Natural Convection Of Air Research Articles

Natural and forced air convection operation in a direct solar dryer assisted by photovoltaic module for drying of green onion

The aim of this research was to develop a direct solar dryer assisted by photovoltaic module and evaluate its performance under natural and forced convection experiments. The photovoltaic module provides electrical power to eight coolers that allow the renewal of air inside the equipment. In this configuration, the dryer is capable of operating independently of electrical energy distribution grid. Moisture and colorimetric parameters were monitored during drying of green onion. Drying kinetics showed a constant rate period followed by a falling rate period for both operating conditions. The convection experiment showed a higher rate and external mechanisms may have controlled the process. Straight lines were adjusted to constants rate periods with R2 of 0.999. Page and Overhults models best described the falling rate periods, with R2 between 0.932 and 0.999. The effective diffusivity values were 5.15 × 10−9 m2/s and 1.15 × 10−8 m2/s for drying with natural and forced convection runs. The average efficiencies of the solar dryer and the specific energy consumption were 34.2%, 18.3 kWh/kg for the drying process with natural convection, and 38.3%, 16.4 kWh/kg for the drying process with forced convection. Little color variation was observed between fresh and dried green onions in the two operating conditions, which is consistent with the need to maintain the green color of the material.

Development of a Miniature Loop Heat Pipe for High Heat Flux Devices and Evaluation of Its Heat Transport Performance

This paper describes the design, fabrication, and the heat transfer performance of a miniature loop heat pipe (mLHP) aiming for cooling a small and high heat flux devices. For the design process, a one-dimensional numerical model was developed. Based on the numerical model, an mLHP was fabricated. The mLHP comprises an evaporator (15×16×t3 mm3), a condenser (12×12.5×t2 mm3) and transport lines (O.D.=1.6 mm, L=100 mm). The mLHP uses Shirasu porous glass (SPG) as a wick and is mainly made of stainless steel. The heat transport performance was investigated in horizontal attitude, anti-gravity (top heat mode) and gravity-assisted (bottom heat mode) with working fluid of water. When the condenser was dissipated by natural air convection, the mLHP performed 5-W (20-W/cm2) heat transport at 170-℃ heat source temperature in horizontal attitude. The thermal resistance was 7.9℃/W. In the bottom heat mode, the lowest thermal resistance of 2.6℃/W was obtained due to the gravity assist. Additionally, the mLHP was tested under the condition that the evaporator side was kept horizontal but the condenser side was bent upwards by 90 degree around the middle of transport lines. Consequently, in low heat load condition the bent mLHP showed the same performance with the bottom heat mode, but shifted to the horizontal mode as the heat load increased. Thus, the heat transport performance was comprehensively investigated for considering various uses.

Experimental investigation on discharging characteristics of supercooled CH3COONa·3H2O-Na2S2O3·5H2O mixtures triggered by local cooling with Peltier effect

Latent heat stored in the supercooled salt hydrate liquid can be released at the desired time by triggered crystallization. In this paper, the melted sodium acetate trihydrate (SAT, melting point Tm = 55.7 °C) and sodium thiosulfate pentahydrate (STP, Tm = 48.9 °C) are cooled by natural convection of room air with respective supercooling degrees of 33 °C and 28 °C. Experiments are performed on the effect of local cooling by semiconductor refrigerator on triggering crystallization of supercooled SAT, STP and their binary mixtures with different mass ratios. The results show that SAT has higher discharging temperature and longer discharging time than STP; but STP has less phase separation, better cycling stability and shorter induction period during the crystallization than SAT after repeated melting-freezing cycles. Moreover, the phase separation problem of SAT could be significantly alleviated with the addition of STP. With the increase of SAT content in the SAT-STP mixtures, the induction period during the crystallization tends to firstly increase and then decrease. The tested curve of the maximum discharging temperatures presents as a V-shaped trend and roughly corresponds with the binary phase diagram. The eutectic mixture of SAT-STP (mSAT:mSTP = 28:72, the eutectic temperature of 40.8 °C) can be triggered in 4 min and shows a discharging temperature peak measured at 39 °C. The discharging time of 3.3 h with temperature above 30 °C is longer than that of single-component salt hydrates. The obtained results are helpful for system design and practical application of solar energy storage based on supercooled phase change materials.

