Experimental Study Of Natural Convection Research Articles

Experimental study of natural convection and cooling capacity of closed crushed-rock layers affected by summer rainfall in cold regions

Climate-induced rises in air temperature and humidity on the Qinghai-Tibet Plateau modify the hydrothermal conditions of closed crushed-rock embankments (CRE). This alternation arises from the infiltration of summer rainfall, which is closely correlated with the cooling performance of the crushed-rock layers (CRL). Nevertheless, the cooling performance of the closed CRL during the summer rainfall season remains unexplored. In these present, physical models were employed to examine the convective properties and hydrothermal response processes. The results reveal that summer rainfall greatly diminishes the natural convection intensity within the closed CRL. Conversely, cycle-average temperatures showed an upward trend under these conditions. Moreover, based on fluctuations in the thawing/freezing index, the cooling capacity of the CRL weakens post-summer rainfall, with a maximum decline of 27.3% compared with the initial cycle. Additionally, the humidity in the subsoil layer experiences a stepwise growth followed by a gradual decrease owing to concentrated rainwater percolation. After enduring four rainfall events, the volumetric water content in the subsoil layer presents an upward trajectory. Overall, infiltrated water introduces additional heat, modifying the hydrothermal and convective properties of closed CRE, consequently impairing the cooling efficacy of the CRL.

Numerical and Experimental Study of Natural Convection in a Tunnel Greenhouse Located in South West Algeria (Adrar Region)

In this work, we propose to experimentally and numerically study the natural convection in laminar regime in an agricultural greenhouse located in South West of Algeria and more precisely in the Adrar area. The numerical study is two-dimensional and was carried out on a tunnel greenhouse with an area of 180m2 located in Adrar in the southwest of Algeria (Latitude: 27°52′27″N, longitude: 0°17′37″W, the laltitude above sea level is 257 m), with polyethylene cover and houses two rows of tomato plants. The experimental study was made during the winter or flowering period of tomato plants (February) when the temperature difference outside the greenhouse is maximum: T min = 3℃ at night and T max = 20℃ the day. We used a calculate code based on the finite element method to numerically simulate the phenomenon of heat transfer inside the greenhouse. The results of the numerical simulation are in the form of isotherms, streamlines and variations in temperature and speed in the greenhouse. The value of the temperature calculated by numerical simulation at the position where the sensor has been placed will be compared with that measured by the sensor. It was concluded that to have a favorable environment for the growth of tomatoes, we must keep the openings closed especially during the night without needing a heating system, especially in this region characterized by a hyper arid climate.

Experimental Study of Natural Convection in a Closed Cavity (Static Type Domestic Fridge)

This work provides an analysis of the thermal flow and behavior of the (load-free) refrigerator compartment. The main goal was to compare the thermal behavior inside the refrigerator cavity to the freezer door (home refrigerator) effect and install a fan on the freezer door while neglecting the heat transmitted by thermal radiation. Moreover, the velocity distribution, temperature, and velocity path lines are theoretically studied. This was observed without affecting the shelves inside the cabinet and the egg and butter places on the refrigerator door as they were removed and the aluminum door replaced with a glass door. This study aims to expand our knowledge about the temperature and flow fields of this refrigerator model. Finally, the development of this work highlights the importance of numerical simulation in the search for improvements in the design of this refrigerator model, which may assist refrigerator manufacturers.

An experimental study of natural convection on a vertical surface with neighboring isothermal and isoflux heating.

Natural convection on a vertical surface composing of isothermal and isoflux sections is widely present at envelope of buildings and in nature. The interaction between boundary layer flows adjacent to isothermal and isoflux sections of the vertical surface can result in different heat and mass transfer rates. The non-dimensional governing parameters for such a flow configuration are analyzed in this study. Shadowgraph visualization and thermistor measurement are carried out to observe the isothermal boundary layer flow (TBLF) and isoflux boundary layer flow (FBLF). In particular, the flow configurations are focused for which the FBLF and TBLF have the same and the opposing flow direction. The flow configurations are characterized under different regimes with TBLF dominance, FBLF dominance and comparable TBLF and FBLF, and the transient features are discussed.

