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• Evaporation-induced natural convection of ethanol and butanol vapors is studied. • Refractive index fields are measured using Background Oriented Schlieren. • Validity of using similarity of temperature and vapor density fields is analyzed. • Similarity is shown to be violated for fluids with Lewis number larger than 2. • Both downward and upward convection is observed for butanol evaporation. Natural convection driven by evaporation of heavier-than-air fluids from the surface of the heated horizontal and vertical flat plates is studied both experimentally and numerically. The combined buoyancy effects of temperature and vapor density are shown to result in either downward or upward flow depending on the plate temperature and evaporating substance volatility and molar mass. For some substances (e.g., butanol) flow reversals are observed: solutal buoyancy is dominant for low and high temperatures of the heated plate leading to downward flow, whereas at moderate temperatures thermal buoyancy prevails resulting in upward convection. Refractive index fields are measured using Background Oriented Schlieren (BOS). Similarity between the temperature and vapor density fields associated with heat and mass transfer analogy is used to obtain temperature and vapor density fields from experimental data. The validity of this approach is analyzed. Good agreement between experimental temperature and vapor density distributions and numerical simulations is obtained for ethanol vapor, which has Lewis number about 1.8. For butanol vapor, which has Lewis number about 2.5, similarity is violated and temperature and vapor density contributions to refractive index cannot be separated. This is also true for validity of simplified modeling using total expansion coefficient. Similarity is also shown to be violated for hot liquid evaporation from a tank with conductive walls. In this case boundary conditions are dissimilar and upward convective flow of warm clean air near the outer surface of the tank walls is observed, which entrains the heavy vapor. [ABSTRACT FROM AUTHOR]

• The impact of tube tilt angle and fin spacing on natural convection heat transfer was experimentally investigated. • Nusselt number increases and volumetric heat flux density reduces with fin spacing. • Nusselt number and volumetric heat flux density reduce with tube tilt angle. • Correlation for heat transfer was proposed. The natural convection heat transfer of finned oval tubes was studied for different tube tilt angles (0° to 40°), fin spacing (6 m m to 16 m m) and Rayleigh numbers (11 , 000 to 130 , 000). Fin efficiency was determined by temperature measurements along the fin surface and temperature gradient calculations. Nusselt number and volumetric heat flux density were chosen as assessment parameters for the thermal performance. A comparison of the experimental data with correlations from literature was made and good agreement was found. Furthermore, the uncertainty of the measurements was evaluated. In the horizontal tube orientation (0°) the Nusselt number increases with fin spacing. However, the fin efficiency and the volumetric heat flux density reduce. The tilt angle of the longitudinal tube axis was found to have an essential impact on the thermal performance, in particular when the fin spacing is high. For the higher fin spacing values the horizontal orientation gives highest Nusselt number and volumetric heat flux density. At tube tilt angle of 40° the thermal performance becomes lowest for all fin spacing values. When the fin spacing is low, the effect of tube tilt angle is minor. From the experimental results correlations between Nusselt number, Rayleigh number, fin spacing and tube tilt angle are proposed to assist the future design of heat exchanger with tilted finned oval tubes. [ABSTRACT FROM AUTHOR]

• Influence of curvature on heat transfer and regime transition studied experimentally. • Local surface temperature and PIV data collected from 5 heater sizes. • Continuous Nusselt correlations developed using a novel method to scale curvature. This experimental study characterizes the effect of radius of curvature on heat transfer and boundary layer regime transition from laminar to turbulent flow on vertical heated cylinders with a heat flux boundary. Spatially-resolved temperature and velocity data was gathered from a series of five heated cylinders with unique diameters using distributed temperature sensors, thermocouples, and particle image velocimetry. A novel geometric relation has been derived to quantify the influence of curvature on heat transfer and regime transition which has proven to be in good agreement with empirical results. Advanced experimental and analysis methods have been implemented to yield the first continuous, local, curvature-dependent Nusselt correlation for the laminar, transition, and turbulent regimes for natural convection from vertical heated cylinders. As part of this correlation, empirical curvature-dependent dimensionless expressions for predicting regime transition have been developed from three separate data types and are used to define the bounds of the Nusselt correlation regimes. [ABSTRACT FROM AUTHOR]

Highlights • The average heat-transfer coefficient is much higher when the plate is horizontal. • The average heat-transfer coefficient decreases with increasing orientation angle. • The average heat-transfer coefficient increases with increasing oscillation frequencies. Abstract Vibration effects on thermal performances of the rectangular flat plate under natural convection condition are experimentally investigated in both horizontal and slightly inclined from horizontal orientations in multiple angles. The effects of Rayleigh number and vibrational Reynolds number on the average heat-transfer coefficient are also examined. The Aluminium made plate was subjected to sinusoidal vibration in the vertical plane. The test sample was heated under a constant heat flux. The amplitude of vibration was varied from 1.5 to 7.5 mm and the frequency of vibration from 0 to 16 Hz. From the results of the experiments, it is observed that the average heat-transfer coefficient increases linearly with increasing Rayleigh number for different orientation angles. It was also found that the average heat-transfer coefficient is much higher for the cases when the plate is horizontal, and it decreases when the orientation angle value was increased. Such as the average heat-transfer coefficient decreased by approximately 13% for 30° orientation case. The measured result further showed that the average heat-transfer coefficient in the vertical position is lower than the average heat-transfer coefficient in the horizontal position and slightly higher than the other orientation angles (i.e. 30°, 45° and 60°). Finally, it was found that the increases in oscillation frequencies lead to an increase in the average heat-transfer coefficient and the maximum increase was obtained in the horizontal position and higher frequencies. However, the average heat-transfer coefficient decreases with increasing the vibration frequencies when the plate was in the vertical position. [ABSTRACT FROM AUTHOR]