Natural Convection From A Vertical Cylinder Of Finite-Height Embedded in a Darcian Fluid-Saturated Porous Medium.(Dept.M)

Experimental study was carried out for the steady laminar natural convection of air in an inclined annulus porous medium, with constant heat flux heated inner cylinder and an isothermally cooled outer cylinder. The primary objective of the study is to show the effect of the inclination angle of the annulus are: diameters of 21/45mm. gab width=12mm. length of 492mm. and an aspect ratio (length/gab width) = 41. The annulus is filled with spherical beads of diameters d = 6.3 mm with porosity of 0.45. The operating parameters are: heat flux 4≤Q≤324W, Rayleigh number 10≤Ra≤550 and inclination angle 0≤0≤90o. The results show great influence of the local and mean Nusselt number. The average Nusselt number is correlated with both the inclination angle 0 and the Rayleigh number.

How Does Gas-Phase CO2 Evolve in the Headspace of Champagne Glasses?

The chemical space perceived by a consumer of champagne or other sparkling wines is progressively modified all along tasting. Real-time monitoring of gas-phase CO2 concentration was performed, through a CO2-diode laser sensor, along a two-dimensional array of nine points in the headspace of three types of glasses poured with champagne. Two original glasses with distinct headspace volumes were compared with the standard INAO tasting glass. For each of the three glass types, a kind of temperature-dependent CO2 fingerprint was revealed and discussed as a function of the glass geometry and headspace volume. Moreover, a simple model was developed, which considers the rate of decrease of the concentration of gas-phase CO2 in the headspace of a glass after the pouring process as being mainly ruled by natural air convection in ambient air. The timescale which controls the rate of decrease of gas-phase CO2 was found to highly depend on the ratio of the headspace volume to the open aperture of the glass.

Fork-end heat pipe for passive air cooling of spent nuclear fuel pool

When a station blackout accident occurs in a nuclear power plant, active cooling of spent nuclear fuels stored in a water pool using pumps is not available. As a result, the stagnant water in the pool gradually heats up via decay heat from the spent fuels and evaporates, thus exposing the spent fuels to air. This rapidly overheats the spent fuels and produces hydrogen gas due to oxidation of the zircaloy cladding. Accumulation of hydrogen gas can result in harmful detonation in the spent fuel storage building similar to what occurred in the Fukushima accident. In this study, a new concept of a gravity-assisted heat pipe that removes decay heat of spent fuels in a water pool by natural convection of ambient air is proposed. In the design, the condenser at the top of the heat pipe consists of several finned tubes for passive air cooling while the evaporator at the bottom is a single rod submerged in the spent fuel pool, which is called fork-end heat pipe (FEHP). A theoretical analysis model of a FEHP was developed and a laboratory-scale specimen was manufactured and tested for validation. It was found that the developed theoretical analysis model reasonably predicted the heat transfer rate of the FEHP specimen at diverse operating conditions and the operating limit point where the heat transfer rate is maximized due to the counter-current flow limit phenomenon.

