Natural Convection Mode Research Articles

Kinetics and modelling of the heat and mass transfer during the solar drying of cassava slices under natural convection mode

Abstract The kinetics and modelling of heat and mass transfer during the solar drying of cassava slices integrated with the pebble's sensible thermal storage material (PSTSM) were investigated. Fifteen (15) thin layer drying models were adopted to model the drying curves following standard criteria for fitness. The maximum temperatures were 52.5 °C and 55 °C for plywood drying chamber (PLC) and Perspex glass chamber (PGC) respectively. It took both drying systems with Perspex and plywood chambers 38 hrs to reduce the moisture content of cassava slices from 56% w.b. to 10.62 and 15.20% w.b. respectively. The effective moisture diffusivity (De) for the system with Perspex glass and plywood chambers were 6.28 × 10−10 m2/s and 4.53 × 10−10 m2/s respectively. The activation energy values were 20.56 kJ/mol and 20.82 kJ/mol for the system with Perspex and plywood respectively. The mass transfer coefficient values for the cassava slices dried in Perspex and plywood were 1.70 × 10−6 and 1.67 × 10−6 m/s respectively. At the same time, the heat transfer coefficients were 2.63 and 2.08 W/m2. K. The Midilli et al. and modified Henderson and Pabis models were adjudged the best to describe the solar drying of cassava slices using the Perspex and plywood drying chamber respectively. Therefore, the obtained results would be useful in the design of solar drying equipment for agricultural products under natural convection mode.

Experimental ginger drying by a novel mixed-mode vertical solar dryer under partial and fully loaded conditions

An innovative and compact mixed-mode vertical solar dryer (MVSD) with a circular solar collector is developed and tested for ginger slices drying under partial and fully loaded conditions. Ginger slices samples of 4, 5 and 6 mm thick were used for natural convective drying in MVSD under partially loaded condition. The thickness of the ginger slices was observed to affect the performance of the MVSD. The drying rate and thermal efficiency were observed to be highest for 5 mm thick ginger slices sample under partially loaded condition. The MVSD was further tested with full loading capacity for 5 mm thick ginger slices drying under natural and forced convection modes (NC and FC modes). The drying behavior of ginger slices was well described by Midilli-Kucuk model. Heat transfer coefficients and efficiency (thermal and exergy) were observed to be higher under forced convection mode. Embodied energy and energy payback time were observed as 554.44 and 585.53 kWh; and 2.85 and 2.99 years under NC and FC modes, respectively. The values of annualized capital cost and payback period were observed as INR 389.41 and 408.17; and 1.85 and 6.86 years under NC and FC modes, respectively. The quality of the ginger dried under FC mode was found comparatively better than that of under NC mode.

Evaluation of the Performance of the ATESTA Forced Convection Dryer when Drying Pineapples

Objective: Evaluating the performance of the ATESTA [Solar Energy and Appropriate Technology Workshop] forced convection dryer, developed at the Albert Schweitzer Ecological Center of Burkina [CEAS Burkina], on the drying of pineapple slices in Togo. Method: For this purpose, two ATESTA dryers were setup in a company in Lomé, Togo, one operating in forced convection and the other, the witness, in natural convection mode. Drying tests of pineapple slices were carried out with these two dryers. Data were collected on water loss, organoleptic parameters and energy consumption during the drying process Findings: The results showed that the water loss rate increased by 11.4% as well as the first choice dry matter content by 36% and an energy saving of 21% percent of natural convection to forced convection. This performance of the ATESTA forced convection dryer increases the economic profit of USD [United States Dollar] 2.68/kg or USD 45.52/drying cycle and USD 2678.40/t of dried pineapples. The results of microbiological analyses revealed that the total flora [1.2.102 CFU/g] [Colony-Forming Unit per gram] yeast and mold [6.4.101 CFU/g] content of pineapple slices dried by the ATESTA forced convection dryer were in line with the limit values of the Directive 2000/13/EC [European Parliament and the Council]. Novelty: Performance tests on the forced convection dryer have made a more efficient dryer available to pineapple processors. Forced convection can also be adapted to existing dryers. Keywords: Performance, ATESTA dryer, Togo, Forced convection, Dried prineapple