Experimental study of natural convection from vertical cylinders with branched pin fins

In this study, the natural convection heat transfer phenomena from vertical cylinders with branched pin fins are experimentally investigated. Experiments are conducted to measure the heat transfer rates from finned cylinders to the surrounding air at various cylinder wall temperatures, fin pitch angles, and fin heights. The experimental results indicate that the thermal resistance of vertical cylinders with branched pin fins is 20% smaller than that of conventional vertical cylinders with plate fins. The thermal resistance per unit fin volume of the former is 40% smaller than that of conventional finned cylinders. Moreover, a correlation for the Nusselt number is obtained from the calculated thermal resistance values. The error between the predictions from the correlation and experimental results is less than 20%. In addition, the effects of fin length and fin pitch angle on the thermal resistance are investigated using the proposed correlation.

Experimental study of natural convection in a V-shape-section cavity

Natural convection in a V-shaped section cavity heated from below and cooled from above is investigated experimentally for the first time in the literature. Temperature measurements using fast-response thermistors and flow visualization using the shadowgraph technology have been performed. The natural convection development in the cavity including the conductional, the transitional, and the fully developed stages is described. It is experimentally proven that the scaling law of the thermal boundary layer thickness is δT ∼ (κt)1/2 and the scaling law of the plume velocity is vp ∼ κRa7/15/l. Furthermore, the occurrence of Hopf bifurcation in the transition to chaos and the flow structure are also identified experimentally. The power spectral density of the temperature time series reveals that the dominant frequency of the oscillations of the unsteady flow depends on the Rayleigh number, which is quantified.

Experimental and numerical natural convection in an asymmetrically heated double vertical facade

The present work addresses the numerical and experimental study of natural convection inside an asymmetrically heated open double vertical facade. Two heating cases were considered independently, the constant heat flux (Neumann condition) and the constant temperature (Dirichlet condition). The double facade has been modeled using a vertical two-dimensional channel with one wall being maintained at the heating condition and the other one insulated. The boundary conditions at the inlet and outlet were controlled through the addition of adiabatic walls upstream and downstream of the studied area. The airflow is assumed to be laminar and permanent. This study is conducted for several modified Rayleigh numbers ranging from 102 ≤ Ram ≤107 and different aspect ratios (A = 25, 12.5, 8.34, 6.25, and 5). Various parameters have been evaluated and highlighted, namely velocity and temperature profiles. In the first part of this study, the radiative heat transfer is not considered, comparison results give excellent agreement with the experimental work of Webb and Hill [18]. Similarly, the streamline results show a return flow through the outlet of the channel starting from a modified Raleigh value of Ram = 104.In the second case of this study and for isothermal conditions, the radiative transfer is taken into account with mutual radiative heat exchanges between the surfaces and for a transparent not participating medium. The comparison results between the experimental and numerical heat fluxes along the heated plate gives a very good agreement, as well as for the mean Nusselt values for 2.28 × 102≤Ram≤8.22 × 105.

An experimental study of natural convection in vertical annulus with helical fin

ABSTRACTExperimental results of natural convection in an open-ended concentric and eccentric annulus with helical fin on internal tube are presented. The boundary condition investigated is constant temperature for the inner tube while the outer tube is insulated. The effect of pitches and fin diameter under several different constant temperatures applied on the inner tube at concentric and different eccentric conditions are investigated. Helical fins with 2 and 4 mm diameters with 15, 30, and 45 mm pitches have been used in this experimental study. Constant temperature for the inner tube is achieved by passing saturated steam at different pressure through it.

