Showing papers in "Thermal science and engineering in 2020"

Renewable hydrogen implementations for combined energy storage, transportation and stationary applications

Abstract: The purpose of this paper is to discuss the potential of hydrogen obtained from renewable sources for energy generation and storage systems. The first part of analysis will address such issues as various methods of green hydrogen production, storage and transportation. The review of hydrogen generation methods will be followed by the critical analysis and the selection of production method. This selection is justified by the results of the comparative research on alternative green hydrogen generation technologies with focus on their environmental impacts and costs. The comparative analysis includes the biomass-based methods as well as water splitting and photo-catalysis methods while water electrolysis is taken as a benchmark. Hydrogen storage and transportation issues will be further discussed in purpose to form the list of recommended solutions. In the second part of the paper the technology readiness and technical feasibility for joint hydrogen applications will be analysed. This will include the energy storage and production systems based on renewable hydrogen in combination with hydrogen usage in mobility systems as well as the stationary applications in buildings such as combined heat and power (CHP) plants or fuel cell electric generators. Based on the analysis of the selected case studies the author will discuss the role of hydrogen for the carbon emission reduction with the stress on the real value of carbon footprint of hydrogen depending on the gas source, storage, transportation and applications.

Natural convection and entropy production in hybrid nanofluid filled-annular elliptical cavity with internal heat generation or absorption

Abstract: This study is an attempt to understand the characteristics of heat transfer by natural convection, flow, and entropy generation of Cu-Al2O3/H2O based hybrid nanoliquid filled-annulus delimited by two elliptic cylinders considering internal heat generation or absorption (IHG/A) phenomenon The buoyancy-driven flow is induced by a thermal gradient between isothermal and differentially heated inner and outer cylinders A numerical solution of the governing equations in the dimensionless and non-primitive form is performed using the technique of finite volume discretization Impacts of diverse parameters of the study such as copper-alumina nanoparticles volumic concentration, Rayleigh number, and dimensionless internal heat generation or absorption parameter on the thermohydrodynamic characteristic and entropy generation are examined An analysis of the results showed that the combined effects of internal heat generation/absorption and hybrid nanoliquid significantly alter the hydrothermal characteristics, heat transfer rate, and entropy generation within the annulus

Transient simulation of finned heat sinks embedded with PCM for electronics cooling

Abstract: A shear usage of electronic mobile devices during gaming, Web browsing and high-power consuming applications, the portable devices get considerably hot enough to be damaged. Previously, active cooling methods adopted to cool the electronic devices had some limitations.

Development of indirect type solar dryer and experiments for estimation of drying parameters of apple and watermelon

Abstract: In the present paper, an indirect type solar dryer (ITSD) has been developed for apple and watermelon drying. The performance of ITSD and drying kinetics of watermelon and apple slices have been analyzed. From the experimental analysis, diffusion coefficient, surface transfer coefficients and activation energy of apple and watermelon were estimated. Experimental outcomes showed that the temperature inside the drying cabinet varied with time because of frequent variation in solar intensity. The average thermal efficiency of the collector and dryer was 54.5% and 25.39% during apple drying and 56.3% and 28.76% for watermelon drying, respectively. Moisture content of apple decreased from 6.16 to 0.799 kg/kg of dry basis (db) and that of watermelon reduced from 10.76 to 0.496 kg/kg of db. Drying curve was fitted with different models of existing studies. Average effective moisture diffusivity was estimated and it was 4.28 × 10−9 m2/s and 4.01 × 10−9 m2/s for apple and watermelon, respectively. Mass transfer coefficient is found to be in the range 1.584 × 10−4 to 3.158 × 10−3 m/s for apple and 5.17 × 10−4 to 4.98 × 10−3 m/s for watermelon. Heat transfer coefficient was in the range of 0.16 to 3.19 W/m2 K and 0.52 to 5.04 W/m2 K for apple and watermelon, respectively. Activation energy for apple and watermelon is 17.34 and 18.71 kJ/mol, respectively.

A review of printed circuit heat exchangers for helium and supercritical CO2 Brayton cycles

Abstract: Printed circuit heat exchangers (PCHEs) are a promising technology for helium and supercritical CO2 Brayton cycles due to their highly compact construction, very high heat transfer coefficients, capability to withstand high pressures and wide range of operating temperatures. The purpose of this review is to provide a comprehensive understanding of the performance of PCHEs based on available literature and survey of heat exchangers currently available on the market. First, the fundamental principles, including material selection, manufacturing and assembly, are introduced. Then, PCHEs with different flow passages are summarized and analysed along with their heat transfer and pressure drop characteristics. Next, geometric design optimisation of PCHEs is summarised and discussed, taking into consideration the complex relationships between heat transfer enhancement and pressure drop penalty, compactness and fluid inventory as well as capital cost. Finally, knowledge gaps are identified and suggestions for further research to address these for a wider range of applications are presented. The review covers relatively new heat exchangers on the market as well as designs that are still under development. Although extensive work has already been done in this field, and PCHEs are well established in the petrochemical industry, significantly more work is needed to increase their attractiveness for a wider range of applications. This work should be aimed at the optimisation of flow passage configurations in terms of thermohydraulic performance, complexity and manufacturing costs, development and selection of materials to increase further the range of high temperature and pressure operation, and the development of more generalised correlations for performance prediction and overall design optimisation.

