Showing papers in "Heat and Mass Transfer in 2016"
TL;DR: In this paper, experiments were conducted in a natural convection solar greenhouse dryer using different sensible heat storage materials (concrete, sand and rock-bed) in order to study their thermal performance.
Abstract: Experiments were conducted in a natural convection solar greenhouse dryer using different sensible heat storage materials (concrete, sand and rock-bed) in order to study their thermal performance. For both sand and rock-bed, 4″ thickness was found to be optimum as it provides better drying environment both during day and night. The dryer reduced the moisture content of coconuts from 52 (w.b.) to 7 % (w.b.) using concrete as heat storage material in 78 h saving 55 % of drying time compared to open sun drying which takes 174 h for reducing the moisture content to the same level. The sand took 66 h saving 62 % of drying time whereas rock-bed took only 53 h thereby saving 69 % of drying time compared to open sun drying. The efficiency of the dryer was found to be 9.5, 11 and 11.65 % using concrete, sand and rock-bed respectively.
87 citations
TL;DR: Theoretical study on hydromagnetic heat transfer in viscous fluid on continuously stretching non-isothermal surface, with linear variation of surface temperature or heat flux has been carried out as mentioned in this paper.
Abstract: Theoretical study on hydromagnetic heat transfer in dusty viscous fluid on continuously stretching non-isothermal surface, with linear variation of surface temperature or heat flux has been carried out. Effects of Hall current, Darcy porous medium, thermal radiation and non-uniform heat source/sink are taken into the account. The sheet is considered to be permeable to allow fluid suction or blowing, and stretching with a surface velocity varied according to a linear. Two cases of the temperature boundary conditions were considered at the surface namely, PST and PHF cases. The governing partial differential equations are transferred to a system of non-linear ordinary differential equations by employing suitable similarity transformations and then they are solved numerically. Effects of various pertinent parameters on flow and heat transfer for both phases is analyzed and discussed through graphs in detail. The values of skin friction and Nusselt number for different governing parameters are also tabulated. Comparison of the present results with known numerical results is presented and an excellent agreement is found.
85 citations
TL;DR: In this article, the influence of different parameters on the stability of spherical CuO nanoparticles dispersed in water, ethylene glycol and 50:50 water/ethylene glycol binary mixture as different base fluids for heat transfer applications was investigated.
Abstract: The main aim of this work is to investigate the influence of different parameters on the stability of spherical CuO nanoparticles dispersed in water, ethylene glycol and 50:50 water/ethylene glycol binary mixture as different base fluids for heat transfer applications. Nano-fluids were prepared using two-step method at weight concentration of 0.1–0.4 %. Time-settlement experiments were established to examine the stability of nano-fluids. Quality tests were also performed to investigate the morphology, purity and size of nanoparticles. In order to stabilize the nano-fluids, stirring, pH control and sonication were utilized. The criteria for assessing the stability of nano-fluids were zeta potential and time-settlement experiments. Results demonstrated that the ethylene glycol can be the best medium for dispersing the CuO nanoparticle in comparison with water and water/EG binary mixture. The best condition for achieving the most stable nano-fluid was also introduced. Role of sonication time, stirring time and addition of surfactant on the stability of nano-fluids were investigated and briefly discussed.
83 citations
TL;DR: In this article, the effect of drying temperature on drying behavior and mass transfer parameters of lemon slices was investigated in a laboratory air ventilated oven dryer at temperatures of 50, 60 and 75 °C.
Abstract: In this study, the effect of drying temperature on drying behaviour and mass transfer parameters of lemon slices was investigated. The drying experiments were conducted in a laboratory air ventilated oven dryer at temperatures of 50, 60 and 75 °C. It was observed that the drying temperature affected the drying time and drying rate significantly. Drying rate curves revealed that the process at the temperature levels taken place in the falling rate period entirely. The usefulness of eight thin layer models to simulate the drying kinetics was evaluated and the Midilli and Kucuk model showed the best fit to experimental drying curves. The effective moisture diffusivity was determined on the basis of Fick’s second law and obtained to be 1.62 × 10−11, 3.25 × 10−11 and 8.11 × 10−11 m2 s−1 for the temperatures of 50, 60 and 75 °C, respectively. The activation energy and Arrhenius constant were calculated to be 60.08 kJ mol−1 and 0.08511 m2 s−1, respectively. The average value of convective mass transfer coefficient for the drying temperatures of 50, 60 and 75 °C was calculated to be 5.71 × 10−7, 1.62 × 10−6 and 2.53 × 10−6 m s−1, respectively.