Performance investigation of natural and forced convection cabinet solar dryer for ginger drying

In this paper, a Cabinet type solar dryer has been designed and developed with natural and forced convection air circulation. This paper aimed to study the performance of solar dryer for fruits to analyze the moisture content loss and weight loss of Ginger during the drying process by comparing with natural convection and forced convection solar dryers. The Objective of the current experimentation is to present the several parameters of the solar cabinet dryer in a table and graphical form. Study of drying behaviour of Ginger drying has been done with natural and forced air circulation. In this work 4000 gm ginger used for drying and various calibrated instruments used for the measurement. Various tabular data has been prepared on ginger drying with natural and forced Convection. Analysis of per day weight removal of ginger and moisture removal has been done. After analysis, it has been found that the initial moisture content in the ginger is 80% for 4000 gm weight. After drying for three days 830gm in natural air circulation and 620gm in forced air circulation. The Design and specification of the solar Dryer are shown here. Drying has been done in the region Ahmedabad (23.0732N, 72.5515E) Gujarat, India. Comparison of temperature indicated at inlet–outlet of solar dryer in Natural and forced convection mode is represented.

Numerical Simulation of Natural Air Convection in Inclined Eccentric Hemispheres Enclosure

The subject of natural convection heat transfer is motivated by a wide range of applications in engineering technology. The hemispherical cavity is a part of basic geometries although it is not widely studied. The effect of inclinaison on natural convection fluid motions in the gap between two eccentric hemispheres is numerically studied. The inner hemisphere is subjected to a heat flux of a constant density and the outer one is maintened isothermal. The walls separating the two hemispheres are thermally adiabatic. Equations are formulated with vorticity and stream-functions variables. It is also assumed the fluid incompressible and obeys the approximation of Boussinesq. These equations are written by using bispherical coordinates system and solved by using a finite difference method. The results show the topology of flow is strongly dependent on the inclinaison because the flow can change from a unicellular regime to a multicellular regime by varying the inclination from 0 to π. By increasing the Rayleigh number (103Ra7), the flow intensifies. The results are shown in terms of streamlines and isotherms during their transient evolution.

Thermal Simulation of Power Lithium-ion Battery Module

Thermal management of power lithium-ion battery modules is very important to avoid thermal problems such as overheating and out of control, the study of thermal behavior of battery modules can provide guidance for the design and optimization of modules and thermal management. In this paper, a 3d thermal model of the power lithium-ion battery module is established based on STARCCM+ by using computational fluid dynamics (CFD) method, and a grid independence simulation test is used to determine the number of grids, the temperature distribution is analyzed under the condition of 1C charge current. The research results show that the internal temperature rises gradually with the charge going on, the temperature distribution of the cells is basically symmetrical. When the heat transfer coefficient is 5W/(m2⋅K) and the natural convective air inlet temperature is 300K, the module temperature uniformity is good. But because of the maximum temperature slightly higher than the temperature of thermal runaway, additional cooling methods need to be considered to cool the battery.

Natural Convection Air cooling of Electronic Components in Partially top vented Enclosures.(Dept.M)

The study of natural convection from a heat source located at the bottom of partially top vented enclosure is presented. Air cooling for electronic component situated at the bottom of a top vented enclosure is studied theoretically. The flow is assumed to be laminar, steady and of constant physical properties. The process is described by continuity, momentum and energy partial differential equations, which are be expressed in Cartesian coordinates system. Due to the nature of the studied problem and with proper transformation of the problem dependent and independent variables, these governing equations are transformed to a set of dimensionless partial differential equations. This set of differential equations is transformed to set of difference equations by the implantation of finite difference technique. Accordingly, one can obtain the solutions for this problem, which are obtained by application of the well known Gauss Siedel iteration method. A computer program is developed to solve the present proposed mathematical model. According to this solution, the values of Nusselt number, for different values of Rayleigh number are obtained. Comparisons between the present obtained results and those results obtained in previous theoretical results are performed for parametric variations of the vent opening size and shape. Also a correlation for Nusselt number as a function of Rayleigh number and equivalent diameter of the vent is proposed.

