Showing papers on "Latent heat published in 1990"

A Theory for Long-Lived Mesoscale Convective Systems

Abstract: It is proposed that certain long-lived mesoscale convective systems maintain themselves through an interaction between quasi-balanced vertical motions and the diabatic effects of moist convection. Latent heat release, evaporation and melting of precipitation, and thermal radiation are all shown to contribute to the creation of a positive potential vorticity anomaly in the lower troposphere. This anomaly can interact with a sheared environment so as to induce further lifting of low-level air and subsequent release of conditional instability.

Physical properties of foods and food processing systems

Abstract: Units and Dimensions Density and Specific Gravity Properties of Fluids, Hydrostatics and Dynamics Viscosity Solid Rheology and Texture Surface Properties Introduction to Thermodynamic and Thermal Properties Sensible and Latent Heat Changes Heat Transfer Mechanisms Unsteady-state Heat Transfer Properties of Gases and Vapours Electrical Properties Diffusion and Mass Transfer Bibliography and references Index.

Observed Effects of Landscape Variability on Convective Clouds

Abstract: A case is presented in which clouds are observed to form first over a mesoscale-size area (100 x 300 km) of harvested wheat in Oklahoma, where the ground temperature is warmer than adjoining areas dominated by growing vegetation. In addition, clouds are suppressed over relatively long bands downwind of small man-made lakes and areas characterized by heavy tree cover. The observed variability of cloud relative to landscape type is compared with that simulated with a one-dimensional boundary-layer model. Clouds form earliest over regions characterized by high, sensible heat flux, and are suppressed over regions characterized by high, latent heat flux during relatively dry atmospheric conditions. This observation has significance in gaining understanding of the feedback mechanisms of land modification on climate, as well as understanding relatively short-range weather forecasting.

Infrared measurement of canopy temperature and detection of plant water stress

Abstract: Leaf temperature is directly determined by leaf energy and water balance. If diminished water availability decreases latent heat flux at the leaf surface, a complementary increase of sensible heat will occur and create a larger temperature difference between foliage and air. Radiation, air temperature, humidity and wind speed modify leaf temperature and may mask indications of water stress. The position, inclination and orientation of leaves within the canopy also produce considerable variation of leaf temperature. These factors were incorporated in a linear transpiration model, using physical and physiological characteristics of cotton. Water stress was simulated by imposing a limit value for stomatal conductance. The energy balance equation was solved as a function of angle between leaf and solar beam, to determine leaf temperature frequency distribution. The results show that stress induced temperature rise occurs over a small percentage of the total leaf area. Detection of moderate stress requires a normalizing procedure which takes into account meteorological conditions. The leaf temperature distribution is a better indicator of stress than the average value.

Estimates of Evapotranspiration with a One- and Two-Layer Model of Heat Transfer over Partial Canopy Cover

Abstract: One of the applications of remotely sensed surface temperature is to determine the latent heat flux (LE) or evapotranspiration (ET) from held to regional scales A common approach has been to use surface-air temperature differences in a bulk resistance equation for estimating sensible beat flux, H, and to subsequently solve for LE as a residual in the one-dimensional energy balance equation This approach has been successfully applied over uniform terrain with nearly full, actively transpiring vegetative cover; however, serious discrepancies between estimated and measured ET have been observed when there is partial canopy cover In an attempt to improve the estimates of H and as a result compute more accurate values of ET over partial canopy cover, one- and two-layer resistance models are developed to account for some of the factors causing the poor agreement between computed and measured ET The utility of these two approaches for estimating ET at the field scale is tested with remotely sensed a

A Prediction Model for Snowmelt, Snow Surface Temperature and Freezing Depth Using a Heat Balance Method

Abstract: A snowmelt model based on a heat balance method has been developed. This model takes into account both the heat balance at the snow surface and that of the entire snow cover and simultaneously predicts the snow surface temperature and freezing depth. Observed or estimated incident radiation data are required for operation of the model. Calculated amounts of snowmelt and snow surface temperatures were in agreement with those observed. Dependency of snowmelt on several parameters including maximum liquid water content, thermal conductivity and albedo of the snow was examined. It was found that as liquid water content or thermal conductivity increases, snowmelt decreases. Albedo is very influential in evaluating snowmelt. Runoff from a basin having an area of 583 km2 was estimated using the present model, and was verified by the inflow data to a dam.