66 citations
TL;DR: In this paper, the heat transfer characteristics of supercritical carbon dioxide (S-CO2) turbulent flow were investigated experimentally in a horizontal circular pipe with an inner diameter of 8.7 mm.
Abstract: The heat transfer characteristics of supercritical carbon dioxide (S-CO2) turbulent flow were investigated experimentally in a horizontal circular pipe with an inner diameter of 8.7 mm. Local convection coefficients and Nusselt numbers of the flow were obtained at different locations along the pipe with a constant heat flux ranging from 16 to 64 kW/m2. Experiments were performed for fluid mass flow rate ranging from 0.011 to 0.017 kg/s, an inlet fluid temperature ranging from 24 to 28 °C, and a flow pressure ranging from 7.5 to 9.0 MPa to investigate their effects on the convection heat transfer in the pipe. Both enhancement as well as deterioration in the heat transfer coefficient was observed for the flow conditions examined in this work. Experimental results were then compared with the widely used empirical correlation for pipe flow. Three commonly used buoyancy parameters were utilized to investigate their applicability in the present test conditions. Results indicate that all the parameters show a strong presence of buoyancy effects in the present test conditions. The trend and magnitude of these parameters, however, do not agree with the trend and magnitude of heat transfer enhancement and deterioration along the pipe.
61 citations
TL;DR: In this article, the authors investigated the drying characteristics and quality attributes of apple slices at different air temperature levels (50, 60 and 70 °C) and a constant air velocity (1.5 m s−1).
Abstract: The main objectives of the present study were to investigate the drying characteristics and quality attributes of apple slices. The samples were dried at different air temperature levels (50, 60 and 70 °C) and a constant air velocity (1.5 m s−1). It was observed that the drying air temperature affected the dehydration rate significantly. The usefulness of eight different mathematical models to simulate the experimental drying curves was evaluated and the Midilli model provided the best simulation of the samples drying kinetics. The effective moisture diffusivity was determined to be 7.03 × 10−10, 8.48 × 10−10 and 1.08 × 10−9 m2 s−1 for drying air temperatures of 50, 60 and 70 °C, respectively. The shrinkage values of the dried samples at air temperatures of 50, 60 and 70 °C were 74.70, 82.35 and 80.78 %, respectively. The maximum value of rehydration ratio (4.527) and also the minimum value of ∆E (11.27) were obtained for the slices dried at 70 °C.
59 citations
TL;DR: In this paper, the authors investigated the thermal conductivity and viscosity of a hybrid nanofluid containing tetramethylammonium hydroxide (TMAH) coated Fe3O4 nanoparticles and gum arabic (GA) coated carbon nanotubes (CNTs).
Abstract: The aim of this paper is to investigate the thermal conductivity and viscosity of a hybrid nanofluid containing tetramethylammonium hydroxide (TMAH) coated Fe3O4 nanoparticles and gum arabic (GA) coated carbon nanotubes (CNTs), experimentally. The magnetic nanoparticles and CNTs are physically attached as the result of interaction between the TMAH and GA molecules. The morphology and structure of the samples are characterized with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experiments are carried out in the magnetic nanoparticles volume concentration range of 0.1–0.9 %, CNT volume concentration range of 0.05–1.35 % and the temperature range of 25–55 °C. The viscosity of the hybrid nanofluid increases with the increase of volume concentration, while it decreases with the increase of temperature. Besides, results show that hybrid nanofluid behaves as a shear thinning fluid. Furthermore, it is observed that the thermal conductivity of the hybrid nanofluid enhances with temperature and volume concentration.
59 citations
TL;DR: In this article, the effects of nanostructure on rapid boiling of water that is suddenly heated by a hot copper plate were studied. And the results showed that the nanostructures have significant effects on energy transfer from solid copper plate to liquid water and phase change process from liquid water to vapor.