Flow and heat transfer measurements in natural convection in coarse-grained porous media

This paper reports on an experimental study of natural convection in an enclosure that is heated at the bottom and cooled at the top, filled with a packed bed of relatively large solid spheres. Nusselt numbers were measured for various sphere conductivities, spheres sizes and sphere packings for Rayleigh numbers varying between 107 and 109. The Nusselt number measurements showed that at lower Rayleigh numbers, the heat transfer is lower than that for pure Rayleigh-Bénard convection, with the difference depending on packing type, size, and conductivity of the spheres. However, at high Rayleigh numbers, there exists an asymptotic regime where the convective contribution of the total heat transfer for all sphere conductivities, sizes, and packing types collapse on a single curve which is very close to the curve for pure Rayleigh-Bénard convection. Refractive index-matching of the fluid and the solid spheres enabled the use of particle image velocimetry and liquid crystal thermography to obtain highly resolved velocity and temperature fields. The comparison of the velocity and temperature fields for the two heat transfer regimes showed that the velocity magnitudes inside the pores in the core region are much higher in the asymptotic regime than those in the low Rayleigh number regime, which lead to a deeper penetration of cold and hot fluid elements and higher heat transfer.

An experimental study of natural convection in a distribution transformer slice model

The aim of this work is to obtain experimental information on the thermal and fluid dynamics of a distribution transformer. The transformer of interest is of the oil natural – air natural type and our interest is centered in the natural circulation that is produced in the oil volume. An experimental device was built to simulate a representative slice of the transformer and a purpose built PIV setup was used to obtain the flow velocity field. The device overall dimensions and the refrigerating oil are the same as those of commercial transformers of 1000 KVA (Tubos Trans Electric S.A. 1000 KVA Standard Distribution Transformer) Natural circulation is produced by a heating element simulating the heat produced by copper losses in the transformer. A double glass window allows visual access for flow visualization and PIV capture. The slice comprises two fins and the air side is bounded by two isolating boards, forming three air channels. Two reference powers were tested for the experiment: The power corresponding to design operating conditions and a lower power to observe the sensibility to power variations. The result reported in this work shows features of the flow pattern that give important information on the sort of turbulent mixing to be expected. The transient evolution was measured and the thermal characteristic time of the system was estimated. It was found that the convection during the start-up transient is significantly stronger than during steady state operation.

Experimental study of natural convection from an array of square fins

In this research, we experimentally investigate natural convection heat transfer of three finned-tube exchangers with an array of square fins and a fin spacing of 5, 9, and 14 mm. The exchangers are in a 5.8m×4m×3m control room where temperature and humidity are automatically controlled with heating, cooling, humidifying, and dehumidifying equipment. We changed the surface temperature of the center tube by varying the input power of the heating element from 8.4 to 56.7 Watts. To measure the temperature of various points on each exchanger, we used ten T-type thermocouples and conduct four experiments for each exchanger and measured its temperature under the steady-state condition. We also calculated average natural heat transfer coefficient. The results show that amount of radiation heat transfer to the surroundings is about 10%, and the contribution of natural convection is approximately 80% of the total input power to the heating element for all experiments. In addition, average natural heat transfer coefficient increases to a certain value and then decreases when fin spacing increases. Finally, from the results of experiments, we established an empirical relationship in an equation for predicting Nusselt number when the Rayleigh number is between 6.5 and 1335, based on the fin spacing.

Experimental study of natural convection in PV roof solar collector

The aim of this paper is to propose the PV roof solar collector (PV-RSC) to investigate the natural convection heat transfer and estimated the convective heat transfer coefficient in the channel. The experimental set-up was composed of a PV panel on the upper layer and the lower layer is aluminum plate of the channel. The inclination angle and air gap of channel were fixed at 30° and 15cm, respectively. The channel width is 0.7m, and length is 1.2m. The data analysis were confirmed the effect of radiative exchange influent to natural convection within the channel. On the basis of the experimental results, an empirical formula is found; the Nu as a function of Ras sin30, that is Nus=0.3282 (Ras sin30)0.2249. The correlation obtained to range 3×108<Ras sin30<7×108. A comparison between PV-RSC and normal PV panel, it was confirmed that the PV-RSC could be generated electric power than that normal PV panel by about 30W; and also the percentage of power generation increase was rising about 25% throughout the day.

