Enclosure Height Research Articles

On the thermal performance of an internally finned three-dimensional cubic enclosure in natural convection

The present paper evaluates the effect of the aspect ratio and horizontal length of a high conductivity rectangular fin attached to the hot wall of a three-dimensional differentially heated cubic enclosure in laminar natural convection. The objective is the augmentation of the heat transfer delivered from the heated wall to the fluid when the volume fraction of the fin is fixed. Two different values for the fin volume were considered: (i) a large fin that occupies 10 percent of the cubic enclosure, and (ii), a much smaller fin that occupies only 0.1 percent of the enclosure total volume. The finite element technique was applied for solving the coupled steady-state velocity and temperature fields in the 3-D domain in the range 10 3 < Ra < 10 5 , where Ra is the Rayleigh number based on the enclosure height. The numerical results showed that for an enclosure assisted by a large volume fraction fin, the fin aspect ratio does not play an important role, and the average heat flux transferred to the fluid increases monotonically with the fin horizontal length. For a cubic enclosure assisted by a small volume fraction fin, the average heat flux delivered to the fluid increases with the aspect ratio of the fin, and with the horizontal length of the fin. A scale analysis was used to predict the domain in which the fin geometry plays a significant role, i.e., when optimization opportunities are present.

一戸建て住宅戸外空間の囲繞形態とプライバシー感に関する研究

Recently, it can be feeled that residential section landscape is more opened visually. The purpose of this research is to analyze the enclosure form of the detached housing outdoor space, especially (1) present states of enclosure form, (2) whether these enclosure form are affirmative or not, (3) a feeling of privacy according to the stage of openness. Investigation is being done with a questionnaire method for the housing more than 200m^2 in area. The main contents of investigation are the Real and Ideal conditions of ground height, description and height of enclosure, a feeling of privacy at the garden and inside of the house.

Natural convection heat transfer of microemulsion phase-change-material slurry in rectangular cavities heated from below and cooled from above

An experimental study has been conducted in dealing with natural convection heat transfer characteristics of microemulsion slurry in rectangular enclosures. The microemulsion slurry used in the present experiment was composed of water, surfactant, and fine particles of phase-change-material (PCM). The PCM mass concentration of the microemulsion slurry was varied from a maximum 30 mass% to a diluted minimum 5 mass%, and the experiments have been done separately in three subdivided temperature ranges of the dispersed PCM particles in a solid phase, two phases (coexistence of solid and liquid) and a liquid phase. The results showed that the Nusselt number increased slightly with the PCM mass concentration for the slurry in solid phase. In the phase change temperature range, the Nusselt number increased with an increase in PCM mass concentration of the slurry at low Rayleigh numbers, while it decreased with increasing PCM mass concentration of the slurry at high Rayleigh numbers. There was not much difference in natural heat transfer characteristics of the PCM slurry with low PCM concentrations (<10 mass%), however, the difference was getting greater with increasing the PCM concentration, especially for the enclosure at a lower aspect ratio (width/height of the rectangular enclosure). The enclosure height was varied from 5.5 to 24.6 mm under a fixed width and depth of 120 mm. Hence, the experiments were performed for a wide range of modified Rayleigh number from 3 × 10 2 to 1.0 × 10 7. The correlation generalized for the PCM slurry in a single phase was derived in the form of Nu=0.22(1− C 1 C m e − C 2AR ) Ra 1/(3 n+1) , where C 1 and C 2 were the optimum fitting constants obtained by the least square method. While the PCM was in a phase changing region, the correlation could be expressed as Nu=0.22(1− C 1 C m e − C 2AR ) Ra 1/(3 n+1) Ste −0.25, where the Ste was the modified Stefan number.

Natural convection heat transfer in enclosures with microemulsion phase change material slurry