Abstract: Molecular dynamic simulations are performed to study the effects of nanostructure on rapid boiling of water that is suddenly heated by a hot copper plate. The results show that the nanostructure has significant effects on energy transfer from solid copper plate to liquid water and phase change process from liquid water to vapor. The liquid water on the solid surface rapidly boil after contacting with an extremely hot copper plate and consequently a cluster of liquid water moves upward during phase change. The temperature of the water film when it separates from solid surface and its final temperature when the system is at equilibrium strongly depend on the size of the nanostructure. These temperatures increase with increasing size of nanostructure. Furthermore, a non-vaporized molecular layer is formed on the surface of the copper plate even continuous heat flux is passing into water domain through the plate.
57 citations
TL;DR: In this article, a unique experimental set-up was fabricated to carry out axial heat flow steady state experiments for the assessment of thermal contact conductance (TCC) at the interface of two materials.
Abstract: A unique experimental set-up was fabricated to carry out axial heat flow steady state experiments for the assessment of thermal contact conductance (TCC) at the interface of two materials. Three different materials (copper, brass and stainless steel) were selected for the experiments considering their mechanical and thermal properties. Heat transfer experiments were performed in vacuum environment (0.045 torr) to find out solid spot contact conductance for nominally flat surfaces with different surface roughness (1–5 μm) for each specimen under several load conditions (0.6–15 MPa). A precise estimation of TCC for the interface of sets of similar materials was one of the most important results of this research. The effects of the surface roughness, the material properties and the load conditions (nominal interface pressure) have been studied and documented. Furthermore, the experimental results of solid spot contact conductance were compared with four theoretical models, showing their limitations to make a precise estimation of the TCC in the range of the used parameters.
57 citations
TL;DR: In this paper, the effects of some parametric values on convective heat transfer coefficients and thin layer drying process of ginger slices were investigated by using cyclone type convective dryer.
Abstract: In the present work, the effects of some parametric values on convective heat transfer coefficients and the thin layer drying process of ginger slices were investigated. Drying was done in the laboratory by using cyclone type convective dryer. The drying air temperature was varied as 40, 50, 60 and 70 °C and the air velocity is 0.8, 1.5 and 3 m/s. All drying experiments had only falling rate period. The drying data were fitted to the twelve mathematical models and performance of these models was investigated by comparing the determination of coefficient (R
2), reduced Chi-square (χ
2) and root mean square error between the observed and predicted moisture ratios. The effective moisture diffusivity and activation energy were calculated using an infinite series solution of Fick’s diffusion equation. The average effective moisture diffusivity values and activation energy values varied from 2.807 × 10−10 to 6.977 × 10−10 m2/s and 19.313–22.722 kJ/mol over the drying air temperature and velocity range, respectively. Experimental data was used to evaluate the values of constants in Nusselt number expression by using linear regression analysis and consequently, convective heat transfer coefficients were determined in forced convection mode. Convective heat transfer coefficient of ginger slices showed changes in ranges 0.33–2.11 W/m2 °C.
56 citations
TL;DR: In this article, the effect of the variability of different parameters such as relaxation time, metabolic heat source, spatial heat source and different type boundary conditions on temperature distribution in different type of the tissues like muscle, tumor, fat, dermis and subcutaneous based on three models were analyzed and discussed in detail.
Abstract: In this paper, the temperature distribution in a finite biological tissue in presence of metabolic and external heat source when the surface subjected to different type of boundary conditions is studied. Classical Fourier, single-phase-lag (SPL) and dual-phase-lag (DPL) models were developed for bio-heat transfer in biological tissues. The analytical solution obtained for all the three models using Laplace transform technique and results are compared. The effect of the variability of different parameters such as relaxation time, metabolic heat source, spatial heat source, different type boundary conditions on temperature distribution in different type of the tissues like muscle, tumor, fat, dermis and subcutaneous based on three models are analyzed and discussed in detail. The result obtained in three models is compared with experimental observation of Stolwijk and Hardy (Pflug Arch 291:129–162, 1966). It has been observe that the DPL bio-heat transfer model provides better result in comparison of other two models. The value of metabolic and spatial heat source in boundary condition of first, second and third kind for different type of thermal therapies are evaluated.
TL;DR: In this article, the carboxylated water is used with nano-sized silver, copper and aluminium oxide nanotubes and tested for heat transfer performance in an air cooled heat exchanger similar to an automotive radiator.