Analysis of entropy generation in biomimetic electroosmotic nanofluid pumping through a curved channel with joule dissipation

Abstract: Biomimetic designs are increasingly filtering into new areas of technology in recent years. Such systems exploit characteristics intrinsic to nature to achieve enhanced adaptivity and efficiency in engineering applications. Peristaltic propulsion is an example of such characteristics and in the current article it is explored as a feasible mechanism for deployment in electrokinetic pumping of nanofluids through a curved distensible conduit as a model for a bioinspired smart device. The unsteady mass, momentum, energy and nanoparticle concentration conservation equations for a Newtonian aqueous ionic fluid under an axial electrical field are formulated and simplified using lubrication approximations and low zeta potential (Debye Huckel linearization). A dilute nanofluid is assumed with Brownian motion and thermophoretic body forces present. The reduced non-dimensional conservation equations are solved with the symbolic software, Mathematica 9 via the NDSolve algorithm for velocity, temperature, nano-particle concentration distributions for low zeta potential. An entropy generation analysis is also conducted. The influence of curvature parameter, maximum electroosmotic velocity (Helmholtz-Smoluchowski velocity), inverse EDL thickness parameter, zeta potential ratio and Joule heating parameter on transport characteristics is evaluated with the aid of graphs and contour plots. Temperature profiles are elevated with positive Joule heating and reduced with negative Joule heating whereas the opposite behaviour is observed for the nano-particle concentrations.

Performance evaluation of square pyramid solar still with various vertical wick materials – An experimental approach

Abstract: The demand for fresh water is increasing rapidly, but its supply has been typically decreasing over the past decades. The scarcity of drinking water can be resolved through the desalination of seawater. The present study deals with analysing the performance of a square pyramid solar still (SPSS) in the presence of various vertical wick materials by experimentally. To increase the distillate productivity of the conventional SPSS, various types of wick materials were placed vertically in the water basin. Initially, the experiments were conducted for different basin water depths (2, 3, 4, 5, and 6 cm) to calculate the effective basin water depth. The results indicated that the water depth was 2 cm, which provided 14.4%, 23.1%, 31.3%, and 39.6% higher productivity than 3, 4, 5, and 6 cm basin water depths, respectively. In addition, experiments were conducted for various wick materials, like polyester, terry cotton, jute cloth, and woollen fabric, to enhance SPSS productivity at 2 cm basin water depth. The evaporation rate increased due to the capillary action of the wick material. The experimental results proved that the SPSS productivity was 9.4%, 20.9%, and 33.1% higher by using woollen fabrics than of jute, terry cotton and polyester, respectively.

The impact of adding nano-Al2O3 and nano-ZnO to Iraqi diesel fuel in terms of compression ignition engines' performance and emitted pollutants

Abstract: Iraq's diesel fuel has high sulphur content due to the failure of refineries and the underlying quality of the feed crude oil. Aluminum oxide and zinc oxide nanoparticles were added to conventional Iraqi diesel to evaluate on the performance and pollution of a diesel engine not equipped with exhaust gas post-treatment equipment. Before the experiments, the fuel was mixed with the nanoparticles in specified quantities (50 ppm and 100 ppm), and samples of the mixtures were subjected to several tests. Measurements of the mixtures’ thermophysical properties showed that the density, viscosity, and thermal conductivity of the suspension increased. Stability tests showed that these mixtures remained stable for at least two months indicating the success of the mixing process. The specific fuel consumption decreased and thermal efficiency increased when nanoparticles were added. This effect increased as a function of nanoparticle dose. Carbon monoxide, unburnt hydrocarbons, and all kinds of particulate matters, sulphur dioxide and hydrogen sulphate decreased with nanoparticle addition. NOx has increased because of higher generated temperature inside the combustion chamber as a result for nanoparticles addition. The results indicated that adding 100 ppm of nano-alumina gave the best results: It reduced specific fuel consumption by 8% and increased the brake thermal efficiency by 6%. The 100 ppm nano-alumina decreased CO, HC, TSP, SO2, and H2S by 17%, 17%, 26%, 19% and 19%, respectively. The NOx increased by 10%. The study results indicate a significant improvement in engine performance and a clear reduction in most emissions. In the future, the method proposed by the study will play an important role in improving unmodified diesel engine outputs using Iraqi diesel with nanoparticles addition.