Highlights • Natural convection in differentially-heated vertical closed cavity. • Whole field temperature and heat transfer rates. • Interferometry-based non-intrusive measurements and numerical simulations. • Steady to periodic flow transition. Abstract Natural convection in closed cavities has been a subject of intensive research in the past. Compared to numerical studies, the number of experimental works reported in this area have been relatively scarce. Of the limited number of experimental studies available in the literature, a majority have made use of invasive probes, which inherently disturb the flow. In the present work, real-time experimental measurements are carried out using one of the non-invasive techniques (Mach Zehnder interferometry), that provides the whole field temperature distribution of the fluid layer. Experiments are conducted in a differentially-heated vertical closed cavity of aspect ratio three with air as the working fluid. The vertical side walls of the cavity have been subjected to three temperature differences (Δ T = 10, 20 and 30 ° C) (Ra = 9.7 × 105, 1.8 × 106 and 2.5 × 106). Transient numerical simulations have also been performed using COMSOL Multiphysics 5.2 and a detailed comparison of experimental and simulation results has been presented in the form of temperature contours, spatial distribution of Nusselt number and spatially-averaged heat transfer rates as a function of Rayleigh number. The interferometric measurements highlighted the importance of corner flows which affect the heat transfer rates between the two thermally active walls of the cavity. Buoyancy-induced flow patterns inside the cavity, as interpreted through interferometric measurements, have further been corroborated through smoke-based visualization technique as well as through the results of numerical simulations. Maximum heat transfer rates have been observed in the corners of the differentially heated cavity. Possible flow transitions have been captured by performing the spectral analysis of the interferometry-based transient data. Based on this analysis, Ra = 1.8 × 106 was found to be greater than the critical Rayleigh number wherein the flow instabilities with two dominant frequencies were to be clearly seen. [ABSTRACT FROM AUTHOR]

Highlights • The natural convection mechanism of an air-filled heated square cavity has been investigated. • Experimental tests using a PIV system and a set of thermocouples has been carried out. • Numerical simulations have been performed by a commercial CFD code. • Good agreement of the numerical velocity with the experimental measurements has been found. Abstract In the present work, the natural convection mechanism of air in a square cavity with active side and inferior walls has been investigated both experimentally and numerically. The experimental method developed by some of the authors has been tested against new boundary condition. In addition, the combined radiative-convective heat transfer method has been investigated. The position relative to the gravity vector and the temperature difference between hot and cold walls were analysed: four different inclination angles (ϑ = 0, ϑ = π/6, ϑ = π/3, ϑ = π/2) of the cavity and five different temperatures (104 ≤ Ra ≤ 105) of the hot walls were tested for a total of 20 different cases. The velocity fields and the effective temperature of the walls have been measured by, respectively, a Particle Image Velocimetry (PIV) system and a set of thermocouples. It has been assessed that both the average and maximum velocities increase with the increasing of the Rayleigh number and of the tilting angle. The velocity field is characterized by two vortices, with a symmetry axe passing from the cavity centre when ϑ = 0; by increasing the tilting angle the symmetry tends to disappear leading to the presence of only one vortex for ϑ = π/2. Numerical simulations with both pure convection and combined radiative-convective model have been performed. For the model validation, both the maximum and the average values of the velocity field on the cavity have been taken into account. The results are in good agreement with the experimental measurements. The addition of the radiance contribution does not influence the results. [ABSTRACT FROM AUTHOR]

The deployment of phase change materials (PCMs) in practical applications is in large part limited by the low thermal conductivity of the available PCMs. Hence, in this study, enhancement in the melting rate of PCM by addition of fins in rectangular enclosures was experimentally investigated under different inclination angles. The melting process of lauric acid in side-heated enclosures with different numbers of fins was evaluated for inclination angles of 90° (vertical), 45° and 0° (horizontal). In order to visualize the melting process, the enclosure was fabricated from transparent acrylic material except one side made of an aluminum plate to heat the enclosure isothermally. Solid-liquid interface positions and temperature history of the PCM were recorded and applied to calculate the liquid fractions and heat transfer rates. The evolution of the interface in the inclined enclosures revealed that the development of vortical flow structures in the liquid PCM by decreasing the inclination angle significantly improves the melting rate. It was found that for both finned and unfinned enclosures the melting rate increases by reducing the inclination angle. Heat transfer enhancements obtained by the 0-fin horizontal and 3-fin vertical enclosures compared to the 0-fin vertical enclosure were 115% and 56%, respectively. This thermal behavior proposes that simply inclining the enclosure can be more effective to enhance the melting rate than adding fins to the vertical enclosure. Among the different cases studied, the 3-fin horizontal enclosure showed the maximum heat transfer rate and consequently the minimum melting time. The present investigation can contribute to a better understanding of PCM melting in the inclined finned enclosures and provide much-needed benchmark data for the future numerical simulations. [ABSTRACT FROM AUTHOR]

In this work, an inverse methodology is developed for estimating the local heat transfer coefficients on a vertical plate embedded with the three discrete heat sources, under steady state natural convection, with the temperatures measured at the adiabatic surface without disturbing the fluid flow, using simple conduction/surrogate model and Bayesian inference. Liquid crystal thermography (LCT), an optical measurement method based on the colour-temperature relationship of thermochromic liquid crystal sheet (TLC) is used to determine the temperature field of the adiabatic surface. Bayesian framework with Metropolis Hastings-Markov chain Monte Carlo (MH-MCMC) sampling method is considered for exploring the posterior distribution to estimate the parameters in terms of point estimates like mean, Maximum a posteriori (MAP) and standard deviation. A parity plot between simulated (using retrieved parameters) and measured TLC temperatures shows good agreement. [ABSTRACT FROM AUTHOR]

• Natural convection heat transfers of helical coils were measured varying pitches and turns. • Ring bundles were proved enough to simulate helical coils for ring-by-ring heat transfer. • Heated plumes from lower rings impaired heat transfer of upper ones for small pitches. • Increased pitches and turns enhanced heat transfers by initial velocities and superpositions. • Optimal pitch range for the helical coil design was estimated. A helical coil is gaining interest as the part of a compact heat exchanger since it allows large heat transfer area. Many small modular reactor designs adopt the helical coil steam generator. We measured the local and total heat transfers of the helical coil using ring bundles to confirm the influences of pitches and turns. The pitch to coil diameter ratio was varied from 1.08 to 3.97 and the turns from 2 to 12, with the Ra of 3.63 × 107. Based on the analogy between heat and mass transfers, mass transfer experiments were performed replacing heat transfer ones. For a small pitch, the heated plume from the lower ring provides preheated plume to the upper ring, resulting in an impaired heat transfer. However, for a large pitch, initial velocity is provided, enhancing the heat transfer. The comprehensive heat transfer influences also depend on the coil turns. The measurements of total heat transfer showed the peak at around the pitch to coil diameter ratio range from 1.50 to 2.02 regardless of the number of coil turns. [ABSTRACT FROM AUTHOR]