Modeling Multicomponent Aerosol Particle Growth By Vapor Condensation

Abstract: A new “moving-sectional” method is presented to solve the dynamics of multicomponent aerosol particle growth by vapor condensation in a closed system. The method controls numerical diffusion and stiffness by adapting the method of characteristics to a sectional representation of the aerosol. An exact solution for a closed system is presented, and the “moving-sectional” method gives excellent agreement when tested against this solution. Four cases are presented to demonstrate the importance of the solute, Kelvin, and latent heat effects.

Thermal Performance of a Heat Storage Module Using PCM’s With Different Melting Temperature: Experimental

Abstract: A latent heat storage module was constructed, consisting of 45 cylindrical capsules fixed vertically in 15 rows. The capsules, made of 0.335-m long copper tubes having external diameters of 31.8 mm, were fixed in an insulated rectangular duct. Three commercial waxes having melting temperatures of 44°C, 53°C, and 64°C were selected. Each of the three sets of 15 tubes was filled with different wax. For comparison purposes, experiments were also done with a single commercial wax, having a melting temperature of 53°C, in all the tubes. During heat charge, hot air flowed across the capsules such that the melting temperature of the waxes decreased in the flow direction. Air flow direction was reversed during heat discharge. Experimental measurements showed some improvement in the heat transfer rates during both heat charge and discharge when three types of PCM’s were used. There was no improvement in the heat transfer rate during the sensible heat storage period, while a maximum increase of 15 percent was observed during the latent heat period. Theoretical predictions for the performance of the storage module were in reasonable agreement with the experimental measurements.

Characteristics of Cumulus Band Clouds off the Coast of Hawaii

Abstract: We have analyzed aircraft observations from seventeen cumulus cells within cloud bands observed off the east coast of Hawaii during the Joint Hawaii Warm Rain Project (JHWRP) of 1985. Low level convergence generated by the encounter of the trade winds and the island determines the location of the initial convection. However, the upward momentum below cloud base seems to be less important in the subsequent evolution of the bands than buoyancy associated with latent heat release. Entrainment into the clouds occurs at all levels, and almost all cloudy parcels below the trade inversion are moving upwards. While evaporative cooling does not seem to enhance entrainment below the inversion, it does play a role in the descent of cloud top air to the bottom of the inversion. Despite the existence of undiluted cores, the average thermodynamic characteristics of the clouds below the inversion appear well described by a very simple, constant lateral entrainment rate parcel model. We have used in-cloud and ne...

Boundary-layer heat and moisture budgets from fife

Abstract: Aircraft stacks were flown upwind and downwind of the First ISLSCP Field Experiment (FIFE) site in Kansas to measure the heat and moisture budgets of the boundary layer under fairly clear skies for four daytime periods. In this paper, we evaluate the terms in the conservation equation. The vertical flux divergence and advection do not account for the difference between surface and low-level aircraft flux estimates. Budget estimates of the surface fluxes using the aircraft data agree well with surface flux measurements, but extrapolation of the aircraft fluxes gives surface fluxes that are too low. With the 5 km cutoff filter used, the aircraft underestimate for sensible heat flux is about 40%, and for the latent heat flux about 30%. Part of the underestimation is attributable to long-wavelength contributions (longer than the 5 km filter), but more investigation is needed.

The effect of weather variability on the energy balance of a lake in the hudson bay lowlands, canada

Abstract: Evaporation is an important component of the water balance of lakes in the Hudson Bay Lowlands, but the amount of summer evaporation in this area is not well known. Hourly summertime estimates of evaporation, from a small tundra lake near Churchill, Manitoba, are compared to equilibrium evaporation estimates of the Penman model. For the entire measurement period a = 1.35. For individual days a ranges from 1.0 to 2.0 and for individual hours from 1.0 to 4.0. Local advection is primarily responsible for the large fluxes of latent heat. The latent heat flux exceeds available radiant energy over the summer, with temperature inversions occurring over the lake on the majority of days. The advective enhancement of lake evaporation responds to daily weather variations. The classification of these variations will improve operational estimates of lake evaporation.