Effects of Dufour and Soret mechanisms on MHD mixed convective-radiative non-Newtonian liquid flow and heat transfer over a porous sheet

Abstract: The mixed convection heat transfer combined with thermal radiation of a viscoelastic liquid circulation driven by a porous accelerating sheet under the inclined uniform magnetic field impact is studied. The considered phenomenon is modeled by the set of nonlinear differential equations affected by the boundary conditions. These equations are reduced into coupled nonlinear ordinary differential equations with the proper choice of similarity transformations. Further, the analytical solution is determined for transformed system. Also, the governing equations and the solutions are obtained for all considered functions. The temperature distribution impacted by the thermal radiation is very important in industrial fields as in accelerating sheet where cooling of the liquid plays a vital role in obtaining the desired outcome. In engineering applications like extrusion processes, metal spinning, dye casting of metals and polymer production this type of flow problem exists and where the maximum temperature and difference prediction should be controlled. Analysis has been performed for a wide range of radiation number (0 ≤ Nr ≤ 1), Richardson number (0.5 ≤ Λ ≤ 6), and magnetic field inclination angle (0 ≤ τ ≤ π/2).

An experimental investigation into engine characteristics fueled with Lal ambari biodiesel and its blends

Abstract: Nowadays the production of biodiesel from non-edible vegetable oil feedstock is attaining more attention than edible vegetable oil for the replacement of diesel fuel. The novelty of this experimental paper is that it discussed the new biodiesel feedstock as a substitute fuel for a CI engine that has not drawn much consideration among other researchers. Diesel, Lal ambari biodiesel and its blends were used for conducting a short-term test and their effects on CI engine characteristics under different loading conditions have been evaluated. Engine tests have led to the assessment of the performance parameters such as exhaust gas temperature, brake thermal efficiency and brake specific fuel emission characteristics corresponding to oxides of nitrogen, carbon dioxide and smoke and combustion characteristics. These include heat release rate, the maximum rate of pressure rise, cylinder pressure and ignition delay; they were recorded and analyzed furthermore to decide the ideal fuel blend. Results show that the BTE for blend LA20 decreased by 2.5%, while the BSFC increased by 4.4% more than diesel at full load condition. Reduction in smoke emission was recorded for LA20 by 16.6%, while on the other hand, CO2 emissions increased by 3.5% compared to diesel fuel at full load condition. The HRR and cylinder pressure was lower for biodiesel, and its blends at full load condition. The experimental result proved that Lal ambari biodiesel and its blends could be a potential alternative fuel for a CI engine.

Experimental investigation and performance evaluation of a novel solar dryer integrated with a combination of SHS and PCM for drying chilli in the Himalayan region

Abstract: This work presents an experimental analysis of a novel indirect solar dryer for drying chillies in the Western Himalayas. Iron scrap assorted with gravels are kept on the absorber plate and engine oil transported through the copper tube, have been applied as sensible heat storage (SHS) medium in the solar air collector (SAC). Paraffin wax has been employed as a phase change material (PCM) in the drying compartment. The performance evaluation of the drying system has been carried out on the basis of physical, thermal and quality attributes. The moisture level of chillies is reduced to 4.85% from 86.50% (wet basis) in 21 h as compared to 96 h and 150 h by using solar drying without thermal storage and in the natural sun, respectively. Outcomes of this work conclude that the dehydrating time is reduced by 78.12% and 86.00% as against drying conducted without the use of SHS and PCM, and natural sun; respectively. Using this drying system, the colour, texture, pungency (sensory parameters) and rehydration capacity of the dehydrated chillies have been found better. From the experimental results vitamin C, capsaicin, carotene and thiamin of the chillies were found 49.25 mg, 0.52%, 341.5 µg and 0.912 mg which were 32.25 mg, 0.23%, 217.8 µg and 0.534 mg, in case of natural sun drying.

Effect of modified surfaces on bubble dynamics and pool boiling heat transfer enhancement: A review

Abstract: The vast application of boiling phenomena in industrial processes has led scientists to seek methods to increase boiling heat transfer. This requires a precise prediction of the boiling heat transfer coefficient (HTC) between the heater surface and the boiling fluid. Considering the complexity of the heat transfer mechanism, the prediction of this phenomenon under different conditions can be difficult. In pool boiling process, a liquid is heated to its boiling point through a heating surface and phase transition from the liquid state to the vapor state will occur. Therefore, the physical and chemical properties of the liquid, vapor and surface are important in this process.“ In this paper, a review of methods for increasing the boiling heat transfer carried out in three main parts: 1) Surface textures 2) Surface characteristics 3) Surface structures. The effect of modification of these parts on the boiling process parameters, especially the bubbles dynamics, the heat transfer coefficient and critical heat flux (CHF) were investigated which are the most important criteria for evaluating the boiling. The purpose of this study is to obtain a relatively comprehensive knowledge of the enhancement techniques in pool boiling heat transfer in recent years. Empirical correlations and models containing surface specifications are also reviewed and presented for the prediction of boiling characteristics.