Abstract: This paper summarizes a recent work on anti-corrosive and enhanced heat transfer properties of carboxylated water based nanofluids. DI water mixed with Sebacic acid (C10H18O4) as carboxylate additive is dispersed with multi walled carbon nanotubes and tested for corrosion and heat transfer characteristics. Corrosion studies made as per ASTM D 1384 show that carboxylate water dispersed with MWCNTs is resistant to corrosion and hence suitable for automotive environment. In addition to MWCNTs, carboxylated water dispersed with nano sized silver, copper and Aluminium oxide are also tested for corrosion performance but found to be giving considerable corrosion in automotive environment. The stability of MWCNT based nanofluids in terms of zeta potential is found to be good with carboxylated water compared to DI water. Significant improvement is observed in the thermal conductivity of nanofluids dispersed with MWCNTs. There is a slight increase in viscosity and marginal decrease in the specific heat of nanofluids with addition of carboxylate as well as MWCNTs. The carboxylated water is dispersed with very low mass concentration of multi walled carbon nano tubes at 0.025, 0.05 and 0.1 % and tested for heat transfer performance. The heat transfer studies are made in Reynolds number range of 2500–6000 in the developing flow regime. The heat transfer performance of nanofluids is carried out on an air cooled heat exchanger similar to an automotive radiator with incoming air velocities across radiator maintained at 5, 10 and 15 m/s. The coolant side overall heat transfer coefficient and overall heat transfer coefficient have improved markedly. It is also found that the velocity of air and flow rate of coolant plays an important role in enhancement of overall heat transfer coefficient. Stanton number correlation for the entire data has been developed. It is found that the wall temperature gradients play an important role in the enhancement of heat transfer when nanofluids are used.
TL;DR: In this paper, a passive thermal regulation technique with fins in conjunction with cotton wicks is developed for controlling the temperature of PV module during its operation and experiments were conducted with the developed technique in the location of Tiruchirappalli (786°E and 108°N), Tamil Nadu, India with flat 25 Wp PV module and its viability was confirmed.
Abstract: A passive thermal regulation technique with fins in conjunction with cotton wicks is developed in the present work for controlling the temperature of PV module during its operation Experiments were conducted with the developed technique in the location of Tiruchirappalli (786°E and 108°N), Tamil Nadu, India with flat 25 Wp PV module and its viability was confirmed The PV module temperature got reduced by 12 % while the electrical yield is increased by 14 % with the help of the developed cooling system Basic energy balance equation applicable for PV module was used to evaluate the module temperatures and a fair agreement was obtained between the theoretical and experimental values for the cases of with cooling and without cooling
TL;DR: In this article, a closed form approximate solution to nonlinear heat (mass) diffusion equation with power-law nonlinearity of the thermal diffusivity has been developed by the integral-balance method avoiding the commonly used linearization by the Kirchhoff transformation.
Abstract: Closed form approximate solutions to nonlinear heat (mass) diffusion equation with power-law nonlinearity of the thermal (mass) diffusivity have been developed by the integral-balance method avoiding the commonly used linearization by the Kirchhoff transformation. The main improvement of the solution is based on the double-integra- tion technique and a new approach to the space derivative. Solutions to Dirichlet and Neumann boundary condition problems have been developed and benchmarked against exact numerical and approximate analytical solutions avail- able in the literature. List of symbols a Thermal diffusivity (m 2 /s) a0 Thermal diffusivity of the linear problem (m = 0) (m 2 /s) b Coefficient in Eq. (24b) which should be defined trough the ini - tial condition �(t = 0) = 0 C p Specific heat capacity (J/kg)
TL;DR: In this paper, a theoretical approach for exergy analysis of the combined infrared-convective drying process using a simple heat and mass transfer model is proposed, and the applicability of the developed model to actual drying processes is proved using an illustrative example for a typical food.
Abstract: Optimal design and performance of a combined infrared-convective drying system with respect to the energy issue is extremely put through the application of advanced engineering analyses.
This article proposes a theoretical approach for exergy analysis of the combined infrared-convective drying process using a simple heat and mass transfer model. The applicability of the developed model to actual drying processes was proved using an illustrative example for a typical food.
TL;DR: In this paper, the effective moisture diffusivity, activation energy, and energy consumption of rough rice in a batch fluidized bed dryer were evaluated, and it was shown that the effective diffusivities increased with drying air temperature and superficial fluidization velocity and was found to vary from 4.78 −10−11 to 1.364 −10 −10 m2/s.
