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Cooling enhances the energy conversion efficiency and output of photovoltaic (PV) panels. In this work, the effects of natural convection, forced convection, and evaporative cooling on the performance of polycrystalline PV panels were investigated. The output and efficiency of a cooled PV panel were monitored and compared to those of an uncooled PV panel under the same conditions. The cooling was conducted using an insulated channel installed below a PV panel. Natural convection cooling was investigated for various channel air gaps (H = 30, 60, 90, and 120 mm). Natural convection currents in the cooling channels were capable of cooling the panel with wide air gaps. In forced convection cooling, the air was introduced by fans installed at the bottom opening of the cooling channel with various air velocities (ua = 1, 2, and 3 m/s). Evaporative natural convection cooling was performed by a wetted fabric along the lower surface of the cooling channel, whereas evaporative forced convection cooling by pushing air along the wetted lower surface of the channel. The experimental data showed that the panel efficiency and output increased due to cooling. The experimental results of natural convection cooling revealed that the use of an air gap of 120 mm to cool the solar panel contributed to an increase in the panel daily energy production and efficiency by 1.7% and 1.2%, respectively. For forced convection cooling, using air at a speed of 3 m/s increased the daily energy production by 4.4% and the efficiency by 4%. Natural convection evaporative cooling increased the daily energy production and the efficiency by 3.6% and 2.7%, respectively. Forced convection evaporative cooling contributed, at a speed of 2 m/s, to an increase in the daily energy production by 3.8% and an increase in efficiency of 3.8%.

Nowadays, progress in direct absorption solar collectors requires further investigations to realize heat transfer occurring in a horizontal annulus cavity exposed to external heat from different angles. An annulus cavity filled with nano-fluid is numerically simulated by using Ansys-Fluent for Rayleigh number equals 105. The external wall of two-dimensional model for the cavity is equally divided into six parts to simulate heating from isothermal walls placed in different angles. The internal wall has constant low temperature simulating a pipe conveys cold fluid and acting as heat sink. Hot isothermal walls set in different angles to simulate several heating cases according to hot element position. “Upper”, “lower”, “upper-right”, “lower-right”, “upper & upper-right”, “lower & lower-right”, “lower-right & upper-right”, “upper-half”, and “lower-half” are the cases investigated to assess heating effectiveness for all possible directions of heating. It is found that heating in “lower” and “lower-right” cases is about six time that of “upper” case. In addition, heating improves slightly in the case of “lower-right” compared with that of “lower” heating. Heating effectiveness from two or three parts in the upper side appears less than heating of two or three parts from the lower side. Enlarging heating area not always reflects multiplication of heat transfer amount, natural convection on the cold wall decreases significantly due to restricted or weakened stream of fluid motion near to cold wall in poor heating cases. In conclusion, choosing correct direction of heating in annulus cavity demonstrates effective heating compared to that earned by enlarging heating area.

The present contribution is a numerical investigation of the natural convection between two circular objects, where the first one is hot and the second is cold. Both cylinders are placed in an enclosed adiabatic cavity filled with copper-water nanofluid. The cylinders are arranged horizontally in the middle of cavity height. The main target of this paper it to examine the effects of geometrical configurations and the thermo-physical characteristics of nanofluid on the fluid motion and heat transfer rates. The study is conducted for various parameters: Rayleigh number (Ra = 103 to 105), nanoparticle volume fraction (φ = 0 to 10%), the diameter of cylinders (d/H = 0.2 to 0.4), and the gap spacing between cylinder (S/H = 0.25 to 0.7). Also, two cavity shapes are studied (square and circular). The average Nusselt number of the cylinders is computed and plotted as function of the studied parameters. It was concluded that the cavity shape and the particle volume fraction have a negligible effect on the heat transfer rate, whereas the distance between cylinders and the cylinder diameter have a remarkable effect on the flow patterns and convective heat transfer.

An indoor thermal environment is affected by various heat elements, such as heat transfer through walls, solar radiation, and heat emissions from people, lighting and equipment. To promote both local thermal comfort and building energy efficiency, demand-oriented ventilation (such as personalized ventilation) has been developed. When using this method, a good understanding on indoor temperature distribution becomes necessary. For this purpose, an index known as Contribution Ratio of Indoor Climate (CRI) has been developed through extraction from the calculation results of Computational Fluid Dynamics (CFD). This index can be used to analyze the independent contribution of each heat element to indoor temperature distribution. In this paper, a complete and detailed introduction of the CRI is given, including its basic premises, definitions, and mathematical meaning. Particularly, calculation method of the CRI in natural convection airflow fields is further developed. Two cases (forced and natural convection airflow fields) have been carried out in different scenarios, with results showing that the CRI of a heat source had higher values in the area around itself. Also, it had a larger influence range in forced convection airflow field because of the convective airflow, while relatively larger CRI values only appear in the area above the heat source in the natural convection airflow field because of the heat plume. As a useful index for understanding the form of indoor temperature field, the CRI has guiding significance for regulating air-conditioning/ventilation systems to build better indoor thermal environment.

