Critical size of newborn homeotherms

1977 ◽  
Vol 42 (4) ◽  
pp. 571-577 ◽  
Author(s):  
R. T. Balmer ◽  
A. D. Strobusch
Keyword(s):  
Heat Transfer ◽  
Body Size ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Critical Radius ◽  
Critical Size ◽  
The Body ◽  
Birth Size ◽  
Critical Weight

It is shown that for cylindrical and spherical bodies there is a critical radius below which the addition of any form of insulation to the body will increase rather than decrease the cooling of the body. It is proposed, therefore, that it would be thermally detrimental to newborn homeotherms to be born with a protective covering (fur or down) if their body size were less than this critical size, and consequently that the degree of natal covering is not necessarily related to the overall development of the species when the birth size is less than this critical size. A critical weight is derived from the critical radius for basically spherical animals which compares favorably with typical birth weights of various altricial homeotherms. The effect of the overall conductive-convective heat transfer caused by a basically cylindrical animal rolling up into a ball is also discussed.

Numerical Study of Natural Convection Around an Isolated Horizontal Cylinder and Determination of Critical Radius Effect With a Variable Heat Transfer Coefficient

2009 ◽  
Author(s):  
S¸. O¨zgu¨r Atayılmaz ◽  
Hakan Demir ◽  
O¨zden Agra
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Critical Radius ◽  
Convective Heat Transfer Coefficient ◽  
Horizontal Cylinder ◽  
Insulation Thickness

Natural convection heat transfer from an insulated horizontal cylinder is studied analytically and numerically. Curved surfaces such as circular cylinder which has a radius smaller than a certain critical size, adding insulation to the surface increases the heat transfer form the surface. This phenomenon occurs if the effects of the increase of the outer surface area on the heat transfer are higher than the decrease by the total thermal resistance of the insulated cylinder. The critical radius is represented as a function of thermal conductivity of the object and convective heat transfer coefficient in the textbooks on heat transfer. This is only valid if both thermal conductivity and convective heat transfer coefficient are constant. In fact, the convective heat transfer coefficient varies with outer diameter of the cylinder while thermal conductivity can be taken as constant. Therefore, a numerical and an analytical study were performed in order to investigate the effects of variable heat transfer coefficient on determining the critical radius. For this aim an isolated horizontal cylinder having different insulation thickness and a constant thermal conductivity was modeled and solved numerically using FLUENT CFD software. Also the same problem was solved analytically and numerical and analytical results were compared. The variation of the total heat transfer from cylinder surface according to insulation thickness is obtained. It is found that the standard critical radius criterion led to significant errors compared to numerical results.

Mechanisms of Convective Heat Transfer of Nanofluids

2008 ◽  
Author(s):  
Dongsheng Wen
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Single Channel ◽  
Effective Properties ◽  
Effective Property ◽  
The Body ◽  
Two Phase ◽  
Continuous Equation

Research on nanofluids has progressed rapidly since its enhanced thermal conductivity was identified about a decade ago. Much evidence shows that the enhancement of convective heat transfer is much higher than that of thermal conductivity only. The mechanism of such enhancement, however, is still unclear. This work reviews the mechanisms of convective heat transfer of nanofluids in a single channel, and identifies two most likely mechanisms: the modification of effective properties and the migration of nanoparticles under flow conditions. A numerical simulation based on a combined Euler and Lagrange method is investigated in this work to illustrate the feature of nanoparticle migration and the drawback of the effective property approach. The motion of discrete nanoparticles is determined by the Lagrangian trajectory method based on the Newton’s second law that includes influence of the body force, various hydrodynamic forces, and the Brownian and thermophoresis forces. The coupling of discrete particles with continuous flow is realized through the modification of the source term of the continuous equation. It concludes that the two-phase flow nature of nanofluids, especially the nanoparticle migration and the resultant non-uniform particle and effective property profile, needs to be considered to properly model the convective heat transfer.