Abstract: The present work was an attempt to assess the effective moisture diffusivity, activation energy, and energy consumption of rough rice in a batch fluidized bed dryer. Drying experiments were conducted at drying air temperatures of 50, 60, and 70 °C, superficial fluidization velocities of 2.3, 2.5, and 2.8 m/s, and solids holdup of 1.32 kg. Drying kinetics showed that the whole fluidized bed drying of rough rice occurred in the falling rate period. The effective moisture diffusivity was described by an Arrhenius equation. The evaluated effective moisture diffusivity increased with drying air temperature and superficial fluidization velocity and was found to vary from 4.78 × 10−11 to 1.364 × 10−10 m2/s with R2 higher than 0.9643. The activation energy and the pre-exponential factor of Arrhenius equation were found to be in the range of 36.59–44.31 kJ/mol and 4.71 × 10−5–7.15 × 10−4 m2/s, respectively. Both maximum values of the specific energy consumption of 74.73 MJ/kg and the total energy need of 12.43 MJ were obtained at 60 °C drying air temperature and 2.8 m/s superficial fluidization velocity. Both minimum values of the specific energy consumption of 29.98 MJ/kg and the total energy need of 4.85 MJ were obtained under drying air temperature of 70 °C and superficial fluidization velocity of 2.3 m/s.
TL;DR: In this paper, the effects of different acids on the thermal performance of oxidized carbon nanotubes (CNT)/water nanofluids in a two-phase closed thermosyphon were studied.
Abstract: In this paper, the effects of different acids on the thermal performance of oxidized carbon nanotubes (CNT)/water nanofluids in a two-phase closed thermosyphon were studied. The structures morphology and functionalization degree were studied concurrently. The results indicated that strong oxidants increased dispersivity of CNT in the nanofluids. In other words, as the number of COOH groups increased in the nanofluids, an upward trend was also observed in the thermal efficiency of the thermosyphon.
TL;DR: In this paper, the effect of the working tube angle, inlet pressure and number of nozzles on the performance of the vortex tube was investigated in the range of 0-120°.
Abstract: The working tube is a main part of vortex tube which the compressed fluid is injected into this part tangentially. An appropriate design of working tube geometry leads to better efficiency and performance of vortex tube. In the experimental investigation, the parameters are focused on the working tube angle, inlet pressure and number of nozzles. The effect of the working tube angle is investigated in the range of θ = 0–120°. The experimental tests show that we have an optimum model between θ = 0 and θ = 20°. The most objective of this investigation is the demonstration of the successful use of CFD in order to develop a design tool that can be utilized with confidence over a range of operating conditions and geometries, thereby providing a powerful tool that can be used to optimize vortex tube design as well as assess its utility in the field of new applications and industries. A computational fluid dynamics model was employed to predict the performances of the air flow inside the vortex tube. The numerical investigation was done by full 3D steady state CFD-simulation using FLUENT6.3.26. This model utilizes the Reynolds stress model to solve the flow equations. Experiments were also conducted to validate results obtained for the numerical simulation. First purpose of numerical study in this case was validation with experimental data to confirm these results and the second was the optimization of experimental model to achieve the highest efficiency.
TL;DR: The results show that the complex behavior of PHPs can be successfully described by the non-linear structure of this simulator, and the optimum operating point of the system is obtained by using genetic algorithm (GA).
Abstract: In this paper, a novel approach has been presented to simulate and optimize the pulsating heat pipes (PHPs). The used pulsating heat pipe setup was designed and constructed for this study. Due to the lack of a general mathematical model for exact analysis of the PHPs, a method has been applied for simulation and optimization using the natural algorithms. In this way, the simulator consists of a kind of multilayer perceptron neural network, which is trained by experimental results obtained from our PHP setup. The results show that the complex behavior of PHPs can be successfully described by the non-linear structure of this simulator. The input variables of the neural network are input heat flux to evaporator (q″), filling ratio (FR) and inclined angle (IA) and its output is thermal resistance of PHP. Finally, based upon the simulation results and considering the heat pipe’s operating constraints, the optimum operating point of the system is obtained by using genetic algorithm (GA). The experimental results show that the optimum FR (38.25 %), input heat flux to evaporator (39.93 W) and IA (55°) that obtained from GA are acceptable.
