Experimental Investigation Of Natural Convection Research Articles

Experimental investigation of natural convection of Fe3O4-water nanofluid in a cubic cavity

Convective heat transfer of nanofluids has been an active research topic in recent times. But the relative amount of research in natural convection of nanofluids is scarce in comparison with forced convection. In this study the effect of Fe3O4/water nanofluid on the natural convection behavior in a differentially heated cubic cavity of characteristic length 30 mm has been investigated. The nanofluids are synthesized using a two-step approach and their thermophysical properties like thermal conductivity and viscosity are measured in the temperature ranging from 20 to 60 ° C . The effect of nanoparticle volume fraction on heat transfer performance is studied with concentration ranging from 0–0.8% by volume. The cavity is made up of Poly-lactic acid (PLA) with two sides covered with copper plates. One of the copper plates is kept in contact with ambient atmosphere while variable heat flux using silica gel heater is applied on the other copper plate. All previous results have shown either monotonous deterioration or an increment followed by a continuous decrease in heat transfer coefficient and Nusselt number with increase in concentration. The results of this study have shown that for constant input heat flux the Nusselt number decreases monotonously. Further it is observed that for constant Rayleigh number there is an increase in Nusselt number with increase in concentration.

Experimental investigation of natural convection bubbly flows along a vertical heated plate with mixed wettability surfaces

Experimental investigation of natural convection bubbly flows along a vertical heated plate with mixed wettability surfaces

Experimental investigation of natural convection and gas mixing behaviors driven by outer surface cooling with and without density stratification consisting of an air-helium gas mixture in a large-scale enclosed vessel

This paper describes an experimental investigation of natural convection driven by outer surface cooling in the presence of density stratification consisting of an air and helium (as mimic gas of hydrogen) gas mixture in an enclosed vessel. The unique cooling system of the Containment InteGral effects Measurement Apparatus (CIGMA) is used, and findings reveal that the cooling location relative to the stratification plays an important role in determining the interaction behavior of the heat and mass transfer in the enclosed vessel. When the cooling region is narrower than the stratification thickness, the density-stratified region expands to the lower part while decreasing in concentration (stratification dissolution). When the cooling region is wider than the stratification thickness, the stratification is gradually eroded from the bottom with decreasing layer thickness (stratification breakup). This knowledge is useful for understanding the interaction behavior of heat and mass transfer during severe accidents in nuclear power plants.

Convective heat transport in Viscoplastic material due to localized heating: An Experimental approach

Viscoplastic materials are found extensively both in natural and manmade form. In this work experimental investigation of natural convection in viscoplastic fluid with partially heated bottom wall and continuous cooling of top wall in a square cross section enclosure has been carried out. Carbopol Ultrez 20 gel of various concentrations has been used as sample viscoplastic fluid. Conduction and convection phase of heat transport are identified. The results are presented in terms of temperature distribution across the fluid for different gel concentrations and heat input. The average Nusselt numbers are also discussed for different conditions. Onset of convection is delayed and convection strength is weakened with increase in test fluid yield stress. Steady state temperature difference between hot and cold wall shows linear behaviour with heat input for conduction regime and non-linear behaviour in convection regime. Fluid temperature in enclosure shows sharp gradient closure to thermally active walls.

Experimental Investigation of Natural Convection into a Horizontal Annular Tube with Porous Medium Effects


 
 
 
 In this work, an experimental investigation has been done for heat transfer by natural-convection through a horizontal concentric annulus with porous media effects. The porous structure in gap spacing consists of a glass balls and replaced by plastic (PVC) balls with different sizes. The outer surface of outer tube is isothermally cooled while the outer surface of inner tube is heated with constant heat flux condition. The inner tube is heated with different supplied electrical power levels. Four different radius ratios of annulus are used. The effects of porous media material, particles size and annulus radius ratio on heat dissipation in terms of average Nusselt number have been analyzed. The experimental results show that the average Nusselt number increases with increasing annulus radius ratio and particle diameter for same porous media material. Furthermore, two empirical correlations of average Nusselt number with average Rayleigh number for glass and PVC particles are developed. The present experimental results are compared with previously works and good correspondence is showed. 
 