This paper has dealt with the natural convection heat transfer characteristics of microemulsion slurry composed of water, fine particles of phase change material (PCM) in rectangular enclosures. The microemulsion slurry exhibited non-Newtonian pseudoplastic fluid behavior, and the phase changing process can show dramatically variations in both thermophysical and rheological properties with temperature. The experiments have been carried out separately in three subdivided regions in which the state of PCM in microemulsion is in only solid phase, two phases (coexistence of solid and liquid phases) or only liquid phase. The complicated heat transfer characteristics of natural convection have appeared in the phase changing region. The phase change phenomenon of the PCM enhanced the heat transfer in natural convection, and the Nusselt number was generalized by introducing a modified Stefan number. However, the Nusselt number did not show a linear output with the height of the enclosure, since a top conduction lid or stagnant layer was induced over a certain height of the enclosure. The Nusselt number increased with a decrease in aspect ratio (width/height of the rectangular enclosure) even including the side-wall effect. However, the microemulsion was more viscous while the PCM was in the solid phase, the side-wall effect on heat transfer was greater for the PCM in the solid region than that for the PCM in the liquid region. The correlation generalized for the PCM in a single phase is \( Nu = 1/3(1 - C_1 )Ra^{{1 \over {3.5n + 1}}} , \) where C 1 = e –0.09AR for the PCM in solid phase and C 1 = e –0.33AR for the PCM in liquid phase. For the PCM in the phase changing region, the correlation can be expressed as \( Nu = CRa^{{1 \over {7n + 2}}} Ste^{ - (1.9 - 1.65n)} , \) where C = 1.22 – 0.035AR for AR > 10 and C = 0.55 – 16.4e –1.1AR for AR < 10. The enclosure height used in the present experiments was varied from H = 5.5 [mm] to 30.4 [mm] at the fixed width W = 120 [mm] and depth D = 120 [mm]. The experiments were done in the range of modified Rayleigh number 7.0 × 102 ≤ Ra ≤ 3.0 × 106, while the enclosure aspect ratio AR varied from 3.9 to 21.8.

Climate warming experiments: Design of a mesocosm heating system

Research into the impacts of climate change on lakes requires novel experimental methods that enable realistic tests of the effects of increased water temperatures on communities. This article describes the design of a heating system that has been used in situ to study the effects of an increase in lake surface temperatures on littoral communities. Water within four 700‐L enclosures was heated using a propane‐fuelled heat exchange system. Hot water was circulated through a network of heat exchange pipes nested in the bottom of enclosures and temperature within the enclosures was controlled electronically by regulating water flow through a series of valves. The system performed well, with temperatures within warmed enclosures paralleling diurnal fluctuations within control enclosures. The average temperature difference between warm and control enclosures of 4.5°C was close to our target temperature difference of 5°C. Strengths of the experimental system are discussed and potential improvements, including improved heat retention and a design adjustment to facilitate repair in the event of lightning damage are noted. The system is adaptable to larger and smaller volumes, different temperature regimes, and can be adapted for use in pelagic systems.

Natural convection in a shallow cylindrical annuli

This paper examines natural convection in the shallow annular gap between two concentric circular cylinders. Asymptotic solutions are obtained in the limit as the aspect ratio ϵ (defined as the ratio of the enclosure height to the gap width) goes to 0. It is shown that the solution at O( ϵ n ) can only be completely specified by examining the governing equations at O( ϵ n+2 ). Solutions are obtained, and Nusselt number correlations are presented, when the dimensionless radius of the inner cylinder δ is of O(1/ ϵ) and when δ is of O(1). The results indicate that curvature effects profoundly influence the nature of convection in shallow annular enclosures.

Noninvasive assessment of adrenal activity associated with husbandry and behavioral factors in the North American clouded leopard population

AbstractThe North American clouded leopard (Neofelis nebulosa) population is far from self‐sustaining. Breeding success is poor and behavioral problems (i.e., fur‐plucking, tail‐chewing, excessive hiding or pacing, and intersexual aggression that results in mate killing) are common. This study was undertaken to investigate whether some of these problems may be indicators of chronic stress (as reflected by persistently elevated glucocorticoid levels) and whether they are associated with specific management factors. A fecal corticoid metabolite assay was validated to monitor adrenal activity in clouded leopards. Adrenocorticotropic hormone (ACTH) challenges conducted in four clouded leopards established the biological relevance of the assay system. Fecal corticoid concentrations increased 14‐fold above baseline within 24 hours after ACTH administration. Adrenal activity then was monitored in 72 (36 males; 36 females) clouded leopards (65% of the North American Species Survival Plan population) during a 6‐week period and compared to husbandry and behavior data. There was a significant (P &lt; 0.01) gender difference in fecal corticoid concentrations, with females producing higher concentrations than males. Multiple regression analyses revealed negative associations (P &lt; 0.01) between enclosure height, number of hours keepers spent with each animal per week, and corticoid concentrations. A positive correlation (P &lt; 0.001) was found between the number of keepers caring for an individual and corticoid concentrations. Higher fecal corticoid concentrations (P ≤ 0.05) were measured in clouded leopards kept on public display or near potential predators compared to individuals maintained off exhibit or in the absence of predators. Individuals that performed self‐injuring behaviors also had elevated fecal corticoids (P &lt; 0.01). Spearman‐rank correlation analysis of keeper ratings and hormone data revealed positive associations (P ≤ 0.05) between some behaviors (pace, sleep, hide, and fearful/tense) and fecal corticoid concentrations. Overall these results indicate that noninvasive fecal corticoid monitoring has enormous potential for investigating how management and behavioral problems are related to animal well‐being. If conducted under carefully controlled experimental paradigms, this technique could allow researchers and managers to identify problem areas of captive management for clouded leopards (e.g., enclosure height, keeper time) and evaluate the efficacy of strategies designed to promote animal welfare and increased reproductive success. Zoo Biol 21:77–98, 2002. © 2002 Wiley‐Liss, Inc.