Experimental study on heat transfer limit of high temperature potassium heat pipe for advanced reactors

Heat pipe cooled reactor (HPR) is widely applied in various environment due to its remarkable advantages compared with other nuclear power systems, such as high reliability, low noise level and compact structure. As one of the essential components in HPR, the high temperature heat pipe has an important effect on HPR operation performance. In this paper, an experimental study was carried to investigate the thermal–hydraulic characteristics and heat transfer limit of potassium heat pipe. Under the steady-state conditions, heating power and inclination angle of heat pipe were taken as sensitivity parameters. By increasing heating power, the heat transfer capacity was enhanced. For inclination angle, an increase in inclination angle would enhance heat transfer for low filling ratio heat pipe, while it does not make obvious effect on high filling ratio heat pipe. As for heat transfer limit, by analyzing the axial wall temperature distribution and predicting the vapor flow pressure drop, the particular phenomena is classified to four heat transfer limits including viscosity, sonic, entrainment and condensation limit. When heat pipe is cooled by natural air convection, the accurate prediction of condensation limit under effect of inclination is performed by air convection heat transfer relationship. Because of the friction force influence on viscosity limit when operating at low temperature state, the temperature at inlet of condenser section is 11℃ lower than adjacent position. The same reason for sonic limit when operating at low temperature state, the temperature at outlet of evaporator section is 17 ℃ higher than adjacent position. The occurrence of local dryout which lead evaporator temperature increasing sharply to 632 ℃ is entrainment limit. The experimental results provide guidance for the engineering application of high temperature heat pipe.

The assessment of lining structure impact on radon behaviour inside selected underground workings under the cour d\u2019honneur of Ksi\u0105\u017c castle

The results based on 2-year long measurements 01 Jan. 2016–2031 Dec. 2017 have been used for discussing the influence of tunnel lining on the size of 222Rn activity concentration and the impact of the employed rock mass insulation on natural convective air exchange. In April, air movement started when the temperature was at least 7 °C lower than the mean inside. Between May and October, an increase to 9 °C above the underground temperature resulted in an increase of radon concentration. An unconstrained convection process did not start until November and it continued until the end of March. The reinforced concrete lining insulated the fractured and absorptive rock mass. The roof and the sidewall lining had little impact on air movement process.

Instability of natural convection of air in a laterally heated cube with perfectly insulated horizontal boundaries and perfectly conducting spanwise boundaries

Primary and secondary instabilities of buoyancy convection in a laterally heated cube with perfectly thermally insulated horizontal boundaries and perfectly thermally conducting spanwise boundaries are studied. It is revealed that en route to unsteadiness, the flow undergoes a steady symmetry-breaking pitchfork bifurcation. With a further increase of the Grashof number, the nonsymmetric flow bifurcates into an oscillatory state via a Hopf bifurcation.

A comparative study of cooling schemes for laminated lithium-ion batteries

The thermal characteristics of batteries in electric vehicles are important for the vehicle performance and safety. In this work, various battery thermal management methods for laminated lithium-ion batteries are evaluated by experiments and numerical simulations. First, a simulation scheme is proposed and verified by the results of the experiments conducted in this study. Then, five thermal management methods, including air natural convection, natural or forced convection with fins sandwiched between battery cells, forced convection with slotted fins, and natural convection with fins and phase change material (PCM) are numerically investigated. The results show that 3- mm thick copper fins greatly improve the cooling effect. At low discharge rates, natural convection with fins meets the cooling requirements. At high discharge rates, forced convection with fins or natural convection with fins and PCM is necessary to meet the cooling requirements. By replacing the copper fins with aluminum fins, the fin thickness should be increased by 1 mm to meet the cooling requirements, while the battery pack total weight is decreased by 41%, indicating that aluminum has potential in battery thermal management systems in electric vehicles.