Thermal and fluid aspects of dual piezoelectric jet pump of double chambers

The rapid development of electronics has led to a significant reduction in the size of electronic devices, while increasing their power consumption. This has resulted in the need for small-scale thermal management solutions. Piezoelectric pump is one of the efficient devices for local thermal management. This paper presents a novel configuration of a piezoelectric pump, which separates the pump chamber with a rigid wall to create two independent pulsating flows. This configuration aims to reduce eddies within each chamber. Furthermore, the proposed design incorporates two pairs of open-sided channels to guide the pulsating flow towards the hot target. The electrostatics physic of the piezoelectric patches along with energy and momentum equations in three dimensions are manipulating mathematically and solved numerically using the finite element method (FEM) and the Arbitrary Lagrangian-Eulerian (ALE) technique. The implicit Large Eddy Simulation (iLES) turbulence model is adopted for this purpose. Several parameters, including frequency, voltage, and the radius of the piezoelectric patch are investigated to better understanding their influence on both the zero-net volumetric flow rate and the heat transfer process. The findings indicate that by separating the pump chamber, it becomes feasible to obtain a 3D solution with the iLES using a reasonable mesh size. Nusselt number increases with all the examined parameters, where it enhances by 42.48 % at V = 100 Volt when the pump is switched on compared to natural convection mode. The peak-peak volumetric flowrates rises by 50 % and 350 % when the applied voltage is raised from 50 to 100 Volts and the frequency is raised from 20 to 50.50 kHz, respectively.

Modeling and Control of an Indirect Solar Dryer in Forced Convection Mode Using System Identification and Model Predictive Control

In nonlinear process, system modeling is vital part to predict and examine the performance of a plant. Here, indirect solar dryer performed in forced convection mode taken as a plant. Several experimental tests were performed by drying tomato slices, both in forced as well as in natural convection mode to study the dynamic characteristics of it. The efficiency of indirect solar dryer was better with forced convection than the natural convection mode. In this investigation System Identification technique is used to predict the model equation for forced convection. Designing a conventional control strategy to ensure efficiency is cumbersome due to the plant’s multivariable nature and nonlinear dynamics. The regular proportional-integral-derivative (PID) controller provides unproductive control action for nonlinear system. A model predictive controller (MPC) is intended for the proposed system ensuring efficiency of the indirect solar dryer. The MPC parameters must be selected appropriately to ensure optimal performance. The Matlab platform was used to get the both MPC algorithms and conventional PID performance.

Modelling solar water desalination system using natural convection

In recent times, solar energy has emerged as one of the most promising energy sources. Numerous scholars and researchers are exploring various applications of solar energy. This presented work focused on solar water desalination system to evaluate its thermal performance in natural convection mode. Three-dimensional Computational fluid dynamics CFD model is built in Comsol Multiphysics 6.1. Numerical model simulations have been carried out by using Heat transfer in Solid &Fluid and Laminar flow interfaces. The surface temperature varies between 19.5℃ and 102℃ while the velocity magnitude was between 0.05 m/s and 0.5 m/s. Variations of the surface temperature, velocity magnitude, pressure, convective and conductive heat flux magnitudes in Solar water desalination system are presented in the discussion section of the article.

Exploring the performance of an indirect solar dryer by combining three augmentation approaches (trapezoidal absorber, shot blasting, and pebble stone)