The selection of appropriate geometrical characteristics of enclosure has salient influence on performance of different thermal engineering processes and devices like microelectronics, heat exchangers, power engines, boilers, solar collectors, nuclear reactors and so forth. Specifically, the triangular cavity of different aspect ratios are used to obtain multi-objective optimization and gaining excellence in thermal performance of micro channels. Subsequently, the installation of cold cylinder in a triangular enclosure are extensively used to remove high energy dissipation in micro heat sinks and heat exchangers. So, the purpose of current effort is to probe thermal characteristics of power-law liquid describing features of shear thinning and thickening materials containing applications in lubrication and polymer industry. Heat transfer is generated by the consideration of natural convection process generated due to consideration of cold walls and cylinder with provision of non-uniform heating at base wall. No-slip velocity conditions are supposed at all walls of isosceles triangle cavity. Solution of attained differential system is simulated by capitalizing finite element based COMSOL Multiphysics software (Version 5.6). Domain discretization by distributing into triangular and rectangular elements is conceded and equations at element levels are discretized by executing weak formulation. The validation of adopted numerical procedure is established by making agreement with formerly available works in both statistical and graphical approaches. Streamlines and isotherms are plotted and discussed against various parametric regimes. This study reveals noticeable influence of involved physical parameters on average and local heat flux coefficients, kinetic energy and cutlines against involved physical parameters are also evaluated. It is inferred thorough the analysis that with uplift in Rayleigh number (Ra) produces enrichment in kinetic energy and local heat transfer coefficients whereas reverse pattern is depicted against power-law index.(n).

High Explosive Anti-Tank (HEAT) warheads and ammunitions are frequently produced by explosive casting inside an axis-symmetric mold with an inverted conical geometry in the basis. In order to prevent manufacturing defects, the solidification process must be controlled. In this study, a dimensionless solidification model has been proposed to investigate the heat transfer considering the natural convection inside the liquid explosive and the numerical simulations were performed by using COMSOL Multiphysics and Modeling Software, employing trinitrotoluene (TNT) thermophysical properties. The effect of three different boundary conditions on the top of the mold have been evaluated: convection, adiabatic and isothermal. It has been observed that solidification process was faster for convection case and slower for isothermal case, while an intermediary total solidification time value was found for adiabatic case. Moreover, liquid explosive was completely surrounded by solid explosive during the solidification process for convection case and also for adiabatic case through the end of the process. Otherwise, it was not observed for isothermal case. The natural convection effects promoted a vortex inside the liquid explosive, accelerating the heat transfer process. It has been concluded that isothermal mold top boundary condition should be preferred to prevent manufacturing defects, avoiding high thermal stress.

A procedure has been developed to determine the geometrical parameters of fuel assemblies (FA) by an ultrasonic pulse-echo technique used for all types of light-water reactor FAs. The measurement of geometrical parameters is achieved through the pairwise installation of ultrasonic transducers opposite the FA spacer grid faces at a distance of not more than a half of the transducer acoustic field near-region length such that the acoustic axes of the pairwise transducers are parallel to each other. The advantages of the presented technique is that it enables monitoring of any FA modifications, including the VVER reactor assemblies with a different number of spacer grids. The paper presents a mathematical model of the acoustic path developed in a geometrical acoustics approximation and its verification results. The model was used for computational and experimental studies of the ultrasonic test technique, and engineering formulas have been developed to calculate the errors of the transducer-measured distance to the FA surface. A code has been developed to simulate the FA form change monitoring and can be used to design new monitoring systems. The developed technique to determine the VVER-1000 FA geometrical parameters was introduced at units 1 and 2 of the Temelin NPP, the Czech Republic, for the TVSA-T FA form change monitoring. The successful use of the proposed technique makes it possible to recommend it for use in inspection benches at other NPPs.

Because of the rapid advancement of microdevices technologies, electroosmotic flow (EOF) has gotten a lot of coverage in recent years. EOF has a range of benefits over other micropumps, including the ability to create continuous pulse-free flows and elimination of moving parts. Our aim is to investigate the unsteady electroosmotic flow (EOF) of an electrolyte solution of generalized fractional second grade hybrid nanofluid (Cu-TiO 2 / Water) confined between vertical two coaxial tubes. The energy equation will be altered by the fractional Cattaneo model. The finite Hankel technique and Laplace transformation will be employed to acquire the solution of the mathematical model. Since obtaining the inversion of Laplace of the transformed functions in the closed-form is a challenging task, the numerical inversion algorithm will be used. The results showed that the hybrid nanofluid became more viscous with increasing the volume concentration which reduces the electroosmotic (EO) velocity and the flow rate. Moreover, the electroosmotic flow is boosted by the free convection force.

Solid–liquid phase change has been widely used in practice, but most of the existing literature focuses on Newtonian phase change materials, and there are few studies on power-law phase change mate...

The proposed study focuses on the effect of double diffusive convection on the flow structures of power-law fluids between two coaxial cylinders of finite aspect ratios with end caps. We adopted a ...

In this investigation, a comprehensive and accurate numerical analysis of the local thermal non-equilibrium effects on the natural convection characteristics in a horizontal elliptical porous annulus saturated with nanofluid has been carried out using finite volume technique. The internal surface is isothermally heated while the external one is fixed at uniform lower temperature. The nanofluid phase and the solid phase of the porous structure are hors local thermal equilibrium situation. The heat and the hydrodynamics equations in their dimensionless form under Darcy-Brinkman Forchheimer model were solved computationally by the finite volume technique using the standard SIMPLER algorithm. The results have been examined for various porous medium properties under the LTNE condition. The standard 0.05 significance level was used to identify the local thermal equilibrium within the system. According to the obtained results, the higher the Darcy number and the smaller the non-dimensional heat transfer coefficient for the solid/nanofluid interface, the modified thermal conductivity ratio and the media’s porosity, the higher are the LTNE effects. In addition, the spatial distribution contours of the LTNE sources within the porous annulus were schematized.