• Free convection heat transfer from a ribbed vertical plate was experimentally studied. • The effect of rib size, pitch and truncation is investigated. • Continuous ribs negatively affect heat transfer performance, compared to a smooth plate. • Staggered truncated ribs significantly enhance heat transfer from the baseplate. • An optimal length of rib segments for truncated ribs was found. Buoyancy-induced flows over ribbed vertical surfaces involve complex thermal and dynamic interactions between the mainstream and the surface texture, yielding contrasting effects on the heat transfer performance of the heated plate; proper analysis of the overall effect on the heat transfer rate is essential for efficient operation and optimization purposes. The present work pursues an insight into the different factors controlling this problem. Natural convection heat transfer from a vertical plate of 0.5 m height, regularly roughened with wooden transverse square ribs, is experimentally investigated. The surface temperature of the baseplate is varied so that a range of the plate Rayleigh number (Ra) from 3.4 × 108 to 4.9 × 108 is covered. The density of the roughness pattern and the rib pitch-to-height ratio (P/e) are varied by changing the number of ribs attached to the surface (from 10 to 40 rib rows) and using three different square cross-sections (of side lengths 2, 3, or 5 mm). The experimental work relies on the schlieren optical technique, through which the thermal boundary layer is visualized and the Nusselt number distribution is acquired. Analysis of the results reveals that enhancement of the local Nusselt number, relative to a corresponding smooth surface, may be attained only at the central part of the inter-rib region; this occurs exclusively for relatively large values of P/e. At a later stage, the effectiveness of rib truncation in enhancing the heat transfer from the baseplate is explored. Three staggered arrangements are considered, by varying the number of rib segments per row, and heat transfer enhancement, sensitive to the number of rib segments per row, is found. This paper also provides insight into the role of thermal-field disturbances close to turbulent transition, and sheds light on the potential of truncated ribs to amplify such perturbations. [ABSTRACT FROM AUTHOR]

Reliable measurements of thermal and velocity fields were experimentally investigated under different natural circulation intensities in a dual channel facility designed and developed with a representative geometry of prismatic modular reactor core. Experiments were conducted under steady state conditions to obtain statistically stationary time series signals of temperature and velocity. Insignificant radial temperature variation was observed along the down-comer channel implying that the majority of supplied heat is removed through the upper plenum. Velocity results obtained emphasize the delicate nature of natural circulation phenomena in terms of flow destabilization, and recirculation penetration length. Observed air velocity distribution reveals that downward velocity is proportional to the extent of cooling applied to the setup. The calculated values of the dimensionless group Froude number ( Fr ) in the range of 2 to 10 indicate that air is dominated by momentum and is exiting the channel as a jet. Quantification of turbulent intensities implies that flow destabilization is noticeable along the down-comer channel particularly for the case of high cooling intensity. For 5 ° C cooling water temperature, turbulent intensity ( v rms / v ) peaks at the mid-channel (z/L = 0.5) reaching a value of 0.5. This value of turbulent intensity is much higher than the inlet (z/L = 0.96) and outlet (z/L = 0.04) turbulent intensities of 0.2 and 0.06 respectively. This destabilization is because of the flow reversal and heat conduction effects through stainless steel flanges connecting both channels. Current results provide detailed velocity and temperature variation which can be useful for validating computational fluid dynamics codes. [ABSTRACT FROM AUTHOR]

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 5.8 m × 4 m × 3 m 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. [ABSTRACT FROM AUTHOR]

Experimental study on natural convection in a nanofluid-filled differentially heated cavity has been presented. Experiments have been conducted in an octagonal cavity with water and Al 2 O 3 /water-based dilute nanofluids of varying concentrations. A Mach Zehnder interferometer has been employed to record the projection data of convective field from four view angles (0, 45, 90 and 135°). The two-dimensional temperature fields determined through the quantitative analysis of the interferometric images have been employed to reconstruct the local field information using the principles of tomography. Results of the three-dimensional reconstructions clearly brought out the mechanisms of possible fluid movement in the differentially-heated cavity and its dependence on nanoparticle concentration. Near wall surface distributions of temperature fields as well as local Nusselt numbers indicated towards possible breakdown of large scale fluid structures into small length scale multiple roll-like circulatory loops at higher volumetric concentrations of dilute nanofluids. Local disturbances thus induced in the fluid layer due to the addition of nanoparticles resulted into better fluid mixing, which in turn led to an enhancement in the heat transfer rates with increasing concentration of nanofluids. Surface plots of Nusselt numbers obtained in the vicinity of the two thermally active walls of the cavity established an overall energy balance in the fluid layer. [ABSTRACT FROM AUTHOR]

Natural convection in metal foam heat sinks with open slots was investigated experimentally. The test sample was made in-house by attaching several foam strips having fixed width of 10 mm and length of 100 mm onto a substrate of 100 × 100 × 4 mm 3 in size. A total of 29 test samples with different slot widths ( s = 0/2.86/5/8/12.5/20 mm) and foam heights ( H = 10/20/40/60/80 mm) were tested under both horizontal and vertical orientations for each sample. The zero width of open slots corresponds to a heat sink with a single foam block as considered in open literature, while the non-zero open slot width 2.86/5/8/12.5/20 mm corresponds to 8/7/6/5/4 foam strips in a heat sink, respectively. Compared to a single foam block, the foam with open slots are more permeable for air penetrating through the heat sink. More foam strips means more heat transfer area available but less permeation of the heat sink. Experimental results revealed that there exists an optimum open slot width (5–8 mm) for maximum heat transfer coefficient at given heat sink volume. With optimum open slot width, the heat transfer coefficient is enhanced by 14.9%, 21.3%, 37.6%, and 38.2% relative to single foam block ( s = 0 mm) at the foam height of 10 mm, 20 mm, 40 mm, and 80 mm, respectively. [ABSTRACT FROM AUTHOR]

Turbulent natural convection between two horizontal concentric cylinders with different orientation and axis ratios for the outer cylinder with both open-ended has been investigated experimentally and numerically. A Constant heat flux was imposed at the outer cylinder at five different heat fluxes 503, 751, 1002, 1251 and 1508 W/m 2, while the inner cylinder was insulated. The hydraulic diameter was kept constant at 60 mm. The Rayleigh number Ra was varied from 1.017 × 10 7 to 3.25 × 10 7. The outer cylinder orientation angle, ϕ was varied from 0° to 90° with 15° increment. The influence of the axis ratio (major to minor axis) for the outer cylinder was studied at range from 1.0 to 2.0. A finite difference method for the governing equations related with the standard k-ε model has been utilized. The surface temperature distribution at outer cylinder, the axial velocity and average Nusselt number were estimated at different orientation angles and axis ratios of the outer cylinder. Turbulent intensity and axial velocity distribution were estimated numerically. Both experimental and numerical results for the average Nusselt number show good agreement. The results show that the heat transfer enhancement starts as the outer cylinder orientation angle increase more than the critical angle. The maximum increase in the average Nusselt number reaches 25% at vertical position and axis ratio at 2 for the outer cylinder. [ABSTRACT FROM AUTHOR]