The shelf life of food products can be increased by reducing their moisture content with the aid of solar dryers. However, the poor efficiency of solar collectors increases the time and energy required for drying the food crops. Hence, the present work aims to overcome the above bottleneck by employing corrugated shot-blasted absorbers in solar air heaters and comparing their performance with flat plate solar air heaters. Subsequently, the drying kinetics of bitter gourd and tapioca cassava were subject to experimental investigation using indirect solar dryers in their natural mode, aiming to assess the dryers' overall performance. The experiments revealed that the average thermal efficiencies of the SAHs equipped with flat plate absorber plates and corrugation with shot-blasted absorber plate treatment displayed variations ranging from 39.05 to 53.12 % while maintaining a constant MFR of 0.02 kg/s. The average exergy efficiency of the FPAP is 1.103 % and the CSB absorber plate is 1.755 % with a constant flow rate of 0.02 kg/s. The research findings indicate that the CSBAP (surface-improved SAH) demonstrates a higher heat-absorbing capacity when compared to the FPAP SAH. The utilization of the CSBAP design along with the incorporation of pebble stone results in a greater ability to efficiently absorb and retain heat when compared to the traditional FPAP design with a shot-blasted surface. The inclusion of pebble stones in the drying process, as observed in Case II for Tapioca cassava, led to a remarkable improvement in drying efficiency by approximately 36 % when compared to Case I. Furthermore, the drying efficiency for bitter gourd in Case IV experienced a notable improvement of around 30 % when pebble stones were integrated into the drying process, in contrast to Case III. Eventually, the experimental results specified that the carbon credits accrued for the CO2 mitigated by the FPAP and CSBAP systems in the natural convection mode were calculated to be approximately 226.48 and 308.11 Rate $/year, respectively. These alterations synergistically contribute to enhancing the SAH's effectiveness and effectively utilizing solar heat directly contributes to reducing greenhouse gas emissions and, subsequently, the carbon footprint.

Effects of circumferential fin on cooling performance improvement of forced air-cooled battery pack

In demand for clean energy sources, batteries have a vital role to play. Batteries are the heart of many electrical energy storage systems, as in electric vehicles (EVs). The high internal heat generation from a battery is the main hindrance to battery performance and safety. The high-temperature environment impacts performance; therefore, maintaining the battery temperature within desired limits is essential. In this work, we focus on the increasing effectivity of forced air cooling for high-capacity batteries at high discharge rates, decreasing auxiliary power consumption without compromising the compactness of the battery pack. The novelty of this work is that we have developed a hybrid BTMS consisting of circumferential fins and forced air cooling. Using a circumferential fin clad to the cell body has the advantage of increasing the surface area of the battery pack so that the ability of air to extract heat from the battery pack is improved and the auxiliary power consumption is reduced. The experiments use thin circumferential fins of 0.5 mm thickness mounted on cell bodies and discharged at various rates. The numerical model has been developed and tested using ANSYS FLUENT software. Using a single circumferential fin (Model 1) improves cooling performance by 3.14 %, and using three circumferential fins (Model 2) improves cooling performance by 7.32 % in the natural convection mode compared to the uncooled model. The improvement achieved in forced air cooling with a velocity of 2 m/s and three circumferential fins cooling performance is improved by 18.71 %. It is observed that using fins, the overall increase in weight of the system is by 3.58 %.

Thermal analysis on a novel natural convection flat plate collector with perforated tilted transparent insulation material parallel slats (TIM-PS) in Mediterranean climate

The purpose of this research is the evaluation of heat transfer by natural convection in an innovative flat plate solar collector operating in natural convection mode. The design consists of inclined, perforated slats of transparent insulation material installed between the absorber and the glass cover. Hourly climatic data on the four specific days of solstices and equinoxes was used to investigate the behavior of the novel collector design in the city of Monastir; Tunisia. A comprehensive process, ANSYS FLUENT, was implemented to properly evaluate the thermal performance of the collector. In order to obtain accurate thermal properties of the smart collector façade unit containing the TIM-PS to exploit in the simulation, a validated three-dimensional finite volume model developed using the CFD software was used to solve the conductive, convective and radiative heat transfer properties of the system. We have explored in depth the effect of optical characteristics of the TIM-PS on the collector performance. Present research focuses on the analysis of thermal efficiency profile of flat plate collector with innovative façade in comparison with conventional collector for different climatic conditions. The analysis indicated that innovative façade collector’s efficiency is highly influenced by solar intensity. It was found that TIM-PS façade can effectively reduce the convective heat losses through the front of collector. The simulation results show that perforated tilted TIM-PS offer better performance than conventional façade of collector during all the seasons of the year. Simulations performed for one year predict that with careful selection of the parallel slats properties, the new façade system can generate an efficiency up to 83% in June, 76% in September, 53% in March and 42% in December. The results of the thermal evaluation of this research will help guide the future development of this system, as we have demonstrated that the innovative façade collector can be used all year round. In conclusion, the TIM-PS have great potential and a very wide range of applications in the field of flat plate solar collector.