Modern commercial aircraft can experience significant solar loading during turnaround when the aircraft is stationary on the tarmac. With the increased usage of composite materials, a change in the aircraft thermal environment results due to the substantial difference in metal and composite thermal properties. In this paper, an experimental study is undertaken to compare the thermal environment established in a CFRP and aluminium wingbox compartment due to solar loading for levels of 81 W/m2 , 396 W/m2 , 700 W/m2 and two further cases with a constant wing skin temperature of 100 ◦C. It was found that conduction through the vertical front and rear spars established a complex but stable flow environment comprising of four counter-rotating circulations which interact strongly with the spar walls, a criterion using scale analysis was also established to determine whether the induced flow would persist. Nusselt number measurements were carried out for each circulation and compared to existing correlations from the literature for differentially heated and top wall heated cavities. The measurements provide aircraft thermal designers with representative values for the heat transfer coefficient which can be used in the thermal modeling of aircraft wing structures.

In this paper, improvements in the basic physical laws of natural convection heat transfer were implemented in two major respects by incorporating recent research findings in this field. A preferred transition criterion was adopted in this paper to correlate all of the experimental data. Since transition correlations are primarily flow stability problems, the Grashof number, instead of the Rayleigh number, was found to be the preferred criterion. Furthermore, in the case of natural convection heat transfer from a horizontal cylinder, a series of experimental data in the high-Rayleigh-number regions recently became available. These data made it possible to establish new reliable correlations and also to test the validity of previous correlations. It is concluded that the previous correlation for a horizontal cylinder in high-Rayleigh-number regions was based on unreliable experimental results. The transition correlation for a horizontal cylinder occurred at much higher values of Rayleigh number than the previous recommendation. In the case of natural convection heat transfer from a vertical plate, more accurate property values for air under pressurized conditions are now available. This made it possible to replot the reliable data of Saunders. From this result and the experimental result of Warner and Arpaci, a new set of basic correlations in natural convection heat transfer for laminar, transitional, and turbulent regimes are recommended. These recommendations reflect a better understanding of the basic physical laws in the field of heat convection. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 293–300, 2001

This is a review of a new class of fundamental results concerning the optimal spacing of parallel heat generating boards arranged in a stack of fixed volume. The cooling is by single-phase laminar flow, in natural convection or forced convection. Several board models will be considered: smooth boards with uniform temperature or uniform heat flux, flush mounted heat sources, protruding heat sources, and a board cooled in its own (insulated) parallel plate channel.

The thermophysical properties of the nanofluids override the physical properties of conventional fluids due to the high thermal conductivity of the nanoparticles. The applications of nanofluids have varied use in the era of bio-engineering and thermal sciences. Moreover, the nanoparticles attributed the significances in human blood arteries. Therefore, the present analysis manifests with the free convection of electrically conducting nanofluid embedding with resistive forces such that the magnetic field and permeability of the medium. The augmentation in the heat transfer properties is obtained because of the inclusion of radiative heat energy and absorption diffusion. Moreover, the nanoparticles volume fraction is focused with implementation of chemical reaction. Numerical treatment using the code in-build MATLAB code pdepeis employed for the transformed unsteady flow problem. The graphical computations against flow parameters are worked with justified physical importance. A declining change in nanoparticles is examined due to Prandtl number which is more progressive for steady case. The decreasing change in velocity is more dominant for nanoparticles as compared to pure fluid. The nanoparticles concentration reduces for effective variation of chemical reaction constant and Lewis number.

Numerical investigation on natural convection heat transfer of Tiwari - Das model nanofluid inside a square cavity with thermal radiation and magnetic field is carried out in this analysis. Ethylene Glycol E G is considered as base fluid and T i O 2 (Titanium Oxide) considered as nanoparticles for the present investigation. The side horizontal walls of cavity are assumed to be adiabatic and isothermal conditions on both sides walls are considered in this analysis. The finite difference method is implemented to solve the governing non-linear partial differential equations representing momentum and temperature equations. The sway of volume fraction parameter ( 0.01 ≤ ϕ ≤ 0.09 ) , magnetic field parameter ( 1.0 ≤ M ≤ 3.0 ) , Rayleigh number ( 100 ≤ R a ≤ 1000 ) , radiation parameter ( 0.1 ≤ R ≤ 0.9 ) , Reynolds number ( 0 . 1 ≤ R e ≤ 0 . 5 ) and Prandtl number ( 5.2 ≤ P r ≤ 7.2 ) on T i O 2 - E G nanofluid flow and heat transfer is illustrated through graphs. Furthermore, the codes of average Nusselt number with dissimilar values of pertinent parameters are also calculated and results are depicted through graphs. The result shows that, temperature of T i O 2 - E G nanofluid escalates inside the cavity with higher values of (M). Higher heat can be transferred from hot wall to cold wall when radiation parameter (R) intensifies.

The problem of transient free convection in an open cavity containing two elastic thin fins is studied in this paper. The left surface is heated, and the right surface is in opening, which is in contact with the environment air. The dimensionless forms of the governing equations for continuity, momentum and heat transfer in the fluid and structures domains are written in Arbitrary Lagrangian–Eulerian (ALE) formulation. The governing equations are solved by the finite element method (FEM). The governing parameters considered are the oscillating fin direction, fin amplitude and length with several flexibility values and aperture sizes. The results show that a maximum heat transfer enhancement occurs at the suitable parameters between the aperture size and the fins oscillating direction.