Thermodynamic and thermoeconomic analysis of three cascade power plants coupled with RO desalination unit, driven by a salinity-gradient solar pond

Abstract: Solar-based organic Rankine cycles (ORCs) for power supply of reverse osmosis (RO) desalination units are the most energy-efficient technologies for brackish water and seawater desalination within the small-to-medium electricity ranges ( 500 k W ). To collect and absorb solar energy, various collectors are devised, where among all, solar pond (SP) has been considered as a low-cost solar collector for a long working period. Meantime, conventional ORC/RO systems have a low condensing pressure and density, and consequently a turbine with large volume should be designed. The best solution is design of a desalination system powered by a cascade ORC or KC (Kalina cycle), using thermal heat of a SP for long-term applications. With this regard, three new cascade systems based on ORC and KC are proposed and thermodynamic and thermoeconomic analysis of the proposed systems are carried out. It is found that the ORC/ORC/RO system has the highest net power, power efficiency, and exergy efficiency as well as lowest SUCP (sum unit cost of product) and total exergy destruction. However, the total exergy rate of loss of the ORC/ORC/RO system was around 20% higher than that of the ORC/KC/RO system and approximately 10.81% lower than that of the KC/KC/RO system. Among all components, three components of condenser, cascade heat exchanger, and vapor generator had the highest exergy destruction in the ORC/KC/RO, ORC/ORC/RO, and KC/KC/RO systems, respectively. In addition, among all systems, the ORC/ORC/RO system had the lowest product cost rate and loss cost rate of 2185.83 $/h and 309.65 $/h, respectively. The results of case study simulation for the Bushehr Port indicated that the time between May and August can be the most appropriate time in a year in order to extract more thermal heat from the solar pond for the proposed power/desalination systems. Furthermore, the most economical month for the proposed power/desalination was June due to the low value of SUCP (72.42 $/kWh) since more fresh water was produced in this month.

Experimental investigation on the thermal performance of compact heat exchanger and the rheological properties of low concentration mono and hybrid nanofluids containing Al2O3 and CuO nanoparticles

Abstract: The present work focused on preparing and studying the fluid and thermal properties such as density, viscosity, specific heat capacity and thermal conductivity of low concentration hybrid nanofluids. The present work also focused on comparing these properties and the thermal enhancements with other low concentration mono nanofluids. The nanofluids used for present work are Al2O3-CuO, Al2O3 and CuO combined with a 60:40 mixing ratio of distilled water and ethylene glycol acting as the base fluid. The volume concentration at which the nanofluids are prepared are fixed 0.02%, 0.04% and 0.06%. The findings show that the thermal conductivity of the hybrid samples is higher compare to its mono counterparts. The thermal conductivity of Al2O3-CuO showed enhancements by 2.3% when compared to CuO and by 3.6% when compared to Al2O3. By using the prepared samples as a working fluid in a compact fin and tube heat exchanger test rig, an experimental investigation is conducted. This is to observe any form of improvements in terms of heat transfer by comparing the experimental results of the hybrid nanofluids with its respective mono counterparts. Based on the experiments conducted, it is found that the use of Al2O3-CuO hybrid nanofluids as a working fluid in a compact heat exchanger test rig showed an increase in both Nusselt number and average heat transfer coefficients by 6.7% and 7.2% respectively when compared to CuO nanofluids and by 17.9% and 12.1% respectively when compared to Al2O3 nanofluids.

Combination of Co3O4 deposited rGO hybrid nanofluids and longitudinal strip inserts: Thermal properties, heat transfer, friction factor, and thermal performance evaluations

Abstract: The reduced-graphene oxide/cobalt oxide hybrid nanoparticles were prepared based on the in-situ/chemical co-precipitation technique, and they were analyzed by transmission electron microscope, x-ray diffraction, and magnetometer techniques. The hybrid nanofluids were prepared with particle loadings of 0.05%, 0.1%, and 0.2% by dispersing synthesized reduced-graphene oxide/cobalt oxide in distilled water and their physical properties were measured. The thermal performance of the nanofluids was studied, when they flow in the turbulent regime through a circular tube. The thermal performance was also evaluated when straight (longitudinal) strip inserts with aspect ratios of 1, 2, and 4, were used inside the circular. These straight strip inserts by increasing the flow turbulence intensity act as turbulators. Results indicate that with a dilution of 0.2% concentration of hybrid nanoparticles in water, the Nusselt number is enhanced by 25.65%, and it is further enhanced by 110.56% with a straight strip insert of aspect ratio 1. The use of hybrid nanofluids and straight strip inserts leads to a slight penalty in fluid friction. For 0.2% concentration of hybrid nanoparticles in water, the penalty in friction factor is 11%, and it is further increased to 69.8% with 0.2% particle loadings and a straight strip insert of aspect ratio1. Moreover, the thermal performance factor of hybrid nanofluids with and without straight strip inserts presents values higher than 1, which shows the benefit of the prepared hybrid nanofluids in a turbulent flow. A general form of regression equations are developed based on the experimental data.