Experimental study of natural convection in an annulus between two eccentric horizontal square ducts

Free convection in an open-ended horizontal eccentric annulus between two eccentric horizontal square ducts with different eccentricities has been experimentally studied with several heat fluxes. Constant heat flux for outer duct and insulated inner square rod boundary conditions were used. The wall temperature of the outer duct was measured along the length for different eccentricities. The eccentricity ratio was changed (E=−1, −0.5, 0, 0.5, 1) in both upper (negative eccentricity) and lower direction (positive eccentricity). Results indicate that with the increase of eccentricity in both directions, the convection heat transfer coefficient decreases. Also, the lower heat transfer rate is found to be at the middle section between both ends led to maximum surface temperature. The generated Rayleigh number due to the heat flux was changed from 4×105 to 2.9×106. Empirical relations are derived from the average Nusselt number as a function of the Rayleigh number and the eccentricity ratio.

Theoretical and experimental study of natural convection with surface thermal radiation in a side open cavity

In this work a theoretical and experimental study of heat transfer by natural convection and thermal radiation on a solar open cubic cavity-type receiver is presented. The theoretical study consists on solving the laminar natural convection and the surface thermal radiation on a square open cavity at one end. The overall continuity, momentum, and energy equations in primitive variables are solved numerically by using the finite-volume method and the SIMPLEC algorithm. The thermophysical properties of the fluid are considered, for the first case, as temperature dependent in all the governing equations, and for the second case, constant, except for the density at the buoyancy term (Boussinesq approximation), with the purpose of comparing the results of both theoretical models with experimentally obtained results. Numerical calculations are conducted for Rayleigh number (Ra) values in the range of 104–106. The temperature difference between the hot wall and the bulk fluid (ΔT) is varied between 10 and 400 K, and is represented as a dimensionless temperature difference (φ) for the purpose of generalization of the trends observed. Experimental results include air temperature measurements inside the receiver. These results are compared with theoretically obtained air temperatures, and the average deviation between both results is around 3.0%, when using the model with variable thermophysical properties, and is around 5.4% when using the Boussinesq approximation.

Experimental study of natural convection from vertical cylinders with branched fins

In this paper, we experimentally investigate natural convection from vertical cylinders with branched plate fins. Various branch angles, fin numbers, and base temperatures are examined. From the experimental data, we propose a correlation for estimating the Nusselt number, which is applicable when the Rayleigh number, branch angle, and fin number are in the ranges 100,000–600,000, 10–40°, and 9–36, respectively. Using the correlation, we prepare a contour map depicting the thermal resistance as a function of the fin number and fin thickness. The contour map shows that the fin number and fin thickness at which the thermal resistance is minimized exist. Finally, we compare the thermal resistances of cylinders with branched fins and conventional plate fins and find that cylinders with branched fins exhibit thermal resistances up to 36% lower than those of cylinders with plate fins.

The experimental study of natural convection within the space of a bundle of rectangular sections

Bundles of rectangular steel sections are examples of porous charge that can be found in the industrial practice of heat treatment. Due to voids within the sections, this type of charge is characterized by porosity even exceeding 85%. This makes the thermal properties of these elements, as expressed by the effective thermal conductivity, kef, are totally different from the thermal conductivity of the solid steel. The value of the kef coefficient is determined, inter alia, by the intensity of gas convection within the sections. In view of the geometry of the system under examination, this phenomenon should be treated as convection in horizontal gaps.The presented paper describes experimental studies concerned with the qualitative and quantitative analysis of natural convection occurring within the space of unidirectionally heated rectangular steel sections. For qualitative examination, the Schlieren method was used. While for quantitative examination, was a Poensgen apparatus employed.The primary goal of the studies was to determine the natural convection occurring within the space of heated section bundles and the effect of this phenomenon on the intensity of overall heat flow. The tests were performed for square sections, dimensions 40×20mm, wall thickness 2mm. The quality tests, despite the fact that the critical value of the Rayleigh number was exceeded in the heated sections, did not demonstrate a proceeding convection. This situation was confirmed by the quantitative tests. The latter showed that the direction of the section heating, which determines the possible occurrence of the convection phenomenon or its lack, does not affect the intensity of the total heat flow. It was also pointed out that, for the range of the Rayleigh number which occurs during the heating of square sections of the side dimensions up to 80mm in a chamber furnace, the heat flow related to the occurrence of free convection can be expressed as an intensification of the thermal conductivity in the liquid. For that purpose, one can apply a simple criterion dependence resulting from the similarity theory and the dimensional analysis.