Natural convection induced by absorption of solar radiation in the near shore of lakes and reservoirs with a sloping bed is investigated experimentally utilising an open top triangular tank and a light source to simulate solar radiation. The present experimental investigation aims to examine the flow quantitatively for experimental parameters up to Rayleigh numbers one order of magnitude greater than that reported by Lei and Patterson (2002). Concurrent shadowgraph/Particle Image Velocimetry (PIV) technique is used to characterise the flow quantitatively for a number of experimental cases. Three temporal flow stages, initiation, transitional and quasi steady are identified and three spatial sub-regions, conductive, stable convection and unstable convective, are observed, consistent with previous scaling and numerical results. Scales for the onset of instability, velocities along the slope and the movement of the plumes are also verified by the experiments. [ABSTRACT FROM AUTHOR]

We present an experimental investigation for an evaporating ethanol meniscus formed inside a capillary tube of 1 mm. We show the effect of an imposed airflow on the temperature distribution of the meniscus when the airflow speed and direction are changed. We used an infrared camera to map the temperature in time and space at the meniscus pinned at the tube mouth. When airflow is blown around the tube mouth, the natural convection plume is either reduced in size and kept in the same direction ( assisting flow), or bended upwards ( transverse flow), or even inverted completely in an ascending plume ( opposing flow). The measured evaporation rate is compared with the one derived from a simplified analysis showing a good agreement for 0.2 and 0.5 m s −1 airspeed, but less accurate for the other cases. Some flow visualization of the Marangoni flow inside the tube is also presented. [ABSTRACT FROM AUTHOR]

[Display omitted] • Near-imaging of early phases of natural convection from small heating plates. • Linear growth over time of thermal boundary layer thickness. • Bifurcated shape of initial thermal plume produced depending on heat flux-based Rayleigh number. Herein, the evolution of initial thermal plumes (TPs) from small heating plates in water was investigated using near-infrared (NIR) imaging. The imaging system comprised a telecentric uniform illumination system and an NIR camera. The system employed the temperature dependence of the absorbance of water at a wavelength of 1150 nm for the measurements. Two-dimensional absorbance images of a 10 mm thick water cell were captured. A 1 mm thick circular or square steel plate was placed at the bottom and heated via induction. The images revealed that the thickness of the thermal boundary layer, followed by the rise in the TP, increased linearly with time, and several patterns of the TP appeared depending on the heat-flux-based Rayleigh number, Ra*. For a small Ra *, a TP with a mushroom-shaped cap was formed. At the critical value of Ra *, the boundary-layer flow on the plate surface was destabilized, leading to a bifurcated TP and an umbrella-shaped cap above it. These results were validated by comparing them with those of numerical simulations. Furthermore, the relationships between the ascent speed of the TP, plate size, and heat flux were assessed quantitatively. [ABSTRACT FROM AUTHOR]

This paper reports the results of an experimental investigation of free convection from electrically-heated short horizontal circular cylinder of small aspect ratio (L/d = 8) to five liquids of large Prandtl numbers. Three of these fluids are Newtonian and two are weakly power-law shear thinning fluids. Experiments are conducted, under constant heat flux, at various bulk temperatures (40–175 °C) and heat fluxes (0.66–104 kW/m 2 ) that correspond to heat generation from about 0.7 to 113 MW/m 3 . Effects of oil bulk temperatures, oil properties and heat generation parameter, on the free convection results, are investigated. The results indicated that Newtonian fluids attain higher free convection heat transfer coefficient than the power-law fluids by 16% or more. Nusselt number (Nu = hd/k) of the laminar free convection data is correlated in terms of the power-law index and in classical and non-classical forms. Correlations cover Grashof number (Gr ∗ = gβqd 4 /kυ 2 ) from 33 to 1.68 × 10 7 , Rayleigh number (Ra ∗ = gβqd 4 /kυα) from 6.1 × 10 4 to 6.88 × 10 8 , Prandtl numbers (Pr = cpμ/k) from 30 to 1850, Prandtl numbers ratio (Pr b /Pr w ) from 1.03 to 133 and heat generation parameter from 0.001 to 0.009. The developed non-classical correlation that is based on the presented experimental data is the first correlation to consider the heat generation parameter. It predicts the experimental Nu within ±5% whereas the classical form correlation predicts Nu with uncertainty over ±20%. [ABSTRACT FROM AUTHOR]

Three-dimensional natural convection in a rectangular cuboid filled by two immiscible fluids of water and air at different liquid height ratios has been studied experimentally and numerically. An uniform flux heater is located on one of the side walls and the other side-walls are insulated with insulator materials. Top and bottom walls are set at a constant cold temperature. Measurements are carried out for different liquid height ratios of AR = 0.5, 0.625, 0.75 and 0.875 and Rayleigh numbers of Ra = 1.4 × 10 8 , 2.79 × 10 8 , 6.98 × 10 8 and 8.37 × 10 8 . Height, width and length of the cuboid enclosure are 400 mm, 200 mm and 400 mm respectively. The surface temperatures of the heated wall are measured using the LM-35 electronic sensors while twenty cylindrical watertight slots for inserting the PT100 temperature sensor are placed on midline (Z = 0.5) of opposite side wall of the heated wall, so the sensor probe can slides through the slots horizontally. They are distributed on the midline (Z = 0.5) as a vertical array with same distances. Also in this paper, a 3D computational work has been carried out to analyze the natural convection numerically. The finite volume approach is utilized for numerical solutions of the continuity, momentum and energy transport equations. The numerical and experimental results are compared and a good consistency was observed. Measurements and numerical results of the temperature distributions between vertical and horizontal walls for different fluid interface height ratios and Rayleigh numbers are presented. The variation of average Nusselt number along the heater with Rayleigh number and liquid interface aspect ratio are obtained. An uncertainty analysis is also done for experimental data. [ABSTRACT FROM AUTHOR]