Batch-type solar collector is a tilt-able water-heating solar-thermal system comprising an isosceles-trapezoidal enclosure housing a circular-cylinder absorber at the bottom wall, insulated side-walls, and flat-top glazing. Free convection of air inside such trapezoidal enclosure is studied numerically by varying the tilt angle of the whole enclosure from 0o-70o. The influence of tilt on the flow field has been demonstrated by plotting the streamlines and isotherms. The present study successfully identifies the importance of enclosure-tilt in quantifying heat-loss between absorber and glazing by developing a computational correlation between Nusselt and Rayleigh as a function of tilt. The correlation trend is non-monotonic over the range of angular Rayleigh numbers numerically experimented with having a peak around angular Rayleigh number 3×105 corresponding to the tilt-angle 30 degrees. The irregular-shaped cavity implies that the heat-transfer correlations already existing for regular-shaped cavities may not be used otherwise they will draw implausible conclusions and this argument identifies the novelty of the present study.

In this paper, we present a numerical study of turbulent natural convection in a square cavity differentially heated and filled with nanofluid and subjected to an inclined magnetic field. The standard k-e model was used as the turbulence model. The transport equations were discretized by the finite volume method using the SIMPLE algorithm. The influence of the Rayleigh number, the Hartmann number, the orientation angle of the applied magnetic field, the type of nanoparticles as well as the volume fraction of nanoparticles, on the hydrodynamic and thermal characteristics of the nanofluid was illustrated and discussed in terms of streamlines, isotherms and mean Nusselt number. The results obtained show that the heat transfer rate increases with increasing Rayleigh number and orientation angle of the magnetic field but it decreases with increasing Hartmann number. In addition, heat transfer improves with increasing volume fraction and with the use of Al2O3 nanoparticles.

The effect of nanoparticle shape on the natural convection heat transfer of Cu-Al2O3/water hybrid nanofluid inside a U-shaped enclosure is presented in this paper. The governing equations are transformed into the dimensionless form using dimensionless variables. A three-node triangular finite element method is used with the Newton-Raphson method to solve the problem numerically. The streamlines and isotherms as well as the local and average Nusselt numbers are presented for the fluid flow with Rayleigh number of 104 to 106.It is found that blade nanoparticle shape produces the highest heat transfer rate while sphere is the lowest.

The temperature rapid rise phenomenon often happens in the side-by-side frost-free refrigerators during the damper-off cycle, which leads to a decrease in damper-off duration and an increase in energy consumption. A three-dimensional transient model of natural convection during the damper-off cycle is established and experimentally validated to analyze the refrigerating compartment (RC) temperature rapid rise phenomenon. The results show that the violent natural convection airflow driven by the non-uniform temperature distribution will lead to the significant variation of RC temperature. The hot air rises from bottom into top layer along the three passages, which include the front gap (FG) between shelves and door boxes, the side gap (SG) between the right-side wall and door boxes and the rear gap (RG) between shelves and the rear wall. The air enthalpy along FG into the first layer is the dominant factor of the temperature rapid rise, accounting for almost 64.4% of the total inflow air enthalpy. The net air enthalpy flow along FG is larger than other passages in the first 15s and decreases rapidly as the temperature distribution becomes uniform.

We report the complex evaporation kinetics of saline sessile droplets on surfaces with elevated temperatures. Our previous studies show that on non-heated substrates, saline sessile droplets evaporate faster compared to the water counterparts. In the present study, we discover that on heated surfaces, the saline droplets evaporate slower than the water counterpart, thereby posing a counter-intuitive phenomenon. The reduction in the evaporation rates is directly dependent on the salt concentration and the surface wettability. Natural convection around the droplet and thermal modulation of surface tension is found to be inadequate to explain the mechanisms. Flow visualizations using particle image velocimetry (PIV) reveal that the morphed advection within the saline droplets is a probable reason behind the arrested evaporation. Infrared thermography is employed to map the thermal state of the droplets. A thermo-solutal Marangoni based scaling analysis is put forward and the major governing non-dimensional numbers have been accounted for in the analysis. It is observed that the Marangoni flow and internal advection borne of thermal and solutal gradients are competitive, thereby leading to the overall decay of internal circulation velocity compared to the equivalent pure water case, which reduces the evaporation rates. The theoretically proposed advection velocities conform to the experimental results. This study sheds rich insight on a novel species transport behaviour in saline droplets.

The performance investigation of natural and forced convection indirect type solar dryers was performed by drying ivy gourd. The natural convection setup was modified by fitting a trapezoidal duct and central processing unit fans run by photovoltaic panels to promote forced convection. The data from the experiment was used to estimate collector and dryer efficiencies, energies, effective moisture diffusivity, transfer coefficients, specific energy consumption and specific moisture extraction rate. The average actual heat supplied was 776.6 and 997.76 W for natural and forced convections, respectively. The average collector efficiency for the same was 62.56% and 77.2%, respectively. The average drying efficiency for the same was 6.62% and 7.8%, respectively. The average values of activation energy, mass transfer, heat transfer and diffusion coefficients were estimated to be 39.85 and 35.54 kJ/mol, 7.06 × 10−9 and 8.35 × 10−9 m2/s, 0.0033 and 0.0043 m/s, and 3.85 and 4.93 W/m2 K, respectively, for natural and forced convection setups, respectively. The specific energy consumption and specific moisture extraction rate were evaluated where forced convection setup gave the best results. The drying correlations were developed for mass transfer, heat transfer and diffusion coefficients. Uncertainty analysis was performed to check the authenticity of the results.