Unsteady magneto-hydrodynamic transport of rotating Maxwell nanofluid flow on a stretching sheet with Cattaneo–Christov double diffusion and activation energy

Abstract: A description of magnetohydrodynamic effects on the transient rotational flow of Maxwell nanofluids is considered. The temperature and concentration distributions are associated with Cattaneo- Christove double diffusion, Brownian motion and thermophoresis. The diffusion of chemically reactive specie is investigated with Arrhenius activation energy. The governing equations in the three-dimensional form are transmuted into dimensionless two-dimensional form with the implementation of suitable scaling transformations. The variational finite element procedure is harnessed and coded in Matlab script to obtain the numerical solution of the coupled non-linear partial differential problem. The varying patterns of velocities, skin friction coefficients, Nusselt number, Sherwood number, fluid temperature and concentration functions are computed to reveal the physical nature of this study. It is observed that higher inputs of the parameters for magnetic force, Deborah number, rotational fluid, and cause to slow the primary as well as secondary velocities but they raise the temperature like thermophoresis and Brownian motion does. However, the thermal relaxation parameter reduces the nanofluid temperature. The local heat transfer rate reduces against Nt, rotational, and Nb parameters, and it is higher for Prandtl number. The current FEM (finite element method) solutions have been approved widely with the recently published results, showing an excellent correlation. The examination has significant applications in the food industry and relevance to energy systems, biomedical, and modern technologies of aerospace systems.

Geometric optimization of a double pipe heat exchanger with combined vortex generator and twisted tape: A CFD and response surface methodology (RSM) study

Abstract: In this research, a numerical investigation is done on the effect of employing the new combined vortex generators, the twisted tape turbulator and Al2O3-H2O nanofluid as the involved base fluid. Such study is carried out on the behavior of the heat transfer rate and the pressure drop of a double pipe heat exchanger. Accordingly, the response surface methodology (RSM) grounded on the central composite design (CCD) is used for acquiring the optimized geometry of the combined vortex generator and twisted tape turbulator. In order to have the maximum Nusselt number and minimum friction factor, twenty cases with different pitches ratio P i l = 0.09 - 0.18 , angles ( θ = 0 - 30 °) and Reynolds numbers (Re = 5000 - 20000 ) are examined. The Results show that the pitch ratio has a predominant effect on the Nusselt number and the friction factor, which causes an efficiency increase up to five times compared to the original one. In addition, by decreasing the angle of the vortex generators in the new combined turbulator, both Nusselt number and the friction factor are increased.

Next generation biofuels derived from thermal and chemical conversion of the Greek transport sector

Abstract: In Greece today, only biodiesel is produced to cover domestic consumption requirements. Next-generation biofuels are not produced in an industrial scale except in the context of research programs and industry partnerships with the academic community and research institutions. In this paper nine next generation biofuels are proposed to decarbonize the Greek transport sector i.e. advanced bioethanol, biomethanol, advanced biodiesel, hydrothermal upgraded biodiesel, hydrotreated vegetable oil biodiesel, algal biofuels, bio-hydrogen, biosynthetic natural gas and bio-dimethyl-ether. The oil industry of the future will incorporate all possible sources of raw materials (crude oil, natural gas and biomass) for fuel production utilizing a variety of thermal and chemical processes. There is a considerable potential of lignocellulosic biomass, mainly from cereal crops i.e. wheat, maize, barley and forest residues. Integrating biomass processing to biorefineries, both energy and high added value byproducts are produced.