 
 
 
 

Experimental investigation of natural convection in a vertical rib-roughened channel with asymmetric heating

An experimental study in an open-ended vertical channel is carried out in order to describe the fluid dynamics and heat transfer of transient free convection inside a vertical rib-roughened channel asymmetrically heated at various uniform heat fluxes (650, 700, and 780W/m2) corresponding to various modified Rayleigh numbers (3.65×106, 3.93×106 and 4.4×106). Two ribs are symmetrically located on each wall. The investigations focused more specifically on the influence of the ribs positions inside the channel and the modified Rayleigh number (Ra*) both in steady-state regime and during the transitional phase occurring just after the start of the heating on the flow structure and the heat transfer performance. The results showed the appearance of large-scale flow instabilities which will develop and propagate until the development of the pocket-like vortex (reversed flow). Also, the formation and breakup of recirculation eddies, vortex banishment, besides that a separation and shifting of the boundary layer from one wall to another are identified. The best position of the ribs for heat extraction depends on the magnitude of the Rayleigh number. In that case, the top position is the optimal position for the small and the moderate modified Rayleigh numbers.

Experimental investigation of natural convection near a wall containing phase change material

An experimental investigation is conducted to characterize the natural convection heat transfer along panels which contain phase change materials (PCM). A dedicated configuration has been defined for this study. In this configuration, the natural convection heat transfer is triggered by a temperature scan in the ambient air. The temperature scan is responsible for the energy storage and release in the PCM panels. The corresponding experimental set-up was built and several melting and solidification tests were performed, with different values of the ambient temperature scan speed. The tested PCM panel is the Energain®wallboard by DuPont™. The system first reaches a stationary regime when the PCM is fully solid or liquid. During the stationary regime, the wall to air temperature difference and the convection heat flux are constant. The phase change causes an increase of these quantities. It is characterized by a first dynamic stage, a wall temperature slowdown/retrogression process, and a second dynamic stage. The two dynamic stages are well correlated to the PCM thermal characteristics. However, the temperature slowdown/retrogression phenomenon is unexpected. It consists in an extreme slowdown of the wall surface temperature evolution, which turns into a retrogression during the solidification experiments. The temperature slowdown/retrogression has never been observed before with PCM wallboards.

Experimental investigation of natural convection in a supercritical binary fluid

We experimentally investigate natural convection of a supercritical nitrogen/argon (0.9/0.1 in molar fraction) binary fluid in a bottom-heated cavity with an aspect ratio of 2.5 in the present study. We obtain the development process of natural convection by the holographic interferometry technique, which is divided into three phases: Stable thermal boundary layer (TBL) phase, Developing phase, and Stable flow phase. After thermal perturbation applied at the bottom, the TBL is thickened and then loses stability and thermal plumes are generated, which signifies the onset of natural convection. Thereafter, natural convection gradually develops to a stable state. As the heat input increases, the experimental results show that the convection grows more intense and spreads deeper into the bulk of the fluid. The TBL in supercritical binary fluid is hydrodynamically more unstable and the natural convection develops faster than the case of a pseudo-pure fluid due to the existence of the Soret effect (SE) and the Dufour effect (DE). The SE and DE are analyzed by comparing the temperature variation in the bulk of a supercritical nitrogen/argon binary fluid with the case of its pseudo-pure fluid counterpart, showing that they enhance the heat transfer in the fluid and further accelerate the development of natural convection.

Numerical and Experimental Investigation of Natural Convection in Open-Ended Channels with Application to Building Integrated Photovoltaic (BIPV) Systems

Numerical and experimental investigations of the flow and heat transfer in open-ended channel formed by the double skin façade have been undertaken in order to improve understanding of the phenomena and to apply it to passive cooling of building integrated photovoltaic systems. Both uniform heating and non-uniform heating configurations in which heat sources alternated with unheated zones on both skins were studied. Different periodic and asymmetric heating modes have been considered for the same aspect ratio 1/15 of wall distance to wall height and for periodicity 1/15 and 4/15 of heated/unheated zones and heat input, 220 W/m2. In computational study three dimensional transient LES simulation was carried out. It is shown that in comparison to uniformly heating configuration, non-uniformly heating configuration enhances both convective heat transfer and chimney effect.

Experimental Investigation of Natural Convection in Inclined Channel in Presence of Electrostatic Field

The influence of external electrohydrodynamic forces on natural convection from a flat plate has been studied. Temperature distribution in the aforementioned channel at α = 45°, 90°, and 150° angles in different 10-, 12-, and 15-kV electrostatic fields has been reviewed. Corona wind and the shape of the created vortex by electrohydrodynamics were detected. The results showed that heat transfer was enhanced by increasing the electrostatic field in all angles. In the presence of the field, enhancement of heat transfer of the plate at acute angles is better than for obtuse ones. The optimal angle in terms of heat transfer was 45°, noting heat transfer improvement by 61%.