Thermofluidic modeling and temperature monitoring of Li-ion battery energy storage system

The battery energy storage system (BESS) is widely used in the power grid and renewable energy generation. With respect to a lithium-ion battery module of a practical BESS with the air-cooling thermal management system, a thermofluidic model is developed to investigate its thermal behavior. The thermal model for a single battery is established and experimentally validated first, and then the thermofluidic model for this battery module is developed based on the single battery thermal model and validated by experimental data also. Simulations are performed considering the practical frequency regulation condition and some hypothetical situations with unexpectedly high heat generation rates. Thermal behaviors of the battery module under different conditions are carefully analyzed. Results show that the fans-on forced air convection cooling reduces the maximum temperature by 0.4 K and makes the maximum temperature appear in a different battery of the BESS module compared with the natural air convection cooling. Especially, the temperature monitoring strategy is studied. Due to technical and cost constraints, only a limited number of temperature monitoring points can be set and the temperature that can be monitored is the battery surface temperature or some connecting bars’ surface temperature. The rationality of the locations of different temperature monitoring points is analyzed. The obtained results clearly indicate that enough caution must be paid when setting the temperature monitoring points in practical BESSs. The bad temperature-monitoring strategy may largely underestimate the maximum temperature (by as much as 3 K under the normal frequency regulation condition, and probably much larger under harsh working conditions) in the BESS and fails at reflecting the thermal status of the BESS.

Establishment of Intrinsic Permeability of Coarse Open-Graded Materials: Review and Analysis of Existing Data from Natural Air Convection Tests

This paper presents a review and analysis of large-scale air convection tests and the establishment of intrinsic permeability in coarse open-graded materials. Natural air convection can make a significant contribution to heat transfer during cooling periods. In seasonally freezing environments this can result in excessive frost penetration and subsequent frost-related problems. Intrinsic permeability largely defines the onset of convective heat transfer in granular materials. Conventional methods for measuring intrinsic permeability cannot be applied to very coarse materials. Large-scale laboratory experiments on natural air convection can serve as an alternative method for determining this crucial parameter. This paper gives an overview of four different experimental test setups for measuring natural air convection, all differing in physical shape, boundary conditions and heat flux/temperature measurement devices. Comparison between these is difficult because the air convection pattern can differ and in some cases the shape and number of convection cells cannot be validated. Most of the studies available in the literature use theoretical equations to approximate intrinsic permeability. A method based on the analytical Nu-Ra number relationship is employed to establish the values of intrinsic permeability. Tests that provide enough data to enable the use of the Nu-Ra relationship are very limited. The overall results show a reasonable correlation between experiment-based intrinsic permeability and theoretical approximation. However, several issues must be addressed: first, differences may exist between the intrinsic permeability of natural and of crushed materials due to the shape effect. Second, the method used is in theory valid only for two-dimensional air convection within a square enclosure heated from below. Yet the results show that this method could be extended to other conditions with a certain degree of confidence. Third, a good estimate of intrinsic permeability is possible only with accurate experimental measurement.

Natural convection in a vertical channel with open-cell foams

Open-cell foams are promising to enhance heat transfer in many applications, such as electronics cooling. Though heat transfer coefficients are low, natural convection in open-cell foams is used in low-end applications, as consumer electronics or switching devices. In this paper air natural convection in a vertical channel equipped with an open-cell foam, with a symmetric uniform heat flux boundary condition at the vertical sides, is investigated numerically. The foam is modeled as a continuous porous medium under the assumption of Local Thermal Non-Equilibrium (LTNE) between solid and fluid phases. Mass, momentum and energy equations are solved numerically under the appropriate boundary conditions by employing a commercial finite element code. The velocity of the fluid and the temperature of the solid as well as the global heat transfer coefficient, for different thermo-physical properties and morphologies of the foam, and for various channel aspect ratios, are predicted.

