufactured components onto a single electronic module to enhance functionality and operating characteristics. As a consequence of HI, the next generation of electronic chips have regions of localized hotspots or cores, translating to a region of extremely high temperature if not adequately cooled. The mitigation of hotspots demands advanced thermal management cooling schemes compared to the conventional ones. This study investigates the combination of impingement jet array of liquid water and non-uniform hotspot targeted micro pin fins, printed on chips, as a potential heat transfer augmentation technique. The configuration comprises four 2cm×2cm chips (Total chip area is 16 cm2) with eight hotspots on each. Liquid water is employed as the coolant in the present study. A detailed numerical parametric study and optimization were carried out using a supervised machine learning algorithm. The multi-objective optimization is performed to optimize both the thermal and flow resistances simultaneously. The results from the detailed optimization reveal that optimal fin parameters for the regions of hotspot and background could be significantly different. The optimal pin fin configuration resulted in a minimum thermal resistance of 0.208 K.cm2/W (0.013K/W) at a constrained pressure drop of about 10 kPa.

A Review on Study of Shell and Tube Heat Exchanger

Heat exchangers are used in a variety of industrial and technical settings. The design of heat exchangers is fairly sophisticated, since it necessitates an accurate study of heat transfer rate and pressure drop estimates, as well as considerations like as long-term performance and cost. Whenever heat transfer inserts are employyed, Along with the rise in heat transfer rate, the pressure drop increases as well. The pressure has risen.Energy conservation is critical for both industrial development and environmental impact reduction. Because of current energy prices, the need for energy conservation is rising. More resourceful gadgets can help reduce energy costs. In the refrigeration, automotive, chemical, and process sectors, heat exchangers are the most significant device. As a result, there is a demand in the industrial sector for heat exchangers that are less expensive, more efficient, and smaller. Inserts in channels are widely employed as part of a passive heat transfer augmentation technique.

Heat transfer augmentation of triplex type latent heat thermal energy storage using combined eccentricity and longitudinal fin

The universe has shifted its focus from non-renewable to renewable energy sources from the last decade. The limited availability of fossil fuels and pollution is the reason behind the utilization of renewable sources. A major renewable energy source, solar energy, requires efficient thermal energy storage due to its intermittent nature. Phase change material (PCM) can store a large amount of heat by changing its phases. However, the low thermal conductivity of PCM makes its usage very limited. Hence, PCM's heat transfer augmentation technique using eccentricity and fin is studied in the present research. First of all, positive (lower displacement of the inner tube) and negative eccentricity (upper displacement of the inner tube) are studied in triplex type latent heat thermal energy storage (TTLHTES). Among the eccentricities of -15 mm to +15 mm, the eccentricity of +10 mm is found most effective in melting with a 27.63% reduction in melting time, while -3 mm eccentricity is found most effective in solidification with a 12.82% reduction in solidification time. The combination of fins with eccentricity is examined for melting and solidification enhancement. The combination of fin and eccentricity efficiently reduces melting time, with a maximum decrement of 67.37% with +3 mm eccentric TTLHTES with four fins. However, the only fin without any eccentricity is found more effective in solidification enhancement. The solidification time is reduced maximum by 46.15% using four longitudinal fins and no eccentricity. It can be concluded that fin and eccentricity must be coupled when melting time is required to enhance. However, only fin must be utilized when solidification is more critical.

A comprehensive review on compound heat transfer enhancement using passive techniques in a heat exchanger

The heat transfer is a very interesting field for efficient, useful, eco-friendly reasons. The economic & environmental factors demand that we get the maximum possible efficiencies from thermal power plants possible only by making more efficient heat exchangers. Increasing the effectiveness of new and existing heat exchangers is a typical criterion of engineers involved with the design of heat exchangers. Augmenting process plant efficiency and finding new ways to reduce energy requirements is always an important design priority. Therefore, the heat transfer augmentation plays a substantial role in improving energy efficiency and development of high-performance thermal systems. The heat transfer augmentation techniques are classified as passive, active and compound techniques. The current article presents an extensive review of experimental and numerical studies on passive compound heat transfer augmentation techniques (PCHTAT), combining two or more of the current techniques can be employed all together to produce an enhancement larger than that produced by using only one augmentation. The compound technique arrangement are (i.e. helical-ribbed tube with double twisted tape inserts, Dimpled tube fitted with a twisted tape swirl generator, non-uniform wire coil, and twisted tape inserts. etc.) which covers the laminar and turbulent flow regions. Interactions between different enhancement methods will increase the overall heat transfer coefficient of the thermal systems than that produced by using only one augmentation technique.