An experimental study of natural convection in open-cell aluminum foam

Natural convecton n air-saturated alumnum foam has been measured. A carefully designed experimental setup is built for his ask. The calibraton is done by comparing he results of a flat plate wh literature data, revealing excellent agreement. The nvestigated foams have a pore densiy of 10 and 20 PPI. The bondng of the foam is performed via brazing, or by applying a single epoxy which is enriched wh highly conductve alumna particles. The Rayleigh number is varied between 2500 and 6000, wh he rato of he surface area o he perimeter of he substrate as characteristc length. The foam height is varied between 12 and 25.4 mm.A major difference between both he bondng methods is observed. The brazed samples showed a beter heat ransfer n all cases. Furthermore, when ncreasing he foam height, a clear augmentaton of he heat ransfer is observed. Based on hese results, a correlaton is presented.

An Experimental Study of Natural Convection in Porous Media Heated from below

The present experimental study is carried out using a rectangular cavity packed with porous medium such as iron balls or glass balls of different diameters and hence different porosity. The lower plate is maintained at a uniform temperature Th, which is higher than the temperature Tc of upper plate. The experiment is performed for both closed and open systems in vertical as well as horizontal orientation of the test box. When ( Th - Tc ) is increased beyond a certain value, convective motion takes place in the fluid. The results of the experiment indicate that a porous medium can transport more energy than a saturated fluid alone if the porous matrix is highly permeable and thermal conductivity of solid particles is greater than that of fluid. This experiment also shows that convective effect is more in open system i.e., free boundary at the top. It is also observed that the time taken to reach the thermal steady state is less in the horizontal orientation as compared to the vertical orientation. In this study, the relationship between Rayleigh and Nusselt numbers has been investigated in addition to the expected temperature profile. It is found that the magnitude of temperature for iron balls is about 10% more as compared to glass balls and about 18% higher for vertical orientation as compared to horizontal orientation.

Experimental study of natural convection in fluid-superposed porous layers heated locally from below

An experimental study of natural convection in fluid-superposed porous layers heated locally from below is reported. Measurements are made in a rectangular chamber with 3 mm DIA glass beads as the porous layer and distilled water as the saturating fluid. The effects of the heater-to-cavity length ratio and the porous layer-to-cavity height ratio on the overall heat transfer coefficients are reported. Average heat transfer coefficients over the heated surface increase with a decrease in porous layer-to-cavity height ratio, but no clear effect of heater-to-cavity length ratio is seen. Temperature profiles in the domain reveal a plume like flow with a single pair of circulating cells and evidence of convective motion inside the porous layer.

An Experimental Study of Natural Convection in Vertical, Open-Ended, Concentric, and Eccentric Annular Channels

The effects of eccentricity on the natural convection heat transfer from a vertical open-ended cylindrical annulus with diameter ratio of 1.63 and aspect ratio of 18:1 have been investigated experimentally. Within the range of present conditions, and with the possible exclusion of the highest eccentricities, it was found that the flow was thermally fully developed in a considerable section of the apparatus, as indicated by the linear variation of wall temperature with height. This made it possible to estimate the mass flow rate from the wall temperature gradient in the mid-section of the annulus, and use it to calculate the bulk Reynolds number, which was found to be weakly sensitive to eccentricity for a constant wall heat flux and to increase with increased wall heat flux. With the exception of the very low eccentricity range in which it was insensitive to eccentricity, the overall heat transfer rate diminished monotonically with increasing eccentricity. Plots of the local azimuthal variation of the Nusselt number showed that, at low eccentricities, the heat transfer rate increased near the wider gap but decreased near the narrower gap. The average Nusselt number was found to decrease measurably with increasing eccentricity and to increase slightly with increasing heat flux within the examined range. In contrast, the Grashof number was found to be much more sensitive to changes in heat flux and only had a weak dependence on eccentricity.