Transient combined laminar free convection and radiation in a rectangular enclosure

1976 ◽  
Vol 78 (1) ◽  
pp. 65-85 ◽  
Author(s):  
D. W. Larson ◽  
R. Viskanta
Keyword(s):  
Wall Temperature ◽  
Initial Temperature ◽  
Radiation Transfer ◽  
Heated Wall ◽  
Dimensionless Time ◽  
Rectangular Enclosure ◽  
Transfer Equations ◽  
Temperature Heat ◽  
Order Of Magnitude ◽  
Laminar Natural Convection

The mass, momentum and energy-transfer equations are solved to determine the response of a rectangular enclosure to a fire or other high-temperature heat source. The effects of non-participating radiation, wall heat conduction, and laminar natural convection are examined. The results indicate that radiation dominates the heat transfer in the enclosure and alters the convective flow patterns significantly. At a dimensionless time of 5·0 the surface of the wall opposite a vertical heated wall has achieved over 99% of the hot-wall temperature when radiation is included but has yet to change from the initial temperature for pure convection in the enclosure. At the same time the air at the centre of the enclosure achieves 33% and 13% of the hot-wall temperature with and without radiation, respectively. For a hot upper wall the convection velocities are not only opposite in direction but an order of magnitude larger when radiation transfer between the walls is included.

scholarly journals Observational manifestations of ‘cosmological dinosaurs’ at redshifts z ∼ 20

2021 ◽  
Vol 503 (2) ◽  
pp. 3081-3088
Author(s):  
V K Dubrovich ◽  
Yu N Eroshenko ◽  
S I Grachev
Keyword(s):  
Black Hole ◽  
Atomic Hydrogen ◽  
Radiation Transfer ◽  
Ring Width ◽  
High Mass ◽  
Primordial Black Hole ◽  
Array Type ◽  
Local Sources ◽  
Order Of Magnitude ◽  
Very High

ABSTRACT We consider a primordial black hole of very high mass, $10^9\!-\!10^{10}\, \mathrm{M}_\odot$, surrounded by the dark matter and bayonic halo at redshifts z ∼ 20 without any local sources of energy release. Such heavy and concentrated objects in the early Universe were previously called ‘cosmological dinosaurs’. Spectral distribution and spatial variation of the brightness in the 21-cm line of atomic hydrogen are calculated with the theory of radiation transfer. It is shown that a narrow and deep absorption arises in the form of the spherical shell around the primordial black hole at the certain radius. The parameters of this shell depend almost exclusively on the mass of the black hole. The angular diameter 18 arcsec of the absorption ring at z ∼ 20 is well within the current technical possibilities of the Square Kilometre Array type telescopes. But the observation of the ring width itself requires an order of magnitude better resolution.

Highly Subcooled Flow Boiling: A Model for Estimating Heat Transfer Behind Sliding Bubbles in a Narrow Channel

2012 ◽  
Author(s):  
Arif B. Ozer ◽  
Donald K. Hollingsworth ◽  
Larry. C. Witte
Keyword(s):  
Heat Transfer ◽  
Flow Field ◽  
Heat Generation ◽  
Wall Temperature ◽  
Flow Boiling ◽  
Narrow Channel ◽  
Heated Wall ◽  
Uniform Heat ◽  
Proposed Model ◽  
Sliding Bubbles

A quenching/diffusion analytical model has been developed for predicting the wall temperature and wall heat flux behind bubbles sliding in a confined narrow channel. The model is based on the concept of a well-mixed liquid region that enhances the heat transfer near the heated wall behind the bubble. Heat transfer in the liquid is treated as a one-dimensional transient conduction process until the flow field recovers back to its undisturbed level prior to bubble passage. The model is compared to experimental heat transfer results obtained in a high-aspect-ratio (1.2×23mm) rectangular, horizontal channel with one wide wall forming a uniform-heat-generation boundary and the other designed for optical access to the flow field. The working fluid was Novec™ 649. A thermochromic liquid crystal coating was applied to the outside of the uniform-heat-generation boundary, so that wall temperature variations could be obtained and heat transfer coefficients and Nusselt numbers could be obtained. The experiments were focused on high inlet subcooling, typically 15–50°C. The model is able to capture the elevated heat transfer rates measured in the channel without the need to consider nucleate boiling from the surface or microlayer evaporation from the sliding bubbles. Surface temperatures and wall heat fluxes were estimated for 17 different experimental conditions using the proposed model. Results agreed with the measured values within ±15% accuracy. The insight gathered from comparing the results of the proposed model to experimental results provides the basis for a better understanding of the physics of subcooled bubbly flow in narrow channels.