The unsteady natural-convection of Al2O3-water nanofluid form heated open T-pipe inside a cavity has been investigated by incompressible smoothed particle hydrodynamic (ISPH) method using non-homogenous two-phase Buongiorno?s model. Different lengths and heights of T-pipe shape are considered. The side walls of the cavity are kept at cool temperature Tc and the horizontal walls are thermally insulated. The Lagrangian description of the controlling governing equations is discretized and solved using improved ISPH method. In this study, ISPH method is improved using kernel renormalization function for boundary treatment plus modification in the source term of pressure Poisson equation. The source term of pressure Poisson equation contains the velocity divergence plus density invariance multiply by relaxation coefficient. The calculations are performed for variable lengths of T-open pipe (0.2 ? Lb ? 0.6), variable widths of T-open pipe (0.02 ? Wb ? 0.16), (0.02 ? Wt ? 0.16), and variable concentration of nanoparticles volume fraction (1% ? ?avg ? 10%). The obtained results showed that the maximum values of the stream function are reduced by 80.8% when ?avg is increased from 1-10%. Additionally, as lengths and widths of the T-pipe are raised, the average Nusselt numbers at the vertical walls are enhanced.

This study presents experimental investigation into a novel UV cabinet type solar dryer designed for drying banana slices at the Chhattisgarh Swami Vivekanand Technical University (21 27′N, 81 43′ E). An experiment for the performance assessment of banana drying was conducted at the clear sky conditions. The solar dryer consists of 6 trays with the one drying chamber. The same technique is used for forced convection drying using the fan and the PV panel. Experimentally evaluated local metrological conditions such as solar radiation, atmospheric temperature, wind speed and air humidity. In the six trays for drying, 2 kg of banana slices were evenly divided and after a period of 16 h of drying, banana slices dried in the forced convection to moisture content of 13.75 % from an initial moisture content of 86.2%. On the other hand, natural convection drying reaches to 17.5% of the final moisture content, in the same time. Moisture ratio for the forced and natural convection drying varies in the range of 0.79–0.001 and 0.85–0.05, respectively. For the forced convection drying of banana slices the optimal and minimum drying efficiency of the designed solar dryer are 16.32 % and 0.29% whereas 13.75% and 0.15% of drying efficiency for natural convection drying are measured. The more efficient is forced convection drying in comparison with natural convection drying.

The thermophysical properties of bedrock are of primary importance when designing borehole thermal energy systems. We present a novel use of the Enhanced Thermal Response Test (ETRT) to determine bedrock thermal conductivity, natural convection, and drill hole thermal resistance as a function of depth in crystalline bedrock. Bedrock was heated with a 228-m-long hybrid cable containing copper wires and fiber optics for temperature monitoring. A reference fiber optic cable was installed along the whole length of the studied drill hole. For groundwater-filled boreholes, the ETRT offers a means to estimate the magnitude of buoyancy-driven natural convection. We estimated that the heating power in the ETRT should not exceed 20 Wm-1 for the thermal conductivities to be determined with sufficient accuracy. According our results, the accuracy of the ETRT can be significantly improved if the test is performed with a hybrid fiber optic cable combined with a reference fiber optic cable. Thermal resistance can be more accurately determined if a reference fiber optic cable is used. The most important achievement of this method is that compared to other measurement methods, the effective thermal conductivity of bedrock can be simultaneously determined along the entire length of the drill hole.

The paper addresses numerically a study on thermal management of a cavity-shaped thermal system providing localized heating at the bottom. For this fundamental study, different fluids namely air, water, ethylene glycol (EG), and nanofluid are considered in a square cavity from the application point of view. The upper halves of the sidewalls, and top wall act as the isothermal heat sink. The heater acts as an isothermal heat source. Four different configurations are studied in this work: (a) Case-1: central heating with one heater (b) Case-2: corner heating with one heater (c) Case-3: corner heating with two heaters of equal length (d) Case-4: central heating with two heaters of equal length. For all the configurations, the total length of heaters is fixed. The other walls are maintained in an adiabatic condition. The finite volume approach is employed to solve the governing differential equations numerically. The simulation is executed for a spectrum of Prandtl numbers covering the most commonly used working fluids, keeping the Rayleigh number and the volumetric concentration of nanoparticles fixed. The streamlines and isotherms are employed for the visualization of flow and thermal field. The transmission of heat by free convection is characterized by the Nusselt number. The irreversibility in the flow domain is studied using the contours of the entropy generation rate. The aim of this work is to find out the heater location and fluid for which the rate of heat transmission is maximum. The results reveal that the maximum heat removal rate occurs for central heating with a double heater (Case-4) and the minimum heat removal rate takes place for corner heating with a double heater (Case-3). It is noted that the heat transmission rate is maximum for nanofluid.

The Coiflet wavelet solutions are given to the classic cavity flow problem describing natural convection in cavities under the effect of an inclined external magnetic field. The governing partial differential equations are solved by a brand-new and very efficient computational method, namely, the Coiflet wavelet-homotopy method. This approach not only inherits the capability of homotopy analysis to deal with strongly nonlinear problems, but also maintains the orthogonality and high-precision local expression property of the wavelet method. The Coiflet wavelet solutions are obtained via a very efficient homotopy iteration. Rigid comparison with previous studies for a wide range of various physical parameters shows excellent computational efficiency and accuracy of the proposed method.