Natural convection from a discrete heat source on the bottom of a horizontal enclosure

Steady, natural convection from a discrete flush-mounted rectangular heat source on the bottom of a horizontal enclosure is studied numerically. Three-dimensional form of Navier–Stokes equations are solved by using multigrid technique. Rayleigh number based on the enclosure height is varied from 103 until unstable flow is predicted for a fixed Prandtl number of 0.71. Aspect ratio of the source is varied until it fully covered the entire width of the bottom plate. The enclosure is cooled from above and insulated from the bottom. Effect of vertical boundary conditions on the rate of heat transfer from the heat source is studied. It is found that the rate of heat transfer is not so sensitive to the vertical wall boundary conditions. The limit of the maximum Rayleigh number to obtain a convergent solution decreases as the aspect ratio of the source is increased. The variation of Nusselt number as a function of Rayleigh number and aspect ratio of the source is reported.

Deposition of waste water into deep mines

Transport phenomena in contaminated water from waste disposal sites deposited into deep mines are analysed in the paper. Critical transport path for pollutant migration by diffusion of contaminants and buoyancy-driven convection of contaminated water is assumed to be in a vertical cylindrical enclosure. In the first approximation, both transport phenomena are modeled and then numerically simulated separately. The models presented describe ion diffusion and axisymmetric turbulent convection in a vertical cylinder with the radius R=3·5 m heated from the sidewalls. One-dimensional diffusion equation is used for simulating the ion diffusion in an enclosure of height H=1000 m. Three low-Reynolds number two-equation turbulence models contained in the FIDAP 7·5 software package were tested in our computational simulations of the natural convection. Two typical case studies of buoyancy-driven convection represent the aspect ratios H/R=14·3 and H/R=286. The former corresponds to the initial state just after deposition of the surplus water, the latter to the whole height of the cylindrical enclosure filled by the underground water. The numerical results obtained show that ion diffusion is not dangerous as far as the contamination of subsurface water is concerned. On the other hand, it is concluded that the buoyancy-driven convection in the enclosures with the above aspect ratios can lead to the transport of contaminants into subsurface water and hence the protection of receiving waters from contamination must be designed. Copyright © 1999 John Wiley & Sons, Ltd.

Natural Convection Air Cooling of a Discrete Heat Source on a Conducting Board in a Shallow Horizontal Enclosure

Experiments were performed to measure the heat transfer coefficient on the surface of a square flush heat source mounted at the center of an FR-4 plate in a small horizontal enclosure. The plate area was six times larger than the heat source area. Four cases were considered: the plate facing upwards and downwards, and the backside either insulated or convecting. The heat transfer coefficients exhibited distinct behavior at high aspect ratio in which the dominant length scales were related to the source. At intermediate aspect ratios, the length scales of both the source and the enclosure were relevant, and at small aspect ratios a conduction limit was observed. The heat transfer coefficients at high aspect ratios exceeded the prior correlations by 14 percent for upward facing isolated plates when the ratio of heat source area to perimeter was used as the significant length scale, but the dependence on Ra1/4 was consistent. For the downward facing case, the data exceeded the values for a uniformly heated isolated plate by 68 percent. Classical correlations for shallow differentially heated horizontal enclosures were not satisfactory in describing the dependence on enclosure height.