Experimental study on a novel compact cooling system for cylindrical lithium-ion battery module

Lithium-ion batteries generally require adequate thermal management to achieve optimal performance and avoid thermal runaway. Compactness and light-weight are of critical importance to the lithium-ion power battery system. A novel compact cooling system for thermal management of cylindrical lithium-ion battery packs is proposed, which is a hybrid of phase change material and heat pipe cooling systems. The heat pipe and phase-change-material tube are manufactured with aluminum (which is light in weight) and are specially designed to fully utilize the empty space in-between cylindrical batteries in tight contact. Some annular thin fins are integrally molded to the heat pipe condensation section to enhance convective heat removal to the ambient. The phase change material used is paraffin and the working fluid of heat pipe is acetone. A module contains 40 18650-type batteries, 13 phase-change-material tubes and 14 heat pipes is assembled. Experiments conducted testify the effectiveness of the hybrid thermal management system and reveal as well the effects of heat pipe and phase-change-material tube. It is found that the well-equipped hybrid cooling system can control the highest temperature and the maximum temperature difference in the battery module at about 47.7 °C and 2.5 °C respectively, during 2C discharge process and under the condition of natural air convection to 25 °C ambient. The heat pipe is found to be more effective at lowering the battery temperature rise while the phase-change-material tube appears to be more effective at reducing the temperature non-uniformity in the battery module. Moreover, the effects of phase-change-material tube are found to be sensitive to the ambient temperature; the phase-change-material tube appears to be most effective if the ambient temperature makes the battery module in operating experience a temperature range that can rightly ensure a complete melting of the phase change material.

Overall efficiency improvement of photovoltaic-thermal air collector: numerical and experimental investigation in the desert climate of Ouargla region

In the Ouargla region, the desert area of Algeria, photovoltaic fields (PV) suffer from hard climate conditions with high-temperature levels. This temperature level causes a significant fall of PV cells efficiency which requires an integrated cooling system. For achieving this purpose, a thermal part based on airflow provided along a straight channel under the PV module (150 W) is added. It extracts the accumulated heat by air natural convection, then, the airflow passes through an upper glass extension (0.56 m) to reinforce the heat collection. The evaluation of the whole system performance is experimentally conducted by performing several variations of operating parameters and air channel depth. This photovoltaic-thermal (PV/T) system has modeled by a set of balanced energy equations that are resolved numerically using Matlab software. The experimental results show that the increase in the channel depth causes a significant reduction of thermal efficiency and a slight effect on the electrical one. The numerical data are compared and validated by the experimental results, where the characteristic curves (efficiencies, polarization, powers, temperatures) show good concordance with experimental data. The root means square of percentage deviation (RMSD) is between 1.75% and 16.25%. For a channel depth of 10 cm, the energy and exergy efficiency reach their mean values of 58.5% and 14.7%, respectively. The glass extension of 1.6 m gives a net improvement of 5% in the overall energy efficiency.

Theoretical Investigation of Natural Convection Heat Transfer Through Rectangular Enclosure. (Dept. M. (Power ))

Theoretical investigations on natural convection heat transfer have been carried out inside rectangular enclosures filled with air or water. Heat transfer and fluid flow due to buoyancy forces in an enclosure are achieved. The vertical walls are insulated while the top wall is cooled and the bottom wall is heated under uniform heat flux. Steady laminar natural convection in air or water-filled, 2-D rectangular enclosure is studied numerically. The finite difference technique is used to solve the mass, momentum and energy transfer governing equations. The computational results are presented in the form of isotherm and streamline plots. Simulations are performed for several values of both the width-to-height aspect ratio of the enclosure in the range between 0.66 and 8, and the modified Rayleigh number based on the enclosure height in the range between and whose influences upon the flow patterns, the temperature distributions, and the heat transfer rate are analyzed and discussed. The heat transfer rate from heated enclosure increases as the aspect ratio (A) increases until A=2 and then remains constant with further increase in aspect ratio. The heat transfer rate increases as the modified Rayleigh number increases. Nusselt numbers for both air or water filled enclosures are correlated with both aspect ratio (A) and modified Rayleigh number ( ). In order to validate the numerical code for the present study, present results have been compared with the benchmark results of previous researches.