Experimental Investigations of Natural Convection in Wire-Woven Bulk Kagome

Natural convection in an open and highly anisotropic cellular material, wire-woven bulk Kagome (WBK) is experimentally characterized. A series of pressure drop and heat transfer experiments are conducted. In particular, effect of inclination angle on the heat transfer rate for porous-cored heat sinks (WBK and metallic foams), as well as smooth plate was experimentally investigated. There exists an optimal inclination angle for smooth plate and porous-cored heat sinks in association with the maximum heat transfer rate (Nusselt number). An advance in optimal inclinational angle was observed in porous media during the full inclination angle range from \(0^{\circ }\) (horizontal) to \(90^{\circ }\) (vertical) due to the enhanced lateral conduction and a further advance was found in WBK specimen. A strong anisotropic heat transfer existed in terms of different orientations of WBK specimen: O-B orientation with a bigger cross-sectional surface area density (blockage ratio) reveals a higher heat transfer rate than O-A orientation. Further, in comparison with isotropic metallic foams with a given porosity, the WBK which is positioned at vertical orientation can dissipate more heat than the foams due to the higher permeability resulting from less pressure drop in the WBK formed by an assembly of cylindrical wires.

Experimental investigation of natural convection in an enclosure with partial partitions at different angles

Natural convection heat transfer in a partially partitioned enclosure has been investigated experimentally using Mach-Zehnder Interferometry technique. The top and bottom of the enclosure are insulated while one of the vertical walls is heated isothermally. The partitions are made of wood fiber and are attached to the heated wall with angles changing from 30? to 150? in different experiments. The length of each partition is equal to the width of the enclosure, therefore dividing the enclosure to isolated cells only at 90?. At other angles the cells are interconnected near the cold wall. Rayleigh number based on the enclosure width is changed from 3500 to 32000. Results for the local and the average Nusselt numbers at the heated wall of the enclosure are presented and discussed for various partition angles and Rayleigh numbers. It is found that, at each Rayleigh number, there exists an optimum inclination angle which minimizes the average Nusselt number.

Experimental investigation of natural convection about drift-emplaced waste canisters

Cylindrical waste canisters placed horizontally in the underground drifts of a nuclear repository will emit decay heat by convection to the surrounding air and by radiation heat transfer. In drifts with no forced air circulation, natural convection about the waste canisters will play an important role in controlling the temperature distribution inside the canisters and on the drift wall. Experiments were conducted to visualize the flow about model canisters and to measure the steady-state temperature distribution about the canister. The results show an enhancement in heat transfer when compared to heat flow between infinitely long eccentric horizontal cylinders. A discussion of the observed flow regimes as a function of Rayleigh number is provided to describe the various experimental results.

Experimental investigation of natural convection inside a upper part of vertical converging air channel using the Schlieren technique

The article concerns the selected application problem of gradient optical technique for the analysis of development of the thermal boundary layer. The recording of thermal boundary layer development was carried out using the modified Schlieren system. The modifications introduced to the optical measuring systems allowed the extension of the information carrier measuring range along with a reduction in the susceptibility of the systems to misalignment. As the information carrier, a collimated 25-mW He–Ne laser beam was used. The recording of the distribution of optical density of information carrier around the experimental model under analysis was carried out using a CCD matrix with the image acquisition card. The experimental model under consideration was a converging air channel in the form of a space enclosed by two inclined heating surfaces. The measurements of convective heat transfer were carried out for different angles of inclination (18°, 30° and 45°) and thermal conditions (320K, 370K and 490K) of the heating surfaces. The obtained results of experimental analysis were approximated with the power relationship Nux=C(Rax)a.

Experimental study of flow and heat transfer characteristics of natural convection in an enclosure with horizontal parallel heated plates

We conducted an experimental investigation of natural convection in an enclosure containing horizontal parallel heated plates. Our experimental setup was as follows: Two heated plates were arranged vertically in an enclosure. The temperature of the ceiling was maintained at a constant low temperature, while the other sides satisfied thermal insulation boundary conditions. We examined flow characteristics in the enclosure and heat transfer around the horizontal heated plates. In the region between the cooled ceiling and the upper heated surface, vortex motion occurred when the accompanying flow from the lower region interfered with the ascending flow. Heat transfer around the heated surface was enhanced by this vortex motion. In other regions, the flow circulated along the heated surfaces in all cases. Generally speaking, for each flow pattern, the Nusselt number was proportional to some power of the modified Rayleigh number.