Numerical investigations of heat transfer enhancement in a house shaped-corrugated channel: Combination of nanofluid and geometrical parameters

Abstract: Improving the heat transfer rate is one of the main issues at the design stage of different thermal devices for various industries. In this research, a numerical simulation is performed to investigate the combined influences of nanofluid and various parameters designs of a house-shaped corrugated channel on the thermal and hydraulic performance under uniform heat flux of 10 kW/m2 and Reynolds number range of 10,000–30,000. In respect to the fluid medium, SiO2 nanoparticles are used and investigated with volume fraction up to 0.08. The impacts of geometrical parameters including height-to-width ratio (h/W), pitch-to-length ratio (p/L), and house ratio (e/r) on thermal and hydraulic characteristics are evaluated. The findings show that the (h/W) ratio has more influence on heat transfer promotion than the (p/L) ratio. At Reynolds number 30,000, there is a 16.63% increment in average Nusselt number due to a decrease of the (p/L) ratio from 0.175 to 0.075, while the increment 92.28% is achieved by an increase of the (h/W) ratio from 0.0 to 0.05. Heat transfer increases with roof height (r) and decreases with the vertical height of the house-shaped corrugation (e). The findings detect that a h/W of 0.05 with a p/L of 0.075 and e/r = 0.6667 are optimum parameters that showed significant improvement in thermal performance. Moreover, new correlations for the Nusselt number and friction factor were developed and reported. Using nanofluid with the current channel is a useful source of reference to enhance thermal performance and produce more compact heat exchangers.

Modelling and simulation of a hybrid solar-electrical dryer of wood integrated with latent heat thermal energy storage system

Abstract: The main objective of this study is to investigate numerically a novel design of an indirect hybrid solar-electrical dryer of wood integrated with latent heat thermal energy storage system. The studied wood dryer system is composed mainly of a drying chamber, a solar air collector, a thermal energy storage system with Phase Change Material (PCM) as storage medium and an electrical heater. Two numerical models for both drying chamber and thermal energy storage system are developed and validated with existing experimental data. These models are coupled with TRNSYS software standard library and a global model for the dryer system is presented. An optimization study of the thermal storage system is performed and the optimal amount of PCM and the optimal number of tubes were determined. The thermal performance of the developed dryer system is investigated under weather conditions of Tangier site in Morocco. Obtained results show that using the thermal storage system in the dryer system ensures a continuous wood drying process and the temperature of the drying chamber is higher than that of the ambient air by about 4–20 °C, all the night. Performance analysis of the dryer system shows that throughout the year, the drying time in the proposed dryer system with latent heat storage does not exceed 5 days. However, it varies between 7.5 days during summer and 12 days in winter in the case of the dryer without heat storage system.

Numerical study of mixed bio-convection associated with a micropolar fluid

Abstract: In the present study, the nanoliquid flow, heat and mass transfer over a vertical stretching sheet under the impact of motile microorganisms are investigated, numerically. Analysis is performed using the two-component four-equation non-homogeneous equilibrium nanoliquid model. At the same time, the nanoliquid is modeled as a micropolar non-Newtonian fluid. Formulated partial differential governing equations are transformed to ordinary differential equations and solved by the fourth-order Runge-Kutta method with the iterative Newton-Raphson technique. Analysis is conducted for a wide range of control parameters and heat transfer enhancement is obtained for high values of the Richardson number, bioconvection Lewis number and bioconvection Peclet number. It has been revealed that with increasing microorganisms concentration difference parameter (Ω), the angular velocity of fluid particles slightly increases and the temperature reduces and increases with increment in Ω and the bioconvection Rayleigh number (Rb), respectively. The results also showed that increasing bioconvection Lewis number (Lb) and Peclet number (Pe) leads to a decrease in the coupled stress; moreover, an increment in the vortex viscosity parameter (Δ) intensifies the penetration of microorganisms from the sheet to the boundary layer and amplifies the density number of motile microorganisms.

A detailed review on CO2 two-phase ejector flow modeling

Abstract: Ejector-equipped vapor-compression systems for refrigeration and cooling, relying solely on CO 2 (R744) as a natural working fluid, are perceived to be an eco-friendly and highly efficient solution for many applications. However, the complexity of two-phase ejector flows makes it very challenging to find realiable and efficient ejector designs. Improved design methods are necessary in order to achieve higher performance in R744 units compared to the traditional compressor-based systems with refrigerants that put a high strain on the environment. Consequently, the development of advanced models and tools for an accurate design of the R744 ejectors has been a highly prioritized research topic. To the best of the authors’ knowledge, the current status of R744 ejector models and their limitations has not been thoroughly evaluated yet. To summarise the current state of the art and knowledge gaps, this work presents an exhaustive overview of the available numerical models applied to R744 two-phase ejectors, i.e. multiphase flow modeling, turbulence aspects, numerical solution methods, applications of models, to further encourage the adoption of R744 vapor-compression solutions. Finally, a thorough discussion of different focus points for future research as well as the main challenges in the field is presented.