TL;DR: In this article, an experimental study on Silver-oil nanofluid was carried out in order to present the laminar convective heat transfer coefficient and friction factor in a concentric annulus with constant heat flux boundary condition.
Abstract: In this work an experimental study on Silver-oil nanofluid was carried out in order to present the laminar convective heat transfer coefficient and friction factor in a concentric annulus with constant heat flux boundary condition. Silver-oil nanofluid prepared by Electrical Explosion of Wire technique with no nanoparticles agglomeration during nanofluid preparation process and experiments. The average sizes of particles were 20 nm. Nanofluids with various particle Volume fractions of 0.011, 0.044 and 0.171 vol% were employed. The nanofluid flowing between the tubes is heated by an electrical heating coil wrapped around it. The effects of different parameters such as flow Reynolds number, tube diameter ratio and nanofluid particle concentration on heat transfer coefficient are studied. Results show that, heat transfer coefficient increased by using nanofluid instead of pure oil. Maximum enhancement of heat transfer coefficient occurs in 0.171 vol%. In addition the results showed that, there are slight increases in pressure drop of nanofluid by increasing the nanoparticle concentration of nanofluid in compared to pure oil.
TL;DR: In this paper, the authors calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data, and found that there is an increased heat transfer with the increase in nano particle concentration and temperature.
Abstract: The microchannels are device used to remove high heat fluxes from smaller area In this experimental research work the heat transfer performance of nanofluids of Al2O3/water and CuO/water were compared The important character of such fluids is the enhanced thermal conductivity, in comparison with base fluid without considerable alteration in physical and chemical properties The effect of forced convective heat transfer coefficient was calculated using serpentine shaped microchannel heat exchanger Furthermore we calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data The heat transfer coefficient for different particle concentration and temperature were analysed using forced convection heat transfer using nanofluids The findings indicate considerable enhancement in convective heat transfer coefficient of the nanofluids as compared to the basefluid The results also shows that CuO/water nanofluid has increased heat transfer coefficient compared with Al2O3/water and base fluids Moreover the experimental results indicate there is increased forced convective heat transfer coefficient with the increase in nano particle concentration
TL;DR: In this article, the problem of boundary layer stagnation-point flow and heat transfer of a Williamson nanofluid on a linear stretching/shrinking sheet with convective boundary condition is studied.
Abstract: In
this paper, the problem of boundary layer stagnation-point flow and heat transfer of a Williamson nanofluid on a linear stretching/shrinking sheet with convective boundary condition is studied The effects of Brownian motion and thermophoresis are considered in the energy equation The governing partial differential equations are first transformed into set of ordinary differential equations, which are then solved numerically using Runge–Kutta–Felhberg fourth–fifth order method with Shooting technique The characteristics of the flow and heat transfer as well as skin friction and Nusselt number for various prevailing parameters are presented graphically and discussed in detail A comparison with the earlier reported results has been done and an excellent agreement is shown It is found that dual solutions exist for the shrinking sheet case Further, it is observed that the thermal boundary layer thickness increases with increase in Williamson parameter for both solutions
TL;DR: In this paper, the authors modeled the coal goaf as a 3D porous medium based on stress distribution and simulated the variation of O2 distribution influenced by CO2 or N2 injection.
Abstract: To prevent coal spontaneous combustion in mines, it is paramount to understand O2 gas distribution under condition of inert gas injection into goaf. In this study, the goaf was modeled as a 3-D porous medium based on stress distribution. The variation of O2 distribution influenced by CO2 or N2 injection was simulated based on the multi-component gases transport and the Navier–Stokes equations using Fluent. The numerical results without inert gas injection were compared with field measurements to validate the simulation model. Simulations with inert gas injection show that CO2 gas mainly accumulates at the goaf floor level; however, a notable portion of N2 gas moves upward. The evolution of the spontaneous combustion risky zone with continuous inert gas injection can be classified into three phases: slow inerting phase, rapid accelerating inerting phase, and stable inerting phase. The asphyxia zone with CO2 injection is about 1.25–2.4 times larger than that with N2 injection. The efficacy of preventing and putting out mine fires is strongly related with the inert gas injecting position. Ideal injections are located in the oxidation zone or the transitional zone between oxidation zone and heat dissipation zone.