In this study, heat transfer utilizing non-Newtonian fluids with a power-law model in a rectangular enclosure containing a hot obstacle was verified experimentally. The impact of aspect proportion (AR) on fluid flow characteristics and heat transfer, obstacle shapes, and various amounts of carboxymethylcellulose was explored with distilled water which in turn controls the index of powers-law (n). The findings show that the increases in the Number of Rayleigh, lead to the greater the Number of Nusselt, and the greater the non-Newtonian behavior, increased the heat transfer. There is also a noticeable impact when changing the shape of an obstacle on heat transfer as the cylindrical shape enhances heat transfer greater than the cuboid shape. As for aspect proportion, it also affects heat transfer inversely.

This paper reports a numerical study of double-diffusive natural convection through an annular space delimited by a square cylinder on the outside and a cylindrical cylinder on the inside covered by a porous layer. The Darcy-Brinkmann-Forchheimer is used for modeling flow in both fluid and porous areas. The annular space is partially or completely filled with an isotropic porous medium. A finite volume method, using the Patankar-Spalding technique is used for solving the discretization of the dimensionless equations governing the problem. The effects of simultaneously applied thermal and solutal buoyancy forces on heat and mass transfer are shown in the results for a large range of buoyancy ratios N, Rayleigh number, and thermal conductivity. Streamlines, isotherms, and iso-concentrations are presented to analyze the flow structure transition from mass species dominated to thermal dominated flow. Results show that the buoyancy ratio can change the flow pattern and the increased thermal conductivity ratio can improve heat and mass transfer. A good agreement was obtained between the present results and those published were found.

Naturally ventilated double skin facade (NVDSF) utilizes solar radiation to introduce natural ventilation through a double-skin facade cavity with vents. It is an easy-to-implement green technology that can be widely applied to new or existing buildings. However, its implementation in buildings is still limited as the coupled radiation and natural convection pose challenges in predicting ventilation performance. Moreover, the diversity of real-life applications raises a great need for a universally applicable method that can account for realistic design factors. Hence, an in-depth theoretical analysis model is required to facilitate the application of NVDSFs in buildings. This study proposes two new analytical models for NVDSFs that can directly predict ventilation rates with simple inputs. Numerical simulations are also conducted to validate the developed analytical models. It demonstrated that the experimentally validated numerical model can offer accurate predictions of natural convection and radiation for the case that involved complicated environmental, structural, and material factors. Most importantly, the realistic scenarios, NVDSFs with exterior vent louvers are studied both analytically and numerically. Different optical properties from two glazing materials, regular glass and low-e glass, are also tested with the proposed analytical models. Through validation, the discrepancies of the predictions by the proposed analytical models are −5% to −9% for ‘no-louver’ NVDSF for two different glazing materials (i.e., regular and low-e glass), and 13%–27% for NVDSFs with adjustable louver angles 45°–150°.

This research presents an investigation of laminar two-dimensional double-diffusive free convection and entropy formation in an inclined enclosure under the influence of an inclined magnetic field. The performance of natural convective heat transfer can be improved by doing modifications in enclosure geometry that impact the flow structure. We have considered a dome-shaped enclosure to examine the heat and mass transfer performance. The enclosure is saturated with Cu-water nanofluid and the two sidewalls of the enclosure are maintained at constant temperature Tc(

This study aims to analyze the effect of fin geometry on the thermal performance of longitudinally finned-tube horizontal latent heat thermal energy storage (LHTES) systems. The longitudinal fins with different fin heights, thickness, and numbers were applied in the horizontal shell-and-tube LHTES system and the paraffin was packed in the annulus. The melting fronts, average temperature, and velocity profiles of Phase Change Material (PCM) were graphically illustrated and compared for charging processes. It was found that the incorporation of longitudinal fins in a small quantity (2.85% of total volume) could reduce the complete melting time by 34% in comparison to the bare-tube configuration. Melting characteristics were compared using average liquid fraction, average PCM temperature, and stored energy. It was recommended that the fin height should not exceed half of the annulus gap. The optimum fin volume was identified by comparing the enhancement ratio for examined configurations. Furthermore, for the same fin volume configurations, this study evaluated (i) whether a large number of short fins are beneficial or a smaller number of long fins and (ii) whether thick short fins are beneficial or thin long fins? Comparison results showed that a small number of long fins were more beneficial than that of more short fins and the melting characteristics of thin long fins were superior to that of the thick short fins. Finally, the optimum fin configuration was justified, and the priority sequence was suggested for the design of longitudinally finned-tube energy storage systems.

The PCM composited with metal foam (PCM-porous system) as heat sink to cool photovoltaic (PV) panel is a potential application. However, there has little attention on the differences in cooling effect of PCM-porous systems when PV panels install with different inclination angles. To fill this research gap, the charging processes of PCM-porous systems with different inclined angles (φ = 0°, 30°, 60°, 90°) have been investigated numerically and experimentally in the current work. Meanwhile, the influences of different Rayleigh numbers (Ra = 1.4568 × 106, 1.8210 × 108, 1.1654 × 1010) have been considered. Results show that the inclined angle has little impact on the PCM-porous system with small porosity (e = 85%, 90%), which is much different from the PCM-porous system with larger porosity (e = 95%). In the discussion, an interesting phenomenon has been found that the weak natural convection of liquid PCM in porous media induced by the inclined cavity plays a negative role in melting process, which is obvious in PCM-porous system with small porosity (e = 85%, 90%). But this negative effect is not necessarily a bad thing due to that it can extend the duration time to cool the PV panel. The maximum duration time of PCM-porous system with e = 85% or 90% is 2 times of system with e = 95% (φ≠0°).