Performance analysis of natural and forced convection mixed mode UV tent house solar dryer for potato drying

ABSTRACT A solar dryer that uses solar radiation and a mixed mode of operation was installed. The dryer includes a flat plate collector with 1.12 m2 area covered by 5.0 mm glass. It has two trays for holding potato slices in the drying chamber. Meteorological data, including solar intensity, relative humidity, atmospheric temperature, and wind speed, were recorded. The initial moisture content of the potato slices, which were 2.5 mm thick, was 85.90%. In the forced convection drying mode, the moisture content decreased to 14.10% in 8 hours, while the natural convection mode required 14 hours to reach 14.75%. Forced and natural convection dryings were much faster compared to open sun drying. Air moves through the collector at a speed of 1.1 m/s during forced convection. The moisture ratio varied from 0.4750 to 0.0010 for forced convection drying and from 0.4095 to 0.02646 for natural convection drying. The maximum dryer efficiency was 32.95% for forced convection drying and 26.62% for natural convection drying. The little difference attained between natural and forced convections can be due to the fact that the experiments are not made in the same moment.

Heat Transfer Coefficient for a Glazed Transpired Solar Collector in Natural Convection Mode

A glazed transpired solar collector (GTC) was set up to operate in natural convection mode. Collector air temperature and velocity were measured at specific points in the collector. Solar radiation, ambient temperature, collector cover glass and absorber temperature were also measured. Using dimensional analysis and the measured data, a heat transfer correlation was obtained for the collector which will enable the heat transfer coefficient to be determined. The GTC heat transfer correlation based on influence of pitch is . The GTC absorber plate hole diameter was 3.0 mm and the pitches ranged from 15 mm to 35 mm. The Nusselt number correlation for influence of pitch, P for predicted all Nusselt numbers well with maximum error of 5.2%. The predicted Nusselt numbers using the correlation of this work were found to be in close agreement with a similar model which was developed based on the influence of absorber hole diameter. Furthermore, the correlation of this work is valuable for future studies on optimization of pitch for a given hole size in the application of the GTC for air heating and crop drying.

Geometrical effects of pins in a center cleared heat sink on thermal management of electronic systems

Thermal management of electronic components is becoming popular due to the extensive applications of electronic components in modern machines and automobiles. Natural convection heat transfer is preferred for electronic cooling in the unavailability of power supply or where high energy efficiency is desirable. Pin fin heat sinks offer effective cooling of such electronic devices even with the natural convection mode. Center-cleared pin-fin heat sinks offer better thermal management of a heated surface compared to conventional pin-fin heat sinks. The present work numerically investigates the thermal characteristics of center cleared solid cylinder, solid tapered, hollow tapered, and perforated hollow tapered pin fin heat sinks under natural convection. The numerical results are discussed in the form of steady state base temperature, heat transfer coefficient, mass-multiplied thermal resistance, volumetric heat dissipation rate, and effectiveness of different pin fin heat sink designs under varying power input from 2.15 to 10.75 W. The minimum mass-multiplied thermal resistance (RThM) is found to be 0.11 K.kg/W and the maximum volumetric heat dissipation rate is 1750.3 KW/m3 corresponding to the perforated hollow tapered pin fin heat sink design.