An investigation of a latent heat storage porous bed and condensing flow through it

Abstract: In this work the transient analysis of the behavior of a packed bed of encapsulated phase change material (PCM) and the condensing flow through it is presented. The rigorous model used assumes no local thermal equilibrium between the bed particles and working fluid, and incorporates the inertia effects int eh momentum transport by the use of the Ergun-Forchheimer equation. Condensation in the working fluid is investigated. Thermal charging of the packed bed is analyzed and compared for a sensible heat storage material as well as for different latent heat storage materials (PCMs).

Thermal forcing on the tropical Pacific from satellite data

Abstract: Monthly fields of latent heat flux and surface solar irradiance derived from spaceborne sensors were combined to estimate the surface thermal forcing on the tropical Pacific from 1980 to 1983. The annual cycle and the anomalies associated with 1982-1983 ENSO episode were examined. Along the equator and during the early phase of the episode the reduction of solar irradiance into the ocean is found to be compensated to some extent by the decrease in evaporative cooling. The distribution of correlation coefficients demonstrated that outside the equatorial waveguides, surface thermal forcing plays a significant role in the seasonal change of sea surface temperature and surface solar irradiance is main driving force. Significant correlation is also found between anomalous latent heat flux and anomalous change of sea surface temperature over a broad area in the equatorial and southern tropical Pacific, indicating that surface thermal forcing may play a larger role than expected in the anomalous sea surface temperature change, particularly in the reestablishment of the cold tongue at end of the episode.

An observational aircraft-based study of sea-breeze frontogenesis

Abstract: In the summer of 1988/89 flights were carried out in the Coorong coastal area of South Australia to investigate sea-breeze fronts. The flights yielded data sets of the structure of the fronts in the cross-frontal direction with a spatial resolution of approximately 3 m. The study is focused on the budgets of sensible and latent heat in the vicinity of the front and on frontogenesis/frontolysis processes which are closely related to budget considerations.

A Tropical Squall Line Observed during the COPT 81 Experiment in West Africa. Part III: Heat and Moisture Budgets

Abstract: The role of moist convective processes in the heat and moisture budgets of the 22 June 1981 tropical squall line is investigated. Detailed kinematic structure from Doppler radar observations, and thermodynamic and microphysical fields diagnosed from a steady-state model respectively presented in Parts I and II of this paper, are used to estimate the total apparent heat source and moisture sink. The relative contributions of the system components (the convective-scale component associated with the leading convective region, and the mesoscale component of the trailing stratiform region) and of the physical processes (latent heat release and eddy transports) are examined. Consistent results are examined which are obtained in qualitative agreement with those of previous studies. Condensation and evaporation, through the release of latent heat, are the dominant terms in the total apparent heating profile although the melting process is also important. The total apparent moisture sink is also dominated...

Energy sources for Triton's geyser-like plumes

Abstract: Four geyser-like plumes were discovered near Triton's south pole in areas now in permanent sunlight. Because Triton's southern hemisphere is nearing a maximum summer solstice, insolation as a driver or a trigger for Triton's geyser-like plumes is an attractive hypothesis. Trapping of solar radiation in a translucent, low-conductivity surface layer (in a solid-state greenhouse), which is subsequently released in the form of latent heat of sublimation, could provide the required energy. Both the classical solid-state greenhouse consisting of exponentially absorbed insolation in a gray, translucent layer of solid nitrogen, and the 'super' greenhouse consisting of a relatively transparent solid-nitrogen layer over an opaque, absorbing layer are plausible candidates. Geothermal heat may also play a part if assisted by the added energy input of seasonal cycles of insolation.

Investigation of Biogeophysical Feedback on the African Climate Using a Two-Dimensional Model

Abstract: A numerical scheme is specifically designed to develop a time-dependent climate model to ensure the conservation of mass, momentum, energy, and water vapor, in order to study the biogeophysical feedback for the climate of Africa. A vegetation layer is incorporated in the present two-dimensional climate model. Using the coupled climate-vegetation model, two tests were performed involving the removal and expansion of the Sahara Desert. Results show that variations in the surface conditions produce a significant feedback to the climate system. It is noted that the simulation responses to the temperature and zonal wind in the case of an expanded desert agree with the climatological data for African dry years. Perturbed simulations have also been performed by changing the albedo only, without allowing the variation in the vegetation layer. It is shown that the variation in latent heat release is significant and is related to changes in the vegetation cover. As a result, precipitation and cloud cover are reduced.