CFD analysis of a nanofluid-based microchannel heat sink

Abstract: Effective thermal management is a key for the continuous development of electronics, which are characteristics of modern life. It has a great effect on the lifetime, durability and reliability of these systems. A liquid-cooled microchannel heat sink is a compacted cooling part that used to provide better heat dissipation rates and low temperatures in electronic components. Nanofluids have been introduced as effective coolants to be employed in this type of heat sink to increase the heat dissipation rate. However, a comparative assessment of the thermal performance between commonly used nanofluids and water as coolants for microchannel heat sinks is still lacking. For this purpose, a computational fluid dynamics (CFD), non-isothermal, three-dimensional detailed model has been developed to simulate and analyze the fluid flow and heat transfer physiognomies. The results show that examining performance parameters as functions of Reynolds number is misleading since the thermophysical properties are different among each coolant, and that employing nanofluids in a microchannel heat sink is impractical as water is cheaper and safer.

CFD analysis of heat transfer enhancement in plate-fin heat sinks with fillet profile: investigation of new designs

Abstract: In the last decades, attempts have been made on developing more advanced effective cooling technologies of electronic and microelectronic equipment, however, the heat dissipation is still the main challenge for enhancing the cooling efficiency of heat sinks in a highly competitive electronic industry. In current study, the investigation develops a new thermal design for plate-fin heat sinks with fillet profile so that the removed material from the base, to generate fillet, is attached to the plate-fins in the form of half-round pins. To evaluation of thermal performance of the proposed designs, a numerical study has been carried out based on commercially available computational fluid dynamic (CFD) codes. In particular, different arrangements of half-round pins (which are attached to the plate-fins) subject to both parallel and impinging flow have been compared in terms of their thermal efficiency. The study has shown that the plate-fin heat sinks with corrugated half-round pins in vertical arrangement subject to parallel flow and plate-fin heat sinks with symmetrical half-round hollow pins in vertical arrangement subject to impinging flow show superior thermal performance over other configurations. The former design provides approximately 25.1% and 29% reduction in the base temperature and thermal resistance respectively along with approximately 34.48% increase in Nusselt number rather than the conventional plate-fins with fillet profile. For the latter design, approximately 22.6% and 25.7% reduction in the base temperature and thermal resistance were achieved respectively while Nusselt number sees approximately 31.6% higher values. Therefore, these designs have a promising potential to be applied for cooling the electronic devices.

A perspective on the current and future roles of additive manufacturing in process engineering, with an emphasis on heat transfer

Abstract: This review provides an overview of the current state of additive manufacturing in the context of process engineering. The results and opinions from a wide range of public domain reports has been compiled, and used to formulate a new perspective in the ways additive manufacturing has been exploited for heat transfer, and what some of the future opportunities will be moving forward. In addition, this review identifies several specific challenge areas for heat transfer, as well as various broad challenge areas affecting additive manufacturing applications in general. Finally, this review discusses three case studies from the process intensification group at Newcastle University, to put into context how additive manufacturing addresses problems in the pursuit of meeting new intensification objectives. These include: (1) the miniaturisation of the TORBED® technology for screening adsorbents for carbon capture, (2) proposing novel heat pipe wick geometries that could potentially fully optimise the thermal performance of heat pipes, and (3) producing complex reactor geometries to improve the scalability of flow chemistry.

Investigating the influence of oxygenated fuel on particulate size distribution and NOX control in a common-rail diesel engine at rated EGR levels

Abstract: The effect of butanol-diesel blend (B20) and different rates of exhaust gas recirculation (EGR) on particulate matter (PM) characterisation and NOX emissions were investigated in this experimental work. The different rates of EGR (0%, 15% and 30%) and oxygenated fuel (B20) were used in this study. The brake thermal efficiency (BTE) of the engine increased under without used EGR technique compared to the presence of EGR during combustion process for B20 and diesel fuel. Further, higher BTE produced during the combustion of B20 than that produced from combustion of diesel for different rates of EGR. The BSFC slightly increased with B20 combustion compared to the diesel combustion. The cylinder pressure and ROHR improved with combustion of B20 under without using EFR. The results showed that PM reduced from the combustion of B20 more than to the diesel fuel for different rates of EGR. Besides, the average of particle diameter reduced from B20 combustion by 20, 22 and 25 nm compared to the diesel by 31, 37 and 43 nm for 0%, 15% and 30% of EGR, respectively. The soot particles generated from B20 combustion are lower than to the diesel fuel combustion for different rates of EGR. Lower level of NOX emissions can be achieved with using EGR (15% and 30%) than to the without EGR technique, this effect is more clearly with oxygenated fuel. It is indicated that the EGR dramatically increase the THC and CO emissions when exceeds the 30% with no obvious influence on NOX emissions.