Measurements and identification of supercritical pseudo-boiling heat transfer modes based on fiber optic probes and multiscale entropy

Subcritical boiling includes interfacial evaporation and heat transfer induced by bubble dynamics. However, for supercritical heat transfer, direct experimental evidence of the existence of pseudo-evaporation and pseudo-boiling heat transfer, as well as the conversion between them is lacking. In this work, the experimental study of supercritical carbon dioxide pool heat transfer is conducted. The pressure and temperature of the cell are 8–10 MPa and 15 ℃, respectively. As heating element and temperature sensing element, a nickel-chromium alloy wire with a length of 22 mm and diameter of 70 μm is placed horizontally in the high-pressure cell. The fiber optic probe is placed vertically, with its tip 200 μm above the wire. Four heat transfer modes, i.e. natural convection, pseudo-evaporation, transition of evaporation and boiling, and pseudo-boiling, are found to occur sequentially with the increase of heat flux density or wall superheat. Natural convection occurs when the wall temperature is below the pseudo-critical temperature. This work focuses on pseudo-evaporation and pseudo-boiling heat transfer and the transition between them. In the pseudo-evaporation mode, the heat transfer coefficient decreases slightly with wall superheat increasing. The fiber outputs a high frequency signal with small amplitude, and there is no dominant frequency. The multiscale entropy is large, characterizing random signal fluctuations. In the transition of evaporation and boiling mode, the fiber outputs a large-amplitude/low-frequency periodic signal with a significant dominant frequency and small multiscale entropy, representing an ordered periodic pulsating heat transfer. In the pseudo-boiling mode characterized by bubble-like structure, the fiber signal fluctuation amplitude and multiscale entropy are between the counterparts of the first two modes i.e. natural convection mode and pseudo-evaporation mode. The dominant frequency is not obvious. The multiscale entropies in the specific case are calculated under different key parameters, such as dimensionality, time scale factor, and length of origin data. Optimal parameters are selected based on the best separation of heat transfer modes. Finally, pseudo-boiling can be distinguished from pseudo-evaporation by multiscale entropy of 0.9, and from transition of evaporation and boiling by multiscale entropy of 0.5. In this work, direct experimental evidence of supercritical-like boiling is obtained, which deepens the understanding of the supercritical heat transfer mechanism and provides a basis for theoretical studies and engineering applications in future.

Thermal resistance of two and three-layer fabric assemblies for extreme cold weather protective clothing

Two (FA2) and three (FA3) layer fabric assemblies have been fabricated in different configurations using cloth shirting angola, pile acrylic fabric, non-woven polyester fabric (batting), coated nylon fabric and nappa leather for designing and development of multilayer clothing to provide protection in extreme cold weather. The configurations of FA2 are cloth shirting angola, PU coated nylon fabric; pile acrylic fabric, PU coated nylon fabric; cloth shirting angola, nappa leather; and acrylic pile fabric, nappa leather. On the other hand, the configurations of FA3 are cloth shirting angola, non-woven polyester fabric, PU coated nylon fabric; pile acrylic fabric, non-woven polyester fabric, PU coated nylon fabric; cloth shirting angola, non-woven polyester fabric, nappa leather; and pile acrylic fabric, non-woven polyester fabric, nappa leather. Thermal resistance of these multilayer clothing assemblies has been evaluated using high-resolution FLIR thermal imaging camera operating in 8-12µm waveband and PID controlled Acmas hot plate in natural convection mode. The maximum thermal resistance, offered by FA2 and FA3, are 5.59 and 6.24 tog respectively. Rational spatial configurations of FA2 and FA3 are as per the temperature of human body parts, and these may be used in designing and development of protective clothing for extreme cold weather.

Effects of surface roughness in multiple microchannels on mixed convective heat transfer

