Latent heat

About: Latent heat is a research topic. Over the lifetime, 13503 publications have been published within this topic receiving 302811 citations.

Sensible and latent heat loss from the body surface of Holstein cows in a tropical environment

Abstract: The general principles of the mechanisms of heat transfer are well known, but knowledge of the transition between evaporative and non-evaporative heat loss by Holstein cows in field conditions must be improved, especially for low-latitude environments. With this aim 15 Holstein cows managed in open pasture were observed in a tropical region. The latent heat loss from the body surface of the animals was measured by means of a ventilated capsule, while convective heat transfer was estimated by the theory of convection from a horizontal cylinder and by the long-wave radiation exchange based on the Stefan–Boltzmann law. When the air temperature was between 10 and 36°C the sensible heat transfer varied from 160 to –30 W m−2, while the latent heat loss by cutaneous evaporation increased from 30 to 350 W m−2. Heat loss by cutaneous evaporation accounted for 20–30% of the total heat loss when air temperatures ranged from 10 to 20°C. At air temperatures >30°C cutaneous evaporation becomes the main avenue of heat loss, accounting for approximately 85% of the total heat loss, while the rest is lost by respiratory evaporation.

Modelling liquid-solid phase changes with melt convection

Abstract: SUMMARY Methods are described for modelling of phase change processes using the finite element method to simulate freezing and melting including convection in the melt. Evaluation of several enthalpy/specific heat methods and time marching schemes is also included. Suppression of velocities in the solid region is described, and example problems are given. Comparison is made to simulations performed by other researchers using finite difference methods. Substantially different results were found for one of these problems, and this result is shown to be caused by numerical problems in the earlier work. strong effect on the resulting microstructure. A number of researchers have shown reorientation of columnar grains,' alteration of the size and location of equiaxed zones' and macro~egregation,~. all due to melt convection. Mathematical models have been used in attempts to better understand the processes and thus control them. Although the most convenient mathematical models would use analytical solutions to the coupled heat and momentum transport equations, very few such solutions exist for these problems, and none would extend to the realistic problems where complicated geometries and temperature dependent material properties are included. For this reason, nearly all of the efforts in this area have been numerical. There are different types of numerical methods which are appropriate to phase change problems, depending on the kind of material involved. In pure materials, eutectics or congruent melting phases, the liquid-solid interface is sharp and corresponds to an isotherm. For these kinds of problems it may be appropriate to have part of the mesh coincide with the solidification front at all times, and distort the mesh in both phases as the boundary moves. A number of these front- tracking methods have been de~eloped,~, but none exists for three-dimensional problems. For alloys which freeze over a range of temperatures, front-tracking methods are no longer applicable. Instead, what is normally done is to specify the evolution of latent heat over a freezing range as part of the material properties. The phase boundaries are then recovered from the solution as the isotherms corresponding to the beginning and end of the transformation. The two

Simulation of the Temperature, Humidity and Evaporation Profiles in a Leaf Canopy

Abstract: The proposed model synthesizes profiles of temperature, humidity and evaporation in a canopy of leaves from meteorological conditions at canopy top, from the temperature and humidity at the soil surface, from a leaf dimension, from the vertical distribution of leaf area and stomatal resistance, and from observations or extinction coefficients for ventilation and radiation within the canopy under steady-state conditions. The exchange of sensible and latent heat in a canopy stratum is required to be equal to the absorption of radiation by the leaves in that stratum. Further, the difference between strata in their potential for sensible and latent heat exchange is related both to leaf temperature and to the fluxes and diffusive resistances between the leaves. Leaf temperatures, evaporation and sensible heat exchange, and air temperatures within the canopy that meet these requirements were calculated by successive approximation. The microclimate and evaporation of a red clover and of a barley canopy ...