Convection in an Enclosure-Source and Sink Located Along a Single Horizontal Boundary

1978 ◽  
Vol 100 (2) ◽  
pp. 205-211 ◽  
Author(s):  
L. A. Clomburg
Keyword(s):  
Rayleigh Number ◽  
Aspect Ratio ◽  
Numerical Data ◽  
Uniform Heat Flux ◽  
Horizontal Boundary ◽  
Order Of Magnitude ◽  
High Prandtl Number ◽  
Laminar Natural Convection ◽  
Rayleigh Numbers ◽  
Source And Sink

Laminar natural convection in a two-dimensional enclosure with both source (uniform heat flux density) and sink (temperature specified) located on the top horizontal boundary is investigated numerically. Temperature and velocity profiles are presented for a high Prandtl number fluid for length Rayleigh numbers in the range 107 to 109 for length to depth ratios of 1:1 to 4:1. To generalize the results, an order of magnitude analysis is used to determine the dependence of temperature, velocity, and boundary-layer thickness scales on aspect ratio and Rayleigh number. The numerical data are well correlated using these suggested scales. The analysis shows the Nusselt number and the maximum horizontal velocity to depend on the 1/6 and 1/3 powers of the Rayleigh number, independent of aspect ratio.

Laser-Doppler Measurements of the Velocity Along a Heated Vertical Wall of a Rectangular Enclosure

1983 ◽  
Vol 105 (4) ◽  
pp. 782-788 ◽  
Author(s):  
H. Ozoe ◽  
M. Ohmuro ◽  
A. Mouri ◽  
S. Mishima ◽  
H. Sayama ◽  
...  
Keyword(s):  
Numerical Solutions ◽  
Vertical Wall ◽  
Three Dimensional ◽  
Laser Doppler ◽  
Central Core ◽  
Heated Wall ◽  
Critical Test ◽  
Three Dimensional Flow ◽  
Rectangular Enclosure ◽  
First Time

The horizontal and vertical velocity profiles near a heated vertical wall of rectangular enclosure were measured for the laminar regime of natural convection with a laser-Doppler anemometer. The horizontal temperature profiles near the heated wall were measured with a thermocouple. An almost perfect two-dimensional mode of flow was confirmed for the central regime of the box. A minimum in the temperature profile between the hot wall and the thermally stratified central core resulted in a downward flow just outside the boundary layer of upward flow, but the central core was stagnant. Visualization of the flow with a phenolphtalein tracer confirmed the two-dimensionality of the flow along the vertical heated wall and revealed a zone of three-dimensional flow in the form of spiral streaklines along the insulated top plate toward the opposing cooled vertical wall. Measurements such as these provide for the first time the basis for a critical test of the accuracy of numerical solutions.

The Effect of the Top Wall Temperature on the Laminar Natural Convection in Rectangular Cavities With Different Aspect Ratios

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Wenjiang Wu ◽  
Chan Y. Ching
Keyword(s):  
Natural Convection ◽  
Aspect Ratio ◽  
Wall Temperature ◽  
Vertical Wall ◽  
Flow Region ◽  
Temperature Profiles ◽  
Aspect Ratios ◽  
Vertical Walls ◽  
C And N ◽  
Laminar Natural Convection