• Effect of process-induced surface roughness on mixed convection was investigated. • Height, width and average roughness parameters were studied on multiple microchannels. • Surface roughness is an important geometric parameter and affects heat transfer. • Mixed convection increases with decreasing cross-section and increasing roughness. The most widely used type of heat transfer in applications is convection heat transfer, and both natural and forced convection modes have an effect in total heat transfer to a certain extent, whether any of these modes is neglected or not. In this study, the contribution of natural and forced convection effects to total heat transfer in rectangular microchannels was investigated experimentally for determining the effect of manufacturing process-induced surface roughness in multiple microchannels fabricated using stainless steel material. The experiments were conducted with microchannel heat sinks having three different surface roughness values of 1.1 µm, 1.8 µm, and 3.0 µm, three microchannel widths of 300 µm, 500 µm and 700 µm and two heights of 300 µm and 450 µm. During the manufacturing process, surface roughness values were controlled by changing electro-erosion fabrication parameters, and detailed surface maps were generated for the microchannel heat sinks. In the experiments using pure water as the working fluid, constant heat flux was applied to the bottom surface of the microchannel heat sink. The experiments were conducted in the range of 10–80 Reynolds numbers to ensure mixed convection conditions. In conclusion, it was determined that surface roughness had a significant effect on mixed convective heat transfer. An increase in surface roughness from 1.1 µm to 1.8 µm in microchannel heat sinks with cross-sections of 300 µm × 300 µm, 700 µm × 300 µm, 300 µm × 450 µm, and 700 µm × 450 µm s led to an increase of about 24 %, 19 %, 17 %, and 13 %, respectively, in the Nusselt number. Likewise, the increase in surface roughness from 1.8 µm to 3.0 µm resulted in an increase of 17 %, 15 %, 14 %, and 10 %, respectively, in the Nusselt number in the same channel cross-sections. In general, an increase in hydraulic diameter led to a reduction in the effect of surface roughness on overall heat transfer. For a constant microchannel width and height, the effect of surface roughness on mixed convection could be observed only with the increase in the Grashof number. Among the eighteen microchannel heat sinks with different geometric parameters, the best heat transfer results were obtained for the smallest microchannel cross-section of 300 µm × 300 µm and the highest roughness value of 3.0 µm.

Theory of a Glazed Transpired Solar Collector in Natural Convection Mode

Abstract The theory of a glazed transpired collector solar air heater in natural convection mode has been developed. This was aimed at generating a framework for the experimental study of the performance of the collector. The theory involved the definition of some of the collector’s main geometries, the formulation of the energy balance on the collector, and the driving forces within the collector. The dimensional analysis was then applied to the formulations to obtain relationships between important dimensionless groups. The theory of the glazed transpired collector in natural convection mode provides a basis for the development of the collector for application in the areas of space heating and crop drying. This type of collector provides a solution to the challenge of space heating and crop drying at locations where electricity is not available and photovoltaic power is not affordable. It could also contribute to energy savings since it requires no electricity to work.

Irradiation capsule design for neutron coloration of topaz in a WWR-K reactor

This study justifies irradiation capsule design calculations used for efficient coloring of topaz in a WWR-K reactor. Various radiation screens used for removing thermal and epithermal neutrons and their influence on the activation of the main impurities in topaz are considered. Neutron analysis has been performed by means MCNP transport code. It is shown that the use of a sandwich screen composed of boron carbide and tantalum decreases the fraction of thermal neutrons by 24% and increases the fraction of fast neutrons by 15%. These are the optimal neutron conditions for topaz irradiation in a WWR-K reactor. Thermal analysis has been performed by means Comsol code and two approaches were taken: conservative and realistic. A thermo-physical analysis with a conservative approach showed that for boron carbide and tantalum screen the temperatures under forced and natural convection modes were 134°С and 274°С, respectively. The temperature of the case body was 75 °C with forced cooling and 238 °C without cooling. In case of realistic approach, the topaz temperature does not exceed 65°С if regular cooling of the irradiation capsule is ensured. Calculation results showed the importance of the ensure circulation between topaz during irradiation, which makes it possible to reduce the temperature of topaz by almost half.

Study of activation energy and moisture diffusivity of various dipping solutions of ivy gourd using solar dryer.