Experimental Investigations of Natural Convection from Circular Plates at Variable Inclination

EAT transfer from circular disk geometry is very important inmanyapplications.Therehasbeensomeexperimentalresearchdevoted to natural convection heat transfer from stationary circulardisks and plates. For a horizontal plate, Kadambi and Drake [1]proposedthecorrelationforexperimentalresultsobtainedinair.Fawand Dullforce [2] made interferometric studies for a round plate inair, and Schulenberg [3] proposed similarity solutions for a regionaround the stagnation point on the isothermal circular plate withspecifiedtemperatureandspecifiedheatflux.Fujiietal.[4]presenteda theoretical study of natural convection heat transfer fromdownward-facing horizontal surfaces with uniform heat flux. For avertical plate, Lewandowski and Kubski [5] obtained the meancorrelationfromtheresultsof25researchers.Inearlierresearch[6,7]the present authors presented the results of theoretical andexperimental investigations for round horizontal plates facingdownward and for vertical ones. Warneford and Fussey [8] madeexperiments in air for a plate inclined at 85 deg from the vertical.In this paper an experimental study of natural convective heattransfer from an isothermal round plate facing up at arbitrary anglesof inclination has been carried out.

An experimental investigation of natural convection with solidification in a differentially heated cavity

This paper introduces an experimental rig used to produce data for the validation of computational models of natural convection within water in an enclosed cavity. The rig consisted of a rectangular cavity with the two long sides maintained at constant temperature. All other surfaces were insulated and adiabatic except for the top surface which was a free surface with an air gap between the free surface and the insulation. Experimental data in the form of velocity, ice growth rate and profiles are presented at 30 min time steps with a cold wall temperature of −10 °C and hot wall temperature of 5 °C. The data produced has systematic and random errors of ±0.4% and ±0.5%, respectively.

06/00763 Experimental investigation of natural convection from vertical and horizontal helicoidal pipes in HVAC applications: Moawed, M. Energy Conversion and Management, 2005, 46, (18–19), 2996–3013.

“S. Nicola” HVAC plant in Vicenza features innovative and significant energy savings characteristics. It has been set up by a gas engine heat pump (coupled to two condensing boilers) whose performances are here evaluated during three years of operation. Due to a grid expansion, the University received the offer of being connected to the district heating grid. This possibility that is often considered advantageous was economically evaluated. As a result of this, a significant increasing of the building annual energy bill was demonstrated in case of acceptance.

Experimental investigation of natural convection inside horizontal elliptic tube with different angles of attack

Natural convection from the inside surface of a horizontal elliptic tube of axis ratio (major to minor axis) 2:1 with a uniformly heated surface was investigated experimentally. The effect of angle of attack on the heat transfer rate was studied. The angle of attack (α) was varied from 0 (when the major axis was horizontal) to 90 (when the major axis was vertical) with steps of 15. The experiments covered a range of Rayleigh number Ra from 1.45 × 106 to 1.78 × 107. The local and average heat transfer coefficients and Nusselt number were estimated for different angles of attack at different Rayleigh numbers. The results obtained showed that the effect of angle of attack on the heat transfer coefficient is significant. The temperature distributions increase with the increase of axial distance from both ends of the tube until a maximum value at the middle of the tube. Also, the local Nu increases with the increase of α at the same axial distance. The average Num increases with the increase of α at the same Ra. The results obtained were correlated by dimensionless groups and with the available data of the horizontal elliptic tube.

Experimental investigation of natural convection from vertical and horizontal helicoidal pipes in HVAC applications

Natural convection from uniformly heated helicoidal pipes oriented vertically and horizontally is experimentally studied. Four helicoidal pipes of different parameters are presented. The effects of pitch to pipe diameter ratio, coil diameter to pipe diameter ratio and length to pipe diameter ratio on the average heat transfer coefficient are found. The experiments covered a range of Rayleigh number based on tube diameter from 1.5 × 103 to 1.1 × 105. The results showed that the overall average Nusselt number, Num, increases with the increase in pitch to pipe diameter ratio, coil diameter to pipe diameter ratio and length to pipe diameter ratio for vertical helicoidal pipes. For horizontal helicoidal pipes, the overall average Nusselt number, Num, increases with the increase in pitch to pipe diameter ratio and length to pipe diameter ratio, but it decreases with the increase in coil diameter to pipe diameter ratio. New correlations are presented, and they can be used in HVAC applications.