Laminar natural and mixed convection heat transfer from an isothermal horizontal square cylinder was experimentally studied. The study includes the design and construction of a test rig to fulfill the research requirements. The natural convection experiments were established for three different side lengths of the square cylinder (25, 30 and 40 mm) which in atmospheric air give Grashof numbers, Gr of 9.895 × 10 4 , 1.499 × 10 5 and 4.388 × 10 5 . For mixed convection, using the three cylinders, Reynolds number, Re ranged from 48 to 670, and the attack angle, Ψ from 0° to 180° with steps of 30°. The experimental results gave the effects of Grashof, Reynolds and Richardson numbers on the average Nusselt number for air ( Pr = 0.7). The experimental results were correlated and compared with previous work. [ABSTRACT FROM AUTHOR]

This investigation is motivated by interest in the mixing caused by the absorption of solar radiation in the near-shore regions of lakes. In the daytime, incoming solar radiation is absorbed by the water following Beer’s law. In the near-shore region where the water depth is less than the penetration depth of solar radiation, the residual radiation reaches the bottom and is absorbed and re-emitted back into the water body as a bottom heat flux, resulting in an adverse temperature gradient which may become unstable. In this laboratory-scale experimental study, this situation is modelled with a water-filled, open top tank with a black bottom subjected to radiative heating from a halogen theatre spotlight. Concurrent shadowgraph and Particle Image Velocimetry (PIV) are utilised for simultaneous visualisation of the temperature field and flow measurements. Initially, there is no motion and two distinct thermal boundary layers simultaneously grow from the water surface and from the bottom boundary. The bottom boundary layer becomes unstable at a certain thickness, and rising plumes are formed. The plumes transfer heat and cause circulation and vertical mixing in the water body. Subsequently the plumes disappear as the bottom boundary layer is dismantled, the bottom boundary layer is re-established, and the plumes re-formed. This paper will focus on characterising the first and the second stages. Accordingly, the onset of convection, plume rise height, and plume rise velocity are investigated through analysis of the PIV and shadowgraph images and compared with previously published scaling. [ABSTRACT FROM AUTHOR]

Laminar natural convection in a rectangular cavity with three different heat transfer fluids: water, ethylene glycol (EG)–water and air were studied experimentally and numerically. The enclosure has a uniform aspect ratio (AR). The EG–water mixture is made up of 60% EG and 40% water. The main experiments aimed to reach proper thermal boundary conditions for the two differentially heated vertical walls of the cavity. Hence, two heating and cooling heat exchangers with water as the heat transfer fluid were attached to the cavity walls. All other walls were properly insulated. Early experiments revealed that it is hard for the heated and cooled walls to reach a uniform temperature when the cavity is filled with water or EG–water, while a uniform distribution of temperature was achieved when it is simply filled with air. Commercial computational fluid dynamics (CFD) software, ANSYS-FLUENT 15, simulated the entire setup to include two special heat exchangers and the cavity between them to investigate all the transport phenomena. The simulation results were in good agreement with measured data. The distortion of air flow is much higher than with the other two fluids. Water flow inside the cavity is flatter and a big circulation area was captured in the middle of the EG–water fluid flow. The local Nusselt number’s three-dimensional distribution was presented on the walls. When compared to other fluids, the impact of the adiabatic walls in the air flow cavity on the Nusselt number was found to be considerable. Eventually, the roles of energy equation terms were studied. Convective terms were noticeable when compared to thermal diffusion. [ABSTRACT FROM AUTHOR]

The main objective of this experimental study is to quantify and compare the natural convection heat transfer enhancement of perforated fin array with different perforation diameter (4–12 mm) and at different angles of inclination (0–90°). The variables for this natural convection cooling with the help of finned surfaces are orientation and geometry. In this study, the steady state heat transfer from the solid fin and perforated fin arrays are measured. The increase in the heat transfer coefficient was achieved with perforated fins of 12 mm perforation diameter at the angle of orientation 45°, which shows about 32% enhanced heat transfer coefficient as opposed to the solid fin array with nearly 30% saving in material by mass. [ABSTRACT FROM AUTHOR]

• Natural convection of upper and lower spheres and the mutual influences were investigated. • Vertical and horizontal pitches were varied and diameter covered laminar and turbulent. • 4 Distinct effects appeared: stagnant, preheating, initial velocity and chimney-like effects. • The heat transfer of lower sphere was affected by the existence of the upper sphere. • This was evidenced by the visualization of the local heat transfer by copper plating pattern. Natural convection heat transfer of two heating spheres was investigated varying the horizontal and the vertical pitches. Based on the analogy concept, mass transfer experiments were carried out by adopting a copper sulfate-sulfuric acid (CuSO 4 -H 2 SO 4) electroplating system. The sphere diameter was varied from 6 to 40 mm, which correspond to Gr d of 9.10 × 103–2.70 × 106. The heat transfer of two spheres for both in-line and staggered arrangements under laminar and turbulent flow conditions were analyzed by four distinct effects: stagnant, preheating, initial velocity and chimney-like effects. The heat transfer of upper sphere was either enhanced by the initial velocity effect or impaired by the preheating effect. Notable observations were that the heat transfer of lower sphere was also affected by the stagnant flow and chimney effects due to the existence of the upper sphere. This was evidenced by the visualization of the local heat transfer by copper plating pattern. [ABSTRACT FROM AUTHOR]

• Frosting experiments on vertical cold plate at micro scale are carried out. • Four frosting stages are illustrated with three key dividing moments defined. • Five types of frost characteristics in early frosting stage are analyzed. • Variations of frost characteristics could reflect specific frosting stages. Frosting is a common natural phenomenon, and is widely seen in engineering fields. To avoid the drawbacks and take advantage of frost, analyzing the frost characteristics is meaningful. Frosting experiments of a vertical cold plate under natural convection are carried out, with the surface temperature varied at a range of −30 ∼ -10 ℃. The frost characteristics, including the frosting stages, frost layer thickness and surface roughness, dynamic frosting rate, reverse melting and frost density, are analyzed and discussed. Results show that the growth trend of frost thickness at different surface temperatures presents a leaf shape, and the first three frosting stages are within 300 s. Under the surface temperature of −25 ℃, formations of the frost seed, tree and forest advance the time of the branches break and the first reverse melting, thus increase the proportion of the Droplet condensation and coalescence stage to 53.70% and influence the frost layer surface roughness. Periodic reverse melting causes the fluctuations of the frost layer surface roughness and the frost density, and changes the trend of dynamic frosting rate when the first reverse melting occurs. Contributions of this study are expected to provide a more in-depth understanding of frosting and thus provide a reference for frost prediction, suppression and defrosting in engineering applications. [ABSTRACT FROM AUTHOR]