This study aims to establish predictive formulas for the temperature dependence of impurity diffusion coefficient based on a hard-sphere (HS) model and the measurement results. The impurity diffusion coefficients of Sb, Bi, and In in liquid Sn were measured using the shear cell technique and stable density layering at 773 K and 973 K (500 °C and 700 °C, respectively) with suppression of natural convection. The temperature dependence of the impurity diffusion coefficient can be predicted by multiplying the ratio of the solvent to the solute of the following three factors by the self-diffusion coefficient of the solvent as the slope: (i) the square of mean atomic diameter, (ii) the first peak of the pair distribution function calculated by the HS model, and (iii) the square root of the converted atomic weight. If the ratio of the atomic diameter is close to one, the temperature dependence of the impurity diffusion coefficient can also be predicted with an accuracy similar to the abovementioned relationship by multiplying the following two factors by the self-diffusion coefficient of the solvent as the slope: (i) the atomic diameter ratio of the solvent to the solute and (ii) the thermodynamic factor. The predictive formulas based on the HS model showed an accuracy of approximately ±10 pct for the experimental values from 573 K to 973 K (300 °C to 700 °C).

The study of mass transfer in the non-Newtonian fluid is essential in understanding the engine lubrication, the cooling system of electronic devices, and the manufacturing process of the chemical industry. Optimal performance of the practical applications requires the appropriate conditions. The unsteady transient free convective flow of second-grade fluid with mass transfer and wall transpiration is concerned in the present communication. The behavior of the second-grade fluid under the influence of injection or suction is discussed. Suitable non-dimensional variables are utilized to transform the governing equations into non-dimensional governing equations. A Maple solver “pdsolve” that is using the centered implicit scheme of a finite difference method is utilized to solve the dimensionless governing equations numerically. The effects of wall injection or suction parameter, second-grade fluid viscoelastic parameter, Schmidt number, and modified Grashof number on the velocity and concentration profiles are graphically displayed and analyzed. The results show that with increasing wall suction, viscoelastic parameter, and Schmidt number, the velocity and concentration profiles decrease. Whereas, the velocity profiles show an opposite tendency in situations of wall injection. The wall suction has increased the skin friction and also the rate of mass diffusion in the second-grade fluid.

The prime objective of the current study is to examine the effects of third-grade hybrid nanofluid with natural convection utilizing the ferro-particle Fe 3 O 4 and titanium dioxide TiO 2 and sodium alginate (SA) as a host fluid, flowing through vertical parallel plates, under the fuzzy atmosphere. The dimensionless highly nonlinear coupled ordinary differential equations are computed adopting the bvp4c numerical approach. This is an extremely effective technique with a low computational cost. For validation, it is found that as the volume fraction of Fe 3 O 4 + TiO 2 hybrid nanoparticles rises, so does the heat transfer rate. The current and existing results with their comparisons are shown in the form of the tables. The present findings are in good agreement with their previous numerical and analytical results in a crisp atmosphere. The nanoparticles volume fraction of Fe 3 O 4 and TiO 2 is taken as uncertain parameters in terms of triangular fuzzy numbers (TFNs) [0, 0.05, 0.1]. The TFNs are controlled by α − cut and the variability of the uncertainty is studied through triangular membership function (MF).

A new type of nanofluids is nanoencapsulated phase change materials (NEPCMs), where nanoparticles are made of a shell and a core. In the current study, characteristics of free convection flow, entropy generation, and heat transfer of NEPCMs in an enclosure are investigated. The enclosure is an annulus between concentric horizontal circular and square cylinders with a porous medium. The governing equations (i.e., continuity, energy, and momentum) are written in the nondimensional form and then numerically solved by the control volume finite element method (CVFEM). The results of the validation are in good agreement with those of the literature. The effects of decision variables on the entropy generation number and the average Nusselt number are investigated. The outcomes discovered that there is a maximum for Nu ave and a minimum for N gen at θ f = 0.4 for each value of the Stefan number. Also, Nu ave and ECOP increase by 8.8% and 24.8%, respectively, while N gen decreases by 12.8% when ϕ increases from 0 (pure fluid) to 0.05 at θ f = 0.4 .

The present numerical work, based on the finite volume method, deals with the characterization of natural convective flow and thermal fields inside differentially vertical heated square cavities filled with a nanofluid as well as the quantification of the convective exchanges. The investigation is devoted to study the influence of the hybrid nanofluid (Al2O3-Cu / water) on the flow’s general structure with a particular attention to the Nusselt number. An exhaustive parametric study is conducted considering different combinations of Al2O3 and Cu nanoparticles (NPs) dispersed in water for a range of Rayleigh numbers (Ra) and total volume fractions An appropriate agreement with experimental data was observed for the estimation of the hybrid nanofluid thermal conductivity. From the results, it is observed that the heat transfer intensifies by increasing the Ra number and the nanoparticles volume fraction. The hybrid nanofluid seems to be the most efficient nanofluid in comparison with a base fluid and a single nanofluid. This heat transfer enhancement becomes more convincing with the increase of the Cu NPs content (% in volume).