Evaluation of transparent acrylic stepped solar still equipped with internal and external reflectors and copper fins

Abstract: This study experimentally evaluated the effect of combining stepped solar still with copper fins, transparent step walls and internal and external reflectors on its fresh water production and daily efficiency, compared with a conventional single-basin solar still tested under the same environmental conditions of hot arid area in Qena City in Upper Egypt territory, during daytime starting at 7:00 a.m. until 7:00 p.m. in summer conditions. The experimental results revealed that the new stepped solar still attained 129% increase in fresh water production compared with the conventional still. The results also showed that the daily efficiency of the modified stepped solar still reached about 63%, which was found to be higher than that for other stepped solar stills in the literature. Moreover, the daytime-accumulated fresh water per day attained up to 8285 ml/m2/day for an optimum seawater depth of 30 mm in stepped basin, with an additional 630 ml/m2/day of fresh water accumulation during nighttime. Furthermore, the produced fresh water can be used directly in different human applications, due to its acceptable levels of pH, turbidity, conductivity and total dissolved solids (TDS). Therefore, the modified stepped solar still is recommended for the use in remote and hot arid areas.

Hall current, viscous and Joule heating effects on steady radiative 2-D magneto-power-law polymer dynamics from an exponentially stretching sheet with power-law slip velocity : a numerical study

Abstract: A mathematical model is developed for 2-D laminar, incompressible, electrically conducting non-Newtonian (Power-law) fluid boundary layer flow along an exponentially stretching sheet with power-law slip velocity conditions in the presence of Hall currents, transverse magnetic field and radiative flux. The secondary flow has been induced with appliance of Hall current. The distinguish features of Joule heating and viscous dissipation are included in the model since they are known to arise in thermal magnetic polymeric processing. Rosseland’s diffusion model is employed for radiation heat transfer. The non-linear partial differential equations describing the flow (mass, primary momentum, secondary momentum and energy conservation) are transformed into non-linear ordinary differential equations by employing local similarity transformations. The non-dimensional nonlinear formulated set of equations is numerically evaluated with famous shooting algorithm by using MATLAB software. The validation of simulated numerical results has been completed with generalized differential quadrature (GDQ). Extensive visualization of primary and secondary velocities and temperature distributions for the effects of the emerging parameters is presented for both pseudo-plastic fluids (n=0.8) and dilatant fluids (n=1.2). The study is relevant to the manufacturing transport phenomena in electro-conductive polymers (ECPs).

A numerical assessment on heat transfer and flow characteristics of nanofluid in tubes enhanced with a variety of dimple configurations

Abstract: Heat transfer performance and flow characteristics in tubes enhanced with dimples have been studied numerically. Innovative geometrical parameters including dimple filling angle, dimple height, and dimple pitch are presented for dimple shaped roughness applied on tube surfaces. Water and Al2O3/Water nanofluid are considered as working fluids in steady three-dimensional simulations, where Reynolds number ranged from 500 to 4000. Nanoparticle volume fractions are taken to be 1%, 2%, and 4% in this study. Both Newtonian and shear-thinning models are used to describe rheological behavior of the nanofluid. Results show that overall performance of the enhanced tubes increases with decreasing dimple pitch, increasing dimple height, and increasing filling angle in the case of water flowing through the tubes. Moreover, maximum improvement in performance occurs at Re = 2000 while using 4% non-Newtonian (shear-thinning) nanofluid in tube with specifications of 120° dimple filling angle, 2.0 mm dimple height, and dimple pitch equal to one dimple diameter. Highest obtained PEC value is 3.12, which ranks amongst uppermost values regarding internal flows in circular conduits with modified surfaces reported in the literature.

An MHD viscous liquid stagnation point flow and heat transfer with thermal radiation and transpiration

Abstract: The impact of kinematic parameters magnetohydrodynamic (MHD) and thermal radiation on the unsteady flow of a Newtonian liquid through stagnation point due to a linear sheet with mass transpiration is considered. The characteristics of heat and MHD impinging on the sheet are analyzed theoretically. The flow of an electrically conducting liquid through stagnation point has gained considerable interest due to its industrial relevance. In the chemical engineering applications involving cooling of the liquid namely glass blowing, food processing, metal thinning, polymer extrusion, silicon chip manufacturing and applications of similar kind. In all these chemical engineering applications, the interplay between the regulating kinematic parameters and the nature of the fluid is of at most priority. The flow problem is modelled into nonlinear unsteady Navier-Stokes’ partial differential equations. The similarity solution for the velocity distribution is obtained. Depending on the type of boundary heating, the analytical solutions for temperature distribution is derived by means of a power series (Gauss hypergeometric). Temperature distribution for two types of boundary heating processes viz., prescribed time-dependent constant surface temperature (PTDCST) and prescribed time-dependent wall heat flux (PTDWHF) is discussed. There found to exist branching of solutions for both velocity and temperature distribution for certain range of controlling parameters. In fact there exists dual solution for both cases of stretching/shrinking sheet and these are analyzed to see the impacts of various physical parameters on the solution domain. The impact of various regulating parameters on the velocity as well as temperature is analyzed by means of numerous plots.