Analytical Solution for Temperature Distribution in a Multilayer Body With Spatially Varying Convective Heat Transfer Boundary Conditions on Both Ends

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
Long Zhou ◽  
Mohammad Parhizi ◽  
Ankur Jain
Keyword(s):  
Heat Transfer ◽  
Temperature Distribution ◽  
Boundary Conditions ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Thermal Conduction ◽  
The Body ◽  
Theoretical Understanding ◽  
Linear Algebraic Equations ◽  
Spatially Varying

Abstract Analytical modeling of thermal conduction in a multilayer body is of practical importance in several engineering applications such as microelectronics cooling, building insulation, and micro-electromechanical systems. A number of analytical methods have been used in past work to determine multilayer temperature distribution for various boundary conditions. However, there is a lack of work on solving the multilayer thermal conduction problem in the presence of spatially varying convective heat transfer boundary condition. This paper derives the steady-state temperature distribution in a multilayer body with spatially varying convective heat transfer coefficients on both ends of the body. Internal heat generation within each layer and thermal contact resistance between layers are both accounted for. The solution is presented in the form of an eigenfunction series, the coefficients of which are shown to be governed by a set of linear, algebraic equations that can be easily solved. Results are shown to be in good agreement with numerical simulation and with a standard solution for a special case. The model is used to analyze heat transfer for two specific problems of interest involving spatially varying convective heat transfer representative of jet impingement and laminar flow past a flat plate. In addition to enhancing the theoretical understanding of multilayer heat transfer, this work also contributes toward design and optimization of practical engineering systems comprising multilayer bodies.

Convective heat transfer measured directly with a heat flux sensor

1990 ◽  
Vol 68 (3) ◽  
pp. 1275-1281 ◽  
Author(s):  
U. Danielsson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Convective Heat Transfer Coefficient ◽  
The Body ◽  
Heat Flux Sensor ◽  
Calibration Constant

A heat flux disk has been developed that directly measures the convective heat transfer in W/m2. When the sensor is calibrated on an aluminum cylinder, the calibration constant obtained is greatest in still air. As air movement increases, the calibration constant is reduced with increasing convective heat transfer coefficient, 0.5%.W-1.m2.K. The influence of wind on the calibration value is greatly reduced when the sensor is attached to a surface with lower thermal conductivity. The local convective heat transfer coefficient (hc) of the human body was measured. The leg acts in a manner similar to that of a cylinder, with the highest hc value at the front facing the wind and the lowest approximately 90 degrees from the wind, and in the wake a value is obtained that is close to the average hc value of the leg. When hc is measured at several angles and positions all over the body, the results indicate that the body acts approximately as a cylinder with a hc value related to the wind speed as hc = 8.6.v0.6 W.m-2.K-1, where v is velocity.

Experimental study of convective heat transfer coefficient for the human body in water

1977 ◽  
Vol 42 (1) ◽  
pp. 93-100 ◽  
Author(s):  
C. Boutelier ◽  
L. Bougues ◽  
J. Timbal
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Body Shape ◽  
Cold Water ◽  
Convective Heat Transfer Coefficient ◽  
The Body ◽  
Partitional Calorimetry

The steady-state convective heat transfer coefficient in water has been determined by partitional calorimetry for 17 nude subjects. Four water velocities were investigated: 0, 0.05, 0.10, and 0.25 m-s-1; and the water temperature ranged from 33.7 to 18 degrees C. In still water, hc varied from 43 W-m-2-degrees C-1 in thermoneutral conditions and a shivering rate less than 90 W-m-2 to 54 W-m-2-degrees C-1 in cold water with a shiver rate greater than 110 W-m-2. The equation, hc=0.09 (Gr-Pr)0.275, give a good approximation of this coefficient. In stirred water and for the same limits of shivering, hc can be expressed as a power function of the velocity: hc = 272.9 v0.5 and hc = 497.1 v0.65, respectively. These equations show that the flow is laminar in thermoneutral conditions and intermediate between laminar and turbulent in cold water. A study of the influence of skinfold on the magnitude of hc shows that higher values of this coefficient were obtained for thin subjects than for fat ones, concomitant with more intense shivering. The utilization of a theoretical physical model for computations of hc gave excessively high values because such methods do not embody the body shape factor and reduction of water flow adjacent to the skin.

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