The effect of the top wall temperature on the laminar natural convection in air-filled rectangular cavities driven by a temperature difference across the vertical walls was investigated for three different aspect ratios of 0.5, 1.0, and 2.0. The temperature distributions along the heated vertical wall were measured, and the flow patterns in the cavities were visualized. The experiments were performed for a global Grashof number of approximately 1.8×108 and nondimensional top wall temperatures from 0.52 (insulated) to 1.42. As the top wall was heated, the flow separated from the top wall with an undulating flow region in the corner of the cavity, which resulted in a nonuniformity in the temperature profiles in this region. The location and extent of the undulation in the flow are primarily determined by the top wall temperature and nearly independent of the aspect ratio of the cavity. The local Nusselt number was correlated with the local Rayleigh number for all three cavities in the form of Nu=C⋅Ran, but the values of the constants C and n changed with the aspect ratio.

Experimental and Numerical Investigation on Natural Convection in Horizontal Channels Partially Filled With Aluminium Foam and Heated From Below

2016 ◽  
Author(s):  
Bernardo Buonomo ◽  
Oronzio Manca ◽  
Sergio Nardini ◽  
Alessandra Diana
Keyword(s):  
Heat Flux ◽  
Natural Convection ◽  
Wall Temperature ◽  
Aluminum Foam ◽  
Rectangular Channel ◽  
Lower Surface ◽  
Heated Wall ◽  
Working Fluid ◽  
Bottom Plate ◽  
Uniform Heat Flux

Natural convection in horizontal rectangular channel without or with aluminum foam is experimentally and numerically investigated. In the case with aluminum foam the channel is partially filled. In both cases, the bottom wall of the channel is heated at a uniform heat flux and the upper wall is unheated and it is not thermally insulated to the external ambient. The experiments are performed with working fluid air. Different values of wall heat flux at lower surface are considered in order to obtain some Grashof numbers and different heated wall temperature distributions. Two different aluminum foams are considered in the experimental investigation, one from “M-pore”, with 10 and 30 pore per inch (PPI), and the other one from “ERG”, with 10, 20 and 40 PPI. The numerical simulation is carried out by a simplified two-dimensional model. It is found that the heat transfer is better when the channel is partially filled and the emissivity is low, whereas the heated wall temperature values are higher when the channel is partially filled and the heated bottom plate has high emissivity. The investigation is achieved also by flow visualization which is carried out to identify the main flow shape and development and the transition region along the channel. The visualization of results, both experimental and numerical, grants the description of secondary motions in the channel.

The Experimental Study of Single Droplet Impinging on an Inclined Heated Wall

2020 ◽  
Author(s):  
Wenlong Tian ◽  
Huang Zhang ◽  
Qianfeng Liu ◽  
Guang Hu ◽  
Wen He
Keyword(s):  
Inclination Angle ◽  
Wall Temperature ◽  
Weber Number ◽  
High Speed ◽  
Conservation Equation ◽  
Heated Wall ◽  
Single Droplet ◽  
Droplet Spreading ◽  
Energy Conservation Equation ◽  
Inclination Angles

Abstract We investigated a single droplet impinging on an inclined heated wall with different inclination angles. A high-speed camera was used to observe this impinging process at 10000 frames / second. The phenomena of the droplet spreading, shrinking, rebounding, boiling, break-up and splashing were observed. The effects of the wall temperature (Tw, 40–262 °C), the Weber number of the droplet (Wed, 0.66–589) and the wall inclination angle (α, 0–45.6°) on the spreading behavior of the droplet after impinging on the wall were analyzed. Energy conservation equation was used to analyze experimental results. The results show that increasing the inclination angle of the wall is beneficial to the forward spreading of the droplet along the wall, but not to the downward spreading. When α > 45.6°, the droplet will break through the flow resistance of the wall and slide down all the time. The increase of the wall temperature and the Weber number of the droplet is beneficial to the backward and forward spreading of the droplet. Increasing α, increasing Tw and decreasing Wed will promote the shrinking of the droplet after spreading. In addition, the experimental phenomenon also shows that the larger the wall inclination and the higher the wall temperature, the easier the droplet will break away from the wall.

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