A note on energy saving in heated enclosures

When a closed space is heated, thermal stratification can be established such that the temperature at floor level is lower than that near the roof. There are two methods by which, during heating, the temperature at floor level can be raised to provide a convenient microclimate at the lower region. The first is by providing a surplus of heat to the space without destroying the thermal stratification, and the second is by mixing mechanically the air (while heating) to establish a uniform temperature distribution. A simple one-dimensional model is developed to calculate the energy required by each method. The results show that air mixing requires much less energy than surplus heating and is therefore a more economical method. In both methods, surplus heating and air mixing, the energy needed increases with the initial vertical temperature gradient existing in the heated enclosure and with the enclosure height.

Temperature distribution and fluid flow in an enclosure with localised heating and cooling

Preliminary experimental results are reported for a temperature distribution inside a water filled cubicle enclosure. The enclosure is subjected to a localised heating and cooling of the same wall where a heater plate is located below the cooling window and results in a colliding boundary layer situation. The three dimensional temperature distribution and velocity distribution were measured for various values of Rayleigh number, ranging from 7.6 × 10 10 to 1.4 × 10 11 (based upon the temperature difference between the heater and window and enclosure height). The velocity field was determined quantitatively by means of particle tracking. Preliminary results are presented here for velocities and temperature for Ra = 1.4 × 10 11. The important findings are that the vast majority of the enclosure was found to be stably stratified and that the collision of the opposing boundary layers did not result in a strong jet-like intrusion into the enclosure, but rather the collision process resulted in a warm and a cold stream issuing from the collision region.

Experimental Studies of Natural Convection in Partitioned Enclosures With a Trombe Wall Geometry

Natural convection heat transfer was investigated in a scaled test facility of a Trombe wall geometry. A silicone oil was employed as the convecting medium to obtain large Rayleigh numbers (up to 1.5 × 1010, based on enclosure height) characteristic of full-scale Trombe wall in a passive solar building. The main objectives were to study effects of Trombe wall nonisothermality and location on heat transfer, fluid temperature and fluid flow patterns. As expected, Nusselt numbers were slightly larger on the Trombe wall space side than on the living space side. Nusselt numbers increased slightly as the mass transfer gaps in the Trombe wall were increased. The results were verified, for the zero gap case, by comparing with previous studies. Physical understanding of the convection process was enhanced by flow visualization data. The information obtained should be useful to designers in optimizing overall building performance for passive solar heating.

Validation Of Zone Models: Entrainment Rates And Convective Heat Losses From Model Atria Tests

Detailed experiments of smoke layer development in Atria revealed the need for refining the relationships for 1) air entrainment rates into low Froude number ( large diameter) pool fires, and 2) the convective heat transfer coefficient to the walls. Experiments were conducted in a square cross section enclosure 2 m x 2 m x 5 m high having adjustable size openings near the bottom ( for the fires air supply) and at the ceiling ( for smoke exhaust) (see Fig. 1). Steady state conditions were established prior to the measurements for methane pool fires produced by a 30 cm diameter sand box burner at two heat release rates, 10 kW and 20 kW, and various combinations of inlet and outlet vent sizes. Temperature measurements by a vertical thermocouple rack were used to determine the height of the smoke layer. Exhaust flow rates were precisely measured by using the tracer gas method, which in the present experiments included two gases: C02 and O2 (simultaneous CO measurements showed no detectable carbon monoxide levels, i.e., the combustion was complete). The pressure difference distribution and then neutral plane were determined by differential pressure transducers along the enclosure height. The redundancy and accuracy of the measurements in this project were essential for the development and estab1ishment of a new entrainment relationship for the buoyant plume of low Froude number (large diameter) pool fires the normalized entrainment rate varies proportionally to Z/D as it was proposed in a previous publication. Moreover, the convective heat transfer coefficient can be determined by using a constant Stanton number, while currently used correlations in zone models failed to reproduce the experimental results

Effect of corrugation frequencies on natural convective heat transfer and flow characteristics in a square enclosure of vee-corrugated vertical walls

A numerical investigation has been carried out on natural convective heat transfer and fluid flow in a square cavity with vee-corrugated vertical surfaces. This study covers the range of corrugation frequency from 1 to 3 and Grashof number from 103 to 105. The corrugation amplitude has been fixed at 5% of the enclosure height. The vorticity stream function formulation with the control volume based finite element method has been used to analyse the effects of corrugation frequency and Grashof number. The investigation shows that the overall heat transfer through the enclosure increases with the increase of corrugation for low Grashof number; but the trend is reversed for high Grashof number.