In this work, experimental study using holographic interferometry was performed to investigate heat transfer in an open ended cavity between two horizontal parallel circular plates. The lower plate was upward-facing and isothermally heated whereas the upper plate was maintained at ambient temperature. Air was used as the heat transfer medium. To achieve accurate reconstruction of the axisymmetric temperature field, wavelet transformation and a proposed matrix form of the modified Fourier–Hankel were used for the phase extraction and the Abel inversion respectively. Isotherms were mapped in the cavity for several values of the plates spacing and the heated plate temperature. We provide an experimental correlation of the Nusselt number to the Rayleigh number (Ra) based on the heated plate-to-ambient temperature difference and the cavity height. Ra and aspect ratio ranges covered in this work extends from 7 × 10 3 to 4.5 × 10 5 and 0.35 to 1 respectively. [ABSTRACT FROM AUTHOR]

• Heat transfers of a single heating sphere in packed bed was investigated varying its position. • For natural convection, position influence on the packed bed heat transfer was not observed. • For forced convection, local heat transfers were affected by axial and radial positions. • The transition to turbulent flow regime occurred at ∼0.06 m/s for all sphere diameters. • Forced convection correlation for the local heat transfer in the packed bed was developed. Natural and forced convection experiments on a single heating sphere in a packed bed were performed by varying the axial and radial positions of the heating sphere. Based on the analogy between heat and mass transfer, mass transfer experiments were conducted using a copper electroplating system. The sphere diameter was varied from 0.004 to 0.010 m, which correspond to the Ra d of 5.43 × 106–8.48 × 107. The Sc was 2,014. In the forced convection experiments, the range of superficial velocity was 0.01 to 0.58 m/s, which correspond the Re d of 43–5,076. For natural convective flow, the influence of sphere position on the heat transfer of single heating sphere in the packed bed was not observed. However, for forced convective flow, the local heat transfer inside the bed was affected by the variation of the sphere position. The heat transfer increased at the downstream layer of bed due to the intensified turbulence level by the porous structure. The heat transfer near wall region was also enhanced due to the high local porosity. Based on the experimental results, we developed the correlation of local heat transfer for forced convection by adding the multiplier to the existing correlation. [ABSTRACT FROM AUTHOR]

In this study, heat and mass transfer during natural condensation of humid air over a horizontal annular finned-tube is investigated experimentally. The finned-tube is made by extruded process and fins are circular with rectangular cross-section. Outer diameter of central tube, fin diameter, fin thickness and fin density are 25.4 mm, 0.4 mm, 56.0 mm and 433 FPM (fins per meter), respectively. The test case is placed in a specially constructed insulated test room to control test conditions and simulate pure natural dehumidification. Ambient air temperature and relative humidity are controlled by heating, cooling and humidifying equipment. Ethylene glycol-water solution is used as refrigerant to control and keep fin base temperature constant. Experiments are performed for ambient air temperature of 25 °C, 30 °C and 35 °C, relative humidity of 40%, 50%, 60% and 70% and fin base temperature of 4 °C, 6 °C and 8 °C. Observations indicate that the water drops form mainly on tip of the fins and for the conducted tests, no condensation is observed between the fins which is mainly due to small fin spacing (2.0 mm) and lack of convective heat and mass transfer between the fins. The test results also show that condensation and heat transfer rate depend mainly on the temperature and relative humidity of the surrounding air and fin base temperature. The increase in ambient air temperature and relative humidity and the decrease in fin base temperature lead to increase in heat and mass transfer rate. Furthermore, based on the results, new correlations are developed for heat and mass transfer during natural dehumidification over compact annular finned-tubes. [ABSTRACT FROM AUTHOR]

Highlights: [•] Free convection in a vertical channel with opposing buoyancy forces is studied. [•] Flow visualization is used to obtain the streamlines/streaklines. [•] Laser interferometry is used to obtain the temperature-field. [•] The opposing buoyancy forces produce instability inside the channel. [Copyright &y& Elsevier]

Highlights: [•] We propose a correlation for the Nusselt number based on experimental data. [•] The correlation is applicable for vertical cylinders with branched fins. [•] Branched fins have lower thermal resistances compared to plate fins. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Highlights: [•] The effect of fluid on the thermal stratification was researched. [•] The hydrostatic pressure and saturation pressure affect the thermal stratification. [•] The developing speed of stratification layer affects the temperature gradients. [Copyright &y& Elsevier]

Highlights: [•] Natural convection heat transfer in a vertical and heated annulus is studied. [•] Measurements are carried out for four electric powers and three radius ratios. [•] A correlation is proposed to determine average Nusselt number for radius ratios. [•] The results are in a good agreement with previous experimental studies. [Copyright &y& Elsevier]

Abstract: An experimental apparatus dedicated to the study of the reverse flow in a vertical channel with active walls differentially heated is presented. This study explores the influence of two parameters over this air flow, namely the Rayleigh number (6×109 up to 4.5×1010) and the aspect ratio (10 up to 12). The axial velocity u and fluctuations u rms are obtained from laser doppler velocimetry at different positions along the channel axis. As results, conditions of existence of the reverse flow are precised and partially qualified. Three classes of downflows have been confirmed according to an analysis of the velocity histogram: absence, intermittence and permanence. The maximum velocity has been found to vary linearly with downstream position in the channel. An original analysis locates the center of the vortex-like structure and provides the width and the maximum tangential velocity, which are evaluated against the global parameters of the experiment. [Copyright &y& Elsevier]

Abstract: A near-field, unsteady buoyant plume exhibits puffing behaviour, characterised by the periodic formation of large-scale vortical structures (puffs). It has been observed in numerical simulations of pure thermal plumes with a finite area source that the periodic formation of puffs is associated with an instability of the lapping flow that develops over the heated region away from the plume centreline, producing bulge-like structures. Experiments using a shadowgraph technique and two-dimensional, two-component particle image velocimetry are conducted to validate our numerical results for the near-field, unsteady behaviours of a pure thermal planar plume, with water as the working fluid. The formations of bulges in the lapping flow and associated puffs are also observed in transient flow fields obtained in the experiments. Further, quantitative comparison of mean velocity field and oscillation frequencies obtained in the experiments and numerical simulation shows reasonable agreement. This experimental study confirms the important effect of the lapping flow instability on the near-field unsteady behaviour, which has been highlighted in numerical simulations. [Copyright &y& Elsevier]