Enhancing the melting of phase change material using a fins–nanoparticle combination in a triplex tube heat exchanger

A combination of heat transfer augmentation techniques is highly necessary to enhance the performance of Thermal Energy Storage (TES) systems employed in a wide range of applications. The major issue is that many of the Phase Change Materials (PCMs) possess low thermal conductivity (k≤0.2 W/m K), resulting in an inefficient melting process. Triplex Tube Heat Exchanger (TTHX) based TES system is both numerically and experimentally studied using Paraffin (RT82) with Alumina (Al2O3) nanoparticles that has a charging temperature in the range of 78.15–82.15 ∘C. The experimental findings indicate that the Paraffin is not completely melted within the required time of four hours for the inside heating method at 97 ∘C. The Paraffin is successfully melted for both sides heating at 90 ∘C in lesser time and average temperature than the outside heating. With different charging temperatures, the Paraffin melting was consumed a short time for the non steady state at the mass flow rate of 29.4 kg/min, compared with the 16.2 and 37.5 kg/min for inner and outer tubes. Other outcomes were that with the fins–nanoparticle combination, an improved performance for melting the Paraffin, compared with those that occurred without nanoparticle. Furthermore, in the numerical study, compared with the pure Paraffin case, the melting time was minimized for TTHX with longitudinal fins (12%) and TTHX with triangular fins (22%) for the PCM having 10% nanoparticle, respectively. Close agreement is found between the numerical and experimental findings.

Experimental Study of Heat Transfer Enhancement by Turbulence Generator in Side Pipe

The enhancement of heat transfer exchangers is an important matter of concern for achieving more efficient thermal energy conversion systems. The use of modified twisted tape (TT) inserts as a passive heat transfer augmentation technique is an efficient way of improving heat transfer. Twisted tape insert is one of the passive heat transfer enhancement techniques that are widely used in various applications for heat transfer, such as air conditioning and cooling systems, heat recovery systems, Processes, procedures for food and milk, plants for chemical processes. To increase the heat transfer rate of the tubular heat exchanger, in this study a small-scale experimental setup was performed. The heat improvement of the heat exchanger is done using twisted tape inserts with a twist ratio (y=5.5). The effects of two-dimensional parameters namely Nusselt number & friction factor on tubular heat exchanger effectiveness are studied. The turbulent flow was generated by the insertion into the test pipe of the twisted tape inserts, producing a high turbulence rate in the pipe, resulting in an increase in turbulence. And hence the enhancement of heat transfer and drop in pressure. The length and width of the insert were 1000 mm and 21.5 mm respectively. The diameter of the test pipe outside & inside is 23 mm and 22 mm respectively. The length of the test section is 2000 mm. The study is carried out for the Reynold number regime (500-6500). The results show that The maximum enhancement for Nu number at Re 500 was 32% that of the plain tube.

Heat Transfer Augmentation Using Water-in-Glass Evacuated Tube Coupled with Parabolic Trough in Rack Dryer in the Drying of Capsicum Frutescens

Open drying is practiced for drying Capsicum frutescens in the cottage industries of India. In order to improve drying, an experimental set-up of solar rack dryer is constructed and studied different heat transfer augmentation techniques. A passive heating coil extended from the header of a water-in-glass evacuated tube coupled with parabolic trough is used to augment the heat in the collector section of the dryer. The drying rate in rack dryer is found to be 62% higher than that of open air drying of green chillies. Moreover, the drying rate in augmented rack dryer is found to be 7% higher than that of rack dryer. In addition to this, an effective design for the dryer cabinet with respect to air circulation is arrived through computational fluid dynamics analysis. Through this new design, the drying rate is improved further by 3%. Based on energy analysis, the specific energy consumption during the drying of this particular product using the augmented dryer is found to be 1.27 kWh/kg. The exergy efficiency of the drying chamber is found to be in the range of 4%-45% with an average of 16% for Capsicum frutescens.