TL;DR: In this paper, reduced graphene oxide (rGO) is synthesized from graphite using modified Hummer and chemical reduction methods, and different concentrations of 0.01, 0.1, and 0.3 g/l of rGO/water nanofluids are prepared by dispersing the flakes in DI water.
Abstract: In the present study, reduced graphene oxide (rGO) is synthesized from graphite using modified Hummer and chemical reduction methods. Various characterizations techniques are carried out to study the in-plane crystallite size, number of layers, presence of functional groups and surface morphology. Different concentrations of 0.01, 0.1, and 0.3 g/l of rGO/water nanofluids are prepared by dispersing the flakes in DI water. The colloidal stability of 0.3 g/l concentration is measured after 5 days using Zetasizer and found to be stable. The rGO/water nanofluids are then used to study the effect on the enhancement of critical heat flux (CHF) in pool boiling heat transfer. Results indicate an enhancement in CHF ranging from 145 to 245 % for the tested concentrations. The mechanisms of CHF enhancement are analyzed based on surface wettability, surface roughness, and porous layer thickness. The macrolayer dryout model sufficiently supports the mechanism of CHF enhancement of thin wire with rGO deposits, which is not reported yet.
TL;DR: In this article, a numerical simulation is performed to investigate the heat transfer and pressure drop characteristics of three-row inline tube bundles as a part of a heat exchanger (Re = 1000, Pr = 4.29), where two pairs of delta winglet-type vortex generators are installed beside the first row and between the first and second rows of the tube bundles.
Abstract: A numerical simulation is performed to investigate the heat transfer and pressure drop characteristics of three-row inline tube bundles as a part of a heat exchanger (Re = 1000, Pr = 4.29). To enhance heat transfer, two pairs of delta winglet-type vortex generators (VGs) installed beside the first row and between the first and second rows of the tube bundles. The diameter of the second row of the tubes is chosen smaller than those of the first and third. A comprehensive study on the effects of various geometrical parameters such as transverse and longitudinal positions of VGs, length and height of VGs and angle of attack of the delta winglets is performed to augment heat transfer. Based on this study the best values of these design parameters are determined. The results showed that the best model increases the convective heat transfer ratio and thermal performance factor about 59 and 43 %, respectively, in compare with the geometry without VG.
TL;DR: In this paper, the authors investigated the influence of drying air parameters (i.e. temperature, velocity and relative humidity) on effective diffusivity and convective mass transfer coefficient of apple slices.
Abstract: To efficiently design both new drying process and equipment and/or to improve the existing systems, accurate values of mass transfer characteristics are necessary. The present study aimed to investigate the influence of drying air parameters (i.e. temperature, velocity and relative humidity) on effective diffusivity and convective mass transfer coefficient of apple slices. The Dincer and Dost model was used to determine the mass transfer characteristics. The obtained Biot number indicated that the moisture transfer in the apple slices was controlled by both internal and external resistance. The effective diffusivity and mass transfer coefficient values obtained to be in the ranges of 7.13 × 10−11–7.66 × 10−10 and 1.46 × 10−7–3.39 × 10−7 m s−1, respectively and the both of them increased with increasing drying air temperature and velocity, and decreasing relative humidity. The validation of the model showed that the model predicted the experimental drying curves of the samples with a good accuracy.
TL;DR: In this paper, a novel biomimetic flow channel for flow field designs is proposed based on Murray's Law to improve the performance of a proton exchange membrane fuel cell (PEMFC) stack.
Abstract: Bipolar plate is one of the many important components of proton exchange membrane fuel cell (PEMFC) stacks as it supplies fuel and oxidant to the membrane-electrode assembly (MEA), removes water, collects produced current and provides mechanical support for the single cells in the stack. The flow field design of a bipolar plate greatly affects the performance of a PEMFC. It must uniformly distribute the reactant gases over the MEA and prevent product water flooding. This paper aims at improving the fuel cell performance by optimizing flow field designs and flow channel configurations. To achieve this, a novel biomimetic flow channel for flow field designs is proposed based on Murray’s Law. Computational fluid dynamics based simulations were performed to compare three different designs (parallel, serpentine and biomimetic channel, respectively) in terms of current density distribution, power density distribution, pressure distribution, temperature distribution, and hydrogen mass fraction distribution. It was found that flow field designs with biomimetic flow channel perform better than that with convectional flow channel under the same operating conditions.