Abstract: In this paper, the heat transfer characteristics of the natural convection of water near its density maximum in a horizontal annulus had been experimentally investigated. The annulus was formed with two copper cylindrical walls, which were concentric or eccentric. The temperature of the outer cylinder was kept at around 0°C and that of the inner cylinder was ranged from 0 to 12°C. In order to enhance heat transfer, three-dimensional external fins were applied on the inner wall. The effects of the gap width and the eccentricity of annulus and the fin geometric parameters on heat transfer were studied. The results show that the two turning points of the average Nusselt number on the inner wall appear at the temperature differences close to 4°C and 8°C, respectively. With the increase of temperature difference, the average Nusselt number firstly increases, and then decreases, finally increases again. The minimum value of the average Nusselt number locates near the temperature difference 8°C. Additionally, it increases with the increase of the gap width of annulus and the eccentricity. Furthermore, when three-dimensional external fins were applied, the average Nusselt number increases with the increase of the fin axial pitch, whereas it decreases with the increase of the fin width. In the range of experimental values, the influences of the fin height and circular pitch are slight. Finally, the experimental empirical correlations of heat transfer on the inner wall are obtained. [Copyright &y& Elsevier]

Abstract: The paper reported an experimental investigation of natural convection flow in a two-dimensional cavity partially filled with a vertical porous layer. The porous layer consisted of circular cylindrical rods installed horizontally along the left wall of the cavity in a regular array. The porosity and permeability were kept constant but the Rayleigh number varied to simulate different flow conditions. The range of Rayleigh number was from 2.806×104 to 1.053×106. A saline solution having a refractive index similar to that rods and cavity was selected as the working fluid. Particle Image Velocimetry was used to conduct detailed velocity measurement in the cavity, in particular the velocity in the porous and at the porous/fluid interface. From the measurement, the value of slip velocity at the interface was determined. It was found that the slip velocity made dimensionlessly by the shear rate at the interface was independent of interfacial height in mostly flow region, and also the slip coefficient. As a result, an average slip coefficient, defined as a mean value of the coefficient along the direction of interfacial height, was presented in a range of 0.307 through 2.53 for the experiment. [Copyright &y& Elsevier]

Abstract: Natural convection heat transfer from a heat generating horizontal cylinder enclosed in a square cavity, where a temperature difference exists across its vertical walls has been experimentally investigated for the range and a Pr of 0.71. Temperature and cylinder Nusselt number measurements were taken for a range of . is defined as a ratio of cylinder and cavity Grashof numbers. It has been found that at the lower values of , the heat transfer from the cylinder compares well with correlations available in literature. As increases however, it deviates away and the overall heat transfer from the cylinder is increased when compared to these correlations due to the interaction from the cavity. 2D-PIV measurements of the flow structures inside the compartment were conducted. They show an increased interaction between the flow structures generated by the cylinder and by the cavity with increasing , corresponding to the increase in the heat transfer from the cylinder. It is observed that the recirculation generated by the temperature gradient imposed on the cavity is broken down as the plume from the cylinder becomes stronger and a transition process is observed, whereby the flow transitions from being dominated by the temperature difference across the cavity to that dominated by the temperature difference due to the cylinder. [Copyright &y& Elsevier]

Abstract: The operating temperature of building integrated photovoltaic cells is a key factor in terms of energy efficiency. In order to cool them by natural convection, they are mounted in a double-skin façade configuration, meaning that they are separated from the building wall by a ventilated air channel. Identifying and analysing the physical phenomena governing the air flow and heat transfer in this vertical channel are therefore essential to improve cell-cooling. This analysis can only be performed by detailed numerical simulations coupled with experimental studies to validate the results. In the present work, an experimental model is developed in order to study the coupling between the flow and heat transfer of a natural convection flow in a vertical channel. An original approach is taken, using water as the working fluid to avoid radiative heat transfer and obtain a pure convective flow. First the experimental set-up, the boundary conditions and instruments used to obtain reference data are described. Then simultaneous temperature and velocity measurements are provided. Finally, an analysis of the mean velocity and fluctuations profiles shows a change of the flow behaviour occurring at a sufficiently high Rayleigh number. [Copyright &y& Elsevier]

Abstract: An experimental investigation into the three-dimensional flow characteristics of the double-diffusive system was carried out using enclosures of a fixed unit height/span aspect ratio (10mm×10mm) but various widths: 2.5cm, 5cm, 10cm, 23cm and 46cm in the third direction. The ranges of the Rayleigh number and buoyancy ratio of the experiments are 0.89×108–2.67×108 and 0.59–1.18 respectively. It has been shown that an enclosure of a shorter width experiences a shorter mixing time compared to one of a greater width. This result contrasts the dominant presumption in the literature that the end-walls have a stabilizing effect, which is stronger in a shorter enclosure, producing higher flow resistance leading to a longer mixing time. The result can be explained by the observation that a three-dimensional flow is caused by the presence of the front/back walls, which enhances the mixing and destabilizes the flow system. This effect is stronger in a shorter enclosure. A close investigation of the time series and the probability density distribution of the fluid temperature suggests that the fluctuations of the temperature are mostly caused by the intermittent appearance of fluid pockets of discretely different temperatures which significantly contributes to the mixing of the two layers. In addition, the flow visualisation study shows that mixing mechanisms are distinctly different in the fast, intermediate and slow mixing cases. [Copyright &y& Elsevier]

Abstract: Heat transfer between multilayer fibrous materials under unsteady-state conditions is regarded as an intricate and unpredictable process, and the system thermal response with different layer stacking sequences is investigated in this paper. All six possible sequences for three-layer fabrics are measured by an apparatus set up in our lab and simulated by the finite element method, taking account of the effects caused by the internal natural convection and boundary heat loss. It is found that the fabric contacting the hot heat source is the key layer in affecting the system thermal response during the dynamic unsteady-state conditions, and its volumetric heat capacity is the major factor in determining the results. In addition, the effect of the inner natural convection in the multilayer system resulted from the buoyancy is marginal, and negligible under common boundary conditions. In general this study demonstrates that the layer stacking sequences should be considered as an important factor in thermal response while designing a multilayer thermal protective suit. [Copyright &y& Elsevier]

Abstract: In this paper, we propose a correlation for estimating the Nusselt number for natural convection from cylinders with vertically oriented plate fins. For this purpose, extensive experimental investigations are performed for various fin numbers, fin heights, and base temperatures. The present correlation is applicable when the Rayleigh number, fin-height-to-cylinder-diameter ratio, and fin number are in the range 100,000–600,000, 1/6–1/2, and 9–72, respectively. From the correlation, a contour map depicting the thermal resistance as a function of the fin number and fin thickness is presented. The contour map shows the fin number and fin thickness at which the thermal resistance is minimized. [Copyright &y& Elsevier]