In this paper, a modulated permanent helical magnetic field is imposed to improve segregation during directional solidification in a binary Sn–3.5 wt pct Pb alloy. The magnetic force created by this magnetic field significantly interferes with heat, mass and momentum transfer of molten metal during solidification. Our experimental results show that this magnetic field, with an appropriate modulation frequency, can not only promote columnar-to-equiaxed transition (CET) but also significantly reduce solute segregation otherwise caused by conventional helical magnetic fields. Our analysis demonstrates the advantages of this approach over natural convection and conventional electromagnetically forced convection.

Background The exploration of natural convection which is one the substantial types of convective heat transmission in various applications for instance heat exchangers and geothermal systems along with nanofluids (Nanofluids have greater thermal conductivity in comparison to the conventional fluids) engrossed all researchers’ attention. Methods This study is dedicated to the inspection of the free convection of nanofluid as well as entropy generation inside a porous cavity loaded with nano-encapsulated phase change materials (NEPCMs). The wavy bottom section of the enclosure may be subject to a constant heat flux due to the transmitted sunlight comes from a parabolic trough solar collector. The volumetric heat source/sink is comprised in the governing equation. The robust finite element method (FEM) is deployed to handle the transformed governing equations. Findings The numerical simulation of the streamlines and isotherms associated with velocity distribution for diverse factors are displayed. Further, the significant behavior of the contributing parameters on the Nusselt and Bejan numbers are represented. The results demonstrate that the various profiles of wavy bottom section could affect the heat transmission features as well as fluid flow remarkably. Furthermore, it is noteworthy that all the profiles of entropy enhance with increasing the amplitude with respect to the increasing undulation number for the existence of various Rayleigh number.

Background Free convection and second law scrutiny of nano-encapsulated phase change material (NEPCM) suspension along with entropy production inside a circular cold cylinder involving a wavy hot baffle is a significant thermal management aspect subject to various industrial applications. Phase change material (PCM) undergoes a solid-liquid phase mutation at a particular fusion temperature, and absorbs/releases an appreciable amount of energy because of the latent heat of phase mutation. Hence, NEPCMs would be prospective owing to their capability to enhance the working liquids’ performance, keeping the system at a particular cooling temperature. Methods In order to simulatethe free convection along with entropy generation of NEPCMs inside a circular cold cylinder entails a wavy hot baffle under CattaneoChristov heat flux model(Altered Fourier theory) and magnetic field, the finite element method (FEM) could be utilized to solve the governing equations. In this study, the amplitude of baffle could be changeable while its undulation number is fixed at 2. Findings Amplifying Raylegh number intensifies streamlines, isotherms, horizontal and vertical velocities, total entropy generation whittles down local Bejan number. Higher magnetic field strength is responsible for slow movement of NEPCMs and augments local Bejan number. Growth of baffle size yields squeezes the streamlines, horizontal and vertical velocities and intensified tilted isotherms.

Accurate estimate of transpiration is fundamental to develop water-saving management practices and optimize irrigation management. Penman–Monteith (P–M) model based on aerodynamic resistance (ra) estimated from heat transfer coefficient method and Perrier logarithm method were used to calculate the hourly and daily grapevine transpiration in solar greenhouse. The results show that the daytime canopy resistance (rc) of grapevines was basically maintained between 200 and 250 s m−1 during the two growing seasons. The main convection type in the greenhouse was free convection (68.8–87.5%), and supplemented by mixed convection (12.5–31.3%). The intensity of free convection gradually increased with the growth stage. ra1 and ra2 calculated under the free and mixed convection conditions had no noticeable difference and the variation trend was the same, but both were higher than ra′ calculated by the Perrier logarithmic method. The modified P–M models based on the three aerodynamic resistance methods all overestimated transpiration at the early growth stage and underestimated transpiration at the middle and late growth stages to different degrees. Overall, the P–M model under the three methods could accurately simulate hourly transpiration with a determination coefficient (R2) and root mean squared error (RSME) greater than 0.89 and 23.4 W m−2. ra based on the heat transfer coefficient correction improved the accuracy of daily transpiration simulation, and the P–M $$_{{r_{a1} }}$$ model based on the free convection condition had the highest precision (R2 ≥ 0.97, RSME ≤ 17.4 W m−2). The P–M model with ra calculated under free convection is recommended to simulate grapevine transpiration in greenhouse in cold regions of Northeast China.

Experimentally determined vitamin degradation kinetic parameters are vital input for process optimization, e.g., of nutrient rich beverages such as milk and fruit juices. The standard method of obtaining these parameters is to subject a test-tube of the nutrient (in its food matrix) to a water bath at elevated temperatures and measure remaining concentration as a function of time. It is well known that accurate kinetic fitting requires measuring the temperature inside the test tube as a function of time (due to lag it is not identical to the water-bath temperature), and this is included in contemporary studies. However, the importance of axial temperature gradients for valid kinetic parameter estimations has not been discussed to the same extent. This contribution combines experiments and physical modelling to show how an axial temperature gradient evolves in a test tube (R = 13 mm, Z = 75 mm), due to free convection currents (T∞ – T0 = 57–71 °C). Consequently, the test tube temperature is not uniform in such an experiment. Thus, the positioning of the thermocouple in a test-tube-in-water bath experiment is essential for ensuring a valid estimation of the kinetic parameters. Simulations suggest that positioning it half-way between top and bottom is a suitable best practice.