The study is aimed to enhance the shelf life of ivy gourd through the solar drying method in open, forced, and natural convection mode. Ivy gourd is treated as the primary agent to prepare medicines and the stems, leaves and flowers are used to cure diseases related to diabetics, ulcer and skin. The normal shelf life is 2-3days and it can be increased up to 6months with an effective drying process. The experiment is intended to find the best drying process among the open, natural, and forced convection mode with an initial dipping method with ascorbic acid, lemon juice, sugar solution, honey solutions individually, and a control sample (without dipping). A 3kg sample of ivy gourd is dipped in 10g/L of each of the solutions and it is used for the three drying processes individually. The obtained results are indicating that the forced convection method for ascorbic acid is best among the other drying method, with the highest moisture diffusivity is 7.88 × 10-8 m2/s and the lowest activation energy of 21.12kJ/mol. The lemon juice sample is found to have better sensory appeal in terms of colour (darkness) and shrinkage followed by honey, ascorbic acid, and control sample, whereas the honey-dipped sample offers a better taste followed by lemon juice-dipped samples, control, and ascorbic acid-dipped samples, respectively. The influence of dipping solution and drying mechanisms on the functionalities of drying are discussed with suitable illustrations.

Feasibility study to increase the reactor power at natural convection mode in Tehran Research Reactor (TRR) through a hybrid thermal-hydraulic simulation and analysis using the RELAP5 code and Computational Fluid Dynamic (CFD) modeling by ANSYS-FLUENT

Some irradiating applications can be used economically at a low power range and energy consumption. Moreover, removing the decay heat by the natural convection plays a very crucial role in Design Basis Accidents (DBAs) such as Loss of Flow Accident (LOFA) or Station Black Out (SBO). In such cases, the reactor can be operated by natural convection instead of forced convection. Such an operation needs careful thermal-hydraulic analyses to confirm nuclear safety. In this paper, Tehran Research Reactor (TRR) is chosen as the case study. A careful hybrid methodology is taken into account for safety analyses and visualizing the fluid streamlines and temperature profiles assuming both short-term and long-term cooling terms and conditions. Firstly, the RELAP5 code is used for the relevant simulation and safety analysis. Moreover, it is also used to trace any perturbation or oscillation and for the hot spot responses due to the flow regime changes and the reactor dynamic. Then the ANSYS-FLUENT is used for further Computational Fluid Dynamic (CFD) simulation of natural convection flow in detail. A modified 3-D porous media is modeled to simulate and observe the overall behavior of the reactor core in the reactor pool for long-term operations. The Boussinesq approximation is assumed for the buoyancy force. The pressure drop along the porous media is replaced with an accurate model of the Standard Fuel Element (SFE). Moreover, the k- ω turbulence model is also included in the Navier-Stokes equations to fix approximations. The relevant correlations have been also chosen for the heat transfer from the reactor pool to ambient air by convection, radiation, and surface evaporation. Finally, an accurate 2-D model of the hot-channel is also used for the accurate single-channel analysis regarding short-term operating conditions. Presented velocity vectors and temperature profiles depict the domain and behavior of the natural convection in both fuel channels and the reactor pool as well. Concerning both short-term and long-term issues, results are quite promising to increase the maximum operating limit of the natural convection mode from 100 kW to 200 kW. Moreover, regarding short-term operations or emergency conditions, this limit can be also increased to 400 kW due to the safety limit of 378 K for the maximum permissible fuel cladding temperature to protect the fuel from long-term corrosions. Keeping this limit also prohibits any perturbation or oscillation as well.

Visualization experiment and numerical study of latent heat storage unit using micro-heat pipe arrays: Melting process

This study conducts experimental visualization and numerical study on the melting of phase change material (PCM) in a rectangular enclosure of a latent heat storage unit based on micro-heat pipe arrays. The solid–liquid interface and temperature distribution in the melting of PCM are recorded by photographic observation and data measurement. Numerical results based on the enthalpy–porosity model are validated by using experimental results. The effects of enclosure geometry on the melting process of PCM are numerically explored. The transition point of a pure conduction mode into the natural convection mode is analyzed by comparing the results obtained in the numerical simulation with and without considering thermal convection. The correlations of the transient heat transfer and the melting rate in the pure conduction and natural convection modes are determined through the analysis of the different heat transfer characteristics in both stages. The results are expected to provide guidance for the design of latent heat storage units using micro-heat pipe arrays in engineering applications.