Abstract: This paper focuses on an experimental study of the natural convection unsteadiness occurring in an air-filled cavity having two opposite walls respectively heated and cooled at constant and uniform temperature. The other walls are made of insulating materials. The cavity can be tilted in order to obtain an angle of inclination, θ, from 0° (heating from below) to 180° (heating from above). For these extreme angles, the two active walls are horizontal. For each investigated angle of inclination, instantaneous temperature measurements have been carried out in selected areas where temperature signal should became unsteady first, when the temperature difference between the two plates was increased. The location where fluctuations are close to the maximum was then identified. A critical value of temperature difference, ΔT, was calculated after extrapolation of PSD obtained for several values of ΔT. It was highlighted, that for ΔT lower than this critical value, the power spectral density was close to 0 and grew like (ΔT)0.5 for ΔT greater than this critical value. The same critical value was obtained when increasing or decreasing ΔT, showing that it corresponds to a supercritical bifurcation. Spectral analyses reveal that for all the angles of tilt, the first bifurcation appears at the specific and unique value 8.5×107 of a modified Rayleigh number, defined from H ∗ equal to H/[tan(θ/2)]0.5, with H referring to the height of the cavity. In addition, for all the tilt angles, the measured frequencies describe the successive bifurcations occurring until turbulence is encountered with the temperature increase. The exception to this is the angle of 180° where the flow remains always in a steady state. Visualizations have been performed in order to characterise flow patterns obtained between two successive bifurcations. Different class of natural convection flows were then identified, showing Rayleigh–Bénard flow types, boundary layer flows, or typical structures obtained with stably stratified fluids. Time oscillations are also due to completely different modes of unsteadiness: thermal instabilities (gravity or Kelvin – Helmholtz waves) or dynamical instabilities (Tollmien–Schlichting progressive waves). [Copyright &y& Elsevier]

Abstract: The present study is an experimental and numerical analysis on natural convection of air in square enclosures with partially active side walls. The experimental study is carried out both through the holographic interferometry in order to obtain the average Nusselt numbers at different Rayleigh numbers. The temperature distributions in the air and the heat transfer coefficients are measured by a holographic interferometry and compared with the numerical results that are obtained with the finite volume code Fluent 12.1.4. The aim of this comparison is to investigate the influence of the size and number heat sources on the natural convective heat transfer in a square cavity. [Copyright &y& Elsevier]

Abstract: The heat transfer characteristics of the natural convection regime through a vertical rectangular channel simulating a cooling channel of typical material testing reactor have been experimentally investigated. Experiments are performed on demineralized water as coolant passing under atmospheric pressure through narrow rectangular channel of 80cm length, 7cm width and 2.7mm gap thickness under different heat fluxes ranging from 2.7kW/m2 to 26.8kW/m2 that covers almost all possible heat fluxes in the single-phase liquid. The measured local Nusselt number values are compared with the predictions of two correlations for both natural and combined natural and forced convection regimes respectively. The comparison shows the need for developing new correlation to represent the obtained experimental results and to be utilized for the prediction of the local Nusselt number in thermal–hydraulics and safety analysis of the reactor. The developed correlation fits the experimental data within 5.8% relative standard deviation. [Copyright &y& Elsevier]

Abstract: Natural convective flow and heat transfer in an inclined quadrantal cavity is studied experimentally and numerically. The particle tracing method is used to visualize the fluid motion in the enclosure. Numerical solutions are obtained via a commercial CFD package, Fluent. The working fluid is distilled water. The effects of the inclination angle, and the Rayleigh number, Ra on fluid flow and heat transfer are investigated for the range of angle of inclination between 0°⩽ ⩽360°, and Ra from 105 to 107. It is disclosed that heat transfer changes dramatically according to the inclination angle which affects convection currents inside, i.e. flow physics inside. A fairly good agreement is observed between the experimental and numerical results. [Copyright &y& Elsevier]

Abstract: The problem of determination of the turbulence onset in natural convection on heated inclined plates in an air environment has been experimentally revisited. The transition has been detected by using hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity fluctuations (measured through turbulence intensity) start to grow. Experiments have shown that the onset depends not only on the Grashof number defined in terms of the temperature difference between the heated plate and the surrounding air. A correlation between dimensionless Grashof and Reynolds numbers has been obtained, fitting quite well the experimental data. [ABSTRACT FROM AUTHOR]

Abstract: The transient natural convection in an inclined enclosure filled with water is studied experimentally for the time-periodically-varying wall temperature on one side wall and constant average temperature on the opposing side wall. This system has no temperature difference between the opposing two side walls in time-averaged sense. The temperatures of two opposing walls and the heat flux across the enclosure are measured by a heat flux meter. Based on the experimental results, the effects of time-periodically-varying wall temperature and inclined angles of the enclosure on heat transfer characteristics are studied. The experimental results show that, with the upper wall temperature oscillating, the heat flux across the enclosure is also periodically varied with time, and the net heat flux is from the lower wall to the upper wall. Numerical computations are also conducted and numerical results are qualitatively assured by the experimental measurements. [ABSTRACT FROM AUTHOR]

Abstract: The effect of a vertical adiabatic wall on the natural convection heat transfer from vertical array of attached cylinders, which can be considered as wavy surface, was investigated experimentally and numerically. The experiments were carried out using Mach-Zehnder interferometer and the commercial FLUENT code was used for numerical study. This paper focuses on the effect of wall-wavy surface spacing and Rayleigh number variation on the local and average free convection heat transfer coefficients from the each cylinder and the wavy surface. Rayleigh number ranges from 2400 to 10,000 and from 300,000 to 1,250,000 based on cylinder diameter and wavy surface height respectively. The local and average Nusselt numbers were determined for the different Rayleigh numbers, and the ratio of wall- wavy surface spacing to cylinder diameter 0.75, 1, 1.5, 2, 3, 4, 5, and ∞. Results are indicated with a single correlation which gives the average Nusselt number as a function of the ratio of the wall-wavy surface spacing to cylinder diameter and the Rayleigh numbers. There is an optimum distance between the wall and wavy surface in which the Nusselt number attain its maximum value. This optimum distance depends on the Rayleigh number. [Copyright &y& Elsevier]