An Algorithm for Retrieving Water Vapor Profiles in Clear and Cloudy Atmospheres from 183 GHz Radiometric Measurements: Simulation Studies

Abstract: The latent heat represented by atmospheric water vapor is extremely important to the energetics of the earth system. Future satellites (NOAA and DMSP) will carry microwave radiometers designed to measure the profile of water vapor globally. The problem of retrieving water vapor from the measurements is highly nonlinear even in clear atmospheres and the addition of clouds only makes it more so. In this paper, an algorithm with several novel features, which will retrieve water vapor profiles from microwave radiometric measurements even in the presence of clouds, is developed. Simulations with this algorithm show a vertical resolution on the order of 3 km and that clouds are well handled in many, but not all, circumstances. The most surprising result is that clouds can actually improve the vertical resolution of the retrieval.

Method of and means for using a two-phase fluid for generating power in a rankine cycle power plant

Abstract: A method for using a two-phase fluid includes separating the fluid into its two phases, oen of which is a hot gas containing energy in the form of latent heat, and one of which is a hot liquid containing energy in the form of sensible heat; converting sensible heat in the liquid to sensible heat in a working fluid for producing preheated working fluid; and transferring latent heat in the gas to the preheated working fluid for vaporizing the same at substantially constant temperature and pressure.

Growth Instability During Nonuniform Directional Solidification of Pure Metals

Abstract: Solidification of a pure metal against a flat mold surface is examined using a nonuniform thermohypoelastic beam. Heat is removed with a spatially periodic heat flux superimposed onto uniform cooling, which leads to irregular growth of the casting. The associated nonuniform temperature field also produces nonuniform deformation in the casting which in time may lead to air gap nucleation along the mold/casting interface. Air gap nucleation is defined as the circumstance where the local mold/casting interface pressure falls to zero. Unstable growth occurs when such air gaps nucleate beneath thickness minima, thus further reducing heat transfer at these positions, and dramatically amplifying the existing thickness nonuniformity. The time to onset of instability depends upon material properties such as density, latent heat, thermal conductivity, elastic constants, and coefficient of thermal expansion as well as upon process parameters such as cooling rate, fluid pressure, and wavelength of the periodic heat flux.

On the Mechanism of Forced-Convection Subcooled Nucleate Boiling

Abstract: In highly subcooled nucleate boiling the bubbles grow and collapse while sliding along the heated surface, so that there is no net vapor transport away from the surface. A long-standing question exists as to whether conduction and convection between the bubbles, or latent heat transport through the bubbles, is the dominant heat transfer mechanism. It is shown here by simple calculations that the sliding of the bubble results in augmentation of microlayer evaporation under the bubble by a factor of two or more over a stationary bubble with a continuous microlayer. It appears therefore that the latent heat transport mechanism may be dominant at large bulk subcoolings.

Cooling process and apparatus

Abstract: Material to be frozen is subjected to a cooling process which involves the efficient removal of latent heat of freezing. This can be achieved by subjecting the material being frozen to a greater rate of heat extraction when the latent heat is being given up than when the then solid material is being subsequently cooled further. Efficient removal of latent heat is also facilitated by inducing nucleation of the frozen liquid. Nucleation can be initiated acoustically and/or chemically. The invention, which has particular application in the frozen food industry and in the cryopreservation of biological material, allows shorter freezing times and/or improved quality or viability of the frozen product.

Thermo‐mechanical finite element model of casting systems

Abstract: A coupled finite element model is developed to simulate the metal casting process. We present a new method for capturing the solidification zone and obtaining the corresponding rate of phase change. The latent energy release is predicted by the heat conduction rate and introduced by following the enthalpy-temperature curve. The influence of mould-metal contact is considered in calculating the heat flux through the mould-metal interface. A viscoplastic model is employed to predict the gap opening and evolution of plastification. Four examples are presented to demonstrate certain numerical aspects and the capability of the model for industrial applications.

Residential hybrid air conditioning system

Abstract: A residential-type hybrid air conditioning system having a conventional refrigeration subsystem which handles system sensible heat loads and a liquid desiccant dehumidification subsystem which handles system latent heat loads additionally incorporates an evaporative cooler which cooperates with and receives heat from the dehumidification subsystem to increase the performance efficiency of the total system.