TL;DR: In this article, the drying and rehydration kinetics of freeze-dried kiwi slices were determined using a nonlinear regression analysis using statistical software SPSS 16.0.
Abstract: The aim of this study was to determine the drying and rehydration kinetics of freeze dried kiwi slices. Well-known thin layer drying models (Lewis, Page, Modified Page I, Henderson and Pabis, Modified Henderson and Pabis, Logarithmic, Midilli, Modified Midilli, Two-term, Two-term Exponential, Modified Two-term Exponential, and Wang and Singh) were fitted to the experimental data. A nonlinear regression analysis was used to evaluate the parameters of the selected models using statistical software SPSS 16.0. For the freeze drying process of the kiwi slices, the highest R2 value (0.997), and the lowest RMSE (0.018) as well as the χ2 (0.0004) values were obtained from the Two-term Exponential model. The effective moisture diffusivity (Deff) of the freeze dried kiwi slices was calculated with the Fick’s diffusion model as 7.302 × 10−10 m2/s. The rehydration behavior was determined using distilled water at different solid–liquid ratios at room temperature (18 ± 1 °C) using Peleg’s model. The kinetics of the total soluble solid loss was also determined.
TL;DR: In this article, the velocity distribution function of the kinetic boundary condition (KBC) at the vapor-liquid interface has been investigated using molecular dynamics simulations, and it is shown that reflecting molecules in the normal direction to the interface have slightly faster and significantly slower average velocities than that of the Maxwell distribution at the liquid temperature, respectively.
Abstract: Using molecular dynamics simulations, the present study investigates the precise characteristics of evaporating and reflecting monatomic molecules (argon) composing a kinetic boundary condition (KBC) in a vapor–liquid equilibria. We counted the evaporating and reflecting molecules utilizing two boundaries (vapor and liquid boundaries) proposed by the previous studies (Meland et al. in Phys Fluids 16:223–243, 2004; Gu et al. in Fluid Phase Equilib 297:77–89, 2010). In the present study, we improved the method using the two boundaries incorporating the concept of the spontaneously evaporating molecular mass flux. The present method allows us to count the evaporating and reflecting molecules easily, to investigate the detail motion of the evaporating and reflecting molecules, and also to evaluate the velocity distribution function of the KBC at the vapor–liquid interface, appropriately. From the results, we confirm that the evaporating and reflecting molecules in the normal direction to the interface have slightly faster and significantly slower average velocities than that of the Maxwell distribution at the liquid temperature, respectively. Also, the stall time of the reflecting molecules at the interphase that is the region in the vicinity of the vapor–liquid interface is much shorter than those of the evaporating molecules. Furthermore, we discuss our method for constructing the KBC that incorporates condensation and evaporation coefficients. Based on these results, we suggest that the proposed method is appropriate for investigating KBC in various nonequilibrium states or multi-component systems.
TL;DR: In this paper, the Dincer and Dost model was used to determine the moisture transfer parameters and predict the drying curves of the mushroom slices, and it was observed that the entire drying process took place in the falling drying rate period.
Abstract: An accurate understanding of moisture transfer parameters, including moisture diffusivity and moisture transfer coefficient, is essential for efficient mass transfer analysis and to design new dryers or improve existing drying equipments. The main objective of the present study was to carry out an experimental and theoretical investigation of mushroom
slices drying and determine the mass transfer characteristics of the samples dried under different conditions. The mushroom slices with two thicknesses of 3 and 5 mm were dried at air temperatures of 40, 50 and 60 °C and air flow rates of 1 and 1.5 m s−1. The Dincer and Dost model was used to determine the moisture transfer parameters and predict the drying curves. It was observed that the entire drying process took place in the falling drying rate period. The obtained lag factor and Biot number indicated that the moisture transfer in the samples was controlled by both internal and external resistance. The effective moisture diffusivity and the moisture transfer coefficient increased with increasing air temperature, air flow rate and samples thickness and varied in the ranges of 6.5175 × 10−10 to 1.6726 × 10−9 m2 s−1 and 2.7715 × 10−7 to 3.5512 × 10−7 m s−1, respectively. The validation of the Dincer and Dost model indicated a good capability of the model to describe the drying curves of the mushroom slices.