Interzonal Natural Convective Heat and Mass Flow Through Doorway-Like Apertures in Buildings: Experimental Results

This paper presents the results of full-scale experiments in a realistic building to evaluate natural convective heat and mass transfer through doorway-like apertures under small temperature differentials. The zone-to-zone temperature differences were nominally between 1°C and 2.5°C. Heat transfer correlations, coefficient of discharge, and thermal stratification are reported for air (Pr = 0.71), an enclosure aspect ratio of 0.26, aperture height relative to the enclosure height in the range of 0.75 to 1, and aperture width relative to the enclosure width in the range 0.29 to 0.79. In general, the results extends the validity of previous theoretical and experimental work in the literature to large doorway-like apertures and small temperature differentials across the aperture typical of residential building conditions.

Natural convection in rectangular enclosures with adiabatic fins attached on the heated wall

The heat transfer by natural convection in vertical and inclined rectangular enclosures with fins attached to the heated wall is numerically studied using the energy and Navier-Stokes equations with the Boussinesq approximation. The range of study covers 104≤Ra≤2×105,A=H′/L′=2.5 to ∞,B=l′/L′=0 to 1,C=h′/L′=0.25 to 2 andPr=0.72. The inclination angle from the vertical was from 0 to 60 degree. The variation of the local Nusselt numberNuloc along the enclosure height and the average Nusselt numberNu as a function ofRa are computed. Streamlines and isotherms in the enclosure are produced. The results show thatB is an important parameter affecting the heat transfer through the cold wall of the enclosure. The heat transfer is reduced for decreasingC and it passes from a maximum for an inclination angle. The results show that the heat transfer can generally be reduced using appropriate geometrical parameters in comparison with a similar enclosure without fins.

Natural Convection in a Partitioned Cubic Enclosure

Numerical solutions are obtained for fluid flow and heat transfer in a cubic enclosure with a vertical adiabatic partition. The two zones of the enclosure are connected by a single rectangular opening. The partition is oriented parallel to the isothermal sidewalls, one of which is heated and the other cooled while the remaining walls are adiabatic. Results have been presented for air for the Rayleigh numbers in the range 104−107. The width of the opening is held fixed while the height, relative to the enclosure height, is varied from 0.25 to 0.75. The effects of various parameters on the flow structure and heat transfer are investigated. The results of the three-dimensional simulation have also been compared with those for the corresponding two-dimensional configurations.

Natural Convection From a Heat Source in a Top-Vented Enclosure

Natural convection from a heated disk situated at the bottom of a top-vented enclosure was studied experimentally. The experiments were performed in water (Pr ≅ 5) for parametric variations of the vent opening size, inner enclosure height, and disk-to-enclosure-wall tempearture difference (Rayleigh number). For comparison purposes, baseline data were obtained for an unvented enclosure and for the “infinite” case (no enclosure). The heat transfer data were supplemented by cross-vent temperature measurements and by flow visualization using the thymol-blue electrochemical technique. The experiments demonstrated that, for the range of parameters considered, the average Nusselt numbers could be correlated using a single correlation to within 8 percent. It was also found that the presence of the enclosure (vented or unvented) acted to reduce the Nusselt number, especially at the lower Rayleigh numbers. Flow visualization experiments revealed an unstable flow pattern in the vicinity of the vent that fluctuated in a nonperiodic manner between four basic modes. Temperature measurements revealed asymmetric mean cross-vent temperature profiles, with the mean temperature level increasing with decreasing vent size. The intensity of the temperature fluctuations in the vent opening also increased with decreasing vent size.

Natural convection in an externally heated enclosure containing a local heat source

Numerical analysis is carried out for unsteady and steady, two-dimensional, laminar, natural convection heat transfer in an externally heated enclosure containing a local internal heat source. Results are obtained for Rae = 103-106, Ra* = 3-3000, A* = 0.0333, Pr = 0.7 and (Xs, Ys) = (0.5, 0.09091). The transient control regime, the equivalent relative intensity of the internal heat source, and the transient equivalent relative intensity of the heat source are defined. Velocity vector plots and temperature profiles in the enclosure are given, and the average and local Nusselt numbers are presented. Nomenclature A = area of enclosure As = area of local heat source A* = area ratio, AS/A H = height of enclosure k - thermal conductivity Nu = Nusselt number, Eq. (13) P - dimensionless pressure (p + gpy)H2/pot2 p = pressure Pr = Prandlt number